Thermal Denitration of Ammonium Nitrate Solution in a Fluidized-Bed Reactor

Ammonium diuranate (ADU) filtrate, which contains mainly ammonium nitrate (80-100 g/L), is generated during hydrometallurgical processing of uranium. This filtrate stream poses a disposal problem because of its high nitrate content and residual radioactivity. Fluidized-bed thermal denitration is considered as a suitable chemical-free disposal option for the aqueous waste nitrate stream. Hence, investigations to explore the decomposition of ammonium nitrate in a fluidized bed have been carried out. To enable theoretical analysis and performance evaluation of the process, a mathematical model was developed. The model is based on two-phase theory of a bubbling fluidized bed. Model calculations were used to predict the axial concentration profile of ammonium nitrate in the emulsion and bubble phases and the axial temperature profiles of gas bubbles, emulsion gas, and emulsion particles. The mechanism of decomposition of ammonium nitrate in a fluidized bed was explored, and the conversion of ammonium nitrate was estimated. Model predictions were compared with experimental data available from a bench-scale plant. Good agreement was obtained between the model predictions and the experimental measurements. A steady-state parametric study indicated that conversion is enhanced with an increase in bed temperature and feed concentration. It was found that operation at higher feed concentration leads to local hot spots. The required reaction-zone length for complete conversion of ammonium nitrate vapor in the emulsion phase was found to decrease significantly with increased bed temperature. No marked effect of u/u(mf) on conversion was observed. Optimum values of process parameters to maximize the conversion were derived.