Page:The American Cyclopædia (1879) Volume XI.djvu/434

 422 METALLURGY valuable material attendant upon the additional manipulation required. To decide this ques- tion, for instance, with regard to an ore that is to be treated by smelting, it must be deter- mined whether the expense of smelting the whole mass of the ore mined would be greater than that of first separating its worthless or injurious contents, and then smelting with bet- ter results the smaller quantity of concentrated materials ; and also whether in the mechanical separation the loss of valuable material would be so great as to counterbalance the saving in smelting expenses, and the possible gain in purity of product and in completeness of ex- traction. It is also necessary to determine spe- cially in every case how far the process of pre- liminary treatment shall be carried. It would not be desirable, even if practicable, to remove every trace of gangue, since some earthy ma- terial is requisite for the formation of slag to protect the metal in the hearth of the furnace from oxidation. Simple concentration removes a portion of the worthless gangue, and divides the ore into two parts, usually called " head- ings " and " tailings." Headings may be made pure only by a loss of valuable material in the tailings; and, vice versa, the tailings can be made entirely worthless only by leaving con- siderable gangue in the headings. The best sys- tems therefore involve the formation of one or more classes of so-called "middlings," or in- termediate grades, which may be subsequently separated again. This principle is also impor- tant in the treatment of a material containing different ores which cannot be advantageously smelted together. Thus zinc blende, which is very commonly associated with silver-bearing galena, is often poor in silver, and moreover seriously embarrasses the lead-smelting, not only by requiring extra fuel for its own volatili- zation or fusion in the slag, but also by taking up and removing in its vapors or slags a larger portion of silver than it originally contained. Yet this mineral, if separated, can under fa- vorable circumstances be profitably treated by itself. The mechanical separation of minerals depends either upon their magnetic properties or their specific gravity. The former prin- ciple has been employed to a limited ex- tent in the separation of magnetic iron ores, in a finely divided state, from their accompany- ing gangue. Both permanent magnets and electro-magnets have been employed ; but the process can scarcely be said to. have proved an economical success, or to be now in practical operation outside of the laboratory. Separa- tion by specific gravity is performed in air and water. So-called dry concentrators are chiefly used in localities where water is scarce. The simplest form is that of a bowl or hide, in which auriferous dirt is placed and tossed in the wind, the lighter earthy particles being Mown away, while the heavier sands con- taining the gold return to the vessel. Other air concentrators are winnowing channels, in which the material is separated by a draught of air, the heavier particles falling first to the floor, and the lighter ones being carried for longer distances ; the result is a distribution of the materials on the floor of the channels in the order of their specific gravity. The most complete ore-dressing machines employing air are those in which the current of air is replaced by impulses, somewhat after the manner of the water jigs to be mentioned presently. The appa- ratus usually employed in dressing ores involves the use of water as a medium, and depends upon the relative periods occupied by bodies of dif- ferent size, shape, and gravity in falling through water. The most favorable condition for sep- arating ores would be the employment of a liquid exceeding in specific gravity one portion, and exceeded by the other portion, of the ma- terials to be treated. The former would then float, while the latter would sink to the bottom, as in the preparation of fine porcelain clays. But in almost all cases the minerals to be sep- arated sink in water, and a separation must be based on their different rates of sinking, de- pendent upon variations of specific gravity in different materials, and of size and shape of particles in the same material. In many ma- chines for the separation of minerals by water, those particles which actually fall at equal rates will be brought together ; hence, if it is desired to bring together particles of the same mineral (that is, of the same specific gravity) only, the disturbing effect of difference in size must be avoided. In other words, a careful sizing of the material must precede its separation accord- ing to gravity. A given quartz sphere will be 4 times as large in diameter, 68 times in vol- ume, and 23 times in weight, as a galena sphere that falls through water at the same rate ; and if such a piece of quartz were present with such a piece of galena in the material under treat- ment, they would not be separated. The sepa- ration is not usually effected in still water. In some machines the ore is made to fall against a rising current, or against impulses given by the motion of pistons ; in others, the action of a stream of water passing through a trough or over an inclined plane is employed. The si- zing is frequently effected beforehand by sieves, but the inclined plane itself may produce a sep- aration according to the size of the particles, by reason of the greater rapidity of the upper surface of the stream, and the greater effect produced by the current upon larger particles. The shape has here an important influence, as determining the retardation of the particle by rolling or sliding friction. The apparatus in which still water is employed includes various kinds of settling tanks. The upward current and impulse is characteristic of the machines known as jigs ; while the thin stream of water passing over an inclined surface is a feature of the huddle, the plane table, the rotary table, and the percussion table. The most universal- ly serviceable machine for ore dressing is the "jig" or "jigger." This was originally a sim- ple improvement upon the treatment of ore by