Class 12 Chemistry Notes Chapter 6 (General Principles and Processes of Isolation of Elements) – Examplar Problems Book

Alright class, let's dive into Chapter 6: 'General Principles and Processes of Isolation of Elements'. This chapter, often simply called Metallurgy, is crucial not just for your board exams but also forms a significant part of the syllabus for various competitive government exams. It deals with the science and technology of extracting metals from their natural sources and refining them.

1. Occurrence of Metals

• Minerals: Naturally occurring chemical substances in the earth's crust, obtained by mining. They contain metals in combined or free states.
• Ores: Minerals from which a metal can be extracted conveniently and economically. 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.

Common Types of Ores:

• Oxide Ores: Bauxite (Al₂O₃.xH₂O), Haematite (Fe₂O₃), Magnetite (Fe₃O₄), Cuprite (Cu₂O), Zincite (ZnO), Cassiterite (SnO₂).
• Sulphide Ores: Galena (PbS), Zinc Blende (ZnS), Copper Pyrites (CuFeS₂), Cinnabar (HgS), Argentite (Ag₂S).
• Carbonate Ores: Limestone (CaCO₃), Calamine (ZnCO₃), Siderite (FeCO₃), Malachite (CuCO₃.Cu(OH)₂).
• Halide Ores: Rock Salt (NaCl), Horn Silver (AgCl), Cryolite (Na₃AlF₆), Fluorspar (CaF₂).
• Sulphate Ores: Gypsum (CaSO₄.2H₂O), Epsom Salt (MgSO₄.7H₂O), Anglesite (PbSO₄).
• Native Ores: Metals found in their free or elemental state (e.g., Gold, Silver, Platinum, Copper).

2. Steps in Metallurgical Processes

The extraction and isolation of metals generally involve three major steps:
I. Concentration of the Ore
II. Isolation of the metal from its concentrated ore
III. Purification or Refining of the metal

I. Concentration of the Ore (Beneficiation or Dressing)

This involves the removal of unwanted gangue particles from the ore. The choice of method depends on the physical and chemical properties of the ore and the gangue.

• (a) Hydraulic Washing (Gravity Separation):

  • Principle: Difference in specific gravities (densities) of the ore and gangue particles.
  • Process: Ore is washed with an upward stream of running water. Lighter gangue particles are washed away, leaving heavier ore particles behind.
  • Used for: Heavy oxide ores like Haematite (Fe₂O₃), Cassiterite (SnO₂), and native ores like Gold.

• (b) Magnetic Separation:

  • Principle: Difference in magnetic properties of the ore or gangue.
  • Process: Powdered ore is passed over a conveyor belt moving over a magnetic roller. Magnetic components are attracted and fall nearer to the roller, while non-magnetic components fall further away.
  • Used when: Either the ore (e.g., Magnetite Fe₃O₄, Chromite FeCr₂O₄) or the gangue (e.g., Wolframite (FeWO₄) impurity in Cassiterite (SnO₂)) is magnetic.

• (c) Froth Flotation:

  • Principle: Difference in wetting properties of ore and gangue particles by water and oil. Sulphide ore particles are preferentially wetted by oil (pine oil, fatty acids, xanthates - collectors), while gangue particles are wetted by water.
  • Process: A suspension of powdered ore is made with water. Collectors and Froth Stabilizers (cresols, aniline) are added. Air is agitated through the mixture. Ore particles stick to the oil froth and rise to the surface, while gangue settles down.
  • Used primarily for: Sulphide ores (ZnS, CuFeS₂, PbS).
  • Depressants: Sometimes used to separate two different sulphide ores. E.g., NaCN is used as a depressant to prevent ZnS from coming to the froth, allowing PbS to be separated first in an ore containing both ZnS and PbS. NaCN forms a complex Na₂[Zn(CN)₄] on the surface of ZnS.
  • Activators: Can enhance the flotation of a specific mineral. E.g., CuSO₄ activates ZnS flotation after PbS has been removed.

