Extraction of Metals — Reduction, Electrolysis, Refining

Metals don’t exist as pure elements in nature (with rare exceptions like gold and silver). They exist as minerals — combined with oxygen, sulfur, carbonate, or silica. Extracting a pure metal from its ore requires reversing the natural combination, which always takes energy.

The choice of extraction method depends on the metal’s reactivity. Highly reactive metals (Na, Al, Mg) need electrolysis — chemical methods aren’t powerful enough. Less reactive metals (Fe, Zn) can be extracted by reduction with carbon. The least reactive metals (Cu, Ag, Hg) can even be extracted by just roasting in air.

This chapter covers a significant 5–8 marks in CBSE Class 12 and appears regularly in JEE. Understanding the logic behind each method is more important than memorising steps.

Key Terms & Definitions

Ore: A naturally occurring mineral from which a metal can be extracted profitably.

Gangue (Matrix): Impurities present along with the ore in the earth’s crust.

Beneficiation (Concentration of Ore): The process of removing gangue from ore. Methods: gravity separation, froth flotation, magnetic separation, leaching.

Calcination: Heating ore in absence of air to remove water and volatile impurities. Used for carbonate ores (CaCO₃ → CaO + CO₂) and hydrated ores.

Roasting: Heating ore in excess air to convert sulfide ores to oxides. Example: $2\text{ZnS} + 3\text{O}_2 \rightarrow 2\text{ZnO} + 2\text{SO}_2$ .

Smelting: Reduction of metal oxide to metal by heating with carbon/coke. Used for iron, zinc, lead.

Thermite Reaction: Reduction of metal oxide using aluminium powder. The reaction is exothermic and produces very high temperatures. $\text{Fe}_2\text{O}_3 + 2\text{Al} \rightarrow \text{Al}_2\text{O}_3 + 2\text{Fe}$

Electrorefining: Purification of crude metal using electrolysis (impure metal = anode, pure metal = cathode).

The Reactivity Series and Extraction

The extraction method is directly linked to where the metal sits in the reactivity series:

Reactivity	Metals	Extraction Method
Very high	K, Na, Ca, Mg, Al	Electrolysis
Medium	Zn, Fe, Pb, Sn	Carbon reduction
Low	Cu, Hg, Ag	Chemical reduction or roasting
Very low	Au, Pt	Found native (no extraction needed)

Why this correlation? More reactive metals have stronger metal-oxygen bonds. These require more energy to break — only electrolysis can supply this. Less reactive metals have weaker bonds that carbon can break.

Extraction of Aluminium — Electrolysis of Bauxite (Hall-Héroult Process)

Aluminium ore is bauxite ( $\text{Al}_2\text{O}_3 \cdot 2\text{H}_2\text{O}$ ).

Step 1: Concentration — Bayer’s Process

Bauxite is treated with hot NaOH solution under pressure:

\text{Al}_2\text{O}_3 + 2\text{NaOH} \rightarrow 2\text{NaAlO}_2 + \text{H}_2\text{O}

Gangue (SiO₂, Fe₂O₃, TiO₂) does not dissolve. The solution is filtered, and $\text{Al(OH)}_3$ is precipitated by dilution.

Heated to give pure $\text{Al}_2\text{O}_3$ (alumina).

Step 2: Electrolysis (Hall-Héroult)

Pure $\text{Al}_2\text{O}_3$ has a very high melting point (2072°C). It is dissolved in molten cryolite ( $\text{Na}_3\text{AlF}_6$ ) which lowers the melting point to ~950°C. Fluorspar ( $\text{CaF}_2$ ) is added to increase conductivity.

Cathode (−): $\text{Al}^{3+} + 3e^- \rightarrow \text{Al}$ (molten aluminium sinks to bottom)
Anode (+): $2\text{O}^{2-} \rightarrow \text{O}_2 + 4e^-$ (oxygen gas evolves and reacts with carbon anode)

Carbon anodes are consumed and must be replaced periodically.

JEE/CBSE Favourite: “Why is cryolite used in Hall-Héroult process?” — Answer: (1) Lowers melting point of Al₂O₃, and (2) increases electrical conductivity of the melt.

