3 Ferrous and Non-Ferrous Foundries
Ferrous and non-ferrous foundries specialize in melting and casting metal into desired shapes. Foundry products are most often used in automobiles, plumbing fixtures, train locomotives, airplanes and as metal pieces in other kinds of equipment. Independent foundries are classified under SIC code 3300; however, many specialty or smaller production foundries often operate within larger plants classified under other SIC codes.
In 1990, iron and steel accounted for 84% of metals cast (McKinley, 1994). The remaining 15% of foundry operations come from aluminum, copper, zinc and lead production. The foundry industry currently produces 11 million tons of metal product per year, with a shipment value of $19 billion. Almost 200,000 people are employed in over 3,000 foundries in the United States. Although the large iron and steel foundries produce billions of dollars in metal each year and provide many jobs, most foundries have far smaller budgets and employ less than 100 people.
The first step in metal casting (Figure 3) involves the creation of a mold into which the molten metal will be poured and cooled. The materials used to make the molds depend on the type of metal being cast and the desired shape of the final product. Sand is the most common molding material; however, metals, investment materials, and other compounds may also be used.
Green sand mold are used in 85% of foundries. Green sand is a mixture of sand, clay, carbonaceous material and water (Figure 4). The sand provides the structure for the mold, the clay binds the sand together and the carbonaceous materials prevent rust. Water is used to activate the clay. The green sand mixture is packed around a pattern of the metal piece and allowed to harden. The mold is carefully removed from the pattern and prepared for the molten metal.
Sand molds are used only once. Molten metal is poured into the mold and allowed to cool. After cooling, the mold is broken away from the metal piece in a process called shakeout. Most of the sand from green sand molds is reused to make future molds.
Sand mixtures are also often used to create cores. Cores are pieces that fit into the mold to create detailed internal passages in the metal piece. Cores must be strong and hard to withstand the molten metal, and collapsible so they can be removed from the metal piece after it has cooled. To obtain these properties, resins or chemical binders are usually added to sand mixtures. Depending on the binder used, molds may be either air or thermally set.
Other molding materials include chemically bonded sand, metal or refractories. These materials are used in the remaining 15% of foundry applications. Shell molds use chemically bonded sand to make the molds. Permanent metal molds may be used in foundries that produce large quantities of the same piece. Investment molds are made from ceramic substances called refractories. They are used in high precision metal castings.
Foundries melt metals in one of several types of furnaces depending on the type of metal being used (Table 1). Furnaces types include cupolas, electric arc, induction, hearth or reverberatory and crucible. Because of the different nature of metals, different inputs are required and different pollution is released from each type.
Cupola furnaces are the oldest type of furnaces used in foundries. They are tall and roughly cylindrical and are most often used for melting iron and ferro alloys. . Alternating layers of metal and ferro alloys, coke, and limestone are fed into the furnace from the top. Coke makes up 8 - 16% of the total charge to provide the heat that melts the metal (USEPA, 1992). Limestone is added to react with impurities in the metal and floats to the top of the metal as it melts. As in steel melting, this limestone/impurities combination is called slag. By floating on top of the metal while it melts, the slag protects the metal from oxidation.
Cupola furnaces are lined with refractories, or hard, heat resistant substances such as fire clay, bricks or blocks. The refractory protects the furnace shell from abrasion, heat and oxidation. Over time the refractory breaks down and eventually becomes part of the slag.
Cupola furnaces are usually attached to emissions control systems to capture air emissions. Usually, the air emission systems use either high energy wet scrubbers that use water to remove air pollution from the gas stream or dry baghouse systems that use fabric filters to capture the emissions.
Electric arc furnaces are often used in large steel foundries and steel mills. The metal is charged into the furnace, with additives to make recovery of slag easier, and heat to melt the metal is produced with an electric arc from three carbon or granite electrodes. The electric arc furnace is lined with refractories which slowly decompose and are removed with slag. Electric arc furnaces also usually employ air emissions equipment to capture most air pollution.
Induction furnaces are the most widely used type of furnace for melting iron and are increasingly popular for melting non-ferrous metals (USEPA, 1992). They are popular because they provide excellent metallurgical control and are relatively pollution free. Coreless induction furnaces are used for smaller (5-10 ton) operations. In coreless induction furnaces, refractory lined crucibles are surrounded by water-cooled, copper coils.
For larger quantities, channel induction furnaces are used. In these furnaces the copper coils are surrounded by inductors to promote metal melting. Channel furnaces are commonly used to hold the molten metal prior to casting.
Induction furnaces use alternating currents to create heat and melt the metal. The refractories are usually made of silica, alumina or magnesia. They break down over time and become part of the slag.
