The Elko Prince mill
At the Elko Prince mill, Nevada, the rockbreaker product is fed to a 4-ft. Marcy mill using steel balls for grinding. This in turn discharges into a 5 by 14-ft. tube mill. The mill is presumed to have a capacity of 50 tons a day.
The combinations of two tube mills or a ball mill and a tube mill in circuit with a FIG. 12. Two-stage tube mill flowsheet, classifier, of which Fig. 12 is a flowsheet, offer a solution of two-stage grinding with mills of moderate length which have the advantage of decreased weight and the possibility of using roller bearings, thus decreasing the amount of power required to rotate them. The idea embodies sound metallurgical principles and as we are liable to see many future mills designed along these lines the installation at the Nevada Packard Mines Co. Fig. 13 is of particular interest. I quote from R. Freitag, the designer and builder:
11 We have in operation a tube mill unit composed of two mills, mill No. 1 is 6 ft. in diameter and 5 ft. long. This mill takes the roll crusher feed which has been crushed to pass a 2-mesh No. 12 wire screen. This material is fed dry by means of a sheet slat apron feeder, solution being added in the feed box, and by the agitation caused by the scoop is thoroughly pulped. The discharge is delivered to a Dorr classifier from which the slime is discharged to the first Dorr thickener and the oversize to the second mill which is 6 ft. in diameter and 10 ft. long. The discharge from this mill is delivered to the same Dorr classifier for classification. I must say that this grinding unit is doing wonderful work and it has demonstrated to my satisfaction that the days of the long mills are past. The mills handle a feed of 100 tons easily and we have had a 120-ton feed on the mills without crowding them. We are grinding to 80 per cent, through 200-mesh."
The screen on the discharge end of No. 1 mill was discarded, being unnecessary.
I understand this is not a hard quartz ore but one well suited for this class of machine.
In the Alaska-Juneau mill we see the initial crushing being done in 8 by 6-ft. ball mills with forged chrome-steel balls and the final grinding in tube mills, 6 ft. diameter and 12 ft. long, which embodies this idea of stage grinding with ball and tube mills. Fig. 14 illustrates this type of short tube mill.
The mill installed by the United Gold Mines Co., near Sumpter, Oregon, late in 1916 contains two tube mills each 5X8 ft., the first taking the ore from rolls without screening, the pulp going to a Dorr classifier, while the second tube is in closed circuit with the same classifier. Thus we see the short tube mill gaining popularity. The capacity of these two tubes with a coarse feed of fairly soft ore using local stream pebbles is about 40 tons a day.
At the Santa Gertrudis "tube mill grinding is done in two stages, using 5-ft. diameter mills throughout, the primary series being 16 ft. long and the secondary series 20 ft. and 22 ft. Comparison of this system with single-stage grinding has failed to show conclusive results in its favor. A slight benefit is apparent, but insufficient to warrant a repetition of this refinement of design unless in conjunction with water concentration not required with this ore."
Had the primary series of tube mills been of greater diameter than 5 ft. and of shorter length say a 6 X 10 ft. mill delivering a coarser product, the advantage of grinding in two stages would have been more apparent . In section III we have shown that to obtain the maximum capacity from a tube mill it is necessary to feed some coarse material with the fines so in any system of two-stage grinding the first mill must either grind rather coarse, or the second mill be fed with some coarse material unless the diameters of the two mills are proportioned to the size of ore they are intended to grind.