Extraction of Valuable Elements from Red Mud with a Focus on Using Liquid Media—A Review

Pasechnik L.A., Skachkov V.M., Chufarov A.Y., Suntsov A.Y., Yatsenko S.P.

Storage of red mud – bauxite processing waste – leads to serious environmental problems due to its high alkalinity and particle dispersity. Full or partial utilization or recycling of red mud could reduce the harmful effect on the environment. Scandium is the most valuable ingredient of red mud, yet it's extraction is poorly commercialized due to its high cost. The new efficient extraction technologies promise an ensured supply of scandium and a significant drop in cost. Here, scandium concentrate, extracted from leachate after carbonate treatment of red mud, was subjected to sulfatisation by H 2 SO 4 to separate silica from water-soluble sulfates. To recover and selectively separate scandium from other impurity metals, the crystallization of two complex scandium and ammonium sulfates – NH 4 Sc(SO 4 ) 2 and (NH 4 ) 3 Sc(SO 4 ) 3 – is proposed. The solubilities of these sulfatoscandiates in water, established by isothermal method, are 33.4 and 72.4 g/L, respectively. For the less soluble NH 4 Sc(SO 4 ) 2 a further considerable reduction of solubility has been observed in H 2 SO 4 solutions of concentration above 3.5 М in the presence of 0.5 М NH 4 Cl at 20 ± 1 °C. More than 99% of scandium in the form of micron-sized NH 4 Sc(SO 4 ) 2 crystals has been recovered from a multicomponent liquid at 5–6 М H 2 SO 4 and 0.5 М NH 4 Cl. The product contains extremely low levels of impurities. The precipitation of NH 4 Sc(SO 4 ) 2 offers a much higher selectivity in separation of Sc from the other main constituents, as demonstrated by the large separation coefficients between scandium and other metals β Sc/M (e.g., for the couple with aluminum β Sc/Al = 4280). The recrystallization product after calcination at 1000°С contains 99% Sc 2 O 3 . • Sulfuric dry digestion is applied in Sc extraction without silicon dissolution. • NH 4 Sc(SO 4 ) 2 and (NH 4 ) 3 Sc(SO 4 ) 3 are crystallization products from sulfate media. • A decline of NH 4 Sc(SO 4 ) 2 solubility is observed with increasing acidity. • Efficiency of selective precipitation of Sc(III) in solid form, NH 4 Sc(SO 4 ) 2 , reaches 99%. • Attractive separation coefficients for Sc among Al, Fe Ti, Zr and Ca are obtained.

Agrawal S., Dhawan N.

Red mud is an inevitable and under-utilized byproduct of the aluminium industry. The current study evaluates the effect of mechanical and thermal activation on the acid leaching of red mud. The acid leaching of the pretreated product is followed by acid-base treatment to precipitate the dissolved aluminium values from solution and enrich the residue with Fe and Ti values. Mild acid leaching yielded ~76% aluminium dissolution with 81% silicon removal. The direct acid leaching resulted in dissociation of sodium aluminosilicate; meanwhile, gibbsite phase remains in the residue. The mechanical milling for 1 h enhanced the aluminium dissolution to 89.5% with silicon and iron dissolution of 98.5 and 9.6%, respectively. The silica and iron oxide values were separated before HCl leaching by thermal pretreatment step using microwave heating with NaOH and carbon additives respectively. The sodium silicate and sodium aluminate phases formed during alkali thermal treatment resulted in ~33% silicon and ~ 27% aluminium removal by water washing. Meanwhile, HCl leaching of the water wash residue resulted in 82% aluminium dissolution with 70% recovery and residue containing ~52% Fe2O3, and 28% TiO2 is obtained. The carbothermal reduction yielded iron-rich magnetic concentrate having 48% iron grade with 20% metallic iron, and ~ 83% iron recovery. Leaching of the non-magnetic fraction containing hercynite and fayalite as the major aluminium and silicon bearing phases resulted in 81% aluminium dissolution with overall aluminium recovery of only 37%. Mullite formation is also observed in the precipitate with a high Al/Si molar ratio. The carbothermal route was found as an energy-efficient route, whereas the mechanical milling route yielded maximum Al extraction of ~90% with Fe and Ti content of 41.1% and 16.8% in the residue.

