With the widespread use of lithium-ion batteries, the cumulative amount of used lithium-ion batteries is also increasing year by year. Since waste lithium-ion batteries contain a large amount of valuable metals, the recovery of valuable metals has become one of the current research hotspots. The research uses electrometallurgical technology, and the main methods used are cyclic voltammetry, square wave voltammetry, chronoamperometry and open circuit potential. The electrochemical reduction behavior of Ni³⁺ in NaCl-CaCl₂ molten salt was studied, and the electrochemical reduction behavior was further verified by using a Mo cavity electrode. It is determined that the reduction process of Ni³⁺ in LiNiO₂ is mainly divided into two steps: LiNiO₂ → NiO → Ni. Through the analysis of electrolysis products under different conditions, when the current value of LiNiO₂ is not less than 0.03 A, the electrolysis product after 10 h is metallic Ni. When the current reaches 0.07 A, the current efficiency is 77.9%, while the Li⁺ in LiNiO₂ is enriched in NaCl-CaCl₂ molten salt. The method realizes the separation and extraction of the valuable metal Ni in the waste lithium-ion battery.

Secondary zinc oxide (SZO), which comes from the zinc industry, is an important secondary resource of zinc and other valuable metals. In this study, the production feasibility and rationality of a cleaner zinc recovery process using SZO and a hydrometallurgical method were described. Zinc extraction is promoted by the addition of ammonium bicarbonate to a NH₃–H₂O system, and the maximum recovery of zinc could be close to 80% at the optimum leaching conditions of a stirring rate of 400 rpm, an ammonia/ammonium ratio of 7:3, a total ammonia concentration of 4 mol/L, and a liquid/solid ratio of 7 mL/g for 30 min at 35 °C. The kinetics of leaching were modeled using the shrinking core model of constant-size particles, and the rate-controlling step was determined to be the diffusion through the product layer. The apparent activation energy of the reaction was estimated to be 11.04 KJ·mol⁻¹, while the order of reaction with respect to total ammonia concentration was 1.53 and the liquid/solid ratio was 2.26. The analysis results of the initial residue and the leached residue indicated that lead was transferred from PbCl₂ to PbCO₃ and that ZnFe₂O₄ was not leached in the NH₃-NH₄HCO₃-H₂O system.

With the continuous development of society, the number of spent lithium-ion batteries has also increased, and the recovery of valuable metals such as Ni, Co, and Li has become the main research direction of many scholars. In this paper, the extraction process of lithium that enters the molten salt after LiCoO₂ electrolysis is studied. Oxalic acid and phosphate are added to molten salt containing lithium ions to realize the two-part precipitation method to extract lithium. The influence of pH value, temperature, reaction time, and oxalic acid (or phosphate) addition on the process of oxalic acid calcium removal and phosphate lithium precipitation is analyzed. The results show that the calcium removal rate of oxalic acid has reached 99.72% (Initial conditions: PH = 7.0, T = 70 °C, t = 1.5 h, n(H₂C₂O₄):n(Ca²⁺) = 1.2:1). The precipitation of Li₃PO₄ obtained in the phosphate extraction experiment of lithium is as high as 88.44% (Initial conditions: PH = 8.0, T = 70 °C, t = 1.5 h, n(actual dosage of Na₃PO₄):n(theoretical dosage of Na₃PO₄) = 1.2:1). The obtained lithium phosphate crystals show regular spherical particles, which can be seen by SEM.

The electrochemical reduction mechanism of Mn in LiMn₂O₄ in molten salt was studied. The results show that in the NaCl-CaCl₂ molten salt, the process of reducing from Mn (IV) to manganese is: Mn (IV)→Mn (III)→Mn (II)→Mn. LiMn₂O₄ reacts with molten salt to form CaMn₂O₄ after being placed in molten salt for 1 h. The reaction of reducing CaMn₂O₄ to Mn is divided into two steps: Mn (III)→Mn (II)→Mn. The results of constant voltage deoxidation experiments under different conditions show that the intermediate products of LiMn₂O₄ reduction to Mn are CaMn₂O₄, MnO, and (MnO)_x(CaO)_(1−x). As the reaction progresses, x gradually decreases, and finally the Mn element is completely reduced under the conditions of 3 V for 9 h. The CaO in the product can be removed by washing the sample with deionized water at 0 °C.

An Fe/FeAl₂O₄ composite was prepared with Fe-Fe₂O₃-Al₂O₃ powder by a hot press sintering method. The mass ratio was 6:1:2, sintering pressure was 30 MPa, and holding time was 120 min. The raw materials for the powder particles were respectively 1 µm (Fe), 0.5 µm (Fe₂O₃), and 1 µm (Al₂O₃) in diameter. The effect of sintering temperature on the microstructure and mechanical properties of Fe/FeAl₂O₄ composite was studied. The results showed that Fe/FeAl₂O₄ composite was formed by in situ reaction at 1300 °C–1500 °C. With the increased sintering temperature, the microstructure and mechanical properties of the Fe/FeAl₂O₄ composite showed a change law that initially became better and then became worse. The best microstructure and optimal mechanical properties were obtained at 1400 °C. At this temperature, the grain size of Fe and FeAl₂O₄ phases in Fe/FeAl₂O₄ composite was uniform, the relative density was 96.7%, and the Vickers hardness and bending strength were 1.88 GPa and 280.0 MPa, respectively. The wettability between Fe and FeAl₂O₄ was enhanced with increased sintering temperature. And then the densification process was accelerated. Finally, the microstructure and mechanical properties of the Fe/FeAl₂O₄ composite were improved.

The electrochemical reduction process of ZnFe₂O₄ in NaCl-CaCl₂ melts was studied. Thermodynamic analysis shows that the reduction process of ZnFe₂O₄ is carried out in multiple steps, and it is difficult to reduce Fe³⁺ to Fe in one step. Electrochemical tests revealed that the reduction process of ZnFe₂O₄ includes three steps: First, Fe³⁺ is reduced to Fe in two steps, then Zn²⁺ is reduced to Zn in one step. The reduction of Fe³⁺ on the Mo electrode is a reversible process controlled by diffusion, while the reduction of Zn²⁺ is an irreversible process controlled by diffusion. The influence of electrolysis voltage and temperature on the process of electric deoxidation has also been studied. It is indicated that properly increasing the temperature is conducive to the diffusion of oxygen ions, thereby increasing the deoxidation rate. With the gradual increase of voltage, the reduction process of ZnFe₂O₄ is ZnFe₂O₄ → FeO + ZnO → Fe + ZnO → Fe + Zn.