The influence of particle properties on filtration

If the particle proper-ties could be specified for a filtration, the target properties would be for the particles to have as large a size as possible, be as near to spherical as possible, and have a monosize distribution. Their largeness would minimise the interactions at the particle-liquid interface, minimise the effects of ageing, effectively eliminate many cloth related problems such as bleeding and blinding and the need for very frequent cleaning, allow high fluid flow rates through the cake, and reduce the liquid volume retained in the cake when it is discharged. In practice, the feed is invariably composed of a size distribution and the smallest particles in the distribution control many aspects of the filter cycle. The smallest particles are the ones that bleed through the filter cloth in the initial stages of filtration, they accumulate in the cake layers nearest to the filter cloth, they contribute most to the specific surface of the particles and to the specific resistance of the filter cake, and they interact most strongly with ions or other substances (for example, polymers) in the Solution to create the compressibility effect. The particle size that is most important for characterising a filtration is rarely the mean size (50% size) in a distribution; the finer particles in the distribution usually "control" the filtration, and when comparing the likely effects of particle size on filtration it is preferable to compare the 5 or 10% sizes. Even small increases in the number of these finer particles can significantly reduce filtration rates. So there are a variety of operational related reasons to eliminate the fines in the feed, due to their slowing effect on the rates of filtration, washing and deliquoring and being a cause of higher moisture contents in filter cakes. (c) 2007 Elsevier B.V. All rights reserved.