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LEACHABILITY OF METALS FROM MINERAL PROCESSING WASTE
Citation:
Bishop, P. AND P. Gong. LEACHABILITY OF METALS FROM MINERAL PROCESSING WASTE. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-04/051, 2004.
Impact/Purpose:
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Description:
This report was submitted by the University of Cincinnati (UC) in fulfillment of Contract No. 68-C7-0057 under the sponsorship of the United States Environmental Protection Agency (EPA). This report covers a period from August 2000 through November 2001; laboratory work was completed as of August 2001. This report evaluates the leaching of oxanions and other materials from mineral processing wastes, using the TCLP test as well as several other alternative leaching tests (Generalized Acid Neutralization Capacity (GANC), Constant pH 5.0 Leaching Test, Constant pH Leaching Test at Various pHs, and Variable Mass Leaching Test).
Three actual mineral processing wastes were selected for evaluation. These wastes came from (1) fluvial tailings from the Arkansas River, three miles from the mining district in Leadville, CO., (2) the Anaconda copper mine in Yerington, NV., and (3) from a slag pile at a lead and zinc smelter near the village of Dearing, Montgomery county, in southeastern Kansas.
We found that mineral processing wastes vary widely in composition and characteristics. The three samples tested were soil-like and varied in characteristics, with one being classified as a loamy sand one a sandy loam, and the third a silty sand. Unfortunately, since this study focused on leaching of oxoanions, none of the samples contained significant quantities of these materials. The Yerington, NV sample was the highest, with 209 mg/kg As and 156 mg/kg Se. All three samples had TCLP leachate concentrations well below the allowable limits, except for cadmium in the Dearing waste. The Leadville and Yerington wastes would be deemed suitable for municipal landfill disposal, based on the TCLP test. The amount leached during the TCLP test varied from element to element and from waste to waste, but variations were relatively minor. Less than 10 percent of most metals leached. For the oxoanions, very little arsenic, cadmium or vanadium leached, while the amount of molybdenum and selenium leached varied from waste to waste. However, the concentrations of these last two in the wastes were very low to begin with. Oxoanion leaching reached an equilibrium fairly quickly, generally within one day. There was an immediate high degree of leaching, probably caused by surface washoff, followed by a constant arsenic concentration. Thus, the 18-24 hour leaching time used in the TCLP test is probably appropriate. In general, the amounts of metals leached under the Constant pH 5.0 leaching test conditions and the TCLP leaching test are of the same order of magnitude, although there is some variability between them. The concentrations of oxoanions leached under the two leaching conditions are quite similar. Leaching tests run at several constant pH values between pH 3.0 and 9.0 were used to evaluate the influence of pH on metal leaching. Results indicate that 24 hours of leaching is sufficient, and that many of the elements were below detection limits at high pH values. This is not surprising, because most metals have a lower solubility at pH values above neutral compared with below neutral, although this may not be the case for metals in the anionic form. Most of the oxoanions were also non-detectable, though, above pH 5.0.
Based on the results of this study, using this set of MPW wastes, it is concluded that the TCLP test is an adequate as any of the other methods studies for estimating potential risk from the leaching of MPW wastes. The test procedure is simpler than other available tests, and the results obtained from the TCLP test are comparable to those from other more demanding test procedures the TCLP test pH of 5.0 appears to be appropriate for mPW wastes, as is the 18-24 hour leaching period. It must be reiterated, though, that two of the three wastes evaluated did not contain high levels of oxoanions, and the one that did exhibited a very low degree of leaching. Other wastes might exhibit different leaching behaviors. It is recommended that use of the tCLP test for mineral processing wastes be continued. A wider range of MPW wastes, that have been shown to leach oxoanions, should be evaluated, though.