Explain Bayer’s process of concentration of bauxite with chemical equation

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.