PART VI. WHITENING AND POLISHING

Transcription

1 PART VI. WHITENING AND POLISHING In the process of whitening, the silver skin and the bran layer of the brown rice are removed. In the process of polishing the whitened rice, the bran particles still sticking to the surface of the rice are removed, and the surface of the rice is slightly polished to give it a shinier appearance. Polishing, therefore, always takes place after completion of the whitening process. Some confusion exists about the words used for these processes. Whitening is sometimes called "polishing" or "milling". Polishing is sometimes called "refining" or "grinding". Three kinds of whitening machines are widely used in the rice processing industry: (1) the vertical abrasive whitening cone; (2) the horizontal abrasive whitening machine; and (3) the horizontal jet pearler. The vertical whitening machine in its present form is based on an initial European design. It is made by many manufacturers all over the world and is still widely used for the production of undermilled rice as well as for first-class. rice milled for quality-oriented export markets. The horizontal whitening machine developed in Japan after World War II is well suited to the short-grain Japanese varieties of rice; however, its introduction in other countries of the Far East and Asia has been delayed because sensitive adjustments are necessary to accommodate medium and long paddy varieties When this machine is fully adjustable to all paddy lengths its introduction as a substitute for the vertical cone-type whitener will have bright prospects. The horizontal jet pearler is basically a different design than the horizontal abrasive whitening machine. Actually the machine is partly a whitening machine and partly a polishing machine, with the whitening performance emphasized. It is called a jet pearler because it blows an airstream through the rice being processed in the machine. It was introduced about 10 years ago and is slowly gaining popularity. Two types of polishing machines are used in the rice mill industry: (1) the vertical cone-type polisher; and (2) the horizontal polisher. The vertical cone-type polisher looks similar to the vertical abrasive whitening cone; however, the abrasive coating is replaced by leather strips and no rubber brakes are installed. The horizontal polisher, which is slowly gaining popularity, was developed in Japan about 10 years ago and is actually based on the same principle as the vertical cone-shaped machine. 1. THE VERTICAL WHITENING CONE This machine basically consists of a cone-shaped cast iron cylinder with an abrasive coating. The cone is fixed on a vertical shaft that rotates either clockwise or counterclockwise. A wire screen that has a mesh size depending on the variety of paddy to be hulled is fixed around the entire cone. The average distance between the cone coating and the screen is about 10 mm. At regular intervals the wire screen is divided into segments by adjustable rubber brakes. These brakes are mm wide depending on the size of the machine. The number of rubber brakes depends on the diameter of the cone and the preference of the manufacturer. In Germany, the number of brakes is determined by the following formula: n = (D/100) - 2 where D = the cone diameter in millimetres. For example if D = 800 mm then n = (800/100) - 2 = 6. Therefore, the number of brakes is six. In Italy a different formula is used: n = D/100. For example if D = 800 mm the number of brakes is 800/100 = 8. 1

2 The rotating whitening cone (Fig. 153) is vertically adjustable so the clearance between the abrasive coating of the cone and the wire screen can be adjusted. This adjustment depends on the variety of rice, the condition of the grain, the process method, and the wear of the coating. The rubber brakes (Fig. 153, no. 4) in the wirescreen frame are adjustable through a simple handwheel adjustment, and their clearance with the cone surface is only about 2-3 mm. The brown rice is fed into the centre of the machine through a small hopper. A vertically adjustable cylindrical sleeve (Fig. 153, no. 1) regulates the capacity and equal distribution of the brown rice over the entire surface of the rotating cone. By centrifugal force the rice is fed between the cone and the wire screen. If no rubber brakes were installed, the rice would very quickly pass through the free space between the cone and wire screen and nothing would happen. However, the presence of the rubber brakes prevents the immediate discharge of the rice. The resistance built up by these brakes brings the grain under pressure and presses it against the abrasive coating of the cone and against the wire screen. This friction removes part of the bran layer. The bran passes through the wire screen and drops into the cone housing. The partly or fully whitened rice leaves the cone, falls into a self-unloading discharge spout (Fig. 153, no. 5), and is fed to a bin for further processing. The bran is scraped from the bottom of the cone housing by a rotating scraper (Fig. 153, no. 6) and is unloaded into a spout for bran discharge (Fig. 153, no. 7). Fig The vertical abrasive whitening cone. The rotating scraper is driven by a gear wheel (Fig. 153, no. 8) fixed on a vertical shaft that is driven by a flat-belt transmission from the vertical cone shaft (Fig. 153, no. 9). Air is sucked through this machine to cool the grain and at the same time to remove part of the bran (Fig. 153, no. 10). This makes it necessary to blow the air to a cyclone for bran separation. Vertical adjustment of the rotating cone is done manually: The entire shaft-cone assembly is moved by moving the housing of the shaft end-bearing (Fig. 153, no. 11). A simple handwheel adjustment, which is mounted on the base of the cone frame, is used to lift or lower the steel bar that supports the bearing housing. The supporting 2