Zhao Y., Zheng Y., He H., Sun Z., Li A.

During hydrochloric acid leaching of bauxite reaction residue (BRR) produced from coagulant industry, the effects of hydrochloric acid concentration, liquid solid ratio and silica remove on the extraction of aluminum were studied, at atmospheric pressure (85 °C). The aluminum dissolution rate under the optimal conditions (8 mol L−1, L S−1 5:1) was only 44.15 %. After the leached residue was treated by alkali to selectively remove silica and then leached with hydrochloric acid at the same conditions, the aluminum extraction rate rarely changes, even if the silica removal rate reaches 81.52 %. Direct leaching after mechanical activation for 10 h, resulted in the extraction rate increased by 5.05 %. However, compared with the optimal condition (8 mol L−1, 5:1, 10 h) under atmospheric pressure, the aluminum extraction rate was only 49.2 %, while up to 84.23 % of aluminum was extracted from BRR at high-pressure (210 °C). In addition, the leaching experiment and kinetics of aluminum were analyzed in-depth under high-pressure. The apparent activation energy of aluminum dissolution was found to be 20.82 kJ mol−1, and the dissolution process is determined by liquid diffusion. Obtained leachate has been well recycled without waste discharged, which provides a theoretical basis for the BRR utilization.

Wang S., Jin H., Deng Y., Xiao Y.

Red mud is a solid waste produced during the bauxite refining of alumina. In recent years, environmental problems caused by the accumulation of red mud have become increasingly serious. In order to understand the status of red mud recovery in recent years, this article uses a comprehensive literature database to classify and statistically analyze red mud-related publications from 2010 to 2019. The results show that research on the comprehensive utilization of red mud is mainly found in three fields: the construction and chemical industry, the environmental protection and agriculture industry, and the valuable elements extraction industry. A brief report is also made on the related research of red mud in the fields of cement, concrete, glass, ceramics, adsorbents, geopolymers, catalysts, composite materials, sewage treatment, waste gas treatment, soil improvement, and valuable element recovery. The current industrial consumption of red mud in China is measured, and some suggestions for solving the red mud problem are put forward.

Zhang X., Zhou K., Wu Y., Lei Q., Peng C., Chen W.

Red mud is a byproduct of alumina refining of bauxite ores, and is a significant source for extracting scandium. However, a large amount of iron in red mud makes it difficult to recover scandium because Fe(III) and Sc(III) have similar physicochemical properties. In this study, a new method was developed for selective separation of iron and scandium in acid leachate of red mud using D201 resin. Theoretical calculations indicate that the ferric species mainly exists as FeCl 3 or FeCl 4 − at chloride concentration above 6.65 mol/L, while scandium still exists as ScCl 2 + , making it possible to selectively separate iron from scandium through anion resin adsorption. The factors affecting the adsorption of iron and scandium such as chloride concentration, resin dosage, adsorption time, and temperature were evaluated in batch experiments. The Langmuir model was successfully applied to both iron and scandium adsorption, and the maximum adsorption capacities of iron and scandium are 147.06 and 0.95 mg/g, respectively, indicating a significant difference between iron and scandium. Raman analysis further demonstrates that the iron is adsorbed onto D201 resin as FeCl 4 − anion. In chloride-rich solution, Fe(III) existed as FeCl 4 ‒ or FeCl 3 species, both of which could be efficiently adsorbed by D201 resin, while scandium remained as ScCl 2 + in the solution. • A new approach was proposed for the separation of Fe and Sc from red mud. • The forms of Fe(III) and Sc(III) species in high concentration chloride solution were clarified. • Iron ions can be effectively removed from red mud by D201 resin with low scandium loss. • Iron ions in the resin can be easily eluted by dilute hydrochloric acid.

Swain B., Akcil A., Lee J.

