The continuous production of 3 to 4.5 mm NPK-granules is achieved by spraying an appropriate mixture onto recycle or reflux (undersize) product in a rotating drum. Hot air is used to evaporate the water. An extensive study was performed on an industrial granulator to evaluate mass and energy balances, to define the average residence time of the reflux-particles and to develop a model for particle growth. The average residence time was measured from tracer experiments. The mechanism of granulation follows the particle growth principle of drying and layering. Model equations developed by Nienow for a batch fluidized bed granulation were modified to predict continuous operations. The model equation defines the thickness of the coating layer in terms of the initial particle size and mass flow rate of the reflux particles, the mass flow rate of the sprayed mixture, and both the average residence time of the particles and their contact time with the spray. The resulting equation is 3A/d(p0) + 12A(2)/2A(p0)(2) + 12A(3)/3d(p0)(3) =30M(1-epsilon)t(c)/R tau Predictions using the model equations were compared with experimental findings. An acceptable degree of fitting was obtained for all operating conditions, although the model underestimates the growth rate of smaller particles. The use of the model is further illustrated to predict the effects from variations in reflux. granulometry, rotational speed of the drum or increased throughput. (c) 2006 Elsevier B.V. All rights reserved.