scope:

Scope and definitions

The measurement methods described below apply to the mechanical separation of solid particles from suspensions, achieved by the effect of a driving pressure differential. The solid particles are collected on the surface of a porous filtering medium where they form a discretely disperse, porous accumulation with hydraulically interconnected pores.

The liquid passing through the filter medium is called filtrate. To allow the application of cake-filtration methods, the concentration of solid particles in the suspension, the particle size distribution and the pore size of the filter medium must be matched in such a manner as to obtain sufficient particle separation and, thus, cake formation on the filter medium.

As a rule, the suspension contains particles which are smaller in diameter than the aperture crosssection of the pores in the filter medium. Solid bridges across the pores of the filter medium must form for particle separation to be as complete as possible.

Particles which are not collected, passing through the filter medium undesirably, are often called solid loss.

In subsequent process steps, any residue of the suspension liquid left in the pores of the filter cake, including any dissolved substances, can be reduced by washing and/or deliquoring.

For the purpose of this guideline, washing means substituting the suspension liquid originally present in the pores of the filter cake by a liquid that can be mixed with the former at the molecular level.

Filter cake deliquoring is by displacing the liquid filling the pores by means of mechanical compression of the cake and by the resulting reduction of the pore volume, or by means of gas or vapour which penetrates the pores, driving out the liquid.

Complete deliquoring of filter cakes is not physically possible using mechanical methods, which means that an entirely dry solid can only be produced by a subsequent thermal process.

As for particle sizes, the cake filtration methods are applied in the range from 10^-6 m and 10^-3 m.

The application is limited to the side of smaller particles by the generation of high flow resistances in the filter cake. A suitable pretreatment of the suspension by means of agglomeration allows to shift the range of application of cake filtration towards smaller primary particles.

As a matter of principle, no physical limitation is encountered to the side of larger particles; in the case of particles with diameters in excess of several millimetres, liquid separation is usually achieved by drainage under the effect of gravity.

The measurement methods described below are based on the assumption that the rheological properties of the suspension liquid are Newtonian and that any flows are laminar; these restrictions, however, are no mandatory prerequisites.