Spray prilling techniques

For a variety of applications, spherical particles are required. Many of these are associated with the field of powder metallurgy (Chapter 7). While it is relatively easy to produce spherical particles from low-melting materials by conventional techniques, in which melt droplets are produced and solidified in spray (prilling) towers [B.97], refractory solids in general and, specifically, high-melting metals can not be converted by this simple technique. However, if the solid is available in powder form (Section 13.3, refs. 82, 85, 89, 90, 94), various methods are available to produce spherical particles by agglomeration. 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

Table III. It is obvious from the data in Table III that the housefly and the mosquito, in both the adult and larval stage, are susceptible to insecticides of the DDT type. However, the extravagant claims that DFDT is far superior to DDT as a contact insecticide against flies are not borne out by the results of controlled laboratory tests. The Peet-Grady testing technique used by Prill (92) would indicate that in the presence of added pyrethrins DDT is definitely superior to DFDT when applied as a spray. On the other hand, DFDT gave higher percentage kills than DDT when flies were placed under a Petri dish and held in contact with deposits of the compounds on glass surfaces. A comparison of the activity of these compounds against adult mosquitoes has not been reported.

Table VI. The knockdown power of the organofluorine insecticides has been determined principally against members of the Diptera order and by a limited number of workers. There is general agreement that DFDT acts more rapidly than DDT, at least against those species with which they have been compared. Prill 92) found that twice the amount of DFDT compared to DDT was required to give the same knockdown against houseflies when tested by a space spray technique with added pyrethrins. The forced contact method of Fay and Buckner 27) revealed that without added pyrethrins DFDT was a more powerful knockdown agent than DDT.

Globulation is the formation of droplets of solution, slurry, or melt followed by solidification by prilling, spray drying, or fluid bed techniques. Control of particle size is best achieved in fluidized beds. 

The spray crystallization of a solution has some features in common with prilling, the melt crystallization technique described in section 8.2.2. 

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