Alumina extraction

—> alumina retains its strength at high temperatures

—> it’s chemically and physically stable at these high temperatures

—> can be used in linings for furnaces, kilns and reactors 

Key terms : 

Refractory material : material that is physically and chemically stable at very high temperatures, for example, over 3000°C 

—> alumina is extracted from bauxite ore using the Bayer process

1. Bauxite is crushed to form grains

2. Mixed with liquor and crushed again to make a slurry

3. Bauxite contains silica - is removed through a desilication process

4. Hot Na(OH) (caustic soda) is added to dissolve the aluminium bearing minerals : gibbsite, bohmite and diaspore (all present in the bauxite)

• forms a sodium aluminate supersaturated solution
• process called - digestion

—> change based on the composition of the bauxite ores

—> high gibbsite content : 140°C

—> high bohimtic content : between 200-280°C

—> the slurry is cooled to 106°C

—> it’s then clarified to desperate the solids from the liquor sedimentation

—> chemical additives, flocculant are added to aid the sedimentation process

—> the bauxite residue is transferred to the wash tanks to collect the caustic soda (hot NaOH used in the digestion process) 

Key terms : 

Flocculants : substance that causes particles to change and to settle out of a liquid. 

Sedimentation : small solid particles settling at the bottom of a liquid 

—> the liquor is further separated from the bauxite residue through a series of filters to ensure all products

—> the liquor is supersaturated with alumina - is recovers through crystallisation

• the liquor is cooled resulting in the formation of small crystals of aluminium trihydroxide (which will grow to form larger crystals)
• the remaining liquor is heated through a series of heat exchangers and cooled in a series of flash tanks - the condensate is reused in the process

—> these crystals formed during percipitation (crystallisation) are classified in to size using gravity classification tanks

—> filter cakes are fed into the calciners where they are heated to temperatures up to 1100°C to drive off moisture to produce solid alumina 

—> Calcination reaction : Al(OH)3 -> Al2O3 + H2O 

Key terms : 

Calcination : heating to high temperature to remove free and chemically bonded water