Control of Powder Properties

Spray drying can be used to remove water from a slurry or to create agglomerates of fine particles contained within the slurry. Agglomeration is achieved by using a binder (discussed below in this section).

Most of the applications of spray drying in the metals industry require the formation of free-flowing agglomerates. Many of these powders are used for producing pressed parts. Therefore, the agglomerate size distribution and bulk density are the two most important properties of spray-dried powders.

Agglomerate size distribution is a function of atomization conditions and the properties of the slurry. Generally, a lower solids content yields a finer average agglomerate size. The maximum attainable solids content varies with material, but usually can be increased by using deflocculating or suspending agents.

Bulk density is a function of the solids content of the slurry, inlet temperature, and slurry additives. Lower bulk densities generally are achieved from slurries with low solids contents. Also, excessive inlet temperatures can cause lower bulk densities. Rapid evaporation of the liquids causes the partially dried droplets to expand rapidly, thus decreasing density. Introduction of frothing agents may entrap air in the slurry, which also leads to lower bulk densities. Typically, frothing agents are not added in metallurgical applications.

Moisture content of a powder can be controlled by the inlet and outlet temperature of the spray dryer in conjunction with the slurry feed rate. Moisture levels below 0.1% are possible. For a given airflow and inlet temperature, outlet temperature is controlled by the rate of slurry feed and the solids content. With higher percentages of solids, less water must be evaporated, which leads to higher throughput of dry product. If a product can withstand higher inlet temperatures, throughput can be increased (see Fig. 6).

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01 23456789 10 Production rate of bone-dry powder

Fig. 6 Relationship of evaporation capacity of spray dryers to production rate and slurry solids concentration. Source: Ref 3

Binders for Agglomeration. Suitable binder materials must be homogeneously dispersable (preferably soluble) in the liquid used to form the slurry. When dry, binders must form a coating and/or adhere to the material being agglomerated. They must impart the required strength and crush resistance to the particle for subsequent handling. In addition to the liquid, solids, and binders used to formulate a slurry, various other additives may be necessary. The following are typical components of spray drying slurries:

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