In Situ Recovery & Remediation of Metals 0873354869, 9780873354868

Citation preview

IN SITU R REMEDIATION ECOVERY & OF METALS

By Drummond Earley III Color map hydraulic potential from –2.5 log (white) bar to 1.5 log bar (red) mid –0.5 log bar (yellow) Contour hydraulic potential from –2.5 log bar by interval of 0.25

10 m (32.8 ft)

Hydraulic Potential (log bar) 10 m (32.8 ft)

–2.5

By Drummond Earley III

–0.5 1 bar = 14.5 psi

1.5

IN SITU RECOVERY &

REMEDIATION OF METALS

Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

IN SITU RECOVERY &

REMEDIATION OF METALS By Drummond Earley III

Published by the Society for Mining, Metallurgy & Exploration

Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Society for Mining, Metallurgy & Exploration (SME) 12999 E. Adam Aircraft Circle Englewood, Colorado, USA 80112 (303) 948‑4200 / (800) 763‑3132 www.smenet.org The Society for Mining, Metallurgy & Exploration (SME) is a professional society whose more than 15,000 members represent all professionals serving the minerals industry in more than 100 countries. SME members include engineers, geologists, metallurgists, educators, students, and researchers. SME advances the worldwide mining and underground construction community through information exchange and professional development. Information contained in this work has been obtained by SME from sources believed to be reliable. However, neither SME nor its authors and editors guaran‑ tee the accuracy or completeness of any information published herein, and neither SME nor its authors and editors shall be responsible for any errors, omissions, or damages arising out of use of this information. This work is published with the understanding that SME and its authors and editors are supplying information but are not attempting to render engineering or other professional services. Any state‑ ment or views presented herein are those of individual authors and editors and are not necessarily those of SME. The mention of trade names for commercial products does not imply the approval or endorsement of SME. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. ISBN 978-0-87335-486-8 eBook 978-0-87335-487-5 Copyright © 2020 Society for Mining, Metallurgy & Exploration Inc. Electronic edition published 2020. Library of Congress Control Number: 2020932263 All Rights Reserved. Printed in the United States of America.

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Contents

Preface������������������������������������������������������������������������������������������������������������������������������������������������������vii Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix Chapter 1

Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Chapter 2

Ore Formation by Aqueous Solutions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Chapter 3

Flow and Reactive Transport in Geologic Media. . . . . . . . . . . . . . . . . . . . . . . . . . 23

Chapter 4

Geochemistry and Hydrometallurgy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Chapter 5

Drilling and Well-Field Technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Chapter 6

Advanced Rock Mass and Ore Characterization. . . . . . . . . . . . . . . . . . . . . . . . . . 81

Chapter 7

In Situ Copper Sulfide Recovery Project. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Chapter 8

Economics and Permitting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

Chapter 9

Remediation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

Chapter 10 Future Directions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 References����������������������������������������������������������������������������������������������������������������������������������������������� 141 Index ��������������������������������������������������������������������������������������������������������������������������������������������������������� 151



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Preface The only preface line I remember from any book is in Frederick J. Sawkins’ first edition of Metal Deposits in Relation to Plate Tectonics, which starts with the following phrase: “I attempt this volume with no small degree of trepidation.” Quoting this line with respect to in situ recovery is an understatement. It is diffi‑ cult to justify in situ extraction to the public as it is an inherently seclusive tech‑ nology that impacts the environment, but no more so than other natural resource development activities we see every day. As restrictions and technical challenges to traditional mining grow, the need for less-visible and low-waste sustainable mineral extraction technologies has never been more needed. In addition, new advances in material science have created demand for exotic electronic materi‑ als such as rare earth elements. Renewable energy based on electricity has also increased demand for copper, lithium, and other so-called battery metals (Co, Ni, etc.). Moreover, mining fiercely competes with other industries, agriculture, and municipalities for scarce water resources. The need to reduce water consump‑ tion is also driving innovations in mining and interest in mining alternatives. Shortly before finishing my doctorate in geology at the University of Minnesota, my professional career officially began when I landed a job as a geologist with the now-defunct U.S. Bureau of Mines. My current moti‑ vation for this book primarily stems from my experience as a geologist and geochemist working on research and development and commercial mining projects and recognition that there is a need for less-invasive technologies for mineral extraction. Paraphrasing Lewis Wade, the former director of the U.S. Bureau of Mines Twin Cities Research Center, “miners in the future will be using a scalpel rather than dynamite.” The scope of our research and development covered the nano- to mac‑ roscale aspects of in situ recovery and remediation (ISRR). I was initially hired as a mineralogist and petrologist to characterize ore samples before and after leaching. We used traditional microscopy, scanning electron microscopy, electron microprobe analysis, and transmission electron microscopy to detect the movement of leach solutions through macro- and microscopic pores and fractures in the rock. This work heightened my interest in coupled fluid flow and reaction, otherwise known in environmental vernacular as reactive transport. The reactions and reaction products resulted in changes in porosity and

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viii Preface

permeability, which is another subject that fascinated me. In addition, as a geologist, I could see the connection between the ore-forming process and in situ recovery. In a sense, we were reversing the processes that created the ore deposit. These insights helped explain observed well-field scale phenomenon and metal recovery characteristics during leaching tests and pilot studies that the Bureau conducted in the 1990s. In addition, pilot-scale tests were also being conducted by mining companies. Ultimately, permeability, well per‑ formance, metal dissolution rates, and metal grades in solution dictate the feasibility and economics of in situ recovery. I decided to call this book In Situ Recovery and Remediation of Metals because the traditional term in situ mining is hamstrung by the inclusion of the word mining, which has many different historic connotations and visualizations to different people. In addition, in situ recovery is the recent vernacular more commonly used than in situ mining to describe modern commercial projects. The real objective is the extraction of metals that can be achieved in some cases without traditional mining technologies. I also prefer to avoid resistance to my ideas on both sides of the debate by not using the term mining. Regardless of where one is on the spectrum, there is a practical need for new extractive technologies that reduce or eliminate traditional mining meth‑ ods. This is because of the simple fact that the mineral resources of the future are deeper and lower grade than ever before, and we are reaching the limit of surface and underground mining technologies in some cases. In South Africa, for example, mines are being shuttered not because the ore has played out, but because it extends to depths too extreme for human workers. Recently, the Chuquicamata open pit mine in Chile was closed, and operations went underground because of the technical limitations of maintaining slope stabil‑ ity in mega open pits. I also decided to include remediation in my book’s title because in situ extraction, even in its least invasive form, likely requires some remedial activ‑ ity at the end of the extraction phase to protect adjacent groundwater and other resources. Indeed, the application of in situ technologies for environ‑ mental remediation has become apparent so that the technology and advances of in situ metal and mineral extraction can also be applied to remediation. For example, at the Berkeley pit, rather than being treated and disposed, the acidic pit lake water is now used to produce copper. This is a concept that has been utilized for several decades at operating mines, but is now being embraced at Superfund sites by the U.S. Environmental Protection Agency, which regulates metals as contaminants rather than as commodities. This book is intended for a wide audience: geologists, hydrologists, mining engineers, hydrometallurgists, and others with interest in this topic. As such, I Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Preface

ix

have tried to maintain a writing style that is familiar with the diverse audience but is likely to deviate from the standard texts in any one discipline. The content is not encyclopedic but more conceptual; several papers about in situ mining projects and pilot studies have been summarized and cataloged. The concepts introduced, however, are supported with examples and numerical rigor so that the practitioner can apply the principles to real-life projects. I show that standard chemical engineering approaches, such as the contin‑ uously stirred tank reactor model, miss many important processes and likely overestimate production rates. By the same token, hydrogeologists who may simplify reactive transport to partition models might also be disappointed in prediction of recovery and fluid composition. As a geologist, my approach is likely to be more appealing to the reader with geoscience training. However, the projects that I worked on were highly multidisciplinary, and I have tried to integrate engineering-style writing and provide enough detail for project planning and scheduling. Most of the project information contained in this book comes from U.S. Bureau of Mines and other governmental research projects that I have been involved with because recent commercial operations are subject to confiden‑ tiality agreements and are still in the development stage. Conceptual and computer models are used to demonstrate some principles rather than actual examples to circumvent these restrictions. However, most models used in this book have potential real-world applications. Finally, I add my own thoughts on the future of in situ mining. Some may reject my notions, but that is both the privilege and burden that any author must bear.

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Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Acknowledgments First and foremost, I want to acknowledge all of my colleagues and friends at the former U.S. Bureau of Mines (USBM; 1911–1996) who willingly and with sac‑ rifice gave their best efforts and ideas to improve the health and safety of miners and to ensure a secure supply of sustainably produced minerals for the benefit of all U.S. citizens. Indeed, the health and safety technologies and standards devel‑ oped by USBM have been used worldwide and have saved countless lives. It is difficult to estimate the impact that USBM has had on global mineral produc‑ tion, but it is a formidable task to conduct an Internet search on any mineral commodity or mining and mineral processing–related technology that does not include references to USBM and its publications. I especially want to thank Robert D. Schmidt (“RD”) who has been a great inspiration, mentor, and friend. RD led the in situ mining hydrology group at USBM’s Twin Cities Research Center (TCRC). He developed many innovations in hydrologic modeling of in situ mining systems and conducted milestone field research at working in situ mines. He is a true visionary of the importance of in situ mining and a key individual in the development of commercial in situ copper recovery. Without him, this book would not have been written. So many other colleagues at TCRC also provided support and informa‑ tion that made this book possible, including Stephen Paulson, Jon Ahlness, William Larson, Diane Marozas, Daryl Tweeton, Perry Jones, Stephen Crum, Susan Brink, Bernie Saini-Eidukat, Michael Boucher, Rolland Blake, Sterling Cook, Linda Dahl, Allan Granthem, Harland Kuhlman, Daniel Millenacker, and Stephen Swan. We had the benefit of brave leadership during difficult times when the fate of USBM was being deliberated, so special thanks go to Lewis Wade, director of TCRC, and Rhea Gram, director of USBM in its final years. Many other colleagues at our sister research centers also sup‑ ported this work, including William Tolley, Loren Redden, Allan Isaacson, and Robert Reisinger. Multiple mining companies and officials supported our research during our tenure at their sites. I offer special thanks for the support of Hank Kreis of Asarco Santa Cruz, Kermit Behnke of Cyprus Minerals Tohono, and James Brister of Cyprus Minerals Mineral Park. I have also had the privilege to

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xii Acknowledgments

work on several commercial in situ recovery projects, owing to the support, recommendation, and advice of Daniel Johnson, John McCartney, Rolland Goodgame, Stephen Twyerould, and Benoit Bissonnette. I thank them for our many discussions and brainstorming sessions. My gratitude is extended to my advisor James Stout of the University of Minnesota—the best teacher and role model for a budding geologist. His limitless scientific curiosity of how the earth works was contagious to me and countless other students. My fellow graduate student, project lead, and friend Michael Berndt has also been a huge inspiration to me as his rigor and intel‑ lect is exemplary for any scientist in any field. His contributions toward the understanding of gold and other metal mobility in hydrothermal systems will be used by countless generations of economic geologists to come. In addition, his work at the Minnesota Department of Natural Resources is critical for environmental scientists in understanding the fate and transport of metals. I am indebted to the Society for Mining, Metallurgy & Exploration for supporting this book and all the hard work of the SME book publishing professionals. Finally, I thank my daughter, Marissa, for her patience while I spent many evenings and weekends writing this book. She has taught me that it is okay to go outside and play once in a while!

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CH AP TER   1

Background Most metal ore deposits are formed over the course of thousands and millions of years by the action of flowing fluids and chemical reactions within the interior of the earth and at its surface. As discussed further in Chapter 2, fluid motion and movement is driven by the energy of tectonism and the hydrologic cycle. In situ recovery and remediation (ISRR) mimic and accelerate the natural processes of flow and reaction in the earth for the recovery of useful minerals from ore for human consumption and the subsequent restoration and stabilization of the residual rock mass (Figure 1.1). All rocks, sediments, soils, and so forth contain a finite number of voids within the bulk mass that allow the flow of fluids by gravity and pressure gradients. For simplicity, unconsolidated and consolidated earth materials are referred to as rocks in this book unless it is important to differentiate. The point of using the single term rock is to avoid using highly formal terms, such as geologic media, which are constructed by academic authors who are more interested in the mathematical aspects of fluid flow through natural and artificial porous materials. Using the single term rock also avoids differentiation from soft- and hard-rock ore and mineral deposits. Most of the applications discussed in this book focus on hard rock–hosted ore deposits, but the principles are common to soft rock deposits. In some ore bodies, originally competent rock has been so heavily fractured and brecciated by tectonism that its hydraulic properties are closer to unconsolidated sediments like gravels, sands, and clays. Conversely, some unconsolidated sediments have been cemented by mineral precipitates to form hard dense rock. As is often the case, ore deposits form in tectonically active areas where rocks have multiple episodes of fracturing and mineralization. Hence, there is a continuum of unconsolidated to consolidated rock that may host ore deposits with highly variable physical properties. Some voids in rocks are interstices that result from packing of mineral or amorphous particles. This is easy to envision for the packing of particles that approximate spheres as in a beach-sand deposit composed of well-rounded quartz grains. However, the shape of the voids becomes more complex and heterogeneous when the particles have different shapes and sizes. The shape

1

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2 CHAPTER 1

Solvent-Extraction Electrowinning Plant

Ore Mineralization Ore Deposit

Leach Solution FIGURE 1.1  In situ recovery and remediation well-field system

and volume of interstices change as the particles become consolidated by burial under hundreds and thousands of meters or feet of overlying sediment in an oceanic or lake basin. Processes, such as diagenesis, may result in the chemical precipitation of minerals in the interstices. In volcanic rocks, there may be isolated voids in the amorphous glass matrix formed by gases sealed by the quenched lava or magma. Voids are also formed when rocks are broken or fractured by shrinkage during cooling, tectonic activity, earthquakes, and other forces within the earth. Voids in rock are also called pores and rocks can be regarded as porous in the broadest sense. Connected pores form a property of the rock called porosity (percentage of void spaces), which allows the flow of water and fluids through the rock. The resistance to flow is a measure of the rock’s permeability (Freeze and Cherry 1979). Rocks with low resistance to flow are relatively permeable whereas materials with very high resistance to flow are called impermeable even if the absolute permeability is not zero. Permeability is a function of porosity, but the relationship depends on the detailed characteristics and connectedness of the porosity. Porosity is a physical parameter that is represented by the fraction or percentage of void spaces per unit volume of the bulk rock and usually does not exceed 40%–50%. Conversely, permeability is a physical parameter that varies over many orders of magnitude in rocks and has many different units of measure constructed by various disciplines interested Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Background

3

in fluid flow. Most readers have some knowledge of porosity and permeability of earth materials, and more detailed working definitions are provided in the following “Terminology” section. Chapter 3 briefly introduces the concepts of fluid flow and reactive transport in rocks. ISRR for uranium has been commercially successful for more than four decades, and roughly half of the U produced in the world comes from in situ recovery operations (Seredkin et al. 2016). One common mineral in U ore deposits is uraninite (UO2), which is soluble in air-saturated ammonium bicarbonate solutions that are circulated through the ore using injection and recovery wells during ISRR (Figure 1.1). This technology is especially well suited for porous sandstone–hosted roll-front deposits in the western United States and similar sedimentary deposits elsewhere (Jensen and Bateman 1979). The flow of fluids through rocks results in chemical reactions that dissolve minerals in one location and deposit minerals in another location when physical and chemical conditions change. Many sediment-hosted uranium deposits are formed by leaching of relatively low concentration sources such as uranium from granites by surface infiltration of oxygenated meteoric water, which slowly dissolves primary uranium‑bearing minerals. The U-bearing water percolates through unsaturated rock until it reaches the water table when it mixes with groundwater and flows downgradient. Eventually, the U precipitates in mineral form when it encounters an oxygen-poor environment. At the Oklo mine in Gabon, Africa, uranium was enriched by similar processes approximately two billion years ago in the Precambrian era (Zoellner 2009). Because the proportion of the radioactive isotope U-235 was higher at that time, the ore grade reached the critical concentration for spontaneous fission. The rate of water flow through the ore was enough to control the fission reactions while supplying new fuel so that the natural nuclear reactor operated for about a million years with no human intervention. Because there are many excellent books and technical articles that summarize ISRR applications for uranium, this commodity is not covered in this book. Uranium already commands a significant amount of research interest and is largely outside the realm of more typical commercial applications of ISRR given the high degree of regulation and government subsidies. The emphasis of this book is on nonfuel and nonnuclear metals. Furthermore, this book focuses on emerging and future applications of ISRR with emphasis on copper because of the realization of commercial ISRR projects for that metal. In addition, the knowledge base related to the geochemistry and hydrometallurgy of copper leaching is well developed, and examples from that industry are used in Chapters 4 and 5. However, the lines between energy and industrial metals have become somewhat blurred by the accelerating use of copper Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

4 CHAPTER 1

and other metals in renewable energy technologies. For example, lithium has become the focal commodity for renewables as high-capacity lithium-ion batteries are needed to store and transport electricity produced by renewable technologies. More than half of the current lithium production in the world comes from brine extraction, which can be considered a natural ISRR system in that natural leaching of lithium from porous rock results in elevated lithium concentrations in the brines (USGS 2017). Multivalent vanadium batteries are also being developed for storage of electricity from high-production renewable energy facilities, and vanadium can be produced as a by-product of ISRR uranium mining. Even rare earth elements, which are relatively immobile in aqueous fluids are mined by ISRR technologies in China (Haschke et al. 2016). Modern civilization is reliant on technology that requires a wide variety of metals needed for electronics, energy storage systems that can power them, and advanced structural materials. The demand for metals can only be met by recovery and processing of raw minerals from the earth and recycling. Even 100% recovery of existing metals cannot fulfill the demand for expanding populations with growing economies and consumption (Rankin 2011). Market capitalization of the top 150 minerals companies worldwide was approximately equal to that of Walmart in 2001 (MMSD 2002), which demonstrates that minerals as well as other natural resources are being produced at relatively low cost despite exponential demand for material products. The demand has been met by technological advances, increasing the scale of mining operations, and reducing capital and operational costs. Most of the world’s metal resources come from a few thousand mines scattered around the globe (Rankin 2011). By comparison, there are more than half of a million farms in the world (FAO 2014), and many farms rely on mines for phosphorous potassium and other mineral nutrients owing to depletion of soils. Even as the demand for metals has increased, so has the resistance to mining and other development of natural resources (MMSD 2002). This resistance includes additional environmental laws, further growth of the regulatory expansion of existing laws, and societal rejection of new development projects through political and economic pressure. This has the effect of exporting natural resource extraction projects to less-developed countries and importation of raw and processed commodities by developed countries (Kesler 1994). Globalization of economies and increasing pressure from international entities, such as the World Bank, are also affecting the pace and longevity of natural resource development in less-developed countries. Although the sustainable development movement is improving the environmental safety and social responsibility of projects around the globe, the demand for minerals Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Background

5

and metals is increasing (Rankin 2011). Fortunately, technological advancements have allowed natural resources to be developed more efficiently while still meeting higher standards. Von Below (1993) wrote that sustainable development of mineral resources can be achieved only through continuous exploration, technological innovation, and environmental rehabilitation. Chapter 6 provides a summary on the state of the art of rock mass characterization techniques that have been and can be applied toward ISRR in the future. Current metal demands are largely driven by supply and demand, but there is still concern for countries to maintain a critical and strategic supply of metals. The U.S. Geological Survey (USGS 2017) published a list of 35 critical and strategic minerals, most of which are metals that are used in electronics and energy storage manufacturing or other sectors that support them. Copper and precious metals are not included on the critical list as they are not deemed vulnerable to supply disruption but are also vital materials for advanced electronics technology and new energy systems. Only nonfuel metals are included, but many of the listed minerals are metals needed for energy storage in batteries or other storage media. Hence the interdependency of technology and energy on metals availability is also increasing. The critical and strategic list only focuses on vulnerability with respect to current and predicted supply and demand for minerals in the United States and especially for military applications. However, the list does not fully consider environmental and economic trends in minerals and metals production and demand around the globe and may underestimate the criticality of supply of a historically reliable commodity, such as copper, as evidenced by the rise in price in the early 2000s from increasing demand driven by economic growth of populous countries in Asia (NRC 2008). Given the lightning speed of advancements in technology, materials science, and renewable energy plus the rapidly changing world economy and politics, it is impossible to predict the future criticality and availability of individual or classes of metals with a high degree of accuracy. Supply demand and price forecasting is beyond the scope of this book but does factor into the economic evaluation of ISRR projects as discussed in Chapters 7 and 8. A significant influence affecting worldwide availability of minerals is associated with the impacts of mining operations on water resources. Although localized, the impacts of mining on the quality of water resources are long lasting. Metal mining has historically relied on pervasive blasting and excavation of ore deposits to extract metal values. These land disturbances have often resulted in permanent alteration of groundwater levels near pits, adits, waste dumps, and tailings impoundments. The lowering of natural groundwater elevations through mining activities exposes pyrite and other sulfide minerals to unsaturated hydrologic conditions and to rapid weathering, thereby Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

6 CHAPTER 1

greatly increasing the potential for long-term production of acid rock drainage (ARD) and contamination of surface and groundwater by acidity and metal migration (INAP 2018). In arid climates, reduced rainfall decreases the amount of contact water at mine sites, but it is sometimes difficult to reclaim fragile ecosystems or to use artificial wetlands to passivate mining wastes and to treat ARD. A project that investigated the application of in situ recovery from copper sulfide ore at a closed open-pit mine to prevent ARD is summarized in Chapter 7. In addition, in situ remediation is covered briefly in Chapter 9 and other sections of this book along with in situ recovery of metals because remediation is likely to be required to treat residual leach solutions and residues created by in situ recovery operations. In Chapter 10, the future of ISRR is discussed. Certainly, deep in situ subsurface technology will become even more important in the future to develop not only oil and gas and metals but also water resources. Many of the older open pit mines have exhausted shallow reserves (Jamasmie 2018) and some of the newer copper mining projects are developing underground operations that target deeper and higher grade ore to offset the high cost of tunneling at depth (NRC 2008). Recovery of selected metals is already economical from brine, and brine is emerging as a new water resource for the future. Today brine is seen as a nuisance for deep oil and gas production, but it may emerge as the more desirable resource as shallow aquifers become depleted. Of course, brine requires treatment for beneficial use and water will be more expensive to produce. It may be that the mineral resources in brines will be codeveloped as by-products of development. ISRR will not be a panacea for metal commodities for the generations who may find this book useful, but like renewable energy, ISRR may become a significant supplemental source of metals as resources from existing mines dwindle. TERMINOLOGY The following list defines and introduces some of the most important terms for ISRR that are used throughout the remainder of the book, which hopefully become clear as the reader encounters them. Ore Ore is earth material from a deposit of sediments or a rock formation (ore body) that contains minerals that can be extracted at a profit (Kesler 1994). Usually the minerals are concentrated in the ore relative to typical earth materials. The concentration of the mineral or metal is called grade and ore is defined by a minimum cutoff grade where the minerals and metals are too dilute for economic extraction. The cutoff grade can change depending on economic factors and technology. Hence the cutoff grade for in situ recovery Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Background

7

may be different than for conventional mining. The standard terms reserves and resources (Kesler 1994) have the same meaning for in situ recovery but may have different tonnages than conventional mining because of different economic projections. Standard State Standard state is used in chemistry to provide a reference state for a system so that its physical and chemical properties can be described and compared to its properties at nonstandard-state conditions. The most common standard state is at a pressure of 100 KPa (1 bar) and temperature of 25°C (77°F), which are the average conditions you might find in a laboratory occupied by scientists and engineers. Chemists are very familiar with the concept of a standard state, but hydrologists often imply standard state conditions for water when referring to the hydraulic conductivity of a rock mass, which is specific to flow of water in the rock at standard state conditions. Because ISRR conditions and fluids may deviate from standard state conditions, the reader should bear this concept in mind. In many cases, the differences from standard state conditions are not significant, but ISRR solutions may have high densities owing to high dissolved constituent loads and this affects the flow of the aqueous fluid. Porosity Porosity is the volume of voids per unit bulk volume of sediment or rock and is a dimensionless ratio (Freeze and Cherry 1979). For unconsolidated granular media, the concept of an equivalent porous media (EPM) is widely used in groundwater hydrology. Fractured rocks are often considered to be EPM at regional scales and if the fracture density is high enough. However, at the well-field scale of ISRR, the concept of an EPM may not be valid unless the fracture density is very high. Pore dimensions are usually on the same order as grain size or fracture aperture (i.e., 1–10–3 mm) except in sediments or rocks dominated by silt and clays where the pore dimensions may be much smaller than the particles. Porosity may vary from 0 to 50% depending on the rock type and degree of fracturing. Permeability Permeability is the intrinsic property of a rock or EPM to transmit fluids and is related to hydraulic conductivity (K  ) by the properties of pure water under unit gradient (Freeze and Cherry 1979). That is, hydraulic conductivity is related to the intrinsic property permeability, which is specific to the rock at standard state conditions. The preferred units of permeability vary depending on application but square meter, square centimeter, and darcy are the most common. Hydraulic conductivity is usually presented in velocity units such Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

