Selection of rice grinding sand roller

Rice milling is a processing process in which the rice grains are forced to collide, rub and roll with each other with the help of the rotating milling roller, and the rice grains are partially or completely removed by the grinding and friction, so that the rice grains can meet the predetermined quality requirements. According to the different materials of the roller, the commonly used rice mill can be roughly divided into two structural forms: sand roller rice mill and iron roller rice mill. Compared with the iron roller rice mill, the sand roller rice mill has soft impact, rolling and milling functions, low rice breaking rate, low power consumption, high rice yield and high production efficiency. Therefore, the sand roller rice mill is the most widely used rice milling machine in rice processing and production enterprises. The rice grinding sand roller is a key working part that is frequently replaced in the sand roller rice mill. In a certain range, the quality of the rice grinding sand roller also seriously affects the technical and economic performance indicators such as the output, rice yield, power consumption and rice breaking rate of the rice mill and the economic benefits of the enterprise. How to improve the economic benefits of rice processing and production enterprises is a common concern of rice processing and production enterprises and grain processing science and technology workers.

1 Structure of sand roller at present, the abrasives suitable for rice processing and production enterprises are mainly black silicon carbide and green silicon carbide. Because black silicon carbide is cheap and has sufficient supply, black silicon carbide is the main abrasive for manufacturing rice grinding sand roller at present. Abrasive belongs to barren material and has no binding ability. To make it a rice grinding sand roller with grinding ability, it must use binding agent. At present, the binding agents available for rice grinding sand roller are mainly ceramic binding agent and resin binding agent. The rice grinding sand roll composed of ceramic binder and abrasive is called ceramic rice grinding sand roll, and the rice grinding sand roll composed of resin binder and abrasive is called resin rice grinding sand roll. Ceramic rice grinding sand roller is a mixture of abrasive and ceramic binder (the main component of which is inorganic salt) sintered at high temperature above 1200 ℃ and has a dense structure. It has the advantages of high temperature resistance, wear resistance, acid and alkali resistance and long service life; But it is brittle, easy to damage, poor self sharpening performance and expensive. Because of the complex price and manufacturing process, ceramic rice grinding sand roller has not been widely used at present, so resin rice grinding sand roller is still widely used in rice processing practice and production. Resin rice grinding sand roller is a porous mixture made of resin binder (mainly phenolic resin or epoxy resin) and abrasive (mainly black silicon carbide) sintered at high temperature of 180 ℃. Due to its low sintering temperature, loose structure, easy falling off of abrasive grains, good self sharpening performance, and continuous generation of sharp edge multi angular abrasive grains during the working process, the grinding effect of the sand roller on rice grains is enhanced and the rice milling efficiency is improved. Therefore, the resin rice grinding sand roller (hereinafter referred to as the sand roller for ease of description) has the advantages of no damage to rice grains, low rice crushing rate, high rice yield rate, high rice milling efficiency and low power consumption, It is especially suitable for processing raw grain with poor quality and powdered plasmid. It can be seen that the sand roller widely used in rice processing and production enterprises is a mixture of abrasive, binder and void.

