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>Application of Nanotechnology in Mining Processes Nanotechnology has revolutionized processes in many industries but its application in the mining industry has not been widely discussed. This unique book provides an overview of the successful implementation of nanotechnology in some of the key environmental and beneficiation mining processes. This book explores extensively the potential of nanotechnology to revolutionize the mining industry which has been relying for a very long on processes with limited efficiencies. The nine specialized chapters focus on applying nanoflotation to improve mineral processing, effective extraction of metals from leachates or pregnant solutions using nanoscale supramolecular hosts, and development of nano-adsorbents or nano-based strategies for the remediation or valorization of AMD. The application of nanotechnology in mining has so far received little attention from the industry and researchers and this groundbreaking book features critical issues so far under-reported in the literature: * Application of nanotechnology in mineral processing for the enhancement of froth flotation * Development of smart nanomaterials and application for the treatment of acid mine drainage * Recovery of values from pregnant solutions using nanoadsorbents * Valorization of AMD through formation of multipurpose nanoproducts. Audience Industrial interest will be from mining plant operators, environmental managers, water treatment plants managers, and operators. Researchers in nanotechnology, environmental science, mining, and metallurgy engineering will find the book valuable, as will government entities such as regulatory bodies officers and environmentalists.
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Table of Contents
Cover
Title Page
Copyright
Preface
1 Modified Dendrimer Nanoparticles for Effective and Sustainable Recovery of Rare Earth Element from Acid Rock Drainage
1.1 Introduction
1.2 Rare-Earth Element Occurrence in Acid Mine Drainage
1.3 Dendrimer as Extraction Agent of Rare Earth Element in AMD
1.4 Designed a Recovery System for REE from AMD
1.5 Challenges and Opportunities for the Future of Metal Mining
1.6 Conclusion
Acknowledgment
References
2 Cellulose-Based Nanomaterials for Treatment of Acid Mine Drainage-Contaminated Waters
2.1 Introduction
2.2 Cellulose
2.3 Synthesis of CNFs and CNCs
2.4 Cellulose Composites
2.5 Valorization of AMD-Contaminated Water and the Possible Uses of Recovered Elements
2.6 Conclusion
References
3 Application of Nanomaterials for Remediation of Pollutants from Mine Water Effluents
3.1 Introduction
3.2 Existing Treatment Methods of Mine Water and Their Limitations
3.3 Nanoremediation of Mine Water
3.4 Application of Nanomaterials for Mine Water Remediation
3.5 Conclusions and Future Perspectives
References
4 Application of Nanofiltration in Mine-Influenced Water Treatment: A Review with a Focus on South Africa
Abbreviations
4.1 Introduction
4.2 Nanofiltration for Mine-Influenced Water Treatment
4.3 Large-Scale Operations Using Nanofiltration or Reverse Osmosis
4.4 Some Perspectives and Research Directions
References
5 Recovery of Gold from Thiosulfate Leaching Solutions with Magnetic Nanoparticles
Abbreviations
5.1 Introduction
5.2 Recovery of Precious Metals with Magnetic Nanohydrometallurgy
5.3 Synthesis and Functionalization of Magnetic Nanoparticles
5.4 Characterization of Magnetic Nanoparticles
5.5 Recovery of Gold from Thiosulfate Leaching Solutions
5.6 Gold Elution and Reuse of the Adsorbent
5.7 Industrial Scale-Up and Challenges
5.8 Environmental Concerns and Toxicity of MNPs
References
6 Recovery of Na
2
CO
3
and Nano CaCO
3
from Na
2
SO
4
and CaSO
4
Wastes
6.1 Introduction
6.2 Literature Survey
6.3 Materials and Methods
6.4 Results and Discussion
6.5 Conclusions
Acknowledgments
References
7 Recovery of Drinking Water and Nanosized Fe
2
O
3
Pigment from Iron Rich Acid Mine Water
7.1 Introduction
7.2 Literature Review
7.3 Materials and Methods
7.4 Results and Discussion
7.5 Conclusion
7.6 Recommendation
Acknowledgments
References
8 Advances of Nanotechnology Applications in Mineral Froth Flotation Technology
Abbreviations
8.1 Introduction to Froth Flotation
8.2 Current Developments of Nanotechnology in the Mineral Froth Flotation Process
8.3 Intellectual Property (IP) and Commercialization of Nanotechnology in Mineral Froth Flotation Technology
8.4 Current Research Gaps
8.5 Conclusion
References
9 Nanoscale Materials for Mineral Froth Flotation: Synthesis and Implications of Nanoscale Material Design Strategies on Flotation Performance
9.1 Introduction
9.2 Classification of Minerals
9.3 Synthesis and Characterization of Nanoscale Materials
9.4 Nanoflotation Reagents and Mineral Particle Interaction in the Flotation Environment
9.5 Nanotoxicology
9.6 Conclusion
References
Index
End User License Agreement
Guide
Cover
Table of Contents
Title page
Copyright
Preface
Begin Reading
Index
End User License Agreement
List of Illustrations
Chapter 1
Figure 1.1 Schematic showing the pathway of AMD formation, its dispersion into t...
Figure 1.2 Schematic of the different types of remediation techniques for ARD ne...
Figure 1.3 Schematic of magnetic PAMAM succinamic dendrimer nanoparticle.
Figure 1.4 Systematic representation of the proposed processes for the recovery ...
Chapter 2
Figure 2.1 Cellulose structure with the inter-and intra-molecular hydrogen bonds...
Figure 2.2 Schematic of cellulose nanofibril and nanocrystal isolation from biom...
Figure 2.3 Tempo radical with nitrosonium ion as the oxidized form of TEMPO and ...
Figure 2.4 TEMPO-mediated oxidation (a) with TEMPO radical (b) under alkaline co...
Figure 2.5 Proposed mechanism for oxidation of cellulose by TEMPO/Laccase/O2 sys...
Figure 2.6 Schematic diagram of the synthesis of cellulose cationization with th...
Figure 2.7 Synthesis of chitosan from chitin.
Figure 2.8 Summary of the use of sludge from AMD.
Chapter 3
Figure 3.1 Structure of layered double hydroxide [80].
Figure 3.2 Schematic of the chemical formation of metal organic frameworks (MOFs...
Figure 3.3 Several materials derived from the two-dimensional structure of graph...
Chapter 4
Figure 4.1 Structure of the mine water management hierarchy in South Africa. (Ad...
Figure 4.2 Schematic illustration of pressure-driven membrane processes for wate...
Figure 4.3 Schematic illustrating the retention of multi-charged ions (e.g., Fe3...
Figure 4.4 Schematic representation of ion transport (illustrated by arrows) for...
