Alumina is used for the production of aluminium metal, through the Hall–Héroult electrochemical smelting process.
It is also used in applications such as industrial and medical ceramics, sandpapers, pigments, cosmetics and pharmaceuticals.
The Bayer Process is the most economic means of obtaining alumina from bauxite. Other processes for obtaining alumina from metal ores are also in use in some refineries, particularly in China and Russia, although these make up a relatively small percentage of global production.
The process stages are:
The bauxite is washed and crushed, reducing the particle size and increasing the available surface area for the digestion stage. Lime and “spent liquor” (caustic soda returned from the precipitation stage) are added at the mills to make a pumpable slurry.
Bauxites that have high levels of silica (SiO2) go through a process to remove this impurity. Silica can cause problems with scale formation and quality of the final product.
A hot caustic soda (NaOH) solution is used to dissolve the aluminium-bearing minerals in the bauxite (gibbsite, böhmite and diaspore) to form a sodium aluminate supersaturated solution or “pregnant liquor”.
Al(OH)3 + Na+ + OH– → Al(OH)4– + Na+
Böhmite and Diaspore:
AlO(OH) + Na+ + OH– + H2O → Al(OH)4– + Na+
Conditions within the digester (caustic concentration, temperature and pressure) are set according to the properties of the bauxite ore. Ores with a high gibbsite content can be processed at 140°C, while böhmitic bauxites require temperatures between 200 and 280°C. The pressure is not important for the process as such, but is defined by the steam saturation pressure of the process. At 240°C the pressure is approximately 3.5 MPa.
The slurry is then cooled in a series of flash tanks to around 106°C at atmospheric pressure and by flashing off steam. This steam is used to preheat spent liquor. In some high temperature digestion refineries, higher quality bauxite (trihydrate) is injected into the flash train to boost production. This “sweetening ” process also reduces the energy usage per tonne of production.
Although higher temperatures are often theoretically advantageous, there are several potential disadvantages, including the possibility of oxides other than alumina dissolving into the caustic liquor.
The first stage of clarification is to separate the solids (bauxite residue) from the pregnant liquor (sodium aluminate remains in solution) via sedimentation. Chemical additives (flocculants) are added to assist the sedimentation process. The bauxite residue sinks to the bottom of the settling tanks, then is transferred to the washing tanks, where it undergoes a series of washing stages to recover the caustic soda (which is reused in the digestion process).
Further separation of the pregnant liquor from the bauxite residue is performed utilising a series of security filters. The purpose of the security filters is to ensure that the final product is not contaminated with impurities present in the residue.
Depending on the requirements of the residue storage facility, further thickening, filtration and/or neutralisation stages are employed prior to it being pumped to the bauxite residue disposal area.
In this stage, the alumina is recovered by crystallisation from the pregnant liquor, which is supersaturated in sodium aluminate.
The crystalisation process is driven by progressive cooling of the pregnant liquor, resulting in the formation of small crystals of aluminium trihydroxite (Al(OH)3, commonly known as “hydrate”), which then grow and agglomerate to form larger crystals. The precipitation reaction is the reverse of the gibbsite dissolution reaction in the digestion stage:
Al(OH)4– + Na+ → Al(OH)3 + Na+ + OH–
The spent liquor is heated through a series of heat exchangers and subsequently cooled in a series of flash tanks. The condensate formed in the heaters is re-used in the process, for instance as boiler feed water or for washing bauxite residue. The remaining caustic soda is washed and recycled back into the digestion process.
The gibbsite crystals formed in precipitation are classified into size ranges. This is normally done using cyclones or gravity classification tanks (a series of thickeners utilising the same principles as settlers / washers on the clarification stage). The coarse size crystals are destined for calcination after being separated from spent liquor utilising vacuum filtration, where the solids are washed with hot water.
The fine crystals, after being washed to remove organic impurities, are returned to the precipitation stage as fine seed to be agglomerated.
The filter cake is fed into calciners where they are roasted at temperatures of up to 1100°C to drive off free moisture and chemically-connected water, producing alumina solids. There are different calcination technologies in use, including gas suspension calciners, fluidised bed calciners and rotary kilns.
The following equation describes the calcination reaction:
2Al(OH)3 → Al2O3 + 3H2O
Alumina, a white powder, is the product of this step and the final product of the Bayer Process, ready for shipment to aluminium smelters or the chemical industry.
The Bayer Process was invented and patented in 1887 by Austrian scientist Karl Josef Bayer.
Two to three tonnes of bauxite are required to produce one tonne of alumina.
90% of the global alumina supply of around 90 million tonnes is used in aluminium production.
Alumina refineries tend to be located close to bauxite mines and/or ports for efficient transport of raw materials and of the final product.