Processing Quartz and Other Silicas

Silica, the main component of silicates, is widely used as mentioned earlier. In its crystalline and noncrystalline polymorphs, silica is used industrially as a raw material for glasses, ceramics, foundry molds, in the production of silicon, and more recently in technical applications such as quartz oscillators and optical waveguides for long­distance telecommunications. Of the crystalline forms, only a-quartz is commonly used as sand or as natural and synthetic single crystals. Cristobalite is often utilized as the synthetic phase in glass-ceramics.

Beyond the abundant natural sources of quartz and other silicas, techniques for synthetic production of these materials have provided a significantly wider range of applications [27,34]. Large, high-quality crystals of quartz can be grown by the well – established technique of hydrothermal growth in an autoclave filled with a solution of sodium carbonate at elevated temperature and pressure. Quartz particles are fed into the bottom of the growing chamber, while seed crystals are fed into the top in a metal frame. A temperature gradient establishes a greater solubility at the higher-temperature bottom of the chamber, leading to a continuous transfer of material upward to the growing single crystals. Uniform quality crystals are routinely produced with well – controlled shapes and sizes. Specific seed crystal orientations are used to produce desired products such as particular oscillator configurations.

Vitreous silica has a unique set of properties for applications where optical trans­mission, chemical inertness, and thermal stability are crucial. The abundance of vitreous silica in nature is widespread in biogenic sources such as sponges and diatoms, in crystalline opals, and as glass cycled by organisms through the environment (e. g., silicification of plant tissues for structural integrity and protection from insects [35]). This important glass can also be readily found in abiogenic sources such as volcanic glasses, resulting from extensive quenched magmas, tektites (spherical or teardrop­shaped silicate glass bodies linked with impact craters), and lechatelierite (pure silica glass), resulting from lightening strikes of unconsolidated sand or soil that form fulgurites. Glassy silica is also formed by a combination of temperature and pressure resulting from meteoritic impact [27].

Vitreous silica is high purity SiO2 glass that can withstand service temperatures above 1,000°C. As a metastable phase of silica, vitreous silica can be readily obtained in nature and synthetically. Silica glass can be produced in a pure and stable form, displaying useful properties, but is rigid and difficult to shape even at 2,000°C. Hence, it is not accessible to mass production plastic-forming methods. However, techniques have been developed to produce vitreous silica in various shapes and sizes [36-39].

First, quartz crystals can be melted to produce silica glass by either the Osram proc­ess or the Heraeus method [27]. In the Osram technique, fragmented quartz is fed to a tubular furnace and melted in a crucible protected by an inert gas, where tubing is drawn from the bottom of the crucible. In the Heraeus method, quartz crystals are fed in an oxy-hydrogen flame through a rotating fused quartz tube and withdrawn slowly from the burner as clear fused (vitreous) silica accumulates. The quartz crystals are generally washed in hydrofluoric acid and distilled water to remove surface impurities, followed by drying and heating to ~800°C, before being immersed in distilled water. Purity of the natural sand is very important in glass and ceramic materials, and transition metal oxides should not exceed 200 ppm.

In vapor phase hydrolysis [37,38], synthetic vitreous silica is prepared from silicon tetrachloride by oxidation or hydrolysis in a methane-oxygen flame. The resulting soot is sintered to form silica glass. Water, formed from the oxidation of methane, subsequently combines with the chloride, leading to the production of hydrochloric acid and oxygen. Subsequent work on these materials can lead to a variety of useful products, including telescope mirror blanks, lamp tubing, crucibles, and optical fibers (the largest commercial use for vitreous silica in telecommunications).

Finally, vitreous silica can be manufactured by the sol-gel technique developed by Zarzycki [39]. Gels are formed by the destabilization of colloidal sols or by the hydrolysis of metal organic compounds. This latter routine is the most common tech­nique that yields a silica-alcohol-water gel. Subsequently, the gel is dried and fused to produce silica glass. The manufacture of 3D articles by this method is limited due to the difficulty in drying porous gels without large shrinkage and cracking and the associated high costs of the raw materials. Silica gel can also be used as a drying agent and as supports for chromatography and catalysis [6].