Sieving

Sieving - Advantages and Disadvantages of Sieving

Sieving is done by using a device to separate different sizes of materials. The material is subjected to a mechanical force of vertical and horizontal movement, with the help of container of mesh or perforated bottom through which the material is shaken or poured.

In the day to day life, there are times when we need to separate a useful substance from a mixture. This is done by using various methods of which one of them is by sieving

In cooking, a sifter is used to divide and break up clusters in dry ingredients such as flour, as well as to combine and ventilate them. A strainer is a form of sieve used to separate solids from the liquid. An example of a sieve in use in the kitchen in everyday life is a tea filter, which is used to separate tea leaves from water or a pepper grinder and shaker, which allows a certain range of particles of pepper to pass through and fall on the egg and toast. Sieves work because large particles cannot pass through the holes in a sieve, but small particles can. Sieving is often used in preparing food, but it also has other uses.
Low investment cost, high accuracy, and ease of handling make sieve analysis a commonly used procedure for determining the particle size.

Sieving should not be confused with Winnowing. We must understand that winnowing is the separation of solid-solid separated by size and weight whereas sieving is the separation solid-liquid judged by shape and size. Winnowing is used to separate heavier and lighter components of a mixture by wind or blowing air.

Example: -separation of heavier grain particles from lighter husk particles using a winnow.

In the scientific terms, Sieving means an instrument with a perforated or meshed bottom, used for untying coarse from fine parts of loose matter, for straining liquids, etc., particularly one with fine meshes or perforations and circular frame.
A real-life example of a sieve is a tea strainer, which is used to separate tea leaves from the water. The sieve prevents the large particles which cannot pass through the holes in the sieve and retains them on the sieve.

In Civil Engineering, a sieve analysis (or gradation test) is a practice or procedure aided by vibration is used to assess the particle size distribution (also called gradation) of a granular material by allowing the material to pass through a series of sieves of progressively smaller mesh size and weighing the amount of material that is retained by each sieve as a fraction of the whole mass.

Sieving in Civil Engineering is whereby a graduation test is performed on a sample of aggregate (granulated matter) in a laboratory. A distinctive sieve study includes a nested column of sieves with wire mesh cloth or screen. The weight of each sieve sample is then divided by the total weight to give a percentage maintained on each sieve.

There are two types of sieving, Dry Sieving and Wet Sieving and it can be done mechanically or manually depending upon the quantity and size of the particle that must be separated.

Dry Sieving is the preferred method for particle size range between 40┬Ám and 125mm.

If dry sieving cannot produce an acceptable degree of separation between the individual fraction, then Wet Sieving will be preferred to get better results.

Factors in Sieving

Many factors need to be taken into consideration in selecting the appropriate method of sieving that must be applied
Depending upon

1. The type of particles,
2. The number of particles and
3. The size and density of particles and material that need to be separated.

Different types of sieves with different size of holes are used to separate the course from the fine aggregates.

Applications of various types of sieves

Half Height Sieves: This is used when the quantity of the material is to be sieved small.

Microplate Sieves: This is used when the material is of very fine particle size.

Wet Washing Sieves: The material is washed and agitated with enough water to separate the fines. This is in the ranges of 20 microns

Extra Depth Sieves: This is used for the large quantity of the material, is used extensively by the construction industry.

Air Jet Sieves: This is used where the sample is dispersed by an air blown out of a rotating nozzle. The measuring range may be extended to ten microns

Grain Sieves: These sieves are used worldwide for testing grains and cereals.

Advantages of Sieving

1. It does not require much skill and is a traditional method used for separation
2. It is a very cheap method as we do not need the installation of sophisticated equipment
3. Takes lesser time in comparison to other ways

Disadvantages of Sieving

1. Since the efficiency of this method is dependent on having a different particle size, it cannot be used to separate a mixture which contains particles of the same size say flour and chalk powder.

2. It is practically impossible to use this for measuring sprays or emulsions and cohesive as well as agglomerated materials such as clay.