The original Frank Mary / Mine and Smelter patents disclosed the grate discharge principle. This mill type embodies the principle of steep pulp gradient. Quick discharge operation. A grate assembly at the discharge en d of the mill permits retention of a higher ball charge (up to 50%) than is possible in an overflow mill which has a practical limit of around 40%. Behind the grates is a lifting compartment which picks up the ground material as it passes through the grates and transports it out of the mill.

This quick removal of ground material reduces overgrinding to a minimum. The fact that all material leaving the mill must pass though the grates and in so doing must become enmeshed in the ball charge minimizes the possibility of tramp oversize material passing though the mill without being ground. In dry ball mill grinding, a grate mill should be used because dry material will not overflow as well as wet material.

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