CaF2, 4.56 per cent SiO2, and 15.70 per cent CaCO3, while the sorted screen oversize saved analyzed 70.81 per cent CaF2, 0.78 per cent SiO2, and 28.09 per cent CaCO3.

In general the method of milling hard ores used in the larger mills of the Illinois-Kentucky district embodies the following principles: The mine-run ore is fed into a revolving screen or washing trommel with a water spray to clean off the larger lumps of ore. The coarse ore drops on a picking belt from which is picked waste and No. 1 and No. 2 grades of lump ore. The ore remaining on the belt goes to one or more crushers, below which the undersize from the washing trommel joins it. This crushed ore is then fed through a series of sizing trommels, of which the coarsest has usually not larger than five-eighths-inch holes and the finest from three-sixteenths-inch to one-sixteenth-inch holes. The oversize from the coarsest screen is returned to crushing rolls for regrinding, and the undersize from the finest screen is usually sent, after partial dewatering, to concentrating tables. Other screen products are used as jig feeds. Several types of jigs are used, but the five-cell jig of the Harz type is most common.

MILL OF FRANKLIN FLUORSPAR CO.

Figure 10 is a flow sheet of a concentrating mill of the Franklin Fluorspar Co. at Marion, Ky. This mill was formerly the property of the Kentucky Fluorspar Co.

TREATMENT OF ORES CONTAINING GALENA

When ores containing galena are milled, a lead product is usually obtained from the first cell of all jigs. In the coarser sizes this product may not be pure enough for shipment; it is then recrushed and re-treated on the same jigs or on a separate lead jig. The product from the second cell of all jigs may be a lead middling product, which is recrushed and rejigged with the impure lead concentrates from the first cell.

In fluorspar jigging no attempt is usually made to treat separately the products of the screen and the hutch. The screen and hutch discharges both empty into the same launder. The products of the third, fourth, and fifth cells (and of the first and second cells when the ore contains no lead) constitute the finished gravelgrade fluorspar and are sent to the gravel-spar stock bins. The product from the first cell of the lead jig is a finished lead concentrate; from the second cell come lead middlings which are reground and rejigged; the other products of the lead jig are re

and from the lead jig is recrushed with the lead-jig middlings. product. Jig waste from the spar jigs is sent to a waste dump

and retabled, and, if there is sphalerite in the ore, a low-grade zinc

and about 12' long

+1/16 inch - 18 inch

+'/8 inch - 1/4 inch

Chute

No. 1 jig

7-cell Harz

No.1 cell Other cells

Lead Hutch & Tailings

product screen

Waste

Chute

No.2 jig 7-cell Hertz

Products
Same as No.1 jig

Fluorspar
product

Fluorspar
product

Screw conveyor

concentrating tables. From the tables are obtained finished lead and fluorspar products, tailings, middlings which are recrushed crushed in fine rolls passed through classifiers and sent to

Elevator

Dewatering box

FIGURE 10.-Flow sheet of fluorspar concentrating mill of Franklin Fluorspar Co., Marion, Ky.

This method of milling successfully separates fluorspar and galena and removes most of the calcite and silica, but will not separate satisfactorily sphalerite and barite. In the Illinois-Kentucky district barite does not occur in large enough quantity to constitute a serious problem; in fact, the largest mines are free from barite. Many attempts have been made to devise a process for separating barite and fluorspar, but none have been successful. In the central Kentucky district and at some of the western mines, such as at Wagon Wheel Gap, Colo., and Tonuco, N. Mex., barite occurs in important quantities.

MILL AT WAGON WHEEL GAP, COLO.

The aerial tramway buckets discharge on a grizzly at the mill, a general view of which is shown in Figure 11. The fines, 1 inch

FIGURE 11.-Mill of American Fluorspar Co., Wagon Wheel Gap, Colo.

and less, pass through to the mill bin and the lumps to two sorting tables, where the waste is picked out. Disposal of the jig products is variable, depending on the character of the material produced. The tables shown are only used intermittently and are not an integral part of the continuous flow sheet. (Fig. 12.) The separation of the barite has been the principal difficulty in the milling operations and so far it has not been successfully accomplished by mechanical means. Analyses of carload shipments for a number of years averaged about as follows: CaF2 86 per cent, BaSO4 5 per cent, SiO, 4.5 per cent, Al,O, 2.5 per cent, and CaCO, 1 per

cent.

3

A Dorr thickener 10 feet deep by 36 feet in diameter has been constructed but has never been used, although it was used to dewater the slimes from the mill so as not to pollute the stream. Power is supplied at the mill by a 100-horsepower Corliss-type engine and

steam is generated by two boilers, one 150 horsepower and the other 80 horsepower, both of the return tubular type. Coal is purchased from the mines of the San Luis Valley district, with a railroad haul of approximately 100 miles.

FIGURE 12.-Flow sheet of American Fluorspar Co. mill, Wagon Wheel Gap, Colo.

SEPARATION OF SPHALERITE

In parts of some mines in the Illinois-Kentucky district sphalerite or zinc sulphide occurs in considerable quantities. The ordinary methods of jigging the table concentration will not separate fluorspar

99113°-274

and sphalerite successfully. Sphalerite is an injurious impurity in fluorspar concentrates and fluorspar in sphalerite concentrates. Hence a very complete separation must be made. The only large producer that has so far been troubled with this problem is the Fairview Fluorspar & Lead Co., near Rosiclare, Ill. The zinciferous fluorspar is practically confined to one section of this mine, and the company has been stock-piling this ore for several years.

This problem has been studied at the Bureau of Mines experiment station at Rolla, and a report prepared by John Gross.8

The Fairview Fluorspar & Lead Co. (now the Franklin Fluorspar Co.) also studied this problem and finally decided to erect an experi

FIGURE 13.-Flotation mill, Fairview Fluorspar & Lead Co. (now the Franklin Fluorspar Co.), near Rosiclare, Ill.

mental plant to treat this ore by flotation on a commercial scale. This plant was completed in 1921 (see fig. 13) and a few experimental runs made when the demand for fluorspar declined sharply, and further work was suspended. The flow sheet, Figure 14, shows the general methods used in this mill, but it is not definite and final, as much experimenting was done later. Test runs indicated that a good zinc product could be made by flotation for several hours at a time, but that for some unknown reason a favorable condition could not be maintained indefinitely. This problem was solved later, and commercial zinc products were made. The tailings from flotation machines were not high enough in fluorspar to be worth saving.

8 Gross, John, Separation of sphalerite, silica, and calcite from fluorspar: Reports of Investigations, Serial 2264, Bureau of Mines, July, 1921, 3 pp.