raw material plays a specific role. Most of them, as oxides, form the building blocks of the glass network as formers or modifiers (see Chapters 20.4 and 2.5), but among the latter some act as fluxes (alkalis), strengthening agents (alkaline earths), refiners (Na sulfate, Sb, Sn, As oxides), reducers (coke, slags), or even as oxidizing, O2‐releasing (BaNO3), or coloring (Fe, Cr, Mn, Co, etc.) agents.
Raw materials react in specific ways within the batch. Some are strongly hygroscopic, influencing as such the rheology and homogeneity of the still solid batch. As a matter of fact, water is present in most of the raw materials used to produce glass as either free water (moisture) or bound in the crystal structure of minerals. This essential and unavoidable component is crucial in raw‐material management since it minimizes the formation of dust at both the batch plant and the dog‐house (entrance of the furnace) levels, but it may also contribute to the formation of lumps made of the most hygroscopic materials, increasing the heterogeneity of the batch at the very beginning of melting. Furthermore, as a result, the batch may contain up to few wt % of water, whereas there are less than 1000 ppm H2O in the final glass. Removing the water in excess may cost much in terms of both energy and furnace refractories, which may be corroded by acids such as HF and HCl formed when water reacts with other volatile components of the batch.
Without taking into account the formation of intermediate products, the overall meltability of the raw materials is highly variable [3]: H2O is released at around 100 °C; the deshydroxilation of OH‐bearing minerals takes place at 400–800 °C; carbonates release large quantities of CO2 at 700–900 °C; feldspars melt below 1200 °C; the other silicates are dissolved in the pre‐existing glass melt above that range; bauxite has an even stronger refractory character, needing temperature above 1300 °C to be digested by the surrounding liquid (cf. the DSC thermogram of Figure 3, in Chapter. 1.5).
Suppliers of raw materials process their products through several steps to match their customers' specifications [4]. First, rocks containing the desired raw material(s) are blasted or excavated. The bulk material so extracted is then retrieved, crushed, ground, screened, and sorted to achieve the required grain size, washed, dried, or dewatered before being stockpiled and transported in big bags or in bulk. In some cases a physical and/or chemical beneficiation stage may be needed to achieve the required specifications, especially to remove unwanted impurities. All these steps can have an impact on the final quality of the raw materials in terms of presence of impurities and heterogeneities.
2.2 Grain Size
The particle (or grain) size distribution (PSD) is a crucial parameter of individual raw materials. The required PSD may be costly to achieve. It primarily depends on the hardness of the bulk material, which in turn roughly correlates with its melting temperature [5]. As examples, K‐feldspar has a hardness of 6 (out of a maximum of 10) on the Mohs scale and melts at about 1200 °C, quartz has a hardness of 7 and melts above 1700 °C (in the form of cristobalite), whereas corundum (α‐Al2O3) has a hardness of 9 and melts above 2000 °C. Hence, the glassmaker determines the final PSD of the raw material as a compromise between meltability, furnace technology, and price (cost) while also limiting the unnecessary fines that generate dust and furnace carryovers. For specific applications, the glassmaker may in addition request the supplier of raw material to cut the lower end of the PSD to get totally rid of dust from fines.
The sieve PSD curves of a variety of important raw materials are compared in Figure 2 to illustrate their variations with composition and overall batch meltability. The median diameter representing 50% of a sieved raw material is termed D50. For quartz, it ranges from 200 to 300 μm when sand is used for standard window or bottle glass but is much lower at 50–100 μm for the flour, for instance, used as SiO2‐carrier for E‐glass fiber, a peraluminous, boron‐bearing, alkaline earth silicate (Chapter 1.5). At the other end, the D50 of limestone and dolomite may exceed 1 mm and that of basalt for insulating glass applications may even be 10 times larger because chemical heterogeneities are in this case much smaller than within a mixture of raw materials.
Table 1 Natural and synthetic raw materials compositions and prices.
| Oxide | Raw material | Bulk chemistry | Overall mineralogy | Sp – Fr – It – De | Price €/T* |
|---|---|---|---|---|---|
| SiO2 | Quartz‐sand | >95 % SiO2; H2O, Al2O3, RO, R2O, Fe2O3 | Quartz, free‐water, mica, feldspars | Arena – Sable – Sabbia – Sand | 20–200€/T |
| Sandstone | >95 % SiO2; H2O, Al2O3, RO, R2O, Fe2O3 | Quartz, mica, feldspars, FeTi‐oxides, free‐water | Arenisca – Grès – Arenaria – Sandstein | ||
| Quartzite | >95 % SiO2; H2O, Al2O3, RO, R2O, Fe2O3 | Quartz, mica, feldspars, FeTi‐oxides | Cuarcita – Quartzite – Quarzite – Quarzit | ||
| Al2O3, R2O | Feldspar (concentrates from greywacke, arkose, pegmatite, granite, etc.) | 17–20 % Al2O3; 11–15 % R2O; <65 % SiO2; H2O; Fe2O3, TiO2, CaO | Alkali‐feldspars [(K,Na)AlSi3O8: orthoclase, microcline, sanidine, albite, and their solid solutions], quartz (15–20%), micas. Li‐rich (up to 1.5 wt %) contain spodumene, petalite, or lepidolite (Li‐mica), mainly. | Feldespato – Feldspath – Feldspato – Feldspat | 80–150€/T |
| Nepheline(−syenite) | 20–26 % Al2O3; 15–18 % R2O; <56 % SiO2; H2O; Fe2O3, TiO2, CaO | Alkali‐feldspars [(K,Na)AlSi3O8: microcline, sanidine, albite, and their solid solutions], alkali‐feldspatoids [(K,Na)AlSiO4: nepheline, kalsilite, and their solid solutions], micas, titanite, perovskite, garnet, zircon, apatite, REE‐silicates. Silica undersaturated = no quartz | Nefelina – Néphéline – Nefelina – Nephelin | 100–130€/T | |
| Phonolite | 20–26 % Al2O3; 15–18 % R2O; <56 % SiO2; H2O; Fe2O3, TiO2, CaO |
Alkali‐feldspars [(K,Na)AlSi3O8: sanidine, albite, and their solid,solutions], alkali‐feldspatoids [(K,Na)AlSiO4: nepheline, kalsilite,
|
