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| Aluminium Information. Emissions from reduction cells also include organics, carbon monoxide and sulfur oxides. These emission factors are not presented in this document due to lack of data. Small amounts of organics are released by PB pots, and larger amounts are emitted from HSS and VSS pots. Invertical cells, these organics are incinerated in integral gas burners. Sulfur oxides originate fromsulfur in the anode coke and pitch.Emissions from anode baking ovens include the products of fuel combustion; high boiling temperature organics from the cracking, distillation, and oxidation of paste binder pitch; sulfur dioxide from the sulfur in carbon paste, primarily from the petroleum coke; fluorides from recycled anode butts; and other particulate matter. Emission factors for these components are not included in this document due to insufficient data. High molecular weight organics and other emissions from the anode paste are released from HSS and VSS cells. These emissions can be ducted to gas burners to be oxidized, or they can be collected and recycled or sold. If the heavy tars are not properly collected, they can cause plugging of exhaust ducts, fans and emission control equipment. A variety of control devices have been used to remove emissions from reduction cells and anode baking furnaces. To control gaseous and particulate fluorides and particulate emissions, one or more types of wet scrubbers (spray tower and chambers, quench towers, floating beds, packed beds, venturi) have been applied to all three types of reduction cells and to anode baking furnaces. In addition, particulate control methods such as wet and dry electrostatic precipitators (ESP), multiple cyclones and dry alumina scrubbers (fluid bed, injected, and coated filter types) are used on all three cell types and with anode baking furnaces. Recovery of fluorides is important as even small amounts of fluorides in the air can severely damage vegetation. Ingestion by animals causes severe health problems. In a material balance study of fluoride use in a pot room, it was found that approximately 65 pounds of an original 87pounds of fluoride added to the pots were released as emissions.13Pot cathode linings absorb about 20 pounds, and 1.6 pounds adhere to anode butts. Approximately 54 pounds of fluoride are captured for recovery. Of that amount, only 16 pounds were returned to the pots, and about 34pounds were in the scrubber water discharge. Collection efficiency of this older recovery system was found to be 83.4 percent. The fluoride adsorption system is becoming more prevalent and is used on all three cell types. This system uses a fluidized bed of alumina, which has a high affinity for fluoride, to capture gaseous and particulate fluorides. The pot off gases are passed through the crystalline form of alumina, which was generated using the Bayer process. A fabric filter is operated downstream from the fluidized bed to capture the alumina dust entrained in the exhaust gases passing through the fluidized bed. Both the alumina used in the fluidized bed and that captured by the fabric filter are used as feed stock for the reduction cells, thus effectively recycling the fluorides. Wet ESPs approach adsorption in particulate removal efficiency, but they must be coupled to a wet scrubber or coated bag house to catch hydrogen fluoride. Scrubber systems also remove a portion of the SO2emissions. These emissions could bereduced by wet scrubbing or by reducing the original quantity of sulfur in the coke and pitch, by calcining the coke prior to manufacturing anodes. Molten aluminum may be batch treated in furnaces to remove gaseous impurities, and active metals such as sodium and magnesium. One process consists of adding a flux of chloride and fluoride salts and then bubbling chlorine gas, usually mixed with an inert gas, through the molten mixture. Chlorine reacts with the impurities to form hydrochloric acid, Al203and metal chloride emissions. A dross forms on the molten aluminum and is removed before casting. Naphthalene and phenol have been found in small quantities during source tests at electrolytic reduction. Both are listed as hazardous air pollutants (HAPs). Sources of fugitive particulate emissions in the primary aluminum industry are bauxite grinding, materials handling, anode baking, and the three types of reduction cells (see Table 2.3-1).These fugitive emissions have particulate size distributions similar to those presented in Table 2.3-2. 2.4REVIEW OF REFERENCES Pacific Environmental Services (PES) contacted the following sources to obtain the most up-to-date information on process descriptions and emissions for this industry: 1)ALCOA, Pittsburgh, PA. 2)U.S. EPA Region 6, Dallas, TX. 3)U.S. EPA Region 4, Atlanta, GA. 4)Noranda Aluminum, Inc., New Madrid, MO. 5)Reynolds Metal Company, Richmond, VA. 6)U.S. EPA Region 10, Seattle, WA. 7)The Aluminum Association, Washington, DC. |