Mixing Action Is Essentially Diffusive

The drum blender (Figure 7.11) consists of a horizontal cylinder rotating about its axis. Its mixing action is essentially diffusive. The powder to be mixed is placed inside the drum. As the drum rotates, the powder is lifted up, until the angle of repose (see Chapter 2) is exceeded. At that point, the powder falls back on the rest of the bulk and enters a new cycle of lifting and falling. Diffusive mixing takes place during the residence of the powder in air, while falling. Continuous operation can be made possible by tilting the drum.

Figure 7.11. Cross-section of drum blender

Tumbler mixers also belong to the category of diffusive mixers. In the double-cone tumbler (Figure 7.12), the powder undergoes cycles of expansion and compaction as the vessel rotates. In the V-shaped tumbler (Figure 7.13), the powder is subjected to cycles of division and assembly. Convectional elements such as rotating or stationary flow distortion bars (intensifier bars) are sometimes installed in both types of mixers.

Figure 7.12. Double-cone tumbler mixer

Figure 7.13. V-shaped tumbler mixer