Organization: Chinese Academy of Sciences Institute of Mechanics Chinese Abstract: This paper introduces the airflow mill and classifier system for dry industrial production of silicon carbide fine powder. The system is advanced in technology, graded cutting particle size sharpness, regulation and adaptability, The products have high productivity, low wear and less environmental pollution, and the products have fully met the requirements of national standards and international standards. The aerodynamic characteristics of the classification system are analyzed in this paper, and the factors affecting the classification performance are discussed.
original:
With the development of industrial production, scientific and technological progress and increasing market demand, higher requirements have been put forward for the preparation of powder materials, especially fine powders, including high productivity, low energy consumption, narrow particle size distribution and low pollution. This requires a new concept and advanced technology to develop processes and organize production.
Due to the high hardness of the abrasive itself, the strict and narrow particle size distribution must reach the range defined by the national standard, which makes the pulverization and classification of the abrasive fine powder, which is recognized as one of the difficult powder processing industries. With a single airflow pulverization, not only the particle size distribution is difficult to control, but also the energy consumption is high, and the pulverization is caused, which is extremely uneconomical. Therefore, it is necessary to timely separate the granules that meet the requirements from the pulverized material. For more than a decade, domestic and foreign attempts have been made to produce abrasives by airflow pulverization and classification systems, and some results have been obtained [1]. Based on the successful development of QLM-630 airflow mill, our company has developed the SFJ series of classifiers in recent years and successfully applied these two systems to the annual production of thousands of tons of green silicon carbide industrial production, replacing Traditional wet process. The production practice of more than one year in Mudanjiang has proved that the system is stable, reliable, high in productivity, small in wear and easy to operate. The particle size distribution of the product fully meets the requirements of domestic and international standards, reaching the international advanced level in the 1990s. Good social and economic benefits, the system also passed the high-tech product identification of the Chinese Academy of Sciences [2], showing that this technology has a good prospect of promotion and application. This paper focuses on the analysis of the aerodynamic characteristics of the grading system, and discusses the impact of system operating conditions on the grading performance.
1. System working principle and structural characteristics
1.1 Working principle
At present, the advanced classifiers of Zui at home and abroad all adopt the type of rotating parts, which is because of its good regulation performance and can accurately classify the particle size of fine powder. For graders with rotating parts, the movement of particles and particle groups during the classification process is extremely complicated, so far there is no accurate and perfect theory to describe them. The working principle is based on: in the flow field formed by the rotating impeller, the particles Mainly affected by centrifugal force and gas resistance, particles subjected to large centrifugal force are easily separated, and particles with large resistance are easily taken out by the airflow. A simplified two-dimensional model of single particle motion that has been used to date [3, 4] is to use centrifugal force and resistance balance to obtain a grading particle size, ie, a cutting particle size dc.
for:
(1) Considering the structure of the classifier and the operating parameters of the system, the formula (1) has the following relationship:
(2) In equations (1) to (2):
R, Vt, Vr: respectively the tangential and radial velocities of the grading circle radius R in the flow field;
R0, b, h, z: the radius of the impeller, the thickness, the height and the number of the impeller;
Q: the air volume of the system operation;
h, rg, rs: the viscosity, density and density of solid particles, respectively, because the air density rg is much lower than the solid particle density.
A simple functional relationship for cutting particle size can be expressed as:
It can be seen from the above formula that the size of the cutting particle diameter is mainly inversely proportional to the rotation speed n of the classification wheel, and is proportional to the Q1/2 of the system operation air volume. In order to meet the requirements for fine grading of fine powder, after the structure of the classifier - coefficient k - is determined, the system operating parameters must be well debugged.
1.2. Structural features
For the requirements of users with annual output of thousands of tons and various specifications of silicon carbide micro-powder, the process flow we use is shown in Figure 1. It is two independent non-online and open-loop operation systems.
