Concentration of Ores
Concentration of Ores
Ores are naturally occurring rocks that contain metal or metal compounds in sufficient amounts to make it worthwhile extracting them. However, most of the ores are not pure, they are contaminated with a large amount of impurities such as soil, sand, etc., which are collectively known as gangue. The process of removal of gangue from the ore is technically known as concentration or ore dressing and the purified ore is called concentrated ore.
Methods of Concentration
The concentration of ore can be done by various methods which depend on the physical and chemical properties of the ore, the gangue particles, and the separation techniques. Below are some of the common methods of ore concentration:
1. Hydraulic Washing (Gravity Separation)
In this method, the light (low specific gravity) and fine gangue particles are washed away by a stream of water, leaving behind the heavier metals. This is based on the differences in gravities of the ore and the gangue particles.
2. Magnetic Separation
This method is used when one of the components of the mixture is magnetic. The magnetic material is separated from the non-magnetic material by the use of a magnetic separator.
3. Froth Flotation
This method is used for the concentration of sulphide ores. The ore is powdered and a suspension is created in water. To this are added, Collectors and Froth Stabilizers. Collectors (pine oils, fatty acids, etc) enhance the non-wettability of the mineral particles and froth stabilizers (cresols, aniline) stabilize the froth. The mineral particles become wet by oils while the gangue particles by water.
4. Leaching
In this method, the ore is treated with a suitable reagent that dissolves the ore but not the impurities. The solution containing the metal is collected, which is later extracted by various chemical methods.
Table of Differences
| Method | Principle | Used for | Advantages | Disadvantages |
|---|---|---|---|---|
| Hydraulic Washing | Gravity Separation | Oxide and carbonate ores | Simple and cost-effective | Not suitable for fine particles |
| Magnetic Separation | Magnetic properties | Iron ores, chromite, cassiterite | Effective for magnetic materials | Not applicable to non-magnetic ores |
| Froth Flotation | Differential wetting properties | Sulphide ores | High efficiency, can treat fine particles | Requires chemicals, complex process |
| Leaching | Chemical reaction | Bauxite (Aluminium), Gold | Can extract metals at low concentrations | Requires specific conditions, can be slow |
Examples
Example 1: Hydraulic Washing
An example of hydraulic washing is the washing of bauxite (Aluminium ore) to remove the clay and silica impurities.
Example 2: Magnetic Separation
An example of magnetic separation is the separation of iron from its ore. The crushed iron ore is passed over a conveyor belt with a magnet above it.
Example 3: Froth Flotation
An example of froth flotation is the concentration of copper sulphide (CuFeS2). The ore is mixed with water and chemicals to form a slurry, air is bubbled through the mixture, and the copper sulphide particles attach to the bubbles and float to the surface.
Example 4: Leaching
An example of leaching is the extraction of gold from its ore using cyanide solution (NaCN or KCN). The reaction can be represented as:
$$ 4 \text{Au} + 8 \text{NaCN} + 2\text{H}_2\text{O} + \text{O}_2 \rightarrow 4 \text{Na}[ \text{Au(CN)}_2] + 4 \text{NaOH} $$
Conclusion
The concentration of ores is a vital step in the process of metal extraction. It helps in removing the impurities and improving the quality of the ore for further processing. The choice of the concentration method depends on the nature of the ore and the required purity of the final product. Each method has its own advantages and limitations, and they are chosen based on the specific properties of the ore and the desired outcome.