Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 60

Another crucial feature the glassmaker has to consider for raw‐material management is the mass budget. It is common knowledge that to produce 1 ton of new, cullet‐free glass, one needs around 1.2 tons of raw materials. The ~20 wt % mass loss is mostly due to CO₂ released by Na, Ca, and Mg carbonates. Besides free or bound water, raw materials may in addition contain other volatile components such as unbatched carbon, fluorine, chlorine, sulfur, or boron. When present in traces, these components are not detected through standard chemical analyses (Chapter 5.1) but can nonetheless be quantified as loss on ignition (LOI) above 1000 °C. It is an important specification negotiated with the raw‐material supplier since the glassmaker needs it to calculate the total mass budget of the process, including the chimney emissions that are most frequently submitted to regulatory obligations.

Because the large majority of industrial glasses are oxides, they are manufactured under oxidizing conditions. The oxygen budget is a key element to control the combustion process and, thus, melting temperatures, as well as the overall color and optical transmission of the glass, which markedly depend on redox conditions ([6], Chapter 5.6). This feature is particularly important for applications such as solar panels, and is essential for glasses made with O₂‐sensitive raw materials such as coloring agents and those that contain a significant fraction of multivalent elements.

For these reasons the glassmaker must take particular care of the total chemical oxygen demand (COD) whose proper analysis is mandatory for certain raw materials entering the batch calculation. The cullet may, for instance, contain significant amounts of organic components, such as PVC, paper, or any other residues from the downstream industrial chain. Metals, besides being detrimental to the overall quality of the glass, are oxygen sinks so that they may locally shift the overall redox budget of the process when they are oxidized. For iron as a coloring agent, for example, one cannot use iron metal, whatever its grain size, but iron oxides instead. Bearing only Fe³⁺, hematite (Fe₂O₃) is generally selected along with magnetite [Fe₃O₄] whose mixed Fe³⁺, Fe²⁺ valence states make it suitable for reduced compositions such as amber‐glass bottles. In contrast, wüstite [FeO_{1 − x}] is generally of very limited use.

Finally, the apparent density is another factor needing to be specified simply because it must be known to dimension the silos where the raw materials are stockpiled at the plant site. As an example, the bulk density of quartz is 2.65 g/cm³. Depending on grain shapes and contact angles, that of dry quartz sand is in contrast lower than 2 as a result of a high open porosity. Practically this means that a sand‐storage silo must be at least 32% bigger than estimated from the quartz density.