Steam/Liquid Injection
Polymer Usage
In rare applications, the addition of polymer is indispensable in achieving adequate screw
press performance. Polymers are added to dilute waste streams, especially to those
containing very small size suspended solids. The long chain molecules of the polymer will
flocculate the solids, agglomerating them to the point where they can be pressed. Under the
right conditions, dramatic improvement can be achieved in press throughput, press cake
moisture, and press liquor clarity.
Press Aid
Some materials press best if a press aid is mixed into the material to be pressed. Typical
press aids are rice hulls, cottonseed hulls, cellulose fiber, and ground newspaper. Ground
wood is the best, but most expensive, press aid.
Press aids are most used in producing juice from deciduous fruit. The press aid gives the
press something to get a bite on. Press aids also tend to hold back fines (short fibers) and
prevent them from going through the screen with the press liquor. If apples are fed into a
press, apple sauce will come through the screen. However, if a press aid is added to the
apples, then apple juice will come through the screen.
Typically, the amount of press aid used is only 1% to 3% by weight of the flow going
through the press. This will look like more than such a small percentage because press aids
have a much lower bulk density than the wet materials that are being pressed, so the ratio by
volume will be much higher.
Hydrated Lime, Gypsum, and Alum
Lime (calcium hydroxide) must be added to citrus peel before it can be pressed. The lime
breaks down the pectin or cell walls so that the press can remove moisture. Less than 1% by
weight is used. A reaction time of several minutes must be allowed prior to pressing. Lime
has been used successfully in the same manner with potato, onion, tomato, and pineapple
waste. It works well on acidic materials such as strawberries and coffee bean pulp. Vincent
offers lime dosing equipment.
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.
Cleaning
Commonly, material is cleared from a press by stopping the inbound flow, setting the
discharge cone in the withdrawn position, and running the press for a few minutes until no
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