TENORM: Copper Mining and Production Wastes
Soils and rock in copper mining areas may contain naturally occurring radioactive materialsnaturally occurring radioactive materialsMaterials found in nature that emit ionizing radiation that have not been moved or concentrated by human activity. (NORM):
Mining and extraction of copper by common surface or underground methods can concentrate and expose radionuclides in the waste rock (tailingstailingsThe remaining portion of a metal-bearing ore after some or all of a metal, such as uranium, has been extracted.). Another extraction method, known as in-situ leaching, can transport uranium and thorium into ground water or surface water at the site.
Both of these methods expose and/or concentrate NORM, transforming it into TENORM (Technologically Enhanced Naturally Occurring Radioactive Materials).
Copper mining wastes constitute the largest quantity of metal mining and processing wastes generated in the United States. The copper industry is primarily located in the arid west, with Arizona accounting for nearly 61% of the domestic copper production in 2005. Background soil radiation levels in Arizona are highly variable due to the widespread occurrence of uranium-rich source rocks. This results in a broad range of radiation levels in copper mine wastes. The variation depends on several factors:
- Regional geology.
- Mineral assemblage.
- Geologic formation.
The amount of marketable copper produced is small compared to the original material mined. Several hundred metric tons of ore must be handled for each metric ton of copper metal produced, thus generating large waste quantities.
Because of the large waste volumes associated with copper production, the processing facilities are usually located near the mines. Copper production processes include:
- Solvent extraction.
- MillingMillingIndustrial operations that process ores to concentrate desired minerals. According to the Atomic Energy Act, uranium milling is any activity that results in generation of byproduct material during the production of special nuclear material..
- Physical separation.
The vast quantities of ore, overburdenoverburdenSoil and rocks that have been moved out of the way to get to ore are called "overburden." In areas where there are high concentrations of radionuclides in the rock, overburden may have some enhanced radioactivity, but not enough to mine and process. and rock are separated at the mine site and the rock is hauled to the waste site. The ore is crushed, mixed with other low-grade ore, and chemically leached to remove the copper.
During the leaching process, weak acids are allowed to seep through copper-bearing rocks dissolving copper and any radionuclides present in the soil. The leached liquid containing the dissolved copper, known as a pregnant leach solution (PLS), is later collected and further processed to extract the copper.
The liquid leachate may lead to TENORM contamination in the surrounding environment by seeping into the ground water at abandoned and active copper mining sites. Because of these concerns, radiological surveys were conducted by the Arizona Department of Environmental Quality (ADEQ) Exit at selected sites throughout the state of Arizona. Measurements determined radiation levels in ground water, surface water and soils.
The ADEQ surveys show that radionuclideradionuclideRadioactive forms of elements are called radionuclides. Radium-226, Cesium-137, and Strontium-90 are examples of radionuclides. concentrations vary greatly from one area to the next. Some sites exhibit radiation levels at or near background levels while others are above federal maximum contamination levels (MCLs) and Arizona state guidelines for ground water and surface water contamination. Elevated levels of radioactivityradioactivityThe emission of ionizing radiation released by a source in a given time period. The units used to measure radioactivity are curie (Ci) and becquerel (Bq). were also found in soil and sediment samples. More detailed surveys of leach wastes are needed to characterize potential TENORM contamination, particularly in mines located in Arizona and New Mexico.
During in-situ leaching, rather than removing soil and rock to reach copper deposits, acids are injected into ore bodies via wells. The PLS is captured in production wells and pumped to a leach plant where the copper is later recovered. High levels of TENORM have been found in the PLS of two in-situ leach operations in Arizona.
Dump leaching refers to leaching that takes place on an unlined surface. The leach solution flows by gravity through the dump pile to a collection point, usually a pond, constructed at the bottom of the dump.
In contrast to dump leaching, heap leaching refers to the application of chemicals to low-grade ore that has been crushed and deposited on a specially designed pad. The pad is lined with synthetic or natural materials such as polyethylene or compacted clay. Copper heap leach operations are much smaller than copper dump leaches. On the average, heaps contain between 100,000 and 500,000 metric tons of ore.
Once the PLS is collected it is sent to a solvent extraction (SX) plant to remove the copper. SX is a two-step process: In the first step the PLS is mixed with an organic solvent that selectively binds to copper. The copper is then physically separated from the rest of the solution. The now barren solution, known as raffinateraffinateWhat is left behind after the more soluble portion of a material is removed. A liquid from which impurities have been removed by solvent extraction., contains all the remaining elements, including any radionuclides that were dissolved in the PLS. In the second step, the raffinate is recycled back to the leach dumps as a leaching solution and the organic-copper solution is combined with an acidic mixture that strips the copper from the organic component.
Recycling raffinate back to the leach operations may exacerbate the occurrence of TENORM at copper mining sites because it can contain concentrated amounts of radionuclides. Analysis of water samples taken at the ASARCO Santa Cruz In-Situ Copper Project by the ADEQ show levels of uranium, radium, and radon to be 2,870 picocuries per liter (pCi/L), 193 pCi/L, and 2,410 pCi/L respectively in the raffinate streams, all well above their natural crustal abundance.
