General Principles and Processes of Isolation of Metals

SYNOPSIS



METALLURGY


The compound of a metal found in nature is called a mineral. The minerals from which metals can be economically and conveniently extracted are called ores. An ore is usually contaminated with earthy or undesired materials known as gangue.

1. Native ores contain the metal in the free state. Silver, gold and platinum occur as native ores.
2. Oxidised ores consist of oxides or oxysalts, such as carbonates, phosphates, sulphates and silicates of metals.
3. Sulphurised ores consist of sulphides of metals like iron, lead, zinc and mercury.
4. Halide ores consist of halides of metals.
   
   

   Metal	Ore	Composition
   Aluminium	Bauxite	AlOX(OH)3-2X [where 0< X<1] Al2O3
   	Diaspore	Al2O3.H2O
   	Corundum	Al2O3
   	Kaolinite (a form of clay)	[Al2(OH)4Si2O5]
   Iron	Haematite	Fe2O3
   	Magnetite	Fe3O4
   	Siderite	FeCO3
   	Iron pyrite	FeS­2
   	Limonite	Fe2O3.3H2O
   Copper	Copper pyrite	CuFeS2
   	Cuprite	Cu2S
   	Malachite	CuCO3.Cu(OH)2
   	Azurite	2CuCO3.Cu(OH)2
   Zinc	Zinc blende or sphalerite	ZnS
   	Calamine	ZnCO3
   	Zincite	ZnO
   Lead	Galena	PbS
   	Anglesite	PbSO4
   	Cerussite	PbCO3
   Magnesium	Carnallite	KCl.MgCl2.6H2O (K2MgCl4.6H2O)
   	Magnesite	MgCO3
   	Dolomite	MgCO3,CaCO3
   	Epsom salt (Epsomite)	MgSO4 7H2O
   	Langbeinite	K2Mg2(SO4)3
   Tin	Cassiterite (Tin stone)	SnO2
   Silver	Silver glance (Argentite)	Ag2S
   	Chlorargyrite (Horn silver)	AgCl

   
   
   Metallurgy:
   

   The scientific and technological process used for the extraction/isolation of the metal from its ore.
   Isolation and extraction of metals from their ores involve the following major steps:

1. Crushing and grinding: The ore is first crushed by jaw crushers and ground to a powder.
2. Concentration:
   
   1. Hydraulic washing or gravity separation or levigation method
   2. Electromagnetic separation
   3. Froth flotation process
   4. Leaching
   
   
3. Extraction of crude metal from concentrated ore:
   The removal of unwanted useless impurities from the ore is called dressing, concentration or benefaction of ore.
   Isolation of metals from their concentrated ores involves two major steps:
   
   
1. Conversion to oxide:
   Calcination: Process of strongly heating the concentrated ore in a limited supply of air or in the absence of air.
   Roasting: Process of strongly heating the concentrated ore (generally sulphide ore) in excess of air or O2 below its melting point.
   Smelting: In many extraction processes, an oxide is added deliberately to combine with other impurities and form a stable molten phase which is immiscible with the molten metal called slag. This process is termed smelting.
   Slag formation: 
   2CuFeS₂ + 4O₂ Cu₂S + 2FeO + 3SO₂ 
   
   

   CaCO₃ CaO+CO₂
   CaO + SiO₂ CaSiO₃ (fusible slag)
   6CaO + P4O₁₀ 2Ca₃(PO4 )2 (fusible slag: Thomas slag)

2. 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:
   Fe₂O₃ + 3CO 2Fe + 3CO₂
   

   Reduction by other metals:
   The process is known as Goldschmidt or aluminothermic process and the reaction is known as thermite reaction.
   Cr2O3 + Al 2Cr + Al₂O₃
   
   Magnesium reduction method: 
   
   

   Self-reduction method:
   This method is also called auto-reduction method or air reduction method.
   Cu₂S + 3O₂ 3Cu₂O + 2SO₂
   2Cu₂O + Cu₂S 6Cu + SO₂

   Electrolytic reduction:
   1. In aqueous solution: Copper and zinc are obtained by electrolysis of aqueous solution of their sulphates.
   2. In fused melts: Aluminum is obtained by electrolysis of a fused mixture of AI₂O₃ and cryolite Na₃[AIF₆].
   
