Filter Aids

Filter aids are used to assist filtration of colloids, extra fine particles, gel like, and highly compactible materials, which are difficult to be filtered due to low filtrate rate, unacceptable filtrate clarity, high cake moisture content, or serious filter medium clogging. They can be used as “precoat” on the surface of filter media or in conjunction with feeding suspension as “body feed” (or “admix”), or a combination of “precoat” and “body feed”.

A filter aid material is characterized by the following properties:

1. Porous particulate structure with irregular shapes(Figure 1);
2. Rigid particle and in-compactible filter cake;
3. Excellent dispersing and suspending properties;
4. Low bulk density;
5. Chemical stable and inert within operating conditions.

Figure 1. Rice Hull Ash Porous Structure (Courtesy of Agrilectric Research Company)

Principle types of filter aids include Diatomaceous Earth(DE), Perlite, cellulose, and Rice Hull Ash(RHA). Diatomaceous Earth filter aids are mined from deposits of ancient aqueous plants from salt or fresh water resources. Perlite filter aids are obtained from expanding perlitic rock. Rice Hull Ash filter aids are produced from combustion of a renewable biomaterial – rice hulls. Properties of the four types of filter aids are shown in the following table(Svarovsky, 1990).

* Properties of RHA vary depending on different production process. RHA listed in this table is from suspension burning process.

All four types of filter aids come with different grades of products. Normally, perlite and cellulose are used for coarse filtration applications. DE and RHA can be used for both coarse and precise filtration. RHA is similar to some grades of DE products in their chemical composition, particle size, shape and particular porous structure. Properties of RHA and DE filter aids vary from different mining sources or combustion processes, and different particle processing processes. Fundamental research and test are essential for new product development and existing product selection for new applications.

References:

Celite Filter Aids, Celite Corporation Brochure

Filter Aids News, Agrilectric Research Newsletter, Vol. 1, No. 1, 2005

Ladislav Svarovsky, Solid-Liquid Separation, Third Edition, Butterworths, & Co. Publisher Ltd., 1990

Li, Wenping, Carl Kiser, Quintin Richard, “Quality Test of Rice Hull Ash Filter Aids”, Paper presented at the American Filtration & Separations Society 2005 International Topical Conferences & Exposition, September 19-22, Ann Arbor, Michigan

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