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such that NH3 and CO2 are in the end “totally” recycled within the process.
4.2 Raw Materials with Very Low Iron Contents
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).
4.3 Globetrotting Raw Materials
Like other capital‐intensive activities, glassmaking plants traditionally are local industries so as to minimize transportation costs of both raw materials and finished products. Although not necessarily rare geologically, however, certain minerals are not commonly found in commercially exploitable amounts with the consequence that they have to be procured globally, national policies sometimes applying heavy, protectionist custom fees to limit exportations. This situation applies, for instance, to lithium, boron, and some aluminum carriers.
In glassmaking, lithium occasionally serves as an additive (flux) for the production of standard soda‐lime silica glasses, but it is mainly used for glass‐ceramics to form the β‐spodumene [LiAlSi2O6] phase that gives them very low thermal expansion coefficients (Chapter 7.11). But the price of Li2O raw materials has been boosted – it has actually almost tripled – during the last decade, driven by the dramatically increasing demand for Li‐ion batteries (Chapter 9.5). Among the available Li2O raw materials (Table 1), Li from brines is mostly used to manufacture Li‐carbonate or hydroxide (battery‐grade raw materials), whereas mineral Li is incorporated into glass and ceramics. Concentrates of both minerals, spodumene and petalite, are actually crucial sources for the glass industry [17, 18], which does not require as high a purity as Li‐ion battery makers. These Li‐silicate sources are abundantly available in Australia, China, the United States, and Canada, but much rarer and mostly unexploited in Europe (Austria, Finland, Ireland, Portugal, and Spain) where most of the glass‐ceramic industries are in fact located.
As for boron, this element is important for the production of reinforcement fibers (Chapter 1.5) and for insulation (Chapter 9.3) and textile glasses. Borates are found in Turkey, the United States, China, Russia, and South America. Turkey is the world's biggest producer and holds the largest reserves. The United States ranks second both in terms of reserves production, with about 40% of the market [19]. In this case too, the heterogeneous distribution of boron sources translates into high transportation costs as a component of the raw‐material supplies.
Figure 7 Increases of iron contamination caused by grinding of quartz made with steel‐bearing jaws to shift the particle size distribution. (a) Iron contents associated with the PSD curves of quartz sand and flour ground. (b) Alignment of steel particles in the magnetic field of the separator during the deferrization step of raw‐material beneficiation. Picture scale of 0.5 m.
Concerning aluminum, bauxites and laterites are used as Al‐carrier in standard industrial glasses. The main and largest exploited ores, representing more than 90% of the reserves, are located in Western Africa (especially Guinea), Brazil, Central America and the Caribbean (Jamaica, Trinidad and Tobago, Suriname) and Australia [20]. In this market, however, glassmakers are very distant followers of alumina‐ceramics makers and especially of metallic aluminum producers whose needs are several orders of magnitude higher than theirs. In instances when relatively high contents of both aluminum and alkalis are required, it may be advantageous to use instead nepheline syenite, a rock consisting mainly of alkali feldspars and nepheline [NaAlSiO4], which is exploited and exported from Norway, Russia (Kola Peninsula), South‐Africa, Brazil, India, and China.
5 Perspectives
More than 100 million tons of glass (container, flat, fiber, and specialty) are produced yearly. In a society moving toward a CO2‐less or ‐free economy, extra‐clear glasses will play a key role for the development of an efficient solar energy market. As the availability of extra‐pure raw materials is not infinite [16], however, beneficiation techniques will need to be improved in order to meet cost‐efficient requirements adapted to the forthcoming societal challenges. In terms of both volumes and quality, middle‐ and long‐term availability of raw materials is a major challenge for sustainable, cost‐related production. But a successful low‐carbon society implies the fast development of infrastructures and commodity products, which contribute to the overall industrial minerals’ demand in direct competition with the glass raw materials supply chain [16]. Governmental initiatives, such as the European Union ERA‐MIN Program [21] and the EIT Raw Materials, intend to build an EU‐wide network linking industry, academia, and research institutes capable of sustaining the domestic supply chain of non‐energy mineral resources. In parallel, efforts are made to improve batch recipes through either the use of standard raw materials with lower energy consumption [22], or totally new ways exploiting the huge potential of the recycling supply chain. Food and agriculture wastes, for instance, could allow making glass with “exotic” sources such as eggshells for Ca‐carbonate, banana peels as K‐carrier, or rice husk for silica [23]. Major changes are likely on the way.
References
1 1 Barton, J. and Guillemet, C. (2005). Le verre: Science et Technologie. Les Ullis: EDP Sciences.
2 2 Cable, M. (2010). Bontemps on Glass Making: The Guide du Verrier of Georges Bontemps. Sheffield: Society of Glass Technology.
3 3 Gouillart, E., Toplis, M.J., Grynberg, J. et al. (2012). In situ synchrotron microtomography reveals multiple reaction pathways
