Copper - Making Pure Copper

Extremely pure copper (greater than 99.95%), called electrolytic copper, can be made by electrolysis. The high purity is needed because most copper is used to make electrical equipment, and small amounts of impurity metals in copper can seriously reduce its ability to conduct electricity. Even 0.05% of arsenic impurity in copper, for example, will reduce its conductivity by 15%. Electric wires must therefore be made of very pure copper, especially if the electricity is to be carried for many miles through high-voltage transmission lines.

To purify copper electrolytically, the impure copper metal is made the anode (the positive electrode) in an electrolytic cell. A thin sheet of previously purified copper is used as the cathode (the negative electrode). The electrolyte (the current-carrying liquid in between the electrodes) is a solution of copper sulfate and sulfuric acid. When current is passed through the cell, positively charged copper ions (Cu²⁺) are pulled out of the anode into the liquid, and are attracted to the negative cathode, where they lose their positive charges and stick tightly as neutral atoms of pure copper metal. As the electrolysis goes on, the impure copper anode dissolves away and pure copper builds up as a thicker and thicker coating on the cathode. Positive ions of impurity metals such as iron, nickel, arsenic and zinc also leave the anode and go into the solution, but they remain in the liquid because the voltage is purposely kept too low to neutralize them at the cathode. Other impurities, such as platinum, silver and gold, are also released from the anode, but they are not soluble in the solution and simply fall to the bottom, where they are collected as a very valuable sludge. In fact, the silver and gold sludge is usually valuable enough to pay for the large amount of electricity that the electrolytic process uses.