First, the bauxite ore is crushed mechanically. Then the slurry is produced by mixing it with caustic soda in a grinding mill. Then the slurry is pumped into a digester under pressure of 340Kpa and heated to 110-270C. This process takes half an hour to several hours. To be sure all aluminium compounds are dissolved, additional caustic soda may be added.
The hot slurry is passed through a flash tank to lower the pressure and recover heat. Then it is pumped into a clarification tank where impurities settle to the bottom. The residue (called “red mud”) that settles in the bottom of the tank consists of fine sand, iron oxide and oxide of other elements.
After impurities have settled out, the remaining liquid is pumped through a series of cloths filters to be sure no impurities remain. The seed crystals of alumina hydrate are added to a series of precipitation tanks where they attach to dissolved alumina and precipitate. Then the crystals are removed and transferred in to a kiln. Finally, the washed crystals are heated to remove water and leave only dry white alumina crystals.
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