Abstract Significantly unexploited red mud contains 0.5–1.7 kg/ton of rare earth metals (REMs) and 14–45% Fe, 5–14% Al, 1–9% Si, 1–6% Na, and 2–12% Ti which is currently being stockpiled. Reasonably, the red mud can be a suitable secondary resource of REMs, and other valuable metals, if not for the contained base metals. Stockpiled red mud locks down these technologically important and supply chain critical metals. Though quantitatively minor, the value perspective of REMs in red mud is significant. Global red mud inventory could reach 4 billion tons by 2020 and being generated 120–150 million tons per year worldwide. Analysis indicated that industrial-scale valorization could unlock approximately $4.3 trillion worth of REMs from stockpiled red mud. The current review discusses the valorization of red mud through metal value recovery which is an industrial waste circular economy challenge. Unlike other reported reviews, the current review intensively discusses circular economy challenges and opportunities associated with the red mud valorization. Finally, a suitable conceptual sequential metal recovery process has been proposed based on a comprehensive review of various processes which can technically be a feasible process for sequential recovery of various metal values from the red mud. Comprehensive cutting-edge research, broader risk assessment, and cost and benefit analysis of the proposed process can ensure broader valorization. Broader valorization can address several issues through the proposed process like; (i) metal value recovery, (ii) REMs circular economy, (iii) closed-loop process with the industry, and (iv) lower the futuristic carbon economy, simultaneously. Graphical abstract

Zhu X., Li W., Xing B., Zhang Y.

The extraction of Sc by acid leaching with CaF2 and solvent extraction with P507 from red mud was proposed. The influence of acid leaching and solvent extraction on recovery of Sc was investigated. The CaF2 can obviously improve the leaching efficiency of Sc and reduce the acid consumption. The leaching efficiency of Sc increases from 74% to 92% and the dosage of acid reduces under suitable conditions by adding 5% CaF2. The minerals in red mud can easily be decomposed and leached into the acid solution with CaF2 through analysis of XRD pattern. The particles of red mud become smaller and multihole. The Sc can be selectively extracted with 10% P507 at the pH value of 0.1 from the acid leaching solution. More than 98% of Sc and less than 10% of Al and Fe are extracted. The Sc2O3 with purity of 99% is obtained after the process of reverse extraction with NaOH, H2SO4 dissolution, precipitation by oxalic acid and roasting at 750 °C.

Pasechnik L.A., Skachkov V.M., Bogdanova E.A., Chufarov A.Y., Kellerman D.G., Medyankina I.S., Yatsenko S.P.

The Bayer bauxite residue (red mud, RM) is environmentally hostile and hazardous to human health. Due to a high content of iron, RM is utilized here by hydrothermal digestion in an original one-stage method of hematite into magnetite conversion during recovery of alumina. The changes in the content of alumina, hematite, and magnetite are confirmed using XRD, SEM, chemical analysis and magnetometry. The yield of alumina reaches 75.4% and more during digestion with addition of lime and FeSO4. The formation of magnetite involves the interaction between ferrous ions generated from FeSO4 and dissolved from hematite of RM in the Bayer liquor and hematite reduction by ions-reductants from initial RM during repeated digestion. The conversion efficiency of hematite reaches 52% through the Bayer digestion at the molar ratio Fe(II)/Fetot in RM 0.17 with following separation of magnetic fraction of RM (MFRM) containing 33% Fe3O4. The saturation magnetization of MFRM grows to 29.5 emu/g compared with 2.90 emu/g for parent RM without noticeable amount of magnetite. The yield of the magnetic fraction increases with increasing iron(II) dosage. The magnetite concentrate isolated from parent RM by the enrichment method with 10.5% Fetot extraction and with mass yield of this concentrate of only 6% has the highest value, 65.5 emu/g, with 34% Fe3O4 content almost similarly to that in MFRM. The particle aggregation is found to be the same in all samples and may not critically affect their magnetic properties. On the whole, we have demonstrated the feasibility of RM digestion for the recovery of alumina and synthesis of magnetite to improve the enrichment procedure, as well as the use of wastes as a technogenic feedstock.

Zeng H., Lyu F., Sun W., Zhang H., Wang L., Wang Y.

