The mixing of solid alternative fuel particles in cement raw materials was studied experimentally by visual observation in a pilot scale rotary kiln. Fuel particles were placed on top of the raw material bed prior to the experiment. The percentage of particles visible above the bed as a function of time was evaluated with the bed predominantly in the rolling bed mode. Experiments were conducted to investigate the effects of fuel particle size and shape, fuel particle density, rotary kiln fill degree and rotational speed. Large fuel particles and low-density fuel particles appeared more on top of the bed than smaller particles and high-density fuel particles. Fuel particle dimensions and sphericity were important parameters for the percentage of visible particles. Increasing bed fill degree and/or increasing rotational speed decreased the percentage of particles visible on top of the bed. Results can be up-scaled to industrial conditions in cement rotary kilns and show that even relatively large fuel particles will predominantly be covered by raw material after less than 30 s in the rotary kiln. This affects the heating and combustion mechanisms for the fuel particles. (C) 2011 Elsevier B.V. All rights reserved.