Diatomite, filter aids

Gas chromatography (GC) employs a gaseous mobile phase, known as the carrier gas. In gas-liquid chromatography (GLC) the stationary phase is a liquid held on the surface and in the pores of a nominally inert solid support. By far the most commonly used support is diatomaceous silica, in the form of pink crushed firebrick, white diatomite filter aids or proprietary variants. Typical surface areas of 0.5-4 m2/g give an equivalent film thickness of 0.05-1 pm for normal liquid/support loadings of 5-50 per cent by mass. 

Kenite. [Witco] Diatomite filter aid, anticddng t ents, absorbent, bulking agent, ct tioner, extender pigment, ter, opadfier. 

Table 5.2 Typical chemical analyses of diatomite filter aids [Celite ]...

Diatomite (Kiesielguhr) USA, Spain, Denmark, and France 2,000,000 Diatomite filter-aids 850-930... 

In diatomaceous-earth filtration, the powdered filter aid is built upon a relatively loose septum to screen out suspended soHds. The filter becomes clogged, and pressure losses become excessive backwashing is then necessary. The smallest removable particle is 0.5—1 p.m (see Diatomite). 

There are two principal ways in which finished diatomite products are used in manufacturing plants either as a filter aid (see Filtration), where the diatomite is expendable, or as a filler, where the diatomite becomes a component and remains as part of the manufactured product. As of 1990, the use of diatomite products was 71% filtration, 15% fillers, and 14% other (7). 

Filtration. Diatomite is used as a filter aid for appHcations with difficult-to-filter soflds to improve permeabiUty of the filter cake, to prevent the blinding of filter elements, and where high clarity is required such as in the poHsh filtration of wine (qv) or beer (qv) before bottling. It is also used in sugar (qv) refining, water treatment, and in the production of fmit juices (qv) and industrial chemicals. 

Typically, a filter cake or precoat is built up on the filter septa to prevent blinding, short filter cycle times, and cosdy cleaning of the septa. Then diatomite is added as body feed to the Hquid to be filtered so that the permeabiHty of the filter cake may be maintained. Filler aid permeabiHty of diatomite ranges from 0.06 to 30 lnF. At the end of the filter cycle the filtrate is clear and the soHds are retained in the soHd or semisoHd diatomite filter cake. The type and amount of diatomite for precoat and body feed are normally deterrnined by pilot studies (18,19). 

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. 

Diatomaceous Silica Filter aids of diatomaceous silica have a dry bulk density of 128 to 320 kg/m (8 to 20 Ib/fU), contain paiiicies mostly smaller than 50 [Lm, and produce a cake with porosity in the range of 0.9 (volume of voids/total filter-cake volume). The high porosity (compared with a porosity of 0.38 for randomly packed uniform spheres and 0.2 to 0.3 for a typical filter cake) is indicative of its filter-aid ability Different methods of processing the crude diatomite result in a series of filter aids having a wide range of permeability. 

Filter aids should have low specific surface, since hydraulic resistance results from frictional losses incurred as liquid flows past particle surfaces. Specific surface is inversely proportional to particle size. The rate of particle dispersity and the subsequent difference in specific surface determines the deviations in filter aid quality from one material to another. For example, most of the diatomite species have approximately the same porosity however, the coarser materials experience a smaller hydraulic resistance and have much less specific surface than the finer particle sizes. 

Charcoal is not only employed in activated form for decoloring and adsorbing dissolved admixtures but also in its unactivated form as a filter aid. It can be used in suspensions consisting of aggressive liquids (e.g., strong acids and alkalies). As with sawdust, it can be used to separate solids that may be roasted. On combustion, the charcoal leaves a residue of roughly 2 percent ash. Particles of charcoal are porous and form cakes of high density but that have a lesser retention ability than does diatomite. 

Filtration—After an adsorbent has selectively captured the impurities, it must be removed from the oil before it becomes a catalyst for color development or other undesirable reactions. Filtration, the separation method most often used for spent bleaching media removal, is the process of passing a fluid through a permeable filter material to separate particles from the fluid. Examples of the filtration materials used are filter paper, filter cloth, filter screen, and membranes. Filter aid, such as diatomite, perlite, or cellulose, are usually used in conjunction with the permeable filters for surface protection. Traditionally, either plate and frame or pressure-leaf filters have been used for spent bleaching media removal. Currently, self-cleaning, closed filters that operate on an automated cycle are available. 

Any substances that can enhance the filtration efficiency are termed a filter aid. Diatomaceous earth is the most common filter aid for the precoat filtration system. An efficient, economical filter aid must (a) have rigid, intricately shaped, porous, individual particles (b) form a highly permeable, stable, incompressible filter cake (c) remove even the finest solids at high rates of flow and (d) he as chemically inert and essentially insoluble in the Uquid being filtered. Commercial diatomaceous earth, such as Celite diatomite, meets these requirements due to the wide variety of intricately shaped particles and inert composition that makes it practically insoluble in all hut a few liquids. 

The manufacturer of filter aids produces filter aid grades in a wide range of particle sizes to meet practically any industrial filtration requirement. The relative flow rates of these grades are determined by a standard filtration test and are shown in Fig. 4. Typical properties of some commercial filter aids are shown in Tables 1-3. Filter Cel, the finest grade, shown in Fig. 4, giving the highest clarity and lowest flow rate, is a natural diatomite, which has been selectively quarried, dried, milled, and air-classified. To make coarser, faster flow rate filter aids, Filter-Cel is calcined and air-classified. These straight... 

Dicalite. TM for a group of products made from either diatomite or perlite, used in filters and filter-aids. 

There is macroporous silica diatomite or diatomeceous earth as biological deposits of sea algae diatom . This is used as an industrial filter aid and heat insulator. The inside space of the bamboo stems and the rice straws contain silicas of high purity. 

For amorphous materials, sludges or other poor filtering products, improved filtration characteristics and/or filtrate clarity are enhanced with the use of filter aids. Slurry additives such as diatomaceous silica or perlite (pulverized rock), are employed to aid filtration. Diatomite is a sedimentary rock containing skeletons of unicellular plant organisms (diatoms). These... 

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