Extraction of metals (O-level chemistry)

General principles of extraction of metals

Occurrence of elements in nature

Elements occur in two state

1. Native or free especially less reactive, Au, Ag, Cu, Pt.
2. Combined state: generally, the reactive metals occur as oxides, sulphide, phosphates, silicates, carbonates, phosphates, chlorides and nitrates

Minerals and ores

Minerals are compound of metals which are found in the earth and in which the metals are present in combined state. Minerals have a chemical composition and structure. Ores are those minerals from which the metals can be extracted economically and conveniently. Thus, all minerals are not ores but all ores are minerals.

Methods of extraction of metals

The method of extraction of metals depends on metal’s position in electrochemical series.

i.e. Potassium, sodium, calcium,  magnesium, aluminium, carbon, zinc, iron, copper

Elements above carbon are extraction are extracted by electrolysis while those below carbon are extracted by reduction with carbon of carbon monoxide.

Extraction of Sodium

It is manufactured by electrolysis of fused sodium chloride in the Downs’ cell ) (MPt. 800⁰C)

At first there were four difficulties

• It was expensive to keep the electrolyte over 800⁰C
• The molten sodium chloride was corrosive
•      Sodium is fairly soluble in its molten chloride at 800⁰C
• The vapor pressure of sodium at 800⁰C is very high, about ½ atmosphere of 400mmHg.

The difficulties were all overcome by adding calcium chloride to sodium chloride to form a mixture that melts below 600⁰C.  It is cheaper to maintain this temperature, the mixture is non corrosive, sodium is almost insoluble and the vapor pressure is about 15mmHg.

The iron gauze cylinder between anode and cathode prevent the sodium and chlorine from mixing. Liquid sodium leaves the cell in a raised pipe high enough for the low density metal but not high enough for the higher density chloride mixture to overflow continuously into the receiver

The reactions are

At cathode:  Na⁺ (l) + e →Na(l)

Anode;         2Cl^– – 2e → Cl₂ (g)

Sodium is collected in dry nitrogen to protect it from reacting with air.

Uses of sodium

• Molten sodium is used as a coolant in some types of nuclear reactor because of its good thermal conductivity, low melting point and its higher boiling point than that of water
• Sodium wire is used in electrical circuit for special application
• Sodium vapor lamps are used for street lighting
• Sodium cyanide is used in extraction of silver and gold

Iron and its compounds

Iron, which is the second most abundant metal occurring in the earth crust, is extracted from its oxides, haematite, Fe₂O₃ and magnetite, Fe₃O₄, and from the siderite, FeCO₃. Iron pyrites, FeS₂, is not considered an important ore of iron.

The extraction of iron is carried out in a blast furnace that can vary in size between 2 and 600 metres high and up to 10 meters in diameter.

Itis constructed from steel with the inner region lined with bricks. Iron ore, lime stone and coke in the correct proportions are fed into the top of the furnace through a cone and hopper arrangement. Preheated air at a temperature of about 600⁰C is injected into the furnace through a number of pipes called tyères (pronounced “Tweers”).  The tuyères are fed from a “bustle” pipe encircling the blast furnace. The blast furnace is provided with two holes which are plugged with clay; molten iron is tapped from the lower one and molted slag from the upper one.  The production of iron is a continuous process and depending on its size, a blast furnace can be producing from 1000 to 1800 tons of iron every 24 hours.

The energy and reducing agent required for the smelting of iron are obtained by the combustion of coke, the temperature of the charge increasing steadily as it falls through the ascending combustion gases:

                2C (s) + O₂ (g)  →2CO (g)

At a temperature of about 700 ⁰C the iron ore is reduced to spongy iron by the carbon monoxide.

                 Fe₂O₃ (s) + 3CO (g) ↔ 2Fe (s) + 3CO₂ (g)

The limestone decomposes at 500 ⁰C and the calcium oxide reacts with sandy impurities to form a slag of calcium silicate. More carbon monoxide is produced by the reduction of carbon dioxide:

                 CaCO₃ (s) → CaO (s) + CO₂ (g)

                 CaO (s) + SiO₂ (s)  → CaSiO₃ (l) 

                              impurity       slag

                 C (s)   + CO₂ (g)  → 2CO (g)

Uses of iron

In form of steel it used to make bridges, doors and window flame, cutlery  and so on

Properties of iron

1. It reacts with oxygen to form iron (III) oxide.

2Fe (s) + 2O2(g)     →         2Fe₂O₃(s)

• It reacts with sulphur to form black iron (II) sulphide

Fe(s) + S(s)     →       FeS(s)

• It reacts with chlorine to form iron (III) chloride

2Fe(s) + 3Cl₂ (g)   →        FeCl₃(s)

• It reacts with hydrogen chloride gas to form iron (II) chloride

Fe(s)  + 2 HCl(g)    →        FeCl₂(s)

• Reacts reversibly with steam to form hydrogen and triirontetroxide

3Fe (s) + H₂O(g)    →        Fe₃O₄(s) + 4H₂(g)

• It reacts with dilute acid to liberate hydrogen

Fe(s) + 2H⁺(aq)     →        Fe²⁺(aq) + H₂(g)

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Extraction of metals

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