A five-parameter model for residence time distribution of low-density particles in a bed of high-density particles in a rotary kiln is derived based on combinations of flow regimes in several sub-regions. The parameters of the model are the number of turnover stages, the cascade volume fraction, the Peclet number, the dead volume fraction in cascade region, and the cross-flow ratio. The effect of feed rate of solids, rotational speed and inclination of the kiln on the model parameters is evaluated.