8 CHAPTER 1

as meter per second, centimeter per second, or foot per day, and so forth. Rocks with hydraulic conductivities less than 10–6 or 10–7 cm/s (2.8 × 10–3 or 2.8 × 10–4 ft/d) are considered impermeable, and permeable rocks have conductivities as high as 102 cm/s (2.8 × 105  ft/d). Darcy’s law is a widely used formula for calculating the bulk flow through an EPM (Stephens 1996): V = Ki

(Equation 1.1)

where i is the hydraulic gradient or change in elevation potential (head) per unit length. Aquifer Aquifer is a hydrologic term for a water-bearing horizon that contains groundwater resources that may be beneficially used for culinary or irrigation purposes. Groundwater that cannot be used because of water quality or production limitations is usually not considered to be an aquifer, but there are no definitive limits in practice. For an Underground Injection Control, or UIC, permit that is required by the U.S. Environmental Protection Agency for ISRR, an aquifer may be declassified if it contains greater than 10,000 ppm total dissolved solids. Reservoir The term reservoir is more commonly used in the oil and gas industry to define a reserve or resource of the commodity that is contained in the pores of the rock. In this book, reservoir is used to differentiate a porous rock formation filled with fluid from an aquifer. A reservoir may also contain brine or other fluid that is not groundwater that can be used for domestic consumption and agriculture. Representative Element Volume The assumption of the connectedness of pores and open fractures that contain a continuous fluid phase (continuum) and the representative elemental volume (REV) of that continuum are hydrologic concepts as important for understanding ISRR systems as they are for flow in aquifers (Bear and Verruijt 1990). Bear and Verruijt (1990) provided a very detailed description of the basis theory of fluid flow through a complex network of voids bounded by solid interfaces that is used to derive mathematical representations. The REV averages the complexities of the microscopic complexity of the aquifer matrix so that the porous medium can be attributed to hydraulic properties. The REV can also be used to represent the average properties of a porous ore body to include both hydraulic properties and solid ore particles in contact and reacting with the fluid or leach solution. The aquifer and ore body may Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Background

9

also have distinct units or regions with their own REV if the physical and chemical properties are distinct. Just as one core sample assay is unlikely to provide a representative grade for the entire ore body, it is also unlikely to be representative of its hydraulic properties. It may take multiple boreholes to define the ore body or a portion thereof. Pore Volume For a specific volume of an aquifer, ore body, or reservoir, the term pore volume is used to define the volume of water or fluid that is contained in pores. This term is used when discussing well-field operations and defining the volume of fluid that has to be pumped or injected to drain or fill the pores of the aquifer or reservoir volume. In situ recovery operations also use the term priming to describe the volume and time required to flood or displace the reservoir or ore body with leach solution. Lixiviant Lixiviant is a chemical solution—usually aqueous—that dissolves the target ore mineral so that metals can be recovered in solution and refined at the surface. Leaching Leaching is the dissolution of an ore or gangue mineral by a chemical solution or lixiviant. Most often, the lixiviant is an aqueous solution containing chemical species or ligands that facilitate the dissolution and transport of metals as dissolved ions. The metal-bearing fluid generated by leaching is referred to as the pregnant leach solution (PLS). Recovery The fraction or percentage of the total metal mass in the target ore zone successfully extracted from the ore deposit is considered the recovery. Some small losses may occur at the processing plant owing to recirculation of spent leach solution. The term recovery is also used to represent the time-dependent extraction of metal from a leaching test based on its original or head grade and sample mass. Recovery curve is calculated based on the flow and concentration of metal (grade in solution or solution grade) in PLS. Well A well is an excavation or borehole made by digging or drilling that reaches from the surface of the earth to an underlying body of water or other fluid or rock formation of interest. The function of the well is to allow access to that fluid and to retrieve fluids and/or minerals from the earth. The borehole may contain a casing that is cemented to the rock wall of the borehole to prevent Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

10 CHAPTER 1

caving and the unwanted release of fluids being pumped to the surface or injected into the subsurface. Injection Rate Injection rate is the volumetric rate of leach solution (lixiviant) injected to a well. The injection rate is the flow from an injection flow, and pumping rate is the discharge from a pumped well. Production Production is the volumetric rate of loaded or PLS flowing to the surface from the recovery or production well by pumping, artesian flow air lift, and so forth. Production rate can also refer to the rate of metal produced, which is the rate of flow times the metal concentration or grade in solution. Well Pattern Well pattern is usually defined by the two-dimensional array of injection and recovery wells that may be systematic and symmetrical but usually not random. Cone of Depression The lowering of water levels or hydraulic potential near a well or well field that is withdrawing water or other fluid is the cone of depression. Water molecules entering the cone of depression flow toward the well. Reaction Rate Although highly soluble salts leach very rapidly, ore minerals usually dissolve more slowly according to a reaction rate that may vary with the composition of the lixiviant and other physical factors such as temperature and texture. Sweep Efficiency Sweep efficiency is a term that originated in the oil and gas industry but is often used in the ISRR literature to define the distribution of the lixiviant through the pores of the ore body. Sweep efficiency is often related to potential metal recovery, but in subsequent chapters, the term effective sweep efficiency is introduced to distinguish flow distribution from solution–mineral contact. Containment Containment describes control of the leach solutions within a well field or control volume. An aquifer may be confined by impermeable rock or layered formations above and below a permeable formation. Unconfined aquifers have a saturated or phreatic surface that is not bounded by low-permeability rock or sediment above the aquifer. Similarly, an ore body that is saturated Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Background

11

may be confined or unconfined. Confined systems are favorable for ISRR because the low-permeability rock above and below the ore body contains the lixiviant and protects adjacent aquifers that may be above or below the ore. Excursion is the accidental release of leach solutions outside the intended control volume. Containment can be achieved by a net production of solutions from a well field where leach solutions are being injected and PLSs are being recovered. The difference in production minus injection is balanced by groundwater inflow to the well field, which prevents excursions. Other natural (low-permeability rock) or engineered impermeable barriers can be used for containment during ISRR. A clay-rich shale is one example of a natural impermeable barrier or aquiclude in a confined aquifer system. A grout curtain is one example of an engineered impermeable barrier.

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CH AP TER   2

Ore Formation by Aqueous Solutions Mineral deposits (ores) can be thought of as geologic accidents in time where the right set of physical and chemical conditions were present for a long enough period to concentrate minerals in an accessible location for the practical and economic extraction by humans. Because in situ recovery and remediation (ISRR) is based on reactive transport processes that form metal ore deposits, it is, in a sense, a reactivation of that process for the purposes of metal recovery or sequestration. Several different classification schemes for ore deposits exist, but the most important information for ISRR purposes is the process or processes that form them (Table 2.1). Although economic geologists have been develop­ ing theories of ore deposit formation by these processes for several centuries (Jensen and Bateman 1979), it is only relatively recently that we have been able to quantify the physical and chemical conditions of ore deposit forma­ tion (Brimhall and Crerar 1987) and perform computer simulations of the complex flow and reactive transport processes that formed them (Lichtner et al. 1996). In many ore deposits, two or more processes may have been active over the geologic history of the deposit. Many primary or hypogene nonferrous Table 2.1  Critical processes in forming metal ore deposits Process Example Ore Metals Magmatic concentration Ni, Cr, platinum group Sublimation Cu, Zn (S nonmetal) Contact metasomatism See hydrothermal Hydrothermal Cu, Mo, Pb, Zn, Co, Cd, Au, Ag Supergene Cu, Au, Ag Sedimentation and Mn, Cu, Zn, Pb, Au, Ag diagenesis Microbiological Cu, Zn, Pb, Au, Ag Evaporation Li Residual/Mechanical Sn, Al, rare earth element Metamorphism Mostly nonmetallic, see contact metasomatism Adapted from Jensen and Bateman 1979



13

ISRR Potential Low High Medium Medium High High Medium or low High Medium or low Low

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14 CHAPTER 2

Gold Enclosed in Impermeable Pyrite Host Rock

Mountain Belt

Pyrite

Ore Body

Quartz Filling Fracture

Fractures Groundwaters

Hot Rocks or Magma

Groundwaters

FIGURE 2.1  Hydrothermal sulfide ore deposit formation

metal ore deposits are formed by aqueous solutions heated by igneous intru­ sions and chemically altered by magmatic volatiles, including superheated water (Figure 2.1). The initial concentration of metals may be enhanced by weathering and supergene processes at low temperatures near the surface of the earth by the action of rainfall (meteoric water), infiltration, and erosion (Figure 2.2). Some metals, such as nickel, chrome, and platinum group elements, are sourced from ore minerals that precipitate directly from the magma and are concentrated in layers as magma cools slowly and ore minerals crystallize and concentrate at the liquidus boundary. Second to water, the next most impor­ tant magmatic volatile is sulfur, which forms ligands that allow hydrother­ mal solutions to transport metals such as base and precious metals. Reduced sulfur is also the precipitating agent as the aqueous solutions cool. Metals are generally more soluble at elevated temperatures but then precipitate as sulfides as the solution migrates from the heat source and cools. In some systems, chloride and carbon dioxide are also important in hydrothermal and supergene transport. Boiling near the surface of the earth at relatively low pressures can also release sulfur and other volatile gases that destabilize metal complexes and cause precipitation. Precious metal deposits in epithermal systems are formed by this process (Jensen and Bateman 1979; Sawkins 1984). Supergene enrich­ ment may result from the fluctuation of groundwater levels and changes

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15

Ore Formation by Aqueous Solutions

Gold Enclosed by Permeable Iron Oxide Erosion Shattered Groundwater

Quartz Vein and Host Rock Gold Migrates into Open Fracture

Fractures Geologic Forces

FIGURE 2.2  Supergene ore deposit formation

in redox conditions along the groundwater flow path. The latter process is responsible for the formation of uranium roll-front deposits, which have been favorable ore deposits for ISRR. Tectonic overprinting may result in secondary fracturing and chemical processes that can enhance or reduce permeability. HYDROTHERMAL ORE DEPOSITS Most ore-forming hydrothermal fluid temperatures generally range from 100° to 500°C (212° to 932°F), below the minimum melting temperature of granites at high water content), depending on depth and pressure. For reference, the average geothermal gradient is about 25°–30°C per kilometer (124°–139°F per mile) but can be much steeper in tectonically and volcan­ ically active areas. Many of these systems are driven by convection such that the quenched aqueous solution returns to the heat source and picks up sulfide, heat, buoyancy, and momentum away from the source to cool and deposit and concentrate more metal sulfides. The convection cell acts as a conveyor belt of metals and sulfur derived from magma and depositing met­ als and hydrothermal gangue minerals at the end of the line. The fluids may permeate the igneous intrusion itself or surrounding sedimentary volcanic, metamorphic, and older igneous country rock. Cooling of the system results in shrinkage and fracturing of the rock intrusions, which creates permeability and new flow paths for the hydrother­ mal solutions. In most hydrothermal ore deposits, there is a superposition

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16 CHAPTER 2

of mineralized and unmineralized veins from different events (Titley 1972). So fractured rock, like a broken bone, may heal by these processes only to be refractured by episodic intrusion and tectonic forces. Precipitation of sulfides, silica, and other gangue minerals at the cool­ ing margins of the hydrothermal cell reduces permeability and advective transport. The focus of hydrothermal fluids shifts to higher permeability zones or contracts to the core where solubility and pore pressure is high and perme­ability is maintained. The density of fracturing during hydrothermal ore deposition varies depending on the distance from the source and style of alteration but generally ranges from one fracture per centimeter to one fracture per meter (approximately 5 to 0.05 fractures per foot; Figure 2.3). Figure 2.4 is a photograph of a core sample of fractured host rock with veins of copper oxide ore, and Figure 2.5 shows a picture of a core sample taken under a petrographic microscope. The observable fractures in the core sample taper into hairline fractures that are not visible without magnification. The tectonic forces that create mineral deposits are enormous and are often released in punctuated periods of Earth’s history. Volcanic eruptions sometimes occur directly above nascent metal ore bodies. In the United States in 1980, Mount St. Helens, one of the world’s best-known eruptions and the largest historic eruption, released the energy of approximately 1,000 Hiroshima bombs, equivalent to 24 × 106 tons of TNT (USGS 2000). Some large mineralized districts are associated with thousands of such eruptions

Supergene Alteration Hypogene Alteration

Decreasing Temperature

Precipitation

Phreatic Zone

Cooling Magma Hydrothermal Fluids

Chlorite Smectite

Enriched Blanket

Water Table Low-Grade Core K-Alteration n = 0.1–0.2

Kaolinite Sericite

Leached Capping Iron Oxides

Skarn

Pyrite Shell Qtz + Sericite + Py Phyllic Alteration n = 0.3–1.5 (cm–1)

Vertical Faulting

Low Pyrite Shell Propylitic Alteration n = 0.1–0.01

Decreasing Temperature n = fracture density (cm–1 = 0.33 ft)

Adapted from Titley 1972

FIGURE 2.3  Cross section of primary and secondary alteration zones in a porphyry copper deposit Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Ore Formation by Aqueous Solutions

17

FIGURE 2.4  Copper oxide ore core specimen, Santa Cruz deposit, Arizona

FIGURE 2.5  Photomicrograph of a core specimen from the Santa Cruz deposit, Arizona (field of view ~10 mm [0.4 in.])

occurring over millions of years. By contrast, one of the largest nonnuclear explosions in history was triggered for mining and took place in the Old Reliable copper mine in Arizona. It used 2 × 103 tons of explosives (Sisemore 1973). The intent was to liberate copper minerals and generate porosity and permeability for in situ recovery (Bartlett 1992). It has been estimated that the pressure–volume mechanical energy in magma generated during the release of metal-bearing, superheated water is Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

18 CHAPTER 2

on the order of a 10-megaton bomb for each cubic kilometer (0.24 cubic miles) of magma (Brimhall and Crerar 1987). This energy is derived from the threefold volume expansion of water exsolved from cooling magma. The hydrostatic pressures generated by water expansion can overcome one or more kilobars (equal to or greater than 100,000 KPa or approximately 14,500 psi) of lithostatic pressure. These extreme pressure gradients can result in large fluxes through rocks with millidarcy-range permeabilities over the course of tens of thousands of years (Steefel 1992). Obviously by comparison, it is more efficient to take advantage of the tectonic energy already spent to form ore deposits than to match or overcome those forces for the extraction of metals for beneficial use. In some ore bodies, some of the original pore space and fracture net­ works that distributed ore minerals are still open and interconnected enough for fluids to flow through them. The vuggy quartz vein specimens that host gemstones found in rock shops are clear examples. In other ore bodies, the ore minerals and precipitated silica (i.e., quartz veins), micas, and clays have completely closed the conduits such that there is little if any remnant perme­ ability. However, later tectonic activity, low-grade metamorphism, diagenesis, karst processes, and weathering may recreate or destroy permeability. Many ore deposits are formed in highly tectonic areas and have undergone multi­ ple periods of deformation, faulting, and fracturing after ore deposition. The destructive force of earthquakes in the subsurface can readily be appreciated from the effects of single strong events on buildings and infrastructure. Hence, fracture flow must be addressed in ISRR because most ore bodies are fractured, to one degree or another, including clastic sedimentary rocks, such as sand­ stones, which are typically conceptualized as porous media. Microbiological processes may be secondary or primary in concentrat­ ing metals in lower temperature hydrothermal deposits and in supergene ore deposits. Modern examples of primary ore deposits, which are created through microbiological processes, are the so-called black smokers formed from the reduction of sulfate in heated seawater to sulfide by hyperthermo­ philic bacteria (Hannington et al. 1995). The expulsion and quenching of hot metal-bearing seawater results in the creation of multimetallic sulfides that rise 1 m (3.3 ft) or more above the ocean floor (Figure 2.6). It is thought that the physical, chemical, and microbiological processes active in the for­ mation of black smoker metal deposits (observed at the modern seafloor via deep-diving submersibles) may have produced an important class of deposits known as massive sulfide deposits for most of Earth’s history (Hannington et al. 1995). In the formation of black smokers and massive sulfides, dissolved sulfide leaches metals in ocean-floor basalts, which are transported by hydrothermal Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Ore Formation by Aqueous Solutions

19

Source: NOAA 2000

FIGURE 2.6  Black smoker hydrothermal vent, Juan de Fuca Ridge, northeast Pacific

solutions circulating in convection cells. Convection is driven by mid-ocean ridge spreading along the margins of oceanic plates and heat from upwelling magma rising from the mantle, which heats downwelling cold seawater to temperatures of up to 400°C (752°F). The metal sulfides are deposited at depths of 4 km (approximately 2.5 mi) below the surface of the ocean when the hydrothermal solutions vent into the ocean and cool when they mix with the near-freezing temperature seawater at this depth. Autotrophic bacteria use the heat and electron sources for energy, driv­ ing a food chain that results in unique biological communities at this depth (Hessler and Kaharl 1995). The fracture zones that hydrothermal fluids flow through are interconnected along tectonic ridges that spread at rates of up to 10 cm (3.9 in.) per year (Hannington et al. 1995), allowing the bacteria to move from cooling and dying systems to newly formed systems thereby renewing the metal deposition cycle. The fracture zones are transient, and permeability conditions are very dynamic as new fractures form by tectonic extension of the crust followed by a series of basalt seawater reactions that may trigger precipitation of minerals and seal fractures. Growths of filamentous bacteria can also cause clogging of fluid pathways. Other chemical processes sometimes leach minerals and redistribute pores and permeability within the system (Hannington et al. 1995). Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

20 CHAPTER 2

Manganese, which is more mobile in the more oxidized and acidic sea­ water, formed by mixing with cooling hydrothermal solutions, precipitates more slowly, moves off-ridge, and forms manganese nodules with sediments at the bottom of the ocean. As these primary hydrothermal sulfide deposits cool, oxidize, and weather, they generate cold acidic solutions that may leach and redistribute metals, thereby producing supergene oxide ores and iron oxide–rich caps or gossans (Hannington et al. 1995). Sulfide oxidation in deep oceanic environments with relatively low dissolved oxygen may be cat­ alyzed by iron-oxidizing bacteria, such as Thiobacillus ferrooxidans, which are also a key catalyst in the formation of acid rock drainage. SUPERGENE ORE DEPOSITS Secondary or supergene ore deposits are formed by redistribution and concen­ tration of metals from primary ores from infiltration of meteoric water and relatively low-temperature (near standard state conditions of 25°C [77°F], 100 KPa [14.5 psi]) dissolution of sulfides and other minerals (Figure 2.2). Because many base metals are transition metals, oxidation of both the metal and sulfide results in dissolution and transport of metal and sulfate by down­ ward-moving solutions (Brimhall and Crerar 1987). In addition, hypogene pyrite weathers and may increase acidity that results in leaching of the parent hypogene ore. The reaction removes sulfur and often leaves a porous iron oxide reaction product (Figure 2.7) that increases porosity and permeability in the weathered or oxide zone. The weathering process and exfoliation of rock mass at the surface also enhance porosity and permeabil­ ity in the soil zone down through the underlying unsaturated par­ ent rock. However, illuviation and chemical precipitation near and below the phreatic zone can decrease porosity and permeabil­ ity, so there are both positive and Source: Earley et al. 1990 negative feedback mechanisms FIGURE 2.7  Scanning electron microscope that affect hydraulic properties backscatter image of porous iron oxide in a during weathering (Brimhall weathered sulfide ore sample (scale = 10 µm)

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Ore Formation by Aqueous Solutions

21

and Crerar 1987). In general, oxide and supergene ore is a favorable target for ISRR. Some refractory metals are enriched in the solid residue during weather­ ing as meteoric water dissolves and transports unwanted material. The rela­ tively low solubility of aluminum, tin, rare earth elements (REEs), and other metals results in the concentration of the residual soils and regolith during weathering. This forms laterites and lag deposits (Jensen and Bateman 1979), where unwanted components are chemically and physically removed by infil­ trating meteoric water, which concentrates the metals of interest. These types of deposits may be difficult to develop using ISRR because of their refractory properties. However, some of the REE deposits in China have favorable ore mineralogy for leaching, and some ores are mined in situ by surface applica­ tion of strong acids (Haschke et al. 2016). In arid environments, evaporation of meteoric water leads to concentra­ tion of important metals, such as lithium, to economic grades from brines. The brines are formed from slow-moving groundwater that slowly dissolves the rock matrix, and then they are concentrated at the surface or in shallow subsurface reservoirs. Closed basins and salars in arid areas of the earth are the primary geomorphic features that generate metal-bearing brines. The evapo­ concentration process is artificially enhanced in the recovery of lithium and other metals. Placer deposits are created from flowing high-gradient streams as hypogene and supergene ore is exposed and eroded by weathering. Hence, aqueous fluids are involved, in many different processes and conditions, in forming a majority of metal deposits. Banded iron formations (BIFs), which contain the vast majority of the world’s iron resources in Precambrian-age rocks, were formed by biologically driven chemical precipitation of iron leached from basalts and other crustal rocks by seawater, which was largely anoxic except in shallow littoral environ­ ments until about 1 billion years ago. Because of the relatively shallow depths and ease of surface mining, low permeability, low solubility, and abundance of these resources, they have never been considered for ISRR, but research on in situ leaching of layered manganese deposits associated with BIFs shows promise (Paulman 1994). Prior to the late 1800s, when large-scale iron mining and iron trade by rail and ship began, relatively young bog iron formations were used as local sources of iron for forges. Bog iron was produced from upwelling-reduced groundwater that dissolves ferrous iron from bedrock. This source of iron was produced by a natural ISRR system in enough quantities to sustain pre­ industrial populations and was renewable to the degree that demand was kept relatively small. Obviously, these small deposits cannot supply the increasing

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22 CHAPTER 2

demands of the industrialized world with exponentially expanding popula­ tions and consumption. Aluminum deposits are residual laterite deposits formed by intense weathering of a parent rock moderately rich in refractory aluminum. The weathering dissolves mobile components, such as Ca, Na, and K, and leaves behind aluminum-rich clays and oxide minerals that are not mobile in mete­ oric water as it moves through rock. As in BIFs, aluminum laterites are not likely targets for ISRR because of the relatively shallow depths and ease of surface mining, low permeability, low solubility, and abundance of these resources. However, aluminum can be solubilized in acidic leaching solutions used for copper leaching and has become a problematic contaminant in sim­ ilar solutions used for ISRR (Isaacson and Redden 1994).

