Copper Refining: From Ore to Market

By Leia Toovey- Exclusive to Copper Investing News

Mined rock typically contains less than one percent copper, therefore, to obtain the end result of market-ready copper, mined rock must undergo a variety of physical and chemical processing steps. Through refining, the unwanted material is progressively removed, concentrating the copper up 99.99 percent purity, the standard for Grade “A” copper.

After mining, the first major step in getting copper market-ready is concentration. This crucial step, which is generally conducted at or very near the mine (to save on transportation costs), involves grinding up the rocks to roughly separate the copper from the waste rock.  The copper is concentrated further by slurrying 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 phase, the copper concentration is 24-40 percent.

After the concentration step, the next phase is refining, which typically takes place away from the mine, at a refining plant/smelter. The details of the refining process depend on the type of minerals the copper is bound with. Copper ores rich in sulfides are processed via pyrometallurgy, while copper ores rich in oxides are refined through hydrometallurgy.

Pyrometallurgy

In pyrometallurgy, the concentrate is dried and then 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 bottom, matte layer, contains the copper, and the top, slag layer, 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 which react with the metal, resulting in the formation of converted copper, called “blister copper.” The blister copper is recovered, and then subject 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. The left-over 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 percent pure.

Hydrometallurgy

In the hydrometallurgical process, copper oxide ores are leached with sulfuric acid. After leaching, the metal undergoes further refining by one of a few processes. The least-common method is cementation, in which 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. The more commonly employed method is solvent extraction. In solvent extraction, an organic solvent in which copper is soluble is introduced. As the copper enters the organic-copper solution it separates from the impurities/unwanted material.  Next, sulfuric acid is added to strip the copper from the organic solvent into an electrolytic solution. The solution is then subject to another process- called electrowinning, which, simply put, results in the plating of copper onto a cathode. The cathodes can be sold as-is or made into rods or starting sheets for other electrolytic cells.

Getting copper to market

Mining companies sell copper in one of two ways- by concentrate, or by cathode. As mentioned above, copper concentrate is most often refined away from the mine site. Concentrate producers sell a concentrate powder containing 24- 40 percent copper metal content to a smelter/refiner. The selling terms are unique to each smelter, but the general formula involves the smelter paying the miner about 96 percent of the metal value based on metal content contained, minus the treatment charges (“TC”) and refining charges (“RC”). TCs are charged on a dollar per tonne of concentrate treated basis and RCs on a dollar per pound of metal refined. The charges fluctuate with the market but are often fixed on an annual basis.

The concentration of the copper ore is indicated by the miner, and maybe spot-checked, by a third party when en route to the refiner. Penalties may be assessed against copper concentrates according to the level of deleterious elements such as  lead or tungsten. Most smelters have strict limitations on the permissible concentrations of impurities. If the concentrate producer does not meet these requirements, they will be subject to financial penalties. Miners may also receive a credit for “valuable” minerals- such as gold and silver, minus any TCs and RCs, which are levied, separately, on these metals.

Smelters generally operate as a toll, but they may also sell the metal on behalf of the miner. All the risk (and reward) of fluctuating copper prices falls on the miner’s shoulders. Copper concentrate is traded either via spot contracts or under long term contracts as an intermediate product. The miner is paid the price at the time that the smelter-refiner makes the sale, not at the price on the date of delivery of the concentrate. The price is agreed to be at a fixed date in the future, typically 90 days from time of delivery to the smelter, therefore, if copper prices decline, it is the miner that takes the hit. TC /RCs are dynamic, and tend to rise when there is a high availability of copper ore.