Hearth furnaces are used in batch melting of non-ferrous metals. The hearth can be heated by either electric or natural gas methods. Hearth furnaces are used to produce small quantities of metal, usually for art and similar industries.
After metal has been melted, it is poured into a mold and allowed to cool. To remove the mold, sand castings enter a process called shakeout where the sand mold is shaken from the metal piece. During the process dust and smoke are collected by dust control equipment. Permanent molds are pried from the metal pieces without being destroyed. Investment molds and shell molds are destroyed during removal, creating solid waste.
Any additional parts used to hold the piece during casting are removed. The metal piece is cleaned using steel shot, grit or other mechanical cleaners to remove any remaining casting sand, metal flash or oxide.
A surface coating may be applied to the metal piece at the foundry; however, such coating is usually done at metal finishing plants. Further discussion of metal finishing can be found in The Pollution Prevention for the Metal Finishing Industry.
The waste products produced by foundries directly relate to the metal type, the furnace type and the molding technology used. For example, foundries that use sand molds generate the most waste from sand. Nonferrous foundries and steel foundries may produce hazardous waste because of the lead, zinc, cadmium and other metal present in the waste. Cupola furnaces produce more air pollution than induction furnaces due to coke use and sand castings produce more solid waste than permanent molds because of the sand fines that cannot be reused.
By volume, gaseous waste is the largest waste source from foundries (Dieter, 1995). Air emissions come from the binder systems used in mold making, the vapors from metal melting and airborne sand used in the pouring and shakeout steps. Air emissions have not been well quantified; however, they generally contain metals, semi-volatile and volatile organic compounds. They mainly come from the melting procedures. Pouring and cooling steps contribute about 16% of the total organic and semi-volatile wastes from foundries (Shah, 1995).
Most of the gaseous metal emissions are captured in the emissions control systems attached to furnaces, shakeout and cleaning areas of the foundry. Cupola furnaces contributed more metallic air emissions than other furnace types. Metal emissions from induction furnaces are very small. The core and mold making processes produce almost insignificant levels of metal emissions. Emissions from the pouring process depend on the metal temperature. The hotter the metals, the more metal emissions (Shah, 1995).
Organic air emissions come largely from unreacted components of resins, solvents and catalysts. They come primarily from the core and mold making steps and are not well quantified (Shah, 1995). OSHA standards have been the primary reason for monitoring air emissions in the past. However, with the Clean Air Act and its amendments as well as increasing regulations from the EPA, more air emissions studies are being done.
Liquid pollution makes up a small portion of the total waste stream from foundries (Dieter, 1995). Liquid waste comes from non-contact cooling water used to cool metal and other work pieces or from wet scrubber air emission systems. Water runoff from floor cleaning and other maintenance procedures may also produce liquid waste. However, volumes of liquid waste are relatively small and do not pose a large pollution problem for foundries. Some plants have water treatment facilities to remove contaminants for water reuse.
Solid waste makes up a large portion of the pollution from foundries. On-quarter to one ton of solid waste per one ton of castings is expected (Shah, 1995). The waste comes from sand, slag, emissions control dust and spent refractories. Sand waste from foundries using sand molds has been identified as the most pressing waste problem in foundries (Twarog, 1992). Molding and core sand make up 66-88% of the total waste from ferrous foundries (USEPA, 1992).
Green foundry sand is routinely reused. After the sand is removed from the metal piece, it can easily be remolded. However, sand fines develop with reuse. These particles are too small to be effective in molds and have to be removed and often landfilled.
Sand that is chemically bound to make cores or shell molds is more difficult to reuse effectively and may be landfilled after a single use. Sand recovery methods, as discussed later, have been investigated with mixed results.
Sand wastes from brass and bronze foundries pose further waste problems as they are often hazardous. Lead, copper, nickel, and zinc may be found in the sand in sufficient levels to require further treatment before disposal. If metal levels are sufficent, recovery methods may be employed.
Although investment castings are not as widely used as sand castings, they also produce solid waste, as they are usually destroyed when removed from a work piece. Spent molds are non-hazardous unless heavy metal alloy constituents are present. Spent wax, used as patterns for the molds, also contribute to solid waste. The patterns are removed by melting the wax and can usually be reused.
Finished metal pieces are often cleaned in abrasion cleaning systems. The abrasive cleaners and the sand they remove from the metal pieces contribute to solid waste. Grinding wheels and floor sweepings also add solid waste. These wastes are collected and usually landfilled.
Baghouse air emission control systems are one of the most frequently used technology for controling air emissions in foundries. Air is pumped into the baghouse where particulates accumulate on a fabric filter. The system is efficient for removing particles above or below 0.1 - 0.3 micrometers (Shah, 1995). Other types of air emissions control systems may also be used including wet scrubbers, absorption and adsorption systems, combustion and electrostatic precipitation. All systems produce a solid waste from the air emissions and release the cleaned air.
