One of the most common techniques for particle size determination is sieving. Sieving is a simple method. It is low cost and several samples from the original specimen can be prepared for several uses. However, sieving is time consuming and results can be obtained only for a very limited number of particle sizes. Results from sieving typically vary due to several factors such as the method of moving the sieve, the period of operation, the number of particles on the sieve and some physical properties such as the shape or the stickiness of the sample. Furthermore, the actual size of the mesh gaps of the sieves can have large variations from the nominal size. Due to these limitations, this technique is being widely replaced by light scattering methods, especially for sizing particles smaller than a few millimetres.
Figure 1. Static Laser Scattering Equipment.
Technique for Measuring Particle Size
Static laser light scattering can be used to measure particle size ranging from approximately 10-20 nm up to a few millimetres. When particles are illuminated by a laser beam, light scattering is observed and their size can be determined from the angular intensity distribution. The physical theories that support this calculation are the Fraunhofer theory for rather large particles and the Mie theory which applies both to large and small particles.
Particles are defined “small” when their diameter is not larger than the wavelength of the illuminating laser light. Typically, Laser Particle Sizers use laser light with a wavelength between 500 and 700 nm. Therefore, the transition between the Fraunhofer and the Mie limit takes place in the region 0.5-1 μm. For the sake of completeness, it must be said that the Mie and Fraunhofer limits may not only depend on the particle size, but also on the sample material and the specific application.
The Need for Dispersion
It is possible that particles are found in the form of agglomerates. Agglomerates need to be dispersed and the clusters need to be separated. Generally, two different classes of dispersion are available: wet dispersion and dry dispersion.
During the wet dispersion, the sample powder or suspension is added to a closed circuit filled with an appropriate liquid. This mixture is pumped continuously through a measuring cell where the laser beam can illuminate the particle ensemble. During the pumping in the measuring circuit, ultrasound is applied to the system enabling destruction of the agglomerates. Single, separated particles are produced. The quantity of material added to the measuring circuit must be controlled carefully since multiple scattering processes may alter the result of the measurement.
Multiple scattering refers to the fact that the light initially scattered by a particle is then scattered on a second particle before leaving the measurement cell. To ensure that the correct amount of material is used, the beam obscuration is observed while feeding the sample material to the system. The beam obscuration provides the percentage of light that is scattered away from its original path. A value of 10-15% has proven to be a good value for the beam obscuration ensuring a reliable measurement. Figure 2 shows the volume of sample material needed to obtain 10% or 20% of beam obscuration in a FRITSCH ANALYSETTE 22 MicroTec plus Laser Particle Sizer as a function of the particle size.
Figure 2. Calculated total volume of sample material needed to get a beam obscuration of 10 and 20%. The calculation was performed using the Mie theory with optical parameters of Alumina. For a particle size smaller than about 1 μm the required amount of sample material would be significantly influenced by the refractive index of the material.
Large particles require a much larger amount of material than small particles. For a particle smaller than 0.5 μm in size, the amount of material required increases again. The exact value depends not only on the particle size, but also on the refractive index of the material, which is not indicated in Figure 2. One of the limits of wet dispersion is that it is difficult to measure certain materials in liquid. They may dissolve in water or in other organic solvents, or face chemical reactions. In such cases the dry measurement is a valuable alternative.
In dry dispersion the material is accelerated in an air stream through a so called Venturi nozzle and expands quickly behind the nozzle. The highly turbulent stream rotates the agglomerates quickly so that they collide with other agglomerates and particles. This causes the agglomerates to de-aggregate: single particles can then be measured.
However, compared to the introduction of ultrasound in water, this process is less effective, limiting the dry dispersion measurement to particle size of the order of a few micrometers. Also, this method strongly depends on the physical properties of the material. Wet, fat containing, as well as sticky materials are of course much more difficult to disperse in an air stream when compared to dry and easy flowing materials. To improve the efficiency of the dry dispersion process, some instruments incorporate baffle plates, which the material stream is accelerated onto. When hitting these baffle plates, agglomerates are destroyed effectively, but unfortunately especially for soft material, milling of the primary particles also occur. In this case, the resulting particle size distribution depends on the pressure of the air used to accelerate the material stream. In applications that do not require the agglomerates to be destroyed, a falling chute can be used to feed the sample material to the measuring zone. The continuous feeding of the material is established via a vibration feeder and the particles simply fall down into the measuring instrument. They can then be collected or drawn out with a vacuum cleaner.
Figure 3 shows two measurements of iron powder using a falling chute in an ANALYSETTE 22 MicroTec plus Laser Particle Sizer.
Figure 3. Two dry measurements of iron powder using the falling chute in a FRITSCH ANALYSETTE 22 MicroTec plus. The finer sample is the sieve fraction between 125μm and 355μm while the more coarse sample is the sieve fraction between 500 μm and 1.4 mm.
This information has been sourced, reviewed and adapted from materials provided by FRITSCH GMBH - Milling and Sizing.
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