Question
What are the four major methods of ore concentration? For each method, explain the principle and give examples of ores where it is used. How do you decide which concentration method to use for a given ore?
(JEE Main + CBSE Board pattern)
Solution — Step by Step
| Method | Principle | Best For | Example Ore |
|---|---|---|---|
| Gravity separation (hydraulic washing) | Difference in density between ore and gangue | Heavy oxide ores | Tin ore (cassiterite, SnO₂), iron ore (haematite) |
| Magnetic separation | One component is magnetic, the other is not | Magnetic ores or magnetic gangue | Chromite (magnetic) from siliceous gangue; tin ore from wolframite (magnetic gangue) |
| Froth flotation | Difference in wettability — sulphide ores are hydrophobic, gangue is hydrophilic | Sulphide ores | Copper pyrite (CuFeS₂), galena (PbS), zinc blende (ZnS) |
| Chemical leaching | Selective dissolution of ore in a chemical reagent | Specific ores where physical methods fail | Bauxite (Bayer process with NaOH), gold (cyanide leaching), silver |
This is the most frequently tested method:
- Crushed ore + water + pine oil (frothing agent) + collectors (xanthates) in a tank
- Air is blown through — creates froth
- Sulphide ore particles are hydrophobic — they attach to air bubbles and rise with the froth
- Gangue particles are hydrophilic — they get wet and sink
- Froth with ore is skimmed off and collected
Depressants can selectively separate two sulphide ores. Example: NaCN depresses ZnS while PbS floats — allowing separation of galena from zinc blende.
To choose the right method, ask:
- Is the ore a sulphide? → Froth flotation
- Is there a density difference? → Gravity separation
- Is the ore or gangue magnetic? → Magnetic separation
- Can the ore be selectively dissolved? → Chemical leaching
graph TD
A["Ore to Concentrate"] --> B{"Sulphide ore?"}
B -->|Yes| C["Froth Flotation"]
B -->|No| D{"Density difference significant?"}
D -->|Yes| E["Gravity Separation"]
D -->|No| F{"Magnetic component?"}
F -->|Yes| G["Magnetic Separation"]
F -->|No| H["Chemical Leaching"]
C --> C1["CuFeS2, PbS, ZnS"]
E --> E1["SnO2, Fe2O3"]
H --> H1["Al2O3 Bauxite, Au"]
style A fill:#fbbf24,stroke:#000,stroke-width:2px
style C fill:#86efac,stroke:#000Why This Works
Each concentration method exploits a specific physical or chemical property that differs between the ore and the gangue. Gravity uses density. Magnetic separation uses magnetic susceptibility. Froth flotation uses surface wettability (hydrophobic vs hydrophilic). Leaching uses chemical solubility. The choice depends on which property shows the biggest difference for the ore in question.
Common Mistake
Students often apply froth flotation to oxide ores. Froth flotation works primarily for sulphide ores because sulphide minerals are naturally hydrophobic (they do not get wet easily). Oxide ores are hydrophilic and would sink with the gangue. To use froth flotation for oxide ores, you would first need to convert them to sulphides using Na₂S — but this is rare and usually not tested.
For JEE: know that depressants in froth flotation can selectively separate mixed sulphide ores. NaCN depresses ZnS (by forming Na₂[Zn(CN)₄] on surface) while allowing PbS to float. This selective flotation concept is a JEE Main favourite.