General Principles of Isolation of ElementsNEET Chemistry · Class 12 · NCERT Chapter 15

A mineral is any naturally occurring compound of a metal found in Earth's crust. An ore is a mineral from which the metal can be extracted profitably (economically). All ores are minerals, but not all minerals are ores. For example, both bauxite (Al₂O₃.2H₂O) and corundum (Al₂O₃) are minerals of aluminium. Bauxite is the ore used commercially because it has a high enough aluminium content and is easy to process. Corundum is too hard and too pure to be processed economically at scale. The key question that makes a mineral an ore is: can you get the metal out at a profit?

Froth flotation separates sulphide ore particles from gangue (waste rock) by using the difference in the ability of the two surfaces to be wetted by water (hydrophobicity). The ore powder is mixed with water and pine oil, and air is blown through the mixture to create froth. Pine oil (or other frother/collector combination) preferentially wets the sulphide ore particles and makes them hydrophobic (water-repelling). These hydrophobic particles attach to the air bubbles in the froth and float to the surface. The gangue (silicates, etc.) is hydrophilic (water-loving) and sinks to the bottom. The froth carrying the ore is skimmed off. Specific collectors like sodium ethyl xanthate increase the selectivity. Depressants like NaCN can suppress one sulphide (e.g. ZnS) while allowing another (e.g. PbS) to float, enabling selective separation of mixed sulphide ores.

The Ellingham diagram is a graph of standard Gibbs free energy of formation (ΔfG°) of metal oxides plotted against temperature. Each metal oxide has its own line. A line lower in the diagram means the oxide is more stable (more negative ΔfG°) and the metal is harder to extract. To decide if metal A can reduce the oxide of metal B: if the line for metal A lies below the line for metal B's oxide at a given temperature, then the reaction (A + BOx → AOx + B) is spontaneous (negative ΔG). In simpler terms, a metal whose oxide line is lower can reduce the oxide whose line is higher. Carbon is special: the line for 2C + O₂ → 2CO has a negative slope (goes downward with increasing temperature) because entropy increases sharply. At high enough temperature, the carbon line drops below almost all metal oxide lines, making carbon (coke) a universal reductant at high temperatures. The C + O₂ → CO₂ line is nearly horizontal (ΔS ≈ 0, same moles of gas on both sides).

Whether coke (carbon) can reduce a metal oxide depends on the Ellingham diagram: coke reduces the metal oxide only where the C→CO line lies below the metal oxide line (i.e., ΔG for the reduction becomes negative). For Fe₂O₃: the C→CO line crosses below the Fe₂O₃ line at around 700°C. Above this temperature, carbon spontaneously reduces Fe₂O₃ to Fe. For Al₂O₃: the Al₂O₃ line is far below the C→CO line at all practical temperatures (even above 1500°C). This means Al₂O₃ is more stable than any carbon oxide that would form. Carbon cannot reduce Al₂O₃ because the reduction is thermodynamically unfavourable at any achievable temperature. That is why aluminium must be extracted by electrolysis (Hall-Heroult process) instead. Similarly, MgO and CaO have very low Ellingham lines and cannot be reduced by carbon.

Zone refining is a method of purifying metals (especially semiconductors) to very high purity. A narrow molten zone is moved slowly from one end of an impure metal rod to the other. Impurities are more soluble in the molten phase than in the solid phase, so they concentrate in the molten zone and travel with it to one end of the rod. After several passes, the impurities accumulate at one end (which is cut off and discarded), leaving a very pure metal rod behind. Zone refining is used for semiconductors that need extremely high purity: germanium (Ge), silicon (Si), gallium (Ga), and indium (In). NEET frequently asks "which method is used to purify semiconductors?". The answer is zone refining.

Aluminium cannot be extracted by carbon reduction because Al₂O₃ is too stable (its Ellingham line is below the C→CO line at all practical temperatures). Instead, electrolytic reduction is used. The Hall-Heroult process works as follows: alumina (Al₂O₃) is dissolved in molten cryolite (Na₃AlF₆) at about 1000°C. Cryolite lowers the melting point of Al₂O₃ from 2045°C to around 950°C, making the process economically feasible. Small amounts of AlF₃ and CaF₂ are added to lower the melting point further and improve conductivity. Carbon (graphite) electrodes are used. At the cathode: Al³⁺ + 3e⁻ → Al (molten aluminium is deposited at the bottom of the cell). At the anode: 2O²⁻ → O₂ + 4e⁻ (oxygen is released, which reacts with the carbon anode and slowly burns it, so the carbon anode must be replaced periodically). The cell operates continuously; pure liquid aluminium is tapped from the bottom.

Both are pre-treatment steps that convert an ore to its oxide before reduction, but they apply to different types of ores. Roasting is heating a sulphide ore strongly in the presence of excess air. The sulphide is converted to an oxide: 2ZnS + 3O₂ → 2ZnO + 2SO₂. Roasting is used for sulphide ores (e.g. ZnS, CuFeS₂). The SO₂ produced is a by-product (used to make H₂SO₄). Calcination is heating a carbonate or hydroxide ore strongly in the absence or limited supply of air. The ore thermally decomposes to give the oxide: ZnCO₃ → ZnO + CO₂, and Al(OH)₃ → Al₂O₃ + H₂O. Calcination is used for carbonate ores (calamine ZnCO₃, siderite FeCO₃, malachite CuCO₃.Cu(OH)₂) and hydroxide ores (gibbsite Al(OH)₃ from bauxite). Memory hook: Roasting uses air (oxygen present), Calcination uses heat alone.

This is explained directly by the Ellingham diagram. The stability of a metal oxide is shown by how low its ΔfG° line sits on the diagram. MgO has a very low (very negative ΔfG°) line throughout the temperature range, lower than the C→CO line even at 2000°C. This means magnesium has a stronger affinity for oxygen than carbon does, so carbon cannot displace magnesium from MgO at any practical temperature. ZnO has a higher ΔfG° line. The C→2CO line (negative slope) crosses below the ZnO line at around 950°C. Above this temperature, carbon reduction of ZnO becomes spontaneous: ZnO + C → Zn + CO. Zinc is therefore extracted by carbon reduction (in a retort furnace) at about 1200°C followed by distillation to purify the product. For magnesium, electrolysis of molten MgCl₂ is used instead.