Rare-Earth Metal Recovery for Green Technologies: Methods and Applications 3030381056, 9783030381059

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Rajesh Kumar Jyothi  Editor

Rare-Earth Metal Recovery for Green Technologies Methods and Applications

Rare-Earth Metal Recovery for Green Technologies

Rajesh Kumar Jyothi Editor

Rare-Earth Metal Recovery for Green Technologies Methods and Applications

Editor Rajesh Kumar Jyothi Korea Institute of Geoscience and Mineral Resources (KIGAM) Daejeon, Republic of Korea

ISBN 978-3-030-38105-9    ISBN 978-3-030-38106-6 (eBook) https://doi.org/10.1007/978-3-030-38106-6 © Springer Nature Switzerland AG 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

The so-called fourth industrial revolution started in the twenty-first century, and it is continuously developing the massive high-tech electric and electronic industry. However, the flourishing of this type of industry brings with it the drawback of the generation of a huge amount of manufacturing waste alongside the generation of electrical and electronic goods that will be eventually disposed in landfills. At the same time, this “human made” waste contains a variety of toxic metals blended with valuable and scarce metals that when improperly disposed can damage/is damaging the environment. Except for a few nations, all over the world, the natural resources are very limited, and growing population demands a continuous increase in hightech industries to fulfill their needs in present and future. This book, based on the “Resources to Materials” concept, explores primary resources such as minerals or ores and secondary resources such as solid and urban waste. Chapter 1 will discuss the rare earths demand and applicability as well as criticality for the present and future industry while showing the necessity of rare earths utilization for green and clean energy applications. Mineral processing of the rare earth ores is discussed in Chap. 2, followed by the thermodynamics of the rare earths production, discussed in Chap. 3. Chapter 4 explores in elaborate detail the fundamental principles and practices of hydrometallurgical techniques such as solvent extraction (liquid–liquid extraction) and supported liquid membrane process for extraction and separation of rare earths. Chapter 5 describes another hydrometallurgical technique, adsorption, as a promising recently developed strategy for removal of rare earths and precious metals using low-cost natural adsorbents. Recovery of rare earth elements from secondary resources such as solid wastes is discussed in Chap. 6. Chapter 7 presents the fabrication of nanostructured materials with rare earth elements for bioanalytical applications. Chapter 8 deals with the rare earth magnetic materials and progresses in the fabrication technologies and magnetic properties. This book focuses exclusively on the rare earth metal recovery for green technologies by hydrometallurgical techniques. Daejeon, South Korea  Rajesh Kumar Jyothi

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Contents

1 Introduction of Rare Earth Metal Recovery for Green and Clean Energy Technologies����������������������������������������������    1 Ana Belen Cueva Sola, Pankaj Kumar Parhi, Thriveni Thenepalli, and Rajesh Kumar Jyothi 2 Mineral Processing of Rare Earth Ores��������������������������������������������������    9 Surya Kanta Das, Shivakumar I. Angadi, Tonmoy Kundu, and Suddhasatwa Basu 3 Thermodynamic Aspects for Rare Earth Metal Production������������������   39 Sanjay Agarwal, Hong In Kim, Kyung-Ho Park, and Jin-Young Lee 4 Fundamental Principle and Practices of Solvent Extraction (SX) and Supported Liquid Membrane (SLM) Process for Extraction and Separation of Rare Earth Metal(s)������������   57 Pankaj Kumar Parhi, Saroj Sekhar Behera, Dindayal Mandal, Debadutta Das, and Ranjan Kumar Mohapatra 5 Recent Strategies on Adsorptive Removal of Precious Metals and Rare Earths Using Low-Cost Natural Adsorbents��������������   87 Janardhan Reddy Koduru, Lakshmi Prasanna Lingamdinne, Suresh Kumar Kailasa, Thriveni Thenepalli, Yoon-Young Chang, and Jae-Kyu Yang 6 Investigation on Extraction and Recovery of Rare Earth Elements from Secondary Solid Wastes����������������������������������������  111 Saroj Sekhar Behera, Ranjan Kumar Mohapatra, Debadutta Das, and Pankaj Kumar Parhi 7 Fabrication of Nanostructured Materials with Rare-Earth Elements for Bioanalytical Applications��������������������  137 Suresh Kumar Kailasa, Janardhan Reddy Koduru, and Thriveni Thenepalli

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8 Rare Earth-Based Magnetic Materials: Progresses in the Fabrication Technologies and Magnetic Properties ��������������������������������������������������������������������������  153 Rambabu Kuchi and Dongsoo Kim Index������������������������������������������������������������������������������������������������������������������  163

Contributors

Sanjay  Agarwal  Metal Extraction and Recycling Division, CSIR-National Metallurgical Laboratory, Jamshedpur, India Shivakumar  I.  Angadi  CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India Suddhasatwa  Basu  CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India Saroj  Sekhar  Behera  School of Chemical Technology and School of Biotechnology, Kalinga Institute of Industrial Technology (KIIT) Deemed to be University, Bhubaneswar, Odisha, India Yoon-Young  Chang  Department of Environmental Engineering, Kwangwoon University, Seoul, Republic of Korea Surya  Kanta  Das  CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India Debadutta  Das  Department of Chemistry, Sukanti Degree College, Subarnapur, Odisha, India Rajesh Kumar Jyothi  Convergence Research Center for Development of Mineral Resources, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, South Korea Suresh Kumar Kailasa  Department of Applied Chemistry, S. V. National Institute of Technology, Surat, Gujarat, India Hong  In Kim  Convergence Research Center for Development of Mineral Resources, Korea Institute of Geoscience and Mineral Resources, Daejeon, South Korea Dongsoo  Kim  Convergence Research Center for Development of Mineral Resources, Korea Institute of Geoscience and Mineral Resources, Daejeon, South Korea ix

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Contributors

Powder and Ceramics Division, Korea Institute of Materials Science, Changwon, Gyeongnam, South Korea Janardhan  Reddy  Koduru  Department of Environmental Kwangwoon University, Seoul, Republic of Korea

Engineering,

Rambabu  Kuchi  Convergence Research Center for Development of Mineral Resources, Korea Institute of Geoscience and Mineral Resources, Daejeon, South Korea Powder and Ceramics Division, Korea Institute of Materials Science, Changwon, Gyeongnam, South Korea Tonmoy  Kundu  CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India Jin-Young  Lee  Convergence Research Center for Development of Mineral Resources, Korea Institute of Geoscience and Mineral Resources, Daejeon, South Korea Lakshmi  Prasanna  Lingamdinne  Department of Environmental Engineering, Kwangwoon University, Seoul, Republic of Korea Dindayal  Mandal  School of Biotechnology, KIIT Deemed to be University, Bhubaneswar, Odisha, India Ranjan  Kumar  Mohapatra  Department of Chemistry, Government College of Engineering, Keonjhar, Odisha, India School of Chemical Technology and School of Biotechnology, Kalinga Institute of Industrial Technology (KIIT) Deemed to be University, Bhubaneswar, Odisha, India Pankaj Kumar Parhi  Convergence Research Center for Development of Mineral Resources, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, South Korea School of Chemical Technology and School of Biotechnology, Kalinga Institute of Industrial Technology (KIIT) Deemed to be University, Bhubaneswar, Odisha, India Kyung-Ho  Park  Convergence Research Center for Development of Mineral Resources, Korea Institute of Geoscience and Mineral Resources, Daejeon, South Korea Ana Belen Cueva Sola  Convergence Research Center for Development of Mineral Resources, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, South Korea Thriveni  Thenepalli  Mineral Resources Division, Center for Carbon Mineralization, Korea Institute of Geosciences and Mineral Resources (KIGAM), Daejeon, South Korea Jae-Kyu  Yang  Department of Environmental University, Seoul, Republic of Korea

Engineering,

Kwangwoon

About the Editor

Rajesh  Kumar  Jyothi  is working as a Principal Researcher at Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, Republic of Korea. He received his B.S. degree with First Rank and specialization in Geology, Physics, and Chemistry from S.B.V.R. Degree College, Badvel affiliated to Sri Venkateswara University (Tirupati, Andhra Pradesh, India) in 1996. Later, he shifted to University College, Sri Venkateswara University, Tirupati 517 502, Andhra Pradesh, India, for graduation and completed his M.S. with a specialization in Inorganic Chemistry in 1999 and Ph.D. in Chemistry (subject of research is hydrometallurgy) awarded by Sri Venkateswara University and R & D work carried out in Indian Institute of Chemical Technology (IICT), CSIR, Hyderabad, Telangana, India. Dr. R.K. Jyothi received the following awards and honors: • Received the Award as one of the team members for Top 100 Excellence R & D Projects from the Ministry of Science & ICT, Govt. of Korea, 2017 • Korean National News Channels MBC & YTN Science telecast about his career achievements in KIGAM, Korea, on June 14th and 15th, 2016 • KIGAM monthly bulletin published article on his achievements under Creative People, 07-08/2016 • Received the Award as one of the team members for Industrial Technology Transfer from KIGAM, 2014 • Received Award of the Best Scientific and Technological Innovation from WasteEng12 Organizing Committee at Porto, Portugal, 2012 • First foreignerto get a permanent position at Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, Korea, 2009 • Junior Scientist Award received from NESA (National Environmental Science Academy), Delhi, India, 2009 xi

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About the Editor

• Post-doctoral fellowship awarded by KIGAM, Daejeon, Korea, 2007–09 • Senior Research Fellowship Award received from CSIR, New Delhi, India, 2004–06 • Best Research Scholar Award received from Sri Venkateswara University, Tirupati, India, 2003 • Recognized as an Outstanding Reviewer by ELSEVIER Journals such as Hydrometallurgy, Minerals Engineering, Separation and Purification Technology, Journal of Cleaner Production, Waste Management, Journal of Industrial Engineering and Chemistry, and Journal of Photochemistry and Photobiology B: Biology He is highly motivated and is an active participant in several professional organizations as follows: Member: • Canadian Institute of Mining, Metallurgy and Petroleum (CIM) (AM 705541) • American Chemical Society (ACS), Washington DC, USA (AM 30204118) • Royal Society of Chemistry (RSC), Cambridge, UK (e-M 475520525 & MRSC 510732) • The American Association for the Advancement of Science, "Triple A-S" (AAAS), USA (AM 40888071) • The Minerals, Metals & Materials Society (TMS), Warrendale, USA (AM 489534) • Brazilian Metallurgical, Materials and Mining Association—ABM, Brazil (AM 24053) • Korean Society for Geoscience & Engineering, Seoul, Korea (AM 1214) Life member: • • • • • •

Korean Institute of Resources Recycling, Seoul, Korea (LM 476) Chemical Research Society of India (CRSI), Bangalore, India (LM 1591) Indian Institute of Metals (IIM), Kolkata, India (LM 49377) Indian Institute of Mineral Engineers (IIME), Jamshedpur, India (LM 885) Indian Society of Analytical Scientists (ISAS), Mumbai, India (LM 1801) National Environmental Science Academy (NESA), Delhi, India (LM 1323)

As an International Subject Expert for Funding Agencies: • Swiss National Science Foundation (http://www.snsf.ch) • FWO, Belgium Govt. Fellowships and Financing: Referee for Postdoctoral Fellowship Assignments (http://www.fwo.be/en/) • National Center of Science and Technology Evaluation, Ministry of Education and Science, Almaty, Republic of Kazakhstan (http://www. ncste.kz/en) Editorial board member: • Journal of the Korean Institute of Resources Recycling, KIRR, Korea, ISSN: 1225-8326, 2012–2016

About the Editor

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• Sustainability, ISSN 2071-1050; CODEN: SUSTDE, (https://www.mdpi. com/journal/sustainability/editors) • Guest Editor for Special Issue on “Sustainable Development and Recycling of Rare Earth Resources,” MDPI Publishers, Switzerland ­(https://www. mdpi.com/journal/sustainability/special_issues/Rare_Earth_Resources) • Book Editor for NOVA Publishers, USA, VDM Publishers Germany, and Springer Nature, USA • Assistant Editor in Chemistry, De Gruyter Open, Poland, 2012–13 He visited several countries including Australia, Singapore, China, Taiwan, Japan, the USA, Canada, Thailand, Germany, Czech Republic, Portugal, and Korea as an invited researcher to present his research findings in international conferences. He has published ~192 research articles, review/general articles in peer-reviewed research journals, and presented/contributed his research in National or International symposiums. Written two books on liquid–liquid extraction and edited one book Mineral Processing: Methods, Applications and Technology. Several patents were filled in various countries including South Korea (9), World (1), European (1), and the USA (1).

Chapter 1

Introduction of Rare Earth Metal Recovery for Green and Clean Energy Technologies Ana Belen Cueva Sola, Pankaj Kumar Parhi, Thriveni Thenepalli, and Rajesh Kumar Jyothi

The demand and supply of high-tech rare earth elements (REEs) accomplishing Nd, Tb, Eu, Y, Dy, Pr, La, and Ce, in global scenario, is one of the thrust area exclusively in extractive metallurgy domain. Rare earth metals play a critical role in the development of high-end smart materials for their extensive usages in electronic devices as well as several other sectors. Specifically, rare earths are widely used currently in various fields such as electric and electronics devices, chemical industry, computers, televisions, glass, alloys, various types of catalyst for petroleum-refining industry, phosphors for energy-efficient lighting, etc. (Working Group on Defining Critical Raw Materials 2010; Barteková and Kemp 2016; Rollat et al. 2016). During the last decades, the rare earth trade and commercialization have been led by China creating a monopoly in the supply chain and some risk and instability in terms of shortage and nonavailability of these precious resources (US Department of Energy 2010; Mancheri 2015; Tsamis and Coyne 2015; Weng et al. 2016). Rare earths are a group of 17 elements, which are abundant in the earth’s crust, but the mining sites contain very low concentration of them to be profitable for extraction (Jha et  al. 2016). The most abundant rare earth elements in the crust are cerium, yttrium,

A. B. C. Sola · R. K. Jyothi (*) Convergence Research Center for Development of Mineral Resources, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, South Korea e-mail: [email protected] P. K. Parhi Convergence Research Center for Development of Mineral Resources, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, South Korea School of Chemical Technology and School of Biotechnology, Kalinga Institute of Industrial Technology (KIIT) Deemed to be University, Bhubaneswar, Odisha, India T. Thenepalli Mineral Resources Division, Center for Carbon Mineralization, Korea Institute of Geosciences and Mineral Resources (KIGAM), Daejeon, South Korea © Springer Nature Switzerland AG 2020 R. K. Jyothi (ed.), Rare-Earth Metal Recovery for Green Technologies, https://doi.org/10.1007/978-3-030-38106-6_1

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lanthanum, and neodymium with abundances similar to commonly mined and used metals such as nickel and zinc; however, they are usually found in deposits with such low concentration that extraction becomes almost impossible (Dutta et  al. 2016; Jha et  al. 2016). The US Department of Energy (2010) has created a plot where they place the supply risk of rare earths against the importance for clean energy development; from this graph it is deducted that for clean energy development europium, yttrium, terbium neodymium, and dysprosium are critical materials, while the supply risk is very high due mostly to the monopoly of the market. In addition, at this time, there are no substitutes for the different applications of the REE materials; the development of technologies to substitute the usage of REEs will improve flexibility, while meeting the needs of clean energy (US Department of Energy 2010; Barteková and Kemp 2016; Dutta et al. 2016). Finally, recycling REE waste and scraps is a promising alternative to reduce the demand and the necessity of extraction from ores. In consequences of time, there have been enormous efforts being put forward by scientists, academicians, and industrialist to develop the technologies for recovering of these REEs from both primary and secondary sources routed through physical, chemical, and bioprocessing approaches. As per the reports (Behera and Parhi 2016; Behera et al. 2019; Parhi et al. 2015), there is a limited reserve of REEs in natural sources besides the major deposit (more than 90% of total earth deposit) in countries like China which led to explore the technologies, while exploiting these REEs from the secondary sources. The key REE-bearing sources include CRT lamp phosphor, waste magnet, wind turbine, PCBs, LEDs, EVM, spent batteries, and fly ash. In the twenty-first century, the biggest challenge is how the recycling of REEs from the above secondary sources can be optimized on an economic and technological basis with product life cycles with an objective toward sustainable metal management. In contrast to the conventional approaches (energy intensive), the development of green technology for recycling of REEs from secondary sources could be promising for resolving the issues that pertain to socioeconomics as well as environment. In the near future, it is estimated that 26% of the demand of REEs will be to cover the permanent magnet market followed by metal alloys 19%, polishing 16.5%, catalysts 15%, glass/phosphors 6%, and ceramics/others 5.5% (Working Group on Defining Critical Raw Materials 2010; Weng et al. 2016). While it is clear that the demand for automobiles, single-life electronics, catalysts, and green lighting will increase in the following decade, the demand for REEs will also continue to increase especially in the permanent magnet and battery industries. In 2017 it was estimated that China possesses approximately 40% of the world mine reserves of REs with 22 million metric tons and produces around 80% (105 thousand metric tons annually) of the metals sold around the world (US Department of Energy 2010; Han et al. 2015; Barteková and Kemp 2016; Dutta et  al. 2016; Klossek et  al. 2016). Other countries with high RE reserves are Brazil, Russia, Vietnam, and India, but their production of the metals is insignificant compared to China and its biggest competitor Australia with 15% of the production (20 thousand metric tons annually) of these precious metals (International Resource Panel of United Nations Environment Programme 2013; Ali 2014).

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Currently the research in energy and transportation is going toward more s­ ustainable, green, and clean sources of energy. That is why the prediction of the utilization of rare earth metals, especially neodymium and dysprosium, is vital for the development and installation of wind turbines onshore and offshore as well as for all electric vehicles that are being developed and are starting to commercialize. In the case of electric and hybrid vehicles, the average weight of permanent magnets is 1 kg, from which 31% consists of neodymium and 4.5–6% consists of dysprosium, while in the case of wind turbines, the weight of permanent magnets per MW generated is 200 kg from which 31% is neodymium and 2–4% is dysprosium (US Department of Energy 2010; Working Group on Defining Critical Raw Materials 2010; Tsamis and Coyne 2015). The continuous supply of rare earths for green energy represents one of the biggest challenges for the current research and development fields. Though the investigations reported till date does not address extensively the development of complete clean energy technologies, there are several scopes out of these studies that rely on further exploration. Unlike regular pyrometallurgical and hydrometallurgical processes, presently the adoption of green technologies has dragged more attention. Even though the current monopoly supply and the low substitutability of the rare earth metal-based technologies build a scenario where supply can be at risk at any point for clean technology development, there is a resource that has not yet been exploited in its full capacity, and it has become a major topic for research in the recent years: the recycling and recovery of REEs from secondary resources and waste (Binnemans and Jones 2014; Binnemans et al. 2015; Gutiérrez-Gutiérrez et al. 2015; Jha et al. 2016). To reduce the demand and the necessity of reliable supply of REEs from ores, recycling is one of the techniques that can support and alleviate this market necessity. Rare earth materials can be found in many end-of-life products such as phosphors where europium, terbium, yttrium, cerium, gadolinium, and lanthanum can be recycled and recovered (US Department of Energy 2010; Working Group on Defining Critical Raw Materials 2010; Tunsu et al. 2015). Used permanent magnets are one of the most important sources from where dysprosium, neodymium, terbium, and praseodymium can be recycled (Du and Graedel 2011; Dutta et al. 2016). Finally, from nickel metal hybrid batteries, it is possible to reprocess lanthanum, cerium, and neodymium. In addition, magnet swarfs and rejected magnets are an important source of REEs that can be used for recovery of the metals, while different industrial residues also possess REEs that could be reprocessed, not only alleviating the pressure in the production of REEs from ores but also reducing the landfill and disposal necessities for these wastes (Binnemans et al. 2015; Tsamis and Coyne 2015; Tunsu et al. 2015; Dutta et al. 2016; Jha et al. 2016). While recycling of REEs becomes a hot topic, the applications for green technologies also gain great attention and provide a big niche for rare earth metal utilization for effective and green energy production, while demand of these technologies is projected to grow significantly in the short and medium term (US Department of Energy 2010; Working Group on Defining Critical Raw Materials 2010; Rollat et al. 2016). Rare earths are utilized in various green and clean energy sectors (Fig. 1.1).

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Fig. 1.1  Rare earth applications in green and clean energy technologies

The main clean energy technologies that depend deeply on rare earth metals are wind turbines, electric vehicles, and fluorescent lightings. In the case of wind turbines and electric vehicles, one key constituent is permanent magnets, which produce a stable magnetic field without the necessity of an external power source (Du and Graedel 2011; Tsamis and Coyne 2015; Tunsu et al. 2015). Permanent magnet generators are used in wind turbines to transform wind energy into electricity, and in the case of electric and hybrid vehicles, permanent magnet motors help convert the energy stored in the batteries into mechanical power (US Department of Energy 2010; Tsamis and Coyne 2015; Rollat et al. 2016). Another important application for rare earths in the electric vehicle field is in the case of batteries, where lanthanum, cerium, praseodymium, and neodymium play a key role in the production of lightweight and energy-efficient batteries (US Department of Energy 2010; Working Group on Defining Critical Raw Materials 2010; Tsamis and Coyne 2015). Fluorescent lighting using rare earth phosphors is one of the proposed solutions for the improvement in lighting efficiency, which will lead to a reduction in the demand of energy overall. The main rare earth elements used for phosphors in fluorescent lighting are lanthanum, cerium, europium, terbium, and dysprosium; therefore the continuous and stable supply of these elements is a key concern globally (Han et al. 2015; Mancheri 2015; Weng et al. 2016). In the transformation toward a greener,

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energy-efficient society, rare earth metals play a key role in the human life. Rare earths can be found from hybrid and electric vehicles to house lighting and many electric and electronic devices that are used in daily life (Gutiérrez-Gutiérrez et al. 2015; Tunsu et  al. 2015; Dutta et  al. 2016). Color displays in television screens, speakers, smartphones, optical glasses used in cameras and telescopes, computer displays, and hard drives are some of the examples of commonly used items that rely on a stable and continuous supply of rare earth metals. The necessity of a greater and broader research toward substitutability and recycling of rare earths from secondary sources is and should be one of the priorities of the current scientific world. The recycling technology of RREs follows two major stages of operations: (i) upstream and (ii) downstream. In the upstream stage of hydro-processing approach, leaching is an integral part for extracting the RE metal values from solid to aqueous phase. Presently, the usages of green organic reagents are more preferred over the mineral acids or alkalis (Behera and Parhi 2016). Nevertheless, the incorporation of ultrasound assistance and microwave assistance is certainly becoming significant over conventional leaching process (Behera et al. 2019). In this way the exploration of technology with adoption of above reagents/processes ascertains a potential futuristic scope in REE recycling technologies. On the other hand in downstream operation, selective separation and purifications of REEs are carried out by either of these methods: solvent extraction, liquid membrane, ion exchange, and adsorption processes. The SX method is more often chosen owing to its larger prospective of commercialization scale and in which several functionalized organic solvents are used for separating out either of the REEs (Das et al. 2018). Over the past decades, it was realized to use green ionic liquids (ILs) other than the commercial organic reagents for extraction and separation of REEs from numerous secondary-based leached liquors (Parhi et  al. 2019). Subsequently pure form of rare earth oxides/ metal as such is obtained by precipitation process where major process is followed through green processing approach to end up on yielding a very high pure RE metal product(s). The major advantages of the green technology include low-energy consumption, less toxic emission, environment-friendly approach, and development of clean REE products. The recycling subject becomes significant in Korea as well as all over the world due to three major issues: (1) protection of the environment from waste (at the time of manufacturing used wastes), (2) landfill problem issues, and (3) limited natural resources. Five major factors are influencing the recycling subject: (1) metal stocks in society, (2) recycling rates of the metals, (3) environmental risks and challenges, (4) future demand scenarios, and (5) critical metal policy options (Metal Recycling Opportunities, UNEP Book). Why recycling needed for especially critical rare earths is population densities, it will causes to increase of demand as well as landfill problem issues arises due to wastes generation. Two major land countries such as Canada and USA and two highly populated countries such as China and India were compared with Korea (South) having less population densities (Fig. 1.2).

