Unit 1 Metallurgy: Complete Study Guide & Interactive

Volume-I: Unit 1 — Metallurgy

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Minerals vs Ores

Mineral	Ore
Naturally occurring substances in the earth's crust that contain metals in the free state or as compounds.	Minerals from which metals can be conveniently and economically extracted.
Contains a low percentage of the metal of interest.	Contains a high percentage of the metal of interest.
All minerals are not ores.	All ores are minerals.
Example: China Clay (Mineral of Aluminium).	Example: Bauxite (Ore of Aluminium).

Three Core Steps in Metallurgy

Metallurgy is the process of extracting metals from their ores. It involves three sequential steps:

1. Concentration of the Ore: Removal of impurities (gangue).
2. Extraction of Crude Metal: Obtaining the metal by reduction processes (e.g., smelting).
3. Refining of Crude Metal: Purification to obtain highly pure metal.

1. Gravity Separation / Hydraulic Washing

Principle: Based on the differences in specific gravity between the ore and the gangue particles. High specific gravity (heavier) ore particles settle down, while low specific gravity (lighter) gangue particles are washed away by running water.

Application: Used primarily for heavy oxide ores and native metals.

• Native Gold
• Hematite (Fe₂O₃)
• Tin stone (SnO₂)

2. Froth Flotation Process

Principle: Used to concentrate sulphide ores (e.g., Galena PbS, Zinc Blende ZnS). It depends on the preferential wetting of ore particles by oil and gangue particles by water.

Reagents:

• Frothing Agent: Pine oil (creates froth).
• Collecting Agent: Sodium ethyl xanthate (makes ore particles water-repellent so they attach to air bubbles).
• Depressing Agent: Sodium Cyanide (NaCN) (used to selectively separate mixtures of sulphide ores by preventing one from forming froth).
• pH Regulator: Sodium carbonate (Na₂CO₃).

Process: Air is blown through a suspension of powdered ore, water, and pine oil. Ore particles float to the surface as froth, while gangue settles at the bottom.

3. Magnetic Separation Process

Principle: Based on the differences in magnetic properties between the ore components and impurities.

Process: Powdered ore is passed over a conveyor belt moving over a magnetic roller. Magnetic particles are attracted and fall closer to the roller, while non-magnetic particles fall away.

Examples:

• Separation of magnetic Wolframite (FeWO₄/MnWO₄) from non-magnetic Tinstone (SnO₂).
• Separation of magnetic Magnetite (Fe₃O₄) from non-magnetic Silica (SiO₂).

4. Leaching Methods

Principle: The crushed ore is dissolved in a suitable chemical solvent. The metal converts into a soluble salt/complex while the impurities remain insoluble.

Types of Leaching:

• Cyanide Leaching (Gold): Gold forms a soluble complex with aerated dilute NaCN solution.
  4Au + 8NaCN + O₂ + 2H₂O → 4Na[Au(CN)₂] + 4NaOH
  Gold is later recovered by Cementation (adding Zinc):
  2Na[Au(CN)₂] + Zn → Na₂[Zn(CN)₄] + 2Au ↓
• Alkali Leaching (Bayer Process for Bauxite):
  Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O
• Acid Leaching: Sulphide ores like ZnS are treated with hot aqueous sulphuric acid to form soluble sulphates.
  2ZnS + 2H₂SO₄ + O₂ → 2ZnSO₄ + 2S + 2H₂O

Roasting vs Calcination

Roasting	Calcination
Heating the ore strongly in the presence of excess air or oxygen.	Heating the ore strongly in the absence or limited supply of air.
Applied typically to sulphide ores to convert them to oxides.	Applied typically to carbonate/hydrated ores to convert them to oxides.
Reaction: 2ZnS + 3O2 → 2ZnO + 2SO2 ↑	Reactions: CaCO3 → CaO + CO2 ↑ ZnCO3 → ZnO + CO2 ↑

Chemical Terms: Slag, Flux, and Gangue

Gangue: Unwanted non-metallic, sandy, or rocky impurities associated with an ore (e.g., SiO₂).

Flux: A chemical substance added during smelting to react with gangue and convert non-fusible impurities into fusible materials.

• Quick lime (CaO): Basic flux added to remove acidic silica gangue.
• Silica (SiO₂): Acidic flux added during copper extraction to remove iron oxide impurities.

Flux + Gangue → Slag

CaO (Basic Flux) + SiO₂ (Acidic Gangue) → CaSiO₃ (Slag)

Methods of Reduction

1. Smelting

The concentrated oxide ore is heated with a reducing agent (like C or CO) above the melting point of the metal.

Fe₂O₃ + 3CO → 2Fe + 3CO₂

ZnO + C → Zn + CO

2. Aluminothermic Process

Used for oxides like chromic oxide (Cr₂O₃) that cannot be easily reduced by carbon. It uses aluminium powder as a reducing agent, ignited using a mixture of BaO₂ and Magnesium powder.

Cr₂O₃ + 2Al → 2Cr + Al₂O₃ + Heat

3. Auto Reduction

Ores of certain metals require no additional reducing agent. Simple roasting of the sulphide ore yields the crude metal directly.

HgS (Cinnabar) + O₂ → Hg + SO₂ ↑

Industrial Case Study: Extraction of Copper from Copper Pyrites (CuFeS₂)

1. Concentration: Done via Froth Flotation.
2. Roasting: Ore is heated with air.
   2CuFeS₂ + O₂ → 2FeS + Cu₂S + SO₂
3. Smelting: Silica flux is added to form iron silicate slag, leaving behind a molten mixture of Cu₂S and FeS called Copper Matte.
   FeO + SiO₂ → FeSiO₃ (Slag)
4. Bessemerisation: Air blown into the matte triggers auto-reduction to yield metallic copper.
   2Cu₂S + 3O₂ → 2Cu₂O + 3SO₂
   2Cu₂O + Cu₂S → 6Cu (Blister Copper) + SO₂

1. Electrolytic Refining

Principle: Chemical deposition on electrodes using electrical current.

