Bayer process
Encyclopedia
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% alumina, Al2O3, the rest being a mixture of silica, various iron oxide
s, and titanium dioxide
. The alumina 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. This converts the alumina to aluminium hydroxide
, Al(OH)3, which dissolves in the hydroxide solution according to the chemical equation
:
The other components of bauxite do not dissolve. The solution is clarified by filtering off the solid impurities. 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
as a white, fluffy solid. Then, when heated to 980°C (calcined), the aluminium hydroxide decomposes to alumina, giving off water vapor in the process:
A large amount of the alumina so produced is then subsequently smelted in the Hall–Héroult process in order to produce aluminium.
. 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, 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.
Bauxite
Bauxite is an aluminium ore and is the main source of aluminium. This form of rock consists mostly of the minerals gibbsite Al3, boehmite γ-AlO, and diaspore α-AlO, in a mixture with the two iron oxides goethite and hematite, the clay mineral kaolinite, and small amounts of anatase TiO2...
to produce alumina (aluminium oxide).
Bauxite, the most important ore of aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
, contains only 30–54% alumina, Al2O3, the rest being a mixture of silica, various iron oxide
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
s, and titanium dioxide
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
. The alumina 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. This converts the alumina to aluminium hydroxide
Aluminium hydroxide
Aluminium hydroxide, Al3, ATH, sometimes erroneously called Hydrate of alumina, is found in nature as the mineral gibbsite and its three, much more rare forms, polymorphs: bayerite, doyleite and nordstrandite. Closely related are aluminium oxide hydroxide, AlO, and aluminium oxide, Al2O3,...
, Al(OH)3, which dissolves in the hydroxide solution according to the chemical equation
Chemical equation
A chemical equation is the symbolic representation of a chemical reaction where the reactant entities are given on the left hand side and the product entities on the right hand side. The coefficients next to the symbols and formulae of entities are the absolute values of the stoichiometric numbers...
:
- Al2O3 + 2 OH−HydroxideHydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
+ 3 H2OWaterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
→ 2 [Al(OH)4]−
The other components of bauxite do not dissolve. The solution is clarified by filtering off the solid impurities. The mixture of solid impurities is called red mud
Red mud
Red mud is a solid waste product of the Bayer process, the principal industrial means of refining bauxite in order to provide alumina as raw material for the electrolysis of aluminium by the Hall–Héroult process. A typical plant produces one to two times as much red mud as alumina...
, and presents a disposal problem. Next, the hydroxide solution is cooled, and the dissolved aluminium hydroxide precipitates
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
as a white, fluffy solid. Then, when heated to 980°C (calcined), the aluminium hydroxide decomposes to alumina, giving off water vapor in the process:
- 2 Al(OH)3Aluminium hydroxideAluminium hydroxide, Al3, ATH, sometimes erroneously called Hydrate of alumina, is found in nature as the mineral gibbsite and its three, much more rare forms, polymorphs: bayerite, doyleite and nordstrandite. Closely related are aluminium oxide hydroxide, AlO, and aluminium oxide, Al2O3,...
→ Al2O3 + 3 H2OWaterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
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 Karl BayerKarl Bayer
Karl Josef Bayer was an Austrian chemist who invented the Bayer process of extracting alumina from bauxite, essential to this day to the economical production of aluminium....
. Working in Saint Petersburg, Russia to develop a method for supplying alumina to the textile industry (it was used as a mordant
Mordant
A mordant is a substance used to set dyes on fabrics or tissue sections by forming a coordination complex with the dye which then attaches to the fabric or tissue. It may be used for dyeing fabrics, or for intensifying stains in cell or tissue preparations. The term mordant comes from the Latin...
in dyeing cotton), Bayer discovered in 1887 that the aluminium hydroxide
Aluminium hydroxide
Aluminium hydroxide, Al3, ATH, sometimes erroneously called Hydrate of alumina, is found in nature as the mineral gibbsite and its three, much more rare forms, polymorphs: bayerite, doyleite and nordstrandite. Closely related are aluminium oxide hydroxide, AlO, and aluminium oxide, Al2O3,...
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, Louis Le Chatelier
Louis Le Chatelier
Louis Le Chatelier was a French chemist and industrialist who developed a method for producing aluminium from bauxite in 1855. The method was later superseded by the Bayer process. His son was the well known chemist Henri Louis Le Chatelier. His name is inscribed on the Eiffel tower.-References:...
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
Aluminium hydroxide
Aluminium hydroxide, Al3, ATH, sometimes erroneously called Hydrate of alumina, is found in nature as the mineral gibbsite and its three, much more rare forms, polymorphs: bayerite, doyleite and nordstrandite. Closely related are aluminium oxide hydroxide, AlO, and aluminium oxide, Al2O3,...
by carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, 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
Hydrometallurgy
Hydrometallurgy is part of the field of extractive metallurgy involving the use of aqueous chemistry for the recovery of metals from ores, concentrates, and recycled or residual materials...
.
Today, the process is virtually unchanged and it produces nearly all the world's alumina supply as an intermediate in aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
production.
See also
- Hall–Héroult process
- Aluminium separation