The average copper content of copper ores is only 0.6%, and commercial copper ores are mainly sulfide ores, especially chalcopyrite (CuFeS 2 ), followed by chalcopyrite (Cu 2 S).[30] The ore is crushed and concentrated by froth flotation or bioleaching. [30] The ore is crushed and concentrated by froth flotation or bioleaching to increase the copper content to 10 to 15 percent. The ore is then flash smelted with silica to remove the iron as slag. This process takes advantage of the fact that iron sulfides are more readily converted to oxides, which then react with silica to form a silicate slag that floats on the surface of the hot melt. The resulting copper matte is composed of cuprous sulfide, which is converted to cuprous oxide by roasting:
2 Cu 2 S + 3 O 2 → 2 Cu 2 O + 2 SO 2
After further heating, cuprous oxide is converted to crude copper:
2 Cu 2 O → 4 Cu + O 2
This process converts only half of the sulfides into oxides, and the resulting oxides then oxidize and remove the remaining sulfides. The resulting product is electrolytically refined, and the gold and platinum contained in the anode sludge can still be utilized. This step takes advantage of the ease with which copper oxides can be reduced to metal monomers. Natural gas is first blown on the crude copper to remove most of the remaining oxides, and then the product is electrolytically refined to give pure copper.
