Reactions of alkali vapours with silica based refractory in glass furnaces, thermodynamics and mass transfer

The attack of silica crowns by alkali vapours, such as NaOH, originating from the molten glass, has been investigated by thermodynamic modelling in combination with mass transfer relations and laboratory experiments. Volatile alkali compounds from the melt react with silica, forming a low viscosity alkali silicate slag. Silica loss by sodium attack is mainly determined by: (a) the chemical activity of sodium oxide in the slag and (b) mass transfer in the gas phase boundary layer adjacent to the silica surface. The chemical activity is influenced by the calcium oxide in the binding phases of the silica refractory. Severe silica loss by alkali attack at certain conditions was observed in laboratory studies and industrial glass furnaces. This corrosion process was modelled for different temperatures and for different sodium hydroxide vapour pressures in the furnace atmosphere. The NaOH vapour pressure in the combustion space of a furnace, beyond which the silica starts to react, depends strongly on temperature and on the CaO content of the silica. The most severe conditions for silica attack are: (1) relatively low silica crown temperatures; (2) high alkali vapour pressures; (3) high CaO content of the silica; (4) high gas velocities along the refractory surface.