Hydraulic washing of the ore
Hydraulic washing is a type of gravity separation, which is based on gravity differences between the ore
and the gangue particles. In this process, the lighter gangue particles are washed away by a stream of water, leaving behind
the heavier ore.
Concentration of ore is a step involved in the process of extraction of metal from its ore. Unwanted rocks and grit are eliminated from mineral ore in this process. Hydraulic washing, froth floatation process, magnetic separation and chemical separation are some of the techniques used for concentrating or purification of ores.
Hydraulic washing:
Hydraulic washing process is done by washing the ores with streams of water. If an ore is heavier or denser than the gangue,
then the gangue particles are washed way with the stream. The heavier or denser ore particles remain behind and can be collected.
Hydraulic washing is done for ores that have tin or lead, as they are found to be heavier than the gangue.
Froth–floating process
The pulverized ore is treated with an oil detergent mixture, then stirred and aerated to create a froth that
separates the detergent coated mineral from the gangue. The mineral rich froth is collected for further processing.
Froth – floating process:
This process is used for sulfide ores. Sulfide ores are first ground to powder and water is added. Then pine oil is added and
the emulsion is agitated by passing compressed air. Oil and froth float on the surface along with the sulfide ore. The gangue
particles being insoluble in oil remain at the bottom of the water tank. The froth is removed and allowed to settle down. This is called the froth – floating process. This process is used for sulfide ores of Cu, Pb and Zn.
Magnetic separation
In this process the ore is pulverized and rolled on the magnetic roller. Magnetic particles are attracted
towards belt leaving behind the non magnetic particles.
Magnetic separation:
This method of concentration can be applied when the gangue and the ore particles have different magnetic properties. For example,
if the ore particles are magnetic in nature and if the gangue particles are non – magnetic, then a strong magnet can be
brought and the magnetic ore particles can be sucked out from the powdered ore. The powdered ore is poured over a conveyer belt.
One of the rollers of the belt is made out of magnet. The magnetic roller makes the magnetic ore particles stick on the belt and
these are moved at a distance before they are collected. The gangue particles, being non – magnetic in nature, do not get
attracted to the roller and fall in a heap below the roller itself. Iron ores like magnetite, chromite and manganese ore like
pyrolusite are concentrated by this process. Sometimes, a reverse situation may occur: the ore is non magnetic and the gangue is
magnetic. In this case also magnetic separation may be used for concentration of the non – magnetic ore.
Bayer's process
Bayers process is the most economic means of obtaining bauxite. Two to three tonnes of bauxite are
required to produce on tonne of alumina. Crude ore is treated with NaOH forming aluminate which on further treatment with HCl
produces alumina.
Chemical separation:
Different chemical affinity between the ore and the gangue is utilized for concentrating the ore. One example shown below will
make this process clear.
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Separation of Al from bauxite ore: This process is known as Bayer's process. Bauxite ore is reacted
with hot NaOH. A water soluble sodium aluminate is formed. The gangue is insoluble in NaOH as well as water. The sodium
aluminate formed is removed by adding water and filtering the solution.
- The solution containing sodium aluminate is treated with HCl. A precipitate of aluminum hydroxide is obtained.
The precipitate is separated and dried, then heated. Pure aluminum oxide is obtained. The oxide can be reduced to obtain pure Al.