Introduction to Metallurgical Processes

Q: What are the main metallurgical processes?

Metallurgical processes are the steps that give a pure metal from its ore: concentration to remove gangue, calcination or roasting to form the oxide, reduction of the oxide to crude metal with carbon, carbon monoxide, another metal or electrolysis, and finally refining. The route chosen depends on the type of ore and how reactive the metal is.

Q: What is the difference between a mineral and an ore?

A mineral is any naturally occurring compound of a metal found in the Earth's crust. An ore is a mineral from which the metal can be extracted conveniently and profitably. So all ores are minerals but not all minerals are ores: bauxite is an ore of aluminium, while clay is only a mineral.

Q: What is the difference between calcination and roasting?

Calcination heats an ore strongly in the absence or a limited supply of air; it suits carbonate and hydrated ores and drives off carbon dioxide or water. Roasting heats an ore, usually a sulphide, in excess air below its melting point; it oxidises the sulphide to the oxide and releases sulphur dioxide.

Q: What does an Ellingham diagram show?

An Ellingham diagram plots the standard Gibbs energy of formation of oxides, per mole of oxygen, against temperature. An element can reduce the oxide of any element whose line lies above its own at that temperature. It explains why coke reduces iron oxide above about 1000 K and why aluminium needs electrolysis.

Q: Why is aluminium not extracted by reducing alumina with carbon?

Alumina is so stable that its Ellingham line lies below the carbon to carbon monoxide line up to about 2000 K. Carbon would work only at extremely high temperatures, where it also forms aluminium carbide, so the process would be costly and impure. Electrolysis of alumina dissolved in molten cryolite is used instead.

Q: What is the difference between the Mond process and the van Arkel method?

Both are vapour phase refining methods. In the Mond process impure nickel reacts with carbon monoxide at 330 to 350 K to form volatile nickel tetracarbonyl, which decomposes at 450 to 470 K to pure nickel. The van Arkel method converts zirconium or titanium to a volatile iodide that decomposes on a hot tungsten filament.

Q: Is metallurgy in the JEE Main and NEET syllabus in 2026?

No. General Principles and Processes of Isolation of Metals was dropped from the JEE Main syllabus in 2024, and NEET removed metallurgy from its Class 12 chemistry syllabus at the same time. Both 2026 syllabi keep it out. The topic remains part of JEE Advanced under Isolation of Metals.

Q: How is metallurgy asked in JEE Advanced?

JEE Advanced lists ores and concentration, the thermodynamic principles for iron, copper and zinc, the electrochemical principle for aluminium, the cyanide process for silver and gold, and refining. Questions often combine an Ellingham diagram with a named process, a froth flotation depressant, or a match-the-column on ores and methods.

ChemistryGeneral Principles And Processes Of Isolation Of ElementsFor JEE aspirants

Metallurgical processes are the steps that turn an ore into a pure metal: concentration of the ore, conversion to the oxide by calcination or roasting, reduction of the oxide to the crude metal, and refining. The right method depends on the type of ore and on thermodynamics. The Ellingham diagram shows when carbon, carbon monoxide or another metal can reduce an oxide, and very reactive metals need electrolysis. Metallurgical processes are tested in JEE Advanced under Isolation of Metals.

On this page1Ores2Steps3Concentration4Oxide5Flux and slag6Reduction7Ellingham8Electrochemistry9Refining10Furnaces11Alloys12Mind map13Examples

Key Formulas - Quick Reference

Ore: a mineral from which the metal can be extracted profitably. All ores are minerals; not all minerals are ores.
★ Must learn $Gangue + Flux \to Slag$ : acidic flux ( $Si O_{2}$ ) for basic gangue ( $FeO$ , $CaO$ ); basic flux ( $CaO$ ) for acidic gangue ( $Si O_{2}$ ).
Concentration: sulphides by froth flotation; heavy oxides by gravity; magnetic ores by magnetic separation; Al, Ag, Au by leaching.
★ Must learnCalcination: heat without air (carbonates, hydrated oxides). Roasting: heat in excess air below the melting point (sulphides).
$Δ G^{\circ} = Δ H^{\circ} - T Δ S^{\circ}$ ; a reduction is feasible only if $Δ_{r} G^{\circ} < 0$ .
★ Must learnEllingham rule (per mol $O_{2}$ ): $Δ_{r} G^{\circ} = Δ_{f} G_{(reductant oxide)}^{\circ} - Δ_{f} G_{(metal oxide)}^{\circ}$ ; the lower line reduces the oxide above it.
★ Must learn $Δ G^{\circ} = - n F E^{\circ}$ : the more negative $E^{\circ}$ of $M^{n +} /M$ , the harder the reduction; Na, Mg, Ca, Al need electrolysis.
Self-reduction: $C u_{2} S + 2C u_{2} O \to 6Cu + S O_{2}$
★ Must learnRefining: distillation (Zn, Hg), liquation (Sn), zone (Si, Ge, Ga), Mond (Ni), van Arkel (Zr, Ti), electrolytic (Cu, Ag, Zn).

1. Occurrence of Metals: Minerals, Ores and Gangue

1.1 Native and combined metals

The Earth's crust is the biggest source of metals, and sea water holds dissolved salts of sodium, magnesium and potassium. Metals of low reactivity have little affinity for oxygen and moisture, so they occur in the free or native state: gold, platinum, silver and some copper. Reactive metals occur only in the combined state, as oxides, sulphides, carbonates, halides, silicates and other compounds.

The sixteen most abundant elements (crust, oceans and air taken together, classic Clarke values) are:

Element (Z)	% by mass	Element (Z)	% by mass
Oxygen (8)	49.5	Hydrogen (1)	0.9
Silicon (14)	25.7	Titanium (22)	0.6
Aluminium (13)	7.5	Chlorine (17)	0.2
Iron (26)	4.7	Phosphorus (15)	0.1
Calcium (20)	3.4	Manganese (25)	0.09
Sodium (11)	2.6	Carbon (6)	0.08
Potassium (19)	2.4	Sulphur (16)	0.06
Magnesium (12)	1.9	Barium (56)	0.04

Together they make up about 99.8 % of the total. Aluminium is the most abundant metal and iron the second. Oxygen and silicon lead the list, which is why oxides and silicates are the commonest minerals.

1.2 Minerals, ores and gangue

Mineral: a naturally occurring chemical substance in the Earth's crust, obtained by mining.
Ore: a mineral from which a metal can be extracted conveniently and profitably.
Gangue (or matrix): the earthy impurities (sand, clay, rock, quartz, mica, feldspar) that come mixed with the ore.

Bauxite ( $A l_{2} O_{3} \cdot 2 H_{2} O$ ) and clay ( $A l_{2} O_{3} \cdot 2Si O_{2} \cdot 2 H_{2} O$ ) both contain aluminium, but aluminium can be extracted profitably only from bauxite. So bauxite is an ore of aluminium, while clay is just a mineral. All ores are minerals, but not all minerals are ores.

1.3 Types of ores

Ores are grouped by the anion they contain. The metal extracted is shown in brackets.

