We take many metals for granted. Aluminum, copper, iron, tin, zinc, and magnesium are a few examples. All of these metals exist naturally as metal ores, in which the metal exists as in ionic form as cations. The extraction and purification of these metals all depend on the process of reducing these metal ions to atoms. Reduction may occur by reacting the ore with an appropriate reducing agent and/or by the process of electrolyis,which involves forcing a current through an electrochemical cell to produce a chemical change that would otherwise not occur spotaneously. The device used in electrolysis is called the electrolytic cell which is very similar to an electrochemical cell. The main difference is that an electrochemical cell (also called a galvanic cell) produces electrical energy (a current ) when an oxidation-reduction reaction occurs, while an electrolytic cell uses electrical energy to cause an oxidation-reduction reaction.
An example of the process described above is the refinement of copper. A common copper containing ore is chalcocite (Cu2S). This ore is first treated at high temperatures (by a process called, roasting) or by blowing oxygen through the melted ore. As the oxygen comes in contact with the ionic complex it causes the reduction of the copper (I) ions in the ore to copper metal. At the same time sulfur is oxidized to sulfur dioxide as shown in the following equation.
Cu2S + O2 -----> 2 Cu (s) + SO2 (g)
The copper metal that is formed by this reduction process still contains a small amount of impurities such as zinc, iron, silver, and gold. The impure copper is further refined via electrolysis. In this process, as shown below, the impurities are removed in one of two ways. The more electropositive zinc and iron are oxidized into their respective ions and enter into solution. The Noble metals, silver and gold, are not oxidized at the anode, but settle out as metal atoms in a "sludge" as the impure copper anode dissolves. The copper obtained by this refinement is about 99.5% pure, and the amount of silver and gold recovered in the process is often sufficient to pay for the cost the electricity required for the electrolytic process.
