The screening is a mechanical physical process by which solid particles of different size separate the passing them through a sieve. A sieve is a tool that has pores generally of equal size throughout its surface.
The particles with a size greater than the pore or hole of the screen remain in it; while the smallest, with diameters smaller than the pores, freely penetrate the surface.
In addition to being a sieve, this device or instrument is also known as a strainer, sieve, sieve or filter. In fact, the word ‘strainer’ is derived from the Latin ‘ colum ‘, which means sieve.
The sieve can be made of metal, plastic, silicone, ceramic; and it can be a screen, mesh or woven net. The size of the pores of the screen is variable, and will depend on the nature and size of the particles to be separated.
There are different sized sieves, strainers, or simple household utensils that are used to separate or aerate wheat flour.
With strainers or filters that function as a sieve, solid products contained in liquids are separated. As for example in the preparation of coffee, or when blending and straining a guava juice to eliminate the seeds.
In the food, pharmaceutical and chemical industries, sieving is a fundamental part of the production and quality control of powdered and grain products.
In the construction area, sieving is used to separate the sand by removing large particles. It is also used in the study of the structure of soils, determining the size of the particles that compose it.
What is sieving?
Sieving is a physical-mechanical method by which solid particles present in heterogeneous mixtures can be separated.
By means of a simple or straightforward procedure, the larger particles of the mixture are retained on the surface of the sieve. Only particles whose diameter is smaller than the holes or holes in the sieve will pass through this instrument.
For the sieving process to take place, there must be a certain movement between the mixture to be separated and the surface of the sieve. Thus, coincidences are established between the pores or holes of the sieve with the particles that can pass through them.
The purpose of sieving is to separate the solid particles according to their size. By applying a single sieving process, the mixture will be separated into two more homogeneous fractions.
There is the fraction of particles retained on the surface of the sieve known as the coarse or reject fraction; and the fraction that passes through the orifices thereof, called the fine or sieving fraction.
Efficiency and solid aspects
A sieving can be considered efficient as long as it allows to achieve the greatest separation of the mixture.
To carry out the sieving process it is important to consider the nature of the mixture to be sieved. The selection of the sieving surface, the inclination of the sieve, its movement, the sieving time, among other aspects.
The shape of the solid particles to be separated can be spherical, acicular or fibrous. Spherical particles in general tend to exhibit better separation or sifting than those particles that are fibrous.
There are particles that tend to form aggregates or lumps, which are treated as larger particles: as in the case of flour.
When sifting the flour, by friction between the particles, the aggregates or lumps that may exist in the mixture are dissolved. Larger flour components, such as bran, remain in the strainer during the sieving process.
The opportunity or probability of sifting can be increased in the screening process by taking into account several variables. The inclination of the sieve surface is important: there is an optimal inclination according to the size and surface of the particles.
In general, sieving is done by gravity. Small particles pass freely through the surface of the sieve. The sieving increases if the surface of the sieve is subjected to some type of movement, among other factors.
At an industrial level, there are processes that take these aspects into account and cause the particles to be forced through the sieve. Very often they use sieving equipment moving on the surface of the sieve to optimize the result.
Types of separation
The classification of the sieving is carried out depending on the humidity of the mixture to be separated; It can be sieved dry or wet.
It is carried out for those mixtures of particles that by nature have little or no humidity. It also refers to the sieving of substances or mixtures of particles that were previously subjected to drying processes.
This type of sieving is more frequent, it is used in products that are sold in dry powder.
In this type of sieving, water or other fluids are added to the mixture. This is done in order to facilitate the dragging or passing of the finer particles through the sieve.
The screening carried out by this method or route is more efficient; but in general it is the least used. When this type of sieving is used in products whose final presentation is dry, they require drying after the sieving.
Types of surfaces used
It is important to consider the opening, span, or pore diameter of the sieve surface. Metal screens are available that offer openings as large as 1 micron (1µ) in length. The size and shape of the pores will depend on the weft or warp of the screen fabric, bars or plates.
The types of surfaces with which the screens are constructed can include metallic, silk, or plastic fabrics. They can be made from perforated metal plates, metal bar grids, wire, plastic and even bamboo.
Among the most commonly used metals are stainless steels.
Types of equipment used in this technique
The equipment with which the screening procedure is carried out are varied; They include grates, drums, agitated, vibrating and oscillating screens.
Instruments, machines or equipment can be classified depending on whether or not the surface of the sieve exhibits movement. Based on this criterion, the equipment can be fixed or mobile.
Among the fixed are the equipment whose sieving surface is still, static. Among these there are sieves with inclined grates, stationary bars and those formed by generally stationary metallic fabrics, among others.
Among those classified as mobile are sieves that have back and forth movements, oscillatory, rotary and vibratory movements.
Sifting can be single, double or multiple. There are various screening systems: parallel or in line, in series or cascade.
The following is a brief description of some of the frequently used sieving or screening equipment or machines.
Formed by steel bars with predetermined spaces between them, used for sifting rocks or minerals before being crushed.
The grids can be stationary, that is, they remain still and do not need any mechanism to make them move; or vibratory, which perform the screening process with a forward and backward or circular movement, and therefore require a mechanical force.
Formed by a cylindrical frame open at the ends, they surround a cloth of wire or plates with holes of a certain diameter. There are drums for manual use, others that can be rotatable at an industrial level.
Sieves with mechanical agitation
Generally formed by a rectangular frame that supports a cloth, wire mesh or perforated plate. This sieve is slightly inclined, with a support of flexible springs that allow agitation.
They print the sieving process faster and offer greater performance in the selection of particles by size. There are sieves with mechanical vibration and others with electrical vibration.
There is a wide variety of sieves: horizontal or vertical rotation, oscillating motion sieves, electromagnetic vibration, centrifugal sieves, among others.
-In the construction area, sieving is used to separate sand, specifically in the cement industry. This process is essential to evaluate the particle size distribution of a granular material. This procedure is called grading and is very useful in civil engineering.
-In soil study laboratories, sieves with graduated pore size are used. These sieves allow to separate and study the different components of the soil based on the size of their components.
-There is great diversity in filtration systems that work with the principle of sieving to separate solid particles from liquid media.
-At industrial level, there are duplex or double filter filters inserted along water, fuel or oil pipes. These double filters, or double basket, allow to eliminate polluting particles from the system of these fluids by double sieving.
-In the food industry, the sieving process forms part of the quality control that will allow the product to be marketed. Sifting is used to obtain smooth textures during the preparation of foods such as sauces, soups and custards.
-Sifting is also used to decorate desserts by sprinkling for example icing sugar, cocoa powder, among others.
-One of the examples of sieving is applied to the separation of wheat flour before making a cake. The wheat flour is passed through a homemade strainer in order to retain the lumps or some foreign particles.
-There are several food products that are sieved at an industrial level, such as semolina, corn flour, grains, among others.
-In laboratories where the study of the structure of soils is carried out, sieving is used in granulometry. With this technique, granulometric curves are elaborated that allow them to know the size of the particles that make up the soil.
-Strainers or large bowls with holes are also used to sift or strain pasta, vegetables or rice. The cocktails are sieved through strainers to remove ice from the drinks.
-Double or even multiple sieving is used at the level of water treatment, separating the solids present in the wastewater. First, a sieving is carried out for coarse elements and later for finer particles.
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