Type	Important ores and minerals
Native	Cu, Ag, Au, Hg, Pt, Pd, Bi, Sb, As
Oxide	bauxite $A l_{2} O_{3} \cdot 2 H_{2} O$ (Al); corundum $A l_{2} O_{3}$ (Al); cuprite $C u_{2} O$ (Cu); haematite $F e_{2} O_{3}$ (Fe); magnetite $F e_{3} O_{4}$ (Fe); limonite $F e_{2} O_{3} \cdot 3 H_{2} O$ (Fe); cassiterite $Sn O_{2}$ (Sn); pyrolusite $Mn O_{2}$ (Mn); zincite $ZnO$ (Zn); chromite $FeO \cdot C r_{2} O_{3}$ (Cr); pitchblende $U_{3} O_{8}$ (U); spinel $MgA l_{2} O_{4}$
Sulphide	copper pyrites (chalcopyrite) $CuFe S_{2}$ (Cu); copper glance $C u_{2} S$ (Cu); galena $PbS$ (Pb); zinc blende $ZnS$ (Zn); cinnabar $HgS$ (Hg); argentite $A g_{2} S$ (Ag); millerite $NiS$ (Ni); iron pyrites $Fe S_{2}$ (used for $S O_{2}$ , not for iron)
Carbonate	limestone $CaC O_{3}$ (Ca); magnesite $MgC O_{3}$ (Mg); dolomite $CaC O_{3} \cdot MgC O_{3}$ (Mg); calamine $ZnC O_{3}$ (Zn); malachite $CuC O_{3} \cdot Cu (OH)_{2}$ (Cu); azurite $2CuC O_{3} \cdot Cu (OH)_{2}$ (Cu); cerussite $PbC O_{3}$ (Pb); siderite $FeC O_{3}$ (Fe)
Halide	rock salt $NaCl$ (Na); sylvine $KCl$ (K); carnallite $KCl \cdot MgC l_{2} \cdot 6 H_{2} O$ (Mg, K); horn silver $AgCl$ (Ag); cryolite $N a_{3} Al F_{6}$ (Al); fluorspar $Ca F_{2}$ (Ca)
Sulphate	gypsum $CaS O_{4} \cdot 2 H_{2} O$ (Ca); Epsom salt $MgS O_{4} \cdot 7 H_{2} O$ (Mg); anglesite $PbS O_{4}$ (Pb); barytes $BaS O_{4}$ (Ba)
Silicate	feldspar $KAlS i_{3} O_{8}$ (K, Al); mica $K_{2} O \cdot 3A l_{2} O_{3} \cdot 6Si O_{2} \cdot 2 H_{2} O$ ; asbestos $CaSi O_{3} \cdot 3MgSi O_{3}$ ; talc $M g_{3} S i_{4} O_{10} (OH)_{2}$ ; willemite $Z n_{2} Si O_{4}$ (Zn); beryl $3BeO \cdot A l_{2} O_{3} \cdot 6Si O_{2}$ (Be)
Nitrate	Chile saltpetre $NaN O_{3}$ (Na); Indian saltpetre $KN O_{3}$ (K)
Phosphate	phosphorite $C a_{3} (P O_{4})_{2}$ (P); monazite, a phosphate of Ce, La and Th

2. Steps in Metallurgy

Metallurgy is the whole process of obtaining a pure metal, or an alloy with the properties we want, from its ore. No single method suits every metal: the route depends on the nature of the ore and on how reactive the metal is. As a rule:

Noble metals (Ag, Au) are leached with cyanide solution (hydrometallurgy) or taken up by mercury (amalgamation).
Moderately reactive heavy metals (Fe, Cu, Zn, Pb, Sn) are roasted or calcined and then smelted with carbon.
Highly reactive metals (Na, K, Ca, Mg, Al) are obtained by electrolysis of their fused chlorides, oxides or hydroxides.

Whatever the route, the extraction follows the same four steps (Figure 1).

Figure 1: The four steps of metallurgy. Each step removes a different kind of impurity, which is why they run in this order: concentrate, convert to oxide, reduce, refine.

3. Concentration of Ores

Removing the gangue from the ore is called concentration, dressing or beneficiation. The ore is first crushed and ground to a powder. The method then uses a physical or chemical difference between the ore and the gangue.

3.1 Hand picking

Large lumps of rock are picked out by hand when the ore and the gangue look very different, for example the stony impurities in haematite.

3.2 Hydraulic washing (levigation or gravity separation)

This method uses the difference in density. The powdered ore is washed with a running stream of water, usually in a hydraulic classifier. The lighter gangue is carried away while the heavier ore particles settle. It suits oxide and carbonate ores such as haematite and cassiterite, and native gold.

3.3 Froth flotation

Froth flotation is used mainly for sulphide ores. It works on the different wetting of the particles: sulphide ore particles are wetted preferentially by oil, while gangue particles are wetted by water. The powdered ore is mixed with water to form a pulp, and small amounts of three kinds of reagent are added:

Collectors such as pine oil, fatty acids and xanthates (sodium ethyl or amyl xanthate) coat the ore particles and make them water-repellent, so they cling to air bubbles.
Froth stabilisers such as cresols and aniline stop the froth from collapsing. Pine oil also acts as the foaming agent (frother).
Modifiers: an activator such as $CuS O_{4}$ makes a mineral float; a depressant such as NaCN or KCN stops a mineral from floating; lime or $N a_{2} C O_{3}$ adjusts the pH.

A rotating paddle churns the pulp while air is blown in. The oil-coated ore particles stick to the air bubbles and rise into the froth, which overflows and is skimmed off and dried. The gangue sinks to the bottom. Adsorption of the collector on the mineral surface is what makes the process work (Figure 2).

Figure 2: Froth flotation separates by wetting, not by density: the oil-wetted sulphide ore rides the air bubbles into the froth while the water-wetted gangue sinks.

Separating two sulphides with a depressant. Galena ( $PbS$ ) is often found with zinc blende ( $ZnS$ ) and pyrites. Adding NaCN depresses ZnS: cyanide forms a zinc cyanide complex, $N a_{2} [Zn (CN)_{4}]$ , on the surface of ZnS, so ZnS stays in the pulp and only PbS floats. After the PbS froth is removed, $CuS O_{4}$ is added to activate ZnS, which is then floated separately.

3.4 Magnetic separation

This is used when either the ore or the gangue is magnetic. The powdered ore falls on a belt moving over a magnetic roller; the magnetic particles are held longer and drop into a separate heap. Chromite ( $FeC r_{2} O_{4}$ ), magnetite ( $F e_{3} O_{4}$ ) and pyrolusite ( $Mn O_{2}$ ) are separated from non-magnetic siliceous gangue this way, and magnetic wolframite ( $FeW O_{4}$ ) is removed from non-magnetic cassiterite ( $Sn O_{2}$ ).

Figure 3 shows a typical magnetic separator.

Figure 3: Magnetic separation. The roller holds magnetic grains a little longer, so they drop into their own heap, away from the non-magnetic gangue.

3.5 Electrostatic separation

Particles that conduct electricity well become charged in an electrostatic field and are repelled by the electrode of like charge; poor conductors are not. Lead sulphide (a good conductor) is separated from zinc sulphide (a poor conductor) in this way.

3.6 Leaching

Leaching is a chemical method: a reagent dissolves the ore but not the gangue. Bauxite is leached with hot concentrated NaOH (Bayer's process), which dissolves $A l_{2} O_{3}$ but leaves $F e_{2} O_{3}$ behind:

A l_{2} O_{3} (s) + 2NaOH (aq) + 3 H_{2} O (l) 473-523 K, 35-36 bar 2Na [Al (OH)_{4}] (aq)

Silver and gold are leached with dilute NaCN or KCN solution in the presence of air, which oxidises the metal into a soluble cyano complex:

4Au (s) + 8C N^{-} (aq) + 2 H_{2} O (l) + O_{2} (g) 4 [Au (CN)_{2}]^{-} (aq) + 4O H^{-} (aq)

A g_{2} S + 4NaCN ⇌ 2Na [Ag (CN)_{2}] + N a_{2} S

Low-grade copper ores are leached with dilute acid or with bacteria, and copper is recovered from the solution with scrap iron or hydrogen.

Ore or property	Method	Examples
Lumps look different from rock	Hand picking	haematite
Ore much denser than gangue	Hydraulic washing	haematite, cassiterite, native gold
Sulphide ore	Froth flotation	$CuFe S_{2}$ , $PbS$ , $ZnS$ , $HgS$ , $A g_{2} S$
Ore or gangue magnetic	Magnetic separation	$F e_{3} O_{4}$ , $FeC r_{2} O_{4}$ , $FeW O_{4}$ from $Sn O_{2}$
Ore conducts, gangue does not	Electrostatic	$PbS$ from $ZnS$
Ore dissolves in a reagent	Leaching	bauxite (NaOH), Ag and Au (NaCN)

Exam Trick

Match the method to the difference. Different density: wash it. Different wetting: float it. Different magnetism: use a magnet. Different solubility: leach it. If the question says sulphide, the answer is almost always froth flotation.

Key idea

Every concentration method uses one difference between ore and gangue: density, wetting, magnetism, conductivity or solubility.

4. Conversion to Oxide: Calcination and Roasting

Oxides are easier to reduce than carbonates or sulphides, so the concentrated ore is first converted to the oxide.

4.1 Calcination

In calcination the ore is heated strongly, below its melting point, in the absence or a limited supply of air (air only carries away heat and moisture). The name comes from calcite. Carbonate and hydrated oxide ores lose $C O_{2}$ or water, volatile matter is driven off, and the mass becomes porous:

CaC O_{3} (s) Δ CaO (s) + C O_{2} (g)

MgC O_{3} \cdot CaC O_{3} Δ MgO + CaO + 2C O_{2}

ZnC O_{3} (s) Δ ZnO (s) + C O_{2} (g)

CuC O_{3} \cdot Cu (OH)_{2} Δ 2CuO + H_{2} O + C O_{2}

F e_{2} O_{3} \cdot 3 H_{2} O Δ F e_{2} O_{3} + 3 H_{2} O

A l_{2} O_{3} \cdot 2 H_{2} O Δ A l_{2} O_{3} + 2 H_{2} O

Calcination is carried out in a reverberatory furnace or a rotary kiln.

