It is almost impossible to imagine a world without copper, writes Dineo Phoshoko.
The World Copper Factbook 2017, compiled by the International Copper Study Group (ICSG) in 2017, defines copper as a “malleable and ductile metallic element”. Copper is also considered to be a good source for conducting heat and electricity.
The metal has various uses, some of which are unique to copper. Transport, appliances, electronics, and power generation are among the end uses of the final product.
In an article for the Copper Development Association, Vin Calcutt highlights the fact that copper minerals and ores are located in igneous and sedimentary rocks. As such, copper can be mined using both underground and opencast mining methods. In the article, titled “Introduction to Copper: Mining & Extraction”, Calcutt explains that copper mined underground usually has higher quality compared to copper mined using opencast mining methods.
Conventional copper processing
The copper mining industry has seen many innovations in the extraction process throughout the years. Before today’s technology, copper was extracted using conventional methods of leach, solvent extraction and electrowinning (L/SX/EW).
In a paper written for the Journal of the South African Institute of Mining and Metallurgy, GA Kordosky outlines the three methods used in the extraction of copper before the emergence of technological innovations.
The paper titled “Copper recovery using leach/solvent extraction/electrowinning technology: Forty years of innovation, 2.2 million tonnes of copper annually”, Kordosky mentions that in 1968, there were only two practices for copper leaching. The first process was the vat leaching of high-grade copper oxide ore. The process was followed by electrowinning of copper from the leach solution — which produces low-quality copper cathode at a high cost. A decrease in the tonnage of high-grade oxide ores in 1968 resulted in a decline in the vat-leaching process.
The second copper leaching process is the heap and dump leaching method. In his paper, Kordosky explains that the “heap and dump leaching of low-grade oxide and/or sulphide ore was followed by precipitation of low-quality copper from the leach solution on scrap iron”. This process was practiced on oxide ore that was either too low-grade for vat leaching, or low-grade sulphide ore that needed to be mined to expose the underlying high-grade sulphide ore. In those days, recovering copper from leaching low-grade copper ores was very beneficial because understanding the leaching process required minimal effort. Kordosky explained that both these processes — vat and copper leaching — used diluted sulphuric acid.
Improvement in copper mining processes
In that same year, Rancher’s Bluebird copper solvent extraction plant was built for copper operation in Arizona, US. With the plant came technological innovation that raised the bar on improving copper extraction methods. The plant illustrated that L/SX/EW technology had the ability to produce large quantities of good quality cathode copper consistently on a daily basis. The plant also proved to be profitable to an extent that the copper industry became aware and interested in copper L/SX/EW.
Improvement in L/SX/EW technology was inevitable. In Kordasky’s paper, he writes that “the first improvement in copper SX reagents came when LIX 64N was added as makeup to the Ranchers plant in late 1968”. LIX 64N had several advantages with greater extractive strength, faster kinetics, faster phase separation, and lower entrainment, among others. Kordosky explained that the benefits of LIX 64N resulted in reagent properties that broadened the range of copper leach liquors that could be successfully treated by solvent extraction. This had a knock-on effect on the SX plant, as capital costs were reduced. Decreased organic losses and tankhouse bleeds lowered overall operating costs. Adding LIX 860-1 to plants that were already using LIX 64N enabled the plants to upgrade plant performance and flexibility quickly and conveniently.
Various companies with other improvements to L/SX/EW technology were introduced to the market over time, each with their unique benefits. Among them was the reagent SME 529, which was an alternative to LIX 64N. Unfortunately, “the poor properties of the side products from the manufacture of this reagent overrode the very good properties of the extractant molecule widely used” — resulting in limited use of the reagent in the market. LIX 65N, SME 529, P-1, and LIX 84-1 were other technologies introduced to the market.
From a leaching perspective, copper producers felt that this method was an important source of copper, especially after seeing the cost-effectiveness of purifying and concentrating copper from leach liquors. The Thin Layer (TL) acid cure leaching process was one of the copper leaching processes and in his paper, Kordosky explains that Sociedad Minera Pudahuel (SM) was the first plant to practice the TL leaching for copper in 1978. Through this process, the plant was able to achieve “high copper recovery from both the oxide and sulphuric portions of their ore low soluble silica in the pregnant leach liquor, and an overall water/acid balance to give a zero-discharge plant”. By leaching the tails from their TL operation for an additional 45 days, SMP was able to prove that total copper recovery from the chalcocite/bornite portion of their mixed oxide sulphide ore could reach 85%. From then, the significance of bacteria leaching metal sulphides was recognised.
Kordosky mentions that in 1968, Ranchers installed flotation cells to remove entrained organic from the pregnant electrolyte, resulting in improved copper quality — which fell into the category of electrowinning (EW). Seven years later, there was a breakthrough in EW with the registration of Baghdad cathode on the Comex, followed by the registration of Anamax cathode on the London Metal Exchange several years later. One of the methods of implementing cathode through EW was a cathode press to straighten two-day cathodes grown on copper starter sheets. The results of this process were higher current efficiencies and improved copper quality.
Another innovation in EW was modern copper EW tankhouse coupled with copper SX. The results were a “93–95% current efficiency while producing 60% to 80% more copper per unit of tankhouse area than the early EW tankhouses”..
Copper mining and extraction today
Kordosky’s paper eludes to copper extraction innovation that has paved the way for new copper extraction methods and technologies today. Copper extraction has since improved where today, big plants are able to process large tonnes of copper at the highest quality, using smart technological innovations to do so. Elements of conventional copper processing still remain, even with the age of technological innovation.
The University of Arizona’s Superfund Research Programme outlines the complex processes of copper processing today. As complex as the processes are, up to 99.99% of pure copper can be reached, despite mining ore of less than 1%.
According to the research programme, there are two different methods of processing copper, depending on the type of chemistry ores of the copper. The first method is hydrometallurgy, which is more suitable for copper oxide. Copper oxides are found closer to the surface and are ideal for opencast mining method. The disadvantage of copper oxides is that they have low-grade ore, with a lower concentration of copper. The research programme highlights that it is possible to make a profit from mining copper oxides, even though they require additional ore extraction and processing.
The hydrometallurgy method of processing copper oxide relies on using water-based solutions to extract and purify copper oxides at ordinary temperatures. Three steps are involved in this process: heap leaching, solvent extraction, and electrowinning.
On the other hand, copper sulphide ores are less abundant; however, they contain higher amounts of copper. The disadvantage that comes with mining sulphide ores is that the processing costs are very high. Sulphide ores are worth mining in the end because more copper can be extracted from the ores.
For sulphide ores, pyrometallurgy processing is used where the metals are extracted and purified using heat applications. Physical steps and high temperatures are required to purify the copper from sulphide ores. This process requires four steps, namely froth flotation, thickening, smelting, and electrolysis.
Another unconventional method of processing copper is recycling. The World Copper Factbook 2017 mentions that copper is one of the few materials that do not degrade or lose their physical properties during the recycling process. In 2015, the ICSG estimated that up to 29% of used copper came through recycled copper. New and old copper scrap or copper alloys can be melted, re-purified, and recycled into new components.
The United States Geological Survey estimates that there are about 720 million tonnes of identified and unidentified copper resources. This can only mean more innovation in copper mining, and processing can be expected in going forward in the copper mining industry.