Suspending agents high molecular weight

• Sodium carboxymethyl cellulose

• Methyl cellulose

• Ethylene oxide polymers

Plasticizers may be used with binding materials that are hard or brittle and that tend to crack during drying. Suspending agents may be needed to prevent solids from settling within the slurry. Deflocculating agents aid in the formation of slurries by preventing the agglomeration of fine particles. Wetting agents also may be used to maintain solids in suspension. Some slurries have a tendency to foam during mixing. Antifoaming agents or defoamers may be used to control this action. Chemical activators also may be used as additives to aid in subsequent sintering or processing of powders.

Atomization Techniques and Agglomerate Size Distribution. Three standard techniques are used to atomize slurries for spray drying:

• Single-fluid nozzle atomization

• Centrifugal (rotating disk) atomization

• Two-fluid nozzle atomization

Table 1 gives the advantages and disadvantages of these techniques for atomization of slurries. Table 2 gives the relative agglomerate sizes produced by these techniques.

Table 1 Advantages and disadvantages of various spray drying atomization systems

Atomization system

Advantages

Disadvantages

Centrifugal (rotating disk)

High feed rates, less downtime, low-pressure pumps, never plugs

Larger diameter dryer required, coarse agglomerate size not obtainable

Single-fluid nozzle

Large agglomerate capability, smaller dryers can be used

Downtime due to part wear and plugging, high pressure pumps

Two-fluid nozzle

Less part wear

Broad agglomerate size distribution, compressed air needed

Table 2 Relative agglomerate sizes produced by various atomizing systems

Atomizing system

Mean

agglomerate

size, Cm

Centrifugal (rotating disk)

High speed

25-100

Medium speed

50-200

Low speed

100-300

Single-fluid nozzle

High pressure

25-100

Medium pressure

50-200

Low pressure

100-300

Very low pressure

200-600

Two-fluid nozzle

High pressure

10-50

Medium pressure

25-100

Low pressure

As shown in Table 2, the largest agglomerate sizes (600 /'m) are achieved by the single-fluid nozzle. The centrifugal (rotating disk) atomizer yields agglomerate sizes up to 300 /Jm, and the two-fluid nozzle produces agglomerates only up to about 200 /'m in size. Centrifugal atomization yields the narrowest agglomerate size distribution range, followed by single-fluid atomization and two-fluid atomization.

Centrifugal atomization entails the introduction of the slurry into a horizontally rotating disk that is equipped with vanes or holes through which the slurry exits. The slurry is atomized into fine droplets, the average size of which is a function of the design and peripheral speed of the wheel. Most wheel designs incorporate a wear-resistant material for extended life.

Single-fluid atomization is capable of producing the largest diameter agglomerates. However, because of the high pressures required to force the slurry through a single, small orifice to produce the desired droplet size, considerable downtime may be required to replace worn parts and to unplug the nozzle. Use of a nozzle in the fountain spray position extends the residence time of the droplets in the chamber. As a result, smaller dryers can be used for single-fluid atomization of materials that are not heat-sensitive.

Two-fluid atomization, which uses a pressurized air blast to break up a slurry stream into droplets, produces the widest range of agglomerate sizes of all three atomization techniques. However, it is a relatively easy technique for producing atomized droplets. Wear problems do not exist because high pressures are not employed. The orifices used are typically larger than those used for single-fluid atomization; therefore, plugging is not as serve a problem. This technique does, however, require the use and additional expense of pressurized air.

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