On the determination and application of local filtration properties

Detta är en avhandling från Luleå : Luleå tekniska universitet

Sammanfattning: Filtration is a mechanical unit-operation used for the separation of liquid and solid. In this work, the three sub-operations cake build-up, expression and washing have been studied. One difficulty often encountered in the design of filtration equipment is that the materials being filtered form more or less compressible filter cakes. The use of local filtration properties is necessary in order to predict the behaviour of compressible filter cakes. Consequently, it is of great importance that more knowledge of local filtration properties is gained in order to design filtration equipment more accurately and to increase the efficiency of the filtration unit-operation. In the experimental part of this thesis, a number of different filtration test equipment and materials have been used. Piston presses were used to determine local filtration properties of calcium silicate, green liquor sludge, kaolin, lignin sludges and titanium dioxide. A piston press, with pressure transducers and a solidosity measurement system using gamma-rays, was constructed to simultaneously measure the local solidosity and local static pressure. From these simultaneous measurements, it was possible to obtain a good fit for constitutive relationships such as solidosity-solid compressive pressure, and specific filtration resistance-solid compressive pressure. The materials were found to form more or less compressible filter cakes. Values of the average specific filtration resistance, obtained from the classical filtration equation, were found to differ from the corresponding values calculated from local filtration properties. The use of constitutive relationships, based on local experimental measurements, improves accuracy in the design of filtration equipment. Various models were used in the evaluation of the expression phase. The classical expression models, i.e. the Terzaghi and Terzaghi-Voigt models, assume that the parameters in the models are constants. They also assume that the pressure profile at the transition between the cake build-up and expression phase follows a sine-relationship; this pressure profile was determined using kaolin and lignin sludges from the experiments. It was found that the sinusoidal relationship was a reasonable approximation for kaolin whereas large deviations occurred for lignin sludges. The classical Terzaghi model did not fit the experimental data for green liquor sludge, kaolin and lignin sludge; a better fit was obtained if the experimentally- determined pressure profile was used in the Terzaghi model. The Terzaghi- Voigt model was found to fit the experimental data, however, it was noted that the often used simplification of the Terzaghi-Voigt model does not always hold, especially for highly compressible filter cakes. Additionally, an extension of the Terzaghi model, which takes into account the fact that compressible filter cakes have non-constant filtration properties, was developed. The new models for the expression phase make it possible to use local filtration data and increase the knowledge of the expression phase. The use of these models makes the expression models more reliable when applied to the design and control of equipment. The influence of torsion shear on the dewatering of magnetite was studied. The pH was adjusted to investigate the effect of the double layer forces. The experimental results showed that the magnetic forces between the particles had the same magnitude as the double layer forces; this result was confirmed by a simple model. The magnetic forces do thus, determine the filtration process to a large extent. It was observed that sedimentation was present in the experiments. A model for the cake build-up phase which takes sedimentation into account was developed and described the experiments successfully. From the expression phase, it was found that the Terzaghi model could explain the experiments adequately. In sulphate pulp mills, green liquor sludge is to be found in the recovery cycle. Non-process elements may be removed from these pulp mills via green liquor sludge. The separation of green liquor sludge is known to be difficult. A study of four different green liquor sludges was carried out in which the filtration properties of green liquor sludge were determined. It was found that the filtration properties varied greatly between the sludges. Furthermore, it was found, and demonstrated, that the addition of lime mud to an unfiltered green liquor suspension made the separation and washing of green liquor sludge easier to perform.

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