Project ended 1997. Final Report submitted March 1998
This research is focussed on the filtration of sub-l urn particles where inter-particle forces start to become significant and affect both the rates of filtration and dewatering and the properties of the filter cakes formed. Existing models for filtration fail to recognise the existence of particle-particle interactions, nor do they account for chemical or physical interactions of the deposited particles with the filter septum. To take these factors into account, existing models rely entirely on empiricisms developed after experiments have been done to quantify the factors. Consequently, the models are unable to describe the effects of changes in basic properties of the feed on the performance of separation equipment.
This work develops a new approach to the analysis of filtration data, taking account of fundamental properties of the suspensions being separated. The background theory is developed in two parts. Firstly, the initial deposition of solids onto the filter septum is analysed by taking account of fluctuations in the particle velocity near the septum. Secondly, the dynamics of cake formation from suspensions of interacting particles is modelled, taking account of (a) changes of the random fluctuations of particle velocities caused by local direction and magnitude changes of the fluid velocity, and (b) the skeletal structure stress in the cake by using expressions for the two-particle interactive potential, thereby avoiding the need to introduce empiricism. The approaches taken to the two stages are wholly consistent with one another.
The simplest theories for constant pressure filtration of non-interacting particles conclude that the filtrate volume produced a PO*‘. This work shows that the filtrate volume produced during the initial stages of cake formation depends on the range of the zeta potential. For soft sphere interactions, the filtrate volume produced is a p2 and for hard sphere interactions filtrate volume a p. During cake formation, it is shown that the two-particle interactive potential lies between the two limiting analytical expressions usually quoted in the colloid science literature. Filtration rates predicted from the model agree well with experimental data for a wide range of feed suspension concentrations; the model also enables calculation of the distributions of skeletal stress, liquid pressures, and fluid and particle velocities through the cake thickness. Effects of timdamental properties such as ionic strength, pH, ion valency and particle size on filtration can be assessed using the model.