Ventilation.-Ventilation in fluorspar mines is usually not important. No poisonous or explosive gases are found, and the natural ventilation is usually good. In a few of the larger mines a small portable fan may be used in the driving of drifts on the deeper levels a considerable distance in advance of stoping.

Lighting. In virtually all fluorspar mines in the United States. the miners use open-flame carbide lamps. In the largest mines electric lights are used in the shaft. stations, pump rooms, and main haulage ways. In the smaller mines no lighting is provided other than that from the miners' lamps.

Pumping.-Pumping is an important problem at nearly all of the shaft mines of the Illinois-Kentucky district. In the largest mines on the Rosiclare vein in Illinois, pumping is one of the largest single items of expense. The vein is near the Ohio River and not only must ordinary underground water be handled but also seepage from the river, and occasionally during floods river water enters the shafts from the top. As depth increases drainage becomes more difficult and more expensive.

Hoisting equipment.-Hoisting equipment of almost every type may be found in fluorspar mining. In the prospecting stages hoisting may be by hand windlass or horse whim. In the Kentucky field many small mines have small hoists belted to oil engines that use gasoline, kerosene, or crude oil for fuel. The oil engine seems to have nearly displaced the old donkey-engine type of steam hoist. At the largest mines large double-drum, steam-driven hoists or, at a few places, electric hoists are used.

Surface plant and equipment. At the smaller mines the surface equipment is small, usually consisting only of the hoist and perhaps an air compressor, but at the larger mines it is extensive. A few mines have large well-equipped shops, where all small repairs and many large repairs are made, and mine cars, skips, and similar equipment used about the mine are built. These mines also have carpenter shops where all mine timber is sawed and framed. All drill sharpening is done by automatic drill sharpeners.

MILLING

TYPES OF ORE AND NATURE OF IMPURITIES

The methods of concentrating or milling fluorspar depend upon the nature and amount of the associated impurities and upon the type of the ore. The impurities or associated minerals may be roughly divided into: (1) Those which have no harmful effect in the common methods of utilization and may thus be classed as diluents; and (2) impurities which are really injurious in the processes in which

fluorspar is used, and which therefore must be entirely eliminated or reduced to a very low percentage. Opinions differ as to the effect of some impurities, as noted in a discussion of this subject under "Utilization," but the following classification is accurate enough for milling purposes.

1. Harmless impurities or diluents.-Calcite or any from of calcium carbonate; silica in any form; silicates and alumina-silicates, such as feldspar, which do not contain large amounts of the metals; wall rocks, such as granite, slate, shale, and so on; clay and sand.

2. Injurious impurities.-Barite; galena; sphalerite; pyrite; all other sulphides and sulphates; all other lead and zinc minerals; all iron compounds (in glass and enamel grade spar).

Although the so-called harmless impurities act merely as diluents they are not tolerated in large quantities in merchantable fluorspar; thus the lowest marketable grade of fluorspar is ordinarily the gravel grade, which contains at least 80 per cent calcium fluoride and not over 6 per cent silica. The remainder is generally calcium carbonate. The problem of milling therefore resolves itself into elimination of all injurious impurities and raising the grade of the ore to meet the lowest specifications at least.

Successful concentration of fluorspar depends as much upon its type and physical nature as upon the impurities. In general there are three principal types of ore: (1) Residual and (or) disintegrated ore mixed with sand and clay; (2) massive crystalline ores, in which the fluorspar is easily separable from the gangue minerals; and (3) hard mixed ores in which the fluorspar is intimately associated with other vein minerals or with fragments of wall rocks (breccias).

Ores of the first type are most easily concentrated. Such ores, containing as low as 25 per cent fluorspar, may often be easily and cheaply concentrated to a high-grade product.

Massive crystalline ores may sometimes be so mined and prepared by simple hand sorting that mechanical concentration is unnecessary. If concentration is necessary, simple jigging is ordinarily ample. In the Illinois-Kentucky district, where the chief impurity is calcite, it is usually considered that the poorest grade of ore of this type which can be milled profitably at present prices must contain at least 50 per cent calcium fluoride. If the gangue was mostly silica, ores of as low grade as 50 per cent could hardly be treated economically.

When fluorspar is intimately associated with gangue minerals, particularly silica and the silicates-barite, sphalerite, and so oncommercial separation is often impossible by present known methods.

TYPES OF CONCENTRATING MILLS

FOR DISINTEGRATED OR RESIDUAL ORES

Disintegrated or residual ores mixed only with sand or clay usually need very simple treatment. Sometimes residual ores occur in large lumps, as near Cave in Rock, Ill.; more often the crystalline fluorspar of such deposits has disintegrated mechanically, forming a fluorspar gravel mixed with sand and clay. If the lumps have not disintegrated, which is rather unusual, the clay at the surface may have been removed by natural drainage so that simple hand sorting yields a commercial product.

