General Principles And Processes Of Isolation Of Elements Class 12 Notes

Key Terminology in Metallurgy

Metallurgy

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

Mineral

A naturally occurring chemical substance, generally inorganic, found in the Earth's crust.

Ore

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

Gangue (Matrix)

The undesirable earthy and rocky impurities associated with the ore, such as sand, clay, and siliceous matter.

Flux

A substance added during smelting to remove non-fusible gangue by forming a fusible slag. For acidic gangue, a basic flux is used, and vice-versa.

Slag

The fusible product formed when flux combines with gangue during smelting. It is lighter than the molten metal and floats on its surface.

Calcination

Heating an ore (usually carbonates or hydroxides) strongly in the absence or limited supply of air, below its melting point, to remove volatile impurities like CO2 or H2O.

Roasting

Heating an ore (usually sulfides) strongly in a regular supply of air, below its melting point, to convert it into its oxide, releasing gases like SO2.

Overview of Metallurgical Processes

The extraction and isolation of metals from their ores involve a series of steps, collectively known as metallurgy. These processes are designed to purify the metal by removing unwanted impurities and then reducing the metal compound to its elemental form. The choice of specific steps depends heavily on the nature of the ore, the reactivity of the metal, and the type of impurities present.

Broadly, the entire process can be divided into three main stages:

1. Concentration of Ore (Beneficiation): This initial step involves removing physical impurities like sand, clay, and rocky matter (gangue) from the ore. Various physical and chemical methods are employed, depending on the properties of the ore and the gangue. Common methods include hydraulic washing, magnetic separation, froth flotation (for sulfide ores), and leaching (a chemical method for specific ores like bauxite).

1. Extraction of Crude Metal from Concentrated Ore: This stage focuses on converting the concentrated ore into its metallic form. It typically involves two sub-steps:

• Conversion to oxide: Most metals are easier to reduce from their oxide forms. Thus, carbonate and hydroxide ores are converted to oxides by calcination, while sulfide ores are converted to oxides by roasting. This step also helps in removing volatile impurities.
• Reduction of metal oxide: The metal oxide is then reduced to the crude metal. The choice of reducing agent (e.g., carbon, carbon monoxide, more reactive metals like aluminium, or even self-reduction) depends on the thermodynamics of the reduction process, often analyzed using Ellingham diagrams. For highly reactive metals, electrolytic reduction is frequently used.

1. Refining of Crude Metal: The metal obtained after reduction is often impure and is known as crude metal. The final step involves purifying this crude metal to obtain a metal of desired purity. Numerous refining methods exist, each suitable for specific metals and types of impurities. These include distillation, liquation, electrolytic refining, zone refining, vapour phase refining (e.g., Mond's process for Nickel, Van Arkel method for Zirconium/Titanium), and chromatographic methods. Each method exploits differences in properties between the metal and its impurities.

Key Steps for Ore Concentration

1. Hydraulic Washing (Gravity Separation) — Based on the difference in specific gravities of the ore and the gangue particles. Heavier ore particles settle, while lighter gangue particles are washed away by a stream of water. Used for oxide ores like haematite (iron ore) and tin stone.
2. Magnetic Separation — Used when either the ore or the gangue is magnetic. The crushed ore is passed over a magnetic roller. Magnetic particles are attracted to the roller and form a separate heap, while non-magnetic particles fall off earlier. Used for magnetite (iron ore), chromite, pyrolusite, and cassiterite.
3. Froth Flotation Method — Primarily used for sulfide ores (e.g., galena, sphalerite). Based on the preferential wetting of ore particles by oil (e.g., pine oil) and gangue particles by water. Air is blown through the mixture, creating froth that carries the oil-wetted ore particles to the surface, while gangue settles at the bottom. Depressants (e.g., NaCN for ZnS-PbS ore) can be used to prevent one sulfide from forming froth.
4. Leaching (Chemical Method) — The ore is treated with a suitable reagent that selectively dissolves the ore, leaving impurities behind. The metal is then recovered from the solution by precipitation or reduction. Examples: Leaching of bauxite for alumina (Baeyer's process) using NaOH, leaching of gold and silver ores using NaCN solution.

Thermodynamic Principles of Metallurgy: Ellingham Diagram

The Ellingham Diagram is a graphical representation of the standard Gibbs free energy change (ΔG°) for the formation of metal oxides as a function of temperature. It provides a powerful tool for predicting the feasibility of reduction reactions in metallurgy. The fundamental principle governing reduction is that a metal can reduce the oxide of another metal if the Gibbs free energy change (ΔG) for the overall reduction reaction is negative.

Key features and interpretations of Ellingham Diagrams:

• Slope of lines: Most lines for metal oxides have a positive slope because ΔS for the formation of metal oxides (M + O₂ → MOₓ) is negative (gas to solid/liquid). However, the line for C → CO has a negative slope, and C → CO₂ has almost zero slope. This explains why carbon monoxide (CO) is a good reducing agent at lower temperatures, and carbon (C) is effective at higher temperatures.
• Relative positions of lines: A metal can reduce the oxide of another metal if its ΔG° line for oxide formation lies below the ΔG° line of the oxide to be reduced. This indicates that the former metal's oxide is more stable (has a more negative ΔG° of formation) than the latter at that temperature.
• Cross-over points: Where two lines cross, the relative stability of the oxides changes. Below the crossover point, the oxide whose line is lower is more stable. Above the crossover, the other oxide becomes more stable, meaning the metal from the lower line can reduce the metal from the upper line.

