Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
% of the total batch for tinted soda‐lime silica glasses used for packaging, and up to 30 % for standard‐window and windshield glass production. In this case, the culprit is metallic aluminum from soda‐drink cans that pollutes household cullet or from framework residues of window cullet coming from building demolishing sites [10]. Through the redox reaction [12]
(2)
metallic silicon forms while hydrogen is liberated by the reduction of OH‐group from the glass network. The result is a sub‐mm‐sized silicon bead surrounded by H2‐rich gas inclusions (Figure 6), which are virtually indestructible because they are digested at rates of the order of a few μm per hour. Appropriate cullet management is thus needed to reduce this risk in glass production. For similar reasons, the absence of ceramic, porcelain, and glass‐ceramic shards polluting the external cullet must also be checked carefully to avoid stones, knots, and slowing down of the production.
Another special case arises from the presence of nickel in raw materials. In sulfate‐fined glasses, reduction of NiO yields nickel metal, which can react with trace amounts of sulfur to form small inclusions of NiS [millerite] [13]. The problem here is that this sulphide in principle undergoes near 390 °C a phase change from a high‐temperature α‐polymorph to a denser low‐temperature β‐polymorph. The kinetics are slow enough, however, that the phase transition can take place at room temperature several days to several months or even years after tempering (Chapter 3.12). When it eventually does so, the 2–4% volume expansion generates cracking and sometimes the explosion of the finished glass product. Hence, both Ni metal and oxides are nowadays proscribed in raw‐material specifications.
Figure 6 A sub‐mm‐sized silicon bead surrounded by H2‐rich gas inclusions in a soda‐lime silica glass, resulting from aluminum‐metal contamination of recycled cullet.
3.4 The Problem of Dolomite Decrepitation
Even when all chemical, physical, and mineralogical specifications are respected, some raw materials may pose special difficulties upon heating. Decrepitation is such a special case affecting mainly dolomite [14] and, albeit to a lesser extent, limestone. It occurs at 300–400 °C, hence well before the onset of decarbonation and decomposition of the (Ca, Mg) carbonate. Although it is still poorly understood [14], decrepitation appears to result from a sudden change in the overall PSD of dolomite, and an overall increase of fines, when dolomite grains locally burst into smaller grains. The decrepitation factor is defined as the increment in the fraction of grains smaller than 60 μm after heating at 1000 °C. It can range from a few to several 10% depending on geological history and, in particular, on the thermal pathway followed by the dolomite after its formation. High‐decrepitation dolomites are detrimental to the glass process because they contribute to further increase in dust and carryover in the furnace atmosphere. The latter in turn contribute to clogging phenomena at the level of regenerator chambers, drastically decreasing their energy‐recovery efficiency. Furthermore, dolomite dusts may increase the overall wear of the furnace‐superstructure refractories through the formation of new Mg‐bearing phases that decrease their overall durability. When a local supply of good‐quality dolomite is lacking, glassmakers may thus find safer to produce Mg‐free glass.
4 Special Raw Materials
4.1 Sodium Carbonate
Originally known as natron, sodium carbonate hydrates have been the main flux used ever since the beginnings of glassmaking. As exemplified by the celebrated Wadi el Natroun deposits in Egypt (Chapter 10.3), very pure Na2CO3 hydrates did form naturally but such deposits currently satisfy only about 30 % of 50 million tons used annually in industry, half of which for glassmaking. The cheapest source of sodium would be NaCl, but this salt is quite unfit for production of oxidized glasses in view of chlorinated emissions that would in particular dramatically degrade the wearing resistance of the furnace refractories. Soda [NaOH] would be a much better alternative, but its price on an Na2O basis is twice that of synthetic Na2CO3 as manufactured with the Solvay process [15], which currently supplies 59% of the market.
Sodium is most conveniently added to the batch as Na2CO3, whereas CaCO3 is the most common source of carbonate ions. Hence, the goal of the Solvay process is to achieve the overall reaction:
(3)
Now, this reaction could not proceed directly in the solid state even if its Gibbs free energy of about 100 kJ/mol were not positive. But Na2CO3 is readily obtained through heating of NaHCO3 precipitates at around 200 °C:
(4)
The trick of the Solvay process thus is to produce sodium bicarbonate in an aqueous solution from an NaCl brine with the reaction
(5)
which, as a by‐product, yields calcium chloride, a valuable compound:
(6)
The first step then consists in producing a solution of ammonium bicarbonate with
(7)
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
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