Study On The Movement Characteristics And The Whitening Behavior Of Rice Grains In A Friction Type Mill

The relationship between the production and consumption of paddy in our country is always tight equilibrium.About 85%~90% of the paddy are milled and then used as commodity rice.The84% of the commodity rice is sold to feed residents,while 14% are used in industry or feeding livestock.The paddy and its products are one of the biggest and the most stable grain consumption in our country.Milling plays a significant role in the paddy processing and the better milling quality secures the safety of food.The distinction of whitening behavior and improvement of milling quality are two main problems that restrict the development of milling machinery.In specific,the whitening behavior and reducing the breakage occurs in the milling chamber are bothering the milling industry for a long time.The traditional design methods of mills rely on the experience and most of the basic researches about milling rules are qualitative.The investigation of whitening behavior could bring the breakthrough of the design and optimization of the mill structure,which aims at decreasing the breakage occurs,while the related research can hardly be found.Therefore,it is necessary to understand the whitening behavior in the milling chamber.In this paper,the investigation is to understand the whitening process of rice grains for a vertical friction type mill.To clarify the structure of frictional mill,whitening behavior,the dynamic changes of kernel physical properties,the flow characteristics of rice kernel in each part of the mill and effects of operation parameters on the milling qualities,the investigation are carried out as follows.The analyses that conclude whitening strength,flow properties,force and the contact collision of rice kernel,etc,provide the theoretical foundation for the investigation of whitening behavior and characteristics of movement during the milling.And above analyses are completed based on the granular theory,physical properties of agricultural materials and Hertz contact theory.The milling experiments are carried out in a laboratory-scale vertical mill,and then the physical properties such as length,width,thickness,aspect ratio,volume,equivalent diameter,surface area,sphericity,bulk density,true density,porosity,thousand seed weight and friction coefficient are measured.The milling level can be separated based on the difference of physical properties of different milled rice grains.The correlation is established between the whitening behavior and the change of physical properties of rice kernels.The regression model are obtained using several statistical approaches.The investigation aims at providing the support for the macro understanding of whitening behavior and characteristics of movement.The investigation ultimately aims to explore the microscopic whitening behavior.The discrete element model of the rice kernel is established and the proper contact force model is chosen.The DEM parameters of simulations are obtained and the other contact parameters which are difficult to measure in the experiment are calibrated.In present study,the rolling friction coefficient of rice grains is calibrated by the combination of accumulation test and picture processing.The calibrated value is 0.023.Meanwhile,the parameters of rolling shaft are determined by taking the whitening pressure theory as the guidance.To investigate the microscopic whitening behavior and obtain the optimal structural parameters of the rice mill,the flow characteristics are analyzed during feeding,conveying and milling processes.It indicates that structural parameter of rice mill have a significant effect on the feeding and conveying processes.The inlet has a greater effect on the uniformity of flow pattern and the distribution of particle flow during the conveying,while the rotational speed mainly influences the efficiency and power of conveying.When the cutting edge of square feed inlet has the deflection angle of 17 degree that is same as direction of the rolling shaft,the uniformity of flow pattern can be achieved.The relationship between macro performances of screw conveying and microscopic flow characteristics of rice particles can be described based on the method of polynomial fitting.Milling of rice particles mainly occurs in the center of the milling area(referring as the section of ridge of milling roll).The spatial distribution of collision energy may reflect the wear mass loss.Operation parameters can significantly affect flow properties of rice particles,which the outlet opening size has a significant effect on the collision frequency while the rotation speed mainly affects the collision energy.By taking a comprehensive consideration for flow properties in the feeding,conveying and milling process,the better combination of vertical rice mill,namely type of inlet E,rotational speed of 1400 rpm,the outlet opening size of 4.4 mm is chosen in this paper.Based on above optimal parameters of vertical rice mill,the milling experiments are carried out to explore the effect of operation parameters,including filling level,milling time and the weight applied on the outlet.The head rice yield as the key indicators of milling characteristic impacts on the economic benefits of milling process industry and milling power as the main cost has been an important issue for rice processing enterprises,thus reducing the energy consumption has an important economic value.The whiteness of milled rice has a huge effect on the price for selling.Therefore,the head rice yield,milling power,whiteness are taken as the evaluation indexes.The degree of milling and the temperature rise are taken as the restrictive indexes.The effects of operational parameters on the milling quality are investigated and the regression models are established between parameters and evaluation indexes.And then the optimization results are obtained under the restrictive condition,which refers to milling level of rice grains is satisfied to the grade ? and temperature rise is below 20 ?.The result shows that filling level only has the significant effect on the milling power while not significant effects on the others.The decrease of the whitening pressure and increase of the milling time can improve the milling quality in the actual rice milling process.To decrease the milling power,the weight applied on the outlet should be lower when the capacity is large,while the weight should be higher when the capacity is small.To meet the requirement of consumers which prefer to choose the rice with higher whiteness value,the milling time should be long enough to remove the bran totally.After milling experiments used the vertical rice mill,as the results,the correlation between whiteness and degree of milling is the most significant.The best operation parameter of the vertical rice mill is the combination of weight applied on the outlet of 320 g,a filling level of 35 % and milling time of 30 s.This study can provide the reference for the whitening theory and the design of the vertical friction type rice mill which aims to increase the head rice yield.