General Principles of Isolation of ElementsNEET Chemistry · Class 12 · NCERT Chapter 15

Ores and Minerals

The Earth's crust contains many elements, but almost all of them occur as compounds (oxides, sulphides, carbonates, silicates, halides) rather than as free metals. Only a few metals like gold and platinum are found in the free (native) state.

Mineral vs Ore

A mineral is any naturally occurring compound of a metal in the Earth's crust. An ore is a mineral from which a metal can be extracted economically and profitably. The key word is "economically."

Example: Aluminium has two common minerals, bauxite (Al₂O₃.2H₂O) and corundum (Al₂O₃). Bauxite is the ore because it has a high enough Al content and is easy enough to process to be profitable. Corundum is too hard and dense to be processed economically. So bauxite is an ore; corundum is just a mineral.

All ores are minerals, but not all minerals are ores.

Gangue (Matrix)

The unwanted rocky material mixed with an ore in the Earth is called gangue (or matrix). Before extracting the metal, you must first remove the gangue from the ore. This step is called concentration (or beneficiation or dressing of ore).

Important Ores Table

NEET tip: NEET frequently asks you to identify the ore of a metal or to match an ore with its type. Commit this table to memory. The sulphide ores and oxide ores are most commonly tested.

Concentration of Ores

Before you can extract the metal, you must increase the proportion of ore mineral and remove the gangue. This is called concentration (also called beneficiation or ore dressing). There are four main methods.

1. Gravity Separation (Hydraulic Washing)

This method works when the ore particles are denser than the gangue. A stream of water washes over the crushed ore on a vibrating platform. The lighter gangue particles are carried away by the water; the heavier ore particles remain behind. Used for cassiterite (SnO₂, tinstone), which is denser than its silicate gangue.

2. Magnetic Separation

Used when the ore or the gangue is magnetic. The crushed ore mixture is fed onto a belt running over an electromagnetic drum. Magnetic particles are attracted and held; non-magnetic particles fall off.

• Magnetic ore, non-magnetic gangue: e.g. magnetite (Fe₃O₄) and chromite (FeCr₂O₄) are separated from silicate gangue.
• Non-magnetic ore, magnetic gangue: e.g. cassiterite (SnO₂, non-magnetic) is separated from wolframite (FeWO₄, magnetic). The wolframite sticks to the drum; the cassiterite falls off.

3. Froth Flotation

Used for sulphide ores (e.g. PbS, ZnS, CuFeS₂). The method exploits the difference in the ability of ore and gangue particles to be wetted by water (hydrophobicity vs hydrophilicity).

How it works:

1. Finely crushed ore is added to water in a large flotation tank. Pine oil (or cresol) is added as the frother to create stable froth.
2. Collectors like sodium ethyl xanthate (NaC₂H₅OCS₂) are added. These preferentially coat the sulphide ore particles and make them hydrophobic (water-repelling).
3. Air is blown through the mixture. The hydrophobic ore particles attach to the air bubbles and rise to the surface with the froth.
4. The silica/silicate gangue is hydrophilic (water-loving) and remains in the water (sinks).
5. The froth carrying the ore is skimmed off and the ore is recovered.

Depressants in Froth Flotation

When two sulphide ores are mixed (e.g. ZnS and PbS together), you can separate them selectively by adding a depressant:

• NaCN (sodium cyanide) acts as a depressant for ZnS. CN⁻ ions form a complex with Zn²⁺ on the ZnS surface ([Zn(CN)₄]²⁻), making ZnS hydrophilic so it sinks. PbS remains hydrophobic and floats, allowing separation.

4. Leaching (Chemical Method)

The ore is dissolved in a suitable chemical solvent, leaving the gangue behind. The metal is then recovered from the solution.

