West Indies Alumina Company ( WINDALCO ) . once Jamalcan. is a joint venture between the UC Rusal and the Government of Jamaica. UC Rusal is the bulk stockholder ( 93 % interest ) and manages the joint venture. The Government of Jamaica owns the staying 7 % portion. Windalco comprises two aluminum oxide workss – Ewarton Works in St. Catherine and Kirkvine Works in Manchester. Alumina is a all right white stuff similar in visual aspect to salt.
The Bayer procedure dissolves the aluminium constituent of bauxite ore in Na hydrated oxide ( acerb sodium carbonate ) ; removes drosss from the solution ; and precipitates alumina tri hydrate which is so calcined to aluminum oxide. A Bayer Process works is chiefly a device for warming and chilling a big rhenium go arounding watercourse of acerb sodium carbonate solution. Bauxite is added at the high temperature point. ruddy clay is separated at an intermediate temperature. and aluminum oxide is precipitated at the low temperature point in the rhythm. Bauxite normally consist of two signifiers of aluminum oxide – a Monday hydrate signifier Boehmite ( Al2O3. H2O ) and a tri hydrate signifier Gibbsite ( Al2O3. 3H2O ) .
1. Digestion OF BAUXITE
Pisolitic. monohydrate-grade bauxite sized to a upper limit of 20mm. is land in 10 Millss ( each with one compartment of rods and one of balls ) to let better solid liquid contact during digestion. Recycled acerb sodium carbonate solution is added to bring forth a pump able slurry. and calcium hydroxide is introduced for phosphate control and clay conditioning. Desilication:
The silica constituent of the bauxite is chemically attacked by acerb sodium carbonate. doing aluminum oxide and sodium carbonate losingss by uniting to organize solid desilication merchandises. To de silicate the slurry prior to digestion. it is heated and held at atmospheric force per unit area in pre-treatment armored combat vehicles. cut downing the build-up of graduated table in armored combat vehicles and pipes. Most desilication merchandises pass out with the clay waste as Na aluminum silicate compounds. Digestion:
The works has three digestion units. The monohydrate slurry is pumped by high force per unit area pumps through two agitated. perpendicular digester vass runing in series. Assorted with steam and acerb solution. aluminum oxide in the bauxite forms a concentrated Na aluminate solution go forthing un dissolved drosss. chiefly inert Fe and Ti oxides and silica compounds. Chemical reaction conditions to pull out the monohydrate aluminum oxide are about 250°C and a force per unit area about 3500 kPa. achieved by steam generated at 5000 kPa in coal-burning boilers.
Under these conditions. the chemical reactions are rapid: –
2NaOH + Al2O3. 3H2O – & gt ; 2NaAlO2 + 4H2O
2NaOH + Al2O3. H2O – & gt ; 2NaAlO2 + 2H2O
By sizing the vas to optimum holding clip. about 97 % of the entire available aluminum oxide is extracted and the silica content of spirits is reduced. Heat Recovery:
After digestion about 30 % of the bauxite mass remains in suspension as a thin ruddy clay slurry of silicates. and oxides of Fe and Ti. The mud-laden spirits go forthing the digestion vas is flash-cooled to atmospheric boiling point by fluxing through a series of flash vass which operate at in turn lower force per unit areas. The brassy steam generated is used to preheat incoming acerb spirits in cannular heat money changers located parallel to the flash armored combat vehicle line. Condensate from the heat money changers is used for boiler provender H2O and rinsing waste clay. Sweetening:
The tri hydrate bauxite has separate grinding and pre-treatment installations. During the base on balls through the brassy armored combat vehicles. this extra bauxite slurry with high tri hydrate aluminum oxide content is injected to maximize the alumina content of the spirits watercourse. This occurs in the appropriate flash vass when the slurry from the digesters has been cooled to less than 200°C. 2.
