With the development of science and technology, the powder industry will become one of the most important basic industries in twenty-first Century.
Particle size is one of the main indexes of powder material. It has a direct influence on the performance and performance of the product. It is widely concerned in all walks of life. For example, the granularity of the cement determines the setting time of the cement. The particle size of the antibacterial powder determines the effect of the post spinning. The granularity of the paraffin is determined as a addition. The writing fluency of ink after adding ink to the ink. Therefore, the test of particle size has attracted more and more attention.
There are many methods for granularity testing, and there are hundreds of statistics. At present, there are five commonly used methods of settlement, laser, screening, image and resistance. In addition, there are several testing methods commonly used in specific industries and fields.
Sieving method
Sieving is one of the most traditional methods of particle size measurement. It is to test the granularity of particles through different sizes of sieve holes. The screening method is divided into two kinds of dry sieves and wet sieves. A single sieve can be used to control the passing rate of single particle size particles, and a number of sieves can be used to measure the passing rate of multiple particle size particles at the same time, and the percentage is calculated.
Sieving methods include manual screen, vibrating screen, negative pressure screen, automatic screen and so on. Whether particles can be related to grain size and screening time and other factors, different industries have their own screening standards.
Advantages: low cost, easy to use.
Disadvantages: it is difficult to measure dry powder less than 400 mesh (38um). The longer the measuring time is, the smaller the results are; the jet or the emulsion can not be measured; some strange results are obtained when measuring needle like samples, such as the sifting allowance before and after processing.
Settlement method
Sedimentation method is a method of measuring particle size distribution according to the settling velocity of particles with different particle sizes in liquid. The basic process is to put the sample into a certain liquid to make a certain concentration of suspension, and the particles in the suspension will settle down under the action of gravity or centrifugal force. The settling velocity of different particle size is different. The settling velocity of large particles is faster, and the settling velocity of small particles is slower.
In actual operation, the final settlement velocity of the test particles is very difficult, so all the settling instruments are other physical parameters related to the final settlement velocity, such as pressure, density, weight, concentration or light transmittance. Then the particle size distribution of the particles is obtained. The settlement method is divided into two kinds of gravity settlement and centrifugal settlement. The test range of gravity settlement is usually 0.5 ~ 100 mu m, and the size range of the centrifuge settlement can be measured from 0.05 to 5 mu. The settling particle meter generally adopts the method of combining gravity settlement with centrifugal settlement.
Advantages: intuitive principle, high resolution, low price and low running cost.
Disadvantages: slow measurement speed, can not handle different density mixture. The results are influenced by environmental factors (such as temperature) and human factors.
Laser method
Laser diffraction (also called small angle forward scattering) is the most mature and most widely used method in the scattering laser particle measurement technology. By measuring the distribution of the scattered light energy in a small angle in front of the particle, the particle size is obtained by the classical Mie scattering theory and the Fraunhofer theory applicable to large particles. Size and distribution. For larger particle size, the small angle forward scattering method is also called diffraction method because of the diffraction mainly in the forward small angle range.
The laser diffraction method has wide application, wide measurement range, accurate measurement, high accuracy, good repeatability, fast measuring speed, less physical parameters and on-line measurement.
Advantages: the test range is wide (the best measurement range of the laser particle size meter is 0.04-2000um, the general also can reach 0.1-300um), the test speed is fast (1-3 minutes / times), the automation is high, the operation is easy, the repeatability and the authenticity are good, the dry powder sample can be tested, the measurement of mixed powder, emulsion and fog drop can be measured.
Disadvantages: it is not suitable to measure samples with narrow particle size distribution, and the resolution is relatively low.
Resistance method
The resistance method, also known as Kurt method, is a particle size test method invented by a person named Kurt in the United States. This method is based on the particles in a small hole in a small moment, occupying a small part of the small pore space and open the small pore of the conductive liquid, the principle of small micropores at both ends of the resistance changes the principle of testing the size distribution. The resistance at both ends of the hole is directly proportional to the volume of the particles. When the particle size particles of different sizes continuously pass through the small micropores, the resistance signals of different sizes will be produced continuously at both ends of the small micropores. The particle size distribution can be obtained by processing these resistance signals by computer.