Bayer’s Process of Concentration of Bauxite:
This process is applied when bauxite contains more Fe2O3 am less SiO2. This finely ground calcined ore is digested with 45% NaOH solution in an autoclave under 80 lbs pressure at 150°C for ~ 8 hours.
When Al2O3 and SiO2 go into solution as NaAlO2 and Na2iO3respectively. The hot solution is filtered from the insoluble impurities known as red mud. The filtrate is then diluted with water, slowly cooled, and agitated at 20-25°C for several hours with freshly precipitated hydrated alumina. When NaAlO2 undergoes hydrolysis, hydrated alumina is precipitated and Na2SiO3 remains in solution. It is filtered off, washed, and calcined at 1100°C to get pure Al2O3.
Al2O3 + 2NaOH = 2NaAlO2 + H2O
SiO2 + 2NaOH = Na2SiO3 + H2O
2NaAlO2 + 4H2O = 2Al(OH)3 + 2NaOH
Bauxite with high silica content can’t be purified by Bayer’s process since much Al is lost due to the formation of insoluble sod, alumino silicate. Bayer’s process is best adapted to bauxite containing 55-60% Al2O3 and relatively low (<7%) SiO2 content.