Page:The American Cyclopædia (1879) Volume XI.djvu/435

 METALLURGY 423 hand on a sieve under water. By plunging the sieve down suddenly under water, and allow- ing the particles to come again to rest upon it, a separation is effected; and if the stuff has been sized, and the operation has been repeated often enough, the denser particles are found in strata under the less dense. By scraping off the upper layers horizontally, ore and gangue may be separated. The first improvement was that of imparting motion by machinery to the sieve, but it was afterward found more con- venient and effective to employ a submerged stationary sieve, and impart a vertical oscilla- tory motion to the water. This is done by using pistons or elastic diaphragms placed in the sides of the box, or on the top of a lower chamber full of water, and communicating with a lower box through the sieve. An additional feature of recent mechanical jigs is the con- tinuous discharge, by means of which the dif- ferent classes of separated material are removed without interrupting the operation. It was formerly supposed that coarsely crushed ores only could be effectively treated by jigging ; and since ores so crushed usually contain a large proportion of fragments composed of adhe- ring gangue and ore, and therefore possessing a specific gravity different from both gangue and ore, it was supposed that the jig could not be employed for the most delicate separation. But the improvements which have been made in this apparatus permit the treatment by it of much finer material than was formerly practicable, and at the same time have greatly reduced the hand labor and consequently the expense involved in the process. The result has been a great extension of the application of the jig, and the gradual abandonment to a large extent of the more cumbrous buddies and tables, which were formerly considered neces- sary for the treatment of the finest sands. The " dolly tub " is a very simple machine for separating the particles of crushed ore. It con- sists of a cylindrical vessel filled with water, in which the ore is rotated by means of revolv- ing arms. When rotation has been maintained long enough, according to the quality and size of the material, the ore is allowed to settle, while the workman jars the table by blows upon the side of the tub with an iron bar, to prevent adhesion upon the inner surface. This machine has been elaborated by Hund, Kit- tinger, and others, and provided with a con- tinuous discharge. In the machines already described, the material to be worked must pre- viously be carefully sized, but. sometimes it is already too fine to be accurately or rapidly sized in sieves or "trommels," and for this reason resort is had to a different treatment. The material is first separated into "equal-falling" portions, grains constituting each portion being of such relative size and specific gravity that they will sink through water in equal times. Each of these portions is then treated alone upon a machine capable of separating the particles ac- cording to their specific gravity. For classify- ing the equal-falling particles, various machines are employed, in which use is made either of a horizontal stream of water of decreasing rapid- ity and of considerable depth, or a compara- tively shallow, smooth stream, or a vertically ascending column with decreasing rapidity. The Spitzkasten or pointed box employs the first of these agencies. These boxes are hop- per-shaped, and several of them of different sizes are connected. The water carrying the ore flows into and over the first box, and the heavier particles settle, while the lighter flow on to the second box, and so on. The rapidity of the current is diminished by varying the breadth of the boxes. The vertically ascending column of water is employed in the so-called Spitzlutten, a system of conical boxes, in which the water does not flow over as a covering cur- rent, but enters at the bottom. Both the flow- ing current and the ascending current have been combined in some recent forms of point- ed boxes. The riffle, so frequently used in placer gold mining, involves the same princi- ple. The material, having been classified by any of the machines just mentioned into por- tions of equal-falling particles, must be treat- ed further in order to separate each of these portions according to specific gravity; and for this purpose machines must be employed in which the particles will be affected more in proportion to size than in proportion to weight. Of equal-falling grains, the smallest are of course the densest ; hence, the smaller will be mainly ore, the larger mainly gangue. A very thin, smooth stream of water, passing over a plane surface, exerts different forces upon large and small grains lying in the cur- rent. The friction on the layer of water next to the bottom is much greater than on the layer above. Hence large grains, the tops of which protrude into the layer above, will be acted upon by a much more rapid current, and will be moved forward, while the smaller grains lying in the lowest layer are unaffected. This is incidentally also a separation according to gravity, since the large grains are specifically lightest. It is essential that the stream should be thin; a deep stream acts upon all points very nearly alike. Another requisite condi- tion is a proper velocity, which depends upon the inclination of the plane. If too nearly horizontal, the current will not move even the coarser particles, and if too steeply inclined it will be so violent as to sweep away fine and coarse alike. The amount of material held in suspension must also be regulated; if the water is too muddy, it will not be free to act on the separate grains, and the grains will act on each other. Keeping the water perfectly clear will effect the most complete sizing, but this condition is unfavorable to the quantity of work performed. The economical medium is found by practical experiment. Among the machines employed for this purpose are the plane table, the buddle, and the percussion table. The first requires little description. It