[Technology] There are many methods for particle size testing. Which one is the most accurate?

Guide

Techniques and methods for testing particle size mainly include screening method, sedimentation method, microscopic method, light scattering method, electric resistance method (Coulter counting method), specific surface area method, ultrasonic attenuation method, etc. among many test methods. Microscopy is a direct measurement of the absolute geometry of the particles. The particle size measured by the other methods is not the absolute geometric concept size, but the so-called "equivalent particle size".

Techniques and methods for testing particle size mainly include sieving method, sedimentation method, microscopic method, light scattering method, electric resistance method (Coulter counting method), specific surface area method, ultrasonic attenuation method, and the like. Among the many test methods, except that the microscopic method directly measures the absolute geometry of the particles, the particle size measured by the other methods is not the absolute geometric concept size, but the so-called "equivalent particle size".

In this paper, a variety of particle size test methods, such as microscopy, sedimentation, electrical resistance, laser diffraction and specific surface area methods, are used to conduct comparative experiments to study the similarities and differences of the results and analyze the causes, and to find out the existing particle size test for engineering applications. The technique is more realistic and more accurate in reflecting the particle size and its distribution.

1 particle size test method

1.1 Microscope image method

The microscope image method can simultaneously observe the morphology of the particles and intuitively measure the geometry of the particles. It is often used as a calibration or calibration for other measurement methods. This type of instrument consists of a microscope, a CCD camera (or a digital camera). ), graphics capture card, computer (image analyzer) and other components. It works by the CCD camera to transfer the magnified image of the microscope to the computer, and then through the special analysis software to process and calculate the image, resulting in particles Particle size and particle size distribution. This method reduces human error and improves test speed, but it requires high sample preparation, complicated operation and expensive equipment. The measurement results of the microscopic image method mainly characterize the two-dimensional size (length and width) of the particles, and the height cannot be characterized.

1.2 settlement method

The sedimentation method is based on the principle that the sedimentation of the particles in the liquid is in accordance with Stokes' law, and the particle size of the particles is calculated according to the final sedimentation velocity of the particles in the liquid. In actual operation, the final sedimentation velocity of the test particles is relatively high. Difficulty, therefore, all settlers measure other physical parameters related to the final settling velocity, such as pressure, density, weight, concentration, or light transmission. Furthermore, the particle size distribution of the particles is obtained. The sedimentation method is divided into two types: gravity sedimentation and centrifugal sedimentation. The test range of gravity sedimentation is usually 0.5 to 100 μm, and the particle size range of centrifugal sedimentation can be measured from 0.05 to 5 Μm. At present, the sedimentation type particle analyzer generally adopts a combination of gravity sedimentation and centrifugal sedimentation.

1.3 Resistance method

Resistance method known as the Coulter counter method, suitable for a relatively narrow particle size distribution measurements it works is relatively simple: the particles suspended in the electrolytic solution is made of ruby under negative pressure through a small The hole, two resistive sensors composed of platinum electrodes are respectively immersed in the small holes, and the resistance between the electrodes increases when the particles pass through the small holes, generating a voltage pulse. The amplitude of the pulse corresponds to the volume of the particles and the corresponding particles. The number of pulses corresponds to the number of particles. Counting all the measured pulses and determining their amplitudes, the particle size is obtained, and the particle distribution is counted. The resistance method is the highest resolution so far. Particle size analysis technology.

1.4 Laser diffraction method

Laser diffraction (also known as small-angle forward scattering) is one of the most mature and widely used methods in scattering laser granulation. It measures the distribution of scattered light energy in a small angular range of the forward direction. Using the classical Mie scattering theory and the Fraunhofer theory applicable to large particles, the size and distribution of the particle size are obtained. For particles with larger particle sizes, the diffraction is due to scattering in the forward small angle range. Mainly, therefore, the small-angle forward scattering method is also called the diffraction method. The laser diffraction method has wide applicability, wide particle size measurement range, accurate measurement, high precision, good repeatability, fast measurement speed, and less physical parameters to be provided. It can be measured online, etc., so it is widely used.

1.5 specific surface area method

The particle size of the particle group can be indirectly expressed by the specific surface area. The specific surface area is the sum of the surface areas of the unit mass particles. By measuring the specific surface area Sw of the particles, it is converted into the diameter of the uniform spherical particles having the same specific surface area value. The method of measuring the particle diameter is referred to as a specific surface area method, and the obtained particle diameter is referred to as a specific surface area diameter.

2 Experimental results and analysis

Different particle size testers have different equivalent particle diameters due to different working principles. The laser method measures the equivalent volume diameter, the resistance method measures the equivalent resistance diameter, and the sedimentation method measures the equivalent sedimentation. The velocity path (also known as Stokes diameter), the specific surface area method measures the diameter of the sphere of equivalent surface area. In this study, the microscope method measures the Feret diameter (the distance between two parallel lines along a certain direction). To use and compare the granularity data of different test methods, it is necessary to understand the differences and causes between these methods, and to compare and analyze the measurement results.

