A selective hydrometallurgical method for scandium recovery from a real red mud leachate: A comparative study

Salman A.D., Juzsakova T., Mohsen S., Abdullah T.A., Le P., Sebestyen V., Sluser B., Cretescu I.

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 extractants 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).

He L., Xu Q., Li W., Dong Q., Sun W.

The recovery of rare earth elements (REEs) from NdFeB slurry by traditional hydrometallurgy has been limited becuase a large number of REEs are lost during separation together with iron. In this paper, a simple and sustainable method is proposed to efficiently separate and recover REEs and iron from NdFeB slurry. REEs were recovered by one-step selective precipitation in phosphoric acid, and the dissolved iron was recovered by oxalic acid. Phosphoric acid leaching results show that under the conditions of 4 mol/L phosphoric acid, 80 °C, L/S of 30:1 and 90 min, the leaching efficiencies of Fe and REEs reach 98.76% and 1.09%, respectively. While the rest of REEs remained in the leaching residue in the form of REEPO 4 · n H 2 O precipitation. Subsequently, the mixed rare earth oxide (rare earth oxalate roasted at 800 °C) and FeC 2 O 4 ·2H 2 O are obtained by oxalic acid precipitation with purities of 99.49% and 97.17% from the REEPO 4 · n H 2 O dissolving solution and the phosphoric acid leaching solution. Moreover, the phosphoric acid is regenerated while recovering iron, and it can be reused in the phosphoric acid leaching step after removing the impurity C 2 O 4 2− . In summary, this work provides an efficient and environmentally friendly method for recovering REEs and iron from NdFeB slurry waste. Phosphoric acid can effectively separate and recover rare earth and iron from NdFeB waste. Under the optimal conditions, the recovery and purity of iron and rare earth are very high. • Phosphoric acid can effectively separate and recover REEs and iron from NdFeB waste in one-step. • A flow sheet for producing high-purity FeC 2 O 4 ·2H 2 O and rare earth oxide is proposed. • Phosphoric acid can be reused after purification.

Habibi H., Piruzian D., Shakibania S., Pourkarimi Z., Mokmeli M.

• Recovery of RM using a combination of physical and chemical processes was studied. • Fe was effectively separated by carbothermal reduction and magnetic separation. • Carbothermal reduction of RM adversely affected the leaching of Ti and REEs . Red mud is a waste generated during the Bayer process in the production of alumina. Recovery of metallic components of the red mud has both environmental and economic benefits. In this study, the effect of carbothermal reduction on the physical and chemical separation of red mud components, namely Ti, REEs, Fe, and Al, were investigated. At first, the impact of carbothermal reduction on the efficiency and selectivity of iron separation from the bulk material through magnetic separation was studied. At the optimum conditions of T = 1350 °C, using a ring-shaped mixing regime of coal and red mud, coal/red mud ratio of 3, t = 120 mins, and soda ash/red mud ratio of 0.2, the iron content of the red mud was effectively metalized and separated from Ti, REEs, and Al. Recovery of iron in the magnetic separation concentrate was 91%, with Fe content of 81%, while about 90% of Ti and Al and 80% of REEs were retained in the non-magnetic product. For the untreated sample, however, iron separation was insignificant mostly due to the low iron metallization. Finally, the dissolution behavior of Ti, REEs, Fe, and Al for both treated and untreated samples using H 2 SO 4 , HCl, and HNO 3 were studied. Different dissolution behavior of the REEs and Ti were observed and correlated to the mineralogical composition of the sample. Overall, carbothermal reduction has shown to positively affect the magnetic separation of iron but negatively affected the Ti and REEs dissolution efficiency.

Chen Y., Ma S., Ning S., Zhong Y., Wang X., Fujita T., Wei Y.

