Issue link: https://www.e-digitaleditions.com/i/1380119
14 / June 2021 powderbulk.com PBE AGGLOMERATION ADVISOR Providing a steady liquid stream is usually not a problem, but pro- viding a stable powder feedrate or ensuring a constant solids-to-liquid ratio may be a challenge with some powders. Mass-flow hoppers, in which the hopper walls are steep enough and low enough to allow powder to flow along the walls, are more likely to provide steady solids discharge rates. In addition, the size of the hopper outlet must be large enough to prevent a cohesive arch from developing and stopping flow. Methods for designing mass-flow hoppers are discussed in the article "Hopper Design Principles" by G. Mehos and D. Morgan. 2 While an outlet diameter greater than the minimum will prevent cohesive arching, it may not nec- essarily be large enough to allow the desired steady discharge rate, specifically if the powder is fine. A Avoiding erratic powder feedrates into agglomeration equipment liquid bridges will begin to form at contact points between particles. This is known as the pendular state of saturation. Moisture is attracted to the interfaces between powder particles by capillary forces. As the saturation level increases, the powder approaches the funicular state, where all internal solid sur- faces become surrounded by liquid. At this point, the mixture becomes more paste-like, the tensional forces disappear, and the agglom- erates become significantly weaker. When the powder becomes fully saturated, it reaches the capillary state, and at higher moisture levels, the mixture begins to behave as a slurry. The powder saturation states are shown in Figure 1. 1 Producing an agglomerate with the optimal saturation state is crit- ical and requires careful control of the liquid and solids feedrates. A gglomeration is the process of converting fine powder particles into larger ones. This is often desirable to increase a material's bulk density, reduce dust, and improve flowability. In wet agglomeration, the fine par- ticles are wetted with a liquid, preferably water, which acts as a binder between the particles. Wet agglomeration equipment includes pin mixers, plough mixers, disk pelletizers, fluidized beds, and other technologies. The optimal amount of liquid to add to a powder — the amount that gives green (or new) agglom- erates their greatest strength — is typically 40 to 90 percent of the powder's full saturation. The sat- uration level is the fraction of the total void space in the powder that can be filled with a liquid. When the liquid is added to a dry powder, Greg Mehos, director Greg Mehos & Associates FIGURE 1 Saturation states Dry powder Pendular state Funicular state Capillary state