At the Braden Copper Co. the ore is agitated with pine-tar oil in Hardinge conical mills. These mills are all run in closed circuits with the object of producing the least possible -+- 60-mesh solid in the finished pulp. The following screen analysis will show the class of work done by the Hardinge mills:

At the reconstructed Anaconda mill the ore is ground in 10 by 4-ft. Hardinge mills and 8 by 12-ft. tube mills in closed circuit with classifiers. The following screen analysis is typical of the flotation feed :

This is practically a 60-mesh product. In this case the oil is added ahead of the grinders and agitated in the tube mill. At the Lewis mill, Battle Mountain, the grinder is of the Marcy type of mill, diameter 6 ft. and 3 ft. long lined with manganesesteel step liners. The capacity of the mill is 100 tons a day to pass 100-mesh screen. The oil mixture is fed into the mill with the ore.

At the Engels Copper Mining Co., Plumas Co., California, the rolls deliver a J^-in. product to 6 by 5-ft. ball mills using forgedsteel balls 2 to 5 in. in diameter. The overflow from the classifier passes direct to the flotation machines while the oversize from the ball mills passes to a 7 by 10-ft. tube mill working in a closed circuit with a classifier. A screen analysis shows that 5 per cent, will remain on 100-mesh and 65 per cent, will pass 150-mesh screen. Half the oil is added in the tube mill. The circulating load in the tube mill circuit varies from 400 to 500 tons a day.

At the Inspiration Copper Co. the ore is ground with oil and water in 8 by 6-ft. Marcy mills to about 40-mesh screen, tonnage 425 tons per 24 hr. per ball mill. Steel ball (chrone) consumption 1.79 Ib. per ton ground. Power consumption 9.86 kilowatthr. per ton.

At Maimi, Hardinge mills are used: at the Brittania copper mine a tube mill 7 by 12 is used while at the Gold Hunter mill the grinding is done in a 5 by 14-ft. tube mill.

At the Magma Copper Co.'s plant, Arizona, a Symons 24-in. disc crusher is used for intermediate crushing. The fine grinding is accomplished by a Marcy chrome-steel ball mill and a Chalmers and Williams tube mill, the former reducing to 12-mesh and the latter to 60. The Marcy mill product goes to the tube mill to be reground, thence to the flotation system.

These few notes on oil flotation are sufficient to indicate the growing importance of the tube mill in the metallurgical treatment of base as well as precious metal-ores. It may be that the tube mill will in future take the place of the intermediate crushers as well as hold the place it fills so well at the present time, that of taking the product from the intermediate crushers and finishing it for flotation, amalgamation or cyanidation. The use of chromeor manganese-steel balls will make this possibility an accomplishment, judging by present indications.