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Overview On Extraction and Separation of Rare Earth Elements from Red Mud: Focus on Scandium
Abstract
The paper provides an overview of the methods used for processing of red mud to extract rare earth elements (REEs). Red mud is a toxic and highly alkaline waste. Several methods have been adopted and being practiced all over the world for the processing of red mud. Complex processing of red mud is cost-effective since red mud contains iron, aluminum, titanium, calcium, rare earth metals etc. It has been observed that the acid leaching of red mud can almost completely recover the rare earth elements in the solution with various individual techniques and also a combination of them. Therefore, the choice of extraction method depends on the form in which the element occurs in the solution. However, relatively low concentrations of rare earth in the solution and significant amount of impurities increase the cost of getting the final commercial products. To ensure the cost-effectiveness of the process involving rare earth’s extraction from red mud, it is necessary to increase their content by several times. This article presents the various studies that have been carried out in these aspects and the possibility of making this resource a sustainable one for REE extraction with a special focus on scandium replenishment.
... Red mud is an environmental liability due to its high alkalinity, complex composition, fine particle size, and large production volume [5][6][7][8]. Red mud slurry has a pH value typically in a range of 10 to 12 [7,8]. Owing to its high alkalinity, red mud is considered hazardous waste. ...
... Red mud is an environmental liability due to its high alkalinity, complex composition, fine particle size, and large production volume [5][6][7][8]. Red mud slurry has a pH value typically in a range of 10 to 12 [7,8]. Owing to its high alkalinity, red mud is considered hazardous waste. ...
... According to Fig. 1-a, Fe, Al, Ca, and Ti containing compounds constituted the major phases of the sample. Iron is mostly observed as hematite (Fe 2 O 3 ) and chamosite ((Fe 2+ , Mg) 5 Al (AlSi 3 O 10 ) (OH) 8 ). Most of the Ti of the sample was in the form of TiO 2 as two different forms of anatase and rutile. ...
... During alumina production using the Bayer process, a solid waste known as red mud is generated (Akcil et al. 2018). The annual production of red mud is estimated to be around 100 to 150 million tons (Evans 2016). ...
... One of the most common methods of disposing of red mud is storing it in open-air dams or discharging it into the ocean (Borra et al. 2017). The high pH of red mud (11 to 12.5) makes its proper disposal a major environmental challenge (Chun et al. 2014;Xue et al. 2016;Akcil et al. 2018;Narayanan et al. 2018). Besides its environmental risks, the disposal of red mud is costly and occupies large land areas (Lockwood et al. 2015;Wen et al. 2016;Swain et al. 2020). ...
... The presence of Fe, especially in the form of ferric, can significantly reduce the final purity of the Ti product. Solvent extraction is a widely used method for the concentration and recovery of REEs (Akcil et al. 2018). Organophosphorus-based extractants are the most common extractant group used for the solvent extraction of REEs. ...
In this study, a combination of physical and chemical separation processes was used to recover the metallic components of red mud. At first, the impact of carbothermal reduction on magnetic separation of iron was studied. Low magnetic properties of iron minerals resulted in insignificant separation of iron from other components in the non-carbothermally reduced sample. Various carbothermal reduction parameters were optimized to maximize iron separation from other components. The optimum conditions were found T = 1350 °C, t = 120 min, coal/red mud ratio of 3, reaction time of 120 min, and the soda ash/red mud ratio of 0.2. Under the optimum condition, the iron recovery of the magnetic product was observed 91% with 81% Fe content, while the non-magnetic product has contained 90% of Ti and Al and 80% of rare earth elements (REEs). Following the physical separation of iron, the chemical separation of remaining red mud components was investigated using leaching with sulfuric, hydrochloric, and nitric acids. The leaching experiments were performed on two samples, treated red mud with carbothermal reduction and an untreated sample. The untreated sample had a higher dissolution efficiency for Ti and REEs than the carbothermally reduced sample. Different dissolution behavior of the red mud components was explained by samples’ mineralogy. In the end, considering the obtained results, various scenarios for the recovery of red mud components were evaluated from technical and environmental aspects.
... The REEs contained in bauxite residues are at a content of 500-1700 mg/kg [7,8], and the primary REEs are lanthanum(La), cerium(Ce), neodymium(Nd), praseodymium(Pr), scandium(Sc), and yttrium(Y) [9]. Bauxite residues have provided potential opportunities for the production of REEs apart from the use of ion adsorption clay deposits [10]. The REEs in bauxite residues are generally extracted through acid leaching followed by a solvent extraction process. ...
... In the actual extraction process, both an ion exchange reaction and a solvation process could occur. The reaction can be represented by Equation (10). The equilibrium constant K was calculated according to Equation (11) and further transferred into Equation (12). ...
Bauxite residue is a hazardous solid waste produced in the alumina production process and has also become a significant rare earth resource. The extraction behaviors of La, Ce, Sc and Y solubilized in the phosphoric acidic leachate of bauxite residue were investigated in this study with di-(2-ethylhexyl) phosphoric acid as the extractant. With a relatively low concentration of 2% at an aqueous solution pH of 1.5, 90% Sc and 98% Y were extracted by di-(2-ethylhexyl) phosphoric acid. Less than 5% La and Ce and impurities of Fe, Al, Ti and Ca were extracted in this situation. As the concentration of di-(2-ethylhexyl) phosphoric acid increased to 20%, almost all the Sc and Y were extracted and the extraction ratios of La and Ce were 87% and 95%, respectively. A good separation of REEs against impurities was simultaneously obtained in the solvent extraction process and their separation coefficients were much higher than 1. A stepwise extraction process was proposed and established to extract Sc/Y and La/Ce sequentially from the phosphoric acidic leachate. It was further revealed that the Sc and Y in the acidic leachate were extracted by di-(2-ethylhexyl) phosphoric acid through an ion exchange process, and that the extraction of La and Ce was due to an antagonistic process where both an ion exchange reaction and a solvation reaction occurred.
... Scarce Sc deposits are monopolized worldwide which drives prices up and threatens global technological and sustainable development. Therefore, the development of effective, profitable, and eco-friendly Sc mining technologies is an urgent challenge of modern science [8,9]. ...
... Sc purification is a many-stage process, which makes it expensive and difficult to obtain in high amounts. Furthermore, the separation of REEs from each other is a very difficult process because of their highly similar physicochemical properties, ionic radii, valence, and magnetic properties [8,46]. However, biosorption of Sc by different biological systems is marginally better than that of other REEs. ...
Scandium (Sc) plays a special role in high-tech industries because of its wide application in green, space, and defense technologies. However, Sc mining and purification are problematic due to political, technological, and environmental difficulties. The deficit of this element limits global technological development. One sustainable solution to this problem is to use microorganisms to extract Sc from ore and waste, as well as to concentrate and separate it from other elements. Sc also demonstrates attractive metabolic effects on microbes that is of great interest in white biotechnology. Sc increases the production of proteins and secondary metabolites and activates poorly expressed genes. This review offers a comprehensive analysis of current knowledge on the application of Sc–microorganism interactions in promising biotechnologies, its perspectives, and future challenges.
... At present, the source of scandium is comprehensively recovered from titanium dioxide waste liquid, red mud (bauxite residue), tungsten acid slag, and so on, the production, and application of scandium is also greatly restricted (Xiao et al. 2020b;Chenna et al. 2015;Akcil et al. 2018). The development of effective methods of separation and extraction of scandium from poorer raw materials should be considered as a strategy for metals' circular economy that also diminish the environmental impact of these residues. ...
... Red mud is a by-product of bauxite processing through the Bayer process (Zhao, Miller and Wang 2010). Throughout production of alumina by the Bayer process, 95-100% of the scandium contained in bauxite remains in red mud (Akcil et al. 2018). The scandium content of the red mud varies between 15 and 170 ppm, depending on the source of bauxite, which is a potentially valuable scandium resource (Avdibegovic, Regadío and Binnemans 2017). ...
The huge reserves of red mud have a significant potential source of scandium. The scandium can be recovered from red mud by direct acid leaching but the extraction yields are generally low, and the leaching efficiency of iron and silicon is higher. Therefore, the highly selective process steps (a combined sulfation-roasting-water leaching process) were studied to extract scandium from red mud, and in order to minimize costs and waste generated. This study focuses on the phase transformation mechanism and leaching behavior of Sc, Na, Al, Fe, Ti, Ca, and Si in the red mud sulfation-roasting-water leaching process. The results show that the maximum Sc leaching efficiency of 91.98% was obtained at optimum conditions: the roasted at 1023 K for 60 min, the ratio of sulfuric acid to red mud of 0.9 mL/g, the leaching temperature is 323 K, the liquid-to-solid ratio of 5 mL/g, and stirred constantly at 200 rpm leach for 2 h. While the leaching efficiencies of Ca, Na, Fe, Al, Si, and Ti are 21.05%, 93.26%, 1.21%, 9.51%, 1.42%, and 0, respectively. The sulfation-roasting-water leaching process proves to be a promising technique and commercially viable process that allows selective extraction of scandium from red mud, which may contribute to provide options for the treatment of red mud produced continuously by the aluminum industry.
... Thus, extracting REEs and vanadium is an inevitable approach to realize the resourceful and valuable utilization of RM. The contents of neodymium, lanthanum and cerium in RM is 2-6 times than those in earth's crust (Akcil et al., 2018). REEs are mainly present as symbiotic (Lu et al., 2018). ...
As a bulk solid waste with high alkalinity, red mud (RM) not only occupies a large amount of land and requires high maintenance costs, but also unavoidably generates serious hazards to the surrounding ecological environment. The comprehensive treatment of RM has become an enormous challenge for the green, low-carbon and high-quality development of the global alumina industry. To minimize the RM destruction to the ecology and the waste of secondary resources, the sustainable utilization of RM was widely investigated in the past decades, especially for the recovery of valuable metals. This paper systematically summarized the research status of recycling valuable metals (Al, Fe, Na, Ti, Sc, Ga, V and RE) from RM in recent years. The recycling technology mainly includes physical beneficiation, hydrometallurgy, pyrometallurgy and electrodialysis. The technical principles and characteristics as well as the current problems of various recovery processes from RM were comprehensively introduced, and the future development directions of sustainable utilization were also prospected. The advantages and disadvantages based on the different aspects of recovery efficiency, energy consumption and environmental impact were also discussed. The proposal of new technologies for the harmless, high-value and full utilization of RM is beneficial to the future research on the comprehensive utilization of bulk industrial solid wastes.