Red mud (RM), also called bauxite residue, is a strong alkaline industrial waste generated during the alumina production process. The annual production of RM in China is large, but its average utilization rate is low (only 4%). High generation and low consumption make the disposal of RM mainly by stockpiling, which has caused serious heavy metal pollution and radioactive contamination. In this paper, the various industrial utilization methods of RM in China during the past 60 years have been introduced. Moreover, some recent industrial progresses were referred. The results show that RM can be widely used in building materials, valuable metals extraction, and some novel utilization methods, such as silica-calcium fertilizer, inorganic polymer material and desulfurizer. Most of the industrial utilization methods of RM have been used until now and some successfully applied to other aluminum plants, providing some feasible routes for a large amount utilization of RM. Some industrial utilization methods (such as oil well cement and calcium silicon fertilizer) have not been used due to some problems that cannot be ignored, but it provided a lot of valuable experience and was helpful for the subsequent RM utilization. Moreover, some novel and feasible RM utilization methods were proposed and successfully industrialized, which showed that RM has a broader application prospect. Many actual practices showed that the best way to safely dispose of RM was to develop technology that could consume large amounts of RM or transform it into secondary resources, which may need more time and effort.

Archambo M., Kawatra S.K.

ABSTRACT Red mud is generated at a rate of up to 175.5 million tons per year. The global stockpile of red mud is near 4 billion tons. This material is hazardous with pH values from 11 to 13. Reduction of this waste is critical. Current industry practices for disposal of red mud involve different stockpiling techniques on valuable land area or disposing into critical bodies of water. This review studies processes which can reduce the negative environmental impact of red mud in an economic way. For instance, neutralization of red mud with CO2 can decrease the pH from 12.5 to 7. Treatment of red mud by this method lessens the negative environmental impact and prepares it for further processing for utilization. The current utilization rate of red mud is very low, only about 3 million tons per year are used as an additive for cement and construction. Red mud contains a large quantity of valuable minerals that can be extracted to both reduce the amount of red mud and provide value to the waste. This review investigates novel methods for treating red mud and extracting minerals like iron, titanium, and rare earth elements using a variety of smelting, direct reduction, and leaching processes. For example, the iron nugget process is a single step method to reduce iron oxides to metallic iron and separate them from red mud. Iron nuggets produced from red mud have an iron grade above 90%, which is comparable to pig iron generated by the blast furnace.

Hertel T., Pontikes Y.

The use of the alumina production waste bauxite residue (BR) in alkali-activated materials (AAM), such as geopolymers, inorganic polymers and hybrid binders, is attracting considerable interest. This work provides an in-depth review of the growing body of literature in the field of alkali-activated binders from BR and spent Bayer liquor, i.e. another possible resource from an alumina refinery. Additionally, an assessment of the possible role of these resources in alkali-activated materials is made. The review is approaching the topic from the alumina refinery perspective and is not only presenting and analysing published work, but is synthesising these efforts in a framework that emphasises the potential and the challenges. Moreover, this work tries to map the path towards a realistic valorisation, that acknowledges true and perceived risks (e.g. radioactivity, leaching, legislation). Besides the trends in scientific literature, also industrial-relevant literature (i.e. patents) are visualised using text analyses (data mining), and some emerging pilot-plant efforts are discussed as well. General barriers for an industrial implementation of BR valorisation and their relevance in view of AAM are discussed, including options on how they could be potentially lifted. We identified a multitude of BR valorisation routes in AAM, in which BR (or spent Bayer liquor) acts mostly as an inert filler, (partially) reactive material or a supplier of alkalis. Several previously defined barriers concerning an industrial use of BR can be addressed by its use in AAM and can be even advantageous, such as the intrinsic alkalinity of the residue. However, further pilot plant scale work is needed to assess whether the suggested options can withstand real-life requirements and it is recommended to direct research efforts towards economic, life cycle and environmental assessments.

Reddy P.S., Reddy N.G., Serjun V.Z., Mohanty B., Das S.K., Reddy K.R., Rao B.H.

