ChemistryICSE Class 10 · Metallurgy

Ores and Concentration of Ores: Iron, Aluminium and Zinc (ICSE Class 10)

Updated 15 October 2026 · Fact-checked

An ore is a mineral from which a metal can be extracted economically and conveniently. The ore is crushed and then concentrated by removing gangue: the hydrolytic (gravity) method for heavy ores, magnetic separation for magnetic iron ores, and froth flotation for sulphide ores such as zinc blende.

Understand Ores and Concentration of Ores

Minerals and ores. Metals in the earth's crust usually occur as compounds called minerals. A mineral from which a metal can be extracted economically and conveniently is an ore. So every ore is a mineral, but not every mineral is an ore: clay contains aluminium, but extracting aluminium from it is too costly, so bauxite is used instead. The earthy, rocky impurities mixed with an ore are the gangue (also called matrix).

Common ores in the syllabus.

Iron: haematite, Fe₂O₃; magnetite, Fe₃O₄.
Aluminium: bauxite, Al₂O₃·2H₂O (chief ore); cryolite, Na₃AlF₆; corundum, Al₂O₃.
Zinc: zinc blende, ZnS (chief ore); calamine, ZnCO₃.

Pulverisation. The large lumps of ore are crushed and ground to a fine powder, so that ore and gangue particles separate and later chemical steps act quickly.

Concentration (dressing). The gangue is removed using a property in which ore and gangue differ:

1. Hydrolytic method (gravity separation). The powdered ore is washed with a stream of water on a sloping, vibrating table. The heavier ore particles settle; the lighter gangue is washed away. Suitable for heavy oxide ores such as haematite.
2. Magnetic separation. Powdered ore falls on a conveyor belt that passes over a magnetic roller. Magnetic particles cling to the belt longer and drop close to the roller; non-magnetic gangue falls off further away, forming a separate heap. Used for iron ores: magnetite, and haematite (ICSE papers in 2024 and 2025 gave haematite as the ore concentrated this way).
3. Froth flotation. The powder is mixed with water and a little pine oil, and air is blown through to make froth. Sulphide ore particles are wetted by the oil and carried up in the froth, which is skimmed off; the gangue is wetted by water and sinks. Used for zinc blende.

Chemical concentration (leaching). Bauxite is concentrated chemically by Baeyer's process: alumina dissolves in sodium hydroxide while iron oxide does not (see Extraction of Aluminium).

After concentration, the ore is converted into the oxide by roasting or calcination and then reduced.

Ores and Concentration of Ores: formulas and key facts

Ores of iron, aluminium and zinc: Haematite Fe₂O₃
Magnetite Fe₃O₄
Bauxite Al₂O₃·2H₂O
Cryolite Na₃AlF₆
Corundum Al₂O₃
Zinc blende ZnS
Calamine ZnCO₃
Concentration method for each ore: Zinc blende (sulphide) froth flotation
Magnetite and haematite (iron ores) magnetic separation
Heavy oxide ores such as haematite hydrolytic washing
Bauxite chemical method (Baeyer's process)
Property each method uses: Hydrolytic method difference in density
Magnetic separation ore is attracted by a magnet, gangue is not
Froth flotation ore wetted by oil, gangue wetted by water
Percentage of metal in an ore: $% metal = \frac{mass of metal in 1 mol of ore}{gram molecular mass of ore} \times 100$

Haematite	Fe₂O₃
Magnetite	Fe₃O₄
Bauxite	Al₂O₃·2H₂O
Cryolite	Na₃AlF₆
Corundum	Al₂O₃
Zinc blende	ZnS
Calamine	ZnCO₃

Zinc blende (sulphide)	froth flotation
Magnetite and haematite (iron ores)	magnetic separation
Heavy oxide ores such as haematite	hydrolytic washing
Bauxite	chemical method (Baeyer's process)

Hydrolytic method	difference in density
Magnetic separation	ore is attracted by a magnet, gangue is not
Froth flotation	ore wetted by oil, gangue wetted by water

How to solve Ores and Concentration of Ores questions

Matching an ore to its concentration method is about one question: how does the ore differ from its gangue?

