Agglomeration unwanted

Generally, particle attrition is unwanted. However, at times controlled attrition is desirable. For example, in coking units where agglomeration due to wet particles is frequent, jets are used to attrit particles to control particle size [Dunlop, Griffin, and Moser, J. Chem. Eng. Prog. 54 39-43 (1958)]. 

At the contact points of particles roughness peaks may melt due to heat caused by friction and/or pressure. In such a case liquid bridges develop which solidify quickly if no further energy is supplied. This mechanism, called partial melting, is often responsible for unwanted agglomeration and caking of substances with low melting points. 

This means, however, that an unwanted densification of agglomerates occurs in the deeper bed. Therefore, the drum loading is normally kept constant which requires a reduction of the residence time according to... 

The type of movement required for mixing particulate matter also produces ideal conditions for agglomeration by coalescence. Therefore, unwanted agglomeration is often observed in powder mixers, especially if the particle size of the solids is small and/or a small amount of moisture is present. Considerable problems can arise if components of the bulk mass have different particle sizes because, in that case, the smaller fractions may selectively agglomerate, thus making it impossible to obtain an ideal mixture. Such selective agglomeration is of particular concern in the pharmaceutical industry where an extremely small amount of a finely divided active substance must often be mixed uniformly and reliably with a relatively large amount of inert filler material. 

Table 24, Wanted and unwanted agglomeration in various product areas...

Attrition [n.j The unwanted break-down of agglomerates. (See also abrasion, erosion.)... 

Bag set Typical in the fertilizer industry unwanted agglomeration of particulate solids in a closed bag during storage. Mostly caused by recrystallization of dissolved materials. 

Separation During separation unwanted agglomeration may occur and must be avoided if a particle collective is to be divided into well defined classes. The separation curve is a measure for separation quality. In a diagram the degree of separation (the percentage amounts of particles above and below the desired separation size) is plotted versus the particle size. Fig. 5.54 is a qualitative presentation of several separation curves. 

Unwanted agglomeration is often observed if the particulate materials are soluble or if chemical reactions can occur, particularly in the presence of moisture. These phenomena are very common in the fertilizer industry and are called caking if they occur in bulk masses or bag-set if material solidifies in bags. 

The answer to what can be done to avoid or, at least, lessen caking is complex but generally the same as in all other cases where unwanted adhesion or agglomeration occurs Detect the binding mechanism that is responsible and the parameters that influence the process and then try to reduce their effect. In the following some examples will be discussed briefly. 

As mentioned several times before (see Sections 5.3, 5.3.2, 5.4, and Chapter 6) and indicated in Chapters 2 and 3, the natural adhesion of small particles is the most basic agglomeration phenomenon. Therefore, it is that which is most often responsible for unwanted agglomeration (see Section 5.5) and the conglomeration of fine particles that is frequently observed in nature. 

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