Chapter: General Principles and Processes of Isolation of Elements (Metallurgy) – Detailed Notes for NEET/JEE Mains

1. Introduction to Metallurgy

• Metallurgy: The branch of science and technology concerned with the properties of metals and their production and purification from ores.
• Minerals: Naturally occurring chemical substances found in the Earth’s crust, which contain metals in their native state or in combined states.
• Ores: Minerals from which metals can be economically and conveniently extracted. All ores are minerals, but not all minerals are ores.
• Gangue (Matrix): The unwanted rocky or earthy impurities (e.g., sand, clay, silicates) associated with the ore.
• Flux: A substance added to the ore during smelting to remove non-fusible gangue as fusible slag.
  • Acidic flux (e.g., SiO2​): Used to remove basic gangue (e.g., FeO, CaO).
  • Basic flux (e.g., CaO, MgCO3​): Used to remove acidic gangue (e.g., SiO2​, P4​O10​).
  • Gangue+Flux→Slag (fusible)
• Slag: The fusible product formed when flux combines with gangue. It is lighter than the molten metal and floats on top, protecting the metal from oxidation.

2. General Steps in Metallurgical Operations

The entire process of obtaining a pure metal from its ore consists of several steps:

A. Crushing and Grinding (Pulverization)

• The ore is crushed into small pieces using jaw crushers and then ground into fine powder using ball mills or stamp mills.

B. Concentration (Beneficiation or Ore Dressing)

• The process of removing unwanted earthy and rocky materials (gangue) from the ore. It increases the concentration of the valuable mineral.

1. Hydraulic Washing (Gravity Separation or Levigation):
   • Principle: Based on the difference in specific gravities of the ore particles and the gangue particles.
   • Process: Powdered ore is agitated with water. Heavier ore particles settle down, while lighter gangue particles are washed away.
   • Used for: Oxide ores (e.g., Haematite, Tin stone), and native metal ores (e.g., Gold).
2. Magnetic Separation:
   • Principle: Based on the difference in magnetic properties of the ore and the gangue. One component must be magnetic, and the other non-magnetic.
   • Process: Powdered ore is passed over a magnetic roller. Magnetic particles are attracted to the roller and fall in a separate heap, while non-magnetic particles fall earlier.
   • Used for: Magnetic ores (e.g., Chromite FeCr2​O4​, Pyrolusite MnO2​, Magnetite Fe3​O4​) and for removing magnetic impurities from non-magnetic ores (e.g., wolframite from cassiterite (tin stone)).
3. Froth Floatation Process:
   • Principle: Based on the difference in wetting properties of ore and gangue particles. Sulphide ores are preferentially wetted by oil (pine oil, fatty acids, xanthates), while gangue is wetted by water.
   • Process: Powdered ore is mixed with water, frothing agents (pine oil, eucalyptus oil), collectors (potassium ethyl xanthate), and froth stabilizers (cresol, aniline). Air is blown through the mixture. Sulphide ore particles get attached to air bubbles, rise to the surface as froth, and are skimmed off. Gangue settles at the bottom.
   • Depressants: Used to separate two sulphide ores. E.g., NaCN used as a depressant to separate ZnS from PbS. It selectively prevents ZnS from coming with the froth.
   • Used for: Sulphide ores (e.g., ZnS, PbS, CuFeS2​).
4. Leaching (Chemical Method):
   • Principle: The ore is soluble in a suitable chemical reagent, but the gangue is not.
   • Process: Powdered ore is treated with a specific chemical solvent that dissolves the valuable metal compound, forming a soluble complex, while impurities remain undissolved. The metal is then recovered from the solution.
   • Used for: Precious metals (Ag, Au), and highly reactive metals (Al).
     • Leaching of Bauxite (for Aluminium):
       • Baeyer’s Process: Bauxite (Al2​O3​⋅2H2​O) is digested with concentrated NaOH solution at 473−523 K and 35−36 bar pressure. Al2​O3​ dissolves to form sodium aluminate, while impurities (Fe2​O3​, SiO2​, TiO2​) remain insoluble.
         • Al2​O3​(s)+2NaOH(aq)+3H2​O(l)→2Na[Al(OH)4​](aq)
       • Sodium aluminate solution is filtered and neutralized by passing CO2​ gas, which precipitates hydrated alumina.
         • 2Na[Al(OH)4​](aq)+CO2​(g)→Al2​O3​⋅xH2​O(s)+2NaHCO3​(aq)
       • The hydrated alumina is then filtered, dried, and calcined (heated) to get pure alumina (Al2​O3​).
         • Al2​O3​⋅xH2​O(s)1473 K​Al2​O3​(s)+xH2​O(g)
     • Leaching of Gold/Silver (Cyanide Process):
       • Finely powdered ore is treated with a dilute solution of NaCN or KCN in the presence of air. The metal is oxidized and forms a soluble complex.
         • 4M(s)+8CN−(aq)+O2​(g)+2H2​O(l)→4[M(CN)2​]−(aq)+4OH−(aq) (M = Au or Ag)
       • The metal is then recovered by displacement using a more electropositive metal (e.g., Zinc).
         • 2[M(CN)2​]−(aq)+Zn(s)→2M(s)+[Zn(CN)4​]2−(aq)

