Bayer Process: From Bauxite to Alumina
Bayer process consists of the extraction of alumina from bauxite. The process for the production of alumina was discovered in 1888 by Karl Josef Bayer. In conjunction with the Hall–Héroult process, it provided the basis of the modern process of aluminium production.
Whereas the Hall–Héroult process is the reduction of alumina (aluminium oxide) to aluminium by electrolysis, the Bayer process is a process in which aluminium oxide is produced from bauxite through a pressure leaching operation using sodium hydroxide solution.
This aluminium hydroxide is calcined to obtain pure alumina, which is used in the electrolysis process to produce primary aluminium. For each tonne of aluminium produced, about 2 tonnes of alumina is required, which is produced from 4 tonnes of bauxite.
Prior to the Bayer process, the bauxite needs to be crushed and ground to small particles. Crude bauxite ore is crushed or ground in ball mills. This crushed bauxite is first washed to remove all the fine particles coming mainly from the clay.
After washing, the bauxite needs to be dried out to facilitate grinding. The drying is carried out in a rotary kiln at a temperature below the dehydration temperature of the corresponding aluminum hydroxides.
Bayer process involves four main steps:
- digestion,
- settling,
- precipitation, and
- calcination.
A general schema of Bayer process in the production of primary aluminum is displayed in this figure. First, the bauxite is digested with caustic soda, also called the leaching of bauxite. During this process, the alumina is dissolved in sodium hydroxide forming hydroxide aluminate, and the insoluble particles form a red mud.
After this step, bauxite residue is separated from the aluminum-containing liquor by a process known as settling or clarification. Then, a precipitation process of the pure aluminum hydroxide from the solution is carried out.
Finally, to obtain alumina from hydroxide, a calcination step is performed in rotary kilns or in fluidized beds to dehydrate the aluminum hydrate crystals, and the liquor evaporates and it is recycled for leaching. The calcination can be expressed by the equation shown in the figure, where aluminium hydroxide is heated to about 1000 ⁰C and the product is 99,9% pure alumina.
Source:
Bayer Process / Diego Lozano, Gabriela M. Martinez-Cazares, Miguel Bocanegra //Encyclopedia of Aluminium and Its Alloys, 2018