⛏️ Metallurgy – Full Breakdown

1) What is Metallurgy?

Definition: The scientific and technological process of extracting a metal from its ore and then purifying it for use.

The complete journey of a metal:

1. Mining — getting the ore out of the ground
2. Enrichment / Concentration — removing unwanted materials (gangue) from ore
3. Extraction — converting the enriched ore into the free metal
4. Purification / Refining — making the metal pure
5. Alloying — mixing with other metals for better properties (optional)

2) Occurrence of Metals

Native state: Metals found free in nature (uncombined). Only the least reactive metals: Au, Ag, Pt, Cu (sometimes).

Combined state: Most metals are found as compounds (minerals). When a mineral can be profitably used to extract a metal, it's called an ore.

Gangue (Matrix)

The unwanted impurities (sand, soil, rocky material) present in the ore along with the metal compound. The first step is always to remove gangue.

3) Types of Ores (with Common Examples)

4) Enrichment of Ores (Concentration)

Removing gangue to increase the % of metal compound in the ore. The method depends on the physical properties of the ore vs gangue.

a) Hydraulic Washing (Gravity Separation)

Used for: Oxide ores (haematite, cassiterite)
Principle: Ore particles are heavier than gangue. A stream of water washes away the lighter gangue, leaving behind the heavier ore.

b) Magnetic Separation

Used for: When one component is magnetic and the other isn't
Example: Magnetite (Fe₃O₄, magnetic) separated from non-magnetic gangue. Also used to remove Fe impurities from tin ore (SnO₂, non-magnetic).

c) Froth Floatation Process

Used for: Sulphide ores (ZnS, PbS, Cu₂S)
Principle: Sulphide particles are wet by oil but not by water. Gangue is wet by water.

Process:

• Crushed ore mixed with water + pine oil (collector — makes sulphide particles oil-repellent... wait, wrong. Pine oil makes sulphide particles hydrophobic → they attach to air bubbles)
• Air is blown in → froth forms
• Ore particles stick to the froth (bubbles) and rise to the top
• Gangue settles at the bottom
• Froth is skimmed off → concentrated ore

Key terms in froth floatation:

• Collectors (pine oil, eucalyptus oil) — coat ore particles, make them hydrophobic
• Froth stabilisers (cresol, aniline) — keep the froth stable so it doesn't collapse
• Depressants (NaCN, KCN) — sometimes used to prevent one sulphide from floating while allowing another (e.g., separating ZnS and PbS)

d) Chemical Method (Leaching)

Used for: Bauxite (Al₂O₃·2H₂O)
Bayer's Process:

• Bauxite is treated with hot NaOH (concentrated)
• Al₂O₃ dissolves: Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O (sodium aluminate)
• Impurities (Fe₂O₃, SiO₂, TiO₂) remain undissolved → filtered out
• NaAlO₂ is diluted and CO₂ is passed → Al(OH)₃ precipitates out
• Al(OH)₃ is heated → pure Al₂O₃ (alumina) is obtained

5) Extraction of Metals — Based on Reactivity Series

The method of extraction depends on where the metal sits in the reactivity series. This is the single most important concept in the chapter.

6) Thermodynamic Processes — Calcination, Roasting & Smelting

a) Calcination

Heating in limited or controlled air (not enough for complete combustion).

Used for: Carbonate ores

b) Roasting

Heating in excess air (plenty of oxygen).

Used for: Sulphide ores

c) Smelting (Reduction)

The oxide obtained from calcination/roasting is reduced to the free metal. Done in a furnace using a reducing agent (C, CO, Al, or electrolysis).

Flux and Slag

• Flux: A substance added to the furnace to combine with gangue and remove it
• Slag: The product formed when flux combines with gangue (melts easily, floats on top of molten metal, tapped off separately)

Why use flux? Gangue has high melting point and won't separate easily. Flux + gangue → slag which has a lower melting point and can be easily removed.

7) Extraction of Specific Metals — Step by Step

a) Aluminium — Hall-Héroult Process

Ore: Bauxite (Al₂O₃·2H₂O)
Step 1 — Purification: Bayer's process (leaching with hot NaOH) → pure Al₂O₃ (alumina)

Step 2 — Electrolytic reduction (Hall-Héroult):

• Al₂O₃ is dissolved in molten cryolite (Na₃AlF₆) — lowers melting point from 2000°C to ~950°C and improves conductivity

b) Iron — Blast Furnace

Ore: Haematite (Fe₂O₃) or Magnetite (Fe₃O₄)

Charge (raw materials fed from top):

• Haematite (Fe₂O₃) — the ore
• Coke (C) — fuel + reducing agent
• Limestone (CaCO₃) — flux (removes SiO₂ gangue as slag)

Hot air blast from bottom → coke burns → high temperature throughout.

