A mechanistic model for the granulation of particulate materials with a wide size distribution in a large-scale continuous drum is presented. It takes cognizance of the effect of relevant process variables: feed size distribution, moisture content, binders such as lime, residence time distribution, feed rate, etc. The model is based on the auto-layering mechanism of granule growth and incorporates a piecewise linear model for granulation kinetics. Laboratory scale tests on a batch drum are used to provide kinetic parameters. The size-dependent residence time distribution of agglomerating mass in the continuous drum is represented by a combination of mixed and plug flow regimes operating in parallel. The model is customized for a continuous drum in an iron ore fines sintering plant. The predicted granule size distributions are in good agreement with the plant data under widely varying operating conditions. The modelling framework provides scope for modifying the individual modules for drum residence time distribution or the granulation mechanism and growth kinetics.