... Therefore, there has been numerous research on the extraction of REEs and titanium from red mud. Literature reviews indicate that most of the extractions of REEs from red mud have been experimentally investigated using hydrometallurgy methods involving leaching in acid solution followed by retrieval of REEs from the solution using precipitation, solvent extraction or ion exchange adsorption (Zhang et al. 2016;Akcil et al. 2018;Archambo and Kawatra 2021). Recovery of titanium and gallium from red mud have also been experimentally investigated via the hydrometallurgy route (Mehta and Patel 1951;Kasliwal and Sai 1999;Agatzini-Leonardou et al. 2008;Ghorbani and Fakhariyan 2013;Abdulvaliyev et al. 2015;Huang et al. 2016;Alkan et al. 2018;Xue et al. 2019). ...
The Western Indonesia Bauxite Province in Kalimantan forms a lateritic bauxite region with a complex history and poorly known sustainable metal contents within the bauxite residue. Bauxite residue produced using the Bayer process contains notable scandium. We present new geochemistry, mineralogical, and geological data from the lateritic bauxite and red mud from the active mine and deposit, which aims to investigate the behavior of critical elements during weathering. The geochemical analysis and translated isocon results have shown that the content of scandium in red mud is higher than the average concentration of crustal rocks and is concentrated in the ferrite layer and bauxite residue. A positive correlation between the existence of iron oxyhydroxide mineral in residual iron-rich layer and red muds with the rare earth elements (REE) and scandium concentrations may be interpreted as a scavenging effect of mobile REE. The weathering and leaching processes in bauxite allows the adsorption of the trivalent scandium cation (Sc ³⁺ ) on goethite and are followed by the ionic substitution with other trivalent cations in the crystal of hematite. The study illustrates the importance of understanding processes during weathering and laterization for geochemical processes and rare earth elements exploration in tropical areas.
Thematic collection: This article is part of the Geochemical processes related to mined, milled, or natural metal deposits collection available at: https://www.lyellcollection.org/topic/collections/geochemical-processes-related-to-mined-milled-or-natural-metal-deposits
Supplementary material: https://doi.org/10.6084/m9.figshare.c.6689139
... Similar results can be facilitated using microwaves (Agrawal et al., 2019) or pre-washing with sulphuric acid (H 2 SO 4 ) or sulphate solutions (Meng et al., 2020). From further sintering J o u r n a l P r e -p r o o f processes (Akcil et al., 2018) and alkaline leaching, REEs can also be recovered (Borra et al., 2015). ...
Bauxite residues known as “Red Mud” (RM) are the principal waste of caustic digestion of bauxite from the Bayer Process, whose costs of disposal are expensive and cover 5% of the total costs of extraction and processing for aluminium production. Nevertheless, this material can be considered an important source of high-value elements, such as rare earths (REEs) and metals, Fe, Ti, Al and others. In this work, the focus has been on the recovery of iron in the form of the compound Fe(II) oxalate. Four types of acids have been used (HCl, H2SO4, H3PO4, H2C2O4) for iron extraction from RM coming from Montenegro. Hydrochloric acid shows a higher iron extraction capacity, reaching an iron extraction yield in solution of 22.6%. Sulfuric and phosphoric acid, instead, interacted with RM leading to the formation of sulfonate and phosphate species, inhibiting the leaching ability of individual species. Oxalic acid showed the least amount of iron ions extracted but formed a stable ionic complex in solution, Fe2(C2O4)3∙2H2O. Starting from this complex it was possible to recover the corresponding salt by a reduction and precipitation process. Through a pre-treatment with HCl and a subsequent treatment with oxalic acid, it was possible to obtain a better yield of iron oxalate. Starting from the laboratory scale, a CHEMCAD plant was conceptualized with a yield higher than 16% per pass (repeatable 3 times with a global iron yield >50%) and obtaining iron(II) oxalate dihydrate with purity up to 96%wt. In a holistic view of the problem, the proposed process can operate in parallel with other procedures proposed in the literature for the recovery of other valuable substances from red mud.
... In addition to REE-containing orebodies, there are potential alternative sources of REEs that could address the current lack of local supplies while enabling waste valorization. Some of these sources include products of coal combustion (coal fly ash and bottom ash), phosphogypsum [55], red mud (aluminum processing residue) [2], and electronic waste [3]. Among these potential sources, coal fly ash has received the most research attention over the past 20 years for a variety of reasons [57,37,43,59]. ...
... Largely dispersed but not actually rare, Sc is the first, lightest and the most expensive transition metal, and with 20-30 µg/g natural abundance in earth's crust is ranked the 50th most abundant element (Altinsel et al., 2018). However, Sc does not often occur as a single deposit (USGS, 2010) and it can be recovered from secondary sources such as mine tailings including bauxite residue (Akcil et al., 2018;Li et al., 2018;Zhu et al., 2017). Since Sc does not bind to common oreforming anions, it is found in more than 100 minerals at low concentrations (USGS, 2020). ...
... Al and Ti are recovered using hydrometallurgy, here different leaching and digesting agents are used to extract metals, and after that, concentration/precipitation steps are commenced (Ghorbani and Fakhariyan 2013). Likewise, valuable rare earth elements like Sc have a remarkably higher concentration in red mud than in their natural ore (Akcil et al. 2018;Lei et al. 2021). European Commission classified Sc as a critical raw material (CRM) due to high demand and low availability (Ochsenkuehn-Petropoulou et al. 2018;Cusack et al. 2019). ...
The mining industry has powered the human endeavor to make life more innovative, flexible, and comfortable. However, it has also led to concerns due to the increasing amount of mining and associated industrial waste. Special attention is highly desired for its proper management and safe disposal in the environment. The problem has only augmented with the increase in the mining costs because of the investments needed for ecological remediation after the mining operation. It is pertinent that the targeted technologies need to be developed to utilize mining and associated industrial waste as a secondary resource to ensure sustainable mining operations. Every perceived waste is a valuable resource that is needed to be utilized to create additional value. In this review, the case of alkaline bauxite residue (red mud)—alumina refinery waste has been discussed at length. The highlight of the proposed work is to understand the importance of alkaliphile-assisted biomining—a sustainable alternative to conventional metal recovery processes. Along with the recovery of metals, pH reduction of red mud is possible through biomining, which ultimately paves the way for its complete utilization. The unique adaptation strategies of alkaliphiles make them more suitable for biomining of red mud through bioleaching, biosorption, and bioaccumulation, which have been discussed here. Furthermore, we have focused on the potential of the indigenous microflora of red mud for metal recovery in addition to its neutralization. The study of indigenous alkaliphiles from red mud, including its isolation and propagation, is crucial for the industrial-scale application of alkaliphile-based technology and has been emphasized.
... Meanwhile, red mud is a typical secondary resource, containing Fe, Al, Na, Ti, Cr, Sc, V, Ga, and other valuable elements [10]. Unfortunately, it is difficult to conduct red mud effectively by conventional beneficiation methods (i.e., gravity separation, magnetic separation, floatation, and their combination process) because of its complex material composition, micro-fine grain size, and high alkalinity [11][12][13][14][15]. The reduction, recycling, harmlessness and full component utilization of red mud can not only solve the serious environmental pollution caused by the above-mentioned red mud stockpiling, but also alleviate the resource guarantee dilemma of China's high dependence on imports of iron ore under the current serious shortage of domestic iron ore resources, reduce China's dependence on foreign iron ore, and promote the sustainable, healthy and coordinated development of China's national economy [16]. ...
In order to simulate the pre-reduction behavior of ore powder by coal gas produced by smelting reduction of coal-based electric furnace, an efficient and clean utilization technique for red mud based on fluidized bed carbon monoxide reduction was developed in the present study. Experimental results indicated that a metallization rate of 68.08 % and reduction degree of 78.72 was obtained under the optimal conditions of reduction temperature of 800 °C, CO concentration of 85 %, and reduction time of 30 min. Pre-reduced materials can be used as raw materials for electric furnace melting reduction. The order in which iron oxides were reduced is only related to temperature, and at 800 °C, iron oxides were reduced in the order of Fe2O3 to Fe3O4 to FeO to Fe. During different stages of prereduction, the surface structure of ore particles changes, which is related to the metallization rate of iron oxides in red mud.
... Generally, Sc is obtained as a by-product of the processing of various ores, or it is extracted from previously processed tailings or residues, such as RM (Anawati and Azimi, 2019). Scandium accounts for approximately 90% of the commercial worth of the REEs found in RM, and its high concentration (50-120 mg/kg) makes it a suitable ore for this element (Reid et al., 2017;Akcil et al., 2018). These concentrations are significant when compared to that of the natural element in the Earth's crust of 23 mg/kg and more than that detected in bauxite ore Botelho Junior et al., 2020). ...
The aim of this study was to recover Sc as the main product and Fe as a by-product from Hungarian bauxite residue/red mud (RM) waste material by solvent extraction (SX). Moreover, a new technique was developed for the selective separation of Sc and Fe from real RM leachates. The presence of high Fe content (∼38%) in RM makes it difficult to recover Sc because of the similarity of their physicochemical properties. Pyrometallurgical and hydrometallurgical methods were applied to remove the Fe prior to SX. Two protocols based on organophosphorus compounds (OPCs) were proposed, and the main extractants were evaluated: bis(2-ethylhexyl) phosphoric acid (D2EHPA/P204) and tributyl phosphate (TBP). The results showed that SX using diethyl ether and tri-n-octylamine (N235) was efficient in extracting Fe(III) from the HCl leachate as HFeC14. Over 97% of Sc was extracted by D2EHPA extractant under the following conditions; 0.05 mol/L of D2EHPA concentration, A/O phase ratio of 3:1, pH 0–1, 10 min of shaking time, and a temperature of 25 °C. Sc(OH)3 as a precipitate was efficiently obtained by stripping from the D2EHPA organic phase by 2.5 mol/L of NaOH with a stripping efficiency of 95%. In the TBP system, 99% of Sc was extracted under the following conditions: 12.5% vol of TBP, an A/O phase ratio of 3:1, 10 min of shaking time, and a temperature of 25 °C. The Sc contained in the TBP organic phase could be efficiently stripped by 1 mol/L of HCl with a stripping efficiency of 92.85%.
... Various acids are used to dissolve multiple elements (iron, aluminum, titanium, and rare earth elements [REEs]) in red mud in accordance with the acid dosage, leaching time, and temperature [13,14]. However, multielement dissolution complicates and increases the cost of downstream separation processes [15,16]. In addition, the high-pressure hydrochemical treatment of red mud has been performed [17]. ...
