SPI Chemistry · Metals and Non-Metals

Ores and Extraction of Metals

A metal comes from its ore in steps: the ore is concentrated, turned into an oxide, reduced to the metal, and the metal is then treated to give it the hardness needed.

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Why this matters

Four questions in the bank, two EASY and two MODERATE, one of them from the 2022 paper (the name for soaking bauxite in sodium hydroxide). The others ask the metal in bauxite, how iron pyrite is concentrated, and the name for cooling hot iron slowly. All four are names to recall.

Concept 1 of 4: Minerals, ores and gangue

Metals are found in the earth's crust as compounds called minerals. A mineral from which a metal can be taken out cheaply is an ore. The sand and rock that come with it are the gangue.

Definition

A mineral is a natural compound of a metal. An ore is a mineral from which the metal can be extracted profitably. Every ore is a mineral; not every mineral is an ore.
The unwanted earth, sand and rock mixed with an ore is the gangue.
Ores are mostly oxides, sulphides or carbonates. Gold and platinum are found free.

Ore	Formula	Metal
Bauxite	$\mathrm{Al_2O_3 \cdot 2H_2O}$	Aluminium The chief ore of aluminium.
Haematite	$\mathrm{Fe_2O_3}$	Iron
Magnetite	$\mathrm{Fe_3O_4}$	Iron
Zinc blende	$\mathrm{ZnS}$	Zinc
Calamine	$\mathrm{ZnCO_3}$	Zinc
Copper pyrites	$\mathrm{CuFeS_2}$	Copper
Cinnabar	$\mathrm{HgS}$	Mercury
Rock salt	$\mathrm{NaCl}$	Sodium

Iron pyrites,

\mathrm{FeS_2}

, contains iron but is mined for its sulphur; iron is extracted from haematite.

Practice this conceptself-check · 5 quick reps

The same idea in a practice question:

Practice questionEasy

Example 1 · Metals and Non-Metals · Ores and Extraction of Metals

Bauxite is an ore of which metal?

Calling every mineral an ore

A mineral is an ore only if the metal can be extracted from it profitably. Every ore is a mineral, but the reverse is not true.

Pairing an ore with the wrong metal

Bauxite is aluminium, haematite and magnetite are iron, zinc blende and calamine are zinc, cinnabar is mercury. Copper pyrites contains iron too, but it is the ore of copper.

Concept 2 of 4: Concentrating an ore

Before a metal can be taken out, most of the gangue must go. Each method uses some difference between the ore and the gangue: weight, magnetism, wetting by oil, or dissolving in a chemical.

Definition

Hydraulic washing (gravity separation): a stream of water washes away the lighter gangue; the heavy ore stays. Used for heavy oxide ores.
Magnetic separation: a magnet picks out a magnetic ore such as magnetite from the gangue.
Froth flotation: for sulphide ores. The powdered ore is mixed with water and pine oil and air is blown in. The ore is wetted by oil and rises with the froth; the gangue, wetted by water, sinks.
Leaching: the ore is dissolved in a suitable chemical and the gangue is left behind. Bauxite dissolves in sodium hydroxide (Bayer's process); gold and silver ores dissolve in sodium cyanide.

Method	Based on	Used for
Hydraulic washing	Ore heavier than gangue	Heavy oxide ores
Magnetic separation	Ore is magnetic	Magnetite
Froth flotation	Ore wetted by oil, gangue by water	Sulphide ores: zinc blende, copper pyrites, iron pyrites Sulphide ore means froth flotation.
Leaching	Ore dissolves in a chemical	Bauxite (sodium hydroxide), gold and silver (sodium cyanide)

Leaching is the only chemical method of the four; the other three are physical.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

SPI · 2022 · SPI Entrance Examination 2022 · Q98Moderate

Example 2 · Metals and Non-Metals · Ores and Extraction of Metals

The process of soaking bauxite, the aluminium ore, in aqueous NaOH, to separate alumina is called____.

Using froth flotation for an oxide ore

Froth flotation suits sulphide ores, which oil wets. A heavy oxide ore is concentrated by hydraulic washing, and a magnetic one by a magnet.

Confusing leaching with washing

Hydraulic washing only carries the gangue away with water. Leaching dissolves the ore itself in a chemical, such as bauxite in sodium hydroxide.

Drill 1 more on concentrating an ore

Concept 3 of 4: Roasting, calcination and reduction

Once the ore is concentrated, it is first turned into an oxide, because an oxide is easiest to reduce. Then the oxygen is taken away, and how that is done depends on how reactive the metal is.