In order to study the repeatability, accuracy and mutual difference of the test results of these methods, this paper compares the measurement results by selecting one subsample into the instrument and repeating the test three times. The body sample is repeatedly tested on different instruments (microscopy is only tested once for a long time). The above six samples are measured by five different particle size testing techniques. D50 is listed in Table 1.

The SEM photographs of the six powders are shown in Figure 1. As can be seen from Figure 1, the spheroids and monodispersity of the polystyrene spheres are the best, and the diameter of the spheres is relatively uniform (Fig. 1(a)). SB040102 Glass micro The particle size distribution of the beads is narrow and close to monodisperse. Most of the particles are spherical and mixed with a very small amount of non-spherical particles (Fig. 1(b)). The sphericity of SB2004 glass beads is good, but the beads are fine. The particle size distribution is wide and does not have monodispersity (Fig. 1(c)). The tungsten powder is a block particle with a narrow particle size range (Fig. 1(d)), and the Al2O3 powder and SiO2 powder are irregular. Shaped particles. Al2O3 powder particles are irregularly shaped and porous particles, similar to honeycomb (Fig. 1(e)); there are more small particles below 10 μm and large particles of 60 μm in SiO2 powder (Fig. 1) .

It can be seen from the test results from Table 1:

(1) Comparing the results of the three most commonly used particle size analyzers, the laser particle size analyzer (laser diffraction method), the Coulter counter (resistance method), and the centrifugal particle size analyzer (sedimentation method), The standard deviation of the diameter D50 value is statistically analyzed: the standard deviation of the laser particle size analyzer is 0.008; the standard deviation of the Coulter counter is 0.097; the standard deviation of the centrifugal particle size analyzer is 0.219. It can be seen that the standard deviation of the measured D50 value of the laser particle size analyzer is the smallest, the measurement result has the best repeatability and high precision, and the measurement repeatability and measurement accuracy of the centrifugal particle size analyzer are relatively the worst.

(2) The results of D50 values ​​measured by electric resistance method, laser diffraction method, sedimentation method and microscopy method are comparable, indicating that the monodisperse spherical powder has the median diameter measured by these four methods. Comparability; spherical powder SB040102 glass microbead powder with monodispersity and spherical powder SB2004 glass microsphere powder without monodispersity are comparable by the resistance method, laser method and microscopic method. However, the results measured by the sedimentation method are too large, and the relative error is greater than 5%. The results are not comparable. This shows that although the spherical powder is the same, if the particle size range is too large, only some of the measurement methods are measured. The positional diameter is comparable. The median diameter measured by non-spherical powders with different principle instruments is generally not comparable.

(3) The particle size measured by the specific surface area method is much smaller than the particle diameters measured by the other four methods. For example, the measurement of the specific surface area diameter of the porous A 2 O 3 particles is better than that of other methods. It is less than one order of magnitude. The main reason for the smaller surface area is that the powder particles are more or less finely cracked or connected with the outside, and the rich internal surface area of ​​some particles is even larger than the external surface area; In addition, the uneven surface of the particles also makes the measured specific surface area larger, and the particle size result is smaller when substituted into the formula d = 6 / ρSw. In addition, the non-spherical shape of the particles is also one of the factors affecting the measurement results. The particle size measured by the specific surface area method is incomparable with the particle size measured by other methods.

(4) For the three spherical powders, the measurement results of the microscopy method are consistent with the results of other methods, and the D50 values ​​of the polystyrene spheres measured by the four commonly used methods are consistent, and the relative deviation is less than 5%. The results are comparable; the median diameter D50 of the two glass microsphere spherical powders is consistent with the results of the median diameter D50 measured by electric resistance, laser diffraction and microscopy, and the results are comparable, but the sedimentation method and these The deviation of the measurement results is greater than 5%, which is incomparable; for the irregularly shaped SiO2 powder and Al2O3 powder, the D50 value measured by microscopy is not comparable with the D50 value measured by other methods, especially the D50 deviation of SiO2 powder. Larger. The main reason for the deviation is that the SiO2 powder contains a considerable part of particles with a particle size of less than 10 μm (accumulated content of nearly 30%), and the largest particle size is larger than 60 μm. In the same microscope image, it is necessary to measure large, The particle size of small particles is small compared to small particles, and the resolution is not high enough, which leads to errors in the measurement results. Therefore, microscopic method is not suitable for measuring powders with large particle size range and irregular shape. Particle size.

4 Conclusion

(1) The results of the laser diffraction test are reproducible, high precision and strong applicability.

(2) The median diameter D50 values ​​of monodisperse spherical particles measured by laser diffraction, electric resistance, sedimentation and microscopy are comparable.

(3) For spherical powders with a wide particle size range, only the median diameter D50 measured by some test methods is comparable; for non-spherical powders, the median diameter value D50 measured by different principle instruments generally does not have Comparability.

(4) The particle size value measured by the specific surface area method is smaller than the particle size measured by other test techniques, and is incomparable with the measured values ​​of other test methods.

(5) Microscopy is not suitable for measuring the particle size of large particle size and irregular shape.