Rotary kilns are commonly employed to thermally process granular materials. While kiln rotation promotes particle mixing and heat transfer, it also leads to de-mixing through segregation of finer or denser particles, creating a "kidney" within the bed. Experience and experimental evidence indicate that rotationinduced mixing is insufficient to eliminate radial thermal gradients within the bed, but it is unclear as to whether particle segregation plays a significant role in the development of these gradients. This article presents experimental data obtained by heating sand of varying particle-size distributions within a 0.4-m ID batch rotary kiln to bed temperatures up to 775 °C. The results suggest that segregation has little influence on heat transfer within the bed and that radial thermal gradients are primarily the result of inadequate particle mixing. In order to scale up the experimental results, a mathematical model for heat transfer within the transverse plane was developed. A key variable for the model is the mixinginduced conductivity applied to the active layer, for which an empirical value is derived by fitting model predictions to the experimental data. Based on several assumptions for scaling of mixing conductivity with kiln size, model predictions for radial thermal gradients are presented for industrial-scale kilns in the 2-to 4-m ID range.

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