Highly efficient recovery and purification of scandium from the waste sulfuric acid solution from titanium dioxide production by solvent extraction

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%.

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.

Wang L., Wang P., Chen W., Wang Q., Lu H.

Scandium (Sc) is the most expensive rare earth element and is widely considered as one of the most critical materials for various emerging applications such as the third-generation Solid Oxide Fuel Cell (SOFC) battery. The global Sc demand is expected to increase rapidly, while its supply is mainly constrained by limited mineral reserve and environmental burdens associated with its production. There are few Sc deposits distributed in the world, with small reserves and complicated extraction processes. A large amount of Sc is mainly concentrated in tailings and has not been extracted in large quantities. Besides, detailed assessments on the potential environmental impacts of rare earths production, and Sc in particular, are very rare. This study aims to fill this gap to assess the environmental impacts of the production of Sc2O3, where the technical processes are modelled based on a recent project of Sc recovery from rare earths tailings in Bayan Obo mine. The environmental impacts of 5 major processes and 22 sub-processes involved in the beneficiation stage and the smelting stage have been evaluated with the synergies of various inventory data. It is found that the beneficiation stage accounts for 88% of the overall impact, around 56% of which is contributed by the secondary separation process. Among the ten assessed categories, Human toxicity non-cancer (HTNC) and Global Warming Air (GWA) are the top two significant impacts caused by Sc2O3 production. Through comparative analysis with published results, it’s found that the environmental impact of Sc2O3 production from the tailings of Bayan Obo mine are much lower than those of direct production from the mixed rare earths ores. Our hotspot analysis identifies that the consumption of steam in Fe separation (II) and oxalic acid in oxalic acid precipitation are the major contributors to the total environmental impacts of Sc2O3 production. Thus, strategies to reduce their use and improve environmental performance in their production are highly encouraged. This study can provide a basis for sustainable raw material sourcing for the clean development of the third-generation SOFC battery.

Li S., Dong L., Wei Z., Sheng G., Du K., Hu B.

Herein, we developed the invasive plant-derived biochar (IPB) functionalized with CaAl-LDH at five mass ratios using a physical mixture method, assessed their adsorption perform for Eu(III), and explored the relative mechanisms. Results show that the IPB successfully loaded CaAl-LDH in five composites and their Eu(III) sorption affinities were strongly affected by solution pH, contact time, temperature, and the mass ratio of LDH and IPB. All the sorpiton process for Eu(III) occurred on the heterogeneous surface of five composites and the boundary layer diffusion limited the chemical sorption rate. Interestingly, the CaAl-LDH/IPB composite with high ratio of IPB had higher sorption capacity than the one with high ratio of LDH due to larger porosity of the former. Three mechanisms containing ion exchange between Al and Eu ions, surface complexation with carboxyl- and oxygen-containing functional groups, and precipitation were involved in the Eu(III) sorption, but the dominant sorption mechanism for each CaAl-LDH/IPB composite differed with different mass ratio of CaAl-LDH and IPB. In composite with more IPB (e.g., CaAl-LDH/IPB-13), both ion exchange and surface complexes dominated the sorption process and the intensity of Eu 3+ was identified with the one of Eu 2 O 3. Whereas in composites with high LDH, ion exchange dominated the sorption and the intensity of Eu 3+ was obviously higher than the one of Eu 2 O 3 . This research will provide a new perspective for the application of the LDH/biochar materials.

Zhou J., Yu Q., Huang Y., Meng J., Chen Y., Ning S., Wang X., Wei Y., Yin X., Liang J.