3 bar is controlled by either one or two handwheels. If controlled by one, one end of the supporting bar is hinged to the base of the cone frame. The peripheral speed of the cone should be about 13 m/s, making the speed of rotation of the shaft a function of the diameter of the cone. The larger the diameter of the cone, the lower the speed of the shaft (Fig. 154). Example: If D = cone diameter = 700 mm (0.7 m), and V = peripheral speed = 13 m/s. V = x D x n (m/s) 60 n = 60 x V = 60 x 13 = 355 rpm. x D 3.14 x 0.7 Quite often, whitening cones are not properly used. When the machine is only half-filled with rice, the rubber brake will not be able to build up a constant pressure and the rice will jump into the open space between the cone and the wire screen. This will unnecessarily break the rice, and the abrasive coating and rubber brakes will wear unevenly, making proper adjustment and performance of the machine practically impossible. Therefore, it is always necessary to run this type of machine with a fully loaded cone. Because the whitening cone must run without any vibration, it is important to have this heavy part of the machine properly fixed to the top of the main shaft. To secure a perfect positioning of the cone on this shaft, the top end of the shaft is tapered (Fig. 155,A). However, a small impurity on this part of the shaft or the slightest damage to the tapered surface will make correct assembling impossible and vibration of the cone will result, causing excessive breakage of the rice. This risk is reduced when the taper of the shaft and cone is limited to two small areas, which still secure the same positioning of the cone on the shaft (Fig. 155, B). When the cone is lifted the clearance between the rubber brakes and the cone is increased and must be readjusted. A similar adjustment is necessary if the rubber brakes wear. The rubber brake assembly, therefore, is made easily accessible. By opening the doors of the cone housing the required adjustment can be made by a simple handwheel (Fig. 156). Fig Peripheral speed curve for whitening cone (V = 13 m/s). 3

4 Fig Tapered end of shaft supporting whitening cone. (A) Complete taper. (B) Taper limited to two small areas to minimize the chance of a small impurity or damage to the tapered surface interfering with proper assembly. The replacement of worn brakes has also been made simple, although there are different methods of fixing the rubber on the clamps. The most commonly used system abroad is an external swallow-tail connection between the rubber and wood that secures a perfect clamping, leaves a maximum of solid rubber available for the operation, and makes it possible to use the wooden clamps continuously (Fig. 157, A). The internal swallow-tail connection is also used (Fig. 157, B); however, less solid rubber is made available here and quite often the swallow-tailed wooden clamp is damaged beyond repair. In the Philippines the rubber brake is normally glued on to the wood (Fig. 157, C). This system is not recommended. New rubber brakes are straight square bars (Fig. 158, A). However, during the whitening process, because of the friction on the rubber when the rice passes between the brake and the cone, the brakes wear out and follow the pattern of the cone (Fig. 158, B). Eventually both the top and bottom parts of the brakes will have a so-called "nose" that disturbs the free flow of the rice and is responsible for unnecessary breakage (Fig. 158, B). These "noses" should be removed regularly: The wire-screen casing, which follows the shape of the cone is built of three to eight segments, depending on the size of the cone and the preference of the manufacturer. Sometimes the brake assembly is part of the wirescreen segment and is fixed in the frame of these segments (Fig. 159, A). In another system the brake assembly is permanently fixed in the frame of the cone housing and the wire-screen segments fit between the assemblies (Fig. 159, B). The advantage of this system is that the screens can be replaced without disturbing the adjustment of the brakes. Screen repair or replacement is often necessary. Because of the friction caused by the rice, the wire screen wears out and rice pours through the hole in the screen. This can be determined by checking the bran discharge regularly. When rice is mixed with the bran, it can be presumed that at least one screen segment needs repair or replacement. This replacement can only be done when the machine is empty and not running, so a quick replacement system is recommended. 4

5 Fig Simple adjustment mechanism for rubber brake. Fig Methods for attaching rubber to wooden clamps. (A) External swallow tail. (B) Internal swallow tail. (C) Glue. Fig (A) New rubber brakes are straight and square. (B) The brakes eventually develop "noses" that must be removed to avoid unnecessary rice breakage. 5

