General Principles and Processes of Isolation of Elements – Class 12 Chemistry Notes

Unit 6 – General Principles and

Processes of Isolation of Elements

6.1 Occurrence of Metals
6.2 Concentration of Ores
6.3 Extraction of Crude Metal from Concentrated Ore
6.4 Thermodynamic Principles of Metallurgy
6.5 Electrochemical Principles of Metallurgy
6.6 Oxidation and Reduction
6.7 Refining

6.8 Uses of Aluminium, Copper, Zinc and Iron

I. Introduction to Isolation of Elements

The provided sources cover Unit 6: General Principles and Processes of Isolation of Elements, detailing various methods for extracting and purifying metals.

II. Key Metallurgical Processes

Metallurgy encompasses various steps to obtain pure metals from their ores.

A. Concentration of Ores

This initial step involves removing undesirable materials (gangue) from the ore.

Froth Flotation Process
- Principle: This method is primarily used for concentrating sulphide ores. It works by selectively making mineral particles non-wettable by water and thus preferentially attached to oil droplets and air bubbles, rising to the froth, while gangue particles remain in the water.
- Role of Substances:
  - Collectors: Enhance the non-wettability of the mineral particles, allowing them to attach to froth.
  - Froth Stabilisers: Increase the stability of the froth, for example, cresols.
  - Depressants: Used to separate two sulphide ores by preventing certain types of particles from coming into the froth.
- Separation of Sulphide Ores (Example: ZnS and PbS): Two sulphide ores can be separated by adjusting the proportion of oil to water or by using depressants. For instance, NaCN is used as a depressant in the case of an ore containing ZnS and PbS. NaCN forms a complex with ZnS, preventing it from floating with the froth, while PbS remains in the froth.
- Examples of Ores Concentrated: Galena (PbS) and Copper pyrites (CuFeS₂).

B. Conversion of Ore to Oxide

Many metallurgical processes require the ore to be in oxide form as oxides are generally easier to reduce than sulphides [30, 37, 55(b)].

Calcination: This process involves heating the ore in the absence or limited supply of air.
- Reactions occurring during calcination:
  - Decomposition of carbonates, e.g., CaCO₃ → CaO + CO₂.
  - Dehydration of hydrated oxides, e.g., Al₂O₃·xH₂O → Al₂O₃ + xH₂O.
- Reason for converting sulphide ores to oxides: Sulphides are not easily reduced, whereas oxides are easily reduced [30, 37, 55(b)].

C. Reduction of Metal Oxide (and other compounds)

This step involves converting the metal compound (usually an oxide) into the elemental metal.

