Metallurgy: minerals, ores, gangue, flux, slag and concentration of ores
An ore is a mineral that gives metal cheaply. First its gangue is removed by gravity, froth flotation, magnetism or leaching. Later, flux + gangue → slag.
Key terms
| Term | Meaning | Example |
|---|---|---|
| Mineral | A naturally occurring compound of a metal in the earth's crust | Clay (Al₂O₃·2SiO₂·2H₂O) is a mineral of aluminium |
| Ore | A mineral from which a metal can be extracted easily and profitably | Bauxite (Al₂O₃·2H₂O) is the ore of aluminium |
| Gangue (matrix) | Rocky and earthy impurities, such as sand, mixed with the ore | Silica, SiO₂ |
| Flux | A substance added to remove the gangue by turning it into a fusible (easily melted) product | CaO (from limestone) removes acidic gangue SiO₂; SiO₂ removes basic gangue FeO |
| Slag | The fusible product formed when flux reacts with gangue | CaO + SiO₂ → CaSiO₃; FeO + SiO₂ → FeSiO₃ |
| Smelting | Reducing the roasted ore to molten metal at a high temperature | Iron in a blast furnace |
Ores of aluminium, copper and iron
| Metal | Ores and formulae |
|---|---|
| Aluminium | Bauxite Al₂O₃·2H₂O, cryolite Na₃AlF₆, corundum Al₂O₃ |
| Copper | Copper pyrite CuFeS₂, copper glance Cu₂S, cuprite Cu₂O |
| Iron | Haematite Fe₂O₃, magnetite Fe₃O₄, iron pyrite FeS₂ |
Steps of metallurgy
Four ways to concentrate an ore
| Method | Principle | Used for |
|---|---|---|
| Gravity separation (hydraulic washing) | Ore particles are heavier than gangue. A stream of water washes the lighter gangue away. | Oxide ores: haematite (Fe₂O₃), tin stone (SnO₂) |
| Froth flotation | Powdered ore is mixed with water and pine oil. Ore particles are wetted by oil, gangue by water. Air blown in makes a froth that carries the ore to the top; gangue sinks. | Sulphide ores: copper pyrite (CuFeS₂), zinc blende (ZnS) |
| Magnetic separation | Crushed ore falls on a belt moving over a magnetic roller. The magnetic part falls close to the roller, the rest farther away. | Tin stone (non-magnetic) separated from wolframite (magnetic) |
| Leaching (chemical method) | The ore dissolves in a suitable solvent; the gangue does not. | Bauxite in sodium hydroxide solution; gold and silver in sodium cyanide solution |
How to draw the froth flotation cell
- 1Draw a tall rectangular tank, three-quarters full. Label it water + pine oil + powdered ore.
- 2Draw a pipe entering near the bottom. Label it compressed air.
- 3Draw a rotating paddle in the middle. Label it stirrer.
- 4Draw a bubbly layer on top with dots in it. Label it froth with ore particles.
- 5Draw a spout at the top edge. Label it froth (concentrated ore) out.
- 6Shade particles at the bottom. Label them gangue.
India mines bauxite in Odisha and iron ore in Odisha, Chhattisgarh and Karnataka. Every steel tumbler and aluminium vessel in your kitchen began as an ore that was concentrated first.
1 marks: “The ore concentrated by froth flotation is: (a) bauxite (b) haematite (c) zinc blende (d) magnetite” (c) zinc blende, a sulphide ore.
2 marks: “Differentiate between a mineral and an ore.” A mineral is a natural compound of a metal. An ore is a mineral from which the metal can be extracted easily and profitably. Bauxite is an ore of aluminium; clay is only a mineral.
7 marks: “What is concentration of an ore? Explain the four methods of concentration, with a diagram of froth flotation.” Define. Gravity separation, froth flotation (labelled diagram), magnetic separation and leaching: principle and one example each.
Heavy ore? Wash it. Sulphide? Froth it. Magnetic? Roll it. Soluble? Leach it.