Chapter 5
Figure 5.1 Schematic representation of a magnetic solid phase extraction process...
Figure 5.2 Schematic illustration of the crystal structure of magnetite (the bla...
Figure 5.3 Schematic illustration of the crystal structure of maghemite (the bla...
Figure 5.4 Chemical structures of linear and branched polyethylenimine.
Figure 5.5 TEM images of bare MNPs (a) and PEI-MNPs (b). (Reprinted from [14] wi...
Figure 5.6 TEM Images of bare MNPs (a) and PEI-MNPs (b). (Reprinted from [12] wi...
Figure 5.7 Characterization results of bare MNPs, PEI-MNPs and gold adsorbed PEI...
Figure 5.8 Field-dependent magnetization of bare MNPs, PEI-MNPs and Au-PEI-MNPs ...
Figure 5.9 Composition of Au, Cu, S and Ca in a synthetic leaching solution.
Figure 5.10 Gold adsorption kinetics onto PEI-MNPs in calcium thiosulfate leachi...
Figure 5.11 Effect of PEI-MNPs concentration on gold adsorption in calcium thios...
Figure 5.12 Gold concentration (%) change vs adsorption time (mins) (a) at 3.7 m...
Figure 5.13 Combined effect of time and PEI-MNPs dosage on gold adsorption (a) a...
Chapter 6
Figure 6.1 Uses for Na
2
CO
3
[22].
Figure 6.2 Process configuration of the Solvay process [28].
Figure 6.3 Flow configuration of the modified Solvay process [32, 33].
Figure 6.4 Concentration of NaHS by freeze crystallization [36].
Chapter 7
Figure 7.1 Process configuration of the ROC process (courtesy of J P Maree).
Figure 7.2 The Wader mine water treatment demonstration plant (courtesy of J P M...
Figure 7.3 Schematic diagram of the Wader mine water treatment demonstration pla...
Figure 7.4 Acid mine water pond (Kopseer Dam).
Figure 7.5 From left: Top Dam water and sludges produced at pH 3.5, pH 4.5 and p...
Figure 7.6 Settling rate of Fe(OH)
3
at pH 3.2, Al(OH)
3
at pH 4.5 and remaining s...
Figure 7.7 Sludge separation with a centrifuge.
Figure 7.8 Effect of temperature on Fe(OH)
3
.
Figure 7.9 Pigments produced from iron-rich mine water (left).
Figure 7.10 The elemental compositions of the synthesized pigments: (a) goethite...
Figure 7.11 The HR-FESEM images showing the morphological properties of the synt...
Figure 7.12 Flow diagram for treatment of iron(III)-rich water (Process configur...
Figure 7.13 Flow diagram for treatment of iron(II)-rich water (Process configura...
Figure 7.14 Process configuration for processing of tailings and/or tailings lea...
Chapter 9
Figure 9.1 SEM mircrograph of silver-polysulfone electrospun nanofibers generate...
Figure 9.2 (a) SEM micrograph, (b) EDS spectrum of nano zero valent iron (nZVI) ...
Figure 9.3 Proposed generic steps for the development and testing of nanoflotati...
List of Tables
Chapter 1
Table 1.1 Different types of sulfide-bearing ore bodies, % content in ore body, ...
Table 1.2 Potential toxic elements and their effect in humans.
Table 1.3 Different types of applications and the uses of REEs.
Table 1.4 Recovery of REE using different techniques.
Table 1.5 Use of PAMAM dendrimer for the removal of metal ions.
Chapter 2
Table 2.1 Categories, nomenclature, shape, and size distribution of the nanocell...
Table 2.2 A summary of the difference between the commonly used mineral acids in...
Table 2.3 Anion and cation removal using cellulose-based materials.
Chapter 3
Table 3.1 Application of nanoparticles in remediation of pollutants from mine wa...
Chapter 4
Table 4.1 Examples of bench-scale studies and pilot plants using NF (and RO) mem...
Chapter 5
Table 5.1 Some of the main nanoparticle synthesis methods and the properties of ...
Chapter 6
Table 6.1 Physical properties of Na
2
CO
3
[26].
Table 6.2 Determination of CaSO
4
.2H
2
O conversion to CaS via mass loss.
Table 6.3 Chemical compositions resulting from Na
2
SO
4
being reacted with CaS [35...
Table 6.4 Chemical compositions resulting from Na
2
SO
4
being reacted with Ca(HS)
2
...
Table 6.5 Solubility of chemicals [35].
Table 6.6 Effect of CaS concentration on the soluble sulfide fraction.
Table 6.7 Comparison between CaS and Ca(HS)
2
when reacted with CO
2
.
Table 6.8 Behavior of various parameters when CO
2
is added stepwise to Na
2
S(aq) ...
Table 6.9 Chemical compositions when CO
2
was added stepwise to 350 mmol/L (25.2 ...
Table 6.10 Chemical compositions when H2S was added stepwise to 350 mmol/L (25.2...
Table 6.11 Separation of Ca(HS)
2
and CaCO
3
separation through CO
2
addition.
Table 6.12 Recovery of CaCO
3
from Ca(HS)
2
and Na
2
CO
3
with syringe filter.
Chapter 7
Table 7.1 Chemical compositions of water in the Trench, Top Dam and Kopseer Dam.
Table 7.2 ImproChem liquid coagulants.
Table 7.3 ImproChem powder flocculants.
Table 7.4 Solubilities of various alkalis and alkali products. (http://ftpmirror...
Table 7.5 Sludge settling rates with Na
2
CO
3
neutralization.
Table 7.6 Effect of various parameters on the rate of neutralization, settling r...
Table 7.7 Freeze crystallization of acid mine water.
Table 7.8 Feasibility of up-concentration of leachate with freeze crystallizatio...
Table 7.9 Feasibility of treatment of iron(III)-rich water.
Table 7.10 Feasibility of treatment of iron(II)-rich water.
Table 7.11 Comparison between the cost of Top Dam and Kopseer Dam water.
Chapter 8
Table 8.1 List of patents related to applications of nanobubbles and flotation r...
Chapter 9
Table 9.1 Mineral classification hierarchies.
Table 9.2 Classification framework for minerals based on the dominant anion or a...
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Application of Nanotechnology in Mining Processes
Beneficiation and Sustainability
Edited by
Elvis Fosso-Kankeu
Electrical and Mining Engineering Department, University of South Africa
Martin Mkandawire
School of Science and Technology, Cape Breton University, Nova Scotia, Canada
and
Bhekie Mamba
College of Science, Engineering and Technology, University of South Africa
This edition first published 2022 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2022 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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ISBN 978-1-119-86499-8
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Preface
Nanotechnology, initially expected to revolutionize processes in industries, has affected fields in engineering in different ways. For example, the application of nanotechnology in mining processes such as minerals processing and hydrometallurgy has received limited attention so far.