• Aluminium (Bayer process): Bauxite (Al₂O₃.2H₂O) is dissolved in hot concentrated NaOH solution. Al₂O₃ dissolves as sodium aluminate: Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O. The impurities (Fe₂O₃, SiO₂) do not dissolve. The solution is then diluted and Al(OH)₃ precipitates, which is calcined to give pure Al₂O₃.
• Silver and Gold: The ore is treated with dilute NaCN solution in the presence of air (O₂). Ag and Au dissolve as complex anions: 4Ag + 8NaCN + 2H₂O + O₂ → 4Na[Ag(CN)₂] + 4NaOH. The metal is then recovered by displacement with a more reactive metal: 2Na[Ag(CN)₂] + Zn → Na₂[Zn(CN)₄] + 2Ag.

Thermodynamic Principles of Metallurgy

Once the ore is concentrated, you need to extract the metal from the metal compound (oxide, sulphide, or carbonate). The key question is: which reductant should you use and at what temperature?

Thermodynamics gives the answer through the concept of Gibbs free energy (ΔG). A reaction is spontaneous if ΔG is negative. The Ellingham diagram is a visual tool that makes this easy.

The Ellingham Diagram

The Ellingham diagram plots the standard Gibbs free energy of formation (ΔfG°) of metal oxides against temperature. Each metal oxide has its own line.

Key rules for reading the Ellingham diagram

1. Lower line = more stable oxide. A metal oxide with a more negative ΔfG° (lower on the diagram) is more thermodynamically stable. The metal has a stronger affinity for oxygen.
2. Slope of lines: Most metal oxide lines go upward (positive slope) with increasing temperature. This is because the reaction M + O₂ → MO₂ involves a decrease in entropy (solid + gas → solid), so ΔS is negative. As T increases, −TΔS becomes more positive, making ΔG less negative (less stable).
3. Changes in slope: At the melting point or boiling point of the metal, the slope changes because the entropy of the metal changes. This appears as a kink in the line.
4. Using the diagram for reduction: If you want to reduce metal oxide B using metal A, the reaction A + BOₓ → AOₓ + B is spontaneous when the line for A (i.e. for A + O₂ → AOₓ) lies below the line for B at that temperature.

Carbon as a Reductant (Why Coke is So Useful)

Carbon has two oxidation reactions with oxygen:

• C + O₂ → CO₂: The line is nearly horizontal (ΔS ≈ 0 because equal moles of gas on both sides).
• 2C + O₂ → 2CO: The line has a steep negative slope (ΔG becomes more negative with increasing temperature). This is because 1 mol of gas (O₂) produces 2 mol of gas (CO), so ΔS is large and positive. At high temperature, −TΔS dominates and ΔG becomes very negative.

Because the C→CO line has a negative slope, it crosses below the lines of many metal oxides at high temperatures. Above the crossover temperature, carbon can reduce that metal oxide. This is why coke is used in blast furnaces and retort furnaces.

Important Crossover Temperatures

Key NEET point: The most common Ellingham question asks why coke cannot reduce Al₂O₃ or MgO. The answer is that the Al₂O₃ and MgO lines lie below the C→CO line at all practical temperatures, meaning those oxides are more stable than CO. There is no temperature at which the reduction would be spontaneous.

Thermit Reaction (Aluminothermy)

Aluminium can reduce certain metal oxides because the Al₂O₃ line lies below those metal oxide lines in the Ellingham diagram. The reaction with Fe₂O₃ is called the thermit reaction:

2Al + Fe₂O₃ → Al₂O₃ + 2Fe (ΔH ≈ −848 kJ/mol)

This is highly exothermic and produces temperatures around 2500°C, melting the iron produced. It is used in thermit welding for rail tracks and pipelines (no electricity needed, compact, portable).

Extraction by Reduction

After concentration and conversion to the oxide (by roasting or calcination), the next step is to reduce the metal oxide to the free metal. There are three main routes.

Pre-Treatment: Roasting vs Calcination

1. Pyrometallurgy (Smelting)

The oxide ore is heated with a reducing agent (coke, CO, or H₂) at high temperature. The reducing agent removes oxygen from the metal oxide, leaving the free metal.

Carbon (coke) reduction: Most common. Used for Fe, Zn, Sn, Pb.

