Abstract: When testing powder particle size, the dispersion conditions of the sample are the main factors that affect the accuracy of the results. In order to reduce the error caused by different sample dispersion conditions, LS230 laser particle size analyzer, slag powder and glass powder were used to study the concentration of powder solution, temperature, dispersion time, dispersion medium, type of dispersant and their concentration The influence of the results of the particle size determination, thus obtaining the optimal dispersion conditions of the sample for particle size analysis.
In real life, there are many fields that are closely related to particle size analysis, including the field of building materials. For example, for concrete, the particle size of its main materials such as cement, slag, fly ash and other materials has a great influence on its performance. Generally speaking, the smaller the particle size of these powder materials, the more effective they can exert their activity in concrete Or effective, so as to improve the strength of the concrete or improve its durability. Therefore, it is meaningful to control the particle size of the main raw materials before preparing the concrete.
The predecessors have done a lot of research on particle size analysis [1] ~ [3], but mainly used sedimentation, centrifugal and optical particle size analyzers, and their combination with building materials is rare. This article uses the LS230 laser particle size analyzer, which uses the principle of light scattering, fast speed, wide measurement range, long-term, continuous measurement results of a variety of standard samples (L300, G15, GB500) show that the instrument measured The data is reliable and the repeatability of multiple measurements is good. In the process of particle size measurement, the instrument itself has little influence on the measurement results, so the accuracy of the particle size measurement results mainly depends on the dispersion conditions of the sample. Taking the glass powder and slag powder as examples, the author conducted an experimental study on the dispersion conditions of the sample before the particle size analysis test to find the optimal dispersion conditions of the sample for the particle size analysis (the measurement results in this article are measured in 3 times). The position diameter D50-that is, the average value of the diameter of the particles corresponding to 50% of the total particles on the cumulative percentage curve is given).
1 test
1.1 Test instruments and materials
Instruments: LS230 laser particle size analyzer; T660 / h ultrasonic disperser; METTLER AE240 electronic balance.
Material: slag powder; glass powder.
1.2 Sampling and sample preparation methods
Sampling: First use the quartile method to divide the sample, and then use the sampler to extract the sample from the obtained sample. The number should at least meet the test requirements.
Sample preparation: The two powders are dried before the test, and then formulated into a dilute solution. The prepared solution is subjected to ultrasonic dispersion as required, and particle size analysis is performed immediately after dispersion.
2 Test results and discussion
2.1 Effect of powder solution concentration on the measurement results
Using distilled water as the dispersing medium, without adding surfactants, 6 samples of slag powder sample solutions of different concentrations were prepared and dispersed with an ultrasonic disperser for 5 minutes, and then the particle size was measured.
It can be seen from Table 1 that when the powder sample concentration is small, the measured particle size is small and the particle size distribution range is also narrow (as can be seen from its particle size distribution curve); when the powder sample concentration is large Because of complex scattering and easy particle agglomeration, the measured particle size is relatively large and the particle size distribution range is wide (as can be seen from its particle size distribution curve), the test results have a large error. However, the above results do not mean that the smaller the concentration of the powder sample, the better, because when the concentration is small to a certain extent, the particles in the sample have been greatly reduced. Too few particles will produce larger sampling and measurement random errors. The samples are not representative, so the lower limit of concentration should also be controlled during measurement [1]. Based on the analysis of the test results, the optimal concentration of the slag powder is 0.90 ～ 1.10 g / L under the premise of satisfying the minimum sample amount required for the test (so that the shading rate reaches about 10%). The D50 value is very close. This result is not universally applicable to other powders, and the appropriate concentration of different types of powders needs to be determined through experiments.
2.2 The effect of dispersion time on the measurement results
Using distilled water as the dispersing medium, without adding surfactant, prepare 6 samples of slag powder sample solution with a concentration of 1.00 g / L, and disperse them with an ultrasonic disperser for different times, and then perform particle size measurement. In addition, the different power of the ultrasonic disperser also leads to different dispersion effects, which will not be detailed in this article.
2.3 The influence of the dispersion medium on the measurement results
When testing the particle size of the powder, the selected dispersion medium should not only infiltrate the powder, but also have a low cost, non-toxic and non-corrosive. The commonly used dispersion media are water, water + glycerin, ethanol, ethanol + water, ethanol + glycerin, cycloethanol, etc. When the powder is coarser, water or water plus glycerin can be used as the dispersion medium. Add water as the dispersion medium. The infiltration effect of ethanol is stronger than that of water, which makes it easier for particles to be fully dispersed.
2.4 The effect of the type and concentration of dispersant on the measurement result
Choosing the right dispersant is the hotspot of current research, and the most used dispersant is surfactant. The main types of surfactants are: anionic surfactants, cationic surfactants, amphoteric surfactants, nonionic surfactants, special types of surfactants, etc. [4]. Powders are usually charged in water, and the addition of surfactants with the same charge prevents the surface adsorption due to the mutual repulsion between the charges, which can achieve the purpose of dispersing the powder. Different surfactants have different dispersing effects on different types of powders. In the determination, the dispersing effects of several surfactants should be compared, and finally an optimal surfactant should be determined [5].
Using distilled water as the dispersing medium, prepare 6 samples of glass powder sample solutions of the same concentration, add different types of dispersing agents, disperse them with an ultrasonic disperser for 5 minutes, and then measure the particle size.
For glass powders, the more suitable dispersants are sodium dodecylbenzenesulfonate and polyacrylamide.
The concentration of the dispersant also has a certain influence on the measurement results, and should be controlled when used. Taking polyacrylamide as an example, the dispersion effect of glass powder under different concentrations of dispersant is compared. During the test, it was found that when the concentration of the dispersant was too high, flocculation occurred in the solution (this is also one of the reasons for the increase in the particle size measurement result). When polyacrylamide is used to disperse glass powder, the optimal concentration of dispersant is about 1.0-2.0 g / L.
2.5 The influence of the temperature of the powder sample solution on the measurement result
Using distilled water as the dispersing medium without surfactants, prepare 6 slag powder sample solutions of the same concentration and disperse them with an ultrasonic disperser for 5 min, and test them at different temperatures. The particle size measurement results are shown in Figure 2. It can be seen from Figure 2 that as the temperature increases, the particle size tends to become smaller. This is because the high temperature is beneficial to the dispersion of particles, and the low temperature makes the particles easy to agglomerate, which increases the measurement error. However, the test temperature should not be too high, because the trend of particle size reduction above 35 ℃ is not obvious, and the particle size analyzer is not suitable for operation at high temperatures. This shows that the temperature of the powder sample solution should be kept in the range of 20 ~ 35 ℃.
3 Conclusion
1. For slag powder, the best powder concentration is 0.90 ~ 1.10 g / L, and the best dispersion time is 5 ~ 10 min.
2. When testing the particle size of the powder, the selected dispersion medium should be able to achieve the best dispersion of the powder. The dispersion effect of ethanol on the finer powder is significantly better than that of distilled water.
3. For glass powder, the more suitable dispersants are sodium dodecylbenzene sulfonate and polyacrylamide; when polyacrylamide is used to disperse glass powder, the optimal concentration of the dispersant is about 1.0 ～ 2.0 g / L.
4. The temperature of the powder sample solution should be kept within the range of 20 ~ 35 ℃.
5. In addition to the above factors, the shape factor and refractive index (including the refractive index of the dispersion medium) of the sample particles should be mastered before the test, so as to select an appropriate optical model and minimize the measurement error.
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