Natural silica diatomaceous earths

Figures 13.7 and 13.8 show the efficiency of various antiblocking agents. S5mthetic silica (Figure 13.8) is the most efficient antiblocking agent, followed by natural silica (diatomaceous earth in Figure 13.7), and talc (Figure 13.8). Even...

Synonyms Amorphous silica Diatomaceous earth, natural Diatomaceous silica Diatomite Infusorial earth Kieselguhr... 

Diatomaceous earth also influences optical properties of films such as haze and optical clarity. The abrasive nature of diatomaceous earth is another reason for its diminishing use. Abrasion of machine parts requires replacement, and it is a substantial part of expenditures. Finally, crystalline forms of silica, also present in diatomaceous earth, are considered to be dangerous for human health, which further decreases interest in its application. Section 12.11.3 contains comparison of the most important antiblocking agents from the point of view of optical properties, antiblocking effect, and health and safety. 

Silica itself (the simplest silicate ) has found tittle use in PVC because of its abra-sivity (Mohs hardness of 7) and its acidity (pH 6-6.5), having been superseded as an antiblocking agent by zeolite and various silicates. An exception is diatomaceous amorphous silica, which, in synthetic forms (Celite ) and as naturally occurring diatomaceous earth, is often used as the carrier in dry Uquid dispersions. Diatomite is less abrasive than other forms of silica (Mohs hardness of 5-6) and is pH-neutral. It is an excellent solid carrier for polymeric plasticizers of viscosity high enough to make handling difficult. 

MicrocrystaUine Silicas. Various microcrystalline (cryptocrystalline) materials such as flint, chert, and diatomaceous earth are found ia nature (see Diatomite). These may arise from amorphous silica, often of biogenic origin, which undergoes compaction and microcrysta11i2ation over geologic time. 

The liquid stationary phase in a GLC packed column is adsorbed on the surface of a solid substrate (also called the support). This material must be inert and finely divided (powdered). The typical diameter of a substrate particle is 125 to 250 ft, creating a 60- to 100-mesh material. These particles are of two general types diatomaceous earth and Teflon . Diatomaceous earth, the decayed silica skeletons of algae, is most commonly referred to by the manufacturer s (Johns Manville s) trade name, Chromosorb . Various types of Chromosorb, which have had different pretreatment procedures applied, are available, such as Chromosorb P, Chromosorb W, and Chromosorb 101-104. The nature of the stationary phase as well as the nature of the substrate material are both usually specified in a chromatography literature procedure, and columns are tagged to indicate each of these as well. 

In a study of diatomaceous earth workers, those employed in the quarry for more than 5 years and exposed only to natural diatomaceous earth had no significant roentgenologic changes. Of others employed for more than 5 years in the milling process and exposed to calcined material, 17% had simple pneumoconiosis and 23% had the confluent form, probably the result of fibrogenic action of the crystalline silica formed by calcination of the naturally occurring mineral. ... 

The 2003 ACGIH proposed threshold limit value-time-weighted average (TLV-TWA) for amorphous silica, natural diatomaceous earth, is lOmg/m for the inhalable particulate and 3mg/m for respirable dust containing no asbestos and <1% quartz. 

In addition to the above crystalline phases sdica also exists in a few micro-crystadine forms. Such micro crystalline or cryptocrystalline silicas occur in nature and include diatomaceous earth, flint, and chert. They are mostly of biogenic origin forming from compaction of amorphous silica over geologic time. 

Approximately 40% of synthetic amorphous silica production is in Europe, followed by North America at 30%, and Japan at 12%. Although deposits of naturally occurring amorphous silicas are found in all areas of the wodd, the most significant commercial exploitation is of diatomaceous earth in industrialized countries (see Diatomite). This is because of the high cost of transportation relative to the cost of the material. Woddwide manufacturers of amorphous silica products are listed in Table 2. 

This is naturally occurring amorphous silicic acid from the skeletons of diatoms and hence is often referred to as diatomaceous earth. It has less adsorptive properties than silica. 

SYNS AMORPHOUS silica CELITE D.E. DIATOMACEOUS EARTH, NATURAL DIATOMACEOUS SILICA DIATOMITE ... 

Another common form of silica is diatomaceous earth, also known as kieselguhr. 6 This is a naturally occurring material composed of fossilized diatoms. It contains 70-90% Si02, has 0.2-0.7 im pores and a surface area of about 15-40 m /g. It is a common support for nickel catalysts used in large scale reactions. 

The common availability of silica is not the sole reason for its extensive use. Probably, it is the chemical inertness and durability of silica which determined its popularity. The fillers discussed here include not only natural minerals but also a variety of synthetic products. Natural products can be divided into crystalline and amorphous. Crystalline silica fillers include sands, ground silica (or silica flour), and a form of quartz - tripoli, whereas the amorphous types include diatomaceous earth. 

Until 1985 silicas were usually divided into two groups natural and synthetic. Most of the naturally occurring SiC>2 modifications are crystalline, but this group also includes amorphous or mostly amorphous products, such as diatomaceous earth or kieselguhr. However, when the observations made at the end of the introduction are considered, this classification no longer proves sufficient, as fly ashes and silica fume are also synthetically produced, although not deliberately. These airborne dusts are not harmless (22). 

Inorganic colorants listed in 21CFR 178.3297 include aluminum, aluminum hydrate, potassium silicate, aluminum silicate, barium sulfate, bentonite, calcium carbonate, calcium silicate, calcium sulfate, carbon black (channel process, prepared by the impingement process from stripped natural gas), chromium oxide green Cr203, cobalt aluminate (with restrictions), diatomaceous earth, iron oxides, kaolin (modified for use in olefin polymers in amounts up to 40%), magnesium oxides, magnesium silicate (talc), sienna, silica, titanium dioxide, titanium dioxide-barium sulfate, ultramarines, zinc carbonate (limited use), zinc chromate (less than 10%), zinc oxide (limited use), and zinc sulfide (less than 10%). 

This section strives to bring the reader up to date on the practical applications of silica. We include four chapters that were previously published in The Colloid Chemistry of Silica and supplement them with diverse, yet fundamental discussions on the more current uses of silica. Excluded from our coverage of the uses of silica are commercially important, naturally occurring siliceous materials such as sand, quartz, opal, diatomaceous earth, and chert. 

The aim of this chapter is more limited the emphasis is to provide an overview of the uses of synthetic silica gels and precipitated silicas that are of significant commercial importance. Excluded from this discussion are naturally occurring silicas, including products such as diatomaceous earth and so-called amorphous silica minerals (which are actually microcrystalline). Also excluded are fumed and arc silicas, forms of synthetic silica made at high temperature (in contrast to silica gels and precipitated silicas, which are generally made in aqueous solution and... 

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