The MODSIM mineral processing plant simulator is used to find optimal combinations of target separation specific gravities for the separation units in a typical coal washing plant. The plant is set up to wash three size fractions separately in parallel streams. Two alternative flowsheets are examined: the first includes a dense-medium vessel, a dense-medium cyclone and a 2-stuge water-only cyclone, and the second includes a dense-medium vessel, a shaking table, a spiral separator and a bank of flotation cells. A simple graphical construction is used to find the optimal separation densities using data thai is generated by the simulator. Results are presented as optimal ash content - total yield curves for the combined clean coal products.