Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 63

Melting quality first and foremost depends on appropriate digestion rates. Batch stones can, for instance, readily occur when market requirements push glassmakers to increase pull rates and, consequently, to reduce the average residence time of the raw materials in the furnace. They also form in case of errors in batch calculation or of scale malfunction. The problem is illustrated with the overall texture and microstructure of the silica batch stone shown in Figure 3. Noteworthy are former quartz grains, whose shape have been preserved although they have been totally replaced by cristobalite (as a pseudomorph) from which newly formed tridymite lath‐shaped crystals have grown radially in a groundmass of silica glass [10]. These textural features are typical of an insufficiently dispersed quartz sand within the batch. Regardless of the actual pull rate, the overall moisture content and distribution can provoke the formation of lumps when grains stick together at the batch plant and quickly sinter at the dog‐house level, preventing natural convection within the melter from properly stirring the melt under formation. Moisture is of special concern in regions facing very wet or very cold (ice) seasons. Preheating or protecting the raw materials yard can then be an effective, but somewhat costly solution.

Figure 3 Silica batch stone in a soda‐lime silica glass, resulting from incomplete digestion of a lump of quartz grains as viewed under an optical microscope (a) and as observed in a thin section under transmitted light (b), where rounded grains of cristobalite formed as quartz pseudomorph are sluggishly digested while generating radially growing tridymite laths in a vitreous groundmass showing an overall open‐porosity.

Quartz batch stones can also result from an inadequate overall PSD when other raw materials contain up to several weight % of free silica as impurities, whose size distribution differs from that of quartz sand. Limestone, dolomite, and feldspars are typical examples, the two carbonates having a d_max for quartz as high as 2 mm or more (Figure 2). Given the aforementioned digestion rates of quartz, such large grains will end up as unmolten stones in the production process.

Figure 4 Undissolved chromite crystal in a soda‐lime silica glass as seen under a binocular microscope (a) and in a polished thin section photo under reflected light (b). The chromite grain shows a preserved FeCr₂O₄ core, exsolution lamellae, and radially growing eskolaite Cr₂O₃ laths in the outer shell.

Refractory minerals, of course, raise special difficulties as illustrated in Figure 4 by an incompletely dissolved chromite inclusion. Its core is preserved as FeCr₂O₄, but it is surrounded by newly formed laths of eskolaite [Cr₂O₃] radially growing from it according to the decomposition reaction:

(1)

Although Fe²⁺ then diffuses in the melt, the highly refractory eskolaite crystals (melting at 2435 °C) passivate the chromite core, which will thus remain throughout the production process.

Incomplete digestion can alternatively cause the existence of vitreous 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.

Figure 5 A feldspar knot with about 20 wt % Al₂O₃, enclosing bubbles (gas inclusions) 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)

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.