- IIT-JEE Syllabus
Commercially important ores of iron, copper, lead, magnesium, aluminium , tin and silver . Carbon reduction process (iron and tin), Self reduction process (copper and lead), Electrolytic reduction process (magnesium and aluminium), Cyanide process (silver and gold).
The earth’s crust is the main source of metals. The occurrence of metal in native or in combined state in the earth’s crust along with a number of rocky and other impurities depends upon the chemical nature of metals. Metals having less electropositive character have less affinity for oxygen, moisture and occur in free or metallic or native state i.e., in uncombined state e.g. Au, Pt , Ag etc. On the other hand metals with higher electropositive character occurs in combined state i.e., as compounds.
The compound of a metal found in nature is called a mineral. A mineral may be a single compound or a complex mixture. Those minerals from which metal can be economically extracted are called ores. Thus all ores are minerals but all mineral are not ores. For e.g. copper occurs in nature in the form of several minerals like Cu2O, Cu2S, CuFeS2, but copper pyrites is considered as the most economical mineral for the extraction of the metal. Hence copper pyrites is the chief ore of copper.
Ores may be divided into four groups
- i) Native Ores: These ores contain the metal in free state eg. Silver gold etc. These are usually formed in the company of rock or alluvial impurities like clay, sand etc.
- ii) Oxidised Ores: These ores consist of oxides or oxysalts (eg. carbonates, phosphate) and silicate of metal. Important oxide ore includes, Fe2O3, Al2O3.2H2O etc. and important cabonate ores are limestone (CaCO3), Calamine (ZnCO3) etc.
iii) Sulphurised Ores: These ores consist of sulfides of metals like iron, lead, mercury etc. Examples are iron pyrites (FeS2). galena (PbS), Cinnabar (HgS)
- iv) Halide ores: Metallic halides very few in nature. Chlorides are most common examples include horn silver (AgCl) carnallite KCl. MgCl2.6H2O and fluorspar (CaF2) etc.
|It is the branch of engineering employed for the extraction of metals economically on a large scale from their respective ores. Before going into the description of extraction process we should study at least the Ellingham diagram regarding the stability of different oxides.|
Less the ΔG0 value i.e. more the ΔG0 is negative more will be the stability of that oxide. It is clearly observed from the diagram ΔG0 for the reaction C + is always negative after 800°C.
Thus for Zn + …(1)
And C+ …(2)
The reduction of ZnO to Zn will be spontaneous at higher temperature.
= – 57 – (–42) = – 15 kcal / mole
ΔG0 = negative, so the reduction of ZnO by coke is spontaneous at 1200°C. This is why in all the carbon reduction process the temperature is very high called as pryometallurgy. But at 25°C ΔG0CO – ΔG0ZnO = – 33 – (–76) = + 43 kcal/mole. So, no reactants, Hg, Ag crosses
ΔG0 = 0 line at higher temperature. Thus on heating HgO ⎯→ Hg + O2
or Ag2O ⎯→ 2Ag + O2 conversion takes place.
The eligibility of metal as reducing agent in reduction of other metal oxides can be deciphered from this diagram. Carbon line slopes downward at around 710°C. Hence C can be used as a reducing agent at higher temperature. The graph shows the temperature when the oxide line cuts the C-line. This is the temperature of reduction of the oxide by C. Thus MgO is reduced at 1650°C, ZnO at 950°C, FeO at 725°C.
Certain melting processes
C – reduction process: Fe2O3 + C → Fe3O4 + CO
Fe3O4 + C → FeO + CO
FeO + C → Fe + CO
ZnO + C → Zn + CO etc
Self reduction process
Cu2O + Cu2S → Cu + SO2
For carbonate ores
ZnCO3 → ZnO + CO2
ZnO + C → Zn + CO↑
Hydrometallurgy: By this technique at normal temperature the metal can be extracted.
For example: Ag, Au are precipitated by this preparation.
2Na[Ag(CN)2] (aq) + Zn(s) ⎯→ Na2[Zn(CN)4] + 2Ag↓
2Na [Au(CN)2] aq. + Zn(s) ⎯→ Na2[Zn(CN)4] + 2Au↓
Electrolytic Reduction: This is applied for the active metal cloride or Al2O3
MX ⎯→ M+ + X–
At cathode M+ + e ⎯→M
At anode X– – e ⎯→ X