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Fig. 1.2  Population densities of countries such as Korea, Canada, the USA, China, and India (data adopted from https://data.worldbank.org/indicator/EN.POP.DNST?locations=CA-US-KR-JPTH-CN-IN-RU-BR-OE)

Fig. 1.3  Korea’s strategy on resources recycling policy and implementation of the R3 model

Recycling policy implementation in Korea started from year 1986 (Waste Management Act) and year 1992 Act on the Promotion of Saving and Recycling of Resources, followed by Waste Disposal Facilities Assistance Act of 1995. In new millennium (twenty-first century), two acts were implemented such as construction waste recycling (2003) and resource recycling of waste electrical and electronic equipment (WEEE) as well as automobile vehicles (http://eng.me.go.kr/eng/web/ index.do?menuId=50). Overall Korea’s strategy on resource recycling policy was represented in Fig. 1.3. Hydrometallurgy, the aqueous processing of metals, is the oldest technique in metallurgy. Prof. Michel L. Free (Free 2013) discussed in his book on hydrometallurgy

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about historical events in chemical processing of the metals in aqueous media from year 1556 separation of silver from gold leaf with chemical methodology. The established twenty-first-century innovative technology in hydrometallurgical processing was pressure oxidation of copper concentrate (Free 2013). Hydrometallurgy research and development subject deals the primary step in the leaching process of the solid samples (primary ores/minerals and secondary solid wastes). In the leaching process, various techniques can be applied such as oxidation and roasting, acid or alkaline leaching, pressure, bacterial leaching, etc. Leach liquor preparation by leaching process further was followed by solution purification by various chemical methodologies such as precipitation, liquid-liquid extraction, adsorption, ion exchange, membrane process, cementation, crystallization, etc. The final part is the metal recovery by electrochemical methods such as electrowinning and electrorefining. Hydrometallurgy subject was interlinked with various accepts such as whole flow sheet development, engineering, and environmental issues. Process modeling is the key procedure involved in thermodynamics, kinetics, and heat transfer subjects. Hydrometallurgy had a major disadvantage which is waste water generation; it needs dewatering and water balancing to minimize the water usage and recycle the water and effluent treatment (Mooniman et al. 2005).

References Ali, S.  H. (2014). Social and environmental impact of the rare earth industries. Resources, 3, 123–134. https://doi.org/10.3390/resources3010123. Barteková, E., & Kemp, R. (2016). National strategies for securing a stable supply of rare earths in different world regions. Resources Policy, 49, 153–164. https://doi.org/10.1016/j. resourpol.2016.05.003. Behera, S. S., Panda, S. K., Mandal, D., & Parhi, P. K. (2019). Ultrasound and microwave assisted leaching of neodymium from waste magnet using organic solvent. Hydrometallurgy, 185, 61–70. https://doi.org/10.1016/j.hydromet.2019.02.003. Behera, S.  S., & Parhi, P.  K. (2016). Leaching kinetics study of neodymium from the scrap magnet using acetic acid. Separation and Purification Technology, 160, 59–66. https://doi. org/10.1016/j.seppur.2016.01.014. Binnemans, K., & Jones, P.  T. (2014). Perspectives for the recovery of rare earths from end-­ of-­ life fluorescent lamps. Journal of Rare Earths, 32, 195–200. https://doi.org/10.1016/ S1002-0721(14)60051-X. Binnemans, K., Jones, P. T., Blanpain, B., et al. (2015). Towards zero-waste valorization of rare-­ earth-­containing industrial process residues: A critical review. Journal of Cleaner Production, 99, 17–38. https://doi.org/10.1016/j.jclepro.2015.02.089. Das, S., Behera, S.  S., Murmu, B.  M., Mohapatra, R.  K., Mandal, D., Samanray, R., & Parhi, P. K. (2018). Extraction of scandium (III) from acidic solutions using organo-phosphoric acid reagents: A comparative study. Separation and Purification Technology, 202, 248–258. https:// doi.org/10.1016/j.seppur.2018.03.023. Du, X., & Graedel, T. E. (2011). Global rare earth in-use stocks in NdFeB permanent magnets. Journal of Industrial Ecology, 15, 836–843. https://doi.org/10.1111/j.1530-9290.2011.00362.x. Dutta, T., Kim, K. H., Uchimiya, M., et al. (2016). Global demand for rare earth resources and strategies for green mining. Environmental Research, 150, 182–190. https://doi.org/10.1016/j. envres.2016.05.052.

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Free, M. L. (2013). Hydrometallurgy: Fundamentals and applications. Hoboken, NJ., ISBN 978-­ 1-­118-23077-0: John Wiley & Sons, Inc.. Gutiérrez-Gutiérrez, S. C., Coulon, F., Jiang, Y., & Wagland, S. (2015). Rare earth elements and critical metal content of extracted landfilled material and potential recovery opportunities. Waste Management, 42, 128–136. https://doi.org/10.1016/j.wasman.2015.04.024. Han, A., Ge, J., & Lei, Y. (2015). An adjustment in regulation policies and its effects on market supply: Game analysis for China’s rare earths. Resources Policy, 46, 30–42. https://doi. org/10.1016/j.resourpol.2015.07.007. International Resource Panel of United Nations Environment Programme. (2013). Metal recycling: Opportunities, limits, infrastructure. Nairobi: UNEP. Jha, M. K., Kumari, A., Panda, R., et al. (2016). Review on hydrometallurgical recovery of rare earth metals. Hydrometallurgy, 165, 2–26. https://doi.org/10.1016/j.hydromet.2016.01.035. Klossek, P., Kullik, J., & van den Boogaart, K.  G. (2016). A systemic approach to the problems of the rare earth market. Resources Policy, 50, 131–140. https://doi.org/10.1016/j. resourpol.2016.09.005. Mancheri, N. A. (2015). World trade in rare earths, Chinese export restrictions, and implications. Resources Policy, 46, 262–271. https://doi.org/10.1016/j.resourpol.2015.10.009. Mooniman, M. B., Sole, K. C., & Kinneberg, D. J. (2005). Challenging the traditional hydrometallurgy curriculum—An industry perspective. Hydrometallurgy, 79, 80–88. Parhi, P.  K., Behera, S.  S., Mohapatra, R.  K., Sahoo, T.  R., Das, D., & Misra, P.  K. (2019). Separation and recovery of Sc(III) from Mg–Sc alloy scrap solution through hollow fiber supported liquid membrane (HFLM) process supported by bi-functional ionic liquid as carrier. Separation Science and Technology, 54, 1478–1488. https://doi.org/10.1080/01496395.2018. 1520730. Parhi, P. K., Park, K. H., Nam, C. W., & Park, J. T. (2015). Liquid-liquid extraction and separation of total rare earth (RE) metals from polymetallic manganese nodule leaching solution. Journal of Rare Earths, 33, 207–213. https://doi.org/10.1016/S1002-0721(14)60404-X. Rollat, A., Guyonnet, D., Planchon, M., & Tuduri, J. (2016). Prospective analysis of the flows of certain rare earths in Europe at the 2020 horizon. Waste Management, 49, 427–436. https://doi. org/10.1016/j.wasman.2016.01.011. Tsamis, A. & Coyne, M. (2015). Recovery of rare earths from electronic wastes: An opportunity for High-Tech SMEs. European Parliament’s Committee on Industry, Research and Energy, IP/A/ITRE/2014-09, PE 518.777, Feb 2015. Tunsu, C., Petranikova, M., Gergorić, M., et al. (2015). Reclaiming rare earth elements from end-­ of-­life products: A review of the perspectives for urban mining using hydrometallurgical unit operations. Hydrometallurgy, 156, 239–258. https://doi.org/10.1016/j.hydromet.2015.06.007. US Department of Energy. (2010). Critical material strategy. European Parliament’s Committee on Industry, Research and Energy, IP/A/ITRE/2014-09, PE 518.777, Feb 2015. Weng, Z., Haque, N., Mudd, G. M., & Jowitt, S. M. (2016). Assessing the energy requirements and global warming potential of the production of rare earth elements. Journal of Cleaner Production, 139, 1282–1297. https://doi.org/10.1016/j.jclepro.2016.08.132. Working Group on Defining Critical Raw Materials. (2010). Critical raw materials for the EU.

Chapter 2

Mineral Processing of Rare Earth Ores Surya Kanta Das, Shivakumar I. Angadi, Tonmoy Kundu, and Suddhasatwa Basu

2.1  Introduction The term rare earth elements (REEs) denotes 15 elements of lanthanide series along with yttrium and scandium as these two also possess similar physical and chemical properties. Conventionally these elements are divided into two groups, such as light rare earth elements (LREE) consisting of lanthanum to europium (atomic number 57–63) and heavy rare earth elements (HREE) gadolinium to lutetium (atomic number 64–71). Yttrium exhibits similar properties as that of heavy rare earth elements; as a result, it is included in the HREE group. Scandium is not classified in the following groups as it bears different properties (Generalic 2019). Classification and rare earth elements, along with physical properties, are listed in Table 2.1. Rare earth elements are widely used in different industries such as chemical, metallurgical, and advanced materials, etc. (Murthy and Mukherjee 2001). Rare earth metals are added to different alloys to impart high-tech properties, strength, malleability, resistance to corrosion, etc. (Abaka-Wood et al. 2016). As many as 250 rare earth minerals were identified in the earth’s crust. However, rare earth metals can be extracted mainly from bastnaesite, monazite, and xenotime. These minerals are very different and found in various forms, such as halides, silicates, phosphates, carbonates, etc. The concentration of REEs in these minerals is not uniform and varies widely (Gupta and Krishnamurthy 2005) in different placer deposits, residual deposits formed by deep weathering of igneous rocks, pegmatites, iron deposits, copper and gold deposits, marine phosphate deposits, etc. Rare earth minerals are also found in lateritic deposits formed due to deep weathering in tropical environments. In addition to these, carbonatites, ion absorption clays, and syenites can also be a potential source for rare earth minerals. The rare earth mineral

S. K. Das · S. I. Angadi (*) · T. Kundu · S. Basu CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India e-mail: [email protected] © Springer Nature Switzerland AG 2020 R. K. Jyothi (ed.), Rare-Earth Metal Recovery for Green Technologies, https://doi.org/10.1007/978-3-030-38106-6_2

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Table 2.1  Rare earth elements and their properties Light rare earth elements

Heavy rare earth elements

Elements Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Yttrium Scandium

Symbol La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Sc

Atomic number 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 39 21

Atomic weight 138.91 140.12 140.91 144.24 145.00 150.36 151.96 157.25 158.93 162.50 164.93 167.26 168.93 173.05 174.97 88.90 44.95

in association with ion absorption clays is a type of deposit found in South China, which accounts for the maximum heavy  REEs production (Yang et  al. 2013). Classification of rare earth minerals based on different types of rock association is discussed in Table 2.2. The principal beneficiation route of different rare earth-bearing minerals includes gravity separation, magnetic separation, electrostatic separation, and froth flotation. Generally, most of the rare earth elements are associated with heavy minerals. Hence gravity concentration is one of the prominent routes to separate heavy rare earth-bearing minerals from associate silicates and other lighter gangue minerals. Most exclusive gravity concentrators used in rare earth mineral beneficiation are shaking tables, spirals, and cone concentrators. The shaking table is used to separate monazite from lighter siliceous gangue. In the case of fine and ultrafine rare earth minerals, enhanced gravity separators such as Knelson concentrator, Falcon concentrator, and Mozley multi-gravity separators are used. These concentrators use centrifugal force to separate heavier from the lighter particles. Magnetic separation is widely used in rare earth mineral beneficiation to separate paramagnetic rare earth minerals from heavy ferromagnetic impurities. Paramagnetic minerals such as monazite and xenotime are separated by using different types of magnetic separators (Gupta and Krishnamurthy 1992). Monazite is associated with other heavy minerals, namely, zircon and rutile, which are nonmagnetic. Separation of paramagnetic monazite from diamagnetic zircon and rutile could be achieved by following magnetic separation. The magnetic fraction contains monazite, as well as impurities such as ilmenite. Ilmenite is electrically conductive, whereas monazite is nonconductive. Therefore, the electrostatic separation process is adopted to separate

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Table 2.2  Different types of rare earth deposits (Long et al. 2010) Rock types Peralkaline igneous rock

Carbonatites

Iron oxide copper-gold Pegmatites

Porphyry molybdenum Metamorphic Stratiform phosphate residual

Paleoplacer

Placer

Association Magmatic (alkali-ultrabasic) Pegmatite dikes (alkali-ultrabasic) Pegmatite dikes (peralkaline) Hydrothermal veins and stockworks Volcanic Metasomatic albitite Magmatic (alkali-ultrabasic) Dikes and dilational veins Hydrothermal veins and stockworks Skarn Carbonate rock replacement Metasomatic fenite Magnetite-apatite replacement Hematite-magnetite breccia Abyssal (heavy rare earth elements) Abyssal (light rare earth elements) Muscovite (rare earth elements) Rare earth elements-allanite-monazite Rare earth elements-euxenite Rare earth elements-gadolinite Miarolitic-rare earth elements-topaz-beryl Miarolitic-rare earth elements-gadolinite-fergusonite Climax-type Migmatized gneiss Uranium-rare earth elements skarn Platform phosphorite Carbonatite-associated Granite-associated laterite Baddeleyite bauxite Karst bauxite Uraniferous pyritic quartz pebble conglomerate Auriferous pyritic quartz pebble conglomerate Shoreline Ti-heavy mineral placer Tin stream placer

Deposit Lovozero, Russia Khibina, Massif, Russia Motzfeldt, Greenland Lemhi Pass, Idaho Brockman, Western Australia Miask, Russia Mountain Pass, California Kangankunde Hill, Malawi Gallinas Mtns, New Mexico Saima, China Bayan Obo, China Magnet Cove, Arkansas Eagle Mountain, California Olympic Dam, South Australia Aldan, Russia Five Mile, Ontario Spruce Pine, North Carolina South Platte, Colorado Topsham, Maine Ytterby, Sweden Mount Antero, Colorado Wasau complex, Wisconsin Climax, Colorado Music Valley, California Mary Kathleen, Queensland Montenegro Mount Weld, Western Australia South China Poços de Caldas, Brazil Montenegro Elliot Lake, Ontario Witwatersrand, South Africa Cooljarloo, Western Australia Malaysia

ilmenite from monazite (Gupta and Krishnamurthy 2005). Pre-concentrated ­monazite is further subjected to froth flotation process for further enrichment by ­removing silicates, calcite, fluorite, apatite, and other impurities present. Most of the rare earth minerals are liberated from the gangue minerals at fine and ultrafine

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particle sizes. Froth flotation is essential since it is efficient for the separation of fine particle size. The physical beneficiation route is adopted to pre-concentrate the rare earth-­ bearing minerals, and subsequently, hydrometallurgical routes are used to extract and separate different rare earth metals. Generally, rare earth minerals consist of two or more rare earth elements, which makes it complex to extract and purify. Extensive research is going on for the separation of complex rare earth oxides.

2.2  Occurrence and Production 2.2.1  Occurrence Rare earth minerals occur as fluorites, oxides, tantalates, carbonates, phosphates, sulfates, borates, silicates, etc. (Qi 2018). List of different rare earth minerals are presented in Table 2.3. The average concentration of REEs in the earth’s crust is found to be 150–220 ppm, which is much higher than the concentration of other base metals such as copper and zinc as they occur in 55 ppm and 70 ppm, respectively. However, REEs are hardly found anywhere as concentrated, which makes it challenging to extract economically. Most of the rare earth deposits are present in non-exploitable areas and found mainly in dispersed forms. There are hardly dozens of minerals from which rare earth metals are extracted. Bastnaesite, monazite, and xenotime are the major minerals from which rare earth metals are extracted economically. Bastnaesite is a carbonate-bearing mineral, whereas xenotime and monazite are phosphate-bearing minerals. Bayan Obo, China, and Mountain Pass, California, are the top two rare earth deposits in the world. Bastnaesite accounts for around 80% of total REE production, whereas xenotime and monazite contribute 4%, and the rest is produced by other secondary rare earth resources (Kanazawa and Kamitani 2006). Monazite is a complex ore of several REEs such as cerium, lanthanum, neodymium, and thorium. Xenotime is an orthophosphate rare earth mineral, and it is majorly found in hydrothermal deposits. For example, the Brown Range HREE district in Western Australia has the hydrothermal deposit of xenotime, which contains a good amount of yttrium, including some amount of uranium and several other REEs (Richter et al. 2018). In addition to these three minerals, many other minerals such as allanite, loparite, britholite, cerite, cerianite, gadolinite, hydroxylbastnaesite, etc. are potential sources of REEs. Presently, carbonatite ores are the primary source of REEs. Apart from this, some of the REE-bearing minerals are also found in different placer deposits, residual deposits caused by deep weathering of igneous rocks, pegmatite rock beds, iron oxide, copper-gold deposits, and in different marine phosphates. Alkaline igneous rocks are formed during the cooling of magmas, which contain rocks of low melting point. These alkaline igneous rocks are rich in some strategic minerals bearing elements

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Table 2.3  Classification of different rare earth minerals (Qi 2018) Types Fluoride minerals

Oxide minerals

Complicated oxide

Tantalate

Carbonate and fluorocarbonate

Phosphate and arsenate mineral

Sulfate mineral Borate mineral Double acid salt mineral Multi-acid salt mineral

Minerals Fluocerite Yttrian Fluorite Tvetite Gagarinite Cerianite Niobian Anatase Loparite RE-bearing perovskite Yttrotungstite Pyrochlore Formanite Fergusonite Aeschynite Polycrase Bastnaesite Donnayite Lanthanite Beiyunoboite Synchysite Monazite Xenotime Chernovite Agardite Chukhrovite Braistschite Kainosite Britholite Cappelenite Saryarkite

Kemmlitzite Gadolinite Hingganite Silicate minerals of Allanite titanium, zirconium, Chevkinite and aluminum

Silicate mineral

Perrierite Tranquillityite

Chemical formula (Ce,La)F3 (Ca,Y)TiSiO5 (Ca,REE)F2 NaCaY(F,Cl)6 (Ce4+,Th)O2 (Ti,Nb)O2 (Ti,REE)O2 (Ce,Na,Ca)(Ti,Nb)O3 (Ca, REE)TiO3

Specific gravity 5.93–6.14 3.17–3.56 3.94 4.11–4.29

3.79–3.97 4.60–4.89

YW2O6(OH)3 (Ca,Na,REE)2Nb2O6(OH,F) YTaO4 (Ce,La,Y)NbO4 (Ce,Ca,Fe,Th)(Ti,Nb)2(O,OH)6 (Y,Ca,Ce,U,Th)(Ti,Nb,Ta)2O6 (Ce,La)(CO3)F Sr3NaCeY(CO3)6.3H2O (Ce,La)2(CO3)3.8H2O

5.96 4.2–6.4 (~ 5.3) 7.03 4.3–5.8 (~5.05) 5.19 4.97–5.04 (~5) 4.95–5.0 (~4.97) 3.3 2.81

Ca(Ce,La)(CO3)2F (Ce,La,Nd,Th)PO4 (Y,Th,U,Dy,Yb,Er,Gd)PO4 Y(AsO4) (La,Ca)Cu6(AsO4)3(OH)6•3(H2O) Ca3(Y,Ce)(AlF6)2(SO4)F · 10H2O 6(Ca, Nar)O. REzOs .l2BzOz.6Hr Ca2(Y,Ce) SiO4O12(CO3)•(H2O) (Y,Ca)5(SiO4)3OH Ba(Y,Ce)6Si3B6O24F2 Ca(Y,Th)Al5(SiO4)2(PO4,SO4)2(OH)7 •6(H2O) SrAl3(AsO4)(SO4)(OH)6 Ce,La,Nd,Y)2FeBe2Si2O10 (Y,REE,Ca)2(Fe2+)Be2[SiO4]2(OH)2 (Ce,Ca,Y,La)2(Al,Fe+3)3(SiO4)3(OH) (Ce,La,Ca,Th)4(Fe++,Mg)2 (Ti,Fe+++)3Si4O2 (Ce,La,Ca)4(Fe++,Mg)2(Ti,Fe+++) 3Si4O22 (Fe2+)8Ti3Zr2 Si3O24

3.89–4.10 5.15 4.75 4.5 3.72 2.27–2.39 (~2.33) 3.4–3.6 4.2–4.7 (~4.45) 4.4 3.11 3.60 4.20 3.5–4.2 4.5 4.37 4.7

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such as zirconium, niobium, strontium, barium, lithium, and some rare earth elements (Long et al. 2010). Sediments deposited near deltas, shorelines, streams of rivers, and alluvial fans are caused due to erosion and concentration of heavy minerals along with rare earth-­bearing monazite and xenotime. Some common metamorphic, igneous, or older sedimentary rocks contain a good amount of monazite. Later, these rocks are displaced by erosion caused by weathering and deposited in a different place. These deposits are called placers and divided mainly into two types. One is ilmenite placers, and another one is cassiterite placers with a good amount of monazite (Long et al. 2010). The Olympic Dam in Australia is an iron oxide copper-gold type of deposit that contains a significant amount of rare earth values along with uranium. In South China and Kazakhstan, there is a special kind of clay that contains rare earth minerals. Though there is very little information about the formation of these unusual deposits, still extraction is being practiced to recover these values.

2.2.2  Production The rare earth deposits are divided into primary and secondary deposits. Rare earth minerals that are associated with the hard rock deposits are called primary deposits, while REE in association with the placer sands is considered as a secondary deposit. 2.2.2.1  Carbonatites Many rare earth minerals are found in carbonatite rocks, and it has been the primary resource of RE minerals for a long time. Carbonatite rock is a typical igneous rock, and primarily it carries lighter rare earth elements such as cerium, lanthanum, praseodymium, and neodymium. Bayan Obo deposit in China and Mountain Pass deposit in California, USA, are well-known carbonatite reserves for the extraction of REMs. Bastnaesite, allanite, and apatite are the most common minerals containing REE values. Monazite is a phosphate rock having calcium and thorium along with REEs in the crystal matrix. 2.2.2.2  Pegmatites Pegmatite rocks are the primary source of heavy rare earth minerals. These rocks are formed due to the cooling of magma. Rapkivi granite area of Southern Finland has a pegmatite deposit which contains REE-rich minerals such as xenotime, monazite, bastnaesite, allanite, thorite, etc. In addition to the heavy rare earth minerals, pegmatite deposits also host light rare earth minerals.

