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or chemistry. Frictional heat from the press can remove organic water, but this
obviously should be avoided. For chemistry, see the Hydrated Lime, Gypsum and
Alum section. For heat, see the Fluid Injection section.
To determine the moisture content of a material (feed to the press, press cake, or
press liquor), a sample should be weighed and dried overnight at a temperature
slightly less than 100
o
C. (If sugars are present use less than 70º C to prevent
caramelizing.) The sample should weigh six or more times the tare weight of the
sample tray or cup.
The moisture content of press cake varies considerably. Tomato press cake will be
90% moisture. Orange peel will be 80%, unless it is reacted with hydrated lime, in
which case it will go down to 72% moisture; add molasses and it will go to 65%.
Dairy and hog manure will come out at 70% moisture, unless there is sand or
sawdust in the sample, which will reduce the moisture content. Cellulose fiber from
a paper mill (knots, screen rejects, primary clarifier underflow) will come out about
50%. However, if secondary (biological) sludge is added, then the moisture
content of the cake will go up considerably. With high ash content in paper mill
samples, moisture may go down to 40%. Moisture contents of only 25% can be
achieved pressing things like sand, eggshell, glass, and plastic chips.
The heat from steam injection can change the chemistry of the material being
pressed so that cake with lower moisture content is produced. This blanching or
parboiling effect works with fish and orange peel, for example.
A quick approximation of what to expect from a screw press is to squeeze as much
water out with your fist, and figure that the press will do a little bit better. A better
way is to twist a ball of the material in a cotton cloth.
COMPRESSION
A screw press achieves compression using several methods: (1) The discharge
cone of the press causes back-pressure on the material being dewatered. The
higher the cone pressure, the greater the liquid removal. (2) The pitch of the flights
of the screw tightens as the material is conveyed through the press. This forces
liquid to go through the screen. (3) The diameter of the shaft of the screw may be
increased progressively, forcing material outward, against the screen. This is a
tapered shaft design.
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