Introduction to basic methods of commonly used granularity testing

There are many methods for granularity testing, with hundreds of statistics. At present, there are five kinds of sedimentation method, laser method, screening method, image method and resistance method, and there are also several common test methods in specific industries and fields.

1. Settlement method:

The sedimentation method is a method for measuring the particle size distribution according to the different sedimentation speeds of particles with different particle sizes in the liquid. Its basic process is to put a sample into a certain liquid to make a suspension with a certain concentration. The particles in the suspension will settle under the action of gravity or centrifugal force. The settling speed of particles with different particle sizes is different, the settling speed of large particles is faster, and the settling speed of small particles is slower. So what is the quantitative relationship between the sedimentation speed of the particles and the particle size, and in what way do they reflect the sedimentation speed of the particles?

â‘ Stokes law: In the gravity field, the particles suspended in the liquid will move under the action of gravity, buoyancy and viscous resistance. The equation of motion is:

This is Stokes's law.

From Stokes' law, we see that the sedimentation velocity is proportional to the square of the particle diameter. For example, two particles with a particle size ratio of 1:10 have a settling rate ratio of 1: 100, which means that the settling rate of fine particles is much slower. In order to accelerate the sedimentation speed of fine particles and shorten the measurement time, most modern sedimentation instruments introduce centrifugal sedimentation. In the state of centrifugal sedimentation, the relationship between particle sedimentation degree and particle size is as follows:

This is the expression of Stokes' law in the centrifugal state. Since the centrifugal rotation speed is above hundreds of revolutions, the centrifugal acceleration ω2r is much greater than the gravity acceleration g, Vc >> V, so under the same particle size, the test time of centrifugal sedimentation will be greatly shortened.

â‘¡ Bill's law:

As mentioned earlier, the sedimentation method tests the particle size distribution based on the sedimentation velocity of the particles. However, it is difficult to directly measure the sedimentation speed of particles. Therefore, in the actual application process, the sedimentation speed of the particles is reflected indirectly by measuring the rate of change of the light intensity of the suspension through different times. So what is the relationship between the rate of change of light intensity and the particle size? Bill is telling us:

Set a series of light intensity values ​​I1D2> D3> ……> Di measured at T1, T2, T3, ... Ti, and substitute these light intensity values ​​and particle size values ​​into formula (5), and then process them by computer Get the particle size distribution.

2. Laser method:

The laser method tests the particle size distribution based on the phenomenon that the particles can cause the laser to diffract or scatter after the laser irradiates the particles. The laser beam generated by the laser beam expands into a parallel beam with a diameter of about 10 mm. In the absence of particles, the parallel light converges on the back focal plane after passing through the Fourier lens. As shown below:

When a certain amount of particles are uniformly placed in a parallel beam by an appropriate method, the parallel light will diverge. Part of the light will propagate outward at an angle to the optical axis. As shown below:

So, what is the relationship between the scattering phenomenon and the particle size? Both theory and experiment prove that the angle of scattered light caused by large particles is small, and the smaller the particles, the greater the angle between the scattered light and the axis. These scattered lights of different angles will form a series of rings with different radii on the focal plane after passing through the lens of the rich surname. The light spots composed of these rings are called Airy spots. Airy spots contain a wealth of particle size information. Simply understand that auras with a large radius correspond to a smaller particle size; auras with a smaller radius correspond to a larger particle size; the intensity of auras with different radii contains the particle size Information on the number of particles. In this way, we place a series of photoelectric receivers on the focal plane, convert the light signals scattered by particles of different sizes into electrical signals, and transmit them to the computer. Mathematical processing of these signals through the Mie scattering theory can obtain the particle size Distributed.

