CORE AND MOLD MATERIALS:
METALLICS REFRACTORIES FUELS FLUXES
GRINDING WHEELS SHOT
INTERNAL PROCESS RECYCLING:
METALLICS MOLDING SAND CORE SAND
SOLID WASTE TO LANDFILL
USED CORE AND MOLDING SAND:
SWEEPINGS, CORE BUTTS
SLAG, REFRACTORIES FLUX, SCRUBBERS
DUST COLLECTOR: PARTICULATES, ABRASIVES, SHOT. ETC.
COMBUSTION GASES, WATER
Some foundry processes have the potential to generate hazardous wastes within the plant. These processes include the following:
Ductile Iron Production/Melting - To reduce the sulfur content of iron, some foundries use calcium carbide desulfurization in the production of ductile iron. The calcium carbide desulfurization slag generated by this process may exhibit the characteristic of reactivity.
Melting Emission Control - Melt materials which contain significant amounts of certain heavy metals (such as lead, cadmium, and chromium) may result in wastes which are classified as hazardous due to EP Toxicity.
Molding - Nonferrous alloy castings, such as brass and bronze, contain lead that may generate wastes which are classified as characteristic hazardous waste due to EP Toxicity.
Core-Making - Some core-making processes use strongly acidic or basic substances for scrubbing the off-gases from the core-making process. Sludges from this scrubbing process may exhibit the characteristic of a corrosive waste.
Waste Management Options
The basic management options for minimizing both the amount and degree of hazardous waste associated with foundry waste are as follows:
Alter product requirements.
Change raw materials.
Improve production process control.
Recycle to the original manufacturing process.
This paper will discuss how some foundries have evaluated the application of these waste management options for calcium carbide desulfurization slag and melt emission control residuals.
CALCIUM CARBIDE DESULFURIZATION SLAG
In the production of ductile iron, if. is often necessary to add a desulfurizing agent in the melt because the input (charge) materials contain excessive amounts of sulfur. One desulfurization agent commonly used is solid calcium carbide (CaC2). Based on investigations by Talballa et al. (1976), calcium carbide is thought to decompose to calcium and graphite. The calcium then reacts with sulfur to form calcium sulfide (CaS). The calcium carbide desulfurization slag is generally removed from the molten iron in the ladle and placed into a hopper. For adequate sulfur removal, calcium carbide must be added in slight excess. Therefore, the slag contains both CaS and CaC2. The calcium carbide desulfurization process is shown on Figure 3.
PROCESS FLOW DIAGRAM
One common practice is to treat the desulfurization slag with water (Stolzenburg, et al., 1985). This is done to generate and release acetylene gas from the unreacted calcium carbide. The other major reaction product is solid calcium hydroxide or lime.
This reaction is typically exothermic, releasing a considerable amount of heat, which is particularly evident when only small quantities of treatment water are used. Under these conditions, acetylene can ignite during the wetting and mixing of the slag. In addition, upward convection currents and steam generation resulting from treatment reactions may cause the release of considerable amounts of fugitive dust.
The calcium sulfide in the slag may release sulfide into solution in the treatment process water. This sulfide will remain in solution at high pH. However, in the neutral to acidic pH range, bisulfide is converted to hydrogen sulfide gas. During typical treatment operations, the alkaline nature of the calcium carbide desulfurization slag prevents the pH from dropping to a point where hydrogen sulfide gas is formed. However, the treatment water contains bisulfide, and therefore has the potential for releasing ^S if the wastewater is neutralized in a settling pond or wastewater treatment plant before the sulfide has been adequately diluted.
Calcium carbide desulfurization slag has a distinctive odor. Since pure acetylene is odorless, the odor must be produced by other trace constituents in the off-gases. A calcium carbide desulfurization slag sample from one ductile foundry was treated with water at a 1:1 solid-to-liquid ratio, and the gas was collected in a Tedlar bag for analysis by GC-MS. Several trace gases were identified, including arsine, divinyl sulfide (C^-CH^S, ethanethiol (ethyl mercaptan), methane, phosphine, and carbon monoxide.
The sum of measured volumes of these gases represented less than 1 percent of the total gas volume generated. However, the solubility of these gases in the reaction water was not accounted for. Any one of these gases (except methane and carbon monoxide), or a combination of them, could contribute to the observed odor. Furthermore, the reaction of the slag with water may give rise to potential health hazards if workers are exposed to excessive concentrations of these gases.