C. Extraction of Crude Metal from Concentrated Ore

This involves two main steps: conversion of ore into metal oxide, and then reduction of metal oxide to metal.

1. Conversion of Ore into Metal Oxide:

• Calcination:
  • Process: Heating the ore strongly in a limited supply of air or in the absence of air below its melting point.
  • Purpose: Removes volatile impurities (like moisture, organic matter, CO2​, SO2​) and converts carbonate and hydroxide ores into oxides.
  • Example: MgCO3​(s)heat​MgO(s)+CO2​(g)
  • Example: Fe2​O3​⋅xH2​O(s)heat​Fe2​O3​(s)+xH2​O(g)
• Roasting:
  • Process: Heating the ore strongly in the presence of excess air below its melting point.
  • Purpose: Converts sulphide ores into oxides and removes impurities like S, As, Sb as volatile oxides.
  • Example: 2ZnS(s)+3O2​(g)heat​2ZnO(s)+2SO2​(g)
  • Example: 2PbS(s)+3O2​(g)heat​2PbO(s)+2SO2​(g)
  • Byproduct: SO2​ gas is produced, which can be used for manufacturing H2​SO4​.

2. Reduction of Metal Oxide to Metal:

• Smelting (Reduction with Carbon):
  • Process: Heating the metal oxide with a reducing agent (usually Carbon, in the form of coke or charcoal) in a furnace.
  • Example (Iron): In a blast furnace, iron oxides are reduced by coke.
    • Fe2​O3​+3CO500-800 K​2Fe+3CO2​ (lower temp)
    • Fe2​O3​+3CO1000-1200 K​2Fe+3CO2​
    • Fe2​O3​+3C>1200 K​2Fe+3CO (higher temp)
    • Slag formation (gangue SiO2​ + flux CaO): CaO+SiO2​→CaSiO3​ (slag).
  • Applicable for: Moderately reactive metals (e.g., Fe, Zn, Cu, Sn).
• Reduction by more Electropositive Metal:
  • Used when carbon cannot reduce the metal oxide or the metal is highly reactive.
  • Thermite Process (Aluminothermy): Reduction of metal oxides with Aluminium powder. Highly exothermic.
    • Cr2​O3​+2Al→2Cr+Al2​O3​
    • MnO2​+2Al→Mn+Al2​O3​
  • Example (Magnesium reduction): TiCl4​+2Mg→Ti+2MgCl2​ (Kroll process for Ti).
• Self-Reduction (Auto-Reduction):
  • Some sulphide ores (e.g., PbS, Cu2​S, HgS) are partially roasted to form oxides, which then react with the remaining sulphide ore to produce the metal and SO2​. No external reducing agent is needed.
    • 2PbS+3O2​Roasting​2PbO+2SO2​
    • 2PbO+PbSheat​3Pb+SO2​
    • Similarly for copper: 2Cu2​O+Cu2​S→6Cu+SO2​
• Electrolytic Reduction (Electrometallurgy):
  • Used for highly reactive metals (e.g., alkali metals, alkaline earth metals, Aluminium).
  • Molten metal halides or oxides are electrolysed.
  • Example (Hall-Héroult Process for Aluminium):
    • Purified alumina (Al2​O3​) is mixed with cryolite (Na3​AlF6​) and fluorspar (CaF2​) to lower the melting point (2000∘C to 1140∘C) and increase conductivity.
    • Anode: Graphite (carbon)
    • Cathode: Carbon lining of the steel tank.
    • Electrolyte: Molten mixture of Al2​O3​, cryolite, and fluorspar.
    • Reactions:
      • Cathode: Al3+(melt)+3e−→Al(l)
      • Anode: C(s)+O2−(melt)→CO(g)+2e− C(s)+2O2−(melt)→CO2​(g)+4e−
    • Molten aluminium is denser and settles at the bottom. Anodes are consumed during the process.

