The Bayer Process was developed by Karl Bayer, an Austrian Chemist, in 1887. This allowed the cheap production of alumina, and along with the Hall-Heroult process reduced the cost of aluminium by 80%.
The process itself is based around extracting aluminium hydrate from bauxite using caustic soda. The aluminium hydrate (commonly referred to as hydrate within the industry) is then precipitation out where it is filtered, and calcined to remove the free and crystallized water and form aluminium oxide (alumina).
Alumina refineries are generally split into 4 areas:
- Digestion – This is the first area where bauxite is milled to an appropriate size, mixed with recycled caustic soda, and heated to 150 degrees to extract as much hydrate as possible. The slurry is also cooled to increase its supersaturation and improve the downstream processes.
- Clarification – This area is used to remove the solid iron remaining in the slurry, to leave a relatively pure solution of caustic and hydrate. The iron product commonly called red mud due to its colour is then washed to recover as much caustic soda as possible before it is disposed of in large red mud ponds.
- Precipitation – The solution is cooled down to encourage crystalization within many large tanks. This can take over 24 hours to maximize yield. The slurry is then classified based on particle size and filtered, where the coarsest product is sent to calcination, and the finer particles are recycled back to precipitation as crystal seed to improve yield.
- Calcination – The hydrate product is heated to approximately ~1000 degrees C to remove the free and crystallized water and form alumina. This is then sent to smelters to produce alumina.
All the other areas within the refinery are used to improve yield and reduce energy consumption:
- Evaporation – due to the open nature of most refineries significant quantities of water are added to the system diluting out the caustic soda. This is removed by vaporizing the filtrate, where the condensate is recycled back to the boilers to be re-used as steam to heat the digestion stream.
- Security filtration – used to ensure the stream to precipitation is free of solids, particularly iron, to protect product quality.
Alumina refineries are very complex in their nature, and a huge amount of potential still exists for improvement. It is very energy intensive, and uses a lot of caustic soda which significantly increases costs. Due to the significant amount of equipment required to undergo this process the use of chemical engineers is numerous to improve individual equipment performance, plan equipment downtime, and identify synergy between systems.