Copper can be purified through electrolysis, a process that uses an electric current to refine the metal.
Electrolytic Refining of Copper
Electrolytic refining is the primary method used to purify copper on a large scale. This process takes advantage of the electrochemical properties of copper and other metals present as impurities.
The Electrolytic Cell
The process involves setting up an electrolytic cell, which consists of the following components:
- Anode: An impure copper sample acts as the anode (positive electrode).
- Cathode: A thin sheet of pure copper serves as the cathode (negative electrode).
- Electrolyte: An acidic solution of copper sulfate (CuSO₄) acts as the electrolyte.
The Process
When an electric current is passed through the electrolytic cell, the following reactions occur:
-
At the Anode (Oxidation): Copper atoms from the impure anode are oxidized and dissolve into the electrolyte as copper ions (Cu²⁺). Impurities that are more easily oxidized than copper also dissolve. However, less easily oxidized metals like gold, silver, and platinum do not dissolve and fall to the bottom of the cell as "anode sludge" which can be recovered and sold, adding to the economic viability of copper refining.
Cu(s) → Cu²⁺(aq) + 2e⁻ -
At the Cathode (Reduction): Copper ions (Cu²⁺) from the electrolyte are reduced and deposited as pure copper atoms onto the cathode.
Cu²⁺(aq) + 2e⁻ → Cu(s)
Selectivity
The process is selective because the voltage is carefully controlled. Only copper ions are reduced at the cathode. Other metal ions from impurities that may be present in the electrolyte remain in solution.
Impurities
As described above, during electrolysis, impurities behave in one of three ways:
- More reactive metals: These dissolve in the electrolyte solution but do not get deposited at the cathode.
- Less reactive metals: These do not dissolve and collect as anode sludge.
- Similar reactivity metals: These dissolve, but the applied voltage ensures only copper deposits on the cathode.
Benefits of Electrolytic Refining
- High Purity: Electrolytic refining produces copper with a very high purity (typically 99.99%).
- Recovery of Valuable Metals: The anode sludge often contains valuable metals like gold, silver, and platinum, which can be recovered.
Summary Table
| Component | Description |
|---|---|
| Anode | Impure copper |
| Cathode | Pure copper |
| Electrolyte | Acidic copper sulfate (CuSO₄) solution |
| Process | Impure copper dissolves at the anode, copper ions migrate through the electrolyte, and pure copper deposits at the cathode. |
| Result | Pure copper at the cathode, and valuable metals recovered from the anode sludge. |
