Defluorination of spent pot lining from aluminum electrolysis using acidic iron-containing solution

Abstract The defluorination characteristics of spent pot lining, which is a hazardous waste from aluminum electrolysis, were studied using acidic iron-containing solution. The results showed that the insoluble fluoride in spent pot lining can be extracted effectively, and the extraction percentage can reach 88.5% through a two-step leaching process, which involved extraction by deionized water and then a 0.20 mol/L Fe3+ solution with initial H+ concentration of 0.48 mol/L, and liquid–solid ratio of 10 mL/g at 80 °C for 30 min. Contributing to the strong complexing ability of Fe3+ and F−, the distribution and concentration of fluorine-containing complexes in solution varied significantly with the increasing concentration of Fe3+, which the excessive Fe3+ concentration promoted the dissolution of insoluble fluoride and inhibited the volatilization of HF gas. Thermodynamic analysis results demonstrated that the insoluble fluoride as Na3AlF6 and CaF2 dissolved and formed to [FeFi]3-i complexes during the leaching process, in which Fe3+ reacted with free fluoride and highly fluoridated species to form stable FeF2+ complexes. The release of insoluble fluoride in spent pot lining was controlled by interface transfer and diffusion across the product layer, which had a corresponding activation energy of 38.2 kJ/mol. The results suggested that acidic iron-containing solution can efficiently and harmlessly extract insoluble fluoride from spent pot lining.