Experimental and theoretical studies on the combustion of waste zirconium sponge in a rotary kiln simulator

Previous experimental studies have indicated that rotary kilns may be suitable combustion systems to incinerate small quantities of off grade zirconium sponge produced during the manufacturing of zirconium for the nuclear industry. This paper describes a mathematical model of zirconium sponge combustion in a rotary kiln environment and specifically examines the use of the bed submodel to analyze detailed zirconium combustion data obtained previously in a rotary kiln simulator. The results of this analysis indicated that the experimentally observed burning rates could be predicted within 25% based on available transport correlations and current understanding of zirconium combustion. Without any adjustment of the physical parameters in the model, both the experimental results and the model predictions indicated that the primary combustion parameters are the bulk oxygen concentration within the kiln and the local kiln bed temperature. Charge size was found to be less significant. A detailed analysis of the theoretical predictions indicates that the zirconnium sponge oxidation rate is controlled by three sequential processes (the convection of oxygen from the bulk gas to the top of the bed, the diffusion of oxygen through the bed to the particle surface, and the diffusion of oxygen through previously formed zirconium dioxide product to the unreacted zirconium metal). Under conditions typical of commercial rotary kiln operation all three of these resistances appear to be significant. Both the experimental data and the model suggest that intrinsic chemical kinetics are fast and not controlling except during the first few minutes after the zirconium is charged. The model assumes that the zirconium oxide product layer increases until it reaches a maximum value after which it remains constant due to continuous formation and abrasion.