1Write the ore's formula: ZnS, ZnCO₃, Fe₂O₃, Fe₃O₄, Al₂O₃·2H₂O.
2Sulphide? Choose froth flotation.
3Iron oxide (magnetic)? Choose magnetic separation; for heavy haematite the hydrolytic method also works.
4Bauxite? Chemical method (Baeyer's process).
5State the property used (wetting by oil, magnetism, density) and describe what happens to ore and gangue.
6For percentage questions, divide the mass of metal in one mole by the molar mass of the ore.

Ores and Concentration of Ores 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.

Choosing between two ores

Suppose a mining company can buy either 500 kg of a rock containing 60% zinc blende (ZnS) or 500 kg of a rock containing 70% calamine (ZnCO₃), the rest being gangue. (Zn = 65, S = 32, C = 12, O = 16)

Which rock contains more zinc, and which concentration method suits the first rock?

Try it first, then show the solutionSolution

Rock 1: ZnS $= 300$ kg; % Zn in ZnS $= \frac{65}{97} \times 100 = 67.01%$ ; zinc $= 300 \times \frac{65}{97} = 201.0$ kg.
Rock 2: ZnCO₃ $= 350$ kg; % Zn in ZnCO₃ $= \frac{65}{125} \times 100 = 52%$ ; zinc $= 350 \times 0.52 = 182$ kg.
Zinc blende is a sulphide, so it is concentrated by froth flotation.

Answer: The zinc blende rock (about 201 kg Zn against 182 kg); it is concentrated by froth flotation.

Memory hook: The richest ore is the one with the most metal per kilogram, not the most mineral.

Tricks to remember Ores and Concentration of Ores

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

Every ore is a mineral, not every mineral an ore
Like every doctor is a person but not every person a doctor: an ore is a mineral that pays.
Use it when: Defining ore with 'economically and conveniently'.
Sulphides surf on froth
Sulphide ores ride up with the pine-oil froth; sand sinks.
Use it when: Froth flotation for zinc blende.
Magnet for iron
Only iron ores (magnetite, haematite) go near the magnet; zinc, aluminium ores never do.
Use it when: Section A MCQs on magnetic separation.

Ores and Concentration of Ores: solved examples

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

Example 1: Definition by its key words · 1 mark · MCQBoard level

Clay and bauxite both contain aluminium, but aluminium is extracted only from bauxite because this is cheap and convenient. Bauxite is therefore described as:
(a) gangue
(b) an alloy
(c) an ore
(d) a flux

Try it first, then show the solutionSolution

Gangue is the impurity mixed with an ore; an alloy is a mixture of metals.
A mineral from which the metal is extracted economically and conveniently is an ore.

Answer: (c) an ore.

Example 2: Formulae of ores · 2 marks · Short answerBoard level

Give the name and formula of (a) the chief ore of zinc, (b) a carbonate ore of zinc, (c) a magnetic ore of iron, (d) the chief ore of aluminium.

Try it first, then show the solutionSolution

(a) Zinc blende, ZnS. (b) Calamine, ZnCO₃.
(c) Magnetite, Fe₃O₄. (d) Bauxite, Al₂O₃·2H₂O.

Answer: ZnS; ZnCO₃; Fe₃O₄; Al₂O₃·2H₂O.

Example 3: How froth flotation works · 3 marks · Short answerHarder board level

Describe the froth flotation method for concentrating zinc blende. State the property it depends on.

Try it first, then show the solutionSolution

Powdered ore is mixed with water and a little pine oil in a tank, and air is blown through to make froth.
Sulphide particles are wetted by the oil and rise with the froth, which is skimmed off and dried.
Gangue particles are wetted by water and sink to the bottom.
Property: ore is preferentially wetted by oil, gangue by water.