   Extraction of aluminium: It involves the following processes:

   1. Purification of bauxite:
      
      
      1.  Bayer’s method:
         (used for red bauxite containing Fe2O3 and silicates as impurities)
         
      2. Hall’s method:
         (used for red bauxite containing Fe2O3 and silicates as impurities)
         
         
      3. Serpeck’s method:
         (used for white bauxite containing silica as impurities)
         
      4. Silicon volatilises at this temperature
          
   2. Electrolytic reduction (Hall–Heroult process):
      2Al₂O₃ + 3C 4Al + 3CO₂
      Cathode: Al³⁺ (melt) + 3e⁻ Al(l)
      Anode: C(s) + O^2- (melt) CO(g) + 2e-
      C(s) + 2O^2-(melt) CO2(g) + Ae-

   Metallurgy of some important metals:
   

   1. Extraction of iron from haematite:
      Reactions involved:
      At 500–800 K (lower temperature range in the blast furnace):
      3Fe₂O₃ + CO 2Fe₃O₄ + CO₂ 
      Fe₃O₄ + CO 3Fe + 4CO₂
      Fe₂O₃ + CO 2FeO + CO₂
      At 900–1500 K (higher temperature range in the blast furnace):
      C + CO₂ 2CO; FeO + CO Fe + CO2
      Limestone is also decomposed to CaO which removes the silicate impurity of the ore as slag. The slag is in the molten state and separates out from iron.
      CaCO₃ CaO + CO2; CaO + SiO₂ CaSiO2

   2. Extraction of copper:
      From copper glance/copper pyrite (self-reduction):
      2CuFeS₂ + 4O₂ Cu₂S + 2FeO + 3SO₂
      Cu₂S + FeO + SiO₂ FeSiO₃ (fusible slag) + Cu2S (matte)
      2FeS + 3O₂ 2FeO + 2SO₂; FeO + SiO₂ FeSiO₃
      2Cu₂O + Cu₂S 6Cu + SO₂ (self-reduction)

   3. Extraction of lead:
      
      

   4. Extraction of zinc from zinc blende:
      The ore is roasted in the presence of excess of air at a temperature of 1200 K.
      2ZnS + 3O₂ 2ZnO + 2SO₂
      Zinc oxide is reduced using coke.

      
      

   5. Extraction of tin from cassiterite: 
      

      SnO₂ + 2C Sn + 2CO
      2Fe + O₂ 2FeO

   6. Extraction of magnesium:
      From sea water (Dow’s process): 
      MgCl2 ⇌ Mg²⁺ + 2Cl^⎺
      At cathode: Mg²⁺ + 2e^⎺ Mg (99% pure)At anode: 2Cl⎺ Cl² + 2e^⎺

   7. Extraction of gold and silver (MacArthur–Forrest cyanide process):

   1. From native ores: Extraction of gold and silver involves leaching the metal with CN-.
      4Au/Ag(s) + 8CN^⎺(aq) + 2H₂O(aq) + O₂(g) 4[Au/Ag(CN)₂] ^⎺ (aq) + 4OH^⎺ (aq)
      2[Au/Ag(CN)₂] ^⎺ (aq) + Zn(s) 2Au/Ag(s) + [Zn(CN)4]^2⎺ (aq)
   2. From argentite ore: 
      
      

      4Na₂S + 5O₂ + 2H₂O 2Na₂SO₄ + 4NaOH + 2S
      AgCN + NaCN Na[Ag(CN)₂] (soluble complex)
      2Na[Ag(CN)2] + Zn (dust) 2Ag↓ + Na₂[Zn(CN)₄]

4. Purification or refining of metals:
   Physical methods:

1. Liquation process: This process is used for the purification of the metal, which itself is readily fusible, but the impurities present in it are not. It is 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 metals which are volatile and the impurities in them are nonvolatile or vice versa. Zn, Cd and Hg are purified by this process.
2. Zone refining method (Fractional crystallisation method): This process is used when metals are required to have very high purity for specific applications. For example, pure Si and Ge are used in semiconductors.

Chemical methods:

1.  Oxidative refining:
   The molten impure metal is subjected to oxidation by various ways. This method is used for refining metals such as Pb, Ag, Cu and Fe.
2. Poling process:
   This method is used for the purification of copper and tin which contain their own oxides as impurities.
   Green wood→ Hydrocarbons →CH₄
   4CuO + CH₄ → 4Cu (pure metal) + CO₂ + 2H₂O
3. Electrolytic refining:
   Metals such as Cu, Ni and Al are refined electrolytically.
4. Vapour phase refining:

1. Extraction of nickel (Mond’s process): 
   H₂O(g) + C → CO(g) + H₂
2. van Arkel–de Boer process: