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

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inclusions called knots in the glassmaker jargon. The example shown in Figure 5 is that of an mm‐sized pocket of alumina‐rich glass within the normal soda‐lime silica matrix, which represents the ghost of a feldspar crystal. As already stated, feldspar minerals melt at temperature below 1200 °C, therefore early in the process, i.e. already at the dog‐house level. When they do so, they generate an alumina‐rich liquid phase whose viscosity of 105 dPa s at 1000 °C is 10 times higher than that of the soda‐lime silica glass [11]. At high pull rates this difference prevents rapid enough interdiffusion from taking place to ensure complete mixing between the two liquids, whence the presence of knots. A simple solution to avoid them when Al‐carriers are feldspars thus is to control the PSD of these minerals. But, with very different PSD ranges, feldspars can also be present as impurities in a variety of materials such as quartz-sand, limestone, and dolomite. Likewise, a proper PSD helps minimizing the presence of alumina‐rich knots in the final product when the Al‐rich phonolite and nepheline syenite rocks are used as raw materials.

Photo depicts a feldspar knot with about 20 wt percent Al2O3, enclosing bubbles in a soda-lime silica glass as seen under a binocular microscope.

      Even cullet may be a source of defects, which has important practical consequences since it accounts for up to 80–90 % 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)equation

      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.

Photo depicts 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.

      4.1 Sodium Carbonate

      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)equation

      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)equation

      The trick of the Solvay process thus is to produce sodium bicarbonate in an aqueous solution from an NaCl brine with the reaction

      (5)equation

      which, as a by‐product, yields calcium chloride, a valuable compound:

      (6)equation

      The first step then consists

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