Answer: Ore rises with the oily froth and is skimmed off; gangue sinks. Depends on different wetting by oil and water.

Example 4: Magnetic separation · 3 marks · Short answerHarder board level

Explain, with a labelled description, how magnetic separation concentrates an iron ore. Why can it not be used for zinc blende?

Try it first, then show the solutionSolution

Powdered ore is dropped on a conveyor belt moving over a magnetic roller.
Magnetic ore particles are held by the roller and fall close to it; non-magnetic gangue falls further away, so two heaps form.
Zinc blende is not magnetic, so it would fall with the gangue; froth flotation is used instead.

Answer: Magnetic ore collects near the roller, gangue further away; zinc blende is non-magnetic.

Example 5: Which iron ore is richer? · 4 marks · Long answerHOTS / competency

Calculate the percentage of iron in (a) haematite, Fe₂O₃, and (b) magnetite, Fe₃O₄. Which is richer in iron? A 200 kg sample of rock contains 80% Fe₂O₃; what is the maximum mass of iron it can give? (Fe = 56, O = 16)

Try it first, then show the solutionSolution

(a) Fe₂O₃: $\frac{112}{160} \times 100 = 70%$ .
(b) Fe₃O₄: $\frac{168}{232} \times 100 = 72.41%$ . Magnetite is richer.
Fe₂O₃ in the rock $= 0.80 \times 200 = 160$ kg.
Iron $= 160 \times 0.70 = 112$ kg.

Answer: (a) 70% (b) 72.41%; magnetite is richer. Maximum iron: 112 kg.

Example 6: Choosing a method for an unknown ore · 3 marks · Short answerHOTS / competency

An ore X is not attracted by a magnet. When roasted in air it gives a gas that turns moist blue litmus red, and its powder rises with froth when mixed with pine oil, water and air. (a) What type of ore is X? (b) Name the concentration method. (c) Suggest X from the syllabus and write the roasting equation.

Try it first, then show the solutionSolution

(a) Floating with oil and giving an acidic gas (SO₂) on roasting shows a sulphide ore.
(b) Froth flotation.
(c) Zinc blende: $2 Z n S + 3 O_{2} \to 2 Z n O + 2 S O_{2}$ .

Answer: (a) Sulphide ore (b) froth flotation (c) zinc blende, ZnS.

Where students lose marks in Ores and Concentration of Ores

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

Mistake: Confusing ore with alloy or matrix
In 2024 (Q1(xv)) examiners reported that candidates confused ore with alloy or matrix.
Do this: Ore: profitable mineral. Matrix (gangue): impurity. Alloy: mixture of metals.
Usually costs: 1 mark.
Mistake: Choosing zinc blende for magnetic separation
Examiners noted this choice in 2024 (Q8(i)); in 2025 (Q1(vii)) candidates chose bauxite, corundum or calamine instead of haematite.
Do this: Only iron ores are magnetic: haematite or magnetite.
Usually costs: 1 mark.
Mistake: Naming the wrong process for bauxite
In 2025 (Q7(ii)) examiners found Baeyer's process confused with the Hall-Héroult process.
Do this: Concentration of bauxite = Baeyer's process (chemical); extraction = Hall-Héroult (electrolysis).
Usually costs: 1 mark.
Mistake: Wrong formula for an ore
Students write ZnCO₃ for zinc blende or Fe₃O₄ for haematite.
Do this: Blende = ZnS (sulphide); calamine = ZnCO₃; haematite = Fe₂O₃; magnetite = Fe₃O₄.
Usually costs: 1 mark.

Exam tips

Learn ore names and formulae as a table; they appear in Section A MCQs almost every year.
In a definition of ore, the words 'economically and conveniently' (or 'profitably') carry the mark.
For froth flotation, mention pine oil and say which particles float and which sink.
Write the property used, not just the method name, when asked 'on what principle'.

