Class 12 Chemistry Notes Chapter 6 (General principles and processes of isolation of elements) – Chemistry-I Book
Detailed Notes with MCQs of Chapter 6: 'General Principles and Processes of Isolation of Elements'. This chapter, often simply called Metallurgy, is fundamental for understanding how we obtain useful metals from the earth's crust. It's a crucial topic for various government exams, testing your knowledge of chemical principles applied to industrial processes.
We will break down the process into logical steps, focusing on the principles behind each technique.
Chapter 6: General Principles and Processes of Isolation of Elements (Metallurgy)
1. Introduction & Basic Terminology
- Minerals: Naturally occurring chemical substances in the earth's crust, obtained by mining.
- Ores: Minerals from which a metal can be extracted profitably and conveniently. Key point: All ores are minerals, but not all minerals are ores.
- Gangue (or Matrix): Earthy or undesirable impurities (like sand, clay, rocks) associated with the ore.
- Metallurgy: The entire scientific and technological process used for the isolation of the metal from its ore.
2. Principal Ores of Some Important Metals (Memorize these!)
- Aluminium (Al): Bauxite (Al₂O₃.xH₂O), Kaolinite (Al₂(Si₂O₅)(OH)₄)
- Iron (Fe): Haematite (Fe₂O₃), Magnetite (Fe₃O₄), Siderite (FeCO₃), Iron Pyrites (FeS₂)
- Copper (Cu): Copper Pyrites (CuFeS₂), Malachite (CuCO₃.Cu(OH)₂), Cuprite (Cu₂O), Copper Glance (Cu₂S)
- Zinc (Zn): Zinc Blende or Sphalerite (ZnS), Calamine (ZnCO₃), Zincite (ZnO)
3. Steps in Metallurgical Processes
The extraction of a pure metal from its ore generally involves three major steps:
I. Concentration of the Ore
II. Isolation of the Metal from its Concentrated Ore (Extraction)
III. Refining of the Impure Metal
I. Concentration of the Ore (Beneficiation or Dressing)
- Purpose: Removal of unwanted gangue particles from the ore. The choice of method depends on the physical and chemical properties of the ore and gangue.
- Methods:
- (a) Hydraulic Washing (Gravity Separation):
- Principle: Difference in densities (specific gravities) of ore and gangue particles.
- Process: Ore is washed with an upward stream of running water. Lighter gangue particles are washed away, heavier ore particles settle down.
- Used for: Oxide ores like Haematite (Fe₂O₃), Cassiterite (SnO₂), and native ores like Gold.
- (b) Magnetic Separation:
- Principle: Difference in magnetic properties of ore or gangue.
- Process: Powdered ore is passed over a conveyor belt moving over a magnetic roller. Magnetic components are attracted and fall closer to the roller, while non-magnetic components fall further away.
- Used for: Separating magnetic ores (e.g., Magnetite Fe₃O₄, Chromite FeCr₂O₄) from non-magnetic gangue, OR non-magnetic ores (e.g., Cassiterite SnO₂) from magnetic impurities (e.g., Wolframite (FeWO₄ + MnWO₄)).
- (c) Froth Flotation:
- Principle: Difference in wetting properties of ore and gangue particles with water and oil. Ore particles (usually sulphides) are preferentially wetted by oil, while gangue particles are wetted by water.
- Process: Powdered ore suspension in water is mixed with Collectors (e.g., pine oils, fatty acids, xanthates - enhance non-wettability of ore) and Froth Stabilizers (e.g., cresols, aniline - stabilize the froth). Air is blown through the mixture. Oil-wetted ore particles rise to the surface along with the froth and are skimmed off. Gangue settles at the bottom.
- Used Primarily for: Sulphide Ores (e.g., ZnS, PbS, CuFeS₂).
- Depressants: Sometimes used to separate two different sulphide ores. E.g., NaCN is used as a depressant to prevent ZnS from forming froth, allowing PbS to be separated first in an ore containing both ZnS and PbS. NaCN forms a complex [Zn(CN)₄]²⁻ on the surface of ZnS.
- (d) Leaching (Chemical Method):
- Principle: Ore is soluble in a suitable solvent (reagent), while impurities are not.