The waste sulfuric acid solution generated from titanium dioxide production is called titanium dioxide waste acid (TWWA), which is an important resource for scandium production. A way including enrichment and purification stages for recovery and purification of Sc from TWWA with 10% P204 - 5% TBP synergistic extraction system was studied. In the enrichment stage, the preferred experimental conditions were obtained, i.e., the A/O ratio: 30, 25 °C, contact time: 30 min. In the following purification stage, the above-obtained enrichment products by stripping with 5 M NaOH were dissolved with 3 M HCl with 1% H 2 O 2 added. The added H 2 O 2 can lead to Ti form peroxy complex ([TiO(H 2 O 2 )] 2+ ) that can effectively inhibit its co-extraction into the oil phase. The optimized purification conditions were the A/O ratio of 5, 30 min and 11 mmol/L Sc concentration. The stripping products obtained from the purification stage were dissolved with 2 M HCl, and then 0.5 M H 2 C 2 O 4 was added to precipitate scandium. After washing and calcining, a product of 99.28% Sc 2 O 3 was obtained. The recovery rate of scandium in the whole process was 90.34%. • A process for recovery of Sc from waste acid was proposed using 10% P204 - 5% TBP - 85% sulfonated kerosene. • An excellent extraction selectivity towards Sc over Fe was achieved in 3 M HCl feed solution. • The co-extraction of Ti was inhibited by adding 1% H 2 O 2 to the enrichment solution in the purification stage. • The scrubbing stage was eliminated so that large volume of scrubbing agent can be avoided. • A 99.28% Sc 2 O 3 product was obtained with a yield of 99.34%.

Agrawal S., Dhawan N.

• Red mud is evaluated as a polymetallic source for Fe, Ti , Al, Sc. • Scandium associated with Ti > Fe > Al and REEs occur as discrete phases or adsorbed on mineral surfaces. • Metallurgical application of red mud is important for the sustainable supply of critical metals. • This exhaustive review reports metal extraction by pyro- and hydro-metallurgical processes, from laboratory to pilot scale. • Proposed route for recovery of metallic values and valorization of red mud. Red mud is an industrial waste generated during alumina production containing residual minerals of bauxite ore. Significant metallic values with appreciable concentration make it a potential polymetallic source. The current red mud management involves storage in artificial ponds/dams or dry stacked in open areas, which poses an environmental risk, and disposal processes are costly. Recently, red mud is utilized for cement production; however, it results in the loss of valuable minerals, which could be strategically advantageous for resource-deficient nations. Red mud utilization for producing concentrate can significantly improve process efficiency of alumina production, reduce industrial liability and environmental impact. The globally generated red muds are compared according to the mineralogy, composition, and associated values. Hematite is the chief constituent in the red mud, with anatase, gibbsite, goethite, quartz, and desilication phases such as cancrinite, sodalite, and olivine in a significant amount. Red mud originated from karst bauxites containing higher rare-earth content as adsorbed ions on the mineral surfaces, isomorphous substitution, and discrete rare-earth minerals such as bastnaesite, xenotime, chuchite, and allanite. Scandium holds 95% of the rare-earth economic value in the red mud and is associated with hematite, goethite, and anatase. The paper presents a critical overview of the laboratory, pilot, and commercial processes employed to recover iron, aluminum, titanium, scandium, and REEs from various red muds. The response parameters such as alumina-to-silica ratio, iron grade, total rare-earth elements were used to determine the appropriate processing route for recovery of metals from red mud. An integrated process is proposed to recover metallic values from the red mud while ensuring minimal waste generation.

Zhou J., Ma S., Chen Y., Ning S., Wei Y., Fujita T.

