Agglomeration spontaneous

Since the fine dry powder agglomerates spontaneously, and for slip casting It Is necessary to make up a reasonably stable suspension, a dispersion process Is necessary. Parfltt has divided the overall process of dispersion Into three stages wetting of the powder, breaking of the dusters and stabilization. 

The surface of the coal particles oxidizes or weathers resulting in cracks, finer particles, and reduced agglomeration all of which may destroy coking properties. If spontaneous heating takes place, the calorific value of the coal is reduced. Hot spots must be carefully dug out and used as quickly as possible. Without spontaneous heating and with good compaction, calorific value losses below 1%/yr have been recorded. 

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. 

Condensation is generally a transient operation in which, as discussed by Ueda and Takashima(106), simultaneous heat and mass transfer are further complicated by the effects of spontaneous condensation in the bulk gaseous phase. After the creation of supersaturation in the vapour phase, nucleation normally occurs which may be homogeneous in special circumstances, but more usually heterogeneous. This process is followed by both crystal growth and agglomeration which lead to the formation of the final crystal product. As a rate process, the condensation of solids from vapours is less well understood than vaporisation(98). Strickland-Constable(107) has described a simple laboratory technique... 

The particle size distribution of an ultraLne material may increase spontaneously if measures are not taken to stabilize the particles. Particle size growth can occur via agglomeration or crystal growth. This particle size increase will result in a decrease of the available surface area, and hence a decrease of the dissolution rate, potentially lowering drug bioavailability. 

As alluded to earlier, even in the absence of a field, large particles spontaneously agglomerate because of interactions of the permanent magnetic dipoles. This interaction energy between particles i and j given by... 

Figure 8. Snapshots from spontaneous agglomeration simulated by using an MD-DPD algorithm.

Papadimitrakopoulos and coworkers reported on transparent Si/ SiOx nanocomposite films, spontaneously adsorbed on glass or quartz substrates from their colloidal suspensions via a sonication-assisted oxidation process [56]. Individual nanosilicon partides (ca. 20 nm) appear to cover a significant part of the substrate along with agglomerates of the order of 50-80 nm in thickness. Kinetic studies indicate a rapid initial adsorption that slows down significantly after 3 h. 

Some colloidal systems such as polymer solutions and surfactant solutions containing micelles are thermodynamically stable and form spontaneously. These types of colloids are called lyophilic colloids. However, most systems encountered contain lyophobic colloids (particles insoluble in the solvent). In the preparation of such lyophobic colloidal dispersions, the presence of a stabilizing substance is essential. Because van der Waals forces usually tend to lead to agglomeration (flocculation) of the particles, stability of such colloids requires that the particles repel one another, either by carrying a net electrostatic charge or by being coated with an adsorbed layer of large molecules compatible with the solvent. 

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