- Process: Ore is treated with a specific reagent which selectively dissolves the ore.
- Examples:
- Bauxite (Al₂O₃) Leaching (Bayer's Process): Ore treated with concentrated NaOH solution at 473-523 K and 35-36 bar pressure. Al₂O₃ dissolves forming sodium aluminate, leaving impurities like SiO₂, Fe₂O₃, TiO₂ behind.
Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2Na[Al(OH)₄](aq)
The solution is filtered, cooled, and neutralized by passing CO₂ gas or seeding with fresh Al(OH)₃ to precipitate hydrated Al₂O₃.
2Na[Al(OH)₄](aq) + CO₂(g) → Al₂O₃.xH₂O(s) + 2NaHCO₃(aq)
Hydrated alumina is filtered, dried and heated (~1473 K) to get pure Al₂O₃ (Alumina).
Al₂O₃.xH₂O(s) --(Heat, 1473K)--> Al₂O₃(s) + xH₂O(g) - Leaching of Silver (Ag) and Gold (Au) (MacArthur-Forrest Cyanide Process): Ore treated with dilute solution of NaCN or KCN in the presence of air (O₂). Metal dissolves forming soluble cyanide complexes.
4M(s) + 8CN⁻(aq) + 2H₂O(aq) + O₂(g) → 4[M(CN)₂]⁻(aq) + 4OH⁻(aq)(where M = Ag or Au)
The metal is recovered from the complex solution by displacement using a more electropositive metal like Zinc (Zn).
2[M(CN)₂]⁻(aq) + Zn(s) → [Zn(CN)₄]²⁻(aq) + 2M(s)
- Bauxite (Al₂O₃) Leaching (Bayer's Process): Ore treated with concentrated NaOH solution at 473-523 K and 35-36 bar pressure. Al₂O₃ dissolves forming sodium aluminate, leaving impurities like SiO₂, Fe₂O₃, TiO₂ behind.
- (a) Hydraulic Washing (Gravity Separation):
II. Isolation (Extraction) of Metal from Concentrated Ore
- Purpose: Convert the concentrated ore into the metallic form. Usually involves reduction.
- Two Steps:
- (a) Conversion to Oxide: It's generally easier to reduce metal oxides than sulphides or carbonates.
- Calcination: Heating the ore strongly, either in a limited supply of air or in the absence of air.
- Used for: Carbonates (decompose to oxide + CO₂), Hydroxides (decompose to oxide + H₂O), Hydrated oxides (lose water). Also removes volatile impurities.
- Examples:
Fe₂O₃.xH₂O(s) --(Heat)--> Fe₂O₃(s) + xH₂O(g)
ZnCO₃(s) --(Heat)--> ZnO(s) + CO₂(g)
CaCO₃.MgCO₃(s) --(Heat)--> CaO(s) + MgO(s) + 2CO₂(g)
- Roasting: Heating the ore strongly in a regular supply of excess air, usually below its melting point.
- Used for: Primarily Sulphide ores (convert to oxides + SO₂). Also removes volatile impurities like As, Sb, S as volatile oxides (As₂O₃, Sb₂O₃, SO₂).
- Examples:
2ZnS(s) + 3O₂(g) --(Heat)--> 2ZnO(s) + 2SO₂(g)
2PbS(s) + 3O₂(g) --(Heat)--> 2PbO(s) + 2SO₂(g)
2Cu₂S(s) + 3O₂(g) --(Heat)--> 2Cu₂O(s) + 2SO₂(g)
- Calcination: Heating the ore strongly, either in a limited supply of air or in the absence of air.
- (b) Reduction of the Oxide to Metal:
- Smelting (Reduction with Carbon): Heating the metal oxide with a suitable reducing agent like Carbon (coke) or Carbon Monoxide (CO) at high temperatures in a furnace (e.g., Blast Furnace).
- Flux: A substance added during smelting to combine with infusible impurities (gangue) to form a fusible mass called Slag. Flux + Gangue → Slag. Slag is lighter and immiscible with the molten metal, allowing easy separation.
- Acidic Flux: (e.g., SiO₂) Used to remove basic impurities (e.g., CaO, FeO).