Extraction of Iron — Blast Furnace

Iron ore is haematite ( $\text{Fe}_2\text{O}_3$ ) or magnetite ( $\text{Fe}_3\text{O}_4$ ).

The blast furnace is a tall cylindrical furnace. Hot air is blasted in from the bottom; ore, coke (carbon), and limestone ( $\text{CaCO}_3$ ) are charged from the top.

Key reactions (zone by zone):

Combustion zone (bottom): Coke burns: $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$

Reduction zone: CO₂ rises and reacts with more coke: $\text{CO}_2 + \text{C} \rightarrow 2\text{CO}$

Main reduction: Iron ore is reduced: $\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2$

Slag formation: Limestone decomposes: $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$

$\text{CaO}$ removes silica gangue: $\text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3$ (slag)

Products: Pig iron (bottom), liquid slag (floats above iron).

Extraction of Copper

Copper ore is mainly copper pyrites ( $\text{CuFeS}_2$ ).

Step 1: Roasting $2\text{CuFeS}_2 + \text{O}_2 \rightarrow \text{Cu}_2\text{S} + 2\text{FeS} + \text{SO}_2$

Step 2: Smelting $\text{Cu}_2\text{S} + \text{O}_2 \rightarrow 2\text{Cu} + \text{SO}_2$ (partial roasting gives blister copper)

Step 3: Electrorefining Blister copper (98% pure) is refined electrolytically:

Anode: Blister copper
Cathode: Pure copper
Electrolyte: Acidified copper sulfate solution

Cu from anode dissolves: $\text{Cu} \rightarrow \text{Cu}^{2+} + 2e^-$

Cu deposits at cathode: $\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$ (pure)

Impurities (Ag, Au, Pt) settle as anode mud — collected and sold.

Important Formulas

Iron extraction: $\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2$

Aluminium extraction: $\text{Al}_2\text{O}_3 \rightarrow 4\text{Al} + 3\text{O}_2$ (electrolysis)

Thermite reaction: $\text{Fe}_2\text{O}_3 + 2\text{Al} \rightarrow \text{Al}_2\text{O}_3 + 2\text{Fe} + \text{heat}$

Copper refining: $\text{Cu} \rightarrow \text{Cu}^{2+} + 2e^-$ (at anode); $\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$ (at cathode)

Exam-Specific Tips

CBSE Class 12 (Metallurgy chapter): Flow chart questions on extraction of aluminium and copper are common. Practice drawing the flowchart: Ore → Concentration → Conversion to oxide → Reduction → Refining. Name the process at each step.

JEE Main: Questions on electrorefining often ask: “Where do precious metals (Au, Ag) collect in copper refining?” Answer: Anode mud (they are less reactive than copper, so they don’t dissolve and fall below the anode).

Common Mistakes to Avoid

Mistake 1: Confusing calcination and roasting. Calcination = heating without air (removes CO₂ and H₂O). Roasting = heating with excess air (converts sulfide to oxide). Bauxite: calcination. ZnS ore: roasting.

Mistake 2: Saying carbon reduces aluminium oxide. Carbon can only reduce metal oxides that are less stable than carbon monoxide. Al₂O₃ is more stable — only electrolysis can reduce it. This is why Al is extracted by electrolysis despite being more abundant than iron.

Mistake 3: Confusing cathode and anode in electrorefining. Impure metal is always the anode (it dissolves). Pure metal deposits at the cathode. A common trick question: “Which electrode increases in mass?” → Cathode (pure metal deposits on it).

Practice Questions

Q1. Why is coke used in the extraction of iron from haematite?

Coke serves two purposes: (1) It burns to generate the heat needed for the furnace. (2) It produces CO (via $\text{CO}_2 + \text{C} \rightarrow 2\text{CO}$ ), which is the actual reducing agent that reduces Fe₂O₃ to Fe.

Q2. What is the role of limestone in the blast furnace?

Limestone ( $\text{CaCO}_3$ ) decomposes to give CaO, which reacts with silica gangue ( $\text{SiO}_2$ ) to form calcium silicate ( $\text{CaSiO}_3$ ) slag. This removes gangue from the iron, allowing it to be collected separately.

