Studies in filtration characteristics of particulate systems



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Average porosities and specific cake resistances of binary mixtures of various particulate systems were studied experimentally. These filtration properties were obtained from compression-permeability cell measurements and sedimentation runs. Particulate systems investigated included residual solids derived from liquified coal. It was shown that a minimum occurred in the porosity function of most binary particulate systems at some composition of the solids. The notion that binary mixtures of particles took on values of porosities which are intermediate between those of the pure components is, therefore, not always correct. This phenomenon of minimum porosity has been attributed to "cavern effects": a process whereby particles of one component gradually "fill" the caverns of the other, without increasing the total volume of the system. The porosity decreases during this "filling in" process. A minimum is thought to be reached when the caverns are "full." Binary particulate mixtures whose porosities could be predicted from those of the pure components in a strictly additive manner were defined as "ideal." Additive implies that the particles of each component enter into the system carrying their original voids. Values of porosities of most real systems were shown to deviate from the ideal values because of "cavern effects." A simple empirical equation was proposed to correct the ideal values for these deviations. The variation of porosity with composition in the solids was also studied by drawing an analogy with the thermodynamics of binary solutions. Partial volumes were defined. Analyses using these volumes yielded cases where the actual porosities were higher than the ideal values. These higher porosities were attributed to the adhesion of particles of one component on the other, forcing them apart and, consequently, increasing their porosity. Partial volumes were, thus, shown to be potential tools for evaluating the performance of filteraids. In cake filtration, filteraids are added to particulate systems to reduce their resistance to liquid flow. However, it was shown that this reduction did not always occur when less that 10% by volume of filteraids were added to the system. This was attributed to the decrease in the porosity function at these compositions.