2  Mineral Processing of Rare Earth Ores Table 2.4  World REE reserves and production (USGS 2017)

Countries United States Australia Brazil Canada China Greenland India Malaysia Malawi Russia South Africa Thailand Vietnam Total

15 Production (in MT) – 14,000 1100 – 105,000 – 1700 300 – 3000 – 800 300 126,000

Reserve (in MT) 1,400,000 3,400,000 22,000,000 830,000 44,000,000 1,500,000 6,900,000 30,000 136,000 18,000,000 860,000 NA 22,000,000 120,000,000

2.2.2.3  Placer Deposits Placer deposits are formed due to the concentration of heavy minerals from the parent rock during the sedimentation process. A number of rare earth minerals are found in different placer deposits. China is the major producer of REEs and has a dominant global market for REEs. Along with China, other countries such as Canada, India, Vietnam, Malawi, Russia, Malaysia, South Africa, and Thailand also have REE reserves and contribute to global reserves and production. Table  2.4 shows the reserves and production of REEs in different countries during 2016 (USGS 2017).

2.3  Beneficiation of Rare Earth Minerals 2.3.1  Gravity Separation Gravity separation is one of the oldest concentration methods used in mineral beneficiation practices. It utilizes differences in the specific gravity of valuable and gangue minerals to achieve the separation. Taggart has proposed a “concentration criterion,” which is widely used to evaluate the feasibility of gravity concentration:



Concentration criterion =

Dh − Df Dl − Df

(2.1)

where Dh stands for density of the heavy mineral, Dl stands for density of the lighter mineral, and Df stands for density of the fluid.

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The concentration criterion is greater than or equal to 2.5 and indicates an easy and effective separation. The separation efficiency gradually decreases with the decrease in the ratio, and no separation is feasible below 1.25. In the beneficiation of rare earth minerals, gravity concentration plays a vital role. Many processing flowsheets incorporate gravity concentrators for the pre-­ concentration of heavy rare earth minerals. Rare earth minerals such as monazite, bastnaesite, and xenotime have a specific gravity of 4.4–5.6 that are associated with the gangue minerals such as quartz, feldspar, calcite, gypsum, mica, etc. In many applications, gravity concentrators are used in the pre-concentration stage, while few applications have them in the final stage of concentration (Abaka-Wood et al. 2018; Gupta and Krishnamurthy 1992). Gravity concentrators such as spirals and shaking tables are extensively used in the pre-concentration stage to discard various silicate impurities. Further, the gravity concentrate would be processed in magnetic separation, electrostatic separation, and froth flotation to enrich the rare earth minerals (Ferron et  al. 1991). However, in Bayan Obo rare earth deposit, processing flowsheet does not incorporate gravity concentrators to pre-concentrate the feed material, as the rare earth mineral (bastnaesite) is associated with the heavy iron-bearing minerals. The lean-grade ores contain rare earth minerals that are present in fine and ultrafine particle sizes and are intricately associated with the gangue minerals. Processing of such type of ores involves grinding to a fine size to impart the liberation of valuable and gangue minerals. The ultrafine particles do not respond to the conventional gravity concentrators. Therefore, enhanced gravity separators such as multi-gravity separator, Knelson concentrator, Falcon concentrator, etc. are used for the recovery of fine and ultrafine REM particles (Falconer 2003). Applications of gravity concentrations in the beneficiation rare earth minerals are discussed hereunder. Nechalacho deposit in Canada (Avalon Rare Metals Inc.) has 183.4 million tons of ore at a grade of 1.27% of total rare earth oxides. The beneficiation flowsheet includes gravity concentration and magnetic separation to enrich total REE values from 2.40 to 7.69%. A combination of Knelson and Falcon concentrators and magnetic separation has resulted in enrichment of REE values to 7.69% (Jordens et al. 2016), which is depicted in Fig. 2.1. Figure 2.2 shows the beneficiation flowsheet practiced in the Sichuan Mianyang deposit, China. The deposit hoists heavy rare earth elements like europium and yttrium in the form of alkaline pegmatite rocks. The feed grade of the carbonatite rock consists of an average grade of 3.7% rare earth minerals. The beneficiation flowsheet extensively consists of gravity concentration and flotation circuits. The ore is ground to 62% passing 200 mesh size and classified into 4 size fractions, and subsequently, individual fractions are subjected to shaking tables for bastnaesite concentration, which results in producing three different grades of 30%, 50%, and 60% with 75% overall recovery values (Li and Yang 2016). Gravity concentration is widely used in the beneficiation of beach sand minerals. The heavy mineral concentration in the beach sand varies from place to place due to the differences in the mineralogical composition of the deposit. Gravity spirals are used in the primary stage to discard a huge amount of silicate impurities. On the

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FEED (2.40 %) Dry Mill

80% -40 µm

(TREO, %)

KT Knelson

FT

KC

Falcon

FC

Fe Mag

RE Mag WHIMS Circuit

FC Non Mag (4.63 %)

KC Oversize

300 µm

Dry Mag Circuit

FC Mag Fe Mag

KC Mag

RE Mag

KC RE Mag (6.81 %)

FC RE Mag (4.36 %)

KC Non-Mag (7.69%)

Flotation

RE Conc.

Final Tails

Fig. 2.1  Nechalacho rare earth beneficiation flowsheet (Jordens et al. 2016)

contrary, a bulk concentrate containing heavy minerals such as ilmenite, rutile, zircon, garnet, monazite, etc. increases to 20–30% from the feed containing 1–4%. Further, secondary- and tertiary-stage separators such as spirals and shaking tables, respectively, were used to improve the heavy mineral grades to 80–90%. The heavy minerals were subsequently separated using electrostatic separation, magnetic separation, and froth flotation techniques (Gupta and Krishnamurthy 1992). This practice is seen in countries such as the USA, India, and Australia. The beneficiation flowsheet of the Idaho placer deposit consists of only gravity concentrators to produce the final monazite concentrate. The flowsheet was developed by keeping an eye on the recovery of gold and monazite. Gold is recovered by the amalgamation method in the middle of the process. Jigs were used to pre-­ concentrate the material, whereas the shaking table was used to produce the final concentrate. Figure  2.3 shows the flowsheet used at the Idaho placer deposit (Ferron et al. 1991).

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ROM Size Reduction

Hydrocyclone Classification

Shaking Table C

T

Shaking Table C

T

Shaking Table C

T

Shaking Table C

Tailing

Rougher Flotation

Tailing

Conc.

Cleaner Flotation

Concentrate (REO = 50 – 60%)

Scavenger Flotation

Tailing

Tailing

Conc.

Fig. 2.2  Beneficiation flowsheet of Mianyang deposit, China (Li and Yang 2016)

The black sands along the Mediterranean coast in Egypt account for a vast reserve of different beach sand minerals along with monazite. Moustafa and Abdelfattah (2010) have proposed a laboratory-scale beneficiation flowsheet to concentrate from the feed assaying 0.2% monazite, which is depicted in Fig.  2.4. The concentrate assays 97% monazite, which has been produced using a combination of gravity, electrostatic, and magnetic separation units. The feed material also contains ilmenite, zircon, rutile, and some silicate-bearing minerals. The processing flowsheet incorporates three stages of gravity concentrators followed by magnetic and electrostatic separators to enrich the concentrate to 85% monazite. The primary gravity concentrator enriches the monazite content and removes major silicate gangue particles. Hence recovered heavy fraction was subjected to the magnetic separator and secondand third-stage gravity tables to enrich monazite content. The proposed flowsheet contains several stages of gravity, magnetic, and electrostatic separators.

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Feed (Dredge Product) Oversize

Trommel

Tails

Undersize

Jigs (Rougher + Cleaner)

Lights

Heavies

Amalgamation

Au - Ag

Jig Re-cleaner I

Heavies

Lights

Jig Re-cleaner II Heavies

Lights

Tabling

Heavies

Lights (Garnet)

Monazite Concentrate Fig. 2.3  Flowsheet involved in Idaho placer deposit (Ferron et al. 1991)

2.3.2  Magnetic Separation Naturally occurring minerals possess different magnetic properties. Based on the behavior of minerals in the magnetic field, they are classified into paramagnetic, ferromagnetic, and diamagnetic. The magnetic separation is based on magnetism acquired by mineral particles when they are placed in the magnetic field. The behavior of particles in the magnetic field is due to the deflection of magnetic dipoles. Dipoles in ferromagnetic and paramagnetic minerals are inline to the magnetic lines of forces, whereas they line up in opposite direction in diamagnetic minerals. In the

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Raw Sands (0.25% Monazite) Screening (1 mm)

Oversize (+1 mm)

Undersize (-1 mm)

Slimes

Desliming Gravity Concentration Concentrate

Tailing

Drying Low-Intensity Cross Belt Magnetic Separator

Non-Magnetic

Magnetic Conc.

Wet Gravity Concentration

Tails (Rejected)

Drying High Tension Separator Non-Conductor (Rutile)

Conductor (Zircon)

Upgrading Zircon Concentrate Circuit

Magnetics (Monazite)

Middling (Monazite)

Wet Gravity Circuit Monazite Concentrate Middling

Tailing

High-Intensity Magnetic Separation Non-Magnetic

Magnetic

Electrostatic Separation

Electrostatic Separation Conducting

Non-Magnetic (Zircon Concentrate)

Non-Conducting

High-Intensity Magnetic Separation Non-Magnetic

Conducting

Non-Conducting (High grade monazite concentrate, 97%)

Magnetic (High grade monazite concentrate, 97%)

Fig. 2.4  Process flowsheet for the beneficiation of Egyptian black sands (Moustafa and Abdelfattah 2010)

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beneficiation circuits of rare earth minerals, magnetic separators are largely used to remove magnetic and other gangue minerals. In rare earth mineral beneficiation, wet high-intensity magnetic separators (WHIMS), low-intensity magnetic separators (LIMS), drum magnetic separators, and dry magnetic separators (for coarser particles) are used. All the rare earth elements have a series of electrons associated with a shielded 4f subshell. These electrons contain a magnetic charge, which makes the rare earth-bearing minerals paramagnetic in nature (Spedding 1975). Magnetic separators are used for different applications in the beneficiation of rare earth minerals. The first application is to remove ferromagnetic impurities, and the second application is to the concentration of paramagnetic rare earth minerals such as xenotime and monazite (Gupta and Krishnamurthy 1992). In the beneficiation of placer beach sand, magnetic separators are used to separate diamagnetic gangue minerals such as zircon and rutile from the paramagnetic REE-bearing monazite. Xenotime being a paramagnetic mineral can also be separated from its associated gangue by magnetic separation. However, froth flotation is preferred instead of magnetic separation due to its higher selectivity at finer particle size. The Bayan Obo deposit consists of paramagnetic bastnaesite in association with ferromagnetic iron-bearing gangue minerals. In view of this, magnetic separation is used to remove iron-bearing gangue minerals (Zhang and Edwards 2012). Sichuan Mianyang rare earth deposit in China is found to be an alkaline carbonate-­ type rare earth deposit, which has an average REO grade of 3.7% (Chi et al. 2001). The processing flowsheet (Fig. 2.5) incorporates gravity concentration and magnetic separation circuit to recover about 55% of bastnaesite. While, Dalucao rare earth deposit, China, incorporates several stages of flotation and magnetic separation stages to separate the diamagnetic gangue. The final concentrate enriches in bastnaesite content to 65%, with an overall recovery of REO values to 55% (Xiong et al. 2018). Wet high-intensity magnetic separators are used to separate diamagnetic feldspar, plagioclase, etc. from REE-bearing minerals such as bastnaesite, allanite, and fergusonite. The bastnaesite has relatively lower magnetic susceptibility than fergusite and allanite. Therefore, bastnaesite can be separated from fergusite and allanite by a lowintensity magnetic separator where bastnaesite will report to a nonmagnetic fraction (Jordens et al. 2014). Raw Ore

Grinding

Flotation

Tailing

Magnetic Separation

REO Concentrate Fig. 2.5  Processing flowsheet at Dalucao rare earth deposit, China (Xiong et al. 2018)

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Changsha Research Institute of Mining and Metallurgy has proposed a beneficiation flowsheet for the Bayan Obo rare earth mineral deposit, China, during the 1990s, which is shown in Fig. 2.6. The process flowsheet includes several stages of magnetic separators and flotation cells. The feed material was ground to 90% ROM Comminution Classification

Low-Intensity Magnetic Separator (Rougher Stage) Mag

Non - Mag

Low-Intensity Magnetic Separator (Cleaner Stage) Non-Mag

High-Intensity Magnetic Separator (1.4 T)

Mag

Non-Mag

Mag High-Intensity Magnetic Separator (0.6 T) Non-Mag Mag REE Rougher Flotation

Fe Reverse Flotation

Tailing

Conc. Fe Conc.

Tailing

REE Scavenger Flotation Tailing

REE Cleaner I Flotation Conc. Tailings REE Cleaner II Flotation

Final REE Conc. (REO > 60%)

Intermediate REE Conc. (REO > 30%)

Fig. 2.6  Beneficiation flowsheet for Bayan Obo deposit ore developed by Changsha Research Institute of Mining and Metallurgy in 1990 (Li and Yang 2014)

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passing 74  μm and subjected to a low-intensity magnetic separator (~0.2  T) to remove the magnetic gangue minerals. The nonmagnetic fraction was further subjected to a high-intensity magnetic separator (1.4 T) to recover hematite and paramagnetic rare earth minerals. The magnetic fraction was again subjected to a high-intensity magnetic separator (0.6 T) where rare earth minerals are separated from magnetic hematite and other iron-bearing minerals. Magnetic separation enriched the product grade to around 9.8–12.0% from the feed containing 6% REO. The nonmagnetic product was subjected to flotation for further enrichment (Li and Yang 2014).

2.3.3  Electrostatic Separation Electrostatic separation is one of the physical beneficiation methods of separation of particles based on their electrical conductivity. The electrical conductivity of minerals is used to classify the minerals into conductors and insulators. However, there are a few limitations of the electrostatic separators that restrict their applications. The separation is largely influenced by the moisture content of the feed material. Also, the separation efficiency significantly reduces if the particle size is finer than 75 μm. The most common electrostatic separator widely used in the mineral processing industries is high-tension roll separator. In rare earth processing plants, electrostatic separators are used for the recovery of monazite from beach sand minerals. The gravity concentrators are used in the primary stage to remove siliceous gangue minerals to generate the bulk concentrate of heavy minerals, which includes monazite along with ilmenite, sillimanite, zircon, garnet, and rutile. Ilmenite, rutile, garnet, and sillimanite all are conducting in nature, whereas monazite and zircon are nonconducting. A typical example of rare earth sand processing flowsheet is shown in Fig. 2.7.

2.3.4  Froth Flotation Froth flotation is an advanced beneficiation technique that focuses on the physiochemical properties of minerals for the separation of fine hydrophobic and hydrophilic particles. The hydrophobic mineral particles attach to the air bubbles and report to the froth product. However, hydrophilic particles remained in the pulp and separated as a non-froth product. Flotation is a complex phenomenon involving the interaction of three phases, such as solid, liquid, and air. The individual-phase interactions such as solid-liquid, liquid-air, and solid-air were also playing a vital role in the flotation. The flotation of rare earth minerals is broadly classified into two categories, such as carbonate mineral (bastnaesite) flotation and phosphate mineral (monazite and xenotime) flotation. The majority of monazite and xenotime minerals are found in placer deposits, and around 60% of them are liberated at finer than 10  μm particle size (Chan 1992). Flotation is the most effective method for the

24

S. K. Das et al. Beach Sands (1–4 % HM) Gravity Concentration

Rubbles, shells, quartz, mica

Pre-concentrate (90% HM)

Magnetic Separator Non-Mag

Mag (Ilmenite)

High Tension Separator Non Conductor

Conductor (Ilmenite)

Mag (Magnetite)

Non-Mag (Zircon, sillimanite)

Air/Wet Table

High Tension Separator

Quartz Mica

LIMS

Non-Mag High Tension Separator

Non-Conductor

Magnetic Separator Mag

Shaking Table

Non-Conductor

High Intensity Magnetic Separator Mag (Monazite)

Conductor (Ilmenite, Rutile Garnet, Silimanite)

Non-Mag (Zircon)

Conductor (Ilmenite, Leucoxene)

High Intensity Magnetic Separator Mag (Monazite)

Non-Mag (Zircon)

Tailing

Conc.

Magnetic Separation Mag (Monazite)

Non-Mag (Zircon)

Fig. 2.7  A typical flowsheet of beach sand processing (Gupta and Krishnamurthy 2005)

recovery of fine particles. The world’s largest rare earth mineral-producing plants operate at Bayan Obo in China, and Mountain Pass in the USA employs the flotation circuit for the recovery of REMs from the carbonatite ores. 2.3.4.1  Surface Chemistry of Rare Earth Minerals Flotation is a three-phase phenomenon consisting of interactions between the solid, liquid, and air phases. The surface charge of mineral particles plays a vital role in the flotation. The solid surface is in contact with the liquid phase, wherein interactions between surface charge and water molecule/ionic species take place. The solid-liquid ionic interactions attain an equilibrium stage through a semipermeable membrane that allows only charged species common to both solid and liquid to pass through. The electric charge at the mineral surface could be regulated with the help of surfactants, namely, collectors, modifiers, activators, etc. The water chemistry also influences the variation in surface charge and its magnitude. Naturally occurring mineral exhibits a specific surface charge. This property is explored for the adsorption of different types of surfactants. Several variables influence the adsorption of surfactant/collector molecules on to the mineral surface. Type of collector, functional group, molecular weight, the chemical environment of the flotation cell, etc. play a

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vital role in the effective separation of the mineral particles. Literature shows that the isoelectric point of monazite and bastnaesite varies widely; for monazite, it is varying from 1.1 to 9.0 (Zhang and Honaker 2017), and for bastnaesite, 4.9 (Pavez et  al. 1996), 9.5 (Pradip and Fuerstenau 2013), 6.3 (Jordens et  al.  2014), and 7.8–8.0 (Ren et al. 1997, 2000) have been observed. 2.3.4.2  Bastnaesite Flotation Flotation is widely used for the beneficiation of bastnaesite all over the world. The surfactants such as fatty acids, hydroxamates, dicarboxylic acids, and organic phosphoric acids are used in many flotation circuits (Pradip 1981; Pradip and Fuerstenau 1983; Ferron et al. 1991; Jun et al. 2003; Ren et al. 2003; Cui and Anderson 2017). Flotation circuits used in different processing plants are discussed in the subsequent sections. Collectors The most common carboxylic acid collectors (fatty acids) are oleic acid, sodium oleate, synthetic fatty acids, tall oils, and some oxidized petroleum derivatives. Though these collectors are less selective toward the flotation, these are familiar to the industrial practices due to their better availability and cheaper price (Bulatovic 2007). Elevated temperature and excessive addition of depressants help in improving the flotation selectivity. Calcite, barite, and celestite are considered as the major gangue minerals associated with bastnaesite. Collectors such as fatty acids and hydroxamates conditioning in slightly elevated temperatures are one of the key features in improving the selectivity toward bastnaesite in Mountain Pass reserve, California. Indeed, hydroxamates possess better selectivity in comparison to fatty acids (Pradip and Fuerstenau 1991). Hydroxamates are much more selective than the carboxylic acid collectors but rarely used commercially due to higher cost. However, the Bayan Obo processing plant uses hydroxamic acids while treating magnetic separation tailings for the bastnaesite flotation. Naphthyl hydroxamic acid is used as a collector and sodium silicate as a depressant to enrich the grade from 12% REO to 55% REO by one rougher, one scavenger, and two cleaner stages of flotation. The secondary concentrate produced is of 34% REO grade, which implies an overall recovery of 72–75% rare earth oxide (Bulatovic 2007). Phosphoric acid esters are not used in the commercial-­ scale rare earth mineral flotation plants like fatty acids and hydroxamates (Rao 2004). Depressants Several depressants are reported in rare earth mineral flotation, and basically, the choice depends upon the reserve and associated gangue minerals. In the case of bastnaesite flotation, sodium carbonate is used for both regulating pH and depressing

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the associated gangue minerals. Sodium carbonate increases the negative charge on barite and calcite, keeping bastnaesite unaffected (Pradip and Fuerstenau  1991; Houot et al. 1991; Smith and Shonnard 1986). Lignin sulfonate is one of the widely accepted depressants for the bastnaesite flotation, and its use in Mountain pass, California, for the depression of calcite and barite, is reported (Houot et al. 1991). Sodium silicate is a very common surfactant for the depression of different silicate minerals. The Bayan Obo processing plant uses sodium silicate as a depressant and hydroxamic acid as a collector (Taikang and Yingnan 1980). Sodium hexafluorosilicate is also used in Bayan Obo to depress fluorite, barite, and calcite as well as acts as an activator of rare earth mineral (Taikang and Yingnan 1980). 2.3.4.3  Monazite Flotation Monazite [(Ce, La)PO4] and xenotime [YPO4] are often found together in many deposits, wherein, monazite occurs as a major mineral and xenotime presents as a minor mineral (Chelgani et al. 2015). Monazite is a cerium- and lanthanum-bearing phosphate and considered as a major source of thorium and a little amount of uranium. Xenotime is a yttrium phosphate and largely contains the heavy REEs (europium and gadolinium) and a lesser amount of lighter REEs (lanthanum to samarium). Monazite is predominantly present in the beach placers as a primary REE source. In Mountain Pass deposit, California, USA, monazite is present as a secondary REE mineral in association with bastnaesite (Castor 2008). Both monazite and xenotime are phosphate-bearing minerals and hence show similar flotation properties. Cheng (1993) reported that the flotability of xenotime is higher than monazite. However, differences in flotability could be a factor of associated gangue minerals. Hence depressants play a major role than the collectors. Most of the gangue minerals present with this feed material are treated as secondary values, which include ilmenite, rutile, zircon, quartz, and other silicate-bearing minerals. Monazite Collectors Sodium oleate or oleic acid has been reported as a well-known collector for monazite flotation (Sorensen and Lundgaard 1966). The slurry pH influences the flotability of monazite (Dixit and Biswas 1969). Experimentally it has been reported that the surface of monazite is chemisorbed by oleate above PZC value (pH 5.3), and the maximum flotability is recorded at pH  8.5–9.0 (Cheng et  al. 1993). Pavez et  al. (1996) observed maximum flotability of monazite at pH 3.0 and also at pH 8 with the sodium oleate collector. The surface charge of monazite at pH 3.0 is positive, and sodium oleate does not ionize at pH 3. Therefore, physical adsorption of sodium oleate on the surface of monazite occurs due to dipole interactions of the charged surface with the collector molecule. On the other hand, at pH  8.0, the monazite surface exhibits a negative charge. The interaction between sodium oleate and monazite surface is capable enough to overcome the electrostatic repulsion between the

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27

negative-charged surface and anion. This suggests chemisorption of sodium oleate on the monazite surface rendering higher floatability. In addition to the fatty acids, hydroxamates are also reported as collectors for the flotation of monazite. Chemisorption of hydroxamates on to the monazite surfaces results in a better selectivity. Potassium octyl hydroxamate (CH3 (CH2)6CONHO-K) has also been used as a collector, and it records the maximum flotability in the pH range 3–8, i.e., about 70% recovery, and it decreases with further increase in pH (Parvez et al. 1996). Effects of pure hydroxamates, as well as commercial hydroxamates on the floatability of monazite, were also studied. It has been found that the commercial hydroxamate (FLOTINOR V3759) has a better impact than the pure potassium octyl hydroxamate (Pavez and Peres 1993). Apart from fatty acids and hydroxamates, a number of collectors are used for the flotation of monazite such as benzoic acid (Ren et al. 2000), phosphoric acid esters (Andrews et  al. 1990), sodium dodecyl sulfate and dodecyl ammonium chloride, etc. (Harada et al. 1993). Monazite Depressants Depressants play a vital role in monazite flotation due to similar flotation behavior of associated gangue minerals and also inferior feed monazite concentration. Sodium silicate, sodium sulfide, and sodium oxalate are major surfactants used to depress associated gangue minerals such as zircon, pyrochlore, rutile, and other siliceous gangue minerals (Abeidu 1972). Sodium silicate is a typical surfactant used for depression of siliceous gangue minerals. It is also reported that the surfactant improves the flotability by activating the surface of the monazite (Beloglazov and Osolodkov 1936). Sodium silicate adsorbs on the surface of the silicates and decreases its flotability. Generally, sodium silicate is used along with fatty acids as collectors. In addition, sodium silicate is also used to depress the iron-bearing minerals in a few applications. Sodium disulfide is found to be one of the most effective surfactants in monazite flotation as it can depress the pyrochlore and zircon and can activate the monazite surface. Abeidu (1972) reported that increased dosage of sodium disulfide increases depression of associated zircon and pyrochlore without interrupting the monazite flotation. Pavez and Peres (1994) reported the role of sodium metasilicate and sodium sulfide for the depression of zircon and rutile during the flotation of monazite. Sodium metasilicate showed better results than sodium sulfide in the presence of collectors such as sodium oleate and different hydroxamates. With the increase in the dosage of sodium metasilicate, there is a significant drop in the flotability of zircon and rutile, not quite affecting the flotability of monazite. Sodium silicate and sodium sulfide are found to be the most common depressants used in monazite flotation. With the optimum dosage of these depressants along with suitable fatty acids or hydroxamates as a collector, it can enrich the grade of monazite as well as improve the process kinetics by activating the monazite surface.

