Concentration

Ore is a impure metal containing large amount of sand and rocky material. The impurities like sand, rocky materials, limestone, mica etc present in the ore is called gangue or matrix. The substance added flux to the ore to remove the matrix results in the formation of a fusible compound called slag. When the matrix is removed we get the concentrated ore. At first the ore is crushed to powder. The pulverized ore is separated by hydraulic washing, froth-floatation process, magnetic or chemical process, depending on the nature of the ore and its impurities. Concentration of the ore is also known as dressing of the ore or enrichment of ore.

Concentration of an ore is the process of removal of impurities from the ore.

This process is based on the difference in the physical or chemical properties between the ore and gangue particles.

Physical Methods of Concentration

Hydraulic Washing (Hydraulic Classifier)

When the ore particles are poured over a vibrating sloped table with grooves and a jet of water is allowed to flow over it, the denser ore particles settle in the grooves while the lighter gangue particles are washed away.

Principle

Difference in the densities of the ore and gangue particles.

Used for

Concentration of heavy oxide ores of lead, tin, iron etc.

Process

Hydraulic Washing

Powdered ore is spread over a sloping wooden platform which vibrates continuously.

Small wedge shaped obstacles are fixed on the platform. A strong jet of water is passed over the platform. Lighter gangue particles are washed away while the heavier ore particles are left behind.

Froth Floatation

Principle

Difference in the wetting properties of the ore and gangue particles.

Used for

Sulphide ores only.

Process

Froth Floatation

• A mixture of water, pine oil, detergent and powdered ore is taken in a tank

• A blast of compressed air is blown through the pipe of a rotating agitator to produce froth

• Sulphide ore particles wetted and coated by pine oil rise up along with the froth (froth being lighter)

• Gangue particles wetted by water sink to the bottom of the tank (water being heavier)

• Froth containing the sulphide ore is transferred to another container, washed, and dried.

Note : Sulphide being more electronegative attracts the covalent oil molecules. The gangue being less electronegative is attracted by the water.

Magnetic Separation

Principle

Difference in the magnetic properties of the ore and gangue particles.

Used for

Magnetic ores like pyrolusite (MnO₂) and chromite (FeO.Cr₂O₃)

Process

Magnetic Separation

• Powdered ore is dropped over a conveyor belt running over two rollers, one of which is magnetic.

• The magnetic ore particles get attracted to the magnetic roller and run along with the conveyor belt for a little longer than the non magnetic gangue.

• Gangue particles drop down first forming a heap.

• Then, the magnetic ore particles drop down forming a separate heap.

• Thus, two separate heaps of ore and gangue particles are formed.