The terraced open pits of these copper deposits are the largest ore mines in the world. Their dimensions often extend over an area of more than one square kilometre with mining depths of several hundred metres. Currently, about 75 % of the primary copper extracted comes from open pits. In many cases, originally sulphide ore deposits have been transformed into oxide minerals by oxidising influences in the area of the earth’s surface. Therefore, in many deposits, mining starts from the earth’s surface with the extraction of oxidic copper minerals in order to gradually reach the area of sulphidic ores with increasing depth. Depending on the nature of these processes, deposits of varying size and richness can be formed.
An ore is a rock in which enough valuable mineral is found to be worth extracting. For copper, this means that a so-called extraction makes sense when there are more than 2 kg of copper in 1,000 kg of ore (0.2 %). Copper minerals are found in various forms, with only a small number being mined. Copper ores occur frequently. For example, copper is extracted from chalcopyrite (chalcopyrite, CuFeS2), chalcocite (chalcopyrite, Cu2S), and less frequently from bornite (variegated chalcopyrite, Cu5FeS4), atacamite (CuCl2- Cu(OH)2), malachite (Cu2[(OH)2|CO3]) and other ores. Already in 2010, 636 copper minerals were known. The sulphide ores are the most commonly used.
The extracted ores have considerably lower copper contents than the pure copper minerals. The ores mined today often contain only about 1 % Cu, in some large mines even only about 0.5 % Cu. The latter can only be economically extracted in cheaper opencast mines using the most modern mining methods. The terraced open-pit copper mines are the largest ore mines in the world. Although much less copper is produced than iron, the amount of rock moved equals that of the entire iron mining industry in the world.
Before the copper ore is processed by smelting or wet metallurgical processes, the so-called smelting, the large amounts of “dead” accompanying rock present in the mining ores are separated from the copper-containing ore parts. The ore is first crushed by ore crushers and ground into powder in mills, whereby the exposure of the individual mineral phases often only takes place at grain sizes <100 μm.
In the case of sulphide copper ores, the ore is enriched to copper concentrates using the flotation process. In this process, minerals are separated from each other in an ore slurry due to different surface properties. For this purpose, finely ground raw ore is slurried in water to which certain chemicals and foaming agents that influence the wettability of the minerals are added; air is injected in a fine distribution at the bottom of the containers. The sulphidic copper mineral particles are hydrophobic, attach themselves to the air bubbles and are lifted by them into the foam accumulating at the surface, while the dewy rock grains – the gangue – sink to the bottom of the container. In this way, ore concentrates are obtained whose copper content is usually between 20 and 30 %. Very rich concentrates contain up to 50 % copper. Since the copper contents of the raw ores are very low and a large amount of “mountains”, i.e. unusable material, is produced during flotation, the factories for processing the raw ores are located close to the deposits for economic reasons. From oxidic copper ores, the copper is normally extracted wet metallurgically. This involves treating the crushed ore in tanks with sulphuric acid and leaching copper out of the ore. This process can also be applied to ore and tailings piles as well as entire deposits, provided the rock allows the necessary penetration of sulphuric acid. The dissolved copper is then precipitated from the leach by suitable extraction processes.
In subsequent smelting, copper concentrates are mainly processed by smelting metallurgy (pyrometallurgy), while oxide copper ores are processed by wet metallurgy (hydrometallurgy). Due to the high investment costs, smelting metallurgical processing is only worthwhile for high capacities of over 50,000 t of copper per year.
The pyrometallurgical process is the predominantly used process and is carried out on sulphide copper ores that have already been enriched by flotation. The location of the smelter is not tied to the proximity of the mine, as the transport of copper concentrates is economical even over long distances. Hydrometallurgical processing, on the other hand, is already economical at smaller capacities, in some cases already at less than 10,000 t of copper per year.
Smelting metallurgical extraction
The extraction of raw copper from copper concentrates, preferably from chalcopyrite (CuFeS2), takes place in several reaction stages. The process used in the past involves the three operations of partial roasting, stone melting, converter operation, while the technique most commonly used today, the flash smelting process, combines melting and roasting in one operation. A third possibility is to combine the three individual operations into one continuous process. Basically, the reaction sequence is the same for all melting metallurgical extraction processes used. It leads via the smelting of the copper concentrate to copper matte (copper content up to 80 %), via the conversion process stage to blister copper (copper content 96 to 99 %) and with subsequent fire refining to anode copper (copper content ≥ 99 %, oxygen content ≤ 0.2 %).