Recovery of scandium (Sc) from various wastes is generally one strategy to solve potential supply shortage problems of Sc. This work proposes a synergistic process for co-recovery of Sc by using the acid titanium white waste acid (TWWA) to leach the basic red mud, to which the acidity was adjusted with concentrated sulfuric acid when necessary. First, the effect of adjusted acidity of TWWA on Sc leaching from red mud was studied when the acidity of TWWA was adjusted to 5 mol/L H 2 SO 4 , which inhibited the formation of silica gel in the leachate with a Sc recovery of 70%. Then, the volume fraction of 10% P204 (di-(2-ethylhexyl) phosphate)-5% TBP (tributyl phosphate)-85% sulfonated kerosene was used to extract Sc from the leachate under the most suitable conditions. Specifically, at an A/O (volumetric liquid phase over oil phase) ratio of 10 for 30 min at room temperature, over 99% Sc was extracted and almost no Fe and Al were co-extracted. The selective scrubbing of 99% Ti was achieved via 5 mol/L H 2 SO 4 –1% H 2 O 2 at 60 °C with almost no scrubbing towards Sc. A solution of 3 mol/L NaOH was then used as the stripping agent, which resulted in solid Sc enrichment with a high stripping efficiency of 99%. The process generated a final Sc-enrichment product with a purity of 17.4% and a total recovery yield of 68.6%. • A novel process for co-recovery of Sc was proposed by using the acid titanium white waste acid to leach the basic red mud. • The formation of silica gel was inhibited in the Sc leaching step by adjusting the acidity. • 10% P204–5% TBP - 85% sulfonated kerosene was selected to recover Sc with an extraction efficiency over 99%. • 3 mol/L NaOH at 60 °C can effectively strip Sc with a stripping efficiency of about 99%. • Scandium enrichment with a purity of 17.4% and a recovery yield of 68.6% was achieved.

Lei Q., He D., Zhou K., Zhang X., Peng C., Chen W.

Selective recovery of scandium and titanium from red mud leaching solution was achieved through neutralization precipitation followed by acid leaching approach. In the neutralization precipitation section, the effects of pH, temperature and reaction time on metal precipitation efficiency were investigated. Under the optimal condition, the precipitation efficiencies of scandium and titanium were 93.74% and 99.47%, respectively. In the acid leaching section, the effects of acidity, temperature, reaction time, and raffinate to acid solution ratio on leaching efficiency were investigated. Under the optimal condition, the leaching efficiencies of scandium and titanium were 99.97% and 5.44%, respectively. The loss of scandium and titanium were 6.3% and 5.9%, respectively. Compared with the traditional extraction procedure of scandium in red mud, this method could separate titanium from scandium effectively, which is beneficial for the purification of the products and improvement of value of the metal. A new approach was proposed to separate and recover Sc and Ti from red mud, and separation of Sc and Ti was remarkable by this method with low scandium loss. • A new approach was proposed to separate and recover Sc and Ti from red mud. • Adsorption co-precipitation of Sc with Ti was verified. • Separation of Sc and Ti was remarkable by this method with low scandium loss. • Sc and Ti can be easily recovered after separation.

Liu Y., Li Q., Zhang T., Wu X., Du J., Zhang G., Zeng L.

The traditional process of extracting V(IV) with di(2-ethylhexyl) phosphoric acid (D2EHPA) uses H 2 SO 4 for V stripping and ammonium salt for V precipitation, leading to excessive consumption of H 2 SO 4 and NH 3 ·H 2 O, which results in a large volume of ammonia‑nitrogen wastewater. To solve this issue, a novel stripping reagent, H 2 O 2 , with the multifunction of oxidation and stripping, and a new method of precipitating vanadium from a solution of vanadium peroxy compounds were introduced to replace the traditional stripping reagent and ammonium salt precipitation, which requires separate stripping, oxidation, neutralisation and precipitation steps. The oxidative stripping of V from the loaded organic system consisting of 10% D2EHPA and 5% tri-butyl phosphate (TBP) in sulfonated kerosene using H 2 O 2 was systematically investigated. The results indicated that 98.7% of the vanadium was stripped by three-stage cross-flow stripping under mild optimal conditions. Moreover, it was found that VO(O 2 ) 2 – in the strip solution decomposed into VO(O 2 ) + under oscillating conditions and the cation was synergistically re-extracted by D2EHPA and TBP, which resulted in a significant decrease in the stripping of vanadium and poisoning of the organic phase. The use of D2EHPA alone as the extractant can effectively prevent the problems encountered when H 2 O 2 is used as the stripping reagent. The strip liquor could be used for direct precipitation of vanadium. A precipitation efficiency of 87% was obtained by adding a small amount of H 2 SO 4 . This new process combines oxidation, stripping and direct precipitation and exhibits the advantages of a short process flowsheet, with reagent savings (~95% decrease in H 2 SO 4 consumption and no NH 3 ·H 2 O consumption compared with traditional processes), and wastewater that is free of ammonia /nitrogen. • The oxidative stripping mechanisms of VO 2+ from mixed solvents of P204 and TBP using H 2 O 2 was systematically investigated. • A novel method on vanadium precipitation from V(V) peroxo compound was proposed. • It was found that the stripped vanadium was synergistically re-extracted by P204 and TBP and the structure of extracted complexes were determined.

