Granularity test basics

1. Definition of Particle Size and Concept of Particle Size Distribution The so-called particle size is the diameter, size or size of the particles. Realistic powder particles, such as talc, calcium carbonate, cement, etc., whose shape is irregular, how is the particle size described? In fact, any particle size measuring instrument to date uses realistic particles with the same sphere. The method of particle comparison measures the particle size, ie "if the particle is a sphere, then it should be (equivalent to) such a large". The scientific definition of particle size is as follows: when the physical property or physical behavior of the measured particle is closest to a homogenous sphere of a certain diameter (or a combination thereof), the diameter of the sphere (or a combination thereof) is taken as the measured The equivalent particle size (or particle size distribution) of the particles.

Instruments of different principles select different physical properties or behaviors of the particles to measure their size. For example, the laser particle size analyzer selects the scattering characteristics of the particles, and the sedimentation characteristics of the particles in the liquid are selected by the sedimentation instrument. Whether the particles can pass through the mesh, and so on. Since different instruments select different physical properties of the same particle as reference quantities for equivalent, when they are used to measure the same irregular particle, the results may be different. Sometimes even the same principle of the instrument, the test conditions are different, the results may be different, for example, the results of the screening measurement is related to the time of the sieve.

Particle size is used to describe the size of a particle. A particle sample has different sizes of particles, and the particle size distribution is used to describe the overall size of the sample particles more comprehensively. The particle size distribution refers to the ratio of particles of various sizes to the total number of particles. It can be represented by a table (granularity distribution table) or a curve. 2. Comparison of several common particle size measuring instruments in modern times

2 Comparison of several common particle size measuring instruments in modern times
2.1 laser particle size analyzer
Principle: According to the scattering phenomenon of light, that is, the smaller the particle, the larger the scattering angle (which can be called static light scattering).
? Theoretical measurement range: 0.05~2000μm
Advantages: large dynamic range, fast measurement speed, easy operation and good repeatability
? Disadvantages: low resolution, it is not suitable to measure samples with good uniformity

2.2 Coulter (resistance method) particle counter
Principle: Principle of small hole resistance
? Theoretical measurement range: 1 ~ 256μm
Advantages: high resolution, good repeatability, and easy operation
? Disadvantages: easy to block holes, small dynamic range, it is not suitable to measure samples with a wide distribution range, such as cement

2.3 Settlement instrument (including gravity sedimentation, centrifugal sedimentation, light penetrating sedimentation, sedimentation pipe, pipette, etc.)
Principle: The principle of settlement, the Stokes principle, measures the size of the particles based on the sedimentation velocity of the particles.
? Theoretical measurement range: centrifugal sedimentation: 0.01 ~ 100μm Other: 2 ~ 100μm
Advantages: intuitive principle and low cost
? Disadvantages: the operation is complicated, the result is greatly affected by the environment and the operator, and the repeatability is poor.

2.4 Dynamic Light Scattering Apparatus (PCS)
Principle: According to the Brownian motion of tiny particles in the liquid, the local particle concentration changes in the solution, which causes the intensity of the scattered light to change with time. By analyzing the autocorrelation of the scattered light, the velocity of the particle is estimated, and finally the particle is detected. size.
? Measurement range: 2nm~2000nm (2μm)

2.5 particle image workstation
Principle: Microscopy combined with digital image processing technology
? Theoretical measurement range: 0.5 ~ 1200μm
Advantages: high resolution, observable particle morphology and state

Moreover, the particle image workstation adopts the direct measurement method, and is not used for the above several test equipments, and does not need to establish a proportional relationship by light scattering, sedimentation time, etc. to test. Direct access to image images of the particles is more intuitive and eliminates test inaccuracies due to problems with the instrument components. The particle image workstation uses a high-definition industrial camera to capture the state of the particle samples to be tested one by one through an optical microscope device. By using a specially developed particle image analysis software to splicing and processing all the images, the projected area of ​​each particle can be obtained. Data such as particle size and size can be obtained by further statistics and calculations. The ability to obtain shape parameters is its greatest advantage. In addition to the particle image workstation, there are no other instruments that can obtain morphological parameters such as sphericity and aspect ratio. And its price is moderate and easy to operate, it is one of the preferred instruments for particle testing in many industries.

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