FACTORS AFFECTING GIBBSITE PRECIPITATION RATE

By lowering the impurities levels in the liquor with processes such as causticisation and liquor burner, the impurities can be replaced by caustic and while maintaining the alumina supersaturation constant, the yield and production can be increased. However, the precipitation rate, the major factor of yield/production depending on liquor composition, varies with impurity concentration. Well known is the increase of precipitation rate with decreased organics concentration. Less well known is the impact of inorganic anions, such as carbonate, sulphate and chloride, on precipitation rate.
The impact of carbonate, sulphate, chloride and organics on precipitation rate is systematically investigated focussing on two gibbsite precipitation mechanism, birth/spread at high growth rate and spiral growth at moderate growth rate. The ionic strength and alumina supersaturation were kept constant to make comparison possible.
The investigation shows the precipitation rate decrease with decreased concentration of carbonate, sulphate and chloride and the precipitation rate increases when the organic concentration is decreased. Decrease of carbonate, sulphate and chloride by 10 g/L, decrease the precipitation rate between 1 and 6%, while a decrease of 10 g/L organics increase the precipitation rate up to 2%. The main cause for the decreased precipitation rate of inorganic anions is the increased hydroxide concentration, which is a consequence of increased caustic concentration to keep the total soda constant. However, the precipitation rate is independent on type of anion when the impurity is expressed in ionic strength, mol/L.