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1.2 Raw Materials for Glassmaking: Properties and Constraints

       Simonpietro Di Pierro

       EMC2 Department, Saint‐Gobain Research Paris, Aubervilliers, France

1 Introduction

      Even though industrial glasses may now be transported across the planet, glassmaking tends to remain a local industry. When selecting their raw materials, glassmakers target specific oxides or elements to fit quantitatively a calculated recipe [1, 2] by paying special attention not only to their composition and quality but, of course, also to their price. Raw materials in effect may represent up to one third of the total cost of glassmaking, strongly depending on local geographical and logistic conditions such as quarry‐to‐plant distance or transportation mode, which can be by waterways, rail, or truck. This concern holds especially true for commodity products which, like hollow ware, are facing fierce competition from metal or plastic materials.

      Natural materials are in principle favored because, like for the mineral resources used in metallurgy or cement industry, their extraction, milling, and transportation are relatively inexpensive operations compared to the fabrication cost of synthetic materials. Some of the raw materials used in glassmaking must nonetheless be man‐made when their natural counterparts are rare, geographically restrained, or of an insufficient purity. As such, their price can dramatically increase the overall batch cost. For instance, synthetic sodium carbonate may constitute up to 50 % of the total batch cost for window or bottle glass, whereas it constitutes less than 14 wt % of the recipe.

      After batch preparation, melting and subsequent chemical homogenization are complex processes whose kinetics depend on a number of factors. The grain size distribution of raw materials is particularly important in this respect as the main starting product, quartz, dissolves into the forming melt at a rate of only a few hundred μm per hour, so that pull rates tremendously decrease with increasing grain size. The impurity content is another fundamental issue. As natural products, most raw materials are far from being pure chemicals. The infusible minerals they may bring in have a very high likelihood of surviving the glassmaking process and, therefore, of causing inacceptable production losses even when present at the ppb levels. Other impurities such as iron metal originate in the initial processing of the raw materials. In any case, impurities may cause raw materials to depart from the specified physical and chemical properties. Production yield and quality can then be strongly impacted.

      Since many parameters may have either positive or negative consequences if not properly mastered, an in‐depth knowledge of the chemical and physical properties of raw materials is necessary. In this chapter, the main chemical, physical, engineering, and economic criteria pertaining to raw materials specifications will thus be reviewed. Because high production yields cannot be obtained without high‐quality and permanently controlled raw materials, attention will be paid to production problems most commonly met if the specifications and management of raw materials are not respected. The focus will be here on the raw materials used for silicate glasses manufactured in very large quantities. For specialty glasses, the reader is referred to the chapters that deal specifically with optical fibers (Chapter 6.4), chalcogenide (Chapter 6.5) or metallic glasses (Chapter 7.10), sol–gel products (Chapter 8.2), or bioglass (Chapter 8.4).

2 Raw‐material Specifications

      2.1 The Specificity of Raw Materials

A very great many different raw materials can be used today for batch preparation (Table 1). On a chemical and mineralogical basis, they can be classified as silicates, carbonates, borates, oxides, and hydroxides. Recycled glass, known as cullet (cf. Chapter 9.9), as well as industrial by‐products such as metallurgical slags can in addition enter the batch preparation as amorphous and chemically complex materials. As apparent in Figure 1, the compositions of these materials do not match those of the main glass products, which may thus need up to a dozen ingredients for batch preparation. Even the most common soda‐lime silica glasses, circled in Figure 1, manufactured for building (windows), automotive (windshields), packaging (bottles), and solar (photo‐voltaic panels) applications, require several of them.

      Each 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.