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CH AP TER   3

Flow and Reactive Transport in Geologic Media

INTRODUCTION In situ recovery and remediation (ISRR) is based on human-controlled flow and reactive transport in geologic media. Hence, the same physicochemical processes that form ore deposits, as summarized in Chapter 2, are in a sense reactivated during ISRR. Although conventional hydrometallurgical extraction using surface vat and heap leach facilities involves flow and reaction processes like those that are discussed in Chapter 4, the rates of input, ore feed characteristics, reaction rates, and so forth are generally more quantified and controlled than in ISRR. Ore deposits are built by geologic processes over the course of thousands, millions, and sometimes billions of years (Kesler 1994). Indeed, many of the processes involved are tectonically driven, coupled flow and reaction processes. Even after extensive drilling and characterization, only fragmental knowledge of the deposit is attained. A deeper understanding of ISRR requires a considerable amount of imagination and mental spelunking where we imagine what it is like traveling through the rock via water-flooded (or partially flooded) pores and fractures in a tiny submarine much like the 1960s movie Fantastic Voyage. Today, using high-speed computers, it is also possible to do virtual spelunking on micro- to macroscales for numerical simulation, to better understand flow and transport phenomenon in geologic media. In this chapter, the reader is introduced to the theory of flow and reactive transport in geologic media. Practical aspects of this theory to ISRR are developed in the chapters that follow. REACTIVE TRANSPORT Our understanding of hydrogeology and the flow of aqueous and other fluids in geologic media has tremendously advanced in the past century as a result of advancements in drilling, pumping, monitoring tracer investigations, geophysics, mathematical modeling, and computer simulation. Given

23

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24 CHAPTER 3

the enormous volume of information and complexities of each potential ore body, it is impossible to summarize all these advancements here that pertain to individual ISRR projects or aspects. The complexities become exponential when reaction and transport are coupled with subsurface flow, which has evolved more recently as a subdiscipline. Despite these complexities, the conceptual flow and reactive transport process represented in Figure 1.1 can be represented mathematically by one equation that expresses the law of mass conservation at constant temperature: 2 ^ θc i h 2t + div _ θJ disp + θJ adv i = R i (Equation 3.1)

where div is the divergence operator in vector mathematics (three-dimensional [3-D] in real systems), q is the porosity, Ci is the concentration of dissolved metal i in solution, 2t is the time differential, Jdisp is the dispersive-diffusive flux, Jadv is the advective flux (i.e., mass flux attributed to fluid flow), and Ri is the total accumulation rate of metal i in solution from dissolution, precipitation, sorption, and other reactions (Steefel 1992). The divergence principle states that the rate of change in the concentration of a component over time depends on how rapidly the advective and dispersive fluxes change in distance (Bethke 2008). Molecular diffusion is more familiar to most than dispersion, which results from interaction of advective flow and diffusion, so the latter is described in more detail later in this chapter. The advection–dispersion Equation 3.1 has been applied to many reactive transport systems; however, the complexities of expressing and solving the equation exponentially increase as the number of metals or components (i ) increases along with variable q, saturation (S ), flow field, rates of reaction, and so forth. In addition, heat transfer may be important in some systems, adding additional complexity to flow and reaction mass balances. The advective–dispersion Equation 3.1 has been written in different forms in other academic works, but the form presented in Steefel (1992) is useful because the rate of change in mass of component i is represented by its concentration or grade in solution in a flow system with porosity q and is balanced by the rate of change in concentration within the rock (ore) source R. Hence, it represents the mass balance between recovery of metal from ore to the grade in solution. Other forms of the advective–diffusion equation are usually oriented to contaminant transport, where the objective is to represent the mass of contaminant in a plume emanating from a high concentration point source to infinite dilution outside the plume. In this form—Equation 3.1—the rate of change in concentration i in the fluid is the isolated term on the left and is equal to the sum of advective and diffusive flux and the rate of change in the mass of component i transferred to the porous

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Flow and Reactive Transport in Geologic Media

25

solid by attenuation reactions. A comprehensive theoretical presentation of multicomponent reactive transport in porous geologic media is provided in Lichtner et al. (1996). APPLICATION OF DARCY’S LAW ISRR is a practical application of human-induced coupled flow and reactive transport processes. Although commercial operations can benefit from theory, more practical and empirical macroscale approaches to quantification and design are necessary. For this purpose, the most successful empirical description of macroscale flow in saturated, equivalent porous media (EPM) is Darcy’s law: Q = K 2h A (Equation 3.2) 2l where Q is the volumetric flow rate (i.e., advective flux), K is the hydraulic conductivity (specific to water at standard state conditions), 2h/2l is the hydraulic gradient, or head, and A is the area of the flux plane (Freeze and Cherry 1979). Darcy’s classic experiments involved inclined columns filled with granular material with a higher inlet held at constant elevation and lower outlet also held at constant elevation such that the potential energy of the fluid is constant at steady state. The radius-to-diameter aspect ratio of the column is small to constrain flow to one dimension, but a negligible amount of fluid movement in the radial direction is occurring. In an idealized one-dimensional system, the components of flow and potential are represented conceptually in Figure 3.1. The difference in the elevations defines the potential energy or head for pure water at standard state conditions. In natural groundwater systems, hydraulic gradients are relatively flat and rarely exceed 0.01 (Stephens 1996). In ISRR systems, the steepness of the hydraulic gradient may be an order of magnitude or more because of the induced pressure head at the injection well and drawdown at the recovery well, which are separated by a few to tens of meters or feet. Hydraulic gradients in saturated systems are usually steeper in Inlet Elevation Hydraulic Gradient

Outlet Elevation

FIGURE 3.1  Darcy’s one-dimensional flow experiment

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26 CHAPTER 3

the horizontal direction, whereas in unsaturated media, vertical gradients are dominant because of gravity-driven flow (Stephens 1996). For other fluids, Q varies with the same K and head. K varies with the texture, packing, and other properties of the granular material. In an idealized, homogeneous, and isotropic two-dimensional confined aquifer of infinite extent, the transmissivity (T) is related to the hydraulic conductivity by T = Kb (Equation 3.3) where b is the thickness of the aquifer (Freeze and Cherry 1979). Darcy’s law provides an estimation for the advective flux term in the advective–dispersive Equation 3.1. Darcy’s law is applicable for slow-moving, laminar flow of Newtonian fluids where turbulence is not a factor and where density changes do not significantly impart any buoyancy-driven flow. The hydraulic conductivity is related to intrinsic permeability by kρg K = μ (Equation 3.4) where k is the intrinsic permeability, r is the density of the fluid (water or other fluids at standard state conditions), g is the gravitational constant, and m is the dynamic viscosity. In most applications, the fluid is assumed to be pure water with standard state r (997 kg/m3 or 62.2 lb/ft3) and m (890 µPa·s or 600 × 10–6 lb/ft·s). Although the assumption that an aqueous lixiviant has the same properties as pure water at standard state condition is a good approximation for many systems, numerical models of coupled flow and reaction solved by modern computers can simulate nonstandard-state systems such as deep geothermal brines and even nonaqueous fluids such as carbon dioxide. In the simplest system for ISRR, the concentration of metal (solution grade Ci ) and flow field remain constant, and the maximum potential rate of metal production (Pi) at the extraction plant is Pi = QC i (Equation 3.5) If the hydraulic conductivity and gradient/head are known, then Q can be calculated and Ci can be estimated from the ore mineral solubility. This condition might be approached in a metal salt ISRR system in a very homogeneous sandstone containing solubility-controlled metal salt ore minerals that leach very quickly. In fact, it is believed that one of the first ISRR enterprises recovered subsurface brine for making culinary salt in China’s Sichuan Province in 250 BCE (Wu and Kim 1994). The ISRR technology they initially developed used strings of bamboo pipe to lift brine to the surface where it was evaporated using solar energy to make salt. Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Flow and Reactive Transport in Geologic Media

27

One can demonstrate the validity of Equation 3.5 by mixing play sand with table salt and putting it into a drip coffee system with a filter. The system delivers a constant rate of water flowing vertically, which quickly saturates with respect to sodium chloride (NaCl) in the pores of the sand–salt mixture (CNaCl ~ 360 g/kg at 25°C [400 oz/ft3 at 77°F]). If the coffeepot is heated at a rate that evaporates the water at the same rate of drip flow (Q), the production of salt in the coffeepot is equal to P. Finer sand results in lower Q for the same drip system because K is smaller. This very simple production equation is used in some otherwise complex ISRR economic models (e.g., Pugliese and Peterson 1991) by specifying Q and Ci independently, although Q is calculated by analytical solution of production well flow for a five-spot well field with a uniform k. However, even in its simplest form, the advective–dispersion states explicitly that the concentration of solutes in solution is affected by solution flow. More subtly, Equation 3.1 implies that solution flow can also be affected by changing q. This is illustrated in more detail in later chapters. Even relatively simple ISRR systems that produce highly soluble commodities, such as from salt deposits and evaporites, are more complex than the conceptual coffeepot analogy and have nonequilibrium or kinetic limitations on P. This limitation can be attributed to diffusion, kinetically controlled ore dissolution, or both. The relative importance of diffusion in an ISRR system can be estimated by the dimensionless Péclet number l Pe = vl D (Equation 3.6) where Pe is the Péclet number, v is the true velocity of the fluid, D is the diffusion coefficient, and l' is the length scale of interest. For ISRR well fields, l' is usually several meters (feet) or the well spacing of the well field (Steefel 1992). However, l' may be on the order of centimeters (inches) or smaller within the ore matrix where diffusion is driven by the chemical gradient from the matrix to the effective pores of the ore body. This may be in the form of diffusion of the metal ion from the ore mineral or particle itself to the pore space as in the shrinking core model (Roman et al. 1974) or from pores within the matrix that have limited connection to other pore spaces. The latter condition is sometimes referred to as the dual-porosity media model. However, in reality, there is a continuum of pore space connections (Bear and Verruijt 1990). The importance of reaction rate (r) or mineral dissolution in an ISRR system can be estimated from the Damköhler-I number: DaI =

Aθ l v rk l (Equation 3.7)

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28 CHAPTER 3

TABLE 3.1  Potential transport regimes for combinations of Pe and DaI DaI >> 1 DaI > 1 DaI < 1

Pe >> 1 Advective; not rate limited Advective; partially rate limited Advective; rate limited

Pe > 1 Advective and diffusive; not rate limited Advective and diffusive; partially rate limited Advective and diffusive; rate limited

Pe < 1 Diffusive; not rate limited Diffusive; partially rate limited Diffusive; rate limited

where DaI is the Damköhler-I number, Aq is the effective surface area of the ore mineral, r is the reaction rate, and l' is again the scale of interest. When DaI is much larger than 1, the concentration of metal is close to the equilibrium concentration, P is solubility limited, and Ci is constant. If DaI is less than 1, then P is rate limited, and Ci is determined by the rate of chemical reaction r. In addition, the rate of diffusion can affect r, which is a function of Ci for most chemical systems where kinetics predominates over equilibrium. For cases where DaI is less than an order of magnitude larger than 1, concentrations are affected by reaction kinetics and fluid flow. Table 3.1 shows a simplified matrix of possible combinations of Pe and DaI. For ISRR transport regimes, the upper left corner of the matrix is most favorable and least favorable in the lower right. The theory of EPM has been enormously successful for groundwater hydrology where the scale of interest is relatively large, involving aquifer-sized regions or even watersheds. Primary aquifers and oil reservoirs are bedded sandstones and other granular sedimentary rocks that have sufficient porosity or storativity and permeability to be significant sources of water and oil. Rollfront uranium deposits are created in porous sandstones, and application of EPM theory for ISRR has been very successful. More recently, EPM theory has been applied to flow and transport in fractured rock. For large scales of interest, this approach has been successful as the representative elemental volume (REV) becomes large such that the number of fractures acts as a connected continuum of pore spaces. For smaller scale phenomenon, EPM theory becomes less valid even for porous rocks as heterogeneities have stronger effects. One example of this is the skin effect of wellbores where local damage to the rock matrix may decrease K near the wellbore. Further discussions of wells and well fields for ISRR are presented in Chapter 5. One important transport phenomenon that is highly influenced by scaling effects in EPM and other geologic media is dispersion. Dispersion is a stochastic phenomenon that occurs as a result of random variations in flow velocity, path, and dissolved constituent mixing within EPM (Gelhar 1993). One analogy is the effect on traffic when a lane closure forces cars moving in the same direction at the same speed to slow down and merge to compensate Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Flow and Reactive Transport in Geologic Media

100 50

29

100 50

50 25

25 0

Exit

Launch

FIGURE 3.2  Pachinko machine

for the loss of one lane. Frustratingly, some drivers sometimes speed ahead and cut into line just before the merge point, so the original sequence of cars is not maintained and polite drivers get even further behind. The game of pachinko, which was popular in the 1970s, is a good analogy for understanding dispersion (Figure 3.2). The metal marbles of pachinko, representing water molecules, are launched to the top of the machine by a spring and trickle downward under gravity through a series of randomly placed pegs. The balls exit the machine via slots at different levels, and more difficult slots to intercept result in a higher score if successful. Even if one attempts to launch each ball with the same velocity, the paths differ because of minute variations in speed, ball weight, and by collision with other balls launched earlier. The distribution of balls in the machine, or concentrations for a given unit area, are different within time and space (two-dimensional for this game). If the balls are all the same color and launched at the same rate, the number of balls exiting the machine per unit time are the same on average. However, if different colored balls (representing dissolved constituents in the mass of plain balls) are launched in sequence over a finite period, the number of balls of a given color exit the machine according to a Gaussian or other distribution depending on the launch sequence and duration (Figure 3.3). In addition, the maximum concentration of the colored balls from the top to the bottom of the game varies with time. The coefficient of dispersivity is a scale-dependent property of the EPM that when added to the coefficient of diffusion (D) equals the coefficient of hydrodynamic dispersion or simply the coefficient of dispersion (Dl ): D l = α l v + D (Equation 3.8) where al is the dispersivity (Freeze and Cherry 1979). Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

30 CHAPTER 3 5

Marbles Exiting Machine

4

3

2

1

0

10

20

30

40

50

60

70

80

90

100

110

120

Time, s Marbles Linear (Average)

Blue Marbles 2 Per Moving Average (Blue Marbles)

FIGURE 3.3  Estimated rate of blue ball arrivals when launched in sequence in a pachinko machine

Dispersion is not of primary interest for water resource evaluation because dispersion of water molecules does not affect the overall capacity of and production from the reservoir. However, dispersion becomes highly important in investigations of contaminant transport in groundwater and in the production of oil and gas plus ISRR of metals and other commodities because dispersion results in a wider distribution of concentrations of solutes and colloids in flowing aqueous solutions than is observed in idealized plug flow conceptualized in Figure 3.1. Obviously, this is important to ISRR because dispersion creates a distribution of concentrations in the well field as the lixiviant flows from injection to production well, even for highly advective non-rate-limited systems (Table 3.1). Many metal deposits are hard-rock hosted and the primary porosity is dominated by fracture flow. As discussed in Chapter 2, primary or hypogene ore deposit formation occurs primarily via fracture flow, and mineral deposition creates veins. Our understanding of fracture flow has advanced tremendously in the past three decades owing to the intensity of research on nuclear waste repositories (Bear et al. 1993) and hydraulic fracturing of tight oil reservoirs. Despite these advances, EPM theory is often applied to fractured media at relatively large scales. However, the characteristics of many natural fracture systems often require that the design of well fields for ISRR requires Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Flow and Reactive Transport in Geologic Media

31

characterization of fractures and consideration of discrete fracture flow especially when well-to-well connections are anticipated at length scales of several meters. This is because highly transmissive zones may result in a condition where DaI < 1 and solution grades may be diluted. Moreover, direct fracture connections between wells may also result in focused flow, poor sweep efficiency, and poor recovery from the ore body. As discussed in Chapter 5, this condition can be avoided with proper well-field design and operation and by application of conformance technologies. The differences between fracture flow and flow in classical porous media can be first examined by comparing the representations on porosity, which is related to permeability. The porosity of classical porous sediments and soils is (Nelson 1985): Vp θs = V # 100 (Equation 3.9) b where Vp is the volume of voids in the REV of the porous rock, and Vb is the total volume of the REV. In a nonclassical porous media, some of the voids are not connected, and qs is the effective porosity of interest or the connected porosity. For fractured rock, the fracture porosity can be represented by (Nelson 1985): a θf = F + a

# 100 (Equation 3.10)

where qf is the fracture porosity, a is the average effective width of the fractures across the REV, and F is the average fracture spacing or distance between individual fractures. Mineralized fractures (i.e., veins) do not contribute to porosity unless the vein filling can be dissolved or is porous, and only the unmineralized or open fracture width should be used in Equation 3.10. Advective flow along a single fracture can be expressed mathematically by (Nelson 1985) 2h # ρg (Equation 3.11) μ 2l where 2h/2l is the hydraulic gradient that describes viscous flow of a Newtonian fluid between two parallel and impermeable plates. One of the important implications of Equation 3.11 is that the rate of flow per unit area is proportional to the cube of the fracture width or aperture. Hence even when F is small or the fracture density is high, flow may be restricted by small fracture aperture. For most applications in ISRR, single fracture flow is not as important as the aggregate flow through all permeable fractures and the rock matrix if Q a3 = A 12F

#

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32 CHAPTER 3

it has sufficient porosity and permeability. Therefore, the total effective flow is described by Darcy’s equation where the combined permeability for a random fracture network is (Kazemi and Gilman 1993) ka = kr + kf Øf(Equation 3.12) where the fracture permeability (kf  ) is (Nelson 1985) a 2 # ρg (Equation 3.13) k f = 12 μ In some texts, the concept of fracture transmissivity is used: T f = K f # a (Equation 3.14) where Kf is the fracture hydraulic conductivity. Hence, the fracture can be thought of as a very thin transmissive aquifer. This concept is generally only used in theoretical studies of fluid flow as fracture networks are usually of more interest for subsurface applications like ISRR. In many hydrology applications for water resource development, combining fracture and matrix hydraulic conductivity is a reasonable assumption. However, the concept of dual porosity is needed for mineral production because the mineral of interest is often within the matrix of the aquifer but accessible only through fractures. In real aquifers and rocks, there is a continuum of porosity connections within a fractured reservoir, and there is usually a bimodal distribution of fracture and matrix porosities and hydraulic conductivities. In fractured sandstone oil and gas reservoirs, for example, fracture storage capacity or porosity is low and conductivity is high, whereas in the matrix, porosity and storage is high and conductivity is low (Kazemi and Gilman 1993). In fractured bedrock host rock, much of the fluid storage capacity and conductivity of the ore body is dominated by fractures. However, the ore mineralization may be disseminated and reside in the low-porosity and low-conductivity matrix or it may be vein hosted along conductive fractures. Hence, the concept of dual porosity is very useful in ISRR to describe the relationships of ore minerals to effective fluid pathways. FLOW FIELD Three-dimensional flow in a homogeneous isotropic EPM is often conceptualized by two components: 1. Lateral flow in the x–y plane 2. Vertical flow in the z direction This is convenient for nearly horizontal stratigraphic units or sedimentary beds and soils that are laterally continuous. Hence, lateral flow is driven by Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Flow and Reactive Transport in Geologic Media

33

pressure potential (∂ϕ) along the principal flow directions, and vertical flow is driven by gravitational forces. The flow field can be expressed by Equations 3.15 and 3.16 (Stephens 1996): 2ϕ 2ϕ q x =− K ^ θ h 2x = q y =− K ^ θ h 2y (Equation 3.15) 2ϕ 2ϕ zl l ^ hb q z =− K ^ θ hb 2z + 2 2z =− K θ 2z + 1 (Equation 3.16) where qx,y,z is the specific discharge or flow per unit area in the three principal directions. These equations are generalized for media of variable saturation where K(q) is the hydraulic conductivity at a water content equal to q and water potentials (2ϕ/2x), (2ϕ/2y), (2ϕ/2z) in the x, y, and z directions, respectively. For saturated conditions, the saturated hydraulic conductivity is used in Equations 3.15 and 3.16, and the pressure potentials are equal to the hydraulic head (2h/2l ). Although contrary to popular opinion, it is possible to conduct ISRR under variably saturated conditions; however, it is more likely that in situ metal recovery targets are relatively deep and saturated. In other cases, it is desirable to flood the ore body and saturate it in the process. Conversely, in situ metal remediation targets may be located in shallow soils and in the vadose zone and require unsaturated flow characterization and analysis. Because unsaturated flow is driven by capillary and gravity forces, it is more difficult to contain leach solutions unless the ore footwall is rock of very low hydraulic conductivity. Most rocks are not completely homogeneous and isotropic, especially in fractured systems, so the hydraulic conductivity is represented by a secondrank tensor (Stephens 1996) that represents the changes in the property in three dimensions. Fortunately, modern computers can readily simulate 3-D flow (saturated or unsaturated) in nonhomogeneous and anisotropic media using finite element and finite difference algorithms if an experienced computer modeler is provided sufficient hydrologic information, time, and budget. Dispersion is usually represented by its longitudinal and transverse coefficients (Bethke 2008): 2C i qD xi =− θD L 2x (Equation 3.17) 2C q Dyi =− θDt 2y i (Equation 3.18)

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34 CHAPTER 3

And dispersion is greatest along, or longitudinal to, the principal direction of flow. Like hydraulic conductivity, the coefficients of dispersion are represented in tensor form to account for 3-D flow in numerical models. The coefficient of lateral dispersion (Dt ) is usually an order of magnitude less than the coefficient of longitudinal dispersion (DL ). Depending on the scale of the system, representation of discrete fracture flow, and so forth, order of magnitude variations in dispersion are common in typical host-rock systems. The overall effect of increasing longitudinal dispersion is to disperse the recovery of tracer or metal at the production well and lower the peak concentration as compared with piston flow, which results in maximum grades in solution and a very narrow period of recovery. For ISRR well fields with well-to-well spacings measuring meters to tens of meters long (or feet to tens of feet long), longitudinal dispersion coefficients may be on the order of centimeters (inches) to several meters (feet). However, in the context of well-field scale containment in regional groundwater flow systems, coefficients of longitudinal dispersion may be tens to hundreds of meters or feet (Lantz and Statham 1994). Lantz and Statham (1994) note that it is often the case that unquantified heterogeneity in the ore deposit, owing to practical limitations on collection of hydraulic data, may be compensated by dispersity in a flow model. As discussed in Chapter 5, dispersion at the margins of the well field may also impact the overall mass balance of metal and other constituents in the leach solution as it is recycled over the course of ISRR operations. DISSOLUTION AND PRECIPITATION REACTIONS Aqueous solutions at near standard state conditions are most commonly used in ISRR, and water is sometimes referred to as “the universal solvent” because of its ability to dissolve many substances (Brimhall and Crerar 1987). Dissolution and precipitation are familiar chemical reactions but are complex at the molecular level. An empirical level analysis of metal dissolution into a lixiviant is sufficient for ISRR project planning, but a deeper level understanding is useful in diagnosing problems with achieving production goals for metal grade in solution and metal recovery. Most dissolution reactions are written so that one or more metals in a mineral are brought into solution by dissociation into metal cations (Me) and anions (An) of valences +zc and –za, respectively, which are held together by electrostatic forces in the solid form: MexAny = XMe[+zc] + YAn[–za]

(Equation 3.19)

where X[+zc] = –Y[–za](Equation 3.20)

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Flow and Reactive Transport in Geologic Media

35

X and Y are the stoichiometric moles of cation Me and anion An in the solid phase, respectively. Metals and minerals that are not highly soluble in pure water may be dissolved by the presence of ligands dissolved in water such that the metal forms chemical bonds with the ligand ion, which break the chemical bonds in the mineral structure. MexAny + NLi–v = MexLin+z–v + YAn–v(Equation 3.21) The metal–ligand complex may be charged and dissolved electrostatically or neutral and dissolved covalently. Polarized and dissociated water molecules support the metal cation and anion in solution up to the limit of solubility, which varies greatly. The dissociation of ions in the solvent may be dominantly electrostatic, covalent, or a mixture. The degree of dissociation of Equation 3.19 for a given mineral or solid in its equilibrium state is represented by its equilibrium constant (Nordstrom and Munoz 1985): K eq =

X Y a Me +zc a An −za a Me x An y (Equation 3.22)

The activity of pure minerals is 1 at standard state conditions, so Equation 3.22 simplifies to

X Y K eq = a Me +zc a An −za (Equation 3.23)

Equation 3.23 is also known as the solubility product for the solid phase Y X (Nordstrom and Munoz 1985). The activities a Me +zc and a An −za are related to X Y the concentrations of Me[+zc] and YAn[–za] in solution ( m Me +zc and m An −za , X moles per kilogram or molal, respectively) by the activity coefficient ( γ Me +z [+zc] for the metal Me ): X X X a Me +zc = γ Me +zc m Me +zc (Equation 3.24)

And ( γ YAn −za for the anion An[–za]) a YAn −za = γ YAn −za m YAn −za (Equation 3.25) Depending on the composition of the solution, pressure, and temperature, the activity coefficient may be close to or far from 1. Therefore, it may not always be correct to assume that the concentration of a metal in solution can be calculated directly from its solubility product. Several publications provide tables of the saturation concentrations of salts and other highly soluble minerals in water at standard state and other selected conditions (e.g., Lide 1992). However, for metal sulfides and oxides, the saturation concentration of the metal is more difficult to calculate, especially for complex leach