Scandium (Sc), a rare earth element, generally lacks independent large-scale deposits and is mainly recovered as a by-product in mineral processing. Herein, we systematically investigated the extraction of Sc by trialkyl phosphine oxide (TRPO) from both a scandium sulfate solution and leaching solution of Sc concentrate generated from titanium white waste acid in the sulfuric medium. The results showed that the optimum extraction conditions of Sc from scandium sulfate by TRPO were as follows: 10% TRPO, 5 mol/L H2SO4, and an A/O ratio of 1 for 20 min. The equation of scandium extraction by TRPO was calculated based on the log[c(TRPO)] - log[D] fitting results coupled with Fourier transform infrared spectroscopy (FT-IR) analysis, to which the extracted complex was deduced to be HSc(SO4)2•2TRPO. Furthermore, for Sc extraction from leaching solution of its concentrate from titanium white waste acid, a relatively high extraction rate of up to 95.5% was obtained on the conditions of 20% TRPO, 7 mol/L H2SO4, 0.5% of H2O2, and at an A/O ratio of 10. Stripping with H2C2O4 produced a high stripping rate of 99.9%, and the extraction and stripping system still presented excellent extraction performances even after five treatment cycles. Through the subsequent calcination of the stripped Sc2(C2O4)3 at 1000 °C for 2 h, we obtained the final product of Sc2O3 with a high purity of over 99.34% with a total recovery yield of Sc as high as 95% in the whole process. Our findings contribute novel insights into the extraction and purification of Sc for its recovery from the secondary wastes.

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.

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.

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

A novel functional silica composite (SiAcP) based on bis(2-methacryloxyethyl) phosphate (BMAOP) was prepared by in situ polymerization method for separating Zr from Sc. The synthesis conditions were optimized. SiAcP resin using DMSO as solvent with the loading rate of 30% and the cross-linking degree of 13% showed relatively good performance. Adsorption and separation behaviors towards Sc(III) and Zr(IV) were studied by both batch and column experiments. The resin exhibited good adsorption towards Zr(IV) that the equilibrium can be obtained within 5 h with the maximum adsorption capacity of 0.3 mmol Zr/g at 318 K. Excellent selectivity towards Zr(IV) over Sc(III) was observed with SFZr/Sc of 48 and 140 in 0.5 M HCl and HNO3 solutions respectively. In the column test, Zr could be easily separated from Sc. Finally, XPS spectra demonstrated that although the adsorption process was primarily dominated by ion-exchange, P=O plays a key role in selective separation of Zr(IV) over Sc(III). All in all, this work not only provides an excellent material for adsorbing Zr(IV) from acidic solution, but also presents a novel method for separating Zr(IV) from Sc(III) by extraction chromatography.

Dai S., Wang N., Qi C., Wang X., Ma Y., Yang L., Liu X., Huang Q., Nie C., Hu B., Wang X.

Radionuclide contamination has become an urgent problem with the development of nuclear power plants. Herein, chemical-decorated core-shell magnetic manganese dioxide (denoted as Fe3O4@C@MnO2) composites were synthesized via transforming KMnO4 to MnO2 on the carbon-covered magnetite (Fe3O4@C) microsphere surface. It was employed to remove U(VI) and Eu(III) ions from aqueous solution under various conditions. The kinetic adsorption data were well simulated by the pseudo-second-order model and adsorption isotherms were fitted well by Langmuir model. Moreover, the maximum uptake capacities were up to 77.71 mg/g for U(VI) and 51.01 mg/g for Eu(III) at pH = 5.0 and T = 298 K. Adsorption behavior was strongly related to pH values but weakly affected by ionic strength, implying that the interaction of U(VI)/Eu(III) with Fe3O4@C@MnO2 was mainly dominated by inner-sphere surface complexation. XPS analysis illustrated that the interaction of Eu(III)/U(VI) with Fe3O4@C@MnO2 was associated with the strong metal bonds (MnO), hydroxyl bonded on metal (Mn-OH) and carboxyl groups (-COOH) by surface complexation and zeta potential results implied that the adsorption process was governed by electrostatic attraction. This research highlighted the outstanding performance of Fe3O4@C@MnO2 in eliminating Eu(III)/U(VI) ions from aqueous solutions, which was of great significance in the future application in radionuclides' pollution treatment.

