Mechanism of the denitrification of secondary aluminum dross (SAD) during water leaching with delayed addition of a low dosage of sodium hydroxide

Abstract

Secondary aluminum dross (SAD) has been identified as a hazardous waste because it contains refractory AlN, fluoride, and other salts. Considering that Al(OH)₃ produced during the hydrolysis of AlN in SAD hinders spontaneous AlN hydrolysis, which is amplified by the partial embedment of AlN in other oxide phases present in SAD particles, the complete removal of AlN from SAD is difficult. Herein, we propose a new catalytic hydrolytic denitrification process of SAD, which requires a low additive dose. By changing the time at which the additive (NaOH) was added to the SAD slurry, the proportion of nitrogen removed from SAD was increased. The additive (NaOH) reacted with Al(OH)₃, thereby mitigating its hindering effect on spontaneous AlN hydrolysis and facilitating the complete hydrolysis of the semi-encapsulated AlN in SAD particles. Concurrently, the reaction between NaOH and Al₂O₃ and SiO₂ phases present in SAD was mitigated, resulting in the maximal retention of residual materials (phases other than Al resources) in the solid phase. A SAD denitrification efficiency of 99.0% was achieved under optimal processing conditions (T = 95 °C, t = 240 min, Liquid-to-solid ratio = 6:1, stirring speed = 400 rpm, and the addition of 7 wt% NaOH at 0.5 h), and a slightly lower efficiency of 98.2% was achieved when 50% lower amount of NaOH (3 wt%) was used. Thus, approximately 50% less additive is required for nitrogen removal from SAD, relative to that used in traditional catalytic water-washing processes.