7. Particle size and width of the particle size distribution (PSD). These attributes often can’t be changed but contribute to clumping. Finer particles have higher specific surface area and more particle/particle contact, resulting in higher shear forces. In addition, the van der Waals’ forces increase rapidly with finer particles. The wider the PSD, the more likely voids around larger particles will fill in with fine particles and boost cohesion. While this is a major factor in clumping, it’s very easy to identify in advance of a problem.
8. Attrition. This is more of a contributing factor for the previously mentioned sources of clumping. Breakage of particles releases energy that’s confined to the solids’ surface. In addition, finer solids will have poorer flowability and higher electrostatic charge. The increase in fines makes the PSD wider and solids easier to bridge.
9. Mechanical deformation of solids. This usually isn’t the primary cause of agglomeration. The normal stresses in a bulk bag or fiber drum are fairly low. However, the ultimate formation of agglomerates often appears as a mechanical failure. Because a solid is defined as something that can support its own weight, most failures stem from shear forces that exceed the solid’s strength. Many of the previous sources of clumping induce a failure that allows for compression of solids to form agglomerate.
10. Vibration. This often is overlooked as a cause of finer solids sifting into voids and increasing compression of bulk solids. When combined with temperature changes, vibration can make solids soften or plasticize, resulting in physical deformation and clumping. However, sometimes vibration can help to prevent mechanical deformation during transport.
As the above highlights, one major factor that repeatedly enters the equation is the presence of excess solvent or solvent vapor. If either afflicts solids, you must focus attention on the solvent source. Water is the most common solvent; moisture causes many clumping problems reported in the chemical and pharmaceutical industries. Four major types of solvent in bulk solids contribute to clumping (see sidebar). No single test can detect all four; methods for determining the amount of solvent may not give a clear indication of the type.
Chemical Processing has published many articles on ways to test the strength of bulk solids and how they gain strength on storage. The majority of these tests focus on setting parameters for design of a bin or chute to keep product moving or to induce flow. Clumping is an afterthought of these methods. The tests can show how a material gains strength upon storage but can’t predict outside of testing conditions (time or temperature) future increase in strength. In many cases the rise may be limited, at least to a solid block of solids. In addition, most methods require specialized and costly equipment, which is the major reason plants don’t conduct such tests prior to experiencing a problem. However, these tests, by determining the time-dependent unconfined yield strength, are some of the best ways to determine when there’ll be a problem.
The three major contenders for test equipment for this property are the Schultze (ASTM D6773) and Jenike (ASTM D6128) shear cells and the Johanson Indicizer. In addition, other devices have been developed for specialized industries, such as the Humboldt Tri-axial Tester for geotechnical assessment. Some of these methods can use samples as small as 20 g — but several runs may be needed to account for product variability. These tests’ major limitation is that, to be assured that material won’t exceed a given strength in the future, trials must cover a wide range of temperature and humidity over the expected storage time. This may not be practical for solids kept in a bag or drum for many months and for replicating shipping conditions.