• ZnO + C → Zn + CO (above 950°C)
• Fe₂O₃ + 3CO → 2Fe + 3CO₂ (in blast furnace, CO is the actual reductant)

Flux and Slag: A flux is a substance added to the furnace to remove gangue as a liquid slag. The flux reacts with the acidic or basic gangue to form a fusible slag, which floats on the molten metal and can be tapped off separately.

• Acidic gangue (SiO₂) requires a basic flux (CaO, limestone): CaO + SiO₂ → CaSiO₃ (slag)
• Basic gangue requires an acidic flux (SiO₂)

2. Hydrometallurgy (Leaching + Displacement)

The ore is leached with a chemical solvent to form a solution of the metal ion. The metal is then recovered by displacement with a more reactive metal or by electrolysis.

Silver recovery from NaCN leach solution: 2Na[Ag(CN)₂] + Zn → Na₂[Zn(CN)₄] + 2Ag

This route avoids high temperatures and is used for noble metals (Ag, Au) and some oxide ores. It is also more environmentally compatible for some ores.

3. Auto-Reduction (Self-Reduction)

Some metals (mainly copper) can be reduced without an external reductant. When the sulphide ore is heated, the sulphide and oxide react with each other to produce the metal:

Cu₂S + 2Cu₂O → 6Cu + SO₂

This happens in the converter during copper smelting. No coke is added; the matte itself acts as the reductant.

Blast Furnace (Iron Extraction)

The blast furnace is a large steel vessel lined with fire-brick. Layers of haematite ore, coke, and limestone are added from the top. Hot air is blasted in at the bottom.

Pig iron (cast iron) contains about 4% carbon and is brittle. Removing carbon gives wrought iron (pure Fe, very ductile). Controlled carbon gives steel (0.1–1.5% C, strong and workable).

Electrolytic Reduction

Highly reactive metals (those near the top of the electrochemical series: Al, Mg, Na, K, Ca) have very stable oxides. Their Ellingham lines lie far below the C→CO line, so carbon reduction is not thermodynamically feasible. You must use electrolysis to provide the energy needed.

Hall-Heroult Process (Aluminium)

Aluminium is the most abundant metal in Earth's crust, but it cannot be extracted by carbon reduction because Al₂O₃ is too stable.

Step 1: Bayer Process (Concentration by Leaching)

1. Bauxite (Al₂O₃.2H₂O) + hot concentrated NaOH → sodium aluminate solution: Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]
2. Fe₂O₃ and SiO₂ gangue do not dissolve and are filtered off ("red mud").
3. The sodium aluminate solution is diluted and seeded with Al(OH)₃ crystals. Al(OH)₃ precipitates: Na[Al(OH)₄] → Al(OH)₃ + NaOH
4. Al(OH)₃ is calcined (heated at 1200°C) to give pure Al₂O₃ (alumina).

Step 2: Hall-Heroult Electrolysis

• Alumina (Al₂O₃) is dissolved in molten cryolite (Na₃AlF₆) at about 950–1000°C. Cryolite lowers the melting point of alumina from 2045°C to about 950°C.
• Small amounts of AlF₃ and CaF₂ are added to improve conductivity and lower MP further.
• Cathode: Carbon lining of the cell. Reaction: Al³⁺ + 3e⁻ → Al (l). Molten aluminium sinks to the bottom and is tapped off.
• Anode: Carbon (graphite) rods. Reaction: 2O²⁻ → O₂ + 4e⁻. The O₂ produced reacts with the carbon anode at high temperature, forming CO₂. The anode is slowly consumed and must be replaced periodically. This is a distinctive feature of the Hall-Heroult cell.

Down's Process (Sodium)

Sodium is extracted by electrolysis of molten NaCl (not aqueous NaCl, because that would give H₂ at the cathode instead of Na).

• CaCl₂ is added to lower the melting point of NaCl from 801°C to about 600°C. A steel gauze diaphragm separates the two electrodes to prevent Na and Cl₂ from recombining.
• Cathode (iron): Na⁺ + e⁻ → Na (molten sodium rises and is collected).
• Anode (graphite): 2Cl⁻ → Cl₂ + 2e⁻ (chlorine gas collected at top).

