Dust Generators

Dry powders are commonly resuspended to form an aerosol by pneumatic means. High concentrations are readily attainable, but stability of output and the presence of agglomerated particles are common problems. The basic requirements of a dust generator are a constant powder feed rate and a method of deagglomerating the powder. Many techniques have been used to develop a stable dust generator, and a recent review of different dry dispersion methods can found in Hinds [58]. Powder feed methods include scraping off the surface layer of a plug of powder such as in the Timbrell dust generator or the Wright dust feeder, vibrating sieve, screw feed, chain feed, and gravity feed mechanisms (59,60). The two methods of deagglomerating powders that have proven successful are the fluidized-bed and air-impaction methods.

The most stable method developed is the fluidized-bed aerosol generator. Initial work by Guichard [61] and Willeke et al. [62] has led to the development of a commercial model based on the prototype by Marple et al. [63]. The commercial version is shown in Fig. 8. Powder is fed to a fluidized bed of bronze beads by

Figure 8 Schematic of a fluidized-bed aerosol generator (TSI Model 3400).

a continuous-bead-chain drive. Other types of metallic or ceramic beads can be used. The powder is deagglomerated by the fluidizing action and carried out of the bed by the airflow. A cyclone is used to remove agglomerated particles, especially when a fine aerosol is preferred. A period of 2-4 hr is required for the generator to reach a stable output. The recommended size ranges from 0.5 to 40 mm, and a mass concentration of up to 100 mg cm"3 is possible. Up to 15 L/min of airflow can be used with the fluidized bed of 100-mm beads. A higher-output fluidized-bed generator was developed by Boucher and Lua [64]. The stability of this improved generator was demonstrated for submicrometer-size basic oxygen furnace dust. The generator used abed of nickel beads of 125-212 mm. The dustfeeding mechanism was a vibrating mesh system that used steel balls above the mesh to provide some milling and deagglomeration of the dust. The time required to reach a stable output was reduced to 30 min, and an aerosol loading of 4 g/m3 was achieved at a flow rate of 70 L/min. Aerosols generated by fluidized beds tend to be highly charged, and an aerosol neutralizer is usually used [65].

Generation of asbestos or other fiber aerosols has been developed using fluidized beds. Early methods, though successful in aerosolizing the fibers, were not capable of a stable output for long durations (e.g. see Ref. 66 and Ref. 67). A stable fibrous aerosol generator was developed by Tanaka and Akiyama [68] using a continuos screw feeder. Glass fibers obtained by milling a binderless filter were mixed well with glass beads with diameters of 210-297 mm in distilled water. The mixture was dried in an oven. This two-component powder was fed into a hopper and transported to the fluidized bed by a screw feed. An overflow channel similar to that used by Guichard [61] maintained a constant bed height. This method of powder preparation avoided any clumping that might arise were the fibers fed directly into the bed. Stability and reproducibility of the generator were demonstrated on an hourly and a weekly basis.

The fluidized-bed method worked well, and it thoroughly deagglomerates dry and relatively noncohesive powders, such as coal, Arizona road dust, silica, copper ore, and potash [63]. The fluidizing action of the bed alone was inadequate for deagglomerating flocks of fibers and cohesive powders, such as dyes, that consist of relatively flat particles [69]. Air-impaction methods worked better.

The Timbrell dust generator was initially developed to generate asbestos fibers and was later modified for other powders [59]. The aerosols were generated by using two revolving rotor blades to cut or scrape an advancing plug of powder. Air jets dispersed the powder, which was scraped off. The stability of the output was controlled primarily by the packing of the powder plug. The particles might not have been thoroughly deagglomerated at higher feed rates because the air-impaction action was not strong. A commercial model was available but is no longer manufactured.

A more widely used dust generator has been the Wright dust feeder [60]. A diagram of the commercially available model is shown in Fig. 9. The powder

Figure 9 Commercial version of the Wright dust feeder. (A) dust chamber, (B) cap, (C) long pinion, (D) gear, (E) small pinion, (F) treaded spindle, (J) inner tube, (G) main spundle, (K) scraper head, (M) impaction plate, (N) spring ring, (O) scraper blade.

plug in chamber A is fed into inner tube 3 by scraper K when the chamber is rotated. Air fed in through opening H carries the dust down inner tube 3. The aerosol is accelerated through the orifice at L, and agglomerates are broken up by impaction plate M. Particle sizes greater than 10 mm are not recommended because of variability in the packing of the dust plug. Airflow rates can vary from 10 to 40L/min, and output mass concentrations are about 10g/m3. The time to reach stable output is approximately 5 min, which is much shorter than for a fluidized bed.

Higher output both in mass loading and flowrates can be achieved with an NBS-II dust generator. The model name is somewhat generic, and the original design was by Dill at the Bureau of Standards [70]. A commercial version was previously available but has been discontinued. Dust in the hopper flows down by the action of gravity, aided by an agitator. The dust is fed into the spaces between the teeth of a rotating cog. Excess dust is removed by the contoured spreader plate. The dust is carried to a slit that covers three consecutive teeth. The dust is sucked up by the compressed-air ejector. Strong turbulent motions within the ejector nozzle serve to deagglomerate the dust. Fewer agglomerates are present in the aerosol at higher operating pressures of the ejector. Particle sizes ranging from 1 to 100 mm can be aerosolized. Airflow rates are 50-90L/min, and aerosol mass concentrations are about 100 g/m3.

Another method of dust generation uses the rotating-brush method. The design follows that of the German standard VDI-3491 (e.g., see Ref. [71]). A rotating brush with a high-velocity airflow of 150m/see is used to disperse the dust from an advancing plug. Particle sizes up to 100 mm can be dispersed. Airflow rates are specified from 10 to 50 L/min. Aerosol loadings of up to 100 g/m3 can be obtained. The duration of operation is limited by the volume of the powder plug and the rate of feed. Cohesive particles, such as carbon black, can be dispersed. Intermittent generation of large agglomerated particles from buildup on the brush has been found [72]. As in all plug feed methods, the stability of the output is determined by the uniformity of the packing of the powder. An additional consideration is the required cleaning of the brush after operation [71]. Fibrous particles can also be dispersed by this method [71].

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