The emissions control dust is collected at almost all stages of foundry production. If it does not contain hazardous wastes, it is usually landfilled. However, steel foundries frequently produce emissions control dust that contains zinc, lead, nickel, cadmium, and chromium, depending on the metal content. Nonferrous emissions control dust may also be classified as hazardous due to copper, aluminum, lead, tin and zinc. Depending on the metals content in the emissions control dust it may be permitted for land fill, or it may require further treatment before disposal. Nonferrous foundry dust often contains sufficient levels of metals to make metal recovery economically favorable.
Slag waste is often very complex chemically and contains a variety of contaminants from the scrap metals. Common components include metal oxides, melted refractories, sand, and coke ash (if coke is used). Fluxes may also be added to help remove the slag from the furnace. Slag may be hazardous if it contains lead, cadmium, or chromium from steel or nonferrous metals melting. Iron foundry slag may be highly reactive if calcium carbide is used to desulfurize the iron. Special handling is required for highly reactive waste.
Green sand can be reused multiple times without significant refinement. The sand is filtered to remove fines that develop from the process. Additional sand is added to account for sand that is lost. Then the sand is remolded for a different metal piece.
Chemically bound sand used for core making and other types of molds is not so easily reused. However, many methods have been developed to recover foundry sand, with mixed success. The object of sand reclamation is to remove residual binders and contaminants from the sand grains so the sand can be reused without affecting the quality of the mold. The sand reclamation process is defined by the American Foundrymen's Society Sand Reclamation and Recovery Committee as "the physical, chemical or thermal treatment of a refractory aggregate to allow its reuse without significantly lowering its original useful properties as required for the application involved."
Four methods for recovering sand have been developed. The method that will be useful depends largely on the type of metal cast, the binders used, and the desired reuse.
Attrition sand reclamation technology spins two streams of sand in opposite directions in the presence of heat. The combination of sand abrasion and binder combustion free the sand particles from some binders. Attrition cannot remove all residual binders, but works well with no-bake binders. The yield from this process is a high strength recycled sand.
Because all binders cannot be removed through attrition, the sand characteristics may be changed. For some casting operations the characteristics may be changed significantly enough that the sand may be be ineffective for furture castings.
Attrition methods of sand reclamation may also produce large quantities of dust. The dust can be captured in air emission control equipment, hence contributing to the total volume of solid waste.
Dry sand reclamation relies on mechanical and pneumatic scrubbers to remove lumps and binders from sand (Figure 5). Mechanical scrubbing moves each sand grain through a sand-to-metal or sand-to-sand interface to remove impurities. Pneumatic scrubbers use air to propel sand between baffles. These scrubbers are particularly good for removing clay from molding sands and binders in systems that are not baked.
Dry reclamation can produce large quantities of dust. These air emissions have to be monitored and captured by control equipment. Dry sand reclamation may also not be capable of removing binders to the extent necessary for reuse in some foundry operations.
Wet reclamation uses water to remove sand binders (Figure 6). The process uses on the different water solubilities of sand and binders to separate the two. Clay bonded systems work well with water reclamation processes because the clays are very soluble in water. Sodium silicate sand binders can also be removed using wet reclamation. The sodium silicate dissolves part of the sand crystal when binding, but can be removed by exposing it to water. After the sand is soaked in a water bath it is dried and reused.
Although wet reclamation was used in the 1950s and 1960s, it has been nearly eliminated as a method of sand recovery. Chemical binders are also no longer sufficiently hydrophilic to dissolve in water. Further, organic resins that do dissolve and other water soluble impurities can cause significant water contamination. The high volume of waste water and strict environmental regulations can make wet sand reclamation too expensive.
Thermal reclamation uses heat in a rotary kiln, multiple-hearth furnaces, or a fluidized bed to combust binders and contaminants (Figure 7). In removing binders, the process can cause sand to change in composition. Combustion products from the fuel used to heat the sand and thermal cracking of the sand crystals may occur. The resulting sand may be significantly different than the original sand. Depending on the type of casting, thermally treated sand may or may not be usable.
Infrared energy can also be used to thermally treat sand. This method may maintain more of the sands original composition, while still destroying binders. Infrared units, called electric sand reclamation units, are in place in the United Kingdom and Canada ("Navistar Goes Infrared," 1993). External blowers push the sand through fluidized beds, allowing the sand to directly contact the infrared radiation which breaks down the binders. The electric sand reclamation units do not produce the combustion products associated with traditional thermal reclamation processes.