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Apart from this, sodium oxalate, starch, quebracho, phosphoric acid esters, and different cationic amines are used with the combination of suitable collectors for the depression of different gangue minerals present along with the monazite (Houot et al. 1991; Ferron et al. 1991). 2.3.4.4  Xenotime Flotation Generally, xenotime flotation is accomplished in two stages. It includes separation of associated gangue minerals to produce a mixed concentrate of monazite and xenotime and then separation of xenotime from monazite by direct flotation (Harada et al. 1993). The common gangue minerals associated with xenotime is as similar to monazite, which consists of different silicate-bearing minerals and heavy beach sand minerals. Amphoteric collectors (F74286) showed a better response than the conventional anionic collectors; studies showed that a concentrate of 34% of Y2O3 content could be achieved from the feed of 27% with 83% recovery (Ozeren and Hutchinson 1990). Recent research works show the applications of sodium oleate as a collector for xenotime; optimum flotability of xenotime could be achieved at the pH range of 7–8 (Cheng et al. 1994). Pereira and Peres (1997) conducted microflotation studies in a modified Hallimond tube with a hydroxamate (Flotinor V3759 Hoechst) as a collector for xenotime, wherein zircon was the major impurity. The authors have performed experiments in a narrow feed size range (−212 + 106 μm), keeping the slurry pH 10; 90% recovery of xenotime was achieved. Dodecyl ammonium chloride (DAC) and sodium dodecyl sulfate (SDS) both are having the same hydrocarbon chain length and were studied as collectors for monazite and xenotime. DAC being a cationic collector has a wider pH range than the anionic collector SDS. It has been found that separation of monazite is achieved at pH 7.5, whereas xenotime was concentrated in the rougher tailings (PZC of monazite and xenotime were measured 5.5 and 7.0, respectively). Depressants have a definite impact on the grade and recovery of xenotime. Generally, sodium silicate is used as the depressant of silicate minerals that are associated with xenotime. Apart from this, sodium fluorosilicate, lignin sulfonate, quebracho, sodium metasilicate, and corn starch are used to depress various gangue minerals associated with xenotime.

2.4  Rare Earth Ore Plant Practices Bastnaesite and monazite are the two major minerals used for the production of rare earth metals. Primarily, bastnaesite and monazite have lighter rare earth elements (LREE) with relatively less amount of heavy rare earth elements (HREE). The presence of major REE with the xenotime is considered as a rich source of light and

2  Mineral Processing of Rare Earth Ores

29

heavy rare earth elements. Though xenotime contribution to the production of rare earth elements is less, the presence of yttrium in it makes it a significant source. In addition to this, fly ash, ion adsorption clays, and loparite are also considered as prominent sources of rare earth elements.

2.4.1  Bastnaesite Processing Flowsheets Most of the references on bastnaesite processing are found from Mountain Pass and Bayan Obo deposits. The processing circuit of these two deposits includes both physical and chemical beneficiation methods. 2.4.1.1  Bastnaesite Processing in Mountain Pass, California, USA Molycorp Inc. is a mining industry that owns Mountain Pass deposit and considered one of the largest producers of rare earth metals. The deposit consists of bastnaesite (8–12%), calcite (40%), barite (25%), strontianite (10%), silica (8%), and other minor minerals. The run-of-mine ore was crushed and ground to 90% finer than 75  μm size to liberate the valuable bastnaesite from the undesired minerals. The ground material was subjected to a flotation circuit, which is consisting of a number of rougher, scavenger, and cleaner stages. The flotation concentrate enriches in REO values to 63%, with the overall recovery varying between 65 and 70%. The beneficiation concentrate was further subjected to selective acid leaching to reduce the gangue minerals. As a result, residue enriches to 70% REO. The residual carbonates were removed from the residue by roasting at appropriate conditions that increase the REO content to 85%. The roasting imparts partial oxidation of Ce3+ to Ce4+. Further, the roasted product was subjected to hydrochloric acid leaching to form rare earth chlorides. The residue obtained from the hydrochloric acid leaching stage carries a maximum portion of Ce4+ along with some fluorides. Fluoride present in the residue was removed in the form of NaF by alkali (NaOH) digestion and washing. The rare earth chloride solution is neutralized by adjusting the pH of the liquor. Precipitates are removed by filtration, and the remaining chloride solution was further evaporated to get the rare earth chlorides. Figure 2.8 shows the flowsheet for the recovery of rare earths from the Mountain Pass ore. 2.4.1.2  Bastnaesite Processing in Bayan Obo, China Bayan Obo, China, is an iron ore deposit, and the mining activity involves open pit mining methods. The mined ore was crushed and ground in a series of size reduction units followed by drum magnetic separators. The magnetic fraction enriched in Fe content to 65% and the nonmagnetic fraction assays 45%. The nonmagnetic

30

S. K. Das et al.

ROM (7% REO)

Comminution Flotation Circuit

Tailings

(Rougher, Scavenger & Cleaner Stages)

Leaching & Filtration (1st Stage)

HCl

CaCl2 solution (Reject)

CO2

Drying & Roasting Leaching & Filtration (2nd Stage)

HCl

Reducing Agent

Residue

Leach Liquor

Alkali Digestion

Neutralisation pH 3.2

Filtration

Filtration

NaF Filtrate

Cerium rich hydroxide cake

Evaporation

Impurities Reject

Rare earth chloride Fig. 2.8  Molycorp flowsheet for the recovery of rare earths from bastnaesite

fraction undergoes the beneficiation of bastnaesite following tabling and flotation circuits to generate different concentrates. (a) Bastnaesite concentrate with >68% REO (b) Monazite concentrate with >60% REO (c) A mixed concentrate containing >50% REO As it is mentioned in the magnetic separation section, Changsha Research Institute of Mining and Metallurgy developed the most effective flowsheet for the processing of rare earth minerals from Bayan Obo rare earth deposits during the 1990s. The beneficiation circuit includes the comminution circuit, low-intensity magnetic separators (LIMS), high-intensity magnetic separators (HIMS), and combined direct and reverse flotation circuit. The run-of-mine ore was subjected to primary and secondary crushing and grinding units to generate 90–95% material finer than 74 μm. Bayan Obo mine has the primary iron ore deposit along with the rare earth minerals such as bastnaesite and monazite. The rare earth minerals of Bayan Obo deposit have a strong association

2  Mineral Processing of Rare Earth Ores

31

with the iron-bearing minerals like hematite and magnetite. The iron-bearing minerals are primary impurities in addition to secondary silicate minerals. The ground ore was subjected to LIMS rougher and cleaner stages. Most of the magnetite were recovered in the cleaner magnetic fraction, and the tailing was further processed in high-intensity magnetic separators (HIMS) with a magnetic field strength of 1.4 T. The Fe-bearing hematite and the rare earth-bearing minerals that possess low magnetic susceptibility were recovered in the magnetic fraction. The separation of REE-bearing minerals from Fe-bearing minerals could be achieved by another stage magnetic separation (0.6 T). The magnetic fraction contains the Fe-bearing minerals, and the nonmagnetic fractions contain most of the rare earth assaying 9.78–12% REO. The magnetic fractions of both LIMS and HIMS were subjected to reverse flotation cells to get the iron ore concentrate. The non-magnetic fraction of the HIMS cleaner stage was treated in flotation cells, wherein, naphthyl hydroxamic acid, J 10, and sodium silicate are used as rare earth mineral collectors, frother, and depressant for silicate minerals, respectively. The slurry density was maintained between 35 and 45%. The flotation circuit consists of one rougher stage, one scavenger stage, and two cleaner stages. The flotation concentrate enriches REO to 55% and 34% for the primary and secondary flotation products, respectively, with overall combined recovery values varying between 72 and 75%. Flotation concentrate was a mixture of bastnaesite and monazite, which was later separated by floating bastnaesite and depressing monazite. Phthalic acid or benzoic acid can be used as the collector for bastnaesite wherein alum was used to depress monazite. The complete processing flowsheet is shown in Fig. 2.6.

2.4.2  Monazite Processing Flowsheets Monazite is considered as the second most important resource for rare earth elements after bastnaesite. Generally, monazite and xenotime are found in various placer deposits across the world. In placer deposits, commercial mining is practiced for major minerals such as ilmenite, rutile, zircon, cassiterite, and other beach sand minerals. The production of rare earth oxides from beach sand involves several steps such as mining, pre-concentration of the heavy minerals, separation of monazite, and finally, chemical routes for the extraction of rare earth elements. 2.4.2.1  Monazite Processing at IREL, India Commercial monazite mining and processing are practiced in Guangdong in China, Mount Weld in Australia, Kangankunde in Malawi, Zandkopsdrift and Steenkampskraal in South Africa. In India, monazite mining and processing are exclusively carried out by the Indian Rare Earths Limited (IREL) from the beach sand deposits near Kerala, Odisha, and Tamil Nadu coasts.

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IREL, India, is engaged in mining and processing of beach sand minerals in the eastern and southern coast of India. The primary activities of IREL include mining, beneficiation, and extraction of rare earth chlorides and oxides. Currently, IREL is exercising mining activities in Chavara in Kerala, Manavalakurichi in Tamil Nadu, and Chhatrapur (Odisha Sand Complex) in Odisha. IREL has also established rare earth extraction plant in Odisha to produce around 11,000 tons of rare earth chloride and other associated products. In Aluva, Kerala, IREL has also constructed a ­high-­purity rare earth plant to produce rare earth oxides or carbonates. In addition to this, IREL is also engaged in mining and processing of ilmenite, rutile, zircon, sillimanite, and garnet as their by-products. A typical beach sand deposit in India constitutes about 15–20% of total heavy minerals which include ilmenite (7.0–9.0%), rutile (0.4–0.45%), zircon (0.3–0.4%), monazite (0.2%), sillimanite (2.5–3.5%), and garnet (5.0–7.0%). The mining and pre-concentration are carried out in dredging and wet upgradation plant (DWUP). Cutter suction dredge or bucket wheel dredge are used for the mining of heavy sand from the placer. The pre-concentration process involves various classification and gravity concentration units. Trommels are used in the primary stage to remove pebbles and lumps from the feed material. Further, the classified undersize material was fed to gravity spirals where it is processed in various stages such as rougher, scavenger, cleaner, and recleaner units. A series of spirals are placed with different designs and sizes to enrich total heavy mineral (THM) value from 15–20% to around 85–90%. The rejects of this DWUP is used for landfill. The pre-concentrate obtained from the DWUP was further processed in heavy upgradation plant (HUP) to enrich the THM value up to 98%. The HUP consists of various gravity concentration units, such as spirals and Wilfley tables. Typical concentrate of HUP constitutes ilmenite (~65%), rutile (~2.8%), zircon (~2.4%), monazite (~1.5%), sillimanite (~6.0%), and garnet (~18%) contents. The HUP concentrate was processed in mineral separation plant (MSP) wherein individual heavy minerals such as ilmenite, rutile, zircon, monazite, sillimanite, and garnet are separated. These minerals exhibit different physical properties, such as electrical conductivity, magnetic susceptibility, specific gravity, etc. In the primary stage, HUP concentrate was subjected to high-tension separator (HTS), wherein conducting (ilmenite and rutile) and nonconducting (garnet, sillimanite, monazite, and zircon) minerals are separated. The conducting fraction was further subjected to a magnetic separator in the second stage to separate magnetic ilmenite from the nonmagnetic rutile. The nonconducting fraction was also subjected to a low-­intensity magnetic separator (LIMS), wherein garnet is recovered as a magnetic product, and the nonmagnetic product mostly consists of zircon, monazite, and sillimanite minerals. Monazite is feebly magnetic in nature, whereas zircon and sillimanite are completely nonmagnetic. Hence, with the application of high-intensity magnetic separator (HIMS), monazite is collected in the magnetic fraction, while zircon and sillimanite remain in the nonmagnetic stream. The separation of zircon and sillimanite was achieved by using gravity concentrators as the specific gravity difference between the two minerals is very distinct. The mineral separation plant has dry and wet gravity separation units such as floatex, spirals, and Wilfley tables that separate zircon and sillimanite. Zircon is collected as a heavy mineral, whereas sillimanite

33

2  Mineral Processing of Rare Earth Ores

remains in the tailing section along with some quartz and other silicates. Further, the tailing was subjected to froth flotation to remove different silicate impurities from the sillimanite. The complete flowsheet is shown in Fig. 2.9.

2.5  Recovery of Rare Earths from Different Sources 2.5.1  Xenotime Xenotime is not as rich as monazite and bastnaesite minerals in rare earth elements. Still, it is an important source as it carries heavy rare earth elements such as terbium and dysprosium. It is also an important source of yttrium. Some other minerals such ROM Dredge and Wet Upgrading Plant (DWUP) Heavies Upgrading Plant (HUP)

Rejects for land filling

Lights reject

Mineral Separation Plant (MSP) High Tension Separator Conducting (Ilmenite & Rutile)

Non-Conducting (Garnet, Monazite, Zircon & Sillimanite)

Low Intensity Magnetic Separation (LIMS)

Magnetic Separator Magnetic (Ilmenite)

Non-Magnetic Non-Magnetic (Rutile) (Monazite, Zircon & Sillimanite)

Magnetic (Garnet)

Magnetic Separator Non-Magnetic (Zircon & Sillimanite)

Magnetic (Monazite)

Dry/Wet Gravity Separator Light

Heavy Zircon

Froth Flotation

Sillimanite

Rejects

Fig. 2.9  IREL flowsheet for the separation of monazite from beach sand minerals

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as euxenite, gadolinite, fergusonite, allanite, samarskite, etc. also contain yttrium, but due to the presence of titanium, niobium, and tantalum elements, it makes it difficult to process. As xenotime is found in placer deposits, it is easy to process, and the concentrate carries 40–60% Y2O3 with a mixture of other rare earth oxides. The processing route of xenotime is similar to that of monazite.

2.5.2  Ion Adsorption Clays Commercial mining and processing of these types of ores are currently exercised only in China. The REO assay in this ore is too low, and it lies around 0.05–0.2%. Therefore, ore beneficiation does not have much role in the processing route. As these deposits occur in the weathered profile, so the open-cast mining methods are adopted. This mined product does not require any further milling and is directly subjected to leaching with sodium chloride or ammonium chloride solution, which results in 90% rare earth dissolution in the ion-exchange process. This is much selective in nature and allows very little impurities leached inside it. Oxalic acid leads to precipitation of dissolved rare earths from the leach liquor, and at the end, calcination of oxalate leads to recover REO oxide with 90% purity. The absence of thorium, uranium, and other radioactive materials makes it much easier to process. Fujian and Jiangxi are some of the deposits of China which bear these types of resources. These deposits are being classified in terms of light, medium, and heavy rare earth minerals. So each of the deposit requires different processing routes.

2.5.3  Loparite Commonwealth of Independent States (CIS) in Eurasia bore some deposits of loparite and is treated as the major source of rare earths. This mineral is a complex mixture of different light rare earth elements (LREE), which are depicted in Table  2.5. Generally, these types of ores contain 25% La2O3, 55% CeO2, 3.5% Pr6O11, 13–15% Nd2O3, and 0.7% (Sm, Eu, Gd)2O3 of the total REO. The high-­ temperature chlorination process is used for the recovery of rare earths from the loparite. The ore is first subjected to size reduction to finer size and then mixed with coke or coal for the preparation of briquettes. At 750–800 °C, the present volatile chlorides such as chlorides of niobium, tantalum, and titanium get distilled, and the chlorides of cerium, rare earths, and thorium remain as residue. It follows a series of precipitation to separate iron, uranium, thorium, and radium. The final solution contains the rare earths which are recovered by conventional methods.

2  Mineral Processing of Rare Earth Ores Table 2.5  REE distribution in loparite mineral, Revda, Russia (Hedrick et al. 1997; Jordens et al. 2013)

35 Elements Lanthanum Cerium Praseodymium Neodymium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Yttrium

Wt., % (~) 25.0 49.7 5.0 15.0 0.7 0.1 0.6 – 0.6 0.7 0.8 0.1 0.2 0.2 1.3

2.5.4  Industrial By-Products Recently, REEs are also recovered from various by-products and industrial wastes. Tailings generated from the processing of brannerite, a uranium mineral, carry some amount of yttrium and heavy rare earths. Many technologies are developed to recover the rare earths from this material, which are practiced in the Elliot Lake area, Canada (Gupta and Krishnamurthy 2005).

2.6  Concluding Remarks The term “rare earth” implies that these minerals are scarce, but the fact is that they are quite abundant in nature compared to many base metals such as copper, tin, lead, zinc, etc. Uneven distribution of the rare earth values in the earth’s crust and technical difficulties of recovering and extraction have made these metals rare in nature. About 80% of the world’s rare earth reserves are confined to a few countries such as China, the USA, Brazil, and the Commonwealth of Independent States (CIS). It has been identified that REEs are found in 200 and odd minerals. However, bastnaesite, monazite, and xenotime are major economic minerals from which rare earth metals are extracted. In the present chapter, an attempt has been made to collate world rare earth reserves and production, various kinds of ores, processing strategies, plant practices, and different rare earth sources. The REE-bearing minerals of economic importance occur in carbonatites, pegmatites, and placer deposits. In most of these

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deposits, rare earth minerals are found in association with silicate and carbonate gangue minerals. Rare earth minerals possess higher specific gravity than the associated gangue minerals. Therefore, gravity concentration plays a crucial role in the recovery of these minerals. Further, the paramagnetic nature of rare earth minerals is explored in many cases to enrich the REE values. Beach sand deposits (placer deposits) contain monazite along with other heavy minerals, which require electrostatic separators to remove other heavy minerals. Froth flotation is one of the prime concentration techniques used in rare earth processing. Major rare earth-producing plants such as Mountain Pass and Bayan Obo follow the froth flotation technique exclusively to enrich the bastnaesite concentrate. There has been a lot of advancement in mineral processing activities to improve the recovery of rare earth minerals from the lean grade ores. The progress in rare earth technology needs to keep pace with the present demand.

References Abaka-Wood, G.B., Addai-Mensah, J., & Skinner, W. (2016). Review of flotation and physical separation of rare earth element minerals. 4th UMaT Biennial International Mining and Mineral Conference. Tarkwa Ghana, MR 55–62. Abaka-Wood, G. B., Quast, K., Zanin, M., Addai-Mensah, J., & Skinner, W. (2018). A study of the feasibility of upgrading rare earth elements minerals from iron-oxide-silicate rich tailings using Knelson concentrator and Wilfley shaking table. Powder Technology, 344, 897–913. Abeidu, A. M. (1972). The separation of monazite from zircon by flotation. Journal of the Less Common Metals, 29, 113–119. Andrews, W. H., Collins, D. N., & Hollick, C. T. (1990). The flotation of rare earths—a contribution to industrial hygiene. Carlton, VIC: Australasian Institute of Mining and Metallurgy. Beloglazov, K. F., & Osolodkov, G. A. (1936). Effect of the alkalinity of the pulp in flotation of apatite. Representative of the Leningrad Ministry Institute, 1, 26–30. Bulatovic, S.  M. (2007). Handbook of flotation reagents. Amsterdam: Elsevier. https://doi. org/10.1016/B978-0-444-53029-5.X5009-6. Castor, S. B. (2008). The Mountain Pass rare-earth carbonatite and associated ultrapotassic rocks, California. The Canadian Mineralogist, 46, 779–806. Chan, T. N. (1992). A new beneficiation process for the treatment of supergene monazite ore. In Rare earths: Extraction, preparation and applications (pp. 77–94). San Diego, CA: TMS and AusIMM. Chelgani, S. C., Rudolph, M., Leistner, T., Gutzmer, J., & Peuker, U. A. (2015). A review of rare earth minerals flotation: Monazite and xenotime. International Journal of Mining Science and Technology, 25, 877–883. Cheng, Ta-Wui. (1993). Surface properties and flotation behaviour of monazite and xenotime. PhD thesis, The University of New South Wales. Cheng, T.-W., Holtham, P.  N., & Tran, T. (1993). Froth flotation of monazite and xenotime. Minerals Engineering, 6, 341–351. https://doi.org/10.1016/0892-6875(93)90014-E. Cheng, T.-W., Partridge, A. C., Tran, T., & Wong, P. L. M. (1994). The surface properties and flotation behaviour of xenotime. Minerals Engineering, 7, 1085–1098. Chi R, Xu S, Zhu G, Xu J, Qiu X. (2001). Light metals 2001: Proceedings of the technical sessions presented by the TMS Aluminium Committee at the 130th TMS annual meeting, 1159–1165, New Orleans, Louisanna, February 2000. Warrendale, PA: Minerals, Metals and Materials Society.