The design of the SFJ type grading system mainframe adopts a vertical rotary impeller structure; the airflow and materials enter the grading chamber from the tangential direction, and the coarse particles are separated in the centrifugal force field where the impeller rotates at a high speed. The system is equipped with auxiliary wind: the grading outdoor jacket introduces the airflow as “blown air (third wind)â€, which makes the fine powder more fully dispersed in the grading chamber; the lower cylinder is provided with “lifting wind (secondary wind)†and The pacing cone can lift the fine particles sandwiched by the large particles and send them to the grading room for further grading to improve the cutting sharpness of the grading. The cyclone separator is used for fine powder collection of the two systems, and the remaining trace dust is collected through a large-area baghouse, and the total amount of dust discharge is in full compliance with environmental protection requirements.
2. Influence of system operating parameters on grading performance
2.1. Classification wheel speed and system air volume
It can be seen from the relationship between the classification particle size dc of the formula (3) and the rotation speed n of the classification wheel, and it is obviously necessary to increase the rotation speed of the classification wheel for the classification of the fine powder. Galk et al. [4] pointed out that for the cutting particle size dc=1mm fine powder, the circumferential speed Vt of the classifying wheel is about 100m/s, and when dc=0.5mm, the Vt is as high as 200m/s. However, the value of n cannot be infinitely increased, and even if it is increased to a large extent, the reduction in the cutting particle diameter dc is very limited. According to the classification principle, under the condition that the speed of the grading wheel is constant, the running air volume Q of the system, including the auxiliary wind, is reasonably adjusted, and when the air volume is small, a smaller cutting particle size can be obtained. This is because the speed of the grading wheel determines the size of the centrifugal force field, and the size of the system air volume affects the resistance of the particles. The adjustment of the operating parameters of the system is essentially to make the particles obtain the largest possible centrifugal force and small resistance.
Two classification systems of type 630 and 200 are given in Fig. 2, and the variation of the cutting particle size dc with the rotation speed n of the classification wheel under different air volume is estimated by (3); the carbonization for several specifications is also marked in the figure. When the silicon fine powder is in production operation, the measured product cuts the particle size. It can be seen from the figure that the system operation results are qualitatively consistent with the theoretical estimates, and are closer when operating with small air volumes. Practice has proved that in order to obtain accurate classification of fine powder, it is uneconomical to simply increase the speed of the classification wheel. It is also necessary to reasonably control the ratio of the auxiliary wind to the main air volume. When the system is operated with relatively small air volume, adjust the speed of the classification wheel to cut the particle size. The change is more sensitive. However, the reduction of the total air volume will lead to a reduction in the amount of processing, which is obviously uneconomical. Therefore, the system should be weighed against the pros and cons when operating, and choose the parameters of the good working conditions.
2.2. Feeding and dispersion of fine powder
Due to its poor fluidity, easy agglomeration and agglomeration, the fine powder causes great difficulty in feeding and dispersing the powder. The commonly used vibration feeding and screw feeder not only have poor uniformity, serious wear and tear, and agglomeration and The agglomeration directly affects the classification effect. It has been shown by theory [3] that for a fine powder of dc=10mm, the surface adhesion is about 105 times of gravity, and when dc=1mm, it reaches 107 times of gravity, thus overcoming the adhesion to enhance the dispersion of fine powder. It is very important for fine grading. The FJJ grading system adopts a combination of vibrating feeder + closing fan + long pipe gas conveying to ensure that the powder entering the grading chamber is uniformly and quantitatively fed. The tertiary air introduced by the graded outdoor jacket further assists in the dispersion of the fine powder, which makes the production operation stable, and the worker is easy to operate. When the amount of the fine powder is as high as 200 kg/h, the quality of the product can be ensured.
