PART II. THE PRECLEANING OF PADDY

Transcription

1 PART II. THE PRECLEANING OF PADDY Prior to the actual milling operation, the paddy received from the farmer is cleaned in precleaning machines. Foreign matter or impurities are removed to protect the processing equipment and improve the final product. The impurities can be divided into large impurities, small impurities, and impurities of about the same size as the paddy grain. Large impurities normally consist of rice straw, panicles, bag string, soil, stones, and sometimes iron parts. Small impurities consist of dust, sand, soil particles, weed seeds, insects, and small stones. Impurities of about the same size as the paddy grains can be empty grains, stones, and iron particles. In the precleaning process use is made of differences in the size, weight, and sometimes length of the impurities compared to the paddy grain. Impurities lighter in weight than paddy can be removed by aspiration or by sieving. Large and small impurities heavier than paddy are removed by sieving; whereas, particles the same size but heavier can be removed by gravity separation. Foreign material about the same weight and size as the paddy grain is difficult to remove, and it is presumed to dis-integrate during the actual milling process. Weed seeds are generally small impurities normally separated through sieves. If they are not separated during the precleaning process, the seeds will not be processed during the milling operation and will finally be mixed with the end product, consequently downgrading the white rice. In those rice mills that produce mainly white rice for high-quality markets, trieurs (rotating grading cylinders) are used to remove the seeds based on differences in their length as compared to paddy grains. Iron parts or particles are removed by sieving, by gravity separation, or by permanent or electro-magnets. 1. OPEN DOUBLE-SIEVE PRECLEANER (FIG. 96) In most of the rice mills in the Philippines, simple precleaning is performed through open double-layer oscillating sieves. The precleaners are driven directly by an eccentric drive from the main transmission shaft. The frequency of oscillation is equal to the speed of rotation of this transmission shaft and varies between 300 and 400 double strokes per minute. The paddy is fed into the top of the machine (A), and passes to the first sieve (1) that has large perforations and retains only large impurities, which are conveyed to the overflow (B). The second sieve (2) with small perforations holds the paddy and allows small impurities to fall to the bottom pan where they are discharged (D). The precleaned paddy, held by the second sieve, is then removed (C). This type of sieve. has many disadvantages: (1) Since it is an open sieve, dust formation is considerable, and in fact this machine is mainly responsible for the very dusty appearance of the cone-type rice mills in the Philippines; (2) None of these precleaners has been equipped with self-cleaning sieves and consequently, most of the bottom sieves with small perforations are clogged, and separation efficiency is reduced considerably. Consequently, many weed seeds are not removed and white rice produced in the cone-type mills of the Philippines holds many weed seeds; (3) Impurities of about the same size as the paddy grains are not separated. The precleaning efficiency of this type of machine, therefore, is insufficient. 1

2 Fig. 96. Open double-sieve precleaner. Fig. 97. Self-cleaning sieve. (A) Sieve frame showing square compartments and different mesh sizes. (B) One or two rubber balls are used in each compartment. (C) Balls strike particles in top mesh and knock them free. 2

3 2. SELF-CLEANING SIEVES (FIG. 97) The introduction of self-cleaning sieves will improve the operation of the open double-sieve precleaner. Self-cleaning sieves are not only used in precleaning machines but also in other machines where sieves with round or slot perforations and wire screens are used in the rice milling process. A self-cleaning sieve consists of a wooden frame dividing the sieve surface into square compartments of about 20 x 20 cm or larger. The top of this frame is covered with the selected perforated sheet or wire screen, and the bottom is covered with a wire screen of large mesh for free discharge of the separated products (A). In the compartments are one or two rubber balls (B) varying between 1.9 and 3.2 cm (0.8 and 1.3 inches) in diameter depending on the size of the machine. During the operation of the machine these balls continuously hammer the top sieve and knock the particles, which normally clog the sieve, out of the perforations (C). Nevertheless, these self-cleaning sieves must be cleaned manually at least once a day. 3. SINGLE-ACTION ASPIRATOR-PRECLEANER (FIG. 98, 99 and 100) To avoid the formation of dust by the pre-cleaner, the machines can be combined with an aspirator (Fig. 98, I) and are known as closed-type aspiratorprecleaners. These units consist of a stationary section and an oscillating section (Fig. 98, II). The stationary section is fixed in a wooden or iron frame and houses a suction fan. The oscillating section hangs in this frame and is driven by an eccentric from a transmission shaft (Fig. 99). The paddy is fed into this machine through an opening in the top (Fig. 98, A). The suction fan draws air through the film of grain and separates all dust and light impurities (Fig. 98, 1). The flow of the grain and the capacity of the suction air can be controlled (Fig. 98, 2). The light impurities drop to the cone-shaped bottom of the aspiration housing for automatic discharge (Fig. 98, F) through double air valves (Fig. 100). The grain falls to a sieve with large perforations, for discharge of large heavy impurities (Fig. 98, B), and is unloaded on the long top sieve of the oscillating section, where more large impurities are removed (Fig. 98, C). Quite often magnets are installed under the guide plate of the short top sieve (Fig. 98, 3). The paddy and remaining impurities fall to the bottom sieve, which allows the small impurities to pass through (Fig. 98, D). The overflow of this sieve is the precleaned paddy (Fig. 98, E). The fan in the top section of this machine blows air and dust in a cyclone for dust separation, making the entire precleaning operation practically dust-free. Fig. 98. Closed-type single-action aspirator-precleaner. 3

