Pore bridging, filter cloths

Problems are usually encountered when pressure systems designed to operate at reasonable levels of particulate concentration are fed with dilute suspensions at the start of filtration. Low concentrations of sohds prevent the bridging effect which ensues when concentrated swarms of sohds are directed towards the pores in the filter medium. This effect is discussed quantitatively elsewhere in this Chapter. Failure to bridge the cloth pores will lead to deposition of particles inside the fabric. 

Rushton, Albert and Rushton, Alan, Size and concentration effects in filter cloth pore bridging , Filtration and Separation, May/June, 274 (1972)... 

An additional benefit of prethickening is reduction in cake resistance. If the feed concentration is low, there is a general tendency of particles to pack together more tightly, thus leading to higher specific resistances. If, however, many particles approach the filter medium at the same time, they may bridge over the pores this reduces penetration into the cloth or the cake underneath and more permeable cakes are thus formed. 

The blocking of the pores of the filter medium by particles is a complex phenomenon, partly because of the complicated nature of the surface structure of the usual types of filter media, and partly because the lines of movement of the particles are not well defined. At the start of filtration, the manner in which the cake forms will lie between two extremes — the penetration of the pores by particles and the shielding of the entry to the pores by the particles forming bridges. Heertjes(11) considered a number of idealised cases in which suspensions of specified pore size distributions were filtered on a cloth with a regular pore distribution. First, it was assumed that an individual particle was capable on its own of blocking a single pore, then, as filtration proceeded, successive pores would be blocked, so that the apparent value of the specific resistance of the filter cake would depend on the amount of solids deposited. 

When the process conditions are such that bridging of the surface pores of the medium is iiqiossible, particles may be deposited within the interstices of the cloth or, particularly for monofllament, bleed through the pores. In multifilament or nonwoven media, internal deposition in the cloth may prove to be permanent, de ite attempts to clean the filter by laundering, back-flushing, etc. 

---