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RICE MILLING SYSTEM

Nitat Tangpinijkul Post-harvest Engineering Research Group

Agricultural Engineering Research Institute Department of Agriculture

Paper prepared for Training Course on “Grain Post-Harvest Technology” during June 29 – July 12, 2008, at Manhattan Klongluang Hotel, Pathumtani, Thailand. * Reprinted from “Rice Post Harvest Technology” IDRC Publication nr.

053e.

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PART I. RICE MILL SYSTEMS 1. INTRODUCTION

Before World War II, paddy (rough rice) was processed in commercial rice mills of different size or grain flow pattern, but all of them were using more or less the same type of machines, specially as far as it concerns the removal of the husks, the separation of paddy from brown rice and the removal of the germs and bran layers.

Since the end of World War II, however, the Japanese rice mill manufacturing industry developed - and initially for local use only - a type of rice mill which is using machines for the removal of the husks, the separation of the paddy from the brown rice and the removal of the germs and bran layers of an entirely different concept. About 20 years ago, the Japanese manufacturing industry started exporting their equipment and nowadays you will find Japanese type rice mills installed in practically all rice producing countries throughout the world. Here too references can be made to a large range of capacities and different grain flow patterns. Normally, we refer to "conventional rice mills" if we talk about the traditional type of rice mills and to "Japanese rice mills" or "rubber-roll type rice mills" if we refer to the installations as now exported by the Japanese Industry. In both cases, we refer to commercial. rice mill installations.

The purpose of this paper is to describe the two principles involved with the use of machine layouts and grain flow projections.

We therefore refer to: Figures 92 - 93 for: A "CONVENTIONAL RICE MILL" Figures 94 - 95 for: A "JAPANESE TYPE RICE MILL" First, a full description of both systems will be given and afterwards the basic

differences of both systems will be summarized.

2. 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 the milled rice is weighed, bagged, sewed and stored for final distribution through the existing trade channels. (item 33) (item 34).

The final product is then graded mixed fully milled polished rice. However, not always polished rice is required and for that reason quite often

the polishing cone and its sieve are bypassed which can normally be done through adjustments of a simple change-valve in the elevator-discharge-spout. (item 22) (item 23) (item 35).

This completes the basic description of the operation of a conventional type rice mill.

Figure 92 refers to the machine layout Figure 93 refers to the grain-flow pattern

3. JAPANESE TYPE RICE MILL There are two leading manufacturers of rice milling machines for commercial

rice mills in Japan and both of them are using more or less the same principles. It will lead to too many details if we should try to discuss the differences in

the machines as produced by both manufacturers. Their names are: "SATAKE ENGINEERING COMPANY" - HIROSHIMA and

"KYOWA-YANMAR ENGINEERING COMPANY"- KOCHI. Our description refers to a Japanese made rice mill of the Satake concept and

the machine layout could refer to a rice mill with a paddy intake capacity of about 1 ton/hour or more.

For smaller rice milling units a more simplified layout is used. Reference is made to the machine numbers as indicated in Figure 94. Description

Uncleaned paddy, with a moisture content of about 14% - still mixed with a wide range of impurities, is unloaded in a receiving hopper and elevated into a holding bin. (Figure 94) (item 1) (item 2) The paddy is then weighed in an automatic hopper scale for recording of the weight of the uncleaned paddy received for processing.

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If out-door weighing facilities are available, this weighing can be done in the warehouse instead of in the rice mill. After weighing the paddy is transferred into a holding bin.

The next operation will be the precleaning of the paddy and here normally a single drum type precleaning machine is used, which machine is connected with a large capacity suction fan for the separation of the light impurities. (item 3). A magnet can be included for the separation of iron particles.

This type of precleaning machine should be able to remove: - large impurities - light small impurities - heavy small impurities - iron The light small impurities or dust is blown by the suction fan into a cyclone

for separation and discharge. (item 4) The large impurities and the heavy small impurities are separated by gravity and discharged in 'sacks or containers. Iron is trapped by magnets and is to be removed manually.

This type of separators does not separate stones which are of about the same size as the paddy grain.

In order to remove these stones use is made of the differences in weight of stones and paddy and therefore the paddy will be passed through a gravity separator or "de-stoner". (item 5) The stones are discharged by this machine in a small iron container. After this final cleaning the paddy is elevated, unloaded in a holding bin and out of this bin weighed for a second time in an automatic hopper scale, so that the actual weight of clean paddy to be processed by the rice mill, can be determined. (item 6).

The weighing of paddy on "unclean" basis and "clean" basis is important for reasons of purchasing and efficiency computations. (item 2) (item 6).

