Purification of spent cathode carbon by mixed molten salt roasting-ultrasonic hot-pressing synergistic leaching: Ultrasound-induced repair of graphite lattice structure

Spent carbon cathode (SCC) is a recyclable hazardous solid waste generated from electrolytic aluminum production. In this study, a combined leaching process involving mixed molten salt roasting and ultrasonic-thermal pressure was proposed to purify graphite and repair the lattice structure of waste cathodes. The results showed that the mixed molten salt system reduced the reaction temperature of the impurities in the waste cathode and that adding an ultrasonic pretreatment step shortened the subsequent hot-pressing acid leaching time. It also lowered the reaction temperature, and the fixed carbon content of the product increased from 99.81% to 99.86%. X-ray diffraction (XRD), Raman spectroscopy, BET, and particle size tests showed that ultrasonic pretreatment did not damage the graphite crystal structure and even promoted the repair of the graphite lattice while destroying the porous structure of the waste cathode and removing impurities between graphite layers. This study provides a solution to the high energy consumption and graphite structure damage during the recycling of waste cathode graphite and achieves a dual effect of impurity removal and structure repair. It also provides new experimental data for designing other waste graphite purification processes.