Mechanism for leaching of fluoride ions from carbon dross generated in high-temperature and low-lithium aluminum electrolytic systems

Carbon dross, a hazardous solid waste generated during aluminum electrolysis, contains large amounts of soluble fluoride ions for the main components of the electrolyte (such as Na3AlF6 and NaF). Response surface methodology (RSM) was used to investigate the mechanism for fluoride ion leaching from carbon dross via water leaching, acid leaching and alkali leaching, and the kinetic and thermodynamic principles of the leaching process were revealed. The RSM predicted the optimum conditions of water leaching, alkali leaching and acid leaching, and the conditions are as follows: temperature, 50°C; shaking speed, 213 r·min-1; particle size, 0.075 mm; shaking speed, 194 r·min-1; liquid-solid ratio, 12.6 mg·L-1; sodium hydroxide concentration, 1.53 mol·L-1; liquid-solid ratio, 25.0 mg·L-1; sulfuric acid concentration, 2.00 mol·L-1; and temperature, 60°C，and actual results which were almost consistent with the predicted results were gained. The fluoride ions in the alkaline and acid leaching solutions were mainly the dissociation products of fluorides such as Na3AlF6, Na5Al3F14 and CaF2, as indicated by thermodynamics calculations. In particular, the fluoride compounds dissolved in alkali solution were Na3AlF6, Na5Al3F14, AlF3, ZrF4, K3AlF6, while the acid solution could dissolve only Na3AlF6 and CaF2. The leaching kinetics experiments showed that the leaching rate fit the unreacted shrinking core model [1-2/3α-(1-α)2/3=kt] and that the leaching process was controlled by internal diffusion. This study provides theoretical guidance for the removal of soluble fluoride ions from carbon dross and will also assist in the separation of electrolytes from carbon dross. Carbon dross, a hazardous waste generated during the aluminum electrolysis production process, contains a large amount of soluble fluoride. Improper storage will lead the fluoride ions pollution in soil, surface water or groundwater under the direct contact between carbon dross and rainfall, snow or surface runoff. The influence of wind will cause carbon dross dust to pollute further areas. With the human body long-term contact with fluoride ion contaminated soil or water, human health will be seriously harmed.