Issue link: https://www.e-digitaleditions.com/i/1197147
January 2020 / 35 type of powder, the powder's ow properties, and the feeder design. Dry agglomeration can be a mis- nomer because the method often requires moisture, although signi - cantly less than the amount needed for wet agglomeration processes. The most popular equipment used in dry agglomeration is a rotary drum agglomerator, shown in Fig- ure 3. In this process, nozzles inside the drum coat particles with liquid to promote adhesion. The major advantage of dry tumble-growth agglomeration over wet agglomeration is reduced dry- ing costs. However, disadvantages include the long residence time required to granulate the powder, ity with the powder, and ability to give an agglomerate a certain amount of strength. Common wet agglomeration equipment includes pin mixers, disk pelletizers, plough mixers, uidized beds, and other technologies. Wet agglomeration processing of ne powders occurs in three stages. The rst stage is mixing and combines the ne powder, liquid, and binder. The particles are joined together to form green or uncured agglomerates, which is the second stage. Nuclei are rst formed and then grow into larger aggregates by layering or coalescing with other particles. The nal stage is drying or curing the agglomerate, which takes place in a separate device. In some cases, nucleation and aggre- gate growth take place in two separate pieces of equipment; for example, agglomerates formed in a pin mixer, like the one in Figure 1, are then fed into a disk pelletizer, like the one shown in Figure 2, to increase the agglomerate's size. Since it is critical to produce a green pellet with an optimum sat- uration state, meticulous control of the solids and liquid feedrates is required. Providing a steady liquid feedrate is generally straightforward and easy to achieve; however, ensur- ing a constant solids feedrate for some powders relies heavily on the ential scanning calorimetry (DSC), a technique used to study the thermal transitions of a polymer, can be used to estimate the temperature required to agglomerate a ne powder by melting or sintering. Many suppliers of agglomeration equipment have test facilities for conducting feasibility studies with pilot-scale agglomerators. Often the equipment is available for rent, and customers can perform thorough in-house evaluations to nd out if the agglomeration process achieves the desired result. Tumble-growth agglomeration Tumble-growth agglomeration can be either wet or dry. In a wet agglomeration process, the ne particles are mixed with a liquid, usually water. In some cases, a sur- factant or other chemical is added to improve the wettability of the solid particles. The optimal amount of liq- uid added to a powder — the level that gives an agglomerate its greatest strength — is typically 40 to 90 per- cent of the powder's full saturation. Binders are frequently added to improve an agglomerate's strength. Examples of organic binders are starch, waxes, and rosin. Inorganic binders include sodium silicate, bentonite, and various aqueous solutions. The choice of binder depends on its cost, compatibil- FIGURE 2 Disk pelletizer Courtesy of Eirich Machines. FIGURE 1 Pin mixer Courtesy of FEECO International. FIGURE 3 Rotary drum agglomerator Courtesy of FEECO International. FIGURE 4 Fluidized-bed agglomerator Courtesy of Freund-Vector Corporation.