In order to conserve natural resources and prevent waste generation, effective utilization of industrial wastes and/or by-products for beneficial engineering applications becomes inevitable. In order to accomplish this, extensive research studies, exploring properties and new applications of waste materials in a sustainable and environmentally friendly manner, have been initiated worldwide. Red mud (RM, also known as bauxite residue) is one of the wastes generated by the aluminium industry and its disposal and utilization have been traditionally hindered due to the extreme alkalinity (pH about 10.5–13.5). To date, no comprehensive review on various properties of RM of different origin and associated challenges in using it as a beneficial engineering material has been performed. The objective of this study is first to critically appraise the current understanding of properties of RM through a comprehensive literature review and detailed laboratory investigations conducted on Indian RM by the authors, to assess and identify the potential engineering applications, and to finally discuss associated challenges in using it in practical applications. Physical, chemical, mineralogical and geotechnical properties of RMs of different origin and production processes are reviewed. Mechanisms behind the pozzolanic reaction of RM under different chemical and mineralogical compositional conditions are discussed. Environmental concerns associated with the use of RM are also raised. Studies relevant to leachability characteristics reveal that most of the measured chemical concentrations are within the permissible regulatory limits. Overall, the review shows that RM disposal and reuse is complicated by its extreme alkalinity, which is also noticed to be influencing multiple engineering properties. But with selected pH amendments, the treated RM is found to have significant potential to be used as an effective and sustainable geomaterial. The assessment is majorly based on the characteristics of Indian RMs; hence the adaptation of the findings to other RMs should be assessed on a case-by-case basis. Moreover, field studies demonstrating the performance of RM in various engineering applications are warranted.

Zhu X., Niu Z., Li W., Zhao H., Tang Q.

A novel process for recovery of aluminum, iron, vanadium, scandium, titanium and silicon from red mud was proposed. The parameters of HCl leaching, solvent extraction, polymerization process, alkali leaching with pressure, and aging process were investigated. The results show that more than 96 % of scandium and vanadium could be leached by using HCl with many aluminum, iron and few titanium and silicon from red mud. More than 97 % of vanadium and scandium was extracted by solvent extraction with mono(2-cthylhexyl)2-cthylhexyl phosphonate (P507), but few other elements from the acid leaching solution. The rich vanadium liquid and rich scandium liquid could be obtained by washing the saturated organic phase. More than 97 % of iron and aluminum could be precipitated as the polyaluminum ferric chloride from the raffinate. The rich titanium material of TiO2 with grade of 62 % was obtained by NaOH leaching with pressure from the acid leaching residue. The white carbon black with purity of 99.5 % was obtained by aging with sulfuric acid solution.

Li X., Li L., Huang Z., Chang Z., Tu Z., Tian L., Du W., Li H., Zhang P., Pan B.

Vielma C.A., Svobodova-Sedlackova A., Chimenos J.M., Fernández A.I., Berlanga C., Rodriguez R., Barreneche C.

Red mud (RM), the primary waste product of the aluminium industry, is notable for its high concentrations of metals and rare earth elements (REE). Efforts have been made to develop extraction methods for REE recovery from RM, aiming to enhance its valorisation and reduce the European reliance on external REE sources—particularly crucial for technological advancements and the transition to renewable energy. However, these methods have only been limited to low technology readiness levels (TRLs), with no economically or technically viable processing routes yet defined to enable large-scale industrialisation within a circular economy model. This study characterised RM samples from the Seydişehir region in Türkiye using different techniques and explored the experimental process for recovering metals and REE. Moreover, the study assessed the global prospective potential of RM based on technical and economic data, as well as the sustainability of the implemented process through the life cycle assessment (LCA) tool. Results showed a total REE concentration of up to 1600 ppm, with Ce, being the most abundant (426 ± 27 ppm), followed by La, Nd, and Sc. Concentration efficiencies for La and Nd ranged between 240–300%. Sc, Y, Ce, La, and Nd have significant usage in European markets and represent prime RM targets for further prospecting. The LCA revealed that the highest global warming potential of the sequential extraction process was attributed to hydroxylamine hydrochloride and hydrogen peroxide. The findings highlight the need to explore alternative, more eco-friendly reagents to improve RM valorisation.