Practice questions on Ores and Concentration of Ores

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 pair of ores could both be concentrated on a magnetic roller?
(a) Zinc blende and calamine
(b) Bauxite and cryolite
(c) Magnetite and haematite
(d) Zinc blende and haematite
Hint
Both members of the pair must be iron ores.
Answer
(c) magnetite and haematite, the two iron ores.
Q2 · 1 mark · Very short answerBoard level
What is gangue?
Hint
Impurities with the ore.
Answer
The earthy and rocky impurities (sand, clay, rock) mixed with an ore.
Q3 · 2 marks · Short answerBoard level
Name one ore each of iron, aluminium and zinc with formulae.
Hint
Any one per metal.
Answer
Iron: haematite, Fe₂O₃ (or magnetite, Fe₃O₄). Aluminium: bauxite, Al₂O₃·2H₂O. Zinc: zinc blende, ZnS (or calamine, ZnCO₃).
Q4 · 1 mark · Very short answerHarder board level
Why is clay not used as an ore of aluminium even though it contains aluminium?
Hint
Definition of an ore.
Answer
Aluminium cannot be extracted from clay economically and conveniently, so clay is a mineral but not an ore.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Zinc blende is concentrated by froth flotation.
Reason (R): Sulphide ore particles are wetted by oil while gangue is wetted by water.
(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
What makes the ore float?
Answer
(a).
Q6 · 3 marks · Short answerHarder board level
State the property on which each depends: (a) hydrolytic method, (b) magnetic separation, (c) froth flotation.
Hint
Density, magnetism, wetting.
Answer
(a) Difference in density: heavy ore settles, light gangue is washed away. (b) Ore is magnetic, gangue is not. (c) Ore is wetted by oil, gangue by water.
Q7 · 2 marks · Short answerHarder board level
Calculate the percentage of zinc in zinc blende and in calamine, and state which is the richer ore. (Zn = 65, S = 32, C = 12, O = 16)
Hint
65 ÷ 97 and 65 ÷ 125.
Answer
ZnS: $\frac{65}{97} \times 100 = 67.01%$ . ZnCO₃: $\frac{65}{125} \times 100 = 52%$ . Zinc blende is richer.
Q8 · 3 marks · Short answerHOTS / competency
Suppose a student puts powdered bauxite into a froth flotation tank and then over a magnetic roller, and finds neither step separates the alumina well. Explain why, and name the method that does work.
Hint
Bauxite is an oxide, and its main impurity is iron oxide.
Answer
Froth flotation suits sulphide ores; bauxite is a hydrated oxide and is not preferentially wetted by oil. Alumina is not magnetic, so the roller does not separate it from gangue. Bauxite is concentrated chemically by Baeyer's process (leaching with NaOH, in which alumina dissolves but iron(III) oxide does not).

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.Formula of calamine?Show answer
ZnCO₃.
2.Oil used in froth flotation?Show answer
Pine oil.
3.Chief ore of aluminium?Show answer
Bauxite, Al₂O₃·2H₂O.
4.What is pulverisation?Show answer
Crushing and grinding the ore into a fine powder.
5.Concentration method for zinc blende?Show answer
Froth flotation.

Ores and Concentration of Ores: frequently asked questions

What is the difference between a mineral and an ore?

A mineral is any naturally occurring compound of a metal. An ore is a mineral from which the metal can be extracted economically and conveniently. All ores are minerals; not all minerals are ores.

Which ores are concentrated by magnetic separation?

Iron ores, which are magnetic: magnetite (Fe₃O₄) and haematite (Fe₂O₃), as the ICSE board papers treat it.

Why is froth flotation used for sulphide ores?

Sulphide particles are wetted by oil and carried up by the froth, while gangue is wetted by water and sinks, so they separate cleanly.

How is bauxite concentrated?

Chemically, by Baeyer's process: it is heated with sodium hydroxide, in which alumina dissolves and iron(III) oxide does not.