For example, the ΔG° line for the formation of CO from carbon lies below the lines for Fe₂O₃ and CuO at certain temperatures. This indicates that carbon (or CO) can reduce these metal oxides effectively. Similarly, the reduction of Al₂O₃ by carbon is difficult because the line for Al₂O₃ is much lower than that of C→CO, meaning a very high temperature (electrolytic reduction is preferred for Al).

Worked Examples in Metal Extraction

• {"title":"Extraction of Iron (Blast Furnace)","bodyMarkdown":"Reduction Reactions:\n1. Lower temperature zone (500-800 K): Fe₂O₃ + 3CO → 2Fe + 3CO₂ (CO acts as a reducing agent)\n2. Higher temperature zone (900-1500 K): Fe₂O₃ + 3C → 2Fe + 3CO (C acts as a reducing agent)\n3. Flux action: CaCO₃ → CaO + CO₂; CaO + SiO₂ (gangue) → CaSiO₃ (slag)"}
• {"title":"Extraction of Copper (Self-reduction)","bodyMarkdown":"Roasting of Copper Glance (Cu₂S):\n2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂\n\nSelf-reduction:\n2Cu₂O + Cu₂S → 6Cu + SO₂\n(Here, Cu₂S acts as a reducing agent for Cu₂O. This is 'auto-reduction' or 'self-reduction'.)"}
• {"title":"Electrolytic Refining of Copper","bodyMarkdown":"Anode: Impure Copper (oxidizes: Cu → Cu²⁺ + 2e⁻)\nCathode: Thin strip of pure Copper (reduces: Cu²⁺ + 2e⁻ → Cu)\nElectrolyte: Acidified CuSO₄ solution.\nImpurities more active than Cu (e.g., Zn, Fe) go into solution as ions. Less active impurities (e.g., Ag, Au, Pt) settle as anode mud."}

Key Points to Remember

• All ores are minerals, but all minerals are not ores. Ores are minerals from which metal can be profitably extracted.
• Froth flotation is suitable for sulfide ores due to their preferential wetting by oil.
• Calcination occurs in the absence of air and removes volatile matter (CO₂, H₂O), while roasting occurs in the presence of air and converts sulfide ores to oxides (produces SO₂).
• Ellingham Diagram helps predict the feasibility of reduction; a metal can reduce an oxide if its ΔG° line lies below the oxide's line.
• Carbon is a good reducing agent at high temperatures, whereas CO is effective at lower temperatures for iron oxides.
• Electrolytic reduction is used for highly reactive metals (e.g., Na, Mg, Al) that cannot be reduced by carbon.
• Zone refining is used for ultra-pure metals (e.g., Si, Ge) and is based on the principle that impurities are more soluble in the molten state than in the solid state.
• Mond's process (Ni) and Van Arkel method (Zr, Ti) are examples of vapour phase refining, forming volatile compounds that decompose on heating.
• Leaching is a chemical method of concentration where the ore dissolves, and impurities do not (e.g., Al from bauxite using NaOH).

Exam Tip: Ellingham Diagram Interpretation

When studying the Ellingham Diagram, pay close attention to the slopes of the lines and their relative positions. A positive slope implies that entropy decreases with oxide formation, favoring higher temperatures for reduction. The crossover points are critical as they indicate the temperature at which the reducing agent becomes more effective. Remember, a more negative ΔG° of formation implies a more stable oxide. For reduction, you need a reaction with a more negative overall ΔG, which often means pairing a reducing agent whose oxide formation line is below the metal oxide you want to reduce. Practice drawing qualitative diagrams for common reductions like iron by carbon or CO.

Practice Questions with Solutions

• Q: Why is froth flotation method preferred for sulfide ores? A: Sulfide ores have a higher affinity for oil and are preferentially wetted by oil, while gangue particles are wetted by water. This difference allows the sulfide particles to be carried with the froth, separating them from the gangue.
• Q: What is the main principle behind the Ellingham diagram for predicting reduction feasibility? A: It's based on Gibbs free energy change (ΔG = ΔH - TΔS). For a reduction to be spontaneous, the overall ΔG for the coupled reaction (oxidation of reductant + reduction of metal oxide) must be negative. A reductant's oxide formation line must lie below that of the metal oxide to be reduced on the diagram.
• Q: Name two metals purified by vapour phase refining methods and their respective processes. A: Nickel (Ni) is purified by Mond's process, and Zirconium (Zr) and Titanium (Ti) are purified by Van Arkel method.
• Q: What is the role of 'flux' in the extraction of iron? A: In the extraction of iron, limestone (CaCO₃) acts as a flux. It decomposes to form CaO, which then combines with the acidic gangue (like SiO₂) to form fusible slag (CaSiO₃), removing impurities from the molten iron.

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