Purified Siliceous Earth

Many of the diatomaceous earths are useful as absorbent and polishing powders. The United States Pharmacopoeia IX recognizes, under the name of Terra Silicea Purificata (Purified Siliceous Earth), a powder consisting of the frustules and fragments of diatoms which has been purified by boiling with diluted hydrochloric acid, washed and calcined. 

Although silicon compounds have been used since the earliest times, the element was not obtainable until advances in technology came about. Minerals containing silicon include sand and silicates that are widely distributed (silicon constitutes approximately 23% of the earth s crust), and they have been used in making glass, pottery, and mortar for many centuries. In addition to these uses, silicon is now highly purified for use in integrated circuits (chips) and as an alloy known as Duriron that has many uses. Silicon has the diamond structure with a density of 2.3 g/cm3. 

Silicon, the second most abundant element in the earth s crust, is obtained by reducing silica sand (Si02) with coke. It is purified for use in the semiconductor industry by zone refining. In the silicates, SiC>4 tetrahedra share common O atoms to give silicon oxoanions with ring, chain, layer, and extended three-dimensional structures. In aluminosilicates, such as KAlSi30g, Al3+ replaces some of the Si4+. 

Note that aqueous flotation (with appropriate detergent eompounds) is used to separate the rare-earth-rich particles from the siliceous materials. After drying, the ore is calcinated (fired in air to form the oxides) and then treated with concentrated HCl. A cerium-rich concentrate is left after the other rare earths have dissolved. We have not shown how the cerium concentrate is processed since the main object is to obtain the more expensive oxides as a final product. The rare earth chloride solution is then adjusted to pH = 1.0, purified with activated carbon to remove some of the heavy metals and then extracted with DEHPA. The first extraction separates the "heavy" and "light" fractions of the rare earths. The resulting extracted solutions are then subjected to further extraction processes to produce the individual products, including La, Pr and Nd. 

Silicon accounts for 27.7% of the mass of the Earth s crust and occurs in a wide variety of silicates with other metals, clays, micas, and sand, which is largely Si02. The element is obtained on a small scale by the reduction of silicon(IV) oxide (Si02) by carbon or calcium carbide. For semiconductor applications very pure silicon is produced by direct reaction of silicon with an HCI/CI2 mixture to give silicon tetrachloride (SiCb), which can be purified by distillation. This is then decomposed on a hot wire in an atmosphere of hydrogen. For ultra-pure samples zone refining is used. Unlike carbon, silicon does not form allotropes but has only the diamond type of structure. 

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