General Principles and Processes of Isolation of Elements Class 12 Notes | YoLearn.ai

Key Principles of Metallurgy

Metallurgy is the scientific and technological process used for the extraction of metals from their ores and their applications. It involves several stages.

• Crushing and Grinding (Pulverization): Ores are broken down into a fine powder.
• Concentration of Ore: Removing unwanted earthy and rocky materials (gangue) from the ore. This increases the metal content.
• Extraction of Crude Metal from Concentrated Ore: This usually involves two steps:
• Conversion to Oxide: Often, sulphide or carbonate ores are converted to oxides (roasting or calcination) because oxides are easier to reduce.
• Reduction of Oxide: The metal oxide is reduced to the crude metal using suitable reducing agents (e.g., carbon, CO, other metals, or electrolysis).
• Refining of Metal: Purifying the crude metal to obtain a metal of desired purity.

Thermodynamic Principles are vital for understanding the feasibility of reduction reactions. Gibbs Free Energy Change (ΔG) plays a central role:

• ΔG = ΔH - TΔS
• For a reaction to be spontaneous (feasible), ΔG must be negative.
• A high negative ΔG indicates a strong reducing agent. The Ellingham Diagram graphically represents the change in ΔG for the formation of various metal oxides at different temperatures, making it a powerful tool to predict the most suitable reducing agent at a given temperature. It highlights that an element can reduce the oxide of another element if its own oxidation line lies below that of the metal oxide to be reduced on the diagram.

Electrochemical Principles are applied when metals with high reduction potentials (like alkali and alkaline earth metals, or Aluminium) cannot be reduced by common chemical reducing agents. In such cases, electrolytic reduction of fused salts or aqueous solutions is employed, where the metal ions gain electrons at the cathode. The choice of reduction method depends on the nature of the metal and its position in the reactivity series.

Definitions of Key Metallurgical Terms

Mineral

A naturally occurring chemical substance obtained from the earth's crust, which contains metals in either free or combined state.

Ore

A mineral from which metal can be extracted economically and conveniently. All ores are minerals, but not all minerals are ores.

Gangue (Matrix)

The unwanted earthy or rocky impurities associated with the ore.

Flux

A substance added to the ore during smelting to remove non-fusible impurities by forming a fusible slag.

Slag

The fusible product formed when a flux reacts with gangue during smelting. It is lighter than the molten metal and floats on top.

Metallurgy

The entire scientific and technological process of extraction of metals from their ores and refining them for use.

Pyrometallurgy

Extraction of metals involving high temperatures, typically involving heating, roasting, and smelting.

Hydrometallurgy

Extraction of metals using aqueous solutions, typically involving leaching the ore with a suitable solvent.

Concentration of Ores: Removing Impurities

The process of removing unwanted materials (gangue) from the ore is known as concentration or ore dressing. The choice of method depends on the physical properties of the ore and the gangue.

• Hydraulic Washing (Gravity Separation): This method is based on the difference in specific gravities of the ore particles and the gangue. Lighter gangue particles are washed away by a stream of water, leaving behind heavier ore particles. It is typically used for oxide ores (e.g., haematite) and native ores (e.g., gold).
• Magnetic Separation: Used when either the ore or the gangue is magnetic. The crushed ore is passed over a magnetic roller, which attracts magnetic particles, separating them from non-magnetic ones. Examples include chromite, pyrolusite (magnetic ores), and cassiterite (non-magnetic, but used to separate from magnetic impurities).
• Froth Flotation Method: Specifically designed for sulphide ores. This method utilizes the difference in wetting properties between sulphide ore particles and gangue particles. The finely powdered ore is mixed with water, pine oil (frothing agent), and collectors (e.g., xanthates, which enhance non-wettability of ore). Air is blown through the mixture, creating froth that carries the hydrophobic ore particles to the surface, while hydrophilic gangue settles at the bottom.
• Leaching: A chemical method of concentration where the ore is soluble in a suitable chemical reagent, but the impurities are not. The ore is treated with a reagent to dissolve the metal or its compound, forming a soluble complex, while impurities remain insoluble. The solution is then separated, and the metal is recovered from the complex.
• Leaching of Aluminium (Bayer's Process): Bauxite (Al₂O₃.xH₂O) is leached with hot concentrated NaOH solution, forming soluble sodium aluminate. Fe₂O₃ and SiO₂ remain insoluble. The solution is then diluted and seeded with fresh Al(OH)₃, precipitating pure hydrated alumina, which is then calcined to yield pure Al₂O₃.
• Leaching of Gold and Silver (Cyanide Process): Gold and silver are leached with a dilute solution of NaCN or KCN in the presence of air (O₂). This forms a soluble cyano complex ([M(CN)₂]⁻), from which the metal is recovered by displacement with a more electropositive metal like zinc (reduction by displacement).

