Metallurgy is the extraction and purification of metals from their ores. CBSE Class 12 and NEET cover the main steps and specific processes. Expect one to two NEET questions a year on metal extraction.
Core Concepts
Steps of metallurgy
- Mining of ore. 2. Concentration (remove gangue). 3. Conversion to oxide (roasting or calcination). 4. Reduction to metal. 5. Refining.
Each step has a specific purpose. Concentration removes the earthy impurities (gangue) that would waste fuel and reduce efficiency in later steps. Conversion to oxide is needed because oxides are easier to reduce than sulphides or carbonates. Reduction is the actual metal-winning step. Refining removes the last traces of impurity.
Concentration methods
Hand picking. Gravity separation (density difference). Froth flotation (for sulphide ores). Magnetic separation (for magnetic ores like magnetite). Leaching (chemical dissolution, used for aluminium and gold).
Froth flotation in detail: The crushed ore is mixed with water and a frothing agent (like pine oil). Air is blown through. Sulphide ore particles are preferentially wetted by oil (hydrophobic) and rise with the froth. Gangue particles are wetted by water (hydrophilic) and sink. This is the most important concentration method for copper, zinc, and lead ores.
Leaching examples:
Bauxite (Al2O3 with Fe2O3 and SiO2 impurities) is dissolved in hot NaOH. Alumina dissolves (amphoteric oxide); iron oxide does not. The filtrate is then treated with CO2 to precipitate pure Al(OH)3, which is calcined to get pure Al2O3.
Gold ore is treated with dilute NaCN solution in the presence of air. Gold dissolves as a soluble cyano complex. The gold is then recovered by adding zinc dust, which displaces gold from the complex.
Roasting and calcination
Roasting — heating in presence of air; converts sulphide ores to oxides (ZnS → ZnO). Calcination — heating in absence of air; converts carbonates to oxides (CaCO3 → CaO).
The key difference: roasting needs air (oxygen reacts with the sulphide), calcination does not. Roasting produces SO2 as a byproduct (environmental concern — causes acid rain). Calcination produces CO2.
Reduction methods
Smelting (carbon reduction) for less active metals like iron, copper, zinc. Electrolysis for active metals like aluminium, sodium. Displacement by more active metals (e.g., iron displaces copper from copper sulphate).
The choice of reduction method depends on the metal’s position in the reactivity series and the thermodynamics of the reduction (Ellingham diagram).
Ellingham diagram insight: A metal can reduce the oxide of another metal if its Ellingham line is lower (more negative ) at that temperature. Carbon’s line crosses many metal oxide lines at high temperatures, which is why carbon reduction works for moderately reactive metals. But carbon cannot reduce Al2O3 — aluminium’s line is below carbon’s at all practical temperatures.
Blast furnace (Iron):
Carbon reduction (Zinc):
Electrolysis (Aluminium — Hall-Heroult):
Thermite (displacement):
Refining
Distillation (volatile metals like Hg, Zn). Electrorefining (Cu, Ni, Ag). Zone refining (very pure Si, Ge). Van Arkel method (Ti, Zr).
Electrorefining in detail: The impure metal is the anode. A thin sheet of pure metal is the cathode. The electrolyte is a salt of the same metal. During electrolysis, the anode dissolves and pure metal deposits on the cathode. Impurities either dissolve in the electrolyte or fall as anode mud (which often contains precious metals like Ag and Au).
Zone refining: A cylindrical rod of impure material is heated at one end by a circular heater. The molten zone moves slowly along the rod. Impurities are more soluble in the liquid phase, so they concentrate in the molten zone and get swept to one end. Multiple passes give extremely pure material (99.9999% — semiconductor grade).
Worked Examples
Iron ore, coke and limestone are fed in. Hot air blown at the bottom burns coke to CO. CO reduces Fe2O3 to iron. Limestone removes silica as slag. Molten iron is tapped at the bottom.
Aluminium extraction. Bauxite is purified (Bayer process) to alumina, dissolved in molten cryolite, then electrolysed. Extremely energy-intensive.
Pure alumina has a melting point of about 2072°C — too high for practical electrolysis. Dissolving alumina in molten cryolite (Na3AlF6) lowers the melting point to about 950°C, saving enormous amounts of energy. Cryolite also improves the conductivity of the melt.
