Foundry Charge Modification Ep Toxicity Test Results

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H.W. LIMIT

TIME (DAYS)

FIGURE 6

the metallurgy problems will determine the feasibility of such an alternative. In general, this method for hazardous waste minimization is generally not used by a majority of foundries.

Recvcle/Beneflclallv Reuse

Another reuse alternative outside of the original process is to reclaim the lead and cadmium which have concentrated in the emission control residuals. The feasibility of this reclamation outside of the original production process largely depends on the concentration of metals within the residual, the cost of recovering the metals, and the market price for the metal. While recycling of these materials outside the original production process has been used in the nonferrous foundry industry (i.e., brass foundry), its application within grey iron foundries is extremely limited. Some foundries have also pursued marketing of the furnace dust as input to brick manufacturing and other consumer product applications, but consumer product liability concerns have limited this option.

Treatment

Various chemical means have been used to reduce the leaching potential of toxic metals. The four types of chemical treatment applicable to emission control residual waste which are EP Toxic for lead or cadmium include the following:

Precipitation

Absorption

Chemical Reduction

Solubility Control

This paper highlights two of the four chemical treatment methods which have been used quite extensively in the foundry industry.

Chemical Reduction

Chemical treatment using metallic iron added to hazardous wastes has proven effective. Figure 7 shows that, at a dosage of 5 percent (by weight) iron filings, lead leaching from a cupola emission control sludge was reduced from 28.6 mg/1 to less than 0.1 mg/1 (Stephens et al., 1984). A dosage of 7.5 percent iron fillings did not significantly increase treatment effectiveness. Cadmium leaching was virtually unaffected by treatment at these dosages. The mechanism proposed to explain the observed decrease in lead leaching is a redox reaction whereby metallic iron reduces divalent lead to metallic lead.

Evidence for a redox mechanism is found in EP Toxicity Test results. The addition of iron filings resulted in a final pH in the EP Toxicity Test slightly higher than untreated samples, indicating that the hydrogen ion may have been neutralized by chemical reduction.

Some success has been achieved using metallic iron to treat wastewater directly. An advantage of using metallic iron instead of iron hydroxide is that less sludge volume is generated. The presence of oxidizing agents in the wastewater can limit the effectiveness of metallic iron treatment.

Solubility Control

Another method of chemical treatment involves controlling the pH of the waste solution so metals will not be soluble when exposed to acidic conditions. Since

FIGURE 7: LEAD AND CADMIUM LEACHATE CONCENTRATIONS FROM CUPOLA EMISSION CONTROL SLUDGE TREATED WITH METALLIC IRON

FIGURE 7: LEAD AND CADMIUM LEACHATE CONCENTRATIONS FROM CUPOLA EMISSION CONTROL SLUDGE TREATED WITH METALLIC IRON

DOSAGE (% IRON FILINGS)

FIGURE 8: LEAD LEACHATE CONCENTRATIONS FROM BRASS FOUNDRY SOLID WASTE TREATED WITH LIME OR MAGNESIUM HYDROXIDE.

BLANK 15% LIME 20% LIME DOSAGE

EP TOXICITY TEST WATER LEACH TEST

LEAD H.W. LIMIT

the solubility of many metals, including lead and cadmium, is pH dependent, waste classified as EP Toxic can sometimes be rendered nonhazardous by adding a material that will maintain a neutral or slightly alkaline pH leaching condition.

One of the simplest and most cost-effective additives for this purpose is slaked lime (Ca(OH)2) . It is well-known that lime is effective in precipitating lead and cadmium as insoluble hydroxides. Excess dosages of lime can result in pH values high enough to redissolve lead. The addition of 15 percent by weight lime to brass foundry solid waste reduced lead leaching from 22 mg/1 to < 0.1 mg/1 (final pH in EP Toxicity Test of 7.8). However, when a water leaching test with no acid added was used, the pH was 11.6, and lead leached at 7.2 mg/1 (Figure 8) . This can pose a limitation to the use of lime because most "natural" leaching at disposal sites for treated foundry wastes is likely to be under leaching conditions with close to neutral pH.

Because of the problem of dissolving lead at higher pH values, magnesium hydroxide (Mg(OH)j) is a more desirable material for pH control than lime (Turpin, et al. , 1985). Magnesium hydroxide is buffered at a maximum pH of approximately 10.5; therefore, the potential for dissolving lead at higher pH's is minimal even if an excess of Mg(0H)2 is added. Adding 10 percent Mg(0H)2 by weight as a 55 percent slurry to the same brass solid waste reduced lead leaching to 0.3 mg/1 (final pH of 8.5 in EP Toxicity Test). Less than 0.1 mg/1 of lead leached in the water leach test (Figure 8).

Magnesium hydroxide slurry has been tested on wastewaters from three foundry cupola emission control systems. Doses ranging from 0.3 to 1.1 grams of Mg(0H)2 per gram of suspended solids resulted in a reduction in lead leaching from the resulting sludge to well below the hazardous waste limits (Figure 9). Reduction of cadmium leaching is dependent on the final pH of the leaching medium in the EP Toxicity Test. Data for the Mg(0H)2 treatment tests indicate that a final pH of eight or more is required for significant reduction in cadmium leaching (Figure 10).

Although magnesium hydroxide is more expensive than lime, it is preferable to lime because it has the benefit of easier pH control, thereby reducing process control requirements. The results of testing show that, while in many cases adding lime or magnesium hydroxide is effective in rendering a hazardous waste nonhazardous, knowledge of the leaching characteristics of the waste material and the treated waste mixture is important to ensure effective treatment and proper disposal. Also, while pH control of solubility appears to be an effective treatment method in the short-term, long-term characteristics of the chemical additive should be evaluated to ensure that the waste will remain nonhazardous over time.

SUMMARY OF OPTIONS

In concept, the best waste minimization options for ferrous foundry melting/emission control residual management appears to be altering the raw materials (i.e., charge modification) or the use of electric induction melting without air pollution control equipment. When these options are not feasible from an economic standpoint, the only remaining option, other than hazardous waste disposal, is to treat the waste using an appropriate form of chemical treatment. The authors have found that developing waste minimization options for melting emission control residuals must be done on a case-by-case basis because of the variability of materials as well as melting operations, air pollution control equipment, and product requirements.

FIGURE 9. LEAD LEACHATE CONCENTRATONS FROM CUPOLA EMISSION CONTROL SLUDGE FROM WASTEWATER TREATED WITH MAGNESIUM HYDROXIDE

^ AVERAGE INITIAL LEAD VALUE = 210mg/l

LEAD H.W. LIMIT

FIGURE 10: Cadmium laachata concentrationĀ« va. final pH In EP Toilclty taata on cupola amlaalon control aludga Irom waatawatar traatad with magnaalum hydroilda.

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