Fig. 9 illustrates the general form of a peb ble mill. The principal dimcnsions are as fol lows: The size of the cylinders range from 30 x 19 inches to 6 x 8 feet; the charge, taking sand as the unit of measure, ranges from 120 to 4,000 pounds; and the size of pulleys range from 24 x 4 inches to 36 x 12 inches in diam eter. These machines require 1 to 18 horse power to drive them in dry-grinding, and from one-half to nine horse-power, in wet-grinding at cylinder speeds ranging from 44 to 13 revo lutions per minute.
Fig. 10 illustrates a tube mill of the ordinary type. The general principle of grinding em ployed is similar to that of the pebble mills, but with the difference that the material to be ground in the tube mill is fed at one end, and delivered as a finished product at the other end, its fineness being regulated simply by the speed at which the material is fed into the machine. As every particle of the material must pass under the grinding action of the entire charge of pebbles, a thorough and uniform grinding is the result, and the use of sieves is unneces sary.
Fig. 11 shows a vertical cross-section of an aAbben ball mill.
Jar-mills, consisting of porcelain jars in which pebbles are rotated together with the material to be ground, are extensively used for the grinding of paints, ink and other chemical compounds. They are composed of one or more grinding jars. Fig. 12 shows a machine of the single jar type. Its outside dimensions are about 12 x 13 inches; it uses a charge of porce lain halls weighing about 22 pounds; and is capable of grinding up to 15 pounds at a charge, when running at 50 revolutions per minute.
Impact pulverizing machines are represented by the ((Max” mills and various forms of ((Ray mond) pulverizers. The principle employed is that of percussion, the working device consist ing of a vertical shaft attached to a carrier pro vided with hardened steel heaters. As the material is fed into the mill it is thrown by cen trifugal force against a hard iron plate, the particles at the same time being thrown into violent contact with each other. Equipped with vacuum air separators they are extensively and economically used for the reduction of all kinds of dry colors and chemicals, and for the threshing, cleaning and separating of tobacco stems.
The mechanical apparatus required to effec tively apply this system of air separation should be capable of satisfying the following condi tions: (1) To expand and rarify the air so that the coarser particles will fall out of the current and allow it to deliver the impalpable powder at the discharge spout. (2) When a large out put of the finely ground material is required per hour, the apparatus should he capable of using an amount of air sufficient to lift the total weight of the charge of raw material. (3) When using a large volume of air, sufficient room should be provided for its expansion and rarefaction so as to obtain a current light enough to carry away the impalpable powder only. (4) The apparatus should be so con structed that the coarse particles will fall out of the light air current gravity into the con tracted portion of the separator where the blast is stronger, and thus pass out through the tail ing spout or into the pulverizer to be re ground, without being accompanied by any of the fine powder. (5) The air space within the
apparatus should approach as near as possible to a perfect vacuum.
As the fineness of the product depends upon the size of the separator, almost any limit of grinding is readily obtained by making the apparatus of sufficient size to produce the proper relative expansion and rarefaction of the air. When it is desired that the finished product should be an extremely fine powder, the use of the large-sized separators will give the best results; but, for products of medium grades of fineness, and large output, the smaller ma chines with fan and dust collector attachments are the most effective.
Fig. 13 illustrates a Raymond impact pul verizer and vacuum separating plant as erected for the cleaning and separating of tobacco stems. The material is fed to the machine by the operator in a manner similar to the feeding of a threshing machine. The perforations of the screen surrounding the pulverizing cylinder vary in size in proportion to the fineness de sired. As the stems come in contact with the rapidly revolving beaters, the particles of the leaf are liberated therefrom, and passing through the screens with the stems are caught by the air current and drawn up into the sepa rator, which expands the air so that all the light pieces of stems drop back and are discharged through the opening at the bottom into the drag elevator. The particles of leaf drop into the centre cone of the separator and discharge from the spout, while the dust is carried through the fan into the dust collector and discharged therefrom.
An-astra Machinery.—Arrastra plants are employed for the reduction of Metallic ores into a fine pulp which is subsequently treated by the patio or some other process and the pure metal separated from the accompanying impuri ties. The bottom of the arrastra is paved with stone 10 x 10 inches square, made of the best granite and set vertically to the depth of about three feet in the ground. The outside is con structed of flat stones which project from 18 to 30 inches above the floor. L.ine shafts driven by engine power • extend over this floor and operate poles or arms which-revolve and move the °dragging stones)" over the granite floor and thus accomplish the reduction: of the ore fed into the arrastra to a fine pulp. These stones are made of the best granite, and range in weight from 11,000 to 25,000 pounds. The are is first prepared by being passed through the jaw-crushers of a primary crushing plant. From these machines it passes through a set of crushing rolls and is dropped automatically into revolving screens which separate the fine pulp frolic the coarser grindings. The last-named material is sent back and passed again through the crushing rolls until all of the pulp is fine enough to be finally treated in the arrastra. Each arrastra requires about six horse-power for its operation, and range in crushing capacity from 6 to 10 tons each, per 24 hours, according to the character of the ore.
For other forms of crushing and winding machinery, and their special application, see articles under the titles BRICK MAKING MA CHINERY, COAL MINING MACHINERY and MIN ING AND MILLING MACHINERY.