Other metals: Magnesium is extracted by electrolysis of molten MgCl₂ (Dow process). Calcium and potassium also need electrolysis of their fused salts.

Refining of Metals

The metal obtained after reduction is usually impure. Refining removes impurities to give a pure metal. The method chosen depends on the properties of the metal and its impurities.

Electrolytic Refining (Detailed)

This is the most important refining method for NEET. Let us take copper as the example.

• Anode: Impure copper (blister copper, 98–99% Cu)
• Cathode: Thin sheet of pure copper
• Electrolyte: Acidic CuSO₄ solution (copper sulphate + H₂SO₄)

What happens at each electrode:

• Anode reaction: Cu → Cu²⁺ + 2e⁻. The impure copper dissolves. Impurities more active than Cu (Fe, Zn, Ni) also dissolve into the solution as Fe²⁺, Zn²⁺, Ni²⁺.
• Cathode reaction: Cu²⁺ + 2e⁻ → Cu. Only Cu²⁺ is preferentially deposited because it has the right reduction potential. The more active metal ions (Fe²⁺, Zn²⁺, Ni²⁺) stay in solution.
• Anode mud: Metals less active than Cu (gold, silver, platinum) do not dissolve at the anode. They fall below the anode as the "anode mud" or "anode slime." This anode mud is commercially valuable (source of precious metals).

The purity of the cathode copper reaches 99.99% (4 nines purity). This is required for electrical wiring.

Zone Refining (Detailed)

Zone refining (developed by W.G. Pfann, 1952) achieves the highest purities achievable and is essential for the semiconductor industry.

1. An impure metal rod is placed in a tube under an inert atmosphere.
2. A narrow circular heater moves slowly from one end of the rod to the other, creating a narrow molten zone.
3. Impurities have a higher solubility in the liquid phase than in the solid phase (their distribution coefficient k < 1). As the molten zone moves forward, the impurities dissolve into the liquid zone and travel with it.
4. Pure solid metal crystallises behind the molten zone as it moves forward.
5. After multiple passes, impurities are concentrated at one end (the end where the heater finished). That end is cut off and discarded.

The remaining rod is extremely pure. Zone refining is used for Ge, Si, Ga, In, Bi, and other elements where ultra-high purity is needed for electronics.

Mond Process for Nickel

Nickel forms a volatile compound, nickel tetracarbonyl Ni(CO)₄, at low temperatures:

Ni (impure) + 4CO → Ni(CO)₄ (gas, at 50–60°C)

The Ni(CO)₄ vapour is then passed to a decomposer at 230°C, where it breaks back down:

Ni(CO)₄ → Ni (pure, deposits) + 4CO

The CO is recycled. This process is very selective: iron impurities form Fe(CO)₅ at a different temperature so can be separated. The pure Ni deposits on a nickel seed. The Mond process gives 99.99% pure nickel.

Van Arkel Method for Titanium and Zirconium

This uses a volatile halide intermediate. For titanium:

Ti (impure) + 2I₂ → TiI₄ (vapour, at 250°C) → heated tungsten filament (1400°C) → Ti (pure) + 2I₂

The iodine is recycled. This method gives extremely pure Ti and Zr, which are needed for aerospace and nuclear applications. The key point is that the metal forms a volatile iodide that can be transported and then thermally decomposed on a hot wire.

Extraction of Specific Metals

NEET questions often focus on specific metals. Know the complete extraction pathway for Cu, Fe, Al, and Zn.

