ALUMINIUM PROCESS

Bayer process chemistry

Extraction

The aluminium-bearing minerals in bauxite - Gibbsite, Böhmite and Diaspore - are selectively extracted from the insoluble components (mostly oxides) by dissolving them in a solution of sodium hydroxide (caustic soda):

Gibbsite:

Al(OH)3 + Na+ + OH- ---> Al(OH)4- + Na+

Böhmite and Diaspore:

AlO(OH) + Na+ + OH - + H2O ---> Al(OH)4- + Na+

Depending on the quality of the ore it may be washed to beneficiate it prior to processing. The ore is crushed and milled to reduce the particle size and make the minerals more available for extraction. It is then combined with the process liquor and sent in a slurry to a heated pressure digester.

Conditions within the digester (concentration, temperature and pressure) are set according to the properties of the bauxite ore. Ores with a high Gibbsite content can be processed at 140oC. Processing of Böhmite on the other hand requires between 200 and 240°c. The pressure is not important for the process, as such but is defined by the steam pressure during the actual process conditions. At 240°c tl the pressure is approximately 35 atmospheres (atm).

Although higher temperatures are often theoretically advantageous there are several disadvantages including corrosion problems and the possibility of oxides other than alumina dissolving into the caustic liquor.

Clarification – Along with the supersaturated caustic solution, there is a large amount of impurities from the bauxite, which is removed through the use of settling tanks and filtration vessels. Some of the caustic solution will be lost here, and can be recovered by washing the bauxite waste (aka red mud) with caustic soda. The red mud is then sent to a waste management area for disposal, while the supersaturated solution contains through the plan

After the extraction stage the insoluble bauxite residue must be separated from the Aluminium-containing liquor by a process known as settling. The liquor is purified as much as possible through filters before being transferred to the precipitators. The insoluble mud from the first settling stage is thickened and washed to recover the caustic soda, which is then recycled back into the main process. 

Precipitation

Crystalline aluminium trihydroxide (Gibbsite), conveniently named "hydrate", is then precipitated from the digestion liquor:

Al(OH)4- + Na+ ---> Al(OH)3 + Na+ + OH-

This is basically the reverse of the extraction process, except that the product's nature is carefully controlled by plant conditions, including seeding or selective nucleation, precipitation temperature and cooling rate. The "hydrate" crystals are then classified into size fractions and fed into a rotary or fluidised bed calcination kiln. Undersize particles are fed back into the precipitation stage.

Calcination

"Hydrate", is calcined to form alumina for the aluminium smelting process. In the calcination process water is driven off to form alumina:

2Al(OH)3 ---> Al2O3 + 3H2O

The calcination process must be carefully controlled since it dictates the properties of the final product.
FLOW DIAGRAM:

Environmental problems in mining and transporting the bauxite

Think about:

• Loss of landscape due to mining, processing and transporting the bauxite.
• Noise and air pollution
  
  
  USES:
  
  

  aluminium is used for	because
  aircraft	light, strong, resists corrosion
  other transport such as ships' superstructures, container vehicle bodies, tube trains (metro trains)	light, strong, resists corrosion
  overhead power cables (with a steel core to strengthen them)	light, resists corrosion, good conductor of electricity
  saucepans	light, resists corrosion, good appearance, good conductor of heat