Wood cellulose, filter aids

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular commercial filter aids diatomaceous silica (also called diatomite, or diatomaceous earth), which is an almost pure silica prepared from deposits of diatom skeletons and expanded perhte, particles of puffed lava that are principally aluminum alkali siheate. Cellulosic fibers (ground wood pulp) are sometimes used when siliceous materials cannot be used but are much more compressible. The use of other less effective aids (e.g., carbon and gypsum) may be justified in special cases. Sometimes a combination or carbon and diatomaceous silica permits adsorption in addition to filter-aid performance. Various other materials, such as salt, fine sand, starch, and precipitated calcium carbonate, are employed in specific industries where they represent either waste material or inexpensive alternatives to conventional filter aids. 

FILTER AIDS. Slimy or very fine solids that form a dense, impermeable cake quickly plug any filter medium that k fine enough to retain them. Practical filtration of such materials requires that the porosity of the cake be increased to permit passage of the liquor at a reasonable rate. This is done by adding a filter aid, such as diatomaceous silica, perlite, purified wood cellulose, or other inert porous solid, to the slurry before filtration. The filter aid may subsequently be separated from the filter cake by dissolving away the solids or by burning out the filter aid. If the solids have no value, they and the filter aid are discarded together. 

The Substrate and Its Modification. Solka Floe—SW40A (Brown Co., Berlin, N. H., USA), a wood (spruce) pulp commonly used as a filter aid has been used as a basic cellulosic material for susceptibility tests and saccharification studies. The material was milled dry in a laboratory porcelain pot mill using glazed porcelain balls of 1 inch (2.65... 

Materials suitable as filter aids include diatomaceous earth, expanded perilitic rock, asbestos, cellulose, non-activated carbon, ashes, ground chalk or mixtures of those materials. Whilst mineral filter aids have been well established in industry for many decades, those based on organic, selfreplenishing raw or even waste materials such as cellulose, wood fibres or plant fibres have been increasing in use. The best situation is when a cheap or reject material can be found elsewhere in the process and is suitable for using as a filter aid, at little or no extra cost. 

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