a given pull rate p in t/h, the nominal overall dwell time of the melt in the furnace is
(1)
where ρ is the density of the melt and L·W·D the volume of the melting tank. Depending on the size of the furnace and the targeted glass quality (in terms of residual bubbles), τNOM ranges from 20 to 40 hours. During this time, the average volume element circles 2–6 times in vortex 1, and about twice in vortex 2. For a detailed analysis of the role of the flow pattern on melting and fining, see [3, 4]. The process of refining (M4) already starts at the descent of vortex 2; it is completed in a subsequent compartment termed refiner, which is thermally separated from the melter. Thermal separation is accomplished either with a vertical wall leaving an opening of about 0.5 × 0.3 m2 cross section (the throat) at half width right above the bottom of container‐glass furnaces (Chapter 1.5), or by an area of moderately narrowed width (the waist) in float glass furnaces (Chapter 1.4). For the sake of glass quality (i.e. homogeneity), it is mandatory to keep the position of the hot spot constant at any pull rate.
4.2 The Chemistry of Melting
The first high‐temperature process is primary melting of the batch. It is typically accomplished within one hour and is characterized by a very high energy demand and the release of CO2 from the carbonated raw materials. For a glass batch, the individual reactions involved are the following ones:
1 Physical melting of salt‐like raw materials, Na2CO3, Na2SO4, NaNO3, NaOH, NaCl, etc.
2 Evaporation of batch and hydrate water in the temperature range 100–600 °C.
3 Decomposition of limestone and dolomite:
4 Formation of a double carbonate: Na2CO3 + CaCO3 → Na2Ca(CO3)2 near 785 °C; in conventional batches, this is a side reaction of only minor importance.
5 Formation of silicate melts. These reactions assume noticeable turnover rates only after actual melting of soda ash:
Formation of ternary melts (eutectic NS–NS2–N2CS3: 821 °C, eutectic
1 Sulphate coal reaction: Na2SO4 + 4CO → Na2S + 4CO2 at approx. 900 °C.Figure 3 Liquidus temperatures and viscosities (dPa·s) of primary melts formed during the early stages of batch melting. (a) Binary salt‐like melts formed from soda ash and another compound. (b) Primary oxide melts in the systems Na2O–SiO2, Na2B4O7–SiO2, and B2O3–SiO2. Invariant points indicated by open circles.
2 Reactions with cullet. Although it appears as a neutral post in the energy balance, cullet vigorously react with soda ash; very fine cullet compete with sand for soda ash, delaying sand dissolution.
The liquidus temperatures and viscosities of the primary melts forming during the early stages of batch melting are plotted in Figure 3 in the cases of binary salt‐like melts formed from soda ash and another compound (Figure 3a) and of the systems Na2O–SiO2, Na2B4O7–SiO2, and B2O3–SiO2 (Figure 3b).
Owing to their extremely low viscosities, the salt‐like primary melts play an important role in bringing about high turnover rates during batch melting. When they are lacking, as in alkali and boron‐free continuous fiber glasses, the products in contrast remain in a granular state until they reach their lowest eutectic temperature (compare with Figure 6b in Chapter 6.1).
These different stages of early batch melting are sketched in Figure 4 for a soda lime silicate glass batch. Before a soda‐ash melt forms, the batch remains in a granular state (Figure 4a). Then, a primary salt‐like, low‐viscosity melt rapidly spreads throughout the batch, thereby wetting the solid grains (Figure 4b). This stage is characterized by a large ratio between the liquid interface and the melt volume. It is predominantly at this time that diffusion paths for oxygen exchange are short and that the surrounding atmosphere effectively interacts with the melt to determine its final redox state.
Upon further melting (Figure 4c, d), the melt becomes increasingly viscous and the ratio of liquid interface to melt volume decreases. In a real batch heap, stages (a) to (d) proceed longitudinally along the L axis (see Figure 2) and vertically from the outside to the inside of the batch. The batch melts from both its top and bottom side, typically in an almost symmetrical way as the heat fluxes from above and below are of the same order of magnitude. The release of gases is in contrast asymmetric since those from the upper parts readily escape whereas those coming from the lower parts remain trapped below the batch. In a successful primary melting process, the majority of solids are digested, only a minor part being released to the rough melt. This requires good batch mixing and a well‐balanced granulometry of the raw materials. The issue can be tested at the lab scale by so‐called batch‐free time crucible tests. In these simple tests, batch samples of 50–100 g are exposed to a laboratory furnace at 1400 °C and the progress of melting is inspected visually after a given time.
Figure 4 Early stages of batch melting, manually sketched after the scanning electron microscopy micrograph. (a) Open‐pore stage with granular solids and gas, the gas composition being dominated by the equilibrium between CO2 and O2 from trapped air and the furnace atmosphere. (b) Closed‐pore stage with the development of a widespread primary liquid, a large ratio s of effective liquid interface (solid/liquid and solid/gas) and liquid volume, and a gas composition dominated by CO2, redox active materials, and polyvalent ions in the primary melt. (c) Reaction‐foam stage characterized by large volumes of granular solids, bubbles, and melt, and by progressive melting of solids and decreasing s ratios. (d) Rough‐melt stage, the melt being the predominant phase coexisting with considerable amounts of bubbles and undissolved grains and showing on top a seam of the primary foam formed.
4.3
