Copper Processing

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.

Solvent extraction is more commonly used to refine copper. An organic solvent in which copper is soluble is introduced. As the copper is more soluble in the organic layer than the aqueous, it enters an organic-copper solution and is separated. Sulfuric acid is added to strip the copper from the organic solvent into an electrolytic solution.

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.

* USEPA, Profile of The Nonferrous Metals Industry. EPA 310-R-95-010.