The Santa Gertrudis mill

At the Santa Gertrudis mill, Mexico, tests were recently made comparing the use of cast-iron balls in the tube mills with the mine rock generally used. " Results thus far obtained indicate a capacity increase of 33 per cent, with finer grinding. Power load shows an increase of 33 per cent., from 65 to 90 hp. per mill. Forged steel balls ordinarily used for such grinding were not obtainable, but it is probable that chilled cast-iron or semi-steel balls and liners will prove more economical taking into account the low cost of locally made castings (2.5 cents per Ib.) as against high first cost plus importation expense of steel balls. Ball wear is 1.7 Ib. per ton milled.

H. E. West, in the Mining Journal, July 31, 1909, gives a series of figures showing the economy of using mine rock instead of imported pebbles in the tube mills at El Oro, Mexico. Using flint pebbles, the consumption was about 8 tons per month for Krupp No. 3, 14 tons for No. 4, and 20 tons for No. 5. With the nine mills in operation this is equivalent to 90 tons per month. At present, with mine rock 40 tons per day is consumed, or 1,200 tons per month (100 tons are maintained in the mills varying from 8 to 16 tons in quartz) ; in other words, the amount of mine rock used is 13 times by weight the amount of imported flints. This is roughly 5 per cent, of the monthly mill run. The pebbles cost 60 pesos per ton, or 5,400 pesos per month. The mine rock averages $6 gold per ton, the increase in the bullion return contributed by the crushed rock being about $6,000 gold per month. Allowing the cost of mining and handling the rock to equal the value of the gold extracted, it is apparent that the cost of the pebbles, or 5,400 pesos is saved per month.

Manganese or chrome-steel balls offer the best substitute for flint pebbles and from present indications the art of tube milling will in the future be more influenced by the use of this kind of grinder than from any other innovation. Steel being from two and onehalf to three times the weight of flint, a mill, to hold the same load of balls, must be made of greater massiveness and strength to withstand this increased weight. If a 4-in. flint pebble with a 5-ft. diameter mill has been found to be an effective grinder for a given ore, we may with the same diameter mill use a 1-in. steel ball with the same effective pressure between the ball surfaces, with this difference that the larger surfaces will retain larger pieces of ore to be ground, so that theoretically a finer product must be fed the mill; this, however, is offset to a great extent by the increased amount of grinding surface by reason of the greater number of balls of lesser diameter. Just how far practice will agree with theory and at what point the compromise between diameter, area and weight will be reached can be indicated only very roughly at present. With the increased weight of tube mills due to the use of steel grinders we may expect a decreased diameter and length, that is, instead of our mills being increased in diameter to 8 or 10 ft. we may expect them to remain at 4 to 5 ft. That a mill of this character will be used to any extent, where the cyanide process is a necessary feature, is doubtful on account of the amount of undesirable fine iron or steel added to the pulp, but for other grinding schemes this objection may not exist. In several places this transition of pebble to ball mill is taking place. Mr. Seeber informs me that at the Winona mill, Michigan, by the use of manganoid-steel balls (and liners) in place of pebbles a greater power ratio has been obtained and the mill capacity increased. He uses a mixture of %-in., IJ^-in., 2-in., 2^-in. and 3-in. diameter balls. The increased capacity has amounted to approximately 15 per cent, with a consumption of 125 to 150 Ib. of steel balls instead of 250 to 300 Ib. of pebbles.