As pre-treatment technique for conditioning slurries prior to dewatering, effectiveness of freezing and thawing methods was examined by using ferrichydroxide slurry and sludge from waterworks. Slurries contained in vertical columns were frozen in a freezer operated at -30°C for various refrigeration times and also in a refrigerating bath controlled in ranging from -5 to -76°C. The relations between the temperature of the slurry and the time were measured. The size distributions of particulate flocs before and after the freezing were measured by an image analyser. It was demonstrated that the floc particles considerably increased in size and also decreased in the ratio of the perimeter to the projected area after freezing and thawing operations. Dewaterability of the conditioned slurries was discussed in terms of the sedimentation tests, the constant pressure filtration tests and the capillary suction time (CST). The ratio of the final equilibrium height to the initial height of sediment, H_∞/H₀, the ratio of the wet to the dry cake mass, m, the average specific filtration resistance, α_av, and CST markedly decreased with freezing and thawing operations. It was shown that these treatments made remarkable improvements in dewaterability.
In order to obtain very strong frocs from the slurry, the slurry should be refrigerated enough until the temperature of the whole parts of the slurry became equal to the refrigeration temperature after the complete freezing of the slurry. Optimum refrigeration time of the slurry in the freezing and thawing operations could be well determined by measuring the temperature variation of the slurry during the freezing. Dewaterability of the conditioned slurry of ferric hydroxide was improved progressively with the lowering ofthe refrigeration temperature between -5°C and -20°C, but a further improvement was not observed between -20°C and -76°C.