In rod mills the movement of the comparatively rigid media involves a certain amount of rolling and cascading which doesn’t resemble the action of a loosely projected ball path. Similar rod action in mills of different diameters my occur at varying percentages of critical speed. Consequently, it has proven best to describe the speed instead of referring to their percentage of critical speed as is done for ball mills of the same diameter. Marcy low pulp level rod mills show best efficiency between a peripheral speed of 400 and 550 feet per minute depending on diameter. Marcy engineers have determined the best mill speeds from practice and have fitted a curve of speed vs. diameter with an equation enabling them to recommend the best speed for any mill design. The two graphs on page 18 show various mill speeds on page 18 show various mill speeds as a function of mill diameter for rod mils and ball mills.
Above are there illustrations showing the action of balls in a mill at different speeds. The action at medium speeds is generally most desirable and efficient for grate and ball mills.
