The mixing phenomena in bulk solid mixers or in other powder handling devices can generally be characterised as stochastic. This stochastic nature emerging during the flow and mixing of bulk solids may highly influence the dynamic behaviour of the process, as well as the results of the reliability of the involved operations. The uncertainty of the state characteristics and results of this type of processes is obvious especially if the stochastic flow phenomena within the particle bed extend to large scales. Earlier experimental and theoretical investigations have pointed out, that it is necessary and important to take into consideration this stochastic nature. For this purpose, a new stochastic model based on cell-decomposition was established. The results are illustrated by some examples.