Extraction of Crude Metal from Concentrated Ore

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Refining of Crude Metals

Crude metals obtained after reduction often contain impurities. Refining is the process of purifying these metals to obtain the desired purity.

• Distillation: Used for low boiling point metals like Zinc (Zn), Cadmium (Cd), and Mercury (Hg). The crude metal is heated, vaporized, and then condensed to obtain pure metal, leaving behind non-volatile impurities.
• Liquation: Used for metals with low melting points (e.g., Tin (Sn), Lead (Pb), Bismuth (Bi)) containing high melting impurities. The crude metal is heated on a sloping hearth, and the pure metal melts and flows down, while infusible impurities remain behind.
• Electrolytic Refining: One of the most important and widely used methods for highly pure metals like Copper (Cu), Zinc (Zn), Aluminium (Al), Nickel (Ni), Silver (Ag), Gold (Au). The crude metal is made the anode, a thin sheet of pure metal is the cathode, and a soluble salt of the metal acts as the electrolyte. Upon passing current, pure metal deposits at the cathode, while less electropositive impurities dissolve from the anode, and more electropositive impurities collect as "anode mud."
• Example (Copper): Anode: Cu → Cu²⁺ + 2e⁻; Cathode: Cu²⁺ + 2e⁻ → Cu
• Zone Refining: Based on the principle that impurities are more soluble in the molten state than in the solid state of the metal. A circular heater slowly moves along a rod of crude metal. As the heater moves, a molten zone is created which moves along, carrying the impurities with it to one end of the rod. This process is repeated several times. Used for producing highly pure semiconductors like Silicon (Si), Germanium (Ge), Gallium (Ga).
• Vapour Phase Refining: The metal is converted into a volatile compound, which is then collected and decomposed to give pure metal. This requires two conditions:

1. The metal should form a volatile compound with a suitable reagent.
2. The volatile compound should be easily decomposable to recover the pure metal.

• Mond's Process (for Nickel): Impure Ni is heated with CO at 330-350 K to form volatile nickel tetracarbonyl [Ni(CO)₄]. This carbonyl is then decomposed at a higher temperature (450-470 K) 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 and Titanium): Impure metal is heated with Iodine to form a volatile metal iodide. The iodide is then decomposed on an electrically heated tungsten filament at a very high temperature (1700 K for Ti, 1800 K for Zr) to obtain pure metal.
• Zr(impure) + 2I₂ → ZrI₄ (volatile)
• ZrI₄ (1800 K, W filament) → Zr(pure) + 2I₂
• Chromatographic Methods: Based on the principle of differential adsorption or partition between a stationary and mobile phase. Used for purification of elements when impurities are in very minute quantities or when elements are available in small amounts.

Ellingham Diagram: A Thermodynamic Tool

The Ellingham Diagram is a graph plotting the standard Gibbs free energy change (ΔG°) for the formation of various metal oxides against temperature. It is a powerful tool to predict the spontaneity and feasibility of a reduction reaction in metallurgy.

• Key Features and Interpretation:
• Slope of the lines: The slope of the line for M + O₂ → MO is positive because ΔS (change in entropy) for such a reaction is negative (gas O₂ is consumed to form solid oxide).
• Intersection Points: The intersection point of two lines indicates the temperature at which the Gibbs free energy change for both reactions is equal. Below this temperature, the oxide corresponding to the lower line is more stable; above this temperature, the other oxide becomes more stable.
• Reducing Agent Selection: An element (e.g., Carbon) can reduce the oxide of another metal if its ΔG° line lies below the ΔG° line of the metal oxide to be reduced on the diagram. This is because the overall ΔG° for the coupled reaction (reduction of metal oxide + oxidation of reducing agent) will be negative.
• Temperature Dependence: The effectiveness of reducing agents often increases with temperature. For instance, carbon becomes a more effective reducing agent at higher temperatures (its line slopes downwards at a steeper rate after the formation of CO, due to increased ΔS).
• Limitations:
• It assumes reactants and products are in equilibrium, which is not always the case in real processes.
• It does not account for reaction kinetics (how fast the reaction proceeds).
• It's based on standard state conditions, and actual conditions may vary.
• Application: Crucial for choosing the appropriate reducing agent and temperature for the extraction of metals like iron from its oxide in a blast furnace. For example, below 1073 K, CO is a better reducing agent than carbon for Fe₂O₃, while above 1073 K, carbon is more effective.

