|Leachability of Metals From Mineral Processing Waste (63 pp, 1.39 MB) (EPA/600/R-04/051) August 2004
This report evaluates the leaching of oxoanions and other materials from mineral processing wastes, using the Toxicity Characteristic Leaching Procedure (TCLP) test. Other leaching tests were used as well: Generalized Acid Neutralization Capacity Test, 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:
The 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. Because this study focused on leaching of oxoanions, none of the samples contained significant quantities of these materials.
The Yerington, NV, sample had the highest amounts of arsenic and selenium with 209 milligrams per kilogram (mg/kg) and 156 mg/kg, respectively. 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 leached molybdenum and selenium varied from waste to waste. However, the concentrations of molybdenum and selenium in the wastes were very low from the beginning. Oxoanion leaching reached 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 leached metals 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 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 higher 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 nondetectable above pH 5.0.
Based on the results of this study and using this set of mineral processing wastes, it is concluded that the TCLP test is as adequate as any of the other methods studied for estimating potential risk from the leaching of mineral processing wastes. The TCLP test procedure is simpler than other available tests and the results are comparable to those from other more demanding test procedures.
The TCLP test pH of 5.0 appears to be appropriate for mineral processing 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 mineral processing wastes that have been shown to leach oxoanions should be evaluated.
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