Copper Extraction

Main ore: Copper pyrites, CuFeS₂

1. Concentration: Froth flotation (sulphide ore; pine oil; collector is sodium ethyl xanthate). Gangue sinks; ore floats.
2. Roasting (partial, limited air):2CuFeS₂ + O₂ → Cu₂S + 2FeS + SO₂. The ore converts to a molten mixture called matte (Cu₂S + FeS).
3. Smelting in reverberatory furnace: Matte is heated with SiO₂ (silica flux). FeS is oxidised: 2FeS + 3O₂ → 2FeO + 2SO₂. FeO + SiO₂ → FeSiO₃ (slag, removed). Remaining Cu₂S goes to the converter.
4. Converter (Bessemerisation): Cu₂S undergoes auto-reduction: 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂; Cu₂S + 2Cu₂O → 6Cu + SO₂. Product: blister copper (98–99% pure, blistered surface from SO₂ bubbles trapped during solidification).
5. Refining: Electrolytic refining. Anode: blister Cu. Cathode: pure Cu. Electrolyte: CuSO₄ (aq). Anode mud contains Au, Ag, Pt.

Iron Extraction (Blast Furnace)

Main ore: Haematite, Fe₂O₃

1. Concentration: Magnetic separation (Fe₂O₃ is magnetic; silicate gangue is not).
2. No separate roasting needed: Haematite is already an oxide ore. Limestone (CaCO₃) is added as flux. The blast furnace processes ore, coke, and limestone together.
3. Smelting in blast furnace: (See the blast furnace zone table in Section 4.) CO reduces Fe₂O₃ step by step. Molten pig iron (4% C) and slag (CaSiO₃) form at the bottom.
4. Steel making: Pig iron is converted to steel by removing excess carbon and impurities using the Basic Oxygen Furnace (BOF) or electric arc furnace.

Aluminium Extraction (Already covered in Section 5)

Ore: Bauxite (Al₂O₃.2H₂O). Steps: Bayer leaching → calcination → Hall-Heroult electrolysis.

Zinc Extraction

Main ore: Zinc blende, ZnS

1. Concentration: Froth flotation.
2. Roasting: 2ZnS + 3O₂ → 2ZnO + 2SO₂ (in excess air, 900°C). ZnO is obtained.
3. Carbon reduction (retort furnace): ZnO + C → Zn + CO (above 1000°C). Zinc is more volatile than the temperature needed; it leaves as zinc vapour. It is condensed in a separate chamber.
4. Refining by distillation: Zinc (b.p. 907°C) can be separated by distillation from less volatile impurities. Gives 99.99% pure Zn.

Worked NEET Problems

Summary Cheat Sheet

Ore Concentration Methods

Ellingham Diagram Quick Reference

• Line lower in diagram = oxide more stable = metal harder to extract
• C→CO line has negative slope (ΔS positive, 1 mol gas → 2 mol gas)
• C→CO₂ line is nearly horizontal (ΔS ≈ 0, equal moles of gas)
• Where C→CO crosses below metal oxide line = carbon can reduce that oxide at that temperature
• Al₂O₃, MgO, CaO lines never crossed by C→CO = must use electrolysis
• Al₂O₃ line below Fe₂O₃ line = Al reduces Fe₂O₃ (thermit reaction)

Refining Method Quick Reference

• Zone refining: Ge, Si, Ga, In (semiconductors, ultra-high purity)
• Distillation: Zn, Hg (volatile metals)
• Liquation: Sn, Bi (low melting point metals)
• Electrolytic: Cu, Ag, Au, Ni, Zn (anode = impure; cathode = pure; anode mud = precious metals)
• Mond process: Ni (forms volatile Ni(CO)₄ at 55°C; decomposes at 230°C)
• Van Arkel: Ti, Zr (forms volatile TiI₄ at 250°C; decomposes on hot wire at 1400°C)

Specific Extraction Summary

NEET Frequency Summary

This chapter contributes about 1–2 questions per year. The most tested topics are:

1. Froth flotation: depressants (NaCN for ZnS), collectors, frothers
2. Ellingham diagram: slope interpretation, which metal oxide carbon can/cannot reduce
3. Zone refining: which metals (Ge, Si), principle (impurities in melt)
4. Hall-Heroult process: role of cryolite, electrode reactions
5. Roasting vs calcination: which pre-treatment for which ore type
6. Electrolytic refining: anode = impure, cathode = pure, anode mud = precious metals
7. Matte in copper metallurgy: Cu₂S + FeS
8. Auto-reduction: Cu₂S + Cu₂O → Cu + SO₂