Quantitative evaluation on AlN transformation and volatilization of chlorides and fluorides in pyrometallurgical treatment for aluminum dross

Abstract

Reaction characteristics in pyrometallurgical treatment for aluminum dross were studied qualitatively and quantitatively by thermal analysis. The transformation of aluminum nitride (AlN), volatilization of chlorides and fluorides in two kinds of dross under both argon (Ar) and Ar-Oxygen (O2) atmospheres were in detail analyzed and compared. Gas species including water vapor (H2O(g)), ammonia (NH3), nitrogen (N2), nitric oxide (NO), and carbon dioxide (CO2) were clearly identified. The study indicated that N2 and small amount of NO were produced from AlN–O2 reaction while NH3 from AlN–H2O(g) reaction, and H2O(g) and CO2 were necessarily from thermal decomposition of hydroxides and carbonates. Calculation indicated that the AlN–O2 reaction, AlN–H2O(g) reaction, and volatilization of chlorides or fluorides were remarkably slow, for around 30 mg raw material with taking about 37 min to finish the AlN–O2 reaction in 500–1000°C, 60 min to the AlN–H2O(g) reaction in 130–500 °C, and 30 min to the volatilization of chlorides or fluorides in 760–1000°C. Kinetics study indicated that the Ginstling-Brounshtein's diffusion model and Jander's diffusion model were most appropriate for describing the kinetics, respectively with an apparent activation energy of 273.2 kJ mol−1 and 307.4 kJ mol−1 for the AlN–O2 reaction, and with that of 182.0 kJ mol−1 and 194.0 kJ mol−1 for AlN–H2O(g) reaction. The study will be beneficial for design, optimization of pyrometallurgical routes of aluminum dross treatment and for evaluating its environmental effect.