TENORM: Copper Mining and Production Wastes
Within the United States, mining and production of copper are primarily located in the West, specifically Arizona, New Mexico, Utah, Nevada and Montana. According to the United States Geological Survey (USGS), production in these states and Michigan account for more than 99% of domestic copper production. Copper is used in many different industries, including building construction, transportation, electronics and consumer goods.
Minerals are concentrated in specific types of geologic formations (ore bodies), which are where mining takes place. These ore bodies may already contain naturally-occurring radioactive materials (NORM). The most frequently-occurring radionuclides (and their decay products) found in copper ore include:
Copper mining and processing methods can expose and/or concentrate NORM, transforming them into Technologically Enhanced Naturally Occurring Radioactive Materials (TENORM).
Mining and Processing Methods
Ores that contain copper in the U.S. are typically mined in large, open-pits. Copper processing facilities are usually located near mining or extraction sites. Significant waste volumes are associated with copper production. Mine wastes may contain radionuclides due to their natural presence in ores and can be a source of TENORM.
Uranium, which may be co-located with copper ore, may also be recovered as a side stream, if economical. However, in the United States this is not a general practice. Learn more about uranium mining.
The production of copper typically includes the following processes:
- Leaching
- Solvent Extraction
- Smelting
Leaching
There are two main leaching methods: heap leaching and in-situ leaching. Heap leaching is the most common method used in the U.S.
When processing copper through heap leaching, vast quantities of ore and overburden
overburdenSoil 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. are mined. The overburden is separated and either hauled to the waste site or stockpiled for future site reclamation. The ore is crushed and stacked into specially designed pads. The pads are lined with synthetic or natural materials such as polyethylene or compacted clay.
Acids are introduced to copper-bearing ore, seeping through and dissolving copper and any other metals, including radionuclides. The liquid that “leaches” through the rocks is collected. This liquid contains the dissolved copper, known as a pregnant leach solution (PLS). The copper present in this liquid is collected and later processed.
A second method is in-situ leaching. Groundwater and certain geochemical conditions must be present in order to utilize this method. During in-situ leaching, rather than physically mining and removing overburden to reach copper deposits, chemicals are introduced into ore bodies using injection wells. The PLS is then captured in production wells, collected and later processed.
Solvent extraction
Once PLS is leached, it is sent to a solvent extraction plant to remove the copper. The PLS is mixed with an organic solvent that binds to the copper and chemically separates the copper from the rest of the liquid. Then, the copper-bearing liquid is combined with another acid to precipitate the copper from the organic material. Another process, called electrowinning, uses electricity to extract copper out of the PLS onto thin metal sheets.
The remaining liquid is known as raffinate, a waste product. Raffinate can contain concentrated amounts of TENORM.
Smelting
Copper smelting involves three steps:
- Roasting: First, ore concentrates are roasted or heated to remove sulfur and moisture.
- Smelting: Then, copper concentrates are mixed with silica (sand) and limestone and heated in a furnace to form two naturally-separating layers. One layer is a waste containing 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: Finally, matte copper is converted into a copper-rich slag, which is then returned to the leaching process and separated from “blister copper," which is sent to another furnace for casting.
Waste Generation
Copper mining wastes make up the largest percentage of metal mining and processing wastes generated in the United States. There is a broad range of TENORM concentrations in copper mining wastes.
Mining and extraction of copper by surface or underground methods can concentrate and expose radionuclides in the waste rock and tailingstailingsThe remaining portion of a metal-bearing ore after some or all of a metal, such as uranium, has been extracted.. Leaching and solvent extraction/electrowinning processes, as well as the practice of recycling raffinate at copper mines, may extract and concentrate soluble radioactive materials. In some cases, levels up to two orders of magnitude over background have been measured. View the report, TENORM in SW Copper Belt of Arizona, on the TENORM Resources webpage for more information and specific measurements.
Because the pyrite and sulfide minerals are in tailings piles, which have an exposed surface area, they may be susceptible to leaching of radionuclides. If pyrites and 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 chemical reaction produces acid mine drainage (AMD), a pollutant that is present at many abandoned mine sites. Learn more about abandoned mine drainage.
Copper mining waste storage piles may be as large as 1,000 acres and typically include three types of waste; tailings, dump and heap leach wastes, and waste rock and overburden.
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. For example:
- In-situ leaching can transport uranium and thorium into groundwater or surface water at the processing site. High levels of TENORM have been found in the PLS of two in-situ leach operations in Arizona.
- Copper smelting and refining facilities produce 2.5 million metric tons (MT) of smelter slag and 1.5 million MT of slag tailings per year. This is a large volume of slag, however, it is very small compared to comparable waste volumes from mining and crushing operations.
More information about copper mining and production slag can be found in the EPA's Report to Congress on Special Wastes from Mineral Processing.
For detailed information and publications about copper mining and production, including statistics about domestic and international mining and use, see the U.S. Geological Survey Copper Information webpage.
Disposal and Reuse
Some wastes from copper mines can be reused. Mixtures of crushed waste rock 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. For more information, see 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).
Raffinate
Raffinate generated at copper mines is generally stored in ponds and recycled back to the leaching operation as a leaching solution. Raffinate can contain TENORM and needs to be properly managed. Groundwater samples collected at selected mine sites in SW Arizona in the early 1990s showed average values at or below drinking water standards, but high values ranged up to 4,801 Bq/m3 (130 pCi/L) for Ra-226, 4,514 Bq/m3 (122 pCi/L) for Ra-228 and total uranium at 7,733 (Bq/m3) 209 pCi/L. Radon levels were as high as 147,260 Bq/m3 (3,980 pCi/L). View the report, TENORM in SW Copper Belt of Arizona, on the TENORM Resources webpage.
Smelter Slag
Smelter slag is initially deposited in piles. These slag piles range in surface area from one to 75 acres, and in height from three to 45 meters. Slag is associated with smelter operations, which may or may not be co-located with the mines. Copper slag can be used either as sandblast grit or an admixture in concrete.
EPA and the States
EPA's Role
The Arizona Department of Environmental Quality (ADEQ) shared the results of radiological surveys with EPA on TENORM emanating from copper mines in mid-1992. These surveys show that radionuclide concentrations vary, from near-background levels to above maximum contaminant levels, across the state, including increases of up to 100 times background levels for all radiochemicals tested except radon-222.
After reviewing the data provided by ADEQ, the EPA then examined 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 on the TENORM Resources webpage.