After weighing the clean paddy is transferred to a holding bin for further processing in the rice mill.

The husks are removed with the use of rubber roll hullers. (item 7) This machine holds two rubberrolls of equal diameter which run at high speed. One roll runs clockwise, one roll runs anti-clockwise. The speed of one roll is about 25% faster than the speed of the second roll. The clearance between these rolls is adjustable. As result of the difference in speed the husks are stripped off without damaging the pericarp. Consequently, this machine does not produce a coarse bran and since the pericarp is still intact, small brown rice particles from cracked grain will not be released.

Breakage through the actual hulling operation is very low. Therefore, we are not concerned about the separation of coarse bran and brokens prior to the aspiration of the husks. However, the Japanese rice mills normally emphasize the separation of immatured grains in two stages.

For that reason, we can summarize that the discharge of the rubberroll huller is a mixture of:

- matured brown rice - immatured brown rice - husks - matured paddy - immatured paddy Since we do not have to separate any coarse bran or small brokens, the

rubberroll huller is normally combined with a husk-aspirator for direct discharge by the huller into this aspirator. (item 7).

The husk aspirator separates the husks from the grain mixture, and blows these husks in a cyclone for final discharge. (item 8).

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The husk-aspirator has provisions to trap immatured paddy grains: which are discharged through a screw conveyor for bagging. Frequently, the rice millers feed these immature grains back into the rubberroll huller by changing the original grain flow pattern. (See Figure 95). The main discharge of the rubberroll-huller-husk aspirator combination is now a mixture of (item 7).

- paddy - about 5-10% - matured brown rice - immatured brown rice This mixture is elevated for unloading in a holding bin. The next step is the

separation of the unhulled paddy from the brown rice. The Japanese use a different type paddy separator namely the "tray-type

paddy separator". (item 9). Here use is made of the differences in length-smoothness and weight between

paddy and brown rice. The machine however, is not fully separating all paddy from the brown rice

and its discharge recognizes three products namely: a. paddy b. paddy still mixed with brown rice c. brown rice The paddy discharged by the paddy separator is conveyed back to the

rubberroll huller for dehusking. (item 10) The mixture of paddy and brown rice is recirculated back into the paddy-separator. (item 11).

The brown rice finally is transferred into a holding bin for further processing. In Japanese rice mills, and mainly as result of quality requirements in Japan,- the immatured brown rice kernels are removed prior to the whitening process. Therefore, the brown rice passes a number of rotating thickness graders for the separation of these immatured brown rice grains. (item 12) The immatured grains are collected in containers.

The matured brown rice is now ready for whitening and discharged in a holding bin. For medium and long grain bold or slender varieties, the Japanese are using two or three horizontal abrasive whitening machines equipped with steel friction pieces for performance control and discharge valve-pressure adjustment for pressure control.

During each pass through one of these abrasive whiteners a certain quantity of germs - points and small rice particles (brewers' rice) is produced but not separated during the actual whitening process (that will be done later).

The brown rice is fed by gravity into the first horizontal abrasive whitening machine for the removal of the pericarp and the outer bran layer. (item 13) Immediately after leaving this machine the now so called undermilled rice is fed into the second horizontal abrasive whitener which machine removes some germs and bran. The discharge is now weakly medium milled still mixed with the brewers' rice. (item 14) This mixture will now be elevated, unloaded in a holding bin and out of this bin the grain is fed into the third horizontal abrasive whitening machine during which pass more germs and bran are removed. (item 15) The product leaving the third abrasive whitener is now strongly medium milled and still mixed with brewers' rice.

The abrasive whiteners, using a sharp abrasive material, make the grain surface looks dull and not very shiny. Therefore, the last whitening pass will be through a friction type whitening machine. This machine consists of a hollow perforated shaft with a steel slotted cylinder running at high speed in hexagonal pressure chamber, lined with a perforated slotted screen with resistance knobs. A discharge valve pressure adjustments controls the pressure on the grain and under pressure, through the hollow perforated shaft and the steel cylinder slots, air is blown through the grain for cooling purposes. The milled rice still mixed with the brewers' rice as discharged by the third abrasive whitener is normally directly fed

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into the horizontal friction type whitening machine, which machine removes more bran and finally produces the fully-milled rice still mixed with the brewers' rice-germs included: (item 16) It is quite normal that the grain flow through these four whitening machines is not interrupted which makes intermediate elevation optional. (between items 14, 15) The fully milled rice discharged by the friction type whiteners, looks more shiny but still loose bran is sticked to the grain. (item 17) For that reason the rice passes a polishing machine - also called "refiner" which is a horizontal drum type frame covered with leather strips and running in a round cylinder with slot perforations. In this machine the loose bran is removed.

