Determining which particle size analysis methods provides the most accurate representation of your product can often be a difficult process. Many times, there isn’t one correct answer and a combination of tools are required to determine the accurate result. Identifying the particle size is an integral part of determining if the milling or sieving process being performed is yielding good results. There are three most common forms of particle size analysis used on powdered materials.
PSD or Particle Size Distribution, has been used extensively in countless applications in various industries to monitor, control, and investigate material properties. Understanding and controlling Particle Size Distribution in materials are critical to:
- product development
- quality control
- toll processing
While there are other methods available, this blog will focus on the 3 most common:
1) RoTap system with sieves
2) Jet Sieve
3) Laser Diffraction
RoTap System with Sieves
The RoTap is a method of particle size analysis that uses a uniform rotary motion and tapping on top of a sieve stack to determine what percentage of powders fall within a specific mesh/micron size range. This is calculated by the amount of material retained on each test sieve after running the RoTap machine. The typical ASTM standard calls for the test to be performed on one hundred grams of powder for a period of fifteen minutes. Sieve analysis is designed to emulate screening on a lab scale and claims to provide accurate results down to 44 microns. For coarse and granular powders, the RoTap/sieve analysis method is the most accurate way to determine particle sizes. The combination of energy and motion of the sieve method allow for particles to pass through the screen and can determine the amount of fines or oversized particles in a certain product fraction. There are drawbacks to this method as well. For angular particles in particular, the RoTap can lead to differing results. Without an outlet on the sieves, angular particles are able to vertically pass through the screen. This leads to a larger percentage of fines measured in the product than there really is. This also indicates less oversize in the product than is actually there. Customers need to determine if they want these angular particles to be in the product or not. Furthermore, our experience indicates the RoTap is limited in accurately measuring fine particle sizes. For fine sizes below 44 microns, the RoTap is not a suitable tool as it does not have the energy needed to pass particles through the screen.
Jet Sieve Method
The Jet Sieve method of determining particle size analysis uses both a vacuum and air knife to determine particle size. It creates both a push and a pull to keep the screen clean and determine the particle size at a specific point. For measuring particle size distribution, the Jet Sieve is not typically used since it would require testing at many points to develop a full distribution. The Jet Sieve is best utilized to measure the quantity of fines in a product at a certain micron size or the percentage of oversize in a product at a specific point. Jet Sieve technology is limited by the fact that it only can be used with one sieve at a time and has difficulty with high aspect ratio and platelet shapes. It can be used for sizes as fine as 20 microns and is great for measuring efficiency of the processing screening machine.
The Laser Diffraction system is one of the most advanced particle size measurement tools available. To measure particles, the system shoots a laser at individual particles and measures the amount of diffraction off the particle to calculate the particle size. Laser analysis is the most flexible type of analysis due to the fact that it is not limited by size and can be measured by both dry and wet measuring systems. The dry system works great for powders that are soluble or agglomerate in water, which would lead to an analysis that shows coarser particles than really are. The wet system can be used with both water and alcohol and is great for powders that are agglomerated. By dispersing the sample in water or alcohol, it allows the particles to become statically unbound and leads to more accurate readings. The liquid combined with the agitation inside the system and the available combination of ultrasonic pulsing can break up almost all agglomerates within the powder.
The laser system can measure sub-micron sizes and is repeatable when it comes to determining the particle size determination methods of feed products. Furthermore, the laser system can accurately determine the percentage of fines remaining in a product at any size. The laser system is a very accurate tool, but it does have limitations. For angular particles, the laser systems can produce very different results. Angular particles tend to pass through the laser the “long way” and this can lead to particles being read as larger than they really are. It is not a great tool for measuring particles that contain high aspect ratio pieces and is much more accurate for spherical and semi-spherical powders. This method will produce the standard powder distribution curve that is seen when analyzing powders. There are many laser systems on the market and while they each have their benefits; their results can vary drastically. Companies should speak with customers to see what laser methods they use before deciding which system to purchase.
While there are many options available on the market, there is no right answer to determining the best method for testing particle size. Companies should use these methods as tools and many times a combination of methods is needed to most accurately determine the particle size. Elcan Industries has a full-scale toll sieving facility where we sift and test the quality of powders every day. Our in-house state of the art lab contains multiple RoTap, Jet Sieves, and a Malvern Mastersizer 3000 for laser analysis. Particle measurements are included with every sieving project and we also offer lab testing services as well.
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