Study And Application Of The Fluidized Bed Jet Grinding And Classifying Technology

In terms of the present developing and research conditions of jet grinding and air classifying technology, most of the focus is concentrated on application and development, the forms of the jet grinding and classifying devices, the impact on the jet grinding and classifying performances by various operational parameters. Whereas, it is not very mature in terms of the principles, the characteristics of jet grinding and classification field and even the research on structural parameters. The studies of jet grinding technology in our country were stemmed from 1980s, most of which are limited to modeling other devices; however, the theoretical study and achievements of the jet grinding and classifying technology are extremely fewer.In this dissertation, jet mill and classifier are classified by the principles of jet grinding and classifying. It's the first time, with the aid of the software—FLUENT, the models of fluidized bed jet mill and classifier are set up as a whole and the flowing condition of jet grinding field and air classifying are analyzed and studies. The result of numerical simulation indicates that the higher the Mach number of nozzle design, the quicker the air speed from the nozzle outlet; and for the straight nozzle, the more diverse the jet spray is. The air speed at the supersonic nozzle outlet and the maximum negative pressure appear at the place about 3-4 mm near the nozzle outlet. The appearance of the shock wave from the nozzle outlet induces the greatly reducing of the jet speed. The air speed at the jet spray center increases rapidly. The analysis of the radical velocity, the tangential velocity and the axial velocity in the air classifying area indicates that the radical velocity into the classifier is uneven; the higher the classifier, the more uneven the radical velocity; the higher classifier has partial air leaking and the position and size of leaking are related with the height of the classifier, which leads to the increase of radical velocity into the classifier. According to the numerical simulation, the ratio of the height of the classifier to the diameter should be controlled under 0.5. Because air gradually enters the classifier in the axial process, the air amount gradually decreases from the bottom to the top of the classifier; thus, the axial air speed gradually decreases from the bottom to the top of the classifier, which causes uneven in particle concentration and its diameter size in different areas of the classifier. At the spot several millimeters directing inside from the outside of blades, the tangential velocity comes to the maximum and the tangential velocity directing outside decays rapidly. When the ratio of the height to the diameter is higher, the leaking air formed has obvious impact on the axial velocity and the tangential velocity in the classifying area. These conclusions have a guiding role to some important structural,parameters, such as the fluent-field distribution of supersonic nozzle outlet and the height of classifier.Through analyzing the operating parameters in supersonic nozzle, what should be considered include not only the velocity in nozzle, but also dynamic pressure of nozzle outlet and grinding pressure in the grind district when supersonic nozzle is designed for jet mill. In the nozzle, to accelerate air at the same Mach number, the higher the pressure of the nozzle inlet, the lower requirements of grinding pressure and the higher the dynamic pressure of nozzle outlet. To ensure the requirements of the designed nozzle outlet velocity, grinding pressure should be approximately equivalent to the pressure of nozzle outlet. If the Mach is too high, and the grinding pressure cannot meet the required negative pressure, which will appear shock wave and jet speed is reduced greatly. In terms of the requirement of grinding pressure, dynamic pressure of nozzle outlet and the diverse degree of outlet air, the designed Mach number of air grinding nozzle should be 1.4-2.05. The use of heat compressed air could get higher air speed and improve the productivity of jet mill. At the same time, high temperature air can effectively affect the grinding effect and characteristics.It's the first time to do the analysis on the particle progress which is accelerated by the high-speed gas in the nozzle is conducive to the kinetic energy of high-speed gas can change into grind energy of particles efficiently. And the research is proved that the exit velocity of nozzle decayed extremely fast. The finer the particles, the faster the speed and the shorter the accelerated distance, vice versa.Whereas, it's important to choose the best accelerated distance of fluidized bed nozzle for the best grinding condition. Accelerated distance of nozzles is related to solids load, particles characteristics(e.g. density, injecting feed size) and the product size requirements. Accelerated distance of nozzles is also related to production rate, size stability and size distribution of product. Accelerated distance of nozzles is 10-20 times than the dimensionless distance, but for tougher particles(≥500μm), accelerated distance is over 20 times than the dimensionless distance.Given the air speed which is in the center of the nozzle is high and the probability of particle collisions is frequent, more particles need to add into the jet center for fluidized bed jet mill designed. The author adopts the same axial nozzle to prove the correctness of the conclusion creatively. Different collision ways have different influences on grind efficiency. Through the comparison of experiment and theory among spiral jet mill, fluidized bet jet mill and target jet mill, it's prove that the single jet efficiency is lower than the opposed jet mill, and the opposed jet mill efficiency is lower than the target jet mill.Operating parameters(e.g. nozzle inlet pressure, feed volume and injecting feed size) have great impacts on the characters of air grinding. It's the first time that, the computing method of mass hold up in jet mill chamber is given and validated, and the influence of upward currents on air grinding and classification in the transmission zone is discussed in this paper. It's also the first time to design and study the structure of new typed fluidized bed jet mill. Adding fresh air to the top and the bottom in the fluidized bed jet mill to increase the speed of upward currents, the way is leading fluidized particles to be boiling and reduce the eddy strength. The aim is to reduce the opportunity of regrinding. It is valuable to produce the particles which are rough, high density and high product rate requirement(given size). At present, the technology has been prevailed, which produced lots of benefits for society and economy.And it's the first time to study the technology which is fluidized bed jet mills to control appearances of particles. In the processing progress, the key factor is to control the probabilities of selective grinding, the principles of particles interactions, collision frequency, power, retention time in the grinding zone. The basic factors affecting particle appearances are that grinding structures, nozzle structures, shapes, locations, fluid properties(e.g. pressure, air flow, jet speed, temperature), particle accelerated distances and locations, gas-solid concentration, classification flow and so on. To some extent, it's proved that it's feasible to change appearances of particles by air grinding.The operational parameters, just like the rotating speed of air classifier, air flow, second wind, gas-solid concentration of classification zone and feed size, have great impacts on the characters of air grinding. Through experimental research, it's proved that the operational parameters are related to the cut size and the grading sharpness. The result is conductive to select parameters and design structures for jet mill. Such as new typed air classifier can meet the requirements of submicron classification. And the research is proved that the ratio of the height of the classifier to the diameter is less, instead the grading accuracy is higher.Theoretical analysis and experimental research to the overheated vapor as media in fluidized bed jet mill, it's proved that the energy consumption of vapor jet mill is far less than air jet mill, but the grinding power of vapor jet mill is far better than air jet mill. The results are conductive to saving energy in practice and developing larger equipments. To ensure overheated vapor to go through supersonic nozzle under the dry vapor state, and to control the inlet pressure strictly, outlet pressure, outlet Mach of the nozzle. It's the first time to use low-grade overheated vapor of electric factory as.working media, to superfining the fly ash. It's proved that this method can meet the requirements of low cost and large scale to apply fly ash practically.At present, fluidized bed jet mills have applied in home and abroad, but the researches in the field are extremely fewer. The internal flow of fluidized bed jet mill is too complicate; it's too difficult to use theory and experiment to do system research. The dissertation has studied and analyzed application of fluidized bed jet mill in industrial technology comprehensively and provided some useful advices for industrial design and applied equipments reasonably.