Issue link: https://www.e-digitaleditions.com/i/1197147
36 / January 2020 powderbulk.com of pellet mills. A flat die mill is similar to an extruder as the pow- der is screw-conveyed against an orifice plate. A cutter at the oppo- site side of the plate is then used to make pellets. In a ring die mill, like the one in Figure 6, the powder is fed into the inside of a cylindri- cal chamber where rollers inside the cylinder compress the powder through the die holes. Cutters out- side the cylinder then cut the pel- lets as they leave the mill. Melting and sintering agglomeration Agglomeration by melting or sin- tering is accomplished using ram extruders and screw or auger extruders. The extruder is usu- ally heated externally, although the temperature of the equipment will rise without the introduction of heat because of friction between the particles themselves and fric- tion between the particles and the extruder walls. Ram extrusion is a batch process. A volume of powder is fed into the ram extruder's barrel, and then a hydraulically driven piston forces — or rams — the materials toward a die. The piston is then reversed, and a new batch of powder is fed into the barrel. Screw extrusion differs in that it's a continuous process. The screw extruder is comprised of a feed section, conveying section, melting or sintering section, and pumping section, as shown in Fig- ure 7. In the screw extruder, fric- tion or a heated barrel increases the temperature of the conveyed powder. Pressure is generated by pumping the materials through a die plate or restricted orifice. The extrudate leaving the die plate or orifice is often immersed in a water bath to reduce the extrudate tem- perature to below its melting point before converting it into pellets. the fluidized-bed agglomerator, as shown in Figure 4. The equipment provides a turbulent environment where particles come in contact with liquid and each other, coalesce, layer, and form agglomerates. If the solids are initially dis- solved or suspended in a lot of liq- uid, agglomeration can be achieved by feeding partially dried particles from a spray dryer into an inte- grated fluidized-bed dryer. Because the drying is incomplete in the spray dryer, particles fed into the fluidized bed are slightly cohesive, and size enlargement will occur during fluid- ization. The final drying also takes place in the fluidized bed. Pressure agglomeration Pressure agglomeration falls in two general categories, roller compaction and die compaction. In a roller com- pactor, like the one shown in Figure 5, mechanical forces are used to press the powder. Fine powder is fed between two counter-rotating rolls, where it is compressed into a sheet or strips. The sheet or strips are then fed through a flake breaker, which shears and crushes the material into manageable sizes. The equipment works best when the powder fed into the compactor is deaerated before it reaches the rolls. This can be accomplished by equip- ping the roller's feed hopper with a screw to compress the material as it is conveyed downward. Another technique is to apply a vacuum on the clean side of gas-permeable hop- per walls or rolls. Die compaction equipment, such as briquetters and pellet mills, pro- duces agglomerates with a desired shape. A briquetter is a roller com- pactor that has pocketed rolls, allow- ing the material to be molded by pressing it into the recessed cavities on the rolls. Pellet mills are devices that force fine powders into dies. Flat dies and ring dies are two common types FIGURE 5 Roller compactor Courtesy of Komarek. FIGURE 6 Ring die mill Courtesy of CPM Roskamp Champion. FIGURE 7 Screw extruder Courtesy of Coperion. the need for seed particles to be fed into the drum during startup, and the high material recycle rates that are generally necessary. Other tumble-growth agglom- eration equipment includes mixers that are equipped with spray noz- zles and fluidized beds, such as