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 30–54% aluminium oxide, (alumina), Al2O3, the rest being a mixture of silica, 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 is digested by washing with a hot solution of sodium hydroxide, NaOH, at 175 °C, under pressure. 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 mixture of solid impurities is called red mud. 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 aluminum 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
Then, when heated to 980°C (calcined), the aluminium hydroxide decomposes to aluminium oxide, giving off water vapor in the process:
For bauxites having more than 10% silica, Bayer process becomes infeasible due to insoluble sodium aluminum silicate being formed, which reduces yield, and another process must be chosen.
A large amount of the aluminium oxide so produced is then subsequently smelted in the Hall–Héroult process in order to produce aluminium.
History of the Bayer process
The Bayer process was invented in 1898 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.
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- Habashi, F. (2005). "A short history of hydrometallurgy". Hydrometallurgy 79: 15–22. doi:10.1016/j.hydromet.2004.01.008.