Flotation
In some cases, where it is not practicable to settle out fine particles, these
can sometimes be floated to the surface by the use of air bubbles. This
technique is known as flotation and it depends upon the relative tendency of
air and water to adhere to the particle surface. The water at the particle
surface must be displaced by air, after which the buoyancy of the air is
sufficient to carry both the particle and the air bubble up through the liquid.
Because it depends for its action upon surface forces, and surface forces can
be greatly changed by the presence of even minute traces of surface active
agents, flotation may be promoted by the use of suitable additives. In some
instances, the air bubbles remain round the solid particles and cause froths.
These are produced in vessels fitted with mechanical agitators, the agitators
whip up the air/liquid mixture and overflow the froth into collecting troughs.
The greatest application of froth flotation is in the concentration of minerals,
but one use in the food industry is in the separation of small particles of fat
from water. Dissolving the air in water under pressure provides the froth.
When the pressure is suddenly released, the air comes out of solution in the
form of fine bubbles which rise and carry the fat with them to surface
scrapers.
Cyclones
Cyclones are often used for the removal of particles of about 10µm or more
in diameter from air streams.
They are also used for separating particles from liquids as well as from
gases
For separating liquid droplets from gases
The cyclone is a settling chamber in the form of a vertical cylinder, so
arranged that the particle-laden air spirals round the cylinder to create
centrifugal forces which throw the particles to the outside walls. Added to the
gravitational forces, the centrifugal action provides reasonably rapid
settlement rates. The spiral path, through the cyclone, provides sufficient
separation time.
In a centrifugal separator, such as a cyclone, for a particle rotating round the
periphery of the cyclone:
Fc = (mv2)/r --------------------- (6)
Where Fc is the centrifugal force acting on the particle, m is the mass of the
particle, v is the tangential velocity of the particle and r is the radius of the
cyclone.
1
, This equation shows that the force on the particle increases as the radius
decreases, for a fixed velocity. Thus, the most efficient cyclones for removing
small particles are those of smallest diameter. The limitations on the
smallness of the diameter are both the capital costs of small diameter
cyclones to provide sufficient output, and also the pressure drops.
Classifiers
Classification implies the sorting of particulate material into size ranges. Use
can be made of the different rates of movement of particles of different sizes
and densities suspended in a fluid and differentially affected by imposed
forces such as gravity and centrifugal fields, by making suitable
arrangements to collect the different fractions as they move to different
regions.
Rotary mechanical classifiers, combining differential settling with centrifugal
action to augment the force of gravity and to channel the size fractions so
that they can be collected, have come into increasing use in flour milling.
One result of this is that because of small differences in sizes, shapes and
densities between starch and protein-rich material after crushing, the flour
can be classified into protein-rich and starch-rich fractions. Rotary
mechanical classifiers can be used for other large particle separation in
gases.
Classification is also employed in direct air dryers, in which use is made of
the density decrease of material on drying. Dry material can be sorted out as
a product and wet material returned for further drying. One such dryer uses
a scroll casing through which the mixed material is passed, the wet particles
pass to the outside of the casing and are recycled while the material in the
centre is removed as dry product.
CENTRIFUGAL SEPARATIONS
Centrifugal separation: the separation by sedimentation of two immiscible
liquids, or a liquid and a solid, depends on the effects of gravity on the
components. Sometimes, this separation may be very slow because the
specific gravities of the components may not be very different, or because of
forces holding the components in association, for example as occur in
emulsions
Also, under circumstances when sedimentation does occur there may not
be a clear demarcation between the components but rather a merging of the
layers. For example, if whole milk is allowed to stand, the cream will rise to
the top and there is eventually a clean separation between the cream and
the skim milk. However, this takes a long time, of the order of one day, and
so it is suitable, perhaps, for the farm kitchen but not for the factory. Much
2
In some cases, where it is not practicable to settle out fine particles, these
can sometimes be floated to the surface by the use of air bubbles. This
technique is known as flotation and it depends upon the relative tendency of
air and water to adhere to the particle surface. The water at the particle
surface must be displaced by air, after which the buoyancy of the air is
sufficient to carry both the particle and the air bubble up through the liquid.
Because it depends for its action upon surface forces, and surface forces can
be greatly changed by the presence of even minute traces of surface active
agents, flotation may be promoted by the use of suitable additives. In some
instances, the air bubbles remain round the solid particles and cause froths.
These are produced in vessels fitted with mechanical agitators, the agitators
whip up the air/liquid mixture and overflow the froth into collecting troughs.
The greatest application of froth flotation is in the concentration of minerals,
but one use in the food industry is in the separation of small particles of fat
from water. Dissolving the air in water under pressure provides the froth.
When the pressure is suddenly released, the air comes out of solution in the
form of fine bubbles which rise and carry the fat with them to surface
scrapers.
Cyclones
Cyclones are often used for the removal of particles of about 10µm or more
in diameter from air streams.
They are also used for separating particles from liquids as well as from
gases
For separating liquid droplets from gases
The cyclone is a settling chamber in the form of a vertical cylinder, so
arranged that the particle-laden air spirals round the cylinder to create
centrifugal forces which throw the particles to the outside walls. Added to the
gravitational forces, the centrifugal action provides reasonably rapid
settlement rates. The spiral path, through the cyclone, provides sufficient
separation time.
In a centrifugal separator, such as a cyclone, for a particle rotating round the
periphery of the cyclone:
Fc = (mv2)/r --------------------- (6)
Where Fc is the centrifugal force acting on the particle, m is the mass of the
particle, v is the tangential velocity of the particle and r is the radius of the
cyclone.
1
, This equation shows that the force on the particle increases as the radius
decreases, for a fixed velocity. Thus, the most efficient cyclones for removing
small particles are those of smallest diameter. The limitations on the
smallness of the diameter are both the capital costs of small diameter
cyclones to provide sufficient output, and also the pressure drops.
Classifiers
Classification implies the sorting of particulate material into size ranges. Use
can be made of the different rates of movement of particles of different sizes
and densities suspended in a fluid and differentially affected by imposed
forces such as gravity and centrifugal fields, by making suitable
arrangements to collect the different fractions as they move to different
regions.
Rotary mechanical classifiers, combining differential settling with centrifugal
action to augment the force of gravity and to channel the size fractions so
that they can be collected, have come into increasing use in flour milling.
One result of this is that because of small differences in sizes, shapes and
densities between starch and protein-rich material after crushing, the flour
can be classified into protein-rich and starch-rich fractions. Rotary
mechanical classifiers can be used for other large particle separation in
gases.
Classification is also employed in direct air dryers, in which use is made of
the density decrease of material on drying. Dry material can be sorted out as
a product and wet material returned for further drying. One such dryer uses
a scroll casing through which the mixed material is passed, the wet particles
pass to the outside of the casing and are recycled while the material in the
centre is removed as dry product.
CENTRIFUGAL SEPARATIONS
Centrifugal separation: the separation by sedimentation of two immiscible
liquids, or a liquid and a solid, depends on the effects of gravity on the
components. Sometimes, this separation may be very slow because the
specific gravities of the components may not be very different, or because of
forces holding the components in association, for example as occur in
emulsions
Also, under circumstances when sedimentation does occur there may not
be a clear demarcation between the components but rather a merging of the
layers. For example, if whole milk is allowed to stand, the cream will rise to
the top and there is eventually a clean separation between the cream and
the skim milk. However, this takes a long time, of the order of one day, and
so it is suitable, perhaps, for the farm kitchen but not for the factory. Much
2
