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Gypsum and alum salts are also effective chemical press aids. They are typically
used in dewatering sugar beet pulp, and they have rarely been found effective on
other materials.
PRESS LIQUOR
A screw press produces relatively "dirty" press liquor as compared to a Filter Press
or Belt Press. Suspended solids will pass through the screen of the screw press
along with the liquid being expressed from the inbound material.
If suspended solids need to be removed from the press liquor, the most common
method is to pump the press liquor either over a static (sidehill) screen or through a
rotary drum screen. Generally, the screen tailings (sludge solids) are fed back into
the screw press along with the flow of inbound material. Most of these fines will be
captured with the solids of the inbound material and end up in the press cake.
Although some of these tailings will once again go through the press screen with
the press liquor, equilibrium of recirculating solids is reached and satisfactory
pressing operation is realized.
If the press liquor is to be concentrated in an evaporator, better screening than can
be achieved with a static screen may be required. Another Vincent machine, the
Fiber Filter, provides premium performance. Decanters or centrifuges may be
required.
SCREEN SELECTION
The screen of the press is made either of wedgewire (slotted screen) or perforated
stainless sheet (round holes).
Screens made of wedgewire come standard with 0.015" to 0.020" slot width; they
are also available with slots that are 0.008" to 0.060" wide. With slot widths less
than 0.012" there is a tendency for the screen to blind (be covered over) with the
material being pressed. However they work well in alcohol and oil separation.
Changing the slot width generally has little impact on the clarity of the press liquor
or the dewatering capacity of the press.
Perforated metal screens are usually a simple sleeve which is held in the screen
assembly. These are less durable but inexpensive to replace.
In contrast, wedgewire screens have slots rather than holes. They are expensive
long-life weldments that must be replaced when excessive damage or wear has
occurred.
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