Samal S.

An overview is presented on the prospective use of red mud as a resource in this review. Various scopes are suggested for the utilization of red mud to maintain a sustainable environment. The potential use of red mud covers the valuable metal recovery that could emphasize the use of red mud as a resource. Red mud could act as reduced slag in the metallurgical field for the extraction of minerals and metals for upscale application. Although many studies have revealed the potential utilization of red mud, most of them are only limited to a lab-scale basis. Therefore, a large-scale investigation on recycling of red mud for the extraction in the area of the metal recovery section will draw attention to the extensive use of red mud. Metal ions of major elements Fe (44 wt.%), Al (18.2 wt.%), Si (14.3 wt.%), Ti (9.3 wt.%), Na (6.2 wt.%), Ca (4.4 wt.%) as major elements and of Mg, V, Mn, Cr, K as minor elements and rare earth elements such as Ce (102 mg/kg), La (56 mg/kg), Sc (47 mg/kg), Nd (45 mg/kg), Sm (9 mg/kg). Moreover, an appropriate in-house metal recovery facility with the alumina industry will come out as a cost–benefit analysis.

Zhou J., Ning S., Meng J., Zhang S., Zhang W., Wang S., Chen Y., Wang X., Wei Y.

Nowadays 80% of scandium in China is obtained from titanium pigments production waste through a complex purification process. The study mainly focused on the purification of Sc from its concentrate generated from titanium pigments production waste by solvent extraction. Several extractants have been tried and 10% D 2 EHPA - 5% TBP - 85% sulfonated kerosene exhibited the best extraction performance towards Sc in 7 mol/L H 2 SO 4 solution, so it was selected as the oil phase. 0.5% of H 2 O 2 was added into the concentrated solution which can effectively inhibit the extraction of Ti. Both the extraction and back extraction parameters are optimized. The preferred extraction conditions were obtained, i.e., acidity: 7 mol/L H 2 SO 4 , the phase ratio A/O: 10, room temperature, mixed contact time: 30 min, Sc concentrate: 10 g/L, that the extraction rate of Sc in the above conditions was nearly 100%. NaOH was used for back extraction with the stripping rate 99% on the following conditions: 5 mol/L NaOH stripping for 30 min at a phase ratio A/O: 1 at 90 °C. Finally, H 2 C 2 O 4 was used to further purify the back extraction product and Sc 2 (C 2 O 4 ) 3 precipitant formed. The final product Sc 2 O 3 with a purity over 99.5% was obtained by calcining Sc 2 (C 2 O 4 ) 3 at 1000 °C for 2 h. A conceptual process for Sc purification was put forward and proved. The total recovery yield of Sc in the whole process is 95%. 10% D 2 EHPA - 5% TBP - 85% sulfonated kerosene was selected with the extraction efficiency of Sc over 99.9%. The usage of H 2 O 2 can promote the extraction of scandium and inhibit the extraction of titanium. NaOH was used for back extraction with the stripping rate of 99%. H 2 C 2 O 4 was used to further purify the back extraction product and Sc 2 (C 2 O 4 ) 3 precipitant formed, which is roast obtained Sc 2 O 3 . Finally, a conceptual process of purification Sc from its concentrate was put forward and proved in the laboratory with 99.5% Sc 2 O 3 product obtained with a yield of 95%. • Several extractants were tried to purify Sc from its concentrate. • 10% D 2 EHPA - 5% TBP - 85% sulfonated kerosene was selected with the extraction efficiency of Sc over 99%. • 0.5% of H 2 O 2 was added into the feed solution to inhibit the extraction of Ti. • 5 mol/L NaOH at 90 °C can effectively strip Sc with the stripping rate of about 99%. • A simple flowsheet for Sc purification was proposed and 99.5% Sc 2 O 3 was obtained with the yield over 95%.