• Anode: Impure metal rod.
• Cathode: Thin sheet of pure metal.
• Electrolyte: Acidified aqueous solution of the metal salt.

Example: Refining of Silver (Ag)

Electrolyte: AgNO₃ + Nitric Acid.

Anode (Oxidation): Ag(s) → Ag⁺(aq) + e⁻

Cathode (Reduction): Ag⁺(aq) + e⁻ → Ag(s)

Pure silver deposits at the cathode, while impurities fall below the anode as anode mud.

2. Zone Refining

Principle: Based on fractional crystallisation. Impurities are significantly more soluble in the molten state of a metal than in its solid state.

Process: A mobile circular heater travels along an impure metal rod, forming a moving molten zone. Impurities dissolve in this molten zone and are pushed to one end of the rod, which is later severed. Done in an inert gas atmosphere to avoid oxidation.

Applications: Production of ultra-pure semiconductors like Silicon (Si), Germanium (Ge), and Gallium (Ga).

3. Vapour Phase Refining

Requirements: The metal must form a volatile compound with an available reagent, and this compound must easily decompose back into the pure metal at elevated temperatures.

Mond Process (Refining of Nickel)

Step 1 (350 K): Ni (Impure) + 4CO → Ni(CO)₄ (Volatile Gas)

Step 2 (460 K): Ni(CO)₄ → Ni (Pure) + 4CO

Van-Arkel Method (Refining of Titanium / Zirconium)

Step 1 (550 K): Ti (Impure) + 2I₂ → TiI₄ (Volatile Gas)

Step 2 (1800 K): TiI₄ → Ti (Pure) + 2I₂

Thermodynamic Principles (Ellingham Diagram)

The metallurgical reduction of an ore depends on the standard Gibbs free energy change (\(\Delta G^\circ\)).

ΔG = ΔH - TΔS

An Ellingham Diagram plots \(\Delta G^\circ\) against temperature for the formation of metal oxides.

Key Applications:

• Lower lines represent more stable metal oxides.
• A metal whose line is lower down in the diagram can reduce the oxide of a metal whose line is placed higher up. For example, Aluminium can reduce Cr₂O₃.
• Helps select the correct reducing agent and temperature range for spontaneous reduction (\(\Delta G < 0\)).

Limitations:

• It only provides information about thermodynamic feasibility, not the kinetics or rate of reaction.
• Assumes equilibrium states, which might not be practically maintained.

Electrochemical Principles & Hall-Héroult Process

For reactive metals, reduction is carried out via electrolysis based on electrochemical factors:

ΔG° = -nFE°

A reaction is feasible if the cell EMF (\(E^\circ\)) is positive, making \(\Delta G^\circ\) negative.

Electrometallurgy of Aluminium (Hall-Héroult Process)

• Cathode: Carbon/Graphite lining of the iron tank.
• Anode: Carbon rods immersed in the solution.
• Electrolyte: 20% Alumina (Al₂O₃) dissolved in molten Cryolite (Na₃AlF₆) with 10% Calcium Fluoride (CaF₂).
• Role of Cryolite & CaF₂: Lowers the high melting point of pure alumina and increases electrical conductivity.

Reactions (Above 1270 K):

Cathode (Reduction): 4Al³⁺ + 12e⁻ → 4Al (molten)

Anode (Oxidation): 6O^2- + 3C → 3CO₂ ↑ + 12e⁻

Overall: 4Al³⁺ + 6O^2- + 3C → 4Al + 3CO₂

Applications of Key Metals

Aluminium (Al)

• Manufacture of aircraft parts and transport design due to its lightness and strength.
• Household cooking utensils and heat exchangers.
• Electrical transmission wires and overhead cables.
• Aluminium foils used extensively for food packaging.

Iron (Fe)

• Construction rods, structural framework, bridges, and buildings.
• Cast iron is utilized to make industrial pipes, valves, and pumps.
• Alloys like stainless steel are vital for cutlery and surgical instruments.

Zinc (Zn)

• Used for galvanising iron/steel sheets to prevent oxidation and rusting.
• Zinc oxide (ZnO) is applied in pigments, paints, rubber manufacturing, and cosmetics.
• Zinc sulphide (ZnS) is crucial for making luminous paints and X-ray screens.

Copper (Cu)

• Widely utilized for manufacturing electrical wires and appliance circuits.
• Minting coins and structural alloys.

Gold (Au)

• Jewellery, ornaments, and monetary coinage.
• Gold nanoparticles are cutting-edge tools in medicine, catalysts, and solar cell electronics.

Test Your Knowledge

Click on an option to check if your answer is correct!

Q1. Which method is uniquely best suited to concentrate sulphide ores like Galena (PbS)?

Gravity Separation Froth Flotation Magnetic Separation Zone Refining

Q2. What role does Cryolite play in the Hall-Héroult electrolytic extraction of Aluminium?

Acts as an oxidizing agent Acts as an acidic flux to remove basic impurities Lowers the melting point of the alumina mixture Acts as a gaseous carrier ligand

Q3. Ultra-pure semiconductor elements (e.g., Ge, Si) are refined via which principle?

Vapour Phase Mond Process Zone Refining / Fractional Crystallisation Electrolytic Anode Mud Deposition Aluminothermic Reduction

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