Sieving: Principles, Mechanisms, Applications & Uses

Sieving is one of the most common and easiest separation methods. In our day-to-day life, many of the things which we use exist as a mixture of two or more pure substances or elements. To separate these pure substances from the mixture we use many techniques and methods in order to use them individually. 

For instance, we don’t use the flour bought from the market directly for making chapati because the flour comes with many impure substances like husk and stones so we use certain separation methods for separating the flour from the impurities attached to it before using it for the final consumption. Sieving is the effective method to separate the tiniest of impurities from pure substances. 

Read more: Handpicking 

Key Terms: Sieve, Impure particles, Separation methods, Strainer, Segregation, Fine particles, Particle size

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What is Sieving?

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A sieve is a device for removing the wanted elements from the material by separating the unwanted elements from the material. It is also known as sifting or fine mesh strainer, the word "sift" is originated and derived from "sieve".  In simple words, sieving refers to the process of separating and segregating the fine particles from the larger particles. 

• This method of separation is used commonly in flour mills where it is required to remove the impurities like stones and husks from wheat. 
• Sieving is also used in our daily home activities when we separate the fine flour particles from the impurities. 
• Also, a sifter is used to break and separate the clumps in dry ingredients like flour, as well as to aerate and combine them during cooking. 
• A strainer is also used for sieving which is used to separate the suspended elements from a liquid through filtration.

There are different forms of sieves used according to the size and form of particles. A sieve that is used for sifting flour contains very small holes. While the coarse particles are broken or separated by grinding against one another and the screen openings. Thus, depending upon the various types of particles, sieves with different types and sizes of holes are used. Not only in cooking but the sieves are also used for separating the stones from sand. Sieving plays a significant role in various industries such as food-producing industries where different forms of sieves are used for the different types of material.

Sieving process

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Mechanism of Separation

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Sieving can be used only for particles with specified sizes. These particles can pass through sieve screens, which have perforated or blended bottoms. While the size of mesh might vary depending on where they are used. Sieving that is used to separate the husk from wheat is in the form of sieve plates with a larger mesh size than those used for flour separation. Therefore, it is a technique for separating particles based on particle size.

Read more: Distillation

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Principle of Sieving

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During sifting, depending on the method used for sieving, the material is moved horizontally or vertically during the process. As a result, the sieve and particles move relative to each other. The selection of a method is determined by the status of dispersion of the sample. The estimation of particle size by sieve analysis is considered to be the oldest and most well-known approach. The volume or mass of the material determines the size of the particle.

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Application of Various Types of Sieving

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There are different types of sieves with various sizes of holes used to separate the fine particles from the mixture of material or things. These varied types include:

1. Vibratory sieving:

The material is propelled upwards by the vibrations of the sieve’s bottom and then the material falls back down due to the forces of gravity. The amplitude shows the vertical vibration elevation of the sieve bottom. In this process as a result of this simultaneous effect, the material is evenly distributed across the entire sieve area. The particles of the material are vertically transported, spin freely, and then return to their statistical orientation.

2. Horizontal sieving:

In this type of sieving, the material or particles are rotating in horizontal rings in a plane in a horizontal sieve shaker. During this process, as a result of the horizontal sifting motion, few particles change their alignment on the filter.

3. Tap sieving:

It is a type of sieving in which a vertical motion is produced by a tapping force which is overlaid on a horizontal movement in a tap sieve shaker. As a result, sieving times are longer. Due to the tapping motion, the particles move with a stronger impulse. The fraction of small particles in specific materials, which is usually higher. While on the other hand, the fraction of tiny particles in light materials like flour is lower.

4. Air jet sieving:

It is a single-sieve sieving machine, which implies that only one sieve is required for each procedure of sieving. In this process, a vacuum cleaner is linked to the sieving tool or equipment, which produces a vacuum in the chamber and draws in fresh air through a revolving nozzle. The increased air stream is blown against the sieve and passes through the tiny slit of the nozzle which scatters the particles.

5. Half Height Sieves:

This form of sieving is used when the quantity of the material is to be sieved small.

6. Microplate Sieves:

This form of sieving is used when the material is in the form of very fine particle size.

7. Wet Washing Sieves:

In this type of sieving, the material is washed and then agitated with water to separate the fine particles. It ranges to about 20 microns.

8. Extra Depth Sieves:

This is used for sieving the material in large quantities. This form of sieving is extensively used by the construction industry.

9. Grain Sieves:

These are worldwide sieving methods used for testing grains and cereals.

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Advantages and Disadvantages of Sieving

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The sieving method of separation has both advantages and disadvantages. Some of them are mentioned below: 

Advantages:

Some of the prominent advantages of sieving are as follows:

• The sieving method of separation takes less time in comparison to other methods.
• It is considered to be the most traditional approach used for separation, which does not require any kind of specific skill.
• The sieving technique is very cost-efficient because it does not require the cost of installation and maintenance.
• It is a convenient and user-friendly method used in construction sites or in grain mills.
• The results of sieved particle sizing are accurate and reproducible.

Disadvantages:

Some of the disadvantages of sieving include as follows:

• This method is efficient only for the material having a different particle size, and it cannot be used to separate a mixture that contains particles of the same size such as the mixture of flour and chalk powder.
• It can only be used for the separation of dry particles
• It cannot be practically used for measuring sprays or emulsions and materials such as clay.

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Things To Remember

• A sieve is a device for removing the wanted elements from the material by separating the unwanted elements from the material. 
• Sieving refers to the process of separating and segregating the fine particles from the larger particles. 
• Depending upon the various types of particles, sieves with different types and sizes of holes are used. 
• Sieving can be used only for particles with specified sizes.
• During sifting, depending on the method used for sieving, the material is moved horizontally or vertically during the process.