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Metal Casting Technologies : December 2007
of sulfur in the coke is absorbed in the liquid metal and the rest is converted to SO2 by combustion in air. Oxides of nitrogen are generally formed in cupola due to the intense combustion conditions prevailing near the tuyeres. On the other hand, CO is a common gaseous compound in the stack gases due to existing thermodynamic conditions inside a cupola. Other melting furnaces such as electric arc furnace and electric induction furnace are comparatively less polluting, out of these two, induction furnace causes minimum pollution. Arc furnaces produce more fumes near the arc during melting than induction furnaces. Therefore more foundries prefer to install induction furnaces as their melting unit, although higher metal temperature and associated benefits are common with this type of furnace. POLLUTION IN POURING AND KNOCK-OUT SECTION The temperature profile inside a mold/core changes as soon as the pouring starts. VOCs start evolving from the mold/core depending on the changing temperature profile, since the evolution of VOCs is favored at a particular medium range of temperature. Below this temperature range, the evaporation of the compounds is negligible, whereas at higher temperature, the compounds break down to harmless elemental forms. The pollutants in the knock-out section are mainly silica dust and other solid particles from binders and additives. POLLUTION IN FETTLING SHOP A lot of fine SPM and fumes are generated in the fettling shop. However the nature and chemical composition of the SPM in the fettling shop is different from those in the mold/ core shop. Silica dust is formed in this shop during chipping and grinding operations, since sand is generally adheres over the surface of iron and steel castings. In addition, metallic particulates are also generated in the shop during these operations. Due to higher density, the metallic SPM tends to settle fast, at the same time the elements such as Cr, Ni etc. are known to be more harmful than silica dust. Various pollutants in a typical foundry are summarized in the above Table. INDIAN SCENARIO The Energy & Resources Institute (TERI), India with support from the Swiss Agency for Development & Co- operation (SDC) had undertaken an effort to address the problems of energy efficiency and environmental compliance in the small-scale foundry sector. To develop an energy efficient and environmentally friendly technology in the small-scale foundries, a demonstration plant was set up in the Howrah foundry cluster in 1998. The divided - blast cupola technology, which is an energy efficient option to the conventional or modified conventional cupola technology, has been demonstrated in this plant along with a venturi-scrubber based pollution control system (PCS). The pollution control system reduced the level of SPM from a range of 1300- 3900 mg/Nm3 to about 50 mg/Nm3. The SO2 emissions were also reduced to 40 mg/Nm3 from the prescribed level of 300 mg/Nm3. The existing emission standards from melting furnaces in foundries by the Central Pollution Control Board (CPCB) are as follows: Note: From May, 2001, the West Bengal Pollution Control Board has prescribed that the emission from cupola of all capacities must meet the limit of 150 mg/Nm3. Process Particulate emissions Harmful VOC emissions Patternmaking Wood dust, resin dust, Metallic particles Solvents from paints, glues and adhesives Mold and Coremaking Sand dusts (silica, zircon or chromite) Phenol, formaldehyde, furfuryl alcohol, toluene, benzene, isocyanates, esters, Di-methyl Ethyl amines etc. Melting Metal dust and fume, dusts from metal treatments and fluxing Organic compounds from the burn-off of oil, grease, paints and plastic contaminants etc. Casting and knock-out of sand molds Silica dust, metal fume Phenol, anilines, naphthalene, aromatics, formaldehyde, toluene, benzene, xylene, butadiene, acrolein, etc. Fettling Silica dust, metal dust, welding fumes N/A Type Pollutant Concen- tration (mg/Nm3) Cupola, less than 3 T/hr. Cupola, 3 T/hr. and above Cupola of all capacities SPM SPM SO2 450 150 300 Arc Furnaces of all capacities SPM 150 Induction Furnaces of all capacities SPM 150 METAL Casting Technologies December 2007 39