Page 16 - Curriculum Visions Dynamic Book

Concentrating bauxite ore
Bauxite is a red rock-like material. It consists
of aluminium oxide and a wide range of unwanted substances. To produce aluminium metal, the
ore first has to be concentrated, thus removing the bulk of the impurities. Then it goes to a refinery, where the pure metal is produced. The concentrating stage is called the Bayer process.
The Bayer process
Chemical reactions work most efficiently if the reactants have a large surface area. In the case of an ore, chemical reactions are made faster by pulverising the ore into a powder. Bauxite powder is mixed with crushed sodium hydroxide (soda ash) and calcium oxide (lime) and then mixed with water.
The chemical reaction takes place at high temperatures and pressures, making the aluminium oxide soluble (as sodium aluminate), so that it can be drained off into settling tanks. Here, any solids, such as pieces of sand and iron oxide, settle out. The alumina is a light brown liquid.
This liquid is drawn off and pumped into tall vats where it is allowed to cool. Inside the vats the liquid is stirred and tiny crystals of alumina form. The stirring causes
the crystals to stick to each other until they are about the size of grains of sugar, then they sink to the bottom. The crystals are taken away and washed clear of any remaining liquid. They are then heat-dried, and the resulting alumina is a grey powder.
EQUATION: Production of alumina
(i) dissolving bauxite in sodium hydroxide
Bauxite + sodium hydroxide + water ➪
Bauxite ore.
Al2O3(ore) + 2NaOH(aq) + 3H2O ➪ 2NaAl(OH)4(aq)+impurities
red mud
(ii) recovering the sodium hydroxide to leave alumina
Sodium aluminate ➪ alumina + sodium hydroxide 2NaAl(OH)4(aq) ➪ Al2O3•3H2O(s) + 2NaOH(aq)
Crushers grinds the bauxite ore to a fine powder.
The pulverised bauxite is mixed with sodium hydroxide (NaOH)
at high pressure and temperature.
sodium aluminate
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