Study On The Microstructure Of The Filter Cake And The Expression Filtration Theory

Filtration and separation are indispensable technology in all industrial fields and the newly-developed science and technological fields of today. Their technological level, equipment quality, scientific operation and low energy consumption are prerequisite for guaranteeing modern industrial production process, realizing the advance and reliability of technological process, improving the quality of products , lowing energy consumption, protecting environment and increasing economic and social returns.With the development of scientific technology and industrial production, more and more attention will be paid to such issues as energy saving, resource exploration, eco-agriculture and environment protection, etc. Fast development of high-tech fields such as biochemical engineering, late-model materials, fine chemical engineering,etc, demands of filtrating and separating equipment not only large handling capacity, but also press function so as to dewater more, and drier,which can save large a amount of energy for the subsequent drying process,or even leave out later drying process, hence significant to full use of resources and realization of circulating economy. Therefore press filtration technology has attracted more and more attention.But, conventional pressure filtration and expression filtration are affected by both macrohydrodynamics and micro- physical chemistry for the reason that solid-liquid two-phase filtration is a flow process in which complex phase fluid flows through porous medium, so we, up to now, have failed to accurately describe filtration and expression process with mathematic language, calculate the property of cakes according to the property of material or predict and describe the quantity, size and distribution of the material contained in filtrate, though great efforts have been done. At present, filtration operation is still a kind of technology largely dependent on practice and experience.This dissertation is aimed at the study of micro structure of the cake and the expression filtration theory under the condition of the laboratory.Through the researches, the following work has already been completed: The tri-dimensional expression filtration theory model and equation in a rectangular coordinate system has been deduced. The expression of u ,pore pressure, has been obtained. More practical calculating methods and formulas of cake expression coefficients have been deduced according to mechanism of the cake expression. Based on the solution to the equation of the tri-dimensional expression filtration, a two-dimensional expression filtration equation and a one -dimensional expression filtration equation, after simplified, have been deduced. In addition, the differences between the filtration ratio in theory and that in experiments, and the differences between the expression ratio in theory and that in the experiments have been compared. The errors of theory values and those of experiments are 20%. After analyzing the influences of the physical property parameters of the suspension,cake structure parameters and operating condition parameters on expression process, the method by which the criterion equation can be obtained to give a complete description of the expression filtration equation by establishing expression filtration criterion model under the expression of one-dimensional constant voltage and by using experiment data to carry out multiple linear regression has been put forward. Pressure-filtration-experiment apparatus and expression-filtration-experiment apparatus are set up to determine the basic physical property parameters of the material by selecting five representative compressible materials. With the pressure experiment apparatus in the lab, pressure filtration process is simulated and pressure filtration experiments of the cake are carried out under low and middle pressures, to obtain such important parameters as filtration specific resistance, compression index and filtration ratio, etc. With the pressure experiment apparatus in the lab, expression filtration process is simulated and expression filtration experiments of the cake have been carried out under low and middle pressures. Operating conditions favorable for filtration expression have been tried. According to the experiments with different starting expression points under the same expression pressures and the experiments when the filtration pressure and the expression pressure are the same or different, it has been proved that different starting expression points have greater influences on expression filtration, as a result, an experiment method has been raised to find out the optimal operating conditions for expression filtration. The experiments show that for 15% (quality fraction) mixed suspension of diatomite and kaolin, the best time for expression is when about 72% of the filtrate quantity in theory has been obtained under pressure filtration, while for 15% (quality fraction) mixed suspension of diatomite and clay , the best time for expression is when about 72% of the filtrate quantity in theory has been obtained under the pressure of 0.25MPa. The methods of stratifying and slicing cakes and Environmental Scanning Electron Microscope (ESEM) have been adopted systematically to measure porosity within the cake layers for the study of the microstructure of the cake. Cake porosity has been measured stratified under both different filtration pressures and different expression pressures, and both at the end of filtration and during the whole course of expression. The cake porosity has also been measured when the cake is expressed at different starting expression points. Hence a thorough understanding of the filtration process, expression process and the inner structure of the cake in different expression cycles. The calculating data got from the traditional pressure tests have been used for comparison and contrast. Through industrial tests and measurement, to find out the optimal experiment method for expression filtration at field (proposed by this paper)has been proved to be manipulable. The industrial tests show that the ahead-of-time expression, compared with the conventional filtration method, can help shorten technological handling time, increase production efficiency and lower cake moisture and cake weight, which is meaningful to increase of production quality, decrease of cake transportation burden, reduction of the cost of the subsequent drying procedure, energy saving and more economic returns.The main originality innovation of this dissertation includes the following aspects: the research makes a study of expression filtration theory, starting with the microstructure of thecake, puts forward that the optimal operating condition for expression filtration should be found out and has gained the method to obtain the starting expression points in the process of expression filtration; it suggests that the methods of stratifying and slicing cakes and Environmental Scanning Electron Microscope (ESEM) be adopted to measure porosity within the cake layers for the study of the microstructure of the cake; and it also puts forward the three dimensional mathematical model for expression filtration, etc. It is believed that the initial research results will be enlightenment to the subsequent study of the expression theory and it will find application in industry.