Unit 5 - Chemistry - Purifiation, extraction and manufacture of useful substances

• Aluminia is a material that retains its strength at high temperatures. 
• It is chemically and physically stable at these high temperatures. 
• It is a refractory material. 

Alumina and aluminium extraction

• Alumina is extracted from bauxite ore using Bayer process. 
• Bauxite is crushed to form grains. 
• It is then mixed with liquor from the later precipitation stage amd crushed again to make a slurry . 
• Bauxite can have a high leel of silica present so it may have to go through the descication process to remove this. 
• Hot causstic soda solutiion is used to dissolve the aluminium-bearing minerals, gibbsite, bohmite and diaspore, in the bauxite, to form a sodium aluminate supersaturated solution. 
• Conditions for this change due to the composition of the bauxide ores. 
• Those with a high gibbsite content need about 140 degree cekcius, while those with a high bohmitic content need a higher temperature between 200 and 280 degree celcius. 
• After digestion, slurry is cooled to around 106 degree celcius. 
• Then the solids are seperated from the liquids via sedimentation. 
• Chemical additives, flocculants are added to help the sedimentation process. 
• Further seperation of the pregnant liquor from the bauxite residue is performing using a series of filters to ensiure that the final product is not contaminated with impurities present in the residue. 
• Precipitation…