The production of glass-ceramics

Physical and chemical properties of glassy and liquid silicates 315 

---

Selenium occurs in the earth s crust as metal selenides [5], and the element and its compounds are used in electronic applications and in the production of glass, ceramics, and stainless steel [11]. In trace amounts, selenium is an essential nutrient, but in higher quantities or when certain species are present, the element becomes toxic. The most important forms of selenium are selenate (Se042-), and selenite (Se032""), and selenide (Se2 ). Selanates are relatively soluble, whereas selenites and elemental selenium are virtually insoluble [12]. 

In addition, the present use of lithium in other sectors such as the production of glass, ceramics, and lubricants and in batteries for electronics also has to be taken into accoimt [38]. In 2010, the production of lithium for these sectors amoimted to 28,0001 [24]. (In 2010 there was no noteworthy production of lithium traction batteries.) From 1990 until 2010, we observed a yearly average increase in production and demand of approximately 10% [24]. Therefore, a 3% growth per year can be considered as a rather conservative assumption. 

A particular advanu e in the production of glass-ceramics is that products demonstrating almost zero shrinkage can be produced. These specific materials are produced on a large scale for industrial, technological, and domestic applications (e.g., kitchenware). 

The reprocessed blast-ftimace slag in the form of granules can be used as fillers for concretes, asphalts, and as additives in the production of cement, bricks and other building materials. As well, reprocessed blast-furnace slag can be poured into forms for the production of glass-ceramic tiles. 

About 100,000 t of titanium dioxide aimuaHy are used as formulation components in the production of glass (qv), ceramics, electroceramics, catalysts, and in the production of mixed-metal oxide pigments. 

Crystalline Silica. Quartz sand is of course the principal raw material for the production of glass (qv). Cristobalite and 3-quartz are used in glass ceramics (qv), ie, ceramics produced by the controlled crystallization of glass. Silica is a main constituent of ceramics (qv). For example, refractory silica brick containing small amounts of A O is used as roof brick for open-hearth furnaces at temperatures >1600° C (see Refractories). Silica sand or flour (ground quartz) is the raw material for soluble silicates, such as sodium silicate, which is consistently ranked as one of the top 50 U.S. industrial chemicals (98) (see Silicon compounds, synthetic inorganic silicates). 

Fig. 11.4.24 The production of glass plates for the Ceran ceramic heating plate as made by Schott.

Litharge and the other lead oxides that are used in the production of glasses and ceramics are obtained primarily through the oxidation of refined (purified) metallic lead. Because metallic lead does not occur naturally in large quantities, it must be extracted from either primary sources (mineral ores) or secondary sources (recycled materials such as lead-acid batteries and cathode ray tubes). The processing required to refine metallic lead can be broken down into three major steps, as seen in Fig. 3 ... 

---