4. Refining of Metals (Purification of Crude Metal)

The crude metal obtained from extraction processes usually contains impurities. Refining processes are used to obtain metals of high purity.

1. Distillation:
   • Principle: Used for low boiling point metals (volatile metals).
   • Process: The crude metal is heated, and the pure metal vaporizes and is then condensed. Non-volatile impurities are left behind.
   • Used for: Zn, Cd, Hg.
2. Liquation:
   • Principle: Used for metals that have a low melting point compared to their impurities.
   • Process: The crude metal is heated on a sloping hearth, where the pure metal melts and flows down, while infusible impurities are left behind.
   • Used for: Sn, Pb, Bi.
3. Electrolytic Refining: (Most important and widely used method)
   • Principle: Based on preferential deposition of pure metal at cathode.
   • Process:
     • Anode: Impure metal (more active metals as impurities get oxidized).
     • Cathode: Thin sheet of pure metal.
     • Electrolyte: Solution of a soluble salt of the same metal.
     • Reactions:
       • Anode: M(impure)→Mn+(aq)+ne− (Metal dissolves) Less reactive metals as impurities settle as anode mud. More reactive metals as impurities also dissolve but remain in solution.
       • Cathode: Mn+(aq)+ne−→M(pure) (Pure metal deposits)
   • Used for: Cu, Zn, Ag, Au, Pb, Al, Ni.
4. Zone Refining:
   • Principle: Based on the principle that impurities are more soluble in the molten state than in the solid state of the metal.
   • Process: A circular mobile heater is moved along the length of a metal rod. The molten zone moves with the heater, and as it moves, pure metal crystallizes out, while impurities concentrate in the molten zone and are eventually swept to one end of the rod. The impure end is cut off.
   • Used for: Obtaining ultra-pure metals (semiconductors) like Ge, Si, B, Ga, In.
5. Vapour Phase Refining:
   • Principle: The metal is converted into a volatile compound, which is then decomposed to yield pure metal.
   • Conditions:
     • The metal should form a volatile compound with the refining agent.
     • The volatile compound should be easily decomposable to yield pure metal.
   • Types:
     • Mond’s Process (for Nickel):
       • Impure Nickel is heated in a stream of CO at 330−350 K to form volatile nickel tetracarbonyl.
         • Ni(impure)+4CO330-350K​Ni(CO)4​(volatile)
       • The nickel tetracarbonyl is then heated to a higher temperature (450−470 K) where it decomposes to pure nickel.
         • Ni(CO)4​450-470K​Ni(pure)+4CO
     • van Arkel Method (for Zirconium and Titanium):
       • Impure metal is heated with iodine to form a volatile metal iodide.
         • Zr(impure)+2I2​heat​ZrI4​(volatile)
       • The metal iodide is then decomposed on a hot tungsten filament (at high temperature, e.g., 1800 K) to yield pure metal.
         • ZrI4​1800K​Zr(pure)+2I2​
6. Chromatographic Methods: (e.g., Column Chromatography)
   • Principle: Based on the difference in adsorption capabilities of the metal and its impurities on an adsorbent.
   • Used for: Purification of elements when impurities are in very small quantities and chemical methods are not suitable.

5. Thermodynamic Principles of Metallurgy (Ellingham Diagram)

• Ellingham Diagram: A graph plotting ΔG∘ vs T for the formation of various metal oxides (and carbon oxides).
• Uses:
  • Predicts the feasibility of reduction of an oxide by another element.
  • A metal can reduce the oxide of another metal if its formation line lies below that of the oxide to be reduced on the Ellingham diagram. (The element forming the more stable oxide (more negative ΔG∘) can reduce the oxide of the element forming a less stable oxide).
  • Intersection points on the diagram indicate the temperature at which ΔG∘=0 for the reduction process. Below this temperature, reduction is not spontaneous; above it, it is.
  • Reduction by Carbon: The line for C→CO (or C→CO2​) is considered. If this line crosses below a metal oxide line, then carbon can reduce that metal oxide above the intersection temperature.
    • The C→CO line has a negative slope (more negative ΔS), so it becomes more favourable at higher temperatures, making carbon a good reducing agent at high temperatures.
  • Limitations:
    • Applies to standard conditions (activities are unity), but in reality, concentrations may differ.
    • Does not predict the rate of reaction.
    • Assumes reactants and products are in equilibrium.