dewatering bins, together with the fine jig concentrate, and the middling is sent to the 10 by 4-ft. Hardinge mills. The overflow from the Anaconda classifiers is sent to the slime thickener division, consisting of 28 by 3ft. Dorr tanks. The spigot product from these tanks is divided; about onehalf is returned to the section and the remainder is sent to the slime plant. The product from the Hardinge mills (Fig. 2) is treated in six simplex

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FIG. 4.-SPOUT FOR FEEDING PEBBLES OR BALLS INTO HARDINGE OR TUBE MILLS.

Dorr classifiers-one classifier to each mill-the overflow going to the flotation division and the classifier sand being returned to the mill.

At the time it was first decided to remodel the concentrator, it was not definitely known whether pebbles or steel balls would be used for grinding. To provide for this uncertainty a compromise was effected. The mills were made 10 by 4 ft. and built sufficiently strong for steel balls in case balls were used. Each mill was equipped with a 225-hp. motor directly connected through a flexible coupling. The mill filled

For description of this thickener plant, see Trans., xlix, p. 470 (1914).

with pebbles takes from 95 to 115 hp. to operate. In case steel balls were used it was planned to put in a false wood lining back of the steel lining in the cylindrical part of the mill to reduce the effective diameter of the mill. This latter plan was finally adopted, and the Hardinge mills will be equipped with the false wood lining, 15 in. thick, in the cylindrical part of the mill, and a Cascade steel lining. With this form of lining, the mill is virtually 72 by 6 ft. and requires about 225 hp. when loaded with steel balls.

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The drawing (Fig. 3) gives the details of the lining. This lining was designed by the American Manganese Steel Co. At first the pebbles, and later the balls, were fed to the mills through the feed scoop. This method of introducing the grinding medium into the mill gave considerable trouble, due to the breaking of the feed boxes caused by the jamming of a pebble or ball between the revolving scoop and the feed box. We tried to obviate this difficulty by various changes in the amount of clearance left between the scoop and the box, but without success. In our particular case this trouble was aggravated by the fact that we had to use 7 ft. diameter scoops, in order to lift back into the mill the sand discharged by the Dorr classifier. Finally a method was tried of feeding the pebbles, or balls through a spout passing through the center of the feed scoop. This device has worked splendidly and all of our mills have since been equipped with it (see Fig. 4).

FLOTATION DIVISION

The flotation division consists of four Minerals Separation machines, each having 15 agitators 3 ft. square, and 14 spitzkästen or floating compartments. The agitators for the Minerals Separation machines are of gun metal and are driven by bevel gears from a line shaft, the direction of rotation of the agitators alternating.

The machines are made of California red wood; the agitator boxes are further lined with hard maple extending about 18 in. from the bottom of the box.

Each machine has an individual drive, power being supplied to the line shaft by a 150-hp. motor running at 385 r.p.m. The speed of the agitators is 225 r.p.m. and as the impellers are 18 in. in diameter the peripheral speed is about 1,060 ft. per minute.

Each machine makes three products, a concentrate, which goes to the dewatering division, a middling which is returned to the head of the machine, and a tailing which goes to waste. The concentrate is taken from the first three to five spitzkästen and the middling from the last nine to eleven. A portion of the pulp is overflowed from the last three

4 A detailed description of these mills, together with grinding data and Dorr classifier data, will be published in a subsequent paper.

spitzkästen together with the froth. About 6 to 8 lb. of 50°Bé. H2SO4 per ton of flotation feed is used together with 2 to 3 lb. of kerosene sludge acid and 1⁄2 to 1 lb. of crude wood creosote. A portion of the wood creosote is added ahead of the Hardinge mill (about 0.03 to 0.05 lb. per ton of feed) and the remainder is added in the sixth agitating compartment. The sulphuric acid and sludge acid are added at the head of the machine. The pulp is heated to from 60° to 70°F., by passing live steam into it at the head of the machine. Three machines are used for treating sand and the fourth for treating current slime from the upper portion of the mill. Each machine has a capacity of about 400 tons per day on sand and 175 tons on slime.

The method of adding the oil and acid is rather unique. The mechanism consists of a revolving disk to which are attached, around the circumference, a number of cups. This disk is set vertically so that its lower edge dips into a pan of acid or oil. As the cups come around they are filled and later discharge their contents into a suitable launder leading to the flotation machine. The disk is driven by the friction of a wheel against another disk attached to the main drive. The wheel is run at constant speed and by varying the point of contact between wheel and disk any speed desired can be given to the main disk and thus the amount of oil or acid added can be regulated. In addition to the speed regulation, the amount of oil or acid fed may be varied by adding or removing cups

or by changing the size of the cups. A photograph of the machine is shown in Fig. 5.

At present (December, 1915) seven sections are operating on the new flowsheet, and the whole mill will be remodeled not later than Jan. 15, 1916. The sections are being remodeled one at a time. All the work is being done by the company's engineering force. Things have been so well organized and systematized that it requires less than 30 days to tear out the old section and install the new equipment, ready for operation.

The following table gives metallurgical data concerning the operation of the mill. Line 1 gives the monthly assay of second-class ore for October, which is the feed to the mill. Line 2 gives the monthly assay of the flotation tailing from the remodeled sections for October. The feed to the flotation machines during the month consisted of reground sand tailing and a portion of the thickened mill slime which was returned to the mill and treated in the fourth machine in each remodeled section. The tailing assay shown below, 0.13 per cent. Cu, is for the total sand and slime tailing produced. The sand tailing alone averaged 0.10 per cent. Cu and the slime tailing averaged about 0.25 per cent. Cu.

No complete analyses of the flotation concentrate are available, as the slime flotation plant is not in operation yet and the round-table concentrate is being mixed with the flotation concentrate from the mill. Following is an estimate of the power consumption per 2,000-ton section, including its proportion of slime treatment.