Innovative technology for defluorination of secondary aluminum dross by alkali leaching

Abstract

Secondary aluminum dross (SAD) contains harmful fluorides, so it requires a harmless treatment before recycling. In this paper, an innovative SAD defluorination technology was developed. It extracts all the soluble sodium fluoride and active cryolite of SAD; besides, it can also stabilize the harmful fluorides towards deep detoxification. The defluorination process gave a novel idea for the comprehensive wet processing treatment of SAD. A NaOH solution was used to leach active fluorides, including cryolite and soluble fluorides, from SAD. The Effects of NaOH concentration, temperature, liquid-to-solid ratio, and leaching time on the fluoride leaching rate were investigated. The leaching rate of active fluorides was 95.8% at a NaOH concentration of 10 wt%, a leaching temperature of 80 °C, a liquid-to-solid ratio of 4, and a leaching time of 20 min. The kinetic analysis showed that the fluoride leaching occurred according to the hybrid control model of solid product layer in-diffusion and interfacial chemical reaction. The apparent activation energy was 39.64 kJ/mol. Meanwhile, 99.9% of fluoride ions in the alkaline solution were converted into CaF2 by adding CaO, stabilizing the active fluorides. The concentration of fluorides in the alkaline solution was 5.944 mg/L, satisfying the Chinese National Standard.