Bauxite ore is the primary source of aluminum hydroxide used in the
Bayer Process. The chemical components of bauxite can be present in
one of three polymorphs of aluminum – gibbsite, boehmite or
diaspore. The mineral of interest is gibbsite, and bauxite deposits
with higher compositions of gibbsite are preferred. After
extraction of gibbsite from bauxite ore, aluminum hydroxide can be
obtained as a fine-grained powder by seeding the solution.
The extraction process to obtain aluminum hydroxide from bauxite
can be detailed as follows:
Step 1: Digestion of bauxite ore
First, the bauxite ore is crushed and heated with sodium hydroxide
at temperatures between 150 and 200 °C. The ‘digested’ bauxite ore
is easier to process.
Step 2: Filtration
In this process, the aluminum oxide from bauxite ore is converted
to sodium aluminate. Filtration enables the separation of soluble
sodium aluminate from insoluble impurities present in the bauxite
ore. Rotary sand traps and flocculants such as starch are used to
discard the solid impurities. The undissolved waste mostly consists
of iron oxides, calcia, silica and unreacted alumina.
The governing equation for the process is:
Al2O3 + 2NaOH = 2NaAlO2 + H2O
Step 3: Seeding
In the final step of this process, the supersaturated solution of
aluminium hydroxide and sodium aluminates is seeded with highly
pure aluminium hydroxide crystals to facilitate the precipitation
of Al(OH)3 crystals. This is an economical process since the heated
solution doesn’t need to be cooled further prior to extraction.
The chemical reaction can be illustrated as follows:
2H2O + NaAlO2 = Al(OH)3 + NaOH
The aluminum hydroxide produced is either converted into aluminium
oxide, a starting reagent for various processes, or used in the
manufacture of other industrially useful chemicals such as
aluminium sulfate and sodium aluminate.