A combined low-temperature sodium salt-assisted roasting and water leaching process was investigated as a cleaning method for the treatment of Al-goethite-containing red mud (AGRM), which is conducive to aluminum recycling and iron mineral enrichment in leaching residue. In this work, the mineralogical characteristics and phase transformation of AGRM roasted at low temperature were evaluated by using an advanced mineral identification and characterization system, thermogravimetric analysis and differential scanning calorimetry, X-ray diffractometry, and backscatter scanning electron microscopy/energy dispersive spectrometry. In addition, the main factors, such as roasting temperature, sodium hydroxide dosage, leaching temperature, and time were investigated. Results revealed that the fraction of aluminum in Al-goethite is up to 73.59% of the total aluminum content in AGRM. The transformation of Al-goethite into Al-hematite occurred at approximately 360 °C, and adding sodium hydroxide can promote the conversion due to the formation of sodium aluminate. Compared with AGRM after roasting at 400 °C for 30 min followed by water leaching at 30 °C for 10 min, the leaching rate of aluminum increased from 0.36% to 90.21% and the grade of TFe in the leaching residue increased from 45.63 wt% to 54.09 wt% after roasting with 25 wt% sodium hydroxide under the same conditions. Given that the enhanced transformation of Al-goethite significantly improved aluminum recovery and the obtained iron-rich leaching residue can be easily co-disposed in the steel industry, thus may achieve the almost zero-waste discharge of AGRM from the Bayer process.
... However, a large number of studies are aimed to find additional scandium resources (Wang et al., 2011). The most promising of which are bauxite ores processing wastes i.e. red mud (Akcil et al., 2018;Ujaczki et al., 2018), vanadium mud (Wang et al., 2013;Cao et al., 2020), and Nb-Ta ores (Purcell et al., 2018;Molchanova et al., 2019). ...
This study contains research on the sorption separation of scandium, zirconium, and titanium from acidic sulfate solutions by a novel monofunctional Purolite RUA21207 ion exchange resin with primary amino groups. The kinetic and equilibrium performance of Sc, Zr, Ti sorption has been investigated. The results revealed that sorption favored at 25 g L⁻¹ H2SO4 in solution when the initial scandium concentration had been no more than 0.9 g L⁻¹. Zirconium and titanium could be selectively eluted from the loaded resin with the solutions of 150 g L⁻¹ KHF2 and 50 g L⁻¹ HCl, respectively.
In this work, the scandium concentrate generated from acid waste of titanium white production has been dissolved in 25 g L⁻¹ H2SO4. Then, the solution has been passed through a column loaded with Purolite RUA21207 resin. Purification of scandium by 147 times that of zirconium and more than twice that of titanium have been achieved. Purolite RUA21207 resin has high potential application in the purification of scandium concentrate from Zr and Ti by sorption and definitely can be used for providing high purity scandium oxide.
... The high concentration of major metals in leached solution of the waste strongly interferes with the extraction of REEs as their separation is problematic [66]. Ion exchange is used for the separation however, cationic contamination of the resins by species such as Fe 3+ , Al 3+ , Ca 2+ decrease the sorption or selectivity of REEs [61]. ...
Bauxite Residue (BR) is an industrial waste generated by the extraction of alumina through the Bayer process. It is usually stored in specially constructed sedimentation ponds. Long term storage notoriously leads to severe environmental issues due to its high alkalinity which affects neighboring communities. It contaminates phreatic and surface waters through the infiltration of metal laden caustic solution and radionuclides. The air also gets polluted through dispersion of particles which threatens surrounding biodiversity. To manage these environmental concerns, it is necessary to engineer the waste residue into value added products. This paper systematically reviews the mineralogical and chemical characterization of the waste and techniques developed for its recycling.
... The bauxite from Kachchh region has all different grades, and therefore, it is important to characterize the bauxite reserves in detail to explore their possible applications [1,13,15]. The increasingly growing markets, for rare-earth elements (REE), their very high prices, and their strategic importance have aroused huge interest in the REE extraction from BR [16][17][18][19][20][21][22][23]. REEs are elements from the lanthanide series, scandium, and yttrium. ...
The mineral deposits in the Kachchh region of Gujarat contain bauxite of all different grades, with a huge amount of low-grade bauxite. The low-grade bauxite from Kachchh is characterized in detail using different physicochemical techniques like X-ray diffraction (XRD), wavelength dispersive X-ray fluorescence (WD-XRF), field emission scanning electron microscopy (FESEM) with energy-dispersive X-ray (EDX) detector, nitrogen adsorption/desorption isotherm at 77 K, and inductive coupled plasma-mass spectrophotometer (ICP-MS). The different mineral phases present in the low-grade bauxite were identified from the X-ray diffraction studies. The different elements present in the mineral were identified and quantified using WD-XRF and EDX measurements. The quantification of rare-earth elements particularly scandium is carried out by ICP-MS analysis. The digestion method has been optimized for the complete mineral digestion and precise measurement of scandium. The low-grade Kachchh bauxite is explored for the possible extraction of alumina and scandium. The ICP-MS analysis shows the presence of approximately 80 ppm of scandium in the low-grade bauxite. The low-grade bauxite has approximately 39% alumina, which is predominantly gibbsite in nature. The alkali digestion conditions were optimized for the maximum dissolution and extraction of gibbsite.
... REE have been found in significant quantities in red mud. REE concentrations in red mud samples can reach up to 500-1700 µg/g (Akcil et al., 2018). Table 4.4 presents individual REE concentrations (µg/g) of red mud from different countries in comparison with the earth's average crust composition . ...
Rare earth elements (REE) have applications in various modern technologies, e.g., semiconductors, mobile phones, magnets. They are categorized as critical raw materials due to their strategic importance in economies and high risks associated with their supply chain. Therefore, more sustainable practices for efficient extraction and recovery of REE from secondary sources are being developed.
This book, Environmental Technologies to Treat Rare Earth Elements Pollution: Principles and Engineering:
presents the fundamentals of the (bio)geochemical cycles of rare earth elements and which imbalances in these cycles result in pollution.
overviews physical, chemical and biological technologies for successful treatment of water, air, soils and sediments contaminated with different rare earth elements.
explores the recovery of value-added products from waste streams laden with rare earth elements, including nanoparticles and quantum dots.
This book is suited for teaching and research purposes as well as professional reference for those working on rare earth elements. In addition, the information provided in this book is helpful to scientists, researchers and practitioners in related fields, such as those working on metal/metalloid microbe interaction and sustainable green approaches for resource recovery from wastes.
... These residues are harmful due to its alkalinity, but also are rich in valuable rare earth metals as to be economically treated. Scandium represents about 95% of the economic value of the REEs present in red mud containing between 130 and 390 ppm [44]. ...
Rare earth elements (REEs) are critical raw materials and are attracting interest because of their applications in novel technologies and green economy. Biohydrometallurgy has been used to extract other base metals; however, biole-aching studies of REE mineral extraction from mineral ores and wastes are yet in their infancy. Mineral ores have been treated with a variety of microorganisms. Phosphate-solubilizing microorganims are particularly relevant in the bioleaching of monazite because transform insoluble phosphate into more soluble form which directly and/or indirectly contributes to their metabolism. The increase of wastes containing REEs turns them into an important alternative source. The application of bioleaching techniques to the treatment of solid wastes might contribute to the conversion towards a more sustainable and environmental friendly economy minimizing the amount of tailings or residues that exert a harmful impact on the environment.
This paper investigates the synergistic extraction performance of Scandium using a mixture of Cyanex572 and Cyanex923 in sulfuric acid medium. The results show that the best synergistic effect is achieved when the pH value of the aqueous phase is 2 and the mole fraction of Cyanex572 is 0.5, with a synergy coefficient value R of 4.78. In addition, the extraction mechanism of this synergistic system is discussed, and the extracted complex is determined to be [Sc(SO 4 ) 0.5 (HL 2 ) 2 B 2 ] by slope analysis. The loaded Scandium can be stripped within two times by 2 mol/L sulfuric acid to obtain a stripping efficiency of 99.4 %. Finally, this synergistic system was applied to extract Scandium, demonstrating the potential application in separation of Scandium from the leachate of laterite nickel ore.
Industry represents a fundamental component of modern society, with the generation of massive amounts of industrial waste being the inevitable result of development activities in recent years. Red mud is an industrial waste generated during alumina production using the Bayer process of refining bauxite ore. It is a highly alkaline waste due to the incomplete removal of NaOH. There are several opinions in both the literature and legislation on the hazards of red mud. According to European and national legislation, this mud is not on the list of hazardous wastes; however, if the list of criteria are taken into account, it can be considered as hazardous. The complex processing of red mud is cost-effective because it contains elements such as iron, manganese, sodium, calcium, magnesium, zinc, strontium, lead, copper, cadmium, bismuth, barium and rare earths, especially scandium. Therefore, the selection of an extraction method depends on the form in which the element is present in solution. Extraction is one of the prospective separation and concentration methods. In this study, we evaluated the kinetic modelling of the solid–liquid acid extraction process of predominantly scandium as well as other elements present in red mud. Therefore, three acids (HCl, HNO3 and H2SO4) at different concentrations (10, 20 and 30%) were targeted for the extraction of Sc(III) from solid red mud. Specific parameters of the kinetics of the extraction process were studied, namely the solid:liquid ratio, initial acid concentration, contact time and temperature. The extraction kinetics of Sc(III) with acids was evaluated using first- and second-order kinetic models, involving kinetic parameters, rate constants, saturation concentration and activation energy. The second-order kinetic model was able to describe the mechanism of Sc(III) extraction from red mud. In addition, this study provides an overview on the mechanism of mass transfer involved in the acid extraction process of Sc(III), thereby enabling the design, optimization and control of large-scale processes for red mud recovery.
The scandium production rate and price data for scandium oxide and scandium metal was extracted from various sources. Data for extractable resources of scandium were found and evaluated by application of estimated yields. The feasible extractable resource is about 6.1 million ton, and with present technology, about 676,000 ton scandium appear as potentially extractable. The potential for scandium extraction is about 1,500 ton/year at present, but only about 45 ton per year was produced in 2022. With improved extraction and access yields, production could be increased to about 450 ton per year, and the scandium resource would increase to about 1.5 million ton. The investigation suggests that there will be an increased demand for scandium metal in the future, but that it is limited by the high price and the lack of a properly functioning market and by a lack of production infrastructure. The scandium market show signs of being disorganized and unstructured. Modelling of future scandium production was done using the WORLD7 integrated assessment model, after development of a scandium module. Simulations show that the price will remain relatively high, but lower than in the past. The most uncertain factor for predicting the price is the size of the demand. The main limitation for making scandium metal is high energy costs and low extraction yields.
