Primary aluminium refining

ALUMINIUM SMELTING AND REFINING

Aluminum is primarily used to produce pistons, engine and body parts for cars, beverage cans, doors, siding and aluminum foil. It may also be used as sheet metal, aluminum plate and foil, rods, bars and wire, aircraft components, windows and door frames. The leading users of aluminum include the container and packaging industry, the transportation industry, and the building and construction industry. Aluminum can either be produced from bauxite ore or from aluminum scrap. Refinement of aluminum ore is sufficiently expensive that the secondary production industry commands much of the market. About 40% of aluminum in the US is recovered for secondary refining.

Due to high energy requirements, the major primary aluminum producers tend to locate in areas with low energy costs, including the Northwest and Ohio River Valley. Secondary producers tend to locate near industrial centers, including southern California and the Great Lakes.

Both primary and secondary aluminum producers refine and melt the aluminum and pour it into bars called ingots. The ingots are shipped to metal casting plants or other shaping plants for molding or rolling.

PRIMARY ALUMINIUM REFINING

Aluminum production from bauxite ore is a three step process. First the alumina is extracted from bauxite ore usually using the Bayer Process. In the Bayer Process, finely crushed bauxite is mixed with sodium hydroxide and placed in a digester. High temperatures and pressures in the digester cause reactions in the ore / sodium hydroxide mixture. The result is dissolved aluminum oxide and ore residue. The residues, which include silicon, lead, titanium, and calcium oxides, form a sludge in the bottom of the digester. The aluminum oxide is evaporated off and condensed. Starches and other ingredients are added to remove any remaining impurities from the oxide.

The solution is then moved to a precipitation tank where the aluminum oxide is crystallized. Aluminum hydroxide and sodium hydrizide are the products of the crystallization. The crystals are washed, vacuum dewatered and sent to a calcinator for further dewatering.

Aluminum oxide from the Bayer Process is then reduced to aluminum metal usually using the Hall-Heroult process. In this process the aluminum oxide is placed in an electrolytic cell with molten cryolite. A carbon rod in the cell is charged and the reaction results in carbon monoxide, carbon dioxide and aluminum. The aluminum sinks to the bottom where it is removed from the tank and sent to a melting or holding furnace.

The molten aluminum is then mixed with desired alloys to obtain specific characteristics and cast into ingots for transport to fabricating shops. In the fabrication shops, the molten aluminum or aluminum alloys are remelted and poured into casts and cooled. Molten aluminum may be further heated to remove oxides, impurities and other active metals such as sodium and magnesium, before casting. Chlorine may also be bubbled through the molten aluminum to further remove impurities.

Дата добавления: 2015-07-20