Field measurements were conducted where desulfurization slag was being treated with water. Arsine and/or phosphine and carbon monoxide were detected with Draeger gas detection tubes; and divinyl sulfide was identified by GC-MS from a grab sample in a gas sampling bottle. It is difficult to differentiate between arsine and phosphine using Draeger tubes. Sampling and analysis of off-gases using NIOSH methods P & CAM 265 and 216 showed that these substances, if present, were below the detection limits of these methods.
All three gases are of particular concern for human health reasons. The TLV (8-hour average) for carbon monoxide is 5.5 mg/M3, and the National Ambient Air Quality Standard (NAAQS) is 10 mg/M3. No standard for divinyl sulfide has been established in the United States. However, in the Soviet Union, where toxicological research on laboratory animals indicates that divinyl sulfide is a central nervous system depressant (Trofimov and Amosova, 1984), a maximum allowable concentration (MAC) of 0.2 mg/M3 has been recommended (Glukharev, et al., 1980).
Solid wastes can be classified as hazardous under the federal Resource Conservation and Recovery Act (RCRA), as either "listed" or "characteristic" wastes. One of the characteristics of a hazardous waste, as defined under 40 CFR 261.23, is reactivity. Desulfurization slag may be considered hazardous by this definition. Specifically, the slag reacts with water to form acetylene gas which may give rise to explosive mixtures as defined in 40 CFR 261.23(3). Also, with a neutral to acidic pH value, the calcium carbide desulfurization slag may emit hydrogen sulfide gas, which may result in its definition as hazardous under 40 CFR 261.23(5). Other gases, which are potentially toxic at high concentrations, may also be generated during treatment of the slag potentially rendering it hazardous by 40 CFR 261.23 (4). For these reasons, analysis and treatment of this type of waste are of great concern to the ductile iron foundry industry.
The requirements for ductile iron have been developed with time and experience. The key property that differentiates grey iron from ductile iron is the sulfur content. There is a large body of information that relates the sulfur content of cast iron to the physical properties. Individual manufacturers have researched the properties needed for their products, and have established specifications for sulfur content and other related properties so that product requirements are met.
Often, the material specifications for products have been established not on the needs for the product but on what the state-of-the-art technology is capable of producing. When this happens, a particular technology is actually being prescribed. What some companies have found is that they need to revisit the needs of the product to determine whether, for example, the sulfur content they have specified is too restrictive. Some companies have found that the product requirements can be adjusted without sacrificing the utility or the durability of the product.
For example, if a higher sulfur content is acceptable, the foundry will not have to go to the far end of what the calcium carbide desulfurization treatment process will produce. Instead of having to add a 20 or 30 percent excess calcium carbide to the molten metal for complete desulfurization, a lower dose could be used, resulting in far less unspent calcium carbide in the waste; therefore, the wastes may not be classified as reactive hazardous waste.
This situation exists within the foundry industry. Not all ductile iron foundries that employ calcium carbide desulfurization generate reactive hazardous waste. Whether the differences are due to more or less stringent product requirements cannot be determined. The quality of the charge metal purchased and the design of the desulfurization process itself also affect waste characteristics. If large excesses of the calcium carbide reagent are not needed, and if the desulfurizing process equipment is efficient, the resulting calcium carbide desulfurization slag will be far less reactive, possibly nonreactive.
The first option is to eliminate the generation of the reactive desulfurization slag by substituting calcium carbide with some other material. A few large foundry companies have made major advancements in new desulfurization technologies over the past years. One such process involves the use of a mixture of calcium oxide, calcium fluoride, and two other materials. One foundry reports that, not only is the product quality satisfactory, but the plant has eliminated the generation of a major problem hazardous waste, and the economics of the process are actually better than calcium carbide desulfurization.
A second option is to alter the charge metal by purchasing scrap that has low sulfur content in the first place. This method is commonly used by steel foundries, since the products they manufacture generally require a lower sulfur content. However, the purchase of high-grade, low-sulfur scrap may or may not work for production of high-quality ductile iron, and the supply of high-quality scrap varies; thus, the economics may favor other options.
Since unspent calcium carbide is what causes the slag to be classified as reactive, a logical method of eliminating the hazardous characteristic would be to control the process better in order to completely react the calcium carbide with the sulfur in the metal.
