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Renin Angiotensin Mechanism.

Filtration: Filter Aids.

 

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:

  1. Sometimes remove the coloured substances by absorbing them.

  2. Sometimes active principles such as alkaloids are absorbed by filter aid.

  3. 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. 
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