6 Fig (A) The brake assembly is part of the wire screen segment and is fixed in the frame of these segments. (B) The brake assembly is permanently fixed in the frame of the cone housing and the wire screen segments fit between the assemblies. Fig Self-centering upper shaft ball-bearing assembly of the whitening cone. In this machine any vibration in the machine components must be avoided to prevent breakage of the rice. In this respect, proper bearing of the shaft and maintenance of these bearings is very important. What has been mentioned about shaft end-bearings of the under-runner disc huller with respect to construction and maintenance also applies to the whitening cones since the same designs are followed. The same applies to the bronze upper bearing, which mainly absorbs radial forces. For larger diameters of the cone this bronze upper bearing can be replaced by a selfcentering ball-bearing assembly (Fig. 160). The entire assembly is clamped on the shaft and moves up and down with the shaft when the whitening cone is adjusted. For this 6

7 reason the bearing assembly has a grooved guide block (Fig. 160, no. 1), that at the same time houses the self-centering ball-bearing (Fig. 160, no. 2). This bearing is clamped on the shaft by a cone-shaped clamp bush (Fig. 160, no. 3). The entire bearing house (Fig. 160, no. 4) is tilled with grease and sealed on the shaft by filterings. The bearing assembly is protected against direct infiltration of bran and rice particles by a steel cover, which is fixed on the shaft. The cone shaft is normally driven by either a flat-belt or a V-belt transmission, similar to those used for the under-runner disc huller. Therefore, what has been mentioned about flat-belt pulley drives, V-belt transmission drives, and belt adjustment problems of the under-runner disc hullers also applies to the whitening cones. However, because three or more whitening machines are usually installed in a mill, the direction of rotation of the machines sometimes requires special attention. When mounted on independent foundations, the direction of rotation is of no importance and can be clockwise or counterclockwise for all machines. However, quite often the frame supporting the cone assembly is a part of the entire rice mill frame that supports practically all the rice-milling machines. A clockwise-rotating machine, in this case a whitening cone, causes a clockwise-directed tension in the frame of the mill. When multiplied by more machines, it may cause an unpleasant vibration of the entire mill frame. This effect can be neutralized by rotating the first machine clockwise, the second machine counter-clockwise, and so on (Fig. 161). In many rice mills the abrasive coating is applied to the cone in a rather primitive way so that it is practically impossible to secure a uniformly shaped working surface. However, very simple and cheap tools can be made that provide a perfectly shaped surface of the cone. In this respect two methods are used. In the first method, with the cone turned upside down, the coating is applied manually and its uniform shape is formed by a rotating scraper (Fig. 162). The second method uses a special cone-shaped mould, following exactly the pattern of the cone (Fig. 163, no. 1). The coating is poured between the mould and the cone body, is rammed by a simple tool (Fig. 163, no. 2) to eliminate air bubbles, and is rounded by a hand scraper (Fig. 163, no. 3). Any correction to the surface of the abrasive coating is done by simple redressing tools, which can be fixed inside the cone housing (Fig. 163, no. 4). The use of these tools is indispensable for achieving smooth and efficient operation of the whitening cone. Fig Direction of rotation of the whitening cones should be alternately clockwise and counterclockwise for multipass whitening. 7

8 Fig Rotating scraper used to shape the whitening cone after it has been recoated. Fig Special tools for the recoating and redressing of the whitening cone: (1) cone shaped mould; (2) simple ramming tool; (3) hand scraper; and (4) redressing tool. In the Philippines a pure emery composition for the coating of the cones is generally used. Normally this composition consists of 100 weight units emery grit 16 (or sometimes 50 weight units of emery grit 16 plus 50 weight units of emery grit 18), 20 weight unit magnesite, and magnesium chloride brine (30 Baume). This composition is not used abroad anymore and has been replaced by a new composition that has both better whitening capacity and efficiency and results in longer life of the coating. The recommended composition includes a new component, silicium carbide (carborundum), whose hardness is about two and one-half times higher than that of emery. The recommended composition for cones is: grit (25% emery grit 16, 50% silicium carbide grit 16, and 25% silicium carbide grit 18); 20% magnesite; and 20% magnesium chloride brine (30 Baume). This composition is for the first and second whitening cones. For the third whitening cone the grit part of the composition can be 25% emery grit 18 plus 75% silicium carbide grit 18. What applies to the positioning of the under-runner disc hullers in a rice mill also applies to the whitening cones; therefore, these machines can be installed on: (1) an elevated foundation (Fig. 164, A); (2) the ground floor level (Fig. 164, B); or (3) working floor level with frame support (Fig. 164, C). A whitening machine removes the bran from the brown rice by friction, and the friction produces heat. When all the bran is removed in one simple whitening pass, the residence time of the grain is increased, and the clearance between cone and screen is reduced. Consequently, friction is applied to the grain, and the grain becomes hot. 8

