Agglomeration is the process of converting fine powder particles into larger particles by the introduction of external forces. Agglomeration is often desirable because the process yields a product that has a higher bulk density,contains less dust, and has improved flowability. Figure 1 shows samples of fine potash powder and powder that has been agglomerated. Fine powders often exhibit flow problems in a hopper, bin or silo, such as flow stoppages, erratic flow, flooding and limited discharge rates. Flow stoppages occur when attractive forces between particles, such as Van der
Waals forces, valence forces and hydrogen bridges, cause a cohesive arch to develop at the vessel outlet. In somecases, powder may only flow in a narrow channel when a feeder or gate isoperated. If the material has enough cohesive strength to become stable as the flow channel empties, flow stoppages will occur when powder along the walls
remains stagnant. Erratic flow results when ratholes collapse, causing the powder to arch as it impacts the out let. (For an illustration of arching andratholing, see the online version of thisarticle at www.che.com.)If a stable rathole forms in a hopper,bin, or silo and fresh powder is added,it may become entrained in the air and become aerated. Because most feeders are designed to handle solids and not fluids, flooding may result when the fluidized material reaches the outlet.
Flooding can also occur when ratholes collapse into an emptying flow channel. A fine powder sometimes cannot