Copper Smelting Process: Historically, the most abundant copper bearing ore was called chalcopyrite, CuFeS2. This ore is first enriched by a process called flotation. Powdered ore is mixed water as a slurry and then further mixed with oil and a chemical called amyl xanthate. Compressed air is then blown through the mixture. The flotation process is dependent on pine oil droplets, but it's really xanthate-coated chalcopyrite grains sticking to air bubbles on their way up and over the tank. Pine oil simply makes the bubbles. Almost any frother would do, but pine oil is cheap and readily degrades in the environment. The less dense oil covered copper sulfide particles are carried to the top in the foam and are then skimmed off.
The next step carried out in an oxygen flash furnace is heated using natural gas to a high temperature. Next is added limestone, sand, and fresh copper ore concentrate (chalcopyrite) is blown into the furnace with oxygen. It is heated to 1100 C. The combustion with oxygen is endothermic, and chalcopyrite is actually the fuel that keeps the furnace hot at that point. The oxygen in the air selectively reacts with the iron to form the iron oxide, FeO, and leaves copper in the form of the sulfide, CuS. Sulfur dioxide is a by product and pollutant unless captured and converted to sulfuric acid.
The silicon dioxide in the sand reacts with the limestone and the iron oxide to form slag, FeSiO3 and CaSiO3. At the same time the excess sulfur in the ore reduces copper(II) sulfide, CuS, to copper (I) sulfide, Cu2S, which melts and flows out of the bottom of the furnace.
The molten copper(I) sulfide, called copper matte, is run into a converter furnace, where air containing oxygen is blown through the copper matte to oxidize the sulfide ions to sulfur dioxide. At the same time some of the sulfide ions reduce the copper (I) ions to impure blister copper metal. A final heating in an anode furnace is used to burn off the remaining oxygen.
An interesting aside is that for every ton of copper produced, 1.5 tons of slag and 2 tons of sulfur dioxide are also produced as waste products. The final step is the conversion of the impure copper to pure copper. This is done by electrolysis where the impure copper is made into the anode electrode and the pure copper is formed at the cathode electrode. The copper electrodes are immersed in a solution of sulfuric acid and copper(II) sulfate. Oxidation occurs at the anode, therefore copper metal is converted to copper (II) ions with the release of two electrons. At the cathode the opposite reaction occurs: copper (II) ions are joined with two electrons to form copper metal. During the transformation of copper metal into copper ions and back to copper metal, the impurities drop to the bottom of the electrolysis cell. Some of the impurities are gold, silver, nickel, platinum which are themselves recovered to be used.
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