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Cui, H., & Anderson, C. G. (2017). Alternative flowsheet for rare earth beneficiation of Bear Lodge ore. Minerals Engineering, 110, 166–178. Dixit, S. G., & Biswas A. K. (1969). Minerals beneficiation – pH-dependence of the flotation and adsorption properties of some beach sand minerals. Trans AIME, 244(3), 173. Falconer, A. (2003). Gravity separation: Old technique/new methods. Physical Separation in Science and Engineering, 12, 31–48. Ferron, C. J., Bulatovic, S. M., & Salter, R. S. (1991). Beneficiation of rare earth oxide minerals. Materials Science Forum, 70–72, 251–270. Generalic, E. (2019). Rare Earth Elements (REE). EniG. Periodic table of the elements. Retrieved from https://www.periodni.com/rare_earth_elements.html Gupta, C.  K., & Krishnamurthy, N. (1992). Extractive metallurgy of rare earths. International Materials Reviews, 37, 197–248. Gupta, C. K., & Krishnamurthy, N. (2005). Extractive metallurgy of rare earths. Boca Raton, FL: CRC Press. Harada, T., Owada, S., Takiuchiand, T., & Kurita, M. (1993). A flotation study for effective separation of the heavy mineral sands. In XVIII international mineral processing congress (pp. 1017). New York. Hedrick, J. B., Sinha, S. P., & Kosynkin, V. D. (1997). Loparite, a rare-earth ore (Ce, Na, Sr, Ca) (Ti, Nb, Ta, Fe+3)O3. Journal of Alloys and Compounds, 250, 467–470. Houot, R., Cuif, J. P., Mottot, Y., & Samama, J. C. (1991). Recovery of rare earth minerals, with emphasis on flotation process. Materials Science Forum, 70–72, 301–324. Jordens, A., Cheng, Y.  P., & Waters, K.  E. (2013). A review of the beneficiation of rare earth element bearing minerals. Minerals Engineering, 41, 97–114. https://doi.org/10.1016/j. mineng.2012.10.017. Jordens, A., Sheridan, R. S., Rowson, N. A., Waters, K. E. (2014). Processing a rare earth mineral deposit using gravity and magnetic separation. Minerals Engineering, 62, 9–18. https://doi. org/10.1016/j.mineng.2013.09.011 Jordens, A., Marion, C., Langlois, R., Grammatikopoulos, T., Rowson, N.  A., & Waters, K.  E. (2016). Beneficiation of the Nechalacho rare earth deposit. Part 1: Gravity and magnetic separation. Minerals Engineering, 99, 111–122. Jun, R., Wenmei, W., Jiake, L., Gaoyun, Z., & Fangqiong, T. (2003). Progress of flotation reagents of rare earth minerals in China. Journal of Rare Earths, 21(1), 1–8. Kanazawa, Y., & Kamitani, M. (2006). Rare earth minerals and resources in the world. Journal of Alloys and Compounds, 408–412, 1339–1343. Li, L. Z. & Yang, X. (2014). China’s rare earth ore deposits and beneficiation techniques. 1st European Rare Earth Resources Conference ERES2014. pp. 26–36. Li, L. Z., & Yang, X. (2016). China’s Rare Earth Resources, Mineralogy, and Beneficiation. In Rare earths industry (pp. 139–150). Amsterdam: Elsevier. Long, K., Gosen, B.S., Foley, N., & Cordier, D. 2010. The principal rare earth elements deposits of the United States. USGS Scientific Investigations Report: 2010–5220. Mineral Commodity Summaries. Moustafa, M. I., & Abdelfattah, N. A. (2010). Physical and chemical beneficiation of the Egyptian beach monazite: Beneficiation of monazite. Resource Geology, 60, 288–299. Murthy, T. K. S., & Mukherjee, T. K. (2001). Processing of rare earth resources, rare earth metals, non-ferrous metals strategy cum source book. New Delhi: TIFAC. Ozeren, M.S. & Hutchinson H. 1990. The selective flotation of xenotime from heavy minerals. The AUSIMM annual conference. Rotorua New Zealand. Pavez, O., Brandao, P. R. G., & Peres, A. E. C. (1996). Adsorption of oleate and octyl-hydroxamate on to rare-earths minerals. Minerals Engineering, 9, 357–366. Pavez, O., & Peres, A. E. C. (1993). Effect of sodium metasilicate and sodium sulphide on the floatability of monazite-zircon-rutile with oleate and hydroxamates. Minerals Engineering, 6, 69–78. Pavez, O., & Peres, A. E. C. (1994). Technical note bench scale flotation of a Brazilian monazite ore. Minerals Engineering, 7, 1561–1564.

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Pereira, C. A., & Peres, A. E. C. (1997). Flotation concentration of a xenotime pre-concentrate. Minerals Engineering, 10, 1291–1295. Pradip, K. (1981). The surface properties and flotation of rare-earths minerals. Ann Arbor, MI: University Microfilms International. Pradip, K., & Fuerstenau, D. W. (1983). The adsorption of hydroxamate on semi-soluble minerals. Part I: Adsorption on barite, Calcite and Bastnaesite. Colloids and Surfaces, 8, 103–119. https://doi.org/10.1016/0166-6622(83)80079-1. Pradip, K., & Fuerstenau, D. W. (1991). The role of inorganic and organic reagents in the flotation separation of rare-earth ores. International Journal of Mineral Processing, 32, 1–22. Pradip, K., & Fuerstenau, D. W. (2013). Design and development of novel flotation reagents for the beneficiation of Mountain Pass rare-earth ore. Mining, Metallurgy & Exploration, 30, 1–9. Qi, D. (2018). Hydrometallurgy of rare earths: extraction and separation. Cambridge, MA: Elsevier. Rao, S. R. (2004). Surface chemistry of froth flotation. Kluwer Academic, New York, USA. https://doi.org/10.1007/978-1-4757-4302-9 Ren, J., Lu, S., Song, S., & Niu, J. (1997). A new collector for rare earth mineral flotation. Minerals Engineering, 10, 1395–1404. Ren, J., Song, S., Lopez-Valdivieso, A., & Lu, S. (2000). Selective flotation of bastnaesite from monazite in rare earth concentrates using potassium alum as depressant. International Journal of Mineral Processing, 59, 237–245. Ren, J., Wang, W. M., Luo, J. K., Zhou, G. Y., & Tang, F. Q. (2003). Progress of flotation reagents of rare earth minerals in China. Journal of Rare Earths, 21, 1–8. Richter, L., Diamond, L.W., Atanasova, P., Banks, D.A., & Gutzmer, J. (2018). Hydrothermal formation of heavy rare earth element (HREE) – xenotime deposits at 100 °C in a sedimentary basin. Geology, 26(3). 263–266. ISSN 0091-7613 https://doi.org/10.1130/G39871.1 Smith, R. W., & Shonnard, D. (1986). Electrokinetic study of the role of modifying agents in flotation of salt-type minerals. AICHE Journal, 32, 865–868. Sorensen, E. & Lundgaard, T.. (1966). Selective flotation of steenstrupine and monazite from Kvanefjeld Lujavrite—Report for the Danish Atomic Energy Commission. Roskilde. Spedding, F. H. (1975). Contributions of the rare earths to science and technology. Symposium on the effects of rare earths on the properties of metals and alloys. ASM, Cincinnati, Ohio, United States of America, 1–11. Taikang, D., & Yingnan, H. (1980) Studies on high grade rare earth flotation technology. Multipurpose Utilization of Mineral Resources, 1, 27–33 (in Chinese) U.S. Geological Survey. (2017). Mineral commodity summaries 2017: U.S. Geological Survey, 202 p. https://doi.org/10.3133/70180197 Xiong, W., Deng, J., Chen, B., Deng, S., & Wei, D. (2018). Flotation-magnetic separation for the beneficiation of rare earth ores. Minerals Engineering, 119, 49–56. Yang, X.  J., Lin, A., Li, X.-L., Wu, Y., Zhou, W., & Chen, Z. (2013). China’s ion-adsorption rare earth resources, mining consequences and preservation. Environmental Development, 8, 131–136. Zhang, J., & Edwards, C. (2012). A review on rare earth mineral processing technology. In 44th Annual Meeting of the Canadian Mineral Processing (pp. 79–102). Ottawa: CIM. Zhang, W., & Honaker, R. (2017). Surface charge of rare earth phosphate (monazite) in aqueous solutions. Powder Technology, 318, 263 – 271. https://doi.org/10.1016/j.powtec.2017.05.048

Chapter 3

Thermodynamic Aspects for Rare Earth Metal Production Sanjay Agarwal, Hong In Kim, Kyung-Ho Park, and Jin-Young Lee

3.1  Introduction The average concentration of rare earth elements in the earth’s crust is about 150–220 ppm (from Wikipedia and Taylor 1964). This value exceeds that of some primary metal productions like 60 ppm copper, 70 ppm zinc, and 75 ppm nickel. Even the rare earth cerium (atomic no 58) is 15,000 times more abundant than gold. Rare earth elements because of their geochemical properties are generally dispersed in the earth’s crust; hence they are rarely concentrated at a place with a minable mineral ore deposit. It was the scarcity of these minerals that led to naming them as rare earths. Rare earth elements (Krishnamurthy and Gupta 2016) are a group of 17 elements consisting of lanthanide elements with scandium and yttrium, which have similar chemical properties and are often found in the same ore deposits. All elements occur in nature except promethium which originates as a part of radioactive decay process. Rare earths are categorized into light rare earths and heavy rare earths. The light rare earths consist of lanthanum, cerium, praseodymium, neodymium, promethium, samarium, and europium. The heavy rare earths consist of gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium, and yttrium. The first rare earths were discovered in 1787 by Lieutenant Carl Axel Arrhenius, an amateur mineralogist of the Swedish royal army at a quarry in the village of Ytterby, near Stockholm in Sweden, and were named ytterbite. Rare earths have similar chemical composition and hence were difficult to separate. Finnish chemist

S. Agarwal (*) Metal Extraction and Recycling Division, CSIR-National Metallurgical Laboratory, Jamshedpur, India H. In Kim · K.-H. Park · J.-Y. Lee Convergence Research Center for Development of Mineral Resources, Korea Institute of Geoscience and Mineral Resources, Daejeon, South Korea © Springer Nature Switzerland AG 2020 R. K. Jyothi (ed.), Rare-Earth Metal Recovery for Green Technologies, https://doi.org/10.1007/978-3-030-38106-6_3

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Johan Gadolin, in 1794, was able to separate the first rare earth oxide from the mineral ytterbite and named it as impure yttrium oxide which was later in 1800 renamed as gadolinite in his honor (Krishnamurthy and Gupta 2016). Since the twentieth century, rare earth elements have become technologically, environmentally, and economically important for all of us across the globe. India (first few countries) started the production of rare earths as early as 1911 from its beach sand having ilmenite, sillimanite, garnet, zircon, monazite, and rutile mineral (Chandrashekar and Sundaresan 2016). India has around 35% of the world’s total beach sand mineral deposits. Indian Rare Earths Limited began the extraction of rare earth minerals from the Indian beach sands. Till the 1940s India and Brazil were the dominant player in the market. Australia and Malaysia started their production of REMs after the 1940s. In the 1950s South Africa became a leading producer of monazite mineral. From the 1960s the USA started its production of rare earths from its bastnasite minerals from the Mountain Pass mines and became the world’s number one producer at that time. Latter in the mid-1980s, the Chinese found that their rare earth minerals associated with iron minerals can be more efficiently and economically extracted, and they carefully planned for it, and China started producing rare earth minerals, metals, and its product in a large way and became the world leading player in the rare earth market since then (Hurst 2010). China gradually strengthened its hold in the world rare earth market like Middle East Asia is having for its oil and gas reserves. Table 3.1 provides us with the data of world reserves of rare earth by different countries (Ministry of Mines, Government of India 2018), and Table 3.2 gives us the data of worldwide production of rare earth oxides by major producing countries during the year 2013–2018 (Major Countries in Rare Earth Mine Production Worldwide from 2013 to 2018 2019 https://www. statistia.com/statistics/268011). Rare earths world reserves are estimated at 121 million tons; China is leading in the reserves and production of rare earth oxides. The other major rare earth producers are Australia, the USA, Burma, Russia, India, Brazil, and others according to the Mineral Commodity Summaries 2018. Table 3.1  World reserves of rare earths (by major countries)

(In million tons of REO content) Country Reserves World’s total reserves 121.06 China 44.00 Vietnam 22.00 Brazil 22.00 Russia 18.00 India 6.90 Australia 3.40 Greenland 1.50 USA 1.40 South Africa 0.86 Canada 0.83 Malawi 0.14 Malaysia 0.03

3  Thermodynamic Aspects for Rare Earth Metal Production

41

Table 3.2  World production of rare earth oxides in MT Country China Australia USA Burma Russia India Brazil Thailand Burundi Vietnam Malaysia

2013 95,000 2000 5500 − 2500 2900 330 − − 220 180

2015 105,000 10,000 4100 − 2500 − 880 760 − − 200

2017 105,000 19,000 − − 2600 1800 1700 1300 − 200 180

2018 120,000 20,000 15,000 5000 2600 1800 1000 1000 1000 400 200

It is the emerging trend in the current scenario that all the evolving green and advanced technologies are mainly depending on the use of rare earth elements. Rare earths are used in electric and hybrid cars, wind turbines, batteries, magnets, high-­ end electronics, ceramics, catalysts, super alloys of steel, high-performance coolants, energy-efficient lamps, solar panels, and military navigation systems; these are very essential for economic viability and for better performances of technologies (Agarwal et al. 2017; Jyothi and Lee 2017). Rare earth elements are necessary for us to develop the green and clean technologies for the upcoming future for a sustainable growth of the world in which we are presently living. Rare earth elements are highly reactive, and they generally tend to form oxides, carbides, nitrides, fluorides, and sulfide (Balachandran 2014). In the Ellingham diagram, the rare earth oxides appear closer and below the calcium oxide line which shows that they are very stable oxides (Krishnamurthy and Gupta 2016). Their reduction to metal is a very tricky and difficult process because of vapor pressure and melting point of rare earth metals. Rare earth minerals are treated by acid/base chemical reaction methods to produce pure rare earth oxides. The carbothermic reduction of rare earth oxides tends to form carbides which are highly endothermic reactions and required huge amount of energy for the process. Individual rare earth metal can be extracted by either fuse salt electrolysis or metallothermic reduction processes. For the extraction of rare earths, the basic understanding of metallurgical thermodynamics is of utmost importance and will be dealt in detail here.

3.2  Thermodynamics of Rare Earth Extraction The term thermodynamics is formed by two Greek words “therme” and “dynamikos” which means “heat” and “movement,” respectively. Thermodynamics deals with heat, temperature, and their relation to energy, work, as well as the equilibrium

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states and variables of the system under study. Thus thermodynamics defines heat of a process that is being transferred from one region to another region down a temperature gradient. Chemical thermodynamics is a tool used to predict whether a chemical reaction is feasible or not. It also does quantitative calculation of the state of equilibrium of a system in terms of composition, pressure, and temperature (Ghosh 2015). Laws of thermodynamics are exact in nature, and hence calculations based on them are in principle reliable. In this chapter we will go through the application of thermodynamic tools for rare earth metal extraction from its minerals/oxides. In extractive metallurgical thermodynamic calculations, maximization of system entropy is the main driving force for energy and mass transformation processes, enabling changes to be quantified and the outcome of reactions predicted. The second law of thermodynamics states that entropy of an isolated system (combination of a subsystem under study and its surroundings) increases during all spontaneous chemical and physical processes (Gasik 2013). This leads to temperature (heat energy), pressure (mechanical energy), and chemical potentials of the component equalization for a particular system under consideration. Equalization of temperature happens through the process of conduction, convection, and radiation within/ through the system or between two systems. Equalization of pressure happens through the deformation of solid phases or through fluid flow within/across the system by the forces acting on them (a liquid or a gas). Equalization of chemical potential in a system happens by mass transfer between different phases of the system and also by diffusion of species from higher chemical potential to lower chemical potential zone in the system or between the system. A simple example is a system of hot gas molecules diffusing in a homogeneous environment. In this system, the molecules tend to move from areas of high concentration to low concentration area, until the concentration is the same everywhere. The overall reaction rate for a process (like in the processing of rare earth minerals/oxides) is controlled by the combination of kinetics of chemical transformation (reactions) and the rate of mass and energy transportation of the process under consideration. The rate of chemical reactions as well as diffusion of particles in the system increases with increasing temperature. The rate at which mass is transported in or through the system by fluid flow depends on the fluidity and mixing (mechanical energy) within the system. The rate of mass transportation in the solid phases depends on the diffusivity of species within and between the phases and active surface area available for diffusion. The driving force for a chemical reaction process to happen at constant temperature (T) and pressure (P) is its tendency to minimize the Gibbs free energy (G). Gibbs energy minimization analysis tool is generally used to predict the possibility and direction of the chemical process and for defining the final equilibrium state of the process. Even if the equilibrium state is not reached, it reveals the extent to which the process is theoretically feasible. In thermodynamic analysis, a phase undergoing transformation in the system is described by the thermodynamic quantities: internal energy (U), enthalpy (H), entropy (S), heat capacity (Cp at constant pressure and Cv at constant volume), Gibbs energy (G), and Helmholtz energy (F).

3  Thermodynamic Aspects for Rare Earth Metal Production

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In spontaneous uncontrolled chemical processes, the entropy tends to increase, which is the main driving force for energy, mass transformation, and transportation. In controlled processes like most of the industrial processes where temperature, pressure, or volume of the system is kept constant or stepwise processes along with changing temperature, pressure, or volume with a constant amount of mass, the direction of individual mass transformation processes and equilibrium can be analyzed by Gibbs free energy G at constant temperature and pressure or by Helmholtz free energy F at constant temperature and volume, respectively. The energy transformation of the system at constant pressure is related to enthalpy as follows: H ( enthalpy ) : dH = TdS + VdP



(3.1)



and at constant volume (when the system is not doing external work by increasing volume against external pressure) to internal energy:

U ( internal energy ) : dU = − PdV + TdS

(3.2)



These quantities are linked to system state variables (T, P, V, S) by fundamental differential equations:

G ( Gibbs energy ) : dG = VdP − SdT



F ( Helmholtz energy ) : dF = − PdV − SdT

(3.3)



(3.4)



Change in the enthalpy (H) of a system at constant pressure P quantifies the change in the thermal energy of the system. Change in the internal energy of a system (U) can be measured by processes involving heat and work. Helmholtz free energy (F) measures the maximum work that a change on the internal energy of a system can do. Thus Helmholtz free energy provides a criterion for equilibrium of a system at constant volume and constant temperature, and Gibbs free energy provides a criterion for equilibrium of a system at constant pressure and constant temperature, and attainment of equilibrium coincides with the system having minimum value of G. The temperature and pressure dependence of the state variables of a single, homogenous phase (pure element, compound, molten or liquid solution, gas mixture) with constant composition can be obtained from total derivatives of these variables: P=− V =−

∂U ∂F ∂U ; T= S= − ∂V ∂V T ∂S

∂H ∂G ∂F ; S=− S= − ∂P ∂P T ∂T

V

=−

V

∂H ; ∂S P

=−



∂G ; ∂T P

(3.5)

As the heat capacity of the material is linked to change in its stored thermal energy (heat value) with temperature, for constant pressure (CP) or volume (CV), conditions can be written as follows:

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CP = CV =

∂H ∂T ∂U ∂T

P

V

=T

=T

∂S ; ∂T P



∂S ; ∂T V



(3.6)

(3.7)

The heat capacity of any material and phase is expressed in the standard polynomial form as a function of temperature:

CP (T ) = A + BT + CT −2 + DT 2 + ET 3 +…

(3.8)



where coefficients A, B, C, D… are tabulated in all the existing thermodynamic databases for many substances. Temperature-dependent enthalpy and entropy of homogeneous phase are written as the sum of integrals of heat capacities of each stable phase plus the sum of the respective enthalpy or entropy of the phase transformation (like melting or structure changes):



0 H ( T ) = H 298 + ∑

∫ Cphase (T ) dT + ∑ ( ∆H )phase P

phase T2 ,phase

0 S ( T ) = S298 + ∑



T1 ,phase

T1 ,phase

∫

phase T2 ,phase

phase

CPphase ( T ) T

 ∆H phase dT + ∑  phase  T  phase

(3.9)



  

(3.10)

0 is standard state reference (298.15 K and 1 bar pressure). T1 and T2 are here H 298 temperature of the specific phase stability range, (ΔH)phase is the enthalpy change value at phase transformation, and Tphase is the temperature of this phase transformation. For example, pure neodymium (Nd) in the temperature range 298–900 K, the heat capacity function (3.8) in J/mol K, is (Bale et al. 2009):



CP ( T ) Nd = 27.085 − 1.112 × 10 −3 T − 69774 T −2 + 1.615 × 10 −5 T 2



(3.11)

For the temperature range 900–1128 K, it is:

CP ( T ) Nd = 22.753 + 8.408 × 10 −3 T + 1.0812 × 10 −5 T 2

(3.12)



For the temperature range 1128–1799 K, it is:

CP ( T ) Nd = 238.18 − 0.157T − 77, 620, 700T −2 + 3.628 × 10 −5 T 2



(3.13)

0 0 (the standard value of H 298 and S298 is zero for pure element) When Nd melts, the heat capacity of liquid Nd becomes nearly constant (48.78 J/ mol K) over 1799 K. However at the melting point, Nd enthalpy is increased by (ΔH)Nd = 5051.74  J/mol  K (latent heat of melting) and entropy by 72.67 J/mol K.

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The Gibbs free energy of any phase can be calculated with known enthalpy and entropy values as follows:

G (T ) = H (T ) – T × S (T )



(3.14)

where H(T) and S(T) are defined by Eqs. (3.9) and (3.10). The partial molar Gibbs energy of the component (known as chemical potential μi) is defined as the change of the molar Gibbs energy with an increasing number of moles of this component ni, keeping T, P, and other component amounts nj constant:

µi = Gi =

∂G |n ,T ,P ∂ni j

(3.15)

Application of thermodynamic analysis to a system under consideration is based on the proper definition of the system and its properties, temperature, pressure, volume, and amount (elemental composition). The thermodynamic analysis of the general metallurgical processes is based on an isothermal-isobaric approach (i.e., mass transformation), and their driving forces and equilibrium state are analyzed at constant temperature and pressure. In actual process, temperature varies with time inside the furnace, and pressure variation is not that significant inside the furnace (except the processes which use vacuum for its operation). This means that thermodynamic analysis of the process based on Gibbs energy minimization should be spatially and timely divided into stages to accurately evaluate the advance of the process till the completion. Chemical potential plays a very important role in the solution thermodynamics. For the metallothermic reduction of rare earth oxides to produce rare earth metals, the change of thermodynamic properties of the phases is accompanied by a combination of chemical reaction between the reductant (metals such as La, Li, Na, Ca, K, Mg, Al) and oxides, formation of molten metal and slag phases, mutual dissolution of oxides, phase transformations (melting of solid phases, generation of metal vapor from the molten phases as a result of chemical reactions between the species), vapor deposition of species, and so on. The direction and extent of equilibrium state of these processes can be predicted by the combination of the molar Gibbs energy of each phase and the material balance at each level of temperature and pressure. For a process to be thermodynamically possible, all the reactions taking place in the process should lead to the decrease of the total Gibbs energy of the reacting system. By calculating the enthalpy analysis of the system, one gets a comprehensive picture of the progress of process (i.e., at a particular temperature, amount, and composition of each phase formation). In the next section, we will deal with important application of thermodynamics for the changes in Gibbs energy with the progress of chemical reaction to form a solution.

