Using sulfuric acid–phosphoric acid leaching to remove aluminum and cathode material metal residues from spent graphite

Abstract

Spent graphite (SG) separated in industrial-scale recycling of spent lithium-ion batteries (LIBs) usually contains various impurities, such as cathode material metals, aluminum, and organic contaminants. Therefore, the re-use of SG may require various purification and regeneration processes to meet battery-grade purity requirements. This study investigated the simultaneous removal of aluminum and cathode material metals (lithium, nickel, manganese, and cobalt) from LIB black mass-derived SG using a sulfuric acid–phosphoric acid mixture for leaching. The effects of phosphoric acid concentration (0–0.7 mol L⁻¹) and reaction time (240–600 min) were evaluated. Cathode material metals were removed with sulfuric acid leaching (97.5–98.9%), but aluminum impurities remained almost completely in the SG (aluminum removal: 17.8%). The simultaneous removal of the studied elements was achieved by a sulfuric acid–phosphoric acid mixture. The removal of studied elements was significantly enhanced by increasing phosphoric acid concentration from 0.1 mol L⁻¹ to 0.5 mol L⁻¹ and by increasing reaction time from 240 min to 600 min. Under the studied conditions (sulfuric acid: 2.75 mol L⁻¹; phosphoric acid: 0.5 mol L⁻¹; time: 600 min; temperature: 100 °C; L/S ratio: 10 mL g⁻¹), aluminum removal was increased to 90.8%, while cathode material metals were removed between 97.7% and 99.5%. The carbon content of the SG sample notably increased from 69.2 to 93.0 wt% with this single-stage acid mixture leaching process. Based on thermogravimetric and X-ray photoelectron spectroscopy analyses, residual impurities in the purified SG sample consisted mostly of organic or other volatile impurities. In addition, lithium phosphate, produced as a side stream in lithium processing, was also examined as an alternative phosphate source in the acid mixture to replace phosphoric acid. Based on the results, the sulfuric acid–lithium phosphate mixture showed equal capability for aluminum and cathode material metal removal to the sulfuric acid–phosphoric acid mixture, increasing carbon content up to 92.3 wt%. This offers a potential application for further lithium phosphate use. The results of this study showed that a sulfuric acid–phosphoric acid mixture has a good capability to purify SG, and side stream-based lithium phosphate can replace phosphoric acid in the mixture.