Einfluß der Prozeßbedingungen auf die Verdampfung bei Kalk-Natron- und Borosilicatgläsern

The evaporation of sodium and boron species from the melts in industrial glass furnaces leads to emissions of particulates (dust) and to furnace atmospheres containing reactive evaporation products. These reactive species, especially alkali vapors, can react with the superstructure refractories causing attack on or corrosion of these materials. The vapors form condensation products during the cooling process in the flue gas channels, recuperators or in regenerators, causing deposition of salt. In some cases, this deposition leads to reduced heat transfer in these heat exchangers or blockage of the flue gas channels. The evaporation processes at the glass melt surface include: - direct evaporation of glass components; - evaporation of compounds formed by a reaction of different glass components in the melt; - evaporation of compounds formed by a reaction of a glass component with gases in the furnace atmosphere. In the last case, the composition of the furnace atmosphere will influence the vapor pressures of the volatilized compounds. Diffusion of gases from the melt into bubbles or evaporation into gas bubbles may take place, especially during the primary fining process. This hardly contributes to the total evaporation losses of boron and alkali species, but it is essential for the SO2 release from the melt during fining. Under industrial conditions, gas flows above the melt may be high. Experimental investigations show that evaporation kinetics are enhanced by higher gas flows especially for evaporating components having high concentration levels in the melt. For minor but volatile components in the melt, diffusion from the bulk of the melt to the surface may limit the evaporation rates, depending on diffusion coefficient and convection flows. Evaporation is drastically enhanced by high glass melt surface temperatures, high gas velocities and high activity coefficient values of components in the melt. The presence of both boron and alkali in melt gives high activity coefficient values for alkali borates in the melt, which leads to high vapor pressures of these compounds. Increasing water vapor pressures will enhance evaporation of HBO2 and NaOH or KOH but will hardly have an impact on KBO2 or NaBO2 evaporation. This means that high water vapor pressures in the furnace atmosphere, as encountered with oxygen firing, will increase NaOH, KOH or HBO2 vapor pressures, however for sodium borosilicate glasses, NaBO2 is the major evaporating species. NaBO2 evaporation is not directly influenced by water vapor. In oxygen-fired furnaces combustion gas volume flows are much less than in air-fired furnaces, the lower gas velocities will suppress the evaporation process. Initially, a high evaporation rate may lead to depletion of volatile components at the surface of the melt. This depletion will slow down the evaporation process from stagnant melts as time proceeds. Thus there may be compensating effects for NaOH, KOH and HBO2 evaporation, but for NaBO2 or KBO2 volatilization, oxygen firing will lead to lower specific (per unit quantity of molten glass) evaporation losses of these components.