2. At present, The manufacturing process of sand roll widely used by rice processing and production enterprises is as follows: batching → mixing → molding → low-temperature strengthening (about 90 ℃) → grinding outer surface (outer circular surface and both ends) → group grinding groove surface (spiral groove, spiral groove and straight groove, spiral groove and inclined straight groove) → manual grinding of groove transition surface (such as: spiral groove and straight groove and arc smooth connection of spiral groove and inclined straight groove) → bucket drilling air hole → high-temperature sintering (about 180 ℃) → correcting static balance → correcting dynamic balance → spraying black silicon carbide abrasive (outer circular surface and groove surface of sand roller) → packaging and warehousing. Specifically, the manufacturing process of the sand roller is to weigh a certain amount of abrasive (black silicon carbide and a small amount of brown corundum, etc.) and an appropriate amount of resin binder according to the predetermined formula, put it into the powder mixer (turbine mixer, horizontal spiral belt mixer or double shaft mixer, etc.) and mix it evenly, then weigh a certain amount of abrasive mixture, And fill it into the mold cavity equipped with the iron core (the cylindrical inner hole surface, keyway surface, outer cylindrical surface and both end surfaces of the iron core have been mechanically cut to the specified size requirements, and at the same time, in order to further improve the binding ability of the iron core and the abrasive mixture, the outer circular surface of the iron core is also processed with many triangular small grooves, etc.). After the press operates, the sand roll blank with appropriate physical and mechanical strength is obtained, The sand roll blank is heated by a heating furnace at a low temperature (usually using a resistance heating furnace, the furnace temperature is about 90 ℃) and maintained for a certain time, and then strengthened into a sand roll blank with a certain physical and mechanical strength. The outer cylindrical surface, both end surfaces and groove shaped working surfaces (spiral groove, spiral groove and straight groove, spiral groove and inclined straight groove) of the sand roll blank are ground, After manually grinding the groove shaped working surface (such as the smooth arc connection of spiral groove and straight groove or spiral groove and inclined straight groove) and drilling the ventilation hole, it enters the resistance heating furnace for high temperature (the furnace temperature is about 180 ° C, because when the temperature exceeds 200 ° C, the resin binder will melt, and when the temperature exceeds 250 ° C, the resin binder will carbonize). After heating and holding for a certain time, it becomes a semi-finished sand roller with appropriate physical and mechanical strength. Finally, the sand roller product is obtained after correcting static balance, correcting dynamic balance, spraying black silicon carbide abrasive (outer cylindrical surface and groove surface of the sand roller) and packaging.

3. Rice grinding process performance of sand roller at present, the sand roller widely used by rice processing and production enterprises complies with the provisions of the national standard GB 5339-1985 (rice grinding wheel)

[1] The abrasive used to make the sand roller complies with the provisions of the national standard GB 2476-1983 abrasive code [2], and the binder used to make the sand roller also complies with the provisions of the national standard GB 2484-1984 abrasive code

[2] 。 3.1 abrasives are the main raw materials of the sand roller. The abrasives widely used in industrial production can generally be divided into natural abrasives (such as quartz, garnet, natural corundum and natural diamond) and artificial abrasives. Artificial abrasives include ordinary abrasives such as corundum and carbide, superhard abrasives such as artificial corundum and cubic boron nitride, and abrasives with low hardness (such as chromium oxide, iron oxide and glass powder). At present, the sand roller widely used in rice milling practice is usually made of artificial abrasives. According to the difference of the types of abrasives, the abrasives suitable for rice processing and production enterprises are mainly black silicon carbide (code C, original code th) and green silicon carbide (code GC, original code TL). Because black silicon carbide is cheaper than green silicon carbide and has sufficient supply, black silicon carbide is the main abrasive for manufacturing sand rollers at present.

3.2 grain size: the grain size of the sand roller refers to the grain size of the abrasive constituting the sand roller. In fact, the particle size is the size of the abrasive particle size. At present, there are 6 grain sizes suitable for rice processing and production enterprises: 24 #, 30 #, 36 #, 40 #, 46 #, and 60 #. The number of grain size is large, indicating that the abrasive is fine; On the contrary, the number of particle size is small, indicating that the abrasive is coarse.

3.3 hardness: the hardness of the sand roller refers to the firmness of the bonding of abrasive particles, which refers to the difficulty of the abrasive particles falling off under the action of external force. If the abrasive particles are easy to fall off, it indicates that the hardness of the sand roller is low; On the contrary, if the abrasive particles are difficult to fall off, it indicates that the hardness of the sand roller is high. At present, the hardness of the sand roll made of abrasives suitable for rice processing and production enterprises has roughly 5 hardness levels.

3.4 binder: almost all the binders used for the sand roller are resin binders (Code: B, the main component is phenolic resin or epoxy resin). The resin binder has high bonding strength, good elasticity and impact resistance. Therefore, the sand roller can work for a long time at high speed, and has the advantages of good self sharpening performance, high rice milling efficiency, high rice yield, low power consumption and good rice quality; But not resistant to high temperature, acid, alkali and oil. Therefore, when the sand roller is stored, it should be well protected against moisture, mildew, freezing and overheating, and it is not allowed to be mixed with acid, alkali and petroleum substances.

