The Marcy mill
The following description of the Marcy mill may be of interest :
" The Marcy ball mill is of the drum type, equipped with the usual spiral feeder and loaded with pebbles or balls as may be required. The drum is divided into two parts the crushing compartment, equipped with lifters to insure the elevation and dropping of the balls to secure maximum crushing efficiency, and a small compartment at the end of the mill, separated from the crushing space by means of a manganesesteel plate and having perforations about % in. wide. In this end compartment there are radial screens and lifters, the lifter forming the bottom of a box the top of which is formed by the screen itself. These are the finishing screens, and material which will pass through them is considered as being of finished size. The material passing through the screens is diverted by arrangement of the compartment so that it will be discharged through the central orifice; while the oversize material which remains on the screen is diverted back into the mill through a proper opening. The screens and lifters form a box, as has been stated, and are placed so that they may be removed from the outside of the mill for rearrangement or cleaning. In order to give the screens maximum efficiency and to increase their life, an arrangement of pipe is made so that water or solution under head can be thrown against them while the screen is passing through the upper half of the circle formed by a cross-section of the mill.
" The action of the lifters, in addition to forming the bottom of the box and assisting in the screening operation, is to maintain a low pulp level in the mill. It is clear that, by constantly removing the solution and pulp from the bottom of the crushing drum, a removal of the material already sufficiently finely ground is accomplished rapidly and no energy is lost in doing work where it is not required. The coarser material is left in a comparatively dry state upon the balls or pebbles, and the crushing and grinding action of the mill can have maximum effect upon it. The accompanying drawing shows a longitudinal view and partial crosssection of the mill, giving a clear idea of its construction and operation."
These illustrations of ball mills are intended to show the relation between the ball and the tube mill of which latter we have two types, the cylindrical tube mill of various lengths and diameters and the Hardinge conical mill. As this latter type of mill has at times been put forward as a competitor of the long cylindrical mill for sliming ores and as our object is to consider the tube mill as an adjunct of the intermediate crusher for further reducing ore, we must either accept or reject the use of the conical mill for this specific purpose.