4.2 Roasting

In roasting the ore, usually a sulphide, is heated in a regular supply of excess air at a temperature below its melting point. Roasting is an oxidation (the old name is de-electronation). It:

converts the sulphide to the oxide, with $S O_{2}$ given off;
oxidises impurities of S, As and Sb to volatile $S O_{2}$ , $A s_{2} O_{3}$ and $S b_{2} O_{3}$ ;
removes moisture and makes the mass porous, so it is reduced more easily.

2ZnS + 3 O_{2} Δ 2ZnO + 2S O_{2}

2PbS + 3 O_{2} Δ 2PbO + 2S O_{2}

2C u_{2} S + 3 O_{2} Δ 2C u_{2} O + 2S O_{2}

4As + 3 O_{2} Δ 2A s_{2} O_{3} ↑

Roasting is done in a reverberatory furnace (sometimes a blast furnace). The $S O_{2}$ is collected and used to make sulphuric acid. Several kinds of roasting are used:

Type	What happens	Example
Oxidising roasting	sulphide to oxide; S, As, Sb leave as volatile oxides	$2ZnS + 3 O_{2} \to 2ZnO + 2S O_{2}$
Partial roasting	only part of the sulphide is oxidised	$2CuFe S_{2} + O_{2} \to C u_{2} S + 2FeS + S O_{2}$
Blast roasting	roasting in a blast of hot air	copper pyrites, galena
Sulphating roasting	sulphide to a water-soluble sulphate, then leached	$ZnS + 2 O_{2} \to ZnS O_{4}$ ; $CuS + 2 O_{2} \to CuS O_{4}$
Chloridising roasting	heated with NaCl in air to give the chloride	$A g_{2} S + 2NaCl \to 2AgCl + N a_{2} S$
Reducing roasting	an oxide is heated with a reducing agent below its melting point	$3F e_{2} O_{3} + CO \to 2F e_{3} O_{4} + C O_{2}$

The silver chloride from chloridising roasting is then reduced by mercury: $2AgCl + 2Hg \to 2Ag + H g_{2} C l_{2}$ .

4.3 Calcination and roasting compared

Feature	Calcination	Roasting
Air	absent or limited	excess air
Ores treated	carbonates, hydrated oxides	sulphides
Gas given off	$C O_{2}$ , $H_{2} O$	$S O_{2}$ (and $A s_{2} O_{3}$ , $S b_{2} O_{3}$ )
Chemical change	thermal decomposition	oxidation
Example	$ZnC O_{3} \to ZnO + C O_{2}$	$2ZnS + 3 O_{2} \to 2ZnO + 2S O_{2}$

Exam Trick

Calcine the Carbonates, Roast the Rotten-egg ores. Sulphides are the "rotten-egg" ores (they give $H_{2} S$ with acids). If the gas given off is $C O_{2}$ or water, the step was calcination; if it is $S O_{2}$ , it was roasting.

Quick Recall: tap to check

Which concentration method suits a sulphide ore?

Froth flotation.

Name a depressant and an activator used in froth flotation.

NaCN depresses ZnS;

CuS O_{4}

activates it.

Calcination or roasting: which for

ZnC O_{3}

, which for ZnS?

Calcination for

ZnC O_{3}

(no air); roasting for ZnS (air).

5. Flux, Slag and Smelting

Some gangue always survives concentration. A flux is added to turn it into a fusible mass called slag. Slag is lighter than the molten metal, floats on it and is tapped off; the slag layer also protects the metal from oxidation.

$Gangue + Flux \to Slag$ . An acidic flux removes basic gangue; a basic flux removes acidic gangue.

Flux	Removes	Reactions
Acidic: $Si O_{2}$ , $P_{4} O_{10}$ , borax	basic gangue: $FeO$ , $CaO$ , $MgO$	$FeO + Si O_{2} \to FeSi O_{3}$ (in copper extraction); $CaO + Si O_{2} \to CaSi O_{3}$
Basic: $CaO$ , $MgO$ , limestone	acidic gangue: $Si O_{2}$ , $P_{4} O_{10}$	$CaO + Si O_{2} \to CaSi O_{3}$ (in iron extraction); $MgO + Si O_{2} \to MgSi O_{3}$ ; $6CaO + P_{4} O_{10} \to 2C a_{3} (P O_{4})_{2}$

The rule for choosing a flux is summed up in Figure 4.

Figure 4: Flux and slag. An acid meets a base: the flux is always chosen to be the opposite of the gangue, and the product is a fusible silicate slag.

Calcium phosphate slag from the basic Bessemer process (Thomas slag) is sold as a phosphate fertiliser, and blast-furnace slag is used to make cement and for road building.

Smelting is the process in which the roasted or calcined ore is melted with a flux, usually together with coke as the reducing agent, in a blast furnace. The metal, or a molten mixture of sulphides called matte ( $C u_{2} S$ with a little FeS in copper extraction), collects below the slag. In iron extraction, limestone is the flux; the lime it gives turns silica into calcium silicate slag:

CaC O_{3} Δ CaO + C O_{2}

CaO + Si O_{2} CaSi O_{3}

6. Reduction of the Oxide to the Metal

The oxide is reduced by heating it with a suitable reducing agent. Which agent is used depends on the thermodynamics of Section 7 and on cost.

As a first guide, the position of the metal in the reactivity series points to the method (Figure 5).

Figure 5: The higher a metal stands in the reactivity series, the more stable its oxide and the stronger the method needed to extract it.

6.1 Reduction by carbon (smelting)

Oxides of the less electropositive metals (Pb, Zn, Fe, Cu, Sn, Mn) are heated with coal or coke, usually with a flux. At smelting temperatures carbon is oxidised mainly to CO:

PbO + C Δ Pb + CO

ZnO + C 1673 K Zn + CO

F e_{2} O_{3} + 3C Δ 2Fe + 3CO

Mn O_{2} + 2C Δ Mn + 2CO

C u_{2} O + C Δ 2Cu + CO

Tin from cassiterite. The ore is concentrated by gravity and magnetic separation (to remove wolframite, $FeW O_{4}$ ), roasted to drive off S and As, then smelted with anthracite and a little limestone in a reverberatory furnace. The crude block tin contains Fe, Pb and Cu and is refined by liquation, poling or electrolysis (electrolyte $SnS O_{4}$ with $H_{2} S O_{4}$ and $H_{2} Si F_{6}$ ).

Sn O_{2} + 2C 1500-1600 K Sn + 2CO

The limestone removes silica as slag:

CaC O_{3} + Si O_{2} CaSi O_{3} + C O_{2}

6.2 Reduction by carbon monoxide

CO formed by incomplete burning of carbon reduces oxides at lower temperatures. In the blast furnace this is called indirect reduction:

F e_{2} O_{3} + 3CO 2Fe + 3C O_{2}

FeO + CO Fe + C O_{2}

PbO + CO Pb + C O_{2}

CuO + CO Cu + C O_{2}

6.3 Reduction by hydrogen

Hydrogen is used for metals that form carbides with carbon (W, Mo) and when a very pure metal is needed. Water gas ( $CO$ + $H_{2}$ ) is also used for nickel.

W O_{3} + 3 H_{2} Δ W + 3 H_{2} O

Mo O_{3} + 3 H_{2} Δ Mo + 3 H_{2} O

NiO + H_{2} Δ Ni + H_{2} O

I n_{2} O_{3} + 3 H_{2} Δ 2In + 3 H_{2} O

6.4 Reduction by aluminium (Goldschmidt aluminothermic process)

Oxides such as $C r_{2} O_{3}$ and $M n_{3} O_{4}$ are not reduced easily by carbon. Aluminium, which forms a far more stable oxide, reduces them with the release of a great deal of heat:

C r_{2} O_{3} + 2Al Δ A l_{2} O_{3} + 2Cr

3M n_{3} O_{4} + 8Al Δ 4A l_{2} O_{3} + 9Mn

F e_{2} O_{3} + 2Al A l_{2} O_{3} + 2Fe Δ H^{\circ} = - 852 kJ

A mixture of the metal oxide and aluminium powder is called thermite ( $F e_{2} O_{3}$ : Al is about 3 : 1 by mass). The set-up is:

The thermite mixture is placed in a graphite crucible.
It is covered with an ignition mixture of aluminium powder and barium peroxide ( $Ba O_{2}$ ), with a magnesium ribbon pushed in as a fuse.
A layer of carbon and a thin layer of feldspar on top keep the heat in and the air out.
The magnesium ribbon is lit; the reaction spreads through the mass and the temperature rises to about 2500-3000 °C, far above the melting point of iron.

