Production of Aluminum Hydroxide from Aluminate Solution Obtained by Red Mud Sinter Processing via Carbonation

This study investigated the processing of an aluminate solution obtained by laboratory leaching of industrial red mud sinter, followed by sequential silica removal and aluminum hydroxide precipitation through carbonation. Autoclave treatment at 145 °C for 60 min, followed by treatment with CaO (8 g/dm3) at 90 °C for 120 min, reduced the SiO2 concentration from 1.10 to 0.03 g/dm3 and increased the silica modulus of the solution from 31.1 to 1076. The effects of solution density (1.10–1.40 g/cm3) and temperature (20–90 °C) on carbonation efficiency were evaluated in terms of Al2O3 recovery and the phase composition of the precipitate. At a solution density of 1.35 g/cm3 and a temperature of 90 °C, Al2O3 recovery reached 90.1%, while the resulting product contained 98.3% gibbsite, Al(OH)3. Increasing the solution density to 1.40 g/cm3 decreased Al2O3 recovery to 47.9% and promoted the formation of NaAlCO3(OH)2, thereby defining the upper limit of the rational solution-density range. The obtained aluminum hydroxide exhibited an average particle size of 46.3 μm. The results clarify the effect of alkaline solution composition on phase formation during carbonation and establish the conditions required to obtain a predominantly gibbsite product.