Mining plays a vital role in the economic development of many countries around the world; it is, therefore, understandable that the technologies applied in mining must ensure cost-effective recovery of values from the ore and minimize the impact of processes on the environment. After extraction of ore minerals, they must be separated from the gangue to be processed for metal extraction via a process such as hydrometallurgy which is less energy demanding and has a limited impact on the environment. Although hydrometallurgy has less impact on the environment than pyrometallurgy, the former still contributes to the discharge of solid wastes containing residual sulphide minerals that can be oxidized to form acid mine drainage in the environment. However, little research has been reported on the application of nanotechnology in three mining processes, vis mineral processing (concentration through flotation), hydrometallurgy (concentration or purification of metals loaded solution) and management of mining liquid wastes to minimize environmental impact.
Ore minerals are generally dispersed in a large volume of gangue minerals, requiring therefore that the rock is crushed to small particles for the beneficiation of valuable minerals through froth flotation, which consists of the floatation of crushed particles in an aqueous solution containing “collector chemical” that can attach to the valuable particles allowing them to remain at the top of bubbling solution and making easier to skim them off. In conventional froth flotation, air bubbles are relatively large and less stable; recent findings have shown that the application of nanoflotation can considerably improve the separation of valuable minerals from gangue minerals through the use of hydrophobic nanoparticles or the formation of nanobubbles using special dispersing pumps.
The concentration and purification processes in hydrometallurgy often require selective extraction from solution. However, conventional techniques such as ion exchange and solvent extraction still have low efficiencies. For example, solvent extraction often results in an unpure solution due to poor coalescence of the organic solvent, which contaminates the aqueous solution, also resulting in the loss of expensive reagents. In contrast, conventional semipermeable membranes made of aggregates of polymers and ion exchangers tend to be non-selective because the absence of atomistic control limits sufficient exposure of sidechains to the solution. Recently, nanoscale supramolecular hosts exhibiting selective, high-capacity and recyclable adsorption potential have been developed and applied to extract metals from leachates or pregnant solutions with great success.
One major impact of mining activities on the environment is the formation of acid mine drainage, a very acidic solution rich in metals that can negatively affect aquatic life. One of the approaches to remediate AMD pollution often consists of removing metals using nano-adsorbents with a very large surface area and, therefore, high adsorption capacity. These nano-adsorbents are also used to extract and separate rare earth elements (REE) from mine effluents. In addition, a new approach focusing on the circular economy promotes the valorization of mine wastes such as AMD, resulting in the production of nano-based materials with economic values.
This book presents nine specialized chapters that focus on applying nanoflotation to improve mineral processing, effective extraction of metals from leachates or pregnant solutions using nanoscale supramolecular hosts, and development of nano-adsorbents or nano-based strategies for the remediation or valorization of AMD.
The editors and the publisher are grateful to the reviewers who have contributed to improving the quality of the book through their constructive comments. The editors also thank the publisher for including this book in their portfolio.
This book will be of interest to researchers from the fields of Environment, Chemistry, Engineering, Mineral processing, Hydrometallurgy and Geochemistry, engineers and environmentalists from the mining industry, as well as the environmental policies makers mostly in the public sector, to name a few. Furthermore, it is our wish that this book assists the readers in improving their experimental and operational processes by implementing the ideas disseminated in the various chapters of this book.
Elvis Fosso-KankeuMartin MkandawireBhekie B. MambaJanuary 2022
1Modified Dendrimer Nanoparticles for Effective and Sustainable Recovery of Rare Earth Element from Acid Rock Drainage
Anyik John Leo1,2*, Innocentia Gugulethu Erdogan1,3, Frans B. Waanders1, Martin Mkandawire1,2, Thabo T.I Nkambule4, Bhekie B. Mamba4 and Elvis Fosso-Kankeu4,5
1Water Pollution Monitoring and Remediation Initiatives Research Group, School of Chemical and Minerals Engineering, North-West University, Potchefstroom, South Africa
2Solid-State Research Group, Department of Chemistry, School of Science and Technology, Cape Breton University, Sydney, Canada
3Faculty of Engineering and the Built Environment, Chemical Engineering Department, Cape Peninsula University of Technology, Cape Town, South Africa
4Institute for Nanotechnology and Water Sustainability (iNanoWS), College of Science Engineering and Technology (CSET), University of South Africa, Florida Science Campus, Johannesburg, South Africa
5Department of Electrical and Mining Engineering, College of Science Engineering and Technology (CSET), University of South Africa, Florida Science Campus, Johannesburg, South Africa
Abstract
Mining supplies key resources necessary for technological advancement to ameliorate challenges imposed by the increase in the human population worldwide. One of the legacies of mining resources is the formation and discharge of acid mine drainage (AMD) during and even after active mining. It is a major environmental concern because it enhances the dissolution and increases the dispersion of contaminants, mostly toxic metals, in the environment. Many countries have now adopted or promulgated legislation that requires mining operators to treat and manage the formation of AMD, costing them a fortune from their profits. AMD can be an alternative source of valuable rare earth elements (REE), but the currently available extraction methods of REE from AMD are inefficient and costly, exceeding by many folds their conventional extraction from ores. Thus, there has been a growing effort to develop a novel and cost-effective method to recover REEs from AMD, in which extraction using polymeric nanomaterials, like Poly(amidoamine) (PAMAM) dendrimers, are growing in prominence. PAMAM dendrimers nanoparticles have high adsorption capacity, contributing highly to metal recovery from most wastewater. However, their application in the recovery of REEs from AMD is hampered by the low pH of the AMD, which protonates the amine functional groups forming cationic charges on the surfaces of the dendrimer nanoparticles. Therefore, designing these materials to adsorb metal ions in an acidic solution is paramount for treating AMD. This chapter discusses designing a cost-effective method for the recovery of REEs from AMD after alkaline treatment, using surface-functionalized magnetic PAMAM dendrimer nanoparticles. The environmental effect and shortcomings of AMD remediation methods will be highlighted as a background motivation in developing this procedure.
