# Sieves - Types, Calibration, Standards, and More

A sieve, also called a sifter, is a device for separating desired elements from unwanted material or for characterizing the particle size distribution of a sample. This is a device typically made-up of using a woven sieve such as a mesh or net or metal. The word "sift" is derived from "sieve". A sieve has very small holes. The sieving process is comparatively inexpensive, simple in concept, and easy to use. Coarse particles are separated or broken up by grinding against one another and sieve openings. Sieves are the most commonly used devices for particle size analysis. Depending upon the types of particles to be separated, sieves with different types of holes are used. Sieving plays an important role in pharmaceutical and food industries where they (often vibrating) are used to prevent the contamination of the product by foreign bodies. The design of the industrial sieve is of primary importance. Each sieve has a specific number that denotes the number of meshes in a length of 2.54 cm (≈1 inch).

## Construction of Sieves:

Based upon application specially constructed sieves are used. Various types of sieves include electroformed sieves, perforated plate sieves, sonic sifters, air jet sieves, wet wash sieves, etc. Generally, pharmaceutical sieves are made up of stainless steel, brass, bronze, etc., and are not coated with any material to avoid wear and tear as well as contamination in the products. Sieves should be non-reactive and resistant to corrosion. The most common choice of material for sieves is iron because it is cheap. Iron has limitations for its use when there is the possibility of corrosion and contamination. This can be avoided by coating iron surfaces with a galvanizing agent. Stainless steel, brass, and phosphorus bronze due to their corrosion resistance, good strength, and non-contaminating qualities are preferred as an alternative to iron. Even non-metals such as nylon and terylene are used if contamination is to be avoided. Sieves with different types of holes viz. size and shapes made in plates as perforations can be used as separate devices. Sieves made-up of woven cloth are used if fine powders are to be separated. This cloth can be made of cotton, nylon, or silk.

## Types of Sieves:

Pharmaceutical sieves are extensively used for size separations from 300 mm down to around 38 µm. The efficiency of sieving decreases rapidly with fineness. Dry sieving is used for material above 5 mm in size and wet sieving is common in use down to 250 µm but is possible to about 40 µm. In its simplest form, a sieve is a surface having many apertures (holes), usually with uniform dimensions. Particles presented to that surface will either pass through or be retained, according to whether the particles are smaller or larger than the dimension of the apertures. There are numerous different types of industrial sieves available, Fig.1. Some of the most common are vibrating sieves, static sieves, trommels, roller sieves, flip-flow sieves, circular sieves, linear sieves, etc.

 Fig.1: Types of Sieves

### (i) Vibrating Sieves:

The commonest sieve type in industrial applications is the vibrating sieve. There are many subtypes of vibrating sieves in use for coarse- and fine-sieving applications. Vibrating sieves have a rectangular sieving surface with feed and oversize discharge at opposite ends. They are used in a variety of sizing, grading, scalping, dewatering, wet sieving, and washing applications. Sieves are vibrated to throw particles off the sieving surface so that they can again be presented to the sieve and convey the particles along with the sieve. Vibrating motion is generally produced by vibrating mechanisms based on eccentric rotating masses with an amplitude of 1.5–5 mm and operating in a range of 700–1000 r.p.m. The right type of vibration also induces stratification of the feed material, which allows the finer particles to work through the layer of particles to the sieve surface while causing larger particles to rise to the top. Vibrating sieves of most types can be manufactured with more than one sieve deck. On multiple-deck systems, the feed is introduced to the top coarse sieve, the undersize falling through to the lower sieve decks, thus producing a range of sized fractions from a single sieve.

### (ii) Inclined or circular motion sieves:

This type of vibrating sieve is widely used for sizing applications. A vertical circular or elliptical vibration is induced mechanically by the rotation of unbalanced weights or flywheels attached usually to a single drive shaft. The amplitude of the throw can be adjusted by adding or removing weight elements bolted to the flywheels. The rotation direction can be contraflow or in-flow. Contraflow slows the material flow more and permits more efficient separation, whereas in-flow permits a greater throughput. Single-shaft sieves must be installed on a slope, usually between 15° and 28° angle to permit the flow of material along with the sieve.

### (iii) Static sieves:

Very coarse material is usually sieved on an inclined sieve called a grizzly sieve. Grizzlies are characterized by parallel steel bars or rails set at a fixed distance apart and installed in line with the flow of ore. The gap between grizzly bars is usually greater than 50 mm and can be as large as 300 mm, with feed top size as large as 1 m. The most common use of grizzlies in mineral processing is for sizing the feed to primary and secondary crushers.

### (iv) Horizontal low-head or linear vibrating sieves:

These sieves have a horizontal or near-horizontal sieving surface and therefore need less headroom than inclined sieves. Horizontal sieves must be vibrated with a linear or an elliptical vibration. The accuracy of particle sizing on horizontal sieves is superior to that on inclined sieves; however, because gravity does not assist the transport of material along with the sieve, they have a lower capacity than inclined sieves. Horizontal sieves are used in sizing applications where sieving efficiency is critical and as drain-and-rinse sieves in heavy medium circuits.