3.5 organization number: the organization number of the sand roller indicates the volume occupancy of abrasive particles in the sand roller, i.e. the abrasive particle rate. The abrasive grains shall be evenly distributed on the surface, radial thickness and axial height of the sand roller, and the microstructure shall be uniform. The organization number of the sand roller reflects the degree of looseness of the sand roller and the number of abrasive particles participating in the grinding action per unit area. The small number of the organization number indicates that the abrasive content of the sand roller is high and the rice milling efficiency is high, so the abrasion of the sand roller is low and the service life is long; On the contrary, the large number of the organization number indicates that the abrasive content of the sand roller is low, the rice milling efficiency is low, the sand roller wears fast and the service life is short. The commonly used organization number and abrasive content of sand roll are shown in Table 3.

3.6 strength the strength of the sand roller refers to the ability of the sand roller to resist its own breaking due to the centrifugal force in the working state of high-speed rotation. In order to ensure that the sand roller is not broken during normal operation, the working speed of the sand roller should not exceed a certain value. The practical production experience shows that the safe working linear speed of the sand roller in the rice processing process should not exceed 20m / s. Obviously, there are many factors that affect the strength of the sand roll. In addition to strictly following the operating procedures during transportation, storage, storage, installation, disassembly and use of the sand roll, the quality factor of the sand roll also has a great impact on the strength of the sand roll. However, when other conditions remain unchanged, the greater the physical and mechanical strength of the bonding agent, the thicker the wall thickness (radial dimension) of the sand roller, the higher the accuracy of the geometric position dimension, and the smaller the static and dynamic imbalance, the higher the strength of the sand roller; On the contrary, the smaller the physical and mechanical strength of the binder, the thinner the wall thickness (radial dimension) of the sand roller, the worse the geometric position and dimensional accuracy, and the greater the static and dynamic imbalance, the lower the strength of the sand roller.

3.7 the roll shaped sand roll can be roughly divided into: special shaped sand roll (such as the double eccentric sand roll of the original widely used mnnf14 rotary screen air jet rice mill) and cylindrical sand roll according to the geometry. The cylindrical sand roll can also be divided into smooth cylindrical sand roll (also known as disk-shaped sand roll) and groove shaped cylindrical sand roll according to the groove shape. The groove shaped cylindrical sand roll can also be divided into straight groove cylindrical sand roll, inclined groove cylindrical sand roll and spiral groove cylindrical sand roll according to the groove shape. Strictly speaking, the straight groove cylindrical sand roller and the inclined groove cylindrical sand roller are also spiral groove cylindrical sand rollers, because when the spiral rise angle is equal to 90 °, the spiral groove cylindrical sand roller becomes a straight groove cylindrical sand roller; When the spiral rise angle is close to 90 °, the spiral groove cylindrical sand roller becomes the inclined groove cylindrical sand roller. According to the relevant knowledge of kinematics and dynamics, it can be seen that: ① due to the particularity of its roll shape (non cylindrical), the moment of inertia of the special-shaped sand roll is larger than that of the cylindrical sand roll with the same specification and the same quality, that is, the electromechanical consumption of the special-shaped sand roll is high; ② In the same cross-section, the uneven gap (gas) between the surface of the special-shaped sand roller and the screen piece and the screen pressing bar leads to the uneven distribution of the whitening pressure, which is easy to damage the rice grains and cause broken rice. Therefore, the imaginary diameter (maximum rotation diameter) of the special-shaped sand roller is usually small in practice and production, and it is appropriate not to exceed 140mm. At present, it has been gradually replaced by the spiral groove cylindrical sand roller. At the same time, because the smooth cylindrical sand roller has a very strong grinding effect on rice grains, it is usually only suitable for the processing of Japonica rice. It can be seen that most of the sand rollers widely used in rice processing and production enterprises are spiral groove cylindrical sand rollers (hereinafter referred to as spiral groove sand rollers for convenience of description). Spiral groove: the spiral groove of the sand roller is composed of positive and negative spiral surfaces. Generally, the spiral surface along the rotation direction of the sand roller is referred to as the forward surface (also referred to as the advancing surface), and the spiral surface against the rotation direction of the sand roller is referred to as the backward surface (also referred to as the auxiliary surface). In the axial section of the sand roll, the angle between the forward surface and the radial direction of the sand roll β It is called the pushing angle, and the included angle between the backward surface and the radial direction of the rice grinding sand roller is γ, Included angle between the forward face and the backward face（ β+γ) It is called groove angle. The size of groove angle is convenient for selecting the specifications of tools (Abrasives) when processing sand rollers, and also indicates the size of groove width. The groove depth is B, the diameter of the spiral groove sand roller is D, and the lead is h. according to the properties of the spiral surface, the spiral rise angle of the spiral groove sand roller can be obtained

π - circumference, taking π = 3.1416.