2.3. Characteristics of QLM Jet Mill and FJJ Grading System in Abrasive Production
According to the requirements of different sizes of abrasive grains at home and abroad, we use the two steps of jet mill pulverization and classifier as shown in Figure 1, to organize and implement the process. Firstly, the raw material coarse powder of about 36 mesh is pulverized in the QLM-630 jet mill to control the parameters of the jet mill, so that the particle size composition of the pulverized mixed powder (semi-finished product) is basically normal distribution, and the content of the coarse grain of zui is not allowed. The coarse particles in 1-2 or D3 do not exceed the standard. In the second step, the mixed powder is sent to the grading system, the grading wheel speed and the system air volume are set, the excess fine particles are removed, the fine granule content is less than a certain percentage, or the fine particles in the D94 are not exceeded, which completely conforms to the abrasive label product. Requirements. In short, the process of grinding the coarse head by the airflow and cutting the fine head by the classifier realizes the dry production of the fine powder abrasive.
The silicon carbide micropowder products produced by QLM-630 type jet mill and FJJ type grading system not only fully meet the GB2477-83 standard used in China's abrasive industry, but also the particle size distribution requirements of fine powder products such as W14, W10, W7 and W5. In Japan's JISR6001-1987 industrial standard for fixing abrasive micro-powders, the particle size distribution requirements of F1000, F1200 and F2000 products, the fine powder yield reached 80%, which was well received by Japanese customers. Figure 3 shows the results of a typical sample by particle size distribution.
1,2—FJJ-630 runs in high air volume and small air volume
3,4---FJJ-200 runs in high air volume and small air volume
Fig. 2 Cutting machine particle size characteristics (SIC)Fig. 3 Particle size distribution of silicon carbide powder
in conclusion
The airflow grinding and grading system used in this paper has the advantages of advanced technology, simple structure, high productivity, convenient operation and maintenance, low energy consumption and low pollution. The performance is much better than the wet process, and it is well promoted in the abrasive industry. prospect. This technology is also suitable for the classification of fine powder in the metallurgical, chemical, building materials, pharmaceutical and other industries.
The grading technology includes both hardware and software. In addition to reasonable design and layout of the structure and manufacturing of the system, the operating parameters of the system must be adjusted strictly and finely. For different materials and different particle size distributions, it is necessary to find the operating conditions of the zui. In view of the complexity of the flow field of the jet mill and grading system, it is necessary to further study the grading principle and optimization performance.
Author: Liu Qi years
original:
With the development of industrial production, scientific and technological progress and increasing market demand, higher requirements have been put forward for the preparation of powder materials, especially fine powders, including high productivity, low energy consumption, narrow particle size distribution and low pollution. This requires a new concept and advanced technology to develop processes and organize production.
Due to the high hardness of the abrasive itself, the strict and narrow particle size distribution must reach the range defined by the national standard, which makes the pulverization and classification of the abrasive fine powder, which is recognized as one of the difficult powder processing industries. With a single airflow pulverization, not only the particle size distribution is difficult to control, but also the energy consumption is high, and the pulverization is caused, which is extremely uneconomical. Therefore, it is necessary to timely separate the granules that meet the requirements from the pulverized material. For more than a decade, domestic and foreign attempts have been made to produce abrasives by airflow pulverization and classification systems, and some results have been obtained [1]. Based on the successful development of QLM-630 airflow mill, our company has developed the SFJ series of classifiers in recent years and successfully applied these two systems to the annual production of thousands of tons of green silicon carbide industrial production, replacing Traditional wet process. The production practice of more than one year in Mudanjiang has proved that the system is stable, reliable, high in productivity, small in wear and easy to operate. The particle size distribution of the product fully meets the requirements of domestic and international standards, reaching the international advanced level in the 1990s. Good social and economic benefits, the system also passed the high-tech product identification of the Chinese Academy of Sciences [2], showing that this technology has a good prospect of promotion and application. This paper focuses on the analysis of the aerodynamic characteristics of the grading system, and discusses the impact of system operating conditions on the grading performance.
