Fine Particles Haunt Me: The Complex World of Particulate Solids

Fine particles usually have a higher charge-to-mass ratio than larger particles, so they stick to the large particles or surfaces. I’m reminded of that every time I wash my car (more on that later.) Rather than write a book, I thought it would be helpful to pull together the basic characteristics of fine particulates and how they interact with larger particles.

1. Size is the basic characteristic of fines. You can determine size using mechanical equipment, such as screens, or through visual methods like a microscope. I recommend that you always examine particles, even large ones, with a microscope and compare that observation to other methods of size characterization. You also can estimate size by physical behavior, such as terminal velocity or even buoyancy. More sophisticated methods may involve the use of light or lasers to extract an equivalent size and even get information on shape of the particle. However, it’s important to know where you plan to use this size/shape information for process design.
2. Shape can be determined while documenting the size, but it is often more important to observe the texture of the surface and look for cracks in the particles. One method to refine the shape characteristic is the use of a scanning electron microscopy (SEM) to find surface irregularities. This information is very helpful when the particles are exposed to abrasion rather than fracture forces. Irregular shapes that turn into sphere-like particles in a process can generate many finer particles or increase growth rates in a crystallization process.
3. Density is often only treated as an average because it is difficult to determine directly in the particles. It can be estimated from composition or slip velocity (i.e. using terminal velocity). Not all the volume of a fine particle has solids in it. Crystallized solids, as well as compacted solids, can have voids that can change density.
4. Composition is elusive. It can be estimated from transmission electron microscopy in combination with SEM or mass spectrometry. However, the most important issue with composition is not the chemical species but rather the location of it, which can affect conductivity of the particle. Particles may not be able to hold a charge in an electrical field if they are highly conductive
5. Void fraction is usually associated with the bulk solids, but we often overlook the amount of volume taken up by fluids in the particle. Those voids offer a pathway for the fluids to migrate to the surface. Depending on the fluid, the density may be altered.
6. Charge-to mass-ratio can usually be estimated from size and composition. It is used to estimate the potential that a fine particle will bind with larger particles or be repelled. As particles move across a surface, they can induce a charge on the particle. If that force is larger than the van der Waals force, the particles will separate.
7. Zeta potential is rarely available, but it is an important indicator of the stability of colloidal suspensions. You can easily measure it using electrophoresis and can use it separate particles by size, charge or binding affinity.

In most cases, size and some general information about shape are attainable. Don’t assume the physical properties are from the bulk solid. This is a mistake, especially when you are introducing a new product to an existing process.

We had a process where the void fraction of the bulk material was rather high. To avoid excess moisture from accumulating in the bag we purged the bulk solids before going into the bag. Unfortunately, the new product had a high void fraction in the fine particles but not in the overall solids. The purge was not long enough to remove all the moisture, so the product clumped up during long-term storage.

Back to washing your car: When you wash your car with a high-velocity water stream, you would expect the fine particles to come off easily. However, the van der Waals forces hold the particles on the surface, so you must wipe them off or use a surfactant to charge the particles and break these forces.