First, the particle size and its representation
In the sieving process, the materials to be processed are scattered material groups of different sizes and shapes. To understand the particle size characteristics of such materials, it is necessary to clarify the concept of particle size and the method of particle size analysis.
(1) Particle size and representation
The so-called particle size is a measure of the size of the ore. It is usually measured in millimeters or micrometers. In practice, the "diameter" of the ore particles is often used to indicate the particle size of the ore particles. The diameter of the regular ore particles is easy to indicate that the diameter of the spherical ore particles is the diameter of the sphere, and the diameter of the cubic ore particles is generally expressed by the side length. The broken material is composed of irregularly shaped loose particles, and the irregular shape of the material, the simplest method usually measures the length a, the width b, the height c, and uses its average value to indicate its diameter d, ie
For fine granules, the method for determining the particle size is: (1) the particle size of the granule passing through the sieve surface, and (2) the particle size by the actual properties of the granule, such as volume or surface area, (3) The particle size is expressed by the behavior of the particles under certain special conditions, such as sedimentation in water under certain conditions. The particle size represented by these methods is called the equivalent diameter. Due to the different characteristics specified, there are many types of equivalent diameters, and some of the more widely used equivalent diameters are listed in the table below. Obviously, the diameter measured for irregularly shaped particles depends on the measurement method used. If the sedimentation analysis is used, the free sedimentation diameter d f and the Stokes diameter d st can be measured, and the projected area and the projected circle diameter can be measured by a microscope, and the sieve diameter can be obtained by a sieve test.
The following table nominal diameter
name | symbol | definition |
d | Spherical diameter | |
Screen diameter | d A | The width of the smallest square hole through which the particles pass |
Surface diameter (S/Ï€) 1/2 | d s | The diameter of the pellet with the same surface area and particle size (~1.28d A ) * |
Volume diameter (6V/Ï€) 1/3 | d v | Diameter of pellets equal in volume to particles (~1.10d A ) * |
Projection area diameter (4A p /Ï€) 1/2 | d a | The diameter of the pellet (~1.4d A ) with the same projected area as the particle when viewed in a direction perpendicular to the plane of stability * |
Force diameter | d d | In a fluid of the same viscosity and velocity, the diameter of the pellet with equal motion resistance to the particle (Rep hours ~d s ) |
Free settling diameter | d f | In a fluid of the same density and viscosity, the diameter of the pellets having the same density and free settling velocity as the particles |
Stokes diameter | d st | Free sedimentation diameter in the laminar flow zone (Rep<0.20) (~0.97d A ) * |
Specific surface diameter d 3 /d s 2 | d vs | The ratio of surface area to volume is the same as the ratio of the surface area to volume of the particles. |
*only represents typical values, the actual value depends on the shape of the particles. [next]
Application of the values related to different diameters, in the study of material crushing and screening problems, using sieving diameter d θ; Gravity beneficiation of coal and the election, and the like may be used volume diameter d v, and the free-falling diameter df Stokes diameter d st and so on; therefore, when discussing particle size, the equivalent diameter used should be indicated.
All of the above are particle size representations of individual particles. For large batches of broken mixture, it is impossible to measure each particle and can only be determined by particle size analysis.
(2) Particle size analysis
There are many methods for particle size analysis. The particle size analysis methods commonly used in coal preparation and beneficiation processes are as follows:
1. Screening analysis Particle size analysis is performed using a set of sieves with different mesh sizes, and can be divided into (n+1) levels by using n-layer sieves. The smallest square mesh through which the ore particles can pass is typically used as the particle size of this grade. The size of the sieve hole on the upper sieve surface is l 1 , and the size of the sieve hole on the adjacent lower sieve surface is l 2 , then the material of this grade between the two sieve faces can be expressed as: <l 1 ,> l 2 . For the particle size composition of materials larger than 0.045 mm, sieve analysis is generally used. Dry sieves are used for grades greater than 100 μm and wet sieves for grades less than 100 μm. This method is simple and easy to operate, but is greatly affected by the shape of the particles.
2. Sedimentation analysis Sediment analysis is the use of different sizes of particles in the water medium to determine the sedimentation velocity into a number of grain size, so this method is also called water analysis. It measures the equivalent spherical diameter with the same settling velocity, apparently it is affected by the difference in material density. The deposition analysis is suitable for precipitating a particle size composition of 1 to 75 μm.
3. Microscopic analysis This method is mainly used to analyze fine materials and directly measure the shape and size of particles. The best measurement range is 0.5~20μm.
In industry, the most widely used method is the screening analysis method.
All of the above are particle size representations of individual particles. For large batches of broken mixture, it is impossible to measure each particle and can only be determined by particle size analysis.
