An optimising design method is presented for designing closed grinding circuits to meet the requirements for the particulate product expressed as a combination of the mean value and standard deviation of the particle size. The grinding circuit is modelled by a discrete mathematical model describing the axial mixing and breakage of particles in the mill and a possible time delay in the recycle line. The design problem is formulated as a nonlinear programming task in which the convective flow velocity and the axial dispersion coefficient of the ground material are selected as design variables. The method proved to be a useful tool for optimising design of grinding systems, and can be applied as a decision aid for process engineers to improve the efficiency of processes and maintaining the quality required for the particular products. (C) 2004 Elsevier B.V. All rights reserved.