Leaching : It involves the treatment of the ore with a suitable reagent as to make it soluble while impurities remain insoluble. The ore is recovered from the solution by suitable chemical method. For example, bauxite ore contains ferric oxide, titanium oxide and silica as impurities. When the powdered ore is digested with an aqueous solution of sodium hydroxide at about 150°C under pressure, the alumina (Al2O3) dissolves forming soluble sodium meaaluminate while ferric oxide (Fe2O3), TiO2 and silica remain as insoluble part.
Al
2O
3 + 2NaOH
2NaAlO
2 + H
2O
Pure alumina is recovered from the filtrate
NaAlO
2 + 2H
2O
Al(OH)
3 + NaOH
2Al(OH)
3Al2O
3 + 3H
2O
Gold and silver are also extracted from their native ores by Leaching (Mac-Arthur Forrest cyanide process). Both silver and gold particles dissolves in dilute solution of sodium cyanide in presence of oxygen of the air forming complex cyanides.
4Ag + 8NaCN + 2H2O + O2 |
 |
4NaAg(CN)2 + 4NaOH |
Sod. argento cyanide
4Au + 8NaCN + 2H
2O + O
24NaAu(CN)
2 + 4NaOH
Ag or Au is recovered from the solution by the addition of electropositive metal like zinc.
2NaAg(CN)
2 + Zn
Na
2Zn(CN)
4 + 2Ag
2NaAu(CN)
2 + Zn
Na
2Zn(CN)
4+ 2Au
2. Second operation––Reduction to free metal
Some of the methods commonly used to get free metal the concentrated ore are given below :
(a) Smelting : This involves the reduction of the ore to the molten metal at a high temperature. For the extraction of less electropositive metals such as Pb, Zn, Fe, Sn, etc., powerful reducing agents such as C, H2, CO, water gas, Na, K, Mg, Al may be used. Some examples are given below :
PbO + CPb + CO
WO3 + 3H2W + 3H2O
CuO + COCu + CO2
TiCl4 + 2MgTi + 2MgCl2
Cr2O3 + 2Al2Cr + Al2O3
Out of these carbon reduction and aluminium reduction are important processes.
(i) Carbon reduction process : It is generally called as smelting. The oxides of the less electropositive metals are reduced by strongly heating them with coal or coke.
PbO + C
Pb + CO
PbO + CO
Pb + CO
2
Similarly,
Fe
2O
3 + 3C
2Fe + 3CO
Fe
2O
3 + 3CO
2Fe + 3CO
2
The ores, even after concentration, contain some gangue. To remove gangue, certain substances are mixed with concentrated ore which combine with gangue to form fusible material which is not soluble in molten metal. The substances used are called fluxes and the fusible material formed during reduction process is called slag. Slag is usually lighter and floats on the surface of the molten metal.
An acidic flux (e.g., silica, borax, etc.) is the chemical substance which removes the basic impurities.
SiO2 + CaO |
|
CaSiO3 |
Acidic flux |
Basic
impurity
(Gangue) |
Slag |
The basic flux (e.g., limestone, magnesite, ferric oxide, etc.) is the chemical substance which removes the acidic impurities.
|
|
MgSiO3 + CO2 |
Basic flux |
Acidic
impurity
(Gangue) |
Slag |
(ii) Reduction by aluminium (Goldschmidt aluminothermic process) : This process is employed in the case of those metals which have very high melting points and are to be extracted from their oxides. Their reduction with carbon is not satisfactory. A mixture of concentrated oxide ore and aluminium powder, commonly called as thermite, is taken in a steel crucible placed in a bed of sand. The reaction is started by the use of an ignition mixture containing magnesium powder and barium peroxide.
Cr2O3 + 2Al 2Cr + Al2O3
3Mn3O4 + 8Al 9Mn + 4Al2O3
Large amount of heat energy is released during reduction, which fuses both the alumina and the metal.
(b) Self reduction process : This process is also called autoreduction process or air reduction process. The sulphide ores of less electropositive metals like Hg, Pb, Cu, etc., are heated in air as to convert part of the ore into oxide or sulphate which then reacts with the remaining sulphide ore to give the metal and sulphur dioxide. No external reducing agent is used in this process.
2HgS + 3O2 2HgO + 2SO2 Extraction of Hg
2HgO + HgS 3Hg + SO2 from cinnabar ore
2PbS + 3O2 2PbO + 2SO2 Extraction of lead
2PbO + PbS 3Pb + SO2 from galena ore