Because it produces molten iron on the spot, the thermite reaction is used to weld railway tracks and repair broken machinery and ship parts.

6.5 Reduction by other metals (Mg, Na, K, Ca, Si)

Very stable halides and oxides are reduced by strongly electropositive metals, usually in a closed vessel or an inert gas:

TiC l_{4} + 2Mg 1100 K, Ar Kroll process Ti + 2MgC l_{2}

TiC l_{4} + 4Na Δ Ti + 4NaCl

K_{2} Ti F_{6} + 4K Δ Ti + 6KF

K_{2} Zr F_{6} + 4Na Δ Zr + 2KF + 4NaF

B_{2} O_{3} + 3Mg Δ 2B + 3MgO

MgC l_{2} + Ca C_{2} Δ Mg + CaC l_{2} + 2C

Silicon (as ferrosilicon) reduces magnesium oxide from calcined dolomite at high temperature under vacuum (Pidgeon process): $2MgO + Si \to 2Mg + Si O_{2}$ .

6.6 Self-reduction (auto-reduction)

Sulphide ores of less reactive metals (Cu, Pb, Hg) need no separate reducing agent. Part of the sulphide is roasted to the oxide or sulphate, which then reacts with the remaining sulphide to give the metal and $S O_{2}$ :

2C u_{2} S + 3 O_{2} 2C u_{2} O + 2S O_{2}

C u_{2} S + 2C u_{2} O 6Cu + S O_{2}

PbS + 2PbO 3Pb + S O_{2}

PbS + PbS O_{4} 2Pb + 2S O_{2}

HgS + 2HgO 3Hg + S O_{2}

Mercury from cinnabar. Cinnabar ( $HgS$ ), the only important ore of mercury, is concentrated by froth flotation and heated in air at 773-873 K. Mercuric oxide is unstable at this temperature (Figure 6), so the metal distils off and is condensed; the overall change is $HgS + O_{2} \to Hg + S O_{2}$ . Crude mercury containing Zn, Sn and Pb is stirred with warm dilute nitric acid. The mercurous nitrate formed oxidises the more reactive metals into solution, for example $Zn + H g_{2} (N O_{3})_{2} \to Zn (N O_{3})_{2} + 2Hg$ . The best purification is distillation under reduced pressure.

6Hg + 8HN O_{3} warm, dilute 3H g_{2} (N O_{3})_{2} + 2NO + 4 H_{2} O

6.7 Hydrometallurgy (displacement from solution)

A more electropositive metal displaces a less electropositive one from a solution of its salt or complex, at room temperature:

2Na [Ag (CN)_{2}] + Zn N a_{2} [Zn (CN)_{4}] + 2Ag ↓

2K [Au (CN)_{2}] + Zn K_{2} [Zn (CN)_{4}] + 2Au ↓

CuS O_{4} + Fe FeS O_{4} + Cu ↓

6.8 Amalgamation

Finely crushed native ores of silver and gold are brought into contact with mercury, which dissolves the noble metal as an amalgam. The mercury is then distilled off and reused, leaving the metal behind.

6.9 Electrolytic reduction

The most reactive metals (Na, K, Ca, Mg, Al) cannot be reduced by any cheap chemical agent, so their fused compounds are electrolysed (Section 8).

6.10 Summary: how common metals are extracted

Metal	Chief ore(s)	Main method	Key reaction
Iron	haematite $F e_{2} O_{3}$ , magnetite $F e_{3} O_{4}$	reduction by CO and coke in a blast furnace	$F e_{2} O_{3} + 3CO \to 2Fe + 3C O_{2}$
Copper	copper pyrites $CuFe S_{2}$ , cuprite $C u_{2} O$	partial roasting, then self-reduction	$C u_{2} S + 2C u_{2} O \to 6Cu + S O_{2}$
Zinc	zinc blende $ZnS$ , calamine $ZnC O_{3}$	reduction of ZnO by coke, or electrolysis of $ZnS O_{4}$	$ZnO + C \to Zn + CO$
Lead	galena $PbS$	self-reduction, or reduction of PbO by carbon	$PbO + C \to Pb + CO$
Tin	cassiterite $Sn O_{2}$	reduction by carbon	$Sn O_{2} + 2C \to Sn + 2CO$
Mercury	cinnabar $HgS$	heating in air	$HgS + O_{2} \to Hg + S O_{2}$
Silver	argentite $A g_{2} S$ , native	cyanide leaching, then zinc	$2 [Ag (CN)_{2}]^{-} + Zn \to [Zn (CN)_{4}]^{2 -} + 2Ag$
Gold	native; traces in Cu and Ag ores	cyanide process, as for silver	$2 [Au (CN)_{2}]^{-} + Zn \to [Zn (CN)_{4}]^{2 -} + 2Au$
Nickel	millerite $NiS$	reduction of NiO by CO, refined by Mond process	$NiO + 5CO \to Ni (CO)_{4} + C O_{2}$
Chromium	chromite $FeO \cdot C r_{2} O_{3}$	reduction of $C r_{2} O_{3}$ by Al	$C r_{2} O_{3} + 2Al \to 2Cr + A l_{2} O_{3}$
Aluminium	bauxite $A l_{2} O_{3} \cdot 2 H_{2} O$	electrolysis of $A l_{2} O_{3}$ in molten cryolite	$2A l_{2} O_{3} + 3C \to 4Al + 3C O_{2}$
Magnesium	carnallite, magnesite $MgC O_{3}$ , sea water	electrolysis of fused $MgC l_{2}$ with KCl or NaCl	$MgC l_{2} \to Mg + C l_{2}$
Calcium	limestone $CaC O_{3}$ , gypsum	electrolysis of fused $CaC l_{2}$ with $Ca F_{2}$	$CaC l_{2} \to Ca + C l_{2}$
Sodium	rock salt $NaCl$	electrolysis of fused NaCl with $CaC l_{2}$	$2NaCl \to 2Na + C l_{2}$
Lithium	spodumene $LiAlS i_{2} O_{6}$ , lepidolite	electrolysis of fused LiCl with KCl	$2LiCl \to 2Li + C l_{2}$

7. Thermodynamic Principles: The Ellingham Diagram

7.1 The Gibbs energy test

Whether a reduction can take place is decided by the change in Gibbs energy:

Δ G^{\circ} = Δ H^{\circ} - T Δ S^{\circ} Δ G^{\circ} = - R T ln K

A reaction is feasible when $Δ G^{\circ}$ is negative, and the more negative it is, the larger the equilibrium constant $K$ . Reducing an oxide, $M_{x} O (s) + C (s) \to xM + CO (g)$ , can be pictured as two coupled changes: the oxide gives up its oxygen (uphill) and the reducing agent takes it (downhill). Written per mole of $O_{2}$ , the overall change is the difference of the two formation energies:

Δ_{r} G^{\circ} = Δ_{f} G_{(C, CO)}^{\circ} - Δ_{f} G_{(M, M_{x} O)}^{\circ}

So the reduction works when the reducing agent forms the more stable oxide (more negative $Δ_{f} G^{\circ}$ ) at that temperature. Coupling an uphill change to a strongly downhill one is the central idea of pyrometallurgy. Heating helps only when it makes $Δ_{r} G^{\circ}$ negative, so we need to know how each $Δ_{f} G^{\circ}$ changes with temperature.

7.2 Reading the Ellingham diagram

An Ellingham diagram plots $Δ_{f} G^{\circ}$ for the formation of oxides against temperature, always per mole of $O_{2}$ (for example $2Zn + O_{2} \to 2ZnO$ or $\frac{4}{3} Al + O_{2} \to \frac{2}{3} A l_{2} O_{3}$ ). Since $Δ H^{\circ}$ and $Δ S^{\circ}$ change little with temperature, each line is nearly straight, with slope $- Δ S^{\circ}$ and intercept $Δ H^{\circ}$ .