Li S., Kim S., Kang C.

A study was carried out for recovery of scandium from KOH sub-molten salt (SMS) leaching cake of fergusonite. Major metals in the cake were rare earths (RE), K, Th, U and Ca, and minor metals were Pb, Nb, Ta, Ti, Fe, Si, Sn, Al, Bi and Mn. Scandium content was about 0.94% in the cake. Experiments for the recovery of scandium involved washing of the cake with water for potassium removal, leaching of scandium from the cake with HCl, solvent extraction with tri-butyl phosphate (TBP) and precipitation of scandium with oxalic acid. Effects of liquid-to-solid ratio and leaching temperature on leaching of scandium as well as impurities such as uranium, thorium and other rare earths such as yttrium and lanthanides were investigated, with the aim of both increasing scandium leaching and selectively pre-separating the impurities. And also, the separation characteristics of scandium and the impurities were evaluated in the solvent extraction with TBP. Finally, we proposed a process for recovery of scandium from the KOH SMS leaching cake of fergusonite.

Nunes da Silva F., Bassaco M.M., Bertuol D.A., Tanabe E.H.

The aim of this work was to develop new materials employing polymeric nanofibers modified with an organic extractant for the extraction of metals. For this purpose, nylon-6 nanofibers were modified with di-(2-ethylhexyl) phosphoric acid (DEHPA) for use in the selective extraction of zinc and nickel. The best extraction efficiencies were 85.5% for Zn and 4.6% for Ni, achieved under the following conditions: pH 2, solid:liquid (S:L) ratio of 1:40, and contact time of 7.5 min. In the stripping, the best efficiencies were 85.9% for Zn and 90.1% for Ni, using a concentration of HCl of 0.5 M, S:L ratio of 1:10, and contact time of 5 min. The nanofibers were evaluated in terms of their stability and capacity for reuse, and it was found that there was no loss of the DEHPA extractant from the nanofibers, while the extraction efficiency remained almost constant. Regarding to environmental aspects of the new technology, the results confirm that nanofibers exhibit the same efficiency of conventional liquid-liquid extraction, without the disadvantage of the use of organic solvents. In this way, this new technology brings an enormous gain in environmental terms, since it drastically reduces the use of hazardous substances.

Ye Q., Li G., Deng B., Luo J., Rao M., Peng Z., Zhang Y., Jiang T.

Acid leaching-solvent extraction is an effective process to extract scandium from scandium-bearing resources. This study was aimed to investigate the extraction behavior of metal ions, including Sc3+, Fe3+, Al3+and Ca2+, in phosphoric acid medium using P204. More than 95% of scandium was selectively extracted under the conditions of pH value of 1.5–1.8, aqueous-organic ratio of 3 and oscillation for 15 min. The impurity elements like Fe3+, Al3+ and Ca2+, were separated via the solvent extraction process using P204 (e.g., separation coefficient of scandium to iron is 298). In stripping process, the majority of co-extracted metal ions can be removed from the organic phase with hydrochloric acid solution. The scandium-bearing organic phase was stripped with 4 mol/L NaOH, wherein the recovery of scandium attained about 95% while that of the co-extracted iron and aluminum were only 3.1% and 1.2%, respectively. It was also confirmed that both P204 and H3PO4 played roles in extraction reaction, and the desirable extraction of scandium in P204 was attributed to the ion exchange between hydrogen ion of PO(OH) and Sc3+ on acidic condition (pH = 0.4–1.5). Impurity elements (Fe3+ and Al3+) also reacted with phosphate anion to form hydrophilic ions, and in turn result in selective extraction of scandium in phosphoric acid leachate using P204.

Jeong D., Kim J., Kwon O., Lim C., Sengodan S., Shin J., Kim G.