Vanadium is a critical metal that has been widely used in a broad variety of applications with almost no metal substitutes. However, the limited availability of its (vanadium) primary resources has raised concerns of supply security. In view of the criticality, recycling vanadium from secondary resources has been identified as a vital supply alternative. This article thus presents a comprehensive overview of metallurgical processes used in the recycling of vanadium from a variety of secondary resources, including spent HDS catalyst, spent SCR catalyst, fly ash, red mud, Bayer’s sludge, alloy scrap, tailings, etc. First, the physicochemical characteristics of these secondary resources are emphasized. Understanding the characteristics of vanadium-bearing secondary resources is important as it determines the recycling route. The metallurgical recycling processes of vanadium, which include aqueous- and thermal processes are discussed in depth, along with the theoretical backgrounds and fundamentals of each process. Also discussed are the industrial-scale processes and trend in research and development (R&D) for the respective secondary resources. Besides highlighting the status of recycling processes, the article also provides prospective directions for such resources.
Bioleaching exhibits high potential for the processing of low-grade complex mineral resources. With the development of the economy and an increase in demand, rare earth elements (REEs) in secondary resources, such as phosphogypsum, red mud and coal-related resources, are gaining more and more attention. In this review, the bioleaching performance of diverse microorganisms is summarized and compared for primary (mainly monazite) and secondary REE resources, based on publications from the past decade. The mineral characteristics of these REE resources are different, as they can be found in phosphate, sulfate, or silicate forms. Correspondingly, microbial species suitable for use in bioleaching differ. The most efficient bioleaching microbe for monazite is Paecilomyces sp., while Acidianus manzaensis is effective in processing red mud. Acidophilic sulfur oxidizers are suitable for processing acidic phosphogypsum. Acidithiobacillus thiooxidans could recover a significant amount of REEs from coal fly ash. In particular, monazite has a high REE content but extremely low bioleaching efficiency compared to that of secondary resources, supporting the understanding that bioleaching approaches are more competitive for minerals with low REE contents. Overall, great progress has been made over the last decade, as considerable REE recovery rates have been achieved, and the main metabolites of microbes were identified. However, numerous challenges still exist. Future efforts should focus on improving biorecovery efficiency, reducing the cost of cell-culture media, and exploring the interaction mechanism between cells and minerals, with an emphasis on mineralogical phase transformations and the molecular regulation mechanisms inside cells during the bioleaching process.
Currently, due to the high rate of development of the rare-earth industry, new sources of raw materials are being mastered and new technologies for obtaining rare-earth metals (REM) are being developed. Studies have shown that REM in kaolinite clays of Alexeevskoe deposit in Kazakhstan and Egypt deposits in Sinai Peninsula (K-Watt, K-Tech) and in Aswan region (KB,KPL) are mainly represented by erbium (Er). Production of Er concentrate is considered as a by-product in a comprehensive middlings processing of kaolinite clays to produce alumina and building materials. The possibility of obtaining Er concentrate by sulfuric acid leaching and sorption concentration methods has been determined. Optimal technological conditions of kaolinite clays leaching is the use of 5% solution of H2SO4, at temperature 50 °C, duration 60 min and L:S ratio = 5. Under these conditions the separation of REM from the main components Fe2O3, Al2O3, SiO2 is achieved. Concentrates were obtained with the content of the sum of REM oxides from 91.3 to 93.4%, in which the relative content of Er was from 64.89 to 90.82%. The results showed that the developed technology can be used for processing of erbium-containing kaolinite clays of various deposits.
The growing stockpiles of bauxite residueBauxite residue and associated environmental hazards require a sophisticated process flowsheet for sustainable residue management and value recoveryRecovery. Considering the association of multiple elements (Fe, Al, Si, Ca, Ti, V, Sc) within bauxite residueBauxite residue, metal extraction is of prime interest. The complex association of different elements and physical and chemical characteristics makes the extraction and purification process expensive and challenging. The present study focuses on developing a novel hydrometallurgical flowsheet for the subsequent recoveryRecovery of base metals and critical elements from bauxite residueBauxite residue. The major elements present in bauxite residueBauxite residue are recovered as high-purityPuritymagnetiteMagnetite, titaniumTitanium dioxide, and aluminaAlumina. At the same time, critical elements (such as V and Sc) are recovered in the liquid stream generated after the recoveryRecovery of base metals.
Rare earths are classified as most important and critical material for US economy and defense by Congress, and a mandate has been set to increase their in-house production, domestic resource utilization and decrease reliance on foreign resources and reserves. They are widely available in earth crust as ore (bastnaesite (La, Ce)FCO3, monazite, (Ce, La, Y, Th)PO4, and xenotime, YPO4), but their so-called economic reserves are sparsely located geographically. They may be produced by various means such as beneficiation (physical, chemical, mechanical, or electrical), reduction (direct or indirect), electrolysis (of aqueous or molten/fused single or mixed salt systems) at high temperature or hydrometallurgy. Out of these, direct reduction, also known as metallothermic reduction (La and Ca reduction), is mostly utilized. Its variant, high temperature electrowinning of fused salts, is also practiced widely. These processes are material- and application-specific. In this study, the author will employ thermodynamics (Ellingham diagrams, free energy of formation, reduction potential, Nernst equation, Pourbaix (Eh-pH) diagrams, E-pO⁻² diagrams), kinetics, and energetic of a chemical reaction (chemical metallurgy) to reduce rare earth oxide/salt to rare earth metals (REO/RES – REM). It is shown that materials and energy requirement vary greatly depending on the type of mineral ore, production facility, and beneficiation/mineral processing method selected. The aim is to reduce dependence on coal deposits. It is anticipated this route will be able to produce rare earths with >35% yield and >98% purity which will be described in subsequent studies and patents.
The main components of metallurgical slags are iron compounds, which are extracted by reduction smelting. There are several types of this process with obtaining various products based on iron and slags of various compositions (aluminum-calcium self-disintegrating), etc. The mode of smelting and cooling of alumocalcium slag, formed during melting in the furnace, should ensure the most complete spontaneous crumbling of it, as well as high rates of extraction of REM from it. Synthetic slags similar in phase composition to industrial samples after smelting of iron-containing ores were selected for the experiments. The simulated samples correspond to the region of primary crystallization of bicalcium silicate on the triple state diagram of CaO – SiO 2 – Al 2 O 3 system. The slag after crumbling was subjected to a sieve analysis using a mechanical sieve. In the experiments, slags with a silicon module k = 2.0 were used, which actively crumbled during their cooling. With an increase in the silicon module, the crumbling deteriorates. It was established that it is impossible to precisely limit the areas of compositions of crumbling slags at certain cooling rates. The conducted studies showed that the crumblability of slags improves as it approaches the center of the bicalcium silicate region. The composition of slags close to composition of the intersection points of the lines 2CaO·SiO 2 – 2CaO·Al 2 O 3 and 2CaO·SiO 2 – 12CaO·7Al 2 O 3 with the lines of the permanent silicon module granulometric composition almost does not depend on the cooling rate. The temperature mode from smelting to cooling affects the slags crumblability. The most promising are slags with a silicon module in the range of 2.85 – 3.00, close to the phase triangle 12CaO·7Al 2 O 3 – 2CaO·SiO 2 – 2CaO·Al 2 O 3 .
This chapter covers a comprehensive overview for recycling of different secondary resources, their generation, metal content, and recycling processes. This chapter depicts the prominence of the various processes for metal extraction from anthropogenic wastes (aluminium cans, e-wastes, LCD screens), and industrial wastes (ferrous and non-ferrous metal wastes, catalysts, effluents, mine tailings, and metallurgical slags. Alongside, the underlying issues like material collection, treatment, and mode of operations have been discussed.KeywordsMaterial recyclingAnthropogenicIndustrialResourcesExtractionEfficiency
Red mud produced from the Bayer process was used for the high temperature metallurgical processing of ilmenite. The major components of the red mud were Fe2O3, Al2O3, Na2O, and TiO2, which play important roles in the flux material. The process temperature of ilmenite slag was lowered from 1794 K (1521°C) to 1687 K (1414°C), as a preliminary study by a high-temperature microscope. Thermochemical calculations using FactSageTM 7.0 confirmed the effect of red mud on the lowered melting temperature of ilmenite slag. The discrepancy between the observed results of the microscope and the calculated phase diagram was due to the slag viscosity, which was significantly affected by Al2O3 in red mud. The reduction of ilmenite at lower temperatures by the addition of red mud was investigated at 1723 K (1450°C). Ilmenite, pseudobrookite, and associated clay minerals were the major phases in the reduced ilmenites which was confirmed by X-ray diffraction (×RD) analysis. Microscopic images obtained by scanning electron microscopy (SEM) showed variations in the pseudobrookite phases according to the amount of fluxed red mud. The benefits of red mud utilization are discussed with regard to the lower energy consumption and recovery of resources.
The current study deals with preliminary chemical activation, which allows transforming the phase composition before gravitational concentration and effective removal of silicon during subsequent alkaline treatment, in order to obtain a raw material suitable for the production of alumina by the Bayer method. Electron-microscopic analysis of the original bauxite showed that the coarse-crystalline fraction was tightly pressed by the fine one. Silicon modulus of the averaged bauxite sample is 3.65. After chemical activation of bauxite in a sodium bicarbonate solution, the fine fraction got separated from the coarse fraction and the phase composition changed i.e. the calcium silicate phase disappeared and the calcite phase was formed. Preliminary chemical activation at temperatures of 90 and 120 оС for 20 min resulted in the increase of the bauxite beneficiation degree by 12.1 and 15.2 %. In order to obtain a quality bauxite concentrate with a silica modulus of more than 7, the coarse-grained bauxite fraction separated after gravity concentration should be subjected to chemical beneficiation in an alkaline solution at a second stage.
Despite their significance in numerous applications, many critical minerals and metals are still considered minor. Since most of them are not found alone in mineral deposits, their co- or by-production depends on the production of base metals and other major commodities. In many cases, the concentration of the minor metals is low enough not to be considered part of the production. Hence, their supply is not always secured, their availability decreases, and their criticality increases. Many researchers have addressed this issue, but no one has set actual impact factors other than economic ones that should determine the production of these minor commodities. This study identified several parameters, the number and diversity of which gave birth to developing a computational tool using a multi-criteria-decision analysis model based on the Analytical Hierarchical Process (AHP) and Python. This unprecedented methodology was applied to evaluate the production status of different commodities in a polymetallic deposit located in Chovdar, Azerbaijan. The evaluation outcomes indicated in quantifiable terms the production potentials for several commodities in the deposit and justified the great perspectives of this tool to evaluate all kinds of polymetallic deposits concerning the co- and by-production of several minor critical raw materials.
