Crossflow microfiltration of concentrated non-Newtonian slurries
Crossflow filtration of non-Newtonian slurries of magnesia and talc suspensions on micron-size membranes has been investigated. Three major membranes PTFE capillary, metal and ceramic tubes were employed and challenged by concentrated slurries more than 25 % by weight. Crossflow velocity, system pressure and slurry concentration are considered in relation to the possible deposit thickness and permeate flux behaviour. A thorough investigation of pressure drop in tubes of similar dimensions to the membranes has also been carried out in order to identify and develop an appropriate model. The experimental results, especially the deposit depth and flux rate are compared with the model prediction which is based on a permeate flux rate with shear stress correlation. Furthermore, diafiltration experiments were carried out and washing efficiency is compared with a stirred tank model of the process. Measurement of pressure drop in tubes confirmed the mathematical model used to determine the diameter open to flow during filtration, i.e. the deposit thickness. Overall crossflow experimental results show reasonable agreement with the predictions of the proposed models. The pseudo steady state flux is shown to correlate with the wall shear stress and the correlation is much improved when the deposit depth on the membrane was taken into account. In most cases the deposit depth and permeate flux rate are correctly predicted once the basic parameters correlation data and slurry rheology were determined. Since the rheology properties can be determined independently, the mathematical model developed is very practical and could be extended to some other systems, for example tube diameter. Hopefully, the translation of these parameters could be used to design and then develop the appropriate equipment when crossflow filtration of concentrated slurries is concerned, and in the design of systems to prevent tube blockage.