Aluminium Hydroxide Manufacturing Process - Caustic Soda in Aluminum Extraction

The Bayer Process and the Role of Caustic Soda

The Bayer Process is an industrial method used to extract alumina (aluminum oxide) from bauxite ore. Alumina is the primary raw material for aluminum production. This process was developed by Karl Joseph Bayer in the late 19th century. Here are the main steps of the Bayer Process in detail:

Crushing and Grinding of Bauxite

– Bauxite ore is extracted from the mine and transported to the plant. In the first step, the bauxite is crushed into smaller pieces to increase its surface area for the caustic soda solution. The crushed bauxite is then ground into a powder.

Dissolution of Bauxite in Caustic Soda Solution

The bauxite powder is mixed with a caustic soda solution at high temperatures (150 to 200°C) and high pressure (4 to 7 bar) in autoclave reactors. In this step, caustic soda reacts with the aluminum in the bauxite to form sodium aluminate (NaAlO2). The primary chemical reaction is as follows:

3Al(OH)3​+NaOH→Na[Al(OH)4​

Clarification

The resulting solution from digestion contains dissolved sodium aluminate and undissolved solid residues (known as red mud). This mixture is transferred to settling tanks. The mixture separates into solid phases (bauxite residue) and liquid (sodium aluminate solution). Filtration separates the solid from the liquid.

Precipitation

The clarified sodium aluminate solution is transferred to precipitation tanks. Here, by lowering the temperature and using aluminum hydroxide seed crystals, dissolved alumina precipitates as aluminum hydroxide (Al(OH)3):

Na[Al(OH)4​]→3Al(OH)3​+NaOH

Filtration and Washing

The precipitated aluminum hydroxide is separated from the solution and washed to remove any remaining caustic soda.

Calcination

The aluminum hydroxide is transferred to calcination kilns and heated at about 1000 to 1100°C to remove water and produce pure alumina:

2Al(OH)3​→Al2​O3​+3H2​O