CaO + SiO₂ → CaSiO₃ (Slag) - Basic Flux: (e.g., CaO from Limestone CaCO₃, MgO) Used to remove acidic impurities (e.g., SiO₂, P₄O₁₀).
SiO₂ + CaO → CaSiO₃ (Slag)
- Acidic Flux: (e.g., SiO₂) Used to remove basic impurities (e.g., CaO, FeO).
- Used for: Extraction of moderately reactive metals like Fe, Zn, Pb, Sn, Cu.
- Examples:
ZnO(s) + C(s) --(1673 K)--> Zn(s) + CO(g)
Fe₂O₃(s) + 3CO(g) --(Blast Furnace)--> 2Fe(l) + 3CO₂(g)
- Flux: A substance added during smelting to combine with infusible impurities (gangue) to form a fusible mass called Slag. Flux + Gangue → Slag. Slag is lighter and immiscible with the molten metal, allowing easy separation.
- Reduction by Other Metals (e.g., Aluminothermy/Thermite Process): Used when the metal oxide is very stable and requires very high temperatures for reduction by carbon (which might form carbides). A more reactive metal (like Al, Mg) is used as the reducing agent. The reaction is highly exothermic.
- Used for: Cr, Mn from their oxides. Also used in thermite welding.
- Examples:
Cr₂O₃(s) + 2Al(s) → Al₂O₃(s) + 2Cr(l) + Heat
3Mn₃O₄(s) + 8Al(s) → 4Al₂O₃(s) + 9Mn(l) + Heat
- Self-Reduction (Auto-Reduction): For less reactive metals like Cu, Pb, Hg. The sulphide ore is partially roasted to form some oxide, which then reacts with the remaining sulphide ore at higher temperatures to give the metal. No external reducing agent is needed.
- Example (Copper):
2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂(Roasting)
2Cu₂O + Cu₂S → 6Cu + SO₂(Self-reduction in Bessemer converter) - Example (Lead):
2PbS + 3O₂ → 2PbO + 2SO₂
PbS + 2PbO → 3Pb + SO₂
- Example (Copper):
- Electrolytic Reduction (Electrometallurgy): Used for highly reactive metals (alkali metals, alkaline earth metals, Al) which have very high negative electrode potentials and cannot be reduced by C or other chemical agents.
- Process: Electrolysis of the metal salt in molten state (fused state). Sometimes, other salts are added to lower the melting point or increase conductivity.
- Example (Aluminium - Hall-Héroult Process): Pure Al₂O₃ is dissolved in molten Cryolite (Na₃AlF₆) with some Fluorspar (CaF₂) (lowers melting point from ~2300 K to ~1140 K and increases conductivity). Electrolysis is carried out using carbon electrodes.
- Cathode (Steel vessel lined with Carbon): Al³⁺(melt) + 3e⁻ → Al(l)
- Anode (Graphite rods): C(s) + O²⁻(melt) → CO(g) + 2e⁻ ; C(s) + 2O²⁻(melt) → CO₂(g) + 4e⁻
- Note: The carbon anodes are consumed during the process as they react with the oxygen produced.
- Example (Sodium): Electrolysis of molten NaCl (Down's Process). CaCl₂ is added to lower M.P.
- Aqueous Solution Electrolysis: Can be used for less reactive metals like Cu, Zn (often in refining).
- Smelting (Reduction with Carbon): Heating the metal oxide with a suitable reducing agent like Carbon (coke) or Carbon Monoxide (CO) at high temperatures in a furnace (e.g., Blast Furnace).
- (a) Conversion to Oxide: It's generally easier to reduce metal oxides than sulphides or carbonates.
Thermodynamic Principles of Metallurgy (Ellingham Diagrams)
- Concept: The feasibility of a reduction process depends on the Gibbs Free Energy change (ΔG) being negative (ΔG = ΔH - TΔS). For reduction, the overall ΔG of the coupled reaction (Oxidation of reducing agent + Reduction of metal oxide) must be negative.
- Ellingham Diagram: A plot of ΔG° (standard Gibbs free energy of formation of oxides) vs Temperature (T).
