Modelling fine grinding in a fluidized bed opposed jet mill Part II: Continuous grinding

An overall model of a continuous grinding process is proposed which combines experimentally-determined breakage kinetics, a model of solids flow in the active zone and the grade efficiency curve of the classifier. This model is developed and tested for an Alpine 100 AFG fluidised bed opposed jet air mill with an integral classifier. The classifier appears to have a predominant role with respect to the product quality and the efficiency of the process but the existence of a draw off results in a dead flux of particles less than 4 mu m. The solids flow in the mill is represented by a model consisting of two zones: a perfectly mixed zone where grinding takes place, followed by a plug flow transport zone where no grinding takes place. The single adjustable parameter model appears to give a good representation of the change in particle size distribution in the process, expressed in terms of the residual fraction (a concept introduced in part I). The a posteriori calculation of the space time in the grinding zone supports the value used for the time range used in the batch grinding kinetics.