2.  In order to transfer material from storage to
process, it is important to know how the
particulate material will flow.
 If the particles tend to agglomerate, poor flow
properties may again be expected.
 Agglomeration arises from interaction between
particles, as a result of which they adhere to
one another to form clusters.

3.  Mechanical interlocking.
 If particles are long and thin in shape
 large masses may become completely interlocked.
 Surface attraction.
 Surface forces, like van der Waals’ forces, may give rise to
substantial bonds between particles, particularly where
particles are very fine (<10 μm),
 their surface per unit volume is high.
 freshly formed surface, such as that resulting from
particle fracture, gives rise to high surface forces.

4.  Plastic welding.
 When irregular particles are in contact, the forces between
the particles will be on extremely small surfaces and the
very high pressures are developed
 Electrostatic attraction.
 Particles may become charged
 significant electrostatic charges may be built
up, particularly on fine solids.

5.  Effect of moisture.
 Moisture will tend to collect near the points of contact
between particles and give rise to surface tension
effects.
 It may dissolve a little of the solid, which then acts as a
bonding agent on subsequent evaporation.
 Temperature fluctuations
 give rise to changes in particle structure and to greater
cohesiveness.

6.  “Glidants” are added
 Very fine powders which reduce interparticle
friction by forming surface layers on particles
 Thus combat effect of friction arising from
surface roughness
 Also reduce effect of electrostatic charges
 But Optimization of particle size is most
important to improve flow properties

7.  Fine particles may be difficult to discharge from
hoppers as particles may cling to the walls.
 Though can be minimised by vibration or
mechanical stirring, but very difficult to overcome
them entirely
 only satisfactory solution is to increase the particle
size by forming them into aggregates.
 In addition, very fine particles give rise to serious
environmental and health problems, particularly
as they may form dust clouds during loading
 in windy conditions disperse over long distances.

8.  A desired particle size may also be achieved by
building up from fine particles
 Such as production of fertiliser granules by
agglomeration.
 Formation of pellets or pills for medicinal
purposes by the compression of a particulate
mass,
 often with the inclusion of a binding agent that will
impart the necessary strength to the pellet.

9.  Prilling
 Urea and ammonium nitrate production
 Cascading conc sol of urea from the top of the
tower
 Air used to cool the sol to form granules
 Seed granules of <0.5mm dia can be built to
product size of 2-3 mm

10.  Pelletizing is the process
Applications of Size of compressing or molding a
Enlargement material into the shape of
a pellet.
 Such as in medicines
(pharmaceuticals) etc.
 Pressure Compaction
 with or without the addition
of a binder

11.  A dust explosion is the fast combustion of dust
particles suspended in the air in an enclosed
location.
 Coal dust explosions are a frequent hazard in
underground coal mines,
 but dust explosions can occur where any
powdered combustible material is present in an
enclosed atmosphere.

12.  There are four necessary conditions for a dust
explosion or deflagration:
1. A combustible dust
2. The dust is suspended in the air at a
high concentration
3. There is an oxidant (typically atmospheric
oxygen)
4. There is an ignition source

13.  electrostatic discharge
 Friction
 hot surfaces, including e.g. overheated bearings
 fire
 However it is often difficult to determine the
exact source of ignition post-explosion.

14.  Many materials which are commonly known
to oxidize can generate a dust explosion
 E.g. coal, sawdust, and magnesium.
 Many mundane materials can even lead to a
dangerous dust cloud
 such as grain, flour, sugar, powdered milk and pollen.
 Powdered metals (such as Al and titanium) can form
explosive suspensions in air.
 The dust can arise from activities such as
transporting grain
 Grain silos do have dust explosions.
 Mining of coal leads to coal dust and flour mills likewise
have large amounts of flour dust as a result of milling.

15.  For combustion, the dust must consist of very
fine particles with a high surface area to
volume ratio,
 Thus making combined surface area of all the
particles very large
 Dust is defined as powders with particles less
than about 500 micrometres in diameter
 but finer dust will present a much greater
hazard than coarse particles by virtue of the
larger total surface area of all the particles.

16.  Below a certain value, the lower explosive limit
(LEL), there is simply insufficient dust to
support an explosion.
 A figure 20% lower than the LEL is considered
safe.
 Similarly, if the fuel/air ratio increases above
the upper explosive limit (UEL) there is
insufficient oxidant to permit combustion to
continue at the necessary rate.

17.  Bulk Density: mass of many particles of the
powdered material divided by the
total volume they occupy
 Dusts have a very large surface area compared
to their mass.
 Since burning can only occur at the surface of a
solid or liquid, where it can react with oxygen,
this causes dusts to be much more flammable
than bulk materials.
 For example, a 1 kg sphere of a material with a density
of 1g/cm3 has a surface area of 0.3 m2.

18.  However, if it was broken up into spherical dust
particles 50µm in diameter (about the size
of flour particles) it would have a surface area of 1600
m²
 This greatly increased surface area allows the material
to burn much faster
 and the extremely small mass of each particle allows it
to catch on fire with much less energy than the bulk
material, (as there is no heat loss to conduction within
the material).
 When this mixture of fuel and air is ignited, especially
in a confined space such as a warehouse or silo, a
significant increase in pressure is created, often more
than sufficient to demolish the structure.

19.  Even materials that are traditionally thought of
as non-flammable, such as Al or Fe, or slow
burning, such as wood, can produce a
powerful explosion when finely divided, and can
be ignited by even a small spark.
 Such metal powders are widely used
in fireworks for their dramatic effects.

20.  Diluting the dust – such as coal dust by stone
dust to the point where it cannot burn
 Use of inert gases (N2, Ar, CO2) instead of O2 in
some industries
 Spraying water on area such as mines.
 Good housekeeping

21.  Oxidant Concentration Reduction
 Deflagration venting
 Deflagration pressure containment
 capable of withstanding the maximum pressures resulting from an
internal deflagration.
 Deflagration suppression
 Technique of detecting and arresting combustion in a confined space
while the combustion is still in its incipient stage
 Deflagration venting through a dust retention and flame-
arresting device
 The dust retention device is for retaining dust when particulate material
is dumped through the device into a container
