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METALLURGY
The compound of a metal found in natures called a mineral. The minerals from which metal can be economically
‘and conveniently extracted are called ores. An ore is usually contaminated with earthy or undesired materials,
known as gangue,
Native ores contain the metal in free state. Silver, gold, platinum etc, occur as native ores.
Oxidised ores consist of oxides or oxysalts (e.g. carbonates, phosphates, sulphates and silicates) of metals.
‘Sulphurised ores consist of sulphides of metals like iron, lead, zinc, mercury etc.
Halide ores consist of halides of metals Metal [ores [composition
[Aluminium Bauxite [AlO\(OH)s >< lwhere 0 Cu,S+2FeO + 380,
CuS + FeO + SiO, —> FeSiO, (Fusbiestes) + Cu,S (mats)
qecmel eT (vpper ayer (ower layer)
Matte also contains a very small amount of iron(l) sulphide
‘To remove unwanted acidic impurities like sand and PO,,, smelting is done in the presence of limestone.
Caco, > Cad + Co,
Ca0 + SiO, —+ CaSiO, (usbie sien)
6CA0 + P,0,, + 2Ca,(PO,), (ust ia - Thomas san)
Reduction of a metal oxide :
‘The free metal is obtained by reduction of a compound, using either a chemical reducing agent or electrolysis, Chemical reduction method :
Reduction with carbon :
PbO + C_— Pb +CO (extraction of lead)
Reduction with CO : In some cases CO produced in the furnace itself is used as a reducing agent.
Fe,0,+3C0—+2Fe +360,
Reduction by other metals
Metalic oxides (Cr and Mn) can be reduced by a highly electropostive metal such as aluminium that liberates
a large amount of energy (1675 kJ/mol) on oxidation to Al,O,, The process is known as Goldschmit or
aluminothermic process and the reaction isknowm as thermite reaction.
61,0, +Al— 26r(0)Al.O,
Magnesium reduction method : Magnesium is used in similar way to reduce oxides, In certain cases where
the oxide istoo stable to reduce, electropositive metals are used to reduce halides.
Ticl, +2 Mg ss
Ti+ 2Mgcl,
Ticl,+4Na M2", Ti+ 4 NaCl
‘Self-reduction method :
‘This method is also called auto-reduction method or air reduction method. Ifthe sulphide ore of some of the
less electropositive metals like Hg, Cu, Pb, Sb, etc. are heated in air, a part of these is changed into oxide or
‘sulphate then that reacts with the remaining part of the sulphide ore to give its metal and SO..
Cu,S +30, —> 3Cu,0+280,
2Cu,0 + Cus —> 6Cu+ SO.
Electrolytic reductior
Itpresents the most powerful method of reduction and gives.a very pure product. Asit isan expensive method
‘compared to chemical methods, itis used either for very reactive metals such as magnesium or aluminum or
for production of samples of high purity
In aqueous solution : Electrolysis can be carried out conveniently and cheaply in aqueous solution that the
products do not react with water. Copper and zinc are obtained by electrolysis of aqueous solution of their
sulphates.
In fused melts : Aluminum is oblained by electrolysis of a fused mixture of Al,O, and cryolite Na,{AIF
Extraction of Aluminium : It involves the following processes
(@) Purification of bauxite
(Bayer’s Method
{used for red bauxke containing Fe,0, and
silcatesasimpurties)
ALO. 7400+ 2NaOH >
2NaAIO, (soluble) +3,
Fe.0, (insoluble) separated as red mud by|
fiteraton solution is dluted with water and
seeded with freshly prepared AXOH), I nduces
the preciptation of ANOH),. KOH), i ftered
leaving behind silicates in solution
NaAlO, +2H.0-> NaOH + A(OH),L
2NyoH),
» ALO,+3H,0
Haire Method
(used ferred bento containing Fe,O,ane
slicates as impurities)
1,0,.2H,0+Naco, =>
2NaNO, (soluble) *C0,+24,0
2NaAIO,-3H.0~C0, >
2AKOH)L=Na.CO,|
2AKOH), #25 AL0,#9H,0
(used forwhite bauxite containing
silica as impurities}
ALO, .2H,0 +30 + Ne
2AIN + 9C0 + 24,0
2AIN + 31,0 -+ AVOH,L + NH,
SIO, + 200 -» 260, + Si
‘Slicone volatlises a this tem
2AKOH), "7" JA,0, +340 (b) Electrolytic reduction (Hall-Heroult process) :
2A10,+3C ——> 4al+3CO,
Cathode AB (melt) +3e° > Al)
‘Anode C{s) + 0% (melt) —+ COW) +2e
C(s) + 20* (melt) —> CO, (g) + 4e-
Metallurgy of some important metals
Extraction of iron from ore haematite :
Reactions involved :
‘At 500 ~ 800 K (lower temperature range in he blast furnace)
3Fe,0,+CO —> 2Fe,0,+C0,
Fe,0,+CO —> 3Fe+4CO,
Fe,0,+CO —> 2Fe0 +00,
‘AL. 900 - 1500 K (higher temperature range in the blast furnace)
c+C0, —> 2c0; Feo +CO ——>Fe+Co,
Limestone is also decomposed tom CaO which removes silicate impurity of the ore as slag, The slag is in
‘molten state and separates out from iron.
