Dust entrainment

We have examined the effect of kiln aerodynamics on fluid mixing and combustion. It is equally important to look at the aerodynamic effect on dust carryover from rotary kilns processing mineral materials. Although the principles behind particle motion are related to granular flow which will be covered in Chapter 4, the interaction of the flow of fluid in the freeboard and the active layer surface of the kiln bed is an aerodynamic phenomenon. 

The two principal underlying phenomena for saltation are (i) that the layer behaves, so far as the flow outside it is concerned, as an aerodynamic roughness whose height is proportional to the thickness of the layer, and (ii) that the concentration of particles within the layer is governed by the condition that the shear stress borne by the fluid decreases, as the surface is approached, to a value just sufficient to ensure that the surface grains are in a mobile state (Owen, 1964). 

The first principle follows the analogy of the aerodynamic behavior of surface roughness and the velocity profile in the fluid outside the saltation layer follows the friction law 

Bagnold (1941) determined two threshold values of p, a lower threshold value for which grain movement is first detected (P = 0.01), and an impact threshold at which saltation would be maintained (P = 0.0064). Hence the shear stress required near the surface to sustain an 

Other correlations for dust entrainment in rotary kilns have appeared in the literature (Li, 1974) but these are not easy to use for 

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Dust entrained in the exit-gas stream is customarily removed in cyclone cohectors. This dust may be discharged back into the process or separately cohected. For expensive materials or extremely fine particles, bag collectors may follow a cyclone collector, provided fabric temperature stability is not hmiting. When toxic gases or solids are present, the exit gas is at a high temperature, the gas is close to saturation as from a steam-tube diyer, or gas recirculation in a sealed system is involved, wet scrubbers may be used independently or following a cyclone. Cyclones and bag collec tors in diying applications frequently require insulation and steam tracing. The exhaust fan should be located downstream from the cohection system. 

In general, this equipment offers an economical heat-transfer area for first cost as well as operating cost. Capacity is hmited primarily by the air velocity which can be used without excessive dust entrainment. Table 12-32 shows hmiting air velocities suitable for various sohds particles. Usually, the equipment is satisfactory for particles larger than 100 mesh in size. [The use of indirect-heated conveyors eliminates the problem of dust entrainment, but capacity is limited by the heat-transfer coefficients obtainable on the deck (see Sec. 11)]. 

For abandoned opencast sites, and for large spoil heaps associated with underground coal mines, landscaping and revegetation are clearly the key to long-term prevention of dust entrainment by the wind (see below). 

Fluidized-bed chlorination was started in 1950. The titanium raw material (with a particle size similar to that of sand) and petroleum coke (with a mean particle size ca. five times that of the Ti02) are reacted with chlorine and oxygen in a brick-lined fluidized-bed reactor (Fig. 16, c) at 800-1200 °C. The raw materials must be as dry as possible to avoid HC1 formation. Since the only losses are those due to dust entrainment the chlorine is 98-100% reacted, and the titanium in the raw material... 

Zender C.S., Bian H., and Newman D. (2003). Mineral dust entrainment and deposition (DEAD) model Description and 1990s dust climatology. /. Geophys. Res., 108(D14), AAC8/1-AAC8/19. 

The only safe way of avoiding formation of dust clouds is to use air velocities at which no entrainment of dust takes place. In band and tray dryers, the control of velocity is easier, but this is difficult to fulfill in dryers that involve suspended-particle processing. In general, the nature of the drying operation is such that dust entrainment is not easy or feasible for various reasons. 

Mobcue Content Dry Basis [kg/kg 0,067 i 00021 Dust Entrained m Gei/ Meterid Total 0.000 ... 

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