Layered perovskite oxides are considered as promising cathode materials for the solid oxide fuel cell (SOFC) due to their high electronic/ionic conductivity and fast oxygen kinetics at low temperature. Many researchers have focused on further improving the electrochemical performance of the layered perovskite material by doping various metal ions into the B-site. Herein, we report that Sc3+ doping into the layered perovskite material, PrBaCo2O5+δ (PBCO), shows a positive effect of increasing electrochemical performances. We confirmed that Sc3+ doping could provide a favorable crystalline structure of layered perovskite for oxygen ion transfer in the lattice with improved Goldschmidt tolerance factor and specific free volume. Consequently, the Sc3+ doped PBCO exhibits a maximum power density of 0.73 W cm−2 at 500 °C, 1.3 times higher than that of PBCO. These results indicate that Sc3+ doping could effectively improve the electrochemical properties of the layered perovskite material, PBCO.

Yu Q., Ning S., Zhang W., Wang X., Wei Y.

A macro porous silica-polymer based TRPO/SiO2-P adsorbent was prepared for recovery of scandium from sulfuric acid solution. The as-prepared TRPO/SiO2-P was characterized by SEM-EDS, TG-DSC, BET, IR. It was found the adsorption of TRPO/SiO2-P towards Sc reached equilibrium in 120 min with the saturated adsorption amount about 13.3 mg/g. It needed about six molecules of TRPO/SiO2-P to uptake 2 molecules of Sc(III), meanwhile three molecules of SO42− were needed for charge balance. Furthermore, TRPO/SiO2-P exhibited high adsorption selectivity towards Sc and almost no adsorption towards the other RE and Al, Fe and little adsorption towards Ti which could be effectively suppressed by H2O2. And column experiments showed clear separation of Sc from Al, Ti, Fe, Y, La and Ce in the effluent, indicating TRPO/SiO2-P could successfully separate Sc from the other metal ions. In a word, TRPO/SiO2-P is a very promising adsorbent for Sc separation from the H2SO4 system.

Zhang W., Zhang T., Lv G., Zhou W., Cao X., Zhu H.

The solvent extraction and separation process of Sc(III) and Fe(III) from a strongly acidic and highly concentrated ferric solution using mixtures of di(2-ethylhexyl) phosphate (D2EHPA) and tri-n-butyl phosphate (TBP) in sulfonated kerosene was studied. The effects of various parameters, including D2EHPA concentration, dosage of TBP, and phase ratio, were investigated for the extraction process. The results indicated that the extraction rate of Sc(III) was 99.72% with 1.09% Fe(III) co-extracted after two stages of counter-current extraction under optimal conditions. Moreover, saturation capacity and slope analysis were used to determine the reaction mechanism. Sc(III) is extracted in the form of HSc(SO4)2·4HL. Further separation of Sc(III) and Fe(III) was realized in a scrubbing and stripping process. First, 98.67% of the co-extracted iron in the loaded organic phase was scrubbed with a dilute HCl solution by a three-stage counter-current scrubbing. Then, 85.00% of Sc(III) can be stripped efficiently with a 2-mol/L NaOH solution saturated in 1-mol/L NaCl by three-stage cross-current stripping.

Zhang M., Shang S., Li H., Tian L., Liu K., Villa-Gomez D., Cao Y., Liu Y.

Li W., Zhou M., Wang N., Gu H.

Zhang M., Tian L., Villa-Gomez D., Zhang L., Yang W., Liu Y., Cao Y.

Yoshida W., Goto M.

Yu J., Ma B., Cao Z., Liu Y., Shi L., Wang C., Chen Y.

Jiang H., Liu J., Chen X., Cao X., Ye X., Shi G.

