Processing and production factors
Starch processing operations
Step 4 Screening
In separating the pulp from the free starch a liberal amount
of water must be added to the pulp as it is delivered by the
rasper, and the resulting suspension stirred vigorously before
screening. Mixing with water can be carried out more or less
separately from screening, but more often the two operations
are combined in "wet screening" - that is' the mass
is rinsed with the excess water on a screen which is in continuous
motion.
4.1 Hand screening
In the smallest mills, screening is done by hand. The rasped
root mass is put in batches on a cloth fastened on four poles
and hanging like a bag above the drain leading directly to the
sedimentation tanks. Spring water or purified river water is
run in from a pipe above the bag, and the pulp is vigorously
stirred with both hands. Sometimes bamboo basketwork is used
to support the screening cloth. The pulp under processing still
contains appreciable amounts of starch and therefore has a certain
value (e.g., as a cattle fodder): in the small mills it is pressed
out by hand, and the lumps obtained are dried on racks in a
well-ventilated place.
4.2 The rotating screen
A simple form of rotating screen consists of a conical frame
of hardwood, fixed on a hollow, horizontal axis, at least 3
m long, covered with ordinary cloth or phosphor-bronze gauze.
Phosphor-bronze is often preferred for its durability, but its
use necessitates frequent brushing in order to remove clogging
pulp particles. The crude pulp is fed into the cone at the narrow
end and by the rotation of the screen, at approximately 50 revolutions
per minute, slowly moves down to the other end, whence it is
conveyed to the pulp tanks. In the meantime, water is sprayed
on it under pressure (e.g., 6 atmospheres) from a number of
openings in the hollow shaft. Thus, by the time the pulp reaches
the lower end of the cone, it is more or less completely washed
out. The rotation screen has the advantage of preventing the
plugging of the meshes of the sieve with gummy materials (they
tend to agglutinate with the fiber as the screen rotates). The
flour milk is caught in a cemented basin stretching out below
the screen over its whole length, and from there runs along
channels into sedimentation tanks or flour tables.
The screen is mounted close to the rasper and at a somewhat
lower level in order to ease the flow of the crude pulp. The
washed-out pulp discharged at the lower end of the screen is
carried off by some form of conveyor to basins outside the factory.
Since its dry matter still consists mostly of starch, this byproduct
after drying and pulverizing is marketed as a fodder.
A more elaborate type of rotating screen is equipped with two
sets of brushes, one set being arranged to convey the fiber
along to the discharge, the other acting as beaters, which at
the same time keep the screens clear to allow the starch milk
to flow away readily. Both sets of brushes are adjustable, so
that excessive wear on the bristles can be taken up and the
maximum life obtained from them. The screens are carried in
aluminum frames which are removable for changing covers. Up
to now a single rotating screen is most generally used in factories
of medium capacity. In larger factories, for economic reasons,
the starch must be extracted from the whole root as thoroughly
as possible with the minimum amount of water. This often implies
a more intricate arrangement of the operation of rasping and
screening as well as more efficient screening devices.
4.3 The shaking screen
In large factories the rotating screen is replaced by the shaking
screen. It consists of a slightly inclined, horizontal frame,
4 m in length and covered with gauze, which is put into a lengthwise
shaking motion in short strokes by means of an eccentric rod.
The fresh pulp, after being mixed with water in distribution
tanks, is conducted by pipes to the higher end of the screen;
during screening, the pulp remaining on top of the screen is
slowly pushed downward by the shaking motion.
It is advantageous to let the suspensions pass a series of shaking
screens of increasing fineness (80-, 150-, and 260-mesh), the
first one retaining the coarse pulp, the others the fine particles.
The pulp remaining on the first of these screens is often subjected
to a second rasping or milling operation and then returned to
the screening station.
Another means of increasing efficiency is to perform the combined
operations of screening and washing the pulp in two stages.
In the first stage' the pulp is vigorously stirred with water
in a washer provided with coarse screens at the bottom and with
paddles in order to obtain thorough mixing during the transport
of the pulp toward the end of the trough. In the second stage,
the crude flour milk from these washers is conducted to a shaking
screen below, which retains the rest of the fine pulp. The operation
is twice repeated with the pulp thus obtained in similar washer-and-screen
units, which may be arranged in a battery.
The complete separation of free starch from pulp is achieved
here by the countercurrent principle. In the third (lower) washer,
the pulp from a first rasping is washed out with flour milk
from the second washer-and-screen unit. The pulp from this first
treatment passes a secondary rasper, whence it is conveyed to
the washer, where it is rinsed with starch milk from the first
(upper) washer-and-screen unit. Finally, the pulp is conveyed
to the upper washer where fresh water is run in.
Efficient rising of the pulp on the screens is promoted by inserting
one or more shallow transverse channels in the surface of the
screen, where the strong whirling movements caused by the shaking
of the screen effectively loosen the starch granules from the
pulp.
4.4 Jet extractors
An efficient machine for the separation of starch from cellulose
fiber is the jet extractor, or the continuous perforated-basket
centrifuge. The starch-pulp slurry is put in a conical basket
and centrifugal action separates the starch dispersion from
the fibrous pulp. Jets of water sprayed on the pulp as it travels
the length of the cone assure complete recovery of the starch.
4.5 The Dorr-Olivier DSM screen
Another type of modern equipment used in the starch industry
for the complete separation and washing of fiber is the Dorr-Oliver
inclined DSM screen, which consists of a stationary screen housing
equipped with a con cave wedge bar-type screen. The suspension
to be screened is fed tangentially either by gravity or under
pressure into the screen-plate and flows in a direction perpendicular
to the bars. Each bar of the screen surface slices off a layer
of liquid of a thickness approximately one fourth the slot width.
Different types of screens, with slot widths ranging from 50
up to 3 mm, are used in the starch industry.
After rasping, the starch-pulp slurry flows down the DSM screen
by gravity and the pulp and starch are separated. As many as
four screens are operated in series to assure that the starch
dispersion is completely separated from the pulp. The pulp from
one screen is discharged into a basin, redispersed with dilution
water, and pumped to the succeeding screen.
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