Leaching kinetics and mechanism of slag produced from smelting-reduction of bauxite for alumina recovery

Abstract

In the present study, we investigate the kinetics and mechanism of the leaching of calcium aluminate slag in Na2CO3 solution for alumina recovery. The slag consists of leachable phases, e.g., 12CaO·7Al2O3, CaO·Al2O3, 5CaO·3Al2O3, and a non-leachable phase that contains Ca-Al-Si-Ti oxides. A series of leaching treatment is done at different temperatures, leaching times, stirring rates, Na2CO3 concentrations, and different setups which includes wet-grinding and shaking-digestion reactor. The highest alumina recovery up to 90.5% is obtained after the slag is leached by 10 wt% Na2CO3 solution, at low temperatures (30–45 °C) within 90 min. It is shown that the rate of alumina recovery is high at the beginning of leaching and is then slow down due to the calcite layer product nucleation and growth at the surface of slag. The wet-grinding leaching and vigorous stirring increase the possibility of the collision between both particles and the stirrer that breaks the calcite layer, yielding less residue agglomeration and better recovery compared to the slow and mild agitations. A surface observation of the slag using electron microscopy shows that the calcite starts to nucleate at the non-leachable phase as the best deposition site, which has the least mass transfer barrier in the system. The apparent activation energy of the leaching reaction is calculated as 10.8–19.9 kJ/mol, which indicates the reaction is diffusion rate-limited as revealed by the applied kinetic models.