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Metal Casting Technologies : December 2007
METAL Casting Technologies December 2007 40 Environmental Report There are nearly 200 foundries in Agra cluster of foundries. The historic Taj Mahal has been affected by the foundry pollutants in the form of degradation of its surface. To save the Taj Mahal from being further damaged by pollutants, the Supreme Court of India directed the Agra district authorities in 1999 to close down 53 iron foundries in and around the Taj Mahal. Most of these foundries were using cupola as their melting unit. At present the foundries are trying to install gas-fired cupola, which is not only less polluting, the energy efficiency is also highest in comparison to other alternative fuels. Recently Taft-Wesman has installed a cokeless cupola in Agra which started operation this year. The trend in use of melting units is towards eco-friendliness and improved energy efficiency. COMMON POLLUTION CONTROL CYSTEMS Particulate matters are of major concern in terms of pollution in various shops in a foundry. Depending upon the sizes of the particulates and their size distribution and the presence of other gaseous pollutants, a suitable PCS or a combination of systems are usually selected to control pollution. Some of the common PCS used in the foundries are discussed below. CYCLONE AND MULTI-CYCLONE Cyclone separators are commonly used for controlling the particulate matter. Cyclone separators are used when the dust loading may be high or the product is relatively coarse. Normally the secondary dust collection system is provided after the cyclone separator to further trap the fines escaping from the cyclone separator. The major drawback of cyclone is that it is not very effective for removing the finer particulate matter (< 10 μm) and the overall efficiency of 85% max can be achieved by this device. Normally the secondary dust collection system is provided after the cyclone separator to further trap the fines escaping from the cyclone separator. The major limitation of the cyclone is that it can not control gaseous emissions such as SO2 from cupola. When volume of air flow is large, single cyclone tends to become inefficient. Therefore it is recommended to use multiple numbers of cyclones thus called multi- cyclone. It has a common body container wherein a number of small cyclones are included, each handling small volumes, thus offering better efficiency. Thus, number of cyclones operating together offer far better performance for larger volume than single one. Whereas cyclone is not capable of meeting the emission standard of 450 mg/Nm3 from cupola or other sources of pollution, multi-cyclone are effective for meeting the emission standard of 450 mg/Nm3 and its overall efficiency approaches up to 95%. WET SCRUBBER Wet scrubbers are used when multiple products are to be handled in a same dust collection system. If the gaseous effluent contains hazardous gases such as SO2 and particulate matter then the wet scrubber is the most ideal air pollution control system. These are highly efficient for meeting the emission standard of 450 mg/Nm3 sub-micron particulates, fumes, smoke and gaseous pollutants with 99% efficiency. This is also extensively used when the gases are at elevated temperatures such as stack gases of cupola and the bag filters may not be suitable for those temperatures. A wide variety of scrubber designs are available. The selection depends on the duty requirements and the criticality of emission norms. Wet scrubbers are offered for the specific air flow rates, operating temperatures and the dust loading. BAG FILTERS Bag filters are extremely efficient particulate collectors for foundry units. Bag filters are used when the dust particles are extremely fine or the emission norms are very critical. The device can remove finer particulates up to 0.2 μm with as high as 99% efficiency. However it is an enegy-intensive device and the hot gases need to be cooled to below 140°C before passing it through the bag filter. Bag filters are offered with automatic bag cleaning devices so that they can be used continuously without any stoppage for cleaning of filter bags. POLLUTION IN FOUNDRIES -- PREVENTION IS BETTER THAN CURE! Pollution control in foundries is given more emphasis rather than reducing the degree of pollution in different shops. We need to reduce or even eliminate pollution from various sources, wherever possible, so that pollution control becomes irrelevant. Although it is practically a very challenging proposition, efforts can be made to minimize pollution in the shops. A few directions are presented here to approach zero- pollution in foundries. ■ In pattern shop, wood flour is generally unavoidable. Therefore, metallic and polymer patterns should be used wherever feasible. ■ In mold and core shop, natural organic binders, which do not produce harmful VOCs should be preferred. GM-BOND might be an answer to this problem. In addition, processes using sand without any binder, such as lost foam casting, V-process etc. are the next alternatives. Relatively less volume of combustion products of EPS emits in lost foam casting and still less volume of gases is generated during pouring in V-process by burning of plastic films. Is it possible to develop a casting process for high volume production, where gaseous emissions are nil during mold/ core preparation and pouring? ■ Gas fired cupola should be used in increasing numbers for cost-effectiveness. Induction furnaces are definitely the best option from pollution from point of view. ■ Use of binders and additives in mold/core and efficient mold/ core coating may minimize the need of surface finishing in the fettling shop and hence minimum pollution. Riserless castings will not require oxy-cutting of risers. As the casting defects are reduced, fumes from salvaging would also reduce.