Liang X., Ji Y.

Red mud, a by-product of alumina production, has a great impact on the environment due to its high alkalinity. In this paper, two-part geopolymer mortar was synthesized by combining red mud and blast furnace slag (BFS) to obtain optimized compressive strength and flexural strength for construction materials. Geopolymer concrete was prepared with the cementitious material in the concrete replaced by geopolymer mortar. Mechanical properties, permeability and microscopic properties of geopolymer concrete were measured. The results showed that the compressive strength grade of concrete prepared with geopolymer concrete can reach 54.43 MPa indicating that the geopolymer concrete can be used as materials for load-bearing members in structures. Due to lower total porosity and better pore structure, the permeability resistance of geopolymer concrete was significantly better than ordinary concrete. Microscopic analysis indicated that a large amount of aluminosilicate reaction products was generated in a geopolymer by the reaction of OH− with the aluminosilicate components in red mud and BFS in a strongly alkaline environment. The surface [SiO4]4− and [AlO4]4− tetrahedrons form chemical bonds through dehydroxylation, which is the direct reason for their high strength and determines their excellent physical and chemical properties.

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.

Zou D., Li H., Chen J., Li D.

High-efficiency and good-selectivity extraction systems are very attractive in the solvent extraction process. A synergistic extraction of scandium (III) from a spent sulfuric acid solution of titanium dioxide production by using a mixture of bis-2-ethylhexyl phosphoric acid (D2EHPA, P204) and primary amine (N1923) has been proposed in this study. The effect of the molar fraction of N1923 in the mixed extractants on the extraction of scandium was studied and the synergistic enhancement coefficient R was calculated. The maximum synergistic coefficient was found to be 12.50 when the H2SO4 concentration was 0.5 mol·L−1 and the molar fraction of N1923 was 0.5. The synergistic extraction mechanism was also discussed. Appropriately 90% scandium could be stripped from the loaded organic phase in the synergistic extraction system when the nitric acid was 6 mol.L−1. The result of FT-IR spectra well verified the conclusion that the synergistic effect owed to the formation of hydrogen bond N H+···O P, causing a much easier dissociation of proton H of P204. Finally, after three-stage counter-current extraction and stripping, Sc2O5 product with a high purity of 90% was obtained from spent sulfuric acid solution by precipitation and calcination.

Botelho Junior A.B., Espinosa D.C., Tenório J.A.

A new step in the Bayer process has been used to recover NaOH and to reduce its content in red mud. After digestion, the residue is separated from the Bayer liquor and goes through press filters, which enables the recovery of NaOH and decreases the moisture content. It is well known that red mud is one of the most important sources of scandium. For this reason, the goal of this work was to characterize a Brazilian red mud (BRM) for scandium recovery. The sample was collected after the press filter system process. Analysis of energy-dispersive X-ray fluorescence (EDXRF), scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS), and X-ray diffraction (XRD) was performed. Microwave digestion using an acid mixture was carried out to determine the concentration of the main elements. Fe2O3 represented 40% of the BRM, and the Sc concentration was 43 mg/kg. The SiO2 content was 24%, which is the highest value found in the literature, whereas the sodium concentration was the lowest. A literature review was carried out to compare results with BRM, as well as the current studies for recovery of scandium by the leaching/ion-exchange process. Scandium recovery using a leaching/ion exchange process may be possible with efficiency higher than 90%. The greatest challenge is the occurrence of silica gel formation during leaching. Graphical abstract

Hu J., Zou D., Chen J., Li D.

