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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