The separation density of a dense media comprising a slurry of magnetite and water in a cyclone for beneficiating coal containing refuse is maintained within predetermined limits by: (a) measuring the total solids in an overflow of the cyclone; (b) comparing the total solids with a predetermined correlation with separation density; and (c) changing the relative proportions of the feed flow of magnetite and water to the cyclone when the separation density as determined by the correlation is outside the predetermined limits.