Q&A: Grains of Wisdom: A Journey Through Particle Size Reduction

Recently, Powder & Bulk Solids presented “Grains of Wisdom: A Journey Through Particle Size Reduction” as part of its DryPro webinar series. In it, static pressure and how to create a better understanding of it with respect to dust collection systems and system design was discussed.

Willow Slusser, business development manager at Custom Processing Services, discussed the fundamentals of particle size reduction, traditional methods vs. modern technologies, practical applications in varied industries, and more.

After the presentation portion of the webinar, a live Q&A took place. Here are the questions and answers.

Q. If I have a product that needs particle size reduction, how do I go about determining which is the best method to achieve my goals? Can you help me with this size reduction?

This is definitely material dependent. I would start thinking about it with these basic principles. The first thing I would look at is material properties. And that’s going to be your compass for figuring out how you need to do your particle size reduction without specific material. That drives everything you’re looking for.

[From the webinar:]

Basic Principles:

• Mechanical Forces – physical force

• Fluid Energy – particle-particle collisions

• Screening & Classification – sort & size

• Temperature & Moisture – conditioning

• Material Properties – consideration of material characteristics

• Particle Size Distribution – specific range

• Agglomeration & Cohesion – sticking and clumping

• Milling Time & Speed – balance

Q. Many of the powders you mention are combustible. Do the different milling technologies have different chances of dust explosions inside the mills?

Yes, a lot of them have combustible dust. So, it’s all about controlling that combustible dust. It’s not really mill dependent; you have to put other controls in place because you’re milling inside a chamber and when it exits is where you want to control your dust.

Q. In your experience, have you seen more success in extruding or compounding multiple ingredients prior to milling vs. simply blending multiple ingredients?

A lot of materials that need to be compounded have a much better chance of staying together during the milling process vs. if you’re just blending two materials together and you go to say, jet mill them, they might mill at different rates or consistencies so you’re not going to keep that homogeneity of the two materials together so it’s always better together, if you can, compound them first and then mill them.

Q. If I have an abrasive material, how can I reduce the particle size without contamination?

That is all about equipment design. There are many different mills that have interchangeable parts on them that are built for more abrasive materials and are called ARs (abrasive resistant). A lot of jet mills have specific classifier wheels that are built for them to be able to mill those abrasive materials and not degrade the equipment.

Q. In your experience, have you been able to modify or control the curve of the particles as well as particle size to influence the distribution?

Yes. If we look at a ball mill vs. a jet mill, a ball mill can create very wide particle distribution curves. You can mimic that on a jet mill; it will move it slightly but depending on your conditions, you can try to mimic that and pull it in and narrow it more.

Q. You referenced Material Properties to determine size reduction technology, what are the important properties or combination of properties?

You’re looking for things like moisture content, how friable is it? If it is not friable material, if it’s like nylon, you might need to cryogenically mill that material, bring that temperature down, and condition the particle to be able to be milled.

Q. Is air classification a good and efficient classification technique for particles between 1-10 microns and make narrow fractions?

It really depends on the material, but really small size ranges might be a little harder for air classification because those finer particles get entrained in the air, so it depends on the material. But if it’s very tight and very fine particles it might not be the best.

Q. Why is bead/wet milling better than a traditional ball mill for precise particle size reduction?

Ball mills, (you can do wet or dry), it’s more of control. Bead mills have a lot more control over the agitator, the flow of the liquid media through the mill. And you can do more efficient cycling through a bead mill than you can with a ball mill.

Q. What is the smallest size recommended for jet milling and is this product dependent or does it not matter what the material is?

It definitely matters what the material is. The material will kind of guide you as to how fine you can get it in a jet mill. It’s a little of a misconception for jet mills. A lot of the time we want to say we can get it down to an average of 2 micron but it’s just an average and it is material dependent. So, if that material is super friable, you might be able to get down to 1 micron or even sub-micron.

Q. How often do you run into situations where multi-milling steps are necessary to achieve the desired particle size and when necessary is more dependent on the material or their desired particle size?

Very material dependent. We run into this pretty often actually. It’s very common when trying to separate out by density — very common in food applications. And it is dependent on particle size but also the density of what you’re trying to separate as well, especially for food products. You might be able to separate out like a higher starch compound from a material. So again, it is very material dependent and what the components are of that material.

Q. How small of a nano-size can a mechanical mill make? (of any type of mill)

Again, very material dependent. This is a harder one. Mechanical mills, if you’re talking about a bead mill, some micron range, very small nano particles can come about that but it’s very material dependent.

Q. Do you have a preference for the technique and tools used for determining the PSD?

Yes, typically laser diffraction is a very good, reliable method. You can see your entire curve. It’s a more expensive piece of equipment. So, if you’re working in the construction industry, you might not want to spend that on laser diffraction. And there, the sieve analysis is very popular. So very easy and you can also get a profile from there by stacking your screens. Typically for our higher precision particle size distributions, we like to go with laser diffraction.

Q. How to check for friability of powder?

If there has been some work done in the past, a lot of the time we like to do some research. Otherwise, trial and error. If we’ve never worked with a specific material or it’s a material that is being developed and formed, we might have to do trialing to figure out how well it grinds.

Q. Smallest PSD jet mills can get for soft and very low bulk density material?

Again, it really is based on the exact material. Even if it is a softer and lower bulk density material, it still might be friable enough to get a smaller PSD, but it depends on what you’re looking for.

Q. When cryogenically milling, how would one determine the optimal temperature for a given material?

This is also some research. Typically, you can look at some of your melt points for your raw material. Typically, when cryogenic milling, we just kind of crank the nitrogen up. We are monitoring but it gets cold. Sometimes all of a sudden it starts to mill and it’s not at the temperature you need it at, but it works for that material.

Q. For jet mill products and size reduction do you also have the capability to control the particle shape and maintain the product’s morphology?

Yes, we absolutely do. There are specific parameters that we can adjust the jet mills to in order to be able to control that shape.

Q. Can you give us an idea of the typical size reduction ratios for the milling technologies you discussed? Do they also generate different particle shapes?

I’m not sure how to answer this one… Just in general for milling technologies for generating different particle shapes, it depends how the material fractures. And that will help control the shape. Using different mechanical mills and putting different forces on it might alter the shape as well.

Q. Have you seen many bead mills, air-classification, and jet milling implemented in Li-ion industries?

Yes, we’ve had multiple deals over the past few years that we have wet media milled LI-ion products for batteries. Air classification, not so much. And then jet milling, yes, we have seen a lot of that come through for jet milling.

View the "Grains of Wisdom: A Journey Through Particle Size Reduction" webinar on demand here.