The manufacturers of this mill furnish the following data : This is an excellent mill for fine-grinding, and a large number of them are in use for comminuting phosphate rock, copper matte, etc. They have lately been introduced for grinding iron ore for magnetic concentration with very good results. Mr. W. II. Hoffman gave the results in cinching with this mill at the Croton magnetic iron mines in a paper read before the American Institute of Mining Engineers, as follows : The screen-block openings were in. wide, and the coarsest material passing through them was less than -h in. thick, while the finest material would be rejected by a 60-mesh screen. The ore entered the mills at a temperature of about 350', under which conditions it was quite friable. and there was no difficulty in grinding 22 tons per hour with a 20-in, mill, and 16 tons in the same time with a 15-in. mill. One set of bushings will grind from 4,000 to 6,000 tons of ore, according to the depth of the chill in the bushing, the cost of each set being $16. The screen blocks for this amount of ore cost 49. At 22 tons per hour the 20-in, mill requires 04 horse-power, and the 15-in. mill 70 horse-power.
The Cyclone Pulverizer consists of a small iron cylinder, in the base of which are two small iron fans, resembling propeller screws, which are rotated in opposite directions at great velocity, creating counter currents of air of great force, which take up the particles of the material with which the machine is fed, grinding them by their impact and attrition. The velocity of the fans ranges from 1,800 to 3,000 revolutions per minute. At the upper part of the machine is a hopper into which the ore to be pulverized is delivered: connected with the hop per is an automatic device which feeds the material regularly into the machine. The degree of fineness of the grinding is regulated by an adjustable fan, which causes a draft of air through the machine. If the product is desired coarse, the draft is increased, and the product thrown into the collecting chamber before it is reduced to a powder ; if the product is desired exces sively fine, the draft is made slight, thereby allowing the material to remain longer in the machine. This machine is adapted for very fine grinding only. Mr. Axel Sahlin. in a paper read before the American Institute of Mining Engineers, October, 1801, stated that puddle slag was ground with this machine at Boonton, N. J., so that the coarsest particles would pass a 225 mesh sieve, at a cost of $1.50 per ton, the capacity of the machine being about 000 lbs. per hour.
The Frisbee-Lucop (Fig. 13) is a centrifugal roller mill in which the rolls are driven around against the inner periphery of a heavy steel ring, set vertically in a suitable iron casing. The ring is immovable ; the rolls, two in number, are held loosely in position between a pair of annular disks, or disk-plates, firmly bolted to a central arm, which latter is keyed to the shaft. Between the disks also are fastened two cylindrical drivers, one for each roll, which pushes or drives it around as the mill revolves. The revolving parts are thus the arm,
disks, drivers, and rolls, all of which have a motion uniform with the shaft ; the rolls have in addition an independent motion around their axes. The separation of the fines from the coarse is effected within the mill, either by air-blast or by suitable screens. The casing on each side of the mill is divided by a vertical screen of suitable fineness, set transversely to the shaft, Fan-blades are attached to the outside of the disks, by which the material, as fed into the mill, is distributed around the inner periphery of the ring in the path of the rolls, to insure an equal amount of work at every point of the face of the ring. Exterior to the screens on each side is a circulating fan, which causes a current of air to pass out ward through the screens, and discharges the fin ished product through chutes passing downward through the bed plate. The casing is divided hori zontally, and the upper and lower halves are held together by hinged bolts in slots cat in the flange of each section. The upper half is hinged to the lower at one side, and can be raised so as to give free access to the interior of the mill for examina tion and replacing worn parts.
A revolving screen of coarse mesh, attached to the shaft inside of the fixed screens, prevents any piece of iron or large fragments of rock from coining in contact with the fine screen, but does not prevent the finished product from passing freely. This re volving screen is unnecessary with quartz or other hard material, and is used only when clay, cement, talc, or some such material is ground, when the mill is entirely full of the rock. The material to be ground is first passed through a crusher to a size of in. or less, as may be convenient, and then fed by a chute directly on the feed shoe. The mill should be used for dry work only. The mill is built in two sizes, 24-in. and 20-in., these dimensions being the inner diameter of the ring. The 24-in. screen mill weighs 5,500 lbs. It will grind, according to its manufacturers, about 2,000 lbs. of quartz per hour to 60-mesh powder, and up to 6,000 lbs. per hour of softer material. Speed. 300 revolutions per minute, requiring from 15 to 18 horse-power, according to hardness of material ground. The 20-in, mill weighs 3,800 lbs. It has a capacity of 1,000 lbs. of hard quartz per hour to 60-mesh powder. Speed, 500 revolutions per minute, requiring about 8 horse-power. In the Frisbee-Lucop blast mills the screens arc omitted, and the pulverized material is separated from the coarse by gravity, being drawn from the mill, as generated, through pipes connected with the top of the casing, and carried to chambers prepared for it, by an exhaust fan or other method of causing a cur rent of air to pass through the mill. This mill is claimed to be especially adapted for grind ing phosphate rock and like substances.