Two probl ems arise. First, the amount of sulfur in the scrap metal varies widely, so the metallurgist never knows exactly what dose is needed to just use up the calcium carbide. Second, there is some inherent inefficiency in contacting the sulfur with the calcium carbide, and some metallurgists contend that an overdose will always be necessary, even if one could predict stoichiometrically exactly how much calcium carbide was needed to reach a desired sulfur content.
Typically, when a foundry begins using calcium carbide for desulfurization, the level of unspent calcium carbide in the slag is very high. This level decreases with time and experience, but often not to a level low enough so the waste could be considered nonreactive. Recent innovations in process control for calcium carbide desulfurization include methods of introducing the material itself as well as forms of the material. The most common form of calcium carbide used is coarse granules. Some companies have experimented with very fine granules, coated granules, and solid rods made of calcium carbide in an effort to control the physics of the reaction more closely. Because there is no established USEPA test procedure for some of the reactivity criteria that come into play with this waste, it is hard to claim clear success; however, at least two calcium carbide manufacturers appear to have come up with important developments for this basic option.
Often, calcium carbide desulfurization slag will contain from 10 percent to 50 percent metal. Because the slagging operations involve raking molten materials off a ladle of 3,000°F metal, it is common to find fairly high metal content in the slag. Several foundries have reclaimed this metal via sorting, while other foundries have experimented with recharging the entire mass of the slag back into the melting furnaces.
When this is done, the calcium hydroxide is included in the melting furnace slag, and the unspent calcium carbide is either used or oxidized in the melting furnace. Little testing has been done to determine the actual fate of the sulfur. Most of it may be included in the slag, but it may also be emitted to the air as sulfur dioxide, or, for foundries with wet emission control systems, it may be dissolved in the water.
Foundries who have tested this method found that, with good control and metering of the recycle, the sulfur does not seem to concentrate in the metal products. Before going forward with this option, a foundry needs to know the environmental effects on all media for this recycling operation. It needs to know what additional contaminants are being placed into the air, the water, and the wastes from the process. But, given that those things are known and turn out favorable for the recycling, other major hurdles are the logistics of the recharge and regulatory constraints.
Because the calcium carbide desulfurization slag contains lime, some foundries have determined that it can be useful in dealing with other problems they may have. Since some foundries generate melt emission control residuals (e.g., furnace dust) that are classified as EP Toxic due to lead and cadmium leaching, they have found that, by blending calcium carbide desulfurization slag with the melt emission control residuals in proper doses, the mixture is often non-EP-Toxic. Because the majority of this treatment effect is accomplished through the lime content of the slag, one must be concerned about overdosing, since raising the pH of the furnace dust too high (above about pH 11.0) will cause lead to leach out. (Lead leaches at both low and high pHs.) RMT has experimented with treatment substances that, when mixed with calcium carbide desulfurization slag, will cause melt emission control residuals to be rendered non-EP-Toxic, and will ensure that the treated waste does not cause problems when disposed in a non-acidic environment.
We have seen that the state-of-the-art methods for spraying or immersing the desulfurization slag in water have the potential to cause difficult air emissions and industrial hygiene problems. Several foundry companies and research groups have evaluated three different basic types of reactors for treatment of calcium carbide desulfurization slag.
The first, a reactor system involving immersion of the slag in water, is really a more controlled version of the methods commonly used by foundries at this time. The British Cast Iron Research Association has done work on these types of reactors over the past 10 years, and has developed prototypes for systems to introduce the slag into the water.
Some groups experimenting with these types of processes have experienced problems with gas control and fires or explosions. Since the latent heat from the desulfurization slag is slow to dissipate, potentially explosive conditions can develop where the personnel operating the treatment systems simply do not wait long enough to pay close enough attention to the condition of the slag before introducing it to the treatment systems. The prototype systems that have been suggested and tried deal with quantities of desulfurization slag, usually less than 200 pounds per batch, and require trained and knowledgeable people to operate them. The foundry industry has generally shied away from water-based reactors for the desulfurization slag based on inherent problems with process control and dependency on personnel.
