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| Aluminium Information. Primary aluminum production is a two-step process that refines alumina and reduces alumina to aluminum metal. It starts with the mining of bauxite ore, a hydrated oxide of aluminum consisting of 36 to 56 percent alumina (Al2O33) and lesser amounts of iron, silicon, and titanium. The Bayer process is used to refine bauxite into alumina. The Hall-Heroult process is used toreduce alumina to aluminum metal by electrolytic reduction. The refining (Bayer process) and the reduction (Hall-Heroult) process are seldom accomplished at the same facility. Details of both processes are discussed below. A schematic diagram of primary aluminum production is shown inFigure 2.2-1.Bayer Process Description In the Bayer process, crude bauxite ore is dried, ground in ball mills, and mixed with a preheated spent sodium hydroxide (NaOH) leaching solution. Lime (CaO) is added to the bauxite mixture to control phosphorus content and improve the solubility of alumina. The resulting slurry mixture is combined with fresh sodium hydroxide and pumped into pressurized digesters operated at 105C to 290C (221F to 554F). Digesters are large tanks that are operated at high temperatures and pressures 413 kPa to 6890 kPa (60 psi to 1,000 psi). After approximately fivehours, the slurry contains sodium aluminate (NaAl2OH) in solution and insoluble red mud. This is cooled to 100C (212F) and a flocculent, such as starch, is added to increase the settling rate ofthe red mud. It is then sent through either a gravity separator or a wet cyclone to remove courses and particles. The overflow from the settling tank contains the alumina in solution, which isfurther clarified by filtration prior to being pumped through a cooling tower. As the solution cools, it becomes supersaturated with sodium aluminate. Fine crystals of alumina trihydrate ([Al2O3•3H2O]) are seeded in the cooled solution, causing the alumina to precipitate out as alumina trihydrate. Washed and filtered, the alumina trihydrate is calcined to produce a crystalline form of alumina which is advantageous for the electrolysis process. The calcine is a course, sandy alumina that has not been fully calcined.This is done intentionally to improve the collection and recycling of fluoride emissions that occur during smelting operations. Hall-Heroult ProcessThe Hall-Heroult process is used to produce aluminum metal by electrolytic reduction of alumina that takes place 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 as the cathode. Electrical resistance to the current passing between the electrodes generates heat that maintains the cell operating temperature. Molten cryolite (Na3AlF6) functions as both the electrolyte and the solvent for the alumina. The electrolytic reduction of Al2O3by the carbon from the electrode occurs as follows:(1)The carbon required for this reaction comes from the electrode, which requires from 0.5 kgto 0.6 kg (1.1 to 1.3 lb) of carbon per kilogram (2.2 lb) of metal. Carbon anodes are continuously depleted by the reaction. In the electrolytic reduction of alumina, the carbon anodes are lowered into the cell and are consumed at a rate of about 2.57 cm (1 in) per day. In theory, only 0.33 kg(0.73 lb) of carbon is required per kg of aluminum. Furnace offgases contain and 10 to 50 percent carbon monoxide, accounting for the difference between theoretical and actual carbon consumption. Molten cryolite functions as both the electrolyte and the solvent for the alumina. Pure cryolite has a melting temperature of 1010C (1850 F). The electrolyte contains fluorospar(CaF2), some AlF3, and lithium in some instances, which along with the dissolved alumina, reduces the melt temperature sufficiently to permit the cells to operate between 940 and 980C (1725 and1795F).Aluminum is deposited at the cathode, where it remains as molten metal below the surface of the cryolite bath. Aluminum metal is tapped every 24 to 48 hours beneath the cryolite cover using a vacuum siphon. The aluminum is then transferred to a reverberatory holding furnace where it is alloyed, fluxed and degassed to remove trace impurities. From the holding furnace, the aluminium is cast or transported in molten state to fabricating plants up to 300 miles away.Three types of aluminum reduction cells are used in electrolytic reduction: prebaked anode cell (PB), horizontal stud Soderberg anode cell (HSS), and vertical stud Soderberg anode cell (VSS). Most of the aluminum produced in the U.S. is processed using the prebaked anode cellprocess. All three aluminum cell configurations require a "paste" (petroleum coke mixed with a pitch binder). Paste preparation includes the crushing, grinding, and screening of coke and