weighed in proportions determined by batch calculations, conveyed to a mixer, and thoroughly mixed. During mixing, typically 2–3% of water is added to suppress dust formation and segregation induced either originally by transportation or subsequently by mixing. In batches containing soda ash, small amounts of this product dissolve in the water before reprecipitating on other batch grains. This process termed “impregnation” actually enhances the kinetics of batch melting.
4 The Conversion of Batch into Melt
4.1 The Basic Importance of Convection
In principle, the melting compartment (the tank) is a shallow basin whose typical dimensions (length L × width W × depth D) are 10 × 6 × 1 m3 for medium‐size container‐glass furnaces and 30 × 10 × 1.4 m3 for float‐glass furnaces. The required energy is delivered in a combustion space right above the melting compartment and transferred to the melt chiefly by black‐body radiation. Additional energy (5–20%) is delivered by direct electrical heating (boosting) of the melt.
The very melting process, i.e. steps M1–M3, takes place in one single box‐shaped compartment as sketched in Figure 2a, b. The sketches illustrate some essential features only (see Chapter 9.7 for details). Spatially, the individual process steps are separated – not in a rigorous but effective way – by two convective vortices. In furnaces with transversal flame direction (so‐called side port fired furnaces; flow pattern Figure 2a), these are mainly generated thermally by an appropriate distribution of fuel input to a series of burners arranged along the L axis above the melt. This results in two well‐developed vortices with a well‐localized hot spot at the position of maximum energy input. In furnaces with longitudinal flame direction (end port fired furnaces; flow pattern Figure 2b), the energy input via combustion along the L axis cannot be controlled at the same distinction as in a side port fired furnace. Here, the flow pattern is typically dominated by a single predominant vortex; the hot spot is shifted toward the end of the furnace. Local stabilization of the vortices is achieved by electrical heating from below, and – for the second type of furnaces – also mechanically by the action of air bubblers or by implementation of a solid barrier (a wall) at the bottom of the tank. As seen from Figure 2, the batch floats on the surface of the molten phase and melts continuously in the L direction, thereby typically covering an area whose length is about L/3, or more than 2L/3 for side and end port firing, respectively. Processes M2 (sand dissolution) and M3 (fining) then take place in the first and second vortices, respectively. Note that the first vortex conveys very hot melt right underneath the batch, helping to satisfy the very high intrinsic energy demand of melting. As for the second, it transports a very hot (hence low‐viscosity) melt along the surface area, thereby facilitating the escape of rising bubbles. This effect is especially pronounced in the pattern shown in Figure 2a while in Figure 2b, a large portion of the melt does not reach the surface at all.
Table 4 Scheme for final batch adjustment with sodium sulphate set to 4 kg/t glass for a targeted redox number R of −24.
| Raw material i | mI(i)a | mII(i)b | mIII(i)c | R(i) | R(i)·mIII(i) |
|---|---|---|---|---|---|
| kg/t glass | kg/t glass | kg/2000 kg sand | |||
| Sand | 674.28 | 674.28 | 2000.00 | ||
| Feldspar | 44.02 | 44.02 | 130.57 | ||
| Calumite | 44.02 | 44.02 | 130.57 | −0.073 | −9.53 |
| Dolomite | 123.39 | 123.39 | 365.99 | ||
| Limestone | 70.61 | 70.61 | 209.44 | ||
| Soda ash | 231.99 | 229.01 | 679.26 | ||
| Sulfate | 4.00 | 11.86 | 0.67 | 7.95 | |
| Carbon | 1.13 | 3.35 | −6.70 | −22.46 | |
| Redox number R = ∑ R i ·m III (i) | −24.04 | ||||
∑ Ri·mIII(i) matches the target value R = −24.
a Batch composition as calculated in Table 2.
b Batch composition with 4 kg of sulfate added, soda ash reduced accordingly.
c Batch composition normalized to 2000 kg of sand; amount of carbon varied until the sum.
Figure 2 Convection cells (vortices) in the melting tank of a glass furnace (vertical projection): (a) float glass furnace (side port firing), transit to the refining zone indicated by the dotted vertical line on the right‐hand side; (b) end port fired container glass furnace, transit to the refiner through a narrow opening at the lower right (the “throat”); D = depth of the