In order to produce copper matte, a mixture of copper sulphide and iron sulphide, copper concentrates are melted in an oxidising process with the addition of slag formers (SiO2). Depending on the process, a greater or lesser part of the necessary melting energy is obtained by partial oxidation of the sulphur to sulphur dioxide and of the iron to iron oxide. The right ratio of copper, iron and sulphur is important for smelting to copper matte. Often, therefore, some of the sulphur must first be removed by ore roasting. Here, the copper matte separates from the resulting, specifically lighter slag (main components: Fe0, Si02, Ca0, Al203; 40 % iron, 1.5 % copper), which floats as a layer on the liquid copper matte. When smelting copper matte, a fundamental distinction is made between bath and flash smelting processes. In the first process, the reactions take place mainly in the liquid phase in the ore flame furnace and in the gas phase in the latter. In the flash smelting process, the downstream settling zone in the lower furnace serves only to separate copper matte, slag and flue dust.
Today, the flash smelting process (Outokumpu process) has become the standard for large capacities: Dried ore concentrate (maximum 1 % moisture), sand aggregates and flue dust from gas cleaning are taken from a bunker and introduced into the flash smelter. In addition, air enriched with oxygen is blown in. In the upper part of the furnace, partial oxidation of the sulphur and iron by the oxygen takes place (partial roasting). The roasting of copper concentrate is a highly exothermic process (CuFeS2 + 13/2 O2 2 CuO + Fe2O3 + 4 SO2 – 971 kJ). The resulting reaction heat is used to further heat the descending concentrate. The particles reach the melting point and sink to the bottom as copper matte or slag drops. The copper matte consists of copper sulphide and iron sulphide (Cu2S + FeS) and contains on average about 65 % copper, 20 % sulphur and 15 % iron. The resulting slag consists of iron oxide and silicates and contains about 40 % iron and 1.5 % copper. This relatively high copper content requires further processing: the slag is therefore fed into an electric furnace. The copper content is reduced by this process to about 0.6 – 0.8 % with the extraction of further copper stone. The waste gas produced here has a temperature of about 1300 °C. It is used in the waste heat boiler. It is used in the waste heat boiler to generate steam or energy. The waste gas itself contains 20 – 40 % SO2. It can be used for sulphuric acid production. The copper matte obtained in the flash smelter and electric furnace is finally fed into the converter in the molten state (approx. 1180 °C) for further processing.
Here, a further reduction of the sulphur and iron content takes place by first oxidising the remaining iron sulphide by blowing air into the liquid copper matte, whereby sulphur is discharged with the exhaust gas as gaseous SO2 and processed to sulphuric acid, while the iron forms a slag with added silicic acid, which is poured off. During the subsequent blowing process, the copper sulphide also decomposes, so that the converter content finally consists of liquid raw copper and a slag with a high copper content. The end product of the converter work is the blister copper with 98 to 99 % copper content. This is then transferred to the anode furnace.
Among the bath smelting processes, the direct processes, which combine all three process steps of roasting, smelting and blowing in one step, are of great importance. The production processes known as the Mitsubishi process and the Noranda process have achieved large-scale technical application in this field. Although the wear of the oxygen/air lances and the quality of the blister copper produced are problematic in the Mitsubishi process, the only continuously and economically operating converter to date has been integrated in this process. While the bath melting processes are well suited for the direct use of return materials, this is only possible to a limited extent with the flash melting processes. This may also determine the selection of the process, as recovery from secondary materials is becoming increasingly important.
The hydrometallurgical or wet-chemical extraction of copper is used for oxide and oxide-sulphide ores. It has a comparatively low but steadily increasing share of global copper production. First, copper concentrate with a copper content of 15-30 % is formed by leaching the ores with sulphuric acid. In the subsequent process of cementation, scrap iron is added to the acidic extract, causing iron to dissolve and the more noble copper to be released as a metal. The resulting crude copper must still be separated from impurities such as iron, lead, tin, zinc, arsenic or antimony in a refining smelter before it goes through the final process of electrolytic refining. The ratio between pyrometallurgical and hydrometallurgical extraction is about 80 % to 20 %.
The introduction of the solvent extraction technique (leaching with sulphuric acid and organic extraction agents) around 1970, followed by the electrowinning process (winnowing electrolysis), decisively improved the hydrometallurgical processing – preferably of oxide copper ores. The SXEW cathodes obtained are not inferior in quality to those obtained by refining electrolysis.
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