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36 CHAPTER 3

solutions with high total dissolved solids, and may require computer software that can simultaneously calculate the activities of all chemical species in an aqueous solution using comprehensive chemical analyses of the leach solution and electrolyte theory (Nordstrom and Munoz 1985; Nordstrom and Nicholson 2017). Many minerals, including the major rock-forming silicates, do not readily dissolve in water at the surface of the earth even though they are not at equilibrium under standard state conditions. Some of these rock-forming minerals only form under very high temperature and pressures and are far from equilibrium at the earth’s surface, but owing to the strong chemical bonds of the elements in some minerals, they only dissolve or weather very slowly. Most gemstones, including diamonds, are also far from equilibrium yet last for thousands of years mounted in jewelry. Only over very long periods of time, usually thousands to tens of thousands of years depending on climate (Nahon 1991), do feldspars and other major rock-forming silicates weather and form clay rock soils. Therefore, the leachability of ore minerals cannot be predicted by equilibrium constants alone. However, very large or very small values of Keq can indicate the relative stability and solubility of ore and gangue minerals and reaction products. As these values vary over many orders of magnitude, the logarithmic (logKeq ) values are usually tablulated in thermodynamic databases. In a sense, in situ recovery seeks to speed the weathering process by altering pH, redox, temperature, pressure, composition, and other parameters of the aqueous solution. In practice, metal concentrations in leach solutions are usually not in equilibrium with the ore minerals and ore grades in solution, and leaching rates are measured directly in laboratory leaching tests. Samples are collected periodically over the test period, and the metal and other chemical parameters are measured in the analytical laboratory. Test data is then used to estimate expected concentrations and leaching rates during ISRR by scaling the laboratory sample size (e.g., length and diameter for column tests) and residence time to the estimated contact volume and residence time for injection well to recovery well flow paths. Hydrometallurgical testing methods are discussed in more detail in Chapter 4. It is rare that metal leaching reactions during ISRR attain perfect equilibrium and instantaneously reach a solubility limited concentration for the mineral source. The reaction rate can be relatively simple or complex depending on the mineral source. A simple first-order reaction rate is expressed by (Bethke 2008): 2n Q rk =− 2t k = A θ k e 1 − K o (Equation 3.26) eq

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Flow and Reactive Transport in Geologic Media

37

where rk is the rate (in moles nk per unit time t), Aq is the surface area of the mineral source, k is the rate constant at standard state conditions, Q is the reaction product

a X a YAn Q = Me a Me An +zc

x

−za

≥ or ≤ Keq(Equation 3.27)

y

and Keq is the equilibrium constant for the reaction as previously defined. Therefore, the rate is dependent on the mineral surface area and the concentration of metal cations and anions in solution. As the concentrations increase to the solubility limit or equilibrium concentration, the rate decreases to zero. During ISRR, if Q approaches or is equal to Keq along a flow path, the leach solution is fully loaded, the metal concentration cannot increase, and the reaction rate goes to zero. In some cases, the solubility limit is first reached with respect to a different mineral than the source. More complex rate laws can account for the strength of the lixiviant or ligands and inhibiting ions. The rate constant k is a function of temperature and to a lesser degree pressure. The temperature dependence can be calculated using the well-known Arrhenius equation (Bethke 2008) but is not discussed further here. As a rule of thumb, increasing temperature generally doubles the rate of metal loading during ISRR for every 10°C (50°F) increase in temperature. In general, the major rock-forming silicate minerals dissolve in weak acids at a much slower rate than carbonate minerals and are considered to be inert for practical purposes, but silicates dissolve more rapidly at higher temperatures. In contrast to solids, gases may dissolve into aqueous solutions and attain equilibrium readily without dissociation as neutral species: O2(g) = O2(aq) ; Keq ~

m O2 (aq) PO2 ( g)

(Equation 3.28)

where PO2(g) is the partial pressure of oxygen in the gas phase (Nordstrom and Munoz 1985). Equation 3.28 assumes that oxygen and other major gases in the atmosphere are close to ideal and the aqueous activity coefficient of neutral species is usually close to 1. Standard chemical properties references also provide dissolved gas concentrations at standard state and other conditions in tabular form (e.g., Lide 1992). Equation 3.28 is meaningful for ISRR because metal oxidation is important in dissolution and precipitation reactions, especially for metal sulfides, in order to oxidize the sulfide anion and destabilize strong metal–sulfide bonds. Because the solubility of oxygen is related to pressure, maintaining sufficient oxygen in solution to achieve the target metal grade in solution may be difficult in relatively low-pressure systems. Furthermore,

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38 CHAPTER 3

oxidation and reduction reactions are often kinetically slow as compared to simple dissociation. Additional reactions that usually attain equilibrium quickly include metal exchange and adsorption reactions with gangue minerals. In the case of clay minerals and organics, the exchange reactions can significantly reduce the metal grade in solution and recovery. This reaction is often referred to as preg-robbing or robbing metal value from the leach solution. Quantitative models of ISRR leaching reactions, grades in solution, and recovery are often derived empirically from batch leaching and column leaching experiments conducted in the laboratory or from small pilot tests. Usually the numerical representations only consider the metal and a few other necessary components like reagent (acid, oxidant, ligand, etc.) consumption. Similarly, contaminant transport models usually only include one or two components and assume that the matrix is inert except for attenuation by sorption or exchange. Other reactions may be taking place during ISRR and impact the efficiency of leaching. Some gangue minerals may consume a reagent or act as preg-robbers. In other cases, the gangue mineral consumption of a reagent may cause chemical precipitation of a mineral that can reduce porosity and permeability. Hence, it is important to consider all significant reactions in the ISRR project and their potential impact on metal recovery and reagent consumption. Empirical models are based on a limited number of laboratory trials and usually fixed flow direction and length from injection to recovery. In fullscale systems, the ISRR system involves many flow paths or streamlines, each having its own chemical evolution from injection to recovery. These different chemistries are mixed at the production well to yield an aggregate leach solution chemistry. As discussed in Chapter 5, the effects of variable flow velocity, path length, and multiple flow paths on pregnant leach solution (PLS) chemistry poses one of the greatest challenges to ISRR well-field design and operation. In heap-leaching applications, numerical models assume relatively rapid advective vertical flow in the pore space surrounding rock fragments, which carries leached metals that diffuse from the interior of the fragment to the flow channel where they are transported by advection to the collection system or liner (Bartlett 1992). In ISRR, there are multiple flow paths in different directions, and the advective flux of metal Me in the principal directions is qAxMe = qxCMe(Equation 3.29) qAyMe = qyCMe(Equation 3.30)

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39

Flow and Reactive Transport in Geologic Media

qAzMe = qzCMe(Equation 3.31) respectively (Bethke 2008). Similarly, there are three dimensions of diffusion and dispersion in an ISRR system. The velocity of fluid along a flow path can be calculated from its Darcy velocity: K 2h 2l  (Equation 3.32) v= θ Many different fluid velocities exist along well-to-well flow paths, so there is a distribution of lixiviant and metal concentrations in time and space because of the different path lengths. This creates different scales of chemical gradients within an ISRR system from individual pores to the entire well field and results in a distribution of arrival times at the production well.

t l = vl 

(Equation 3.33)

The well-to-well chemical gradient is dominated by injection of fresh lixiviant in the injection well and recovery of spent or nearly spent lixiviant and metal-loaded solutions (PLS) at the production well. The metal sources and sources of other nonessential components (Ri terms in Equation 3.1) are located all along the flow path. As these are depleted with time, the reaction front shifts toward the production well until the metal of interest is depleted to a grade lower than the economic limit of maintaining injection and recovery. The flow and reactive transport principles discussed in this chapter can be illustrated by a simple one-dimensional simulation of the natural attenuation and retardation of a migrating hexavalent chromium in an aquifer with pyrite and hematite. This simulation can be conducted by several of the commercial and publicly available computer software applications for reactive transport (Nordstrom and Nicholson 2017). Figure 3.4 shows the simulated progression of the metal reaction front with time in a hypothetical one-dimensional flow path, where hexavalent chromium at a concentration of 1 mg/L enters the aquifer at time zero and is reduced by electron transfer from ferrous iron in hematite and pyrite. The dissolved hexavalent chromium is reduced to a less toxic and soluble trivalent chromium. However, the reaction is not fast enough to attenuate the entire flux of chromium entering the aquifer and breakthrough occurs in 10 years. The reaction also oxidizes both pyrite and magnetite, which results in the formation of hematite (Figure 3.5). In Figure 3.6, Dl is increased by a factor of 10, the reaction front moves more quickly by dispersion, and breakthrough occurs in six years.

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40 CHAPTER 3 1e5 1

Y4

Y 1.2 Y 2

0

Y 10

Y8

Y6

CrO4–, µg/L

1e–5 1e–10 1e–15 1e–20 1e–25 1e–30

0

100

200

300

400

500

600

700

800

900

1,000

X Position, m 1 m = 3.28 ft Constant Discharge = 30 m/y (10 m/y = 32.8 ft/y) Longitudinal Dispersivity = 10 m (32.8 ft) Permeability = 0.3 Darcy (~1 × 10–5 ft/s) Porosity = 0.3

FIGURE 3.4  Natural attenuation of hexavalent chromium plume (1 mg/L) by ferrous iron minerals

Y8 0.01 Y 4

Hematite, wt %

Y 1.2 Y6

0.001

Y 2 Y 10 1e–4

1e–5

0

0

100

200

300

400

500

600

700

800

900 1,000

X Position, m 1 m = 3.28 ft (~1 × 10–5 ft/s)

FIGURE 3.5  Reduction of hexavalent chromium results in oxidation of ferrous iron minerals magnetite (2 wt %) and pyrite (1 wt %) and precipitation of hematite Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

41

Flow and Reactive Transport in Geologic Media

1e5 1

Y 1.2 Y 2 Y 4 Y 6 Y 8 Y 10 0

CrO4–, µg/L

1e–5 1e–10 1e–15 1e–20 1e–25 1e–30

0

100

200

300

400

500

600

700

800

900 1,000

X Position, m 1 m = 3.28 ft Constant Discharge = 30 m/y (100 m/y = 328 ft/y) Longitudinal Dispersivity = 100 m (328 ft) Permeability = 0.3 Darcy (~1 × 10–5 ft/s) Porosity = 0.3

FIGURE 3.6  Natural attenuation of hexavalent chromium plume (1 mg/L) by ferrous iron minerals

In three dimensions, the reaction and depletion fronts form shells around the injection well that expand outward until they reach the production well when the practical recovery of metal has been reached. The shells become thicker and more diffuse with greater Dl . Reaction fronts can also become more complex with greater heterogeneity in hydraulic properties and if there is more than one metal source with different rates of reaction. As discussed in Chapter 5, multiple injection and recovery well-field operations are designed to optimize recovery curves throughout the ore body.

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CH AP TER   4

Geochemistry and Hydrometallurgy

INTRODUCTION Most in situ recovery and remediation (ISRR) applications use fluid-based transport of metals to surface processing plants via wells and pumps, using natural rock pores and fractures as the primary conduits. Some ISRR technology is mechanically based, such as the borehole miner (Savanick and Miller 1997), which is mentioned again in the discussion of future directions in Chapter 10. Fluid-based ISRR uses the same hydrometallurgical concepts and techniques that have been developed for conventional processing of mined ores. In most hydrometallurgical systems that have been used commercially for leaching, the base fluids are aqueous solutions. Air and pure gases, such as oxygen, are sometimes dissolved in the solutions to boost oxidation potential. However, supercritical carbon dioxide (CO2), which is the stable phase of CO2 at the high pressures that may be attained in deep ISRR systems, has been proposed as a base solvent for rare earth elements (REEs) and other metals (Sinclair et al. 2019). The Society for Mining, Metallurgy & Exploration has published two encyclopedic volumes on mineral processing covering the huge array of complex and large-scale processing techniques that have been developed to satisfy our enormous and ever-increasing demand for minerals and metals (Dunne et al. 2019). Summaries of solution mining and in situ leaching are included in the SME Mineral Processing & Extractive Metallurgy Handbook, Volume II (Hiskey et al. 2019) and in Bartlett (1992). Metal recovery from ISRR leach solutions is usually accomplished using similar or modified metallurgical and hydrometallurgical technologies as those used for tanks, vats, or heap-leaching operations. Therefore, they are not extensively discussed in this chapter as there are ample resources in the literature. However, surface processing operations must be compatible with the ISRR processes. For example, solvent extraction and electrowinning (SXEW) technology has been very successful for production and refining of copper (Jergensen 1999) and other metals from pregnant leach solutions (PLSs). However, solvent extraction (SX) produces organic and colloidal particulates

43

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44 CHAPTER 4

that can clog well screens unless filtered or decanted prior to reinjection of raffinate (Isaacson and Redden 1994). Activated carbon and ion exchange resins are also used to selectively extract metals from PLS as well as for water treatment and to provide good filtration of process solutions. SOLUTION–MINERAL CONTACT The primary difference between conventional and ISRR metal recovery is ore comminution—the first stage of conventional ore processing beginning with blasting with explosives (Figure 4.1)—which is far less intensive to nonexistent in an ISRR operation. Much research effort has been focused on rock fragmentation for ISRR (Otterness et al. 1994) using techniques such as blasting, propellants, microwaves, sonic waves, plasmas, and so forth. However, the degree of ore disaggregation by any method is much reduced compared to even that which is accomplished with open cast or underground blasting in conventional mining operations. Because of the lithostatic pressures and absence of free faces within intact rock masses, blasting for ISRR enhancement can only create new fractures and redistribute porosity, and creation of new porosity is minimal. Some attempts at using blasting for permeability enhancement for ISRR have even resulted in reduced permeability by the formation of fine solids near the blast zone (Otterness et al. 1994), which is sometimes located in the well itself that is the target for stimulation. The exception to this is in brownfield ISRR that is located near surface or underground mine workings where overburden displacement is possible as in drill-and-blast open pit bench leaching (Figure 4.2). In these operations, like conventional mining, the in situ rock mass fragments and moves as a function of the properties of the explosive and rock mass and the confinement provided to the explosion gases. In deeper confined ore bodies with no nearby surfaces or workings, the fragments cannot significantly move, and fractures cannot expand and open. In the 1970s, nuclear weapons were tested for fracturing relatively deep oil and gas reservoirs, and large nonnuclear blasts were conducted for in situ recovery of copper by leaching as part of the U.S. government’s Project Plowshare (DOE 2012). Nuclear testing stimulated oil and gas reservoirs to a depth of 2 km (1.24 mi) but resulted in radioactive contamination of the resources and was found to be uneconomical. The large nonnuclear blasts at Old Reliable and Big Mike copper mines in Arizona and Nevada, respectively, were successful in enhancing permeability for leaching with sulfuric acid but required underground and open pit workings for megaton rock mass movement and fragmentation (Bartlett 1992). The average diameter of the rock fragments was approximately 30 cm (11.8 in.), which translates to a lower fracture density than within the primary or hypogene ore zone of a porphyry Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

45

Geochemistry and Hydrometallurgy

FIGURE 4.1  Pit bench blasting prior to acid irrigation for in situ copper leaching

Secondary Copper Extraction Copper Cathode

Mine Pit Gossan Supergene Cu Ore

Irrigation

Processing Plant Pump

Acid Lake Cu+2

Hypogene Cu Ore

FIGURE 4.2  In situ pit bench leaching with in-pit recovery of leach solutions

copper deposit (see Figure 2.3). At Big Mike, the blastholes were up to 60 m (200 ft) deep, but blastholes in most hard-rock mines are only about 10 m (33 ft) deep (Hartman 1987). However, no applications of underground nuclear and large conventional explosions for mining or ISRR have followed under the nuclear test ban and owing to the high degree of risk involved. The creation of fractures in rock in and of itself does not necessarily create porosity and permeability unless the fractures are open or dilated. Hydraulic fracturing, or fracking, which has resulted in new production from tight shale oil and gas reservoirs, employs proppants that are usually high-strength Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

46 CHAPTER 4

silica sand of very even grade to maintain porosity once the fracture has been generated (Economides and Nolte 2000). However, fracking results in relatively few fractures that allow the fluid oil and gas to flow from reservoir pore storage toward the recovery well under hydrostatic pressure. Fracking does not generate enough open microfractures to enhance sweep efficiency to the degree that fluids are more likely to contact disseminated ore minerals in the rock matrix. Crushing and grinding can liberate much more ore mineralization than blasting and is the only recourse when permeability of the natural ore is very low and ore minerals are hopelessly locked in the rock matrix and as inclusions within insoluble gangue minerals. These processes, however, require huge amounts of energy for hard-rock ores and result in the generation of fine waste material (tailings) that have enhanced surface area and reactivity with meteoric water and may generate contaminated drainage that has to be contained and treated. Hence, the primary means for fluid ore particle contact in ISRR is via natural pores in the host rock sometimes augmented in some cases with artificial fracturing near the wellbore to enhance well efficiency and injection or recovery capacity. Acid stimulation can also be used to enhance permeability in carbonate-hosted ores and other partially soluble host rocks (Economides and Nolte 2000). Reservoir engineers and hydrologists have attempted to quantify recovery from porous rocks in terms of sweep efficiency for specific well-field designs and operating parameters based on macroscale flow distribution analysis of continuous porous media. The common definition assumes that the pores are originally filled with given portions of oil, gas, and water that are displaced by another fluid during recovery. A fraction of the original fluid may be left behind because of failure of the invading fluid to move through small pores, hence the fractional sweep efficiency may be less than 1, or less than 100%. Sweep efficiency in ISRR applications is a complex metric to quantify and is a function of microscale to macroscale processes. The probabilistic approach presented in Ganoulis (1994) is helpful in understanding microscopic or pore-scale sweep efficiency. Pore-scale flow can be defined stochastically for a porous media with a heterogeneous distribution of connected pores with a distribution of sizes of radius r (Ganoulis 1994) or idealized two-dimensional fractures of infinite extent with aperture a (Figure 4.3). Advective transport along a fracture becomes ineffective at a critical fracture aperture ac under constant hydraulic gradient and steady-state conditions where the Darcy velocity v approaches zero: v c ? a c ~0 

(Equation 4.1)

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47

Geochemistry and Hydrometallurgy

P1

P2

a≈ac

a>>ac

aac

– lf –

FIGURE 4.3  Well-to-well fracture flow and aperture distribution effect

As the permeability of the rock in Figure 4.3 is a function of fracture aperture (see Equation 3.13), the probability (P) of advective transport (or sweep A) is a function of the pore size density distribution function f  (a) as expressed in Equation 4.2: A = P ^a 2 a ch = 8 a c f ^ah 2a  3

(Equation 4.2)

where the pore density distribution is similar and related to the particle size distribution, or fracture density and network for fracture flow. In situ recovery of metals is also dependent on the distribution of ore minerals that are in proximity to pores and fractures with a > ac so that the dissolved metal can be transported advectively. In equivalent porous media continuum theory, the pores are interconnected such that the entire matrix is in connection with the pores as in a saturated sandstone or other unconsolidated rock. Within consolidated rocks, matrix minerals may be isolated by regions of low porosity and permeability (i.e., a > ac) or no pore and fracture connections altogether. In many cases, the ore minerals are preferentially located within natural pores or fractures as they were deposited by hydrothermal or supergene fluids. Hence, the probability of the lixiviant dissolving an ore mineral along a transmissive flow path is proportional to the probability

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48 CHAPTER 4

of the pore or fracture intersecting the ore mineral i with modal abundance m, assuming the ore mineral is not refractory (non-leachable) in hydrometallurgical terms: M ? P^ a + m i h 

(Equation 4.3)

AM = SWeff 

(Equation 4.4)

As discussed in Chapter 3, the rate of dissolution of an ore mineral is a function of its reactive surface area at the mineral fluid interface, which is a result of mineral texture, structure, time, and so forth. Therefore, Equation 4.3 is only an approximation of the actual recovery potential as the intersection may yield different surface areas depending on geometry, mineral-specific surface area, and reaction mechanisms. As a simplification for discontinuous porous media, the product of unconstructed flow (A) and ore mineral contact (M ) factors yields the potential or effective sweep efficiency (SWeff ), assuming a uniform hydraulic gradient within the rock volume of interest. Equation 4.4 illustrates that the artificial generation of discrete, random fractures to access ore minerals that are usually a few percent or less in modal abundance is not an effective strategy for enhancing sweep efficiency and recovery from an ore body because the probability M is low unless the fracture density and A is very high. In Chapter 5, the effects of hydraulic gradient and flow field on macroscale sweep efficiency are discussed. In theory, it is possible to collect enough core samples to conduct microscopic analysis of pore size, fracture apertures, and mineral texture to determine A and M and their correlations, but pragmatically, it is usually not cost effective to analyze enough samples to represent the entire distribution in an ore body. Core logging, microscopic analysis of core samples, and other petrographic techniques can provide a semiquantitative estimate of these parameters for feasibility analysis supported by laboratory and pilot testing (Brink et al. 1991; Blake and Earley 1990). Such analyses can determine the style of mineralization, which may or may not result in a preferential deposition of ore minerals in available pores or within inaccessible zones of the matrix. Pores that were once active during ore mineralization events may be filled with gangue minerals coeval with subsequent hydrothermal or supergene processes. Petrography and mineralogical characterization techniques used for hydrometallurgical liberation characterization (Bradshaw et al. 2019) can be used and adapted for ISRR (Brink et al. 1991; Blake and Earley 1990). The focus of ore petrographic characterization for ISRR is the relationship of ore minerals to pores and fractures where lixiviants can be dispersed through the rock to contact the target ore minerals. A variety of textural relationships

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Geochemistry and Hydrometallurgy

49

(a)

(c)

(b)

FIGURE 4.4  Microscopic analysis of three gold occurrences: (a) fracture hosted, (b) encapsulated by sulfide, and (c) interstitial between gangue minerals and pores (scale = 10 µm)

of gold mineralization occur in different ore types (Blake and Earley 1990; Figure 4.4) including the following: ■■

In open fractures ■■ Within porous matrix of host rock ■■ Encapsulated by silica, pyrite, and other minerals Petrographic characterization for hydrometallurgical planning uses this information to determine how to most efficiently liberate the ore minerals from the host-rock matrix to achieve the target recovery by leaching, flotation, and so forth. Quantitative scanning electron microscopy is an automated petrographic mapping instrument that is now commonly used with computer software analysis for hydrometallurgical characterization of solution– mineral contact (Cook et al. 2017) and can be used for the same purpose for ISRR projects. ISRR utilizes the natural texture of the host rock to achieve contact with ore minerals. In some projects, permeability modification may Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

50 CHAPTER 4

Source: Earley et al. 1997

FIGURE 4.5  Atacamite-filled fracture in core sample (field of view = 10 mm)

create connected pore access to additional ore minerals for metal recovery, but recoveries are always lower than standard milling technologies. Brink et al. (1991) also show how pre- and post-leach test petrographic characterization can be used to determine solution–mineral contact efficiencies and reaction products. Figures 4.5 and 4.6 show pre- and post-leach core samples that have been subject to acid leaching in a pressure leaching vessel. In Figure 4.5, atacamite, a very soluble copper chloride mineral, is the primary host mineral for copper in the ore and occurs in veins, fractures, and porous iron oxides that are accessible to leach solutions. Atacamite can be leached by sulfuric acid along these zones until breakthrough occurs, and the acidic leach solution short-circuits along the right wall of the cased core. Figure 4.6 is a photomicrograph of the leached zones of porous iron oxide produced by stoichiometric leaching of atacamite. Brink et al. (1991) discuss special pre- and post-leach core sample preparation techniques, such as epoxy impregnation, that can be used to preserve textural relationships like those shown in the figure. Other minerals, such as chrysocolla, do not leach stoichiometrically and leave a secondary reaction product. In the case of chrysocolla, an amorphous silica residue remains after leaching and it contains traces of aluminum iron and copper as shown in Figure 4.7. Copper can also substitute by exchange for iron in iron oxides and interlayer potassium in biotite (Brink et al. 1991). In some ores, disseminated copper in matrix minerals can account for 50% or more of the total copper grade and is not readily accessible to leach solutions via natural pores or fractures (Earley 1994). The leaching chemistry of

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Geochemistry and Hydrometallurgy

51

Source: Earley et al. 1997

FIGURE 4.6  Open voids in leached atacamite ore core sample (field of view = 5 mm)

Source: Earley et al. 1990

FIGURE 4.7  Leached chrysocolla in core sample; amorphous silica and clay shown as unleached residues (field of view = 5 mm)

stoichiometric, nonstoichiometric, and other ancillary reactions during ISRR are discussed further in the next section. LEACHING CHEMISTRY The mineralogy of the ore is one key to ISRR success because if the target ore mineral cannot be dissolved and brought into solution, then there is no

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52 CHAPTER 4

potential for recovery by leaching. Such ore mineralization is often called refractory (Yannopoulos 1991; Marsden and House 2006). Refractory ore may include ore minerals that are relatively insoluble or kinetically dissolve too slowly to economically recover the metal value. Refractory ore can also include ore minerals that are locked in insoluble minerals such as sulfide or silica. For example, refractory gold ore may include gold that is included in pyrite. In another type of refractory ore, there may be gangue minerals or compounds like organics that sorb the metal from leach solutions or compromise the lixiviant chemistry. The term preg-robbing is used for this phenomenon. Some ores may have more than one type of refractory character and are called double refractory. Some of the precious metal ores in the Carlin trend of Nevada fall in this category (Marsden and House 2006). Mineralogical and petrographic characterization techniques have been developed to identify and quantify ore mineralogy and refractory phases. Refractory gold ores may be pretreated prior to leaching by roasting, autoclaving, or bioleaching to unlock ore particles so they can be leached with cyanide (Yannopoulos 1991; Marsden and House 2006). Copper sulfides may also be roasted or autoclaved to make them amenable to leaching (Kuhn and Alley 2019). Bioleaching is also effective in treating refractory copper ores (Miller et al. 2001). Some research has investigated the possibility of in situ autoclaving for copper recovery, but currently, the energy costs are generally considered to be too expensive. However, rock formations are good insulators, and in situ autoclaving or pressure oxidation of primary copper sulfides is generally considered to be technically feasible (Dysinger and Murphy 1994). The concept is similar to in situ coal gasification. High-temperature ISRR (i.e., superheated water conditions) is not discussed further in this book other than to note its future potential. Moderately elevated temperatures may increase leaching kinetics in some systems and are commonly attained in sulfide leaching systems, such as heaps and dumps, from exothermic heat (Murr et al. 1982). However, Bartlett (1992) reports no benefit from elevated temperatures in the in situ flooded leaching of primary copper ore, whereas Trexler et al. (1991) report increased gold extraction from ores during leaching with geothermally heated cyanide solutions. In situ thermal extraction of organic hydrocarbons is a well-established technique for contaminated sites containing refractory organic contaminants, but not for metal contaminants. In general, there are subtle but poorly documented temperature effects in ISRR systems that should be kept in mind if moderate temperature variations from standard state conditions are encountered. For example, recent research has shown that arsenic sorbs more efficiently on iron hydroxides at higher temperatures than at lower temperatures (Earley et al.

