EXTRACTION OF METALS

A metal is an element (except hydrogen) which loses electron(s) to form positive ions.

Physical Properties of metal compared to non-metal

Physical strength is the tensile strength of the metal. Physical strength results from the way atoms are arranged in a substance.

• There are metals with high tensile strength like iron, copper, and aluminium. Other metals have little tensile strength like sodium and potassium.

Chemical strength is the reactivity of the metal. Chemical reactivity depends on the way electrons are arranged in an atom. Potassium and sodium have very low tensile strength but chemically they have the greatest chemical strength.

Chemical properties of metals

Chemical properties of metals depend on the reactivity rate.

1. Reaction with air / oxygen

Almost all metals react with oxygen to form metal oxides. But different metals react with oxygen at different intensities.

• Potassium, sodium, calcium, magnesium, zinc, iron, and lead react in air or oxygen to form oxides.
4Na + O₂ → 2Na₂O
• Lead and copper react with oxygen on heating but they do not burn.
• Less reactive metals like silver, gold, and platinum do not react with oxygen.

2. Reaction of metals with water

Metals react with water to produce metal hydroxide (or metal oxide) and hydrogen gas. The reactions depend on the reactivity of the metal.

• Potassium, sodium, and calcium react with cold water to produce their hydroxides and hydrogen gas.
2Na + 2H₂O → 2NaOH + H₂
• Magnesium, zinc, and iron react with hot water (steam) to produce oxides and hydrogen gas.
Mg + H₂O → MgO + H₂

3. Reactions of metals with dilute acids

• Potassium and sodium react extremely violently with rapid effervescence and splashing (should not be tried).
• Calcium, magnesium, zinc, and iron react slowly with bubbles of hydrogen.
• Copper, silver, gold, and platinum do not react with dilute acids.

4. Reaction of metals with hydrogen

Only a few metals like Na, K, Ca, and Mg react with hydrogen to form metal hydrides.

5. Displacement reactions

A more reactive metal (higher in the reactivity series) can displace a less reactive metal from its compound.

Extraction of Metals

An ore is a naturally occurring mineral from which a metal can be extracted economically.

• Some minerals contain a high percentage of a particular metal and can be extracted from them.
• An ore contains a good percentage of metal and no objectionable impurities.

Therefore, all ores are minerals, but not all minerals are ores.

Extraction of metals is the process of obtaining pure metal from its ore.

The abundance of chemical elements in the Earth's crust:

• Oxygen 45%
• Silicon 27%
• Aluminium 8%
• Iron 5.8%
• Calcium 5.1%
• Magnesium 2.8%
• Sodium 2.3%
• Potassium 1.7%
• Titanium 0.8%
• All others 1.4%

Ores are those minerals from which metals are extracted conveniently and profitably.

Some Common Ores

Methods of Extracting Metals

The methods employed in extracting metals depend on their reactivity. The more reactive the metal, the more difficult it is to extract from its compound.

Stages / Procedures of extracting metals:

Metals can be extracted from their ores by the following processes (especially for moderately reactive metals):

1. Concentration of the ore

This is the removal of unwanted material such as sand, clay, etc., from the ore. It is also known as beneficiation of the ore. Methods include washing with water, magnetic separation, or froth flotation.

2. Conversion of concentrated ore into metal oxide (Roasting or Calcination)

• Roasting: Strongly heating the ore in excess air below its melting point.
2ZnS + 3O₂ → 2ZnO + 2SO₂
2PbS + 3O₂ → 2PbO + 2SO₂
• Calcination: Heating the ore in a limited supply of air below its melting point.
CaCO₃ → CaO + CO₂

3. Reduction of oxides to metals

The process of converting metal oxides into metals is called reduction. The method depends on the reactivity of the metal.

4. Purification of the metal

The metal obtained is usually impure and needs to be purified. Methods include distillation and electrolytic refining.

Extraction of Sodium by Down's Process

Principal ores:

• Sodium chloride (rock salt)
• Sodium nitrate (Chile saltpetre)
• Sodium carbonate (soda ash).

On an industrial scale, sodium is extracted by Down's Process in a Down's cell:

• The cell consists of a steel container lined with firebricks.
• Anode: Graphite rod.
• Cathode: Ring of iron, separated by a steel mesh to prevent sodium and chlorine from recombining.
• Sodium chloride has a high melting point (800°C), so calcium chloride is added to lower it to about 600°C.

Reactions:

At the cathode:

At the anode:

Why is sodium kept under kerosene?

Sodium reacts vigorously with air and catches fire at room temperature, so it is stored under kerosene to prevent contact with air.

Extraction of Iron

Chief iron ores:

1. Haematite (Fe₂O₃)
2. Magnetite (Fe₃O₄)
3. Siderite (FeCO₃)
4. Iron pyrite (FeS₂)
5. Limonite (Fe₂O₃·3H₂O)

Iron is mainly extracted from haematite and magnetite in a blast furnace:

1. Coke reacts with hot air to form carbon dioxide (exothermic reaction):
C + O₂ → CO₂
2. Limestone decomposes into calcium oxide and carbon dioxide:
CaCO₃ → CaO + CO₂
3. Carbon dioxide reacts with more coke to form carbon monoxide (reducing agent):
CO₂ + C → 2CO
4. Carbon monoxide reduces iron oxide to iron:
Fe₂O₃ + 3CO → 2Fe + 3CO₂
5. Calcium oxide reacts with impurities (e.g., silica) to form slag:
CaO + SiO₂ → CaSiO₃

Molten iron and slag are tapped off separately.

Extraction of Aluminium

Main ores:

1. Bauxite (Al₂O₃·2H₂O)
2. Cryolite (Na₃AlF₆)

Stages:

1. Purification of bauxite:

• Treated with sodium hydroxide to remove impurities:
Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O
• Aluminium hydroxide is precipitated and heated to form pure alumina:
2Al(OH)₃ → Al₂O₃ + 3H₂O

2. Electrolysis of purified bauxite:

• Alumina is mixed with cryolite to lower its melting point (from 2050°C to 900°C).
• Reactions:

At the anode:

2O^2- → O₂ + 4e^-

At the cathode:

Al³⁺ + 3e^- → Al

Uses of Aluminium:

1. Food containers (resistant to corrosion, non-toxic).
2. Aircraft bodies (light, strong, corrosion-resistant).
3. Overhead power cables (conducts electricity, lightweight).
4. Window frames (corrosion-resistant).
5. Cooking utensils (high melting point).

Extraction of Copper

Principal ores:

• Copper pyrites (CuFeS₂)
• Cuprite (Cu₂O)
• Copper glance (Cu₂S)
• Malachite (CuCO₃·Cu(OH)₂)

Stages:

1. Roasting of concentrated ore:
2CuFeS₂ + 4O₂ → Cu₂S + 2FeO + 3SO₂
2. Conversion of iron(II) oxide to slag:
FeO + SiO₂ → FeSiO₃
3. Reduction of copper(I) sulphide to copper:
Cu₂S + O₂ → 2Cu + SO₂(g)
4. Purification by electrolysis.

Uses of Copper:

1. Electrical wires (excellent conductor).
2. Alloys (bronze, brass).
3. Cooking utensils (conducts heat, high melting point).
4. Water pipes (corrosion-resistant).
5. Coins and ornaments.

Alloys

A mixture of two or more metals or a metal and a non-metal.