Dissolving of baddeleyite-corundum refractories in optical glass melts

The relative transfer rates from baddeleyite-corundum (bakor) refractories into optical glass melts, of aluminium, zirconium and also iron, which has the highest content of transition metal impurities, was studied. Two bakor refractories with similar composition but differing in their mode of production were used. Tests were performed on a crown glass and two flint glasses. The cylindrical glass specimens were held in holes in the refractories for 24-96 h at 1300-1460 C, depending on the type of glass. The glass was then annealed and analysed for Al, Zr and Fe content by various spectral, proton activation and photocolorimetric methods. The changes in the Al2O3, ZrO2 and Fe2O3 concentrations in the glasses occur in a smooth manner, and the rate of increase in concentration decreases with time. The relative transfer rates of the refractory components into the glass melt could be estimated. The relative transfer rate of ZrO2 into the melt is 2-4 times lower than that of Al2O3. This is clearly due to the different corrosion resistances of the oxides in the melts. The transfer rate of Fe into the melt is not proportional to its content in the refractory, and to the transfer rate for Al2O3, but was 1.5-5 times faster than the latter. Whilst the effect of ZrO2 on the optical constants of the glass can be neglected, the accelerated transfer of iron into the melt must be considered when making high-transparency glasses in small tank furnaces. 3 refs.