1. System working principle and structural characteristics
1.1 Working principle
At present, the advanced classifiers of Zui at home and abroad all adopt the type of rotating parts, which is because of its good regulation performance and can accurately classify the particle size of fine powder. For graders with rotating parts, the movement of particles and particle groups during the classification process is extremely complicated, so far there is no accurate and perfect theory to describe them. The working principle is based on: in the flow field formed by the rotating impeller, the particles Mainly affected by centrifugal force and gas resistance, particles subjected to large centrifugal force are easily separated, and particles with large resistance are easily taken out by the airflow. A simplified two-dimensional model of single particle motion that has been used to date [3, 4] is to use centrifugal force and resistance balance to obtain a grading particle size, ie, a cutting particle size dc.
for:
(1) Considering the structure of the classifier and the operating parameters of the system, the formula (1) has the following relationship:
(2) In equations (1) to (2):
R, Vt, Vr: respectively the tangential and radial velocities of the grading circle radius R in the flow field;
R0, b, h, z: the radius of the impeller, the thickness, the height and the number of the impeller;
Q: the air volume of the system operation;
h, rg, rs: the viscosity, density and density of solid particles, respectively, because the air density rg is much lower than the solid particle density.
A simple functional relationship for cutting particle size can be expressed as:
It can be seen from the above formula that the size of the cutting particle diameter is mainly inversely proportional to the rotation speed n of the classification wheel, and is proportional to the Q1/2 of the system operation air volume. In order to meet the requirements for fine grading of fine powder, after the structure of the classifier - coefficient k - is determined, the system operating parameters must be well debugged.
1.2. Structural features
For the requirements of users with annual output of thousands of tons and various specifications of silicon carbide micro-powder, the process flow we use is shown in Figure 1. It is two independent non-online and open-loop operation systems.
The design of the SFJ type grading system mainframe adopts a vertical rotary impeller structure; the airflow and materials enter the grading chamber from the tangential direction, and the coarse particles are separated in the centrifugal force field where the impeller rotates at a high speed. The system is equipped with auxiliary wind: the grading outdoor jacket introduces the airflow as “blown air (third wind)â€, which makes the fine powder more fully dispersed in the grading chamber; the lower cylinder is provided with “lifting wind (secondary wind)†and The pacing cone can lift the fine particles sandwiched by the large particles and send them to the grading room for further grading to improve the cutting sharpness of the grading. The cyclone separator is used for fine powder collection of the two systems, and the remaining trace dust is collected through a large-area baghouse, and the total amount of dust discharge is in full compliance with environmental protection requirements.
2. Influence of system operating parameters on grading performance
2.1. Classification wheel speed and system air volume
It can be seen from the relationship between the classification particle size dc of the formula (3) and the rotation speed n of the classification wheel, and it is obviously necessary to increase the rotation speed of the classification wheel for the classification of the fine powder. Galk et al. [4] pointed out that for the cutting particle size dc=1mm fine powder, the circumferential speed Vt of the classifying wheel is about 100m/s, and when dc=0.5mm, the Vt is as high as 200m/s. However, the value of n cannot be infinitely increased, and even if it is increased to a large extent, the reduction in the cutting particle diameter dc is very limited. According to the classification principle, under the condition that the speed of the grading wheel is constant, the running air volume Q of the system, including the auxiliary wind, is reasonably adjusted, and when the air volume is small, a smaller cutting particle size can be obtained. This is because the speed of the grading wheel determines the size of the centrifugal force field, and the size of the system air volume affects the resistance of the particles. The adjustment of the operating parameters of the system is essentially to make the particles obtain the largest possible centrifugal force and small resistance.