Kurniawati R., Dahani W., Subandrio S., Palit C., Kisal Albrian Gheraldy S.

Abstract Red mud is classified as hazardous waste due to its high alkalinity and the presence of radioactive materials. This study aims to investigate the leaching properties of major metals and rare earth elements in red mud at pH levels of 2, 7, and 13. We performed XRD, XRF, and ICP analyses to determine the effect of pH on metal leaching in red mud. The findings revealed that the pH of the solution substantially affected the concentration of dissolved metals. Metal dissolution increases dramatically at an acidic pH (pH 2), but metal concentration drops quickly at a neutral pH (pH 7). At alkaline pH (pH 13), Al dissolves significantly, whereas Fe remains reasonably stable. These findings indicate that the red mud processing industry can utilize pH modulation to enhance the leaching effectiveness of specific metals, a crucial step towards developing more efficient and environmentally friendly purification procedures.

Balaram V., Armstrong-Altrin J.S., Khan R.M., Rao B.S.

Zhu M., Wang Y., Zheng C., Luo Y., Li Y., Tan S., Sun Z., Ke Y., Peng C., Min X.

Jarosite waste is a by-product generated from iron removal process in the jarosite process, which typically contains valuable metals including zinc, nickel, cobalt, silver, indium, and lead. Due to the large amount of jarosite and the less efficient and costly methods of recovering residual metals, it is mainly disposed by landfills. However, leachate generated from the landfills can release high concentrations of heavy metals, which contaminate nearby water resources and pose environmental and health risks. In this review, the environmental and resource properties of jarosite waste were briefly summarized. Then those pyrometallurgical, hydrometallurgical and biological methods were discussed. In this review, considering the polymetallic properties and the low content of valuable metal elements of the jarosite waste, it is indicated that these processes had their own benefits and drawbacks such as overall yield, economic and technical constraints, and the necessity for combined processes to recycle multiple metal ions from jarosite wastes. Finally, this paper provided a critical and systematic review of studies on the novel green recycling technology for metals and material preparation based on the jarosite waste. This review can lay a guidance for the near-zero-waste processing of jarosite waste, with a particular focus on the combination of chemical and biological processes and waste-to-materials.

Angelopoulos P.M., Oustadakis P., Anastassakis G., Pissas M., Taxiarchou M.

Balaram V., Santosh M., Satyanarayanan M., Srinivas N., Gupta H.

The energy transition challenges faced by modern civilization have significantly enhanced the demand for critical metals like lithium resulting in improved methods to explore, extract, and utilize these metals. In this comprehensive review, we discuss the different types of lithium resources, factors, and mechanisms controlling lithium enrichment in various geological settings including terrestrial and marine environments. Diverse exploration strategies including geological, geophysical, mineralogical, geochemical, and remote sensing techniques including drone-based techniques for lithium exploration studies in different terranes are summarized. An overview of the mining techniques, including beneficiation and extraction, and their principles, mechanisms, operations, and comparison of the various approaches and compatibility with different types of lithium deposits for obtaining maximum yield are evaluated. Lithium isotopic studies are useful in understanding geological processes such as past weathering events and riverine input into the oceans, as well as in understanding the source of lithium in diverse types of deposits. We also highlight the recent developments in other areas such as recycling, environmental impact, and state-of-the-art analytical techniques for determining lithium in different lithium ore deposits and other geological materials. Our overview provides the latest developments and insights in the various sectors related to lithium and prompt further developments to meet the growing demand for this valuable metal as the world transforms to clean energy.

Wang Y., Zhen Z.

This study aimed to decipher the influence of red mud on the mechanical properties, pore structure, and microstructure of basic magnesium sulfate cements (BMSCs). The results showed that BMSC prepared with an appropriate addition of red mud exhibited improved mechanical properties and yielded the highest compressive strength of 94.54 MPa after curing for 28 days. Adding red mud reduced the total porosity and optimized the pore structure of BMSC. The microstructure and hydration products of the specimens were examined using X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The results illustrate that the addition of 50% red mud did not affect the amount of the main strength phase 5-1-7 produced in BMSC. It could also reduce the residual amount of MgO and the generation of Mg(OH)2. The red mud and the M-S-H gel generated by the reaction between active SiO2 and α-MgO in the red mud together filled the pore structure of BMSC, making its microstructure denser and higher-strength. This study aims to improve the comprehensive use of red mud, and the results show that red mud can improve the mechanical properties of BMSCs, protecting the environment and simultaneously reducing BMSC production costs to create good economic benefits.

