This paper reports the solid-liquid phase equilibria of the CoSO4-H2O and CoSO4-H2SO4-H2O systems at low temperatures. Binary and ternary phase diagrams, including the stable solid phases CoSO4·6H2O and CoSO4·7H2O were established using experimental data and thermodynamic modeling applying the mixed-solvent electrolyte (MSE) model. The results showed that the addition of H2SO4 shifts the eutectic temperature and concentration to lower values for cobalt sulfate and ice crystallization. The trends obtained from the experimental data and the modeling are consistent for the binary CoSO4-H2O system with good agreement, but the ternary CoSO4-H2SO4-H2O system shows some deviations. In general, the MSE model is shown to be reliable for inferring and establishing the phase diagram of the low-temperature system. The phase diagrams are helpful for designing the pathways of cooling crystallization and eutectic freeze crystallization and assessing the performance of the low-temperature crystallization process in the production of CoSO4 hydrates. In addition, some practical examples of cooling crystallization and eutectic freeze crystallization of CoSO4 solutions are provided.

Crystallization of cobalt sulfate within a typical hydrometallurgical process for the recycling of Ni-Mn-Co oxide or Ni-Co-Al oxide Li-ion batteries has been investigated. The cobalt sulfate salt was obtained by eutectic freeze crystallization (EFC) from a synthetic acidic cobalt strip liquor after solvent extraction. The ternary phase diagram of CoSO4–H2SO4–H2O was first established by the mixed-solvent electrolyte (MSE) model to predict and reveal the changes in the system during the freezing process and to assess the conditions required for EFC. Batch EFC experiments were then conducted for the cobalt strip liquor, which contained a low concentration of impurities. It is shown that with suitable control of supersaturation, seeding, and stirring, pure ice and salt crystals can be recovered as separate phases at the eutectic temperatures, with the crystalline salts in the form of a heptahydrate. The crystallization process can be described using the ternary phase diagram, but with certain deviations. The deviations might be related to insufficient data at the low temperatures in the MSE model and acid entrapment in crystals during the crystallization process. Finally, the performance of the EFC process has been compared to that of an evaporative crystallization (EC) using the same strip liquor. It was found that the CoSO4·7H2O product obtained by EFC was of slightly higher quality considering purity and crystal shape compared to that from EC.

Elemental sulfur (S8) is an abundant and inexpensive by-product of petroleum refining. Polymeric sulfur is thermodynamically unstable and depolymerizes back to S8 with time, which limits its applications and causes megatons of sulfur to accumulate in nature. A novel sulfur-oleylamine copolymer, synthesized using the inverse vulcanization method, is reported for the selective recovery of Cu2+ from a complex mixture of transition metals. Adsorption studies have been performed using batch experiments in the simulated aqueous solution containing a mix of metal ions (Mx+= Fe, Al, Mn, Co, Ni and Cu). The effect of different adsorption parameters such as pH, time, adsorbent dose, sulfur content, and desorption have been studied. The results demonstrate that the sulfur-oleylamine copolymer shows high selectivity towards Cu2+, with excellent adsorption efficiency of >98 % in acidic pH (pH≈1) at room temperature, which is of practical relevance in the handling of battery leach liquors obtained from industrially derived blackmass. Finally, the sulfur-oleylamine copolymers were also applied to battery leach liquors with hydrochloric (HCl) or citric acid and which showed Cu2+ adsorption efficiency of >98 %±1 and > 95 %±7, respectively. This work presents a novel way to convert industrial waste into a stable sulfur polymer and demonstrates its use as a promising material for selective recovery of Cu ions from battery waste and industrial effluents in a simple and cost-effective manner. 

In this study, eutectic freeze crystallization (EFC) was investigated to recover NiSO4 and CoSO4 hydrates from aqueous and dilute sulfuric acid solutions of metal sulfates. Binary phase diagrams were established using a combination of thermodynamic modeling and experimental data. The mixed-solvent electrolyte (MSE) model was employed to model solid–liquid phase equilibria. The predicted binary phase diagrams from the model were in good agreement with the experimental results. Experimental eutectic temperatures and eutectic metal sulfate concentrations for the NiSO4-H2O and CoSO4-H2O systems are −3.3 °C and 20.8 wt% and −2.9 °C and 19.3 wt%, respectively. For NiSO4-H2SO4-H2O and CoSO4-H2SO4-H2O systems, the eutectic temperature and eutectic metal sulfate concentration decrease with increasing H2SO4 concentration. Batch experiments were performed to study the EFC of different sulfate solutions, including 25- wt% NiSO4 in H2O, 20- wt% NiSO4 in 0.5 mol/kg H2SO4, 25- wt% CoSO4 in H2O, and 20- wt% CoSO4 in 0.5 mol/kg H2SO4. The results show that controlling the supersaturation allows high-quality ice and salt crystals to be recovered as separate phases under eutectic conditions, with the crystalline salts in the form of heptahydrates. This study shows that EFC can be a promising alternative to evaporative crystallization for recovering NiSO4 and CoSO4 hydrates from sulfate solutions.

