Impure metals are purified using electric current through a process called electrolytic refining.
Electrolytic Refining Explained
Electrolytic refining uses electrolysis to transfer metal atoms from an impure anode (positive electrode) to a pure cathode (negative electrode) through an electrolytic solution. Here's a breakdown:
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Electrolytic Cell: The setup includes a container holding the electrolyte, an anode, and a cathode.
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Anode (Impure Metal): The impure metal to be refined serves as the anode. As electric current passes through the cell, the impure metal atoms are oxidized and dissolve into the electrolyte.
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Cathode (Pure Metal): The cathode is a thin strip or sheet of the pure metal. Metal ions from the electrolyte are reduced at the cathode, depositing as pure metal.
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Electrolyte: The electrolyte is a solution containing soluble salts of the metal being purified. This allows for the transfer of metal ions between the electrodes.
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Impurities: Impurities present in the anode behave differently:
- More Reactive Metals: More reactive metals also dissolve in the electrolyte. However, under appropriate conditions, their ions remain in solution and are not deposited at the cathode.
- Less Reactive Metals: Less reactive metals do not dissolve. Instead, they fall to the bottom of the electrolytic cell as "anode mud," which can be recovered and processed to extract valuable elements.
Process Summary
- Setup: An electrolytic cell is prepared with the impure metal as the anode, pure metal as the cathode, and a suitable electrolyte.
- Electrolysis: An electric current is passed through the cell.
- Anode Dissolution: At the anode, the impure metal is oxidized and dissolves into the electrolyte as metal ions.
- Cathode Deposition: At the cathode, metal ions from the electrolyte are reduced and deposited as pure metal.
- Impurity Management: More reactive impurities remain in the solution, while less reactive impurities settle as anode mud.
Example: Copper Refining
A common example is the electrolytic refining of copper:
- Anode: Impure copper.
- Cathode: Pure copper.
- Electrolyte: Copper sulfate solution ($CuSO_4$).
During electrolysis, copper from the impure anode dissolves into the solution as $Cu^{2+}$ ions and then deposits onto the pure copper cathode. Impurities like gold and silver, which are less reactive than copper, settle as anode mud and can be recovered. Zinc, being more reactive, also dissolves but generally doesn't plate out at the cathode.
Benefits of Electrolytic Refining
- High Purity: Electrolytic refining can achieve very high purity levels, often exceeding 99.99%.
- By-Product Recovery: Valuable by-products can be recovered from the anode mud.
- Versatility: It can be applied to purify various metals, including copper, silver, gold, nickel, and lead.