9 Since friction and heat are responsible for breakage of rice, and are to be avoided, it is recommended that the bran layer be removed in three or even more passes. This reduces the residence time of the rice in the machine and allows the clearance between the cone and screen to be increased. The amount of friction is thereby reduced and as a result the temperature of the rice is lowered. This process is called "multipass whitening" and is aimed at increasing rice mill recovery by reducing the amount of rice breakage (Fig. 165). Multipass whitening produces higher rice mill recovery because there is: (1) an increased head rice yield; (2) a reduced percentage of large brokens, and (3) a reduced percentage of small brokens. Fig Different positions for the whitening cone. (A) Elevated foundation. (B) Ground floor level. (C) Working floor level with supporting frame. Fig Multipass whitening increases rice mill recovery by reducing the amount of rice breakage. Multipass whitening does not involve much more investment than single-pass or double-pass whitening. This is because the capacity of a cone for single-pass whitening is much lower than for multipass whitening because of differences in 9

10 residence time and clearance adjustment. Thus, for a required capacity the size of the cone needed for single-pass whitening is much larger than the size of the whitening cones used for multipass whitening. For example, if a mill requires a capacity of 1200 kg of rice per hour this can be accomplished by: (1) single-pass whitening with one 1000 mm cone (Fig. 166, A); (2) double-pass whitening with two 800-mm cones (Fig. 166, B); or (3) multipass whitening with three 600-mm cones (Fig. 166, C). For cones 600 mm in diameter the capacities for the different whitening processes are: (1) singlepass whitening 700 kg/ha; (2) double-pass whitening 1000 kg/h; and (3) multipass whitening (three cones) 1200 kg/h. Fig Three ways of producing 1200 kg of rice per hour. (A) One 1000-mm cone. (B) Two 800-mm cones. (C) Three 600-mm cones. 2. THE HORIZONTAL WHITENING MACHINE Compared to the sturdy, solidly constructed vertical whitening cone, the horizontal whitening machine developed by the Japanese is more compact in its construction details. Basically the machine consists of a cylindrical abrasive roll (Fig. 167, no. 2) clamped on a horizontal shaft (Fig. 167, no. 1) that runs at high speed (about 1000 rpm) in a cylindrical whitening chamber, which is covered with a slotted perforated sheet (Fig. 167, no. 6). A feeding screw (Fig. 167, no. 4), fixed on the horizontal shaft, feeds the rice into the free space between the abrasive roll and the slotted cylinder. A counter pressure is built up in this space by an adjustable valve (Fig. 167, no. 5) fitted in the rice outlet spout of the machine. The counter pressure of this valve, which is controlled by a weight balance, controls the pressure and friction applied to the rice inside the machine. Over the full length of the cylindrical housing three rows of adjustable steel brakes are installed that can be adjusted from 0 (axial) to 90 (radial) (Fig. 168, no. 1-3). These adjustable brakes direct the position of the rice grain inside the machine during the whitening process to obtain an optimum whitening efficiency. 10

11 For a long grain variety the position of the brakes is 0 (Fig. 169, A). The rice grains are lined up in an axial direction and roll under pressure through the whitening chamber. For short grain varieties the position of the brakes is 90 (Fig. 169, B). In this case the rice is lined up in a radial direction and tumbles through the pressure chamber of the machine. If long grain rice is processed in the machine at the 90 adjustment of the brakes the grain will be badly broken. The brake adjustment has a range between 0 and 90 and the correct angle of adjustment depends on the variety of rice to be processed. This remains a very critical point in the operation of this machine and requires an experienced operator. A disadvantage of this machine is that the clearance cannot be adjusted. Once the diameter of the 250-mm roll is reduced by about 6 mm it must be replaced. The rolls, which are made of grit 32 silicium carbide, cannot be recoated and new supplies from the manufacturers have to be secured continuously. Fig Horizontal whitening machine (Satake). Fig Cylindrical abrasive roll of horizontal whitener showing the slots in the drum and the adjustable steel brakes. 11