Keywords: Acid rock drainage, dendrimers, magnetic iron oxides nanoparticle, potentially toxic elements, rare earth element
1.1 Introduction
The global human population has risen considerably since the industrial revolution and currently stands at above 7.7 billion worldwide, beyond the carrying capacity of the earth [1]. The rapid population growth is imposing tremendous challenges such as the easy spread of disease outbreaks, food scarcity, shortage of infrastructures, and insufficient communication networks, which require resources and technological advancement to ameliorate these predicaments. All resources required for this technological advancement come from mother earth and can be obtained only through two means; if they cannot be harvested (from farming), they should be mined. The mining industry plays a vital role in supplying these key resources but lacks the potential to obtain mineral resources without compromising environmental integrity [2]. Nevertheless, mining cannot be easily halted due to the growing need for mineral resources to support technological advancement required to artificially sustain the ever-increasing human population, which is beyond the earth’s carrying capacity. The mineral resources, including metals, are essential components in the advancement of several technologies, like the production of medications and vaccines, fertilizers for agricultural application to ensure food security, and the manufacture of building materials for construction of mega-infrastructures to improve road networks. They are also used as vital components to manufacture computers and cell phones for better communication networks. In addition, mining is a vital economic activity for many nations, bringing in much-needed foreign exchange earnings and employment [3]. Despite the benefits mentioned above, the legacy of mining activities includes major environmental pollution and heaps or piles of municipal solid waste (MSW) from mining [4]. For example, most valuable minerals like gold (Au), copper (Cu), sulfur (S), zinc (Zn), silver (Ag) or lead (Pb) occurs in sulfidic ore bodies (Table 1.1) with more than one type of mineralization [5]. Once these valuable metals have been extracted from their sulfidic ores, vast volumes of mine water and leftover mining solid wastes and tailings are generated, which contain most of the sulfide mineral, like pyrite. The exposure of the pyrite-containing waste to oxygen and water leads to an acid-generating material, producing acid mine drainage (AMD). The discharge of AMD is attributed to most of the contamination being transported from mining sites to the receiving environments, affecting environmental water quality [6–8] (Figure 1.1). Due to its well-known and publicized ecological impacts, many countries have adopted stringent regulations, such as Section 402 of the Clean Water Act of the Republic of South Africa, enabling mining operators to treat mine water discharging AMD [9–11].
On the one hand, treatment of AMD as required by environmental legislation has serious financial implications for the mining operators, but on the other hand, generation of AMD in former mining sites occurs long after active mining when the responsible mining companies no longer exist. Thus, treatment and remediation of AMD are an economic and financial burden to the mining company if the operation is still active, and to the community and their governments in closed or abandoned mines. Mine water discharging AMD is known to contain a consortium of dissolved elements, including precious metals and rare-earth elements (REE) [12]. Thus, AMD can be a valuable resource, especially of REE and precious metals that can generate more income and compensate for the treatment and remediation expenses of mine water and contaminated mining sites [13]. Furthermore, AMD can be regarded as an initial and natural process of hydrometallurgy of REE. However, extraction of REE from AMD is a big challenge due to the need for highly selective extraction methods, targeting only REE and leaving other metal ions dissolved in AMD to produce the required purity. Thus, recovery of REE from AMD is usually more expensive, resulting in AMD not being attractive as a source of REE. Nevertheless, using polymeric nanomaterials as hydrometallurgical extraction agents is promising to be a cost-effective and efficient method of extracting REE from AMD. One of the agent groups with high potential is Poly(amidoamine) (PAMAM) dendrimers. Consequently, this chapter discusses the application of PAMAM dendrimers as an extraction agent of REE, evaluating their performance potential and the development of methods in which they are applied.
Table 1.1 Different types of sulfide-bearing ore bodies, % content in ore body, uses, world deposit and origin [14, 15].
Sulfide ore bodies and formula
Minerals present in the ore body with % content
Application of the major elements in the ore body
Regions of major deposit of ore bodies in the world
Origin of occurrence
Arsenopyrite
(FeAsS)
Arsenopyrite is the major source of arsenic, with 46% Arsenic, 34.3% Iron, and 19.7% Sulfur in the ore body. It should be noted that the arsenic content is made up of minor gold deposits.
Used in the manufacture of herbicide, alloys, wood preservative, medicine, insecticide, and rat poison.
China, Morocco, Namibia, Russia, Belgium, Iran, and Japan
Hydrothermal veins, pegmatite, contact metamorphism, and metasomatism
Bornite
(Cu
5
FeS
4
)
Source of rich-grade copper metal. The ore body contains 63.3% Copper (Cu), 11.1% Iron (Fe), and 25.6% Sulfur (S)
Major applications are in electrical wires, cables, plumbing, currency, utensils, machinery, alloys, architecture, nutritional supplements, and agricultural fungicides
United States, England, Austria, Zimbabwe, Morocco, Dzhezkazgan, and Kazakhstan
In the zone of secondary supergene enrichment, source of rich copper metal
Chalcopyrite
(CuFeS
2
)
Chalcopyrite is the principal source of copper metal and accounts for approximately 70% of the copper deposits in the world. The ore body content is made up of 34.5% Copper (Cu), 30.5% Iron (Fe), and 35.0% Sulphur (S)
Major applications of copper metal are in electrical wires, cables, plumbing, currency, utensils, machinery, alloys, architecture, nutritional supplements, agricultural fungicides, and space exploration capsules
Chile, China, Peru, United States, DRC, Australia, Russia, Zambia, Canada, and Mexico
Large, massive, irregular veins, disseminated and porphyry deposit at granitic/dioritic intrusive and SEDEX type
Cinnabar (
HgS)
The ore body is a primary source of mercury. It contains 86.2% Mercury (Hg) and 13.8% Sulfur (S).
Used in the manufacturing of industrial chemicals and in electronics, thermometers, medicine, cosmetics, pigments, and fluorescent lamps. Environmentally sensitive due to health and safety regulations
China, Mexico, Kyrgyzstan, Peru, and Tajikistan
Vein-filling by recent volcanic activity and acid-alkaline hot spring
Galena
(PbS)
Galena is the primary source of lead metal and one of the sources of sulfur. It contains 86.6% Lead (Pb) and 13.4% Sulfur (S).
Used as a key ingredient for paint production, used in plumbing materials, bullets, automobile batteries, alloys, sheets, radiation shields, electrodes, ceramic glazes, stained glass, and cosmetics
China, Australia, United States, Peru, Russia, Mexico, and India
Individually or associated with zinc and copper sulfide deposit
Molybdenite
(MoS
2
)
The primary source of molybdenum with 60% Molybdenum (Mo) content and 40% Sulfur (S)
Used to coat stainless steel materials because it is resistant to corrosion, used as lubricant, tools, and high-speed steels, cast iron, electrodes, fertilizers, and for pollution control in power plants
Armenia, Canada, Chile, China, Iran, Mexico, Mongolia, Peru, Russia, and United States
High-temperature hydrothermal ore of chalcopyrite, pyrite, and molybdenite
Pentlandite
(Fe, Ni)
9
S
8
The primary source of nickel with 22% Nickel (Ni), 42% Iron (Fe), and 36% Sulfur (S)
Used in stainless steel, superalloys, electroplating, alnico magnets, coinage, rechargeable batteries, electric guitar strings, 4.6 microphone capsules, and green tinted glass
Australia, Namibia, Canada, and Brazil
The layered maficultramafic intrusion at high magmatic temperature, differential segregation
Pyrite
(FeS
2
)
Common in all rocks and as massive sulfide deposits. It contains 46.6% Iron (Fe) and 53.4% Sulfur (S)
Mainly used to produce sulfur dioxide for paper and sulfuric acid for the chemical industry. Rarely mined for iron content due to complex metallurgy and being uneconomical commercially. Acid drainage and dust explosion are common hazards with pyrite deposits.
