Precipitation of fine aluminium hydroxide from Bayer liquors

The precipitation of fine aluminium hydroxide (Al(OH)3) powders (average particle size < 2 μm) from Bayer plant liquor was investigated, which is important for the production of specialty alumina (Al2O3) products. The process involves first a nucleation stage to form primary seeds by adding an acidic aluminium sulphate solution to a Bayer plant liquor which contains 210 g/L caustic (assayed as Na2CO3) and an alumina/caustic ratio (A/C) of 0.65. Further addition of the plant pregnant liquor to this primary seed slurry produced secondary gibbsite seeds required for the precipitation of high quality fine aluminium hydroxide powder. The precipitation was controlled at 50–60 °C using 0.5–1.0% w/w secondary seed addition with respect to the alumina content in the pregnant liquor.