Question

A slurry is filtered in a plate and frame press containing 12 frames, each 0.3 m square and 25 mm thick. During the first 180 s the pressure difference for filtration is slowly raised to the final value of 400 kN/m2 and, during this period, the rate of filtration is maintained constant. After the initial period, filtration is carried out at constant pressure and the cakes are completely formed in a further 900 s. The cakes are then washed with a pressure difference of 275 kN/m2 for 600 s using thorough washing What is the volume of filtrate collected per cycle and how much wash water is used? A sample of the slurry had previously been tested with a leaf filter of 0.05 m2

2

filtering surface using a vacuum giving a pressure difference of 71.3 kN/m . The volume of filtrate

collected in the first 300 s, was 250 cm3 and, after a further 300 s, an additional 150 cm3 was collected. It may be assumed that the cake is incompressible and that the cloth resistance is the same in the leaf as in the filter press.

Answer 1

For any filtration pressure, the rate of flow is greatest at the beginning of the process since the resistance is minimum.High initial rates of filtration may result in plugging of the pores of the filter cloth and cause a very high resistance to flow.The orientation of the particle in the initial layers may appreciably influence the structure of the whole filter cake.

filter cakes may be divided into two classes- incompressible cakes and compressible cakes. ln the case of an incompressible cake, the resistance to flow of a given volume of cake is not appreciably affected either by the pressure difference across the cake or by the rate of deposition of material. On the other hand, with a compressible cake, increase of the pressure difference or of the rate of flow,causes the formation of a denser cake with a higher resistance. For incompressible cakes 'e' in below equatiom (1) may be taken as constant and the quantity e^3/(5( 1 - e)^2 S^2] is then a property of the particles forming the cake and should be constant for a given material.

(1)

thus become ,(2) where,

It may be noted that, when there is a hydrostatic pressure component such as with a horizontal filter surface, this should be included in the calculation of (-deltaP)
Equation (2) is the basic filtration equation and 'r' is termed the specific resistance which is seen to depend on e and S. for incompressible cakes,'r' is taken as constant, although it depends on rate of deposition, the nature of the particles, and on the forces between the particles .' 'r has the dimensions of L^-2 and the units m^-2 in the SI system.

If the filter cloth and the initial layers of cake are together equivalent to a thickness L of cake as deposited at a later stage in the process and if - deltaP is the pressure drop across the cake and cloth combined, then

(3)