Concentration of Ores: Froth Flotation and More | Class 10

Quick answer: Ores dug from the earth contain unwanted impurities such as sand, soil and rock, called gangue. Removing the gangue is called concentration (or enrichment) of the ore. The method depends on how the ore and gangue differ: hydraulic washing (difference in density), magnetic separation (one is magnetic), froth flotation (sulphide ores are wetted by oil, gangue by water) and leaching (the ore dissolves in a chemical and the gangue does not).

Ores taken from the earth are never pure. They contain large amounts of unwanted material such as soil, sand and rock, called gangue. Heating this raw material directly to extract the metal would waste a lot of fuel and energy.

So before the metal is extracted, the gangue is removed. This first step is called concentration of the ore.

In CBSE Class 10 Science, Chapter 3 (Metals and Non-metals), you need to know what gangue is and that it is removed by separation techniques based on differences in the physical or chemical properties of the ore and the gangue.

Syllabus note: The NCERT Class 10 textbook only states that gangue is removed using separation techniques based on differences in physical or chemical properties. The detailed methods below (hydraulic washing, magnetic separation, froth flotation and leaching) go beyond the Class 10 textbook. They are useful background and help with higher-level questions.

What You Will Learn

• What gangue and concentration of ore mean
• How the choice of method depends on the properties of the ore and the gangue
• Hydraulic washing (gravity separation)
• Magnetic separation
• Froth flotation for sulphide ores
• Leaching, with the purification of bauxite as an example

1. Gangue and Concentration of Ore

Gangue: the unwanted impurities, such as soil, sand and rock, that are mixed with an ore.

Concentration (enrichment) of ore: the removal of gangue from an ore, to increase the proportion of the useful mineral before the metal is extracted.

2. Hydraulic Washing (Gravity Separation)

Principle: the difference in density between heavy ore particles and lighter gangue particles.

How it works:

1. The powdered ore is washed with a stream of running water, often on a sloping or shaking table.
2. The lighter gangue particles are carried away by the water.
3. The heavier ore particles are left behind.

Used for: heavy ores such as some oxide ores, for example tin stone (cassiterite).

3. Magnetic Separation

Principle: the difference in magnetic properties of the ore and the gangue.

How it works:

1. The powdered ore is dropped onto a moving belt that passes over a magnetic roller.
2. Non-magnetic particles fall straight off the end of the belt.
3. Magnetic particles are attracted by the roller, stay on the belt a little longer and fall into a separate pile.

Examples:

• Separating magnetic iron ore (magnetite, $\mathrm{Fe_3O_4}$) from non-magnetic gangue.
• Removing magnetic wolframite impurities from tin stone.

4. Froth Flotation (for Sulphide Ores)

Froth flotation is used mainly for sulphide ores, such as zinc blende ($\mathrm{ZnS}$), galena ($\mathrm{PbS}$) and copper pyrites ($\mathrm{CuFeS_2}$).

Principle: Sulphide ore particles are wetted by oil, while gangue particles are wetted by water.

How it works:

Step 1: The finely powdered ore is mixed with water in a tank.

Step 2: An oil such as pine oil is added. Other chemicals called collectors may be added to make the ore particles repel water even more, and froth stabilisers help the froth last.

Step 3: Air is blown through the mixture and it is stirred, forming a froth.

Step 4: The ore particles, coated with oil, stick to the air bubbles and rise to the top in the froth. The froth is skimmed off and dried to get the concentrated ore.

Step 5: The gangue, wetted by water, sinks to the bottom.

5. Leaching (Chemical Separation)

Leaching is a method of concentration in which the powdered ore is treated with a chemical that dissolves the useful mineral but not the gangue.

Example: Purifying Bauxite

Bauxite (an ore of aluminium, mainly hydrated aluminium oxide) contains impurities such as iron oxide and silica.

Step 1: Powdered bauxite is heated with a concentrated solution of sodium hydroxide. The aluminium oxide dissolves to form soluble sodium aluminate, while impurities such as iron oxide do not dissolve and are filtered off.

$\mathrm{Al_2O_3 + 2NaOH + 3H_2O \longrightarrow 2Na[Al(OH)_4]}$

Step 2: Carbon dioxide is passed through the solution, which precipitates aluminium hydroxide.

$\mathrm{2Na[Al(OH)_4] + CO_2 \longrightarrow 2Al(OH)_3 + Na_2CO_3 + H_2O}$

Step 3: The aluminium hydroxide is filtered and strongly heated to give pure aluminium oxide (alumina).

$\mathrm{2Al(OH)_3 \xrightarrow{\text{heat}} Al_2O_3 + 3H_2O}$

This pure alumina is then used to extract aluminium by electrolysis.

6. Summary Table

7. Exam Tips and Common Mistakes

Exam Tip: If asked which method is used to concentrate sulphide ores, answer froth flotation, and mention that the ore is wetted by oil while the gangue is wetted by water.

Exam Tip: Always define gangue in your answer: the unwanted impurities present in an ore.

Common Mistake: Confusing concentration of ore with extraction of the metal. Concentration only removes the gangue; the metal is extracted in later steps.

Common Mistake: Saying the gangue floats in froth flotation. It is the ore that rises with the froth; the gangue sinks.

Frequently Asked Questions

What is gangue?

Gangue is the unwanted material, such as soil, sand and rock, that is mixed with an ore.

What is meant by concentration of ore?

It is the removal of gangue from an ore to increase the proportion of the useful mineral before the metal is extracted.

Which method is used for the concentration of sulphide ores?

Froth flotation, because sulphide ore particles are wetted by oil while gangue particles are wetted by water.

What is the principle of magnetic separation?

It uses the difference in magnetic properties: magnetic particles are pulled aside by a magnet, while non-magnetic particles fall away.

What is leaching?

Leaching is the treatment of an ore with a chemical that dissolves the useful mineral but not the gangue, as in the purification of bauxite with sodium hydroxide.