Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 58

A very great many different raw materials can be used today for batch preparation (Table 1). On a chemical and mineralogical basis, they can be classified as silicates, carbonates, borates, oxides, and hydroxides. Recycled glass, known as cullet (cf. Chapter 9.9), as well as industrial by‐products such as metallurgical slags can in addition enter the batch preparation as amorphous and chemically complex materials. As apparent in Figure 1, the compositions of these materials do not match those of the main glass products, which may thus need up to a dozen ingredients for batch preparation. Even the most common soda‐lime silica glasses, circled in Figure 1, manufactured for building (windows), automotive (windshields), packaging (bottles), and solar (photo‐voltaic panels) applications, require several of them.

Each raw material plays a specific role. Most of them, as oxides, form the building blocks of the glass network as formers or modifiers (see Chapters 20.4 and 2.5), but among the latter some act as fluxes (alkalis), strengthening agents (alkaline earths), refiners (Na sulfate, Sb, Sn, As oxides), reducers (coke, slags), or even as oxidizing, O₂‐releasing (BaNO₃), or coloring (Fe, Cr, Mn, Co, etc.) agents.

Raw materials react in specific ways within the batch. Some are strongly hygroscopic, influencing as such the rheology and homogeneity of the still solid batch. As a matter of fact, water is present in most of the raw materials used to produce glass as either free water (moisture) or bound in the crystal structure of minerals. This essential and unavoidable component is crucial in raw‐material management since it minimizes the formation of dust at both the batch plant and the dog‐house (entrance of the furnace) levels, but it may also contribute to the formation of lumps made of the most hygroscopic materials, increasing the heterogeneity of the batch at the very beginning of melting. Furthermore, as a result, the batch may contain up to few wt % of water, whereas there are less than 1000 ppm H₂O in the final glass. Removing the water in excess may cost much in terms of both energy and furnace refractories, which may be corroded by acids such as HF and HCl formed when water reacts with other volatile components of the batch.

Without taking into account the formation of intermediate products, the overall meltability of the raw materials is highly variable [3]: H₂O is released at around 100 °C; the deshydroxilation of OH‐bearing minerals takes place at 400–800 °C; carbonates release large quantities of CO₂ at 700–900 °C; feldspars melt below 1200 °C; the other silicates are dissolved in the pre‐existing glass melt above that range; bauxite has an even stronger refractory character, needing temperature above 1300 °C to be digested by the surrounding liquid (cf. the DSC thermogram of Figure 3, in Chapter. 1.5).

Suppliers of raw materials process their products through several steps to match their customers' specifications [4]. First, rocks containing the desired raw material(s) are blasted or excavated. The bulk material so extracted is then retrieved, crushed, ground, screened, and sorted to achieve the required grain size, washed, dried, or dewatered before being stockpiled and transported in big bags or in bulk. In some cases a physical and/or chemical beneficiation stage may be needed to achieve the required specifications, especially to remove unwanted impurities. All these steps can have an impact on the final quality of the raw materials in terms of presence of impurities and heterogeneities.