13-08-2012, 02:13 PM
Primary Aluminum Production
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General1
Primary aluminum refers to aluminum produced directly from mined ore. The ore is refined and
electrolytically reduced to elemental aluminum. There are 13 companies operating 23 primary aluminum
reduction facilities in the U. S. In 1991, these facilities produced 4.5 million tons of primary aluminum.
Process Description2-3
Primary aluminum production begins with the mining of bauxite ore, a hydrated oxide of aluminum
consisting of 30 to 56 percent alumina (Al2O3) and lesser amounts of iron, silicon, and titanium. The ore is
refined into alumina by the Bayer process. The alumina is then shipped to a primary aluminum plant for
electrolytic reduction to aluminum. The refining and reducing processes are seldom accomplished at the same
facility. A schematic diagram of primary aluminum production is shown in Figure 12.1-1.
Bayer Process Description -
In the Bayer process, crude bauxite ore is dried, ground in ball mills, and mixed with a preheated
spent leaching solution of sodium hydroxide (NaOH). Lime (CaO) is added to control phosphorus content
and to improve the solubility of alumina. The resulting slurry is combined with sodium hydroxide and
pumped into a pressurized digester operated at 221 to 554EF. After approximately 5 hours, the slurry of
sodium aluminate (NaAl2OH) solution and insoluble red mud is cooled to 212EF and sent through either a
gravity separator or a wet cyclone to remove coarse sand particles. A flocculent, such as starch, is added to
increase the settling rate of the red mud. The overflow from the settling tank contains the alumina in solution,
which is further clarified by filtration and then cooled. As the solution cools, it becomes supersaturated with
sodium aluminate. Fine crystals of alumina trihydrate (Al2O3 • 3H2O) are seeded in the solution, causing the
alumina to precipitate out as alumina trihydrate. After being washed and filtered, the alumina trihydrate is
calcined to produce a crystalline form of alumina, which is advantageous for electrolysis.
Hall-Heroult Process -
Crystalline Al2O3 is used in the Hall-Heroult process to produce aluminum metal. Electrolytic
reduction of alumina occurs in shallow rectangular cells, or "pots", which are steel shells lined with carbon.
Carbon electrodes extending into the pot serve as the anodes, and the carbon lining serves as the cathode.
Molten cryolite (Na3AlF6) functions as both the electrolyte and the solvent for the alumina. The electrolytic
reduction of Al2O3 by the carbon from the electrode occurs as follows:
Emissions And Controls2-10
Controlled and uncontrolled emission factors for total particulate matter, gaseous fluoride, and
particulate fluoride are given in Table 12.1-1. Table 12.1-2 gives available data for size-specific particulate
matter emissions for primary aluminum industry processes.
In bauxite grinding, hydrated aluminum oxide calcining, and materials handling operations, various
dry dust collection devices (centrifugal collectors, multiple cyclones, or Electrostatic precipitators (ESPs)
and/or wet scrubbers) have been used. Large amounts of particulate are generated during the calcining of
hydrated aluminum oxide, but the economic value of this dust leads to the use of extensive controls which
reduce emissions to relatively small quantities.