cleaned spent anodes (butts) and blending the coke with a pitch binder in a steam jacketed mixer. For Soderberg anodes, the thick paste mixture is added directly to the anode casings, which are baked in the aluminium reduction cell. In contrast, prebaked (green) anodes are produced and baked as an ancillary operation at a reduction plant. During PB anode manufacturing, the paste mixture is molded into self supporting green anode blocks that are baked in a direct-fired ring furnace or a Reid Hammer furnace. Direct-fired ring furnaces use pitch and tars to isolate and seal off the green anode blocks from the atmosphere during a 28-day baking process. The Reid Hammer furnace, a European process, is extremely gastight and indirectly heated. After baking, steel rods are inserted into the PB anode and sealed with molten iron. These rods become the electrical connections to the PB anode. Prebaked anode cells are preferred over Soderberg cells because they are electrically more efficient and emit fewer volatile organic compounds. Volatile organic vapors from the pitch paste are emitted during anode baking. In addition, PB cells are not constrained by operating requirements and cell design configurations, as are the Soderberg cells. Prebaked cells require less efficient emission control devices. A PB cell operation, however, does require a separate anode and rodding facility, not needed by HSS or VSS aluminum reduction cells. A better understanding of the magnetic fields generated during the electrolytic reduction in PB cells has recently resulted in the production anduse of larger prebaked anodes, resulting in improved efficiency and lower aluminum production costs. Even though HSS or VSS aluminum reduction facilities require less labor and operate continuously (no requirement to remove the anode), the less efficient emission collection devices require higher exhaust capture velocities. Higher capture velocities increases total air flow and emission dilution. This requires larger equipment and higher emission control capital investment for the same production rate by PB cells. Although the prebake cell is the most common reduction cell used in the U.S., the HSS isused more frequently than the VSS cell. The HSS cell uses a "continuous" carbon anode. Green anode paste is periodically added at the top of the anode casing of the pot and is baked by the heat of the cell into a solid carbon mass, as the material moves down the casing. The cell casing is comprised of aluminum or steel sheeting, with a permanent steel skirt and perforated steel channels, through which electrode connections (studs) are inserted horizontally into the anode paste. During reduction, as the baking anode is consumed, the lower row of studs and the bottom channel are removed, and flexible electrical connectors are moved to a higher row of studs. The VSS cell is similar to the HSS cell, except that the studs are mounted vertically in the anode paste. Vertical stud Soderberg cell construction prevents the installation of an integral gascollection device, and hoods are restricted to canopies or skirts at the base of the cells where the hot anodes enter the cell baths. As discussed above, aluminum is periodically removed from the cells and transferred to a reverberatory holding furnace with other cell batches. The operation of aluminum reverberatory furnaces is discussed in detail in Section 12.8 of the AP-42 document. The molten aluminum is alloyed, fluxed and degassed to remove trace impurities. Primary aluminum generally contains less magnesium than recycled scrap aluminum refined by the secondary aluminum recovery industry. Therefore, primary aluminum emissions are generally lower than secondary aluminum emissions.2.3EMISSIONS AND CONTROLSIn bauxite grinding, hydrated aluminum oxide calcining, and materials handling operations, various dry dust collection devices (centrifugal collectors, multiple cyclones, or electrostatic precipitators and/or wet scrubbers) have been used. Large amounts of particulate are generatedduring the calcining of hydrated aluminum oxide, but the economic value of this dust leads to theuse of extensive controls to reduce emissions to relatively small quantities. Emissions from aluminum reduction processes are primarily gaseous hydrogen fluoride and particulate fluorides, alumina, carbon monoxide, carbon dioxide, volatile organics, and sulfur dioxide from the reduction cells. The source of fluoride emissions from reduction cells is the fluoride electrolyte, which contains cryolite (Na5Al3F14), aluminum fluoride (AlF3), and fluorospar(CaF2). Particulate emissions from reduction cells include alumina and carbon from anode dusting, aluminum fluoride, calcium fluoride, cryolite and ferric oxide. |