Italy, Spain, Kazakhstan, and United States
Common in all rocks, and as massive sulfide deposits associated with gold
Sphalerite
(ZnS)
The primary source of zinc with 67% Zinc content and 33% Sulfur (S)
The main applications are in galvanizing, alloys, cosmetics, pharmaceuticals, 3.9 micronutrients for humans, animals, and plants
China, Peru, Australia, United States, Mexico, India, Bolivia, Kazakhstan, Canada, and Sweden
The majority as large SEDEX-type deposits associated with galena, chalcopyrite, and silver
Figure 1.1 Schematic showing the pathway of AMD formation, its dispersion into the environment and entrance into the food chain: The destruction of natural vegetation in search of mineral resources exposes large surface areas to weathering effects. Due to the presence of sulfide materials in these mine tailings, AMD products are formed which can be washed away into nearby streams. Aquatic life suffers the consequences due to an increase in mortality rate; meanwhile, these polluted waters can be used for irrigation and thus will bioaccumulate in plants. Once in the food chain, human lives are affected in the process.
1.2 Rare-Earth Element Occurrence in Acid Mine Drainage
1.2.1 Acid Mine Drainage Generation and Effects
Although the formation of AMD has been historically attributed to mine tailing dams containing sulfide-bearing materials, it can occur naturally in an environment that exposes hefty volumes of sulfide-bearing materials to air and water [16]. However, of the different sulfide ore deposits shown in Table 1.1, pyrite is the most common and by far the most abundant sulfide mineral [17]. Using pyrite as an example for the generation of AMD, the oxidation process is represented by different reactions (1–3) [18]. Pyrite is oxidized to sulphate ions ferrous iron (Fe2+) and protons (H+) at the initial stage when the tailing dam is exposed to atmospheric oxygen in the presence of excess water at neutral pH (1.1).
The formation of Fe2+ is solubilized by the oxidation process and subsequently oxidized to ferric iron (Fe3+) and is the rate-determining step of the overall reaction (1.2).
Ferric cations produced can also oxidize additional pyrite into ferrous ions, and the net effect of these reactions is to produce H+, which increases the acidity of the influent and maintains the solubility of the ferric iron (1.3).
The pH value of AMD is as low as 2–4 and will naturally enhance the rate of dissolution of potentially toxic elements (PTEs), resulting in the tailing dam containing a high content of metal(loids), including sulfate ions in solution. Once leached into the lotic system (river), it will destroy their bicarbonate buffering system and enhance the rate of dissolution of metal ions, which could persist for hundreds of years once initiated, forming an age-long pollution stream with low pH [5, 19]. Consequently, the design lifespan of civil infrastructures such as water reticulation networks and bridges within this environment are shortened, caused by corrosion when metal oxides are formed [20]. In addition, the mortality rate of the aquatic organism within this inhospitable environment increases once these toxic metals accumulate in their organs [21]. More so, when this polluted water enters the irrigational system, the toxic metals accumulate in plants and indirectly enter the food chain. The growth of plants is distorted because, at low pH, plant nutrients such as nitrogen, phosphorus, and potassium necessary for their growth are immobilized, and the calcium and magnesium content becomes deficient. Meanwhile, in the food chain, these toxic metals bioaccumulate and cause deleterious health effects in humans (Table 1.2) due to their non-biodegradability [22, 23].
Table 1.2 Potential toxic elements and their effect in humans.
Elements
Recommended levels in surface or groundwater (ppm)
Health-related issues in humans
Reference
Aluminum (Al)
2.9
Aluminum exposure is a risk factor for the development or acceleration of the onset of Alzheimer’s disease (AD) in humans.
[33]
Arsenic (As)
0.01
Exposure to arsenic causes skin and internal organ cancers, impaired nerve function, kidney and liver damage, or skin lesions.
[34]
Copper (Cu)
2.0
Exposure to excess copper induces oxidative stress, DNA damage and reduced cell proliferation in the human body.
[35]
Iron (Fe)
1.0–3.0
Iron is an essential part of hemoglobin in humans, but its overload causes severe health problems such as liver cancer, diabetes, cirrhosis of the liver, heart diseases and infertility.
[36]
Manganese (Mn)
0.5
Manganese is an essential nutrient to the body, but in excess can also interfere with the normal function of the nervous system to induce motor and cognitive impairments as well as neuropsychiatric symptoms.
[37]
Lead (Pb)
0.01
Exposure to lead causes cardiotoxicity, neurotoxicity, nephrotoxicity, carcinogenicity and genotoxicity in humans.
[38]
Zinc (Zn)
5.0
Zinc is considered an essential mineral in humans as it is necessary to produce hundreds of enzymes throughout the body. The toxicity of Zn in humans differs significantly and varies from acute exposure to chronic exposure. Renal injury, ranging from asymptomatic hematuria to interstitial nephritis or acute tubular necrosis, has also been reported due to acute toxicity, while chronic exposure can lead to sideroblastic anemia and granulocytopenia, and myelodysplastic syndrome.
[39]
Rare earth elements (REEs)
Data not established and are currently unregulated in humans and environment
Despite their extensive application in electronics, exposure to these metals will cause dysfunctional neurological disorders such as reduced intelligence quotient (IQ) in children, bone alteration, genotoxicity and fibrotic tissue injury and antitesticular effects and male sterility in humans.
[40]
Table 1.3 Different types of applications and the uses of REEs.
Types of applications
REEs used and their functions
Reference
1. Medical (La, Ce, Pr, Gd, Nd, Tb, Dy, Ho, Er, Tm, and Yb)
Lanthanum oxide nanoparticles can be used for magnetic resonance imaging (MRI).
Cerium-doped lutetium orthosilicate is used to convert high-energy radiation to visible light for positron emission tomography (PET) imaging.
This test is used to reveal tissue and organ function.
Praseodymium oxide nanoparticles have been synthesized and used for cancer treatment as a radiotherapy technique.
The treatment of skin cancer, as well as hair removal using a laser beam, was achieved by using Neodymium as crystals.
