Powder-Blend Handling Processes

CONCERNS WITH POWDER-BLEND HANDLING PROCESSES 

There are two primary concerns with powder handling that cannot be overlooked when scaling processes achieving reliable flow and maintaining blend uniformity. To address these issues when scaling processes, knowledge of how powders flow and segregate is required. 

A number of problems can develop as powder flows through equipment such as bins, chutes, and press hoppers. If the powder has cohesive strength, 

One of the most important factors in determining whether powder will discharge reliably from bins or hoppers is establishing the flow pattern that will develop as powder is discharged. The flow pattern is also critical in understanding segregation behavior. 

Two flow patterns can develop in a bin or hopper funnel flow and mass flow. In funnel flow (Fig. 1), an active flow channel forms above the outlet, which is surrounded by stagnant material. This is a flrst-in, last-out flow sequence. As the level of powder decreases, stagnant powder may slough into the flow channel if the material is sufficiently free flowing. If the powder is cohesive, a stable rathole may remain. 

---

CONCERNS WITH POWDER-BLEND HANDLING PROCESSES... 

J. W. Carson and T. A. Royal, "In-Bin Blending Improves Process Control," Powder Handl Proc. (Sept. 1992). 

The wet granulation process is the traditional method of manufacture and is frequently used in the pharmaceutical industry. Expertise in wet granulation is widely available, as is the required equipment. The process improves flow and cohesion, reduces dust and crosscontamination, and permits the handling of powder blends without loss of homogeneity. 

Fuel fabrication facilities are located in many parts of the world, as shown in Table 12.2, where the countries and the annual fuel production capacihes are listed. These facilities range from sites which can handle all steps including UF processing, UO2 powder blending, pelletizing lines, and specialized component manufacture, to sites that are responsible for final assembly only. 

Wihns, H. (1992). Blending silos an overview. Powder Handl. Process., 4(3), 293-299. 

Further, the use of aqueous additive systems eliminates dust and compaction problems associated with the handling of powder form additives. Finally, the use of aqueous additive systems provides flexibility for mixing the additive with the resin. The additive can be mixed and dried with the resin still in moist, wet-cake form. It can be blended with other formulation ingredients, or pumped into an extruder or kneader during the compounding step. This flexibility can, in turn, provide still lower-cost processes and improved uniformity of mixing. 

The process flow layout for the twin-screw mixer set up is presented in Figures 3.12 and 3.16. As noted above, the metering screws for feeding powder, rubber, and pellets would each have a different design, each optimized for trouble-free smooth delivery of the blend components. The rate is affected by the ability of the feed port to handle the bulky compounds. In reality, this is often the limiting factor in production of blend compounds that have fluffy fillers. 

---