Diatomite or Kieselguhr has been used as a filter aid for over a century. The ore is a soft, friable siliceous mineral. It is composed of the skeletons of microscopic plants (diatoms) deposited on the bottoms of lakes and oceans probably during the Miocene age. Freshwater DE refers to diatomite that was deposited in a freshwater environment, whereas saltwater DE was formed in an ocean. Most commerical DE is freshwater DE.
Under the microscope the particles of diatomite show up in a variety of forms; symmetrical figures resembling disks, rods, cylinders and snowflakes. The shapes of the diatoms (the morphology), their rigidity and their microstructure is what makes DE such an excellent raw material for the production of filter aids.
There are three types of commercially available DE: Natural DE, which is the material in it’s raw form and is simply dried and sized; calcined DE, which is usually pink or orange in colour, and is processed at temperatures of over 1000F to agglomerate and purify the material; and flux-calcined DE, which is processed at temperatures of over 1500F with a fluxing agent which further agglomerates the particles, and turns the material bright-white.
Natural DE is not recommended for filter aids due to their small size and limited permeability. Calcined DE is normally used for fine filtration where clarity of the filtrate is most critical. Flux-calcined DE is generally used where high flow rates and high solids removal is critical.