Copper Dump Leaching and Management Practices That Minimize the Potential for Environmental Releases
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United States Hazardous Waste Engineering Environmental Protection Research Laboratory Agency Cincinnati OH 45268 Research and Development EPA/600/S2-88/005 Mar. 1988 Project Summary Copper Dump Leaching and Management Practices That Minimize the Potential for Environmental Releases Robert L. Hearn and Robert Hoye The full report presents a is fully documented in a separate description of the magnitude and report of the same title (see Project distribution of copper dump Report ordering information at back). leaching, the design and operation of leaching facilities, the potential for Introduction environmental impact, and Dump leaching refers to percolation management practices that can be leaching of copper from run-of-mine used. to minimize environmental low-grade ores that have been piled on releases. The information contained native ground. These dumps typically in the report was obtained through cover hundreds of acres and contain searches of published and millions of tons of low-grade ore. The unpublished literature and through leaching cycle covers a span of many contact with knowledgeable years. The percentage of copper individuals involved in the dump produced by leaching operations has leaching industry. Ten leaching increased and this trend is expected to operations were visited to acquire continue. Currently, there are 18 firsthand knowledge and site- commercially active copper leaching specific information. operations in the United States with a Seepage from leach dumps and total production capacity of 280,000 process solution collection systems metric tons of copper per year. Fourteen is the most significant potential of these sites are in Arizona. There are mechanism for the release of about 23 inactive and abandoned contaminants. These solutions have leaching sites. low pH and high concentrations of Sections 8002(f) and (p) of the metals and total dissolved solids Resource Conservation and Recovery (TDS). Ground-water impacts have Act (RCRA) and its amendments require been documented. The application the U.S. Environmental Protection and efficiency of standard waste Agency (EPA) to conduct studies on the management practices at dump "adverse effects on human health and leach operations are site specific the environment of the disposal and and are limited by the magnitude of utilization of solid wastes from the these facilities. extraction, beneficiation, and processing This Project Summary was of ores and minerals." The EPA developed by EPA's Hazardous Waste submitted a report to Congress on Engineering Research Laboratory, December 31, 1985, that indicated Cincinnati, OH, to announce key concern with the low pH and the findings of the research project that potentially high concentration of metals found in leachates and leach material environments, however, the amounts of process uses scrap iron to precipitate associated with copper dump leaching. surface-water runoff and ground water copper from the pregnant solution. The The EPA subsequently issued a are greater because of winter snow iron replaces copper in solution, and this regulatory determination on July 3, 1986, accumulation. iron-rich solution is subsequently that expressed continued concern about recycled to the top of the leach dump. mining wastes having high acid- Operating Practices Upon exposure to the atmosphere, the generation potential. Also in this dissolved iron oxidizes to form insoluble determination, the EPA indicated that it Copper leach dumps typically cover salts, which precipitate on the surface of would develop a regulatory program for hundreds of hectares, are more than a the dump and restrict the flow of solution. mining wastes under Subtitle D of RCRA hundred meters high, and contain Solvent extraction uses a complexation millions of metric tons of leach ore. and collect additional information on the mechanism whereby copper is chelated; nature of mining wastes and Copper leach piles as small as 8 the copper is then stripped from the management practices and the potential hectares (Cyprus Johnson) and as large organic carrier by a strong acid solution. for exposure to these wastes. The full as 850 hectares (Bingham Canyon) were Kerosene is a common carrier used in report addresses these issues with observed during this study. Estimates most solvent extraction operations, and it regard to the development, operation, indicate that more than 5.5 billion metric may appear in small quantities in the and closure activities associated with tons of leach ore now exist in copper raffinate recirculated the dump. copper dump leaching operations. leach dumps scattered around the United States and in excess of 40 million metric General Characteristics tons of new material is being added to Environmental Impact Although the number of active mines these dumps annually. Seepage from leach dumps and in the United States has decreased in Dump leaching and heap leaching are solution collection systems is the most recent years, the percentage of primary distinguished by the use of liners. Dump significant potential mechanism for the copper produced by leaching has leaching refers to the leaching of low- release of contamination into the ground increased. low copper prices have grade ore that has been deposited water. One of the primary criteria in siting resulted in the closing of many mining, directly on the ground. The pregnant leaching operations is proximity to the milling, and smelting operations. leach solution is typically collected in mine. In dump leach operations, the Although many of the leaching unlined natural drainage basins. In ground surface is neither lined nor operations associated with these sites contrast, heap leaching refers to the treated in any manner to reduce also have been closed, a significant leaching of ore that has been deposited seepage. Because the leaching solutions number are still active because of the on specially prepared pads. Dump are in direct contact with the earth, some relatively low operating costs associated leaching operations are always continuous solution loss results. Releases with dump leaching. The result has been constructed in the immediate vicinity of can also result from pipe and dam an increase in the percentage of copper the mine site. Leach sites are selected to failures equipment malfunctions, and produced by leaching operations in the minimize haulage costs and to utilize the overflows due to severe storm events. United States. Estimates indicate that by natural drainage patterns of the native The solutions generated in copper 1990 approximately 30 percent of the terrain for collection of the pregnant dump and heap leaching operations copper produced in this country will be liquor solutions. usually have a lower pH and higher recovered by some type of leaching Leaching of copper from massive concentrations of metals and total dumps of sulfide ore is accomplished process. dissolved solids than the natural waters The areas in which copper leaching is bacterial activity and, often, by the surrounding the site. Leach ores contain practiced are similar in general addition of sulfuric acid. Ferric sulfate, pyrites and other naturally occurring characteristics. Most of the active U.S. the major. lixiviant, forms in the presence metal sulfides that oxidize to generate a copper dump and heap leaching sites of oxygen and bacterial activity. The low-pH solution when exposed to air are in the Southwest. The climate in bacteria generate acid in situ, which and microbial activity. The solvent these areas ranges from arid to semi- prevides acid for acid-consuming extraction process also reduces the pH of arid. The topography of these areas reactions, including oxygen reduction. the solution by ion exchange before it is varies from gently rolling hills to Frequently, only makeup water is needed distributed on the leach dump. Generally, mountainous terrain. Vegetation is in copper dump operations because the acidic solutions increase the solubility sparse. The active leaching operations in oxidation of the sulfide minerals and bioavailability of heavy metals Utah and northern and eastern Arizona generates sufficient acid to dissolve the contained in the leach material and rock tend to be located in more mountainous copper and maintain an active bacterial surrounding the dump. terrain than those in southern Arizona population. More effective leaching The water quality around several and western New Mexico. The land reagents have been identified, but they active copper dump leaching operations surrounding many of the active leaching are generally much more expensive and has been affected by leachates that have operations consists primarily of sparsely their impact on the environment is seeped into the ground water. The populated and undisturbed vacant land. uncertain. available ground-water monitoring data Some of the active leaching operations, Copper is recovered from pregnant indicate that some degradation of the however, are relatively close to leach liquors either by cementation or ground water has occurred around residential-and urban areas. by solvent extractionlelectrowinning. several copper dump leaching Very little surface water is found near These processes remove copper from operations. Some seepage of leachates active leaching sites in the Southwest. solution and allow other dissolved into the ground beneath copper leach The depth to ground water, which is the substances to accumulate. The recovery dumps is inevitable. The amount of principal source of water most of these process itself may