ChemistryCBSE Class 12 · General Principles and Processes of Isolation of Elements

Concentration of Ores, Calcination, Roasting and Reduction: Class 12 Chemistry

Updated 18 October 2026 · Fact-checked

Ores are first concentrated to remove gangue: hydraulic washing, magnetic separation, froth floatation (sulphide ores) and leaching (bauxite, silver, gold). The concentrate is converted to the oxide by calcination (removes volatile matter) or roasting (heating in air, for sulphides), and the oxide is reduced to the metal, with electrolysis for highly reactive metals like aluminium.

Understand Concentration of Ores, Calcination, Roasting and Reduction

Terms. Minerals are naturally occurring chemical substances obtained by mining; ores are minerals from which a metal can be extracted profitably; gangue is the earthy material mixed with the ore; metallurgy is the whole process of isolating the metal. Aluminium is the most abundant metal (about 8.3 per cent by weight); iron is second. Bauxite is the ore for aluminium. For iron, oxide ores are chosen because they are abundant and do not produce polluting SO2, unlike iron pyrites.

Concentration (dressing). Removing gangue, chosen by differences in physical properties.

Hydraulic washing: an upward stream of water washes away lighter gangue and leaves heavier ore.
Magnetic separation: used when the ore or the gangue is magnetic. Powdered ore falls on a belt over a magnetic roller.
Froth floatation: for sulphide ores. Powdered ore is suspended in water with collectors (pine oil, fatty acids, xanthates) and froth stabilisers (cresols, aniline). Mineral particles are wetted by oil and gangue by water; air from a rotating paddle makes froth that carries the mineral up. Depressants separate two sulphides: NaCN keeps ZnS out of the froth while PbS rises.
Leaching: dissolving the ore in a suitable solvent. Bauxite is digested with concentrated NaOH at 473 to 523 K and 35 to 36 bar: Al2O3 + 2NaOH + 3H2O gives 2Na[Al(OH)4]. CO2 is passed in and seeding with fresh hydrated alumina precipitates hydrated Al2O3: 2Na[Al(OH)4] + 2CO2 gives Al2O3.xH2O + 2NaHCO3; heating gives pure Al2O3. Silver and gold are leached with dilute NaCN or KCN in air to form soluble cyanide complexes, then displaced by zinc.

Conversion to oxide.

Calcination: heating to remove volatile matter, leaving the oxide, for example ZnCO3 gives ZnO + CO2.
Roasting: heating in a regular supply of air below the melting point of the metal, for sulphide ores: 2ZnS + 3O2 gives 2ZnO + 2SO2; 2PbS + 3O2 gives 2PbO + 2SO2; 2Cu2S + 3O2 gives 2Cu2O + 2SO2.

Reduction. The oxide is reduced by carbon (coke), carbon monoxide or a more reactive metal: ZnO + C gives Zn + CO. For very reactive metals like aluminium, chemical reduction is impractical, so electrolysis of the molten compound is used. The reduction must have a negative Gibbs energy change under the conditions used.

Concentration of Ores, Calcination, Roasting and Reduction: formulas and key facts

Concentration methods: Hydraulic washing density difference
Magnetic separation magnetic property
Froth floatation sulphide ores
Leaching chemical dissolution; Choose by property.
Calcination and roasting: $C a l c ina t i o n : Z n C O 3 - > Z n O + C O 2∣ R o a s t in g : 2 Z n S + 3 O 2 - > 2 Z n O + 2 S O 2$; Roasting needs air, for sulphides.
Leaching of bauxite: $A l 2 O 3 + 2 N a O H + 3 H 2 O - > 2 N a [A l (O H) 4] ∣2 N a [A l (O H) 4] + 2 C O 2 - > A l 2 O 3. x H 2 O + 2 N a H C O 3$; Digestion then seeding.

Hydraulic washing	density difference
Magnetic separation	magnetic property
Froth floatation	sulphide ores
Leaching	chemical dissolution

How to solve Concentration of Ores, Calcination, Roasting and Reduction questions

This chapter is formative (school) assessment only in 2026-27. Questions ask for method selection, equations and mole-mass calculations.

1Identify the ore type: sulphide (froth floatation, roasting), carbonate (calcination), bauxite (leaching).
2Write the balanced equation and check atoms of each element.
3For mass problems, convert masses to moles using the given atomic masses.
4Use mole ratios from the balanced equation.
5Convert back to mass and state units.

Concentration of Ores, Calcination, Roasting and Reduction in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Why fine crushing helps floatation

Suppose a small plant crushes a sulphide ore into a fine powder before mixing it with water and pine oil.

Why is the ore powdered, and what carries the mineral to the top?

