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.