Bayer process

The Bayer process is the principal industrial means of refining bauxite to produce alumina.

Bauxite, the most important ore of aluminium, contains only 30-54% alumina, Al2O3, the rest being a mixture of silica, various iron oxides, and titanium dioxide. The alumina must be purified before it can be refined to aluminium metal. In the Bayer process, bauxite is washed with a hot solution of sodium hydroxide, NaOH, at 175°C (called digestion). This converts the alumina to aluminium hydroxide, Al(OH)3, which dissolves in the hydroxide solution according to the chemical equation


 * Al2O3 + 2 OH- + 3 H2O → 2 [Al(OH)4]-

The other components of bauxite do not dissolve and are filtered from the solution as solid impurities (clarification). The mixture of solid impurities is called red mud, and presents a disposal problem. Next, the hydroxide solution is cooled, and the dissolved aluminium hydroxide precipitates out as a white, fluffy solid (precipitation). When then heated to 1050°C, the aluminium hydroxide decomposes to alumina (calcination), giving off water vapor in the process:


 * 2 Al(OH)3 → Al2O3 + 3 H2O

A large amount of the alumina so produced is then subsequently smelted in the Hall-Héroult process in order to produce aluminium.

History
The Bayer process was invented in 1887 by the Austrian chemist Karl 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, Henry Louis Le Chatelier 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 was abandoned in favor of the Bayer process.

The process began to gain importance in metallurgy together with the invention of the electrolytic aluminium process invented 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 in aluminium production.