The Hardinge conical mill
This is rather a summary way of disposing of the Hardinge conical mill as a slimer but we know from experience that its sphere of usefulness does not go down so far in the scale as 200-mesh and we must look to the cylindrical tube mill for this work.
The accompanying table of tube mill data of the Homestake mill, South Dakota, compiled from earlier records, when none of the tube mill discharge was returned for further grinding, is interesting as containing a comparison of the work of different size mills. It will be noted that a 5 by 14-ft. tube mill has about the same capacity as a 6 by 6-ft. Hardinge conical mill and consumes slightly less horsepower per ton of ore ground to 100- mesh. 2 Each mill discharges its tailing over an amalgam table, and about 40 cts. is recovered per ton ground.
An interesting comparison may be made between a cylindrical tube mill and a conical mill of the same length and major diameter in the case of a discarded 7 by 12-ft. cylindrical mill at the Morning mill of the Federal Mining & Smelting Co., Idaho, which was so changed as to make it into a conical mill. The transition was effected by bolting heavy timbers inside the tube mill, forming a cone. Over the timbers steel rails were spiked or bolted to form a lining which resembled the El Oro ribbed type. The capacity of the mill before the change was 98.64 tons per day, while afterward it was 88.2, a reduction of about 10.5 per cent. The horsepower required before the change was 75.6, and afterward, 65, a saving of about 14 per cent. The revolutions of the mill was not changed, remaining at 22J^ r.p.m. in each case. Moisture in feed before transition was 58.9 per cent, and after, 57.7 per cent.
The following screen analysis shows the work done by the mill both before and after the transition.
Comparing the work done by the two forms of mills we see that the cylindrical mill produced 57.5 per cent. 200 while the conical mill produces 41.20 per cent, through the same mesh. Using the method of comparison explained in another chapter under the heading of " Crushing Efficiencies," we find that the amount of work done before the change is in proportion to that done after the change as 1.3 to 1. If we knew the amount of grinding space occupied by the cone, it might be found that the amount or work done in each case was proportional to the amount of space actually occupied by the pebbles. Considering alone the power factor, that for the cylindrical mill is 1.3 while that for the conical is 1.35. The author states that the power required before the change may be underestimated so that there may not be that difference between the power factors here indicated.
