Thermal diffusivity of soda–lime–silica powder batch and briquettes

The efficiency of heat transfer through glass batch is an important factor affecting the energy efficiency of glass manufacturing and product quality. In this study, we investigated the effective thermal diffusivity and heat flux through a batch blanket of powder raw materials and briquettes, by using a large scale top heated laboratory furnace. Particular focus was placed on the influence of raw material grain size and briquette size on the effective thermal diffusivity. The results obtained for both fine and coarse powder batches indicate that the effective thermal diffusivities are relatively unaffected by raw material grain size, and thus are not influenced by the reaction pathway and initial melt phase formation. On the other hand, analysis of the heat flux derived from conduction heat transfer (Q_c) and heat flux passing through cavities between briquettes (Q_cav) revealed that the briquette size has a strong influence on the effective thermal diffusivity of the batch blanket. The thermal diffusivity observed for flake-type briquettes (2–5·6 mm) is the same as for powder raw materials. The likely reason is that the contact heat resistance increases when many small briquettes are stacked in the batch blanket, and thus the conduction heat transfer is decreased. Clearly, for small briquettes of coarse raw materials (18·6 mm × 13·5 mm × 3·7 mm), the main factor for improving the thermal diffusivity was improving the conduction heat transfer. Additionally, for large briquettes of coarse raw materials (38 mm × 38 mm × 21 mm), the thermal diffusivity can be improved via both conduction and radiant heat transfers through the cavities between briquettes. The results clarified that both Q_c and Qcav can be effectively tuned by adjusting the briquettes size. In addition, the meltable amount per unit area may be increased compared to the powder raw material, by melting the briquettes with a batch blanket of appropriate thickness.