Question
How do we decide which method to use for concentration, reduction, and refining of a metal ore? Create a selection guide based on the type of ore and metal reactivity.
(CBSE 12 + JEE Main)
Solution — Step by Step
| Ore type | Method | Principle |
|---|---|---|
| Oxide ore (heavy) | Gravity separation | Difference in density |
| Sulphide ore | Froth flotation | Difference in wettability |
| Magnetic ore () | Magnetic separation | Magnetic vs non-magnetic |
| Bauxite () | Leaching (Bayer’s process) | Selective dissolution in NaOH |
Froth flotation is the most asked — sulphide particles are hydrophobic (attach to froth), gangue is hydrophilic (sinks).
| Metal position in reactivity series | Reduction method |
|---|---|
| Highly reactive (Na, K, Al, Mg) | Electrolytic reduction |
| Moderately reactive (Fe, Zn, Cu) | Carbon reduction (smelting) |
| Low reactivity (Hg, Cu from sulphide) | Self-reduction / roasting |
| Very low reactivity (Au, Ag) | Found native or cyanide leaching |
The Ellingham diagram shows which oxide is more stable — a metal lower on the diagram can reduce the oxide of a metal higher up.
| Method | Used for | Principle |
|---|---|---|
| Distillation | Zn, Hg (low boiling point metals) | Boiling point difference |
| Liquation | Sn, Pb, Bi (low melting point) | Melting point difference |
| Electrolytic refining | Cu, Al, Zn | Impure anode dissolves, pure metal at cathode |
| Zone refining | Si, Ge, Ga (semiconductors) | Impurities more soluble in melt than solid |
| Van Arkel method | Ti, Zr (ultra-pure) | Decomposition of volatile compound |
flowchart TD
A["Given an ore"] --> B{"Type of ore?"}
B -- Sulphide --> C["Froth Flotation"]
B -- Oxide --> D["Gravity / Magnetic Separation"]
B -- Alumina --> E["Bayer's Leaching"]
C --> F["Roast to oxide"]
D --> F
E --> G["Electrolytic Reduction"]
F --> H{"Metal reactivity?"}
H -- High --> G
H -- Medium --> I["Carbon Reduction / Smelting"]
H -- Low --> J["Self-reduction"]
G --> K["Refining"]
I --> K
J --> K
K --> L{"Which refining method?"}
L -- "Cu, Al" --> M["Electrolytic Refining"]
L -- "Zn, Hg" --> N["Distillation"]
L -- "Si, Ge" --> O["Zone Refining"]Why This Works
Each step exploits a physical or chemical difference between the metal and its impurities. Concentration uses physical properties (density, magnetism, wettability). Reduction uses chemical reactivity — more reactive reducing agents are needed for more reactive metals. Refining uses differences in melting point, boiling point, or electrochemical potential.
The Ellingham diagram is the theoretical backbone — it shows that carbon becomes a better reducing agent at high temperatures (its line has a negative slope), which is why coke works for iron extraction but not for aluminium.
Alternative Method
For JEE MCQs on metallurgy, memorise these specific pairings: Cu is refined electrolytically (impure Cu anode, pure Cu cathode, electrolyte). Al is extracted by Hall-Heroult process (electrolysis of dissolved in cryolite). Fe is extracted in a blast furnace using coke. These three cover most questions.
Common Mistake
Students confuse roasting with calcination. Roasting is heating in AIR (for sulphide ores — ). Calcination is heating in LIMITED or NO air (for carbonate/hydroxide ores — ). The key difference is the presence of oxygen. If the ore contains sulphur, it is roasting. If it contains carbonate, it is calcination.