Filter Aids:
Filter aids are the substances used to prevent the filter medium from becoming blocked and to form an open, porous cake, hence reducing the resistance to flow of the filtrate.
Filter aid forms a surface deposit which screens out the solids and also prevents the plugging of the supporting filter medium.
Characteristics of filter aids:
Should be chemically inert and free from impurities.
Should have low specific gravity, hence can remain suspended in liquids.
Should be porous rather than dense, so that the porous cake can be formed.
Should be recoverable.
Disadvantages:
Sometimes remove the coloured substances by absorbing them.
Sometimes active principles such as alkaloids are absorbed by filter aid.
Rarely, filter aids are a source of contamination such as soluble iron salts, which can provoke degradation of sensitive ingredients.
Handling of filter aids:
Filter aids may be used in either or both two ways:
Pre- coating technique: by forming a pre-coat over the filter medium by filtering a suspension of the filter aid.
Body- mix technique: A small proportion of the filter aid (0.1- 0.5 %) is added to the slurry to be filtered. This slurry is recirculated through the filter until a clear filtrate is obtained; filtration then proceeds to completion.
Different flow rates can be achieved depending on grade of aid
Low flow rate: fine grade filter aids- mainly used for clarity
Fast flow rate: coarse grade filter aids- acceptable filtrate.
Examples of filter aids
Diatomite (Kieselgur) , obtained from natural siliceous deposits.
Perlite is an aluminum silicate.
Cellulose, Asbestos, charcoal, talc, bentonite , fullers earth etc.