The Refinery
At the Worsley refinery, alumina is extracted from bauxite using the Bayer Process.The crushed bauxite arrives by conveyor from the mine site and passes through four main process stages - digestion, clarification, precipitation and finally calcination. The process dissolves alumina in solution, and separates it from the red mud residue.
The finished product is carted by rail about 50km for export through the Port of Bunbury. The refinery produces more than 3.1 million tonnes of alumina each year, and is one of the biggest alumina refineries in the world.
A world-class research and development laboratory, administration centre, maintenance workshops, warehouse and advanced alumina transport and storage facilities also make it the world's most modern and efficient.
Digestion: When the bauxite ore comes off the conveyor it is finely ground in a mill and mixed with hot caustic soda to form a slurry. The slurry is heated to remove any contaminants that can form scale on the inside of the pipes.The alumina is dissolved into the slurry in digestor vessels through a combination of pressure and heat. The temperature and the pressure at which the slurry is put into the digesters depend on the bauxite and on the type of process. The gibbsitic bauxite mined by Worsley does not require the higher temperatures used for other forms of bauxite.
Clarification: The clarification stage separates the alumina dissolved in the liquor from the red mud residue. This is done by settling. Liquor is pumped into settling tanks where a settling agent is added. The mud bonds with the settling agent and sinks to the bottom of the tank where it is removed. The floating liquor is then filtered and sent to precipitation tanks. The red mud residue is washed to recover any alumina not in solution, which is returned to the process. The caustic soda is also washed from the red mud before it is stored in specially built bauxite residue disposal areas. See the video
Precipitation: This stage marks the beginning of the "white side" of the process, where the alumina hydrate-rich solution in the liquor is clean of the mud and is no longer red. The alumina hydrate slowly precipitates from tank to tank as the temperature goes down. The introduction of alumina hydrate, which has already been precipitated, speeds up the process. The liquor is filtered to separate the wet hydrate to be washed to remove final traces of caustic soda and other impurities. The remaining liquor is recycled back to the beginning of the refinery to begin the process again. See the video
Calcination: Washed and filtered hydrate is then dried in hot air at about 600°C before being passed into a gas furnace where it is heated to 900°C. The extreme temperature removes water trapped in the hydrate crystals. The final product resembles a fine, white powder and is in excess of 99 per cent pure alumina. The calcined alumina is cooled and then conveyed using air slides to storage silos ready for rail loading. See the video
The Power House
The Refinery needs approximately 100 megawatts of electricity per day to run the refinery, overland conveyor and mine site. Worsley produces this electricity through its own on-site coal-fired power station and gas-fired co-generation plant based at the Refinery.
Power Station
Worsley Alumina’s power station was build during construction over 20 years ago. It is connected to Western Power’s South West Interconnected Grid which allows Worsley to buy and sell electricity as needed.
The power station contains three coal fired boilers which each produce up to 225 tonnes of superheated steam (10MPa pressure, 510 degrees Celsius) per hour. On average, the refining process uses 740 tonnes of this steam per hour.
It also has four steam turbines producing power to support the power house (18MW), refinery operations (75MW), conveyor and Boddington mine (14MW).
In addition to producing electricity and steam, Worsley’s power stations is also responsible for treating process condensate in the condensate polishing plant. This condensate, once treated becomes very high quality feedwater to the power house boilers and the co-generation plant’s Heat Recovery Steam Generator.
The power station is powered by coal purchased from the town of Collie. Each year, Worsley uses over 750,000 tonnes per year of coal.
Cogeneration Plant
In 2000, Worsley Alumina increased its production capacity by one third creating a demand for more electricity. To meet this demand, Worsley installed a new $90 million gas fired co-generation plant.
The co-generation plant was a private development and is currently owned by Western Power and Origin Energy, and operated by Worsley Alumina. The plant feeds excess electricity into Western Australia’s power grid.
The co-generation plant runs on a gas turbine fed by natural gas brought from the North-West through the Dampier to Bunbury pipeline. It uses exhaust heat generated by the gas turbine to produce high and low pressure steam. Worsley Alumina has an arrangement to buy the steam so it can be diverted into the alumina refining process as well as to the company’s own steam turbines. The steam is constantly recycled. The co-generation plant is capable of producing 120 MW for WA’s public power supply.
Residue management
The alumina refining process produces a fine bauxite residue. For every tonne of alumina refined, 2.5 tonnes of bauxite residue is produced.
The residue is washed and filtered to remove as much caustic soda as possible before it is deposited in specially constructed dams, called Bauxite Residue Disposal Areas.
The dams are constructed as contained systems, to eliminate any leakage. Water from the residue, as well as rain that falls onto the disposal areas, is collected into pipes and pumped to the refinery catchment lake.
Fresh water from below the disposal areas is collected by a another pipe network that is separated from the residue by a layer of clay.
Once each disposal area is full of residue it is left to dry and consolidate. Research began in 1990 to find the best way to rehabilitate each site. Laboratory research and glasshouse experiments were used to determine limiting factors in plant growth.
Fields trials have shown that shrubs and grasses can be successfully re-established on bauxite residue disposal areas. Worsley Alumina is committed to revegetating each area.