Chemical Engineering August 2022 - 27

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In fact, all machine and process specifics can ultimately
be traced back to the solids transport and the associated
differences in the structure of the filter cake. This
is explained in more detail in the following two sections.
Feeding conditions
FIGURE 2. The sequence shown here demonstrates the operating principle of
a pusher centrifuge
the filtrate usually leaving the centrifuge tangentially. On
the drive side, the machines are also very similar at first
glance, as both machines can be designed with one or
two drive motors, with the two-motor drive variant still
dominating in pusher centrifuges despite the advantages
of single-drive systems. Single-drive systems, with
a combined drive and pusher motor, such as the commercial
pusher centrifuge shown in Figure 1, allow the
reserve capacity available for pushing to be used for accelerating
the basket during the startup process, while
the reserve capacity available for the startup process can
be used for pushing during stationary operation [2]. This
allows significant savings to be made in electricity consumption
and installed power.
However, the inner workings of the pusher and the
screen-scroll centrifuge are very different. Pusher centrifuges
usually consist of one to three (usually two) baskets,
with a plate at the bottom, which is mounted on a
pusher shaft running inside a hollow shaft. The pusher
shaft is subjected to an oscillating axial movement so
that the so-called pusher plate performs a pushing
motion at the bottom of the basket, whereby the solid
material is pushed through the basket. Figure 2 shows
the operating principle of a pusher centrifuge with one
basket [3].
In contrast to this, screen-scroll centrifuges have a single
rotating basket, which is usually conical rather than
cylindrical. It is also perforated and lined with screens,
but inside it, there is a coaxial and co-rotating scroll
conveyor that rotates at a slight differential speed to the
basket and thus transports the solids out in a controlled
way. The differential speed is realized by a gearbox [4].
In both cases, the solids are dewatered during transport
through the centrifugal field and can be washed as
needed to enhance product quality. Similarly, the solids
are discharged continuously in both centrifuges.
Thus, the fundamental difference between these two
types of centrifuges is the method of solids transport.
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In both types of centrifuge, the product to be processed
is fed into the machine centrally via a rotating feeding
system. The product is pre-accelerated before it hits
the screen, which retains the solids while allowing the
liquid (filtrate), to pass through.
Pusher centrifuge. In the pusher centrifuge, a ring of
solids (cake ring) is formed, which is pushed in the direction
of the discharge by the pusher plate. The empty
space that is left when the pusher bottom is retracted is
again filled with new product, allowing a new cake ring
to form that pushes the previous one onwards during the
forward motion (Figure 2). To provide continuous transport
of the product, the pusher centrifuge requires a constant
supply of product.
Certain minimum requirements must be met so that
the cake ring is suitable for pushing. First, some preliminary
dewatering is necessary so that the cake is not too
wet and the next cake ring can be pushed forward. If
the cake moisture content was too high, the cake would
splash forward, causing channeling and vibration in the
machine. The speed of preliminary dewatering depends
on the particle properties, such as shape and size, as
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