The magnetic properties of Gadolinium are used to enhance MRI images of tumors and intravenous radio-contrast agents in MRI scans.
A radioisotope Dy-165 has been employed in the treatment of rheumatoid knee effusions.
Holmium-based solid-state lasers have been used for non-invasive medical procedures for treating cancers and kidney stones.
Erbium-based lasers have been used in medical and dental practice.
A radioisotope Tm-167 has been used as a power source in portable X-ray devices.
A radioisotope Yb-176 can be used to produce Lu-177, which is known to be a promising radioisotope for medical applications.
[41]
2. Telecommunication
Neodymium, terbium, and dysprosium are used in smart cell phones to enable them to vibrate.
[42]
3. Electronics
Scandium (Sc) is used in electron-beam tubes in TV.
Yttrium (Y) is used in the manufacture of capacitors, phosphors, microwave filters, glasses, oxygen sensors, radars, lasers and superconductors.
Eu, Tb, Gd, and Ce are used in flat-screen displays.
[43]
4. Automobile
La, Ce, Nd, and Pr are used as catalytic converters to efficiently control pollution in cars.
[44]
5. Weaponry
Yttrium(Y) and Terbium (Tb) are used for laser targeting and weapons in combat vehicles.
[45]
1.2.2 Rare-Earth Elements and Their Importance
Rare earth elements consist of the 15 lanthanide elements including yttrium (Y) and scandium (Sc) on the periodic table. These elements exhibit similarities in geochemical behavior because of their identical stable trivalent oxidation state (except Ce and Eu), systematic decrease in ionic radius and increasing atomic number [24]. Mineral deposits of REE typically occur in low concentrations as oxides or carbonates in a broad array of geologic formations in very few countries, and over 90% of REE production occurs in China [25]. This near-monopoly has created a conceivable handicap for other countries where REEs are not readily produced [10]. The demand for these REEs has increased tremendously over the years due to their extensive application in several scientific advancements (Table 1.3) owing to their unique magnetic, phosphorescent, and catalytic properties [26]. The extraction of these elements from their conventional ores is energy-intensive and alone is insufficient to satisfy the rising demand in the foreseeable future due to their strategic importance in modern technology [27, 28]. Consequently, it has prompted researchers, national governments, and private entities to develop possible techniques for recovering these elements from unconventional sources, such as AMD, to meet the rising demand. In the United States, a significant concentration of REE was found in precipitates formed during acid mine drainage treatment from coal tailings [10]. In Brazil, acid water generated in a uranium mine in the state of Minas Gerais was found to contain a total concentration of 126 ppm of REE significantly higher than acid waters generated from different mines worldwide [29]. Also, in the Guizhou province in southwestern China, the Xingren coalfield mine is reported to contain REE concentrations varying between 0.1 and 0.9 ppm [30]. Although the recovery of rare earth metals from AMD remains a great challenge as it is several orders of magnitude lower in this product than the conventional REE ores, it can be recovered using a low-cost nano-adsorbent if concentrated out of the liquid solution during the neutralization processes [31]. The emergence of nanotechnology has contributed tremendously to economic prosperity by providing solutions to some challenges facing modern-day technology [32]. In this chapter, a state-of-the-art technique will be presented using modified magnetic dendrimer nanoparticles to effectively recover REE from AMD after alkaline treatment. First, the generation of AMD and its environmental effect are highlighted, and then in the following sections the shortcomings of various remediation methods for AMD as background motivation for this method design are discussed.
1.2.3 Classical AMD Remediation and Treatment Methods
The effects of AMD on the environment are enormous, and several remediation technologies have been implemented (Figure 1.2) depending on the geographical conditions and weather effects of the mining sites to ensure environmental compliance. Over the past decades, neutralization techniques involving active and passive methodologies have been employed, and a detailed description of these technologies was reported by Naidu et al., including a few case studies on actual mining sites [46]. However, these technologies still have some shortcomings and call for logical approaches to enhance their effectiveness. Active treatment technologies use neutralizing agents, such as CaCO3, Ca (OH)2, CaO, Na2CO3, NaOH, and NH3, that produce sludge or precipitates heavily laden with metals and REEs, which require suitable disposal methods, making the procedure very expensive [47].
Figure 1.2 Schematic of the different types of remediation techniques for ARD neutralization or prevention.
Similarly, passive treatments, although being the least expensive approach, also make use of neutralizing agents. This technology is effective in mine waste with low acidic content and minimal water flow rate fluctuation. The major drawback of passive technology is that it requires a lengthier operative time for effective remediation, compromising its usage in modern mining activities [48]. Although investigations and pilot analysis have indicated that active and passive approaches can positively handle AMD [49], the continuous use of chemicals to neutralize AMD has raised some environmental concerns, making this procedure unsustainable and costly. In avoiding the use of excess chemicals, Tabelin and coworkers introduced two passivation techniques known as microencapsulation and galvanic microencapsulation techniques that could potentially prevent pyrite oxidation at the point source. The microencapsulation technique uses a redox-reactive organic carrier that is highly sensitive to pyrite in mine tailings to coat pyrite surfaces and thus prevent AMD formation. Meanwhile, galvanic encapsulation is based on galvanic interactions between pyrite and metals with lower rest potentials to suppress the oxidation of sulfide minerals [50]. However, these methods are not sufficient as they only suppress sulfide oxidation on a temporary basis, besides which sulfide oxidation is a natural process that is slow and difficult to prevent. A recent study has shown that active and passive biological remediation techniques offer great advantages, permanently removing sulphate and metals from mine drainage and having a high ability to recover the valuable metals [51]. However, the shortcomings of these methods is the high cost involved and the difficulty in setting up the processes, calling for a more sustainable and cost-effective technique for the remediation of AMD from mining MSWs.
1.3 Dendrimer as Extraction Agent of Rare Earth Element in AMD
Polymeric materials such as dendrimer nanoparticles have been proven to effectively recover metals from wastewater [52, 53]. Dendrimers are three-dimensional, nanosized polymeric material with great surface chemistry consisting of numerous end groups that can be manipulated or functionalized as modules for a thriving nanotechnology industry [54]. Intensive research studies have been conducted to evaluate the feasibility of retrieving REE from unconventional sources using different techniques (Table 1.4). However, these techniques also have their shortcomings involving cost and, therefore, developing an appropriate cost-effective method to extract them from AMD could make it a perfect source of REE. The use of dendrimers as an absorbent for the removal of metals in solution is a growing trend, and structurally, it has a tree-like branched nature consisting of a central core, an interior and exterior branched cell with the potential of forming more branches through a repetitive synthesis of monomers known as generation growth [55]. The higher generation growth of dendrimers is believed to carry more functional groups on their molecular surfaces. This allows them to interact with solid surfaces, acting as an adsorbent or ligands soluble in water to remove solvated toxic metals [56–58]. The properties of dendrimers are dependent mainly on these functional groups, such as amine, carboxyl, or hydroxyl groups, attached to dendrimers, which are used primarily for adsorption reactions of different targets [55, 59]. For instance, amine-terminated dendrimers, such as polyamido-amine dendrimers (PAMAMs), exhibit a high binding affinity for metal ions to their surface via coordination with the amine or acid functionality [60].