Carbon/Carbon Monoxide Reduction (e.g., in Blast Furnace) [6, 7, 8, 9, 11, 12, 13, 14, 22, 23, 28, 29, 31, 32, 34, 42, 55(a), 55(d)]
- Ellingham Diagram Principles: The Ellingham diagram helps predict the feasibility of reduction reactions. A reducing agent (e.g., carbon, carbon monoxide) can reduce a metal oxide if the Gibbs free energy change (ΔG) for the coupled reaction is negative.
  - For FeO, carbon (coke) can reduce FeO to Fe and produce CO at temperatures above point A (a specific temperature on the Ellingham diagram, indicating higher temperatures).
  - Below point A, FeO can be reduced by carbon monoxide only.
  - At temperatures above 1073 K, coke can reduce FeO to Fe because the ΔG for the formation of CO from C is more negative than that for the formation of FeO from Fe (i.e., ΔG(C, CO) < ΔG(Fe, FeO)) [28, 55(a)].
  - At the temperature corresponding to point D on the Ellingham diagram, the ΔG value for the overall reduction reaction of FeO with carbon monoxide is zero.
- Reactions in Blast Furnace for Iron Extraction (from Haematite):
  - In the temperature range of 500-800 K, the following reactions occur:
    - 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂
    - Fe₃O₄ + 4CO → 3Fe + 4CO₂
    - Fe₂O₃ + CO → 2FeO + CO₂
  - Another main reaction is the reduction of Fe₂O₃ by CO: Fe₂O₃ + 3CO → 2Fe + 3CO₂.
  - Flux in Blast Furnace: Limestone is added as a flux. Fluxes are used for making the molten mass more conducting and removing impurities (gangue) by forming slag. For example, CaO (from limestone) reacts with SiO₂ (impurity) to form calcium silicate slag (CaSiO₃): CaO + SiO₂ → CaSiO₃.
- Limitations of Carbon and Hydrogen as Reducing Agents at High Temperatures: Although carbon and hydrogen are effective reducing agents, they are generally not used to reduce metallic oxides at very high temperatures because they can react with the metals to form undesirable carbides and hydrides, respectively [16, 32, 55(d)].
Autoreduction (Self-reduction)
- This process involves the reduction of a metal oxide by the metal sulphide itself.
- Example (Copper Extraction): In the extraction of copper from its sulphide ore, the metal is formed by the reduction of copper(I) oxide (Cu₂O) with copper(I) sulphide (Cu₂S).
  - The reaction is: Cu₂O + ½ Cu₂S → 3Cu + ½ SO₂.
  - Another way this is expressed is 2Cu₂O + Cu₂S → 6Cu + SO₂.
  - The solidified copper obtained from the reverberatory furnace after this process has a blistered appearance due to the evolution of SO₂ gas.
- Note: Zinc generally cannot be extracted by self-reduction.
Hydrometallurgy
- Principle: Involves dissolving the ore in a suitable reagent, followed by precipitation of the metal by a more electropositive metal.
- Extraction of Copper from Low-Grade Ores: Copper is extracted using hydrometallurgy. Low-grade copper ores are leached out using acid or bacteria. The resulting solution containing Cu²⁺ ions is then treated with scrap iron, zinc, or hydrogen gas to precipitate pure copper.
  - Reactions:
    - Cu²⁺(aq) + H₂(g) → Cu(s) + 2H⁺(aq)
    - Cu²⁺(aq) + Fe(s) → Fe²⁺(aq) + Cu(s)
- Extraction of Gold and Silver (Cyanide Process):
  - This process involves leaching the metal with CN⁻ (cyanide) ion in the presence of water and oxygen.
  - The metal is recovered by displacement by a more electropositive metal from the complex ion.
  - Reactions:
    - 4Au(s) + 8CN⁻(aq) + 2H₂O(aq) + O₂(g) → 4[Au(CN)₂]⁻(aq) + 4OH⁻(aq)
    - 2[Au(CN)₂]⁻(aq) + Zn(s) → 2Au(s) + [Zn(CN)₄]²⁻(aq)
  - Role of Zinc: In the recovery step, zinc acts as a reducing agent to displace gold from its complex ion.
Electrolytic Reduction
- Principle: Uses an electric current to reduce metal ions to their elemental form.
- Extraction of Chlorine by Electrolysis of Brine: In the electrolysis of brine (NaCl solution) using inert electrodes:
  - Oxidation of Cl⁻ ion to chlorine gas occurs at the anode.
  - The reaction at the anode is: Cl⁻(aq) → ½Cl₂(g) + e⁻; E⁰ = 1.36V.
  - For the overall reaction, ΔG⁰ has a positive value (+422 kJ) and E⁰ has a negative value (-2.2V). Therefore, an external emf of more than 2.2V is required for the extraction of Cl₂ from brine.
- Metallurgy of Aluminium (Hall-Heroult Process):
  - Electrolytic reduction is used for the extraction of aluminium.
  - Purified Al₂O₃ is mixed with CaF₂ (Cryolite, Na₃AlF₆).
  - The purpose of adding CaF₂ is to lower the melting point of Al₂O₃ and to increase the conductivity of the molten mixture.
  - During the process, the graphite anode is oxidized to carbon monoxide and carbon dioxide.
- Considerations for Electrochemical Method: Two key considerations are the reactivity of the metal produced and the suitability of the electrodes.

III. Refining of Metals

Refining processes are used to obtain metals of very high purity.

