This article was originally published on Copper Investing News on August 9, 2011.
Mined rock typically contains less than 1 percent copper. Therefore, to become a market-ready copper product, it must undergo a variety of physical and chemical processing steps. Through copper refining, unwanted material is progressively removed, concentrating copper up to 99.99 percent purity, the standard for Grade A copper.
After mining, the first major step in getting copper ready for market is concentration. This crucial step, which is generally conducted at or very near mine locations (to save on transportation costs), involves grinding mined ore to roughly separate copper from waste rock.
The copper is concentrated further by slurrying the ground ore with water and chemical reagents. In the slurrying process, air is blown through the mixture, causing the copper to float to the top. The copper is then removed with a skimmer. At the end of this step, copper concentrations are typically between 24 and 40 percent.
After concentration is complete, the next phase is copper refining. That typically takes place away from the mine, at a refining plant/smelter.
The details of the copper refining process depend on the type of minerals the copper is bound with. Copper ore rich in sulfides are processed via pyrometallurgy, while copper ore rich in oxides are refined through hydrometallurgy.
Copper refining: Pyrometallurgy
In pyrometallurgy, concentrate is dried before being heated in a furnace. Chemical reactions that occur during the heating process cause the copper concentrate to segregate into two layers of material: the matte layer and the slag layer. The matte layer, on the bottom, contains the copper, while the slag layer, on the top, contains the impurities.
The slag is discarded and the matte is recovered and moved to a cylindrical vessel called a converter. A variety of chemicals are added to the converter, and these react with the copper. That results in the formation of converted copper, called “blister copper.” The blister copper is recovered, and is then subjected to another process called fire refining.
In fire refining, air and natural gas are blown through the copper to remove any remaining sulfur and oxygen, leaving refined copper behind to be processed into copper cathode.
This 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 a 99 percent pure product.
Copper refining: Hydrometallurgy
In the hydrometallurgical process, copper oxide ore is leached with sulfuric acid, after which the metal may undergo further refining via one of a few processes.
The least common method is cementation, in which an acidic solution of copper is deposited onto scrap iron in an oxidation-reduction reaction. After sufficient amounts of copper have been plated, the copper is then further refined.
The more commonly employed refining method is solvent extraction and electrowinning, or SX/EW. This newer technology became widely adopted in the 1980s, and, according to the Copper Development Association, roughly 20 percent of the world’s copper is produced via this process.
Solvent extraction begins with an organic solvent, which separates copper from impurities and unwanted material. Next, sulfuric acid is added to strip the copper from the organic solvent, producing an electrolytic solution.
This solution is then put through the electrowinning process, which, simply put, plates copper in the solution onto a cathode. This copper cathode can be sold as is, but can also be made into rods or starting sheets for other electrolytic cells.
Getting copper to market
Mining companies may sell copper in concentrate or cathode form. As mentioned above, copper concentrate is most often refined at a different location than company mine sites.
Concentrate producers sell a concentrate powder containing 24 to 40 percent copper to copper smelters/refiners. Selling terms are unique to each smelter, but in general, the smelter pays the miner approximately 96 percent of the value of the contained copper content in the concentrate, minus treatment charges (TC) and refining charges (RC).
TCs are charged per tonne of concentrate treated, while RCs are charged per pound of metal refined. These charges fluctuate with the market, but are often fixed on an annual basis. TCs and RCs are dynamic, and tend to rise when there is a high availability of copper ore.
Miners indicate copper concentrations, although they may be spot checked by a third party when en route to the refiner. Additionally, penalties may be assessed against copper concentrate according to the level of deleterious elements contained, such as lead or tungsten. Most smelters have strict limitations on the permissible concentrations of impurities, and if concentrate producers do not meet these requirements, they will be subject to financial penalties.
Smelters generally operate by charging tolls, but they may also sell refined metal on behalf of miners. All of the risk (and reward) of fluctuating copper prices, then, falls on miners’ shoulders.
Typically, copper concentrate is traded either via spot contracts or under long-term contracts as an intermediate product. For spot contracts, miners are paid according to the copper price at the time that the smelter/refiner makes the sale, not at the copper price on the date of delivery of the concentrate. For longer-term contracts, pricing is based on an agreed-upon copper price for a future date, typically 90 days from time of delivery to the smelter.