Basic knowledge and basic methods of particle size testing

Particle size testing is an experimental work to characterize powder particle size characteristics by specific instruments and methods. Powder is widely used in our daily life and industrial and agricultural production. Such as flour, cement, plastics, paper, rubber, ceramics, medicines, etc. In different application fields, the requirements for powder properties are different. Among all the indicators reflecting powder properties, particle size distribution is the most important indicator in all application fields. Therefore, objectively and truly reflecting the particle size distribution of the powder is a very important task. Let me talk about the basic knowledge and basic methods of granular testing.

First, the basic knowledge of particle size testing

1. Particles: Geometry with a specific shape in a range of sizes. The size mentioned here is generally between millimeters and nanometers, and the particles refer not only to solid particles but also liquid particles such as mist droplets and oil beads.

2. Powder: A group of particles consisting of a large number of particles of different sizes.

3. Particle size: The size of the particles is called the particle size.

4. Particle size distribution: The percentage of particles of different particle sizes reflected by the specific instruments and methods. There are two forms of interval distribution and cumulative distribution. The interval distribution, also known as the differential distribution or frequency distribution, represents the percentage of particles in a series of particle size intervals. The cumulative distribution is also called the integral distribution, which means less than or greater than the percentage of particles of a certain particle size.

5. Representation of particle size distribution:

1 Tabular method: A method of listing the particle size interval distribution and the cumulative distribution by a table method.

2 Graphic method: A method of expressing the particle size distribution in the form of a histogram and a curve in a rectangular frame.

3 Function method: A method of expressing a granularity distribution using a mathematical function. This method is generally used in theoretical research. For example, the famous Rosin-Rammler distribution is the function distribution.

6, particle size and equivalent particle size:

The particle size is the particle diameter. This concept is very simple and clear, so what is the equivalent particle size, what is the relationship between particle size and equivalent particle size? We know that only the sphere has a diameter, other shapes of geometry have no diameter, and the particles that make up the powder are mostly not spherical, but a variety of irregular shapes, with flaky, Needle-like, polygonal, and so on. These complex shaped particles are theoretically not directly represented by the concept of diameter. In practice, diameter is the most intuitive and simplest quantity to describe a particle size. We hope to use such an amount to describe the particle size. Therefore, in the practice of particle size testing, we introduced the equivalent particle size. This concept.

The equivalent particle size means that when a physical property of a particle is the same as or similar to a homogenous spherical particle, we use the diameter of the spherical particle to represent the diameter of the actual particle. Then the particle size of the spherical particles is the equivalent particle size of the actual particles. The equivalent particle size is as follows:

1 Equivalent volume diameter: the diameter of the ball that is the same as the actual particle volume. It is generally considered that the diameter measured by the laser method is an equivalent volume diameter.

2 Equivalent settling speed: the diameter of the ball that is the same as the actual particle settling velocity under the same conditions. The particle size measured by the sedimentation method is the equivalent sedimentation velocity, also known as the Stokes diameter.

3 Equivalent resistance diameter: The diameter of spherical particles that produce the same resistance effect as the actual particles under the same conditions. The particle size measured by the Kurt method is the equivalent resistance diameter.

4 Equivalent injection area diameter: the diameter of the spherical particle which is the same as the actual particle injection area. The particle diameters measured by the directional mirror method and the image method are mostly equivalent projected area diameters.

7. Several key indicators indicating particle size characteristics:

1 D50: The particle size corresponding to a cumulative particle size distribution percentage of a sample of 50%. Its physical meaning is that particles with a particle size larger than it account for 50%, and particles smaller than it also account for 50%. D50 is also called median diameter or median particle size. D50 is commonly used to indicate the average particle size of the powder.

2 D97: The particle size corresponding to the cumulative particle size distribution of a sample of 97%. Its physical meaning is that particles with a particle size smaller than it account for 97%. D97 is commonly used to indicate the particle size index of the powdery end. Other definitions and physical meanings such as D16 and D90 are similar to D97.

3 Specific surface area: The sum of the surface areas of the particles per unit weight. The unit of specific surface area is m2/kg or cm2/g. The specific surface area has a certain relationship with the particle size. The finer the particle size, the larger the specific surface area, but the relationship is not necessarily proportional.
8. Repeatability of particle size test: deviation between multiple measurements of the same sample. Repeatability is the most important indicator of how good a particle size test instrument and method is. Its calculation method is:

Where n is the number of measurements (generally n>=10);
Xi is the typical value of each test result (generally D50 value);
x is the average of the typical values ​​of multiple test results;
σ is the standard deviation;
δ is the repeatability relative error.

Factors affecting the repeatability of the particle size test are factors of the instrument and the method itself; factors in sample preparation; environmental and operational factors. Good repeatability of particle size testing is a basic requirement for instruments and operators.

