Self-driven hydrolysis mechanism of secondary aluminum dross (SAD) in the hydrometallurgical process without any additives

Secondary aluminum dross (SAD) containing high levels of AlN and fluorine/chlorine salt is a typical hazardous waste generated from aluminum electrolytic and recycling processes. Numerous studies have reported the denitrification of SAD by pyrometallurgical or hydrometallurgical techniques. However, these methods were always inefficient due to strict restrictions on raw materials or the risk of secondary pollution. In this study, a new idea of deep denitrification of SAD by self-driven hydrolysis of AlN and the enhancement of mechanical ball milling was proposed. Thermodynamic analysis suggested that Al(OH)3, Si and SiO2 can be dissolved in the alkaline environment generated by the hydrolysis of AlN alone under low liquid–solid ratio (L/S) conditions. The results of the kinetic fit indicated that the denitrification reaction was controlled by the chemical reaction at the early stage, then shifted to the mixing control as the reaction progressed. The experimental results confirmed that Al(OH)3, Si and SiO2 entered the solution as Al(OH)4- and H3SiO4- during the hydrolysis of SAD, then formed aluminosilicate precipitation (feldspar). This reaction promoted the dissolution of Al and Si, which in turn enhanced the removal of nitrogen. Under the optimal process conditions (L/S = 1.5, T = 80 °C, t = 300 min, ball mill speed = 200 rpm), the residual amount of nitrogen in SAD was reduced to 0.39 wt%, and the denitrification rate reached 93.05%.