Coal fly ash (CFA) is a potential mineral resource from which to recover Al and other valuable metals. In this study, a new processing technology for the recovery of Al and Ti from CFA has been developed and comprehensively investigated. The baking process applied in previous work has been improved by using microwave heating and a mixture of H2SO4 + NH4HSO4 as the extractant. This method enhanced the Al and Ti extraction efficiencies, while decreasing energy consumption and gas emissions relative to other acidic baking processes. When employing the optimized baking and leaching parameters (baking conditions: 280 degrees C, 1.2 times the theoretical amount of reagents, 60 min; leaching conditions: 60 degrees C, L/S: 5 g water to 1 g baked ash, 30 min) 82.4% Al and 55.6% Ti could be extracted. Scanning electron microscopy images and X-ray diffraction analysis indicated that most of the mullite (3Al(2)O(3).2SiO(2)) in the CFA was transformed into godovikovite (NH4Al(SO4)(2)) and quartz (SiO2) after microwave-assisted baking. The soluble salts were then leached into solution, while the quartz remained in the residue. Precipitation allowed for the recovery and separation of Al and Ti from the leach solution. Al was selectively recovered via NH4Al(SO4)(2)center dot 12H(2)O precipitation after maintaining the solution at 0 degrees C for 10 h. A high-quality product of alumina was obtained from the NH4Al(SO4)(2)center dot 12H(2)O. After reducing the iron in the solution from Fe3+ to Fe2+, Ti was recovered via hydrolysis after increasing the pH to 3.1. The Ti precipitate contained 44.2% Ti with a small amount of impurity. The developed approach was cleaner and more efficient than those reported to date for the recovery of Al and Ti from stable CFA.

The widespread and increasing use of Li-ion batteries has led to an impending need for recycling solutions. Consequently, recycling of spent Li-ion batteries with energy-efficient, environmentally sustainable strategies has become a research hotspot. In this work, eutectic freeze crystallization (EFC), which requires less energy input than conventional evaporative crystallization (EC), has been investigated as a method for the recovery of Ni and Co sulfates from synthetic acidic strip solution in the recycling of NMC or NCA Li-ion batteries. Two binary sulfate systems have been studied. Batch EFC experiments have been conducted. It is shown that, with suitable control of supersaturation, ice and salt crystals can be recovered as separate phases below the eutectic temperatures. The work shows that EFC is a promising alternative to EC for the recovery of Ni and Co sulfates from spent Li-ion batteries. 

An ultrasound-assisted extraction (leaching) method of valuable metals from discarded lithium-ion batteries (LiBs) is reported. Mild organic citric or acetic acids were used as leaching agents for a more environmentally-friendly recovery of the lithium, nickel, cobalt, and manganese from the discharged and crushed lithium nickel-manganese-cobalt oxide (NMC) LiBs. The extraction was performed with the presence/absence of continuous ultrasound (US) energy supplied by a 110 W ultrasonic bath. The effect of temperature (30-70 °C), reducing agent concentration (H2O2: 0-2.0 vol%), as well as choice of specific acid on the metal dissolution were investigated. The US leaching decreased the leaching time by more than 50% and improved the leached percentage of Li, Mn, Co, and Ni due to the local heat and improved mass transfer between solid and liquid interfaces in the process. The X-ray diffraction results of residues from the US leaching further confirmed an improved dissolution of the crushed layered NMC structure, resulting in the significant improvement of the leached amounts of the valuable metals. Furthermore, it is demonstrated that using citric acid eliminated the need of additional reducing agents and suppressed the dissolution of copper (Cu) due to its inhibitor effect on the Cu surface, i.e. compared with using acetic acid as leaching reagent. Overall, it is shown that recovery of the battery metals can be facilitated and carried out in a more energy-efficient manner at low temperatures (50 °C) using ultrasound to improve metal ions mass transportation in the residue layers of the NMC during the organic acid leaching.

H₂SO₄ was ensured to be the best candidate for Zr leaching from the eudialyte. The resulting sulfuric leach solution consisted of Zr(IV), Nb(V), Hf(IV), Al(III), and Fe(III). It was found that ordinary metal hydroxide precipitation was not feasible for obtaining a relatively pure product due to the co-precipitation of Al(III) and Fe(III). In this reported study, a basic zirconium sulfate precipitation method was investigated to recover Zr from a sulfuric acid leach solution of a eudialyte residue after rare earth elements extraction. Nb precipitated preferentially by adjusting the pH of the solution to around 1.0. After partial removal of SO₄²⁻ by adding 120 g of CaCl₂ per 1L solution, a basic zirconium sulfate precipitate was obtained by adjusting the pH to ~1.6 and maintaining the solution at 75 °C for 60 min. Under the optimum conditions, the loss of Zr during the SO₄²⁻ removal step was only 0.11%, and the yield in the basic zirconium sulfate precipitation step was 96.18%. The precipitate contained 33.77% Zr and 0.59% Hf with low concentrations of Fe and Al. It was found that a high-quality product of ZrO₂ could be obtained from the basic sulfate precipitate. 

Li-ion battery materials have been widely studied over the past decades. The metal salts that serve as starting materials for cathode and production, including Li₂CO₃, NiSO₄, CoSO₄ and MnSO₄, are mainly produced using hydrometallurgical processes. In hydrometallurgy, aqueous precipitation and crystallization are important unit operations. Precipitation is mainly used in the processes of impurity removal, separation and preliminary production, while controlled crystallization can be very important to produce a pure product that separates well from the liquid solution. Precipitation and crystallization are often considered in the development of sustainable technologies, and there is still room for applying novel techniques. This review focuses on precipitation and crystallization applied to the production of metal salts for Li-ion battery materials. A number of novel and promising precipitation and crystallization methods, including eutectic freeze crystallization, antisolvent crystallization, and homogeneous precipitation are discussed. Finally, the application of precipitation and crystallization techniques in hydrometallurgical recycling processes for Li-ion batteries are reviewed.