(2) Particle size analysis
There are many methods for particle size analysis. The particle size analysis methods commonly used in coal preparation and beneficiation processes are as follows:
1. Screening analysis Particle size analysis is performed using a set of sieves with different mesh sizes, and can be divided into (n+1) levels by using n-layer sieves. The smallest square mesh through which the ore particles can pass is typically used as the particle size of this grade. The size of the sieve hole on the upper sieve surface is l 1 , and the size of the sieve hole on the adjacent lower sieve surface is l 2 , then the material of this grade between the two sieve faces can be expressed as: <l 1 ,> l 2 . For the particle size composition of materials larger than 0.045 mm, sieve analysis is generally used. Dry sieves are used for grades greater than 100 μm and wet sieves for grades less than 100 μm. This method is simple and easy to operate, but is greatly affected by the shape of the particles.
2. Sedimentation analysis Sediment analysis is the use of different sizes of particles in the water medium to determine the sedimentation velocity into a number of grain size, so this method is also called water analysis. It measures the equivalent spherical diameter with the same settling velocity, apparently it is affected by the difference in material density. The deposition analysis is suitable for precipitating a particle size composition of 1 to 75 μm.
3. Microscopic analysis This method is mainly used to analyze fine materials and directly measure the shape and size of particles. The best measurement range is 0.5~20μm.
In industry, the most widely used method is the screening analysis method.
The process of dividing a group of broken materials having different particle sizes into a single layer or a plurality of layers of sieve holes uniformly into a plurality of different levels is called screening. In theory, the particles larger than the mesh are left on the sieve surface, which is called the sieve on the sieve surface, and the particles smaller than the sieve (the smallest two-dimensional smaller than the sieve size) pass through the sieve hole, and the sieve called the sieve surface Under the object.
The sieving process of the broken material can be regarded as consisting of two stages: one is that the fine particles smaller than the mesh size reach the sieve surface through the material layer composed of the coarse particles; the other is that the fine particles pass through the sieve holes. In order to complete the above two processes, the most basic condition must be met, that is, there should be relative motion between the material and the screen surface. To this end, the screen box should have appropriate motion characteristics, on the one hand, the material layer on the screen surface is loose; on the other hand, the coarse particles blocked on the screen hole are flashed off, and the fine particle permeable screen is kept clear.
The actual screening process is: a large number of different particle sizes, after the coarse and fine mixed materials enter the sieve surface, only a part of the particles are in contact with the sieve surface, and in the part of the material contacting the sieve surface, not all the fine particles smaller than the sieve pores Most of the particles smaller than the size of the mesh are distributed throughout the entire layer. Due to the movement of the screen box, the layer on the screen surface is loosened, so that the larger gap existing in the large particles is further enlarged, and the small particles take the machine through the gap and transfer to the lower layer. Since the small particle gap is small, large particles cannot pass through, and therefore, the large particles are constantly in position during exercise. Therefore, the original disordered particle group was separated, that is, layered according to the particle size, and the arrangement rule of small particles underneath and coarse particles was formed. The fine particles reaching the sieve surface are sieved through the sieve, and finally the coarse and fine particles are separated to complete the screening process. However, sufficient separation is not available, and during sieving, a portion of the undersize is typically left on the screen.
When the fine particles are sieved, although the particles are smaller than the mesh holes, the ease of their screening is different. It is known from experience that the smaller the particles, the easier the sieve is, and the particles with similar mesh sizes are transparent. Screening is more difficult, and it is more difficult to pass through the large particle gaps in the lower layer of the screen.
The sieving process of the broken material can be regarded as consisting of two stages: one is that the fine particles smaller than the mesh size reach the sieve surface through the material layer composed of the coarse particles; the other is that the fine particles pass through the sieve holes. In order to complete the above two processes, the most basic condition must be met, that is, there should be relative motion between the material and the screen surface. To this end, the screen box should have appropriate motion characteristics, on the one hand, the material layer on the screen surface is loose; on the other hand, the coarse particles blocked on the screen hole are flashed off, and the fine particle permeable screen is kept clear.
The actual screening process is: a large number of different particle sizes, after the coarse and fine mixed materials enter the sieve surface, only a part of the particles are in contact with the sieve surface, and in the part of the material contacting the sieve surface, not all the fine particles smaller than the sieve pores Most of the particles smaller than the size of the mesh are distributed throughout the entire layer. Due to the movement of the screen box, the layer on the screen surface is loosened, so that the larger gap existing in the large particles is further enlarged, and the small particles take the machine through the gap and transfer to the lower layer. Since the small particle gap is small, large particles cannot pass through, and therefore, the large particles are constantly in position during exercise. Therefore, the original disordered particle group was separated, that is, layered according to the particle size, and the arrangement rule of small particles underneath and coarse particles was formed. The fine particles reaching the sieve surface are sieved through the sieve, and finally the coarse and fine particles are separated to complete the screening process. However, sufficient separation is not available, and during sieving, a portion of the undersize is typically left on the screen.
When the fine particles are sieved, although the particles are smaller than the mesh holes, the ease of their screening is different. It is known from experience that the smaller the particles, the easier the sieve is, and the particles with similar mesh sizes are transparent. Screening is more difficult, and it is more difficult to pass through the large particle gaps in the lower layer of the screen.
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