• (d) Leaching (Chemical Method):

  • Principle: Ore is treated with a suitable reagent which dissolves the ore but not the impurities, or vice versa.
  • Examples:
    • Bauxite (Al₂O₃) containing SiO₂, Fe₂O₃, TiO₂ impurities (Bayer's Process): Ore is treated with concentrated NaOH solution at 473-523 K and 35-36 bar pressure. Al₂O₃ dissolves forming sodium aluminate, leaving impurities behind.
      Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2Na[Al(OH)₄](aq)
      SiO₂ also dissolves forming sodium silicate.
      The solution is filtered, cooled, and neutralized by passing CO₂ gas or seeding with fresh Al(OH)₃, precipitating hydrated alumina.
      2Na[Al(OH)₄](aq) + CO₂(g) → Al₂O₃.xH₂O(s) + 2NaHCO₃(aq)
      The precipitate is filtered, dried, and heated strongly (~1473 K) to get pure Al₂O₃.
      Al₂O₃.xH₂O(s) --(Heat)--> Al₂O₃(s) + xH₂O(g)
    • Extraction of Silver (Ag) and Gold (Au) (MacArthur-Forrest Cyanide Process): Ore is treated with dilute NaCN or KCN solution in the presence of air (O₂). The metal dissolves forming a soluble cyanide complex.
      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)

II. Isolation of Metal from Concentrated Ore

This usually involves two steps:

1. Conversion of the concentrated ore into its oxide form.
2. Reduction of the metal oxide to the free metal.

• (1) Conversion to Oxide:

  • Calcination: Heating the ore strongly, usually below its melting point, either in the absence or a limited supply of air.
    • Purpose: Removes moisture (hydrated oxides), removes volatile organic matter, decomposes carbonates into oxides, makes the ore porous.
    • 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 below its melting point in a regular supply of air (or oxygen).
    • Purpose: Primarily used for sulphide ores to convert them into oxides. Removes volatile impurities like Arsenic (As), Sulphur (S), Phosphorus (P) as their volatile oxides (As₂O₃, SO₂, P₄O₁₀). Removes moisture. Makes the ore porous.
    • 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)
    • S + O₂ → SO₂ ; P₄ + 5O₂ → P₄O₁₀ ; 4As + 3O₂ → 2As₂O₃

• (2) Reduction of the Oxide to Metal:

  • Smelting (Reduction with Carbon): The oxide ore is heated with a reducing agent, usually Carbon (coke) or Carbon Monoxide (CO), at high temperatures in a furnace (like a Blast Furnace for Iron). A flux is often added to remove infusible impurities (gangue) as a fusible slag.
    • Flux + Gangue → Slag (Slag is molten, immiscible with molten metal, and lighter).
    • Acidic flux (e.g., SiO₂) removes basic gangue (e.g., CaO, FeO). CaO + SiO₂ → CaSiO₃ (Slag)
    • Basic flux (e.g., CaO, Limestone CaCO₃ which decomposes to CaO) removes acidic gangue (e.g., SiO₂, P₄O₁₀). SiO₂ + CaO → CaSiO₃ (Slag)
    • Examples:
      • ZnO(s) + C(s) --(1673 K)--> Zn(s) + CO(g)
      • Fe₂O₃(s) + 3CO(g) --(Blast Furnace)--> 2Fe(l) + 3CO₂(g)
      • PbO(s) + C(s) --(Heat)--> Pb(s) + CO(g)
  • Thermodynamic Principles (Ellingham Diagrams):
    • Gibbs Free Energy change (ΔG) determines the spontaneity of a reaction: ΔG = ΔH - TΔS. For a reaction to be spontaneous, ΔG must be negative.
    • Ellingham Diagram plots ΔG° (standard Gibbs free energy of formation of oxides) vs Temperature (T).
    • Features: Most lines slope upwards because ΔS is negative for xM + O₂ → MₓO₂ (gas is consumed). Sharp changes in slope indicate phase transitions (melting/boiling). The line for C + O₂ → CO₂ is almost horizontal (ΔS ≈ 0). The line for 2C + O₂ → 2CO slopes downwards (ΔS is positive, 1 mole gas → 2 moles gas).
    • Application: A metal oxide (MₓO) can be reduced by an element (M') if, at a given temperature, the ΔG° for the formation of the oxide of M' is more negative than that for MₓO. In the diagram, the line for M' oxidation must lie below the line for M oxidation. Carbon (as C or CO) can reduce metal oxides where the C/CO oxidation line lies below the metal oxidation line. The temperature above which reduction is feasible corresponds to the point where ΔG° for the overall reduction reaction becomes negative (often indicated by the intersection point of the two lines).
  • Reduction by Other Metals (e.g., Aluminothermy): Highly reactive metals like Al, Mg can reduce oxides of less reactive metals.
    • Cr₂O₃(s) + 2Al(s) --(Heat)--> Al₂O₃(s) + 2Cr(l) (Thermite process - highly exothermic)
    • 3Mn₃O₄(s) + 8Al(s) --(Heat)--> 4Al₂O₃(s) + 9Mn(l)
  • Auto-Reduction (Self-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.
    • 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂ (Partial Roasting)
    • 2Cu₂O + Cu₂S --(High Temp)--> 6Cu + SO₂ (Self-Reduction)
    • Similar reactions occur for PbS and HgS.
  • Electrolytic Reduction (Electrometallurgy): Used for highly electropositive metals (Group 1, 2, Al) which cannot be reduced by carbon.
    • The metal oxides or halides are dissolved in a suitable molten salt electrolyte to lower the melting point and increase conductivity.
    • Example: Hall-Héroult Process for Aluminium:
      • Electrolyte: Molten mixture of purified Al₂O₃ + Cryolite (Na₃AlF₆) + Fluorspar (CaF₂). Cryolite lowers the melting point (from ~2323 K to ~1140 K) and increases conductivity. Fluorspar further lowers the melting point.
      • Temperature: ~1140 - 1220 K
      • Electrodes: Carbon lining of the steel tank (Cathode), Graphite rods (Anode).
      • Reactions:
        • At Cathode: Al³⁺ (melt) + 3e⁻ → Al (l)
        • At Anode: C(s) + O²⁻ (melt) → CO(g) + 2e⁻ ; C(s) + 2O²⁻ (melt) → CO₂(g) + 4e⁻
      • The carbon anodes are consumed during the process (forming CO and CO₂) and need periodic replacement. Overall reaction (approx): 2Al₂O₃ + 3C → 4Al + 3CO₂