Milling and Separation
Higher grade ores are further milled then concentrated by physical separation. The tailings are pumped to the tailings pile and the copper concentrate is transported to a nearby smelter.
Mill tailings may contain radionuclides due to their natural presence in the ore bodies and thus are a potential source of TENORM. Because the pyrite/sulfide minerals are in tailings piles with an exposed surface area, they may be particularly susceptible to leaching of radionuclides. If pyrites/sulfides are exposed to the air and water, they may form sulfuric acid that will mobilize many metals including uranium, which is highly soluble in acid. This process is known as acid mine drainage (AMD), a natural process that occurs at many abandoned mine sites. Learn more about abandoned mine drainage.
Copper smelting involves three steps:
Roasting: Ore concentrates are roasted or heated to remove sulphur and moisture.
Smelting: Copper concentrates are mixed with silica (sand) and limestone then heated in a furnace to form two immiscible (naturally separating) layers. One layer is waste consisting of iron and silica compounds and is discarded as slagslagByproduct left after a desired metal is thermally separated from its raw ore.. Approximately 75 percent of the copper concentrate ends up as slag. The other layer, called "matte copper," consists of copper, iron sulfide and other metals.
Converting: Matte copper is transferred to a converter, where more silica is added to help separate it into a copper-rich slag, which is returned to the crusher, and "blister copper," which is sent to another furnace for casting.
EPA's Report to Congress on Special Wastes from Mineral Processing indicates that 2.5 million metric tons (MT) of smelter slag and 1.5 million MT of slag tailings were generated by copper smelting and refining facilities in the U.S. in 1988. However, the slag waste volume from copper smelting and refining is very small compared to the overburden and tailings waste volumes from mining and beneficiation (e.g., crushing) operations.
The ADEQ detected elevated levels of TENORM in the smelter flue dust at the Magma Copper Company's smelter and concentrator operations in San Manuel Arizona. The exact source of the radiation is unclear, and it may originate from the ore concentrates, or the natural gas used in the smelter, or it may come from some other source.
Copper mining waste piles may be as large as 400 hectares and typically include three types of waste:
- Tailings (33%).
- Dump and heap leach wastes (28%).
- Waste rock and overburden (39%).
Some of the copper mine wastes have been put to use, but on a limited scale. Mixtures of crushed waste rock, including waste rock from the copper mines, have been used to construct embankments, fills, or pavement bases for highways. Some studies have shown that copper tailings can be used in bricks if pyrites are first removed. Read the 1985 EPA Report to Congress, "Wastes from the Extraction and Beneficiation of Metallic Ores, Phosphate Rock, Asbestos, Overburden from Uranium Mining, and Oil Shale" (PDF) (303 pp, 9.5 MB, About PDF).
A typical in-situ facility contains raffinate impoundments and processing facilities for the injectate (a lixiviant (leaching solution) of sulfuric acid with a pH of 2), a PLS impoundment, a SX plant, surface run-on/run-off facilities, an evaporation impoundment, a non-storm water containment impoundment, and ancillary facilities. The mining area is usually divided into discrete mining units. Recovery wells are constructed 50 feet to 200 feet from the injection wells, depending on the permeability of the formation. Once the ore zone has been depleted, it will be rinsed with fresh formation water until the aquifer meets Aquifer Water Quality Standards (AWQS) and Primary Maximum Contaminant Levels (MCLs).
Raffinate generated at copper mines is generally stored in ponds and recycled back to the leaching operation as a leaching solution. Following the closure of a mine, the raffinate must be disposed.
Uranium-enriched raffinate may be considered a resource that can be exploited at relatively low cost through eulex-ion exchange technology. This technology removes the potential contaminants from the environment and contributes to the long-run profitability of the mining operation by reducing remediation costs.
Smelter slag is initially deposited in separate piles. These slag piles range in surface area from one to 30 hectares, and in height from three to 45 meters. As of 1988, slag accumulations in individual piles ranged from 0.5 to 21 million metric tons (MT).
Three copper smelters (San Manuel, White Pine, and Garfield) process all their smelter slag either in a conventional ore concentrator or in a stand-alone slag concentrator. The slag tailings from these operations are co-managed at on-site tailings piles with the tailings from ore beneficiation. Slag tailings ponds range from 142 to 2,270 hectares, with an average size of about 600 hectares. Depths range from about 16 to 61 meters with an average depth of 46 meters. As of 1988, quantities of slag tailings in these ponds ranged from 240,000 MT to 3.4 million MT, with an average of 1.8 million MT.
EPA is conducting a number of studies on various sources of TENORM in the United States. When the Arizona Department of Environmental Quality shared data with EPA on TENORM emanating from copper mines in mid 1992, the Agency began a study of the occurrence and distribution of TENORM at mines in the southwestern copper belt of Arizona.
View the report, "TENORM in SW Copper Belt of Arizona" (PDF) on the TENORM Resources webpage. The data complied in this report show that leaching and related operations may extract and concentrate soluble radioactive materials. The results show increases of up to 100 times background levels for all radiochemicals tested except radon-222.