Sieving is a physical separation technique used to sort materials based on particle size. The process involves introducing a mixture of particles to a screen, mesh, or perforated plate. Agitation is then applied to encourage the material to interact with the openings, which controls the purity and consistency of materials.
How Particle Separation Occurs
The separation process relies on the geometry of the sieve’s mesh and the material’s movement. The mesh size, or aperture, defines the maximum dimension a particle can have to pass through the screen. Particles are mechanically separated into two fractions: the undersize, which passes through the openings, and the oversize, which is retained on the screen surface.
Successful separation requires agitation to continuously present all particles to the mesh openings. Movement methods include a three-dimensional throwing motion, which ensures uniform distribution, or a tapping motion, which helps dislodge stuck particles. Wet sieving uses a liquid medium to wash finer particles through the screen.
Separation is a probabilistic event; a particle smaller than the aperture must encounter an opening and be correctly oriented to pass through. Agitation maximizes the chances of passage. Near-size particles, which are only slightly smaller than the aperture, require more time and movement to successfully navigate the opening.
Diverse Applications of Sieving
Sieving is utilized whenever particle size affects a material’s performance or quality. In analytical laboratories, sieving is the standard technique for determining the particle size distribution of a sample. Researchers use a stack of sieves with progressively smaller mesh openings to sort a sample into precise size fractions, providing quantitative data.
Pharmaceutical Uses
In the pharmaceutical industry, particle size analysis is performed to ensure the correct dissolution rate and bioavailability of drug powders. Sieving also serves as a quality control step to remove foreign contaminants and ensure consistency in products like tablets and capsules.
Industrial Separation
Industrial applications involve high-volume separation of raw materials across various sectors. The construction and mining industries use large-scale vibrating screens to classify aggregates like sand, gravel, and ore based on size. In the food industry, sieving ensures uniformity in products like flour and sugar.
Maximizing Sieving Effectiveness
Efficiency and accuracy require careful control of process variables, including particle shape. Spherical or cubical particles pass through square apertures more easily than elongated or flaky particles. Irregularly shaped materials must be correctly oriented to pass through the mesh, necessitating a longer sieving duration.
Controlling moisture content is important, especially with fine powders. Excess moisture causes fine particles to stick together, or agglomerate, forming clumps too large to pass through the mesh.
This clumping, along with particles lodging in the mesh, leads to “blinding” or “pegging,” which reduces the sieve’s functional open area. Optimizing agitation prevents blinding and ensures separation is complete without causing particle attrition.