Dust, from pellet production

Pellets (e.g. polyethene), granules (e.g. fertilizer) and coarser particulate materials (explosives, media from granular filters), often contain a small dust (sub 50-100 pm) friction, which is undesired. In many cases, customers require the levels of such dust or fibers to be reduced to levels well below 0.5% (in the case of polyethene pellets the level desired is 50 ppm ). While the methods of production and handling can be optimised to prevent the production of such dust, this approach is not always feasible. A number of devices have thus been developed for removing what one may call "fugitive" dust from such products. Some of these are shown in Figure 1. 

In a much smaller scale, the conditions in the die holes during medium pressure agglomeration or pelleting (Chapter 5, Fig. 5-10bl-b6) realize the same densification process as developed in the (long) channel of ram or screw extrusion presses. Therefore, medium pressure agglomerators can be also used for the production of fuel pellets from, for example, shredded paper, saw dust, plastic foil, and similar waste materials [B.97]. Fig. 6.10-27 shows two examples of lose raw materials and the resulting pelleted product. 

Other uses for coal-tar pitch include production as a binder for foundry cores, as a sealant for dry batteries, and in the manufacture of clay pigeons. Pelleted pitch used as the binder in foundry cores is a hard pitch supplied as spherical granules which are formed by a spray-cooling process. Clay pigeons consist of disks molded from a mixture of hard pitch and a mineral filler such as clay or limestone dust. 

Inhalation, dermal, and some oral exposure to tetryl has occurred in workers involved in production and use of tetryl compounds. In the past, workers in munitions plants were exposed to tetryl dust released into workroom air (Cripps 1917 Hardy and Maloof 1950 Hilton and Swanston 1941 Probst et al. 1944 Troup 1946 Witkowski et al. 1942). In one study, air samples taken in 1942 from a small powder house where exploder bags were loaded with weighted tetryl, stemmed, tied, and inspected had tetryl levels ranging from 1 to 18 mg/m (Hardy and Maloof 1950). Workers in these plants were exposed via inhalation of the tetryl dust and by dermal contact with the tetryl powder and pellets. Today, workers engaged in demilitarization operations involving detonation, open- burning, or incineration of tetryl explosives are likely to be exposed to tetryl. The extent of exposure in these workers has not been adequately determined. 

Virgin resin in pellet form contains emulsifiers that give the pellet surface a thin, oily coating that acts to collect dust, preventing it from becoming airborne. Colorant and additive products also contain emulsifiers and/or lubricating waxes (especially when powdered resins are used) that also impart this property to the final product pellet. 

As removed from industrial-scale reactors under ambient conditions, polyethylene is typically a white powdery or granular solid. In most cases, the raw polymer is then melted and selected additives are introduced. (Additives are essential to improve stability and enhance properties of polyethylene. See Chapter 8.) The product is shaped into translucent pellets and supplied in this form to processors. Pelletization increases resin bulk density resulting in more efficient packing and lower shipping costs. It also lowers the possibility of dust explosions while handling. 

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