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Land Utilization                  Bauxite Mining                Ore Processing                 Materials

There are a number of methods of refining bauxite to alumina.  The most common is a chemical process known as the Bayer Process (see process flow diagram), named after an Austrian chemist who invented it in 1889. It requires approximately three tons of wet bauxite to produce one ton of alumina. The process is essentially a simple one but, unfortunately it is complicated in its application. 
 

In the first stage of the Bayer Process the bauxite is mixed with liquid caustic soda, grinded and pumped as a slurry into digesters.   In the digesters, the dissolution of alumina in caustic liquor takes place under high temperatures and pressures. The resulting mixture of dissolved alumina, in the form of sodium aluminate and the undissolved residue, 'red mud', is first allowed to separate in settling tanks. The major constituents of the residue are iron (which gives the residue its characteristic colour), titanium and silica. The partially clear liquid containing the dissolved alumina is then finally clarified by filtering under pressure, through heavy canvas in filter presses. The red mud residue is washed to remove chemicals and stored in ponds or drying beds.

After clarification, the now clear, strong, tea-coloured solution is pumped into precipitator tanks. These may rise to a height of about 30 metres; each holding upward of 4.5 million litres.

The precipitation of alumina trihydrate takes place upon addition of seed (in the form of previously precipitated material). The mixture of solution and precipitating hydrate passes through several tanks in series to provide sufficient time for acceptable yields. After precipitation, the hydrate is classified into seed and product, the seed being recycled to the first tanks, while the product is washed and de-watered in preparation for the calcination process. At this stage in the process, the alumina trihydrate is a damp cream-coloured substance.

The hydrate is fed into brick-lined rotating kilns where it is heated to about 1000^oC to remove the chemically combined water. The heating process takes place while the kiln is revolving, exposing the hydrate crystals to the combustion gases, thus ensuring maximum uniformity of the product. When it emerges from the lower end of the kilns, the dehydrated alumina is cooled before being taken, by belt conveyors, to storage bins. The finished product is a fine white powder somewhat resembling table salt.

Red Mud disposal area

Precipitators

Calcination Area, including
rotary kilns