An efficient recycling process of spent cathode carbon blocks via low temperature molten salt roasting and two stage leaching

Spent cathode carbon (SCC) as a hazardous solid waste from the aluminum electrolysis industry, contains numerous valuable components, but poses a serious threat to ecological environment. Molten salt roasting is an efficient separation and purification method for SCC than directly leaching of hyfrometallurgy. However, the current treatment of SCC by molten salt roasting is under inert atmosphere and high temperature conditions, which causes high production costs and low product economics. Herein, a low-temperature molten salt roasting process with 550 °C and NaOH additive under air atmosphere-followed by water leaching and acid leaching in turn is proposed to deeply purify SCC. A single-factor optimal experiment indicates that most of the impurities, such as Na₃AlF₆, were removed after water leaching and the refractory substances like silica-aluminate, were further transformed by acid leaching, which results in a significant improvement in the purity of the recovered graphite (RG) from 43.55 wt% to 98.67 wt%. DFT calculations confirm that RG have a greater density of states (DOS) near the Fermi energy level and lower migration energy barriers for Li ion, which provides a theoretical foundation for the high-value utilization of RG in anode material for lithium batteries. The primary and second leach filtrates were combined to recover 96.55% fluorine from solution in the form of cryolite under optimum conditions, based on solution equilibrium chemistry calculations of Al and F complexes. The whole process features low cost and environmental protection, and basically completes harmless treatment of SCC and recovery of valuable components.