Bayer Process
The production of alumina form bauxite is carried out by the use of the Bayer Process. The process was invented in 1888 by an Austrian, Karl Joseph Bayer. Bayer discovered that when bauxite was mixed with caustic soda, the alumina content in the bauxite would dissolve and separate itself from the other components, such as iron and silica, which would remain in solid state.
Bauxite Mining
The bauxite is extracted from open pits, then hauled to a loading bay, for further transportation to the alumina plant.
Digestion/slurry mix
Bauxite received is ground with caustic soda liquor and pumped into digestion vessel where it is heated up to temperatures below 100oC and 260oC. This is where the alumina is dissolved from bauxite. The mixture of aluminate in the solution and the undissolved particles go through a heat recovery process and clarification area.
Clarification
The red mud is separated form the aluminate liquor in large settling tanks. After settling the mud is washed to recover soda and alumina and large quantities of soda recovered. The mud is then transferred to the depository.
Refiltration
The clear aluminate (overflow from the settlers), is filtered in Kelly filters where the last portions of iron and lime particles are removed.
Precipitation
The solution is pumped to the precipitation station where seed crystals are added, the solution is cooled and after a long holding time, the aluminum hydrate is precipitated out of the liquid.
Filtration
The alumina hydrate is separated from the spent liquor.
Evaporation
The spent liquor is heated with steam in a shell and tube heater. The pressure and temperature is then reduced which causes the water to evaporate. Fresh caustic is added to achieve concentration.
Calcination
The coarser particles are calcined to alumina in kilns at temperatures about 100oC. The finer particles are returned as seed to precipitation. The end product, alumina, is transported to alumina silos, before being transferred the port.