The Bayer process is the principal industrial means of refining bauxite to produce alumina. The general steps are to wash, digest and filter the bauxite in the hot solution of sodium hydroxide, then send the slurry to the rotary kiln for drying at high temperature to get Al₂O₃. ^[1]

The detailed process can be divided into 4 steps

STEP1: Digestion

In this process, the bauxite slurry is pumped from the holding tanks to the autoclaves (digester vessels) where it is mixed with hot concentrated caustic soda liquor. The digestion temperature and pressure depend on the mineralogical composition of the bauxite. Most bauxitic ores are composed of gibbsite (Al (OH)₃), boehmite (γ- AlO (OH)) and diaspore (α-AlO (OH)).

STEP2: Clarification：

In this step the mixture is clarified to remove impurities. Other than alumina and silica, all other components present in Bauxite do not get dissolved. The solids that are not dissolved get settled down at the bottom forming red mud. This red mud is then discarded from the solution commonly by using rotary sand trap.^[2]

STEP3 Precipitation:

Crystals of aluminum hydroxide (Al (OH) ₃) are recovered in this step: The clear liquor is pumped from the settlers to the precipitators through heat exchangers, which transfer the heat from the solution to the cold spent (processed) liquor. Precipitation of aluminum hydroxide is promoted by seeding the liquor with pure alumina crystals acting as nuclei for the precipitation process. The crystals of Al₂O₃*3H₂O grow and aggregate. The coarser particles are separated from the fine particles and transferred to calcination. The finer particles are filtered from the slurry and then used as seeding (nuclei) crystals.90% of aluminum hydrate is recovered from the liquor.

STEP4 calcination：

90 percentage of the gibbsite manufactured is converted into alumina by washing, drying, and then heating aluminum hydroxide in a rotary klins or fluid flash calciners at temperature 1010-1260 degree Celsius.

2 Al (OH) ₃→Al₂O₃+ 3 H₂O.^[3]

References：

[1] Guichon valves, manufacturer of valves, Alumina-Manufacturing process of alumina, retrieved from

http://guichon-valves.com/faqs/alumina-manufacturing-process-of-alumina/

[2] How products are made, Made how, Volume 5, aluminum, how aluminum  is made ，retrieved from              

http://www.madehow.com/Volume-5/Aluminum.html

[3] Manufacturing of alumina through Bayer process, published byData Research Analyst, Worldofchemicals.com, Retrieved from: https://www.worldofchemicals.com/591/chemistry-articles/manufacturing-of-alumina-through-bayer-process.html