Bauxites that have high levels of silica (SiO2) go through a process to remove this impurity. Silica can cause problems with scale formation and quality of the final product.
A hot caustic soda (NaOH) solution is used to dissolve the aluminium-bearing minerals in the bauxite (gibbsite, böhmite and diaspore) to form a sodium aluminate supersaturated solution or “pregnant liquor”.
Al(OH)3 + Na+ + OH- → Al(OH)4- + Na+
Böhmite and Diaspore:
AlO(OH) + Na+ + OH- + H2O → Al(OH)4- + Na+
Conditions within the digester (caustic concentration, temperature and pressure) are set according to the properties of the bauxite ore. Ores with a high gibbsite content can be processed at 140°C, while böhmitic bauxites require temperatures between 200 and 280°C. The pressure is not important for the process as such, but is defined by the steam saturation pressure of the process. At 240°C the pressure is approximately 3.5 MPa.
The slurry is then cooled in a series of flash tanks to around 106°C at atmospheric pressure and by flashing off steam. This steam is used to preheat spent liquor. In some high temperature digestion refineries, higher quality bauxite (trihydrate) is injected into the flash train to boost production. This "sweetening " process also reduces the energy usage per tonne of production.
Although higher temperatures are often theoretically advantageous, there are several potential disadvantages, including the possibility of oxides other than alumina dissolving into the caustic liquor.