Try it first, then show the solutionSolution

Powder gives a large surface area for wetting by oil.
Mineral particles are wetted by oil; gangue by water.
Air bubbles carry the mineral particles in the froth.

Answer: A fine powder exposes a large surface so the mineral particles are wetted by the oil, while the gangue is wetted by water. Air blown by the paddle forms a froth that carries the mineral particles to the top.

Memory hook: Oil loves mineral, water loves gangue.

Why sulphides are roasted

Suppose a sample of zinc sulphide concentrate is heated in a furnace with a steady supply of air.

State the reaction and the gas that needs control.

Try it first, then show the solutionSolution

2ZnS + 3O2 gives 2ZnO + 2SO2.
SO2 is a polluting gas.

Answer: Roasting converts ZnS to ZnO: 2ZnS + 3O2 gives 2ZnO + 2SO2. The SO2 produced is a pollutant, which is why oxide ores are preferred to pyrites for iron.

Memory hook: Roasting a sulphide makes SO2.

Tricks to remember Concentration of Ores, Calcination, Roasting and Reduction

Short memory aids. Say each one aloud once, then use it on a question the same day.

Calcine = carbonate, Roast = air
Calcination removes volatile matter; roasting uses air for sulphides.
Use it when: Distinction.
Pine oil, xanthate, cresol
Collectors: pine oil, xanthates; stabilisers: cresol, aniline.
Use it when: Froth floatation.
NaCN depresses ZnS
NaCN keeps ZnS down while PbS floats.
Use it when: Separating sulphides.

Concentration of Ores, Calcination, Roasting and Reduction: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Method for sulphide ores · 1 mark · MCQBoard level

Which method concentrates sulphide ores?
(a) hydraulic washing
(b) magnetic separation
(c) froth floatation
(d) leaching with NaOH

Try it first, then show the solutionSolution

Sulphide ores are wetted by oil and carried up in froth: floatation (1 mark).

Answer: (c) froth floatation

Example 2: Calcination versus roasting (2 marks) · 2 marks · Very short answerBoard level

Distinguish between calcination and roasting with one equation each. (2 marks)

Try it first, then show the solutionSolution

Calcination: heating to remove volatile matter, for example ZnCO3 gives ZnO + CO2 (1 mark).
Roasting: heating in air for sulphides, for example 2ZnS + 3O2 gives 2ZnO + 2SO2 (1 mark).

Answer: Calcination is heating to remove volatile matter, as in ZnCO3 giving ZnO + CO2. Roasting is heating in a regular supply of air, used for sulphide ores, as in 2ZnS + 3O2 giving 2ZnO + 2SO2.

Example 3: Mass in roasting and reduction (5 marks) · 5 marks · Long answerHarder board level

194 g of pure ZnS is roasted completely, and the ZnO formed is reduced with carbon (ZnO + C gives Zn + CO). Given atomic masses Zn = 65, S = 32, O = 16, find the mass of ZnO and of Zn. (5 marks)

Try it first, then show the solutionSolution

Molar mass ZnS = 65 + 32 = 97 g/mol; moles = 194 / 97 = 2 mol (1 mark).
2ZnS + 3O2 gives 2ZnO + 2SO2: 2 mol ZnS gives 2 mol ZnO (1 mark).
Molar mass ZnO = 65 + 16 = 81; mass = 2 x 81 = 162 g (1 mark).
ZnO + C gives Zn + CO: 2 mol ZnO gives 2 mol Zn (1 mark).
Mass of Zn = 2 x 65 = 130 g (1 mark).

Answer: Moles of ZnS = 194 / 97 = 2. Roasting gives 2 mol ZnO = 2 x 81 = 162 g. Reduction gives 2 mol Zn = 2 x 65 = 130 g.

Example 4: Leaching of bauxite (4 marks) · 4 marks · Short answerHarder board level

Write the equations for digestion of bauxite with NaOH and for precipitation of alumina by CO2, and explain the role of seeding. (4 marks)

Try it first, then show the solutionSolution

Al2O3 + 2NaOH + 3H2O gives 2Na[Al(OH)4]; conditions 473 to 523 K, 35 to 36 bar (1 mark).
2Na[Al(OH)4] + 2CO2 gives Al2O3.xH2O + 2NaHCO3 (1 mark).
Seeding with fresh hydrated alumina helps precipitate hydrated Al2O3 (1 mark).
Heating the precipitate gives pure Al2O3 (1 mark).

Answer: Digestion: Al2O3 + 2NaOH + 3H2O gives 2Na[Al(OH)4] at 473 to 523 K and 35 to 36 bar, while SiO2 dissolves as silicate. Precipitation: 2Na[Al(OH)4] + 2CO2 gives Al2O3.xH2O + 2NaHCO3, helped by seeding with a little fresh hydrated alumina. Heating the precipitate gives pure Al2O3.

