Basic Principles of Metallurgy > Refining of Metals

The process of purifying impure metal is called refining of metal. The most widely used method for refining impure metals is electrolytic refining, which ensures high purity. Metals such as copper, zinc, tin, lead, nickel, silver, and gold are commonly refined using this method.

Process of Electrolytic Refining: For the refining of an impure metal by electrolysis,

1. Anode: A thick block of impure metal (connected to the positive terminal of the battery).
2. Cathode: A thin strip of pure metal (connected to the negative terminal of the battery).
3. Electrolyte: A water-soluble salt solution of the metal to be refined.

Working Mechanism: When electric current is passed through the electrolyte,

• The impure metal dissolves from the anode and forms metal ions in the solution.
• These metal ions move toward the cathode, gain electrons, and deposit as pure metal.
• Soluble impurities remain in the electrolyte.
• Insoluble impurities settle at the bottom as anode mud.

Electrolytic Setup:

The electrolytic tank contains acidified copper sulphate (CuSO₄) solution as the electrolyte.

• Anode: A thick block of impure copper (connected to the positive terminal).
• Cathode: A thin strip of pure copper (connected to the negative terminal).

Electrolysis Process:

• When electric current is passed, impure copper dissolves from the anode into the electrolyte as Cu²⁺ ions.
• These Cu²⁺ ions migrate to the cathode, where they gain electrons and deposit as pure copper.
• Soluble impurities remain dissolved in the electrolyte.
• Insoluble impurities settle at the bottom of the tank as anode mud.

Electrode Reactions:

1. At Anode (Oxidation): Cu → Cu²⁺ + 2e⁻ (Copper atoms lose electrons and dissolve into the solution as Cu²⁺ ions.)
2. At Cathode (Reduction): Cu²⁺ + 2e⁻ → Cu (Copper ions from the solution gain electrons and deposit as pure copper on the cathode.)

Result:

• Pure copper deposits at the cathode.
• Impurities settle below the anode as anode mud.

Significance of Electrolytic Refining:

• Produces high-purity metal.
• Removes both soluble and insoluble impurities effectively.
• Used for refining valuable metals like silver and gold.

Refining is the process by which crude metal is purified.

• Refining means purifying a crude metal by removing impurities.
• Distillation is used for metals such as zinc and mercury, which have low boiling points.
• Liquation is used for soft metals such as lead and tin, which have low melting points.
• Electrorefining uses electricity to purify metals like copper and silver.
• Pure metal collects at the cathode; impurities settle as anode mud.