Metallurgy
Metallurgy in the Tamil Nadu HSE Second Year syllabus traces the conversion of an ore into a pure metal. Questions usually test the purpose and equation for each step, the choice between roasting and calcination, and named refining processes.
Concentration and pretreatment
- Gravity separation works when ore and gangue differ in density; froth flotation is used for sulphide ores because the ore is wetted by oil while gangue is wetted by water.
- Leaching dissolves the ore selectively: bauxite in hot NaOH, argentite or gold in dilute NaCN.
- Roasting heats a sulphide in excess air to give an oxide and SO2. Calcination heats a carbonate or hydrate in limited air to give an oxide and CO2 or H2O.
Reduction and refining
- Carbon reduction is used for Fe, Zn and Pb; aluminothermy for Mn and Cr; electrolysis for Na and Al.
- Mond process purifies Ni through Ni(CO)4; van Arkel–de Boer purifies Ti or Zr through volatile iodides; zone refining purifies Si and Ge.
Worked example
A zinc sulphide ore is concentrated by froth flotation, then roasted. Why is flotation suitable, and what is the roasting equation? Reasoning: ZnS is a sulphide, so oil wets it and air bubbles lift it; silica gangue stays in water. Roasting in excess air gives ZnO: 2ZnS + 3O2 -> 2ZnO + 2SO2. Calcination would be the wrong label because the ore is a sulphide, not a carbonate.
Traps
- Slag is the product; flux is the substance added.
- Roasting and calcination both give oxides, but their reagents and by-products differ.
- Mond is specific to nickel; do not use it for copper.
- In zone refining the impurity concentrates at the end of the rod, not in the molten zone permanently.