Characterization of fine powders

Fine powders are used in many applications and across many industries such as powdered paints and pigments, ceramics, petrochemicals, plastics, pharmaceuticals, and bulk and fine chemicals, to name a few. In addition, fine powders must often be handled as a waste by-product, such as ash generated in combustion and gasification processes. In order to correctly design a process and process equipment for application and handling of powders, especially fine powders, it is essential to understand how the powder would behave. Many characterization techniques are available for determining the flow properties of powders; however, care must be taken in selecting the most appropriate technique(s). In this work, sample fine powders with a mean particle size between 22 and 31 μm were characterized using a variety of techniques that tested powders under different stress states, ranging from static to dynamic. In the powder coating process, for example, powders must exhibit good fluidization and resist agglomeration in order to produce a smooth paint coating. It was found that dynamic characterization techniques such as fluidized bed expansion were best suited for predicting the fluidization performance, while static characterization techniques such as cohesion were better for predicting agglomeration. It was also found that results from static and dynamic characterization techniques do not necessarily agree, where fine powders that showed good fluidization performance also displayed increased agglomeration, and vice versa. This suggests that flow properties are dependent upon the stress state and that no single technique is suitable for fully characterizing a powder. Extending this example to other industries, both static and dynamic characterization techniques must be employed to completely understand the flow properties of a powder and predict how it will behave under all process conditions.