Particle size explained: Mesh vs Microns, D50 and D98
Understanding particle size data on a data sheet can be challenging. Not everyone measures things the same way!
The terms, for a start can be confusing. Sometimes one sees Mesh sizes, other times one sees Microns. What is the difference?
What is Mesh?
The Mesh system is the old imperial standard for measuring particle size and is still widely used for minerals in countries like India and China, where particle size is measured using screens.
Originally it was the measure of a woven filter cloth’s ability to remove contaminant particles. This system simply counted the number of strands or yarns per inch of woven media. Hence, a 100 Mesh media has 100 yarns per inch of media.
The higher the Mesh size, the smaller the particles that are able to pass through that filter cloth or metal screen. The symbol used for Mesh is #.
This system is not exact because it depends for example, on how thick the strands are. Over time, various Mesh systems were developed such as the Tyler, the American ASTM and the British BSS systems. These are similar but with some variations, so for the purposes of this article I will be using the ASTM system (to see how they compare, click here).
What is a Micron?
The micron is a metric measurement – you could say that it is a more modern measure of particle size. A micron is also called a micrometre, with symbol µm or simply µ or u. We now have instruments like the Sedigraph and the Malvern which give accurate micron readings of particle size. This is especially important for very fine powders where screening is difficult, e.g. < 20 microns.
To get an idea of the size of a micron, I find it is useful to remember that there are 1000 microns in one millimetre, or 0.5mm is 500 microns.
Pluck a piece of hair out of your head (ouch!) and look at it. Your hair is about 50 microns thick. If it was thinner than 40 microns, it would be invisible. Humans just cannot see something that small without using a microscope.
The following chart gives the micron size of some common particles, including the Coronavirus that has changed our world so much:
|
Micron |
Particle |
|
0.12 |
Coronavirus |
|
0.3 |
Smoke, TiO2 |
|
0.5 |
Bacteria |
|
0.7 |
Lung Damaging particles |
|
1.0 |
Atmospheric Dust |
|
1.3 |
Moulds |
|
2 |
Flour mill dust, kaolin |
|
4 |
Cement dust |
|
5 |
Pulverized coal, CaCO3 |
|
7 |
Commercial dust |
|
10 |
Pollen |
|
75 |
Silt |
|
1000 |
Sand |
Source: https://www.liquidfiltration.com/filter-bag-fundamentals
Many mineral producers have now moved away from Mesh sizes to the more accurate micron measurements. However Mesh sizes are still widely used in some countries. So to compare one mineral to another, we need a conversion table.
To assist you, we have put a Mesh-to-Micron Conversion Table onto our website as a free download – click here.
325 Mesh
This is a Mesh size that you see a lot, especially on kaolin data sheets. You may be offered a “325 Mesh kaolin”, for example. What does this mean?
Well, if you go to the conversion table you will find out that 325 Mesh is 45 microns. So a 325 Mesh kaolin means that most of the particles are smaller than 45 microns.
Particles that remain on a 325 Mesh/45 micron screen are usually called the “Residue” or “grit content”, which should be a small amount. It would be written on the data sheet in one of the following ways, which all mean the same thing:
|
% > 325 Mesh |
0.2 |
|
Sieve residue 325 Mesh |
≤ 0.2% |
|
325 # (% Max) |
0.2 |
|
Residue (> 45 micron) |
0.2% |
|
+ 45µm |
0.2% |
|
Retention on BSS 350 Mesh |
0.2% |
You will notice that 325 Mesh on the ASTM system equates to 350 Mesh on the British BSS system.
Similarly, if you are offered a 400 Mesh talc this usually means that 99% of the talc particles are smaller than 37 microns (400 Mesh).
D50 and D98
The D50 (also written D50) is the mean or average particle size of a mineral. So “D50 = 2 microns” means the average particle size of that mineral is 2 microns. It also means that 50% of the particles are smaller than 2 microns, and 50% are bigger.
“D98 = 2 microns” on the other hand, means that 98% of the particles are smaller than 2 microns.
As technology advances we are starting to mill minerals to finer and finer particle sizes, even down to nanoparticles. “Nanoparticles” are generally considered to be those particles smaller than 0.1 micron (100 nanometres). But that’s a topic for another day.