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3.3  Solution Thermodynamics Solution can be defined as a homogeneous phase formed by chemical or phase transformation process at a given temperature and pressure by mixing two or more dissimilar components. Initial state is an unmixed one and final state is a mixed solution state. The reaction can be represented as follows: P ,T



nA Apure + nB Bpure → ( nA + nB ) A − B

(3.16)



Metallurgical solutions can be defined as metallic solutions (like Nd-Fe-B alloy) and nonmetallic solutions (a molten oxide slag solution containing SiO2, Al2O3, MgO, CaO, FeO, etc.). In the case of metallic solution for equilibrium calculations, using Eqs. (3.1)–(3.4) along with Eq. (3.15) for sum of chemical potentials of all the participating species, ∑ ni d µi , the change of Gibbs energy (dG) equals to zero, i hence dG = 0:

VdP + SdT + ∑ ni d µi = 0, i

(3.17)



which is known as Gibbs-Duhem equation. Using Eq. (3.17) it follows that at constant temperature and pressure (dT  =  dP  =  0), thus at equilibrium, the chemical potential of components in all phases is same. After formation of the solution, its Gibbs free energy is not equal to the sum of the free energies of initial mixed components because of the increase of mixing entropy. For solutions having K components with ni moles of each component i, the molar fraction of every component is: Xi =

ni

(3.18)

j =1

∑ nj k



For the formation of 1 mol of the solution, its Gibbs energy is the sum of individual component Gibbs energy, proportional to their molar fractions, plus contribution from the ideal mixing of the solution and the contribution from the nonideality (it is a measure of deviation from ideality known as excess free energy):

G m = ∑ X j G 0j + RT ∑ X j ln X j + ∆G xs j

j

∆G m = RT ∑ X j ln X j + ∆G xs = ∆H m − T ∆S m j

(3.19)



(3.20)

here G 0j is the molar Gibbs energy of the pure component j, Xj is molar fraction of the j-component, R is universal gas constant and has value 8.314 J/mol K, and ΔGxs is the excess Gibbs energy of mixing. Enthalpy and entropy changes of the solution formed are defined in similar lines as of Gibbs energy change. The Gibbs energy of ideal mixing describes the effect of dilution of a substance when mixing two or

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more different components into a phase without interaction between the components assuming completely statistical random mixing. Any deviations from the random mixing of the species or a nonzero thermal effect of the solution formation make the system nonideal (ΔGxs ≠ 0). The excess Gibbs energy of mixing describes the chemical interaction between the species in the (ΔHxs ≠ 0) phase and deviation from the random mixing of the components (ΔSxs ≠ 0) (Gasik 2013). Using Eq. (3.15), the chemical potential of a component in the multicomponent solution is expressed in terms of mole fraction as follows:



j ≠i  ∂G  ∂G   ∂G  µi =  m  = G m + [1 − Xi ]  m  − ∑ Xj  m   ∂ni  n j  ∂Xi  X j / Xk K −1  ∂X j

   X j / Xk

(3.21)

This allows us to calculate the chemical potential for any cross section of the compositional space by derivation of Eq. (3.19). Selection of actual concentration paths for free energy and chemical potential calculations must be taken into account because they are path-dependent state functions, as in a multicomponent system they depend upon on the integration of the path chosen. Chemical potential is directly linked with activity (a) as follows:

µi ( Xi ) = µi0 + RT ln ( ai ( Xi ) )



(3.22)

Activity of a species in the solution is its measure of effective concentration in the solution with respect to its reaction with other species in solution, in the sense that chemical potential of a species depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution. Raoult in 1887 defined activity by relating it to the partial pressure of the dissolved component over a solution versus its reference state as follows: ai ( Xi ) =

Pi ( Xi ) Pi 0

(3.23)

here Pi is the partial pressure of the species in the solution, and P0 is the pressure of this substance in the reference state ai0 = 1 (Ghosh 2015). Note that ai has no meaning without specifying the reference standard state. Many thermodynamic calculations use pure substance as the reference state by default. Effect of pressure is small for condensed phases and its composition is its activity.

(



)

ai = xi

(3.24)

ni , Raoult’s law for ideal solution ∑ ni For nonideal solutions ai = γixi, where γi is activity coefficient, γi = 1 for ideal solutions. γ is a function of temperature, pressure, and composition of the phase stated as γ = f( T, P, Comp.). When the concentration of elements like oxygen, nitrogen, hydrogen, silicon, and manganese presents in very small amount in liquid steel/dilute solution, the

where xi =

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Fig. 3.1  Schematic drawing showing the application of Henry’s law to the solute and Raoult’s law to the solvent

thermodynamic treatment of this is generally done by Henry’s law which states that activity of any component at infinite dilution is equal to its concentration. hi = (wt%)i as shown in Fig. 3.1. The term “dilute” may be used as long as this relationship is valid at infinite dilution, and thus the range of its application may vary from one element to another. If the concentration is expressed in mole fractions Xi, then according to Henry’s law for an infinite dilution solution, hi = Xi. Free energy function has to be a continuous and differentiable function of the quantity of solute in the solution to calculate the chemical potential and activity of the reference state, otherwise there will be errors in the calculations (Ghosh 2015). The knowledge of the activity of a component versus variables like temperature, pressure, and composition allows us to calculate the resulting solution composition to determine favorable process parameters for the reduction of smelting processes. For gaseous mixtures the activity of species is expressed as fugacity. For an ideal gas, the fugacity is numerically equal to the pressure at all pressures. For nonideal gas in the actual process conditions, fugacity can be considered as the corrected pressure. Fugacity is a tool, used to conveniently define the chemical potential for a real gas in similar lines as for an ideal gas. It can be regarded as an idealized measure of the pressure of a gas. Fugacity can be used to describe nonideal gases, liquids, as well as solids. The fugacity coefficient is defined as the ratio of fugacity/ pressure. For gases at low pressure (here ideal gas low is a good approximation), fugacity is almost equal to pressure. Thus the ratio defines how close the real substance behaves like an ideal gas. The understanding and application of activities for all chemical reactions taking place in the system to produce the desired product, at equilibrium, are very important for the system under study.

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3.4  Thermodynamics of Chemical Reactions Consider a chemical reaction between different species as follows:

aA + bB + cC + dD +… = pP + qQ + rR + sS +…,

(3.25)

At constant pressure and temperature, where A, B, C, D… are reactant, P, Q, R, S… are products, and prefixes are the number of g-atom or g-mol of the corresponding species. The free energy change of reaction (Eq. 3.25) may be written as follows: ∆G = ∑ Gproduct − ∑ Greactant

= ( pG p + qGq + rGr + sGs +…) − ( aGa + bGb + cGc + dGd +…)

(3.26)

If the reactants and products are present in their standard state, then the free energy change, ΔG0, may be written as follows:

(

) (

)

∆G 0 = pG p0 + qGq0 + rGr0 + sGs0 +… − aGa0 + bGb0 + cGc0 +…

(3.27)

Subtracting the above two equations, we get:

(

) (

)

(

) (

)

∆G − ∆G 0 =  p GP − G p0 + q Gq − Gq0 +… −  a Ga − Ga0 + b Gb − Gb0 +…

Substituting the value of (G − G0) from Eqs. (3.3) and (3.14):



∆G − ∆G 0 =  p ( RT ln a p ) + q ( RT ln aq ) +… −  a ( RT ln aa ) + b ( RT ln ab ) +…



or ∆G − ∆G 0 = RT ln

aPp. aQq … a Aa. aBb …



or ∆G = ∆G 0 + RT ln

aPp. aQq … a Aa. aBb …

(3.28)

If the reactant and products are in equilibrium with each other, the free energy change ΔG is zero, and hence: ∆G 0 = − RT ln

aPp. aQq … a Aa. aBb …

(3.29)

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 p q  The term  aP . aQ …  is called the thermodynamic equilibrium constant or equiliba b  a A. aB …  rium constant and is denoted as K; hence at equilibrium:

∆G 0 = − RT ln K ,

(3.30)

Thus equilibrium constant of a reaction can be calculated from the knowledge of the standard free energy change of the reaction. The reaction proceeds only if ΔGreaction 0.2 M), although other rare earth metal extraction was enhanced steadily. The co-­extractions of other base metals especially iron (~56%) and other valuable metals such as Cu (~60%) and Ni (~40%) were also reasonably very high at increased H2SO4 concentrations, which can lead to problem downstream in the separation of rare earth metals. Therefore, dilute (0.2 M) H2SO4 of was selected as the suitable condition for leaching of rare earth metals where co-extraction of other base metals (Mn 0.3%, Fe 4.6%, Cu 20.6%, Co 0.2% and Ni 10.4%) is low, though; total rare earth extraction is only ~46%. Unlike the low-grade minerals, other minerals like monazite, bastnaesite, red mud and fly ash are usually leached with HCl and/or HNO3. In addition, majority of the secondary sources such as e-waste, magnet waste and scrap alloys are ideally leached with above mineral acids except the extraction of scandium where NaOH is employed as lixiviant during leaching since the solubility of scandium in alkali phase is equally good other than acid. Though organic acids are also being used for leaching of REEs, their investigation is limited, and the success rate is restricted up to laboratory scale. Thus, the rare earth metal is extracted either with its mixed form or along with other base metal impurities in to the aqueous phase. This urged to develop suitable separation technology for selective separation of REEs from leach liquor in the presence of other REEs and/or base metals. The details of separation and purification process routed through solvent extraction and liquid membrane separation are described in the following section.

4.5  S  olvent Extraction (SX) Method and Its Application in Rare Earth Separation In general, the solvent extraction is a process to separate compounds based on their relative solubility, while two immiscible phases such as aqueous and organic phases are contacted upon each other. In hydrometallurgy separation of metal of interest from the process leach liquor is usually carried out with the help of suitable organic extractant. The overall process is based on the extraction of metal from either of the phase at the appropriate operating condition. Phase transfer of the metal takes place as per Nernst distribution law (discussed in later section). Thus, when a metal-­ bearing aqueous solution is equilibrated with an organic phase (extractant), the metal ion gets extracted into the organic phase. Ideally the metal ions are found in the aqueous phase as hydrated ion; thereby, the metal ion has the least or more tendency to transfer with organic extractant. Therefore, to make the metal ion more extractable in nature, the water of hydration in some is replaced by molecule or ion. This can be attained either by complexing ions with ion of opposite charge to form an ion pair, and as a result, the partial or full replacement of water of hydration around the metal ion in the aqueous phase takes place, or by direct replacement of

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water of hydration with solvent (Parhi 2013). In consequences of this, the target metal to be extracted becomes a hydrophobic species from a hydrophilic species. Nevertheless the above converting approach further helps the transportation metal as sole ion or ion pair with the organic reagent as the extractants employed are of either neutral or ionic (cationic/anionic) through the ion pair association or by substation ion exchange mechanism. Solvent extraction overall process is broadly categorized as (i) extraction and (ii) stripping. Extraction includes the extraction of metal from aqueous or organic phase, whereas stripping is called as back extraction as the metal extracted in the former case is again extracted back from the loaded organic phase to the strip solution. Sometimes an optional stage scrubbing is adopted; it is a process which is basically trailed to recover the trace amount of metal transported during extraction for ensuring the high selectivity of the extraction of target metal. Therefore, it is usually applied before stripping and after the organic phase. The treatment of loaded organic is done with the requisite aqueous solution to remove the unwarranted trace content of impurity. Overall process extraction stages a critical role as a number of factors are responsible for the transport behaviour of metal from aqueous to organic phase. Unlike the base metal extraction, rare earth extraction is of a different kind as all the RREs do have equal extraction tendency owing to their similar chemical and physical properties. The extraction of either of the RRE metals including 4f, 5f and Sc/Y with high selectivity is very rare. Therefore, there is a major challenge for the researchers to achieve a separation of clean and pure rare earth metals from their mixed solutions. Though an ample of studies are being investigated in this domain and a number of technology have been developed, researchers are successful on effective separation of rare earth metals such as Nd, Pr, Dy, Ce and Eu from the secondary sources since these sources contain a number of base metals with some of the above RE elements. On the other hand, while extracting from the primary sources of RRE solution, it is very difficult since those solution contains more than one RE element. Thus to reach out the solution for effective extraction of REEs from primary and secondary sources, the following section discusses details of the basics of solvent extraction process and its adoption on rare earth metal extraction.

4.5.1  S  peciation Chemistry of REEs in Solvent Extraction (SX) and Supported Liquid Membrane (SLM) Process The ionization of complex species is an important feature of tri-positive rare earth, however, any hybrid/orbits from the ions would take part in consulate bonding and the large size of the RE3+ ions. It is possible only for certain types of complexes to be formed. The salient features for complexation of trivalent REE species are: (1) possiblity on attracting the RE cations as a result of their own small size large charge, and chelating abilities will yield complexes, (2) Malabilities of such ­complexes with respect to dissociation will be less than those of the tri-positive transition melted ions, and (3) bonding in all complex species would be predominantly of ionic kind.

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The coordination number is usually in 7, 8, 9 or 10 and only in isolated cases it is 6. Properties that depend directly on the 4f electrons are affected by complexation. The complexes formed by tri-positive rare earth ions include complexes with citric acid, ethylenediaminetetraacetic acid (EDTA) and hydroxyl ethylene diamine triacetic acid (HEDTA). The most important of the chelates are the species derived from the various polyamine polycarboxylic acids. Many of these have been isolated and characterized. These complexes are water soluble. However, the stabilities of the species in solution are of great importance because differences in the stabilities of the complexes have been used in conjunction with ion exchange techniques to effect separation of rare earths from one another (Parhi et al. 2017, 2013a; Huang et al. 2008; Prakorn et al. 2005). The complex formation is pH dependent, and the stabilities of chelates are related to the ionic radii of the rare earth ion. The stabilities of complex species invariably increase from La3+ to Eu3+ or Gd3+, but for cations heavier than gadolinium, the stability may continue to increase, remain nearly constant or pass through a maximum. The tetravalent form of REEs such as cerium (IV) is the only tetrapositive rare earth species that is stable in aqueous solution as well as in solid compounds. The III and IV valence states of cerium are often designated as cerous and ceric, respectively. Ce(IV) is obtained in the solution by treatment of Ce(II) solution with strong oxidizing agents like ozone, peroxydisulphate or bismuthine in nitric acid. Under alkaline conditions oxidation of cerium to +4 states is readily affected by OCI−, H2O2 and O2 (Parhi et al. 2017; Huang et al. 2008). In solutions Ce(IV) can also be obtained by electrolytic oxidation. Cerium (IV) forms phosphates insoluble in 4NHNO3 and iodates insoluble in 6 N HNO3 as well as insoluble oxalate. Ce(IV) is extracted more readily than the RE (III) ions into organic solvents like tri-butyl-­ phosphate; Ce(IV) can be used as an oxidizing agent. The other REE species like Pr(IV) is a powerful oxidizing agent and oxidizes water itself. Therefore, it does not exist in aqueous solution. When Pr(II) salty or oxide is heated in Pr6O11 forms, both Pr(III) and Pr(IV) are present in the oxide.

4.5.2  Principle of Liquid-Liquid Extraction The separation of REE metal(s) by organic extractant is governed by Nernst distribution law. The distribution ratio “D” is given as expressed in Eq. (4.1): D=

M in organic phase [ M ]org = M in aqeous phase [ M ]aq

(4.1)

where “Morg” is present in various differently complexed forms in the aqueous phase and, in the organic phase, [M]aq refers to the concentrations of all “M” species in a given aqueous phase. It is important to distinguish between the distribution constant, KD, which is valid only for a single specified species, and on the other hand the distribution ratio “D”, which may involve sums of species of the kind indicated

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by the index and thus is not constant. The percentage of metal extracted (%E) is dependant in the distribution ratio which is as given in Eq. (4.2). %D =

100 D (1 + D )

(4.2)

In this extraction  process, the REE metal ion-bearing aqueous solution is equilibrated with requisite solvent reagent till to attain the equilibrium. After complete phase disengagement, the aqueous phase was separated and subjected to analysis to analyse the REE metal ion concentration. The concentration of metal present in the organic phase was determined from the difference between its concentration in the aqueous phase before and after extraction. As required, the loaded organic (LO) phase is further stripped with suitable strip solution (acidic/alkaline reagent) and then diluted to requisite times for analysis of meal content in the LO phase. Most of the cases’ phase ratio (A/O) is kept fixed to unity, and experimental studies are ideally performed at ambient condition.

4.5.3  Supported Liquid Membrane Approach The supported liquid membranes (SLMs) consist of hydrophobic membrane supported with a solvent which is contained within the pores of the polymeric solid. In this membrane the solvent and metal ion species from a feed solution are dissolved selectively by combining with an active carrier to form a complex. This complex generally diffuses through the membrane and is then stripped at the other side of the membrane. This carrier-mediated transport of metal is mainly controlled by diffusion process. As per the literature survey till date (Parhi et al. 2012; Parhi and Sarangi 2008), the supported liquid membranes have been proposed as effective methods for the selective separation metal ions using the noble extractants as carrier, though; its application especially in industrial use is limited owing to relative instability and short lifetime. In recent development, the use of ionic liquids as a liquid membrane phase could overcome such issues due to their negligible vapour pressure and the possibility of minimizing their solubility in the surrounding phases by proper selection of the cation and anion. The following section describes the theory, principle and possible usage of SLM using commercial solvents as well as ionic liquids for transportation behaviour of rare earth metals.

4.5.4  Theory and Principle of SLM Approach Membrane separation process is a new method in separation science domain. It has a number of applications especially in the downstream stage, and the number of such applications is still growing. Unlike other membrane methods (microfiltration,

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ultrafiltration, reverse osmosis, electrodialysis, gas separation, evaporation), liquid membrane technology in the twenty-first century gains lots of attention in metallurgical separation process. The major use of LM was reported to be especially on rare earth separation owing to having a number of advantages than the other methods. The liquid membrane is usually supported with a carrier which certainly plays the crucial role for effective and selective extraction of target metal from complex mixtures. In SLM process the membrane is interposed between two aqueous phases which act as a barrier with either of the phase. One of the phases is termed as feed and other one is termed as strip. The carrier being supported over the membrane phase is of organic phase where functionalized extractants are used looking upon the target metal to be extracted from the feed to strip phase. Ideally the separation is achieved as the liquid membrane has the ability to transport one component from the feed phase more readily than other component. The extraction of metal is mainly dependent on the driving force acting on individual components of the feed phase due to the dynamics applied during the mass transport process. The performance of the given membrane is determined by two key parameters, namely, selectivity and the flow through the membrane. The flow behaviour in other words is expressed as the flux or permeation rate as the volume of solution flows through the membrane per unit area and time. Moreover the flux is called as volume flux which can readily be converted to mass flux based on the molecular weight of respective metal. Fick’s law flux (Parhi et al. 2009) equation is in general expressed as given in Eq. (4.3). The details of the parameters associated in the equation are presented in the following section.

4.5.5  Transport Behaviour of Metals Through SLM The mass transfer of RE metal from feed phase to strip phase is explained by the diffusion model (Lozano-Blanco et al. 2011). The driving force is mainly due to the concentration gradient existing between the feed and a strip phase; accordingly, the flux of the metal ion species is determined by Fick’s first law of diffusion. Since during experimentation both feed and strip compartments are continuously stirred, the transport process of the metal from the feed phase to the strip phase does follow the five consecutive steps. 1. Transport of metal from the bulk of the feed solution to the feed-membrane interface. 2. Partition of the substrates between the feed phases (Cf) and the IL immobilized in the organic membrane (Cfi). 3. Diffusion of metal across the liquid membrane to the interface of membrane phase under the action of the concentration gradient. 4. Partition of the substrates between the IL immobilized in the organic membrane (Cri) and the strip phase (Cr). 5. Transport of metal from the membrane interface phase to the bulk strip phase.

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Fig. 4.1 Schematic diagram of HFLM separation for separation of REE (Parhi et al. 2019)

Assuming no accumulation of the metal in the liquid membrane phase, permeation of metal is explained based on Eq. (4.3):

J = P ( Cf − Cr )

(4.3)

where J is the mass flux of solute and P is the permeability of the membrane. Based on the solution-diffusion model, the permeability of a specific compound can be expressed in terms of the partition coefficient of this compound between the liquid membrane phase (ionic liquid) and the feed/strip phase, the diffusion coefficient and the thickness of the membrane as indicated in Eq. (4.4):

P = KD (4.4)

This relationship is between the permeability (P) values of several compounds and the partition coefficient of these respective compounds. The transport behaviour of REEs from feed to strip phase through liquid membrane phase is shown in Fig. 4.1. The REE metal, namely, scandium has been examined using noble ionic liquids such as R4ND as carrier supported over the hollow fibre liquid membrane module.

4.5.6  Extraction Mechanism of REEs in SX and SLM The extractants are of different type based on the functionality, and accordingly, the complexation of extractants with REEs can be of cationic, anionic, chelating and ion pair type, though majority of REE extraction is of cation exchange phenomenon as most of REEs exist in tri-/tetrapositive cationic form in aqueous phase.

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A cation exchanger kind of extractant can be in the form of carboxylic acids, organophosphorus acids and glycol amide acid. Generally these extractants have high efficiency in extraction compared to other classes. Such extractants from cation exchanger do exist in high acidic form (Xie et al. 2014). Usually better REE extraction is observed 99.99% (Tian et al. 2013). According to the scheme, monazite concentrate is first digested with NaOH. After filtration, REM remained in the solid residue as hydroxides are dissolved in HCl or HNO3 solutions. After purification, obtained solutions are subjected to solvent extraction to produce individual rare earth oxides. Chloride media are used to produce a mixture of REM compounds, such as dehydrated rare earth chlorides, which were used to produce misch metal. Nitrate media are used to obtain individual REM oxides. For example, in the first stage of separation, lanthanum (99.99% La2O3) remains in the aqueous phase while a mixture of Ce, Pr, Nd, Sm, etc. loads into the organic phase. The extraction process for simultaneous production of a mixed Ce-Pr-Nd product (for magnet production) and pure Nd oxide was developed in the work (McGill 1997). Rare earth chloride solution generated by leaching oxide with HCl was treated by solvent extraction with P507 in kerosene. In the first solvent extraction circuit, Sm and all heavier rare earths along with Y were loaded into the organic phase. The raffinate containing praseodymium and lighter rare earths were fed in the second solvent extraction circuit with P507 in kerosene. Pr and Nd and part of Ce were partially extracted into the organic phase, and lanthanum completely remained in the aqueous phase from which marketable lanthanum products were produced. Organic phase materials produced in the second extraction circuit was treated by selective stripping to produce high-purity neodymium oxide and a mixture of Ce, Pr and Nd oxides. The authors of the work (Doyle et al. 2000) used a synergistic extraction system to produce different REM-containing products from rare earth sulphate and chloride pregnant leach solutions that result from bastnaesite concentrates. In the work (Huang et al. 2008), a method for the solvent extraction of lanthanum from chloride solutions containing La, Pr and Nd was developed using

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cascade mini-battery unit of mixers and settlers. The parameters of the process depended on extractant (D2EHPA or P507) concentrations, duration of the contacts between the phases, acidity of aqueous phase and other factors. It was shown that the set-up for SX study consists of 22 stages: eight for REM sulphate feed solution, eight for scrubbing and six for stripping. Mixed extractants Multistage mixers settlers was introduced for separation of REEs from aqueous scrub (H2SO4 or HCl) Heavy rare earths (heavier than Gd) La La, Ce, Pr, Nd Sm, Eu, Gd Raw material REM concentrates subgroups separation Pure REM oxides production Oxide Gd Oxide Dy Oxide Er, La, Ce, Nd, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Y, Tm, Yb, Lu with 99.9–99.99% extraction, eight for scrubbing, and six for stripping on yielding high-grade lanthanum oxide (>99.9% La2O3). In a study the selective separation of yttrium in the presence of the base metal like copper using primary aliphatic ammine is attained because of a unique complexation mechanism of rare earth sulphate ion with the Primene JMT. The complexation mechanism is described in aqueous solution; yttrium and the REEs interact with sulphate ion to form three known complexes, MeSO4+, Me(SO4)2− and Me(SO4)33−. The distribution of species depends distinctly upon the sulphate concentration. Total sulphate concentration in copper PLS typically ranges from about 0.3 M to 1.3 M SO42− (de Morais and Ciminelli 2004). At these concentrations, the system would contain predominately Me(SO4)2− and Me(SO4)33−. The first step in the extraction process involves the protonation of the amine as shown in Eq. (4.4). Protonation of the amine with sulphuric acid produces sulphates or bisulphates (Nekovar and Schrotterova 2000). At low pH values the bisulphate ion is the dominant anionic sulphate species (Ritcey and Ashbrook 1984). Moreover from the study, it was assumed that at low Primene JMT concentrations, Me(SO4)33− is the dominant species being extracted. As extractant concentrations increases, the reaction appears to favour of the extraction of Me(SO4)2−. Unlike solvent extraction process and FSSLM process, the HFSLMs get noticeable attraction in recent days owing to its higher surface area per unit of module volume and highly integrated efficiency (Parhi et  al. 2019). HFSLMs gain the importance for its efficient transport ability and separation behaviour with high interfacial area per volume and easy assembly module. Though earlier there was a limited application of HFSLM especially in seawater desalination and medical fields, the hydrophobic liquid membrane (impregnated functionalized organic solvent extractant) is of more demand for clean and selective separation of metals from numerous aqueous solutions. The transportation study of Nd3+ and Sm3+ using saponified P507 was investigated (Zhang et al. 2001, 2002). Wannachod and his co-­ workers had successfully achieved the clean separation of Pr3+ and Nd3+ from mixed RE ions through a single-stage HFSLM system (Wannachod et  al. 2011, 2014a, 2015a, b). After testing the performances of organophosphorus reagents such as D2EHPA, PC88A, Cyanex272 and TOPO, a comprehensive study accomplishing the development of mathematical model on the conservation of mass convection, diffusion, reaction and accumulation has been investigated in order to understand the transport of REE metals through HFSLM (Wannachod et al. 2014b). The mutual separation of trivalent as well as tetravalent REEs from sulphuric acid medium was examined in HFSLM (Ramakul et al. 2012). From this study the mathematical