12 Fig (A) Brakes of the horizontal whitening machine are positioned at 0 for long grain varieties of rice. (B) The correct setting for short grain varieties is THE JET PEARLER The jet pearler is a machine that, in a Japanese designed whitening process, is used for the last pass of the rice. Its objective is to remove the final part of the bran layer and simultaneously cool the grain through an airstream of ambient temperature. It consists mainly of a horizontal, partly hollow, perforated shaft (Fig. 170, no. 1) on which a cast steel cylinder with friction ridges is clamped (Fig. 170, no. 2). Just behind the two ridges the cylinder has a long opening that allows the passage of air. This cylinder runs in a hexagonal chamber (Fig. 170, no. 5) consisting of two halfhexagonal perforated screens with slotted perforations (Fig. 171, A). A feeding screw (Fig. 170, no. 3), mounted on the solid part of the horizontal shaft feeds the rice into the press chamber of the machine. Here a counter pressure is built up by an adjustable valve fitted in the rice outlet spout of the pearler. A strong airstream is blown by a centrifugal blower (Fig. 170, no. 4) through the hollow shaft and the openings of the cast steel cylinder (Fig. 172, B). The air passes through the rice that is rolling under pressure in the chamber of the pearler and removes the freed bran as it leaves the machine through the slotted screens. The quantity of air blown through the machine can be adjusted by an air sleeve mounted at the air intake side of the blower (Fig. 170, no. 6). The clearance between the hexagonal screen and the cast steel cylinder is adjustable by a screw controlling the distance between the two halves of the screen (Fig. 170, no. 7). Although the feeding pressure of the feeding screw is constant, the pressure inside the chamber is variable because the counter pressure caused by the valve in the outlet spout can be varied by adjustment of the counter weights (Fig. 172, A). The actual pressure on the grain is the total of the feeding pressure and the counter pressure. The rice produced by this machine is free of bran, slightly polished, and cool. However, for medium- and long-grain varieties its performance is not as good as for short grain varieties and there is a considerable increase in the amount of brokens. In addition wear on the screen is considerable and replacement cylinders are quite expensive. Both of these parts are supplied by the manufacturer and a regular supply must be available. 12

13 Fig Details of jet pearler (Satake). Fig (A) Hexagonal perforated screen used in jet pearler. (B) Friction ridges of the cast steel cylinder. (C) Hollow perforated shaft that the air stream is blown through. 13

14 Fig (A) Counter pressure is regulated by a counter-weighted valve. Milling pressure is the sum of the feeding and counter pressures (PF = feeding pressure = constant, Pc _ counter pressure = variable, PM = milling pressure = variable). (B) Air is forced through the hollow shaft and openings in the cylinder and removes the bran from the rice. 4. THE VERTICAL POLISHING CONE This machine looks similar to the vertical whitening cone (Fig. 173); however, there are basic differences: (1) the cone itself is made of a simplified steel construction and is covered with wood on which leather strips have been nailed (Fig. 175); (2) no rubber brakes are used; and (3) the speed of rotation of the cone is about 25% lower than the whitening cone. The feeding of the rice into the machine is identical to the method used for the whitening cone. When the rice enters the space between the cone and the wire screen, it is gripped by the leather strips that roll the grains over each other and against the leather and the wire screen. Under slight pressure the remaining bran particles are removed and the rice becomes shinier or more transparent. This machine produces few brokens and its power consumption is slightly lower than a comparable-sized whitening cone. 14

15 Fig Vertical polishing cone. 5. THE HORIZONTAL POLISHER (Refiner) The horizontal polisher was designed in Japan and actually is based on the same principles as the vertical polishing cone (Fig. 174). It consists of a steel cylinder on which a large number of leather strips (8 x 17 cm) are screwed (Fig. 174, no. 1). This cylinder is mounted on a horizontal shaft that rotates inside a cylindrical chamber covered with slotted perforated screens. The polishing process is identical to that of the polishing cone. The rice leaves the machine through an outlet spout and the bran falls through the screens into a V-shaped collector for discharge by a conveyor. The leather strips can easily be replaced and a local supply can be secured. The horsepower consumption of this machine is low compared with polishing cones of the same capacity, for example: a 2000 kg/h vertical polisher requires about 8 hp and a 2000 kg/h horizontal polisher about 5 hp. Fig Horizontal polishing machine (Satake refiner). Fig (A) Cone of vertical polisher is covered with wood that has leather straps nailed to it. (B) Rice is gripped between the leather strips and the screen to remove the remaining bran and to polish the rice. 15