Aluminum Processing
Posted by smculp on November 11, 2008
Aerosol cans are produced though aluminum processing. Aluminum is either made from bauxite ore (primary aluminum refining) or aluminum scrap (secondary aluminum production). Due to the expense of refining aluminum, 40% of aluminum in the US is recovered for secondary refining. Major primary aluminum producers locate in low energy costs areas, such as the Northwest and the Ohio River Valley, due to high-energy requirements involved in primary aluminum production. Secondary producers locate near industrial centers like California and the Great Lakes. The EPA explains how aluminum is processed in specific ways.
Primary Aluminum Refining
First, alumina is extracted from bauxite ore using the Bayer Process [Finely crushed bauxite is mixed with sodium hydroxide and put in a digester and with added heat and pressure, the aluminum oxide is evaporated off and condensed. The aluminum oxide is crystallized in a precipitation tank, and the crystals are washed and dewatered]. The aluminum oxide is reduced to aluminum metal using the Hall-Hercoult process [Aluminum oxide is placed in an electrolytic cell with molten cryolite, and a carbon rod in the cell is charged, creating a reaction. This produces carbon monoxide, carbon dioxide, and aluminum. The aluminum sinks to the bottom and is removed and melted]. The molten aluminum is mixed with alloys and cast into ingots (bars).
Pollution: Air emissions from bauxite and aluminum oxide are usually caught by air emissions equipment, but air emissions from the reduction of aluminum oxide are usually able to escape. These emissions consist of fluorides, alumina, carbon monoxide, and sulfur dioxide.
Secondary Aluminum Production
Scrap aluminum is melted in gas or oil furnaces. Impurities are removed with chlorine or other chemicals.
Pollution: Air emissions depend on the scraps used, but can include hydrogen chloride and metal chlorides, aluminum oxide, and metal compounds. Aluminum fluoride can substitute chlorine to decrease air emissions.
In addition to the pollution created by aluminum processing, aerosol cans also contribute to pollution due to the difficulty of recycling them.
