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Softening ammonia sulfate wastewater is one of the vital issues in the rare earth industry. In this article, a novel softening process using [4,4′-isopropylidenebis (phenoxyacetate)] (H2IPOAA) for Ca2+ and Mg2+ (M2+) is developed. The effect factors, i.e., equilibrium time, pH value of aqueous phase and saponification degree are investigated. The extraction mechanism is proposed to be cation exchange. With a saponification degree of 85%, H2IPOAA is successfully used for the precipitation of Ca2+ and Mg2+ from ammonia sulfate wastewater within 10 min. The process can form precipitated particles with an average size of 35 μm, which are much larger than the precipitates formed by H2C2O4, Na2C2O4 and Na2CO3. The precipitated particles can be easily separated from the raffinate. In addition, H2IPOAA is repeatedly used for five times with a little loss. To a considerable extent, the reuse of H2IPOAA contributes to lowering the cost and reducing the pollution. The concentrations of Ca2+ and Mg2+ in the raffinate extracted by H2IPOAA are up to the MVR standard for wastewater treatment. The economical and efficient softening process meets the need of green chemistry, which is expected to replace traditional industrial processes for water softening based on H2C2O4, Na2C2O4 and Na2CO3.

The synergistic effect of mixed Cyanex 572 and n-octyl diphenyl phosphate for Th(IV) extraction from nitric acid solution has been investigated in this paper. The effect of acidity together with mole fraction of extractant on the synergistic effect was studied. Based on the obtained results, the maximal synergistic enhancement coefficient reaches 12.40 at a ratio of 1:2 of Cyanex 572 and n-octyl diphenyl phosphate. Under the experimental conditions, the phase separation performance, stripping property and selectivity of Th and REE in the mixed extraction system are better than those from individual Cyanex 572 or n-octyl diphenyl phosphate system. The synergistic extraction mechanism for Th(IV) in nitric acid medium is proposed as Th aq 4 + + HA 2 org + 2.5 B org + 3 NO 3 − aq ↔ K ex Th NO 3 3 HA 2 B 2.5 org + H aq + . The mixed extraction system is successfully used for the separation of Th(IV) and REE(III) from the industrial feed solution leached from ion-absorbed type rare earth mineral waste residue. Lower stripping acid, stronger extraction ability, better phase separation performance and selectivity provide the synergistic extraction system with good application prospects.

To remove aluminum (Al) from rare earth element (REE) especially light rare earth element (LREE), octyl-phenoxy isopropionic acid (OPIPA) has been developed for the liquid-liquid extraction separation of Al from REE in this study. Using specific extraction experiment and infrared spectra analysis, the stoichiometry of saponified octyl-phenoxy isopropionic acid (S-OPIPA) to Al is indicated to be 3:1. The results of extraction investigation show that the separation effects of Al and REE arrive best when the initial water solution is pH = 3. The stripping experiment shows that sodium hydroxide is the best stripping agent. The extraction abilities of Al by OPIPA, naphthenic acid (NA), saponified OPIPA (S-OPIPA) and saponified naphthenic acid (S-NA) are compared at the pH values of 1, 2, 3, which indicates the importance of saponification. The loading capacities of S-NA and S-OPIPA reach the maximum values of 1.82 g/L and 1.85 g/L, respectively. It is found that S-NA is easy to be emulsified, while S-OPIPA reveals excellent phase separation behavior for the Al solution with the high concentration. The difference indicates that S-OPIPA is more suitable for the massive removal of Al. In the separation process of simulated REE ore leaching solution, the Al concentration can be lowered to zero after two stages of Al removal with S-OPIPA and S-NA sequentially, and REE recovery rate of 86.43% with a purity of 99.69% is achieved.

Cyanex 572 has been first studied for the extraction of scandium from trivalent rare earths (REs). The extraction mechanism of Cyanex 572 for Sc(III) was proposed to be cation-exchange, and the stoichiometry of Cyanex 572 with Sc(III) was indicated to be close to 3:1. The extractabilities of Sc(III) from leaching solution of tungsten residue with Cyanex 572, P507, Cyanex 923 and TBP were compared at different concentrations, which indicated that Cyanex 572 was the best among them to extract scandium under the same conditions. The loading capacity of Cyanex 572 toward Sc(III) and stripping of loaded Sc(III) from the organic phase were also studied. By simple washing and stripping steps, the composition of scandium was increased more than eight hundred times.

Unlike common solvent extraction and chemical precipitation, a novel extraction-precipitation process for the separation of Li and transition metals using p-tertylphenoxyacetic acid (POAA) is developed. Extraction-precipitation method refers to a method in which the extraction-precipitant can quantitatively extract the metal ions to form solid extracted complex without the requirement of organic solvent or carrier, and the extraction-precipitant can be stripped and recycled. In this study, the extraction-precipitation mechanism and thermodynamic parameters between POAA and manganese (Mn), cobalt (Co), nickel (Ni) were analyzed. Subsequently, the equilibrium time and pH value of the separation process were optimized. Finally, the leach solution of LIB having Li, Ni, Co, Mn with concentration of 3.52 g/L, 7.93 g/L, 5.72 g/L, and 14.16 g/L, respectively, was studied for the separation at pH 4.5. The results indicate that efficient separation of Li and transition metals was possible by one-step precipitation using 0.2 mol/L POAA solution (degree of saponification = 100%) when the ratio of nPOAA/nMn+Co+Ni was 2. The separation factor for the transition metals over lithium (βCoNiMn/βLi) reached as high as 133,014. After the transition metal-loaded POAA was stripped with 2 mol/L sulfuric acid, the stripped solution could be used for the production of ternary material precursor, and the regenerated POAA could be recycled for the extraction-precipitation process. The lithium in the solution was recovered in the form of lithium carbonate (Li2CO3) with a purity level of 97.7%. This high efficiency and sustainable process based on extraction-precipitation has shown potential application for spent LIB recovery.

