Dissolution behavior of overhaul slag from aluminum reduction cell in a cryolite-based molten salt system

Abstract

The harmless and reclamation disposal of overhaul slag, which is a hazardous waste from aluminum electrolysis, is a great challenge. Herein, a synergistic disposal strategy, i.e., adopting the overhaul slag as a raw material together with aluminum oxide in the aluminum electrolysis process to produce Al–Si alloy, was proposed. The dissolution behavior of overhaul slag in a cryolite-based molten salt system was investigated by thermodynamic calculation and dissolution experiments. The liquidus temperature of molten salt was decreased with the addition of SiO2 and Al2O3. This suggests that the synergistic disposal of overhaul slag did not cause an adverse effect on the aluminum electrolysis process. The possible dissolution products were Na3AlF6, Na5Al3F14, SiO2, NaAlSiO4, NaAlSi3O8, Na2SiO3, Na2SiF6, Na2 MgAlF7 and SiF4. The addition of Al2O3 could enhance the solubility and dissolution rate of SiO2 and hence shorten the dissolution time. The dissolution process of overhaul slag satisfied well with the mass transfer model, suggesting that the dissolution rate was controlled by the diffusion process. This work provides a basis for the synergistic disposal of overhaul slag in the aluminum electrolysis process.