Because of the high hardness of abrasives, the strict and narrow distribution of granule diameter should arrive to the limitative range according to the international standard. The smashing and grading about abrasive micro powder has been considered as one of the hardest powder processing industry. If you adopt the single jet milling, the granule distribution can not be controlled and the cost is very high. What’s more, it causes the over smashing and diseconomy. You need to separate the granules meeting with demands from the crushed materials in time. In recent ten years, people at home and abroad have tried to adopt jet milling and grading system to produce adhesives and have gotten some achievements. Production practices have proved that this system runs stably and reliably with high productivity, small abrasion and easy operation. The product’s granule diameter distribution has met with the requirements of domestic and international standards; has arrived to the international level of 90s, and has earned good social and economic benefits. This system has passed the identification of high-tech products by Chinese Academy of Sciences, which shows that this technology has good promoting prospects. This article focus on the analysis of aerodynamic characteristics of grading system and discusses the system’s operation and grading performance.
System working principles
1. Working principles At present, the most advanced grading machines adopt the type with rotating parts. Because this kind of machine can be controlled well and can precisely classified the fine powder granule diameter. Refer to the grading machine with rotated parts, because of the complex movement of granules and granule groups during the classification; we can not describe them exactly and perfectly. Their working theory is that in the flow field formed by rotating impeller, granules are affected by centrifugal force and air resistance at the same time. The granules affected by the larger centrifugal force can be separated easily, and the granules affected by the larger resistance can be carried out by the air. The granule’s cutting diameter is inversely proportional to the rotating speed n of grading wheel, and is proportional to the air volume of system Q1/2. In order to meet with the demands of fine classification about fine powder, you must debug the system operating parameters well before you ensure the grading machine’s structure and coefficientk.
2. The effects of system operating parameters about classification performance Viewing from the relationship between grading granule diameter dc and rotating speed of grading wheel, the classification of fine powders needs the increase of grading wheel’s rotating speed. Galk pointed out that the grading wheel’s circumferential speed is about 100m/s when the powder’s cutting diameter dc=1mm. When dc=0.5mm, Vt reaches to 200m/s. But the value of n can not increase unlimitedly. Although it reaches to the maximum, the decrease of cutting granule diameter is limited. According to the classification theory, under the situation of unchangeable rotating speed of grading wheel, the correct adjustment about system’s air volume including the assistant air is necessary. When the air volume is small, we can get the smaller cutting granule diameter. That is because the rotating speed of grading wheel decides the size of centrifugal force field. The system’s air volume affects the resistance subjected to the granules. The adjustment of system operating parameter is the largest centrifugal force and the smallest resistance to the granules. The results of system operation are accord with theoretical estimates. It is close to the value when the operation is under the small air volume. Practice has proved that, in order to get the exact classification of fine powder, the simple increase of grading wheel’s rotating speed is uneconomical. You should correctly control the proportion of assistant air and main volume of air. When the system runs with small air volume, the grading wheel’s rotating speed is more sensitive to the change of cutting granule diameter. But the decrease of total air volume leads to the decrease of handling capacity which is uneconomical obviously. Consequently, you should weight the pros and cons when the system is running, and choose the best working parameters.
3. Feeding and detracting about fine powders Because of the poor fluidity and easy join and agglomeration, it is very difficult to the feeding and detracting of powders. The ordinary vibration feeder and Screw feeder has poor uniformity, heavy mechanical wear, join and agglomeration directly affect the efficient of classification. Some theories show that, the surface adhesion of powders dc=10mm is 105 times of its gravity. When dc=1mm, it is 107 times of its gravity. Thus, to overcome the fine powders’ surface adhesion is very important to the fine classification. QLF classification adopts method of combining vibration feeder, blower and gas transmission in long tube, which ensures that the powders entering into the grading room are uniform and quantitative feeding. Three times of wind introducing by the outer jacket of grading room is helpful for the separation of fine powders, which makes the production stable and easy to be operated. When the powder handling capacity reaches to 200kg/h, the quality of product also can be guaranteed.
4. The features of QLM type jet milling and QLF type jet milling grading system in the abrasive production According to the different specification demands about granule diameter abroad, we adopt jet mill smashing and grading machine two steps to organize and carry out the process. First of all, smash about 36 heads of raw materials in QLM-20 jet mill, control the parameters of air jet milling, make the mixed powders (semi-finished products) are in the normal school and the content of the most coarse granules is less than 1～2, or the coarse granules counted by D3 are not excessive. Secondly, feed the mixed powders into grading system. Set the rotating speed of grading wheel and system air volume. Wipe off the extra fine granules. Make the content of fine granules is less than a certain percentage, or the coarse granules counted by D94 are not excessive, and completely is accord with the demands of standard abrasive products. In brief, adopt the technology of jet milling by coarse head and cutting by grading machine by fine head, which realizes the production of fine abrasive by dried method. The fine powder products produced by QLM-20 type jet milling machine and QLF type grading machine are not only meet with W14、W10、W7、W5 fine powder product’s granularity distribution demands according to the GB2477-83 standard used by domestic abrasive powder industry, but also meet with F1000、F1200 and FG2000 granularity distribution demands according to the JIS R6001-1987 industrial standard for abrasive fine powders in Japan. The rate of fine powders reaches to 80% and the products have gained good reputation among the customers from India, North Korea, countries in South-Eastern Asia and China.
5. Conclusion The dried production method of abrasive with jet milling plus grading system mentioned in this article has good promoting foreground by its advanced technology, simple structure, high productivity, easy maintenance, low energy cost and low pollution. Its performance is better than the wet technology. This technology is also suitable for the classification of fine powders in refractory materials, ceramic materials, non-metallic minerals, metallurgy, chemicals, building materials, medicine, food, etc. Classification technology includes two aspects hardware and software. Except the right design and distribution about the system structure and manufacturing, you should strictly and subtly to adjust the system operating parameters. Find the best running situation for the different materials and granularity distribution. Because the jet milling and grading system’s flow field is complex, it is necessary to research the grading theory and performance optimizing.