4 Fig. 99. Unmoving (I) and oscillating (II) section of aspirator-precleaner. Fig Double air valves used for automatic discharge. 4. DOUBLE-ACTION ASPIRATOR-PRECLEANER (Fig. 101) This machine is basically the same as the single-action aspirator-precleaner. The only difference is that the precleaned paddy passes a second airstream for final removal of dust. The back part of the unmoving section of the machine (I), housing the suction fan, is equipped with an air compartment through which a controlled airstream is sucked by the blower (4). The air passes first through the precleaned paddy (E), absorbing all remaining light impurities, which will then drop into a coneshaped section at the bottom of the aspirator housing for automatic discharge through double air valves. Being a doubleaction suction unit, the aspirator housing of this machine is equipped with two sets of double air valves for impurity discharge (F). 5. DOUBLE-DRUM TYPE PRECLEANER (Fig. 102) A number of drum-type paddy precleaners have been designed, one of which is a double-drum aspirator-precleaning unit, produced in West Germany. This machine basically consists of two horizontal perforated rotating cylinders and a suction fan. The first cylinder has small perforations through which small heavy impurities are removed (B). At the same time an airstream is sucked through this cylinder, removing most of the light impurities. These impurities drop into the cone-shaped bottom of the fan-housing section for automatic discharge through a rotating air valve (F). The paddy and its large impurities then reach the second horizontal cylinder, this time equipped with a perforated sheeting of larger perforations for the separation of the large impurities (C). The precleaned paddy passes through this cylinder for direct discharge into the rice mill hulling section (E). The fan blows the air and dust into a cyclone for dust separation (D). 4

5 The flow of paddy into this machine (1) and the airstream can be controlled by adjustable valves (2, 3). Fig Closed-type double-action aspirator-precleaner. Fig Double-drum type of precleaner (West Germany). 6. SINGLE-DRUM PRECLEANER (Fig. 103) The Japanese have developed single-drum type precleaners, sometimes with gravity separation of stones. One of the single-drum type precleaners is made in combination with an oscillating sieve. The horizontal rotating cylinder is covered with a wire screen of large mesh for the separation of the large impurities (B). Some of the large impurities, however, are separated by a vibrating inclined sieve prior to the drum cleaning process (1). The paddy and the smaller impurities drop through the wire screen of the drum and 5

6 reach the oscillating sieve as a film of grain. On the way down, an airstream is sucked through the film of grain, removing light impurities and dust for discharge by the suction fan (C). This fan blows the air and dust into a cyclone for the separation of the impurities. The oscillating sieve is a double-layer type. The top sieve has large perforations for the separation of the larger impurities (D) and the bottom sieve has small perforations for the removal of small impurities (E). The overflow of the bottom sieve represents the precleaned paddy (F). Paddy input is controlled by an adjustable valve (2), and equal distribution of the paddy over the full width of the vibrating top sieve and rotating cylinder is secured by a rotating paddle and adjustable flap (3, 4). The wide screen of the cylinder is continuously cleaned by a rotating drum cleaner (5) and a special scraping device on top of the oscillating sieve removes the separated impurities. Fig Single-drum type of precleaner (Japan). 7. STONER (Fig. 104 and 105) Stones of about the same size as the paddy grains are separated by gravity separation methods. The basic principle is that the lighter paddy grains are carried away by the airstream, leaving the stones behind. Different types of machines have been designed in Germany, the USA, and Japan. One of the stoners now introduced in the Philippines is made in Japan. It consists of a perforated screen in an inclined position (Fig. 104). Through a parallel mechanism driven by an eccentric, this sieve makes an upward movement (Fig. 105). As a result of the upward movement of the sieve, little stones are transported to the top of the sieve. Any paddy grains transported upwards with the stones are blown free by a smaller second blower mounted under the sieve (Fig. 104, 5). The stones are collected under a flap (Fig. 104, 1) and are unloaded into the discharge spout by manually lifting this flap (Fig. 104, B). Because different varieties of paddy are to be processed, the inclination of the sieve has been made adjustable. This is done by manually changing the length of bar a-b, which changes the position of the parallel mechanism carrying the sieve, allowing the sieve inclination to be increased or decreased (Fig. 105). 6

7 The paddy is fed into the top of the machine and its flow and equal distribution are secured by a rotating paddle and adjustable valve (Fig. 104, A). Stoners are always used after the main precleaning process, so that only precleaned paddy is actually fed into these machines. Fig Stoner (Japan). Fig Change in length of bar a-b changes inclination of sieve ( a ). 8. MAGNETIC SEPARATOR (Fig. 106) The bulk of the iron particles mixed with the paddy is removed during the normal precleaning operation as large or small impurities. If no special stoner is installed, iron particles of about the same size as the grains can only be removed by magnetic separators. Nevertheless, we have to take into account that bolts and nuts from the paddy processing machines may get loose any time and if not removed can seriously damage the processing equipment. Therefore, magnetic separators are installed not only in precleaning equipment but also in husk separators and the discharge spouts of elevators. There are simple permanent magnets for machines and/or discharge spouts (Fig. 106, A, B). Another type of permanent magnetic separator is a rotating brass cylinder passing over a half-round permanent magnet (Fig. 106, C). The grain film is transported by the rotating cylinder that unloads the grain in the discharge spout (1), but holds the iron particles attracted on its surface because of the magnet. When the 7

8 cylinder is not moving over the magnet, the iron is automatically released and is discharged separately (2). In some of the larger rice mills special high-capacity magnetic separators are installed for mechanical and continuous discharge of the iron. Most of the magnetic separators are of the permanent type; however, electromagnets may also be used. Fig Different types of magnetic separators: A, B simple permanent magnets; C rotating brass cylinder and permanent magnet. 8

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