Copper metal is prepared commercially in various ways. Copper sulfide ores, usually containing only 1% to 2% copper, are concentrated to 20% to 40% copper by the flotation process. They are then usually roasted to remove some of the sulfur and other impurities, and then smelted with iron oxide in either a blast furnace or a reverberatory furnace to produce copper matte, a molten solution of copper sulfide mixed with small amounts of iron sulfide. The matte is transferred to a converter, where it is treated by blowing air through it to remove the sulfur (as sulfur dioxide, a gas) and the iron (as a slag of ferrous oxide). The resulting copper is 98% to 99% pure; it is called blister copper because its surface is blistered by escaping gases when it solidifies during casting.
Most copper is further purified by electrolysis. The blister copper is refined in a furnace and cast into anodes. Thin sheets of pure copper are used as cathodes. A solution of copper sulfate and sulfuric acid is used as the electrolyte. When the anode and cathode are immersed in the electrolyte and an electric current is passed, the anode is dissolved in the electrolyte and pure copper metal is deposited on the cathode. Soluble impurities, usually nickel and arsenic, remain dissolved in the electrolyte. Insoluble impurities, often including silver, gold, and other valuable metals, settle out of the electrolyte; they may be collected and purified.
Copper oxide ores are usually treated by a different process, called leaching, in which the copper in the ore is dissolved in a leaching solution (usually dilute sulfuric acid); pure copper is recovered by electrolysis. Alternatively, the solution is treated with iron to precipitate the so-called cement copper, which is impure.