- Key Features & Interpretations:
- Most ΔG° lines slope upwards because oxygen gas is consumed (ΔS is negative), making the TΔS term more positive (less negative) at higher T.
- The line for C → CO slopes downwards because 2 moles of gas (CO) are formed from 1 mole of solid (C) and 1 mole of gas (O₂), increasing entropy (ΔS is positive).
- A metal M can reduce another metal oxide M'O if the ΔG° line for M → MO lies below the line for M' → M'O at a given temperature. This means ΔG° for the formation of MO is more negative than for M'O, making the overall reduction reaction
M + M'O → MO + M'feasible (ΔG negative). - The intersection point of two lines indicates the temperature above which one reduction becomes more feasible than the other. E.g., Carbon can reduce ZnO above ~1270 K because the C → CO line goes below the Zn → ZnO line.
- Limitations: Assumes equilibrium, doesn't account for reaction kinetics (speed). Provides feasibility, not rate.
Electrochemical Principles of Metallurgy
- Used in electrometallurgy (electrolytic reduction) and electrolytic refining.
- Principle: More reactive metals have large negative E° values, their reduction is difficult. Less reactive metals have positive E° values, easier to reduce.
- In electrolysis of molten salts, the metal ion with higher reduction potential (less negative or more positive) gets deposited first if multiple ions are present.
- In aqueous solutions, reduction of water (producing H₂) or oxidation of water (producing O₂) can compete with metal deposition/anode dissolution. Overpotential sometimes needs consideration.
- Example: Recovery of Ag/Au by displacement with Zn relies on Zn being more electropositive (more negative E°) than Ag/Au.
III. Refining of the Impure Metal
- Purpose: Removal of final traces of impurities to obtain high-purity metal. Method depends on the nature of the metal and impurities.
- Methods:
- (a) Distillation:
- Principle: Useful for metals with low boiling points (volatile metals). Impure metal is heated, pure metal distils over, leaving non-volatile impurities behind.
- Used for: Zinc (Zn), Mercury (Hg), Cadmium (Cd).
- (b) Liquation:
- Principle: Useful for metals with low melting points compared to impurities. Impure metal is heated on a sloping hearth just above its melting point. The pure metal melts and flows down, leaving solid, higher-melting impurities behind.
- Used for: Tin (Sn), Lead (Pb), Bismuth (Bi).
- (c) Electrolytic Refining:
- Principle: Most widely used method for high purity. Based on electrolysis.
- Process:
- Anode: Thick block of impure metal.
- Cathode: Thin sheet of pure metal.
- Electrolyte: Aqueous solution of a soluble salt of the same metal.
- Mechanism: On passing current, pure metal dissolves from the anode and deposits onto the cathode. Less electropositive impurities (e.g., Ag, Au, Pt from Cu refining) settle below the anode as Anode Mud. More electropositive impurities (e.g., Zn, Fe from Cu refining) remain dissolved in the electrolyte.
- Used for: Copper (Cu), Zinc (Zn), Nickel (Ni), Silver (Ag), Gold (Au), Aluminium (Al).
- (d) Zone Refining (Fractional Crystallisation):
- Principle: Impurities are more soluble in the molten state (melt) than in the solid state of the metal.
- Process: A narrow circular mobile heater moves slowly along a rod of impure metal. The heater melts a small zone of the metal. As the heater moves forward, pure metal crystallizes out of the melt, while impurities concentrate in the adjacent molten zone which moves along with the heater to one end of the rod. The process is repeated several times, and the end containing impurities is cut off. Usually carried out in an inert atmosphere.
- Used for: Obtaining ultra-pure metals, especially semiconductors like Germanium (Ge), Silicon (Si), Gallium (Ga), Indium (In), Boron (B).
- (e) Vapour Phase Refining:
- Principle: Metal is converted into a volatile compound, which is then decomposed thermally to get the pure metal. Requires: (i) Metal should form a volatile compound with an available reagent, (ii) The volatile compound should be easily decomposable to recover the pure metal.
- Examples:
- Mond Process for Nickel (Ni): Impure Ni is heated in a stream of Carbon Monoxide (CO) around 330-350 K to form volatile Nickel Tetracarbonyl [Ni(CO)₄]. This vapour is then heated to a higher temperature (450-470 K) where it decomposes to give pure Ni.
