METALLURGY - 2 · KnowledgeBin.org

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.

Al₂O₃ + 2NaOH2NaAlO₂ + H₂O

Pure alumina is recovered from the filtrate

NaAlO₂ + 2H₂OAl(OH)₃ + NaOH
2Al(OH)₃Al2O₃ + 3H₂O

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 + 2H₂O + O₂4NaAu(CN)₂ + 4NaOH

Ag or Au is recovered from the solution by the addition of electropositive metal like zinc.

2NaAg(CN)₂ + ZnNa₂Zn(CN)₄ + 2Ag

2NaAu(CN)₂ + ZnNa₂Zn(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 + CO₂

Similarly,

Fe₂O₃ + 3C

2Fe + 3CO
Fe₂O₃ + 3CO

2Fe + 3CO₂

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.

MgCO3 + SiO₂		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

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