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prediction model results showed that the interface layer thickness will become thinner with a higher flux due to increasing mass transfer coefficient of metals. The selective separation of Pr3+, Nd3+ and Dy3+ with HFSLM module from scrap magnet liquor was resulted even in the presence of non-REEs (Parhi et al. 2013a), followed by the recovery of corresponding REE oxides. Some improved combined SLM (Parhi et  al. 2013a; Wodzki and Sionkowski 1995) (hybrid liquid membrane, HLM (Kislik and Eyal 1996)) kind was used for separation of metals, and it was found to be much stable in extracting REEs than the SLM, where there are two supporting porous membranes that were fixed to separate the organic phase from feed and stripping phases. The membrane used in these cases is either hydrophobic/hydrophilic or ion exchange, so that the micropores in the membrane phase will be filled with either aqueous or organic solution. The objective of utilizing such kind is to support a stable organic solution in the module. The despersed SLM (Pei et al. 2012) or dispersion flat combined liquid membrane (Pei et al. 2014) are used for REEs separation study of metals, where the promising separation of middle and heavy REEs, such as Eu3+, Dy3+ and Tm3+, was successfully attained (Pei et al. 2011b, 2009a, b). The dispersion phase is composed of both membrane phase (carriers in organic solutions) and stripping solution. Concentrated stripping solution with target REEs will be easily obtained after suspending and still standing of the dispersion phase. There have been several research studies for separation of REEs through solvent extraction and HFSLM process using commercial reagents as the extractants. The numbers of commercial reagents used for successful separation of REEs are summarized in Table 4.1. However, the loading ability and selective affinities of these solvent reagents are very low. Therefore, to improve the extraction efficiency and to obtain clean separation of targeted REE metal, the noble green reagent, e.g. ionic Table 4.1  Commercial reagents in REE separation studies Sr. no. Extractant 1 D2EHAG, DODGAA, Versatic 10 2 3

Cyanex 272 D2EHPA

4 5 6 7 8 9

PC88A DNPPA D2EHPA [P444C1COOH]Cl Cyanex 302 Tri-iso-amyl phosphate (TIAP)

10 11 12

Metal(s) Sc, Y, La, Nd, Eu and Dy Pr(III) Nd(III)

Nd(III) Nd(III) La(III) and Ce(III) Sc Sc, Y, La and Gd Sc, Zr, Ce,Nd, Sm, Eu, Y, Lu D2EHPA, Cyanex 272, Ionquest 801, Sc(III) Versatic 10, Primene JMT, LIX54 D2EHPA and Cyanex 272 Sc(III) D2EHPA Nd(III)

Author(s)/year, references Baba et al. (2014) Wannachod et al. (2011) Wannachod et al. (2014b) Wannachod et al. (2015a) Wannachod et al. (2015b) Ramakul et al. (2012) Depuydt et al. (2015) Wu et al. (2007) Kostikova et al. (2005) Wang et al. (2013) Das et al. (2018) Pei et al. (2012)

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liquids, is being developed. The following section describes the successful usage of ILs in rare earth metal extraction both by solvent extraction and supported liquid membrane processes.

4.6.2  Separation of REEs Using Ionic Liquids (ILs) Ionic liquids (ILs) used in metal extraction process is of molten organic salt kind having a wide temperature range of application with multifold loading ability than that of reported commercial solvent reagents. In the view of development of environmentally friendly processes in separation chemistry of metals, the use of ILs is being driven with utmost attraction to the researcher especially in rare earth metal extraction system because of their number of advantages (Behera et al. 2019; Hidayah and Abidin 2018; Tao and Huiqing 2009; Taggart et al. 2018; Peelman et al. 2016; Parhi et al. 2016). In addition to liquid-liquid extraction SX process, the adaptation of ILs as chelating or leaching agents has been noticed for mineral beneficiation and mineral processing prospective (Parhi et al. 2019; Das et al. 2018; Radhika et al. 2010; Xie et al. 2014; Ozevci et al. 2018). The flexibility stemming of ILs resides in the ability to tailor in almost an unlimited manner their constitutive cation and anion moieties to achieve the desired physicochemical properties promoting their affinity towards REE cation species (Behera et al. 2019; Ghosh et al. 2015; Lokshin et al. 2002; Resende and Morais 2010). Amongst the ILs, bifunctional IL extractants (Bif-ILEs) have shown to outperform many commercial extractant reagents (D2EHPA, Cyanex 272, LIX Series, TOPO, TBP) used in SX process as both cationic and anionic moieties strongly contribute for complexation with REE metals (Behera et al. 2019). Such improved performances have been ascribed to synergistic interactions from both cationic and anionic moieties of Bif-­ILEs in REE extraction (Behera and Parhi 2016). The ability of complexation of ILs with REE metal mainly depends on its nature of IL functional groups. The length of the alkyl chain and structure of both cationic and anionic moieties have also an influence on complexation performances (Behera et al. 2019; Park et al. 2012; Parhi et al. 2013a). The extraction behaviours of ILs towards the heavy and REEs are different. Many numbers of bifunctional phosphonium-based ionic liquids (ILs) are introduced for extraction of earth elements (REE) based on density functional theory (DFT) simulations. From this study the affinity for REE extraction is predicted to be increased with increase in atomic number of rare earth element and is well supported with previous studies (Behera et al. 2019; Hidayah and Abidin 2018; Park et al. 2012; Parhi et al. 2013b, 2015). Thus it was evident that the heavier REEs are often prone to form complex with IL than the lighter REEs. The other extraction behaviour of REEs in the presence of counter anion like chloride, sulphate and nitrate showed profound effect on overall extraction of ILs. From the results extraction of REE follows the order REE nitrate species > REE chloride species > REE sulphate species. The most common ILs include imidazolium ([Cnmim] +), pyridinium ([Cnpy]+) and pyrrolidinium ([Cnmpip]+) and are exclusively used for separation of REE. The corresponding anionic species of these ILs are mainly of (tri-fluoromethyl)sulfonyl)

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imide ([NTfn]−), chloride ([Cl]−) and fluorinate ([PFn]−). These ILs can be tuned not only with cation/anion parts but also the alkyl chains attached to it, and that leads to the significant modification for making IL more suitable for metal extraction (Dietz 2006; Domanska 2008; Park et al. 2014). In other way, IL can be customized to create either a hydrophobic or hydrophilic solvent by controlling the structure nature of cation and anion (Durga et al. 2013). For example, the species like NTf2− can reduce the miscibility of TSIL solvent and Cl− enhance the hydrophilicity of solvent phase (Cocalia et al. 2006). However, hydrophobic characteristic of IL is highly preferred in the extraction of REE due to its strong ability on extraction of metal ions from aqueous phase (Lee 2012). The imidazolium [Cnmim]-based extractant shows to have good stability in both oxidative and reductive conditions for selective separation of individual metal ions. It has flexibility in design and can be used in different kinds of metal separation including REE (Durga et al. 2013). Alkyl group of Cnmim is extendable, and the longer the chains the better the hydrophobicity and viscosity (Rout et al. 2012; Aslanov 2011). As per Huddleston et al. 1998, butylmethylimidazolium hexafluorophosphate, [C4mim][PF6], is a green solvent which is more suitable for REE extraction. Membrane based non-dispersive solvent extraction (NDSX) approach was applied for the extraction of Nd(III) from nitrate media and was observed that NDSX was more efficient for the recovery of Nd than that of  HFSLM, Also  the reproducibility of the result was checked (Patil et al. 2011), and after five repeated runs, Nd3+ extraction rate remains unchanged ensuring the good reproducibility of extractant as well as membrane module for effective separation of neodymium. Kumar et al. (2005) have investigated the mass transfer of integrated HFLM process and single unit HFLM module. In consequences, single HFLM module showed weakened performance. SDHFLM process was developed by Pei et  al. (2011a) where technology for quantitative separation of Sm3+ or Dy3+ by P204 and P507, respectively, was achieved. This assures suitable adoption of SD HFLM, while supporting IL as carrier and avoiding the common disadvantages in stability, such as carrier leakage and dissolution loss of extractant during extortion of REEs. The numbers of ILs used in REE separation process are presented in Table 4.2, and the successful separation of either the rare earth metals is achieved, while employing ILs both in SX and SLM methods.

4.6.3  Separation of REEs Using Synergist Extractants In the view of several disadvantages of IL as well as conventional solvent extractants, the attempts are being made for combining the solvents in order to investigate synergist effect on extraction behaviour of metals. The synergist mixture of [A336] and [P204] (Sun et al. 2010), D2EHiBA and [Cnmim][NTf2] (Pathak et al. 2015) and TODGA and [C4mim][NTf2] (Sengupta et al. 2014) shows greater potentials in extraction of REE from aqueous solution. The use of imidazolium has become more common in synergistic extractant (Nakashima et  al. 2003). Generally two types of SE, namely, inner synergistic and physical synergistic SE are considered in

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Table 4.2  Ionic liquids (ILs) in REE separation studies Sr. no. Extractant 1 Triethyl-pentyl-phosphonium bis(trifluoromethyl-­sulfonyl)amide ([P2225] [TFSA]) 2 Methyltrioctylammonium sec-octylphenoxy acetate ([N1888][SOPAA]) and trihexyl(tetradecyl)phosphonium bis(2,4,4trimethylpentyl)phosphinate ([P66614] [BTMPP]), and Cyphos IL 104 3 (Bif-ILEs) tricaprylmethylammonium sec-octylphenoxy acetic acid ([A366] [CA-12]) and tricaprylmethylammonium sec-nonylphenoxy acetic acid ([A336] [CA-100]) 4 Betainium bis(trifluoromethylsulfonyl)imide [Hbet][Tf2N] 5

6 7 8 9 10 11

Trihexyl(tetradecyl)phosphonium benzoate [T66614][BA] and trihexyl(tetradecyl) phosphonium bis(trifluoromethylsulfonyl) imide [T66614][TFSA] [(CH2)nCOOHmim][Tf2N] (n = 3, 5, 7) HNO3-Cyphos IL 104 Betainium bis(trifluoromethylsulfonyl)imide [Hbet][Tf2N] R4ND R4ND, R4NCy Trioctylmethylammonium dioctyl diglycolamate [A336] [DGA]

Metal(s) Pr, Nd and Dy

Lu, Y, Yb, Tm, Er, Ho

Author(s)/year, references Matsumiya et al. (2014) Ma et al. (2017)

Ce, Pr, Nd, Sm, Wang et al. (2011) Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y Onghena and Binnemans (2015)

Sc, Y, La, Ce, Nd, Dy, Fe, Al, Ti, Ca and Na Nd

Panigrahi et al. (2016)

Sc, Y, La Y(III) Sc, Al, Fe

Chen et al. (2017) Liu et al. (2010) Onghena et al. (2017)

Sc(III) Nd and Pr

Parhi et al. (2018) Padhan and Sarangi (2017) Rout and Binnemans (2014)

Nd(III)

SX processes. The inner SE is developed by merging the cation and anion elements, resulting in an advanced extractant (Sun et al. 2010). On the other hand, physical synergistic SE is simply a physical mixing of any two solvents. Synergistic and combination effects on advanced extractant are expected to give better efficiency on the extraction and selectivity with usage of reduced amount of primary solvent. Apart from increasing the extraction efficiency, SE shows the capability to be recycled, thus reducing the consumption of overall reagents used in the processes. The main objective of SE recycling is to recover metal ions from the extractant and reuse the extractant without losing the property of organic solvent. Synergistic extractant of CMPO in [Bmim][PF6] could be recycled up to four folds using the same procedures without losing the nature of the extractant (Nakashima et al. 2005). The combination of DODGAA and [C4mim][NTF2] also exhibited no declination even after three times of usage on fresh stock of aqueous solution (Kubota et al. 2011). These studies assure the successful adoption of SE, and in consequences,

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Fig. 4.4  Schematic flow diagram for separation of REEs by SX and SLM process

less consumption of primary extractant was noticed. In addition, the ability to achieve high separation efficiency and regeneration of the extractant has been achieved. To summarize SE systems (using solvent reagents and ionic liquid) were employed for successful separation of REEs. The conceptual flow diagram for separation and recovery of REEs from numerous primary and secondary sources is proposed based on Fig. 4.4.

4.7  Concluding Remarks and Futuristic Prospective The rare earth metals are of profound interest at present era owing to their economic values and various application prospective. Limited content of REEs in natural sources allured the metallurgist to recover REEs from several secondary sources. Majority of technologies are based on integrated hydrometallurgical technology routed through leaching—solvent extraction/supported liquid membrane separation-­ precipitation. Leaching is commonly applied to dissolve REEs into the respective liquor. The real challenge encountered is of the separation in downstream process.

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The solvent extraction and supported liquid membrane methodologies are exclusively adopted in hydrometallurgical downstream rare earth extraction process. The former one is a well-proven and well-established approach, whereas the latter one has wider futuristic scope as the research studies in this domain is limited. The major advantage of liquid membrane process ascertains the usage of noble reagents like ionic liquids (ILs) as the carrier for selective and effective separation of REEs from numerous leach as well as synthetic solutions. The synergistic system both in SX and SLM method has drawn considerable attention owing to having several advantages, and as a result, it has successfully been applied in separation of REEs. The commercial reagents such as D2EHPA, PC88A, Cyanex 272, TOPO and TBP are successfully used for extraction of rare earth metals including Ce, La, Nd, Pr, Dy, Sm, Eu and others. However, the ILs have shown promising loading ability along with selectivity towards selective separation of either REEs from respective mixed solutions. The principle and chemical extraction mechanism of REEs with organic extractants both in SX and SLM are quite similar. The association of extractant and cationic/anionic species associated in organic phase during extraction is ascertained by slope analysis method. The equilibrium study thus ensures the overall extraction process of extractant for separation of REEs. The kind of association due to extractant (commercial/ Ionic-regents) and targeted metal is well described in this chapter. The extraction/stripping isotherm (McCabe-Thiele plot) especially in SX process predicts the number of stages required on quantitative extraction/ stripping of REEs. On the other hand, the phase ratio (A/O) variation results ensure the effective enrichment of REEs. Overall enrichment factor helps to obtain enriched concentration of REEs in the stripped solution for substantial recovery of rare earth oxide by precipitation method. The influence of pH/acidity/alkalinity and extractant(s) is shown to be significant on overall REE extraction for SX/SLM processes, though other factors such as temperature, diluents, salt and stripping reagent appear to have a partial affect in it. This book chapter mainly attempted to reach out the exclusive application of SX and SLM approach in REE extraction process accomplishing details of extraction principle, mechanism, process optimization and futuristic scopes of investigations. Acknowledgements  Author Pankaj Kumar Parhi would like to thank and acknowledge SERB, DST, Government of India, and DST-Inspire Programme for awarding Young Scientist (CS-­ 076/2014) and Inspire Faculty (IFA12-CH-26) under Start-Up Research Grant and Inspire Faculty Award scheme, respectively. Co-author Saroj Sekhar Behera wishes to acknowledge DST, Government of India, for partial funding of this work.

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Chapter 5

Recent Strategies on Adsorptive Removal of Precious Metals and Rare Earths Using Low-Cost Natural Adsorbents Janardhan Reddy Koduru, Lakshmi Prasanna Lingamdinne, Suresh Kumar Kailasa, Thriveni Thenepalli, Yoon-Young Chang, and Jae-Kyu Yang

5.1  Adsorption and Its Basic Principles 5.1.1  Adsorption Adsorption is a surface phenomenon and is the process in which substance is extracted from one phase and accumulated at an interface of the surface of a second phase (interface accumulation). Adsorption is a surface phenomenon as opposed to absorption where matter changes the solution phase, e.g., gas transfer. That means adsorption is hoarding of molecules on a solid surface at the interaction of phases. Absorption is not a surface phenomenon; it is dissolution of ions from the bulk phase to the material or within a phase, e.g., within an organic phase in contact with an air or a water phase. The following schematic representation (Fig. 5.1) demonstrates the adsorption and absorption phenomena. Here, the material, which is adsorbing the substance on its surface, is called as an adsorbent, for example, activated carbon or ion-exchange resin often used as adsorbents. The material, which molecules adsorbed on the surface of the adsorbent (i.e., solid, liquid, gas, or solute), is known as the adsorbate. Hence, the phenomenon of adsorbing of adsorbate molecules on an adjacent solid surface of the adsorbent is called adsorption. It is a well-developed separation technique for wastewater

J. R. Koduru (*) · L. P. Lingamdinne · Y.-Y. Chang · J.-K. Yang Department of Environmental Engineering, Kwangwoon University, Seoul, Republic of Korea S. K. Kailasa Department of Applied Chemistry, S. V. National Institute of Technology, Surat, Gujarat, India T. Thenepalli Mineral Resources Division, Center for Carbon Mineralization, Korea Institute of Geosciences and Mineral Resources (KIGAM), Daejeon, South Korea © Springer Nature Switzerland AG 2020 R. K. Jyothi (ed.), Rare-Earth Metal Recovery for Green Technologies, https://doi.org/10.1007/978-3-030-38106-6_5

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Fig. 5.1  The schematic representation of the difference in absorption and adsorption

treatment at domestic and industrial effluents. In general, the activated carbons are used as adsorbents for water treatment. The opposite or backward process of adsorption is nothing but called as desorption. If the adsorption and desorption are in equilibrium, it can be represented as A + B ⇌ Aads, where “A” is the adsorbate and “B” is a vacant site on the adsorbent surface. Activated carbon is widely used for adsorption of organic substances and nonpolar adsorbates, and it is used for waste gas and wastewater treatment, since most of its chemical (e.g., surface groups) and physical properties (e.g., pore size distribution and surface area) can tune according to what is needed (https://en.wikipedia. org/wiki/Adsorption). Its usefulness also derives from its large microporous (and sometimes mesoporous) volume and the resulting high surface area.

5.1.2  Adsorption Principles The basic principles or requirements for the occurrence of the adsorption process are the following:

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• The adsorbent or contaminant dislike of water phase—“hydrophobicity” or low solubility. • The contaminants or adsorbate attraction to the adsorbent surface by: –– Van der Waals forces: physical attraction –– Electrostatic forces (surface charge interaction) –– Chemical effects (e.g., π and hydrogen bonding) • A combination of the above two. • The physical and chemical process in which a substance is accumulated at an interface between phases. • Attraction force—adsorbate has lower free energy at the surface of the adsorbent than in the solution. Hence, adsorbate is attracted to the adsorbent from a solution by a physical or chemical mechanism. • Electrostatic attraction—adsorbate is attracted to the adsorbent when surfaces are oppositely charged. It can promote the adsorption of specific ions on the surface of the particles.

5.1.3  Types of Adsorptions The adsorption process can be classified into two types based on the nature of interactions existing between adsorbate and adsorbent. That means as said in the adsorption principles, there is survival of various kinds of forces of interactions between adsorbent surface and adsorbate molecules. Based on the type of strength of interaction living, the adsorption process is classified into physical sorption and chemical sorption. Physical adsorption (physisorption): When the weak forces of van der Waals (dipole) interactions exist between adsorbent and adsorbate molecules, the adsorption process is said to be physical sorption, for example, the hoarding of gas molecules onto the surface of the adsorbent through the dipole or van der Waals forces. • Physical sorption is rapid, and the efficiency is proportional to the specific surface area of the adsorbent. • The adsorption of adsorbent on the adsorbent surface can be carried out by monolayer or multilayer. In general, it takes place with the formation of the multilayer on the surface of the adsorbent. • It is an exothermic process and liberates low enthalpy (0.4 eV). Hence, the high temperature or increase in the heat of a system is favorable for the chemisorption. • It is frequently irreversible. On desorption, the chemical nature of the original adsorbate will have changed in some cases. • Only a monolayer of adsorbate molecules occurs on the adsorbing medium (adsorbent). • Covalent/metallic/ionic states are involved, and the mechanism involves dissociation of molecules. • The surface symmetry is specific. • It is highly dependent on solution pH and ionic strength. The graphical representation of physical and chemical adsorption process is shown in Fig. 5.2.

5.2  Factors Influencing Adsorption Many factors influence adsorption on an adsorbent including the surface area of adsorbent, nature of the adsorbate, hydrogen ion concentration (pH) of the solution, surface charge and zero points of charge (ZPC) of adsorbent, and temperature. Surface area: Adsorption efficacy is directly proportional to the specific surface area. The total surface area available for the particular volume of adsorbent is defined as a specific area. Thus, the fine or nano-size adsorbent shows larger adsorption capacity than that of bulk material.