Clarification OF THE LIQUOR STREAM
Most ruddy clay waste solids are settled from the spirits watercourse in individual deck 40 metre diameter settling armored combat vehicles. Flocculants are added to the colonist provender watercourse to better the rate of clay subsiding and accomplish good lucidity in the overflow spirits. Washers:
The clay is washed with fresh H2O in counter-current lavation trains to retrieve the sodium carbonate and aluminum oxide content in the clay before being pumped to big disposal dikes on Boyne Island. Slaked calcium hydroxide is added to thin acerb spirits in the lavation procedure to take carbonate ( Na2CO3 ) which signifiers by reaction with compounds in bauxite and besides from the ambiance and which reduces the effectivity of spirits to fade out aluminum oxide. Lime regenerates acerb sodium carbonate. leting the indissoluble Ca carbonate to be removed with the waste clay.
Na2CO3 + Ca ( OH ) 2 – & gt ; CaCO3 + 2NaOH
Settlers overflow spirits incorporating hints of all right clay is filtered in Kelly-type changeless force per unit area filters utilizing polypropylene filter fabric. Slaked lime slurry is used to bring forth a filter bar. Mud atoms are held on the filter leaves for remotion and intervention in the clay washers when filters are consecutive taken off line. Heat Interchange:
With all solids removed. the pregnant spirits go forthing the filter country. contains alumina in clear supersaturated solution. It is cooled by brassy vaporization. the steam given off being used to heat exhausted spirits returning to digestion. 3. Precipitation OF ALUMINA HYDRATE
Dissolved aluminum oxide is recovered from the spirits by precipitation of crystals.
Alumina precipitates as the tri hydrate Al2O3. 3H2O in a reaction which is the contrary of the digestion of tri hydrate – 2NaAlO2 + 4H2O – & gt ; Al2O3. 3H2O + 2 NaOH
The cooled pregnant spirits flows to rows of precipitation armored combat vehicles which are seeded with crystalline tri hydrate aluminum oxides. normally of an intermediate or all right atom size to advance crystal growing. Each precipitation armored combat vehicle is agitated. with a keeping clip of about three hours. During the 25-30 hours pass through precipitation. aluminum oxide of assorted crystal sizes is produced. The entry temperature and the temperature gradient across the row. seed rate and acerb concentration are control variables used to accomplish the needed atom size distribution in the merchandise. As right atom size is of import to smelter operations. size is carefully controlled. The QAL precipitation works was designed to run on a uninterrupted footing to bring forth “sandy” or harsh aluminum oxide. Categorization:
The finished mix of crystal sizes is settled from the spirits watercourse and separated into three size scopes in three phases “gravity” categorization armored combat vehicles. The primary classifiers collect the coarse fraction which becomes the merchandise hydrate. The intermediate and all right crystals from the secondary and third classifiers are washed and returned to the precipitation armored combat vehicles as seed. Exhausted Liquor:
Spent acerb spirits basically free from solid floods from the third classifiers and is returned through an vaporization phase where it is re concentrated. heated and recycled to fade out more aluminum oxide in the digesters. Fresh acerb sodium carbonate is added to the watercourse to do up for procedure losingss. 4. Calcination OF ALUMINA
A slurry of harsh hydrate ( Al2O3. 3H2O ) from the primary thickenings is pumped to hydrate storage armored combat vehicles and is filtered and washed on horizontal-table vacuity filters to take procedure spirits. Calcining:
The ensuing filter bar is fed to a series of calcining units – an 1800 metric tons a twenty-four hours go arounding fluidised bed calciner or one of nine rotary kilns each 100m long and 4m in diameter. The feed stuff is calcined to take both free wet and chemically-combined H2O. 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 flaxen aluminum oxide atoms are 90 % + 45 µm ( micrometers ) in size. Cooling:
Rotary or satellite ice chests are used to chill the calcined aluminum oxide from the rotary kilns. and to pre-heat secondary burning air for the kilns. Fluidised-bed ice chests further cut down alumina temperature to less than 90°C before it is discharged on to conveyor belts which carry it to storage edifices where it is stockpiled for cargo.