Write short notes on: [2.5+2.5] i) Hydrometallurgy ii) Zone of refining.

i) Hydrometallurgy

Hydrometallurgy is a branch of metallurgy that involves the use of aqueous solutions to extract metals from their ores. It is widely used for extracting low-grade ores and secondary sources of metals that are not economically viable through traditional pyrometallurgical methods.

Steps Involved in Hydrometallurgy:

1. Leaching:
   • The ore is treated with an aqueous solution (leaching agent) to dissolve the metal.
   • Common leaching agents include:
     • Acids (e.g., sulfuric acid, hydrochloric acid) for oxides and sulfides.
     • Alkalis (e.g., sodium hydroxide, ammonia) for certain metal oxides.
     • Complexing agents (e.g., cyanide for gold/silver, thiourea for copper).
   • Example: Copper is leached using sulfuric acid: $$\mathrm{CuO}+\mathrm{H}{}_2\mathrm{SO}{}_4\to \mathrm{CuSO}{}_4+\mathrm{H}{}_2\mathrm{O}$$

CuSO₄+FeDisplacement (electrolytic or chemical)Cu+FeSO₄

Cu₂S+H₂SO₄Roasting (pre-treatment)CuSO₄+H₂S

CuO+H₂SO₄Leaching (Hydrometallurgy)CuSO₄+H₂O

2. Solution Purification:

   • The leach solution contains impurities (e.g., iron, aluminum, silica).
   • Purification methods include:
     • Precipitation (e.g., adding lime to remove iron as hydroxide).
     • Solvent extraction (using organic solvents to selectively extract metal ions).
     • Ion exchange (to remove unwanted ions).

3. Metal Recovery:

   • The purified solution undergoes electrolysis or chemical reduction to recover the metal.
   • Example:
     • Electrowinning: Copper is recovered by electrolysis of copper sulfate solution. $$\mathrm{Cu}{}^{2+}+2\mathrm{e}{}^{-}\to \mathrm{Cu}(\mathrm{at}\mathrm{cathode})$$
     • Cementation: Zinc is recovered by displacing it with aluminum: $$2\mathrm{Al}+3\mathrm{ZnSO}{}_4\to \mathrm{Al}{}_2(\mathrm{SO}{}_4){}_3+3\mathrm{Zn}$$

4. Waste Treatment:

   • Residual solutions and sludges are treated to prevent environmental pollution (e.g., cyanide detoxification).

Advantages of Hydrometallurgy:

• Suitable for low-grade ores.
• Lower energy consumption compared to pyrometallurgy.
• Environmentally friendly (no SO₂ emissions).
• Can process complex ores selectively.

Disadvantages:

• Slower process than pyrometallurgy.
• Requires large volumes of water and chemicals.
• Corrosion issues with equipment.

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ii) Zone of Refining

Zone refining (or zone melting) is a method used to purify metals and semiconductors by moving a narrow molten zone through a solid ingot. It is particularly useful for producing ultra-pure materials like silicon, germanium, and gallium arsenide.

Principle:

• Impurities in a solid metal are more soluble in the liquid phase than in the solid phase.
• When a molten zone passes through the solid, impurities concentrate in the liquid and are carried forward, leaving the solidified zone purer.

Procedure:

1. Setup:

   • A long rod of impure metal is placed horizontally in a furnace.
   • A small heating coil creates a molten zone at one end.

2. Movement of Molten Zone:

   • The heating coil is slowly moved along the length of the rod.
   • As the molten zone moves, pure metal solidifies behind it, while impurities accumulate at the front.

3. Repetition:

   • The process is repeated multiple times to achieve higher purity.
   • Impurities are eventually pushed to one end of the rod, which is discarded.

Applications:

• Purification of semiconductors (e.g., silicon for solar cells, transistors).
• Production of high-purity metals (e.g., germanium, indium, gallium).
• Used in nuclear and aerospace industries for critical materials.

Advantages:

• Produces ultra-high-purity materials (up to 99.999% purity).
• Suitable for thermally sensitive materials (no chemical reactions involved).
• Can handle small quantities efficiently.

Disadvantages:

• Slow process (time-consuming for large-scale production).
• High energy consumption (requires precise temperature control).
• Limited to materials with a significant difference in solubility between solid and liquid phases.

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