Introduction | Separation Techniques

Definition of Separation techniques:

Separation techniques are physical methods used for separating the constituents of various mixtures. The techniques employed in separating a particular mixture depend on the physical properties of its constituents. These properties include;

• size of the particles
• solubility of the substances in solvents
• Boiling point
• density
• miscibility
• magnetic property
• sublimation
• rate of movement

Types of Separation Techniques:

The techniques are as follows:

Handpicking, sieving, winnowing, filtration, floatation, decantation, centrifugation, evaporation, distillation, fractional distillation, crystallization, fractional crystallization, magnetization, separation of immiscible liquid (Use of separating funnel), sublimation, precipitation, chromatography.

Hand-picking:

This method is used when the particles of the constituents of one of the substances are large enough to be picked with hands. For example-picking stones from rice, beans, etc.

Winnowing:

This method is best described by observing the groundnut seller when removing the light brownish chaff of the groundnut. The seller threshes the nut between his/her hands and blows off the chaff. The chaff will easily blow off by air because it is lighter than the nut. Other examples are blowing chaff from beans, guinea corn, etc.

Sieving:

This is used to separate solid particles of different sizes e.g separation of particles of garri, yam flour from unwanted coarse particles, etc. It is used in mining and also in garri industries.

Magnetization:

Magnetization is used to separate magnetic substances from non-magnetic substances.

Filtration:

This method is used for separating an insoluble solid from a liquid. 

Apparatus – funnel, filter paper, conical flask. For example, separating a mixture of chalk and water or sand and water.

On pouring a suspension of chalk or any insoluble liquid through a filter paper, the water passes through, leaving the insoluble solid (chalk particles) on the filter paper.

The insoluble solid on the filter paper is the residue while the liquid in the conical flask or beaker is the filtrate.

Industrial Application: In water purification plants and breweries and in the purification of pipe-borne water.

Decantation:

Decantation is also used for separating a mixture of an insoluble solid from a liquid. The mixture separates into two distinct layers on standing. A lower solid layer and an upper clear liquid layer. The top liquid portion is carefully poured, leaving the solid particles.

This method is less effective than filtration because it does not ensure total separation.

Centrifugation:

A centrifuge is a machine used to spin test tubes containing suspensions at high speeds. The spinning causes the heavier solid particles to be thrown to the bottom of the test tube, while the clear liquid remains at the upper layer and can be separated by decantation.

Centrifugation is used in hospitals for blood analysis during blood screening.

Floatation:

Floatation method is based on the wide differences in the densities of the constituents of the mixture.

Separation of constituents of mixtures of two solids in which one is heavier and the other lighter when these solids are mixed with liquid (water). The heavier particles sink to the bottom while the lighter one floats on the surface in the foamy suspension of air bubbles e.g. a mixture of coarse sand and wooden cork.

Floatation is used to separate an ore of metal from earthly impurities.

Evaporation:

Evaporation is used to recover a soluble solute from a solution. 

Apparatus – Tripod stand, Wire gauze, Bunsen burner, Beaker, Evaporating dish.

Example: solution of common salt and water.

Procedure:

Pour the solution into an evaporating dish. Heat gently until the solvent is completely evaporated leaving the residue in the evaporation dish.

Use of Water bath (Evaporation):

A water bath is used for steady evaporation. The solvent is evaporated by the heat of the steam given off from boiling water.

The solute (salt) is left behind in the dish, while the solvent escapes into the air as vapour.

Evaporation is used in salt-making industries.

Simple Distillation:

Distillation involves differences in the boiling points of liquids. This method is used to separate a mixture of miscible liquids whose boiling points are widely different e.g. mixture of water and ethanol (with boiling points of 100°C and 78°C respectively). It may also be used to recover a solvent from its solution e.g pure water from river water or seawater.

The process involves heating the mixture of liquids until the more volatile liquid (i.e. the one with the lower boiling point) changes to vapour.

The vapour is cooled by passing it through a condenser as it is collected in a liquid form as a distillate. The other liquid with a higher boiling point is left in the distillation flask.

Distillation is used by gin and water distilleries for the manufacture of gin and distilled water. 

The advantage of distillation over evaporation is that the solvent can eventually be collected.

Fractional Distillation:

This method is used to separate a mixture of miscible liquids with close but different boiling points.

This method of separation is the same as simple distillation. The only difference is that a fractionating column is introduced between the distillation flask and the condenser. As the mixture is boiling the fractions distil over in ascending order of their boiling points (i.e. the ones with lower boiling point distils first before the liquids with higher boiling point e.g. crude oil.

The functional distillation process continues until all the component fractions in the mixture distil over.

Fractional distillation is used in the separation of crude oil. Crude oil contains the following fractions – Petrol, kerosene, diesel oil, gas oil, bitumen, etc.