Ni(impure) + 4CO --(330-350 K)--> Ni(CO)₄(volatile)
Ni(CO)₄ --(450-470 K)--> Ni(pure) + 4CO - Van Arkel Method for Zirconium (Zr) and Titanium (Ti): Impure metal is heated in an evacuated vessel with Iodine (I₂) around 850 K (for Zr) to form volatile covalent iodide. The metal iodide vapour is decomposed by heating electrically on a Tungsten filament at a very high temperature (~1800 K). Pure metal deposits on the filament. The Iodine is reused.
Zr(impure) + 2I₂ --(850 K)--> ZrI₄(volatile)
ZrI₄ --(Tungsten filament, 1800 K)--> Zr(pure) + 2I₂
- Mond Process for Nickel (Ni): Impure Ni is heated in a stream of Carbon Monoxide (CO) around 330-350 K to form volatile Nickel Tetracarbonyl [Ni(CO)₄]. This vapour is then heated to a higher temperature (450-470 K) where it decomposes to give pure Ni.
- (f) Chromatographic Methods:
- Principle: Based on the difference in adsorption of various components of a mixture on an adsorbent material. Used for purification of elements available in minute quantities or having very similar chemical properties. (e.g., Lanthanoids). Less common for bulk industrial refining.
- (a) Distillation:
Practice MCQs for Government Exams:
-
Which of the following ores is concentrated by the Froth Flotation process?
(a) Haematite
(b) Bauxite
(c) Cinnabar
(d) Carnallite -
In the extraction of Aluminium by the Hall-Héroult process, cryolite (Na₃AlF₆) is added to:
(a) Increase the conductivity of the melt only
(b) Lower the melting point of alumina only
(c) Lower the melting point and increase the conductivity of the melt
(d) Act as a reducing agent -
The process of heating an ore strongly in the absence or limited supply of air is known as:
(a) Roasting
(b) Calcination
(c) Smelting
(d) Leaching -
Which method is used for refining volatile metals like Zinc and Mercury?
(a) Liquation
(b) Electrolytic refining
(c) Distillation
(d) Zone refining -
During the smelting process in metallurgy, the role of flux is to:
(a) Reduce the metal oxide
(b) Remove gangue as fusible slag
(c) Increase the melting point of the ore
(d) Oxidize the impurities -
The Mond process is used for the refining of:
(a) Zirconium
(b) Titanium
(c) Nickel
(d) Copper -
According to the Ellingham diagram, a metal M can reduce another metal oxide M'O at a given temperature if:
(a) The ΔG° line for M → MO lies above the line for M' → M'O
(b) The ΔG° line for M → MO lies below the line for M' → M'O
(c) Both ΔG° lines intersect at that temperature
(d) The slope of the M → MO line is positive -
Which of the following metals is extracted by the MacArthur-Forrest cyanide process?
(a) Iron
(b) Aluminium
(c) Copper
(d) Silver -
Zone refining is based on the principle that:
(a) Impurities have a lower boiling point than the pure metal
(b) Impurities are more soluble in the melt than in the solid metal
(c) Impurities have different magnetic properties than the pure metal
(d) Impurities are lighter than the pure metal -
In the extraction of copper from copper pyrites (CuFeS₂), iron is removed as:
(a) FeSO₄
(b) FeSiO₃ (slag)
(c) Fe₂O₃
(d) FeS
Answer Key:
- (c) Cinnabar (HgS) is a sulphide ore.
- (c) Cryolite serves both purposes.
- (b) Calcination definition.
- (c) Distillation is used for volatile metals.
- (b) Definition of flux function.
- (c) Mond process is specific to Nickel.
- (b) Interpretation of Ellingham diagram for reduction feasibility.
- (d) Cyanide process is used for Ag and Au.
- (b) Principle of Zone Refining.
- (b) In the reverberatory furnace/Bessemer converter stage, SiO₂ is added as flux to remove FeO (formed from FeS) as iron silicate slag (FeSiO₃).
Study these notes thoroughly. Pay close attention to the principles, examples, specific reagents, and conditions mentioned for each process. Good luck with your preparation!