The synergistic extraction of scandium(III) with a mixture of Cyanex272 and Cyanex923 in sulfuric acid medium has been investigated in this study. At low H2SO4 concentration, the mixture of Cyanex272 and Cyanex923 exhibited an obvious synergistic extraction for Sc(III). When the concentration of H2SO4 was 0.10 mol/L and the molar fraction of Cyanex272 was 0.6, the maximal synergistic enhancement coefficient was obtained to be 5.20. What’s more, the synergistic extraction mechanism was discussed and the extracted complex was determined combined with FT-IR characterized result. Besides, the effect of temperature on the extraction of Sc(III) was investigated, and the thermodynamic parameters and equilibrium constants of extraction reaction have been calculated. In the meanwhile, the synergistic extraction system indicated of a potential practical application value for the recovery of Sc(III) from the sulfuric acid liquor of nickel laterite ores.

Xue J., Song H., Gou C., Wang H., Cao Z., Bi Q.

Liu X., Xu Y., Zhao Z., Chen X., Li J., He L., Sun F.

A novel lead and bismuth deep-separation extraction-stripping system is proposed for the leaching solution of molybdenite-bismuthinite mixed ore, which creates perfect conditions for the preparation of high-purity bismuth-based materials. Based on the activity and ionic strength theories, the forms of bismuth and lead present in the solution were predicted according to the thermodynamic models of different systems. The results showed that N235 + 2-octyl alcohol + kerosene formed a synergistic extraction system, which realized the synergistic extraction of bismuth, iron and lead with solvent extraction ratios of 98.74 %, 9.34 % and 46.24 %, respectively. In addition, the FTIR spectra of bismuth and lead before and after extraction were recorded to better understand the extraction and vaporization mechanisms. When stripping with water, the stripping ratios of iron and lead were 99.87 % and 99.74 %, respectively, while the loss ratio of bismuth was only 0.01 %. In conclusion, this study will be a key link in the comprehensive recycling process of bismuth resources.

Balaram V.

Tan W., Yang Y., Liang D., Weng W., Chi X., Zhong S.

Efficient removal of iron and aluminum impurities is critical for the extraction of lithium and rubidium from zinnwaldite, a lithium-bearing mineral. In this study, solvent extraction using P507 was employed to remove iron and aluminum from zinnwaldite leaching solutions. However, stripping iron from the organic phase proved challenging due to the strong interaction between iron ions and the extractant. To address this, a novel reduction stripping method was developed using ascorbic acid (AA) as a reductant. This method exploits the reduction of Fe3+ to Fe2+ in the aqueous phase, weakening the binding between iron ions and the organic phase, thus enabling efficient stripping. The optimized process achieved over 99.99% removal of iron and aluminum impurities. Subsequently, rubidium was selectively extracted using t-BAMBP, with a total recovery rate of 88.53%. Scaling-up experiments confirmed the feasibility of the process for industrial applications, demonstrating high efficiency and reagent recyclability. This study offers a promising approach for the efficient extraction and separation of valuable metals from zinnwaldite, with potential for broader applications in metal processing.

Praneeth S., Sakr A.K., Dardona M., Tummala C.M., Roy P.K., Dittrich T.M.

Li F., Li X., Shi L., Li X., Liang D., Wei Y., Fujita T.

Red mud is an important secondary resource for scandium production. The red mud leaching solution containing scandium in this study was derived from environmentally friendly ammonium sulfate roasting and water leaching process. A synergistic extraction with a mixture of di(2-ethylhexyl) phosphate acid (D2EHPA) and tributyl phosphate (TBP) for recovery of scandium from red mud leaching solution is proposed. The effects of D2EHPA concentration, H2SO4 concentration, rare earth elements, dosage of TBP, phase ratio (A/O), contact time and H2O2 concentration on scandium extraction were investigated. The results show that more than 99% scandium is extracted under the optimal conditions while Fe, Al, Ti, Ca and rare earth elements (Ce, Y, La, Nd, Er, etc.) are hardly extracted. The stripping efficiency of Sc reaches above 92.37% under the optimal stripping conditions of 5 mol/L NaOH with an A/O of 1 at 90 °C for 30 min. The proposed technology could provide an effective method for extraction of scandium from red mud leaching solution.