Herein, tartaric acid (TA) was used as a binder to prepare red mud pellets. The effect of TA on the compressive strength and induration mechanism of the pellets containing TA was analyzed via the changes in crystal structure, functional groups, and valence states of the constituent elements. The induration mechanism of TAPE is likely the chemical reaction between TA and partial calcite along with complexation of TA with minerals containing Ca(II), Al(III), and Fe(III). The density functional theory calculation results revealed that the TA-Ca(II) complexation products in red mud were more stable than those of Al(III) and Fe(III).
The growing stockpiles of bauxite residue and associated environmental hazards require a sophisticated recycling system for complete utilization and value recovery. The current application is limited as a raw material for building and construction applications. Considering the association of multiple elements (Fe, Al, Si, Ca, Ti, V, Sc) within bauxite residue, metal extraction is of prime interest with respect to the economic value of the final product. The following study presents a hydrometallurgy-based process flowsheet for the sustainable recovery of base metals and critical elements within bauxite residue. Major elements present in bauxite residue are recovered as materials for industrial application, including high purity magnetite, alumina, titania, silica, and calcium carbonate. Trace elements are recovered in the liquid stream generated after the recovery of base metals.
The leaching behavior of scandium (Sc) from bauxite residues can differ significantly when residues of different geological backgrounds are compared. The mineralogy of the source rock and the physicochemical environment during bauxitization affect the association of Sc in the bauxite i.e., how Sc is distributed amongst different mineral phases and whether it is incorporated in and/or adsorbed onto those phases. The Sc association in the bauxite is in turn crucial for the resulting Sc association in the bauxite residue. In this study systematic leaching experiments were performed on three different bauxite residues using a statistical design of experiments approach. The three bauxite residues compared originated from processing of lateritic and karstic bauxites from Germany, Hungary, and Russia. The recovery of Sc and Fe were determined by ICP-OES measurements. Mineralogical changes were analyzed by X-ray-diffraction and subsequent Rietveld refinement. The effects of various parameters including temperature, acid type, acid concentration, liquid-to-solid ratio and residence time were studied. A response surface model was calculated for the selected case of citric acid leaching of Hungarian bauxite residue. The investigations showed that the type of bauxite residue has a strong influence. The easily leachable fraction of Sc can vary considerably between the types, reaching ~20–25% in German Bauxite residue and ~50% in Russian bauxite residue. Mineralogical investigations revealed that a major part of this fraction was released from secondary phases such as cancrinite and katoite formed during Bayer processing of the bauxite. The effect of temperature on Sc and Fe recovery is strong especially when citric acid is used. Based on the exponential relationship between temperature and Fe-recovery it was found to be particularly important for the selectivity of Sc over Fe. Optimization of the model for a maximum Sc recovery combined with a minimum Fe recovery yielded results of ~28% Sc recovery at <2% Fe recovery at a temperature of 60 °C, a citric acid normality of 1.8, and a liquid-to-solid ratio of 16 ml/g. Our study has shown that detailed knowledge about the Sc association and distribution in bauxite and bauxite residue is key to an efficient and selective leaching of Sc from bauxite residues.
The reduction kinetics of serial phase transitions of iron oxides during reduction to a metallized state with different modes of technical hydrogen supply has been studied and substantiated. The results of the pellets formation when 3-5 % molasses is added to the red mud as a binding reagent are presented. The dependences of the reduction rate of iron oxides on the hydrogen flow rate are obtained. Based on the results of the experiments, a kinetic model was constructed, and with the help of X-ray phase and spectral analysis, it was proved that the agglomerates formed after heat treatment received high strength due to the adhesion of reduced iron particles with red mud particles. The use of a new type of charge materials in melting units will reduce the amount of emissions and dust fractions, as well as increase the metal yield.
A comprehensive process flowsheet is investigated to recover metallic values from red mud with high-purity products such as alumina, silica, ferrous oxalate, titania, and Sc-Ga containing purified solution. The carbonated red mud leached in 1 M hydrochloric acid at 50 °C for 1 h dissolves 44.5% Al and 51% Si, followed by 2 M oxalic acid leaching at 90 °C for 2.5 h yielding 61% Fe and 46–48% Sc and Ga dissolution. High-purity ferrous oxalate product is retrieved from the leach solution, and the Sc and Ga ions report to the solution. Titanium enriched residue is baked with 1 mL/g sulfuric acid at 300 °C for 1 h, forming TiOSO4, Fe2(SO4)3, and Al2(SO4)3 phases, followed by water leaching. 64.5% Ti and ∼ 27% Fe dissolution is attained, and solution contains Fe (7.47 g/L), Ti (3.09 g/L), Al (2.44 g/L), Si (0.17 g/L), Sc (2 mg/L), and Ga (3 mg/L) ions. Thermal hydrolysis of solution recovers titania precipitate (96.9% purity), and the solution after hydrolysis contains Fe, Sc, and Ga values. The overall metal extraction (84% Fe, 79% Ti, 92% Al, 78% Sc, 89 % Ga) in the proposed flowsheet is higher compared to direct acid baking process (51% Fe, 65% Ti, 89% Al, 51% Sc, 48 % Ga). Products recovered include high-purity silica (4.8 wt%), alumina (9.3 wt%), Fe(II) oxalate (20.7 wt%), titania (8.9 wt%), and a final residue yield of 12.6 wt%. Microwave exposure at 2 kW for 7 min improves the dissolution of the Sc (84%) and Ti (85.5%). H2 reduction at 450 °C, 30 min converted hematite to magnetite and increased Fe dissolution to 90%. Red mud's mineralogical and composition modification by magnetic separation, reduction, and microwave exposure were ineffective to improve selective dissolution.
Increasing disruption in the rare earth supply chain creates an urgency to develop alternative resources, in which utilization of coal-based materials presents great potential. Nevertheless, environmental control is a significant challenge in rare earth extraction processes. This study was conducted to contribute to the limited information on removing thorium and uranium from rare earths while coal-based products are used as feedstock. The laboratory studies suggested that the selective precipitation and solvent extraction approach yields the most favorable separation performance. Complete thorium precipitation was achieved around a pH value of 4.8. Due to the close precipitation pH ranges of uranium and rare earths, further separation by solvent extraction was applied to achieve an enhanced separation. Based on a Box-Behnken experimental design, the effect of extractant concentration, pH, strippant concentration, and O/A ratio was investigated. Best separation performance was achieved using 50 v% TBP at a pH of 3.5 with an O/A ratio of 3 and 1 mol/L H2SO4, which resulted in 1.8% uranium and 73.4% rare earth extraction. The
extraction and precipitation behavior of the elements were further assessed with the distribution ratio, separation factor, thermodynamic parameters, and species distribution diagrams to provide a thorough
understanding of the separation mechanisms. The results were statistically analyzed, and a model was developed to predict uranium recovery. The developed experimental protocol was validated using a rare earth oxalate sample produced at the pilot-scale processing facility. Finally, a conceptual process flowsheet was developed to effectively separate radionuclides while producing rare earth oxide products.
Bauxite residue (BR), simultaneously an environmental challenge as well as known to be a secondary resource for resources various valuable metals like Ti, V, Ga, and rare earth metal (REM). Lack of understanding and technology detects BR to be stockpiled which is counterproductive considering the environment, land scarcity, and management of BR inventories. As BR remains unexploited, significant amounts of REMs in BR remain unlocked, which are critical metals from green energy, environmental sustainability, and supply chain bottleneck perspective. Our current investigation analyses the potential of BR as secondary resources and quantity and worth of REM being remains unlocked. The quantitative content of global bauxite, alumina, and BR production during the last 5 decades have been analyzed. Also, plausible BR generation in the next 3 decades has been estimated. Considering the content of REM in BR amount of REM either stockpiled or to be stockpiled along with BR has been analyzed. Our study indicated about 9.14 million tons of REM remain locked in the stockpiled BR, 31.24 million tons of REM remain locked in the bauxite reserve. The worth of worldwide REM oxide remains unexploited in bauxite reserves and locked in stockpiled BR could be approximately $5000 billion, potentially can meet current and project demand of REM abundantly.
The recovery of scandium (Sc) from wastes and various resources using solvent extraction (SX) was discussed in detail. Moreover, the metallurgical extractive procedures for Sc recovery were presented. Acidic and neutral organophosphorus (OPCs) extractants are the most extensively used in industrial activities, considering that they provide the highest extraction efficiency of any of the valuable components. Due to the chemical and physical similarities of the rare earth metals, the separation and purification processes of Sc are difficult tasks. Sc has also been extracted from acidic solutions using carboxylic acids, amines, and acidic β-diketone, among other solvents and chemicals. For improving the extraction efficiencies, the development of mixed extract-ants or synergistic systems for the SX of Sc has been carried out in recent years. Different operational parameters play an important role in the extraction process, such as the type of the aqueous phase and its acidity, the aqueous (A) to organic (O) and solid (S) to liquid (L) phase ratios, as well as the type of the diluents. Sc recovery is now implemented in industrial production using a combination of hydrometallurgical and pyrometallurgical techniques, such as ore pre-treatment, leaching, SX, precipitation, and calcination. The hydrometallurgical methods (acid leaching and SX) were effective for Sc recovery. Furthermore, the OPCs bis(2-ethylhexyl) phosphoric acid (D2EHPA/P204) and tributyl phosphate (TBP) showed interesting potential taking into consideration some co-extracted metals such as Fe(III) and Ti(IV).
Bauxite residue, also known as red mud (RM), from alumina production is the most promising technogenic material for the production of scandium (Sc) and other rare earth elements (REEs). Conveniently, RM is processed by using a strong acid (pH < 2.5), which lead to co-dissolution of iron and other undesirable major components. In this work, for the first time, the possibility of selective extraction of scandium from red mud by using highly diluted acid (pH > 4) in the presence of MgSO4 was shown. The effect of temperature (40–80 °C), time (0–60 min), pH (2–5), and the MgSO4 concentration (12–36 g L−1) on Sc extraction efficiency was evaluated. It was shown that Sc extraction was higher than 63% even at a pH of 4, at 80 °C, after 1 h, while more than 80% could be extracted at a pH of 2. Iron extraction reduced from 7.7 to 0.03% by increasing the pH from 2 to 4. The kinetics study using the shrinking core model (SCM) has shown that diffusion through a product layer is a rate-limiting stage of the process at high temperatures (>60 °C) and low pH (<3), whereas, at lower temperatures and higher pH values, the leaching rate is limited by diffusion through the liquid film.