Two classification systems of type 630 and 200 are given in Fig. 2, and the variation of the cutting particle size dc with the rotation speed n of the classification wheel under different air volume is estimated by (3); the carbonization for several specifications is also marked in the figure. When the silicon fine powder is in production operation, the measured product cuts the particle size. It can be seen from the figure that the system operation results are qualitatively consistent with the theoretical estimates, and are closer when operating with small air volumes. Practice has proved that in order to obtain accurate classification of fine powder, it is uneconomical to simply increase the speed of the classification wheel. It is also necessary to reasonably control the ratio of the auxiliary wind to the main air volume. When the system is operated with relatively small air volume, adjust the speed of the classification wheel to cut the particle size. The change is more sensitive. However, the reduction of the total air volume will lead to a reduction in the amount of processing, which is obviously uneconomical. Therefore, the system should be weighed against the pros and cons when operating, and choose the parameters of the good working conditions.
2.2. Feeding and dispersion of fine powder
Due to its poor fluidity, easy agglomeration and agglomeration, the fine powder causes great difficulty in feeding and dispersing the powder. The commonly used vibration feeding and screw feeder not only have poor uniformity, serious wear and tear, and agglomeration and The agglomeration directly affects the classification effect. It has been shown by theory [3] that for a fine powder of dc=10mm, the surface adhesion is about 105 times of gravity, and when dc=1mm, it reaches 107 times of gravity, thus overcoming the adhesion to enhance the dispersion of fine powder. It is very important for fine grading. The FJJ grading system adopts a combination of vibrating feeder + closing fan + long pipe gas conveying to ensure that the powder entering the grading chamber is uniformly and quantitatively fed. The tertiary air introduced by the graded outdoor jacket further assists in the dispersion of the fine powder, which makes the production operation stable, and the worker is easy to operate. When the amount of the fine powder is as high as 200 kg/h, the quality of the product can be ensured.
2.3. Characteristics of QLM Jet Mill and FJJ Grading System in Abrasive Production
According to the requirements of different sizes of abrasive grains at home and abroad, we use the two steps of jet mill pulverization and classifier as shown in Figure 1, to organize and implement the process. Firstly, the raw material coarse powder of about 36 mesh is pulverized in the QLM-630 jet mill to control the parameters of the jet mill, so that the particle size composition of the pulverized mixed powder (semi-finished product) is basically normal distribution, and the content of the coarse grain of zui is not allowed. The coarse particles in 1-2 or D3 do not exceed the standard. In the second step, the mixed powder is sent to the grading system, the grading wheel speed and the system air volume are set, the excess fine particles are removed, the fine granule content is less than a certain percentage, or the fine particles in the D94 are not exceeded, which completely conforms to the abrasive label product. Requirements. In short, the process of grinding the coarse head by the airflow and cutting the fine head by the classifier realizes the dry production of the fine powder abrasive.
The silicon carbide micropowder products produced by QLM-630 type jet mill and FJJ type grading system not only fully meet the GB2477-83 standard used in China's abrasive industry, but also the particle size distribution requirements of fine powder products such as W14, W10, W7 and W5. In Japan's JISR6001-1987 industrial standard for fixing abrasive micro-powders, the particle size distribution requirements of F1000, F1200 and F2000 products, the fine powder yield reached 80%, which was well received by Japanese customers. Figure 3 shows the results of a typical sample by particle size distribution.
1,2—FJJ-630 runs in high air volume and small air volume
3,4---FJJ-200 runs in high air volume and small air volume
Fig. 2 Cutting machine particle size characteristics (SIC)Fig. 3 Particle size distribution of silicon carbide powder
in conclusion
The airflow grinding and grading system used in this paper has the advantages of advanced technology, simple structure, high productivity, convenient operation and maintenance, low energy consumption and low pollution. The performance is much better than the wet process, and it is well promoted in the abrasive industry. prospect. This technology is also suitable for the classification of fine powder in the metallurgical, chemical, building materials, pharmaceutical and other industries.
The grading technology includes both hardware and software. In addition to reasonable design and layout of the structure and manufacturing of the system, the operating parameters of the system must be adjusted strictly and finely. For different materials and different particle size distributions, it is necessary to find the operating conditions of the zui. In view of the complexity of the flow field of the jet mill and grading system, it is necessary to further study the grading principle and optimization performance.
Author: Liu Qi years
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