Perlite is a generic name for naturally occurring siliceous volcanic rock readily available throughout the world. A unique property of perlite is that it expands up to 20 times its original volume when it is heated to its softening range.

This expansion process is caused by the presence of water in the crude rock. When perlite ore is rapidly heated to above 850 C (1600 F), this water vaporizes and causes the softened rock to expand. Tiny glass-like bubbles are produced which account for the light weight and exceptional physical properties of expanded perlite.

It is these light weight glass-like bubbles that are milled and classified under stringent quality control conditions to produce perlite filter aids. This material exhibits a unique, jagged, interlocking structure with myriads of microscopic channels affording optimum flow rates and clarities for a wide variety of applications. Perlite filter aids do not impart taste, color, or odor to liquids being filtered and they are virtually insoluble in mineral and organic acids at all temperatures. Solubility in strong alkalies varies depending on temperature and contact time.

AIR FILTRATION WITH PERLITE

Many bag house operations are presented with particularly difficult problems because of the material being filtered. For instance, in coals mills, asphalt plants, smelting plants, some incineration operations, paint shops, and certain chemical plants, the "dirty" air contains particulates which can bind up cloth filters, restrict airflows, and decrease the life of bags and cartridges.

Other bag house operations have problems due to bag binding, tacky or sticky or viscous particulates, or problems due to temperature, moisture, or particle size.

In most of these applications, special filtration grades of perlite can be added either when new bags are first installed as an initial "pre-coating", or it can be added to the air flow continuously during the regular operation of the bag house. In either situation, perlite helps protect the bag fibers, improves bag house performance by increasing air flows and decreasing the particle sizes which can escape through the bags.

APPLICATION GUIDES

Perlite should be added to bag houses in the same volumetric proportions as any other conditioning or precoating material would be used. Essentially enough perlite filter powder is added to maintain a very fine "coating" of approximately 1/16 to 1/8 of an inch on the surface of each bag. Once you know the total square footage of the bags being used in the entire baghouse, this number is multiplied by the design thickness of the filter cake (1/16 to 1/8 of an inch divided by 12) to determine the approximatley number of cubic feet needed to condition all the bags. Most perlite manufacturers pack in 2.3-2.4 cubic foot bags so the final cubic footage of material needed would be divided by 2.3 to give the total number of bags of perlite needed.

How and when perlite is used differs with different systems and different problems. Perlite normally is added one of two ways:

• in many systems, new bags are "pre-coated" before they are used for the first time
• in other systems, the perlite is "injected" or added constantly during regular operations

For more precise and more technical information on both how and when to use perlite as a pre-coat filtering product, the baghouse manufacturer and bag supplier should be consulted.

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