Figure 6: Ellingham diagram (lines computed from standard enthalpy and entropy data, with kinks at melting and boiling points). A reducing agent can reduce any oxide whose line lies above its own at that temperature. The falling

C \to CO

line is why hot coke reduces so many oxides.

Metal oxide lines slope upward. Oxygen gas is used up, so $Δ S^{\circ}$ is negative and $Δ G^{\circ}$ becomes less negative as the temperature rises.
A line bends upward at the melting point and more sharply at the boiling point of the metal, where $Δ S^{\circ}$ becomes more negative (see Zn and Mg).
The line for $2C + O_{2} \to 2CO$ slopes downward: one mole of gas gives two, so $Δ S^{\circ}$ is positive. Carbon becomes a better reducing agent the hotter it gets.
$C + O_{2} \to C O_{2}$ is almost flat ( $Δ S^{\circ} \approx 0$ ) and $2CO + O_{2} \to 2C O_{2}$ slopes upward. The three carbon lines meet near 983 K: below it CO is the better reducing agent, above it carbon is.
Where two lines cross, $Δ_{r} G^{\circ} = 0$ for the reduction. Beyond that temperature the element of the lower line can reduce the oxide of the upper line.
A line that crosses $Δ G^{\circ} = 0$ means the oxide decomposes on its own above that temperature (HgO, $A g_{2} O$ ). These metals are obtained just by heating.

Exam Trick

Lower line wins. A lower line means a more stable oxide. At any temperature, an element can reduce the oxide of every element drawn above it, and the bigger the vertical gap, the more negative $Δ_{r} G^{\circ}$ .

7.3 Extraction of iron from its oxides

Iron oxides are reduced in the blast furnace by both CO and coke. In the cooler upper part (500-800 K) the CO/ $C O_{2}$ line lies below the iron oxide lines, so CO does the work:

3F e_{2} O_{3} + CO 2F e_{3} O_{4} + C O_{2}

F e_{3} O_{4} + 4CO 3Fe + 4C O_{2}

F e_{2} O_{3} + CO 2FeO + C O_{2}

In the hotter lower part (900-1500 K) the C/CO line has fallen below the Fe/FeO line (they cross at about 1073 K in NCERT's diagram, about 1000 K in Figure 6), so coke reduces FeO directly and also regenerates CO:

FeO + C Fe + CO

C + C O_{2} 2CO

For the reduction of FeO by carbon, $Δ_{r} G^{\circ} = Δ_{f} G_{(C, CO)}^{\circ} - Δ_{f} G_{(Fe, FeO)}^{\circ}$ per mole of $O_{2}$ , which turns negative only above the crossing point. The full blast-furnace chemistry is covered in Extraction of Iron.

7.4 Extraction of copper from cuprous oxide

The Cu/ $C u_{2} O$ line lies near the top of the diagram, so almost any reducing agent works; hot coke reduces $C u_{2} O$ easily:

C u_{2} O + C Δ 2Cu + CO

Copper, however, is mined as the sulphide, and carbon cannot reduce $C u_{2} S$ . The $Δ_{f} G^{\circ}$ values of $C S_{2}$ and $H_{2} S$ are less negative than that of $C u_{2} S$ , so neither carbon nor hydrogen can take the sulphur away. The sulphide is therefore partly roasted to $C u_{2} O$ and the two react by self-reduction:

C u_{2} S + 2C u_{2} O 6Cu + S O_{2}

7.5 Extraction of zinc from zinc oxide

Zinc blende is roasted and calamine is calcined to give ZnO:

2ZnS + 3 O_{2} Δ 2ZnO + 2S O_{2}

Above the boiling point of zinc (1180 K) the Zn/ZnO line becomes steep, and the C/CO line falls below it at around 1200 K. In practice ZnO is mixed with crushed coke and heated to about 1673 K in fire-clay retorts (Belgian process), which makes the reaction fast:

ZnO + C coke, 1673 K Zn + CO

At this temperature zinc is a vapour, so it distils out and is condensed, leaving the less volatile impurities behind. The crude metal, called spelter (about 98 % Zn), contains Cd, Pb and Fe and is refined by fractional distillation or electrolysis. Zinc is also produced by electrolysis of $ZnS O_{4}$ solution.

7.6 Why aluminium and magnesium need electrolysis

The $A l_{2} O_{3}$ and MgO lines lie far below the C/CO line right up to about 2000 K. Carbon could reduce them only above roughly 2000-2300 K, where it also forms carbides such as $A l_{4} C_{3}$ , and the process would be very costly. These metals are therefore made by electrolysis. The Mg and Al lines cross at about 1623 K (NCERT); below it Mg can reduce $A l_{2} O_{3}$ , above it Al can reduce MgO. The Al line lies below the $C r_{2} O_{3}$ line at all temperatures, which is why the thermite reduction of $C r_{2} O_{3}$ works.

JEE Advanced

Limits of the Ellingham diagram. (1) It is pure thermodynamics: it says whether a reduction can happen, not how fast (zinc is reduced at 1673 K rather than 1200 K partly for speed). (2) It assumes pure solids and gases at 1 bar; in a real furnace the CO/ $C O_{2}$ ratio and metals dissolved in the melt shift the true temperatures. (3) Each line assumes constant $Δ H^{\circ}$ and $Δ S^{\circ}$ , so crossing temperatures are approximate. (4) It covers oxides only; sulphides and chlorides need their own diagrams.

Key idea

A reduction works where the reducing agent's Ellingham line lies below the metal's; carbon gets better with temperature because its CO line falls.

Quick Recall: tap to check

On an Ellingham diagram, which element can reduce which oxide?

The element of the lower line reduces the oxide of any line above it, at that temperature.

Why does the line for

2C + O_{2} \to 2CO

slope downward?

Δ S^{\circ}

is positive: one mole of gas gives two.

Roughly above what temperature can carbon reduce ZnO?

About 1200 K on the diagram; industry uses about 1673 K for speed.

8. Electrochemical Principles

For a reduction in solution or in a melt, Gibbs energy is linked to the electrode potential:

Δ G^{\circ} = - n F E^{\circ}

Here $n$ is the number of electrons transferred and $F = 96500 C mol^{- 1}$ . A positive $E^{\circ}$ for the overall cell reaction makes $Δ G^{\circ}$ negative. This gives two routes:

Displacement. A more reactive metal (more negative $E^{\circ}$ ) reduces the ion of a less reactive one: $C u^{2 +} + Fe \to Cu + F e^{2 +}$ , with $E_{cell}^{\circ} = 0.34 - (- 0.44) = 0.78 V$ . Zinc reduces the silver and gold cyano complexes in the same way. This is the chemistry of hydrometallurgy.
Electrolysis. Metals with very negative $E^{\circ}$ (Ca $- 2.87$ V, Na $- 2.71$ V, Mg $- 2.37$ V, Al $- 1.66$ V) cannot be displaced by any cheap reducing agent, so an external voltage must drive the reduction. Their fused salts are electrolysed; in water, $H_{2}$ would be released at the cathode instead of the metal. A second salt is added to lower the melting point and raise the conductivity.

For fused sodium chloride, and for fused magnesium chloride from carnallite or sea water:

N a^{+} + e^{-} cathode (iron) Na

2C l^{-} anode (graphite) C l_{2} + 2 e^{-}

M g^{2 +} + 2 e^{-} cathode Mg

Metal	Electrolyte	Why the additive
Na	fused NaCl with $CaC l_{2}$ (Downs cell)	$CaC l_{2}$ lowers the melting point to about 873 K
Mg	fused $MgC l_{2}$ with NaCl and $CaC l_{2}$ , 973-1023 K	lowers melting point, raises conductivity
Ca	fused $CaC l_{2}$ with $Ca F_{2}$	$Ca F_{2}$ lowers the melting point
Al	$A l_{2} O_{3}$ dissolved in molten cryolite $N a_{3} Al F_{6}$ (with $Ca F_{2}$ ), about 1173-1223 K	cryolite lowers the melting point and makes the melt conduct
Li	fused LiCl with KCl	KCl lowers the melting point

Extraction by oxidation. The same principle works in reverse for non-metals. Chlorine is made by oxidising chloride in brine, $2C l^{-} + 2 H_{2} O \to 2O H^{-} + H_{2} + C l_{2}$ , for which $Δ G^{\circ} = + 422 kJ$ . Using $Δ G^{\circ} = - n F E^{\circ}$ with $n = 2$ gives $E^{\circ} = - 2.2 V$ , so an external voltage above 2.2 V is needed.

