Modelling and Simulation of an Industrial Scale Continuous Fluidized Bed Roaster

Swagatika Dash, Swati Mohanty, B.K. Mishra

Fluidized beds have been widely used for roasting of zinc concentrate in metallurgical industries. The roasters are designed for a particular grade of ore and particle size. However, frequently the quality and particle size of the feed change, and it is difficult to operate the roaster at its designed throughput, resulting in loss for the plant. Design of continuous fluidized bed primarily depends on experimental data and empirical correlations. Since it may be difficult to carry out experiments in a large scale fluidized bed, numerical simulation is an effective method to study the effect of different parameters on the roaster performance. In the present work, an industrial scale continuous fluidized bed roaster has been modelled to study the flow behaviour of fine particles for different solids feed rate, as well as the conversion of ZnS to ZnO, so that the desired conversion can be obtained with a high throughput. The height of the roaster is 24.5 m, the freeboard diameter is 16.85 m and the bed diameter is 12.5 m. Due to the size and complexity of the roaster, a simplified geometry is created, which is meshed to get almost uniform sized hexagonal mesh. A multi-phase computational fluid dynamics (CFD) simulation, considering four granular phases and one gas phase has been carried out to predict the flow and concentration profile in the roaster. Based on the kinetics of the ZnS to ZnO reaction and the residence time of the solids in different sections of the roaster, the conversion in each section is calculated at 1203 K. The heat liberated and possible rise in temperature in each zone is calculated based on the heat of reaction and sensible heat of the solid and gaseous products. Simulations have been carried out with 21 vol.-% and 26 vol.-% O2 to increase the conversion of ZnS to ZnO when the solid feed rate is increased. The simulation results were validated with the plant data, operated with 21 vol.-% O2. The model can be used to increase the performance of the roaster when there is a change in feed quality.