Another option for foundry sand is recycling. Many industries use sand as a raw material in their processes. As foundry sand is usually not hazardous, it can serve this purpose. Markets for spent foundry sand include manufacturing of: cement, concrete, asphalt, bricks and tiles, flowable fill (permeable, low-strength concrete), geotechnical fill and roadfill, daily landfill cover, and manufactured topsoil and composting. Liability and local legislation must, of course, be considered before selling spent foundry sand.
Slag and emissions control dust constitute the remainder of the solid waste produced by foundries. Not much has been written regarding process modification to reduce these solid wastes. However, if the slag or dust contained sufficient metal content, they can be fed back into the furnaces to reclaim the remaining metal dust. The metals can also be recovered from the dust using electrolytic or other metal recovery techniques. The recovered metal can either be added to the molten metal or sold for other uses.
Heine, Hans J. "Saving Dollars Through Sand Reclamation - Part 1," Foundry Management and Technology. 111:5 (May, 1983), pp. 22-25.
Leidel, Dieter S. "Pollution Prevention and Foundries." Industrial Pollution Prevention Handbook, ed. Harry M. Freedman. 1995.
McKinley, M.D. et al. "Waste Management Study of Foundries Major Waste Streams: Phase II." HWRIC #TR-016. April 1994.
Shah, D.B. and A.V. Phadke. "Lead Removal of Foundry Waste By Solvent Extraction." Journal of Air and Waste Management. 45 (March, 1995), pp. 150-155.
Trombly, J. "Recasting a Dirty Industry." Environmental Science and Technology. 29:1 (1995), pp. 76-78.
Twarog, D.L., et al. "Waste Management Study of Foundries' Major Waste Streams: Phase I." HWRIC Project RRT-16, Waste Management and Research Center, Champaign, Illinois, November, 1992.
USEPA. Metal Casting and Heat Treating Industry." EPA/625/R-92/009. September 1992.
Air Quality Committee (10-E), "Foundries Face Stricter Air Quality, Pollution Monitoring," Modern Casting. May 1990. This article provides a good description of SARA Title III.
Cornett, Michael J., "Eliminating the Waste Stream from Your Cold Box Process." Foundry Management and Technology. 121:12 (December, 1993), pp. 38-40. Good information about the Isocycle process. Discusses cold box sand casting briefly.
Douglas, John. "Electrifying the Foundry Fire," EPRI Journal. October / November 1991, pp. 17 -23. This article discusses electric options to replace coal-fired processes.
East, William, "Solid Waste No Place to Go," Foundry Management and Technology. May 1991. This article discusses the sources of solid waste from foundries.
Fuller, Robert, "Toxicity: Characteristics Leaching Procedure Replaces Extraction Procedure Toxicity," Modern Casting, 80 (November, 1990), pp. 51-53. Discusses changes in EPA methods for determining toxicity characteristics of industrial waste.
Gschwandtner, Gerhard and Susan Fairchild, Emissions Factors for Iron Foundries Criteria and Toxic Pollutants. EPA-600/2-90-044, US Environmental Protection Agency, Washington, DC, August, 1990. Discussion of air pollution sources from foundries.
Ham, R.K. and W.C. Boyle. "Research Reveals Characteristics of Ferrous Foundry Wastes," Modern Casting. February, 1990, pp. 37-41. Discusses the toxicity of liquid foundry wastes.
Jacobs Engineering. Waste Audit Study, Thermal Metal Working Industry, Jacobs Engineering, December, 1990. Discussion of waste streams associated with foundries.
Mosher, Gary E. "EPA Publishes New Land Ban Regulations." Modern Casting. 80:1 (January, 1990), pp. 40-41. Discussion of the Hazardous and Solid Waste Amendments of 1984 as they apply to foundries.
National Renewable Energy Laboratories. "The Foundry Industry" In Technology Partnership.
Washington, DC: Department of Energy. April, 1995. Brief discussion of the industry and the processes. Also a brief discussion of the Metal Casting Competitiveness Research Act of 1990.
Smith, Virginia D. "Foundries and Clean Air Act: Several Unanswered Questions." Foundry Management and Technology. 119:2 (February, 1991), 16-18. This article provides a general description of how the Clean Air Act of 1990 will potentially affect the foundry industry.
Summary of Factors Affecting Compliance by Ferrous Foundries, Volume 1, EPA-340/1-80-020, US Environmental Protection Agency, Washington, DC, January, 1981. This article discusses waste sources associated with foundries.
Trombly, Jeanne. "Recasting a Dirty Industry," Environmental Science and Technology. 29:2 (1995), pp. 76-78. Discussion of foundry air emissions and the Clean Air Act of 1994.
Air Emissions Reduction
Replacing Organic Cleaners with Citrus Based Solvents