Residual gravel fluorspar must be purified by washing. This may be done by washing in single or double log washers, washing in log washers followed by sorting on a picking belt, or log washing and jigging.

The ordinary log washer commonly used in fluorspar milling usually consists of an octagonal wooden log 12 to 18 feet long and about 12 inches in diameter, to which iron blades are attached. The log is set in a wooden trough about 2 feet deep by 2 feet wide and the whole machine inclined at an angle of 1° to 3° from the horizontal. The blades are set at an angle and distributed around the log in a spiral, with each blade 1 to 4 inches ahead of the blade next to it.

The log is usually revolved at 12 to 14 revolutions per minute, but on cleaner ores speeds as high as 40 revolutions per minute are sometimes used. The ore is fed in at the lower end, usually by hand, and a 12 to 2 inch stream of water enters at the upper end of the trough. The action of the blades thoroughly disintegrates the ore and slowly carries it to the upper or discharge end of the washer. The water aids in the disintegration and at the lower end carries off the clay and fine sand in suspension. At the upper or discharge end there is usually placed a small inclined section of screen with about one-fourth-inch holes, upon which the ore drops, allowing a certain amount of drainage. From this screen waste is also sometimes picked. Figure 8 shows a typical single-log washer.

When the ore is particularly dirty or difficult to disintegrate a double-log washer or two single logs connected in tandem may be used. A double-log washer consists of a trough about double the usual width in which two logs are set side by side. Figure 9 shows a double-log washer.

A single-log washer requires about 1% to 2 horsepower and a double-log 3 to 4 horsepower. Small steam engines were formerly used exclusively to drive log washers, but now small gasoline or kerosene engines are more common. A 5-horsepower engine provides a reserve of power and is large enough to do some additional

work about the mine. Fohs' states that at 14 revolutions per minute the 16-foot washer handles about 20 tons in 10 hours (probably crude-ore basis). For soft ores of which the solid part is practically pure fluorspar, at mines where water is plentiful, simple log washing is a very cheap and efficient method of milling. However, it has the disadvantage that fine grains of fluorspar are lost in the overflow or in the undersize from the lip screen at the discharge end. Attempts are sometimes made to save these fines by passing the overflow through settling tanks or even crude classifiers. In this way a low-grade concentrate is made which is mixed in small proportions with the clean, washed product.

If the residual ore is in fairly large lumps mixed with waste rock, clay, sand, and gravel, it may first be passed through a log washer

FIGURE 8.-Typical single-log washer, on La Rue vein near Marion, Ky.

to remove the clay and sand, then over a picking belt, where the coarse waste may be removed.

When the impurities are too fine to be removed from a picking belt and too coarse to be eliminated in the log washer, the logwasher product is sometimes sized by screening and passed over a jig. The one example of this method noted was apparently unsuccessful, for it had been abandoned and simple log washing resumed. The reason for lack of success was apparent, however, for only one jig was used and the jig feed was not properly sized.

The trough, or gravity washer, was formerly used in the western Kentucky district but is now practically abandoned. It consisted of an inclined wooden trough about 1 foot wide, 8 to 10 inches deep, and 25 to 75 feet long, set at 10° to 12° from the horizontal.

7 Fohs, F. Julius, Fluorspar Deposits of Kentucky: Kentucky Geol. Survey. Bull. 9, 1907, p. 113.

At the upper end was an inclined-bottom feed bin, from which the ore was sluiced down the trough with a 12 to 2 inch stream of water. Walk ways along the sides of the trough allowed men to assist the passage of the ore when necessary by prodding or pushing with a hoe or bar. This method was cheap, but not as successful as the log washer, especially for dirty ores; moreover, the loss in fines was greater.

FOR HARD OR MIXED ORES

The log washer is not successful for hard ores or those mixed with calcite, quartz, galena, sphalerite, and so on. For such ores a much more complicated and expensive method is usually necessary.

FIGURE 9.-Typical double-log washer, Nancy Hanks mine, west of Marion, Ky.

Very occasionally the crude ore is so pure that hand sorting yields a marketable product, but such work on a large scale has never been possible. In some of the larger deposits part of the ore could be recovered by simple hand sorting, but as the remainder must be milled a picking belt is used as part of the milling equipment and no attempt is made to select the ore underground. At the Nakaye mine in New Mexico (see p. 126), where the impurities are chiefly calcite and limestone, the larger lumps of ore and waste are separated by hand sorting and the fines are shoveled over a screen with one-half-inch openings. The screen undersize is wasted and the oversize is hand sorted. This method could not be expected to be successful, because fluorspar has a better cleavage and is more brittle than calcite or limestone, therefore the fines would contain more fluorspar than the coarse. That this is actually true is shown by the fact that the fines wasted analyzed 83.52 per cent