Electrolytic reduction

Extracts the metal from its compound.
Fused salt, no water (NaCl, $MgC l_{2}$ , $A l_{2} O_{3}$ in cryolite).
For very reactive metals: Na, Mg, Ca, Al.

Electrolytic refining

Purifies a metal already extracted.
Aqueous salt of the same metal; impure anode, pure cathode.
For Cu, Zn, Ag, Au, Ni, Pb; gives anode mud.

9. Refining of Crude Metals

The metal from the reduction step still contains other metals, non-metals, unreduced oxide and slag. The refining method depends on how the metal differs from its impurities.

9.1 Physical methods

Distillation. Low-boiling metals such as zinc, cadmium and mercury are vaporised and the vapour is condensed; non-volatile impurities stay behind.
Liquation. A metal that melts at a lower temperature than its impurities (Sn, Pb, Bi) is heated on the sloping hearth of a furnace. The pure metal melts and flows down, leaving the infusible impurities (dross) on the hearth.
Fusion. Simply melting the metal drives out dissolved gases, for example oxygen from silver and $S O_{2}$ from copper.
Fractional crystallisation. Platinum and iridium are separated through their ammonium hexachloro salts, which crystallise at different stages. The Pattinson process for silver in lead uses the same idea.
Vacuum arc melting. The crude metal is pressed into an electrode, melted under vacuum in an electric furnace and chilled in a water-cooled copper crucible, so gases and volatile impurities escape. It is used for reactive high-melting metals such as Ti, Zr and Mo.
Chromatographic methods. A solution or gas containing the metal compound is passed through a column of adsorbent such as $A l_{2} O_{3}$ . The components are held to different extents and come out separately. This suits elements present in very small amounts and impurities very similar to the element.

Zone refining gives the purest metals of all. A circular heater melts a narrow zone of a rod of the impure metal and moves slowly along it in an inert atmosphere. Impurities are more soluble in the melt than in the solid, so pure metal crystallises behind the heater and the impurities travel with the molten zone to one end, which is cut off. Repeating the pass brings impurities down to parts per billion. Semiconductors (Ge, Si, B, Ga, In) are refined this way (Figure 7).

Figure 7: Zone refining works because impurities are more soluble in the melt than in the solid, so the moving molten zone sweeps them to the far end of the rod.

9.2 Chemical methods

Poling. Used when the metal contains its own oxide, as in blister copper (with $C u_{2} O$ ) and tin. The molten metal is stirred with poles of green wood; the hydrocarbon gases released, such as methane, reduce the oxide: $4C u_{2} O + C H_{4} \to 8Cu + C O_{2} + 2 H_{2} O$ .
Cupellation. Silver containing lead is heated in a shallow dish of bone ash (a cupel) in a blast of air. Lead is oxidised to litharge, $2Pb + O_{2} \to 2PbO$ , which is blown away or soaked up by the cupel, leaving pure silver.
Oxidation by an air blast (Bessemerisation). Air blown through molten pig iron oxidises Mn, Si and C: $2Mn + O_{2} \to 2MnO$ , $Si + O_{2} \to Si O_{2}$ , $2C + O_{2} \to 2CO$ , and $MnO + Si O_{2} \to MnSi O_{3}$ (slag).

Vapour phase refining needs two things: the metal must form a volatile compound with an available reagent, and the compound must decompose easily to give back the metal (Figure 8).

Ni + 4CO 330-350 K Mond process Ni (CO)_{4}

Ni (CO)_{4} 450-470 K Ni + 4CO

Zr + 2 I_{2} Δ van Arkel method Zr I_{4}

Zr I_{4} W filament, 1800 K Zr + 2 I_{2}

Titanium and hafnium are refined by the van Arkel method in the same way; titanium forms its iodide at about 523 K: $Ti + 2 I_{2} \to Ti I_{4}$ .

Figure 8: Vapour phase refining. The metal leaves its impurities behind as a volatile compound (

Ni (CO)_{4}

Zr I_{4}

), which is then decomposed to the pure metal; the reagent is recycled.

Electrolytic refining is the most widely used method (Cu, Zn, Ag, Au, Pb, Sn, Ni). The impure metal is the anode, a thin sheet of pure metal is the cathode, and a soluble salt of the same metal is the electrolyte. The metal dissolves at the anode and deposits at the cathode:

M anode M^{n +} + n e^{-}

M^{n +} + n e^{-} cathode M

More reactive impurities (Fe, Zn in copper) stay in solution; less reactive ones fall below the anode as anode mud. For copper the electrolyte is acidified $CuS O_{4}$ , and the anode mud contains Sb, Se, Te, Ag, Au and Pt, whose recovery pays for much of the refining.

Method	Principle	Metals refined
Distillation	metal is more volatile than impurities	Zn, Cd, Hg
Liquation	metal melts at a lower temperature	Sn, Pb, Bi
Poling	green wood gases reduce the metal's own oxide	Cu, Sn
Cupellation	impurity (Pb) is oxidised selectively	Ag
Zone refining	impurities more soluble in the melt	Si, Ge, Ga, B, In
Mond process	volatile $Ni (CO)_{4}$ , then decomposed	Ni
van Arkel method	volatile iodide, decomposed on a hot filament	Zr, Ti, Hf
Electrolytic	anode dissolves, cathode receives pure metal	Cu, Zn, Ag, Au, Pb, Ni, Sn
Chromatography	different adsorption on a column	elements in trace amounts

Figure 9 turns the table into four questions.

Figure 9: Choosing a refining method. Each question tests one property that can separate the metal from its impurities; electrolysis is the general fallback.

Mond process

Nickel only.
Reagent: CO.
$Ni (CO)_{4}$ forms at 330-350 K.
Decomposes at 450-470 K.

van Arkel method

Zr, Ti (and Hf).
Reagent: $I_{2}$ .
Tetraiodide forms on heating.
Decomposes on a W filament near 1800 K.

Key idea

Pick the refining method by the property that separates the metal from its impurities: volatility, melting point, a volatile compound, solubility in the melt, or electrode potential.

Quick Recall: tap to check

Which refining method for Ge, for Ni and for Zr?

Zone refining; Mond process; van Arkel method.

What collects as anode mud in copper refining?

Ag, Au and Pt, with Sb, Se and Te.

Poling removes which impurity?

The metal's own oxide, such as

C u_{2} O

in blister copper.

10. Refractory Materials and Furnaces

Refractory materials line furnaces, crucibles and the hotter parts of chimneys. A good refractory withstands very high temperatures without melting or softening, resists sudden changes of temperature, does not crumble under load when hot, and resists the corrosive action of slag.

Type	Examples	Behaviour
Acidic	silica bricks, ganister (a siliceous rock), siliceous sandstone	react with bases; used with acidic slags
Basic	lime, magnesia (from magnesite), dolomite	react with acids; used with basic slags (basic Bessemer)
Neutral	graphite, chromite	react with neither acids nor bases
Semi-neutral	fire clay (about 60 % $Si O_{2}$ , 35 % $A l_{2} O_{3}$ )	mildly acidic

Approximate safe limits: silica up to 1750 °C, bauxite bricks up to 1800 °C, alumina up to 2000 °C, magnesia and chromite bricks up to 2200 °C.

Furnace	Main use
Blast furnace	smelting of iron, lead and copper ores
Reverberatory furnace (flame reflected from the roof onto the charge)	calcination, roasting, smelting, poling
Open-hearth (regenerative) furnace	making steel (Siemens-Martin process)
Muffle furnace (charge heated out of contact with the fuel)	annealing and assaying of gold and silver
Electric arc (Héroult) and induction furnaces	high-quality and alloy steels
Rotary kiln	calcination (lime, alumina)
Bessemer converter	copper matte to blister copper; pig iron to steel

11. Common Alloys

Metallurgy often ends with an alloy rather than a pure metal. The table gives approximate compositions (by mass) of alloys that appear in exams. Steels are covered in Extraction of Iron and aluminium alloys in Extraction of Aluminium.

