Why is refining of aluminum so expensive

First the alumina is extracted from bauxite ore usually using the Bayer Process. 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 a 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.

Air emissions come from a number of sources. The grinding of the bauxite, calcinating the aluminum oxide, and handling materials produce particulates. Air emissions equipment is used extensively to capture these particulates. The particulates may be metal rich. If the metallic content is sufficient, the emissions control dust can be remelted to capture any remaining metals or it may be otherwise reused or sold for its metallic content.

If the dust is not sufficiently metal rich, it usually landfilled. Another source of air emissions from primary aluminum production processes occurs during the reduction of aluminum oxide to aluminum metal.

Hydrogen fluoride gases and particulates, fluorides, alumina, carbon monoxide, sulfur dioxide and volatile organics are produced. Electrolytic baths often use anode pastes in the cell. The paste must be continually fed into the cell through a steel sheet with an opening. This continual feed allows the gas to escape. One method for reducing gas emissions is the use of pre-baked anodes.

Pre-baked anodes must be manufactured in an on-site plant. The pre-baked anodes allow the electrolytic bath to be sealed, allowing gas to be captured. The anodes are then replaced every days, containing the gasses for collection. Aluminium oxide does not dissolve in water, but it does dissolve in molten cryolite. This is an aluminium compound with a lower melting point than aluminium oxide.

The use of cryolite reduces some of the energy costs involved in extracting aluminium. The diagram shows an aluminium oxide electrolysis cell. The negative electrodes cathodes and the positive electrodes anodes are made of graphite, a form of carbon.

During electrolysis:. The oxygen reacts with the carbon in the electrodes, forming carbon dioxide which bubbles off. Although Hall was working in a small U. Skip to content Site Navigation The Atlantic. Popular Latest. The Atlantic Crossword. Sign In Subscribe.

Charles Martin Hall was 22 when he figured out how to create pure globs of aluminum. But the lure of shiny, valuable metal had captured the imagination of both. Aluminum is one of the most plentiful substances on Earth— the most common metal found in the planet's crust.

But aluminum is also a friendly element, and it's often found bound tightly to other elements.