Sc and Y are key rare earth elements and are widely used in lamp phosphors, lasers and high-performance alloys. However, highly efficient extraction and separation of Sc3+ and Y3+ is laborious, harmful, slow, and costly, strongly necessitating more efficient extraction and separation techniques. Here, we produced hydrated Sc3+- and hydrated Y3+-controlled graphene oxide (GO) membranes and find that both hydrated cations were completely self-rejected by the membrane. By combining this self-rejection effect of the larger hydrated Y3+-controlled GO membrane and the rapid passage of the membrane through the smaller hydrated Sc3+, we proposed a strategy to separate Sc3+ and Y3+ by using a hydrated Y3+-controlled GO membrane. The experimental results show that the permeation rate of Sc3+ exceeds that of Y3+ when the separation factor reaches 4.02, which can be attributed to the interlayer sieving effects of the GO membrane. Our finding illustrates the use of a forward osmosis process with a GO membrane for the efficient separation of Sc3+ and Y3+ by interlayer sieving, which provides a new effective and eco-friendly method for the separation of rare earth elements.

Hu X., Zhang X., Dong Y., Chen L., Yang T., ChangfengYi, Gao Q.

The waste sulfuric acid solution emerged as a main emission substance from titanium dioxide production is called titanium white waste acid (TWWA). The disposal of TWWA has been a concern due to its potential impact on the environment. A green way including one-step preparation and purification stages of α-calcium sulfate hemihydrate (α-HH) from TWWA and lime mud via a neutralization reaction in the hydrothermal pressure apparatus was developed. The preferred experimental conditions were obtained, i.e., lime mud/TWWA/sodium citrate ratio: 16/16/1, 140 ℃, 0.5 MPa, reaction time: 10 min. The recovery rate of α-HH in the whole process was 89.5%. The method presented high efficiency and selectivity in conversion to a short-columnar-shaped α-HH under addition of 3.40 × 10−2 mol/L sodium citrate as a crystal modifier. The structure and composition of the obtained α-HH products were confirmed by XRD, TGA, and SEM. Compared with titanium gypsum, the obtained short columnar α-HH with hexaprismatic morphology showed excellent mechanical properties. According to GB/T 9776 − 2008 “Calcined gypsum”, the compressive strength of 7 days of the cemented short-columnar α-HH was about 6.35 MPa, which it meets the strength requirements for building gypsum.

Cao W., Hua J., Jin X., He M., Xin Y., Liu W.

The solid waste and waste acid generated during the production of titanium dioxide contain considerable amount of scandium, which are valuable secondary resources. In this study, the titanium dioxide waste acid was used to leach the scandium-containing solid waste, and the leached solution was pretreated for iron removal by reduction-crystallization process. After that, scandium was recovered from the leached solution by using the P204-TBP co-extraction system. The process parameters were investigated systematically. The results showed that iron powder reduction-crystallization for iron removal at molar ratio of Fe:Fe3+ = 0.25 was most suitable for subsequent extraction, and the scandium extraction efficiency could reach 100% using 15% P204-5% TBP at 25 °C with A/O = 8. This study provided a novel process for treating scandium-bearing solid waste with scandium-bearing waste acid, showing great potential for industrial application.

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.

Han F., Wang M., Liu W., Song W.

The waste acid generated during the production of titanium dioxide contained a high proportion of ferrous ions. Comprehensive utilization of these waste acid involves the recovery of sulfuric acid and iron. However, traditional methods faced difficulty in attaining the desired outcome. Therefore, this study proposed a treatment process primarily based on membrane electrolysis. Initially, selective electrodialysis was employed to recover sulfuric acid from the waste acid. The effects of initial acid concentration, potential, and operating time on the performance of selective electrodialysis were investigated. Subsequently, a three-chamber membrane electrolysis method was utilized to oxidize Fe2+ to Fe3+ in the anode chamber and generate OH– in the cathode chamber. Both compounds were directed to the precipitation chamber through ion exchange membranes, resulting in the formation of iron hydroxide and the successful recovery of iron. The findings revealed that when the waste acid had a high initial concentration, it was recommended to operate at a higher potential. When treating 18 % of the waste acid, 77.5 % of the sulfuric acid can be recovered while 78 % of the ferrous ions were retained. As the acidity decreased, the potential could be reduced, resulting in a higher recovery rate of sulfuric acid and retention rate of Fe2+. By using a current of 150 mA during membrane electrolysis and maintaining a pH of 2.4 in the precipitation chamber, a purity of 94 % iron hydroxide was achieved. The ion flux of the anion exchange membrane decreased by 19 % after 8 h of electrolysis. This reduction could be mitigated by cleaning the membrane with titanium dioxide waste acid with a pH of 0.4 for 30 min. This study is expected to introduce innovative ideas and methods for the efficient and high-purity recovery of resources from titanium dioxide waste acid.

