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Show 2,000 to 3,000 operating days. Spent pot liner is designated as hazardous waste K088 by the Resource Conservation and Recovery Act (RCRA). Recommended solutions for this waste generation include (a) purification of the carbon anode to prevent heavy metal contamination of the pot liner, and (b) thennal treatment or vitrification of the dross and spent potliner to recover aluminum, destroy hazardous organics, and enable recycling of the refractory materials (A. R. TeissierduCros, 1995). 4.6.1.2 Secondary Aluminum. The fabrication begins with the melting of scrap and ingot in a reverberatory furnace or directly from the primary smelting process. Feed that is remelted is further refmed to remove impurities and alloyed with other metals to achieve the required end-use metallurgical properties such as ductility, hardness, or strength. This typically requires metal assays before and after the secondary smelting operation. Reverberatory furnaces are usually directly flfed with natural gas, and furnace temperatures are about 2,400°F. These are the most energy intensive combustion processes in Al fabrication. Fluxing salts and CI gas injection are used during this processing step. Molten metal is poured into a foundry mold for ingots or casting fmal shapes. The cast metal typically undergoes a homogenizing heat treating step before additional processing, e.g., machining or cold rolling. Depending on the final shape desired, the cold metal may be cold- or hot-rolled for sheet product, or remelted in smaller furnaces, typically indirect fired crucible, and investment or die cast to net shape. Alwninum is heat treated at several stages in the fabrication process in small, direct fired to very large muflle furnaces. Typical heat treating steps include homogenization and reheating at about 1,000°F, usually after casting operations; annealing at 460 to 775 OF, usually after hot milling; and aging at 250 to 460 °F, usually after metal working. Heat treating generally requires close control of temporal and spatial temperature distributions within the work and, therefore, accurate measurements and control of furnace temperature. Thennocouple arrays are used, sometimes in conjunction with instrwnented work pieces to control and confinn temperature histories. Natural gas is almost exclusively used in these furnaces . Of lesser interest are aluminunl anodizing and painting (coating) operations since these are less energy intensive, smaller, and more disperse processing operations. These typically involve boilers for providing steam heat for chemical cleaning and plating tanks or direct heat to paint drying booths. Aluminum die casting and foundries and stTip mills are large single point users of energy, since large quantities of metal are heated/melted to high temperatures. Metals are typically batch-melted in pots (indirect fired crucibles) or direct-fired reverberatory furnaces (the latter sometimes with heat recovery). Heat treating and anodizing operations are energy intensive at the plant level, though energy use is widely distributed among a number of unit operations, e.g., heat treating and reheat furnaces and heated chemical tanks. Research and development for improved process control (sensors), emissions (acid and air toxic) control, improved high-temperature insulation packages and heat transfer enhancements, and cost-effective high- and low-grade heat recovery are important areas to explore. 4.6.1.3 Combustion Equipment. Large gas-fired rotary kilns are used in the primary alwninum industry to produce high purity alunlina, which is subsequently reduced electrolytically to aluminum in electrically heated smelters. Nearly all primary processing in kilns is done offshore at the ore mines. Electrically heated smelting is practiced at low cost U.S. hydro sites. These rotary kilns are typically configured like cement kilns with feed preheat sections, and have heat inputs of 50 to 150 MMBtuIhr. However, gas-fired furnaces to produce carbon electrodes used in the smelting process are more important to the U.S. primary metals industry than are rotary kilns. Petroleumderived coke is used to manufacture carbon electrodes. The prebake process requires that the electrodes be temperature-treated in a ring furnace to volatilize residual hydrocarbons and develop structural rigidity. The furnaces are gas fired, recuperated during the cooling cycle, and the most fuel intensive of the process steps practiced at U.S. smelters. In the secondary alunlinum industry, reverberatory furnaces are batch-charged, moderately large (25- MMBtuIhr heat input) furnaces, which operate cyclically. Once metal is charged, burners are set to high fire for melting. The metal is held at internlediate fire, assayed and withdrawn nearly continuously. Melt temperatures are less than 1 ,400°F, requiring high temperature, corrosive resistant refractories for containment. Furnace thennal efficiencies are generally poor, 20 to 300/0. Furnaces are usually gas fired, though the feasibility of pulverized coal firing has been demonstrated. Oxygen enriclunent has also been practiced to improve melter throughput. Thennal efficiencies tend to be low because of the difficulty in applying heat recovery. Cost effectiveness is further 19 |