I love working with industrial gases and liquids. It’s easy to measure the quality of the product; containment is built into the process; and the composition rarely changes as you transport the product. Particulate solids, on the other hand, are difficult to store, move and sample. It’s hard to make a particulate solid without getting undesirable side effects. Unlike liquids and gases, solids can vary in size, shape, discoloration and other physical properties. The major offender is usually fine particles. They can be generated during transport or handling, and be the culprits of clumping and spillage. The most common complaint I hear from customers pertains to fine particles. While we may think about gases and liquids as being flammable or explosion hazards, solids are just as cantankerous and a housekeeping hazard. The problem comes from the particle size distribution, which is also an important property of the bulk solids. Fines can control the flowability of the bulk solids, act as the cement in a process and increase dissolution rates. However, we seem to have more problems with fines than all of the benefits that they offer.
Problems with Crystallization
Most problems with fines are initiated upstream of where they are observed. Take crystallization as an example: rapid cooling generates excess super-saturation and too many nuclei. By itself, this would not be a problem as a fines destruction loop or settling chamber can overcome this condition. The problem is exacerbated when you are working with polymorph chemicals. I’ve seen cases where two polymorphs’ solubility cross at a certain temperature. Rapid cooling can form the wrong polymorph and steal solute from the desired species. Everything looks fine until you put the product in a bag. Upon storage, a small amount of solvent can be released and condense onto the solids. Recrystallization creates a more stable species, and the product is clumped. Often, you’ll need to add a conditioning step to purge the excess moisture or improve control of the cooling rate.
Fixing Fluid-Bed Dryers
In another scenario, the excess fines cause problems in the drying step. I recall a fluid-bed dryer where the product was generated by precipitation. We expect more fines in this type of operation, but the fluid-bed had a dust collection device built into the dryer. Large particles reached an acceptable moisture and the fines were over-dried. Since the dryer output went to an agglomeration step, the moisture smoothed out and helped in the formation of granules. However, fines would accumulate on the filter and were blown back into the bed. The ratio of fines to large particles increased and reduced the minimum fluidization velocity, blowing larger wet particles up to the dust collector. Eventually, the dryer was plugged up. This was fixed by removing the dust collector from inside the dryer.
Combatting Attrition
Attrition can occur in solid-liquid separation, drying and conveying. Centrifuges and liquid filters can attrit particles due to the discharge or feeding mechanism. Some cyclone designs and gas filters can generate fines, but the major culprit is excessive particle-to-particle impact, not particle-to-wall contact. An unexpected source of attrition occurs when the particles are dumped into a silo or tanker. Make sure you control the discharge height when handling a friable material to minimize attrition
Most of what I’ve highlighted above shows the complexity of fine particle origins and how they manifest themselves in the process. I’ve even found screens, whose purpose was to remove fines, make the problem worse. Overloading a screen can cause more particle-to-particle impact and create fines. Improper grounding and selection of hole size increase residence time on the screen or increase solids load. Fine particles have high charge-to-mass ratio, but can be neutralized by adding an air ionizer.
When all else fails, an end-of-the-pipe device can be installed, like a screen, fluid-bed or even a washing step. The mass of solids removed can reduce productivity, unless you can agglomerate the fines and put them in your production. However, this adds another piece of equipment. There are many elutriation devices that can remove the fine particles, but they tend to be expensive and create a disposal/recycle problem. Some people have even tried cyclones as a poor man’s particle separator, but they do not have a sharp cut size like an elutriator.
