Mining and Refining of Copper
Copper ore is mined both underground and on the surface. Large excavations formed by surface mining are called open-pit mines. Most of the copper ores mined today are oxide or sulfide ores.
From the mines, copper ore is taken to mills, where it is crushed and finely ground in preparation for refining. The method of refining varies with the type of ore.
In the case of copper-oxide ores, the copper is usually leached (dissolved) from the ore with a solution of sulfuric acid.
The copper can be recovered from the leaching solution through electrolysis. In this process, a direct electric current is set up between positive and negative electrodes placed in the solution. The negative electrodes, called cathodes, are usually made of thin sheets of pure copper and the positive electrodes, called anodes, are usually made of lead. The electric current causes the copper in the solution to be deposited on the cathodes as a coating of pure copper.
Another method is to pass the solution over scrap iron; a chemical reaction causes the copper to be deposited on the iron. The copper is separated from the iron by methods used to refine copper-sulfide ores (many of which also contain iron).
Copper-sulfide ores are first treated by a process called flotation. In this process, bubbles are produced in a mixture of ground copper ore, water, and chemical reagents. The particles of copper-bearing minerals in the ore stick to the bubbles and float to the top of the mixture, where they can be skimmed off.
The copper-bearing minerals are roasted to drive off a part of the sulfur. The resulting product is smelted, yielding a molten combination of copper sulfate and iron sulfide called matte. Some light impurities in the matte combine to form slag, which is removed. The matte is then poured into a converter, where air is forced through it to burn out the remaining sulfur and to oxidize the iron. At this stage, most of the remaining impurities, including the oxidized iron, float to the top of the matte to form more slag, which is poured off.
The metallic copper that is left at the bottom of the converter is known as blister copper. It is very pure, but further refining is necessary to remove impurities consisting of small amounts of gold, silver, and other precious metals.
This refining is done electrochemically, using a process similar to the one used with oxide ores. In this case, however, the anode is molded from blister copper and decomposes during electrolysis. The direct electric current that flows between the electrodes placed in an electrolytic tank transfers the copper of the blister copper anode onto the cathode. The precious-metal impurities collect at the bottom of the tank.
A significant portion of the copper produced today is refined from copper scrap. Copper produced from scrap is called secondary copper.