Generally speaking, the chemical process is known as the Bayer process. It is used to turn mined Bauxite ore into alumina.
The fist stage is known as Digestion. The Bauxite is ground in mills and then slurried up with hot, caustic soda solution. this dissolves the Alumina that is contained in the Bauxite ore. Other impurities found in the Bauxite such as silica, iron and titanium compounds are not dissolved.
The next stage is known as Clarification. The Bauxite ore and Caustic soda slurry are passed into rows of Settler tanks (also known as thickeners or clarifiers). Usually with the help of a diesel based polymer known as flocculant and a defoamer, the mud and impurities will settle to the bottom of the Thickener, leaving a clear alumina charged 'pregnant liquor' overflow. This is further filtered and passed on to the next stage in the process known as Precipitation.
During Precipitation, the liquor is cooled, concentrated and stirred in open top tanks until it forms crystals. Pure alumina is added to the mixture to encourage alumina trihydrate crystals to form. It is then moved along to the next stage of the process.
In Calcination, the alumina trihydrate crystals are washed, filtered and heated in gas fired kilns (usually at temperatures exceeding 1 100 degress celcius). This removes the 3 molecules of water from the alumina trihydrate. The remaining fine white powder is known as alumina. It is then typically cooled and stored until it is ready to be sent to a Smelting facility to be made into Aluminium.
Typically, you will use 2 tonnes of Bauxite to produce 1 tonne of Alumina. There are a number of side streams of the process but this is just a very brief overview..