Ore concentration methods — gravity, magnetic, froth flotation, leaching selection

Question

How do you decide which concentration method to use for a given ore? Explain gravity separation, magnetic separation, froth flotation, and leaching with examples, and state the selection criteria for each.

Method Selection Decision Tree

flowchart TD
    A["Ore needs concentration"] --> B["Is the ore a sulphide?"]
    B -->|Yes| C["Froth Flotation"]
    B -->|No| D["Is the ore or gangue magnetic?"]
    D -->|Yes| E["Magnetic Separation"]
    D -->|No| F["Is there a large density difference?"]
    F -->|Yes| G["Gravity Separation / Hydraulic Washing"]
    F -->|No| H["Is the ore soluble in a specific solvent?"]
    H -->|Yes| I["Chemical Leaching"]
    C --> J["Example: ZnS, PbS, CuFeS2"]
    E --> K["Example: Chromite from silica, Magnetite"]
    G --> L["Example: Tin ore (cassiterite), Gold dust"]
    I --> M["Example: Bauxite (Bayer's), Gold (cyanide)"]

Solution — Step by Step

Principle: Difference in specific gravity between ore and gangue particles. Heavier ore particles settle faster in a stream of water while lighter gangue is washed away.

When to use: When the ore is significantly denser than the gangue. Works best with oxide ores and heavy metal ores.

Examples:

Cassiterite ( $\text{SnO}_2$ ) — tin ore, much denser than silica gangue
Gold dust — heavy gold particles separated from lighter sand
Haematite ( $\text{Fe}_2\text{O}_3$ ) in some cases

Method: Ore is washed on a vibrating, sloped table (Wilfley table) with running water. Heavy particles settle in grooves; light gangue flows away.

Principle: Difference in magnetic properties between ore and gangue. Either the ore or the gangue must be magnetic.

When to use: When one component is magnetic and the other is not.

Examples:

Chromite ( $\text{FeCr}_2\text{O}_4$ ) — magnetic ore separated from non-magnetic silica gangue
Magnetite ( $\text{Fe}_3\text{O}_4$ ) — strongly magnetic, separated from non-magnetic impurities
Rutile ( $\text{TiO}_2$ ) — non-magnetic ore separated from magnetic ilmenite impurity

Method: Crushed ore is dropped onto a conveyor belt that passes over a magnetic roller. Magnetic particles cling and fall close to the roller; non-magnetic particles fall farther away.

Principle: Difference in wettability of ore and gangue. Sulphide ore particles are naturally hydrophobic (not easily wetted by water), while gangue (silica, clay) is hydrophilic.

When to use: Specifically designed for sulphide ores. This is the most important concentration method for CBSE and JEE.

Examples: Galena ( $\text{PbS}$ ), Sphalerite ( $\text{ZnS}$ ), Chalcopyrite ( $\text{CuFeS}_2$ ), Copper pyrites.

Method: Crushed ore is mixed with water + pine oil (collector) + froth stabiliser. Air is blown in. Sulphide particles attach to oil-coated air bubbles and float to the top as froth. Gangue sinks.

Key chemicals:

Collectors (pine oil, xanthates) — enhance hydrophobicity of ore
Froth stabilisers (cresol) — stabilise the froth
Depressants (NaCN for ZnS) — selectively prevent one sulphide from floating when separating two sulphides

Principle: Selective dissolution of the ore in a suitable solvent, leaving gangue behind.

When to use: When the ore can dissolve in a specific chemical but the gangue cannot.

Examples:

Bauxite ( $\text{Al}_2\text{O}_3 \cdot 2\text{H}_2\text{O}$ ) — leached with concentrated NaOH (Bayer’s process). Alumina dissolves as sodium aluminate; impurities ( $\text{Fe}_2\text{O}_3$ , $\text{SiO}_2$ , $\text{TiO}_2$ ) remain.
Gold — leached with dilute NaCN solution in the presence of air (MacArthur-Forrest process): $4\text{Au} + 8\text{NaCN} + \text{O}_2 + 2\text{H}_2\text{O} \to 4\text{Na[Au(CN)}_2\text{]} + 4\text{NaOH}$

Why This Works

Each method exploits a specific physical or chemical property difference between the ore and gangue. The selection is logical: check sulphide (froth flotation), check magnetic properties (magnetic separation), check density (gravity), and if none work, try dissolving selectively (leaching). JEE and CBSE questions typically give you the ore and ask which method is used — this decision tree covers all cases.

For JEE Main, the most frequently tested topic within concentration is froth flotation — especially the role of depressants. Remember: NaCN depresses ZnS (forms $\text{Na}_2[\text{Zn(CN)}_4]$ on surface, making it hydrophilic) while PbS floats. This is how we separate galena from sphalerite.

Common Mistake

Students often assume froth flotation works for all ores. It is specifically for sulphide ores. Using froth flotation for an oxide ore like bauxite does not work because oxide particles are hydrophilic — they sink instead of floating. For oxide ores, use gravity separation, magnetic separation, or leaching depending on the specific case.

Question

Method Selection Decision Tree

Solution — Step by Step

Why This Works

Common Mistake

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