Manufacturing of alumina through Bayer process
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Manufacturing of alumina through Bayer process


Alumina:

Alumina also known as Aluminium Oxide is an amphoteric oxide comprising of aluminium and oxygen. Alumina is symbolised as Al2O3. Alumina has the appearance of white solid like table salt.

Alumina has a boiling point of 2980 degree Celsius and melting point of 2040 degree Celsius. Its molar mass is 101.96 g/mol.

The Bayer Process:

Alumina is the main components of bauxite, so bauxite is refined in order to produce Alumina. The Bayer process is the principal way for producing alumina by refining Bauxite. Bauxite other than with 30-60percentage of aluminium oxide contains mixture of silica, iron oxides and titanium dioxide.

Bayer process of producing alumina can be divided into following four steps:

i. Digestion: In this step of Bayer process, bauxite ore is crushed, milled and then heated with sodium hydroxide at the temperature of around 150-200 degree Celsius. In the Bauxites, aluminium compounds are present as gibbsite (Al (OH) 3), boehmite (AlOOH) and diaspore (α-AlO (OH)).

ii. Filtration: In this step the mixture is clarified to remove impurities. Other than alumina and silica, all other components present in Bauxite do not get dissolved. The solids which are not dissolved get settled down at the bottom forming red mud. This red mud is then discarded from the solution commonly by using rotary sand trap.

The filtration process converts the aluminium oxide to soluble sodium aluminates, 2NaAlO2, as per the equation:

Al2O3 + 2NaOH → 2NaAlO2 + H2O.

i. Precipitation: The clear remaining mixture is added to precipitators by using heat exchangers, which turns the mixture from heat to cool liquor. Silica is precipitated from the mixture because heating. Crystals of aluminium hydroxide are discovered in this step. Some amount of aluminium hydroxide discovered in this step is used as a water treatment chemical.

ii. Calcination: 90 percentage of the gibbsite manufactured is converted into alumina by washing, drying, and then heating aluminium hydroxide in a rotary klins or fluid flash calciners at temperature 1010-1260 degree Celsius.

2 Al (OH) 3 → Al2O3 + 3 H2O.

More than 90 percent of alumina manufactured is used to produce aluminium by Hall-Heroult Process.

Read more: https://goo.gl/YEdrZL
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