Zhang S., Yan Y., Zhou Q., Fan Y.

The recovery of scandium (Sc) from highly acidic industrial effluents is currently hindered by the use of large quantities of flammable and toxic organic solvents. This study developed an extraction system using ionic liquids (ILs) and phenylphosphinic acid (PPAH) as diluents and an extractant, respectively, to selectively recover Sc from the aqueous phase. The effect of IL chemical structure, aqueous pH and temperature on the extraction of Sc was systematically investigated and the findings revealed that ILs with longer alkyl side chains had reduced Sc extraction ability due to the presence of continuous nonpolar domains formed by the self-aggregation of the IL alkyl side chain. The IL/PPAH system maintained high extraction ability toward Sc across a wide temperature range (288 K to 318 K) and the extraction efficiency of Sc could be improved significantly by increasing the aqueous pH. The extraction process involved proton exchange, resulting in the formation of a metal−ligand complex (Sc(PPA)3).

Akcil A., Swami K., Gardas R., Hazrati E., Dembele S.

Aluminum is produced from its primary bauxite ore through the Bayer process. Although Al is important nowadays in the development of humanity, its production leads to the generation of a huge amount of waste, called red mud. Globally, the estimation of the stock of red mud is about 4 billion tons, with about 10 million tons located in Turkey. The presence of rare-earth elements (REEs) in crucial materials such as red mud makes it a major source of these elements. A number of methods have been developed for treating red mud, which are employed globally to recover valuable products. The application of a suitable method for REE extraction from red mud is a way to overcome the supply risk, contributing to reducing the environmental issues linked to red mud pollution. The current review summarizes the research on red mud processing and examines the viability of recovering REEs from red mud sustainably, utilizing hydrometallurgy and biohydrometallurgy.

Yang T., Li T., Guan Y.

Extraction of low-concentration rare earths is one of the important paths for the sustainability of the rare earth industry. The research and development of materials suitable for the extraction of rare earth ions at low concentrations is of great significance. Currently, the application of solvent extraction in the rare earth production industry is highly mature. However, it is not economical to use it for the extraction of low concentration rare earths with problems of phase separation and organic phase loss. This work innovatively proposes to encapsulate the extractant and diluent in magnetic oleogel microspheres, which are obtained by solubilizing magnetic poly (phenyl divinylbenzene)(PS-DVB) microspheres with the extractant and diluent.. The magnetic PS-DVB microspheres of 3 ∼ 16 μm were prepared by suspension polymerisation, followed by encapsulation of di(2-ethylhexyl)phosphoric acid (D2EHPA) and dichloroethane in magnetic PS-DVB microspheres using emulsion swelling, and then magnetic oil-based gel microspheres containing D2EHPA with a particle size of 10 ∼ 32 μm were obtained. The magnetic oil-based gel microspheres containing D2EHPA consist of about 96 wt% liquid phase and about 4 wt% solid phase, with D2EHPA accounting for more than 4.19 wt%. The magnetic oil-based gel microspheres are very close to small oil droplets and are superparamagnetic. The magnetic oil-based gel microspheres containing D2EHPA could completely extract 60 ppm of scandium ions(Sc3+) under the conditions of O:A = 1:50,1:60,1:70, and the extraction equilibrium times were 6, 9 and 12 min, respectively. This extractant encapsulation scheme inherits the selectivity of traditional extractants while simultaneously preserving the advantages of rapid extraction by solvent extraction.

Shi Z., Ding Y., Ren J., He X., Zhao B., Zhong J., Zhu Y., Wang B., Zhang S.

Liu J., Peng C., Jiang J., Zhang X., He D., Zhou K., Chen W.