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Geochemistry and Hydrometallurgy

53

2019). This effect may have implications for in situ remediation of arsenic and other metals. The geochemistry of the ore and host rock determine whether the metal resource is amenable to ISRR by dissolution with a fluid, usually aqueous based at standard state or earth surface temperatures and pressures. Water by itself is referred to as the universal solvent and can dissolve many important ore minerals without chemical additives. This is because water molecules are polar and can dissociate into H+ (proton) and OH– (hydroxyl) ions (Brimhall and Crerar 1987). The electrostatic attractions and repulsions formed by dissociation of water allow dissolution of minerals to occur via reactions such as the generic Equation 3.19. The reactivity of ore minerals to a prospective lixiviant primarily is dependent on chemistry and crystal chemistry and secondarily is dependent on the mineral habit and texture. Pearson’s (1963) Lewis acid–base theory has been successful in predicting and explaining the transport of metals in ore-forming fluids by chemical bonding with ligands (Brimhall and Crerar 1987). Cations are Lewis acids, whereas anions are Lewis bases and are classified as being hard, soft, or borderline (Langmuir 1997). Hard acids and bases are dominated by ionic bonds, whereas soft acids and bases are dominated by covalent bonds. Borderline acids and bases form a mixture of ionic and covalent bonds with soft bases. For example, the well-known divalent transition metals copper, zinc, and lead in ore deposits are sometimes referred to as chalcophile (sulfur loving) because they are borderline Lewis acids and bond with relatively soft bases such as bisulfide (HS–) in hydrothermal ore-forming fluids (Brimhall and Crerar 1987). Soft Lewis acids and bases, such as gold and silver, predominantly form covalent bonds with ligands such as sulfide compounds and cyanide. In addition, because of the relatively soft or deformable electron cloud of the partially outer orbitals that bonds covalently to other covalent elements, they bond with themselves and occur predominantly in elemental form as solids. Table 4.1 provides a summary of the expected chemical behavior of some key metals. The redox state and ionic form of the metal are important in predicting chemical reactivity in leach solutions. For example, chromium most commonly occurs in ore deposits as the oxide mineral chromite that crystallizes from basaltic magmas. Chromite is very stable and difficult to leach as are some other primary oxides. However, aqueous hexavalent chromium in acidic plating solutions is very mobile and persistent in groundwater if accidentally released into the environment, resulting in difficult remediation. Table 4.1 provides insights to leaching agents of target ore minerals and for remediation of contaminant metals, but the kinetic and multicomponent aspects of ISRR solutions also need to be considered. Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

54 CHAPTER 4

Table 4.1  Chemical classification of some common metals and ligands, hard–soft Lewis acid theory Hard Acids

Borderline Acids

Soft Acids

H+, Li+, Ti+4, Sn+4, Cr+3, Mn+2

Fe+2, Co+2, Ni+2, Cu+2, Zn+2

Cu+, Ag+, Au+, Cd+2, Pd+2

MoO+3, WO+4, Fe+3, Al+3, VO+2

Sn+2, Pb+2, Bi+3

Metals (zero valent)

UO2

—

Hg+2, CH3Hg+

Hard Bases

Borderline Bases

Soft Bases

NH3, H2O, OH–, CO3–2, NO3–

Br–

CN–, H2S, HS–, I–

PO4–3, SO4–2, F–, Cl–

—

Se–2, S2O3–2, NH2

Oxyanions

—

—

+2, Sc+3, Y+3, La+3, actinides

Source: Pearson 1963

Railsback (2007) has compiled a periodic table of the elements for earth scientists that is also useful in predicting the degree of hardness or softness of Lewis acids and bases by mapping the ionic potential (charge divided by ionic radii Z/r) of the most common oxidation states. Some elements may be hard or soft depending on their oxidation state and ionic potential. For example, the desirable properties of some elements, such as chromium, lithium, and REEs, make them very difficult to leach because they have relatively small ionic radii and have high ionic potentials that form strong bonds with oxygen; only strong acids or bases are capable of bringing these elements into solution. Lithium has a large ionic radius and hence charge density, which makes it ideal for batteries. Ironically, gold, which is commonly thought to be relatively inert in its solid form, is readily solubilized in aqueous solutions by many different soft bases as predicted by the earth scientist’s periodic table (Railsback 2007) because of its relatively small ionic potential. Theoretical analysis of metals chemistry, geochemistry, and metallurgy provides a starting point for understanding the potential range of reagents that can be used for ISSR, but cost, availability, stability, and toxicity of lixiviants ultimately dictate whether they are candidates for ISRR. For example, sodium cyanide has been used for more than a century to leach gold from ores, and cyanidation in vats and heaps is by far the primary technique for production in the industry today. Because of its toxicity, however, it will likely never be widely used for in situ recovery. Because the gold industry is based on cyanide, it is very difficult to replicate the infrastructure that is needed to make, distribute, handle, and use for a relatively small number of ISRR operations. In fact, approximately 90% of all the sodium cyanide produced is used in the precious metals extraction industry (Crump 2000). In 2017, Barrick Gold constructed and began the first vat-leaching facility for precious metals based on thiosulfate at its Goldstrike operation (Marsden 2019), but it had to commission and operate its own production plant to do so. Commercial in situ recovery of gold is reported at one site (Seredkin et al. 2016), and Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

55

Geochemistry and Hydrometallurgy

certainly, there are many different alternatives to cyanide (Earley et al. 1991; Yannopoulos 1991; Marsden and House 2006) that can make in situ recovery of gold more common in the future. Conversely, sulfuric acid is manufactured and used on a very large scale in many different industries. Copper mining companies that have leaching operations have options for suppliers who can produce the reagent in quantities that can meet the demand. Sulfuric acid used to be produced in abundance by smelters as a waste product, so it was natural to use for leaching in the early stages of the copper leaching industry. However, with the shutdown of most North American smelters beginning in the 1990s because of stricter air emissions standards, copper producers have turned to more expensive manufacturers of sulfuric acid with fewer impurities. Sulfur dioxide is also an abundant and unwanted waste product generated by energy and manufacturing operations and provides a relatively cheap reagent for manganese leaching (Paulman 1994). Bioleaching, biological mineral processing, and bioremediation technology have advanced rapidly in the past few decades as new technologies for identifying and characterizing microbial strains and processes have resulted in a wealth of new data and information (Kawatra and Natarajan 2001). Technologies for isolating and nurturing desirable microbes and suppressing undesirable microbes have also been developed. Biotechnologies for ISRR include catalysis of redox processes that are inorganically too slow for economic recovery or remediation of metals. The critical biological processes involved in copper sulfide ore leaching in heaps and dumps has been known for several decades (Murr et al. 1982). Chemolithotrophic bacteria, such as Thiobacillus ferrooxidans, facilitate rapid oxidation of ferrous iron to ferric iron, which is needed for copper sulfide leaching. Unfortunately, the same bacteria are also responsible for the generation of acid mine drainage and biofouling of heap-and-dump leaching operations (Murr et al. 1982). Other iron oxidizers such as Gallionella are known to cause biofouling in water wells (Chapelle 1993). Therefore, caution must be used with biotechnology in ISRR because of the relatively low porosity and permeability characteristics of these systems, which increases the risk of biofouling and other potentially unwanted by-products of biotechnologies. Porosity may be created or lost during in situ recovery depending on the ore type and host rock and lixiviant. Some minerals, such as copper carbonates and chlorides, dissolve stoichiometrically in an acid lixiviant via Reactions 4.1 and 4.2 (Iasillo and Schlitt 1994): CuCO3·Cu(OH)2 (malachite) + 4H+ = 2Cu+2 + CO2 + 3H2O (Reaction 4.1) 3CuO·CuCl2·3H2O (atacamite) + H+ = 4Cu+2 + Cl– + 6H2O

(Reaction 4.2)

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56 CHAPTER 4

Other minerals, such as copper silicates and copper sulfides, dissolve nonstoichiometrically via Reactions 4.3 and 4.4: CuSiO3·2H2O (chrysocolla) + 2H+ = Cu+2 + SiO2 (amorphous silica) (Reaction 4.3) + 3H2O Cu2S (chalcocite) + 2H+ + ½O2 = Cu+2 + CuS (covellite) + H2O

(Reaction 4.4)

In Reaction 4.4, as chalcocite is leached by acid, an oxidant produces a reaction product similar to covellite that leaches very slowly under standard state conditions so is considered to be nonstoichiometric in this example. The increase or decrease in porosity during leaching depends on the volume balance of reactants and products. In some cases, gangue minerals also react with the lixiviant to generate porosity or solid precipitates that decrease porosity. The well-known reaction of calcium-bearing gangue minerals with sulfuric acid produces gypsum because of its low solubility, for example, in Reaction 4.5: CaCO3 (calcite) + H2SO4 = CaSO4·H2O (gypsum) + CO2 (gas)

(Reaction 4.5)

Permeability decreased about 50% during in situ leaching of copper oxide ore at the Cyprus Casa Grande underground mine in Arizona as a result of gypsum precipitation and porosity reduction (Earley and Jones 1992). For cyanide leaching of gold ores in heap leach operations, fewer chemical precipitation reactions and porosity changes occur; however, decrepitation of ore can result in the generation of fines that can migrate and plug porosity. This process is likely to be less important during ISRR. However, in the case of massive manganese oxide ore leaching with sulfur dioxide, large volume changes may occur because the ore minerals constitute a large volume of the rock and dissolve primarily by stoichiometric reactions (Paulman 1994) such as MnO2 (pyrolusite) + SO2 + H2O = MnOOH (manganite) + SO3–1 + H+

(Reaction 4.6)

MnOOH (manganite) + SO2 + H+ = Mn+2 + SO3–1 + H2O

(Reaction 4.7)

Similar to chalcocite, manganese oxide mineral leaching can involve a twostep electron transfer process with a solid intermediate phase. Reaction 4.6, however, is faster and the intermediate reaction product may not be present in the leached residue. Porosity and permeability changes can be dramatic as large voids are formed and collapse. Decrepitation of the rock formation from

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leaching reactions in copper ore and other systems can result in the loss of porosity, permeability, and well integrity (Brink et al. 1991). Leach Solution Chemistry Hydrometallurgical leaching operations generate an evolved lixiviant and PLS composition owing to selective metal recovery, recirculation, and regeneration of solutions. Because the surface processing plants for in situ metal recovery are likely the same or similar to traditional leaching facilities, the target metal concentrations in PLS are similar. Cutoff metal grades in solution for various commodities that can be recovered in situ have been estimated by Seredkin et al. (2016). In all leaching operations, the concentrations of unrecovered dissolved metals and other components build up to equilibrium levels as the solution is recirculated over many cycles. Given the same ore body and host rock, the composition of the evolved or equilibrium leach solution during ISRR may be different than for conventional leaching operations. Some of the differences include ■■

Mine water balance, ■■ Solution–mineral contact, ■■ Lixiviant strength, and ■■ Chemical additives. Obviously, the water balance for an ISRR site is different than a conventional mine where water losses or gains caused by evaporation and precipitation are usually greater. Because ISRR uses natural porosity, the solution-tomineral contact surface areas for gangue minerals can be smaller, resulting in less buildup of metals and other constituents in that potential mineral source. Lixiviant strength and chemical additives may need to be specifically adjusted for ISRR, which can result in compositional differences from traditional operations. The composition of the evolved leach solution matters for operational and environmental reasons. For example, the buildup of unwanted constituents can lead to mineral precipitation, which clogs pore space in ISRR wells and well fields. The concentration of contaminants in residual leach solutions and precipitates is also of concern for well-field restoration and compliance with water-quality regulations (Earley and Johnson 2012). Many different hydrometallurgical tests have been used to determine ISRR feasibility (Sinclair and Thompson 2015). Traditional bottle-roll leach tests are used to determine the leachability of ore when all of the ore mineral surface area is exposed to the leach solution as in a tank or vat. This determines the mineralogical limitations for metal recovery and any refractory behavior. Column testing procedures that are used for heap-and-dump

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58 CHAPTER 4

leaching can also be used as indicators of solution–mineral contact for a given crush particle size distribution. However, unless the in situ ore is unconsolidated (exotic copper placer, paleoplacer, etc.), column tests can likely over‑ estimate recovery but have more representative bulk compositions as it is easier to blend samples. For hard rock–hosted ores, core leaching testing procedures have been developed. Some pass solution over whole core pieces within a confined vessel (Sinclair and Thompson 2015), whereas others inject the leach solution directly into the core (Paulson and Kuhlman 1989). Figure 4.8 shows a high-pressure core leaching apparatus where the core specimen is sealed into a steel sleeve with epoxy so that leach solution flows axially through the core specimen. By monitoring the pressure drop across the core, the permeability and time-dependent permeability can be measured using Darcy’s law (see Equation 3.2). After completion of the experiment, the core can be sectioned for post-testing petrographic and chemical analysis. Figure 4.9 shows a sectioned post-leach test specimen of atacamite ore from the U.S. Bureau of Mines– Asarco Santa Cruz joint venture project (Kreis 1994) where a highly visible preferential flow channel developed along the edge of the vessel by leaching of atacamite-filled fractures despite injection into a large-diameter core specimen (HQ) with initially low permeability and no lengthwise fractures. Flow concentration can occur along internal layers, fractures, or lenses that may be

FIGURE 4.8  High-pressure core leaching apparatus and (inset) epoxied core in steel jacket

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Geochemistry and Hydrometallurgy

Flow Channel Developed by Leaching

FIGURE 4.9  Post-leach atacamite ore from core leaching experiment, Santa Cruz deposit, Arizona

slightly more permeable than the bulk specimen and then localizes in space and speeds up the processes of dissolution until breakthrough occurs (Phillips 1991). Earley et al. (1990) also performed petrographic analysis of pre- and post-leach core samples from two different host rocks from the Santa Cruz test site—a granite and a felsic porphyry. Petrographic analysis showed that the granite has less clay, mica, iron oxides, and other reactive minerals than the feldspar porphyry. The effect of the reactive mineralogy in the porphyry is slower copper recovery (Figure 4.10) relative to copper recovery in the granite (Figure 4.11). As discussed in Chapter 3, reactive minerals sorb copper by different reaction mechanisms, which retards copper recovery in the porphyry host rock relative to the granite host rock from the same ore deposit. Regardless of the laboratory testing procedure used, it is difficult, time consuming, and expensive to perform enough tests at an appropriate scale and time frame to simulate well-field scale solution flow and reaction in an ore body, so the results have to be scaled and extrapolated accordingly. In addition, single-pass leach tests cannot simulate the buildup of gangue constituents over time from continuous processing, regeneration, and recycling of leach solutions. Paulson (1994) conducted leach solution recycle experiments on whole core samples from a deep copper oxide ore deposit located in Arizona that contained soluble chrysocolla and atacamite. Isaacson and Redden (1994) also conducted closed-loop tandem column leaching experiments on drill Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Cu Recovery, %

60 CHAPTER 4

100 90 80 70 60 50 40 30 20

100 90 80 70

10

10

0

60 50 40 30 20

0

20

40

60

80

100

120

140

160

0

Time, days Source: Earley et al. 1990

Cu Recovery, %

FIGURE 4.10  Copper recovery curve for porphyry-hosted ore core leaching test 100

100

90

90

80

80

70

70

60

60

50

50

40

40

30

30

20

20

10

10

0

0

0

20

40

60

80

100

120

Time, days Source: Earley et al. 1990

FIGURE 4.11  Copper recovery curve for granite-hosted ore core leaching test

cuttings from the same deposit to simulate leaching along longer flow paths, but the chemical evolution of the leachate may not be representative of the in situ rock mass, owing to the use of cuttings. The host rock for the chrysocolla mineralization was a porphyry that contained a moderate amount of reactive gangue minerals that consume acid. The host rock for the atacamite mineralization was a granite that had relatively low abundance of acid-consuming minerals. The chrysocolla ore had a lower grade than the atacamite ore, but both were relatively high-grade ore containing approximately 2% and 8%

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Geochemistry and Hydrometallurgy

61

total copper, respectively. Hence, relatively high sulfuric acid strengths were used (20–40 g/L H2SO4) over the course of the experiment, and the pH of the injection solution was approximately 0 in both sets of experiments. In addition, relatively high injection pressures up to approximately 100 bars were used because of the relatively low initial permeability of the core (approximately 10–1 and 10–3 for the chrysocolla and atacamite ores, respectively). Copper was removed from the leach solutions collected from the core apparatus using a batch solvent extraction system in the laboratory to simulate the effect in well-field leaching operations (Paulson 1994). In the chrysocolla leach solution recycle tests, the concentrations of Al, Fe, Mg, and K increased to 1–10 g/L of solution while the concentration of sulfate rose to approximately 50 g/L in about 100 days (Paulson 1994). Both Ca and Na quickly attained a steady-state concentration of a few hundred milligrams per liter that remained relatively constant for the duration of the experiment. The buildup of cation gangue metals is attributed to leaching of gangue minerals in the host rock, whereas the buildup of sulfate is a result of sulfuric acid regeneration of depleted leach solutions. The relatively constant concentrations of Ca and Na are likely a result of reaching saturation with respect to sulfate salts gypsum and sodium jarosite, respectively. It is possible that the precipitation of these minerals and dissolution of gangue minerals caused the fluctuations in permeability, which ranged from 0.05 to 0.35. The pH of the effluent collected at the end of the core initially rose to 3 in the experiment but then reached a steady state of 1 after depletion of fast acid-consuming minerals. Approximately 50% of the copper was recovered in the 100-day test. In the atacamite leaching recycle test, the gangue constituents did not build up as quickly but still reached a few hundred milligrams per kilograms in solution for some constituents. The granite host had a less reactive mineralogy. Both chloride and sulfate attained concentrations near 20 g/L of solution before dilution of the recycled leach solution was started to avoid excessive chloride buildup and corrosion (Paulson 1994). Atacamite was the source of the unwanted chloride. The sulfate concentration remained constant and near the injectate concentration until the dilution intervention because makeup acid is not required, owing to the positive acid balance of the solvent extraction circuit for atacamite ore (Paulson 1994). The pH of the effluent remained relatively high (3–4) because of the higher grade of highly leachable copper. Copper recovery reached approximately 50% after about 400 days of leaching and recycle. In completely closed-system leaching operations (i.e., vat or tank), it may be necessary to bleed off leach solutions to control the levels of potentially deleterious constituents such as chloride, total dissolved solids, and soluble Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

62 CHAPTER 4

metals that may precipitate and reduce permeability of the ore (Isaacson and Redden 1994). For example, some ion exchange recovery systems may become inefficient by competition with the buildup of unwanted metals. In full-scale operations, the time interval from the start of operations to steady state varies depending on the mine plan, lixiviant, leaching kinetics, water balance, and so forth. ISRR systems are different from conventional leaching operations in that it is often necessary to pump at an overall rate that is higher than injection to maintain containment of leach solutions by the well-field cone of depression. The excess leach solution is then either treated to water-quality standards and discharged or evaporated or a combination of both. Therefore, it is necessary to predict the steady-state composition of some solution components, other than the target metal(s), in the ISRR system to manage, treat, and discharge water efficiently and in compliance with applicable laws and regulations (Earley and Johnson 2012). Figure 4.12 illustrates the concept using a flow diagram representation. The integrated well field can be represented as a tank reactor of volume V for conceptualization. Then the evolution of a conservative component in the leach solution composition follows a parabolic trajectory according to Equation 4.5 (adapted from Stumm and Morgan 1981): c t = c 0 exp c −

q in + d q in c in q +d m c in mE ; V t + q in + d 1 − exp − V t 

(Equation 4.5)

where qin and cin are the inlet flow and concentration, respectively; c0 is the initial concentration; ct is the outlet concentration at time t in the outlet flow qout, which is equal to qin plus d, the discharge bleed stream. Over multiple volumes (V  ) of leach solution recycling, the concentration is calculated using Equation 4.5 over finite time steps and updating c0 and cin. Figure 4.13 provides concentration curve examples with time for a 100-m (330-ft) thick ore body with a 10,000-m2 (approximately 100,000‑ft2) surface well field operating at a flow rate of 10 m3 (353 ft3) per minute with a 5% and 10% bleed stream. The initial ambient concentration (c0) is 0.01 g/L, and the initial leach solution concentration (cin) is 10 g/L, which is a typical concentration for sulfate in a sulfuric acid lixiviant with makeup acid added to the injected solution. Figure 4.13 shows that the well-field concentration reaches 10 g/L in one year as the well field is primed and the leach solution continues to evolve until the conservative constituent concentration reaches approximately 140 g/L and 90 g/L for a 5% and 10% bleed stream flow, respectively, in 25 years. This algorithm can be used to calculate the concentration of an incidental constituent such as sulfate in a closed-loop copper leaching system. In systems that have a high rate of evaporation, such as heap leach operations, the Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

Geochemistry and Hydrometallurgy

63

Bleed Stream (d) Processing qoutct

qincin

c0

Well Field (V)

Dispersion

Dispersion FIGURE 4.12  Well-field mass flux components 160 140

Concentration, g/L

120

ct 10% ct 5%

100 80 60 40 20 0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Time, years

FIGURE 4.13  Simulated buildup curves for a conservative constituent during in situ well-field operation with solution recycle for 5% and 10% bleed streams

concentration may increase faster to a higher steady-state value. This can be calculated by applying a cin that is adjusted for evapoconcentration at each time step. Because of the buildup of dissolved constituents in recycled copper leach solutions, the total dissolved mass and concentrations of metals can be on the order of grams per liter to hundreds of grams per liter. However, in precious metal leaching circuits, leach solution concentrations are lower because the alkaline cyanide lixiviant is less aggressive (McGregor and Van Zyl 1999). Regardless, it is usually necessary to restore the well field to pre-ISRR conditions by flushing, pumping, and treatment of residual leach solutions containing various impurities. Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

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CH AP TER   5

Drilling and Well-Field Technology

INTRODUCTION Excluding open pit or underground mining, production of subsurface resources is accomplished through drilling and well-field technology. Tremendous technological advances have been made in both areas in the past century primarily because of the profitability of oil and gas production and the necessity for the development of groundwater resources for agriculture. Mineral resource production of some commodities has largely used or adapted technology from these two industries, but some drilling and wellfield technologies have been specifically developed for mineral resources. In situ uranium production has advanced the concept of patterned, injection and recovery well fields with well spacings on the scale of tens of meters (approximately 50 to more than 100 ft) whereas oil, gas, groundwater, and high-temperature geothermal operations may use isolated or relatively scattered wells with interwell spacings of hundreds of meters (thousands of feet) in some fields (e.g., Rassenfoss 2018). Deliberate attempts at in situ recovery of copper began at least a century ago without wells through leaching of copper via irrigation of open pit benches and flooding of underground workings with mine contact water that was naturally acidic owing to sulfide oxidation. Prior to the commercial development of solvent extraction and electrowinning (SXEW) technology in the 1970s, copper in these solutions was precipitated using scrap iron and tin cans and then sent to a roaster for purification as a secondary and low-cost source. The literature usually distinguishes true in situ recovery or greenfield recovery using wells exclusively in an otherwise undeveloped deposit from hybrid in situ recovery associated with preexisting mine workings. The latter situation is sometimes referred to as a brownfield operation using environmental restoration terminology. Most of the known copper and other base metal resources are located near existing mines as mineral resources are concentrated in districts. However, many of the resources are low grade, deep, or otherwise difficult or uneconomic to mine using traditional mining techniques given past and present commodity prices. Hence, in situ recovery