Alloy	Composition (%)	Uses
Brass	Cu 60-80, Zn 20-40	utensils, fittings, wires
Bronze	Cu 75-90, Sn 10-25	statues, coins, medals
Bell metal	Cu 80, Sn 20	bells, gongs
Gun metal	Cu 87, Sn 10, Zn 3	gears, bearings, castings
German silver	Cu 50-60, Zn 20-25, Ni 20-25	cutlery, ornaments, resistance wire
Aluminium bronze	Cu 90, Al 10	imitation gold jewellery, coins, frames
Dutch metal	Cu 80, Zn 20	imitation gold leaf
Muntz metal	Cu 60, Zn 40	ship fittings, bolts
Monel metal	Ni about 67, Cu about 30, Fe and Mn about 3	acid containers, pumps
Constantan	Cu 60, Ni 40	thermocouples, resistance coils
Nichrome	Ni 60-80, Cr 15-20, rest Fe	heating elements
Solder	Pb 50, Sn 50	joining wires and metals
Type metal	Pb 75-80, Sb 15-20, Sn 3-5	printing type
Pewter	Sn 75, Pb 25 (traditional)	vessels, decorative ware
Duralumin	Al 95, Cu 4, Mg 0.5, Mn 0.5	aircraft and automobile parts
Magnalium	Al 90-95, Mg 5-10	balance beams, light instruments, aircraft parts
Electron	Mg 95, Zn 4.5, Cu 0.5	aircraft and automobile parts
Alnico	Al, Ni, Co and Fe	permanent magnets

12. Summary Mind Map

The whole topic fits on one page: four stages, each with its own set of methods (Figure 10).

Figure 10: Mind map of metallurgy. Four stages, each with its own toolbox; the Ellingham diagram decides the reduction step.

13. Solved Examples

Solved Example 1

(i) Name three metals that occur in nature as oxide ores. (ii) If the Earth's atmosphere contained only

O_{2}

and

S O_{2}

and no

C O_{2}

, what kind of minerals would you expect to find?

Solution:

(i) Iron as haematite $F e_{2} O_{3}$ or magnetite $F e_{3} O_{4}$ ; titanium as rutile $Ti O_{2}$ ; tin as cassiterite $Sn O_{2}$ . (Aluminium as bauxite is another answer.)

(ii) With no $C O_{2}$ there could be no carbonate minerals. Metals would occur as oxides and, because $S O_{2}$ and $O_{2}$ are present, as sulphites ( $MS O_{3}$ ) and sulphates ( $MS O_{4}$ ), along with sulphides.

Solved Example 2

An ore that does not contain oxygen is
(A) bauxite
(B) haematite
(C) chalcopyrite
(D) calamine

Solution:

Answer: (C). Chalcopyrite ( $CuFe S_{2}$ ) is a sulphide ore. Bauxite ( $A l_{2} O_{3} \cdot 2 H_{2} O$ ), haematite ( $F e_{2} O_{3}$ ) and calamine ( $ZnC O_{3}$ ) all contain oxygen.

Solved Example 3

Match each metal (List I) with its method of extraction (List II) and the flux or additive used (List III).

List I	List II (method)	List III
(a) Iron	(p) Self-reduction	(w) Cryolite
(b) Silver	(q) Electrolytic reduction	(x) $Si O_{2}$
(c) Copper	(r) Carbon reduction	(y) $CaO$
(d) Aluminium	(s) Cyanide process	(z) None

Solution:

Metal	Method	Flux or additive	Reason
(a) Iron	(r) Carbon reduction	(y) $CaO$	coke and CO reduce the oxide; lime removes silica as $CaSi O_{3}$
(b) Silver	(s) Cyanide process	(z) None	leached as $[Ag (CN)_{2}]^{-}$ , then displaced by Zn
(c) Copper	(p) Self-reduction	(x) $Si O_{2}$	silica removes FeO as $FeSi O_{3}$ ; $C u_{2} S$ + $C u_{2} O$ give Cu
(d) Aluminium	(q) Electrolytic reduction	(w) Cryolite	$A l_{2} O_{3}$ is dissolved in molten cryolite

Solved Example 4

Roasting an ore usually converts the metal into its oxide. Why does roasting of cinnabar,

HgS

, give metallic mercury instead of an oxide?

Solution:

HgS + O_{2} 773-873 K Hg + S O_{2}

Mercury has a weak affinity for oxygen (it lies low in the electrochemical series). On the Ellingham diagram the Hg/HgO line crosses $Δ G^{\circ} = 0$ at about 730 K, so HgO is unstable at roasting temperatures and decomposes, $2HgO \to 2Hg + O_{2}$ . Any HgO that does form also reacts with unroasted sulphide by self-reduction, $HgS + 2HgO \to 3Hg + S O_{2}$ . The mercury vapour distils off and is condensed.

Solved Example 5

Impurities physically associated with minerals are called
(A) slag
(B) flux
(C) alloy
(D) matrix

Solution:

Answer: (D). The earthy impurities mixed with an ore are the gangue or matrix. Slag is what forms when a flux reacts with the gangue during smelting.

Solved Example 6

Write short notes on (i) hydrometallurgy (ii) self-reduction.

Solution:

(i) Hydrometallurgy brings the metal into solution with a suitable reagent (for example sodium cyanide solution in air) and then precipitates it with a more electropositive metal. Silver ore is stirred with dilute NaCN while air is blown through, and silver dissolves as the argentocyanide complex, which zinc then displaces:

4Ag + 8NaCN + O_{2} + 2 H_{2} O 4Na [Ag (CN)_{2}] + 4NaOH

AgCl + 2NaCN Na [Ag (CN)_{2}] + NaCl

2Na [Ag (CN)_{2}] + Zn N a_{2} [Zn (CN)_{4}] + 2Ag ↓

(ii) In self-reduction no separate reducing agent is added. Part of the sulphide ore is roasted to the oxide or sulphate, which reacts with the rest of the sulphide to give the metal and $S O_{2}$ . Copper and lead are obtained this way:

2C u_{2} S + 3 O_{2} 2C u_{2} O + 2S O_{2}

C u_{2} S + 2C u_{2} O 6Cu + S O_{2}

Solved Example 7

Hydrometallurgy is used in the extraction of (more than one correct)
(A) Sn
(B) Au
(C) Hg
(D) Ag

Solution:

Answer: (B) and (D). Gold and silver are leached as cyano complexes and precipitated with zinc. Tin is extracted by carbon reduction and mercury by heating cinnabar in air.

Solved Example 8

Write equations for (a) roasting of galena (b) reduction of

C u_{2} O

with charcoal.

Solution:

(a) Galena is roasted in air:

2PbS + 3 O_{2} Δ 2PbO + 2S O_{2}

(b) Charcoal reduces cuprous oxide on heating:

C u_{2} O + C Δ 2Cu + CO

Solved Example 9

At a certain temperature,

Δ_{f} G^{\circ}

- 827

kJ per mol

O_{2}

for

\frac{4}{3} Al + O_{2} \to \frac{2}{3} A l_{2} O_{3}

and

- 540

kJ per mol

O_{2}

for

\frac{4}{3} Cr + O_{2} \to \frac{2}{3} C r_{2} O_{3}

. Can aluminium reduce

C r_{2} O_{3}

at this temperature?

Solution:

Subtract the chromium equation from the aluminium equation:

\frac{2}{3} C r_{2} O_{3} + \frac{4}{3} Al \to \frac{2}{3} A l_{2} O_{3} + \frac{4}{3} Cr Δ_{r} G^{\circ} = - 827 - (- 540) = - 287 kJ

$Δ_{r} G^{\circ}$ is negative, so the reduction is feasible. Multiplying by $\frac{3}{2}$ : $C r_{2} O_{3} + 2Al \to A l_{2} O_{3} + 2Cr$ , with $Δ_{r} G^{\circ} = - 430.5$ kJ. This is the aluminothermic (thermite) extraction of chromium; on the Ellingham diagram the Al line lies below the Cr line.

Solved Example 10

Although it is thermodynamically possible, magnesium is not used to reduce alumina in the metallurgy of aluminium. Explain with the Ellingham diagram.

Solution:

The Mg/MgO and Al/ $A l_{2} O_{3}$ lines cross at about 1623 K. Below this temperature the MgO line is lower, so Mg can reduce $A l_{2} O_{3}$ ( $Δ_{r} G^{\circ} < 0$ ); above it the order reverses and Al can reduce MgO. In practice magnesium is itself made by electrolysis and costs more than aluminium, and the reaction would need high temperatures with volatile magnesium. Electrolysis of $A l_{2} O_{3}$ in cryolite is cheaper, so Mg is not used.

