Application of air flow pulverization and classification system in the production of silicon carbide powder

Institution: Institute of mechanics, Chinese Academy of Sciences Abstract: This paper introduces the air flow grinding and classifier system for dry process industrial production of silicon carbide fine powder The system is advanced in technology, good in grading and cutting particle size sharpness, strong in regulation and adaptability, high in productivity, small in abrasion and environmental pollution, and the products completely meet the requirements of national and international standards The aerodynamic characteristics of the grading system are analyzed to classify the impact The factors of performance are discussed With the development of industrial production, the progress of science and technology and the increasing market demand, higher requirements are put forward for the preparation of powder materials, especially for the preparation of micro powder, including high productivity, low energy consumption, narrow particle size distribution and low pollution This requires new concepts and advanced technology to develop process flow and organize production Due to the high hardness of the abrasive itself, the strict and narrow particle size distribution must reach the scope limited by the national standard, so the grinding and classification of the abrasive fine powder is recognized as one of the powder processing industries with great difficulty Zui It is not only difficult to control the particle size distribution with a single air flow, but also high energy consumption, which will lead to over comminution and is extremely uneconomical Therefore, it is necessary to separate the qualified particles from the comminuted materials in time In the past ten years, domestic and foreign attempts have been made to produce abrasives by air flow grinding and grading system, and some results have been achieved [1] Based on the successful development of qlm-630 air flow mill, our company has developed SFJ series classifier in recent years, and successfully applied these two systems to the industrial production of thousand ton green silicon carbide, replacing the traditional wet process It has been put into operation in Mudanjiang for more than one year It has been proved that the system is stable, reliable, high productivity, small wear and easy to operate The particle size distribution of the product fully meets the requirements of the domestic and international standards It has reached the international advanced level in the 1990s and achieved good social and economic benefits The system has also passed the appraisal of high and new technology products by the Chinese Academy of Sciences This technology has a good application prospect This paper focuses on the analysis of the aerodynamic characteristics of the classification system, and discusses the influence of the operating conditions of the system on the classification performance 1 Working principle and structural characteristics of the system 1.1 working principle at present, the advanced classifier of Zui at home and abroad all adopts the type with rotating parts, because it has good regulation and control performance and can classify the particle size of fine powder As for the classifier with rotating parts, there is no accurate and perfect theory to describe the movement of particles and particle groups in the classification process Its working principle is based on the following: in the flow field formed by rotating impeller, particles are mainly affected by centrifugal force and gas resistance at the same time, and the particles with larger centrifugal force are easy to be separated, and the particles with large resistance are easy to be gas Flow takes out So far, a simplified two-dimensional model of single particle motion [3,4] has been adopted, which uses the balance of centrifugal force and resistance to obtain the graded particle size, i.e the cut particle size DC as follows: (1) considering the structure and system operation parameters of the classifier, (1) the formula has the following relations: (2) in the formula (1) to (2): R VT, VR: the tangential and radial velocity at the classification circle radius of R in the flow field; R0, B, h, Z: the radius, blade thickness, height and quantity of impeller; Q: the air volume of system operation; h, RG, RS: the viscosity, density of air and the density of solid particles, because the air density RG is much lower than the density of solid particles, it can be ignored The simple functional relationship of cutting particle size can be expressed as: from the above formula, it can be seen that the size of cutting particle size is mainly inversely proportional to the rotating speed n of the grading wheel, and is directly proportional to the Q1 / 2 of the air volume of the system In order to meet the requirements of fine classification of fine powder, after the structure of classifier - coefficient K - is determined, the system operation parameters must be well adjusted 1.2 Structural characteristics in accordance with the requirements of users' annual production of kiloton and various specifications of silicon carbide powder, the process flow we adopted is shown in Figure 1, which is two independent off-line and open-loop operation systems The main engine of SFJ classification system is designed with vertical rotating impeller structure The airflow and materials enter the classification chamber from the tangential direction, and the coarse particles can be separated in the centrifugal force field of high-speed rotation of the impeller The system is equipped with auxiliary air: the air introduced by the jacket outside the classification room is "blowing air (tertiary air)", which makes the powder more fully dispersed in the classification room; the lower cylinder is equipped with "holding air (secondary air)" and gap adjusting cone, which can lift up and float the fine particles carried by the large particles and send them to the classification room for classification again, so as to improve the cutting sharpness of classification The cyclone separator is used for the collection of fine powder in the two systems, and the remaining micro dust is collected through a large-area bag type dust chamber The total amount of dust emission completely meets the environmental protection requirements 2 The influence of the system operation parameters on the classification performance 2.1 The relationship between the speed of the classification wheel and the air volume of the system can be seen from the relationship between (3) type classification particle size DC and the speed of