Liquid suspension, spherical agglomeration

In this section three miscellaneous topics in the area of agglomeration shall be discussed, namely, dry pelletization, spherical agglomeration in liquid suspension, and spontaneous or inadvertent agglomeration of fine particles. 

A simple example of immiscible liquid wetting is the addition of oil to a fine coal suspension to agglomerate and remove the carbon constituents while the inorganic impurities (ash constituents) remain in suspension and are rejected. A number of such coal cleaning processes, such as the Trent Process [3], the Convertol Process [4] and the Spherical Agglomeration Process [5], have been developed and used in this century. As discussed below, developments of the latter process have shown that many other applications are possible for immiscible liquid wetting. 

At certain conditions agglomeration also takes place in turbulently moving liquid suspensions. This is the case if, for example, a small amount of a second liquid is added as a binder. This component must be able to adhere to the solid particles in the suspension but must be immiscible with the suspending liquid. After its addition, spherical agglomerates form during a suitable movement of the liquid. Figure 86(a) to (h) shows schematically apparatuses that were successfully used for agglomeration in suspension. 

Figure 86. Methods used for spherical agglomeration in suspensions , (a) Container with stirrer, (b) container with liquid circulation, (c) inclined disc, (d) horizontal drum, (e) colloid mill , (f) shaking container, (g) vertical mixer, (h) continuous mixer...

Suspensions of these spherical particles are used for spray drying to produce large agglomerates which are used as packings for various separation techniques such as High Performance Liquid Chromatography (HPLC) or Supercritical Fluid Chromatography (SFC). They also serve as supports for catalysts. 

In immiscible liquid agglomeration, particles with a small amount of adsorbed binding liquid on their surfaces collide and coalesce to form larger entities. In the sol-gel process (see also Section 5.3.2, Fig. 5.52), another agglomeration technique that occurs in liquid phase, fine particles are initially suspended in a binder liquid. The suspension is then formed into spherical droplets and the excess binder is removed to solidify the droplets into a particulate product. 

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