- Pages: 6
- Word count: 1278
- Category: Business
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West Indies Alumina Company (WINDALCO), formerly Jamalcan, is a joint venture between the UC Rusal and the Government of Jamaica. UC Rusal is the majority shareholder (93% stake) and manages the joint venture. The Government of Jamaica owns the remaining 7% share. 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 the overflow liquor. Washers:
The mud is washed with fresh water in counter-current washing trains to recover the soda and alumina content in the mud before being pumped to large disposal dams on Boyne Island. Slaked lime is added to dilute caustic liquor in the washing process to remove carbonate (Na2CO3) which forms by reaction with compounds in bauxite and also from the atmosphere and which reduces the effectiveness of liquor to dissolve alumina. Lime regenerates caustic soda, allowing the insoluble calcium carbonate to be removed with the waste mud.
Na2CO3 + Ca(OH)2 –> CaCO3 + 2NaOH
Settlers overflow liquor containing traces of fine mud is filtered in Kelly-type constant pressure filters using polypropylene filter cloth. Slaked lime slurry is used to produce a filter cake. Mud particles are held on the filter leaves for removal and treatment in the mud washers when filters are sequentially taken off line. Heat Interchange:
With all solids removed, the pregnant liquor leaving the filter area, contains alumina in clear supersaturated solution. It is cooled by flash evaporation, the steam given off being used to heat spent liquor returning to digestion. 3. PRECIPITATION OF ALUMINA HYDRATE
Dissolved alumina is recovered from the liquor by precipitation of crystals.
Alumina precipitates as the tri hydrate Al2O3 .3H2O in a reaction which is the reverse of the digestion of tri hydrate – 2NaAlO2 + 4H2O –> Al2O3.3H2O + 2 NaOH
The cooled pregnant liquor flows to rows of precipitation tanks which are seeded with crystalline tri hydrate alumina, usually of an intermediate or fine particle size to promote crystal growth. Each precipitation tank is agitated, with a holding time of about three hours. During the 25-30 hours pass through precipitation, alumina of various crystal sizes is produced. The entry temperature and the temperature gradient across the row, seed rate and caustic concentration are control variables used to achieve the required particle size distribution in the product. As correct particle size is important to smelter operations, sizing is carefully controlled. The QAL precipitation plant was designed to operate on a continuous basis to produce “sandy” or coarse alumina. Classification:
The finished mix of crystal sizes is settled from the liquor stream and separated into three size ranges in three stages “gravity” classification tanks. The primary classifiers collect the coarse fraction which becomes the product hydrate. The intermediate and fine crystals from the secondary and tertiary classifiers are washed and returned to the precipitation tanks as seed. Spent Liquor:
Spent caustic liquor essentially free from solid overflows from the tertiary classifiers and is returned through an evaporation stage where it is re concentrated, heated and recycled to dissolve more alumina in the digesters. Fresh caustic soda is added to the stream to make up for process losses. 4. CALCINATION OF ALUMINA
A slurry of coarse hydrate (Al2O3.3H2O) from the primary thickeners is pumped to hydrate storage tanks and is filtered and washed on horizontal-table vacuum filters to remove process liquor. Calcining:
The resulting filter cake is fed to a series of calcining units – an 1800 tonnes a day circulating fluidised bed calciner or one of nine rotary kilns each 100m long and 4m in diameter. The feed material is calcined to remove both free moisture and chemically-combined water. Firing-zone temperatures above 1100°C are used, achieved by firing with natural gas. The circulating fluidised bed calciner is more energy efficient than the older rotary kilns. Product sandy alumina particles are 90%+ 45 µm (microns) in size. Cooling:
Rotary or satellite coolers are used to cool the calcined alumina from the rotary kilns, and to pre-heat secondary combustion air for the kilns. Fluidised-bed coolers further reduce alumina temperature to less than 90°C before it is discharged on to conveyor belts which carry it to storage buildings where it is stockpiled for shipment.