the classification wheel n obviously, the speed of the classification wheel needs to be increased for the classification of fine powder GalK et al [4] pointed out that for the micro powder with DC = 1mm, the peripheral velocity VT of the grading wheel is about 100m / s, and when DC = 0.5mm, the VT is as high as 200m / s However, the n value can not be increased infinitely, even if it increases to a large extent, the reduction of the cutting particle size DC is also very limited According to the classification principle, when the rotating speed of the classification wheel is constant, the operating air volume Q of the system, including the reasonable adjustment of the auxiliary air, can obtain a smaller cutting particle size when the air volume is small This is because the rotating speed of the grading wheel determines the centrifugal force field, and the air volume of the system affects the resistance of particles In essence, the adjustment of the system operation parameters is to make the particles obtain the maximum centrifugal force and the minimum resistance of Zui In Figure 2, two grading systems, model 630 and Model 200, are given respectively The change of the cutting particle size DC with the grading wheel speed n under different air volume is estimated by formula (3); the measured cutting particle size of the product for several specifications of silicon carbide micro powder is also shown in the figure It can be seen from the figure that the operation result of the system is qualitatively consistent with the theoretical estimate value, and it is relatively close when the wind volume is small It has been proved that in order to obtain the classification of fine powder, it is uneconomical to simply increase the speed of classification wheel, and it is also necessary to control the ratio of auxiliary air and main air volume reasonably When the system operates with relatively small air volume, adjusting the speed of classification wheel is more sensitive to the change of cutting particle size However, the reduction of the total air volume will lead to the reduction of the treatment capacity, which is obviously uneconomical Therefore, the system operation should weigh the advantages and disadvantages, and select the Zui good working condition parameters 2.2 Because of its poor fluidity, easy agglomeration and agglomeration, the feeding and dispersion of fine powder are very difficult The commonly used vibrating feeder and screw feeder not only have poor uniformity, but also have serious wear of parts, and agglomeration and agglomeration directly affect the classification effect It has been theoretically shown that for DC = 10 mm micro powder, its surface adhesion is about 105 times of gravity, and when DC = 1 mm, it is 107 times of gravity Therefore, it is very important for fine classification to overcome the dispersion of micro powder strengthened by adhesion The fjj grading system adopts the method of vibration feeder + fan off + long pipe gas transmission to ensure that the powder entering the grading chamber is fed evenly and quantitatively The tertiary air introduced by the grading Outdoor Jacket further helps the dispersion of the fine powder, which makes the production run stably and the workers easy to operate When the treatment capacity of the fine powder reaches 200kg / h, the quality of the product can also be guaranteed 2.3 The characteristics of QLM air flow grinding and fjj grading system in abrasive production according to the requirements of different specifications of abrasive particle size distribution at home and abroad, we use the two steps of air flow grinding grinding and classifier grading as shown in Figure 1 to organize and implement the technological process First of all, 36 mesh raw material coarse powder is crushed in qlm-630 air flow mill, and all parameters of air flow mill are controlled, so that the particle size composition of comminuted mixed powder (semi-finished product) is basically normal distribution, and the content of Zui coarse particles is not allowed to be more than 1-2, or the content of D3 coarse particles is not allowed to exceed the standard The second step is to send the mixed powder into the grading system, set the rotating speed of the grading wheel and the air volume of the system, remove the redundant fine particles, so that the content of the fine particles is less than a certain percentage, or the fine particles calculated by D94 do not exceed the standard, which fully meets the requirements of abrasive label products In short, the process of air flow grinding and cutting the coarse head and classifier cutting the fine head is adopted to realize the dry production of fine powder abrasives The silicon carbide powder products produced by qlm-630 type air flow mill and fjj type grading system not only fully meet the particle size distribution requirements of W14, W10, W7, W5 and other fine powder products in gb2477-83 standards used by abrasive industry in China, but also meet the particle size distribution requirements of F1000, f1200 and F2000 and other products in jisr6001-1987 industrial standards used for consolidation of abrasive powder in Japan The rate reached 80%, which was highly praised by Japanese customers Figure 3 shows the results of particle size distribution of typical samples 1,2-fjj-630 type operates in large air volume and small air volume, 4 --- operation of model fjj-200 in large air volume and small air volume figure 2 cutting particle size characteristics (SIC) of classifier Figure 3 particle size distribution conclusion of silicon carbide powder In this paper, the dry process of abrasive production with air flow grinding and classification system is advanced in technology, simple in structure, high in productivity, convenient in operation and maintenance, low in energy consumption and pollution, and its performance is much better than that of wet process, so it has a good prospect in abrasive industry This technology is also suitable for the classification of fine powder in metallurgy, chemical industry, building materials, medicine and other industries Classification technology includes hardware and software In addition to the reasonable design and layout of the system structure and manufacturing, the operation parameters of the system must be strictly and carefully adjusted For different materials and different particle size distribution, it is necessary to find the best operation conditions of Zui In view of the complexity of the flow field of the air mill and the classification system, it is necessary to further study the classification principle and optimization performance Author: Liu Qinian