65

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66 CHAPTER 5

using wells has been considered for these satellite or orphan ore deposits of the same district. This chapter considers both greenfield and brownfield types of in situ recovery and remediation (ISRR) settings, recognizing that operations near existing mines are often more favorable technically and socially as mining in previously disturbed areas is often more acceptable by the public than greenfield projects. Preexisting mine workings also offer more opportunities for solution control and distribution than those in greenfield projects. The Berkeley pit is a prime example of this as copper is leached by meteoric water and groundwater inflow to the pit via the formation of acid rock drainage. Montana Resources began processing this water in 2012, and some of the revenue is used to offset remediation of the Berkeley pit Superfund site (Associated Press 2004). At the Mineral Park mine in Arizona, Cyprus Minerals has produced copper by pit wall leaching since 1965 (Earley et al. 1996). At some sites, permeability is enhanced by drilling and blasting pit benches (see Figure 4.1) prior to application of leach solutions using heap leach irrigation equipment (see Figure 4.2). In the production of copper from open pits by bench and highwall leaching and collection, the pit can be conceptualized as a large-diameter recovery well. The capture area and inflow is also increased by fracturing the pit wall rock and stress relief, which increases the skin factor. Hence, an open pit is much more productive than even the largest production well as can be seen by the radius effect of a well function for an unconfined aquifer under the Dupuit assumption (Wang and Anderson 1982): 2 2 h ^ r h − h ^ re h  Q = πK r ln r e

(Equation 5.1)

where Q is the well production or discharge, K is the hydraulic conductivity, h is the head of the aquifer, r is the radius of the well or pit, and re is the distance to the ambient head condition from the center of the system. Because the ratio of r to re is larger for a mine pit than for a well, the natural log function is smaller for the pit and Q is larger than for the same re as a small r well. In situ recovery of subeconomic copper oxide ore extending from open pits has been considered at sites like the Emerald Isle mine, in western Arizona (Ahlness and Larson 1994) and at Mina Sur in the Chuquicamata region of Chile (Pallauta 1985). Both deposits have highly soluble and permeable exotic copper resources that plunge away from the pit and are covered by increasing depths of overburden (Figure 5.1). Highwall slopes have been steepened to maximum limits. In situ recovery projects at these sites Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

67

Drilling and Well-Field Technology

Production Wells

Stripped Overburden

Open Pit Highwall

Mined Ore

Exotic Cu Ore

Leach Solutions

FIGURE 5.1  Hypothetical in situ copper recovery of exotic ore extending from maximum highwall slope of an open pit mine

Overburden Monitor Well

Injection Well

Injection Well

∅* – ∅

h

Ore Body and Resistance Layer Mine Void

Process Plant

Adapted from Schmidt 1989

FIGURE 5.2  Leach solution mound and potentials above underground mine workings collection system

examined drilling injection and recovery wells from the surface. However, an alternative and more efficient design involves flooding of the lower part of the pit and flow of leach solutions through the ore to production wells constructed to intercept pregnant leach solution following the downdip extent of the deposit. As Equation 5.1 predicts, it takes many wells to match the injection capacity of the pit. Similarly, the injection fluid capture capacity of an underground tunnel or shaft is greater than a smaller diameter well (Figure 5.2), which makes block cave leaching highly productive (Schmidt 1989). h Q = A K (Ø* – Ø)

(Equation 5.2)

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68 CHAPTER 5

where Ø* – Ø is the difference in piezometric elevation as shown in Figure 5.2. The area of the hydraulic sink (A) is perpendicular to the direction of flow and is larger for a mine tunnel than a horizontal well. DRILLING A comprehensive discussion of drilling technology is beyond the scope of this book and there are bountiful resources of information (e.g., Doubek et al. 2013). The websites of drilling and oil and gas service companies are also an excellent resource. However, some discussion of drilling for ISRR is needed as drilling is one of, if not the largest, costs of completing an ISRR well field or project. Drilling strategy is very site specific, labor intensive, and difficult to automate, therefore expertise in drilling and drilling technology is vital to the success of an ISRR project. Drilling technology includes direct push, wire line, rotary, percussion hammer and sonic techniques, and variations thereof. Traditional mud rotary drilling technology developed for oil and gas and water resource exploration and development in sedimentary rocks is not well suited to hard-rock mineral deposits because they usually require drilling muds to lubricate the bit to reduce wear and also to prevent fluid loss to the formation when drilling through high-permeability zones (Ahlness et al. 1981). The borehole mud cake and drilling fluids can be removed or destroyed after the borehole is completed, but the process is expensive and time consuming. Rotary drilling is necessary, however, for large-diameter, deep well construction, and flooded reverse-circulation drilling can mitigate some of the issues associated with the use of drilling fluids (Doubek et al. 2013). Diamond drilling rigs used for exploration in hard rock are usually not ideal for well construction as they are usually of too small diameter for production and require drilling fluids that may be lost to the formation and obstruct connected pores and fractures. Usually, mineral exploration programs are not oriented toward gathering ancillary information for ISRR, such as hydraulic conductivity, although they can be adapted to do so if accessible (Schmidt et al. 1995). Traditional oilfield and groundwater resource characterization programs may need to be modified for ISRR to enhance sweep efficiency and solution–mineral contact at macro- and microscales. Percussion hammer and sonic drilling techniques are highly versatile as they can be used in unconsolidated and consolidated formations for well completions. The depth of penetration, however, is limited to approximately 500 m (1,600 ft) or shallower. Directional drilling technology has advanced rapidly over the past several decades and can provide access to ore and fractures in difficult conditions (Doubek et al. 2013). In addition, angled and horizontal wells can better Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

69

Drilling and Well-Field Technology

intercept mineralized high-angle veins and faults (Figure 5.3), which are common in metal hard-rock deposits whereas sedimentary-hosted deposits follow shallowly dipping beds unless strongly folded. Fixed-angle or slant borehole drilling is commonly used in exploration to increase the probability of intercepting high-angle structures and ore bodies with incident angles of 90 to nearly 0 degrees in some situations. However, specialized drilling rigs are usually used for low-angle boreholes. Whipstocks can also be inexpensively used to branch drill strings from main casings and intercept more fractures from a single drill pad (Chamberlain 1979). Multidirectional steered and continuous survey drilling represents the state of the art with respect to modern drilling technology (Rowland et al. 2017). The borehole angle can be changed from vertical to horizontal, provided there is sufficient arc length to accomplish the turn. Boreholes up to 25.4 cm (10 in.) in diameter are possible in hard-rock formations. This technology can be used for mine dewatering (Dowling and Beale 2013) or can be similarly applied to ISRR fluid injection or recovery systems. Completions are very expensive, however, compared to fixed-angle drilling; and likely, traditional drilling and wells can be used in conjunction with judicious use of multidirectional wells. Monitor Recovery Injection Diversion

(115.0, 750.0, 0)

Vertical Fault

Z X

Y

0.00

0.250

0.500

0.750

1.00

Saturation Level

FIGURE 5.3  Simulated injection of fluids into a variably saturated fracture zone with angled injection and recovery wells

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70 CHAPTER 5

In the future, drilling itself may become a mechanical ISRR technology using borehole miner (Savanick and Miller 1997) and other modified drilling equipment. At some high-grade gold mines, drill cuttings containing sufficient grade are collected and separated from drilling fluids and sent to the mill for processing. WELLS Prior to wire-line and rotary drilling technology, most wells for water, oil, and gas were hand dug to access shallow resources and were essentially shafts as human laborers excavated the wells by crawling into the well to reach the bottom for continued excavation. Therefore, the distinction between borehole­ constructed wells and mine workings is a continuum as are the site situations with respect to brownfield versus greenfield ISRR operations. A true greenfield project is somewhere outside any hydrologic or physical effect from an operating or closed mine even though it may be in the same district. Well technology for ISRR has largely been adapted from the oil, gas, and groundwater industries. Groundwater well construction techniques are usually more applicable to ISRR, as oil and gas wells involve production of multiphase fluids from large sedimentary reservoirs; and classic water well references, such as Driscoll (1986), can provide fundamental information. Beal et al. (2013) also provide an excellent discussion tailored for mine sites. Some well technologies and requirements are specific to groundwater monitoring such as nonreactive plastic casing to avoid the inadvertent leaching of metals and other contaminants from well construction materials that may compromise the monitoring objectives. For ISRR, a wide range of well construction has been used from shallow open boreholes to deep multiple diameter and casing completions using stainless steel and/or fiberglass perforated after cementation to avoid casing leakage (Kreis 1994). Figure 5.4 is a schematic diagram of an advanced design production well for the Santa Cruz project conducted in the late 1980s and 1990s by the U.S. Bureau of Mines and Freeport-McMoran. The project targeted a deep copper oxide ore body located in central Arizona near the city of Casa Grande. The multiple cemented casings were required to protect the overlying basin fill aquifer. In addition, stainless steel and fiberglass materials were used to minimize corrosion as the leach solutions had high levels of chloride owing to the target atacamite mineralization. Currently, a single, deep, high technology well may cost a million dollars. Therefore, optimizing well design is critical for the economic feasibility of an ISRR project. In Chapter 7, a project conducted by the U.S. Bureau of Mines and Cyprus Minerals (now part of Freeport-McMoran) describes another in situ copper sulfide recovery project and pilot test that used a low-tech well design Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

71

Drilling and Well-Field Technology

Well Cap to Pipe Connection with Pressure Gauge and Valves

Basin Fill Aquifer

601 m (2,000 ft)

Cemented Borehole 37.5 cm (14¾ in.)

Steel Casing 26 cm (10¾ in.) Fiber-Reinforced Plastic Casing 17.8 cm (7 in.) Granite Pressure Transducer Perforations: Casing and Cement

Borehole Cemented 25 cm (97∕ 8 in.) Submersible Stainless Steel Pump and Cable Cu Oxide 61-m (200-ft)

FIGURE 5.4  Santa Cruz in situ copper mining research project recovery well design

approach. In this project, the wells were completed as simple, open boreholes located within the capture zone of an abandoned open pit. The range of well designs used emphasizes the site-specific nature of ISRR projects. As with groundwater, oil, and gas production, and so forth, well development is very important for ISRR to decrease skin effects that may limit injection rates and production from recovery wells (Bartlett 1992). The basic aspects of groundwater well development are summarized in Driscoll (1986). Deep well completions for oil and gas production sometime require more complicated well development procedures because of the necessity to use drilling muds and other fluids during drilling and casing installation. Acid stimulation can also be used to enhance formation permeability by dissolving readily soluble fracture-filling gangue minerals such as caliche. In addition, the use of a conditioning acid other than sulfuric prior to injection can decrease the formation of gypsum by recovering calcium prior to in situ recovery with sulfuric acid. Calcium removal using biotechnology is also a potential method of well stimulation (Modak et al. 2001). Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

72 CHAPTER 5

WELL FIELDS There has been a lot of attention in the ISRR literature on well-field design usually focused on repeatable geometric patterns of wells such as the five-spot pattern that has become a standard for base design (Figure 5.5). The well spacing may measure meters or tens of meters (tens to hundreds of feet). While this approach facilitates well-field layout, hydraulic design, and economic projections, the idealized flow pattern illustrated in Figure 5.5 is rarely realized except in uniform sandstones. For fractured rock, the anisotropy in hydraulic conductivity can be 2:1 or greater, and the dominant flow can be vertical, horizontal, or inclined. Nonetheless, the five-spot pattern helps to illustrate the approach to well-field design, which should incorporate the hydrogeology of the host-rock formation. For example, if the x:y anisotropy in hydraulic conductivity is 2:1 in an otherwise homogeneous unfractured sandstone (owing to alignment of ellipsoidal grains, etc.), then the optimal design of the symmetrical five-spot pattern with a 50-m (164-ft) injection to recovery, well spacing may look like Figure 5.6. In this distorted five-spot, the well spacing is two times greater in the x direction to account for greater flow velocities such that the travel time from injection to recovery is approximately the same for all wells. Otherwise, the solution grades may be lower than desired for kinetically limited transport in the x direction. As mentioned earlier, sweep efficiency is one metric that has been applied to ISRR well-field design that originated in the oil and gas industry. The original usage of sweep efficiency is the effectiveness of an enhanced oil recovery process that depends on the volume of the reservoir contacted by the injected

10 m (32.8 ft)

Hydraulic Potential, log bar 10 m (32.8 ft)

–2.5 –0.5

1.5

1 bar = 14.5 psi

FIGURE 5.5  Five-spot well field with (inset) representative elemental volume of a simulated hydraulic potential field in a homogeneous host rock

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Drilling and Well-Field Technology

10 m (32.8 ft)

73

Hydraulic Potential, log bar 10 m (32.8 ft)

–2.5

–0.5 1 bar = 14.5 psi

1.5

FIGURE 5.6  Extended five-spot well-field configuration for host rock (x permeability = 2 × y permeability)

fluid (Schlumberger 2019). The equivalent sweep efficiency for ISRR is a function of ■■

Injection, recovery pattern, and well-field configuration; ■■ Reservoir thickness and structure; ■■ Host-rock permeability, porosity, and heterogeneity; ■■ Fracture density and orientation in the reservoir; ■■ Surface area of target ore minerals exposed in connected pores; and ■■ Rate of flow and dispersivity. In oil and gas reservoirs, water flooding is used to displace oil from the reservoir to the production well. Hypothetically, one-dimensional piston flow of one liquid displacing another immiscible liquid in a saturated homogeneous, porous matrix results in a sweep efficiency of 100% when all the pores occupied by the immiscible liquid are replaced by the displacing fluid. However, if the permeability of the formation is too low, then recovery of the immiscible liquid may take too long to be economic even with perfect volumetric sweep. Therefore, the effective sweep efficiency has a time frame that is dependent on the economics of the desired commodity. For ISRR, sweep efficiency is usually used in the context of solution–mineral contact or surface area of mineral exposed to connected pores and the distribution of leach solutions in connected pores. However, target minerals may also dissolve and

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74 CHAPTER 5

release metals by chemical diffusion in dead-end pores that contain stagnant fluid (see Figure 4.3). The practical limit of diffusion is only a few centimeters (1–2 in.) at most for the life of an ISRR well field. Hence for ISRR, the concept of sweep efficiency is complex and usually estimated empirically from laboratory and field leaching tests (e.g., Sutton 2019). Sweep efficiency is usually defined by the hydraulic efficiency of moving an injected fluid evenly through all available pores. However, hydraulic sweep efficiency may be different than the effective sweep efficiency of metal recovery. Realization of the potential macroscale sweep efficiency depends on the well and well-field design and operation. Poor well-field planning and operation can lead to poor sweep efficiency if flow interference patterns create dead zones. Tracer testing at the Santa Cruz project five-spot test well field determined that one of the recovery wells was poorly connected to the injection well, and the well-field operation was adjusted accordingly (Kreis 1994). Operating injection and recovery wells that are directly connected also lead to poor sweep efficiency. During the Santa Cruz in situ copper recovery test project, it was determined that preferential flow was occurring in a fracture above the ore zone, and conformance was achieved by polymer injection (Weber et al. 2000). In Figure 4.3, three hypothetical effective sweep efficiencies can result from the same reservoir system. In the bottom of Figure 4.3, a perfect sandstone reservoir is bisected by a highly permeable fracture that connects the injection and recovery wells. Because 95% of the well injection and discharge flow occurs along the fracture, the hydraulic sweep efficiency is only 5%, as the fracture occupies a nominal volume of the reservoir. If the target mineralization leaches readily and is transport-limited and located preferentially along the fracture, then the effective sweep efficiency of ISRR is 100%. If ore minerals are located only in the sandstone matrix and not along the plane of the fracture, then the effective sweep efficiency is only 5%. If ore minerals occur along the fracture and in the matrix, the sweep efficiency is between 5% and 100%, depending on the proportional deportment. As discussed in Chapter 4, petrographic techniques can be used to estimate the maximum potential effective sweep efficiency of an intact ore deposit. For example, Granthem and Earley (1992) determined by petrographic point counting under an optical microscope that approximately 90% of the fracture-hosted atacamite occurred in microfractures that cannot be seen with the naked eye despite highly visible ore mineralization in some fractures. The permeability of the microfractures is orders of magnitude less than the visible fractures such that the apparent sweep efficiency is much less than the effective sweep efficiency. Injection and recovery can be conducted under steady-state conditions where there is a constant flow injection into dedicated injection wells Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

75

Drilling and Well-Field Technology

balanced by recovery at production wells. In Chapter 9, in situ remediation is discussed and involves only the injection of a fixed volume of a stabilizing chemical reagent that stoichiometrically matches the mass of a contaminant metal. As discussed briefly in Chapter 4, there may be a net surplus of recovery over injection during ISRR to maintain containment. For containment, the net surplus production is usually about 1%–5% but may vary depending on site-specific hydrologic conditions and well-field layout. In horizontal well-field operations, the injection well is pressurized such that a hydraulic gradient is maintained sufficient to attain the target flow from the laterally placed production well. Hence, the general form of the potentiometric surface from injection to recovery well is similar to the tangent function, which may be the phreatic surface for an unconfined ore body. Figure 5.7 illustrates a hypothetical cross section and potential function for a three-dimensional system; the potential or head increases and decreases as the well is approached because of flow convergence. The potential drop is linear in a hypothetical one-dimensional system through a homogeneous equivalent porous media (EPM) as originally conceived by Darcy. For a single well, the potentiometric surface resembles a cone that is inverted for the injection well (Figure 5.8). The image well technique is commonly used in analytical solutions for groundwater flow and wells (Driscoll 1986). Hence the term cone of depression has been used to represent the radial drawdown or potential around a pumping well at steady-state conditions. The cone of depression for a single pumping well also represents the capture zone for an aquifer with no regional gradient. The capture zone becomes

Hydraulic Potential, bar (0, y = 30.5 m)

25 20

15 10

5 0

0

10

20

30

40

50

60

70

80

90

100

110

120

X Position, m 1 bar = 14.5 psi; 1 m = 3.28 ft

FIGURE 5.7  Hydraulic potential for horizontal section through the injection well of Figure 5.6

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76 CHAPTER 5 1,200

1,000

800

600

400

200

0 0

200

400

600 800 Hydraulic Potential, bar

100 m (328 ft) 200 m (656 ft)

–2

–0.5

1,000

1,200

1

1 bar = 14.5 psi

FIGURE 5.8  Computer-generated hydraulic potential field of pumping well capture zone

parabolic and extends in the upgradient direction when there is a regional groundwater gradient (Figures 5.8 and 5.9). For each isolated five-spot pattern within an infinitely extended well field, the injection of each production well is surrounded by an injection well that creates a counterbalancing image potential such that the edges of the five-spot become no-flow boundaries in an ideal EPM with no heterogeneities (Figure 5.5). In real ISRR systems, the potentiometric field of each fivespot is somewhat distorted because of heterogeneities and less than perfect balances of injection, recovery, and wellhead pressures. In vertical leaching operations, the lixiviant is injected at relatively low pressure into the ore body and allowed to flow by gravity to the recovery well or underground collection system as in block cave leaching (Figure 5.2). This is similar to heap leaching, but solutions are usually contained by a natural liner such as an impermeable rock formation. In block cave leaching, the ore formation itself has low permeability until fragmented, hence solutions tend to flow to underlying tunnels. Artificial barriers, such as a lysimeter, can also be used to collect leach solutions so that they can be pumped to the surface. Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

77

Drilling and Well-Field Technology

Hydraulic Potential, bar, 0, y = 610 m

1

0.5

0

–0.5

–1.0

–1.5

0

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

X Position, m 1 bar = 14.5 psi; 1 m = 3.28 ft

FIGURE 5.9  Hydraulic potential for horizontal section through the pumping well of Figure 5.8

Thus, ISRR can be conducted under unsaturated or at least initially unsaturated conditions so long as a collection system is available beneath the ore body. Friedel (1991) discussed the dynamics of in situ copper recovery under variably saturated conditions using low-angle, “fan pattern wells” drilled into the walls of underground mine tunnels at the Cyprus Casa Grande copper mine in Arizona (Figures 5.10 and 5.11). Upward vertical flow can also occur under high injection pressure especially in high-angle fractured ore above a low-permeability rock formation (Davidson et al. 1988). In theory, it may be possible to operate a well-to-well injection and recovery system within a partially saturated ore body by balancing the production and recovery precisely so that saturated conditions are maintained within the ore zone. In practice, this is difficult and can result in some solution losses via vertical gravity flow in fractures and faults (Figure 5.3) unless the underlying rock mass has very low permeability. Wells can also be operated independently or semi-independently using the push-pull technique, which can be used for one or multiple wells in a transient flow condition. Push-pull involves the alternation of injection and recovery of a lixiviant and to do so, it is necessary to complete the well for injection and recovery via a submersible pump or other artificial lift mechanism (e.g., air lift in relatively shallow wells). Push-pull is an effective technique, especially at the beginning of operations, to condition and stimulate the well and to

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78 CHAPTER 5

FIGURE 5.10  Fan well array for in situ copper recovery at the Cyprus Casa Grande underground mine

Surface, Elevation ~1,000 m (3,280 ft) above Mean Sea Level

Overburden

Wellhead Drift

Copper Oxide Ore

Level Drift 335 m (1,100 ft)

Injection/ Recovery Wells

Level Drift Sump 274 m (900 ft)

Hypogene Ore

Adapted from Schmidt et al. 1994

FIGURE 5.11  Diagram of Figure 5.10

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Drilling and Well-Field Technology

79

obtain field-scale information on leaching rates and expected grades in solution. However, completing and equipping each well for injection and recovery is more expensive. Well pressurization for ISRR is limited by the fracture gradient, which is the depth-specific injection pressure at which the rock mass fractures (Schlumberger 2019). A fracture gradient of 15.8 kPa/m (0.7 psi/ft) is often used as a reference for initial well operation planning (Davidson et al. 1988), but the rock-specific fracture gradient is highly dependent on rock type, tectonic history, weathering, degree of saturation, stress conditions, and so forth. A single fracture that connects wells is not desirable for ISRR, hence it is usual to practice injection at pressures that are comfortably less than the fracture limit. However, fractures can dilate under increasing pressure, up to the fracture limit, depending on the rock’s mechanical properties. In a push-pull operation, hydrodilation can be used to move solutions in and out of dead-end fractures and pores and create transient advective flow (see Figure 4.3). This technique can be used in rocks that have low pore and fracture connectivity where the probability of fracture apertures (a) less than the critical aperture (ac) is high (see Chapter 4). The ability of terminal fractures to dilate and dilate over many cycles depends on the rock type, degree of fracturing, lithostatic pressure, and so forth. A study of the mechanical properties of the ore body is required to determine the elastic potential and requisite pressure conditions for push-pull ISRR operation. However, this technique can be used to circumvent short circuiting and dual-porosity transport behavior that can limit metal recovery. Decisions regarding the pattern, spacing, and construction of injection and recovery wells depend on an understanding of how fracture hydraulic conductivity varies with the volume of rock being developed. While a large number of fractures may be encountered in a borehole drilled into fractured rock, it is often the case that only a small percentage of these conduct measurable amounts of water into the borehole during pumping (Bear et al. 1993). Fracture-flow modeling studies have revealed that the hydraulic behavior of fractured rock formations is mainly determined by the geometry of connections within the fracture system (Nelson 1985). Because the probability of fracture connection linearly increases with fracture size, fracture hydraulic conductivity also increases with fracture size. Hydraulic conductivity measurements in fractured rock are therefore scale dependent, as for some sample volume, fractures no longer cut across the entire volume of the sample. The mean fracture transmissivity, therefore, varies with sample size. If for increasing sample volumes, new unique fracture distributions are encountered, there does not exist a representative elementary volume of fractures, with representative hydraulic conductivity. More discussion of rock mass characterization is provided in Chapter 6. Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

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CH AP TER   6

Advanced Rock Mass and Ore Characterization Although well-established rock mass and ore characterization techniques utilized for hydrology, mining, metallurgy, and geotechnical purposes can be used for in situ recovery and remediation (ISRR) projects, they must be tailored for this application and sometimes augmented by specialized techniques and technologies. Exploration programs generate information on deposit host rock, structure, grade, rock quality, and so forth but are oriented to valuation for conventional mining. Evaluation of total and lixiviant soluble metal grades, and rock quality or fracture density, are preliminary indicators of ISRR potential (Sutton 2019). Feasibility studies for promising deposits also conduct metallurgical testing for scoping optimal processing technology and market potential. Some of the traditional hydrometallurgical methods that can be used for ISRR are discussed in Chapter 4. As traditional hydrometallurgical processing requires large amounts of water resources, information on regional hydrology is necessary. Open pit and underground operations also require hydrologic information on de­­watering and geotechnical studies to determine rock strength and structure for pit and underground working design that may be useful for ISRR. ISRR feasibility studies, however, need to conduct both field and laboratory studies to determine well-field scale porosity, permeability, and ore mineral deportment for estimation of ultimate recoveries. As discussed in Chapters 4 and 5, there is some confusion and overlap of the terms sweep efficiency and recovery. Sweep efficiency in the oil and gas industry is primarily focused on fluid distribution and mobilization, whereas sweep efficiency in ISRR also involves microscale fluid mineral contact within the meso- and macroscale well-field flow field. Hence as discussed in Chapter 4, specialized leaching tests are required to determine ultimate metal recovery using natural or modified permeability as compared to comminuted ore. However, larger scale characterization of the near wellbore and well-field rock masses is required to properly apply geological, hydrological, and scale laboratory testing results to ISRR projects.

