CHARACTER OF THE DISCHARGE OPENING
The diameter of the discharge opening affects the capacity of a tube mill by limiting the amount of pulp in the mill to the lower level of the opening and by limiting the load of pebbles to the same level unless a screen or grating is used at the discharge end to keep the pebbles in the mill. The position of the discharge opening or the particular point in the discharge end at which the ore is allowed to escape from the mill affects the capacity by fixing the amount of pulp in the mill and the speed with which it runs through the mill. Suppose the mill has a 6-in. discharge pipe at the center of the discharge end, the amount of pulp leaving the mill will be determined by the difference in level of the pulp at the feed and discharge ends and its fluidity. Now suppose we put a grating at the discharge end (closed at the center if desired) and discharge at the periphery of the mill, the "head" of pulp in the mill will be increased and the discharge will be at a maximum. Suppose now we retain the grating but enclose the discharge end leaving a 6-in. pipe in the center for the discharge and arranging a series of lifters or elevators between the grating and the end, the discharge will still be at a maximum with the advantage that any degree of dilution can be maintained in the mill. This idea is incorporated in some of the latest mills. It was thought at one time that the ore should be allowed to remain in the mill until finally ground to the desired mesh and a high pulp level was maintained. It is now considered advisable to pass the ore through the mill as quickly as possible by keeping a low pulp level and return the oversize for further grinding. The low pulp level assists grinding by allowing the pebbles or balls to fall with less resistance from the pulp and therefore the force of the blow delivered is greater.
Gratings are used in tube mills at or near the discharge end so that the pebbles may be carried above the center thus giving increased grinding efficiency. The mesh varies from 1/2 to 3/4 in., which allows pebbles smaller than this size to escape from the mill. As a matter of experience when pebbles have been worn to 1 in. in diameter, more or less, they suddenly disappear, being ground so as to be undistinguishable from the ore. When a tube mill is emptied of its contents, one seldom sees small pebbles.
Fig. 36 illustrates the grating used at the discharge ends of the tube mills at the Goldfield Consolidated mill made of cast iron, but preferably made of manganese steel.
The peripheral discharge is incorporated in the Marcy mill, Fig. 37, which shows the passage of the pulp through the mill and the level at which the pulp remains by reason of the radial ribs or lifters which removes the pulp as soon as it enters the chamber at the end of the mill.
Chalmers and Williams have added to their tube mills an adjustable quick discharge device, Fig. 38, which consists of a perforated plate, interposed between the end of the tube mill and the body of the mill. Between the perforated plate and the end of the mill are radial blades which are used practically as elevators. The blades are arranged on hinges, so that by changing their inclination with respect to the tube mill end, the space between the perforated plate and the mill end may be more or less closed and the amount of pulp elevated greater or less.
The following tests at the Gold Hunter mill, Idaho, show the value of the adjustable quick discharge fitted to a 5 by 14-ft. tube mill; test No. 1 being for the mill without the discharge in use and test No. 2 being for the mill using the adjustable quick discharge.
