Efficient extraction of valuable metals from spent LiNi_0.5Co_0.2Mn_0.3O₂ cathode materials using sodium bisulfate: Parameter optimization and kinetic study

Spent lithium-ion batteries pose significant environmental hazards but also possess high recycling value. In this study, a process for the recovery of valuable metals from spent LiNi_0.5Co_0.2Mn_0.3O₂ cathode materials (NCM523) was proposed by using NaHSO₄ as the sulfation agent. The thermal decomposition of NaHSO₄ and its sulfation roasting process with spent NCM523 were investigated. The findings revealed that NaHSO₄ started decomposing at approximately 200 °C, releasing Na₂S₂O₇. And significant SO₂ gas was emitted after 400 °C, efficiently reducing and sulfating the valuable metals in NCM523. The sulfation roasting parameters were optimized by examining the influences of roasting temperature and the mass ratio of NaHSO₄ to waste cathode materials on the leaching efficiencies of valuable metals. Experimental results indicated that Li was present as Li₂SO₄ and NaLiSO₄ forms after roasting, while other valuable metals existed in complex forms, mainly as bimetallic composite sulfates. The study found that under optimal conditions, with a NaHSO₄ to waste cathode material mass ratio of 5:1, and roasting at 600 °C for 120 min resulted in the highest leaching efficiencies for valuable metals, with Li at 98.4 %, Co at 97.1 %, Ni at 96.1 %, and Mn at 96 %. Additionally, the non-isothermal kinetic of the roasting process was studied, determining the apparent activation energy of each stage and identifying the controlling steps through the Kissinger-Akahira-Sunose (KAS), Flynn-Wall-Ozawa (FWO), and Starink methods. The mechanisms were elucidated using the Satava-Sestak and Coats-Redfern equations, which identified the fourth stage as the controlling step with an average activation energy of 211.4 kJ/mol, indicating a three-dimensional diffusion model. The results highlighted that utilizing NaHSO₄ roasting for recycling waste cathode materials led to high leaching efficiencies of Li, Co, Ni, and Mn metals at lower temperatures. This aligned well with the principles of efficient and environmentally friendly recycling practices.