4
Solids Processing
The following should be used to answer the Worksheet Questions for this chapter (excepting
Q2 & 4).
4.1 Properties which affect size reduction
Size distribution
Size distribution is the proportion of the particles in different particle size ranges.
The size of the material before size reduction and the size wanted after size reduction will
affect the equipment used, and the setting the equipment needs.
Shape
Shape is the form of the particles, eg Whether they are balls, cubes, egg shaped, plates, or
irregular shapes.
Blocks or lumps of brittle material are fairly easy to crush. Pieces that are flat and thin can be
a problem. They can slip through a crusher endwise and, possibly, just keep being returned to
the crusher.
Structure
Structure is whether the particles are granular, fibrous, etc. Normal granular materials such
as coal, ores, and rocks can be easily crushed using normal impact and/or compressive size
reduction equipment. Fibrous materials need a tearing action in the size reduction
equipment.
Hardness
Hardness is defined as ‘resistance to being scratched’.
The hardness of the material undergoing size reduction will affect wear on the equipment and
how much power it uses . With hard, abrasive materials you need to use a low-speed machine
and to protect the bearings from the dust that size reduction produces. Very hard materials
can need extra heavy-duty equipment. Very soft materials can be ground too fine unless size
reduction is gentle.
Abrasiveness
Abrasiveness is the ability of one material to wear away another material.
Hard materials, coarse particles, and fast movement of equipment make a material abrasive
and that leads to wear in the equipment.
To minimise the wear and downtime of equipment, the surfaces that come in contact with
abrasive material must be resistant.
Stickiness
If the material to undergo size reduction or classification has 5-50% moisture, it will not
move easily. Similarly, materials near their melting point, or containing some liquid could
stick together or to the surfaces of the equipment. The material tends to cake together in
balls and clog the equipment.
4. Solids Processing
If sticky materials must be ground:
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the equipment chosen should be able to be cleaned easily
the feed material might have to be dried or cooled before size reduction
Heat sensitivity
If the material is heat sensitive, the energy input has to be controlled. Both the material and
the equipment may have to be cooled. In dry crushing, cooling can be by suitable airflow. In
wet grinding, the liquid usually quickly removes the heat.
Explosiveness
Finely divided metals in air could be explosion hazards. Materials such as sulfur, starch,
wood flour, cereal dust, coal, hard rubber, and plastics are all potential explosion hazards.
The finer the particle size, the more easily it can be ignited by electric motors, static
electricity, and in many other ways.
Potentially explosive materials can be size reduced if they are:
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ground wet
ground in an inert atmosphere, such as nitrogen or carbon dioxide gas.
4.2 Properties which affect mixing
Particle size distribution
To mix particulate solids, the size ranges of each component should be as similar as possible,
otherwise you will get segregation of the final mix.
Particle shape
The more regular the shape, the more likely that the materials will be hard to blend and that
there will be segregation. Particles that are not regular in shape are easier to mix. Irregular
shaped materials do not slide over each other easily, and after mixing do not segregate.
Bulk density
Bulk density includes the air spaces.
Powders of similar bulk densities are easier to mix together. If the bulk densities are very
different, mixing is harder. The material with the higher bulk density will tend to sink
through the mix and cause segregation.
Particle density
Particle density excludes the air spaces.
Higher density particles are likely to settle more quickly through lighter particles after mixing
has stopped.
Moisture content & stickiness
If the powder has a moisture content between 5-50% or naturally sticky, a different type of
mixer is needed.
4.2
4. Solids Processing
Static electricity
Static electricity can:
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cause sparks that ignite very fine powders, possibly resulting in an explosion
make powder stick to surfaces so it can’t mix in with the bulk of the material
cause particles to repel each other, leading to segregation
Equipment needs to be earthed to remove the charge from the particles when they touch the
surface of the equipment.
Friability
Friability is the ease of crushing or grinding a material. If some of the materials to be mixed
are friable, the mixing needs to be gentle, otherwise further reduction in particle size will
occur.
4.3 Difficult mixtures
If particle size, particle density, and particle shape of the materials to be mixed are similar,
then the rate of mixing depends on the number and proportions of components.
If there are only two or three components, and each is in about the same proportion, mixing
should be quick. A large number of components will take a fairly long mixing time to make
sure they are all evenly distributed throughout the mix.
If some of the components are in small proportions, mixing will take much longer to get the
small proportions evenly distributed in the mix. Mixing in two stages can reduce this
problem.
In the first stage the small percentage component is mixed with some of the main
component, so it forms a larger percentage of the mix. Because it forms a larger percentage of
this mixture it blends in quickly.
The second stage is mixing the first mix into the rest of the main component. This ‘premix’ is
a large percentage of this mixture and blends in quickly.
If moisture or stickiness is a problem, it could be necessary to modify the properties of the
raw materials, e.g. drying, addition of anti-caking agents.
4.3