Author: Liaoning Xinda Talc Group

One of the characteristics of powder is particle size distribution. What is particle size distribution? This concept and its application development originated from the mining industry, where ores undergo various beneficiation processes after crushing. Therefore, the shape and size of particles have a significant impact on beneficiation processes and equipment. Later, the inorganic non-metallic powder industry emerged, gradually introducing the concept of particle size distribution into the powder industry. There have always been many methods for characterizing the irregularity of particles, especially in powder clusters, which can serve as references and inspirations.

1、 Characterization of individual particles:

For individual particles of rules, we can represent them by shape and dimensional size. For example, spherical particles can be expressed by a numerical value of diameter, rectangular prisms by length * width * height, and conical prisms by bottom diameter * height. For approximate regular shapes, the average value can also be used to express them.

2. How to describe non spherical particles, such as irregular particles such as strip-shaped, filamentous, rod-shaped, sheet-like, radial, etc? Due to the vast differences in shapes, it is difficult to express in practice, understand, and study the rules of particle states. Therefore, in academia, another method is used to express it, called the "equivalent method". The so-called "equivalent method" is to convert all irregular particles into spherical shapes and use the spherical diameter as the data representation of the particles uniformly.  

1) Measurement method: By measuring the diameter dimensions of particles in multiple dimensions and averaging the data, an average value is obtained, which is considered as the equivalent diameter of spherical particles.

2) Projection method: It is to project particles from all directions onto a plane, integrate and calculate the area of the plane to convert it into diameter, and finally average the values to obtain the equivalent diameter of spherical particles.

3) Hydraulic settling method: The Stokes formula in fluid mechanics (Vg=d2 (ƿ s - ƿ l)/18 ƞ) states that the settling velocity of particles in a medium is related to their diameter. Therefore, in many cases, calculating the settling velocity of particles in liquid water can yield an equivalent diameter.

4) In certain situations where shape description is required, other data such as aspect ratio, sphericity, and aspect ratio may be used to assist with the shape.

2、 Characterization of group particles (powders):

Regardless of whether the powder is processed by physical or chemical methods, the diameter of each individual particle shows an approximately continuous state from large to small, and without exception, it conforms to the normal distribution law. Therefore, using the equivalent diameter of individual particles clearly cannot describe the particle state of the powder.

In order to describe the characteristics of the powder population, the concept of particle size distribution is introduced. The so-called particle size distribution refers to the proportion of individual particle diameters from large to small in a group. Due to the continuous diameter of particles, it is generally necessary to divide the diameter into segments, such as 100-50um, 50-20um, 20-10um, 10-5um, and 5-0um. These segments are determined based on the actual use and are not fixed. Then, the proportion of particles with different diameters in the total powder is calculated, such as weight ratio, quantity ratio, etc., for example: 100-50um (10%), 50-20um (27%), 20-10um (35%), 10-5um (25%), and 5-0um (8%). This can basically describe the distribution state of powder particle size, but it is still quite complex in practical applications. Therefore, scholars have adopted the concept of cumulative distribution, which can be divided into positive cumulative and negative cumulative, that is, the proportion of particles larger or smaller than a certain diameter. As shown in the example above, positive cumulative can be expressed as;

>0um=100%,>5um=92%,>10um=67%,>20um=37%,>50um=10%,>100um=100%;

Negative accumulation can be expressed as:

<100um=100%,<50um=90%,<20um=63%,<10um=33%,<5um=8%,<0um=0%

The above proportions and accumulations can be divided into multiple segments to form a continuous curve, and then any point on the curve can be taken to find the corresponding values of the diameter and proportion that have accumulated positively and negatively.

3. Usually, we use negative accumulation, such as d97=20um, which means that particles with a diameter less than 20um account for 97% of the powder.

4. In inorganic non-metallic powders, d50 \ d97 (d98) \ d10 is commonly used to express as much as possible, and there are also specific fields that require the proportion of 1um \ 2um \ 8um \ 10um to express. For example, paper coating requires two levels of 2um content greater than 90% and 60%.

5. Nowadays, particle size analyzers can provide proportions and cumulative curves, so it is also possible to query the values corresponding to any cumulative proportion and particle size in the curve, which is called particle size distribution. From describing individual particles to characterizing the particle size distribution of powders, the simplest methods have been used to approach theoretical research in various disciplines.

3、 Determination of particle size distribution:

The method for determining particle size distribution also varies in accuracy depending on the user's requirements.

1. Screening method: Using different sizes of mesh holes for screening, calculate the weight ratio of the sieve above and below. For example, some companies use the 325 target sieve for screening calculation, which is commonly understood as the sieve residue. Different industries, even within the same industry, use different sieve tests. For example, some use 150 mesh, 200 mesh, 400 mesh, 500 mesh, and 600 mesh sieves for sieve residue inspection to determine whether the powder is qualified, while others use sieve residue to classify the fineness of the powder. There is no right or wrong, it is just an agreed upon method.

2. Hydraulic settling method: One method is to thoroughly mix powder and water in a fixed amount, let them stand, and then calculate the descending speed of the clarified water surface as a standard; In addition, after thorough mixing, let it stand in a fixed measuring cylinder for a fixed time (usually more than 2 hours), calculate the final settling volume, and then obtain the wet bulk density as the standard for powder fineness.

3. Bulk density method; Due to the different fineness of powders, the bulk density varies, and bulk density is often used to control and check the fineness of powders. Bulk density tests generally include natural bulk density, compacted bulk density, and wet bulk density.

4. Scraper fineness: Scraper fineness meters come in various specifications and are generally used to detect the maximum particle size. Currently, they are still widely used in the paint industry.

5. Laser diffraction method; Laser diffraction is formed through powder, and then the continuous particle size distribution in the calculation model is calculated. Due to the influence of manufacturing accuracy and calculation model, the laser particle size analyzer test data may have significant differences.

6. X-ray sedimentation method: After mixing powder and water, the sedimentation is accelerated by centrifugal force. The X-ray intensity attenuation after passing through the mixture is used to calculate the sedimentation velocity and particle size distribution state.

Due to the diversity of testing methods and the lack of a unique method and numerical value for particle size characterization, there is a hundred schools of thought to contend. However, as a user, one can judge the particle size distribution of powders from multiple perspectives and methods. With the development of materials and computers, online detection instruments have emerged.