In copper extraction, the sulphide ore is partially roasted to get a mix of Cu2S and Cu2O. When heated further without air, these two react: . The copper compound reduces itself — no external reducing agent needed. This is called self-reduction or auto-reduction.
Common Mistakes
Confusing roasting and calcination. Roasting is with air; calcination is without.
Saying all metals are extracted by carbon reduction. Active metals need electrolysis.
Writing that aluminium is extracted from iron ore. It is extracted from bauxite (Al2O3).
Forgetting the role of limestone in the blast furnace. Limestone (CaCO3) decomposes to CaO, which reacts with silica (SiO2) to form calcium silicate slag — removing the gangue.
Saying electrorefining produces the metal. Electrorefining purifies already-extracted metal. The extraction step (getting metal from ore) comes before refining.
Exam Weightage and Revision
Metallurgy carries 1-2 NEET questions per year and 5-8 marks in CBSE Class 12 boards. JEE asks Ellingham diagram reasoning and numerical questions. The chapter is factual — names, conditions, and equations — which makes it memorisation-heavy but predictable.
| Question Type | NEET Frequency | Example |
|---|---|---|
| Method for a specific metal | Every year | How is aluminium extracted? |
| Roasting vs calcination | Most years | Differentiate with examples |
| Froth flotation principle | Every 2 years | Why does sulphide ore float? |
| Refining method | Occasional | What is zone refining used for? |
| Self-reduction | Occasional | Explain auto-reduction of copper |
For NEET, memorise the extraction of three metals: iron (blast furnace), aluminium (Hall-Heroult), and copper (self-reduction). These three cover most PYQs.
Practice Questions
Q1. Why is froth flotation used specifically for sulphide ores?
Sulphide ore particles are preferentially wetted by oil (hydrophobic), so they attach to air bubbles and rise with the froth. Gangue particles (silicates) are preferentially wetted by water (hydrophilic) and sink. This differential wetting works best for sulphide ores because of the hydrophobic nature of metal sulphides.
Q2. What is the role of coke in the blast furnace?
Coke serves two roles: (1) It is a fuel — burning coke provides the high temperature needed (about 2000°C at the base). (2) It is a reducing agent — coke reacts with CO2 to form CO (), and this CO reduces iron ore to iron. Without coke, neither the heat nor the reducing agent would be available.
Q3. Why is electrolytic reduction used for aluminium but not for iron?
Aluminium is more electropositive than carbon — carbon cannot reduce Al2O3 at any practical temperature (check the Ellingham diagram — Al line is below C line). Electrolysis provides enough energy to reduce Al2O3. Iron oxide, on the other hand, can be easily reduced by carbon (CO) at blast furnace temperatures because Fe line is above C line in the Ellingham diagram.
Q4. In electrorefining of copper, what happens to impurities like silver and gold?
Silver and gold are less reactive than copper, so they do not dissolve at the anode. Instead, they fall to the bottom of the electrolytic cell as “anode mud” or “anode sludge.” This sludge is collected and processed separately to recover the precious metals. This is actually a significant source of gold and silver commercially.
FAQs
Why is metallurgy essentially reduction chemistry?
Metals in ores are in oxidised form (bonded to oxygen, sulphur, etc.). Extracting the metal means reducing it — removing the non-metal and giving the metal its electrons back. Every extraction step is fundamentally a reduction reaction.
What is the Ellingham diagram and why does it matter?
The Ellingham diagram plots (Gibbs free energy) of oxide formation vs temperature for various metals and carbon. A metal can be reduced by any substance whose line lies below it on the diagram. It tells you which reducing agent works at which temperature — the theoretical basis for choosing extraction methods.
Why is aluminium extraction so energy-intensive?
The Al-O bond is extremely strong (Al2O3 has a very negative of formation). Breaking this bond requires a large amount of energy, which is supplied electrically during electrolysis. About 15,000 kWh of electricity is needed per tonne of aluminium — which is why aluminium smelters are always located near cheap power sources.
Three methods to lock — gravity, flotation, magnetic for concentration. Roasting for sulphides, calcination for carbonates. Carbon reduction for less active, electrolysis for more active.
Metallurgy is applied redox chemistry. Every extraction is a reduction, and the right method depends on the metal’s reactivity.