China Nonferrous Metallurgy

Aluminum electrolysis generates a significant amount of spent cathode carbon, which not only contains a substantial amount of carbon but also a considerable amount of soluble fluoride. The improper disposal of this waste not only leads to environmental pollution but also results in the loss of valuable resources. In this paper, in order to solve the problems existing in the treatment processes studied in the current literature, an ultrasonic-assisted water-alkaline leaching method to treat spent cathode carbon. The process was investigated and the process conditions were optimized. The intermediate products were characterized and analyzed, and the effects of water-leaching-alkaline leaching, both with and without ultrasonic assistance, were compared. The following key findings were obtained. The optimal conditions for water leaching are a leaching time of 50min, a temperature of 25℃, a liquid-to-solid ratio of 7∶1, and an ultrasonic power of 300W. Under these conditions, the fluorine removal rate is 32.31%, and the recovered product is sodium fluoride, with a purity of 99.4 wt.%. The optimal process conditions for alkali leaching are 60min of alkali leaching time, 70℃ of alkali leaching temperature, 10∶1 liquid-to-solid ratio, 1.25mol/L initial NaOH solution concentration, and 400W of ultrasonic power. Under these conditions, the fluorine removal rate can reach 92.24%, and the obtained alkali leaching solution is treated with hydrochloric acid to obtain cryolite powder with a purity of 95.7 wt.%. EDS analysis of spent cathode carbon following ultrasonic alkali leaching revealed that it primarily contained only carbon, indicating that ultrasonic water and alkali leaching effectively removed sodium fluoride, cryolite, alumina, and other components. The ultrasonic alkali leaching process was primarily controlled by diffusion, with an apparent activation energy of 18.61kJ/mol. The ultrasonic-enhanced leaching process reduced the processing time by 92.4% compared to the conventional stirring leaching process, while the fluorine removal rate increased by 13.98%. This study demonstrated that the combined ultrasonic water and alkali leaching process effectively facilitates the harmless treatment of spent cathode carbon and the recovery of valuable components.