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 30–60% "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.
Bauxite ore is a mixture of hydrated aluminium oxides and compounds of other elements such as iron. In the Bayer process, bauxite ore is heated in a "pressure vessel along with a "sodium hydroxide solution (caustic soda) at a temperature of 150 to 200 °C. At these temperatures, the "aluminium is dissolved as "sodium aluminate in an extraction process. 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.
The extraction process converts the aluminium oxide in the ore to soluble sodium aluminate, 2NaAlO2, according to the "chemical equation:
This treatment also dissolves silica, but the other components of bauxite do not dissolve. Sometimes "lime is added at this stage to precipitate the silica as "calcium silicate. The solution is clarified by filtering off the solid impurities, commonly with a rotary sand trap and with the aid of a "flocculant such as "starch, to remove the fine particles. The undissolved waste after the aluminium compounds are extracted, "bauxite tailings, contains "iron oxides, "silica, "calcia, "titania and some unreacted alumina. The original process was that the "alkaline solution was cooled and treated by bubbling carbon dioxide through it, a method by which aluminium hydroxide "precipitates:
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:
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.
The left-over or 'spent' sodium aluminate solution is then recycled. This, however, allows "gallium and "vanadium impurities to build up in the liquors, so these can be extracted. The organic impurities on the precipitation of gibbsite can cause high gibbsite impurity content, liquor and gibbsite colouration, caustic losses and an increase in the solutions viscosity and density.
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.
1.9-3.6 tons of bauxite is required to produce 1 ton of aluminum oxide. This is due to a majority of the aluminum in the ore is dissolved in the process. Over 90% (95-96%) of the aluminium oxide produced is used in the "Hall–Héroult process to produce aluminium.
Red mud is the waste product that is produced in the digestion of bauxite with sodium hydroxide. It has high calcium and sodium hydroxide content which makes a complex chemical composition. This makes it very toxic and a possible source of pollution. The amount produced during the process is nothing to overlook. The amount warranted scientists and refiners to find a use for it, which turns out is good for ceramics. Red mud dries into a fine powder that contains iron, aluminum, calcium and sodium. It becomes a health risk when Some plants use the waste to produce aluminum oxides.
In the United States, the waste is disposed in large "impoundments, a sort of reservoir created by a dam. The impoundments are typically lined with clay or synthetic liners. The US does not approve of the use of the waste due to the dangers it risks to the environment. The EPA identified high levels of arsenic and chromium in some red mud samples.
On October 4, 2010, the Ajka alumina plant in Hungary had an "incident where the western dam of its red mud reservoir collapsed. The reservoir was filled with 700,000 m3 of a mixture of red mud and water with a pH of 12. The mixture was released into the valley of Torna river and flooded parts of the city of Devecser and the villages of Kolontár and Somlóvásárhely. The incident resulted in 10 deaths, more than a hundred injuries, and contamination in lakes and rivers.
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. The industrial success of this process caused it to replace the Le Chatelier process which was used to produce alumina from bauxite.
The engineering aspects of the process were improved upon to decrease the cost starting in 1967 in "Germany and "Czechoslovakia. This was done by increasing the heat recovery and using large "autoclaves and precipitation tanks. To more effectively use energy, "heat exchangers and flash tanks were used and larger reactors decreased the amount of heat lost. Efficiency was increased by connecting the autoclaves to make operation more efficient.
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 produces nearly all the world's alumina supply as an intermediate step in aluminium production.