Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов

Figure 2 Sieve particle size distribution (PSD) curves of the main raw materials used in glassmaking (<40 μm laser PSD domain). The D50 is the median diameter representing 50% of a sieved raw material. The steeper the curve, the more homogeneous is the PSD.

      2.3 Operational Parameters

      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.

3 From Raw Materials to Melt

      3.1 Effects of Digestion Kinetics

      The sum of the oxide contents listed in Table 1 (column “bulk chemistry”) is always lower than 100 wt %. The missing few percent mainly relate to impurities. Although it would be tempting to consider them as negligible for the overall glassmaking process, these are actually significant as illustrated by building and automotive glasses, which are manufactured with the float process (Chapter 1.3). Under standard market conditions, a float line averages 600–700 tons of daily production. This pull rate requires to introduce between 500 and 600 tons of quartz sand daily into the melter. With a SiO₂ content of 99%, about 5 tons of impurities are then introduced at the same time. Even with an expensive 99.9% quartz, there remains about half a ton of impurities. These include different minerals, highly dispersed and diluted in the bulk, some of which may simply be incompatible with the glassmaking process because refractory materials are too slowly digested by the surrounding alkali‐rich molten glass.

      Even quartz does not melt during the glassmaking process, but is digested at rates of a few hundred μm per hour at 1400 °C [7]. It follows that 1 mm‐sized quartz grain will need hours to be completely digested. That is why raw‐material suppliers sieve and sort their products, and why the maximum PSD of quartz sand is fixed at less than 1 mm by glassmakers. But dissolution rates are typically as low as a few μm per hour for commonly found heavy minerals such as corundum [α‐Al₂O₃]; zircon [ZrSiO₄]; kyanite, sillimanite, and andalusite [the Al₂SiO₅ polymorphs]; spinel [MgAl₂O₄]; and chromite [FeCr₂O₄]. Now, corundum, for instance, melts at above 2000 °C whereas the glass temperature does not exceed 1500 °C at the hottest point in a melter where the average residence time of the batch is at most a few hours. As a consequence, part of the corundum introduced as mm‐sized grains would survive the glassmaking process and occurs in the final product as tiny inclusions, causing mechanical stress and impairing the optical quality. Such a product would not reach the customer to be discarded instead to the internal recycling circuit or, in the worst cases, to be dumped.

      The impact of such impurities may indeed be very serious. Consider a 20 m², 5‐mm thick float‐glass slab. With a volume of about 0.1 m³, its weight is 200 kg and requires 170 kg of quartz sand to be made. If on average a single mm‐sized grain of chromite was present in 1 kg of the sand, then the 20 m² slab would display 170 dot‐like defects of chromite. It would clearly be unsellable since current specifications dictate that at most one dot‐like defect be present in 100 m² of glass. To meet glassmaking specifications, raw materials thus have to be purified by the suppliers through flotation and other costly operations [4].

      3.2 Quantification of Heavy Minerals

One finds approximately 150 × 10⁶ grains of quartz in 1 kg of fine sand [4]. In such a population it is obvious that one cannot detect one grain of chromite or other impurity by routine chemical analysis. To detect these minerals at the part‐per‐billion (ppb) level, one rather takes advantage of the fact that they are denser, or even much denser than quartz (e.g. corundum: ~4 g/cm³; chromite: ~5 g/cm³). One can thus concentrate them by immersing the test powder in a liquid with a density slightly higher than that of quartz, which will then float while heavier minerals will sink, allowing the harmful ones to be identified and quantified with standard methods such as optical microscopy, Raman spectroscopy, or electron microscopy [8]. For this purpose, the most frequently used liquids have long been bromoform (CHBr₃) and diiodomethane [CH₂I₂, also called methylene iodide], whose room‐temperature densities of 2.9 and 3.3 g/cm³, respectively, can be slightly lowered through mixing with lighter ethanol [C₂H₆O].