Li S., Zhang W.

Phytomining emerges as an innovative technique for extracting rare earth elements (REEs) from soil by employing hyperaccumulators. REE hyperaccumulators were treated using microwave-assisted hydrothermal carbonization (MHTC) in acid-mediated systems to efficiently transfer REEs and other elements into biocrudes and produce high purity and value-added hydrochar. However, the subsequent treatment of biocrudes to recover valuable elements still presents a significant challenge. In this study, a process that combines solvent extraction and struvite precipitation was first developed to address this challenge. In the extraction step, 95.6% of REEs were extracted using 0.05 mol/L di(2-ethylhexyl)phosphoric acid (D2EHPA) with an aqueous to organic (A/O) ratio of 1:1 at pH 3.0. However, 75.1% of Al, 81.2% of Ca, 54.5% of Fe, 61.5% of Mn, and 81.3% of Zn were co-extracted into the organic phase with the REEs. To solve this issue, a subsequent scrubbing step using deionized water was applied, with the removal of over 98% of these impurities, while incurring negligible loss of REEs. After the scrubbing step, over 97% of REEs were ultimately stripped out from the organic phase as REE oxalates using 0.01 mol/L oxalic acid as the stripping agent. Furthermore, phosphorous (P) was found to be retained in the raffinate after the solvent extraction process. 94.4% of the P was recovered by forming struvite precipitate at pH 9.0 and a Mg/P molar ratio of 1.5. In general, high purity and value-added REE products and struvite precipitate were eventually achieved from biocrudes in environmentally friendly and economically viable ways.

Fu Q., Wang J., Chi M., Li S., Zhang K., Xie Y., Jiao G., Yuan W., Li M.

Su X., Liu H., Tian G.

Two novel ILs, [Laur][Tf2N] and [Coca][Tf2N], were developed as extractants. Sc(iii) could be recovered from other REs with high separation and extraction efficiency, allowing ILs to be recycled.

Xiao J., Zhong N., Cheng R., Deng B., Zhang J.

In this study, a scandium concentrate with Sc2O3 content of 66.24 g/t was obtained from V-Ti magnetite tailings by physical concentration, and the main Sc-bearing minerals were augite and hornblende. A novel process of roasting and leaching was proposed to extract scandium from scandium concentrate with titanium dioxide wastewater. Scandium concentrate was pretreated by roasting, and titanium dioxide wastewater was used to directly leach scandium from the roasted ore. The effects of roasting and leaching parameters such as roasting temperature, roasting time, roasting agents, leaching temperature, leaching time, liquid-to-solid ratio, and leaching agents on scandium separation were thoroughly researched in the experimental procedure. The results show that a scandium leaching efficiency of 85.89% was obtained, and the scandium content of leaching residue decreased to 9.31 g/t under the optimal conditions: a roasting temperature of 1123 K, a roasting time of 120 min, a leaching temperature of 343 K, a leaching time of 120 min, and a m (titanium dioxide wastewater)∶m (roasted ore)∶m (ammonium fluoride) ratio of 8∶1∶0.09. The main findings of the scandium separation mechanism show that Sc-bearing minerals can effectively decompose and release scandium element after roasting, and created favorable conditions for scandium leaching with titanium dioxide wastewater to achieve the purpose of scandium recovery.

Daminescu D., Duteanu N., Ciopec M., Negrea A., Negrea P., Nemeş N.S., Pascu B., Lazău R., Berbecea A.

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.