The effect of high concentrations of calcium hydroxide in neutralised synthetic supernatant liquor—Implications for alumina refinery residues

The presence of calcium hydroxide in Bayer residue slurry inhibits the effectiveness of seawater neutralisation. An increase in the slurry pH, after neutralisation, is caused by the dissolution of calcium hydroxide and hydrocalumite (both components are found in the solid residue). At increased calcium hydroxide concentrations, additional hydrocalumite forms. Hydrocalumite has been found to be unstable when the solution pH is less than 10.5, releasing OH− ions and aluminium ions back into solution (pH and aluminium reversion). Through this mechanism, the concentration of calcium hydroxide in alumina refinery residues has an impact on seawater neutralisation process efficiency.