Aluminio

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Aluminum

Aluminum is produced by aluminum ore refining and reduction (primary production) or from recycling aluminum (secondary production). The U.S. aluminum industry is the world’s largest, producing 7.5 million tons in 1996. Aluminum production in the U.S is approximately 52% primary aluminum and 48% secondary aluminum. In 1996, 62.8 billion aluminum cans were recycled in the U.S. ThePacific Northwest aluminum facilities carry 38.6% of the U.S. primary aluminum capacity and have the ability to produce 1.8 million tons of aluminum. The largest markets for aluminum are transportation, packaging (beverage cans), and infrastructure. (Aluminum Association, Inc., 1997). In Washington, primary aluminum production facilities are one of the largest producers of toxic emissions. Thereare currently seven primary aluminum production facilities and one secondary aluminum production facility in Washington. The largest toxic emissions from primary aluminum production are from fluorides and polycyclic organic matter. The largest toxic emissions from secondary aluminum production are chlorine and hydrogen chloride.
Primary Aluminum

Primary aluminum production encompassestwo processes:
I. Refining: grinding and handling of the bauxite ore
II. Reduction: aluminum is removed from crystalline alumina.
The refining process produces mainly particulate emissions and controls are used to recover much of this valuable dust. Since there are no refining processes in Washington and emissions are low and mostly comprised of particulate matter, therefining process will not be included in this report. If needed, more information can be found in AP-421. The rest of this section will focus on the reduction process.

Description of Process

The aluminum enters the reduction process as crystalline alumina (Al2O3) from the refining process. To produce aluminum metal the alumina is electrolytically reduced using the Hall- Heroult Process.Reduction occurs by reacting the alumina with carbon, as in the following reaction:
2 Al203 + 3 C → 4 Al + 3 CO2 ecu. 1

The carbon is in the form of an anode (negatively charged) which is continuously depleted. The
pots (shallow rectangular steel shells) are also lined with carbon, which is positively chargedand
acts as a cathode where the molten aluminum is deposited. Molten cryolite is used as the electrolyte and also as a solvent. The molten aluminum is tapped from the cathode every 24 - 48 hours using a vacuum siphon and is sent to a reverberatory holding furnace where it is alloyed, fluxed, and degassed to remove trace impurities. There are many different methods used to accomplish this. Onemethod is to add chloride and fluoride salts and then bubble chlorine gas through the molten aluminum. The impurities will react with the chlorine to form hydrogen chloride, aluminum oxide, and metal chlorine emissions. There are three types of aluminum reduction cells: the prebaked anode cell, and the horizontal and vertical stud Soderberg anode cell (HSS and VSS). All these cells are prepared withpetroleum coke mixed with pitch binder to make a paste. The coke is crushed, ground, and screened before being mixed with the pitch binder in a steam jacketed mixer. The paste is added directly to the anode casings of the Soderberg cells, but in the prebaked anode cells it is then baked in a directfired ring furnace or an indirectly-heated Reid Hammer furnace. The prebaked anode cells are mostcommon because they are more efficient electrically and they emit fewer organic compounds. There are four pre-baked, two HSS and one VSS facility in Washington. The most common toxic emissions from potlines are fluorides (especially hydrogen fluoride) and polycyclic organic matter. There is little information available to estimate toxic emissions from aluminum facilities. There are emissions factors...
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