III. Refining of Crude Metal

The metal obtained after reduction is usually impure (crude metal). Refining removes residual impurities to achieve desired purity. The method depends on the nature of the metal and the impurities.

• (a) Distillation:

  • Principle: Useful for metals with low boiling points (volatile metals). Impure metal is heated above its boiling point, the vapours are condensed to get pure metal, 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 higher melting point impurities behind.
  • Used for: Tin (Sn), Lead (Pb), Bismuth (Bi).

• (c) Electrolytic Refining:

  • Principle: Based on electrolysis. One of the most common and effective methods.
  • Process:
    • Anode: Thick block of impure metal.
    • Cathode: Thin sheet of pure metal.
    • Electrolyte: Solution of a soluble salt of the same metal (often sulphate).
    • On passing current, pure metal dissolves from the anode and deposits onto the cathode.
    • Anode: M → Mⁿ⁺ + ne⁻
    • Cathode: Mⁿ⁺ + ne⁻ → M
    • Impurities: More electropositive impurities remain in the solution. Less electropositive impurities settle down below the anode as Anode Mud (often contains valuable metals like Ag, Au, Pt, Se, Te).
  • Used for: Copper (Cu), Zinc (Zn), Nickel (Ni), Silver (Ag), 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 metal melts at the heated zone. As the heater moves forward, pure metal crystallises out from the melt, while impurities concentrate in the molten zone which moves along with the heater to one end. The process is repeated several times, and the end containing impurities is cut off. Done in an inert atmosphere.
  • Used for: Producing high purity semiconductors and other metals like Germanium (Ge), Silicon (Si), Boron (B), Gallium (Ga), Indium (In).

• (e) Vapour Phase Refining:

  • Principle: The metal is converted into a volatile compound, which is then decomposed at a higher temperature to give 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 metal.
  • Examples:
    • Mond Process for Nickel (Ni):
      • Impure Ni is heated with Carbon Monoxide (CO) at 330-350 K to form volatile Nickel Tetracarbonyl [Ni(CO)₄].
      • Ni(s) + 4CO(g) --(330-350 K)--> Ni(CO)₄(g)
      • The vapour is heated to 450-470 K, decomposing it to give pure Ni.
      • Ni(CO)₄(g) --(450-470 K)--> Ni(s) + 4CO(g)
    • Van Arkel Method for Zirconium (Zr) and Titanium (Ti):
      • Impure metal is heated with Iodine (I₂) in an evacuated vessel (~850 K for Zr, ~523 K for Ti) to form a volatile covalent iodide. Oxygen and Nitrogen impurities remain reacted with the metal and do not react with Iodine.
      • Zr(s) + 2I₂(g) --(850 K)--> ZrI₄(g)
      • Ti(s) + 2I₂(g) --(523 K)--> TiI₄(g)
      • The gaseous iodide is decomposed by heating electrically on a tungsten filament at high temperature (~1800 K for Zr, ~1700 K for Ti). Pure metal deposits on the filament.
      • ZrI₄(g) --(1800 K)--> Zr(s) + 2I₂(g)
      • TiI₄(g) --(1700 K)--> Ti(s) + 2I₂(g)
      • The I₂ is reused.

