CHOICE OF CYLINDRICAL OR CONICAL MILLS

The only conical tube mill manufactured is the Hardinge conical mill which is made of two cones joined by a cylinder. The length and diameter of the latter designates the size of the mill; for example, an 8-ft. mill with 18-in. face means that the diameter of the cylindrical portion of the mill is 8 ft. and 18 in. long. Fig. 7 illustrates a modern conical mill, which is one of a series of such mills at the Timber Butte mill, Butte, Montana, while Fig. 8 is the style of conical ball mill being installed at the Inspiration Copper Co. Note how rugged the ball mill is compared to the other. The conical mill is used extensively for regrinding ore already crushed by stamps, rolls,, etc., especially where a granular product is required for concentration, or a medium sized product for flotation.

The cylindrical tube mill illustrated in Fig. 9 is made in any diameter desired between 3 and 10 ft. and practically of any length from 4 to 24 ft., these being the limits of present-day practice.

I consider the conical mill to be most useful for grinding to 100-mesh or thereabout while the cylindrical mill, covers this degree of fineness but goes beyond. Therefore, for sliming ores to 200-mesh the cylindrical tube mill must be given preference.

For regrinding a stamp mill product to say 90-mesh the conical mill offers splendid opportunities as it is self-contained and occupies little space. We know the limitation of the stamp mill, that it will not grind economically to the finer meshes ; hence if it is desired to increase the capacity of a mill the best plan is to crush coarse in the stamps and regrind in a tube mill. If the object is to concentrate the ore, the conical mill is a good regrinding machine as it can be made to produce a granular product with little oversize. I would not have it understood that the conical mill alone would be more economical than a short cylinder of large diameter in a closed circuit with a classifier for this method of regrinding has been found both economical and efficient.

There is one point we must bear in mind that while the conical mill has been developed to a high degree of utility the cylindrical tube mill makers are only just awakening to the possibilities inherent in a machine of this character.