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Sieving of the sample material e.g. in

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Sieving Analysis

It is one of the most common methods used to classify solid particles
(powder, granules, etc.) according to their particle size.

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Because solid particles are mainly processed in large amounts,
properties of the bulk particulate material have to be defined as well. These
properties can only be derived from single particle properties to a small
degree and have to be determined separately.

In Sieving process, samples are subjected to vertical and horizontal
stirring as illustrated in Figure 1. the particles are presented to equal-size
apertures that constitute a series of go-no-go gauges. Sieves are referred to
their mesh size. The possibility of a particle to pass through the sieve mesh
is determined by the relation between the particle size to the sieve openings,
the orientation of the particle, and the number of encounters between the
particle and the mesh openings. The proper sieving method is characterized by
the fineness of the sample material e.g. in dry sieving the preferred apertures
size is in the range between 40 µm and 125 mm. 

Figure 1. Sieving apparatus

The determination of
particle size distribution is an essential technique in order to define the
properties of the solid material. The existence of very fine particles in some
processes such as in metallurgy can lead to decrease the efficiency of the
production and diminish the quality of the product. Particles can be classified
according to their sizes as represented in figure 2. It has been concluded from
industrial practices that softer materials produce finer particles.

Classifying is a mechanical separation process for solids mixtures
according to their particle sizes. The method uses geometrical signs (size) or
also the sink rate of the individual Particles for the separation process. In
the ideal case, a classifier separates a feedstock with different particle
sizes in coarse and fine products. Thereby, the coarse materials that are
larger than a certain grain size would be recovered, whereas the fine materials
are those that smaller in grain sizes. 

The most basic example of a classifier is a sieve. In this case, the
separation grain size through the screen mesh width are specified. With the
represented arrangement of sieving, it is possible to create a Feedstock in
several classes of particle sizes. An application example for such an
arrangement (however with larger screen mesh widths) is the separation of
gravel and sand from the crushed material of a quarry.

 

Figure 2: Classification of particles according to
average size

Sorting is a mechanical separation process in which a solids mixture
would be separated according to particle properties such as Density, color,
shape, wettability and magnetisability. With density as a separating
characteristic is a floating-sink sorting system suitable. A solid mixture is
formed into a liquid. The Particles of the mixture with lesser Density than
that of the liquid are floating on the surface, while particles of higher
density sink. One field of application is the Coal processing. In that case,
the coal separated from the surrounding rock.

Size Reduction Processes

In order to conveniently handle solid materials in most of chemical and
industrial processes, the particles size have to be reduced. The most common comminution methods are mainly compression,
tension and shear.

Milling is a mechanical process in which rotary cutters are subjected
into material resulting in finer particles. Several types of millers have been
developed such as multi-edge cutting mill, beater mills, roller mills, ball
mills, and jet mills.

Figure 3: Ball mill, 1 milling balls, 2 particles to be
milled

Crushing is a comminution process that can handle particles up to 1.5 m
in edge length. In the most common cases, crushing is applied as primary
reduction process prior to the main comminution process that leads to smaller
particle sizes, which enhance the transportation of the raw materials. Crushers
are mainly used in quarries for the preparation and production of road
construction materials, as well as in recycling of rubble, road surfacing, and
metal turnings or scrap. Crushers can be classified into four types: jaw
crushers, gyratory (cone) crushers, roll crushers, and impact crushers.

Figure 4: schematic of jaw crusher

 

Size-reducing equipment is based on compression
or impact. Compression is achieved by means of movable jaws, rollers or a
rotary cone. The maximum discharge size is set by the adjustable by a
clearance. Impact based equipment often uses hammers or media. Rollers in most
cases can produce very fine particles. Rollers are used in flour grinding,
where crushing produces particles of various sizes, allowing the separation of
purified flours. The moisture content is important so that, for example, corn
germs can be separated from starch and fibers by roller milling, as the germ is
selectively absorbed by water and processed into flakes while the starch
breaks.

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