• (f) Chromatographic Methods:

  • Principle: Based on the differential adsorption of different components of a mixture on an adsorbent. Used for purification when impurities are chemically similar to the metal, or when the metal is available in small quantities. (Details usually covered in analytical chemistry).

4. Uses of Some Important Metals

• Aluminium: Lightweight, corrosion-resistant. Used in aircraft, utensils, window frames, electrical transmission cables (good conductor), alloys (e.g., duralumin, magnalium).
• Copper: Excellent electrical conductor. Used in electrical wiring, water pipes, calorimeters, alloys (e.g., brass [Cu+Zn], bronze [Cu+Sn]).
• Zinc: Used for galvanizing iron (prevent rusting), in batteries (dry cells), alloys (e.g., brass, german silver). Zinc oxide used in paints, cosmetics.
• Iron: Most widely used metal. Cast iron, wrought iron, steel are its forms/alloys with varying carbon content and properties. Used in construction, machinery, tools, vehicles, magnets.

Now, let's test your understanding with some Multiple Choice Questions (MCQs).

MCQs on General Principles and Processes of Isolation of Elements

1. Which of the following ores is best concentrated by froth flotation?
   (a) Magnetite (Fe₃O₄)
   (b) Cassiterite (SnO₂)
   (c) Galena (PbS)
   (d) Bauxite (Al₂O₃.xH₂O)

2. In the extraction of aluminium by the Hall-Héroult process, cryolite (Na₃AlF₆) is added to:
   (a) Lower the melting point of Al₂O₃ and increase its conductivity.
   (b) Act as a reducing agent for Al₂O₃.
   (c) Remove impurities like SiO₂ as slag.
   (d) Prevent the oxidation of molten aluminium.

3. The process of heating a carbonate ore strongly in the absence or limited supply of air to convert it into metal oxide is known as:
   (a) Roasting
   (b) Smelting
   (c) Calcination
   (d) Reduction

4. Which method is used for refining volatile metals like Zinc and Mercury?
   (a) Liquation
   (b) Distillation
   (c) Zone Refining
   (d) Electrolytic Refining

5. During the electrolytic refining of copper, anode mud may contain:
   (a) Zn, Ni, Fe
   (b) Ag, Au, Pt
   (c) Na, K, Ca
   (d) Al, Mg, Mn

6. The role of a depressant (like NaCN) in the froth flotation process is to:
   (a) Enhance the wetting of ore particles by oil.
   (b) Stabilize the froth formed.
   (c) Selectively prevent one type of sulphide ore from forming froth.
   (d) Increase the solubility of gangue particles.

7. According to the Ellingham diagram, a metal oxide can be reduced by carbon monoxide (CO) at a specific temperature if:
   (a) The ΔG° line for CO oxidation lies above the metal oxide formation line.
   (b) The ΔG° line for CO oxidation lies below the metal oxide formation line.
   (c) Both lines intersect at that temperature.
   (d) The slope of the metal oxide line is negative.

8. The Mond process is used for the refining of:
   (a) Titanium (Ti)
   (b) Zirconium (Zr)
   (c) Nickel (Ni)
   (d) Copper (Cu)

9. Leaching is a process of:
   (a) Reduction of the metal oxide to metal.
   (b) Concentration of ore by dissolving the ore in a suitable reagent.
   (c) Removal of volatile impurities by heating.
   (d) Purification of metal by fractional crystallization.

10. Auto-reduction process is used for the extraction of:
    (a) Fe
    (b) Al
    (c) Cu
    (d) Mg

Answer Key for MCQs:

1. (c) Galena (PbS) - Froth flotation is primarily for sulphide ores.
2. (a) Lower the melting point of Al₂O₃ and increase its conductivity.
3. (c) Calcination - Definition of calcination, especially for carbonates.
4. (b) Distillation - Used for volatile metals.
5. (b) Ag, Au, Pt - Less electropositive metals settle as anode mud.
6. (c) Selectively prevent one type of sulphide ore from forming froth.
7. (b) The ΔG° line for CO oxidation lies below the metal oxide formation line - This makes the overall ΔG for reduction negative.
8. (c) Nickel (Ni) - Mond process involves volatile Ni(CO)₄.
9. (b) Concentration of ore by dissolving the ore in a suitable reagent.
10. (c) Cu - Auto-reduction is common for less reactive metals like Cu, Pb, Hg from their sulphide ores.

Study these notes thoroughly. Understanding the principles behind each step is key for competitive exams. Let me know if any part needs further clarification!