Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 66

Given its strong absorption bands (Chapter 6.2), iron badly needs to be present at the lowest possible concentrations in a variety of glasses for which optical transmission must be optimized. This is, for instance, the case of the sheets protecting silicon wafers from oxidation in solar panels or of the mirrors used for concentrating solar energy in thermal solar plants. Such solar glasses are currently the most transparent available on the market with an optical transmission that can be as high as 91–92%, against values lower than 90 % for standard glasses used in windows or car windshields. Although it might appear small, this difference is in practice significant so that it is worth the subsequent increases in the cost of the raw materials. Here, two factors must be considered, namely the total iron content and the iron redox state. Whereas the latter can be controlled through various process parameters, the former is, of course, determined by the batch composition. Specifically, the total iron content of extra‐clear glasses for solar applications must be below 100 ppm [16], compared to the 600–1000 ppm of clear glass for windows. Natural raw materials with so low iron contents are rare, however, so that suppliers need beneficiation processes to reach them [4]. Grinding then becomes an issue because of potential iron contamination by the steel of the machines. Magnetic separation then is a convenient way to remove any added iron as the last step of raw‐material preparation (Figure 7).