3. Screening method:

Sieving method is one of the most traditional particle size testing methods. It tests the particle size by passing the particles through sieve holes of different sizes. The sieving method is divided into two types: dry sieve and wet sieve. A single sieve can be used to control the passing rate of single particle size particles, or multiple sieve can be used to measure the passing rate of multiple particle size particles simultaneously . There are many methods for sieving, including manual sieving, vibrating sieve, negative pressure sieve, and automatic sieve. Whether the particles can pass through the sieve is related to such factors as particle orientation and sieving time. Different industries have their own sieving method standards.

4. Resistance method:

The electrical resistance method is also called the Kurt method, which is a particle size test method invented by a person named Kurt in the United States. This method is based on the principle that when particles pass through a small micropore, they occupy part of the space in the small micropore and discharge the conductive liquid in the small micropore, so that the resistance at both ends of the small micropore changes. of. The size of the resistance at both ends of the pore is proportional to the volume of the particle. When particles of different sizes continue to pass through the small pores, the two ends of the small pores will continuously generate resistance signals of different sizes, and the particle size distribution can be obtained by processing these resistance signals through a computer. as the picture shows:

The medium used for the particle size test by the Coulter method is usually physiological saline with better conductivity.

5. Microscopic image method:

The micro image method includes a microscope, a CCD camera (or digital camera), a graphics acquisition card, and a computer. Its basic working principle is to transfer the magnified particle image of the microscope to the computer through the CCD camera and graphics acquisition card. The computer performs edge recognition and other processing on these images to calculate the projected area of ​​each particle. According to the equivalent projected area In principle, the particle size of each particle is obtained, and then the number of particles in the set particle size interval is counted to obtain the particle size distribution.

Due to the small number of particles measured by this method at a time, the same sample can be measured multiple times by changing the field of view to improve the authenticity of the test results. In addition to particle size testing, microscopic image methods are also commonly used to observe and test the morphology of particles.

6. Other particle size test methods:

In addition to the above-mentioned particle size testing methods, scraper method, sedimentation bottle method, ventilation method, ultrasonic method and dynamic light scattering method are also commonly used in production and research fields.

(1) Scraper method: Scrape the sample onto the surface of a flat plate and observe the roughness to evaluate whether the particle size of the sample is acceptable. This method is a method adopted by the paint industry. It is a qualitative granularity test method.

(2) Settling bottle method: Its principle is roughly the same as that of the settlement method mentioned before and after. The test process is to first prepare a certain amount of sample and liquid into a suspension in a 500ml or 1000l measuring cylinder, stir well and then take out a certain amount (such as 20ml) as the total weight of the sample, and then calculate each particle settlement according to Stokes law Time, the same amount of suspension is released at a fixed time to represent the corresponding particle size at that time. After drying and weighing the suspension obtained at each moment, the particle size distribution can be calculated. This method is currently used in abrasives and river sediment.

(3) Breathable method: Breathable method is also called Freund's method. First put the sample into a metal tube and compact it, then install this metal tube into a gas circuit to form a closed-loop gas circuit. When the gas in the gas path flows, the gas will pass through the gap between the particles. If the sample is thicker, the gap between the particles is larger, and the gas flow is less hindered; the sample is finer, the gap between the particles is smaller, and the gas flow is hindered. The ventilation method is based on such a principle to test the particle size. This method can only get an average particle size value, and cannot measure the particle size distribution. This method is mainly used in the magnetic materials industry.

(4) Ultrasonic method: A method of measuring particle size distribution through the principle that particles of different particle sizes have different effects on ultrasonic waves. It can directly test high concentration slurry with solid-liquid ratio of 70%. This method is a new technology that is currently being studied by people both at home and abroad. It is said that instruments are already available abroad, but not yet at home.

(5) Dynamic light scattering method: The laser light scattering method mentioned above can be understood as a static light scattering method. When the particles are small to a certain degree, the particles in the liquid are affected by Brownian motion, which shows a random state of motion. The distance and speed of the motion are related to the size of the particles. The particle size distribution can be obtained by identifying the motion state of these particles through related technologies. Dynamic light scattering method is mainly used to measure the particle size distribution of nanomaterials. There are ready-made instruments abroad, but not yet at home.

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