CONVENTIONAL TYPE RICE MILL
The description is a reference to an improved conventional type rice mill and
the layout could refer to conventional rice mills with a paddy intake capacity of about
2 tons paddy/hour or more. For smaller mills the machine layout is normally much
more simplified. Reference is made to the machine numbers as indicated in Figure
92.
Description
Uncleaned dry paddy, still mixed with a large range of impurities is unloaded
in a hopper, and by means of an elevator discharged in a holding bin (Figure 92)
(item 1).
From this bin the paddy is weighed in an automatic hopper scale determining
the weight of the uncleaned paddy. (item 2) This weighing inside the rice mill can be
by-passed if outside weighing facilities for the determination of the intake weights are
available.
After weighing, the paddy is elevated again, unloaded in a hopper and from
this hopper the paddy is. discharged in a precleaner with built-in aspiration for light
impurities, an oscillating double sieve for the heavy impurities and a magnet in order
to trap any iron particles. (item 3) This precleaning machine should be able to
separate:
- light large impurities
- light small impurities
- heavy large impurities
- heavy small impurities
- iron
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The light small impurities, mainly dust, is blown inside a cyclone for
separation and discharge. (item 4) All other impurities are discharged in sacks or
containers.
In general, it will not be possible to separate in this type of precleaning
machine the small stones of about the same size as the paddy grains.
In order to separate these stones the grain now passes a "destoner" or "gravity
separator" which separates the stones from the paddy by making use of the
differences in weights of stones and paddy grains. (item 5).
The paddy then passes a second automatic hopper scale which determines the
weight of the clean paddy which will actually be processed in the rice mill. (item 6) By
having the weight determined two times namely on "uncleaned" basis and on "clean"
basis, it will be possible to determine the purity degree of the paddy for purchasing
and efficiency purposes.. (item 2) (item 6).
The clean paddy is elevated, unloaded in a holding bin for discharge in the
underrunner-disc-huller. (item 7) This huller is called an "underrunner disc huller"
because there are two horizontally installed discs, coated with a ring of abrasive
material of which the top disc is in a fixed position and of which the lower disc is
rotating. The paddy passes through these discs and through an abrasive action the
husks are removed from the brown rice. However, about 10 to 15% of the paddy will
not be dehusked. By using an abrasive coating for the dehusking, the pericarp of the
brown rice will slightly be damaged which produces a coarse bran. Cracked paddy
grains will produce brokens the moment the pericarp will be damaged in addition to
some breakage as result of the dehusking operation.
The huller discharge therefore is a mixture of:
- husks
- brown rice
- paddy
- coarse bran and
- brokens (brown rice)
The huller discharge is unloaded on a double sieve which will first separate
the coarse bran and after that separate the small brokens (brown rice). The coarse
bran - which is a commercial by-product is collected in a sack or container. (item 8).
The same can be done with the small brokens (brown rice) but normally these
brokens are unloaded in the boot by the elevator feeding the first whitening machine.
(item 16).
The huller discharge is now free from coarse bran and small brokens which
products can no longer be blown out with husks.
The huller discharge is now elevated - unloaded in a bin and fed into the
husk-aspirator for the separation of the husks. (item 9) (item 10).
The husks are blown out of the building for discharge and separation in a
cyclone. (item 11).
The product, discharged by the husks - aspirator, is now a mixture of paddy
and brown rice. Normally, the presence of immatured grains is ignored in the
conventional processing methods.
This mixture, - elevated and unloaded in a bin, is now discharged in a
compartment type paddy separator where the paddy is separated from the brown rice
by making combined use of the differences in smoothness, density and bouyance of
paddy and brown rice. (item 12).
The paddy - to be reprocessed in the de-husking section, is now conveyed into
a separate holding bin, and will be dehusked in a separate underrunner disc huller.
(item 13) (item 14) This underrunner disc huller can have the fixed disc coated with a
rubberring instead of abrasive material. Sometimes, a rubber roll huller is used as
return huller.
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The discharge from the second underrunner disc-huller (also called return
huller) is conveyed into the same holding bin feeding the double sieve installed after
the first underrunner disc huller. (item 14) (item 15) (item 8).