Table 1.4 Recovery of REE using different techniques.
Types of REEs
Technique(s) used for recovery/extraction
Gd
Magnetically retrievable imprinted chitosan/carbon nanotube composite reverse osmosis.
Binding on mesoporous silica supports functionalization with diphosphonic acid.
Ion exchange on cesium molybdo vanado phosphate immobilized on platelet SBA-15.
La, Ce, Pr, Nd, Sm, Dy, Ho, Er, Tm
Adsorption on silica gel modified with diglycol amic acid.
Tb
Nano-Mg (OH)
2
reaction column.
Extraction using solvent-impregnated resin containing TOPS 99.
Eu
Sorption on graphene oxide-supported polyaniline composites.
Adsorption on graphene oxide nanosheets.
Nd
Adsorption on carboxymethyl chitosan adsorbents entrapped by silica.
Lu, Yb
Solid-liquid extraction using Tulsion CH-96 resin.
1.3.1 Poly(amidoamine) (PAMAM) Dendrimers
Poly(amido-amine) (PAMAM) was first reported in the 1980s and became the first dendrimer family to be produced and marketed [61]. It is a class of monodisperse, hyperbranched polymer with rich terminal amino functional groups that can be precisely controlled and functionalized with hydroxyl (OH), the carboxylic acid (COOH) or conjugated to hydrocarbon chains [62]. Full generational growth of PAMAM dendrimer can be synthesized with the aid of a microwave by first separately dissolving EDA and multi-ester in methanol, then allowing both solutions to cool down before gradually adding the multi-ester to EDA solution warmed at ambient temperature for several days [63]. PAMAM has gained a lot of research interest as the most widely studied dendrimer due to its unique characteristics such as good biocompatibilities, a high specific surface area, good chemical stabilities, high-capacity chelating agents for metal ions, and many tertiary and primary amine groups in its inner and surface structure [64, 65]. As such, research interest has increased over the years for using PAMAM dendrimers for the adsorptive removal of heavy metals from soil and aqueous solution [66] (Table 1.5). Adsorption is commonly employed due to its simplicity, low cost, and high efficiency.
1.3.2 Principle REE Extraction Using PAMAM
Despite the extensive application of PAMAM dendrimers, no report has outlined their direct use for the recovery of REEs from AMD. This can be attributed to the low pH of the acidic effluent, which instead favors the protonation of the terminal amine functional groups, producing cationic charge on their surfaces to deactivate their adsorptive capacity to metal ions through electric repulsive forces. However, suppose the surfaces of PAMAM dendrimers are functionalized with carboxyl functional groups such as succinic acid. In that case, it will protonate in the acidic effluent to anionic groups, which will increase the binding ability of the polymer with metal ions in the acidic solution via electrostatic attraction. Considerable research interest has been focused on using magnetic nanoparticles (MNPs) as supporting cores for dendrimers to remove heavy metals from wastewater due to their intrinsic magnetic property, which allows for convenient separation through the deployment of an external magnetic field [67]. This feature improves the economic viability of the separation and re-use of dendrimers in a wide range of applications [55]. Therefore, magnetic PAMAM dendrimer nanoparticles will be synthesized, and their surfaces functionalized with succinic acid (PAMAMCOOH@MNPs) (Figure 1.3) and used as a perfect absorbent to recover REEs from AMD.
Table 1.5 Use of PAMAM dendrimer for the removal of metal ions.
Contaminants
Mode of removal
Study outcome
Reference
Cu
2+
and Pb
2+
A single and binary component system using modified carbon nanotubes (CNTs) with four generations of poly-amidoamine dendrimer (PAMAM, G4) for the removal of Cu
2+
and Pb
2+
.
The study was very effective, achieving high adsorption capacities for copper and lead (3333 and 4870 mg/g respectively).
[68]
Mn(II)
silica-gel supported amino- and ester-terminated polyamidoamine dendrimers. The adsorption of the contaminant in solution is dependent on the terminal group.
The results revealed that amino-terminated polyamidoamine dendrimers could be potentially used as promising adsorbents for the effective removal of Mn(II) from an aqueous solution.
[69]
Cd(II), Pb(II) and Cu(II)
As-synthesized magnetic graphene oxide nanocomposite was grafted polyamidoamine dendrimer.
The adsorption capacities of metal ions were 435.85, 326.729 and 353.59 mg g
-1
for Cd(II), Pb(II) and Cu(II), respectively, which proves excellent removal ability of the metal ion from water and wastewater.
[70]
Hg
2+
Carboxyl-terminated hyperbranched poly(amidoamine) dendrimers grafted superparamagnetic nanoparticles with the core-shell structure for selective removal of mercury from the aquatic sample.
As a result, the carboxyl-terminated hyperbranched poly(amidoamine) dendrimers grafted superparamagnetic nanoparticles displayed excellent properties and rebinding ability toward Hg
2+
ions.
[71]
Ni
2+
, Fe
2+
, and Fe
3+
Metal ion remediation using polyamidoamine dendrimers as a chelating agent.
Metal ion removal rates from simulated wastewater were evaluated for these metal ions, and the complexation of Ni
2+
to internal tertiary amine sites occurred more rapidly than that of Fe
3+
, which was more rapid than Fe
2+
.
[72]
Figure 1.3 Schematic of magnetic PAMAM succinamic dendrimer nanoparticle.
1.4 Designed a Recovery System for REE from AMD
1.4.1 Process Overview
Acid mine drainage is a complex effluent characterized by low pH with multiple leached ions depending on their geological formation. The key to recovering these metal ions from waste is based on the fundamental understanding and characterization of the solution chemistry for any given mining site. The use of PAMAM-COOH@MNPs for REEs recovery is very critical as it will stop further production of sludge and adsorb multiple metal ions on their surfaces within the acidic medium. Figure 1.4 is a general representation of the procedure to recover REEs from AMD. However, this design will consider AMD with a high concentration while noting the presence of sulfate ions as the main anion load present in the solution. All the chemical species present in the AMD are mostly available in their oxidized state. The design consists of different stages called reactors, which will be described accordingly with supportive chemical equations to quantify each stage.
