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🧪 Chemistry · Class 12 · NEET & JEE

Isolation of Elements

The science of extracting metals from ores and refining them for use. Covers concentration methods, reduction techniques, refining processes, and the thermodynamic principles that govern metal extraction.

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Reading time~9 min

Revision time~3 min

Last updated2026-07-19

1 Read the chapter ~9 min

🎯 Key Points

4 main extraction steps: mining/crushing → concentration (remove gangue) → reduction → refining
Concentration methods: gravity separation, froth flotation (sulphide ores), magnetic separation, leaching
Reduction methods: carbon reduction (Fe, Cu, Zn), electrolytic reduction (Al, Na, Mg - very reactive metals)
Refining: electrolytic refining (Cu, Ag, Au), zone refining (semiconductors), Mond process (Ni), Van Arkel (Ti, Zr)
Ellingham diagram: a metal can reduce another's oxide only if its own ΔG° line is below the other's at that temperature
Hall-Heroult process: Al extraction by electrolysis of Al₂O₃ dissolved in molten cryolite

An Ellingham diagram plots ΔG° of oxide formation against temperature for different metals; whichever line is LOWER (more negative ΔG°) at a given temperature reduces the oxide of any metal whose line sits above it — the basis of carbon reduction (Fe, Zn) vs electrolytic reduction (Al, Mg, Na) decisions.

Occurrence of Metals

Most metals occur as ores in the Earth's crust. An ore is a mineral from which a metal can be profitably extracted.

Native (free) state: Au, Ag, Pt (unreactive)
As oxides: Fe₂O₃ (haematite), Al₂O₃ (bauxite)
As sulphides: Cu₂S, ZnS, PbS
As carbonates: CaCO₃, ZnCO₃

Steps in Extraction

Mining and crushing: Ore is extracted and ground
Concentration of ore: Remove gangue (unwanted rock)
Reduction: Convert metal compound to metal
Refining: Purify the extracted metal

Concentration Methods

Gravity separation: For dense ores (e.g., gold panning)
Froth flotation: For sulphide ores; ore is preferentially wetted by oil
Magnetic separation: For magnetic ores like magnetite (Fe₃O₄)
Leaching: Chemical dissolution using acid/base/cyanide (e.g., Al₂O₃ in NaOH)

Reduction Methods

Carbon reduction: Fe, Cu, Zn; Fe₂O₃ + 3CO → 2Fe + 3CO₂ (blast furnace)
Electrolytic reduction: Al, Na, Mg (highly reactive metals; Hall-Heroult process for Al)
Thermite reaction: Aluminium reduces metal oxides; used in welding railway tracks

Refining Methods

Electrolytic refining: Cu, Ag, Au (impure metal = anode; pure metal deposits at cathode)
Zone refining: For semiconductors like Si and Ge
Mond process: Refining Ni using Ni(CO)₄

Quick Tips

Aluminium is extracted by electrolysis despite carbon being available (C reduces Al only at very high temperature)
Slag = CaSiO₃; formed in blast furnace by reaction of CaO and SiO₂ (impurity)

Thermodynamic Principle of Reduction: Ellingham Diagram

Ellingham diagram plots standard Gibbs free energy of formation of an oxide (ΔG°) against temperature for different metals
A metal can reduce the oxide of another metal only if its own oxide-formation line lies below that metal's line at the given temperature (more negative ΔG°)
Most ΔG° vs T lines slope upward (less negative ΔG° as T rises) because entropy decreases when gaseous O₂ is consumed to form solid oxide
The carbon line (C to CO) slopes downward, so above about 983°C, carbon can reduce almost any metal oxide, which is why coke is so widely used in extraction at high temperature

Extraction by Reactivity Series

Highly reactive metals (K, Na, Ca, Mg, Al): extracted by electrolytic reduction of molten salts/oxides since chemical reducing agents are not strong enough
Moderately reactive metals (Fe, Zn, Pb, Cu, Sn): extracted by chemical reduction using carbon (coke) or carbon monoxide
Least reactive/noble metals (Ag, Au, Pt): found in native (free) state; extracted by simple physical methods or cyanide leaching (e.g., Ag and Au by MacArthur-Forrest cyanide process)

