Effect of alumina source on ease of melting of glass batch

The selection of raw materials affects the rate of batch-to-glass conversion. In all-electric melters, foam under the batch blanket limits the heat flux from the molten glass, thus slowing the rate of melting. Our study, in which we compare the melting behaviors of three batches formulated to vitrify high-alumina high-level waste, shows that a slowly dissolving refractory component can cause excessive foaming. Faster melting batches with gibbsite [Al(OH)3] or boehmite [AlO(OH)] as an alumina source produced substantially less foaming than a batch with corundum (Al2O3). While gibbsite and boehmite dissolved below 500°C, corundum was still present in the batch up to 900°C; hence, the glass-forming melt lacked alumina in the batch with corundum. The low viscosity of that batch caused the open pores to close prematurely at 660°C, trapping gases and expanding to foam. This would explain the literature-reported slow melting rate of a batch with corundum, as compared to batches with gibbsite and boehmite.