3.8 service life during the working process of the sand roll, the diameter of the sand roll is gradually reduced due to the gradual shedding of the abrasive particles, which is specifically manifested in the formation of pits on the surface of the sand roll, the gradual narrowing and shallowness of the spiral groove, and even the disappearance of the grinding. It seriously affects the rice milling efficiency and output of the rice mill, resulting in the deterioration of rice precision, the increase of broken rice and power consumption. For the normal operation of the rice mill, the worn sand roller should be replaced in time. Generally, the amount of rice processed by the sand roll from the time it is put into production to the time it is replaced is called the service life of the sand roll. Although it is related to many factors such as the formula, roll shape, manufacturing process flow, structure of the whitening chamber, quality of raw grain, rice processing process flow and operation management of the sand roll, the service life of the sand roll is generally required to be not less than 6000t rice processing.

(4) effect of sand roller on rice quality

4.1 grain size because of the wide rice planting area and the complex and changeable climate conditions, the quality of rice is different. Therefore, the grain size of the sand roller should be different when processing raw grain of different quality. If the sand roller with too coarse grains is selected, not only the scratch on the surface of rice grains by the sand roller is deep, the surface of rice grains is rough and uneven, but also it is easy to damage rice grains and cause broken rice; If the sand roller with too fine grain size is selected, the surface of rice grain is smooth and flat, but the milling efficiency is low and the power consumption is high. Therefore, the sand roller with suitable grain size should be selected according to the quality of raw grain.

Specifically, when the multi-stage light rolling process is adopted, the grain size of the rice grinding sand roller should be coarse in the front and fine in the rear. Because the brown rice just entering the rice mill has a thick skin and smooth surface, and has a large capacity of compression, bending and shear resistance, in order to improve the rice grinding efficiency, the sand rollers with 24 # and 30 # grain sizes can be used; The rice milling machine in the rear is mainly used to whiten the rice. Because the surface of the rice grain is rough and the resistance to compression, bending and shearing is weakened, if the coarse grain size sand roller is used again, the rice grain will be damaged and broken. Therefore, the grain size of the sand roller should be smaller. Generally, 46 # and 60 # sand rollers can be used.

When the process flow of one machine (roller) whitening is adopted, the grain size of the sand roller from the feed end to the discharge end should be coarse first and then fine. For example, the sand roller with grain size of 30 # can be used at the feed end and the sand roller with grain size of 46 # can be used at the discharge end.

It is worth noting that the relationship between the grain size of the sand roller and its working speed should also be well mastered. Generally, when the grain size of the sand roller is relatively coarse, the grinding grain has a greater effect on the rice grain. In order not to break the rice, the working speed of the rice grinding sand roller can be lower; On the contrary, when the grain size of the sand roller is fine, the impact and rolling of the abrasive particles on the rice particles are relatively soft. In order to improve the rice milling efficiency, the working speed of the sand roller can be higher.

4.2 hardness if the hardness of the sand roller is too high, the abrasive particles are difficult to fall off, and the self sharpening performance is poor, the rice particles are slippery and not easy to peel, the milling efficiency is low, the power consumption is high, and the rice temperature rises quickly, resulting in the weakening of the physical and mechanical strength of the rice particles and the formation of broken rice; On the other hand, if the hardness of the sand roller is too low, the abrasive grains are easy to fall off, and the abrasion of the rice grinding sand roller is too fast, which not only seriously affects the service life of the sand roller, but also affects the quality grade of the rice when the abrasive grains dropped in the rice grains are mixed in the finished products. Therefore, the hardness of the sand roller seriously affects the rice milling efficiency, rice yield, rice quality and power consumption. Obviously, when processing raw grain or powdered plasmid raw grain with poor quality, the hardness of the sand roll can be lower (such as soft 2, code L) because the physical and mechanical strength of the rice grain is lower; When processing raw grain or horny grain with better quality, the hardness of the sand roll can be higher because of the high physical and mechanical strength of the rice grain (such as medium hard 1, code P). In short, in order to ensure the quality of rice and reduce the amount of broken rice, in practice and production