Hall-Heroult and Other Named Processes

Hall-Heroult process: Purified Al₂O₃ (alumina) is dissolved in molten cryolite (Na₃AlF₆) to lower its melting point, then electrolysed; Al deposits at the cathode, O₂ is released at the carbon anode (which is gradually consumed)
Bayer's process: Bauxite ore is purified by digesting with hot concentrated NaOH; Al₂O₃ dissolves as sodium aluminate, leaving impurities (Fe₂O₃) undissolved, then pure Al₂O₃ is reprecipitated
Van Arkel method: Refines metals like Ti and Zr by converting the impure metal to a volatile iodide, which decomposes on a hot tungsten filament to deposit ultra-pure metal

Refining: Vapour Phase Method and Poling

Mond process (vapour phase refining): Impure Ni heated in CO forms volatile Ni(CO)₄, which decomposes at higher temperature to give pure Ni and CO is recycled
Poling: Molten impure Cu is stirred with green wood poles; hydrocarbon gases released reduce Cu₂O impurity (cuprous oxide) back to pure copper
Liquation: Low-melting impure metal (like Sn) is melted on a sloping hearth so the pure metal flows away from higher-melting impurities

Roasting and Calcination

Roasting: heating a concentrated (usually sulphide) ore strongly in the presence of excess air, below its melting point, to convert it to the oxide and drive off volatile impurities; e.g. 2ZnS + 3O₂ → 2ZnO + 2SO₂
Calcination: heating a carbonate or hydrated ore in the absence (or limited supply) of air to expel CO₂ or water and leave the oxide; e.g. ZnCO₃ → ZnO + CO₂ and Al₂O₃·2H₂O → Al₂O₃ + 2H₂O
Both convert the ore to the oxide because oxides are far easier to reduce to the metal than sulphides or carbonates
Roasting is oxidising (excess air) whereas calcination excludes air — this is the key distinction

Extraction of Iron (Blast Furnace)

Roasted/calcined haematite ore (Fe₂O₃) is mixed with coke and limestone (CaCO₃) and fed into the top of a blast furnace; hot air is blasted in at the bottom
Coke burns to CO₂, which is reduced by more coke to CO — the actual reducing agent: C + O₂ → CO₂; CO₂ + C → 2CO
CO reduces the ore in stages down the furnace: 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂; Fe₃O₄ + CO → 3FeO + CO₂; FeO + CO → Fe + CO₂
Limestone decomposes (CaCO₃ → CaO + CO₂) and the CaO removes silica gangue as fusible slag: CaO + SiO₂ → CaSiO₃ (slag), which floats on the molten iron and is tapped off separately
The product is molten pig iron (about 4% carbon); it is refined to cast iron, wrought iron and steel

Blast furnace cross-section showing roasted ore, coke and limestone charged at the top, preheated air blasted in at the base, temperature zones with the stepwise reduction reactions of iron oxides, and slag and molten iron tapped at the bottom

The blast furnace for extracting iron: roasted haematite (Fe₂O₃), coke and limestone are charged at the top while hot air is blasted in at the base. Rising CO reduces the ore in stages down the furnace as temperature increases (Fe₂O₃ → Fe₃O₄ → FeO → Fe); limestone decomposes and forms CaSiO₃ slag, and molten iron collects at the bottom. Image: OpenStax, CC BY-SA 4.0, via Wikimedia Commons.

Extraction of Copper and Zinc

Copper (from copper pyrites, CuFeS₂): concentrated by froth flotation, then roasted and smelted to give copper matte (Cu₂S + FeS); in a Bessemer converter, a blast of air performs self-reduction: 2Cu₂O + Cu₂S → 6Cu + SO₂, giving blister copper (refined electrolytically)
Zinc (from zinc blende ZnS, or calamine ZnCO₃): the ore is roasted/calcined to ZnO, then reduced with coke: ZnO + C → Zn + CO; zinc distils off (it is volatile) and is refined by fractional distillation or electrolytically
Copper's self-reduction works because copper is a low-reactivity metal, so its sulphide and oxide react together directly without an external reducing agent

Uses of Aluminium, Copper, Zinc and Iron

Aluminium: aircraft and vehicle bodies, wires, foil and utensils (light, corrosion-resistant); thermite welding
Copper: electrical wiring and cables (excellent conductor), alloys (brass, bronze), and heat exchangers
Zinc: galvanising iron to prevent rusting, dry cells, and alloys such as brass
Iron/steel: construction, machinery, tools and transport — the most widely used structural metal

🚀 JEE Advanced Edge

Reading Ellingham diagram crossover points: The Mg line and Al line cross at a certain temperature — below it, Al can reduce MgO; above it, Mg can reduce Al₂O₃. Identifying which metal's line is lower at the EXACT temperature given in a question (not just generally) is the key skill for Ellingham diagram problems.

