Two-stage process for the safe utilization of secondary aluminum dross in combination with the Bayer process

Abstract

Secondary aluminum dross (SAD) is a hazardous waste produced from Al electrolysis, casting, and regeneration. A two-stage process for the safe utilization of SAD was proposed by combining with the Bayer process liquor. In the first stage, approximately 32%, 100%, and 27% of Al, Cl, and F, respectively, from the reactive substances were dissolved into the saturated sodium aluminate solution. A kinetic synergy with a considerable amount of SAD at high temperatures led to an uncontainable reaction, and the frequent addition of SAD at a low dosage was determined for the separation and utilization of H2 and NH3. The inert alumina-bearing compounds and CaF2, which accounted for 50% of the mass of SAD, remained in the residue after the first stage. Efficient conversion of the inert alumina-bearing compounds and elimination of impurities from the Bayer process were both achieved by sintering at 1000 °C for 1 h in the second stage. An alumina extraction efficiency of 99% was obtained at 85 °C after the as-prepared sinter was leached into the saturated sodium aluminate solution. The saturated sodium aluminate solution obtained in the two-stage process was incorporated into the Bayer liquor to produce alumina. Furthermore, cl− and F− were removed by forming sodalite in the desilication process and precipitating CaF2 in the dilute sodium aluminate solution, respectively. Using this novel approach, an estimated 30,000 t/a of SAD has been processed in an alumina refinery in China since 2019. These results provide a novel short process for the safe and efficient utilization of SAD