9, the authenticity of the particle size test:

The usual measuring instruments have indicators of accuracy. Due to the particularity of the granularity test, the meaning of accuracy is usually expressed by authenticity. Since the particle size measured by the particle size test is the equivalent particle size, different equivalent methods may obtain different equivalent particle sizes for the same particle.

It can be seen that the same particle obtained two different results due to different measurement methods. That is to say, an irregularly shaped particle, if it is represented by a numerical value, is not unique, but has a series of values. Each test method is performed on a specific aspect of the particle, and the obtained value is one of a series of values ​​that can represent the particle size, so the results obtained by the same sample using different particle size test methods are The difference is caused by objective reasons. The more complex the shape of the particles, the greater the difference between the results of the different test methods. But this does not mean that the results of the granularity test can be infinite, but it should have a certain degree of authenticity, that is, the actual particle size distribution of the sample should be reflected more realistically. There is no strict standard for authenticity, and it is a qualitative concept. However, some phenomena can be used as the basis for the authenticity of the test results. For example, the measurement results of the standard sample should be within the allowable error range of the nominal value; the pulverized sample should be finer than the powder before pulverization; the large particle content of the sample after grading should be reduced; the result is consistent with industry standards or recognized The method is consistent.

Second, the basic method of particle size testing

There are many methods for particle size testing, and there are hundreds of statistics. At present, there are five kinds of sedimentation methods, laser method, sieving method, image method and electric resistance method. In addition, there are several test methods commonly used in specific industries and fields.

1. Settlement method:

The sedimentation method is a method of measuring the particle size distribution according to the different sedimentation speeds of particles of different particle sizes in a liquid. The basic process is to place the sample in a liquid to make a certain concentration of suspension, and the particles in the suspension will settle under the action of gravity or centrifugal force. The sedimentation velocity of particles with different particle sizes is different, the sedimentation velocity of large particles is faster, and the sedimentation velocity of small particles is slower. Then, what is the quantitative relationship between the sedimentation velocity of the particles and the particle size, and by what way does it reflect the sedimentation velocity of the particles?

1 Stokes' Law: In the gravitational field, particles suspended in a liquid will move due to gravity, buoyancy, and viscous drag. This is Stokes' 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 sedimentation speed ratio of 1:100, which means that the sedimentation speed of the fine particles is much slower. In order to speed up the sedimentation speed of fine particles and shorten the measurement time, most modern settlers are introduced into the centrifugal sedimentation mode.

This is the expression of Stokes' law in the state of centrifugation. Since the centrifugal speed is above several hundred revolutions, the centrifugal acceleration ω2r is much larger than the gravitational acceleration g, Vc>>V, so the test time for centrifugal sedimentation will be greatly shortened under the same particle size.

2 Beer's Law:

As mentioned earlier, the sedimentation method tests the particle size distribution based on the sedimentation velocity of the particles. However, it is very difficult to directly measure the sedimentation rate of the particles. Therefore, in the actual application process, the sedimentation velocity of the particles is indirectly reflected by measuring the rate of change of the light intensity through the suspension at different times. So what is the relationship between the rate of change of light intensity and the particle size? Bill tells us that: Set a series of light intensity values ​​I1D2>D3>...>Di at T1, T2, T3, ... Ti, and substitute these light intensity values ​​and particle size values ​​into equation (5). The particle size distribution can be obtained by computer processing.

2, laser method:

The laser method tests the particle size distribution according to the phenomenon that the laser can cause the laser to diffract or scatter after the laser is irradiated to the particles. The laser light generated by the laser is expanded into a bundle of parallel light having a diameter of about 10 mm. In the absence of particles, the parallel light passes through the Fourier lens and converges onto the back focal plane. As shown below:

Parallel light will scatter when a certain amount of particles are uniformly placed into the parallel beam by a suitable means. A portion of the light will propagate outward at an angle to the optical axis. As shown below:

So what is the relationship between scattering and particle size? Both theory and experiment have proved that the angle of the scattered light caused by the large particles is small, and the smaller the particles, the larger the angle between the astigmatism and the axis. These different angles of scattered light will form a series of halo with different radii in the focal plane through the rich surname lens, and the alternating light and dark spots composed of these halos are called Airy patches. The Airy plaque contains rich granularity information. It is simply understood that the aura with a large radius corresponds to a smaller particle size; the aura with a smaller radius corresponds to a larger particle size; the intensity of the aura with different radii, including the particle size. The amount of information on the number of particles. In this way, we place a series of photoreceivers on the focal plane, convert the optical signals scattered by particles of different particle sizes into electrical signals, and transmit them to a computer. Mathematical processing of these signals by Mie's theory can obtain the granularity. Distributed.

3. Screening method:

Screening is one of the most traditional methods of particle size testing. It is to measure the particle size by passing the particles through mesh openings of different sizes. The sieving method is divided into two types: dry sieve and wet sieve. The single sieve can be used to control the passage rate of single particle size, or multiple sieves can be used to simultaneously measure the passage rate of multiple particle diameters, and calculate the percentage. . The screening method has various methods such as manual sieve, vibrating sieve, negative pressure sieve and fully automatic sieve. Whether the particles can pass through the sieve is related to the orientation of the particles and the screening time. Different industries have their own screening method standards.