Caco, —+Ca0+C0,; — Cad+Si0, —> Casio,
Extraction of copper:
From copper glance / copper pyrite (self reduction) :
2CuFeS, + 40,—+ Cu.S + 2Fe0+ 380,
Cu,S + FeO + SiO, + FeSiO, (tusilesiag) + Cu,S (nate)
2FeS +30, —> 2Fe0 +280, ; FeO+Si0, —> Fesio,
2Cu,S +30, —+ 20u,0+280,; 2Cu,0+ Cu,S ——> 6Cu+ SO, (self reduction)
Extraction of lead :
© 2PS{s) +30, (g) —*+ 2PbO (s) “2+ 2PD(X) + CO, (@)
© sPes(5) "2". pos «6 200 6) Sn + 200
2Fe+ 0, —+ 2FeO
Extraction of Magnesium :
From Sea water (Dow's process) :
‘Sea water contains 0.13% magnesium as chloride and sulphate. itinvolves following steps.
(@ Precipitation of magnesium as magnesium hydroxide by slaked lime.
(©) Preparation of hexahydrated magnesium chloride 0)
“The solution on concentration and crystalsation gives the crystals of MgCl, 6H.O.
(© Preparation of anhydrous magnesium chloride
(© Electrolysis of fused anhydrous MgCl. n presence of NaCl
MgCl, <3 Moi" + 20
Atcathode: Mg” + 2e- ——> Mg(99% pure): At anode = Ce eee
Extraction of gold and silver (MacArthur-Forrest cyanide process)
(a) From native ores : Extraction of gold and silver involves leaching the metal with CN”.
48u1/AG(6)+ 8CN (aq) + 2H, 02g) + 0,46) > 4{Au/Ag(CN),H(aq) + 4OH-(aa)
2[Au/ Ag (CN),} (aq) + Zn(s) —+ 2Au/Ag (s) + [Zn(CN),* (aq)
(b) From argentite ore :
{Ag.S (conc. ore) + 2NaCN “> 2AgcN + Nas
4Na.S +50, +2H.0 > 2Na,SO, + 4NaOH + 2S
AQCN +NaCN ——+ Na[Ag(CN).] (soluble complex)
2NalAg(CN),}+ Zn (dust) —» 2g. + Na,(2n(ON),)
Airy
Purification or Refining of metals :
Physical methods : These methods include the following processes
(1) Liquation process : This process is used forthe purification ofthe metal, which itself is readily fusible, but
the impurities presentin it are not, used for the purification of Sn and Zn, and for removing Pb from Zn-Ag alloy.
(1) Fractional distillation process : This process is used to purify those metals which themselves are volatile
and the impurities in them are nonvolatile and vice-versa. Zn, Cd and Hg are purified by this process.
{i) Zone refining method (Fractional crystallisation method) : This process is used when metals are
Fequited in very high purty for specific application, For example pure Si and Ge are used in semiconductors
Chemical methods : These methods include the folowing methods
() OXIDATIVEREFINING :
‘This method is usually employed for refining metals ike Pb, Ag, Cu, Fe, etc. n this method the moten
impure metal is subjected to oxidation by various ways.
() POLING PROcESs:
This process is used forthe purification of copper and tin which contains the impurities of their own
oxides.
Green wood -» Hydrocarbons -» CH,
4CuO+CH, > 4Cu (pure metal) + CO, + 2H,0
(ll) ELECTROLYTIC REFINING:
‘Some metals such as Cu, Ni, and Al are refined electrolytically.
(IV) VAPOR PHASE REFINING :
(i) Extraction of Nickel (Mond’s process) :The sequence of reaction is
H,0@) +C —> CO() +H,
Nils) + 4COKS) 8, [Ni(CO,] @)
INi(CO),J@) 20 _, Ni+ 4c0(@)
Van Arkel-De Boer process :
Impure Ti+ 21, 2-250" Tie 2,
