Self ignition of layers of metal powder mixtures

The availability of models that predict the thermal stability and spontaneous ignition of metal powders is of great importance to the nuclear industry and powder handling systems in general. The present paper describes the application of a previously published two-dimensional axisymmetric model to the ignition characteristics of zirconium (Zr), tantalum (Ta) and Zr/Ta deposits on a hot surface. Their minimum ignition temperatures and oxidation behavior were investigated by validating the model against extensive experimental data. A nonlinear increase of the minimum ignition temperature was observed for Zr/Ta layers as a function of Zr content. The more reactive material, Zr, determines the thermal sensitivity of Zr/Ta mixtures, even when the Zr content is as low as 30 wt.%. The model successfully predicts the ignition sensitivity of such mixtures with average deviations of ± 4% for Ta loads amounting to less than 80 wt.%. Three main aspects have been highlighted by SEM observations and XRD analyses: i) the diffusion of oxygen through the upper layers is the rate limiting step of the combustion reaction, ii) mechanical stresses are important for tantalum oxide layers and modify oxygen diffusion, and iii) due to lack of oxygen at the bottom of dust deposits, nitration reactions occur.