Sieving is a fundamental technique in the field of process engineering and materials science, used for the separation and classification of solid particles. This process is based on the use of sieves or meshes with specific aperture sizes to separate particles according to their dimensions. Sieving methods are essential in various industries, including pharmaceutical, food, chemical, and mining, as they allow for obtaining products of uniform quality and ensuring production efficiency.
There are several sieving methods, which can be classified based on the mechanisms used for separation. Among the most common methods are manual sieving, vibratory sieving, and air-jet sieving. Each of these methods has specific applications and particular advantages depending on the characteristics of the material to be processed and the objective of the separation.
- Whether the test is performed dry or wet.
- Number of sieves and their apertures.
- Diameter of the sieves.
- Type of sieve and geometric shape (in case of using perforated plates).
Sieving can be performed both manually and mechanically. If sieve shakers are used, the results should be similar to those obtained manually.
- Manual Sieving A reciprocating motion is given to the sieving medium, ensuring that the largest possible amount of the sample crosses the sieve surface, in addition to giving it sharp taps. This facilitates the passage through the apertures of samples that have remained retained (either due to obstruction or size).
- Mechanical Sieving Performed through sieve shakers, commonly divided into 3 different groups:
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Electromagnetic sieve shaker. The vibration is generated by a coil (or an electromagnet), creating a three-dimensional motion that, besides offering vertical motion, also causes a rotational movement of the material over the sieve surface.
Pros: They are appropriate for materials with high density and wet tests.
Cons: Not recommended for samples with low density or high friability index.
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Orbital sieve shaker. Two-dimensional motion transmitted through a motor (but without vibration).
Pros: Recommended for cylindrical and low-density particles (such as cereal grains) and/or those with high friability, as the movement does not damage the particles.
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Tap sieve shaker. These combine an eccentric motion with vertical tapping every 2/3 revolutions.
Pros: It is the system most similar to manually performed sieving.
Cons: These are noisy devices that usually require an exclusive booth.
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Sonic Sieving. Sieve shakers where vibration is transmitted to the samples through low-frequency acoustic waves. Recommended for sieving materials down to 3µm (or difficult-to-sieve materials).
Pros: Fast system.
Cons: Low compatibility of the sieving medium.
- Friability. Some samples can reduce their size during sieving.
- Abrasiveness. Due to this property, samples can end up filing down the sieving medium, affecting the apertures during long sieving times.
- Hygroscopicity. Certain particles absorb moisture, so some samples must be sieved to have minimal contact with the atmosphere.
- Particle profile. Based on this property, results and duration may vary.
- Adherence. Particle spreading depends greatly on this property; the finer the particles, the greater the spreading.
- Magnetism. It can affect the results; it must be considered that some particles can react with each other (agglomeration) or react with the sieve (adhesion).
- Electrostatic properties. Particles can become electrically charged and stick to the sides of the sieve, altering the result.
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Chemical reactivity. Some particles may react due to the atmosphere or the material the sieve is made of.
