Particle Sizing Techniques and Laser Diffraction
The ability to measure the size of very fine particles is an integral part of many industries today. For every material that goes through a grinding or milling process, the final particle size is usually the primary factor that governs product performance or process efficiency. This is why particle sizing has become essential in areas such as the pharmaceutical industry, foods and beverages, cement and building materials and for manufacturers of chemical compounds for industrial and residential uses.
One of the most popular and widely used particle sizing technologies available today is Laser Diffraction. Laser Diffraction has become so popular because it offers a very wide dynamic range; it can be applied to liquid suspensions, dry substances and aerosols and provides a very rapid and reliable measurement process.
The goal of all particle sizing technologies is to provide a systematic and reliable measurement for a range of different sized particles. However it is important to understand that there is a wide range of particle sizing technologies available and no one technology is suitable for every job. Each measurement technology has its own advantages and each is often best suited to specific industries or applications.
It is also important to understand that different measurement technologies can often give different particle results for the same sample. This is because each measurement technique measures a different aspect of the same material.
When reporting particle size, we tend to display a graph showing “particle size” on one axis and “percent of material” on the other axis. However, it is very difficult to describe a multi-dimensional particle using one dimension. In fact there is only one shape that can be described by one dimension, and that is the diameter of a sphere.
For this reason, all particle sizing techniques measure some property of material and then relate this to an “equivalent sphere”.
Some common particle sizing techniques and there reporting methods are as follows:
- Sedimentation techniques – measures the rate at which particles settle in a liquid column and reports the size of a sphere with the same settling rate.
- Sieve technique – measures the mass of material retained on a series of screens and reports the amount of material between spherical hole sizes.
- Aerodynamic sizing technique – measures the behaviour of particles in an airstream and reports the size of a sphere that has the same behaviour.
- Laser diffraction – measures light scattering from a group of particles and reports the size of a sphere that produces the same scattering.
So, with so many different measurement techniques available. The question is: which techniques are the best?
Getting the Answer as Right as Possible
Like most questions in the world of science, the answer has multiple parts. In short, you need to choose the most suitable measurement technology for your product and your process. However, if you had to pick a single technology that can universally applied then it would have to be Laser Diffraction.
Laser diffraction is the best general technique as it can be used with a very wide range of particle sizes and also a very wide range of sample types. Laser diffraction works very well for sprays, dry powders, suspensions and emulsions. The results reported are also displayed in terms of a “volume” distribution, which is the most appropriate description for bulk material properties.
Apart from Laser Diffraction, another technique that is gaining popularity is Image Analysis. Image Analysis is also considered a particle sizing technique, but it does offer one significant difference. It is the only methodology that provides any information on particle “shape”! If particle shape is known to have an influence on product performance, then image analysis may be the most appropriate option.