Figure 1.4 Systematic representation of the proposed processes for the recovery of REEs and water from ARD heavily laden with metals.
1.4.2 Components and Their Functions
1.4.2.1 Reactor 1 – Collection Tank
This is the initial stage of the reaction where the acidic effluent collected from mine drainage is being stored in a tank for filtration. The filtration process will remove all solid materials and coarse impurities such as algae and suspended rock particles.
1.4.2.2 Reactor 2 – Mixing Tank
Immediately after filtration, the acidic effluent will be pumped into the mixing tank for further treatment. At this stage, sulfate ions that are considered the main impurities will be removed to prevent them from reacting with metal ions in the effluent as such, lowering the grade of metals to be recovered. To circumvent the effects of sulphate ions, lime water (calcium hydroxides) from a different tank is pumped simultaneously with PAMAM-COOH@MNPs into the mixing tank. The pH is controlled using the lime water until it reaches the neutral point while stirring the reaction mixture at 400 rpm for 24 hours. The process of controlling the pH using calcium hydroxides is known as stage precipitation, which prevents the sulphate ions from complexing with the REEs within the acidic condition. The introduction of Ca2+ ions will react with the sulfate ligands as the rate of bonding interaction between these two ions is faster than the REE ions in solution due to the high reactivity of Ca2+ ions (Eq. 1.1). Eventually, the proportion of REE ions in the solution will subsequently increase, and its recovery will be enhanced [73]. The introduction of PAMAM-COOH@ MNPs at the initial stage under an acidic condition causes the protonation of the carboxylic functional group on the surfaces of dendrimer producing anions in solution, and since lanthanides are hard Lewis acids and prefer binding to hard donor atoms, such as oxygen [74], will eventually bind with the REE ions on the surfaces of PAMAM-COOH@MNPs via electrostatic force of attraction (Eq. 1.2).
1.4.2.3 Reactor 3 – Separation Tank
From the mixing tank, the solution is then pumped into reactor 3, which is designed with an external magnetic field. This is where the separation process will take place with the deployment of an external magnetic field. Since the dendrimer has the potential to respond to an external magnetic field, it will be separated from the rest of the solution and drained into tank 4 for the final recovery step. The residual effluent will be pumped into tank 5 for further treatment before it can be decanted into the environment or used for consumption and agricultural purposes. It should be noted that the residual effluent is neutral and cannot pose any further harm to the environment. However, it should be treated to ensure that it is safe for consumption.
1.4.2.4 Reactor 4 – Recovery of REEs Metals
Since the surfaces of the dendrimer nanoparticles have a high density of active sites, it will enhance the absorptivity of REEs in the acidic effluent, and at reactor 4, only these materials will be present. However, the REEs can be recovered from the dendrimer nanoparticles by slight acidification to initiate the protonation of these metal ions in the solution (Eq. 1.3). This property makes PAMAM dendrimers promising recyclable chelating agents for the metal ion separation [63]. The individual lanthanides can be extracted using an ion-exchange method where the ionic solution of the lanthanides is flush through a column containing resins and each ion is bonded to the resin with various strengths based on their ion size. After recovery of the lanthanides, the pH of the solution is then adjusted, and the formation of PAMAM-COOH@MNPs is achieved and can be reused.
1.5 Challenges and Opportunities for the Future of Metal Mining
Mining activities provide the many raw materials needed for modern technology, and our future is deeply dependent on them despite their serious environmental and social impact. The use of REEs has skyrocketed, and the future demand is uncertain, which makes it difficult to control and manage their waste as the rush for these minerals hasten mining operators to dig the ore, sell the metals, and, once the deposit is exhausted, walk away and start another mine elsewhere without prior planning to clean up their waste [75]. This attitude has created a lot of unattended landfills around the world for decades now, polluting the environment. Therefore, current mining practices need to change. Moreover, all stakeholders involved need to engage more responsibly by implementing cleaner technologies to minimize their environmental threat. However, mining companies lack these technological innovations to improve the sustainability of their mining operations and require more research endeavours to assist them. However, research has proven that the voluminous tailings dams or landfills after mining activities have become a viable source for some of these precious metals, which can be more economical to recover or recycle than mine ore deposits. The formation of AMD is a viable source for REEs, and despite its environmental consequences, it has presented another option for obtaining these scarce commodities to meet their rising demand. Therefore, the technological ideas to recover REEs from AMD presented in this chapter have opened up future opportunities for mining operators to easily incorporate these cost-effective methodologies in their extraction design to clean up their waste and extract valuable metals to boost their yield and generate more income. The most important step for this technological idea is to engineer an alkaline treatment technology to optimize a high concentration of REEs in the acidic waste before using magnetic dendrimer nanoparticles to aid their recovery. The many functional groups on the surfaces of dendrimers with binding affinity to multiple metal ions are the most intriguing aspect of this technological innovation. However, the difficulty inherent with this procedure will be separating the individual REEs from each other to yield pure single elements, and this will require optimization of factors such as pH to control their recovery.
1.6 Conclusion
The formation of AMD from sulfidic ores during mining activities is an inevitable process that can be enhanced as a secondary source for REEs using an effective alkaline treatment technology engineered to optimize a high concentration of REEs in the waste. Despite the occurrence of metals in rocks and soil as a natural constituent of the Earth’s crust, the leaching of AMD products into the environment will enhance their presence at an elevated concentration significantly higher than the acceptable levels prescribed by the World Health Organization (WHO). Therefore, it is imperative to treat AMD to a useable standard as defined in the water quality guidelines. On the contrary, AMD is a valuable source of REEs which can be recovered to compensate for their shortage in supply needed for advancement of technological developments. Although the recovery of rare earth metals from AMD remains a great challenge, the use of nano-absorbent such as dendrimer has enhanced their recovery, making AMD a future viable source for these precious metals. Dendrimers are an exceptional cost-effective material with surfaces that are easily designed to suit any intended application. The use of magnetic PAMAM dendrimer nanoparticles surface-functionalized with succinic acid presented in this chapter is a pragmatic approach with dual functionality. The succinic functional group attached on the surfaces of the dendrimer will protonate in the acidic effluent to anionic groups, which will increase the binding ability of the polymer with REEs in solution via electrostatic attraction, and its magnetic property will be used to separate the bound metal ions from the solution. This integrated approach will recover REEs from the acidic effluent using a low-cost material that itself is environmentally friendly to generate income for mining operators, thus stopping the pollution cycle.
Acknowledgment
A. J. L. thanks Prof. Elvis Fosso-Kankeu and Prof. Martin Mkandawire for their excellent mentorship and continued support. The authors also acknowledge the support from Prof. Bhekie B. Mamba, the University of South Africa (UNISA), North-West University and Cape Breton University.
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