Example 5: Choosing methods · 4 marks · Case-basedHOTS / competency

A mixture of PbS and ZnS must be separated by froth floatation. (i) Which substance acts as the depressant? (1) (ii) Which sulphide comes up in the froth? (1) (iii) Why is aluminium not obtained by reducing Al2O3 with carbon in practice? (2)

Try it first, then show the solutionSolution

(i) NaCN (1 mark).
(ii) PbS (1 mark).
(iii) Aluminium is highly reactive, so chemical reduction is impractical and electrolysis of the molten compound is used (2 marks).

Answer: (i) NaCN. (ii) PbS rises in the froth while ZnS is kept out. (iii) Aluminium is very reactive, so chemical reduction is not practical; its molten compound is electrolysed instead.

Where students lose marks in Concentration of Ores, Calcination, Roasting and Reduction

The slips examiners see most often, and what to write instead.

Mistake: Confusing calcination with roasting
Both involve heating.
Wrong working
Calcination is done in excess air for sulphides.
Right working
Roasting uses air for sulphides; calcination removes volatile matter.
Do this: Roasting needs air.
Usually costs: 1 mark.
Mistake: Unbalanced roasting equation
Forgetting the 2 : 3 : 2 : 2 ratio.
Wrong working
ZnS + O2 gives ZnO + SO2.
Right working
2ZnS + 3O2 gives 2ZnO + 2SO2.
Do this: Count O atoms: 6 on each side.
Usually costs: 1 mark.
Mistake: Using molar masses without moles
Skipping the mole step.
Wrong working
194 g ZnS gives 162 g ZnO because ratios are the same.
Right working
Convert 194 g to 2 mol, then use mole ratio.
Do this: Always convert to moles.
Usually costs: 2 marks.

Exam tips

This chapter is formative (school) assessment only in 2026-27.
Always balance roasting equations and count oxygen atoms.
Name the method with the ore type.

Practice questions on Concentration of Ores, Calcination, Roasting and Reduction

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which of the following is a collector in froth floatation?
(a) cresol
(b) aniline
(c) pine oil
(d) NaCN
Hint
Makes particles non-wettable by water.
Answer
(c) pine oil
Q2 · 2 marks · Very short answerBoard level
Why are oxide ores preferred to iron pyrites for extracting iron? (2 marks)
Hint
Gas formed in roasting.
Answer
Oxide ores are abundant and do not produce polluting SO2, whereas iron pyrites would.
Q3 · 3 marks · Short answerHarder board level
97 g of ZnS contains how many moles of ZnS, and what mass of ZnO forms on roasting? Given Zn = 65, S = 32, O = 16. (3 marks)
Hint
Molar mass 97.
Answer
Moles = 97 / 97 = 1 mol. 2ZnS gives 2ZnO, so 1 mol gives 1 mol ZnO = 81 g.
Q4 · 5 marks · Long answerHarder board level
Explain with equations how the roasting of copper(I) sulphide and lead sulphide differ in product from the calcination of zinc carbonate, and state why the reduction step usually follows these. (5 marks)
Hint
SO2 versus CO2; oxides reduce easier.
Answer
Roasting: 2Cu2S + 3O2 gives 2Cu2O + 2SO2 and 2PbS + 3O2 gives 2PbO + 2SO2, producing oxides and SO2 in air. Calcination of zinc carbonate: ZnCO3 gives ZnO + CO2, no air needed. Both give oxides, which are easier to reduce, so reduction by carbon or CO follows, for example ZnO + C gives Zn + CO.
Q5 · 1 mark · Assertion-reasonHOTS / competency
Assertion (A): Gold is leached with dilute NaCN solution in the presence of air.
Reason (R): Oxygen is needed to form the soluble cyanide complex.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Hint
Equation includes O2.
Answer
(a) Both A and R are true, and R is the correct explanation of A.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is gangue?Show answer
Earthy or unwanted material mixed with an ore.
2.What is seeding?Show answer
Adding a little fresh hydrated alumina to help precipitate hydrated Al2O3.
3.What is used to leach silver or gold?Show answer
Dilute NaCN or KCN in the presence of air.
4.Which metal is most abundant in the crust?Show answer
Aluminium.

Concentration of Ores, Calcination, Roasting and Reduction: frequently asked questions

What is froth floatation?

A method for sulphide ores in which mineral particles are carried up by froth while gangue stays in water.

What is the difference between roasting and calcination?

Roasting is heating in air for sulphides; calcination removes volatile matter.

How is aluminium extracted?

By electrolysis of the molten compound, since chemical reduction is impractical.