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Nature of the adsorbate: In general, the lower solubility of the solvent leads to the increase of the adsorption efficacy. That means the higher solubility of water increases the strength of the solute-solvent bond that may lead to lower adsorption efficacy. Hence, the solubility of the adsorbent or solute influences the adsorption equilibrium. Hydrogen ion concentration: As we know, the H+ concentration is nothing but called the pH of an aqueous solution. The pH of a solution is an essential factor that significantly influences the adsorption process. For example, most of the colored water from industry contains negatively charged ions that can lead to greater decolorization with carbon materials at higher acidic conditions where the surface of carbon is positive. Moreover, the pH of the adsorbent is also a critical factor that may influence the pH of the liquid. Surface charge and pHZPC: It is a fact that the solution’s pH generally affects the surface charges of the adsorbent and adsorbate. For example, as pH goes down, adsorption efficiency increases for organic material. The surface charge of adsorbent mainly depends on the solution pH and pHZPC. The pH that corresponds to an adsorbent surface charge of zero is called a pH of zero point charge (pHZPC). The surface charge is negative at solution pH > pHZPC and positive at pH  Au(CN)2−>Au[CS(NH2)]2+>Au2(S2O3)23− (Adani et al. 2005; Can et al. 2016; Gallagher et al. 1990), while Ag (as SCN or CN) is in the order Ag( CN)2>AgSCN>Ag[CS(NH2)]2+>CH3COOAg+>Ag(NH3)2+>AgNO3>Ag2SO4>Ag(S 3− as reported by Adani et al. (2005). Yu et al. (2018) reported chemically modi2O3)2 fied activated carbon for adsorption removal of Au. The experimental results conclude that the modification of activated carbon with silver ferrocyanide enhances its adsorption affinity with 3.55 kgt−1 at pH  9.0 followed by diffusion kinetics and heterogeneous Freundlich isotherm (Yu et al. 2018). 5.5.3.2  Inorganic Metal Oxides or Magnetic Metal Oxides The use of magnetic materials in adsorptive applications has enhanced its efficiency in the recovery of PGMs from water solutions. Moreover, the adsorbed material can be easily recovered by an external magnetic field, which results in overcome of the limitation associated with conventional separation techniques. That means that the metal-loaded magnetic materials can be easily isolated by an external magnetic field and recovered using suitable desorbing agents (HCl, thiourea, HNO3, and H2SO4) and reused. Moreover, the magnetic adsorbent can reduce the operational cost and time and minimize the use of further additional isolation steps (Ebrahimzadeh et al. 2010). Homchuen et al. (2016) (Homchuen et al. 2016) reported the potential of magnetite materials for selective adsorptive removal of Pt, Pd, and Rh from acidic chloride solutions. They found that they significantly affect the adsorption removal of the PGMs with solution pH. At pH 6–7, the Pt and Pd show a high adsorption removal, while Rh shows at pH 3–4. Moreover, they found very slow kinetics. As a result, the adsorption mechanism may involve both physical and chemical sorption. Alorro and his colleagues reported various magnetite materials (synthetic and natural) for adsorptive extraction of PGMs (Alorro et  al. 2010). They observed the

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enhanced adsorption affinity and selectivity of synthetic magnetite material for Au, Pt, and Pd. However, the natural magnetite has showed higher adsorption capacity than the synthetic magnetite. They also observed an interesting phenomenon that the uptake of Au increased with increasing time, even more than 24 h for both natural and synthetic magnetite materials. This common phenomenon is not common in regular physical sorption. Moreover, increasing chloride concentration decreased the uptake amount of Au on both magnetite materials studied. To understand the gold complex or mechanism on the surface of magnetite materials, Alorro et  al. (2015) have conducted an electrochemical experiment to explain the adsorption mechanism of Au on magnetic material. The suggested mechanism follows four steps, i.e., (i) AuCl4− ion transport towards the surface of the magnetite from the bulk solution; (ii) then adsorption of AuCl4− ions on the surface of magnetite material by electrostatic interaction; (iii) followed by reduction of gold chloride complex to a metallic Au by electrochemical reduction; and (iv) finally the soluble species transport to the bulk solution phase (Ebrahimzadeh et al. 2010). The magnetic materials’ zeta potential (which is near to neutral pH) that was studied theoretically supported the above mechanism. At acidic pH, the magnetite surface is positive that leads to electrostatic adsorptive removal of gold chloride complexes. However, in higher acidic pH, the dissolution of Fe3+ may increase in the solution. As a result, the dissolved Fe3+ may compete with Au ions for active sites on the surface of the magnetic adsorbent, while in alkaline pH, the surface charge of magnetic materials is negative that leads to electrostatic repulsion between Au and magnetic materials causing decrease in the adsorption of Au. From the above observations, it can be concluded that the best optimum pH range for adsorptive removal of PGMs on magnetite materials is around pH 6–7. Recently superparamagnetic nanoparticles (MNPs), generally indicated as MFe2O4 (M  =  Fe, Co, Cu, Mn, etc.), included magnetite (Fe3O4), maghemite (γ-Fe2O3), and cobalt oxide (Co3O4), having great heed to the adsorptive recovery of metals and PGMs from wastewater (Kraus et al. 2009). The application of MNPs is associated with their superparamagnetic nature along with their unique properties such as high surface area and porosity with excellent dispersion having more significant adsorption efficiency (Giakisikli and Anthemidis 2013). The use of MNPs can reduce the time and required amount of adsorbent with the high ability for metal recovery (Płotka-Wasylka et al. 2016). Moreover, the decrease of adsorbent size to nano-size can increase the number of active sites on the surface of adsorbent that offers high adsorption capacity with a small mass of adsorbent (Mohammed et al. 2017). Moreover, the nano-MNPs have high adsorption capacities than their granule size and natural minerals. For example, Uheida et al. (2006) reported 10 nm Fe3O4 for the adsorptive removal of PGMs from their acidic chloride solutions showing adsorption capacities of 0.103, 0.149, and 0.068 mmol/g, respectively, for Pd(II), Rh(III), and Pt(IV). Besides, it shows fast adsorption kinetics that is around 20 min and more stability with easy recovery of PGMS by using simple eluents including a low concentration of HNO3 (0.5 M) to desorption of Pd(II), Rh(III), and Pt(IV) ions, respectively. However, NaHSO3 (1  M) and NaClO4 (0.5  M) are active desorbing agents for Rh(III) and Pt(IV), respectively. They also found the strong affinity of

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nanomagnetic material for Rh(III) other than Pd(II) and Pt(IV). Moreover, the nano-­ MNPs can be used for adsorptive removal of PGMS even in alkaline pH (10–12) with high adsorption capacities. Mohammadi et al. (2011) (Mohammadi et al. 2011) found the high adsorptive removal capacity, 27.2 and 31.8 mg/g, respectively, for Pd(II) and Rh(III) from its dust samples and Pt-Ir alloys that are at alkaline pH (10–12) by using MNPs. 5.5.3.3  Biosorbents Activated carbons (ACs) have high adsorption efficiency and are the often-used adsorbents on a large scale for the adsorptive removal of PGMs. However, due to the complexity and cost of manufacturing and reactivation process of ACs (Das 2010; Mohan et al. 2014; Perez et al. 2019), the researchers have greater interest to search for alternative adsorbents that offer high efficiency and have more availability with cost-effective biosorbents. Biomaterials including crop residues, sludge, fly ash, microbial/plant/fungi biomass, and derived materials have been tested for adsorptive removal of PGMs (Perez et al. 2019). However, some of the raw biomaterials have achieved less adsorptive removal efficiency than AC (Won et al. 2014). Also, some of the biosorbent applications have been limited due to their low stability and mechanical resistance (Volesky 2001). However, further modified biomaterials could achieve higher stability, higher adsorption efficiency, and excellent selectivity with functional recovery from the aqueous solutions (Perez et al. 2019; Won et al. 2014). The modification of biomaterials is generally carried out through a physical, chemical, or biological way or immobilization of it on to supporter (Fomina and Gadd 2014; Park et al. 2010a). In the literature, there are many reports on raw biomaterials and modified biomaterials used for the adsorptive removal of PGMs. Those reports concluded that the modified biomaterials had shown the highest adsorptive removal capacity than its raw biomaterials (Perez et al. 2019). For example, the altered or cross-linked persimmon tannin gel (Gurung et  al. 2011) and N-aminoguanidine immobilized on persimmon tannin gel (Gurung et  al. 2013; Perez et al. 2019) have achieved higher adsorptive removal for Au(III) than the thick persimmon waste gel (Perez et al. 2019; Xiong et al. 2009) and persimmon tannin powder. There are many examples to the modified biomaterials that are having higher adsorptive removal capacity for PGMs than its raw materials (Donia et al. 2007; Guibal et al. 2002; Gurung et al. 2013; Won et al. 2013). 5.5.3.4  Other Adsorbents Parajuli et al. (2006) have reported eco-friendly adsorptive removal of PGMs using adsorption gels that have primary amine and ethylenediamine functional groups on lignin which were noted as PA-lignin and EN-lignin. Both those adsorption gels selectively and effectively removed the PGMs, including Au(III), Pd(II), and Pt(IV), from acidic chloride solutions. However, Au(III) has shown the highest adsorption

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compared to the other two metals. It followed Langmuir monolayer adsorption on homogeneous adsorbent surfaces. They also noted that the primary adsorption mechanism was due to the formation of ion pairs of metal chloro complex anions and protonated adsorption gels at acidic media (Parajuli et  al. 2006). However, Au(III) is involved in the reduction adsorption mechanism on adsorption gels, and that is the reason why Au(III) shows a high adsorption capacity than Pd(II) and Pt(IV).

5.6  Possible Adsorption Mechanism of PGMs The mechanism of adsorption process mainly depends on the nature of the adsorbent and adsorbate and the experimental conditions of the system. The measured adsorption capacity of an adsorbent for specific adsorbate indicates the performance of the adsorbent. The adsorption capacity of the absorbent mainly depends on the strength of interactions between the adsorbent and adsorbate (Volesky 2001). The main factors that influence the strength of interactions or adsorption capacity included surface physical characteristics (surface functional groups, porous volume, porous diameter, surface area, stability, and selectivity) and chemical characteristics (surface charge and chemical speciation) of adsorbent and operational conditions (pH, temperature, contact time, agitation, occurrence of competing species, and coexistence of other pollutants) (Fomina and Gadd 2014; Park et al. 2010b). The affinity of attractions at the adsorbate and the adsorbent can explain Pearson’s hard-soft-acid-base (HSAB) theory. This HSAB principle says that the hard-hard or soft-soft interactions are stronger than the hard-soft or soft-hard interactions (Li et al. 2019; Perez et al. 2019). That means the hard adsorbate readily binds on the solid adsorbent and soft adsorbent efficiently binds on soft adsorbate. Based on this principle, the hard-hard interactions are ionic, and soft-soft interaction is covalent (Park et  al. 2010b). While the borderline ions may interact with different preferences, it depends upon the particular influence factors of the system. However, these HSAB interactions may also be affected by the other system operational conditions, including metal concentration, competing ions, and pH (Fomina and Gadd 2014). It is a fact that most of the PGMs (Ag+, Au+, Au3+ Pd2+, and Pt4+) are soft acids which can bind efficiently and firmly with soft bases such as imidazole, thiol(sulfhydryl), and thioesters (Li et al. 2019; Perez et al. 2019). It means that the adsorbents or ligands having atoms N and S have more affinity for adsorption removal of PGMs.

5.7  Adsorptive Removal of Rare Earth Elements (REEs) Rare earth elements (REEs) are divided into two categories, namely the light rare earth (LRE) (La, Ce, Pr, Nd, and Pm) and middle-heavy REEs (Sm, Eu, Gd, and Tb). RE metals are used in multisectors such as the mundane (lighter flints, fluorescent lamps), the high-tech (batteries, lasers, super-magnets), and the futuristic

5  Recent Strategies on Adsorptive Removal of Precious Metals and Rare Earths Using… 101

(high-temperature superconductivity, information storage, conservation, and transport of energy) fields, due to their diverse chemical, electrical, metallurgical, magnetic, optical, and catalytic properties (Zhao et  al. 2016). In modern technology, usage of rare earth elements (REEs) has increased in the past years (Bahramifar and Yamini 2005; He et al. 2005; Junk et al. 1974; Shu et al. 2018; Torab-Mostaedi et al. 2015). They are widely used in different sectors such as nuclear energy, metallurgy, medicine, chemical engineering, electronics, and computer manufacturing. Furthermore, gadolinium (Gd), as one middle-heavy REE, is mainly used as a shielding and fluxing device in nuclear power reactors (Junk et al. 1974). As a result, the recovery of REEs is a significant issue that needs appropriate attention. In recent years, many studies have focused on the recovery of REEs and some other fission products from nitric acid solutions of nuclear waste, most of which are based on solvent extraction. Although liquid-liquid solvent extraction has been reported as a successful process for industrial recovery of noble metals and hydrometallurgy, generally, it still needs to be applied by a larger volume of organic solvent and equipment (Shu et al. 2018). However, these methods are not economically attractive. Among the available methods, adsorption has gained more extensive attention because of its simplicity, high efficiency, and low cost. This section of the chapter presents the recently published literature regarding the removal of rare earth metals from aqueous solution by different adsorbents that are summarized in Table 5.2. From this survey it is concluded that the adsorption process proves to be a promising, practical, and cost-effective method for the recovery of rare earth metals. Adsorption was found to be influenced by contact time, adsorbent dose, initial concentration, solution pH, and temperature. Among tested isotherm and kinetic models, Langmuir and pseudo-second-order were found to fit well the experimental data. Future work must be focused on the application of adsorbents on the recovery of REEs from real wastewater. Moreover, batch equilibrium approaches should be accompanied by column studies for better understanding of the mechanism and the behavior between the adsorbent and rare earth metal.

5.8  Conclusions Due to the variety of applications of PGMs, raising the scarcity of resources of PGMs, the separation and recovery of PGMs are excellently worthy. The often-used conventional techniques for reconstruction of PGMs have some disadvantages and limitations including cost, environmental impact, and low efficiencies at trace-level recovery of targets. However, adsorption is an alternative technique for reconstruction of metal due to its simplicity, smooth operation, practicality, and economical benefit with high efficiency. Here, novel low-cost materials including natural and synthetic magnetic metal (iron) oxide and biomaterials used for the recovery of PGMs have been reviewed. Based on the observations, the adsorbents are relatively low-cost inert materials that offer high adsorptive removal and selectivity towards PGMs. Moreover, the surface modification or functionalization of these materials

Adsorbents EDTA-β-cyclodextrin Grapefruit peel Granular grafted hydrogel composites Crab shell Prawn carapace Fish scales Egg shell Corn style Pineapple crown Orange peel Neem sawdust Biohydrogel modified with sporopollenin Biohydrogel modified with xylan Oxidized multiwalled carbon nanotubes Malt spent rootlets Activated carbon Crab shells Chitosan nanoparticles Graphene oxide Sulfonated graphene oxide SBA-15 mesoporous silicas functionalized with N-propylsalicylaldimine SBA-15 mesoporous silicas functionalized with ethylenediaminepropylesalicylaldimine SiO2 SiO2/UF composite material

Maximum adsorption capacity (mg/g) 50.12 159.30 169.49–243.9 90.9 1000 200 166.6 250 142.8 71.4 200 333.3 200 78.12 156 86 3.238 114.9 142.8 125.0 5.1 15.6 0.12 0.23

REEs Ce3+ Ce3+ Ce3+ Ce3+ Ce3+ Ce3+ Ce3+ Ce3+ Ce3+ Ce3+ Ce3+ Ce3+ Ce3+ Dy3+ Eu3+ Eu3+ Eu3+ Eu3+ Eu3+ Eu3+ Eu3+ Eu3+ Eu3+ Eu3+

Table 5.2  Adsorptive removal of REEs by using various adsorbent materials

– –

Isotherms Langmuir Langmuir Langmuir Langmuir Freundlich Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir Freundlich Freundlich Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir, Freundlich Langmuir Naser et al. (2015) Naser et al. (2015)

Dolatyari et al. (2016)

References Zhao et al. (2016) Torab-Mostaedi et al. (2015) Zhu et al. (2015) Varshini and Nilanjana (2014b) Varshini and Nilanjana (2014b) Varshini and Nilanjana (2014b) Varshini and Nilanjana (2014b) Varshini and Nilanjana (2014b) Varshini and Nilanjana (2014b) Varshini and Nilanjana (2014b) Varshini and Nilanjana (2014b) Varshini et al. (2015) Varshini et al. (2015) Koochaki-­Mohammadpour et al. (2014) Anagnostopoulos and Symeopoulos (2013) Anagnostopoulos and Symeopoulos (2013) Cadogan et al. (2014) Cadogan et al. (2014) Yao et al. (2016) Yao et al. (2016) Dolatyari et al. (2016)

102 J. R. Koduru et al.

SiO2/UF impregnated with organophosphorus extractant EDTA-β-cyclodextrin Bone powder Hydroxyapatite Hydroxyapatite Crab shell Prawn carapace Egg shell Corn style Pineapple crown Orange peel Fish scales Neem sawdust Grapefruit peel Pleurotus ostreatus basidiocarps Oxidized multiwalled carbon nanotubes Stichococcus bacillaris Desmodesmus multivariabilis Chlorella vulgaris Scenedesmus acuminutus Chloroidium saccharophilum Chlamydomonas reinhardtii Granular grafted hydrogel composites EDTA-β-cyclodextrin Bone powder Surface-modified mesoporous silica nanoparticles (two-step) (MNSP-N-1) Surface-modified mesoporous silica nanoparticles (one-step) (MNSP-N-2)

– Langmuir Langmuir Freundlich Freundlich Freundlich Freundlich Freundlich Freundlich Freundlich Freundlich Langmuir Freundlich Langmuir Langmuir Freundlich Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir

3.1 55.62 12.7 0.25 0.94 90.9 200.0 100.0 76.9 100.0 125 250 166.6 171.20 54.54 99.01 51.02 100.0 74.60 111.1 129.87 142.86 256.41–333.33 47.78 8.70 56.22 85.38

Eu3+ Eu3+ Eu3+ Eu3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ La3+ Gd3+ Gd3+

Wang et al. (2014) (continued)

Zhao et al. (2016) Butnariu et al. (2015) Granados-Correa et al. (2012) Granados-Correa et al. (2012) Varshini and Nilanjana (2014a) Varshini and Nilanjana (2014a) Varshini and Nilanjana (2014a) Varshini and Nilanjana (2014a) Varshini and Nilanjana (2014a) Varshini and Nilanjana (2014a) Das et al. (2014) Torab-Mostaedi et al. (2015) Işıldar et al. (2019) Koochaki-­Mohammadpour et al. (2014) Birungi and Chirwa (2014) Birungi and Chirwa (2014) Birungi and Chirwa (2014) Birungi and Chirwa (2014) Birungi and Chirwa (2014) Birungi and Chirwa (2014) Zhu et al. (2015) Zhao et al. (2016) Butnariu et al. (2015) Zheng et al. (2015) Zheng et al. (2015)

Naser et al. (2015)

5  Recent Strategies on Adsorptive Removal of Precious Metals and Rare Earths Using… 103

Langmuir Langmuir Langmuir Langmuir Langmuir Langmuir, Sips, Jovanovic, Redlich, Peterson, Toth, Khan, Radke-­Prausnitz, Fritz-­Schlunder, Hill, Koble-­Corrigan, Baudu Langmuir, Freundlich, Sips Langmuir, Freundlich, Sips Langmuir, Freundlich, Sips Langmuir, Freundlich, Sips Langmuir Langmuir Freundlich Freundlich

10.9 323.0 370.0 90.0 350.0 69.75

131.4 111.2 146.4 119.5 66.60 58.80 30.51 45.45

Nd3+ Nd3+ Sm3+ Sm3+ Sm3+ Pr3+

Pr3+ Pr3+ Pr3+ Pr3+ Pr3+ Pr3+ Sc3+ Y3+

Free Sargassum wightii (brown seaweed) Polysulfone-immobilized Sargassum wightii Free Turbinaria conoides (brown seaweed) Polysulfone-immobilized Turbinaria conoides Crab shell Orange peel Lysine-functionalized mesoporous material (Fmoc-SBA-15) NaOH-modified Pleurotus ostreatus

– – –

0.1 0.18 2.8

Nd3+ Nd3+ Nd3+

Langmuir Langmuir

194.73 238.00

Nd3+ Nd3+

Calcium alginate Calcium alginate-polyglutamic acid hybrid gels SiO2 SiO2/UF composite material SiO2/UF impregnated with organophosphorus extractant Bone powder Magnetic nano-hydroxyapatite Magnetic nano-hydroxyapatite Activated biochars from cactus fibers (pH = 3.0) Activated biochars from cactus fibers (pH = 6.5) Green seaweed (Ulva lactuca)

Maximum adsorption capacity (mg/g) Isotherms

REEs

Adsorbents

Table 5.2 (continued)

Hussien and Desouky (2014)

Vijayaraghavan and Jegan (2015) Vijayaraghavan and Jegan (2015) Vijayaraghavan and Jegan (2015) Vijayaraghavan and Jegan (2015) Varshini et al. (2015) Varshini C et al. (2015) Ma et al. (2014)

Vijayaraghavan and Jegan (2015)

Vijayaraghavan (2015)

(Gok 2014) Gok (2014) Hadjittofi et al. (2016) Hadjittofi et al. (2016)

Naser et al. (2015) Naser et al. (2015) Butnariu et al. (2015)

Wang et al. (2014) Naser et al. (2015)

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104 J. R. Koduru et al.

5  Recent Strategies on Adsorptive Removal of Precious Metals and Rare Earths Using… 105

could be enhanced by their adsorptive properties, which offer potential applicability of them for metal recovery from industrial and mineral waste. In addition to those developed technologies tested before it applies for field-scale recovery. Finally, it includes the recent survey on adsorption removal techniques for rare earth metals. It also concludes that future work must be focused on the application of adsorbents on the recovery of REEs from real wastewater. Moreover, batch equilibrium approaches should be accompanied by column studies for better understanding of the mechanism and the behavior between the adsorbent and rare earth metal. Acknowledgments  The author J.R.K. acknowledges the National Research Foundation (NRF) of Korea for financial support through funding by the Ministry of Science, ICT, and Future Planning (MSIP) (2017R1C1B5016656) of the Korean Government.

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Chapter 6

Investigation on Extraction and Recovery of Rare Earth Elements from Secondary Solid Wastes Saroj Sekhar Behera, Ranjan Kumar Mohapatra, Debadutta Das, and Pankaj Kumar Parhi

6.1  Introduction Rare earth elements (REEs) are a group of 17 elements consisting of 15 lanthanides including scandium and yttrium having similar chemical and metallic behavior. Nowadays these elements are becoming most important because of its physical properties such as magnetic, catalytic, photo-physical, optical, thermoelectric, and dielectric. Due to these unique physical properties, these elements have numerous high-tech applications such as electric vehicles, wind turbines, hard disk drives, compact fluorescent lamps, military and aerospace system, and NiMH batteries (Massari and Ruberti 2013; Binnemans et  al. 2013; Antolini and Perez 2011; Kul et  al. 2008; Resende and Morais 2010). The increasing popularity boosts the development of hybrid futuristic invention which must have rare earth elements (REEs) as a major component, raising the demand as well as the price of REEs. According to applications of REEs, European Commission classified the REEs into two classes as light S. S. Behera School of Chemical Technology and School of Biotechnology, Kalinga Institute of Industrial Technology (KIIT) Deemed to be University, Bhubaneswar, Odisha, India R. K. Mohapatra Department of Chemistry, Government College of Engineering, Keonjhar, Odisha, India School of Chemical Technology and School of Biotechnology, Kalinga Institute of Industrial Technology (KIIT) Deemed to be University, Bhubaneswar, Odisha, India D. Das Department of Chemistry, Sukanti Degree College, Subarnapur, Odisha, India P. K. Parhi (*) Convergence Research Center for Development of Mineral Resources, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, South Korea School of Chemical Technology and School of Biotechnology, Kalinga Institute of Industrial Technology (KIIT) Deemed to be University, Bhubaneswar, Odisha, India © Springer Nature Switzerland AG 2020 R. K. Jyothi (ed.), Rare-Earth Metal Recovery for Green Technologies, https://doi.org/10.1007/978-3-030-38106-6_6

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(La-Sm) and heavy (Eu-Lu, Sc, and Y) rare earth elements. Among them, the light rare earth elements belong to the group of most critical raw materials having the highest peak of supply risk (European Commission 2010). Also the US Department of Energy (DOE) categorizes the REEs such as neodymium (Nd), terbium (Tb), dysprosium (Dy), europium (Eu), and yttrium (Y) as most critical elements (Chu 2011). The production of REEs is approximately 100,000–150,000 tons annually. However, only few countries of the world are producing REEs. Among them, China claims to produce more than 90% of all REEs, although these countries possess