The comprehensive recovery of iron and aluminum from iron-rich bauxite residue (IRBR) is of critical importance both in terms of resource utilization and environment protection, which, however, is challenging due to the intertwined phases between Iron and aluminum. In this study, an integrated phase reconstruction approach, consisting of alkali roasting, two-stage column leaching, and carbonation decomposition, was proposed for Fe/Al recovery from IRBR. The results demonstrated that aluminum and sodium were fused into soluble substances such as sodium aluminate (Na7Al3O8, NaAlO2, and Na2O (Al2O3)11) in the alkali roasting process, allowing for the separation of Al and Fe in the subsequent leaching process. Following water/FeCl3 solution leaching, the removal efficiency of aluminum reached 84.66%, and Fe content in the residue could be enriched to 55.56%. Fe can be recycled as iron concentrate, and Al in the leaching solution with 75.95 g/L can be recovered in the form of Al(OH)3 through carbonation decomposition. This work provides an alternative strategy for the recovery of resources from IRBR, with potential implications for the sustainable development of the aluminum industry.

Dawood Salman A., Alardhi S.M., AlJaberi F.Y., Jalhoom M.G., Le P., Al-Humairi S.T., Adelikhah M., Miklós Jakab, Farkas G., Abdulhady Jaber A.

The main aim of this study is to figure out how well cryptand-2.2.1 (C 2.2.1) and cryptand-2.1.1 (C 2.1.1) macrocyclic compounds (MCs) work as novel extractants for scandium (Sc) by using an artificial neural network (ANN) models in MATLAB software. Moreover, C2.2.1 and C2.1.1 have never been evaluated to recover Sc. The independent variables impacting the extraction process (concentration of MC, concentration of Sc, pH, and time), and a nonlinear autoregressive network with exogenous input (NARX) and feed-forward neural network (FFNN) models were used to estimate their optimum values. The greatest obstacle in the selective recovery process of the REEs is the similarity in their physicochemical properties, specifically their ionic radius. The recovery of Sc from the aqueous solution was experimentally evaluated, then the non-linear relationship between those parameters was predictively modeled using (NARX) and (FFNN). To confirm the extraction and stripping efficiency, an atomic absorption spectrophotometer (AAS) was employed. The results of the extraction investigations show that, for the best conditions of 0.008 mol/L MC concentration, 10 min of contact time, pH 2 of the aqueous solution, and 75 mg/L Sc initial concentration, respectively, the C 2.1.1 and C 2.2.1 extractants may reach 99 % of Sc extraction efficiency. Sc was recovered from a multi-element solution of scandium (Sc), yttrium (Y), and lanthanum (La) under these circumstances. Whereas, at a concentration of 0.3 mol/L of hydrochloric acid, the extraction of Sc was 99 %, as opposed to Y 10 % and La 7 %. The Levenberg-Marquardt training algorithm had the best training performance with an mean-squared-error, MSE, of 5.232x10-6 and 6.1387x10-5 for C 2.2.1 and C 2.1.1 respectively. The optimized FFNN architecture of 4-10-1 was constructed for modeling recovery of Sc. The extraction process was well modeled by the FFNN with an R2 of 0.999 for the two MC, indicating that the observed Sc recovery efficiency consistent with the predicted one.

Grudinsky P., Pasechnik L., Yurtaeva A., Dyubanov V., Zinoveev D.

Bauxite residue is a high-iron waste of the alumina industry with significant contents of scandium, aluminum, and titanium. This study focuses on the recovery of Sc, Al, Ti, and Si from iron-depleted bauxite residue (IDBR) into valuable products. Iron depletion was carried out using reduction roasting followed by low-intensity magnetic separation to enrich bauxite residue in Al, Ti, and Sc and reduce an adverse effect of iron on scandium extraction. Hydrochloric high-pressure acid leaching, aluminum precipitation by saturation of the acid leachate, solvent extraction of scandium using di(2-ethylhexyl) phosphoric acid (HDEHP) and tributyl phosphate (TBP), alkaline leaching of the acid residue with subsequent silica precipitation were used to obtain appropriate selective concentrates. As a result, scandium concentrate of 94% Sc2O3, crude alumina of 93% Al2O3, titanium concentrate of 41.5% TiO2, and white carbon of 77% SiO2 were prepared and characterized. Based on the characterization of the treatment stages and the obtained valuable products, the prospect for the application of the suggested flowsheet was discussed.