The bran as produced by the three abrasive whiteners, by the friction type whitener and by the polishing machines is discharged by suction through a control suction fan and blown into a cyclone for separation and discharge. Different grades of bran can be collected if more suctions fans and cyclones will be installed. (item 18) The product produced by the polishing machine is not a final product yet, since the milled rice is still mixed with germs - maybe some loose bran and the brewers' rice.

Therefore, the polisher discharge is transferred to a holding bin and the grain will now pass through an aspirator which machine removes the germs and bran (if any) for collection through a cyclone. (item 19) (item 20).

The product discharged by the aspirator is a mixture of milled rice and brewers' rice.

This mixture now passes a gyrosifter which machine actually is a plansifter, using sieves with round perforation for the separation of the brewers' rice. The brewers' rice is collected in containers. (item 21) The milled rice discharged from the gyrosifter is normally the final ungraded product of the rice mill and is a mixture of head rice, large brokens and small brokens.

When graded milled rice is to be supplied then the same grading procedure is followed as for the conventional method which procedure we shall summarize as follows.

First trieur for small brokens. (item 22). Small brokens discharged in special holding bin. (item 23). Second trieur for large brokens. (item 24). Large brokens discharged in special holding bin. (item 25). Headrice overflow discharged in special holding bin. (item 26). Headrice - large brokens and small brokens discharged in pre-set quantities

on a belt conveyor through three volumetric unloaders or mixers. (item 28 (item 27). The conveyor unloads the grain in a special holding bin with built-in mixing

device. (item 29). Then the milled mixed rice is weighed by an automatic hopper scale and ready

for bagging, sewing and storage. (item 30) (item 31) The final product of the rice mill is then graded mixed fully milled polished rice without immatured grains. (item 32) However, it is possible to by-pass the thickness graders for the separation of the immature brown rice grains. In that case the final product of the rice mill will be graded mixed fully milled polished rice.

Very frequently also the polishing machine is by-passed and as result of this the final product of the rice mill will be graded fully milled rice. (item 33) This completes the description of the operation of a Japanese type rice mill using some of the processing concepts as introduced by Satake Engineering Company.

4. SUMMARY OF THE DIFFERENCES BETWEEN THESE TWO SYSTEMS. Pre-cleaning

Conventional mills normally use aspiration and oscillating flat sieves. Japanese mills are using aspiration, rotating drum-type sieves and oscillating flat sieves. Hulling

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Conventional mills use underrunner disc hullers and return hullers for the paddy separated by the paddy separators. Japanese mills use rubberroll hullers and do not use special return hullers. Coarse Bran Sieve

Necessary in conventional mills for the separation of coarse bran and brokens prior to the aspiration of the husks. In Japanese rice mills, the hullers do not produce the coarse bran and small brokens. Therefore, no coarse bran sieve is necessary and the rubberroll huller can be combined as one unit with the husk aspirator. Husk Aspirators

In the conventional mills very little or no attention is paid to the separation of immatured paddy grains prior to the separation of the husks. In Japanese rice mills the immature paddy grains can be separated as a function of the husk aspirator. Paddy Separator

In the conventional rice mills the compartment type paddy separators are used producing two products namely paddy and brown rice. In the Japanese rice mills tray-type paddy separators are used producing three products namely paddy, brown rice and a mixture of paddy and brown rice which is to be recirculated. Whitening Machines

The conventional rice mills only use the cone-type abrasive whitening machines with vertical shafts. Japanese mills are making use of abrasive whitening machines combined with friction type whitening machines. Both type of machines are having horizontal shafts. The friction type whitening machine has air blown through the shaft and the grain and recently also the abrasive whitening machines are equipped with this cooling system. Brewers' Rice Sieves

The conventional rice mills are separating the brewers' rice after each whitening pass. The Japanese rice mills separate the brewers' rice after completion of the whitening process. This is the result of the fact that in the Japanese mills the discharge of whitening machines is fed by gravity directly into the next whitening machine making whitening a continuous uninterrupted process. Bran Collection

In the conventional rice mill most of the bran is collected by gravity in sacks or containers. In the Japanese rice mills all bran is collected through an aspiration system. General Remark

Many rice mills are a combination of conventional and Japanese milling methods and in man, conventional and Japanese rice mills different machines, machine-arrangements and grain-flow-patterns are introduced.

The description and summary of the differences for both systems as mentioned in this paper are only to be considered as a guide for better understanding or both rice milling systems.

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