Concentration of Ores (Beneficiation)
Master the physical and chemical techniques used to remove gangue from pulverized ores.
Once an ore has been mined and pulverized into a fine powder, the very first task is to remove the unwanted earthy and rocky impurities (gangue). This process of increasing the percentage of the desired metal compound in the ore is called Concentration, Dressing, or Beneficiation.
1. Hydraulic Washing (Gravity Separation)
Principle: This method is based on the difference in specific gravities (densities) of the ore particles and the gangue particles.
Process: The powdered ore is washed with an upward stream of running water (often on a vibrating sloped table called a Wilfley table). The lighter gangue particles are washed away by the water stream, while the heavier ore particles settle down at the bottom.
Used For: Heavy oxide ores. Examples include Haematite ($Fe_2O_3$) and Tin stone / Cassiterite ($SnO_2$).
2. Magnetic Separation
Principle: This method relies on the difference in the magnetic properties of the ore components. If either the ore or the gangue (but not both) is attracted by a magnet, they can be separated.
Figure 1: Magnetic separation of ore and gangue on a conveyor belt.
Classic Example: Separation of non-magnetic Cassiterite / Tin stone ($SnO_2$) from its magnetic impurity Wolframite ($FeWO_4$).
3. Froth Floatation Method (Highest Yield for JEE)
Principle: This highly important method is based on the difference in the wetting characteristics of the ore and the gangue particles.
- The ore particles are wetted by oil.
- The gangue particles are wetted by water.
Used exclusively for: Sulphide Ores (e.g., Zinc blende $ZnS$, Galena $PbS$, Copper pyrites $CuFeS_2$).
Chemical Reagents Used in Froth Floatation
| Reagent Type | Examples | Function |
|---|---|---|
| Collectors | Pine oil, Fatty acids, Xanthates | Enhance the non-wettability of the mineral particles, allowing them to attach to air bubbles. |
| Froth Stabilizers | Cresols, Aniline | Stabilize the froth so it doesn't collapse before it can be skimmed off. |
| Depressants | $NaCN$ (Sodium Cyanide) | Used to separate a mixture of two sulphide ores by selectively preventing one from forming froth. |
When an ore contains both $ZnS$ and $PbS$, $NaCN$ is added as a depressant. It selectively forms a water-soluble complex with Zinc, preventing it from coming to the froth, while allowing $PbS$ to float.
Result: $PbS$ goes into the froth, $ZnS$ remains dissolved in the solution as Sodium tetracyanidozincate(II).
4. Leaching (Chemical Separation)
Principle: Leaching is a chemical method where the powdered ore is treated with a specific chemical reagent (acid, base, or other solvent) in which the ore is soluble, but the gangue is insoluble. The unreacted gangue is filtered out.
A. Leaching of Alumina from Bauxite (Bayer's Process)
Bauxite ore ($Al_2O_3 \cdot xH_2O$) often contains impurities like $SiO_2$, iron oxides, and $TiO_2$. The powdered ore is digested with a concentrated solution of NaOH at 473-523 K. Aluminum oxide dissolves forming Sodium aluminate, leaving impurities behind.
The aluminate in solution is neutralized by passing $CO_2$ gas to precipitate hydrated $Al_2O_3$, which is then filtered, dried, and heated to get pure Alumina.
B. Extraction of Gold & Silver (MacArthur-Forrest Cyanide Process)
Native ores of $Au$ and $Ag$ are leached with a dilute solution of $NaCN$ or $KCN$ in the presence of air ($O_2$). Oxygen acts as an oxidizing agent. The metal forms a soluble cyanide complex.
The pure metal is then recovered by displacement (hydrometallurgy) using a more electropositive metal like Zinc. Zinc acts as a reducing agent.
Mastery Check: Concentration of Ores
10 High-Yield Questions to test your JEE/NEET Preparation
🚀 Up Next: Extraction & Ellingham Diagrams
You've mastered removing the gangue. Now, learn how to convert the concentrated ore into metal oxides and reduce them using Thermodynamics!
Continue to Extraction of Crude Metal
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