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Monday, 11 July 2016

Occurrence and Distribution of Metals,metallurgy of Iron,copper,sodium

Occurrence and Distribution of                     Metals
Ore: deposit that contains enough metal that we can extract economically.
Most metals are found in minerals.
Most important ores are oxide, sulphide and carbonates
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Metallurgy is the science and technology of extracting metals from minerals.
There are five important steps:
1.Mining (getting the ore out of the ground);

2.  Concentrating (preparing it for further   treatment);
3.  Reduction (to obtain the free metal in the zero   oxidation state);
4.  Refining (to obtain the pure metal); and
5.  Mixing with other metals (to form an alloy).

Pyro metallurgy
Pyrometallurgy: using high temperatures to obtain the free metal.
Several steps are employed:
Calcination is heating of ore to cause decomposition and elimination of a volatile product (CO2 or H2O):
PbCO3(s) ® PbO(s) + CO2(g)
Roasting is heating  which causes chemical reactions between the ore and the furnace atmosphere:
2ZnS(s) + 3O2(g) ® 2ZnO(s) + 2SO2(g
2MoS2(s) + 7O2(g) ® 2MoO3(s) + 4SO2(g)
Smelting is a melting process that causes materials to separate into two or more layers.
Slag consists mostly of molten silicates in addition to aluminates, phosphates, fluorides, and other inorganic materials.
Refining is the process during which a crude, impure metal is converted into a pure metal.

The Pyro metallurgy of Iron
Most important sources of iron are haematite Fe2O3 and magnetite Fe3O4.
Reduction occurs in a blast furnace.
The ore, limestone and coke are added to the top of the blast furnace.
Coke is coal that has been heated to drive off the volatile components.
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Coke reacts with oxygen to form CO (the reducing agent):
2C(s) + O2(g) ® 2CO(gDH = -221 kJ
CO is also produced by the reaction of water vapor in the air with C:
C(s) + H2O(g) ® CO(g) + H2(g), DH = +131 kJ
Since this reaction is endothermic, if the blast furnace gets too hot, water vapour is added to cool it down without interrupting the chemistry.
At around 250°C limestone is calcined (heated to decomposition and elimination of volatile).
CaCO3(s) ® CaO(s) + CO2(g)
This reacts with the silicates and other components of the ore to form the slag.
Also around 250°C iron oxides are reduced by CO(g) and H2(g):
Fe3O4(s) + 4CO(g) ® 3Fe(s) + 4CO2(g), DH = -15 kJ
Fe3O4(s) + 4H2(g) ® 3Fe(s) + 4H2O(g), DH = +150 kJ
Molten iron is produced lower down the furnace and removed at the bottom.
If iron is going to be made into steel it is poured directly into a basic oxygen furnace.

Formation of Steel
Oxygen diluted with Ar is used as the oxidizing agent.
When oxygen emerges from the converter, then all the impurities have been oxidized and the iron is poured into a ladle
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Hydrometallurgy is the extraction of metals from ores using water.
Leaching is the selective dissolution of the desired mineral.
Typical leaching agents are dilute acids, bases, salts, and sometimes water.

The Hydrometallurgy of Aluminum
Aluminum is the second most useful metal.
Bauxite is a mineral that contains Al as Al2O3.xH2O.
Major impurities are silicates (SiO2) and iron oxides (Fe2O3).

Bayer process:
The crushed ore is digested in 30% NaOH (by mass) at 150 - 230°C and high pressure (30 atm to prevent boiling).
Al2O3 dissolves:
Al2O3.H2O(s) + 2H2O(l) + 2OH-(aq) ® 2Al(OH)4-(aq)

The silicates and iron oxides do not dissolve and can be filtered from the solution.
The aluminate solution is separated by lowering the pH causing separation of the aluminum hydroxide.
The aluminate hydroxide is calcined to produce the aluminum oxide.

Electrometallurgy of Sodium
Electrometallurgy is the process of obtaining metals through electrolysis.
Two different starting materials: molten salt or aqueous solution?

2H+(aq)   +   2e-      H2 (g)   Ered = 0.00 V
2H2O(l) +   2e-      H2 (g)  + 2OH- (aq)   Ered = - 0.83 V

Water is reduced more easily than metals because the reduction potentials of water under both acidic and basic conditions are more positive than those of:
Na+  Ered = - 2.71 V
Mg2+  Ered = - 2.37 V
Al3+  Ered = - 1.66 V

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Sodium is produced by electrolysis of molten NaCl in a Downs cell.
CaCl2 is used to lower the melting point of NaCl from 804°C to 600°C.
Electrometallurgy of Aluminium
Hall process electrolysis cell is used to produce aluminium.
Al2O3 melts at 2000°C and it is impractical to perform electrolysis on the molten salt. 
Hall: use purified Al2O3 in molten cryolite (Na3AlF6, melting point 1012°C).
Anode:  C(s) + 2O2-(l)   ®  CO2(g) + 4e-
Cathode:   3e- + Al3+(l)   ®   Al(l)
The graphite rods are consumed in the reaction.

Electro refining of Copper
Because of its good conductivity, Cu is used to make electrical wiring.
Impurities reduce conductivity, therefore pure copper is required in the electronics industry.

Slabs of impure Cu are used as anodes, thin sheets of pure Cu are the cathodes. 
Acidic copper sulphate is used as the electrolyte.
The voltage across the electrodes is designed to produce copper at the cathode.
The metallic impurities do not plate out on the cathode.
Metal ions are collected in the sludge at the bottom of the cell.

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