If the paddy is directly milled, it will not only consume time, reduce the yield, produce more broken rice, reduce the rice yield, but also the finished product has poor color, more grains, low purity, and quality. At the same time, the rice husk contains a lot of crude fiber, which must be stripped. Therefore, in the rice mills plant, after cleaning and removing impurities, the rice husk is removed to get brown rice and then milled them.
In the process of rice processing, the process of removing the rice husk of rice is called hulling, and the machine that helps the rice to remove husk is called the rice hulling machinery. The various types of rice hulling machines currently used are limited by mechanical and technological performance, and it is impossible to hull all the rice at one time. Therefore, after hulling, there is not all brown rice, also have paddy, rice husks, and broken rice. The separation refers to separate the paddy, rice husks, and brown rice. The brown rice is extracted for rice milling. The unhusked rice is returned to the hulling machine for hulling again. Some by-products can be separated according to their properties and application, and make reasonable use. The effect of hulling can directly subsequent processes, and can also affect the quality, output, yield, and cost of the finished product. Therefore, it is required to protect the integrity of paddy as much as possible when hulling, reducing the breakage and bursting of rice grains, to improve the rice yield; try to avoid damage to the smooth surface of brown rice, to improve the effect of grain separation. Maintaining a high and stable hulling rate can increase the output of the hulling machine. It is necessary to save power, reduce material consumption, and reduce production costs.
The rice hulling is realized by applying a certain mechanical force from the rice hulling machine based on the characteristics of the rice grain structure. According to the force method and the way of hulling, rice hulling methods can be generally divided into three types: squeezing and rubbing, end pressure rubbing and tearing and impacting.
1. squeezing and rubbing, refers to the method of squeezing and rubbing the two sides of the rice by two working surfaces with different moving speeds, to get rid of the husk. The equipment mainly includes rubber roller hulling machines and roller belt hulling machines.
2. End-pressure rubbing and tearing off the husk refers to the method in which the two ends of the paddy in the length direction are squeezed, rubbed, and torn by two working surfaces that move at unequal speeds. The equipment used is mainly the sand plate hulling machine.
3. The impact hulling refers to the method in which high-speed moving paddy collide with a fixed working surface to get rid of the husk. The equipment mainly includes a centrifugal hulling machine.
The rice husk removed from the paddy is also called the big chaff. Because the rice husk has a small specific gravity, small volume, and poor fluidity, it will affect the process effect of the subsequent processes or normal production if it is not separated in time after hulling. For example, in the separation of rice husk, if a large amount of rice husk is mixed, it will inevitably affect the fluidity of the rice husk mixture, and make it unable to form an automatic classification, which will reduce the separation effect. If too much rice husk is mixed with paddy when returning to the hulling machine, the output of the hulling machine will be reduced and power will be consumed. Therefore, the rice hull separation process must be immediately after the rice hulling process.
The separation of rice husk uses the difference in the physical properties of it and separate them from each other.
The suspension speed of rice husk is quite different from that of paddy and brown rice. Therefore, winnowing is the best way to help separate the husk. Also, the characteristics of large differences of rice husks between relative density, bulk density, the friction coefficient can also be used, which is more conducive to the improvement of the winnowing separation effect and the reduction of energy consumption. Generally, a rice husk separation device is installed in the lower part of the hulling machine. The rice husk separated by winnowing needs to be recycled, which is an important step in the paddy processing process. All the rice husks need to be collected for storage, transportation, and comprehensive utilization, and the discharged air must meet the specified dust-containing standards, so as not to pollute the air and affect environmental hygiene. The commonly used methods of rice husk collection are gravity sedimentation and centrifugal sedimentation.
The Separation of Paddy and Brown Rice
After the rice husk is separated, the remaining brown rice mixture must be separated before the rice is milled. The process of separating the unhusked paddy from the brown rice is called brown rice separation. The pure brown rice is separated and then milled in a rice mill, and the separated paddy is returned to the hulling machine for shelling.
The separation of paddy and brown rice is based on the difference in physical properties such as bulk density, particle size, friction coefficient, suspension speed, relative density, and elasticity, and the automatic classification of the paddy and brown rice mixture during the movement, that is, the paddy floats up and the brown rice is down. Adopting appropriate mechanical motion form and device for separation and sorting. At present, there are two types of grain separation equipment widely used in rice mills: gravity grain separator and grain selection flat rotary screen.
The brown rice separated after separated of paddy and brown rice must be free of paddy (some indicators require that the brown rice contains no more than 40 grains/kg). If the brown rice contains too much paddy, it will affect the effect of the rice milling process and reduce the quality of the finished rice. It can be seen that the separation of paddy and brown rice is an indispensable process in rice processing, and the process requirements are very high.