Suggestions on Waste Gas Treatment of Lost Foam Casting

According to the characteristics of the lost foam casting process, Sanzhuji usually provides two methods of waste gas collection for foundry enterprises: one is to fix the pouring position and move the mold; the other is to move the treatment facility for collection. For dust collection in the whole workshop, the concentration of volatile organic compounds is diluted because a large amount of air is mixed, so the later treatment measures are difficult to be effective.

Current status of lost foam casting
Lost foam casting requires negative pressure technology, mainly using water ring vacuum pump to achieve negative pressure effect, this method has low cost and high efficiency. The waste gas generated during the pouring of lost foam casting will be completely sucked into the water ring vacuum pump to contact with water, and the remaining substances will be discharged through the water ring filtration, which can effectively eliminate the phenomenon of black smoke from casting and improve the working environment. Sanzhuji suggested to use water ring vacuum pump to pump negative pressure to alleviate black smoke pollution of lost foam casting. The use of water ring vacuum pump for negative pressure pumping in lost foam casting is a way of trapping waste gas, which greatly improves the working environment. With other end-of-line treatment measures, the volatile organic compounds emitted into the atmosphere are reduced very little.
Among foundry enterprises adopting the lost foam process, the most used exhaust gas treatment facilities are activated carbon adsorption and photo-oxygen catalytic equipment, and a small number of enterprises use catalytic combustion treatment facilities. Regardless of the treatment facility used, Sanzhuji recommends the following 5 things to be aware of.

1. Water bath treatment has little effect on VOCs
Water bath treatment can only remove a part of particulate matter and small molecular substances, while the main components of volatile organic compounds produced by lost foam casting during the pouring process are benzene, toluene, styrene, xylene and carbon black, etc. Water bath can effectively remove waste gas. However, other volatile organic compounds are almost insoluble in water or slightly soluble in water, and have little effect on the treatment of VOCs.

2. Pretreatment measures should be taken before the terminal treatment facility
The lost foam exhaust gas should be pretreated (dust removal, oil removal, dehumidification) before entering the terminal treatment facility. Particles, carbon black and oily substances generated during the lost foam casting process are easy to block the voids of activated carbon, which greatly reduces the adsorption capacity of activated carbon. If a catalytic combustion facility is used, it will also cover the catalyst or interact with the catalyst, causing the catalyst to lose its activity. Therefore, the dust and water content entering the activated carbon adsorption device or catalytic combustion device should be reduced as much as possible. According to the "Technical Specifications for Industrial Organic Waste Gas Treatment Engineering by Catalytic Combustion Method" (HJ 2027), the concentration of exhaust particles entering the catalytic combustion device should be less than 10 mg/m3, which will not cause significant impact on the catalyst; According to "Technical Specifications for Industrial Organic Waste Gas Treatment Engineering by Adsorption Method" (HJ 2026), the concentration of particulate matter entering the adsorption device should be lower than 1 mg/m3. Therefore, whether activated carbon adsorption or catalytic combustion is used to treat the VOCs produced by casting, efficient pretreatment measures must be taken.

3. Active ignition is the most practical and effective way to deal with
For the lost foam casting process, the easiest and most effective way to remove volatile organic compounds is "active" ignition, which must be emphasized here is "active" ignition. Although it is not actively ignited during the pouring process, the exhaust gas containing VOCs will eventually burn, but the combustion often occurs when the pouring is about to be completed or in the middle of the pouring. At this time, a large amount of volatile organic compounds have been discharged into the air, so operation is required The operator "actively" ignites the gas in the riser and the gas outlet before the spontaneous combustion of the exhaust gas, which should be implemented by the lost foam casting enterprise into its process guide. However, this operation is not suitable for enterprises that use negative pressure to collect exhaust gas, so as not to cause fire or explosion. The "active" ignition operation is also suitable for foundries using green sand and resin sand processes.

4. Operating time of the action
Lost foam casting companies or lost foam process companies mostly implement intermittent production methods, so the running time of the exhaust gas treatment facility is more important. If the treatment facility is running all the time, the production cost will increase; if the treatment facility is running during pouring, the exhaust gas generated later Efficient collection and disposal are not available. The research shows that the exhaust emission of lost foam casting quickly reaches a peak value, and VOCs continue to be generated after the casting is completed, and gradually enter a low-level emission period after 1 hour of casting. Therefore, the treatment facility should be turned on before pouring begins and run continuously for at least 1 h after pouring is completed.

5. Concentration control of lost foam exhaust gas
Appropriate exhaust gas concentration can ensure that the catalytic combustion or activated carbon adsorption system can treat the exhaust gas safely and efficiently, and at the same time, it is beneficial to prolong the service life of the equipment and catalyst. The concentration of organic matter in the exhaust gas entering the adsorption device or catalytic combustion device should be lower than 25% of the lower limit of the explosion limit. Adsorption or catalytic combustion can be carried out only after it has been reduced to 25% of its lower explosive limit. Lost foam casting enterprises should design processing facilities that meet technical specifications according to product types and production arrangements.

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