A second major type of reactor involves thermal destruction of the calcium carbide. At about 1,500°F, both calcium carbide and acetylene are thermally oxidized. Therefore, a system such as a rotary kiln could be used for thermal destruction of the reactivity characteristics. The additional benefit of thermal destruction is that it will also effectively deal with potential sulfide reactivity problems. Large chunks of metals often included in the desulfurization slag will tend to be a problem for many types of thermal units. Concern over air emissions and cost are other hurdles to the use of thermal systems for calcium carbide desulfurization slag.
Some efforts have been made to use the latent heat of the slag (the slag generally runs from 2,600 to 2,900^F when it is generated) by introducing a small flow of oxygen into the very hot slag. Little serious experimentation has been done for this method, since the system again involves a process that is different than what has been used in the past and would appear to be personnel dependent.
A third treatment method involves chemical reaction of the calcium carbide, the acetylene, the other potential off-gases, and the sulfide through use of strong oxidizing agents, such as potassium permanganate and hydrogen peroxide. These chemicals have been shown to be effective in reducing off-gases and in eliminating sulfide reactivity when introduced to the water that is used to treat the slag. However, the chemical doses for achieving all of those goals are very high, and full-scale systems would be very costly.
Some Improvements that have been made to the methods normally used by foundries to treat reactive desulfurization slag include treatment of the waste in a tank rather than in a waste pile. The tanks, generally constructed with three vertical sides and a ramped floor on the fourth side, first involved placing the waste and spraying water into it. Later versions have involved deeper tanks with flat floors that will allow placing hoppers of slag in the bottom of the tanks and immersing them in water by a combination of spraying and flooding. Experiments have shown that, if the slag can be rapidly covered by water, as little as one foot of water over the top of the slag will act as a scrubber; and many of the gases, including acetylene, arsine, and phosphine, will largely be dissolved in the water. When chemical oxidants such as potassium permanganate or hydrogen peroxide are added to the water, the air pollution control effectiveness seems to increase. However, the foundry must be concerned about dissolving large quantities of flammable and toxic gases as well as sulfides, since introducing this water to the plant's wastewater treatment system or to the sanitary sewer could cause the gases to be liberated again at some other point.
Also, there is serious question as to whether such a system would be capable of obtaining a RCRA Part B Permit to operate without formal air emission control systems. These open tank systems are designed to be crude but effective. When one begins to collect flammable and toxic gases over such open tanks and to allow access of fork lift trucks to deliver and retrieve hoppers of slag, the logic of the system falls apart very quickly and one returns to the reactor concept or other options.
The best options for calcium carbide desulfurization slag management appear to lie in altering the raw materials, improving the process control, and recycling to the original manufacturing processes, assuming the product requirements cannot be modified enough to eliminate the generation of reactive desulfurization slag using calcium carbide. Several foundries and suppliers have made major advancements in each of these options, and the next two or three years will tell which of the options becomes most popular.
MELT EMISSION CONTROL RESIDUALS
Producing iron castings requires the metal be melted so it can be poured into the cavity of the mold to produce the desired shape of the casting. Foundries which produce iron castings typically use one of three different processes to melt the iron: cupola, electric arc furnace, or an induction furnace.
Because of Clean Air Act (CAA) requirements, most foundries have been required to install air pollution control equipment over their melting operations to come into compliance with these regulations. Foundries typically use either a dry baghouse or some form of wet scrubber system (e.g., wet cap, venturi, etc. ) to collect the particulates from the melting operation.
A typical materials flow diagram for a cupola melting operation is illustrated on Figure 4. As illustrated on this figure, materials input to the melting operation include scrap metal, fluxes, coke (source of fuel for the cupola), and refractory material. Wastes generated by the process include spent refractories, slag, bottom drop and sweepings (for cupola operations), and either baghouse dust or a wet sludge depending on the type of melting emission control system. Typical pouring temperatures for iron range from 2,040°F to 2,700°F.
Of the waste generated by this process, the one that is often classified as hazardous (EP Toxic) is the melting emission control residuals. Whether or not the melting emission control residual is EP Toxic is largely a function of the incoming quality of the metal which is charged into the furnace and the effi cienc.y of the melting emission control system. Numerous "tramp" metals or minor constituent metals contained in the scrap which is charged into the furnace
FOUNDRY MELT PROCESS SCHEMATIC
METAL FLOW LINE NON-METAL FLOW LINE WASTE FLOW LINE CUPOLA
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