Treatment of carbon residue from aluminum electrolysis cell using Na₂CO₃

Carbon residue in aluminum electrolytic cell is a kind of hazardous waste produced during the smelting and production process of the aluminum industry. Approximately 10 kg of carbon residue is produced for every ton of primary aluminum produced. China’s primary aluminum output was as high as 35.04 million tons in 2019, so its carbon residue production was about 350,000 tons. The accumulation of a large amount of carbon residue wastes electrolyte resources, as well as causes air, soil, and water pollution. Additionally, carbon residue was listed on the National Hazardous Waste List in 2016. Therefore, the treatment of carbon residue needs to be solved urgently. In this experiment, the characteristics of carbon residue were introduced, and it was used as the raw material to study the process feasibility of recovering carbon powder and cryolite by the roasting-water leaching process of carbon residue with Na₂CO₃ as the additive. Na₂CO₃ with a mass ratio of 2.5∶1 was mixed with carbon residue, placed in a crucible-resistance furnace, and then baked at 950 ℃ for 2 h. Test results show that the alumina, cryolite, and sub-cryolite in the carbon residue are consumed by Na₂CO₃, and the mixture after roasting consists of C, Na₂CO₃, NaF, and NaAlO₂. After roasting, the mixture is leached for 1 hour with a pH of 14 and at a leaching temperature of 25 ℃. The purity of the recovered carbon powder after solid-liquid separation can reach 89%. The carbonation method is used to recover F⁻ in the leachate to obtain powdered cryolite with qualified main components. Properly increasing the roasting temperature and extending the holding time can improve the separation efficiency of carbon and electrolyte. Research on economical and effective carbon residue treatment methods can not only solve the environmental pollution caused by carbon residue, but it can also have a profound impact on the sustainable development of society.