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Copper extraction and beneficiation

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This section describes several of the wastes and materials that are generated and/or managed at copper extraction and beneficiation operations and the means by which they are managed. As is noted in the previous section, a variety of wastes and other materials are generated and managed by copper mining operations.

Some, such as waste rock and tailings, are generally considered to be wastes and are managed as such, typically in on-site management units. Even these materials, however, may be used for various purposes (either on- or off-site) in lieu of disposal. Some quantities of waste rock and tailings, for example, may be used as construction or foundation materials at times during a mine’s life. Many other materials that are generated and/or used at mine sites may only occasionally or periodically be managed as wastes. These include mine water removed from underground workings or open pits, which usually is recirculated for on-site use (e.g., as mill/leaching makeup water) but at times can be discharged to surface waters. As another example, leaching solutions are typically regenerated and reused continuously for extended periods. On occasion, however, such during temporary or permanent closure, the solutions are disposed as wastes via land application or other means. Finally, some materials are not considered wastes at all until a particular time in their life cycles. These include spent ore at dump leaching operations: here, only when active leaching for copper recovery ends is the spent ore that comprises the dump considered a waste.

The issue of whether a particular material is a waste clearly depends on the specific circumstances surrounding its generation and management at the time. In addition, some materials that are wastes within the plain meaning of the word are not “solid wastes” as defined under RCRA and thus are not subject to regulation under RCRA. These include, for example, mine water or process wastewater that is discharged pursuant to an NPDES permit. It is emphasized that any questions as to whether a particular material is a waste at a given time should be directed to the appropriate EPA Regional office.

The first subsection below describes several of the more important wastes (as defined under RCRA or otherwise) and nonwastes alike, since either can have important implications for environmental performance of a facility. The next subsection describes the major types of waste units and mine structures that are of most environmental concern during and after the active life of an operation.

Clean Water Act

Under Section 402 of the CWA (33 USC Section 1342), all point source discharges to waters of the United States must be regulated by permit under the National Pollutant Discharge Elimination System (NPDES), with the exception of some storm water discharges covered by the 1987 amendments to the CWA. A point source is defined as any discrete conveyance, natural or man-made, including pipes, ditches, and channels. NPDES permits are issued by EPA or delegated States.

Effluent limits imposed on an NPDES permittee are either technology-based or water-quality-based. The national technology-based effluent guideline limitations have been established for discharges from most active copper mines and mills under the Ore Mining and Dressing Point Source Category (40 CFR Part 440, Subpart J). These regulations govern discharges from all types of copper extraction and beneficiation techniques.

Beneficiation Milling

A secondary and tertiary crusher system further crushes the ore from the coarse ore stockpile. Fine ore, about 0.75 inch in size, is transported by conveyor to the surface mill building. Water is subsequently mixed with the fine ore in each of 16 separate divisions of ball mills in the sulfide mill building. The resulting slurry circulates through a closed system which diverts the finest ore particles (pulp) to the concentrator flotation cells (U.S. EPA 1988b).

The capacity of the Sierrita concentrator is 100,000 tpd (Beard 1990). It uses various inorganic and organic compounds to aid in the extraction of copper and molybdenum. These compounds serve as frothers, flocculents, collectors, flotation modifiers, depressants, leachants, dewatering aids, and water treatment agents. Quantities of organic compounds used at the concentrator are regulated so that these compounds largely remain with the metal concentrate product and are not discarded with process water (U.S. EPA 1989d).

The following inorganic and organic chemical compounds are utilized during ore processing at the Sierrita mill and concentrator: lime, potassium amyl xanthate, allyl ester of amyl xanthate, alkyl sulfonate, Methyl Isobutyl Carbinol (MIBC), petroleum hydrocarbons, anionic polyacrylamides, phosphates, sodium hydrosulfide, sodium sulfosuccinate, and ferric chloride. Frothing, collector, and flocculent reagents are added to the slurry to facilitate the separation of the sulfide-bearing minerals in the flotation cells

The low-grade ore is leached with a dilute solution of in situ-generated sulfuric acid (sulfuric acid is not added for leaching). The lixiviant has been applied by using either infiltration ponds, trickle leach, or rainbird sprinkler methods. The pregnant leach solution (PLS) is collected at the base of the dumps in clay-lined ponds. The PLS, which has fairly high concentrations of dissolved copper, is transported to the precipitation plant via concrete ditches and pipeline. Any excess PLS flows to a double-lined pond with a leak detection system, where it is held for treatment to remove the copper. After the copper has been recovered from the PLS, the barren solution from the cones flows to a sump in the central pump station; from there, it is pumped back to the top of the terraced leach dump piles and recirculated. The pH of this solution ranges from 2.5 to 3.0 (U.S. EPA 1989e; Kennecott 1992).

Each of the PLS ponds have unlined overflow ponds to collect any overflow from the PLS ponds due to a rainfall event or equipment malfunction. The PLS ponds were created by constructing concrete cutoff walls across natural drainages; the walls are keyed into bedrock to prevent subsurface losses. From the ponds, the PLS is conveyed via a main collection canal, which is constructed of epoxy-lined concrete, to the precipitate plant (U.S. EPA 1989e).

Kennecott’s east collection system is “state-of-the-art.” In addition to the main collection canal, a second, emergency overflow canal (constructed of epoxy-lined concrete) collects excess storm water runoff and conveys it to a 500-million-gallon overflow reservoir. This large reservoir is partially lined with clay (i.e., the face of the dam and the bottom of the pond extending away from the dam for several feet are lined). The reservoir is being upgraded to include a plastic liner (Kennecott 1992). This excess storm water is used within the concentrating process. Site personnel have stated that this collection system does not contribute to existing ground water-contamination problems at the site (U.S. EPA 1989e).

Kennecott has two concentrator plants, the Copperton Concentrator and the North Concentrator, with a combined design throughput of 142,000 tons per day (tpd). The Copperton plant, commissioned in 1988 and expanded in 1991, utilizes four conventional semi-autogenous (SAG) mill/ball mill circuits (102,000 to 150,000 tpd) for size reduction with the slurried product feeding a rougher/scavenger froth flotation circuit. Here, copper-, gold-, silver-, and molybdenum-bearing minerals are concentrated. This concentrate is then subjected to subsequent cleaning steps to remove gangue. In addition, the concentrate is further treated in the molybdenite froth flotation circuit where copper, gold, and silver minerals are chemically depressed, recovering a molybdenite concentrate as froth. The copper/gold/silver “tailing” from this step is then thickened and pumped via a 6-inch slurry pipeline to a filter plant, which is adjacent to the smelter some 18 miles away. The molybdenite concentrate is subjected to four further flotation cleaner steps, dried, and then packaged onsite for sale.

The North concentrator utilizes conventional primary, secondary, and tertiary crushing, then a 4-line rod mill/ ball mill circuit (30,000 tpd). This portion of the plant, known as the Bonneville plant, was constructed in 1967. Slurry from the mill is gravity fed 1.8 miles to a froth flotation circuit, constructed in 1984 at the Magna plant. This circuit, although smaller, is identical to the flotation circuit at Copperton. No molybdenite recovery circuit is in use at Magna, but plans call for installation in 1992 (Kennecott 1992). Table 1-8 details the major beneficiation equipment at each plant. Table 1-9 shows reagent type and usage in the flotation process. Reagent type and consumption are the same at both plants.