Spray Drying

Spray drying is a powder-producing process in which a slurry of liquids and solids or a solution is atomized into droplets in a chamber through which heated gases, usually air, are passed. Figure 2 represents a typical spray dryer utilizing a disk atomizer with co-current air flow. Examples of commercially spray-dried powders are shown in Fig. 3. Spray drying is used widely in the pharmaceutical, chemical, and food industries. It is used to a lesser extent in the metals-related industries.

Inlei

Feed material Inlet tern peral u re \

/ Air disperser

Air healer

Air exhaust 10 scrubber

Atomizer

Air healer

Drying chamber

Outlet temperature

Coarse powder discharge

Fine powtfer discharge

Fig. 2 Typical spray dryer arrangement

Fig. 3 Micrographs of commercially spray-dried granules. (a) Ferrite, 75x. (b) Zirconia, 30x

Spray drying offers several advantages over other powder-processing techniques, particularly in applications requiring agglomerates for subsequent pressing and sintering operations. Spray drying also is one of the most economical ways of drying slurries.

Spray drying is a continuous rather than a batch process that allows close control of agglomerate size, bulk density, and moisture content. Agglomerate shape is spherical, facilitating excellent flowability. Particles have very short exposure time. Residence time within the dryer may range from 2 to 20 s, depending on dryer size. Therefore, heat-sensitive materials can be spray dried. Lubricating additives can be added easily for die pressing applications.

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