Bauxite residue, known as “red mud,” is a potential raw material for extracting rare-earth elements (REEs). The main REEs (Sc, Y, La, Ce, Nd, Nb, and Sm) from the raw bauxite are concentrated in RM after the Bayer leaching process. The earlier worldwide studies were focused on the scandium (Sc) extraction from RM by concentrated acids to enhance the extraction degree. This leads to the dissolution of major oxides (Fe2O3 and Al2O3) from RM. This article studies the possibility of selective Sc extraction from alkali fusion red mud (RMF) by diluted nitric acid (HNO3) leaching at pH ≥ 2 to prevent co-dissolution of Fe2O3. RMF samples were analyzed by X-ray fluorescence spectrometry (XRF), X-ray diffraction (XRD), electron probe microanalysis (EPMA), and inductively coupled plasma mass spectrometry (ICP-MS). It was revealed that Sc concentration in RMF can reach up to 140–150 mg kg−1. Sc extraction was 71.2% at RMF leaching by HNO3 at pH 2 and 80 °C during 90 min. The leaching solution contained 8 mg L−1 Sc and a high amount of other REEs in the presence of relatively low concentrations of impurity elements such as Fe, Al, Ti, Ca, etc. The kinetic analysis of experimental data by the shrinking core model showed that Sc leaching process is limited by the interfacial diffusion and the diffusion through the product layer. The apparent activation energy (Ea) was 19.5 kJ/mol. The linear dependence of Sc extraction on magnesium (Mg) extraction was revealed. According to EPMA of RMF, Sc is associated with iron minerals rather than Mg. This allows us to conclude that Mg acts as a leaching agent for the extraction of Sc presented in the RMF in an ion-exchangeable phase.
In an effort to identify new sources of critical raw materials (CRMs) possibility of recovering selected CRMs from Polish coals, chars, and ashes resulting from the combustion of coals and chars was investigated. The samples were collected from pilot fluidized bed gasification systems. The search for CRMs in coal gasification wastes has not been widely reported before. The study used 2 bituminous coal and 1 lignite sample; the concentration of individual critical raw materials (CRMs) was analyzed using the ICP-MS method. The obtained results were compared with Clarke values in coal ash and in the Earth’s crust, and with the adopted cut-off grade. As shown by the analysis, the highest concentrations of CRMs can be found in fly ash, mainly in samples from the eastern part of the Upper Silesian Coal Basin. This applies mostly to Be, Cs, or Sb due to the fact that their concentrations were found to be higher than the Clarke value in the Earth’s crust; the mentioned fly ashes could be used as potential sources of critical elements if appropriate recovery technologies are developed. In addition, the tested materials have elevated Se, Pb, Ni concentrations, but their recovery is currently not economically viable. Compared to the currently adopted cut-off grade levels, there are no critical elements in the analyzed coal gasification waste that could be recovered.
With escalating demand for metals, an increasing global population and rapid technology development, there is a worldwide challenge to secure a sustainable metal supply for industry. Current recycling rates for such metals are extremely low, mainly due to a lack of feasible recovery technologies. As a consequence, valuable metal resources are being landfilled each year from sources such as municipal and industrial solid wastes. The practice of landfilling these resources not only impacts the environment due to potential leaching of metal rich toxic liquids, but also represents a significant long term loss to the economy. Recovery of metals from industrial process wastes, such as bauxite residue and incinerator ashes potentially offers significant quantities of metals to the benefit of the environment and economy alike. The difficulty with their recovery, however is that metals of concern tend to be present in low concentrations within complex matrices and can be technically difficult to extract. Here the extent of unrecovered metals in leachates from bauxite residue and incinerated bottom and fly ashes from municipal solid wastes are quantified and their potential economic value assessed. The potential of saw dust modified biochar and KOH modified hydrochar to remove Vanadium (V) from aqueous solutions in batch study experiments are also assessed, yielding optimum uptakes of 16.5 and 12.3 mg g⁻¹ respectively at a solution pH 4. Finally consideration is given to future research needs to improve the sustainability and overall performance of biosorption of leachate metals.
Heavy metals (HMs) are natural environmental constituents, but their geochemical processes and biochemical equilibrium have been altered by indiscriminate use for human purposes. Due to their toxicity, persistence in the environment and bioaccumulative nature; HMs are well-known environmental contaminants. As result, there is excess release into natural resources such as soil and marine habitats of heavy metals such as cadmium, chromium, arsenic, mercury, lead, nickel, copper, zinc, etc. Their natural sources include the weathering of metal-bearing rocks and volcanic eruptions, while mining and other industrial and agricultural practices include anthropogenic sources. Prolonged exposure and increased accumulation of such heavy metals may have detrimental effects on human life and aquatic biota in terms of health. Finally, the environmental issue of public health concern is the pollution of marine and terrestrial environments with toxic heavy metals. Therefore, because of the rising degree of waste disposal from factories day by day, it is a great concern. Pollution of HMs is therefore a problem and the danger of this environment needs to be recognized.
Biohydrometallurgy recovers metals through microbially mediated processes and has been traditionally applied for the extraction of base metals from low-grade sulfidic ores. New investigations explore its potential for other types of critical resources, such as rare earth elements. In recent times, the interest in rare earth elements (REEs) is growing due to of their applications in novel technologies and green economy. The use of biohydrometallurgy for extracting resources from waste streams is also gaining attention to support innovative mining and promote a circular economy. The increase in wastes containing REEs turns them into a valuable alternative source. Most REE ores and industrial residues do not contain sulfides, and bioleaching processes use autotrophic or heterotrophic microorganisms to generate acids that dissolve the metals. This review gathers information towards the recycling of REE-bearing wastes (fluorescent lamp powder, spent cracking catalysts, e-wastes, etc.) using a more sustainable and environmentally friendly technology that reduces the impact on the environment.
In the light of an expected supply shortage of rare earth elements (REE) measures have to be undertaken for an efficient use in all kinds of technical, medical, and agricultural applications as well as—in particular—in REE recycling from post-use goods and waste materials. Biologically- based methods might offer an alternative and supplement to physico-chemical techniques for REE recovery and recycling. A wide variety of physiologically distinct microbial groups have the potential to be applied for REE bioleaching form solid matrices. This source is largely untapped until today. Depending of the type of organism, the technical process (including a series of influencing factors), the solid to be treated, and the target element, leaching efficiencies of 80 to 90% can be achieved. Bioleaching of REEs can help in reducing the supply risk and market dependency. Additionally, the application of bioleaching techniques for the treatment of solid wastes might contribute to the conversion towards a more sustainable and environmental friendly economy.
It has been said that 'where there's muck there's brass' and now a former Soviet Union scientist is claiming to have demonstrated that some red mud residues are rich in scandium and that chemical beneficiation may make its recovery worthwhile. Scandium has been shown to have significant alloying potential for aluminium particularly in welding applications. And as millions of tonnes of red mud is dumped each year - at great expense - and scandium is worth upwards of $10000/kg there may be sense in the proposal. Aluminium Today publishes this article in the hope that it may generate discussion and welcomes readers comments on the subject.
The world economy is confronted with an increasing supply risk of critical raw materials. In the search for alternative sources, red mud may offer potential in particular for rare earth elements (REEs). Red mud is a by-product resulting from alumina extraction. Depending on the bauxite's origin, red mud may contain considerable amounts of REEs. The extraction of REEs from red mud by selective acid leaching was explored in this study. Hydrochloric (HCl), sulphuric (H 2 SO 4) and nitric (HNO 3) acid were applied for leaching. Citric (C 6 H 8 O 7) and oxalic (C 2 H 2 O 4) acid as small molecular weight organic chelators that can be biologically produced were studied as green alternative to mineral acids. After acidic extraction, REEs were purified by liquid-liquid extraction using di-(2-ethylhexyl)phosphoric acid (D2EHPA).
During direct acid leaching of bauxite residue, high amounts of iron also dissolve at high REEs recovery. In this paper, a sulphation-roasting-leaching process was developed to selectively leach the REEs. The bauxite residue was mixed with water and concentrated H2SO4 followed by roasting and then leaching of the calcined product with water. Several parameters including roasting temperature, duration of roasting, amount of acid were studied under optimised conditions, about 60 wt% of scandium and more than 90 wt% of other REEs can be dissolved with very small amounts of iron (< 1 wt%) and other major elements reporting to the solution.
During acid leaching of bauxite residue (red mud), the increase in dissolution of rare earth elements (REEs) is associated with an increase in iron dissolution, which poses problems in the downstream processing. Therefore, it would be beneficial to remove iron from bauxite residue by smelting reduction. The slag generated in the smelting reduction process could then be further processed for recovery of REEs. Smelting experiments were carried out at temperatures between 1500 °C and 1600 °C. Wollastonite (CaSiO3) was used as a flux and graphite as a reducing agent. Addition of wollastonite decreases the slag melting temperature and the viscosity, facilitating slag-metal separation, whereas a graphite content higher than the optimum level alters the slag chemistry and hinders the slag-metal separation. The optimum conditions were found to be for heating at 1500 °C: 20 wt% of wollastonite and 5 wt% of graphite. More than 85 wt% of the iron was separated from the slag in the form of a nugget. A further 10 wt% of the iron could be extracted from the slag by subsequent grinding and magnetic separation. The slag obtained after iron removal was treated with HCl, HNO3 and H2SO4 acids to extract REEs. Room temperature leaching was found to be not beneficial for REEs extraction. High-temperature leaching enhanced the recovery of REEs. More than 95% of scandium, >70% of REEs and about 70% of titanium could be leached at 90 °C. The selectivity of REEs over iron during slag leaching was clearly improved.
A method for leaching rare earth elements from coal ash in the presence of elemental sulfur-oxidizing communities of acidophilic chemolithotrophic microorganisms was proposed. The optimal parameters determined for rare element leaching in reactors were as follows: temperature, 45°C; initial pH, 2.0; pulp density, 10%; and the coal ash to elemental sulfur ratio, 10 : 1. After ten days of leaching, 52.0, 52.6, and 59.5% of scandium, yttrium, and lanthanum, respectively, were recovered.
