#### Two types of charge motion can be described

N c = / D d Where N c = critical speed rev/min D = mill diameter m D = rod/ball diameter m Mills are driven at speeds of 50 90% of critical speed High speeds = coarse grinding Rod Mill Generally run in open circuit since it produces relatively controlled size reduction Reduction ratio 15 20 1 50mm to 300micron length/diameter Rod diameter 25 150 mm