A preliminary study of gravity filtration

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A methodology based on falling head gravity filtration was developed to allow cake filtration to be rapidly determined in the laboratory. Tests were run on talc and kaolin, and the effects of various filteraids on the filtration resistances of the two materials were studied. Plots of average filtration resistances versus mass fraction of filteraids in the solids mixture were found to fit a general equation for hyperbolas. I . Based on Shirato's Additive Law for ideal mixtures, an equation was developed to calculate the average filtration resistance of a mixture of two solids. The discrepancies between the experimental values of average filtration resistances and the corresponding ones calculated by means of the Additive Law increased with increasing lack of match between the particle size distributions of the two components. Calcium carbonate was unexpectedly found to act as a filteraid when mixed with talc, but not when mixed with kaolin. Calcium carbonate had low positive zeta potentials at pH values close to 7.0 and low negative zeta potentials at pH values above 8.1. Both kaolin and talc had nearly identical negative zeta potentials over a wide range of pH values (4.0 to 10.0). A study of scanning electron microscope pictures of talc and calcium carbonate mixtures revealed highly flocculated, irregular particles of calcium carbonate lodged between unevenly distributed, plate-like particles of talc. Although no information could be drawn about the relative sizes of the pores, it was apparent from the photomicrographs that some interaction had occurred which had changed the shape of the pores of the talc particles.