The Bayer process is the principal industrial means of refining bauxite to produce alumina (aluminium oxide). Bauxite, the most important ore of aluminium, contains only 30–76% [a]], various iron oxides, and titanium dioxide. The aluminium oxide must be purified before it can be refined to aluminium metal.
In the Bayer process, bauxite ore is heated in a pressure vessel along with a sodium hydroxide solution at a temperature of 150 to 200 °C. At these temperatures, the aluminium is dissolved as sodium aluminate. The aluminium compounds in the bauxite may be present as gibbsite(Al(OH)3), boehmite(AlOOH) or diaspore(AlOOH); the different forms of the aluminium component will dictate the extraction conditions. After separation of the residue by filtering, gibbsite (aluminium hydroxide) is precipitated when the liquid is cooled, and then seeded with fine-grained aluminium hydroxide. This converts the aluminium oxide in the ore to soluble sodium aluminate, 2NaAlO2, according to the chemical equation:
- Al2O3 + 2 NaOH → 2 NaAlO2 + H2O
This treatment also dissolves silica, but the other components of bauxite do not dissolve. Sometimes lime is added here, to precipitate the silica as calcium silicate. The solution is clarified by filtering off the solid impurities, commonly with a rotary sand trap, and a flocculant such as starch, to get rid of the fine particles. The undissolved waste, bauxite tailings, after the aluminium compounds are extracted contains iron oxides, silica, calcia, titania and some un-reacted alumina. Originally, the alkaline solution was cooled and treated by bubbling carbon dioxide into it, through which aluminium hydroxide precipitates:
- 2 NaAlO2 + CO2 → 2 Al(OH)3 + Na2CO3 + H2O
But later, this gave way to seeding the supersaturated solution with high-purity aluminium hydroxide (Al(OH)3) crystal, which eliminated the need for cooling the liquid and was more economically feasible:
- 2 H2O + NaAlO2 → Al(OH)3 + NaOH
Some of the aluminium hydroxide produced is used in the manufacture of water treatment chemicals such as aluminium sulfate, PAC (Poly aluminium chloride) or sodium aluminate; a significant amount is also used as a filler in rubber and plastics as a fire retardant. Some 90% of the gibbsite produced is converted into aluminium oxide, Al2O3, by heating in rotary kilns or fluid flash calciners to a temperature in excess of 1000 °C.
For bauxites having more than 10% silica, the Bayer process becomes uneconomic due to insoluble sodium aluminium silicate being formed, which reduces yield, and another process must be chosen.
Over 90% of the aluminium oxide so produced is used in the Hall–Héroult process to produce aluminium.
History of the Bayer process
The Bayer process was invented in 1888 by Carl Josef Bayer. Working in Saint Petersburg, Russia to develop a method for supplying alumina to the textile industry (it was used as a mordant in dyeing cotton), Bayer discovered in 1887 that the aluminium hydroxide that precipitated from alkaline solution was crystalline and could be easily filtered and washed, while that precipitated from acid medium by neutralization was gelatinous and difficult to wash.
A few years earlier, Henri Étienne Sainte-Claire Deville in France developed a method for making alumina by heating bauxite in sodium carbonate, Na2CO3, at 1200 °C, leaching the sodium aluminate formed with water, then precipitating aluminium hydroxide by carbon dioxide, CO2, which was then filtered and dried. This process (known as the Deville process) was abandoned in favor of the Bayer process.
The process began to gain importance in metallurgy together with the invention of the Hall–Héroult electrolytic aluminium process, invented just one year earlier in 1886. Together with the cyanidation process invented in 1887, the Bayer process marks the birth of the modern field of hydrometallurgy.
Today, the process is virtually unchanged and it produces nearly all the world's alumina supply as an intermediate step in aluminium production.
- Harris, Chris; McLachlan, R. (Rosalie); Clark, Colin (1998). Micro reform – impacts on firms: aluminium case study. Melbourne: Industry Commission. ISBN 0-646-33550-2.