Foundry melting emissions contain solid particles ranging from coarse dust to fines of submicron size. Cupola emissions are much coarser than electric furnace emissions, which are generally less than 5 p.
Foundry melting dusts include combustibles containing 20-30% carbonaceous material. Iron oxides account for nearly 60% of collected dusts; silica and miscellaneous metallic oxides account for smaller quantities.
Water curtains and scrubbers are used to remove solids from foundry stack gases. Wet scrubbers also remove acidic compounds. Scrubber water is treated to neutralize acids and to remove solids prior to recirculation. Settled solids are vacuum filtered prior to disposal. Most foundries have a number of scrubbers working on different operations, and all effluents are combined and treated together. In grinding and shakeout areas, the scrubber may be either cyclonic or water curtain, which tolerates dirty feed-water. However, abrasive materials of + 200 mesh should be removed to avoid abrasion of circulating pumps.
For complete solids removal—down to smoke particles from cupola emission gas—high-energy scrubbers such as Venturis are required, which need clean water. Cupola cooling water should also be clean to prevent heat exchange surface fouling. If water is used for slag quenching, a mass of porous particles up to 1 /4 in is produced These usually float. Casting washing produces a slurry with +150 mesh sand. Most of these materials can be separated on a vibrating screen of approximately 50 mesh.
Depending on the recirculation system, grit separators, settling basins, or clarifiers are used. A hydroseparator removes fine sand down to approximately 50 /. Removal of finer solids requires chemical treatment with lime, alum, and possibly a polyelectrolyte to produce clarified effluent containing 10-20 mg/l of suspended solids. Disc, drum, or belt filters are used for dewatering foundry waste solids. Filter rates range from 25-40 lb of dry solids/hr/sq ft.
Some foundries have sand scrubber wastes. This differs from dust collection water as it settles more slowly. Overflow rates of no more than 0.3-0.5 gpm/sq ft can be used. Filtration rates for sand scrubber wastes vary from 3-10 lb of solids/hr/sq ft.
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