A pilot-plant process has been developed based on an innovative laboratory-scale method for the recovery of scandium that exists in economically interesting concentrations in red mud, the main byproduct of alumina production. This method includes acid leaching of the red mud pulp, ion-exchange separation of scandium and lanthanides from the co-leached main elements such as iron, and subsequent liquid−liquid extraction of the eluate for further scandium purification and enrichment. In this work, experimental and theoretical investigation of the pilot-scale leaching process was performed. The following parameters were tested: mode of agitation, solid-to-liquid ratio, acidity of the leachate, number of stages in the process, and pretreatment of red mud with concentrated acids in order to achieve optimum scandium recovery combined with low iron dissolution. Furthermore, by theoretical interpretation of the experimental data, a predictive correlation for the scandium leaching efficiency was developed.
Laboratory-scale research has focused on the recovery of titanium from red mud, which is obtained from bauxite during the Bayer process for alumina production. The leaching process is based on the extraction of this element with diluted sulfuric acid from red mud under atmospheric conditions and without using any preliminary treatment. Statistical design and analysis of experiments were used, in order to determine the main effects and interactions of the leaching process factors, which were: acid normality, temperature and solid to liquid ratio. The titanium recovery efficiency on the basis of red mud weight reached 64.5%. The characterization of the initial red mud, as well as this of the leached residues was carried out by X-ray diffraction, TG-DTA and scanning electron microscopy.
According to European Commission reports published between 2010 - 2013, the development of European economy depends crucially on access to critical raw materials. Following the analysis performed by experts at European level, in 2011 was compiled and published a list of 14 critical raw materials, the so-called EU-14. In 2014 the list was updated with several new elements and one element (tantalum), was withdrawn from the list. The current list, being renamed EU-20, covers 20 critical raw materials including several high tech critical metals. Traditional mine exploitations are concentrated on using the deposits of ore extracted and processed by conventional techniques. The efficiency of metal recovery was variable over time and as a result, a significant amount of metal was discarded, most concentrations exceeding the current minimal permissible threshold. On the other hand, it is necessary the recovery of recyclable waste for reducing the risk of shortage of high tech critical metals. Therefore, it is necessary to develop new technologies for obtaining high tech critical metals, which is applicable to both primary and secondary sources of raw materials. Recovery of high-tech critical metals by processing ore, tailings or mine wastes, and recyclable materials can be successfully done with help of consortia or individual isolates of microorganisms, bacteria or fungi. Microorganisms interact with metals thus altering their physical and chemical condition. Isolation of individual strains and identification of microbial consortia that can be used in the design and development of effective biotechnological processes for the extraction of high tech critical metals is a current challenge of the scientific research in Europe.
In this study, the recovery of gallium (Ga) and aluminum (Al) from the by-product of Bayer process, the electrofilter dust of a calcination plant, was studied. Factorial leaching tests were also designed based on the results of the preliminary tests. Effects of factors and their interactions on the extraction of Ga and Al were demonstrated using Analysis of Variance of the findings. In the factorial design, nitric acid (HNO3) leaching tests up to 43.4% Ga and 35.2% Al were leached from the electrofilter dust. The addition of oxalic acid (H2C2O4) significantly enhanced the sulphuric acid (H2SO4) leaching of the dust with up to 48.3% Ga and 39.6% Al extractions.
Bauxite residue (or red mud) is a waste generated during the Bayer process of alumina production. Its storage is a spatial and environmental concern. Currently, there are no bulk applications of bauxite residue except for minor use in cements and ceramics. Nonetheless, some types of bauxite residues are rich in rare-earth elements (REEs), and the extraction of scandium in particular is of special interest. Leaching experiments on Greek bauxite residue were performed with different acids at different concentrations, liquid-to-solid ratios, leaching times and temperatures. Extraction of the REEs was high for leaching in HCl solutions compared to other acids, but the dissolution of iron was high as well (~60%). The maximum extraction of the REEs was around 80%. Sodium and calcium were completely dissolved during leaching. Dissolution of aluminium, silicon and titanium was between 30 and 50%. The leaching data show a very close association of scandium with the iron oxide phases.
In this study, recovery of vanadium and gallium from solids waste by-products (vanadium sludge and electrofilter dust of calcination plant) of Bayer process was investigated. An efficient purification process wasdevelopedbased on the removal of impurities such as phosphate by water leaching, neutralisation using CO2-enriched air and addition of aluminate solution. Recovery of V2O5 from the purified solution via the precipitation of ammonium metavanadate, its conversion into polyvanadate by the addition of ammonium sulphate and sulphuric acid, respectively, and then the ignition of the latter at 560°C was demonstrated. Effects of various parameters on the purification and precipitation processes were shown. A treatment process involving sintering and two-stage of carbonisation was also demonstrated to produce a Ga-rich precipitate. A gallate solution suitable for electrolysis of Ga was also shown to be prepared from this precipitate. A complete flowsheet was proposed for the treatment of vanadium sludge and electrofilter dust.
Leach solutions and wastes of Bayer process are important resources for metals such as aluminum and vanadium. Despite the fact that vanadium cake is precipitated and removed in the Seydisehir Eti Aluminum Facility (Turkey), it cannot be used due to low metal content and impurities it contains. Within the scope of this study, research and development of environmentally acceptable, technically sound and low-cost chemical leaching and recovery methods were conducted for the recovery of vanadium from the by-product cake of the Bayer process. In the conducted studies, a sample of vanadium cake was used after its detailed characterization. Roasting tests were performed in order to remove the arsenic in the vanadium cake; however, it was found that roasting was not effective in removing the arsenic from the cake. The performance of different reagents were examined in chemical leaching tests (H2O and H2SO4 leaching, H2SO4 leaching with the addition of NaSO3, and NH4F); in the H2SO4 leaching tests performed with the addition of Na2SO3, the concentration of the reagents and the effect of temperature on the efficiency of vanadium recovery (max. 93.09%) were determined with the full factorial experimental design method, the outcomes were evaluated with ANOVA (variance analysis) method, and empirical models were formed. In lab and semi-pilot scale leaching tests, vanadium recoveries were 96.34% and 94.76% respectively. Vanadium was precipitated with NaOH and FeSO4 and almost all vanadium (95.8%) was obtained as Fe3(VO4)2. Cost analysis and economic evaluation have shown the economic feasibility of the leaching and recovery processes proposed.
Bauxite residue (red mud) is a hazardous waste generated from alumina refining industries. Unless managed properly, red mud poses significant risks to the local environment due to its extreme alkalinity and its potential impacts on surface and ground water quality. The ever-increasing generation of red mud poses significant challenges to the aluminium industries from management perspectives given the low proportion that are currently being utilized beneficially. Red mud, in most cases, contains elevated concentrations of iron in addition to aluminium, titanium, sodium and valuable rare earth elements. Given the scarcity of iron supply globally, the iron content of red mud has attracted increasing research interest. This paper presents a critical overview of the current techniques employed for iron recovery from red mud. Information on the recovery of other valuable metals is also reviewed to provide an insight into the full potential usage of red mud as an economic resource rather than a waste. Traditional hydrometallurgy and pyrometallurgy are being investigated continuously. However, in this review several new techniques are introduced that consider the process of iron recovery from red mud. An integrated process which can achieve multiple additional values from red mud is much preferred over the single process methods. The information provided here should help to improve the future management and utilization of red mud.
With an increase in number of waste nickel-metal hydride batteries, and because of the importance of rare earth elements, the recycling of rare earth elements is becoming increasingly important. In this paper, we investigate the effects of temperature, hydrochloric acid concentration, and leaching time to optimize leaching conditions and determine leach kinetics. The results indicate that an increase in temperature, hydrochloric acid concentration, and leaching time enhance the leaching rate of rare earth elements. A maximum rare earth elements recovery of 95.16% was achieved at optimal leaching conditions of 70 °C, solid/liquid ratio of 1:10, 20% hydrochloric acid concentration, −74 μm particle size, and 100 min leaching time. The experimental data were best fitted by a chemical reaction-controlled model. The activation energy was 43.98 kJ/mol and the reaction order for hydrochloric acid concentration was 0.64. The kinetic equation for the leaching process was found to be: 1−(1−x)1/3=A/ρr0[HCl]0.64exp−439,8008.314Tt. After leaching and filtration, by adding saturated oxalic solution to the filtrate, rare earth element oxalates were obtained. After removing impurities by adding ammonia, filtering, washing with dilute hydrochloric acid, and calcining at 810 °C, a final product of 99% pure rare earth oxides was obtained.
Calcium and iron removal from a bauxite ore by Bacillus polymyxa has been demonstrated. Within a period of 7 days, the above organism could remove all the calcium and about 45% of iron from the ore in the presence of 2% sucrose in a Bromfield medium. The highest removal of calcium and iron corresponded with the maximum in extracellular polysaccharide production by the organism. Scanning electron microscopy of the biobeneficiated bauxite surfaces indicated tenacious attachment of the bacteria onto the ore particle. Some calcium and iron removal was observed even in the presence of bacterial metabolites such as polysaccharides, organic acids and slime. However, the calcium removal in the absence of microorganism (by metabolites alone) was found to be 50% of that obtained in its presence. These observations clearly indicate that both a direct mechanism through bacterial attachment to the ore and an indirect mechanism through leaching with metabolites are involved in the biobeneficiation process.
Laboratory-scale experiments were conducted to recover lanthanum and cerium from Indian red mud in sulphuric acid medium. The method includes acid leaching of red mud pulp and subsequent liquid –liquid extraction of the leached metals with different organic extractants, in order to establish the technical feasibility of extraction and separation simultaneously. Maximum Recovery of lanthanum (99.9%) was recorded with 3 M H2SO4 at ambient (35 °C) temperature, S/L ratio of 10 g/L and agitation rate of 200 rpm in 1 h time. Whilst 99.9% cerium recovery was achieved at 75 °C and solid/liquid ratio of 10 g/L in 3 M H2SO4. Significant specificity for complete extraction of lanthanum, cerium and scandium by Cyanex 301 was noted as compared to the solvents such as DEHPA and Cyanex 272.
