Refining of metals

Metal refining is an important part of metallurgy, which focuses on obtaining pure metals from raw ores or impure solutions. This process improves the quality of the metal by removing impurities, which can be useful in terms of enhancing properties such as conductivity, strength or the ability to form alloys. Methods discussed include electrolytic refining, zone refining and vapor phase refining.

Electrolytic refining

Electrolytic refining is a method whereby metals are purified using an electrolytic cell. This process takes advantage of the principles of electrolysis to refine impure metals.

Process description

In electrolytic refining, the impure metal is made the anode. A thin sheet of pure metal is taken as the cathode. The electrolyte is a solution of a salt of the metal. An electric current is passed through the solution which causes the transfer of metal ions from the anode to the cathode. The metal ions are reduced to be deposited as pure metal at the cathode, while the impurities settle down as anode sludge or sludge.

For example, the electrolytic refining of copper involves the following reactions:

    Anode: Cu (impure) → Cu 2+ + 2e -
    Cathode: Cu 2+ + 2e - → Cu (pure)
    

Visual example

In this process, impurities such as iron, zinc and nickel dissolve in the electrolyte, while precious metals such as gold and silver settle as anode sludge. This refining technique is commonly used for metals such as copper, zinc, lead and tin.

Field refinement

Zone refining is a refining method used to refine metals to a very high level of purity. This technique is based on the principle of fractional crystallization and is particularly suitable for refining semiconductors and other materials where extreme purity is required.

Process description

In zone refining, a narrow region of a metal rod is melted using a heat source. This molten zone is moved along the length of the rod. As it moves forward, it accumulates impurities that are more soluble in the molten material than in the solid. Thus, the impurities become concentrated in one region of the metal rod, while the area behind the zone becomes purer.

Imagine refining silicon:

    Solid Si (pure) ← Molten zone (impurities) → Solid Si (impure)
    

Visual example

The zone refining process can be repeated many times to achieve the desired level of purity. It is used extensively in the semiconductor industry to purify elements such as silicon and germanium.

Vapor phase purification

Vapor phase refining is a technique in which metals are purified by converting them into gaseous form and later decomposing them to obtain pure metal.

Process description

This method involves two main steps: (1) the metal is converted into a volatile compound; (2) the compound is decomposed to obtain the pure metal. This technique is highly effective for metals that can be easily vaporized.

The Mond process for nickel purification perfectly illustrates the vapor phase refining method. Nickel is reacted with carbon monoxide to form a volatile compound, nickel tetracarbonyl:

    Ni + 4CO → Ni(CO) 4 (vapor)
    

Nickel carbonyl is then decomposed by heating to yield pure nickel and carbon monoxide gas:

    Ni(CO) 4 → Ni (pure) + 4CO
    

Visual example

    Ti + 2I 2 → Ti 4
    

    TiI 4 → Ti (pure) + 2I 2
    

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