Key Points to Remember

• Ores vs. Minerals: All ores are minerals, but not all minerals are ores (only those from which metals can be extracted profitably).
• Gangue Removal: Concentration methods aim to remove gangue based on differences in physical or chemical properties.
• Calcination vs. Roasting: Calcination (absence of air, for carbonates/hydrated ores); Roasting (presence of air, for sulphide ores). Both convert ores to oxides.
• Ellingham Diagram: A lower ΔG° line for a reducing agent than for the metal oxide indicates feasibility of reduction. Carbon becomes a better reducing agent at higher temperatures due to the formation of CO (increase in entropy).
• Thermodynamic Feasibility: ΔG = ΔH - TΔS. Negative ΔG means a spontaneous reaction.
• Hall-Héroult Process: Electrolytic reduction of alumina (Al₂O₃) dissolved in molten cryolite (Na₃AlF₆) and fluorspar (CaF₂). Anode is graphite, cathode is steel lined with carbon.
• Zone Refining: Based on the principle that impurities are more soluble in the molten state than in the solid state. Used for ultra-pure semiconductors.
• Vapour Phase Refining: Requires formation of a volatile compound and its easy decomposition (e.g., Mond's for Ni, Van Arkel for Zr/Ti).
• Pyrometallurgy: High temperature processes like smelting.
• Hydrometallurgy: Uses aqueous solutions (e.g., leaching of Au/Ag with cyanide).

Worked Examples

• {"heading":"Q: Write the chemical equations for the leaching of bauxite by Bayer's process.","bodyMarkdown":"A: Al₂O₃.xH₂O(s) + 2NaOH(aq) + (3-x)H₂O(l) → 2Na[Al(OH)₄](aq)\nThen, 2Na[Al(OH)₄](aq) + CO₂(g) → Al₂O₃.xH₂O(s) + 2NaHCO₃(aq) (or by seeding with fresh Al(OH)₃ followed by heating)\nAl₂O₃.xH₂O(s) → Al₂O₃(s) + xH₂O(g) (calcination)"}
• {"heading":"Q: Using Ellingham diagram, justify why carbon can reduce ZnO at 1673 K but not at 973 K.","bodyMarkdown":"A: On the Ellingham diagram, the line for Zn + O₂ → ZnO is above the line for C + O₂ → CO₂ (or C + 1/2 O₂ → CO) at 973 K. This means ΔG° for the formation of ZnO is more negative than for CO or CO₂ at 973 K, so ZnO is more stable, and carbon cannot reduce it. However, at 1673 K, the C + O₂ → CO line dips below the Zn + O₂ → ZnO line. This indicates that ΔG° for the formation of CO (or CO₂) becomes more negative than for ZnO, making carbon an effective reducing agent for ZnO at 1673 K."}
• {"heading":"Q: How is nickel purified by Mond's process? Give reactions.","bodyMarkdown":"A: Impure nickel is heated with carbon monoxide (CO) at about 330-350 K to form a volatile complex, nickel tetracarbonyl. This complex is then heated to a higher temperature (450-470 K) where it decomposes to give pure nickel and carbon monoxide, which can be reused.\nNi(impure) + 4CO(g) --(330-350 K)--> Ni(CO)₄(g)\nNi(CO)₄(g) --(450-470 K)--> Ni(pure)(s) + 4CO(g)"}

Exam Strategy: Common Traps and Marking Cues

• Distinguish between Calcination and Roasting: Students often confuse these. Remember Calcination is absence of air, Roasting is presence of air.
• Ellingham Diagram Interpretation: Understand the significance of the slope and intersection points. A negative slope for C to CO conversion is key to its effectiveness at high temperatures. Always relate ΔG to spontaneity.
• Specific Examples for Refining: Know which refining method is suitable for which metal and the underlying principle (e.g., Zone refining for semiconductors, Mond's for Ni, Van Arkel for Zr/Ti, Electrolytic for Cu).
• Balanced Chemical Equations: Ensure all equations for metallurgical processes are balanced and include state symbols where appropriate.
• Role of Flux: Don't just say "removes impurities." Explain how by reacting with gangue to form fusible slag.
• Cryolite in Al extraction: Its role is to lower the melting point of alumina and increase its electrical conductivity, not as a reducing agent.

Quick Revision Check

• Q: What is the main difference between a mineral and an ore? A: A mineral is any naturally occurring chemical substance found in the earth's crust, while an ore is a mineral from which a metal can be extracted economically and conveniently.
• Q: Give an example of a metal refined by zone refining and state the principle behind it. A: Silicon (Si) or Germanium (Ge). The principle is that impurities are more soluble in the molten state than in the solid state of the metal.
• Q: Why is it easier to reduce a metal oxide than a metal sulphide? A: Metal sulphides are thermodynamically more stable than oxides, so their direct reduction is more difficult. Also, the Gibbs free energy of formation of SO₂ is more negative than CS₂, making sulphur removal challenging. Therefore, sulphides are first converted to oxides (roasting) for easier reduction.
• Q: What is the role of depressants in froth flotation process? A: Depressants (e.g., NaCN or KCN) are used in froth flotation to prevent certain sulphide minerals from forming froth with the ore. For example, NaCN selectively prevents ZnS from coming with the froth when PbS is to be concentrated.

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