Refining of Copper
Unrefined or “blister” copper is about 99 percent pure when obtained from the oreNaturally occurring minerals from which metals and other substances can be extracted commercially., but it is desirable to increase this to 99.95 percent if the copper is to be used in electrical wiring. Even small concentrations of impurities noticeably lower copper’s electrical conductivity. Such a high degree of purity can be obtained by electrolytic refining in a cell similar to that shown in Fig. 1.
In such a cell a thin sheet of high-purity Cu serves as the cathodeThe electrode in an electrochemical cell where reduction occurs; the negatively charged electrode in a vacuum tube., and the anodeThe electrode in an electrochemical cell where oxidation occurs. The positively charged electrode in a vacuum tube. is the impure Cu which is to be refined. The electrolyte is a solution of copper(II)sulfate. Some of the impurities are metals such as Fe and Zn which are more easily oxidized than Cu. When current passes through the cell, these impurities go into solution from the anode, along with Cu:
Cu(s) → Cu2+(aq) + 2e–
Fe(s) → Fe2+(aq) + 2e–
Zn(s) → Zn2+(aq) + 2e–
These ions all migrate toward the cathode, but Cu2+(aq) is more readily reduced than Fe2+(aq) or Zn2+(aq) and so it is the only one that plates out. The impurity ions remain in solution. Other impurities, such as Ag, Au, and Pt, are less easily oxidized than Cu. These remain in metallic form and fall to the bottom of the cell, forming “anode sludge” from which they can later be recovered. The great value of Ag, Au, and Pt helps to offset the cost of refining.