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Copper extractionThe most commonly present source of copper ore is chalcopyrite (CuFeS2), which accounts for about 50% of copper production. Chalcopyrite ore is extensively mined in Chile, the United States, Canada, Zambia, and Poland. In the United States, the states of Arizona, Montana, New Mexico, and Utah lead in ore mining. Indeed, the world's largest copper mine, the Berkeley Pit, is located in Butte, Montana.
This article includes the process of copper extraction from chalcopyrite ore into pure metal. Processes for other minerals are mentioned.
Orebody formationTo a large extent, copper oxide and sulfides are naturally separated in nature. As such, once mined the processing of the ore generally does not need to seperate the oxides and sulfides. Copper orebodies are formed when geothermal solutions bring copper dissolved from deep underground near the surface and cool into rock. Copper is usually deposited as copper sulfides or copper metal. During millions of years the mineral deposit may be exposed to oxygen by air penetration, or by oxygen rich water flowing over it. This oxidation alters the mineralogy, changing the minerals from sulfides to sulfates as sulfate in solution. Beneath this oxide zone, some disolved copper enriches the sulfies, making a secondary enrichment, or transitional zone. The primary enrichment was caused by the superheated geothermal solutions. The secondary enrichment replaces iron in the minerals with more copper, further enriching the ore. Typical copper ore deposits are described in terms of their oxide, secondary and primary ore tons and grades. Common Mineral FormsThe most common copper minerals are:
Hydrometallurgical ExtractionOxide ores are readily leached by sulfuric acid, usually using a heap leach or dump leach process in combination with solvent extraction and electrowinning technology (SX-EW). There have been examples where froth flotation was used to concentrate malachite. In general froth flotation is not used to concentrate copper oxide ores, as the cost of leaching is cheap when compared to the cost of grinding and flotation. The implication of this is that copper oxides are more economic to process than copper sulfides. Secondary sulfides are resistent to sulfuric leaching, and so can be termed refractory. High grade secondary sulfides may be concentrated using froth flotation, and subsequently smelted to recover the copper, or else they can be leached using a bacterial oxidation process to oxidize the sulfides to sulfuric acid, which also allows for simultaneous leaching with sulfuric acid. Again, solvent extraction and electrowinning technologies are used to recover the copper from the pregnant leach solution. Primary sulfides, most commonly chalcopyrite, are strongly refractory to sulfuric acid and to bacterial leaching technologies. Many competing hydrometallurgical technologies have been developed in the 1990's to treat low grade primary sulfide copper ores, as of 2005 the pyrometallurgical approach of smelting a froth flotation concentrate is still the only significant industrial process in use. Given that as of 2005 chalcopyrite is the most widely mined copper mineral, the pyrometallurgical approach the most common process for copper extraction. Pyrometallurgical ExtractionThe following is a process of copper extraction from chalcopyrite ore into pure metal. The copper ore is crushed and ground before it is concentrated to between 20 and 40% copper in a flotation process. The next major step in production uses pyrometallurgical processes to convert the copper concentrate to 99% pure copper suitable for electrochemical refining. These high temperature processes first roast the concentrate, then smelt it in a furnace, oxidise and reduce the molten products to progressively remove sulfur, iron, silicon and oxygen to leave behind relatively pure copper. ConcentrationAll copper sulfide ores are concentrated using the froth flotation process. Ground ore is mixed with xanthate reagents (for example, pine oil), which reacts with the copper sulfide mineral to make it hydrophobic on its surface. For cost reasons, oxide ore is often leached instead. The sulfide ore is crushed and ground to increase the surface area of the ore for subsequent processing, namely smelting. The powdered ore is mixed with pine oil (the 'collector chemical') and introduced to a water bath containing surfactant. Air is constantly forced through the slurry and the hydrophobic copper and pine oil mix latches onto and rides the air bubbles to the surface, where it forms a froth and is skimmed off. These skimmings are cleaned of the collector chemical and surfactant, leaving copper concentrate. The remainder is discarded as tailings, or processed to extract other elements. A example collector chemical is potassium amyl xanthate . An example frother chemical is sodium oleate . To improve the process efficency, limestone is used to raise the pH, causing the collector to ionize more and to preferentially bond to chalcopyrite (CuFeS2) and avoid the pyrite (FeS2). This is then sent into a large aerated tank. Jets of air from the base of the tank cause the copper mineral to float. The product from this froth flotation process is known as copper concentrate. When the foam (which is 20%-40% copper) is dried is is known as copper concentrate. Copper concentrate may be treated by either hydrometallurgical methods or sintered before pyrometallurgical methods are used to produce copper metal. It is sometimes traded either via spot contracts or under long term contracts as an intermediate product in its own right. RoastingIn the roaster, the copper concentrate is partially oxidised to produce calcine and sulfur dioxide gas. The reaction which takes place is: 2CuFeS2(s) + 3O2(g) → 2FeO(s) + 2CuS(s) + 2SO2(g) As of 2005, roasting is no longer common in copper concentrate treatment. Direct smelting using the Flash Smelting or El Teniente design of furnace is now used. SmeltingThe calcine is then mixed with silica and limestone and smelted in an exothermic reaction at 1200°C to form a liquid called matte. Several reactions occur. For example iron oxides and sulfides are converted to slag which is floated off the matte. The reactions for this are: FeO(s) + SiO 2 (s) → FeO.SiO2 (l) 2FeS(l) + 3O2 + 2SiO2 (l) → 2FeO.SiO2(l) + 2SO2(g) Conversion to BlisterThe matte, which is produced in the smelter, contains around 70% copper primarily as copper sulfide as well as iron sulfide. The sulfur is removed at high temperature as sulfur dioxide by blowing air through molten matte: CuS(l) + O2(g) → Cu(l) + SO2(g) In a parallel reaction the iron sulfide is converted to slag: 2FeS(l) + 3O2 + 2SiO2 (l) → 2FeO.SiO2(l) + 2SO2(g) This end products of these reactions are sulfur dioxide, more slag and 98% pure copper, known as blister because bubbles of oxygen are present in the sheets which are cast from the liquid product of this furnace. ReductionThe blistered copper is put into an anode furnace to get rid of most of the remaining oxygen. This is done by blowing natural gas through the molten copper oxide. When this flame burns green, indicating the copper oxidation spectrum, the oxygen has mostly been burned off. This creates copper at about 99% pure. RefiningThe copper is then put into sheets which are refined by electrolysis. The copper sheets (anodes) are placed into a solution of copper sulfate and sulfuric acid. The copper then migrates across the solution to the cathode. The reactions are: At the anode: Cu(s) → Cu2+(aq) + 2e- At the cathode: Cu2+(aq) + 2e- → Cu(s) Copper cathode is 99.97% copper in sheets of dimensions: 96 cm x 95 cm x 1 cm, with a mass of about 100 kg. It is a true commodity, deliverable to the metal exchanges in New York, London and Shanghai. A chemical specification for electrolytic grade copper is ASTM B 115-00. See alsoExternal Links
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