ORE-IIRESSING MACHINERY. DRESSING WORKS. (See ORE-CRUSIIING MACHIN dressing is the art of separating the mineral in ore from the worthless rock or gangue, with which it is intermingled, the mineral, thus concentrated, being subsequently treated by the proper metallurgical process. In dressing ores mechanically, there is always a loss in values, varying from 10 per cent, to 50 per cent., or even more, and it is not cus tomary to subject to this form of preparation ores which can be directly treated economi cally by any of the ordinary metallurgical processes. Mechanical dressing is, consequently, only resorted to when the cost of the operation and the loss in values is more than balanced by the saving in freight and in the cost of the subsequent treatment of the ore. gained by the elimination of the worthless gangue.
The method of mechanical ore dressing, in general, consists in crushing the ore to suffi cient degree of fineness to free the particles of valuable mineral from the gangue, and after ward effecting a separation between the two by virtue of the difference in specific gravities. Two classes of crushing machinery are commonly used in every dressing works, viz. : coarse crushing and fine-crushing. The former, of which the well-known Blake crusher is a type, takes the coarse lumps of ore as they come from the mine, and breaks them to a convenient size to be received by the fine-crushing -Machine, which may be a set of Cornish rolls. In most mills there arc two sets of rolls in each crushing system, the final comminution being clone in the second, which are set closer together than the first. Between each crushing machine and the next in series there should be a screen, over which the crushed ore is passed to remove the particles already crushed finely enough, thus relieving the following machines and preventing this ore from being crushed finer than is necessary, an important point, as the fine ore becomes slime, when mixed with water, which will probably give rise to increased loss in the dressing. Similarly, the ore is frequently dumped over a grizzly (a coarse screen composed of parallel steel bars), before being fed to the first crusher. The crushed ore coming from the finishing rolls is passed over a screen, the mesh of which constitutes the standard of crushing of the mill. That which will not pass through this screen is returned to the, rolls: that which passes is sized in preparation for the washing machines. The sizingis done either by screens or hydraulic separators, but generally both systems are used in the same mill. With the former, the operation being technically known as " sizing," the particles of ore are divided into classes of equal size ; in a hydraulic separator the particles of ore settle against an upward current of water, and are thus classified into equal falling grains, the operation being technically known as " sorting." The usual practice in dressing works is to size by screens particles down to about 1 min. in diameter. The fine• particles are sorted. At Lake Superior, where there is a great difference in the specific gravities of the minerals to be separated—native copper and the various siliceous minerals which constitute the gangue— hydraulic classifiers alone are used. Screens, only, may be used in mills doing very coarse work, but never in a well-designed mill intended for fine and close work.
The sized and sorted ore goes from the screens and separators to the washing machines, by which the heavy particles of mineral are separated from the lighter particles of gangue, by virtue of the difference in specific gravities. Washing machines may be divided into two general classes, viz.: sand washing-, represented by the various kinds of jigs; and slime wash ing, of which the various slime tables and huddles are types. The sized ore of which the particles are between 16 min. and 4 nn. in diameter, is commonly designated as pea; between 4 nun. and 1 nun., as and : and finer than 1 nun., as meal. The pea and sand sizes are washed on jigs, the material from each sizing screen being conducted to a jig properly designed and adjusted for that size. The meal sizes, from the hydraulic separator, are washed on the slime machines ; the coarsest meal is worked on jigs, varying from the coarser jigs only in details of design, speed, etc., while the finer meal is conducted to other machines adapted to the size and character of the ore. With the washing machines the operation of dressing is completed, and the concentrates are ready to go to the smelting works, or elsewhere, for further treatment.
The cost of dressing varies, of course, with the capacity of the mill, the character of the ore, and the quality of the work done. The following are a few instances of the best Ameri can practice: At the Atlantic mill, Lake Superior, siliceous copper rock containing from 0.9 per cent. to per cent. native copper has been dressed (1886) at a cost as low as 26.5 cents per ton, about 70 per cent. of the mineral being saved. The cost may be sub divided, assuming the same percentages as in the previous year, about as follows : Labor, 35 per cent.; fuel, per cent.; supplies. etc., per cent. The cost of dressing in this mill in 1890 was cents per ton. At the mill of the St. Joseph Lead Co., at Bonne Terre. Mo., ore was dressed in 1887, according to Prof. H. S. Monroe (Trans. Am. Inst. Min ing Engrs. vol. xvii. 659), at a cost of cents per ton, divided as follows : Labor, cents : repairs, 10 cents ; supplies, cents ; coal, cents. At this mill all the water used has to be pumped to the crusher floor; and all the tailings are carried off in cars, dis advantages under which the Atlantic does not labor, so that in making a comparison between the two, it is only fair to deduct 10 cents per ton, in Professor Monroe's opinion, from the St. Joseph figures. The St. Joseph ore is galena with a magnesian limestone gangue, assaying from per cent. to 8 per cent. lead. The capacity of the mill is 800 tons per day. The loss in tailings amounted to per cent, of the mineral. At the Hecla Con solidated Mining Co.'s mill at Glendale, Mont., ore assaying 7 per cent. lead and 15 oz. silver per ton, was dressed, in 1890, at a cost of 41447 ccuts per ton, 55 per cent. of the lead and 37+ per cent. of the silver being saved. The average cost of dressing low-grade ore in the silver-mining districts of the Rocky Mountains, in small mills, say of 100 tons per day capacity, is probably between 75 cents and $1 per ton.