Local temperature distribution and primary melt formation in a melting batch heap

Abstract

The melting behavior of mass glass batches was studied in a gas-fired lab-scale furnace. Batches were adjusted to different redox numbers by additions of sulfate and coal. In each test, 4 kg of batch were charged onto a cullet melt (7 kg, 600 cm² surface area) which was pre-molten at 1200°C. The batch blankets had an initial height of approximately 5 cm. The vertical distribution of temperatures and electrical conductivides were recorded. The occurrence of primary melt was identified by a sudden increase of conductivity by three orders of magnitude. As expected, primary melt occurred latest in the inner zones of the batch. A comparison of conductivities and temperatures, however, showed that primary melt was not formed along any specific isotherm. Thus, the concept of a uniform melting temperature valid for the entire batch heap as used in several theoretical models is not confirmed. Thermal diffusivities derived from a numerical evaluation of the temperature field also displayed a sudden increase, however, at temperatures systematically higher than the temperatures of primary melt formation.