AbstractAn expression is developed for the passage of material through rotating cylindrical kilns or driers, relating the time of passage to the other important variables—slope, length, diameter, and rate of rotation of the kiln, feed rate and rill angle of the material. The expression is derived from fundamentals, and is in accord with previously published empirical relations. It is thought that the derivation will promote a better understanding of the mechanism of the movement of materials in rotary kilns and drier