Q3. Why must Al₂O₃ be dissolved in cryolite before electrolysis?

Pure Al₂O₃ melts at 2072°C — too high for practical electrolysis. Dissolving it in cryolite lowers the melting point to ~950°C, making the process economically viable. Cryolite also improves electrical conductivity of the melt.

Refining Methods

Method	Principle	Metal refined
Distillation	Low-boiling metal evaporates, impurities stay	Zinc, mercury
Liquation	Low-melting metal flows away from impurities	Tin, bismuth, lead
Electrolytic refining	Pure metal deposits at cathode	Cu, Zn, Ag, Au, Al
Zone refining	Impurities concentrate in molten zone	Si, Ge, Ga (semiconductors)
Van Arkel	Metal iodide decomposes on hot filament	Zr, Ti, V (ultra-pure)
Mond process	Ni(CO) $_4$ formation and decomposition	Nickel

“Match the refining method with the metal” is a staple NEET/JEE question. Zone refining for semiconductors (Si, Ge) and Mond process for nickel are the two most frequently tested.

Ellingham diagram — the thermodynamic map

The Ellingham diagram plots $\Delta G$ (Gibbs free energy of oxide formation) vs temperature for various metals. Key insights:

A metal whose line is lower on the diagram can reduce the oxide of a metal whose line is higher (more positive $\Delta G$ ).
Aluminium’s line is very low, so Al can reduce most metal oxides (thermite reaction).
Carbon’s line has a unique slope change at ~700°C. Above this temperature, CO becomes a better reductant than C itself — this is why CO is the actual reducing agent in the blast furnace, not coke directly.

At any temperature, if $\Delta G_{\text{reductant oxide}} < \Delta G_{\text{metal oxide}}$ , the reduction is thermodynamically feasible.

Example: Al $_2$ O $_3$ line is below Fe $_2$ O $_3$ line at all temperatures $\rightarrow$ Al can reduce Fe $_2$ O $_3$ (thermite reaction works).

Worked Example — Choosing the extraction method

On the Ellingham diagram, the Al $_2$ O $_3$ line lies below the CO line at all practical temperatures. This means carbon cannot reduce Al $_2$ O $_3$ — the reaction is thermodynamically unfavourable. Only electrolysis can supply enough energy to break the strong Al-O bond. This is why Al extraction is energy-intensive (about 15 kWh per kg of Al).

Additional Practice Questions

Q4. What is froth flotation? For which type of ore is it used?

Froth flotation is used to concentrate sulphide ores (like copper pyrites, galena). The ore is mixed with water, pine oil (collector), and froth stabiliser. Air is blown in. Sulphide particles are hydrophobic — they attach to oil-coated bubbles and float to the surface as froth. Gangue sinks because it is hydrophilic.

Q5. What is the role of a flux in smelting?

A flux combines with gangue to form a fusible slag that can be separated easily. For silica gangue, CaO (basic flux) is used: CaO + SiO $_2$ $\rightarrow$ CaSiO $_3$ . For basic gangue (like ite), SiO $_2$ (acidic flux) is used. The flux is always chemically opposite to the gangue.

Q6. Name the method used to refine silicon for semiconductor applications.

Zone refining. A narrow zone of the impure silicon rod is melted and slowly moved along the rod. Impurities preferentially dissolve in the molten zone (they lower the melting point). After multiple passes, impurities concentrate at one end, which is cut off, leaving ultra-pure silicon (99.9999% pure).

FAQs

What is the thermite reaction used for?

The thermite reaction ( $\text{Fe}_2\text{O}_3 + 2\text{Al} \rightarrow \text{Al}_2\text{O}_3 + 2\text{Fe}$ ) produces temperatures above 2500°C. It is used to weld railway tracks in the field (thermite welding). The molten iron produced fills the gap between rail sections.

Why is blister copper called “blister”?

Blister copper is about 98% pure and has a rough, blistered surface caused by SO $_2$ gas escaping during the final stage of smelting. These gas bubbles leave pits and blisters on the surface of the solidified copper.