Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 61

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].