Solved Example 11

An ore containing galena and zinc blende is concentrated by froth flotation with NaCN added. Which statement is correct?
(A) NaCN activates ZnS so that it floats
(B) NaCN depresses ZnS so that only PbS comes up with the froth
(C) NaCN depresses PbS so that ZnS floats
(D) NaCN acts as the frother

Solution:

Answer: (B). Cyanide forms a zinc cyanide complex film on ZnS, so ZnS stays in the pulp while PbS floats. $CuS O_{4}$ is later added to activate ZnS for a second flotation. Pine oil, not NaCN, is the frother.

Solved Example 12

Calculate the mass of aluminium needed to reduce 1.00 kg of

F e_{2} O_{3}

completely, and the heat released (

Δ H^{\circ} = - 852

kJ per mole of

F e_{2} O_{3}

). Molar masses:

F e_{2} O_{3}

159.7, Al 27.0 g mol^-1.

Solution:

F e_{2} O_{3} + 2Al A l_{2} O_{3} + 2Fe

Moles of $F e_{2} O_{3}$ $= \frac{1000}{159.7} = 6.26$ mol. Aluminium needed $= 2 \times 6.26 = 12.5$ mol $= 12.5 \times 27.0 \approx 338$ g.

Mass ratio $F e_{2} O_{3}$ : Al $= 1000 : 338 \approx 3 : 1$ , which is the thermite ratio. Heat released $= 6.26 \times 852 \approx 5.3 \times 1 0^{3}$ kJ, enough to melt the iron produced.

Solved Example 13

Copper is to be recovered from a

CuS O_{4}

leach liquor. Both zinc scrap and iron scrap are available. Which will work, and which is preferred? (

E^{\circ}

Z n^{2 +} /Zn

- 0.76

F e^{2 +} /Fe

- 0.44

C u^{2 +} /Cu

+ 0.34

Solution:

Scrap	Reaction	$E_{cell}^{\circ}$	$Δ G^{\circ} = - 2 F E^{\circ}$
Zinc	$C u^{2 +} + Zn \to Cu + Z n^{2 +}$	1.10 V	$- 212$ kJ
Iron	$C u^{2 +} + Fe \to Cu + F e^{2 +}$	0.78 V	$- 151$ kJ

Both reactions are spontaneous, and zinc has the larger driving force. Iron scrap is much cheaper and plentiful, and it still reduces copper completely, so iron scrap is used.

Solved Example 14

Which ore is calcined rather than roasted?
(A) zinc blende
(B) galena
(C) calamine
(D) copper pyrites

Solution:

Answer: (C). Calamine is a carbonate: $ZnC O_{3} Δ ZnO + C O_{2}$ . The other three are sulphides, which are roasted in air.

Practice Questions

Match: (a) Al (b) Cu (c) Mg (d) Zn (e) Hg with (i) cinnabar (ii) calamine (iii) cryolite (iv) malachite (v) carnallite.Answer: (a)-(iii), (b)-(iv), (c)-(v), (d)-(ii), (e)-(i).
Which of Na, Ag and Fe is extracted by (i) complex formation (ii) reduction with carbon (iii) electrolysis of a fused salt?Answer: (i) Ag (cyanide complex); (ii) Fe; (iii) Na.
Of Cu, Fe, Ag and Al, which is the lightest and which is the least reactive?Answer: Al is the lightest (density about 2.7 g cm^-3); Ag is the least reactive.
(i) What is solvent extraction? (ii) Explain the van Arkel method. (iii) Explain fractional crystallisation.Answer: (i) Transferring a metal compound from water into an immiscible organic solvent in which it is more soluble, leaving impurities behind. (ii) Impure Zr or Ti is heated with iodine to form the volatile tetraiodide, which is decomposed on a tungsten filament at about 1800 K to pure metal. (iii) Separating compounds by their different solubilities, so they crystallise at different stages (Pt and Ir salts; Pattinson process for Ag in Pb).
(i) How does NaCN act as a depressant for ZnS in froth flotation? (ii) Why is a sulphide ore roasted to the oxide before reduction?Answer: (i) It forms a zinc cyanide complex film on ZnS, so ZnS is not wetted by the collector and does not float. (ii) Neither carbon nor hydrogen can reduce sulphides ( $C S_{2}$ and $H_{2} S$ are less stable than metal sulphides), but oxides are readily reduced by C or CO.
You have impure samples of zinc, copper and germanium. Suggest a refining method for each.Answer: Zinc: distillation (or electrolysis); copper: electrolytic refining; germanium: zone refining.
In which climate can tin not be used as a structural metal, and why?Answer: Very cold climates. Below about 13 °C (286 K) white tin slowly changes into brittle, powdery grey tin (tin pest), so tin objects crumble.

Common Mistakes to Avoid

Watch out

Calling every mineral an ore. An ore must give the metal profitably: clay is a mineral of aluminium, bauxite is its ore.
Saying froth flotation works because the ore is lighter. It works by wetting: the sulphide is wetted by oil and sticks to air bubbles.
Mixing up calcination (no air, carbonates, $C O_{2}$ given off) and roasting (excess air, sulphides, $S O_{2}$ given off).
Choosing the wrong flux: silica removes basic gangue such as FeO; lime removes acidic gangue such as $Si O_{2}$ .
Writing $ZnO + C \to Zn + C O_{2}$ , which is not even balanced. At smelting temperatures carbon gives CO: $ZnO + C \to Zn + CO$ .
Reading an Ellingham line per mole of oxide. Every line is per mole of $O_{2}$ ; compare values only on that basis.
Thinking the Ellingham diagram tells how fast a reduction goes. It shows feasibility only, not kinetics.
Assuming carbon can reduce $C u_{2} S$ or ZnS directly. Sulphides are first roasted to oxides.

Frequently Asked Questions

What are the main metallurgical processes?

Metallurgical processes are the steps that give a pure metal from its ore: concentration to remove gangue, calcination or roasting to form the oxide, reduction of the oxide to crude metal with carbon, carbon monoxide, another metal or electrolysis, and finally refining. The route chosen depends on the type of ore and how reactive the metal is.

What is the difference between a mineral and an ore?

A mineral is any naturally occurring compound of a metal found in the Earth's crust. An ore is a mineral from which the metal can be extracted conveniently and profitably. So all ores are minerals but not all minerals are ores: bauxite is an ore of aluminium, while clay is only a mineral.

What is the difference between calcination and roasting?

Calcination heats an ore strongly in the absence or a limited supply of air; it suits carbonate and hydrated ores and drives off carbon dioxide or water. Roasting heats an ore, usually a sulphide, in excess air below its melting point; it oxidises the sulphide to the oxide and releases sulphur dioxide.

What does an Ellingham diagram show?

An Ellingham diagram plots the standard Gibbs energy of formation of oxides, per mole of oxygen, against temperature. An element can reduce the oxide of any element whose line lies above its own at that temperature. It explains why coke reduces iron oxide above about 1000 K and why aluminium needs electrolysis.

Why is aluminium not extracted by reducing alumina with carbon?

Alumina is so stable that its Ellingham line lies below the carbon to carbon monoxide line up to about 2000 K. Carbon would work only at extremely high temperatures, where it also forms aluminium carbide, so the process would be costly and impure. Electrolysis of alumina dissolved in molten cryolite is used instead.

What is the difference between the Mond process and the van Arkel method?

Both are vapour phase refining methods. In the Mond process impure nickel reacts with carbon monoxide at 330 to 350 K to form volatile nickel tetracarbonyl, which decomposes at 450 to 470 K to pure nickel. The van Arkel method converts zirconium or titanium to a volatile iodide that decomposes on a hot tungsten filament.

Is metallurgy in the JEE Main and NEET syllabus in 2026?

No. General Principles and Processes of Isolation of Metals was dropped from the JEE Main syllabus in 2024, and NEET removed metallurgy from its Class 12 chemistry syllabus at the same time. Both 2026 syllabi keep it out. The topic remains part of JEE Advanced under Isolation of Metals.

How is metallurgy asked in JEE Advanced?

JEE Advanced lists ores and concentration, the thermodynamic principles for iron, copper and zinc, the electrochemical principle for aluminium, the cyanide process for silver and gold, and refining. Questions often combine an Ellingham diagram with a named process, a froth flotation depressant, or a match-the-column on ores and methods.

Previous year questions on Introduction to Metallurgical Processes

4 questions from past papers, each with a step-by-step solution.

Ready to master General Principles And Processes Of Isolation Of Elements?

Take a full mock test, practice concept-by-concept, and get an AI-powered rank prediction — all on Fundamenthol.

Practice questions →Take a mock test