+ O2 → Cu + SO2 In a parallel reaction the iron sulfide is converted to slag: 2 FeS + 3 O2 → 2 FeO + 2 SO2 2 FeO + SiO2 → Fe2SiO4 The purity of this product is 98%, it is known as blister because of the broken surface created by the escape of sulfur dioxide gas as blister copper pigs or ingots are cooled. By-products generated in the process are sulfur dioxide and slag. The sulfur dioxide is captured and converted to sulfuric acid and either sold on the open market or used in copper leaching processes. Refining Fire refining The blister copper is put into an anode furnace, a furnace that refines the blister copper to anode-grade copper in two stages by removing most of the remaining sulfur and iron, and then removing oxygen introduced during the first stage. This second stage, often referred to as poling is done by blowing natural gas, or some other reducing agent, 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. Electrolysis The final stage in the production of copper is refining. Refining is achieved by electrolysis, which exploits the easy (low potential) and selective conversion of copper(II) solutions to the metal. The anodes cast from processed blister copper are placed into an aqueous solution of 3–4% copper sulfate and 10–16% sulfuric acid. Cathodes are thin rolled sheets of highly pure copper or, more commonly these days, reusable stainless steel starting sheets (as in the IsaKidd process). A potential of only 0.2–0.4 volts is required for the process to commence. In industrial plants current densities up to 420 A/m2 are possible. At the anode, copper and less noble metals dissolve. More noble metals such as silver, gold, selenium, and tellurium settle to the bottom of the cell as anode slime, which forms a salable by-product. Copper(II) ions migrate through the electrolyte to the cathode. At the cathode, copper metal plates out, but less noble constituents such as arsenic and zinc remain in solution unless a higher voltage is used. The reactions are: At the anode: Cu(s) → Cu2+ + 2e− At the cathode: Cu2+ + 2 e− → Cu(s) Concentrate and copper marketing Copper concentrates produced by mines are sold to smelters and refiners who treat the ore and refine