MECHANICAL SIZING. Classification of Dry Granular Material.— The mechanical classification of dry granular material is the re sult of the action of several natural laws operating in conjunction with a mechanical device so arranged as to permit the balanc ing of the action of one law with another. The combined motion places the relative action of the dry granular material completely under con trol of the mechanical device. The first active law in mechanical sizing is termed the laws of selectiveness. When mixed particles of various sizes of granular material are under agitation upon a level plane, the finer particles sink to the bottom and engage the surface while the coarser particles rise to the top without regard to the specific gravity of such particles. The second active law is called the law of displacement or the displacement of the centres of gravity of particles upon an inclined plane. The relative. difference in the tendency of coarse and fine material of granular shape to move down an inclined plane is due to the difference in the displacement of their centres of gravity upon the same angle of plane. The third active law is the law of friction of mass or the cient of friction? Close observation of the action of these laws as well as the influence of other forces led to the design of a emechanical sizer" simply ar ranged to best meet the requirements of these laws. In order to assist the law of selective ac tion on an inclined plane, a series of several hundred tapered riffles or grooves are placed on the inclined surface of what is termed the deck of the device. This permits the to sink to the bottom of these grooves and forces the discharge of larger particles over the top of the riffle, in accordance with the ac tion of the law of displacement of centres of gravity. This is accomplished by the feeding of dry material to the inclined deck while the same is being agitated forward and backward by a specially designed head motion, which pushes the deck forward at one speed and causes it to return at a higher speed. This sets
up what is termed a progressive action of ma terial on the deck or causes the granular parti cles to travel in a forward direction, under the government, however, of the law of selective action and the law of displacement aided by the law of friction of mass. Under operation, the device is fed with dry ore, sand, grain or the like, the feed engaging a feed board where a preliminary rough separation is accomplished by a modified application of tapered riffles. The feed then engages the table deck at the head of its proper zone and here, because of the length and the great number of riffles, almost any esired number of carefully sized products may he taken off by placing receptacles at the bottom and far edges of the deck, from which falls, when in action, a constant sheet of granu lar material graded carefully from coarse par ticles to fine dust.
Mechanical sizing is applicable to every form of dry granular material such as crushed ore, salt, coal, sand, emery, cereals, unbroken or crushed, or, in fact, there is no field in dry siz ing now filled by metallic screen devices that cannot be filled by a mechanical sizer and often with greater economy and efficiency. This is proved by the fact that metallic screens blind and lose their efficiency while the mechanical device cannot clog or blind and automatically cleans itself. Specific gravity has practically no effect upon mechanical sizing for the reason that granular particles of the same contour and volume, when plated upon an inclined plane, have the same displacement of their centres of gravity with the result that a particle of lead and a particle of sawdust of the same size and shape will discharge from the table at the same point, although the specific gravity of the one is many times that of the other.