Zone Refining
- Principle: This method is based on the principle that impurities are more soluble in the molten metal (melt) than in the solid metal. A molten zone is created and slowly moved along a rod of impure metal; as the zone moves, the impurities concentrate in the molten region and are carried to one end of the rod.
- Applications: Very useful for producing high purity semiconductors like Germanium (Ge) and Silicon (Si).
Vapour Phase Refining [13, 24, 28, 31, 43, 55(e)]
- Basic Requirements: For this method to be effective, two requirements must be met:
  - The metal should form a volatile compound with an available reagent.
  - The volatile compound should be easily decomposable (often by heating), so that the pure metal can be easily recovered.
- Mond Process (for Nickel):
  - Nickel is purified by heating the impure metal with a stream of carbon monoxide to form a volatile nickel tetracarbonyl complex, Ni(CO)₄.
  - Ni(CO)₄ is a volatile compound which decomposes at 460 K to give pure Ni.
- Van Arkel Method (for Zirconium (Zr) and Titanium (Ti)) [9, 13, 20, 24, 30, 31, 38, 46, 51, 55(e)]:
  - Used for refining Zr and Ti.
  - In this method, the crude metal is heated with iodine to form a volatile metal iodide [13, 24, 30, 38, 55(e)].
  - Example for Zirconium: Zr + 2I₂ → ZrI₄.
  - The volatile iodide (e.g., ZrI₄) is then decomposed at a high temperature (e.g., 1800 K for ZrI₄) on a hot filament to give pure metal.
  - Example for Zirconium: ZrI₄ (at 1800 K) → Zr + 2I₂.
Electrolytic Refining
- Used to purify metals like Copper (Cu) and Zinc (Zn).
Other Refining Methods Mentioned:
- Fractional distillation: Used for purification of mercury.
- Liquation: Can be used for purification.
- Chromatography: Can result in colored bands and is a purification method.

IV. Specific Metals, Ores, and Related Concepts

A. Iron (Fe)

Most Abundant Element in Earth's Crust along with Aluminium.
Wrought Iron: Considered the purest form of iron.
- Prepared from cast iron by oxidizing impurities.
- Removal of Impurities: Impurities like sulphur, silicon, and phosphorus can be removed from cast iron. Limestone is added as flux, and these impurities change to their oxides and pass into the slag.
- Furnace Lining: In the preparation of wrought iron by oxidizing impurities in a reverberatory furnace, Haematite (Fe₂O₃) is used to line the furnace. It acts as an oxidizing agent.
Cast Iron: Obtained by remelting pig iron with scrap iron and coke using a hot air blast.

B. Copper (Cu)

Copper Matte: Produced when copper ore is mixed with silica in a reverberatory furnace. It contains sulphides of copper(I) and iron(II) (Cu₂S and FeS). Note: It does not contain ZnS.
Blistered Copper: The solidified copper obtained from the reverberatory furnace has a blistered appearance due to the evolution of SO₂ during extraction.
Ores: Malachite (CuCO₃·Cu(OH)₂) and Copper pyrites.

C. Aluminium (Al)

Most Abundant Element in Earth's Crust along with Iron.
Ores: Bauxite is the primary ore.
- Common impurities present in bauxite are Fe₂O₃ and SiO₂.
Forms of Al₂O₃: Corundum is Al₂O₃. Sapphire is a variety of corundum (Al₂O₃); however, one matching item in the sources lists Sapphire with Co.

D. Zinc (Zn)

Ores: Calamine (ZnCO₃) and Sphalerite (ZnS).

E. Other Notable Metals/Ores

Gold (Au) and Silver (Ag): Their extraction involves leaching with CN⁻ ion.
Nickel (Ni): Purified by Mond process.
Zirconium (Zr) and Titanium (Ti): Purified by van Arkel method [9, 13, 20, 24, 30, 31, 38, 46, 51, 55(e)].
Germanium (Ge) and Silicon (Si): Purified by zone refining for ultrapure applications, especially semiconductors.
Galena: PbS ore concentrated by froth flotation.
Haematite: Fe₂O₃ ore, reducible by carbon.
Magnetite: Fe₃O₄, an iron ore.

V. General Concepts and Terminology

Flux: A substance added during metallurgical processes to make the molten mass more conducting and to remove infusible impurities by forming a fusible slag.
Gangue: The undesirable earthy or stony material associated with the ore.

In essence, isolating elements is like a multi-stage treasure hunt, where the ore is the raw treasure chest. First, you concentrate the treasure (ore dressing) by separating the valuable minerals from the dirt (gangue). Then, you might need to convert the treasure into a form easier to handle, often an oxide, which is like opening a specific lock. Next, you reduce the treasure to its pure metallic form, which is like solving a puzzle, sometimes using fire (carbon reduction), sometimes using water (hydrometallurgy), or even electricity (electrolytic reduction). Finally, to ensure you have the purest gem, you refine it using specialized techniques, like zone refining for perfect crystals or vapor phase refining for metals that love to play hide-and-seek in volatile compounds.

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