4, the resistance method:

The resistance method, also known as the Coulter method, is a particle size test method invented by a man named Coulter in the United States. This method is based on the principle that the particle occupies a part of the space in the small micropore at the moment of passing through a small micropore, and the conductive liquid in the small micropore is discharged, and the resistance across the small micropore changes. of. The size of the resistance across the aperture is proportional to the volume of the particle. When particles of different sizes continuously pass through the small micropores, the two ends of the small micropores will continuously generate different resistance signals, and the particle size distribution can be obtained by processing the resistance signals by a computer.

5, microscopic image method:

The microscopic image method includes a microscope, a CCD camera (or a digital camera), a graphic acquisition card, a computer, and the like. Its basic working principle is to transfer the magnified image of the microscope to the computer through the CCD camera and the graphic 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 projection area. The principle results in the particle size of each particle, and then counts the number of particles in the set particle size interval, and the particle size distribution can be obtained.

Since the number of particles measured in a single method is small, 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 imaging is also commonly used to observe and test the morphology of particles.

6, other particle size test methods:

In addition to the above several particle size test methods, the scraping method, the settling bottle method, the gas permeable method, the ultrasonic method, and the dynamic light scattering method are commonly used in the production and research fields.

(1) Scraper method: The sample is scraped onto the surface of a flat plate and the roughness is observed to evaluate whether the particle size of the sample is acceptable. This method is a method used in the coatings industry. Is a qualitative method of testing granularity.

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

(3) Ventilation method: The ventilation method is also called the Freund's method. The sample is first loaded into a metal tube and compacted, and the metal tube is installed in a gas path to form a closed loop gas path. As the gas in the gas path flows, the gas will pass through the gaps in the particles. If the sample is thick, the gap between the particles is large, and the obstruction of the gas flow side is small; the sample is fine, the gap between the particles is small, and the gas flow is hindered. The gas permeability method is based on such a principle to test the particle size. This method can only obtain 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 the particle size distribution by the principle that particles of different particle sizes have different effects on ultrasonic waves. It can directly test high-concentration slurry with a solid-liquid ratio of 70%. This method is a new technology. At present, there are people at home and abroad to conduct research. It is said that there are instruments in foreign countries, and there is currently no domestic.

(5) Dynamic light scattering method: The laser scattering method described above can be understood as a static light scattering method. When the particles are small to a certain extent, the particles are affected by Brownian motion in the liquid, showing a random motion state, and the moving distance is related to the moving speed and the particle size. The particle size distribution can be obtained by recognizing the motion state of these particles by the related art. Dynamic light scattering is mainly used to measure the particle size distribution of nanomaterials. There are ready-made instruments in foreign countries, which are not available in China.

Third, the status quo and development trend of particle size testing technology

China's research on particle size testing technology started in the 70s. In the early 1980s, the China Particle Society was established. Professor Guo Musun, a member of the Chinese Academy of Sciences, served as the chairman of the board of directors. The committee consisted of particle preparation, particle testing, aerosol and nanomaterials. The establishment of the Particle Society not only promoted the research of particle testing technology, but also promoted the process of industrialization. Later, domestic particle size analyzers were put on the market. After nearly 20 years of development, there are more than ten manufacturers of particle size instruments. In 2002, the production and sales volume is expected to reach more than 500 sets. The market share of domestic particle size analyzers is over 80%. Not only did it end the history of particle-based instruments that were almost entirely dependent on imports until the 1980s, but there was also a certain amount of exports. The main performance indicators of the domestic particle size analyzer have reached the level of the early and mid-1990s.

Compared with foreign advanced particle size analyzers, domestic instruments still have a small test range, a low level of manufacturing technology, and a low level of automation intelligence. Nanoparticle size analyzers and online special particle size analyzers are still blank.

At present, China's powder industry is in a period of vigorous development, and the demand for particle size testing instruments has increased dramatically. Moreover, China has joined the WTO, and foreign markets are gradually opening. In the past 20 years of reform and opening up in China, particle testing technology has made great progress from scratch, which proves that we have a greater development foundation and potential. As long as we continue to make breakthroughs in technology, innovation, and we have relatively low prices, we are fully qualified to become a manufacturing country and a strong country of granular instruments. Not only can it meet domestic needs, but it can also be exported in large quantities. So, what should we improve from those aspects? I think we should do the following work:

1. Train a large number of professionals in particle size testing as soon as possible.

2. Strengthen basic research, including basic theoretical research and applied research.

3. Pay close attention to foreign technology development trends and actively use the latest research results from abroad.

4. Establish national standards and supporting standard samples for various particle size analyzers.

5. Make full use of new technologies and processes in other fields to improve the overall level of particle testing instruments.