Study on the Dissociation of Cryolite in SPL

Abstract

The spent pot lining of aluminum electrolysis (SPL) is a typical harmful solid waste discharged from the production process of primary aluminum. It contains about 30% of fluoride and 0.2% of cyanide, which is a valuable fluorine-containing resource. With SPL as raw material, sodium carbonate was used as a reagent, and dissociation of cryolite was investigated by a baking process. Gibbs free energy of cryolite dissociation was calculated and the reaction mechanism of cryolite was discussed. At the same time, the effects of reagent addition ratio, baking temperature and reaction time on the dissociation of cryolite in SPL were discussed. The results show that under the conditions of a reagent addition ratio of 30–35wt%, baking temperature of 800–850 ℃, and reaction time of 3–3.5 h, the phase of cryolite and mullite in the SPL disappears, cryolite is transformed into sodium fluoride and sodium metaaluminate compounds, and mullite is transformed into nepheline or feldspar. The concentration of fluoride ion in clinker leaching solution increased to 8.84 g/L, almost two times that of primary concentration 4.56 g/L in SPL, and the effect of dissociation was obvious, which will be beneficial to the subsequent recovery of fluorides.