The brown rice discharged from the compartment type paddy separator is
unloaded in a holding bin feeding the first whitening cone coated with abrasive
material. (item 7) (item 12) (item 16).
In the first whitening cone, the pericarp of the brown rice and the outer bran
layer is removed. To a certain extent points - germs and small rice particles are
separated from the brown rice during this first whitening pass.
A dark colored bran is discharged out of the machine through a built-in bran
scraper and by gravity. The discharge of this first whitening cone is a mixture of
undermilled rice and brewers' rice (points, germs and small rice particles).
This discharge of the whitening cone is unloaded on an oscillating sieve with
small round perforation for the separation of the brewers' rice. (item 17).
The overflow of the sieve - namely the undermilled rice is now elevated and
unloaded in a small holding bin.
Out of this bin, the undermilled rice is fed into a second whitening cone for
more bran removal. (item 18) Here again germs, points and small rice particles
(brewers rice) will be produced.
The bran discharged out of the machine - by gravity is lighter in color and is
called "medium bran".
The discharge of the second whitening cone is a mixture of medium milled rice
and brewers' rice.
This mixture is now unloaded on a second oscillating sieve with small round
perforation for the separation of the brewers' rice. (item 19) Normally, the sieve for
separation of brewers' rice after the first whitening pass and the sieve for the
separation of brewers' rice after the second whitening pass are combined into one
double layer sieve for a still independent separation of brewers' rice after each pass.
(item 17) (item 19).
The overflow of this second sieve, called "the medium milled" rice, is
discharged in an elevator for unloading in a holding bin. (item 19).
The medium milled rice now passes the third whitening cone for final bran
removal. (item 20) Here too brewers' rice is produced.
The bran produced by this machine and discharged by gravity is now very
light in color.
The discharge of this third whitening cone is a mixture of fully milled rice and
brewers' rice. This mixture passes the third oscillating sieve with small round
perforation in order to separate the brewers' rice. (item 21).
The milled rice overflow of this third sieve is now elevated in a small holding
bin. This fully milled rice does not look shiny since still a small quantity of loose bran
is sticked to the grains.
For that reason the fully milled rice will pass through a polishing machine of
which the cone-shaped cylinder is covered with leather strips. In this machine the
loose bran is removed and to a very limited extent some points and small rice
particles will be produced as brewers' rice. The light bran is discharged by gravity
and the brewers' rice is removed by passing the polished milled rice over a fourth
oscillating sieve with small round separation. (item 23) (item 16, 18, 20) The three
whitening cones - all three coated with an abrasive material and the polishing cone,
covered with leather strips are connected with an aspiration system for grain cooling.
At the same time, this aspiration system removes some bran which is recovered
through a cyclone. (item 22) (item 24).
Normally, the grain discharge from the polishing machine is considered as the
final product of the rice mill and consists of a mixture of head rice, large brokens and
small brokens.
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This mixture is discharged in an elevator and conveyed into a holding bin, -
by-passing any grading equipment - for final weighing, bagging, sewing and storages.
When, however, a final milled rice of a given composition in terms of head rice,
large brokens and small brokens is to be produced, then grading of rice becomes
necessary.
Under these circumstances, the milled rice discharged by the polishing cone
and its sieve is elevated and is unloaded in a first indented grading cylinder also
called "Trieur". (item 22) (item 23) (item 25).
In this machine, milled rice is graded by length. In this first trieur, we can
separate the small brokens for immediate discharge in a special holding bin. (item
26).
The overflow of the first trieur is a mixture of head rice and large brokens.
This mixture is fed into the second trieur for separation of the large brokens, which
too will be discharged in a special holding bin. (item 27) (item 28) The overflow of the
second trieur, being head rice only, is unloaded in a holding bin for head rice. (item
29).
Under each bin is a volumetric unloader also called a "volumetric mixer". (item
30) The unloading capacity per time unit of each volumetric mixer can be pre-set
upon requirement so that e.g. 80% head rice, 12% large brokens and 8 % small
brokens (or any other combination with a total of 100%) can be discharged.
After having properly set the volumetric mixers, the head rice, large brokens
and the small brokens can now be unloaded simultaneously in pre-set quantities on
a belt conveyor, for elevation into a holding bin with built-in mixing device. (item 31)
(item 32).
After that t