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82 CHAPTER 6

Table 6.1  Advanced rock mass characterization tools and tests for ISRR Tool or Test Borehole packers

Information and Data Permeability, hydraulic conductivity

Borehole televiewer (light or acoustic) Nuclear logging (neutron, geochemical logging tool, etc.) Seismic and sonic borehole logging and tomography Electromagnetic borehole logging and tomography Muon tomography

Lithology, well integrity, fracture density, orientation Lithology, porosity, saturation, rock composition, metal grade Lithology, fracture density and orientation, fluid movements Fluid content and movement

Rock density and fluid movement in deep rock masses Cross-hole aquifer pump testing and Fluid movement, well connectivity hydrologic inverse modeling Tracer testing Fluid movement, effective porosity, dispersion

Packer Inflation

Nelson 1991; Jouanna 1993 Tweeton et al. 1989; Jouanna 1993 Tweeton et al. 1989; Jouanna 1993; Tweeton et al. 1994 Schouten 2018; Bonneville et al. 2017 Schmidt et al. 1994; Vesselinov et al. 2001 Weight and Sonderegger 2001; Wolkersdorfer 2006; Slater et al. 2010

Pressure and Flow Data Acquistion

Water Injection

Transducer Pass-Through

References Schmidt et al. 1995; Doubek et al. 2013 Schmidt et al. 1995; Jouanna 1993

Inflation Lines

Inflatable Packer Pressure Transducer Injection Interval

3 m (9.8 ft)

Inflatable Packer Borehole 15.24 cm (6 in.)

Source: Schmidt et al. 1995

FIGURE 6.1  Typical borehole straddle-packer configuration

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Advanced Rock Mass and Ore Characterization

83

Table 6.1 is a summary list of some advanced rock mass and ore characterization techniques that can be applied to ISRR. In addition to traditional single well slug and pumping hydraulic testing (Doubek et al. 2013), packer and straddle-packer aquifer pump tests (Figure 6.1) are especially useful in determining mesoscale permeability in ore zone intervals, which is highly important for ISRR (Schmidt et al. 1995). Target metals grades are usually variable and so is permeability, especially in fractured host rock, and there may be random or systematic variations. Packer and straddle-packer aquifer pump tests can determine permeability at different depths in the borehole that may correspond with host-rock lithology or geologic structures. With this information, wells and well fields can then be designed to optimize ISRR for site conditions. A schematic diagram of a straddle-packer arrangement is provided in Figure 6.1. Schmidt et al. (1995) used straddle-packer aquifer pump testing to log vertical differences in borehole permeability and transmissivity that corresponded with the gossan (leached cap), supergene (fracture-hosted chalcocite), and hypogene (disseminated chalcopyrite) ore horizons of a porphyry copper-molybdenum deposit at the Mineral Park mine in Arizona (Figure 6.2). The leached cap or gossan zone had relatively high transmissivity as a result of weathering and enhancement of porosity. Supergene ore deposition of copper and clay minerals at the water table resulted in an enrichment blanket with reduced porosity and transmissivity. The hypogene ore had an average transmissivity that was between the average leached cap and supergene zone transmissivities. The hypogene ore contained remnant porosity from porphyry copper system emplacement in partially filled quartz veins and fractures. The figure shows the vertical variations along with acoustic borehole televiewer fracture density at the Mineral Park test site in Arizona, which is correlated with permeability. More details of this ISRR project are provided in Chapter 7. Intensive fracture characterization is also required to determine the true distribution of fracture characteristics, such as the length scale (lf ) of fracture segments of aperture a (see Figure 4.3), and to compensate for geometric effects of fracture density estimates from linear core (Dahl 1989). It is uncommon for ISRR practitioners and geologists to do this for entire deposits with statistically complete sampling frequency for commercial projects, but qualitative estimates can be made. Specialized downhole logging tools developed for the oil and gas industry (sometimes referred to as wire-line logs) can be used to extend information from core sample characterization to larger rock masses surrounding the borehole. Logging tools include seismic, sonic, nuclear, electromagnetic

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84 CHAPTER 6

20 40 60 80 100 120 140

Depth, ft

160 180 200 220 240 260 280 300 320 340 360 380

1E–9

1E–8

1E–7 1E–6 Permeability, m/s

1E–5

1 ft depth = 0.31 m; 1 m/s permeability ~ 32 ft/s

Figure 6.2  Acoustic borehole televiewer diagram of fracture density compared to straddle-packer test permeability measurements

instruments for measuring ambient and induced seismic, sonic, radioactive, and electromagnetic signals from the rock mass. Downhole spinner logs are also used to directly measure fluid flow velocities (Weber et al. 2000). Depths of investigation range from millimeters to tens of meters (tenths of inches to hundreds of feet). Cross-hole tomography is possible with some of these instruments (Tweeton et al. 1989). Borehole logging of fractured rock is especially needed because core recovery is usually poor where rock is highly

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Advanced Rock Mass and Ore Characterization

85

fractured, making the horizons of interest poorly represented. Summaries on some of these techniques are provided in Nelson (1991) and Jouanna (1993). For some metals, such as copper, advanced nuclear logging can directly measure grade by neutron activation as long as proper calibration to laboratory assays are performed (Charbucinski et al. 2004). This specialized logging tool is sometimes referred to as a geochemical logging tool (GLT; Nelson 1991). Figure 6.3 shows an example output from a multielement GLT log. A relatively recent advancement in pulsed neutron activation technology now allows borehole logging without using a radioactive source because fast and thermal neutrons can now be generated using an electric source that can be

Source: Nelson 1991

Figure 6.3  Geochemical logging tool log from a copper oxide deposit

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86 CHAPTER 6

activated and deactivated to protect workers from radiation (Sodern 2019). The depth of investigation into the formation away from the borehole is several centimeters (inches) depending on lithology and whether the borehole is cased or uncased. One potential application of the GLT is the measurement of in situ copper grade in a borehole annulus before and after leaching to determine potential in situ recovery. GLT logging of a well that is capable of injection and recovery determines the head grade. A short-term lixiviant injection and recovery test with the same well produces a copper grade in solution and recovery curve within the intact rock mass surrounding the well bore. This method is potentially more representative of the actual ISRR metal recovery than the column and core leaching tests performed in the laboratory discussed in Chapter 4. Borehole televiewers and acoustic borehole televiewers and logging can supplement core fracture analysis. Although it is possible to collect oriented core for determining fracture aspect (Dahl 1989), borehole televiewer surveys are cheaper and more continuous than oriented core. Borehole televiewers are also used to inspect well completions and for operating well diagnostics. ISRR wells are subject to scaling biofouling and corrosion, and periodic televiewer inspections can identify problems and potential remedies. Multiple borehole testing and hydrologic inverse modeling analysis techniques have been applied to ISRR (Schmidt et al. 1994) and hydrology research (Vesselinov et al. 2001). By monitoring hydraulic head at multiple levels in several monitoring wells, a three-dimensional (3-D) tomographic image of hydraulic gradient can be produced by computer post processing. This information is highly useful for ISRR but is complex and time consuming. Geophysical sensing techniques using seismic waves and electromagnetic radiation have been developed for detection and monitor the subsurface movement of ISRR solutions (Tweeton et al. 1994). Because ISRR leach solutions often have high total dissolved solids and conductivity, they can be traced using electromagnetic sensing. Recently, air- and ground-based electromagnetic geophysical survey services have also been used to map subsurface mine-influenced water and sulfide mineralization at closed mines to depths of nearly 300 m (approximately 1,000 ft) (Hammack et al. 2003). Muon tomography is an emerging technique that can potentially provide more detailed information on in situ rock mass and ore body density (Schouten 2018) and track fluid movement during ISRR (Bonneville et al. 2017) to depths of up to 2 km (approximately 1.25 mi) beneath the surface of the earth. Muons are subatomic, negatively charged particles that are similar to electrons but with much greater mass. Muons are generated in the upper

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Advanced Rock Mass and Ore Characterization

87

atmosphere from cosmic ray interactions, strike Earth with speeds approaching light, and penetrate the earth at multiple angles. The particles can penetrate rock to great depths in the earth because of their speed, mass, and momentum. While sounding futuristic, muon tomography has successfully imaged pyramids (Guarino 2017) to nondestructively find new chambers and has imaged magma movement in volcanoes (Tanaka et al. 2001). Because they have a diffuse source and random trajectories, they can be used to image rock masses with different densities, which creates variations in detection frequency when detectors are placed beneath or adjacent to the mass. Because pore fluids can also change the density of rock mass, fluid movement during injection can be tracked. Detectors that are small and sensitive enough to be placed into boreholes for monitoring fluid injection are being developed by the U.S. Department of Energy (Bonneville et al. 2017) but are currently not commercially available. Tracer testing has been used to determine fluid movement in aquifers for hydrologic, environmental, and ISRR applications (Weight and Sonderegger 2001). If the tracer test is designed correctly, much valuable information can be gained prior to ISRR injection and recovery that cannot be gathered by any other method. For example, direct fluid pathways between wells and anisotropic permeability can be identified to predict potential short circuiting. In addition, estimates of effective porosity and dispersity can be derived. Sodium chloride is one of the most commonly used tracers as it is readily dissolved, highly soluble, chemically conservative (little reaction or sorption potential), and background concentrations are usually low in relatively shallow aquifers. In addition, at high enough concentrations, it can be used to enhance electromagnetic tracing of fluid movement because of enhanced conductivity. Figure 6.4 shows an example of a computer-simulated chloride tracer return in an ISRR seven-spot well pattern in a sandstone host rock. The computer results were generated for tracer test design and to estimate reagent and pumping equipment requirements. Initial breakthrough at the well nearest to the injection well was predicted to occur in about five days. However, in the actual field test, the first arrival of tracer occurred in the nearest production well within one hour and reached peak concentrations within one day. The injection well was completed with a percussion hammer drill rig, and well development was performed with high-pressure gas; postmortem analysis revealed that the pressure in the open borehole exceeded the fracture gradient. A borehole camera revealed that a large nearly horizontal fracture had propagated to the production well as a result. The artificial fracture resulted in a much earlier arrival of the tracer. This example illustrates the importance of drilling and well completion in ISRR performance and

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88 CHAPTER 6

10,000

50 m, 30 m

9,000

38 m, 18 m

8,000

62 m, 14 m

Cl–, mg/kg

7,000 6,000

34 m, 46 m

5,000

82 m, 30 m 66 m, 46 m

4,000

22 m, 30 m

3,000 2,000 1,000 0

50 m, 62 m 0

10

20

30

40

50

60

Time, days 1 mg/kg = 7.2 × 10–5 oz/lb Grid well coordinates in meters (1 m = 3.3 ft) Injection well at 50 m (164 ft) east, 30 m (98 ft) north

Figure 6.4  Simulated breakthrough of chloride tracer in seven-spot well field for a homogeneous sandstone formation

the value of using borehole televiewers and other wire-line logging tools in characterization and diagnostics. Various other tracers can be used to determine hydraulic and chemical characteristics of an ore body or rock mass. For example, stable isotopes can be used to distinguish chemical reaction mechanisms such as biological versus abiotic leaching (Chapelle 1993). Heat can also be used as a tracer by monitoring temperature in wells. Fiber-optic distributed temperature sensing is another emerging application for contaminant transport detection (Slater et al. 2010) that can be applied to ISRR.

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CH AP TER   7

In Situ Copper Sulfide Recovery Project This chapter summarizes key results from the Mineral Park in situ recovery and remediation (ISRR) project conducted by the U.S. Bureau of Mines prior to its closure in 1996. Several of the basic principles and approaches to ISRR, which have been developed to this point, are illustrated in this example. Because this is a project where the U.S. Bureau of Mines worked cooperatively with a mining company and other government and educational institutions to develop in situ recovery technology, the data are somewhat more available than other projects. A recent bibliography of in situ copper recovery projects was compiled by Sinclair and Thompson (2015), and other summaries also provide overviews of some less published projects. Two good summaries of the successful San Manuel commercial project area are also available in the literature (Sutton 2019; Burt et al. 1994). The Mineral Park project falls under the category of brownfield projects located in partially developed ore deposits by conventional mining, but it is unique in that it targeted copper sulfides. Greenfield copper oxide recovery projects are currently being developed in Arizona. INTRODUCTION A pilot-scale in situ copper recovery test was conducted at the Mineral Park copper mine located in the Wallapai mining district of northwestern Arizona near the town of Kingman (Figure 7.1). The multidisciplinary project was conducted under a three-way cooperative agreement between the U.S. Bureau of Reclamation, Columbia University, and the Cyprus Minerals Corporation (now Waterton Global Resource Management). In addition, the U.S. Geological Survey (USGS) and Idaho National Engineering Laboratory (INEL) conducted fracture (Schmidt et al. 1995) and microbiological characterization research studies (Lehman et al. 2001), respectively. The goals of the project partnerships were threefold: 1. Develop a safe and economical process for in situ recovery of copper from relatively shallow and low-grade supergene sulfide copper ore deposits.

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90 CHAPTER 7

Sulfuric Acid Storage (35,000 lb)

Turquoise Mountain 4450

Acid Pipeline

Acid Flow Control/Solenoid Valve Acid/Raffinate Mixing Tank Raffinate Flow Control Conductivity Tank and Flow

4310

Raffinate Pipeline from Electrowinning Plant

4300

Sensors

4400

4350 Process Solution Pipeline

Injection Flow Control and Flow Sensors

4330

4200

4100 B2 B3

B1 Operations W1 A2 Trailer A4 Injection Monitor Trailer length approximately Wells E1 Wells A3 10 m (30 ft) E3 Injection Well Bypass E2 Monitor Well Pipeline Recovery Well Pressure/Flow Sensor Flow Controls Test Site Boundary A1

In Situ Well Operating Area W2

Recovery Wells Pregnant Leach Solution Recovery Tank Pregnant Leach Solution Pipeline to Solvent Extraction Plant

Ithaca Pit, Water Level 4100 ft MSL

N 1 in. = 100 ft

FIGURE 7.1  Mineral Park in situ leaching test site

2. Determine the utility of applying in situ technologies to surface and groundwater control plans for mine closure. 3. Assess the environmental benefits of using in situ technology to reduce long-term acid rock drainage (ARD) pollution potential from inactive and abandoned metal mines. In 1995, because of the closure of the Bureau of Mines’ Twin Cities Research Center, the in situ test facility at Mineral Park was transferred first to the U.S. Bureau of Reclamation, and then to the Earth Engineering Center

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In Situ Copper Sulfide Recovery Project

91

at Columbia University. During the late summer of 1996, a three-month pilot-scale in situ leaching test was conducted at Mineral Park mine. The results of this pilot-scale leaching test are presented here in more detail than other examples because the project quickly ended, owing to the closure of the Bureau of Mines, and extensive publication was not possible. This section summarizes pilot test results along with a brief review of the hydrogeologic and geochemical characterization work presented earlier (Schmidt et al. 1995). The geology and early history of the Mineral Park deposit and mine have been described in detail by Wilkinson (1981). Duval Corporation began development of the Mineral Park deposit in 1964 using conventional open pit mining methods. Between 1964 and 1979, Duval produced more than 272 million kilograms (600 million pounds) of copper and 21 million kilograms (46 million pounds) of molybdenum at Mineral Park. Three large pits were developed on the property during this time. The largest, Ithaca pit, is at the former location of the mountain known as Ithaca Peak. Two large waste dumps and an impoundment containing 60 million tons of mill tailings were also created during these years. In 1981, Duval Corporation ceased conventional open pit mining operations at Mineral Park. During the peak years of activity at Mineral Park, conventional mining and milling practices processed copper (and molybdenum) from only the highest-grade sulfide ore. Lower grade ore and waste rock were piled onto dumps in the form of poorly sorted rubble and finer material. In 1986, the Mineral Park property was sold to Cyprus Minerals Corporation. Cyprus Mineral Park combined mine rehabilitation and closure of some facilities with economical production of copper by leaching from low-grade copper reserves in dumps and pit walls. Copper was recovered from recirculated acidic mine water using drill-and-blast leach-mining methods. With drill-and-blast leaching, low-grade ore on the periphery of pits was broken by blasting (see Figure 4.1). Acidic ferric sulfate leach solution was then distributed on the surface of blasted ore using a network of sprinklers. The leach solution infiltrated the unsaturated rubblized ore and flowed to one of the three nearby pits. The copper-bearing pregnant leach solution (PLS) was recovered by pumping from ponds at the bottom of pits (see Figure 4.2). Recirculation of acidified catchment water at Mineral Park prevented mine discharges from reaching both the surface waters and groundwaters of the Sacramento Valley and lower Colorado River basins as excess inflow water to the mine evaporated in the pits. All of the chalcocite reserves in dumps and pit walls that were amenable to drill-and-blast leaching methods were fully developed, and copper was recovered by a solvent extraction and electrowinning (SXEW) plant built in 1994. Despite the relatively low grade of Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

92 CHAPTER 7

copper in solution, averaging approximately 0.5 g/L, the plant was able to produce copper cathode, and the return-flow raffinate copper concentration was 0.1 g/L. Mercator Mineral Park Holdings bought the property in 2003 and operated the mine to produce copper by leaching and molybdenum ore until the company closed the mine in 2014. In situ recovery at Mineral Park resulted in economical production of copper from reserves that were low grade and too distant from the existing pits and too deep to conventionally mine or mine by drill-and-blast techniques. In addition, this method reduced the long-term acid mine water generation problem that can result from expansive pit development, rubblization of ore, and dewatering of sulfide minerals (Earley et al. 1996). Hence, in situ recovery can extend the economic life of the mine without adding to the burden of waste reclamation. GEOLOGY AND MINERALOGY Located in the central part of the Wallapai mining district of northern Arizona, Mineral Park is a copper-molybdenum deposit within and adjacent to a Laramide quartz monzonite porphyry intrusion. Minable copper reserves have been estimated to exceed 40 million tons of 0.2% or greater (Wilkinson 1981). About 90% of the sulfide minerals at Mineral Park occur as fracture fillings and veins. These vein sets are steeply dipping and trending to the northeast, north, and northwest. The third stage of alteration resulted in the formation of economically important veins of chalcopyrite. At least two periods of uplift since Laramide time have resulted in deep weathering and enrichment of the Mineral Park deposit and oxidation of chalcopyrite by oxygenated recharge water infiltrating from the surface-mobilized copper to deeper horizons. Dissolved minerals have moved downward through steeply dipping fractures until encountering chemically reducing conditions at or just below the groundwater table. Repeated uplift and fluctuation in the groundwater elevations have resulted in the formation of multiple layers of chalcocite precipitation within the enrichment blanket (see Figure 2.3). IN SITU LEACHING TEST SITE The test site at Mineral Park (Figure 7.1) is situated in an altered assemblage of metamorphic and igneous bedrock that is part of the Cerbat Complex. The chalcocite ore at the test site is hosted by Precambrian hornblende metadiorite and biotite schist. The target of in situ leaching is a low-grade chalcocite copper enrichment blanket that lies between 21 and 58 m (69 and 190 ft) below the surface with an average grade of 0.33% copper. Approximately three-fourths of the ore is situated below the ambient groundwater table.

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The in situ leaching test site area is approximately 91-m (300-ft) wide and 183-m (600-ft) long and is situated 90 m (295 ft) from the southern edge of Ithaca pit. The surface elevation of the site is 1,321 m MSL (mean sea level) (4,337 ft). The deepest elevation of Ithaca pit is 1,252 m MSL (4,110 ft). Drill-and-blast leaching areas are located along the Ithaca pit walls north of the site. Leach dumps that drain into Bismark Wash and the tailings impoundment are located west of the site. Turquoise Mountain, elevation 1,457 m MSL (4,780 ft), is located south and southwest of the site. Waste dumps at higher elevations are also located south and east of the site. In September 1993, seven 15.24-cm (6-in.) diameter rotary holes were drilled at the test site to a maximum depth of 91 m (300 ft). Except for a 6–11 m (20–36 ft) cemented surface casing, the holes remained open and uncased. The construction, pattern, and spacing of these seven boreholes, A1, A2, A3, E1, E2, W1, and W2, were intended to provide the maximum opportunity to gain information for characterization and design of an in situ leaching operation. The steep groundwater gradient induced by pumping from Ithaca pit, 90 m north of the test site, was utilized and monitored using this pattern of boreholes. Drill cuttings from these boreholes were assayed to determine the distribution of chalcocite ore beneath the site. The open-hole construction also permitted a borehole televiewer to be used to determine the orientation and intensity of fracturing within the ore zone (Schmidt et al. 1994). The pattern and variable spacing of boreholes allowed for pumping tests to evaluate hydrologic properties that are dependent on region, orientation, or scale of measurement. Hydrologic properties of individual lithologic units (and individual fractures) were evaluated in open boreholes by an arrangement of straddle packers and pumps. Comprehensive chemical analysis of groundwater was conducted using samples obtained during borehole pumping and packer tests. In August 1994, two directionally drilled NX size (4.7-cm [1.85-in.]) core holes were also made at the test site. The two oriented core holes trended S28E and S15W, and dipped 58 and 59 degrees from the horizontal, respectively. The core holes were drilled to intercept the two main fracture sets, a northeast trending set and a northwest trending set, at approximately right angles. Approximately 35 m (100 ft) of oriented core was recovered from each hole. A north–south cross section through the middle of the test site (Figure 7.2) shows the approximate depth of the vertical boreholes, enrichment blanket, and Ithaca pit. The chalcocite ore zone lies between 20 and 58 m (66 and 190 ft) below the surface. It is entirely above the bottom elevation of Ithaca pit. The enrichment blanket is overlain by 15–30 m (49–98 ft) of weathered cap rock. The weathered cap (gossan) is rich in iron oxides that have replaced Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

94 CHAPTER 7

4,600 4,500

Ithaca Peak

Turquoise Mountain In Situ Leaching Test Site

Elevation, ft

4,400 4,300

Oxide Cap Groundwater Level

4,200

Chalcocite

Ithaca Pit

Chalcopyrite

4,100

Injection, Recovery, and Monitor Wells

4,000 0 South

150

300

450

600

750

900

1,050

Distance, m Elevation in feet above mean sea level, horizontal distance in meters, 1 m = 3.3 ft

