OCCURRENCE OF METALS
The earth’s crust is the biggest source of metals besides some soluble salts of metals found in sea water. The mode of occurrence of a metal is largely dependent on its chemical nature. Those metals, which are relatively inert, occur in free or native state (i.e. in uncombined state) but most of the metals are reactive and hence are found in combined state. The naturally occurring inorganic substances, which are obtained by mining, are known as minerals. The mineral has a definite composition. It may be a single compound or a complex mixture. The minerals from which the metals can be conveniently and economically extracted are known as ores. Thus, all ores are minerals, but all minerals are not ores, e.g. cinnabar (HgS) is an ore as well as mineral of mercury but iron pyrites (FeS2) is a mineral of iron but not an ore. The valuable mineral contained in an ore is an ore mineral. The other minerals contained in the mixture, which ordinary are waste materials, constituted the gangue of the ore.
Ore = Ore mineral + Gangue
The chief ores in the ores of economic importance are :
(i) Oxides (ii) sulphides (iii) carbonate (iv) sulphates (v) halides and (vi) silicates.
(a) Native Ores :
These ores contain metal in free state. For example : Silver, gold, platinum, mercury, copper etc. Sometime lumps of pure metals are found known as nuggets.
(b) Combined Ores :
These ores contain metal in combination with oxygen or sulphur or halides etc. The ores & minerals of various metals are
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Metals |
Minerals/ores with their chemical formula |
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1. |
Iron (VIII, transition metal) |
(a) |
Iron pyrite; FeS2 (as sulphide) |
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(b) |
Siderite; FeCO3 (as carbonate) |
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(c) |
Red haematite; Fe2O3 (as oxide) |
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(d) |
Magnetite; Fe3O4 (as oxide) |
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(e) |
Limonite or brown haematite; 2Fe2O3.3H2O (as oxide) |
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2. |
Magnesium (II A, bridge metal) |
(a) |
Cuprite; Cu2O (as oxide) |
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(b) |
Copper pyrites; CuFeS2 (as sulphides) |
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(c) |
Copper glance; Cu2S (as sulphides) |
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(d) |
malachite; CuCO3.Cu(OH)2 |
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(e) |
Azurite;2CuCO3.Cu(OH)2 |
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3. |
Tin (IV A, amphoteric metal) |
(a) |
Cassiterite or Tin stonel SnO2 (as oxide) |
|
(b) |
Stannite; Cu2S.FeS.SnS2 (as sulphide) |
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4. |
Lead (IV A, amphoteric metal) |
(a) |
Galena; PbS (as sulphide) |
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(b) |
Cerussite; PbCO3 (as carbonate) |
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(c) |
Anglessite; PbSO4 (as sulphate) |
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(d) |
White lead; 2Pb(OH)2.PbCO3 (as carbonate) |
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5. |
Magnesium (II A, bridge metal) |
(a) |
Carnalite; KCI.MgCI2.6H2O (as chloride) |
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(b) |
Dolomite; MgCO3.CaCO3 (as carbonate) |
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(c) |
Magnesite; MgCO3 (as carbonate) |
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(d) |
Epsomite; MgSO4.7H2O (as sulphate) |
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(e) |
Kiesserite; MgSO4.H2O (as sulphate) |
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(f) |
Asbestos; CaMg(SiO3)4 |
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6. |
Aluminium (III A, amphoteric metal) |
(a) |
Bauxite; AI2O.2H2O (as oxide) |
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(b) |
Corundum; AI2O3 (as oxide) |
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(c) |
Feldspar; KAISi3O6 (as oxide) |
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(d) |
Clay silicate; AI2O3.2SiO2.2H2O (as oxide) |
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(e) |
Cryolite; 3NaF.AIF3.(Na3AIF6) (as halide) |
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(f) |
Alum; K2O.3AI2(SO4)3.24H2O (as sulphate) |
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(g) |
Diaspore; AI2O3.H2O (as oxide) |
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(h) |
Mica; K2O.3AI2O3.6SiO2.2H2O (as oxide) |
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7. |
Zinc (II B, amphoteric metal) |
(a) |
Zinc blende; ZnS (as sulphide) or sphalerite |
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(b) |
Zincite; ZnO (as oxide) |
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(c) |
Smithsonite, ZnCO3 (as carbonate) |
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(d) |
Hemimorphite or Calamine, Zn2SiO4.H2O (as silicate) |
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8. |
Silver (IB, best reductive metal) |
(a) |
Horn silver (Chlorapatite); AgCI (as halide) |
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(b) |
Lunar caustic; AgNO3 (as nitrate) |
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(c) |
Silver glance or argentite; Ag2S (as sulphide) |
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(d) |
Ruby silver or pyrogyrite; 3Ag2S.Sb2S3 |
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The branch of science that deals with the extraction of metals from their respective ores and the preparation of alloys is called metallurgy. The metallurgy of each metal is an individual problem and the line of treatment depends upon the nature of ore and the chemical properties of the metal. Some common steps involved in the metallurgical operations are
- Crushing and grinding of the ore
- Concentration or dressing of ore
- Extraction of crude metal from concentrated ore
- Reduction of ore to the metallic form and
- Purification of metal