Gupta A., Williams E., Gaustad G.

Hamza M.F., Mira H., Ning S., Yin X., Wang J., Wei Y., Guibal E.

A simple one-pot reaction of maleic anhydride with iminodi(methylphosphonic acid) in presence of N,N'-methylenebisacrylamide allows producing a micron-sized multifunctional sorbent (herein called MPOH-MBA, bearing both carboxylate, phosphonate groups, and tertiary amine groups). These multifunctional groups bring high reactivity for rare earth elements (REEs, hard acids) (soft bases O-bearing ligands, consistently with Pearson's principle) with modulation of positively-charged surface (amine groups for interaction with scandium sulfate species). Effectively, MPOH-MBA shows remarkable sorption capacity for Sc(III) at optimum initial pH (i.e., pH0: 4): qeq,exp reaches up to 5.34 mmol Sc g-1, at room temperature. Langmuir equation fits well experimental profiles (affinity coefficient: 0.677 L mmol-1). Scandium sorption is endothermic: sorption capacity increases up to 6.50 mmol Sc g-1 (at T: 50 °C), while the affinity coefficient increases up to 1.54 L mmol-1. Fast kinetics (equilibrium reached in 20-30 min) are favored by the micron-size of the sorbent (and its good textural properties: 51 m2 g-1 for specific surface area and pore width close to 250 Å). Kinetic profiles are fitted by the pseudo-first order rate equation. The sorbent can be easily regenerated using 0.3 M HCl solution; the loss in sorption is less than 1.5% at the fifth cycle. Main reactive groups (identified by Fourier-transform infrared spectroscopy) are carboxylate and phosphonate groups. In presence of equimolar concentrations of competitor ions: sorption is governed by preference for trivalent metal ions over divalent cations. Among trivalent metal ions rare earth elements (REEs) are preferentially bound (against Al(III) and Fe(III)); in addition, MPOH-MBA sorbs Sc(III) with higher affinity than Nd(III) and Ce(III). The sorbent shows a relative selectivity for scandium and REEs over metal ions present in huge excess (100 to 500 excess), such as Fe(III) and Al(III)), in red mud acidic leachate.

Shoppert A., Valeev D., Diallo M.M., Napol'skikh J., Loginova I.

Bauxite residue (BR) is a by-product of bauxite refining using Bayer process for alumina production, which contains numerous valuable components: iron (Fe), aluminum (Al), and rare earth elements such as scandium (Sc). The main issue of previous research for the Sc extraction from BR was the low extraction efficiency or high Fe co-extraction. This article shows that Fe co-extraction can be significantly reduced after reductive leaching of BR in the presence of Fe(II). This effect was achieved due to the formation of magnetite (Fe3O4) after reductive leaching of bauxite. Magnetite is practically insoluble in H2SO4 at pH > 2. The effect of temperature (T), time (t) and liquid to solid ratio (L:S ratio) on the leaching process was revealed. The results show that under optimum conditions (pH = 2, T = 90 °C, t = 2 h, L:S ratio = 10:1), Sc extraction can reach 57%, with a simultaneous co-extraction of Fe

de Sousa Cordeiro E., Scaratti G., de Souza D.C., Nickel C.D., José H.J., de Fátima Peralta Muniz Moreira R., De Noni A.