Definition

Roasting: a sulphide ore is heated strongly in plenty of air and becomes an oxide, giving off sulphur dioxide.
Calcination: a carbonate ore is heated strongly in limited air and becomes an oxide, giving off carbon dioxide.
Reduction: metals in the middle of the series (zinc, iron, lead) are reduced from their oxides by carbon.
The most reactive metals (potassium, sodium, calcium, magnesium, aluminium) are got by electrolysis of their molten compounds.

Worked example

Zinc is got from zinc blende,

\mathrm{ZnS}

. Write the two steps that turn the concentrated ore into zinc.

Practice this conceptself-check · 4 quick reps

Swapping roasting and calcination

Roasting is for sulphide ores, heated in plenty of air. Calcination is for carbonate ores, heated in limited air. Both end with an oxide.

Concept 4 of 4: Heat treatment of steel

The same piece of steel can be made soft or hard just by how it is cooled after heating. Slow cooling softens it, sudden cooling hardens it, and a gentle reheat takes the edge off the brittleness.

Definition

Annealing: heat the metal, then let it cool slowly and uniformly. It becomes softer and less brittle.
Quenching: heat the metal red hot, then cool it suddenly in water or oil. It becomes hard and brittle.
Tempering: reheat quenched steel to a moderate temperature and cool it slowly. It keeps its hardness but loses much of its brittleness.

Treatment	How it is cooled	Effect
Annealing	Slowly and uniformly	Softer, less brittle Slow cooling is annealing.
Quenching	Suddenly, in water or oil	Hard and brittle
Tempering	Reheated quenched steel, cooled slowly	Hard but less brittle

Tempering always comes after quenching.

Practice this conceptself-check · 3 quick reps

The same idea in a practice question:

Practice questionModerate

Example 4 · Metals and Non-Metals · Ores and Extraction of Metals

Slow and uniform cooling of hot iron in its metallurgy is known as

Mixing up annealing and quenching

Annealing is slow, even cooling and softens the metal. Quenching is sudden cooling and hardens it.

Tempering a metal that was never quenched

Tempering is a second step: it reheats steel that has already been quenched, to reduce its brittleness.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (3)

Minerals, ores and gangue8 rows

Ore	Formula	Metal
Bauxite	$\mathrm{Al_2O_3 \cdot 2H_2O}$	Aluminium The chief ore of aluminium.
Haematite	$\mathrm{Fe_2O_3}$	Iron
Magnetite	$\mathrm{Fe_3O_4}$	Iron
Zinc blende	$\mathrm{ZnS}$	Zinc
Calamine	$\mathrm{ZnCO_3}$	Zinc
Copper pyrites	$\mathrm{CuFeS_2}$	Copper
Cinnabar	$\mathrm{HgS}$	Mercury
Rock salt	$\mathrm{NaCl}$	Sodium

Iron pyrites,

\mathrm{FeS_2}

, contains iron but is mined for its sulphur; iron is extracted from haematite.

Concentrating an ore4 rows

Method	Based on	Used for
Hydraulic washing	Ore heavier than gangue	Heavy oxide ores
Magnetic separation	Ore is magnetic	Magnetite
Froth flotation	Ore wetted by oil, gangue by water	Sulphide ores: zinc blende, copper pyrites, iron pyrites Sulphide ore means froth flotation.
Leaching	Ore dissolves in a chemical	Bauxite (sodium hydroxide), gold and silver (sodium cyanide)

Leaching is the only chemical method of the four; the other three are physical.

Heat treatment of steel3 rows

Treatment	How it is cooled	Effect
Annealing	Slowly and uniformly	Softer, less brittle Slow cooling is annealing.
Quenching	Suddenly, in water or oil	Hard and brittle
Tempering	Reheated quenched steel, cooled slowly	Hard but less brittle

Tempering always comes after quenching.

Watch out for (7)

Calling every mineral an ore
→ Minerals, ores and gangue
Pairing an ore with the wrong metal
→ Minerals, ores and gangue
Using froth flotation for an oxide ore
→ Concentrating an ore
Confusing leaching with washing
→ Concentrating an ore
Swapping roasting and calcination
→ Roasting, calcination and reduction
Mixing up annealing and quenching
→ Heat treatment of steel
Tempering a metal that was never quenched
→ Heat treatment of steel

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