Whenever a particulate solid is mixed with a liquid, which wets the solid surfaces, the three-phase system of solid, liquid and gas will tend to reduce its free energy by the formation of liquid bridges between the particles. The cohesive forces established by the liquid bridges may cause agglomeration and consolidation of the agglomerates insofar as they can resist the disruptive forces caused by the mechanical agitation of the moistened mass.
Figure 1 outlines the liquid states in agglomerates according to Newitt and Conway-Jones (1958). An assembly of uniformly sized, spherical par-
ticks is wetted with increasing amounts of liquid. The relative amount of liquid is expressed by the liquid saturation, i.e. the volume of liquid relative to the volume of pores and voids between solid particles. At low liquid saturations (the pendular liquid state), discrete, lens-shaped liquid bridges are created between the particles. By increasing liquid saturation (the funicular liquid state), the liquid bridges coalesce and gradually fill the voids so that, at high liquid saturations (the capillary state), the particle agglomerate is held together by the capillary suction of the liquid. The limiting liquid saturation between the pendular and funicular liquid states is 25-35%. In the capillary liquid state the saturation exceeds about 80% (Capes, 1980).
The cohesive strength of the agglomerates described in Fig. 1 is due to the pressure deficiency caused by the surface tension of the liquid and the contact angle of the liquid to the solid surface. The strength of mobile liquid bondings was investigated and described by mathematical models for idealized systems by Rumpf and his co-workers (see below). A comprehensive presentation of models for liquid bondings can be found in most reviews on enlargement methods, for example Kapur (1978) and Pietsch (1984).
The mechanical strength of agglomerates has an important role in the formation and growth of agglomerates because the ability of an agglomerate to survive and grow must depend on its strength relative to disruptive forces resulting from agitation. It is difficult, however, to correlate agglomerate formation and growth with the bonding strength. The models for mobile liquid bondings mentioned below describe static systems and do not take into account the dynamic conditions present in agglomeration processes (Ennis et al., 1991). When considering growth kinetics, the strength of agglomerates has to be characterized by more than one parameter. The tensile and shear strengths as well as the strain behaviour determine the agglomerate deformability which is important to the growth kinetics (Kapur, 1978).
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