Mining in Western Australia

The Western Australian mining industry is the largest and most diversified resource industry in Australia. The state encompasses about one-third of the Australian continent and includes rich mineral resources including iron ore,, diamonds, bauxite, uranium, gold, coal and mineral sands As of May 2007, there were 560 commercial mineral projects underway, including 1,222 operating mine sites and 171 operational processing plants. In , Western Australia accounted for 43% of sales and service income for the total Australian mining sector (all minerals, including oil and gas), followed by Queensland at 24%; New South Wales and the Australian Capital Territory contributed 16% of the total. Total revenues were A$ 23.9 billion in the same period. In the Western Australian resources sector accounted for 50% of Australia's total value of mineral and petroleum sales, 67% of Australia's oil and condensate production production, 50% of Australia's mineral production and 31% of Australia's total merchandise exports

Since the early 1900's, the Darling Ranges in Western Australia had been thought to contain considerable deposits of bauxite. However it was not until 1957, following exploration by Western Mining Corporation (now WMC Resources Limited) that these deposits were confirmed, prompting the formation of a joint-venture company to develop an integrated alumina industry. The Aluminium Company of America (now Alcoa Inc.) became a partner, and in 1961 the new venture was granted a 12,619 sq km bauxite mining lease in the Darling Ranges. Alcoa's first mine, at Jarrahdale, began operations in 1963 and for 35 years, until its closure in 1998, was the only source of supply for Alcoa's alumina refinery at Kwinana. During that long partnership, the Jarrahdale mine provided 168 million tonnes of bauxite from which 44.6 million tonnes of alumina were produced. Today, Alcoa has two bauxite mines in Western Australia, at Huntly and Willowdale. The Huntly mine, established in the early 1970's to supply the Pinjarra refinery, now also supplies the ore for Kwinana. This expanded capacity makes Huntly the biggest bauxite mine in the world. The Willowdale mine was established in 1984 and supplies Alcoa's Wagerup refinery.

Caring for the future Alcoa leads the world in mine rehabilitation. Self-sustaining jarrah forest eco-systems now flourish where Alcoa once mined bauxite in Western Australia. A section of this rehabilitated forest at Jarrahdale is today enjoyed by recreational mountain bikers, using designated cycle tracks and riding bikes made from aluminium. Find out more about Alcoa's Award Winning Biodiversity Rehabilitation

Bauxite is washed, ground and dissolved in caustic soda (sodium hydroxide) at high pressure and temperature. The resulting liquor contains a solution of sodium aluminate and undissolved bauxite residues containing iron, silicon, and titanium. These residues sink gradually to the bottom of the tank and are removed. They are known colloquially as "red mud". aluminum.org/production/refining/index.html The clear sodium aluminate solution is pumped into a huge tank called a precipitator. Fine particles of alumina are added to seed the precipitation of pure alumina particles as the liquor cools. The particles sink to the bottom of the tank, are removed, and are then passed through a rotary or fluidised calciner at 1100°C to drive off the chemically combined water. The result is a white powder, pure alumina. The caustic soda is returned to the start of the process and used again. More information about the Chemistry of the Process is available.

The Bayer process involves four steps: digestion, clarification, precipitation, and calcination. In the first step, bauxite is ground, mixed with a solution of caustic soda (sodium hydroxide) and pumped into large pressure tanks called digesters, where the ore is dissolved under steam beat and pressure. The sodium hydroxide reacts with the hydrated aluminium oxide (Al 2 O 3 3H 2 O or Al 2 O 3 H 2 O) of bauxite to form a saturated solution of sodium aluminate (Na[Al(OH)] 4 ); insoluble impurities, called red mud because of their iron oxide content, settle to the bottom. During clarification, the mixture is passed through a series of pressurereducing tanks (called blow-off tanks) and filter presses. Filters catch the red mud, which, except at plants where lower grade ores are refined, is discarded.

A resultant green liquor passes through filters into cooling towers (beat exchangers) and then into tall, silo-like precipitators. Sizable amounts of hydrated alumina (Al 2 O 3 3H 2 O) crystals are added to the solution in the precipitators as "seeding" to hasten crystal Separation. The seed crystals attract other crystals and form groups physically heavy enough to settle out of solution. This precipitate of aluminium hydrate (Al(OH) 3 ) crystals is filtered out, washed to remove impurities, and heated in long, rotary kilns at temperatures in excess of 1800°F (980°C). Free water and water that is chemically combined or fluidized-bed calciners with the aluminium hydrate are driven off, leaving commercially pure alumina – or aluminium oxide – (Al 2 O 3 ) a superdry, fine white powder similar to sugar in appearance and consistency. It is still half aluminium and half oxygen by weight, bonded so firmly that neither chemicals nor heat alone can separate them.

Aluminium smelting In the smelting process, alumina undergoes electrolysis in electric reduction furnaces, commonly referred to as pots, to separate the aluminium and the oxygen. The process is known after its discoverers as the Hall-Heroult process. The pots are large, shallow steel tanks lines with carbon and connected electrically in series. In each pot an electric current flows from carbon anodes, through a molten bath containing alumina, to a carbon cathode lining, and then on to the next carbon anode in the next pot. The electric current forces the oxygen in the alumina to separate, and form with the carbon into carbon dioxide at the top of each furnace pot. The molten aluminium forms at the bottom. The aluminium is siphoned into larger holding furnaces where other impurities are removed and alloy elements such as magnesium, silicon or manganese are added. The molten aluminium is cast into various forms and sent to fabrication plants to be rolled or extruded (stretched) or to a foundry for casting. Over 90% of alumina is refined into aluminium.

Hall Heroult Process