5 Copper Processing
The copper processing industry refines copper from metal ores or scrap copper. The leading consumers of copper are wire mills and brass mills, which use the copper to produce copper wire and copper alloys, respectively. End uses of copper include construction materials, electronic products, and transportation equipment. Once refined, copper can be used as a powder in automotive, aerospace, electrical and electronics equipment, in anti-fouling compounds, various chemicals and medical processes. Compounds of copper include fungicides, wood preservatives, copper plating, pigments, electronic applications and specialized chemicals.
Copper can be produced as either a primary product or as a co-product of gold, lead, zinc or silver. It is mined in both the Northern and Southern Hemisphere and primarily consumed in the Northern Hemisphere with the U.S. as a primary producer and consumer.
Copper is mined in open pits and below ground. The ore usually contains less than 1% copper and is often associated with sulfide minerals. The ore is ground, concentrated, and slurried with water and chemical reagents. Air blown through the mixture attaches to the copper, causing it to float to the top of the slurry. The copper is then removed with a skimmer. The tailings remain and are dewatered and disposed of in tailing ponds. The water is recovered and recycled.
One of two processing methods are used to refine concentrated copper. Pyrometallurgy, or smelting, is used on ore with copper sulfide and iron sulfide minerals. The concentrate is dried and fed into a furnace. The minerals are partially oxidized and melted, resulting in segregated layers. The matte layer refers to the iron-copper sulfide mixture which sinks to the bottom. The slag, which refers to the remaining impurities, floats on top of the matte. The slag is discarded on site or sold as railroad ballast and sand blasting grit. Sulfur dioxide gases are also collected and made into sulfuric acid for use in hydrometallurgical leaching (discussed below) or sold off-site.
The matte is recovered and moved to the converter, a cylindrical vessel into which the copper is poured. Air, lime and silica are added to react with the metal oxide. Scrap copper may also be added. Iron slag is removed and often recycled back into the furnace. Sulfur dioxide is captured and converted into sulfuric acid. The converted copper, known as "blister copper," is recovered.
The blister copper then undergoes "fire refining." Air and natural gas are blown through the copper to remove any remaining sulfur and oxygen. The copper is cast into copper anodes and placed in an electrolytic cell. Once charged, the pure copper collects on the cathode and is removed as 99% pure. The copper can be sold to wire-rod mills or further processed into rods. Anode slime refers to impurities that sink to the bottom of the electrolytic cell.
The second method for refining copper is called the hydrometallurgical process. This process begins with oxidized copper ores or oxidized copper wastes. The oxidized material is leached with sulfuric acid from the smelting process. The sulfuric acid is percolated through piles of oxidized metal and collected with acid resistant liners.
Further refining may be performed using one of two processes. In cementation, the
acidic solution of copper is deposited on to scrap iron in an oxidation-reduction
reaction. After sufficient amounts of copper have been plated, the copper is further
refined using the pyrometallurgical process. However, this process is rarely used.
In the electrolytic process, called electrowinning, the copper plates out onto the cathode. The cathodes are sold as-is or made into rods on-site or made into starting sheets for other electrolytic cells.
All remaining organics and acids are reused. Further, sulfur is fixed throughout the process to meet Clean Air Act Standards. If the sulfur content of the gas is over 4%, the sulfur compounds are made into sulfuric acid for use in the process or for sale to fertilizer manufacturers. Slurries with less than 4% sulfur are classified as RCRA hazardous wastes because of sulfur, cadmium, lead and other metals.
Secondary copper processing involves two steps: metal pretreatment and smelting. Pretreatment includes cleaning and concentrating the copper. Concentrating is done manually or mechanically and includes sorting, stripping, shredding and magnetic separation. The metal can be further refined using pyrometallurgical methods — including sweating, insulation burning, or drying — or hydrometallurgical methods — including flotation and leaching. The concentrated metal is then smelted. Generally, copper is fire refined, similar to primary copper smelting operations although the exact procedure depends on the quality of copper scrap.
Primary and secondary copper processing produce similar pollutants with similar pollution prevention opportunities. Air emissions include particulates and sulfur dioxide. Particulate air emissions usually include iron and copper oxides, but many contain other metal oxides, sulfates or sulfuric acid. Particulates are usually captured using emissions control equipment. Depending on the composition of the emissions some recovery of heavy metals may be possible.
In addition, secondary copper processing produces air emissions from the removal of excess oils and cutting fluids. The air emissions are usually captured using baghouses. After-burners may also be used to fully combust products.
Sulfur dioxide is usually captured using single stage electrostatic precipitation. Once captured, the sulfur dioxide is converted into sulfuric acid and sold or reused in process.
Liquid wastes from the copper processing plant include large quantities of water. Most of the water can be reused with minimal refinement. The leaching process creates some sulfuric acid liquid waste. The sulfuric acid is almost always directly reused. Electrolytic refining procedures also produce some liquid waste. This waste is usually sent to waste water treatment facilities and discharged.
USEPA, Profile of The Nonferrous Metals Industry. EPA 310-R-95-010. This document discusses uses, processes and pollution prevention opportunities associated with copper processing.