The use of the concept of maximum technical efficiency as a criterion for optimum setting of a mineral separation stage is discussed. The graphical assessment of an optimum is illustrated by the use of minerals separation curves for flotation of pentlandite and chalcopyrite. For multi-stage circuits it is shown that in some circumstances an often used recycle rule requiring matching streams is in accord with optimizing criteria. However, the rule cannot always be applied, and in the case of flotation systems it may be misleading. Also for flotation, distinctions between optimum setting of a stage and optimum setting of a system are important. An attempt is made to analyze some commonly occurring flowsheet features in terms of optimizing by maximizing technical efficiency. Specifically, cleaner-scavenger operation, bulked recycle streams and open-circuit re-treat systems are assessed by reference to some operations in Australia. The assessment suggests that (within analysis limitations imposed by use of simple optimizing criteria) it may be best to stay with well-established conventional circuitry.