Investigation of hard-to-filter materials -Relating local filtration properties to particle interaction
Filtration as a means of separating solids from liquids is an important unit operation employed
in a range of different industrial sectors, e.g. forest products, mineral, chemical process and
pharmaceutical industries to name but a few. Accurate and applicable models for the filtration
unit operation are imperative if industrial filtration equipment is to be designed correctly.
Easy-to-filter, incompressible or near-incompressible materials can currently be modelled
satisfactorily, thereby facilitating scale-up and design. There is, however, a lack of good
models for compressible cakes that are formed by hard-to-filter materials. Efforts attempting
to model filtration have often been based on average filtration properties; although providing
certain information regarding filtration, this approach is inferior to that of using models based
on local data. Models for filtration often contain lumped parameters, i.e. the classical filtration
equation describes the filtration properties of particles using the average specific filtration
resistance parameter. An alternative would be to base their filtration properties on the
interactions experienced during filtration. Important interactions that should be considered
between particles include surface forces, material bridges between particles and interlocking.
This work investigates hard-to-filter materials. Two materials are used. A model material,
titanium dioxide, is used to investigate the effects of particle interactions on filtration
behaviour. Although all of the interactions mentioned above may be of interest, depending on
the filtration situation, this thesis focuses on electrostatic interactions. The separation of green
liquor, a process that is of great interest in the pulp and paper industry, is also investigated.
Green liquor dregs form hard-to-filter, compressible cakes.
As far as titanium dioxide is concerned, local filtration properties are investigated for different
particle interactions. The inter-particle interactions are changed by controlling the ζ-potential.
Both local solidosity and pressure are measured, and a local specific filtration resistance
calculated from this data. It could be concluded that the compressibility of the filtration cake
is changed substantially by altering the ζ-potential. Several published constitutive
relationships for filtration are applied; they yield very similar parameters that are in good
accordance to the characterization of the material.
Local filtration properties