The growing concern over environmental contamination by pharmaceutical compounds has drawn the attention of the scientific community and regulatory bodies worldwide. It is estimated that a wide variety of drugs, including analgesics, antibiotics, hormones, and antidepressants, are widely detected in bodies of water, soil, and even in drinking water supply systems. This contamination represents not only a direct threat to human health and the environment but also challenges conventional water and wastewater treatment systems, which are often ineffective in completely removing these compounds. In this context, advanced oxidation processes (AOPs) emerge as a promising approach for degrading persistent organic contaminants such as pharmaceutical compounds. AOPs are highly effective techniques that involve the generation of reactive oxygen species, such as hydroxyl radicals, capable of oxidizing a wide range of organic pollutants, transforming them into less toxic and more easily treatable products. The importance of using red mud (RM) as a catalyst in AOPs for the treatment of pharmaceutical compounds is underscored by its abundance as a by-product of the aluminum industry and its unique ability to catalyze oxidation reactions. RM is the residue resulting from the Bayer process for refining bauxite to obtain aluminum. Its porous structure and chemical composition rich in metallic oxides confer catalytic properties that can be exploited in the degradation of organic contaminants. In this review, we explore the application of RM as a catalyst in AOPs for the treatment of pharmaceutical compounds. We investigate the mechanisms involved, the methods of preparation and modification of RM for optimizing its catalytic efficiency, as well as review case studies and significant results in the scientific literature. By addressing these aspects, we aim to provide important insights for future research and developments in this area of environmental science and process engineering.

Alagarsamy G., Sruthi K., Selvakumar R., Sivasubramanian R.

Herein, we report a novel method for the electrochemical deposition of TiO2 on Al substrate through the formation of peroxotitanium complex without any post heat treatment process. The alkaline treated titanium precursors were used as an electrolyte and TiO2 was deposited onto aluminium substrate by both cathodic reduction and electrophoretic deposition through the formation of peroxotitanium complex. The electrodeposition process was optimized by varying the pH, concentration of the electrolyte, deposition voltage, time and area of the electrode respectively. The optimized condition to obtain a maximum yield is as follows: voltage (4.5 V), time (2.5 h), pH (5) and area of the electrode 3 × 3 cm2. The deposits were then characterized using various material characterization techniques such as high-resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy respectively. The morphological analysis showed the deposition of nanocrystalline TiO2 with irregular morphology on Al substrate. The AFM revealed a non-uniform deposition of TiO2 with high surface roughness (average roughness: 44.60 nm). The XRD pattern and Raman spectrum confirmed the presence of anatase phase of TiO2. The obtained TiO2 were then subjected to photoelectrochemical studies by coating over an ITO electrode. The nanocrystalline TiO2 showed excellent photocurrent response which could be due to the short hole retrieval and fast electron transport across the electrode-electrolyte interface. The proposed method can be promising towards the industrial application for the extraction and recovery of TiO2 from the mineral ore residues and spent mining waste.

Jeremy E., Sanwani E., Chaerun S.K., Mubarok M.Z.

Napol’skikh J., Shoppert A., Loginova I., Kirillov S., Valeev D.

Rare earth elements (REEs) and Sc are concentrated in aluminum production byproducts. The novel REEs recovery approach, which involves leaching with acid at a pH > 3 in the presence of MgSO4, results in the formation of a pregnant leach solution (PLS) with a low concentration of iron (Fe) and titanium (Ti) and a large number of valuable elements. This work studies the application of chelating resin Puromet МТS9580 in the sorption recovery of Sc from sulfate solutions. To analyze the static Sc sorption data, Langmuir, Freundlich, and Temkin isotherm models were used. The Langmuir isotherm model was the best fitted to the experimental data, with a coefficient of determination (R2) of 0.983. The dynamic adsorption experiment was conducted using a PLS and a simulated solution without contaminants. Adsorption of Sc from the simulated solution was better fitted to the Thomas model with a Sc capacity greater than 6.4 mg mL−1. Because Ti had a gradual decrease in C/C0, which the Thomas model was unable to simulate, the modified dose-response (MDR) model fitted better with PLS with a Sc capacity greater than 3.8 mg mL−1. The NaHCO3 solution (200 g L−1) effectively desorbed Sc (>98%) from simulated and PLS solutions after 1.5 h of stirring in a batch mode. After 1.5 h of desorption, the concentration of Sc in the desorption solution was 461.5 mg L−1, while the concentration of Mg and Ti was lower than 200 mg L−1 and 50 mg L−1, respectively.