Modeling the kinetics of volatilization from glass melts

A model description for the evaporation kinetics from glass melts in direct contact with static atmospheres or flowing gas phases is presented. The derived models and equations are based on the solution of the second Ficks' diffusion law and quasi-steady-state mass transfer relations, taking into account changing melt/gas-phase boundary concentrations. Mass transfer relations for turbulent and laminar gas flows are given. In almost static humid atmospheres, the depletion of sodium oxide at the glass melt surface, during evaporation of sodium hydroxide from soda-lime silicate melts, appears to be moderate. Exposed to flowing gas phases, the increased mass transfer in the gaseous phase will enhance the evaporation from the melt. But at high temperatures, high gas flow rates, and reactive evaporation from a melt, exposed to high water vapor pressure levels, the diffusion processes in the melt will affect the sodium evaporation losses. Consequently sodium oxide surface concentrations of static melts may become increasingly deviating from the parent melt composition at such high evaporation rates. Measured sodium and lead evaporation rates for static soda-lime silicate and lead silicate melts are compared with the results of modeling of volatilization kinetics in gas streams. Most evaporation processes in glass furnaces can be described by the assumption of static glass melt volume elements, each exposed during a certain average time period to a furnace atmosphere with laminar or turbulent gas flow conditions.