The extraction of scandium from an Australian red mud by selective acid leaching was explored and preliminary leaching tests showed that diluted sulphuric acid can be used to leach scandium from the red mud. The recovery of scandium from a synthetic leach solution of the red mud using solvent extraction was studied. A number of extractants were investigated for the extraction of scandium and its separation from the other metals in the synthetic leach solution. It was found that amongst the three acidic organophosphorus extractants studied, D2EHPA performed best. With the organic system consisting of 0.05 M D2EHPA and 0.05 M TBP in Shellsol D70 under an A/O ratio of 5:1 at pH 0.25 and 40 °C, over 99% scandium was extracted and almost no iron and aluminium were co-extracted. The scandium extracted can be stripped from the D2EHPA/TBP system with 5 M NaOH to obtain Sc(OH)3 product. A conceptual flowsheet for the recovery of scandium from red mud is proposed.
Supply of some critical raw materials by European industry is becoming more and more difficult. After the case of natural textile fibres, in particular cotton, and timber, over the last few years the problem of rare earths (REs) availability has also risen. The 97% of the global supply of rare earth metals (REMs) is produced by China, that has recently done copious cuts of its exports, apparently in order to protect its environment. This fact has greatly increased the REs prices, causing tension and uncertainty among the world hi-tech markets. Many of these materials, in fact, have very few effective substitutes and low recycling rates too. In addition, their natural reserves of rare earths are concentrated in a small number of countries (China, Brazil, US, Russia, Democratic Republic of Congo). REMs are a group of 17 elements particularly used in many new electronic and advanced components: such as fuel cells, mobile phones, displays, hi-capacity batteries, permanent magnets for wind power generation, green energy devices, etc. Many analysts foresee much more requests in the next decades.
The article reviews the cathodic process of gallium ion reduction in alkaline solutions. The solution composition influence on the gallium anion reduction kinetics was analyzed by measuring the polarization curves on a dropping mercury electrode. Itwas found that the cathodic process rate is proportional to the specific adsorption of background cations in the sequence Na+ b K+ b Li+ b Cs+ b La3+. A higher rate of reduction of gallium anion present in alkaline solution of lithiumcation is a result of participation of thewatermolecules fromthe hydration shell of Li+ as a proton donor. In the presence of polyvalent lanthanum cations in alkaline solution, the gallium anion reduction rate increases sharply. This is related to a shift in the ƒÕ1-potential and participation of hydrated cations La(H2O)3 + n. The presence of surface-active agents,which have no proton.donor properties, in the solution, complicates the reduction reaction. Certain patterns of themechanism of the galliumion discharge reaction in alkaline solutions allowqualifying it as the second group of anions, and the slowstage of the reduction reaction comes down to simultaneous transfer of an electron and proton to the discharging anion. The estimated charge of the discharging gallium anion, which is equal to .0.24 in the transition state of the reaction, is indicative of formation of associates with background electrolyte cations [Me+ c GaO2], [Me+ c GaO(OH)2] by gallate anions in alkaline solutions („‚„N 12).
Red mud is the major waste material produced during alumina production following the Bayers process. Depending on the quality of the raw material processed, 1–2.5 tons of red mud is generated per ton of alumina produced. The treatment and disposal of this residue is a major operation in an alumina plant. A lot of research and developmental activities are going on throughout the world to find effective utilization of red mud, which involves various product developments. This article attempts to review these developments.
A process is described for the enrichment of titanium dioxide in red mud. The procedure employed is leaching the red mud with hydrochloric acid followed by roasting the leached residue with sodium carbonate. The kinetics of leaching of various constituents of red mud were obtained experimentally in a stirred batch reactor. The variables include acid to mud ratio and temperature. The data obtained were analysed using the shrinking core model and Jander's equation. The effect of roasting time and temperature on percentage dissolution of alumina in leached residue was studied using a full factorial search and optimized conditions were obtained.
By using X-ray microanalysis, the mechanism of sorption of rare earth elements (REE) and their localization in cells of Candida utilis were found to depend on the metal ion speciation in solution, the permeability of the cytoplasmic membrane (CPM), and elemental composition of cells. Sorption capacity of the yeast cells increased with the increase in the pH of solution, which is connected with the extent of metal hydrolysis. Cells with native permeability of CPM did not sorb either scandium at pH values below 4.5 or lanthanum and samarium at pH values below 5.0. Such cells accumulate rare earth elements on surface structures. Only the cells with impaired CPM could sorb REE from the acid solutions. In this case, REE were accumulated inside the cells due to the interaction with phosphorus-containing compounds; the amount of sorbed REE depended on the content of phosphorus in the yeast cells. The yeast cells were shown to have extremely high affinity to scandium which thus can be selectively sorbed from solutions containing other REE, iron, and aluminum.
The aim of this work is to investigate biological leaching of rare earth elements (REEs) and radioactive elements from red mud, and to evaluate the radioactivity of the bioleached red mud used for construction materials. A filamentous, acid-producing fungi named RM-10, identified as Penicillium tricolor, is isolated from red mud. In our bioleaching experiments by using RM-10, a total concentration of 2% (w/v) red mud under one-step bioleaching process was generally found to give the maximum leaching ratios of the REEs and radioactive elements. However, the highest extraction yields are achieved under two-step bioleaching process at 10% (w/v) pulp density. At pulp densities of 2% and 5% (w/v), red mud processed under both one- and two-step bioleaching can meet the radioactivity regulations in China.
This study presents a rapid and selective method for the recovery of lanthanides and yttrium, existing in economically interesting concentrations, from red mud, the byproduct of the alumina production. The leaching process is based on the extraction of these elements with diluted nitric acid from red mud under moderate conditions and without using any preliminary treatment. Several parameters such as leaching agents, contact time, temperature, pressure and solid to liquid ratio were investigated in order to achieve an optimum recovery. The process followed here was selected taking into account its efficiency for the selective recovery of yttrium and lanthanides, but also its suitability for the subsequent liquid-liquid extraction of the leaching solution for the separation of the individual lanthanides. The achieved recovery percentages were for Y about 90%, for the investigated heavy lanthanides (Dy, Er, Yb) up to 70%, for the middle ones (Nd, Sm, Eu, Gd) up to 50% and for the group of light lanthanides (La, Ce, Pr) up to 30%. Most of the main elements of the red mud and especially iron are hardly dissolved by this leaching process, leading to an interference free determination of the lanthanides and yttrium in the leachate by inductively coupled plasma-atomic emission spectroscopy (ICP-AES).
Scandium, existing in economically interesting concentrations, in red mud, the byproduct of the alumina production, could be selectively separated from the main and minor elements, as well as from yttrium and the lanthanides by the development of a combined ion exchange-solvent extraction method. After a suitable borate/carbonate fusion of red mud, the sample solution was passed through the ion exchanger Dowex 50W-X8 and the main elements, such as Fe, Al, Ca, Si, Ti, Na as well as the minor ones Ni, Mn, Cr, V were removed by elution with 1.75 M HCl. Sc, Y and the lanthanides were quantitatively obtained by a subsequent elution with 6 M HCl. Using as extractant di(2-ethylhexyl)phosphoric acid in hexane, scandium could be selectively and nearly quantitatively extracted in the organic phase, while yttrium and the lanthanides remained in the aqueous phase. By suitable backstripping of the organic phase, scandium was finely quantitatively recovered in high purity in the aqueous phase. Experimental conditions, including the pH of the extracted solutions, the ratio of aqueous to organic phase and the contact time were investigated in order to achieve high distribution ratios and a selective extraction. In this way all spectral interferences for the inductively coupled plasma atomic emission spectrometric determination of scandium were eliminated and a very low detection limit in the ng/g range was achieved, allowing the determination of very low levels of scandium in complex matrices. The validity of the described process was tested on the bauxitic reference material BX-N from ANRT (Association Nationale de la Recherche Technique, France) and the procedure was applied on red mud samples coming from the Greek alumina production.
Activated carbon (AC) was modified by tri-butyl phosphate (TBP) for selectively extracting scandium from red mud and characterized by BET (Brunauer-Emmett-Teller) surface area. The modified AC had a preferential adsorption to scandium. The influences of adsorbent dosage, adsorption temperature, and time on adsorption capacity and selectivity to scandium were examined. An optimum adsorbent dosage (∼6.25 g/L), adsorption temperature (308 K), and adsorption time (40 min) were figured out. A pseudo-second-order kinetics model was employed for describing the adsorption process of scandium.
India has fairly rich reserves of rare and refractory metals. Abundant sources of ilmenite, rutile, zircon and rare earths are found in the placer deposits of the southern and eastern coasts of the country. Columbite-tantalite occur in mica and the mining belts of Bihar and cassiterite deposits are found in Bastar (Madhya Pradesh). Vanadium as a minor associate occurs in bauxites and in the vast deposits of titaniferrous magnetites.
Over the years, research and development and pilot plant works in many research organisations in India have built up a sound technological base in the country for process metallurgy of many refractory and rare earth metals starting from their indigenous sources. The present paper provides a comprehensive view of the developments that have taken place till now on the processing of various refractory and rare earth metals with particular reference to the extensive work carried out at the Department of Atomic Energy. The coverage includes mineral beneficiation, separation of individual elements, preparation of pure intermediates, techniques of reduction to metal and final purification. The paper also reviews some of the recent developments that have been taken place in these fields and the potential application of these metals in the foreseeable future.
An analytical procedure for the rapid, accurate and reproducible determination of lanthanides, yttrium and scandium in iron-aluminum rich matrices as bauxites and red mud from alumina production was developed. After a suitable dissolution, the samples were directly analysed for these elements by inductively coupled plasma atomic emission spectroscopy (ICP-AES), without using any separation or preconcentration step. Some of the investigated elements were also directly determined in the solid samples by x-ray fluorescence analysis (XRF). The optimum conditions and analytical wavelengths were selected by both methods for each investigated element, after detailed studying of the spectral interferences from the major elements from the matrix and the interlanthanide-interferences. In the unique reference bauxitic material BX-N, for which only some proposed values exist, the concentration of nearly all lanthanides, yttrium and scandium Sc could be determined. The precision and accuracy of the described ICP-AES and XRF methods for these elements were tested on the geological standard materials SY-2, SY-3 (Canadian syenites) and the proposed values of BX-N. In this study the enrichment factor of the above mentioned elements in red mud were also determined in comparison to the feed bauxites and the constancy of the concentration level in productions samples, followed up over three years. The obtained results for the lanthanides were evaluated on the basis of chondrite normalized distribution patterns.
The extraction of scandium and uranium from red mud resulting from alumina production with the ampholite resins AFI-21 and AFI-22 was studied. These resins are of great interest due to their high ability to form complexes and their good specific sorption selectivity. The aspects studied include loading capacity, kinetics and elution. The ion-exchange technology for the treatment of red mud was developed and tested at a pilot plant scale. The prospects for developing an industrial process are discussed.