with a comparatively narrow face, so that the difference in velocity between the two ends of the roll is very small, and there is consequently little slip.

47. Modern Chilian Mill.-As has been explained in the previous article, the radial roller mills do not crush the ore primarily, but both crush and triturate, or grind, it. There are certain cases in which this triturating action is very desirable, as, for instance, in the preparation of clay material, and

FIG. 40

hence most, if not all, of the clay mills are of the Chilian mill type.

In Old Mexico and other locations where rich gold and silver ores occur in regions containing practically no water, it is often necessary to reduce the ore by the patio process, and for this purpose the material is ground in Chilian mills driven by mule power. The mills were originally of the old crude type, but at present the greater number of them are of improved construction, and many of them are similar to that illustrated in Fig. 40, which shows a plan and section of a Chilian mill provided with three. rollers. Silver ores intended for pan amalgamation may also be

ground or pulverized in Chilian mills, and a large number of millmen prefer this style of machine, even for use with gold ores which are to be amalgamated on plates outside of the machine. They have also been employed for the regrinding of middlings in concentrating works, but are not as well adapted for this purpose as the Huntington mill. The Chilian mill usually has but two crushing rolls or wheels.

48. Centrifugal Roller Mills. The second type of roller mill is termed centrifugal, because the crushing action is largely due to the centrifugal action of the rollers, which are hung so that they are free to swing outwards in a radial direction, the rapid revolution of the spindle causing the rollers to press against a hardened ring die in the side of the pan, immediately below the discharge screens. These mills are used for wet crushing and amalgamation, also for regrinding the intermediate products in concentration mills. The type is represented by the Huntington roller mill, which

will be described in detail, as its success and extensive adoption in gold metallurgy have placed it upon a high level as a crushing and amalgamating machine.

49. Huntington Mill.-The Huntington mill, shown in Fig. 41, consists of an iron pan, through the center of which passes the vertical shaft g. The arms a extend horizontally and terminate in a ring b, from which the rollers c are hung so as to swing freely in a radial direction on the yoke e, which is shown in detail in Fig. 42. also free to revolve on their own axes.

The rollers are

In front of each roller is a scraper ƒ, which keeps the ore from packing. The rollers are hung so that they clear the bottom of the pan by about an inch. The central shaft, revolving at a rate of from 45 to 75 revolutions per minute, causes the rollers to swing outwards against the ring d, the pressure varying as the square of the speed of the mill. The crushing power of the mill, being equal to the product of the pressure and the velocity, consequently

varies as the cube of the velocity; that is, twice the speed

gives eight times the crushing power.

The ore, which has been previously broken to inch in diameter or smaller, is fed into the hopper / on the side of the

pan. The rollers and scrapers throw it out to the rim of the pan, and as fast as it is crushed it passes through the discharge screens and trough shown to the left of the figure on to the amalgamated plates, not shown in the figure. The greater part of the gold, on being liberated from the gangue, sinks to the bottom of the pan, and is caught there by the quicksilver, of which there are from 17 to 25 pounds. The clearance of the rollers prevents their "flouring" the mercury, and at the same time they are close enough to keep the surface of the mercury agitated and in the best possible condition for amalgamating.

This mill is particularly adapted for the treatment of brittle sulphide ores, which under the stamps are apt to slime.

It is made in three sizes, 34, 5, and 6 feet in diameter, the second being the most commonly used. A 5-foot mill will crush from 10 to 20 tons of rock in 24 hours, through a 30-mesh screen, at an expenditure of from 10 to 12 horsepower. The first cost of the mill is considerably less than for a stamp mill of the same capacity, and the parts are fairly cheap and easily replaceable. The power per ton of ore crushed is much less than for the stamp mill, the pulp is in better condition for concentration, and the loss of mercury is minimized. Its disadvantages are that the wear on the parts is great-particularly on screens, dies, and shoesand that the corroding action of the acid in some mine water and in decomposing pyritic ores soon renders the machine. unfit for use. This latter source of trouble is also present in the stamp battery, but the great thickness of its permanent parts makes it of less moment than in the Huntington mill. Huntington mills have an especial field in the regrinding of intermediate products in concentrating mills.

50. The Kinkead Mill. This mill is simply a large automatic mortar-and-pestle arrangement. The mill is shown in Fig. 43. The upper portion a of the spindle is vertical and revolves in fixed bearings directly over the center of the pan, which is shaped as shown in the figure.