FIGURE 7.2  Mineral Park mine test site and Ithaca pit cross section

pyrite. The rotary boreholes were drilled through the gossan and enrichment blanket and extended 25–60 m (82–197 ft) into the primary chalcopyrite ore underlying it. The chalcocite enrichment blanket is composed of two to three layers of higher grade ore (0.4%–1.3% Cu), from 6- to 12-m (20- to 40-ft) thick, separated by thinner layers of lower grade ore ( 0.3%), and primary ore—appeared densely fractured, although the highest concentration of borehole fractures and highest fracture transmissivity was found in the unmineralized oxide cap (e.g., Figure 7.3). It is difficult to see a correlation between fracture intensity and ore zone lithologies, although there is some indication that fracture intensity in the enrichment zone may be somewhat diminished, relative to the primary ore underlying it. The most distinct fractures are northeast trending, and northwest dipping. But some northwest trending fractures were observed in both holes; they did not predominate. The absence of northeast trending fractures (visible to the acoustic televiewer) in the enrichment blanket suggests that mineral-rich fractures may be invisible because of associated clay fillings. Up to 20 packer injection tests were conducted in each borehole. Seventytwo packer tests were conducted in boreholes A3, W1, E1, and E2. The injection pressures during these tests ranged from 14 to 828 kPa (2 to 120 psi) and flow rates ranged from 0 to 76 L/min (0 to 28.6 gpm). As an initial guide to packer placement, televiewer logs were used to locate intervals of dense fracturing. However, most 3-m (9.8-ft) intervals were so intensively fractured that it was not practical to isolate individual fractures with the straddle packers. The distribution of transmissivity relative to lithologic units is summarized in Table 7.1. The highest transmissivity value is associated with the weathered oxide cap, which averages 1.6 × 10–5 m2/s (1.7 × 10–4 ft2/s), whereas the average transmissivity of the enrichment zone is more than an order of magnitude TABLE 7.1  Transmissivity by mineralization Mean, Mineralization Count m2/s* Gossan 16 1.6e-5 Chalcocite 19 7.3e-7 (supergene) Chalcopyrite 38 2.5e-7 (hypogene) * 1 m2/s = 11 ft2/s. Adapted from Schmidt et al. 1995

Standard Deviation 0.799 0.753

Minimum, m2/s 1.6e-6 3.0e-8

Maximum, m2/s 1.1e-3 8.0e-6

Effective Permeability, mD (millidarcys) 491  23

0.969

1.2e-8

1.0e-5

  8

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96 CHAPTER 7

4,320

4,320

4,310

4,310

4,300

4,300

4,290

4,290

Leached Cap

4,280 4,270

4,270

4,260

4,260

4,250

4,250

4,240

4,240

Fracture Hosted Chalcocite

4,230

Elevation, ft MSL (1 ft = 0.31 m)

4,280

4,220

Ambient Groundwater Elevation

4,210 4,200

4,230 4,220 4,210 4,200 4,190

4,190

4,180

4,180

Disseminated Chalcopyrite

4,170

4,170

4,160

4,160

4,150

4,150

4,140

4,140

4,130

4,130

4,120

4,120

4,110

4,110

4,100 0 0.8 1.5

Total Copper, wt %

4,100 1E-9 1E-5

Transmissivity, m2/s

Source: Schmidt et al. 1995

FIGURE 7.3  Borehole copper assay, transmissivity profile of A3 from straddle-packer testing results

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In Situ Copper Sulfide Recovery Project

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lower. On average, the transmissivity of the primary chalcopyrite ore zone is about the same as the enrichment layer; however, pumping and packer tests revealed that a very small number of fractures in the primary ore supplied almost all of the flow to the borehole. The greater variability of fracture transmissivity within the primary ore zone is indicated in Table 7.1 by a larger standard deviation. The transmissivity profile for one representative borehole, A3 (see Figure 6.2), plots the elevation of the packer interval versus the logarithm of transmissivity. The corresponding copper assay is based on samples of drill cuttings taken at 1.5-m (5-ft) intervals in the borehole. The lithologic boundaries between leached cap rock, enrichment blanket, and primary ore is inferred from the assay data. The ambient groundwater level in the borehole is also noted on this profile. WELL SLUG POTASSIUM BROMIDE TRACER TEST An important hydrologic concern associated with the shallow-injection deep-recovery (SIDR) design involves the capability of leach solution that is injected into the shallow gossan layer to move downward through the chalcocite enrichment blanket as it also moves laterally between injection and recovery wells. To evaluate this aspect of the design, a SIDR tracer test was performed at the test site prior to injecting leach solution. The tracer experiment was conducted as a slug test in May 1995 to determine effective porosity and dispersivity at the Mineral Park test site. Tracer was injected as a slug in borehole B1 during continuous pumping from borehole B2. Figure 7.4 shows the tracer test setup. The B2 pumping rate was fixed at approximately 5.5 L/min (1.45 gpm). The pump in B2 was set 55 m (180 ft) from the surface. A packer was set in B1 at 58 m (189 ft) and inflated. A lower packer was also installed in B1 at the start of the test; however, it was deflated within a few minutes after injection of the tracer. Straddle-packer injection tests in B1 and B2 show that almost all of the hydraulic conductivity in B1 was below 56 m (185 ft) and below 53 m (175 ft) in B2. Steady-state drawdown conditions were observed in both B2 and B1 within three hours after the start of pumping. The steady-state groundwater elevation in B2 was approximately 9 m (30 ft) from the bottom of the borehole (60.7 m [199 ft]). In B1, it was 22 m (72 ft) from the bottom, which had a total depth of 62 m (204 ft). Four hours after the start of pumping, 103 L (27 gal) of tracer, which was approximately 1.9 g/L (1,868 ppm) potassium bromide (KBr), was injected in B1 in the interval between the two packers. Approximately 15 minutes was required for complete tracer injection. Almost immediately after the injection was completed, the lower packer was released, exposing a 5.5-m (18-ft) interval of the 15.2-cm (6-in.) borehole below the inflated packer to the tracer Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

98 CHAPTER 7

B2

B1 Ambient Water Level 56.7 m (186 ft)

58.6 m (189.0 ft)

Pump Packers Flow Not to Scale

4.6 m (14.7 ft) FIGURE 7.4  B1 to B2 well tracer test

18 16

Concentration KBr, ppm

14 12

Peak Arrival Time

10

Upper Packer Release

Lower Packer Release

8 6 4 2 0

0

200

400

1 ppm = approximately 1 mg/L

600

800

1,000

1,200

1,400

1,600

Time, minutes

FIGURE 7.5  Bromide tracer breakthrough and concentration curve at B2

solution. The start time for the tracer test was the time of release for this lower packer (44 minutes). The upper packer remained inflated during the entire test. At the end of the test, after more than 22 hours of well operations, the upper packer was also released. Head conditions below the packer in B1 returned to pre-injection levels within 30 minutes after injection of the slug. Tracer concentration in B2

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In Situ Copper Sulfide Recovery Project

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water samples was monitored in the field for approximately 24 hours after injection using a bromide ion–specific electrode. The tracer test results shown in Figure 7.5 were determined by laboratory analysis of B2 water samples. To obtain an estimate of effective porosity from tracer test results, an advective flow model was used to simulate the flow of tracer from B1 to B2 during pumping. The advective model used the following solution for two-dimensional, steady-state flow from a single well located at the origin given by Q Φ = 2π ln ` Rr j + Φ 0 (Equation 7.1) where F = potential function, defined at any point in a confined aquifer by F = kHf, where kH is transmissivity (hydraulic conductivity × thickness) and f is piezometric head Q = well pumping rate r = arbitrary radial distance from the well R = radial distance from B2 to a steady-state constant head boundary F0 = potential value at the radial boundary The discharge Qr (discharge over an unspecified thickness) at a point that is a distance r from the pumped well is given by Q Q r = dΦ = 2π $ a 1r k (Equation 7.2) dr Average discharge along any arbitrary flow path leading to the well (discretized in n equal segments) is therefore Q r = 1n

n

/

i=1

d Φi (Equation 7.3) dri

Specific discharge qr is the discharge per unit thickness and the velocity of a particle following its trajectory along a flow path and is related to seepage (average) velocity (vr) by the following formula: q r = v r θ (Equation 7.4) where q is effective porosity. Because of microscopic variation in the flow path, the travel time of an average water particle was greater than predicted by the specific discharge. Seepage velocity v r = ΔL (Equation 7.5) Δt

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100 CHAPTER 7

is the time required for an average water particle to move a distance interval DL during a time interval Dt. Seepage velocity is obtained (in a slug tracer test) by measuring peak tracer concentration arrival time. The ambient water level in B1 and B2 was 33.8 m (111 ft) from the surface. The B2 flow rate Q was fixed at 5.5 L/min (1.45 gpm). Transmissivity kH was manipulated in the model until simulated head at the locations of boreholes B1 and B2 matched actual steady-state head conditions. The calibrated value of transmissivity that gave B2 drawdown of 56.7 m (186 ft) and B1 drawdown of 40 m (132 ft) was 4.2 × 10–6 m2/s (4.5 × 10–5 ft2/s). After calibrating with respect to transmissivity, the model was used to estimate specific discharge along a radial flow path between tracer injection borehole B1 and pumped borehole B2. Specific discharge qr along the flow path in Figure 7.4 was estimated to be qr = 0.127 cm/min (0.25 ft/h). Tracer breakthrough (Figure 7.5) indicates arrival time for peak concentration of the tracer slug occurred between 443 and 692 minutes after injection, therefore actual tracer velocity (Vr) is between 1.0 cm/min (2 ft/h) and 0.065 cm/min (1.3 ft/h). Effective fracture porosity (h) is then estimated to be between h = 0.013 = 1.3% and h = 0.02 = 2.0%. It is assumed that the porosity measurement represents effective fracture (rather than matrix) porosity. Because of a pump malfunction, it was not possible to evacuate groundwater below the packer in the B1 borehole before injecting the tracer, as was originally planned. As a result, there was some dilution of tracer with groundwater in the borehole. The KBr concentration of the injected slug was approximately 1.87 g/L (1,867.5 ppm) (total = 192 g [6.8 oz]). The estimated volume of groundwater in the B1 injection pipe and borehole (below the packer) just prior to tracer injection was 128 L (34 gal). Therefore, the actual tracer concentration in the B1 borehole was (103/230.7) × 1,867.5 = 833.8 ppm (approximately 0.83 g/L). The cumulative tracer recovery curve is obtained by multiplying bromide concentration in by flow rate (5.5 L/min [1.45 gpm]) and then integrating the rate curve. The cumulative recovery calculation shows that approximately 105 g (0.23 lb) of bromide tracer (about 55% of the total injected) was recovered from B2 after 24 hours of pumping. The tracer test also provides the opportunity to evaluate dispersivity on a scale of measurement that is important for the in situ test site. However, it is recognized that dispersion is a scale-dependent property of fractured and porous media that increases if a tracer plume encounters new scales of heterogeneity in hydraulic conductivity as it expands (De Marsily 1986; Gelhar 1993).

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101

In Situ Copper Sulfide Recovery Project

E1(40-70)-III: Gossan Leach Experiment Fe+3, ppm

800 600 400

25 g/L 50 g/L

200 0

0

50

1 ppm ~ 1 mg/L

100

150

200

Time, hours

FIGURE 7.6  Ferric iron batch leaching experiment

A procedure involving type curve matching for two well radial-flow tracer tests is used for evaluation of dispersivity (Dl ), Péclet number (Pe), and porosity (q). The method is presented by Davis et al. (1985). The type curve shown in Figure 7.6 is used to determine Pe and Dl . The vertical axis of the type curve is dimensionless concentration, defined as 2 bθC C D = πr m 

(Equation 7.6)

where pr 2bh = volume of cylinder defined by the withdrawal of water from borehole B2 b = thickness D = characteristic distance (distance between boreholes B1 and B2) C = measured concentration of bromide at time t m = mass of tracer injected in B1 The horizontal axis of the type curve is dimensionless time, defined as Qt  (Equation 7.7) πr 2 bθ where t is the time of sampling from B2, and Q is the pumping rate from B2. Matching the log-log plot of the tracer breakthrough curve in Figure 7.5 to the type curve in Figure 7.6 yields a Péclet number (Pe = 2.0), and Dl is determined to be 2.24 m (7.35 ft). A match point is chosen on both tracer concentration and tracer type curves. The match point coordinates are t = 1,000 for tR = 0.5, and C = 1.5e-5 for CD = 0.85. The equation for reduced time is used, where all values except qb are known so that tr =

θb =

Qt = 17.4 cm (0.57 ft) πr 2 t r

(Equation 7.8)

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102 CHAPTER 7

If an approximately 9-m (29.5-ft)-thick zone conducts tracer from B1 to B2, then porosity h = 0.02. Alternatively, if a narrow fracture zone is conducting tracer, then the individual fracture porosity is much larger. To verify the type curve method, the dimensionless concentration equation for CD is used to calculate CD = 0.857. The dispersivity, Dl = 2.43 m (7.8 ft), is on the same order of magnitude as values reported in Gelhar et al. (1992) for tests on a scale of approximately 5 m (16.4 ft). LABORATORY LEACHING TESTS One of the first detailed studies of chalcocite leaching chemistry was conducted by Sullivan (1930). This study showed that chalcocite leaching is most accurately represented by dividing two reaction steps: Cu2S + 2Fe+3 = CuS + Cu+2 + 2Fe+2 CuS + 2Fe+3 = Cu+2 + So + 2Fe+2

(Reaction 7.1) (Reaction 7.2)

At 25°C (77°F), step 1 proceeds relatively fast, whereas step 2 proceeds relatively slowly. This is because the oxidation of S–2 is much slower than oxidation of Cu+1 at ambient temperatures. Sullivan also showed that as the temperature is modestly increased, the reaction rate of step 2 dramatically increases until it is nearly equal to that of step 1 at 50°C (122°F). Sullivan (1930) characterized the leaching residues and verified that a poorly crystalline copper monosulfide (CuS) compound and elemental S are produced during the first stage of chalcocite leaching with acidic ferric sulfate solutions. This leaching behavior has been verified in many studies. Batch and column leaching experiments were undertaken to determine the specific leaching characteristics of the Mineral Park ore. Most leaching studies are designed to investigate leaching characteristics under unsaturated conditions, whereas in situ mining usually takes place in saturated or nearly saturated ore. Bacterial iron oxidation has been the mainstay of copper sulfide leaching. However, in situ bioleaching has limited efficiency in saturated and intact ore bodies. Microbiological investigations conducted at the Mineral Park test site by INEL have shown that chemolithotrophic, iron-oxidizing bacteria are present in core samples from the deposit, but their activity is very low in saturated sulfide ore because of the lack of oxygen and carbon dioxide (Lehman et al. 2001). In addition, heterotrophs were present in the ore that could facilitate sulfate reduction and metal precipitation following closure of the site. Oxygen and carbon dioxide required for bioleaching are usually supplied in unsaturated leaching conditions in heaps and dumps by convective airflow through broken ore (Brierley and Brierley 1999). These gases could be injected with the lixiviant, but both are permeability limited. Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

In Situ Copper Sulfide Recovery Project

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The experiments conducted on samples from the Mineral Park test site were designed to test the potential for producing ferric ion by leaching it from the gossan. This is important in deposits that have not been subjected to conventional excavation because of airflow limitations. Ore piles and mine pits that result in locally unsaturated conditions promote bacterial iron oxidation and bioleaching. The in situ ferric iron leaching process is an alternative, but the process consumes acid to dissolve iron oxide minerals, and an abundant supply of cheap sulfuric acid is required. For example, batch leaching experimental results shown in Figure 7.6 indicate that ferric ion can be relatively quickly leached from a shallow rotary cutting collected from the gossan horizon of hole E1 (a pure 25 g/L and 50 g/L sulfuric acid solution was used). Although copper recovery from chalcocite requires ferric ion or other oxidants, excess ferric iron does not speed the recovery of first-stage copper; the stoichiometric Fe+3/Cu ratio must be equal to or greater than 2 to achieve the maximum copper loadings. Sequential batch and column leaching experiments indicated that copper grades in solution and recovery rates comparable to the existing drill-and-blast operations could be achieved under saturated test conditions using ferric iron leached from the gossan zone. A sequential column test was performed on rotary cuttings from hole E1. Three columns were loaded with cutting splits from each 1.5-m (5-ft) interval. The first column was loaded with splits from the gossan horizon, the second was loaded with splits from the enrichment blanket, and the third column was loaded with splits from the enrichment blanket (Figure 7.7A). The columns were packed with each split in order of increasing depth from bottom to top. Screen analysis of this material showed that 50%–70% of this material was between 8 and 150 mesh. Examination of core samples suggested that most of the chalcocite was fracture hosted, therefore rotary cutting probably liberated more than 90% of the supergene ore for leaching. The columns were connected in series by Teflon tubing. This simulated the shallow-injection, deep-recovery leaching design where fluids flow from the gossan down through the enrichment blanket. Hypogene ore sampling ports were installed between the columns to facilitate interval sampling and tracking of evolved chemistry as the solution flowed from the gossan through the hypogene ore. Pressure gauges were installed at the top and bottom of the core to determine permeability changes with leaching. A threaded plunger was installed through the top of the column flanges to compress the material during loading and settling. The plunger was sealed against the column wall with an O-ring. The system was designed to operate at pressures higher than 551.6 kPa (80 psi). The columns were saturated and flushed with distilled water prior to acid injection. Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

104 CHAPTER 7

Copper, ppm

50 g/L Inj. 10 g/L Inj. Rest and Monitor

Rest

10 g/L Inj. 10 g/L Cure Col 1 DI Soak 50 g/L Inj. 1,000

(b)

Rest 50 g/L Inj. Rest 10 g/L Inj. Rest

(a)

Column 3

800

Column 2

600

Column 1

400 200 0 4/5/96

4/25/96

1 ppm = ~1 mg/L

5/15/96

6/4/96

6/24/96

7/14/96

Date

FIGURE 7.7  Experimental copper concentrations from sequential batch experiments, (a) showing tandem columns with flow from left to right through gossan, supergene ore, and hypogene ore and (b) copper grades in parts per million

Figure 7.7B shows the history of copper grades during the experiment. A pure 10 g/L sulfuric acid (H2SO4) solution was injected into column 1. This allowed ferric iron to dissolve from the gossan material in a manner similar to the ferric cure process (Bartlett 1992). Then 50 g/L solution was injected into the entire system at a rate of 0.16 mL/min (0.0054 oz/min). Acid breakthrough in the columns was observed by the marked agglomeration of clay seams. The high ionic strength of the solution caused smectite and other clays to shrink and flocculate. These observations suggest that increased flow through clay-filled veins can occur during in situ leaching of intact ore. Injection solution acid concentrations and rest periods were alternated over the course of the experiment, but the injection rate was always 0.16 mL/min Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

In Situ Copper Sulfide Recovery Project

105

(0.0054 oz/min). Initial breakthrough of acid was followed closely by a peak in copper concentrations at approximately 2,800 mg/L (2,800 ppm) in column 2 followed by a lesser peak of approximately 1,300 mg/L (1,300 ppm) in column 3. This peak was probably caused by leaching of relatively soluble copper in the form of oxides, exchangeable copper, and first-stage chalcocite leaching. Copper grades then diminished to less than approximately 450 mg/L (450 ppm) for the duration of the experiment. Longer and longer rest periods were required to achieve the maximum copper grade as the experiment progressed. The grade was relatively insensitive to the concentration of acid in the injection solution and to the introduction of raffinate into the system on April 29, 1996. After that date, raffinate was used to concoct the acid solution instead of distilled water. The concentration of copper in the raffinate was approximately 100 mg/L (100 ppm), which was reflected in the rise of copper concentration in column 1 to approximately 150–200 mg/L (about 150–200 ppm) after that date. The early recovery of approximately 50% of the copper was anticipated in light of the work of Sullivan (1930) who showed that first-stage chalcocite leaching is kinetically fast compared to second-stage leaching. Second-stage chalcocite leaching is attributed to the slow oxidation of the CuS product of first-stage chalcocite leaching. The pH of the experimental PLS was controlled to less than 2 by injection of 50 or 10 g/L H2SO4 solution. At this acid strength, no evidence for precipitation of solids or permeability reduction was observed despite the high TDS in the PLS solution. The acid-neutralizing capacity of the ore was nearly exhausted by the end of the experiment, and natural attenuation of leach solutions only occurred as the fluid migrated downgradient and through fresh rock. Therefore, the buffer zone around an in situ facility must be large enough to buffer the entire volume of residual leach solution. PILOT TEST DESIGN AND RESULTS FOR IN SITU LEACHING Site characterization and testing data were evaluated and used for well-field design. The design for in situ recovery of copper from the chalcocite enrichment blanket at Mineral Park was SIDR (Figure 7.8) with no blasting and rubblization of the chalcocite ore prior to application of leach solution. The uncased wells drilled into the undisturbed ore were used to inject leach solution such that the phreatic surface rose into the gossan, which flowed via gravity and hydraulic head through the supergene and hypogene ore. Pumped recovery wells were used instead of the pits, to recover copper-bearing solutions from the saturated ore and to prevent losses downgradient. However, in the event of solution losses, the leach solution could be contained by the Ithaca pit at the test site (Figure 7.8) as the capture zone of the pit includes the test site. Copyright © 2020 Society for Mining, Metallurgy & Exploration. All rights reserved.

106 CHAPTER 7

The SIDR design takes advantage of the highly directional nature of fracturing in the chalcocite zone. Fracture orientations predominantly trend northeast and steeply dip to the northwest at approximately 50 degrees from the horizontal. The anisotropy and heterogeneity in permeability that results from fracture orientation and the layering of gossan and enrichment zones favors an in situ hydrologic design in which vertical movement of leach solution through the enrichment blanket is induced by use of shallow injection wells and deep recovery wells. Eight wells were used for injection and recovery of leach solution (Figure 7.1). Wells A2, B1, B3, and E1 were used for injection of leach solution, and wells E2, E3, A4, and W2 (all 15 cm [6 in.] in diameter) were used for recovery. The location of other test site facilities situated along the edge of Ithaca pit, including acid storage and mixing tanks, piping, control, and monitoring systems are also shown in Figure 7.1. A 13,000-L (3,500-gal) sulfuric acid storage tank was located on a bench 180 m (590 ft) west and 32 m (105 ft) upgradient from the injection and recovery well pattern. Recycled ARD (raffinate) was then piped from the Mineral Park solvent extraction plant to level control and acid mixing tanks located 120 m (394 ft) west and 10 m (35 ft) upgradient from the wells. The raffinate tank was connected to the mixing tank. Acid flowed from the storage tank through a Teflon solenoid valve to the mixing tank. The valve operated in a time-proportional on–off mode that was under automatic set-point control. Operation of the solenoid valve was Turquoise Mountain

4,550

4,450

Elevation, ft MSL

Shallow Injection into Leached Cap 4,350

Deep Recovery from Below Enrichment Blanket

4,250

Leached Cap

4,150

Fracture Hosted Chalcocite > 0.3 %

Ithaca Pit

Disseminated Chalcopyrite

4,050 0

30

60

1 m = 3.1 ft; 1 ft = 0.31 m

90

120

150

N 45 E 180

210

Distance, m

FIGURE 7.8  Shallow-injection deep-recovery ISRR design

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240

In Situ Copper Sulfide Recovery Project

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controlled by solution conductivity measurements taken at the outlet of the mixing tank. Conductivity of the raffinate coming from the electrowinning plant was relatively constant at 25 mS/cm. Conductivity of approximately 60 mS/cm was achieved when 10 g/L acid was added to raffinate, and 50 g/L acid added to raffinate yielded conductivity of approximately 200 mS/cm. The time-proportional controller turned on the solenoid valve for some fraction of the cycle time depending on the acid demand. Process solution from the mixing tank was piped to an injection manifold, which distributed flow to the four injection wells (Figure 7.9). Impellertype flow transmitters were installed on each of the injection pipes. Process solution was injected through a pipe that extended 30 m (98 ft) down each of the four injection wells. Injection wells were not sealed or pressurized at the surface. Four 316 stainless steel submersible pumps were installed in recovery wells W2, E2, A3, and A4 at depths between 44 and 70 m (145 and 230 ft). The maximum rated capacity of the pumps was 75 L/min (20 gpm) with 60 m (197 ft) of head. The chemical processes involved in SIDR in situ leaching are the same as other chalcocite leach-mining operations that use ferric iron in solution to oxidize the chalcocite before copper is dissolved. The most important distinction between conventional leaching of chalcocite and SIDR in situ leaching involves the saturated condition of the ore during and subsequent to leaching. During heap, dump, and drill-and-blast leaching, the ore is only partially saturated. Under partially saturated conditions, bacteria such as Thiobacillus ferrooxidans catalyze the oxidation of ferrous iron to ferric iron. In the SIDR design, the ore is always saturated. Because bacteria are not active enough to

FIGURE 7.9  Injection line manifold leading to injection wells at the Mineral Park test site

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108 CHAPTER 7

catalyze the oxidation of ferrous iron, higher acid concentrations are used to dissolve iron oxide minerals (hematite and goethite) in the gossan layer and generate ferric iron into solution. The high transmissivity of the gossan layer causes the solution to initially spread laterally and form a shallow plume above the enrichment blanket. Pumping from wells completed below the chalcocite layer induces vertical leakage of solution through the enrichment layer along steeply inclined fractures (Figure 7.9). Chemical precipitation within the ore zone is minimized by using a relatively low pH (