furnaces whose production capacities range from a few tons to 1000 t per day.
Regardless of this diversity, fusion involves the same steps that will be successively described in this chapter. The first is the careful preparation of the batch from the appropriate raw materials. The second takes place at high temperatures through the various reactions that lead to a melt through complete dissolution of even the most refractory starting materials. The third step aims at producing a homogenous, bubble‐free product by physical and chemical fining. Finally, this chapter will briefly discuss the economically and environmentally important energetics of the fusion process. A review of earlier work dealing with these issues is the feature article by Cable [1].
2 Overview of Industrial Processes
A continuously operated industrial melting process can be split into two distinct room‐ and high‐temperature parts within which well‐defined different steps may generally be identified. Their main features are as follows:
Preparation (at room temperature)
| P1 | Acquisition and storage of raw materials |
| P2 | Chemical analysis of raw materials |
| P3 | Calculation of the proportions of the batch raw materials |
| P4 | Weighing and then mixing of the batch raw materials |
| P5 | Intermediate storage of the batch in a buffer silo |
| P6 | Batch charging |
Glass melting (at high temperatures)
| M1 | Primary batch‐to‐melt conversion, yielding a rough melt still containing considerable amounts of gas bubbles and undissolved solids Time demand: about one hour Intrinsic energy demand: about 2000 MJ/t of produced glass (3.6 GJ = 1 kWh) Temperature range: 600–1200 °C |
| M2 | Sand dissolution (comprising the dissolution of any other crystalline solids) Temperature range: 1200–1400 °C Intrinsic energy demand (mostly for heating up the melt): approx. 280 MJ/t of glass |
| M3 | Fining, i.e. physical removal of residual bubbles via the thermochemical generation of an adequately high‐volume fraction of large bubbles of a fining gas Temperature range: 1400–1500 °C Intrinsic energy demand (for heating‐up): approx. 140 MJ |
| M4 | Refining, thermal, and chemical homogenization, whereby refining denotes the resorption of residual gas bubbles upon steady cooling Target temperature: 1350 °C Heat released: −220 MJ. |
At the end of step M4, the melt is finally conveyed to the forming area where it is transformed into hollow ware (Chapter 1.5), or flat glass (Chapter 1.4), or any other type of product. To maintain a high glass quality, the filling level of the melting compartment must be kept constant. Therefore, the sequence of process steps P1 to M4 must be well balanced logistically. This constraint puts a stringent time interval to act for online corrections to steps P4–P6; the buffer silo between steps P5 and P6 thus serves the sole purpose of widening this interval. Along the path from P6 to M4, no action for correction is possible at all.
3 Batch Preparation
3.1 Raw Materials
The fusion of a glass of a given composition requires a suitable set of raw materials (Chapter 1.2). These are selected according to their availability and quality, which, in turn, determine their price. The issue of availability may be complex since it includes geological occurrence (for natural materials), production capacity (for manufactured materials), infrastructure for recycling and upgrading (for cullet) as well as transport distance, number of tenders, political stability at the source, etc. Quality is likewise a manifold issue as it concerns chemical composition (from impurities to the main component), mineralogical composition (special attention being paid to side minerals difficult to melt), and grain size distribution (with a particular concern to under‐ and oversized grain fractions). Among chemical impurities, iron is a critical factor. It is present in virtually every natural raw material but is generally tolerated in glass only at very low levels (Table 1) except, of course, when it is itself a major component of the product as in fire‐resistant glass fibers (Chapter 9.3). Here, the iron content is given in terms of stoichiometric Fe2O3 irrespective of its actual valence state. Yet, iron is generally present as Fe2+ and Fe3+ whose relative abundances depend on the redox state of the melt. Owing to the strong absorption bands of both cations, iron has a strong impact on the color of the glass even at low concentrations (Chapter 6.2). And because of its strong absorption in the 600–4000 μm wavelength range, which is that of the heat radiation in the furnace, Fe2+ acts as a blinding agent to limit tightly the transparency of the melt to IR radiation (Figure 1). In Figure 1, the furnace radiation is illustrated by a back‐body‐type curve. This is an oversimplification. The actual flame radiation spectrum in a furnace is characterized by strong emission lines of the H2O and CO2 molecules in the flame (H2O: 0.9–1.1, 1.8–2.0, 2.5–3.2 μm; CO2: 2.7–3.0, 4.2–4.7 μm) and by black‐body radiation from soot particles. The radiation enters the melt directly to a certain extent; however, chiefly via emission (emission coefficient ε ≈ 0.5) and diffuse reflection from the top lining (the crown) of the furnace. The curve shown in Figure 1 is an envelope of the actual radiation only. Irrespective of the above details, furnaces in which glasses with high Fe2+ contents are melted thus exhibit large vertical temperature gradients and low bottom temperatures; heat transfer from the combustion space has then to be brought about by the convective motion of the melt. By contrast, melts with very low amounts of Fe2+ weakly absorb energy from the combustion space since Fe3+ does not influence IR absorption. As a consequence, they display high temperatures at the bottom of furnaces. Controlling the redox state of the melt thus is important not only for color generation but also for furnace operation, a general conclusion that also applies for instance to green glasses colored by Cr3+.
Table 1 Maximum iron contents in various types of glasses, given in ppm of stoichiometric ferric iron (Fe2O3).
| Glass type | ppm Fe2O3 |
|---|---|
| Optical glass | 10 |
| Ultra‐white glass | 100 |
| Continuous fibers | 200 |
| Flint container glass |
|
