Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 62

One finds approximately 150 × 10⁶ grains of quartz in 1 kg of fine sand [4]. In such a population it is obvious that one cannot detect one grain of chromite or other impurity by routine chemical analysis. To detect these minerals at the part‐per‐billion (ppb) level, one rather takes advantage of the fact that they are denser, or even much denser than quartz (e.g. corundum: ~4 g/cm³; chromite: ~5 g/cm³). One can thus concentrate them by immersing the test powder in a liquid with a density slightly higher than that of quartz, which will then float while heavier minerals will sink, allowing the harmful ones to be identified and quantified with standard methods such as optical microscopy, Raman spectroscopy, or electron microscopy [8]. For this purpose, the most frequently used liquids have long been bromoform (CHBr₃) and diiodomethane [CH₂I₂, also called methylene iodide], whose room‐temperature densities of 2.9 and 3.3 g/cm³, respectively, can be slightly lowered through mixing with lighter ethanol [C₂H₆O]. Because strict precautions must be observed to handle these toxic liquids, however, sodium polytungstate [SPT, 3 Na₂WO₄·9 WO₃·H₂O] has become an efficient alternative [9], thanks to the fact that this salt can readily dissolve in distilled water to yield liquids with a maximum density of 3.1 g/cm³.

Proper sampling of raw materials [4] thus is needed to guarantee their conformity with regard to possible geological heterogeneity and product variability at the quarry level. It relies on suitable quartering techniques to obtain a true fingerprint of the mineralogy of all raw materials from which incorporation of harmful species may be ruled out or at least minimized. In other words, heavy‐mineral content is an overwhelming and crucial specification concerning the physical and chemical properties that must be guaranteed by a producer of raw materials, especially when fabrication of a new glass has to be tested. Not complying with these specifications can generate long‐lasting yield drops and large financial losses for the glassmaker. Since glassmakers constantly need to diversify and secure their supplies of raw materials, heavy‐mineral characterization must routinely be operated by well‐equipped internal or academic laboratories.

Of course, the bulk chemistry must also be determined on a daily basis at the plant by XRF, ICP‐MS, or wet chemistry (cf. Chapter 5.1) to monitor the variability of moisture and oxide content, especially for multielement raw materials, and thus to allow batch adjustments needed to keep the glass recipe constant to be calculated (cf. Chapter 1.3). As indicated above, the PSD, LOI, and COD parameters must in addition be included in the almost daily control of raw materials at the plant. In this way it is possible to anticipate possible drifts away from the targeted specifications of the raw‐material feed. As for the overall meltability, energy demand, and expected quality, they may be tested less routinely through differential scanning calorimetry (DSC) measurements, while it should be compulsory to test the actual batch incrementally, first in the laboratory (few kg), then in a pilot furnace (~1 ton), and finally at the industrial scale (<1000 tons).