Granularity testingBasic knowledge
1. Particle: A geometric object with a specific shape within a size range. The size referred to here is generally between millimeters and nanometers, and particles not only refer to solid particles, but also liquid particles such as droplets and oil droplets.
2. Fenxiu: a group of particles composed of particles of different sizes.
3. Particle size: The size of a particle is called its particle size.
4. Particle size distribution: The percentage of particles of different sizes in the total amount of powder reflected by specific instruments and methods. There are two forms of distribution: interval distribution and cumulative distribution. Interval distribution, also known as differential distribution or frequency distribution, represents the percentage content of particles in a series of particle size intervals. Cumulative distribution, also known as integral distribution, represents the percentage of particles smaller or larger than a certain particle size.
5. The representation method of granularity distribution:
① Table method: A method of listing the particle size interval distribution and cumulative distribution one by one using a table.
② Graphic method: a method of representing particle size distribution in the form of histograms and curves in a right angle scale system.
③ Function method: a method of representing particle size distribution using mathematical functions. This method is generally used in theoretical research. The Rosin Rammler distribution is a function distribution.
6. Particle size and equivalent particle size:
Particle size refers to the diameter of particles. This concept is very simple and clear. So what is equivalent particle size and what is the relationship between particle size and equivalent particle size? We know that only spherical bodies have a diameter, while geometric bodies of other shapes do not have a diameter. Moreover, the particles that make up powders are mostly irregular shapes, such as sheet shaped, needle shaped, polygonal, and so on. Theoretically, the size of these complex shaped particles cannot be directly represented by the concept of diameter. In practical work, diameter is an intuitive and simple quantity that describes the size of a particle, and we hope to use such a quantity to describe particle size. Therefore, in the practice of particle size testing, we introduced the concept of equivalent particle size.
Equivalent particle size refers to the diameter of a spherical particle that represents the actual diameter of a particle when its physical properties are the same or similar to those of a homogeneous spherical particle. So the particle size of this spherical particle is the equivalent particle size of the actual particle. There are several specific equivalent particle sizes:
① Equivalent volume diameter: the diameter of a sphere that is the same as the actual particle volume. It is generally believed that the diameter measured by laser method is the equivalent volume diameter.
② Equivalent settling velocity diameter: the diameter of a sphere that is equal to the actual settling velocity of particles under the same conditions. The particle size measured by the settling method is the equivalent settling velocity diameter, also known as the Stokes diameter.
③ Equivalent resistance diameter: The diameter of a spherical particle that produces the same resistance effect as the actual particle under the same conditions. The particle size measured by the Coulter method is the equivalent resistance diameter.
④ Equivalent input area diameter: the diameter of a spherical particle that has the same input area as the actual particle. The particle size measured by the visible mirror method and image method is mostly the equivalent projected area diameter.
7. Several key indicators that represent granularity characteristics:
① D50: The particle size corresponding to the cumulative particle size distribution percentage of a sample reaching 50%. Its physical meaning is that particles larger than it account for 50%, and particles smaller than it also account for 50%. D50 is also known as median diameter or median particle size. D50 is commonly used to represent the average particle size of powders.
② D97: The particle size corresponding to a sample with a cumulative particle size distribution of 97%. Its physical meaning is that particles with a diameter smaller than it account for 97%. D97 is commonly used to indicate the particle size index of the coarse end of the powder.
The definitions and physical meanings of other parameters such as D16 and D90 are similar to D97.
③ Specific surface area: The sum of the surface areas of particles per unit weight. The unit of specific surface area is m2/kg or cm2/g. There is a certain relationship between specific surface area and particle size. The finer the particle size, the larger the specific surface area, but this relationship is not necessarily proportional.
8. Repeatability of particle size testing: the deviation between multiple measurements of the same sample. The repeatability index is an important indicator for measuring the quality of a granularity testing instrument and method. Its calculation method is:
Among them, n is the number of measurements (generally n>=10);
X i is the typical value of each test result (usually D50 value);
X is the average of typical values obtained from multiple tests;
σ is the standard deviation;
δ is the relative error of repeatability.
The repeatability of particle size testing is influenced by the instrument and method itself; Factors related to sample preparation; Environment and Operation
Factors related to various aspects, etc. Good repeatability is a fundamental requirement for both instruments and operators in particle size testing.
9. Authenticity of granularity testing:
Typical measuring instruments have sexual indicators. Due to the particularity of granularity testing, authenticity is usually used to represent the meaning of sexuality. Due to the fact that the particle size measured by particle size testing is an equivalent particle size, different equivalent methods may result in different equivalent particle sizes for the same particle.
It can be seen that due to different measurement methods, the same particle obtained two different results. That is to say, for an irregularly shaped particle, if its size is represented by a numerical value, the value is not *, but a series of values. And each testing method is aimed at a specific aspect of the particles, and the obtained value is one of a series of values that can represent the particle size. Therefore, the difference in results obtained by different particle size testing methods for the same sample is due to objective reasons. The more complex the shape of the particles, the greater the difference in results between different testing methods. But this does not mean that the particle size test results can be boundless, but rather should have a certain degree of authenticity, that is, they should reflect the actual particle size distribution of the sample more realistically. There is currently no strict standard for authenticity, it is a qualitative concept. But some phenomena can serve as a basis for the authenticity of test results. For example, the measurement results of the instrument on the standard sample should be within the allowable error range of the nominal value; The sample after pulverization should be finer than before pulverization; The content of large particles in the graded sample should be reduced; The results are consistent with industry standards or * methods.