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As the name implies, material is discharged from an overflow mill by overflowing the trunnion opening. In this type of mill the balls are kept in the mill, by keeping the top of the ball charge below the level of the trunnion opening. When operating an overflow mill with a higher charge escaping balls are returned to the mill by a reverse spiral in the trunnion liner or are restrained by a ball retaining grate.
As can be seen from the tables on pages 10-11 and 14-15 grate mill will draw more power and produce more product than its overflow counterpart of the same dimensions. Available data showed the grate mill grids at less power per ton of o re than the same sized overflow mill.
Thus savings on power consumption are available by selecting the grate design. The grate mill by virtue of its steeper pulp gradient has a more rapid turnover of material than its overflow counterpart. Material ground to the appropriate size is removed quickly and less overgrinding is apt to occur than in an equivalent size overflow mill. Thus, a grate mill is a more efficient means of grinding than an overflow mill whose higher retention time may result in overgrinding. The initial cost of a grate mill may be slightly higher than the initial cost of an equivalent size overflow mill. Therefore, in determining which type of mill to use, the economic trade off between power savings, operational benefits and initial capital outlay must be analyzed.
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