| Summary: | 碩士 === 中原大學 === 化學工程學系 === 87 === In solid-liquid separation processes, it is often necessary using preatments such as coagulation and/or flocculation for slurries or sludges before dewatered by filter press or expression. However, the subsequent mechanical dewatering is frequently impeded by hydraulic resistance when the sludges contain large quantities of fine or gelatinous particles. Therefore, although with conditioning prior to dewatering, the unacceptable dewatering rate is still a troublesome problem in industrial processes. Recently, a number of studies have been undertaken concerning the application of electric field to enhance the sludge dewatering, however very few paid attention to the effect of conditioning on the performance of electrosmotic-dewatering.
The electroosmosis of sludges depends on the zeta potential, particles size, ionic strength and porosity in sludge beds etc.. It is well known that those important parameters are profoundly affected by the coagulation and flocculation conditions. In order to study the effect of the physico-chemical properties of sludges on the electroosmotic phenomena, the electroosmosis through beds packed with slurry under various coagulation and flocculation conditions are investigated in the first part of this study. Then, the dewatering performance of slurry beds with the combination of a mechanical pressure and electric field was also studied
When the kaolin slurry was coagulated with KCl electrolyte or alum, the particle zeta potential will varied from negative to positive associated with the increasing of coagulant dose. The electroosmotic flux measured indicated that even near the isoelectric points there is still a considerable electroosmotic flux, and that the directions of electroosmotic flow all are toward cathode even when the particles have positively potential values. The latter in quantitativity is opposite to the well known theoretical relationship. When the slurry was flocculated with cationic polymer, there is no observable electroosmotic flux when the particles are neutrally charged and the directions of electroosmotic flow change associated with the polarity of the particles. Therefore, it can be concluded that the conditioning condition gives a significant effect on the subsequent electroosmotic dewatering process.
The results from dewatering experiments only with electric fields showed that, after the electric field is imposed on the slurry bed, the dewatering rate increases very fast to a peak and then a large decline occur for the case with constant current. When the dewatering was operated by a combination of mechanical pressure with electric field, the dewatering at the initial stage is completely contributed by the action of the former, but the subsequent dewatering almost controlled by the electroosmosis. After the comparison of dewatering capacity by simultaneous action of electric field and mechanical pressure with that by mechanical pressure action first and then, when almost no visible dewatering rate was achieved, electric field was added, it was found that the latter operation generally can give a higher dewatering capacity and energy efficiency.
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