Filter aid

- Filter aid are powdery substances, such as kieselguhr or perlite, which are deposited on a filter (tissue or layer) & as a result of their shape & arrangement make filtration possible
- Filter aids can not be used without a supporting filter
- Filter aids used in the present are Kieselgurh and Perlite

Kieselguhr
- Is the term used for fossils of diatoms (single cell algae containing silicon dioxide, SiO2) of which there are more than 15,000 types in the sea
- They must have existed for millions of year in isolated seas or regions of the sea in such quantities that their fossils in the course of time covered the bottom of the sea in a very thick layer
- Depend on their processing there are three groups of guhrs;
- Dried kieselguhrs
- The material is dried at about 300 oC & then milled
- So called fine guhrs are produced in this way
- Ignited & calcine guhrs
- The raw material is heated in large rotating ovens at 600 to 800 oC & then screened
- This produces depending on the combustion temperature & sieving, fine & medium guhrs
- Guhrs ignited with a flux additive
- These are produced by recombustion at 1000 oC of the already ignited guhrs with the addition of CaCO3 or other flux additive & the diatom skeletons are thereby cemented together into larger structures which are require for precoating
- The white guhrs are formed in this way
- The finer the guhr, the better it clarifies but at the same time the lower the filtration speed
- An important feature for the economic use of kieselguhr is the wet density
- By this meant the volume which the guhr occupies under pressure
- Kieselguhrs with a wet density less than 300 g/l are most suitable for filtration
- With higher wet densities must expect to use more guhr & to obtain a higher pressure increase
- Kieselguhr usage can vary between 80 to 200 g/hl
- Kieselguhr is not only very expensive, it also result in costs for disposal of the kieselguhr slurry

Perlite
- Is a material of volcanic origin & consists principally of aluminium silicate
- Raw perlite is heated to 800 oC
- The water contained in it expands & leads to swelling & bursting
- The glassy structures produced are milled
- This give rise to a very light, loose powder which weighs 20 to 40% less than kieselguhr
- At low pH values perlite release chalk & iron