It’s hard to make a particulate solid without getting some undesirable components. Unlike liquids and gases, solids can vary in size and suffer from contamination, discoloration and other variations in physical properties. The major offenders usually are fine particles. Think about it: Don’t your customers complain mostly about fines in the product? These particles also can pose a safety problem as well as a nuisance or housekeeping issue.
Before looking at the methods to minimize fines, let’s consider the many ways you produce these troublesome particles:
Particle formation. The problem can start in the particle formation step (crystallization, reaction or extraction). Excess supersaturation or the lack of nucleation control can allow fine particles to persist through the process. I’ve mentioned this issue in previous columns — “Conquer Crystallization Challenges,” and “Don’t Let Phase Changes Faze You.”
Upstream processing. Attrition can occur in solid/liquid separation, drying, conveying and storage. I’ve even seen attrition in liquid/liquid separation processes where crystals have formed due to the immiscibility of the chemical in a solvent. Centrifuges are a common source of attrition due to their discharge mechanism. Filters aren’t necessarily the solution for the separation, though. While they may seem a gentler option, filters can cause attrition from solids hitting the cloth or solid surface due to a poorly designed inlet.
Drying is a common source of attrition. Mechanical drying devices toss the particles around, allowing them to hit each other. Surprisingly, little attrition stems from sliding of the solids; most arises by particle/particle impact. Some particles have internal solvent left over from upstream processing. The heat applied during drying may cause particles to explode, especially if the rate of heating is excessive. You can fix this by changing the crystallization process as well as cutting the heating rate.
Conveying and storage seem to run together as a source of attrition. Again, the major culprit is particle/particle impact. In some cyclone and elbow research, we presumed that the particles mainly broke up along the walls of the cyclone or pipe. However, we discovered that particles landing on each other was a worse source of attrition. Make sure you control the discharge height when handling a friable material.
To solve the upstream processing issues, you must put on your detective hat because there are many sources of fines in your product. After you’ve pinpointed the source of your fine particles, you’ll need to decide if it’s more economical to solve that problem or resort to end-of-pipeline treatment. Many good choices exist for eliminating fines. However, some of these can require an expensive installation and reduce your productivity. So, let’s look first at two common and relatively low-cost options.
Screening is an obvious choice but can make the problem worse. If you already have a screen, check the grounding or look at adding an air ionizer to remove the charge that fine particles often carry. Overloading of a screen can cause more particle/particle impact and create fines.
Fluidization is a common way to elutriate fines from the larger particles and can be built into the drying process. However, it has limited value where a sharp separation is desired. When the fines content is high, particles bunch together and then settle at a faster velocity. Also, the mass of solids removed can reduce productivity unless you can agglomerate the fines and return them to your production process, which adds another piece of equipment.
Many other elutriation devices can remove fine particles but they tend to be more expensive. Some people have tried cyclones as a poor-man’s particle separator but cyclones don’t provide a sharp cut in particle size. The elutriator uses a narrow flow channel and gas flowing against the particles to give a relatively sharp cut in size. Air ionization can help in rare cases.
You also can consider some specialized ways to remove fines such as washing with a solvent or flowing the bulk solids over a sticky surface. For the most part, I prefer elutriation because of its long track record in solving the excess fines problem. It retains customers.