A novel synergistic system of combined ammonium and phosphonium type ionic liquids is reported for the first time, which displays pronounced synergistic effect in the extraction of Lu(III). The synergistic enhancement coefficients from different mole fractions of [P66614][EHEHP] and [N1888][BTMPP] were calculated. Extraction mechanism of the combined functionalized ionic liquids for Lu(III) was shown to be ion association. Thermodynamic functions of the synergistic extraction system were obtained. The loaded Lu(III) in the synergistic system could be fully stripped using hydrochloric acid. The novel synergistic extraction provides the possibility of separating heavy lanthanides, which reveals theoretical and practical importance in REEs separation.

In this article, some novel extractants were synthesized for the separation of Th and rare earth (RE). Among the synthesized neutral organophosphorus extractants, n-octyl phosphate diphenyl (ODP) revealed excellent extraction efficiencies and selectivities for Th(IV). Moreover, loading capacity of ODP for Th(IV) (0.015 mol/L) reached 130 mg/L, the loading capacities and speed were higher and faster than those of P507, TBP and C272, respectively. Furthermore, the loaded Th(IV) could be fully stripped by sulfuric acid and hydrochloric acid. The novel extractant ODP has been indicated to be an efficient extractant for the separation of Th, REs and Fe from ion-adsorption type RE minerals, which reveals significances in the fields of resource, environment and energy.

In this paper, a process for the separation of thorium (Th) and rare earth elements (REEs) from radioactive waste residues of ion-absorbed type rare earth minerals using Cyanex® 572 (C572) as a new extractant has been developed. The extraction and stripping properties of Th and REEs with Cyanex® 572 were studied systematically. The separation factor of Th to Lu, β Th/Lu, are higher than 80 and 1.46 × 10 3 , when the equilibrium HCl concentration is larger than 0.3 and 1.0 mol/L, respectively. The separation selectivity of Th and REEs showed good performance in the extraction system consisting of C572 and isooctanol in 260# kerosene with the β Th/ΣREEs value of 4.38 × 10 4 . The leaching rates of Th and REEs from the roasted waste residues by HCl were found to be 69.6% and 92.1%, respectively. Recovery of Th and REEs can be achieved by total 10 stages of extraction, scrubbing and stripping section.

To develop sustainable and efficient separation technology for heavy rare earth elements (HREEs), the extraction of Lu(III) and neighboring HREEs using EHEHP type acid–base coupling bifunctional ionic liquid (ABC-BIL) was studied for the first time. Selectivity and stripping performances of the ABC-BIL based system were investigated. Primary mechanism of the ABC-BIL based system was indicated to be ion-association. The EHEHP type ABC-BIL contributes to avoid the saponification wastewater from HEH[EHP]. Higher extractabilities of the ABC-BIL for HREEs, i.e., Ho(III), Y(III), Er(III), Tm(III), Yb(III), and Lu(III), are pronounced advantages of the IL based extracting system than traditional HEH[EHP] system.

Ionic liquid (IL) based on tetraheptylammonium cooking oil (TC) was used for the first time to recovery rare earth elements (REEs) from leaching liquor of ion-adsorbed-type rare earths ores. Firstly, extraction performances of tetraheptylammonium oleate (TO) and tetraheptylammonium linoleate (TL) type ILs were investigated. It was shown that these ILs had good extraction performances, such as short equilibrium time less than 1 min, good extraction capacity of 0.225 mol/L and high stripping efficiency of 99%. Additionally, REEs extraction occurred in the solutions with high acidity (pH = 0.24) because the cation (C7H15)4N+ of TO and TL could form a strong amine salt with hydrogen ion to decrease acidity of the feed solution. Also, an ion association mechanism of REEs extraction was proposed by FT-IR spectra, 1H NMR investigation and slope analysis method. Secondly, TC was synthesized in water bath by mixing tetraheptylammonium bromide with saponified peanut oil. Then, comparative study of commercial extractants showed that TC had the highest REEs recovery yield of 100% from the leaching liquor. The sustainability, biocompatibility and non-toxicity, as well as simple synthesis route and well extraction performances make these cooking oil based ILs promising for REEs recovery from the leaching liquor.

Magnesium sulfate (MgSO4), a leaching agent for ion-absorption rare earth (RE) ores, can reduce the production of ammonia nitrogen wastewater and supply magnesium (Mg) for soil. Three novel solid extractants, i.e., 2,2′-(1,4-phenylenebis(oxy))dioctanoic acid (PPBOA), 2,2′-(1,2-phenylenebis(oxy))dioctanoic acid (OPBOA) and 2,2′-(1,3-phenylenebis(oxy))dioctanoic acid (MPBOA) were synthesized and used to enrich RE by extraction-precipitation strategy, which showed better selectivity and faster equilibrium. The precipitation rates of PPBOA, OPBOA and MPBOA on RE could reach 95.1%, 93.5% and 96.7% at the large liquid/solid phase ratio (450/1). Meanwhile, the purities of RE in the stripping solutions reached 96.1 wt%, 94.1 wt% and 96.2 wt%. Chemical oxygen demand (COD) values of raffinates from OPBOA and MPBOA were 56 mg/L and 69 mg/L, being lower than that of China national emission standard (100 mg/L). The concentration of enriched RE solution by MPBOA was 238.0 g/L, solid extractant and MgSO4 raffinate could be recycled.