Why electrolytic reduction is needed for very reactive metals: Carbon CANNOT reduce the oxides of K, Na, Ca, Mg, Al under normal conditions because these metals' oxide-formation ΔG° is more negative than carbon's even at very high temperatures (their Ellingham lines stay below carbon's line) — these metals are simply too eager to stay as oxides for carbon's reducing power to overcome them, so a stronger driving force (electrical energy) is required.

Worked problem: Explain why the thermite reaction (2Al + Fe₂O₃ → Al₂O₃ + 2Fe) is highly exothermic and self-sustaining. Approach: Al's oxide-formation ΔG° is significantly more negative than Fe's at the reaction temperature (Al's Ellingham line lies well below Fe's), so the reaction releases a large amount of heat — enough to melt the iron product directly, which is why this reaction is used for on-site welding of railway tracks without external heating.

2 Revise ~3 min before the exam

📐 Formula Sheet

Concentration of ore: hydraulic washing (density), froth flotation (sulfides), magnetic separation, leaching (chemical)
Roasting: heat sulfide ore in air → oxide (e.g. 2ZnS + 3O₂ → 2ZnO + 2SO₂)
Calcination: heat carbonate/hydroxide ore in limited air → oxide (e.g. CaCO₃ → CaO + CO₂)
Reduction: oxide → metal, using C/CO (blast furnace), or a more reactive metal (thermite: Al reduces Fe₂O₃)
Ellingham diagram: plots ΔG° vs T; a reductant works where its line lies below the oxide's
Electrolytic reduction: for very reactive metals (Na, Al, Mg) that carbon cannot reduce
Refining: electrolytic (Cu), zone refining (Si, Ge — ultra-pure), Mond process (Ni), van Arkel (Ti, Zr)
Flux + gangue → slag: e.g. CaO + SiO₂ → CaSiO₃

3 Practice apply it

✍️ Worked Examples

Example 1 — Roasting vs calcination
Q: Would you roast or calcine (a) ZnS and (b) ZnCO₃?
Step 1 — Roasting is for sulfide ores, heated in air. ZnS is a sulfide ⇒ roast it: 2ZnS + 3O₂ → 2ZnO + 2SO₂.
Step 2 — Calcination is for carbonates/hydroxides, heated in limited air. ZnCO₃ is a carbonate ⇒ calcine it: ZnCO₃ → ZnO + CO₂.
Step 3 — Both routes converge on the oxide ZnO, ready for reduction.
Answer: roast ZnS, calcine ZnCO₃. Key idea: both steps aim to reach the oxide, which is easiest to reduce.

Example 2 — Choosing a reducing agent
Q: Why is aluminium, not carbon, used to reduce Cr₂O₃ and Fe₂O₃ in the thermite process?
Step 1 — On the Ellingham diagram, aluminium's oxidation line lies below those of Cr and Fe over the relevant range.
Step 2 — This means Al has a stronger affinity for oxygen and can pull it away from Cr₂O₃ and Fe₂O₃.
Step 3 — The reaction is highly exothermic: 2Al + Fe₂O₃ → 2Fe + Al₂O₃ + heat, producing molten iron.
Answer: aluminium reduces these oxides because it binds oxygen more strongly — and the heat released melts the metal for welding. Note: carbon would need impractically high temperatures here.

Example 3 — Zone refining
Q: On what principle does zone refining purify silicon for semiconductors?
Step 1 — Impurities are more soluble in the molten phase than in the solid.
Step 2 — A heated zone is moved slowly along the rod, carrying the impurities in the melt toward one end.
Step 3 — Repeating this sweeps impurities to the end, which is cut off, leaving ultra-pure silicon.
Answer: it exploits the difference in impurity solubility between molten and solid phases. Note: this is how semiconductor-grade purity is achieved.