Windalco comprises two alumina plants – Ewarton Works in St. Catherine and Kirkvine Works in Manchester.
Alumina is a fine white material similar in appearance to salt.
The Bayer process dissolves the aluminum component of bauxite ore in sodium hydroxide (caustic soda); removes impurities from the solution; and precipitates alumina tri hydrate which is then calcined to aluminum oxide. A Bayer Process plant is principally a device for heating and cooling a large re circulating stream of caustic soda solution. Bauxite is added at the high temperature point, red mud is separated at an intermediate temperature, and alumina is precipitated at the low temperature point in the cycle. Bauxite usually consist of two forms of alumina - a mon hydrate form Boehmite (Al2O3.H2O) and a tri hydrate form Gibbsite (Al2O3.3H2O).
1. DIGESTION OF BAUXITE
Pisolitic, monohydrate-grade bauxite sized to a maximum of 20mm, is ground in 10 mills (each with one compartment of rods and one of balls) to allow better solid liquid contact during digestion. Recycled caustic soda solution is added to produce a pump able slurry, and lime is introduced for phosphate control and mud conditioning. Desilication:
The silica component of the bauxite is chemically attacked by caustic soda, causing alumina and soda losses by combining to form solid desilication products. To de silicate the slurry prior to digestion, it is heated and held at atmospheric pressure in pre-treatment tanks, reducing the build-up of scale in tanks and pipes. Most desilication products pass out with the mud waste as sodium aluminium silicate compounds. Digestion:
The plant has three digestion units. The monohydrate slurry is pumped by high pressure pumps through two agitated, vertical digester vessels operating in series. Mixed with steam and caustic solution, alumina in the bauxite forms a concentrated sodium aluminate solution leaving un dissolved impurities, principally inert iron and titanium oxides and silica compounds. Reaction conditions to extract the monohydrate alumina are about 250°C and a pressure about 3500 kPa, achieved by steam generated at 5000 kPa in coal-fired boilers. |
Under these conditions, the chemical reactions are rapid:-
2NaOH + Al2O3.3H2O --> 2NaAlO2 + 4H2O
2NaOH + Al2O3.H2O --> 2NaAlO2 + 2H2O
By sizing the vessel to optimum holding time, about 97% of the total available alumina is extracted and the silica content of liquor is reduced. Heat Recovery:
After digestion about 30% of the bauxite mass remains in suspension as a thin red mud slurry of silicates, and oxides of iron and titanium. The mud-laden liquor leaving the digestion vessel is flash-cooled to atmospheric boiling point by flowing through a series of flash vessels which operate at successively lower pressures. The flash steam generated is used to preheat incoming caustic liquor in tubular heat exchangers located parallel to the flash tank line. Condensate from the heat exchangers is used for boiler feed water and washing waste mud. Sweetening:
The tri hydrate bauxite has separate grinding and pre-treatment facilities. During the pass through the flash tanks, this additional bauxite slurry with high tri hydrate alumina content is injected to maximise the alumina content of the liquor stream. This occurs in the appropriate flash vessels when the slurry from the digesters has been cooled to less than 200°C. 2. CLARIFICATION OF THE LIQUOR STREAM
Most red mud waste solids are settled from the liquor stream in single deck 40 meter diameter settling tanks. Flocculants are added to the settler feed stream to improve the rate of mud settling and achieve good clarity in...