O Level Revision : Integrated Science - Science in Industry - Metals

 Iron and copper

This unit deals with the occurrence and extraction of two important metals namely copper and iron.

IRON

Occurrence in Zimbabwe Iron mines in Zimbabwe

Iron can occur in two forms:

1. a) haematite = iron (III) oxide                                        Red Cliff in Kwekwe
2. b) iron pyrite = iron sulphide                                          Buchwa in Mberengwa

Extraction

Iron is extracted using the blast furnace.

a) Raw materials and their functions

• Iron ore      -  source of iron.
• Limestone  -  flux/removal of impurities.
• Coke           -  fuel.
• Hot air        -  source of oxygen.

b) Chemistry of the blast furnace

(i)   Lower down the furnace closer to tuyeres carbon from coke combines with oxygen from hot air to produce carbon dioxide.

Carbon (coke)    +     oxygen →  carbon dioxide

(ii)  Further up the furnace carbon dioxide is reduced to carbon monoxide by coke.

Carbon dioxide     +     carbon (coke)  → carbon monoxide

(iii) Carbon monoxide reduces iron oxide to iron carbon dioxide.

Iron (III)  oxide  +  carbon monoxide  → iron  +  carbon dioxide

This is called indirect reduction. Direct reduction: the carbon in coke can reduce iron oxide directly.

Iron (III) oxide  +  carbon → carbon monoxide  +  iron

(iv) Limestone is decomposed by heat to form calcium oxide and carbon dioxide. (heat)

Calcium carbonate (limestone)  → calcium oxide  +  carbon dioxide

(v)  Calcium oxide combines with silica to form calcium silicate which floats on top of

molten iron as slag.

Calcium oxide  +  silicon  → calcium silicate

A Blast Furnace

Purification of iron

• It is purified by the oxygen lance process.
• Oxygen under high pressure is blown into a furnace.
• This oxidizes impurities such as phosphorous and carbon. These impurities leave in gaseous form.
• Silica/sand reacts with calcium oxide to form slag.

Alloys of iron

• Iron can react with other substances in the environment.
• To prevent this it is made into alloys or coated.
• An alloy is a mixture of metals.

 Composition, properties and uses of some alloys

COPPER

Occurrence in Zimbabwe

Copper occurs as an ore called chalcopyrite, which is a sulphide of copper with iron (CuFeS ).

The ore is also referred to as copper pyrites.

Copper mines in Zimbabwe

• Alaska.
• Mhangura - now non operational.

Extraction

Four main stages are involved in the extraction of copper, concentration by floatation, roasting, reduction and electrolytic refining. Before the following stages, copper is first ground into powder through crushing.

a) Concentration by floatation

• Crushed ore is agitated in an oil-water mixture by means of a jet of pressurized air.
• The copper bearing ore sticks to the froth at the top while impurities (clay and waste rock) settle at the base of the tank.
• The concentrated ore is skimmed off and taken for the next procedure.

b) Roasting

• The concentrated ore is roasted in air.
• The copper (II) sulphide component of the ore is converted to copper (I) sulphide.
• At the same time the iron (II) sulphide part turns into iron (II) oxide because iron is more susceptible to oxidation by oxygen than copper is:

Copper pyrites + oxygen + copper (I) sulphide →  iron (II) oxide  +  sulphur dioxide

• Sulphur dioxide produced is a valuable by product for the manufacture of sulphuric acid.
• Iron (II) oxide combines with silica to forma slag and is removed (iron silicate).
• Copper (I) sulphide is oxidized to convert some of it to copper (I) oxide.

Copper (I) sulphide     +     oxygen →  copper (l) oxide    +     sulphur dioxide.

c) Reduction

• The mixture of copper (I) oxide and copper (I) sulphide is reduced to copper. Copper (I) sulphide    +    copper (l) oxide                 copper    +     sulphur dioxide
• Crude copper called blister copper is produced.

d) Electrolytic refining

• The blister copper is refined by electrolysis.
• The blister copper is made the positive electrode while the negative is a very thin sheet of pure copper.
• Aqueous copper (II) sulphate forms the electrolyte whose conductivity is improved by acidifying, it with dilute sulphric acid.
• Some impurities are dissolved by the acid while those that do not dissolve settle at the bottom of the positive electrode.
• The positive electrode gets thinner while the negative gets thicker during the process.

Properties and uses of copper

• Copper has certain properties that make it suitable for its uses.

 Properties of copper and its uses

Alloys of copper

• Copper can react with other substances in the environment. To prevent this it is made into alloys.

 Properties and uses of copper alloys

Coating processes

• Materials are coated to prevent corrosion and for decoration.

 Methods used to coat materials and how they prevent corrosion

Application of coating process