Chemical Engineering December 2010 - 37

tion. Where the screw retains the still
liquid slurry at the smaller diameter,
it pro-actively conveys the product at
the large diameter, the friction angle
of which has increased due to the reduced
residual moisture.
As a result of this effect, and through
Figure 7. This inside view of a two-stage pusher centrifuge shows the rotating parts
in green and the rotating and oscillating parts in red. The bearing housing is blue
Vibrating basket. The next modification
of the sliding discharge centrifuge
is the vibrating basket centrifuge.
Here, the opening angle of the conical
basket is first selected in such a way
that no sliding action or product transport
will take place. However, by superimposing
an axial vibrating movement
to the rotation, the friction can
be overcome for a short while through
the impulse that is transferred to the
product by the vibration of the shaft
and the basket. This results in a stepwise
transport through the basket.
The vibrating basket centrifuges
stand out with their substantial solids
throughput rates of up to 350 ton/h.
The centrifugal force (C-value), however,
is limited to approximately 150
times the earth's gravitational acceleration
(150g), as otherwise the transport
impulse can no longer be generated
by means of vibration.
The main field of application of vibrating
basket centrifuges is in the
treatment of fine coal.
Tumblers. The most complex way to
generate a transport impulse is found
in the tumbler centrifuge (Figure 6).
Here again basically the same principle
is applied as in a sliding centrifuge,
using a conical basket. In this
case, however, the basket is mounted
via a U-joint and an articulated hollow
shaft at a slight angle to the actual
axis of rotation. Through a different
speed of the inner drive shaft
to that of the outer hollow shaft, the
angular offset wanders independent
of the rotation. The movement of the
basket can be seen to wobble like a
child's spinning top.
In doing so, every point on the basket
periodically passes through the area of
the slightest inclination and the area
of the greatest inclination, relative to
the surface of the screen. In the area
of the greatest inclination, the product
starts to slide; in the area of the slightest
inclination, the product is slowed
down again until it stops. There is
therefore no cake that is moved as an
intact surface, as in the vibration basket
centrifuge, or as a cake ring, as in
the pusher centrifuge. Instead, there
is a cake that is constantly subjected
to a rotating wave movement.
Since the intensity of the transport
impulse increases to the same degree
as the centrifugal acceleration, the
tumbler centrifuge is not subjected to
the constraints of a vibrating basket
centrifuge. C-values of up to 8,000g
are possible, and throughput rates of
up to 300 ton/h can be achieved.
The tumbler centrifuge is used today
for dewatering carnallite, for example.
Worm/screen. The conical-shaped
basket used in the above centrifuges
are also used in the worm/screen centrifuge.
In the conical basket, there is
also a conical screw that rotates in the
same direction as the basket, but at a
slighter differential speed, resulting
in a forced transport of the product in
the centrifugal field.
In this case, it is not necessary to
exactly coordinate the basket angle
to the product friction angle because
the screw takes on a regulating functhe
forced product transport, the
worm/screen centrifuge is relatively
insensitive to feed fluctuations and interruptions
in the product supply.
Typical applications are the filtration
of iron sulfate and polystyrene.
Pusher. The pusher centrifuge also
has a forced product transport. This
centrifuge has no conical basket. Its
cylindrical basket is fixed to a hollow
shaft and has a plate on the basket
bottom that is mounted on a pusher
rod, which runs inside the hollow
shaft. The pusher rod oscillates axially
so that the plate, known as the pusher
plate, performs a pushing motion at
the bottom of the basket.
The product that is fed centrally
through a rotating feed system, dewaters
in the feed zone on the screen. The
cake ring that then forms is pushed
towards the discharge by the pusher
plate. The vacant space that appears
when the pusher bottom is driven back
is again filled with new product, allowing
a new cake ring to develop that,
during the forward motion, pushes the
previous one further forward.
For a continuous product transport,
the pusher centrifuge requires a constant
topping up with product. The
pusher centrifuge therefore relies on
the feed conditions being kept as constant
as possible. Through the cake
ring that is moved over the screen as a
compact block, the throughput of particles
is very low for a continuous operating
filtration centrifuge, as most
of the fine particles are retained in the
cake. In a modern pusher centrifuge
throughput rates of up to 150 ton/h
can be reached.
After the single-stage pusher centrifuge
was introduced, there soon
followed two-stage pusher centrifuges
(Figure 7). Here, a second, shorter
inner basket with smaller diameter is
installed between the pusher plate and
the basket. In the two-stage pusher
centrifuge the pusher movement -
that in the single-stage machine only
takes place between the pusher plate
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Chemical Engineering December 2010

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