Reactions in different zones (top to bottom):

Products:

• Molten iron (called pig iron or cast iron) — contains 2-5% C, very hard but brittle. Tapped from bottom.
• Slag (CaSiO₃) — floats on top of iron, tapped separately. Used for road construction, cement.
• Gases (CO, CO₂, N₂) — exit from top, used to preheat air blast.

Types of Iron

c) Zinc

Ore: Zinc blende (ZnS) or Calamine (ZnCO₃)

1. Concentration: Froth floatation (for ZnS)

4. Purification: Zinc vapour is distilled and condensed. Impure Zn contains Cd, Pb, Fe — separated by fractional distillation.

d) Copper

Ore: Copper glance (Cu₂S)

1. Concentration: Froth floatation

4. Electrolytic refining: To get 99.98% pure copper.

e) Lead

Ore: Galena (PbS)

1. Concentration: Froth floatation

4. Purification: Electrolytic refining or liquation (Pb melts at low temperature, impurities remain solid).

8) Electrolytic Refining

The final purification step for many metals (Cu, Ag, Au, Zn, Pb).

• Anode: Impure metal
• Cathode: Pure metal strip
• Electrolyte: Aqueous solution of the metal's salt (e.g., CuSO₄ for copper)

9) Alloys — Why We Make Them

Definition: A homogeneous mixture of two or more metals (or a metal + non-metal).

Why alloy? Pure metals are often too soft, weak, or reactive. Alloying improves properties.

10) Corrosion & Rusting

Rusting of Iron

Iron + O₂ + H₂O → Hydrated iron(III) oxide (Fe₂O₃·xH₂O) — rust

Conditions required: Both oxygen AND water must be present. If either is absent, rusting doesn't happen. (This is a classic experiment — nail in boiled water + oil layer → no rust. Nail in dry air with CaCl₂ → no rust. Nail in normal air + water → rust.)

Prevention of Rusting

• Painting / Oiling / Greasing — barrier prevents O₂ + H₂O contact
• Galvanisation — coating with Zn. Zn is more reactive than Fe, so Zn corrodes first (sacrificial protection)
• Tin plating / Chrome plating — coating with less reactive metal (Sn, Cr)
• Alloying — making stainless steel (Cr + Ni prevent rust)
• Sacrificial anode protection — attaching a more reactive metal block (Mg, Zn) that corrodes instead of the iron structure (used for ships, underground pipelines)

11) Top ICSE Reasoning Questions

Q1: Why is cryolite added to Al₂O₃ in the Hall-Héroult process?

Answer: Al₂O₃ has a very high melting point (~2000°C). Cryolite (Na₃AlF₆) lowers the melting point to ~950°C and improves the electrical conductivity of the mixture, making the process more energy-efficient and economical.

Q2: Why are carbon anodes replaced periodically in Al extraction?

Answer: The oxygen liberated at the anode (2O²⁻ → O₂ + 4e⁻) reacts with the carbon anode to form CO and CO₂ gases. This continuously burns away the carbon anodes, so they need regular replacement.

Q3: Why is limestone added to the blast furnace?

Answer: Limestone (CaCO₃) acts as a flux. It decomposes to CaO, which reacts with SiO₂ (acidic gangue in the iron ore) to form CaSiO₃ (slag). Slag melts easily and is removed separately, leaving behind pure iron.

Q4: Differentiate between calcination and roasting.

Answer: Calcination — heating in limited / controlled air, for carbonate/hydroxide ores, releases CO₂, done below melting point. Roasting — heating in excess air, for sulphide ores, releases SO₂, the ore may melt partially.

Q5: What is self-reduction in copper extraction?

Answer: In the Bessemer converter, the remaining Cu₂S reacts with Cu₂O produced during roasting: 2Cu₂O + Cu₂S → 6Cu + SO₂↑. No external reducing agent is needed because Cu₂O and Cu₂S reduce each other. This is called self-reduction.

Q6: Why does iron rust but aluminium doesn't, even though Al is more reactive?

Answer: Al reacts with O₂ to form a thin, adherent, protective layer of Al₂O₃ on its surface. This layer prevents further oxidation by blocking O₂ and H₂O. Iron oxide (rust) is flaky and porous — it falls off and exposes fresh iron to further rusting.

Q7: What is the role of collectors in froth floatation?

Answer: Collectors (like pine oil) coat the sulphide ore particles, making their surface hydrophobic (water-repellent). This causes the ore particles to attach to air bubbles and rise to the surface as froth, separating them from gangue which remains in the water.

Q8: A nail is placed in (a) boiled water with oil layer, (b) dry air with CaCl₂, (c) normal water exposed to air. In which case does rusting occur?

Answer: Rusting occurs only in (c). In (a), boiling removes dissolved oxygen and oil layer prevents air from entering. In (b), CaCl₂ absorbs moisture, keeping air dry. Both O₂ and H₂O are required for rusting — neither (a) nor (b) has both.