### (v) Banana or multi slope sieves:

These sieves become widely used in high-tonnage sizing applications where both efficiency and capacity are important. Banana sieves typically have a variable slope of around 40–30° at the feed end of the sieve, reducing to around 0–15° at the discharge end in increments of 3.5–5°. The steep sections of the sieve cause the feed material to flow rapidly at the feed end of the sieve. The resulting thin bed of particles stratifies more quickly and therefore has a faster sieving rate for the very fine material than would be possible on a slower moving thick bed. Toward the discharge end of the sieve, the slope decreases to slow down the remaining material, enabling more efficient sieving of the near-size material. The capacity of banana sieves is significantly greater and is reported to be up to three or four times that of conventional vibrating sieves.

### (vi) Rotary scrubbers:

These sieves are cost-effective washing units that are an integral part of a material handling system to upgrade a wide variety of primary crushed hard rock and ore, including iron ore. A rotary scrubber is a cylindrical drum with internal lifters, typically supported by trunnion rollers at either end. These are high-capacity, high-retention time machines primarily used to remove water-soluble clays, deleterious materials, and coatings from a wide variety of hard rock and ore. Rotary scrubber designs include solid shell and combination scrubber sieves and are available in a variety of diameters and lengths. Drive transmission choices to include chain and sprocket, gear and pinion, or pedestal mount hydraulic motor.

### (vii) Test Sieves:

Test sieves are measuring devices used to determine the size and size distribution of particles in a material sample using wire mesh of different openings to separate particles of different sizes. Test sieves come in different materials such as brass, stainless steel, or brass frames with stainless steel mesh. Diameters include 3", 8", and 12" with mesh sizes ranging from 4 mm to 38 microns. When stacked on a sieve shaker, the top test sieve has the largest mesh size and the bottom one the smallest mesh size. The sieve stack consists of a pan at the bottom and covers at the top.

## Calibration of Sieves Sieves

used for sieving are to be cleaned regularly. The frequent use can cause changes in mesh openings but much of the damage sustained to working sieves occurs during cleaning. Often, the operator hurries to clear the mesh of residual particles by strongly tapping the frame. This tapping can distort the mesh. Operators also use brushes to remove residual particles after use. This process often distorts sections of the sieve mesh. These alterations of the sieve may change the quality of products in terms of size hence sieves are calibrated intermittently.

## Standards for Sieves

Sieving is the separation of fine material from coarse material using a meshed or perforated surface. The technique was used since early Egyptian days as a way to size grains. These early sieves were made of woven reeds and grasses. Today the sieve test is the technique used most often for analyzing particle-size distribution. Although at first look the sieving process appears to be elementary, in practice, there is a science and art involved in producing reliable and consistent results. To better understand sieving, several areas of sieve specifications should be explained and some of them are given in Table.1. Sieves used in pharmaceuticals must comply with the standards given in Pharmacopoeia, Table.2.

Table.1: I.P. Standards for Sieves

 1 Sieve (mesh) Number The sieve number is several meshes in the length of 2.54 cm in each transverse direction parallel to the wires. Mesh is arranged in multiple configurations. Mesh can be a square pattern, long-slotted rectangular pattern, circular pattern, or diamond pattern. 2 Nominal size of the aperture This term is the distance between the wires. It represents the length of the side of a square aperture. The relation between the nominal size of aperture and size in mm or micrometer is given in I.P. 3 Nominal diameter of the wire The diameter of wires used to make sieves gives strength to avoid distortion in the meshes. A wire with a specific diameter is used for a particular sieve. 4 Approximate percent sieving area This standard expresses the area of meshes in percent to the total area of the sieve. The approximate sieving area range from 35 to 40% of the sieve area. This area is suitable to give enough strength to sieve while sieving. It depends on the diameter of the wire. 5 Aperture tolerance average percent During making or while using some variations in the aperture size takes place. This variation is expressed as aperture tolerance average percent. It is small for coarse sieves while greater for small size sieves.

Table.2: Specifications for Wire and Aperture Size of Sieves as Per I.P.

 Sieve No. Nominal aperture size (mm) Nominal wire diameter (mm) Standard wire gauge Approximate screen area (%) Average aperture tolerance (%) 6 3.812 1.422 17.0 44 3.2 8 2.057 1.118 18.5 42 3.3 10 1.676 0.864 20.5 44 3.3 22 0.699 0.457 26.0 36 4.0 25 0.599 0.417 27.0 35 4.2 30 0.500 0.345 29.0 35 4.4 36 0.422 0.284 31.5 36 4.9 44 0.353 0.224 34.5 38 4.8 60 0.251 0.173 37.0 35 5.3 85 0.178 0.122 40.0 35 5.9 100 0.152 0.102 42.0 35 6.2 200 0.076 0.051 47.0 36 8.2

Table.3 presents the different international sieve standards and the corresponding sieve types. There are several sieve aperture progression ratios commonly available depending on the different international standards. In the USA, a progression ratio of 21/2 is used. This ratio corresponds to successive particle groups of 2 : 1 particle surface ratio. The progression rate of 21/3 (100.1) has been adopted by the French which corresponds to successive particle groups of 2:1 particle volume ratio. The progression ratios of 100.1 and 100.05 are recommended for narrow size distributions.

Table.3: International Sieve Standards

 Country Standard Sieve type Great Britain BS 410 Woven wire USA ASTM E11 Woven wire ASTM E161-607 Micromesh (electroformed) Germany DIN 4188 Woven wire DIN 4187 Perforated plate France AFNOR NFX 11-501 Woven wire International ISO R565 1972(E) Woven wire, Perforated plate

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