possible to give the bottoms of the bins a 35° rather than a 45° slope, and hence a somewhat larger storage capacity may be had when iron linings are employed.

50. Water Tanks.-In most mills the water supply runs into wooden or iron tanks--usually circular and from 8 to 20 feet in diameter-and is drawn from them as desired. By this means a practically constant head or pressure is obtained, and there is always a reserve supply of several thousand gallons-enough to run the mill for several hours if necessary. Some mills use two tanks, one of which is filling while the other is in use. These tanks are usually set outside of the mill, on the ground. In cold countries, however, this is not always practicable, as the tanks would freeze up during the cold weather. In such cases, the tanks should be put in a separate room with its floor sills independent of the rest of the framework of the mill; or if set in the main building, they should at least be set on independent timbers; for if the tanks are set on the mill timbers, the jar of the crusher and other machinery is communicated to the water in the tanks, sets it in rhythmic motion, and the vibration of this immense weight of water is transmitted to the mill timbers, and will, sooner or later, if continued, rack the building to pieces.

51. Amalgamating Mills.-The general arrangement of the rock breakers and ore bins is practically the same for all classes of gold and silver mills. Below the bins, however, the machinery and arrangement vary with the amount and nature of the work required of the mill. Thus, the machinery of a concentrating mill differs in kind and in arrangement from that of an amalgamating mill, as will be seen by comparing Figs. 6 and 8. Of course, all mills should be designed to make the operation as nearly as possible continuous and automatic. For instance, in an amalgamating mill the ore bins discharge directly on the feed floors of the stamp battery, Huntington mill, or whatever fine-crushing machine is used; or, if automatic feeders are employed, into the hoppers of the

feeders. The battery or mill discharges on to the apron plates and the pulp flows from them directly on to any subsequent gold- or amalgam-saving apparatus that may be used, and which is on a level 3 or 4 feet lower than the battery floor. If the mill is a combined amalgamation and concentration mill, as in Fig. 6, the concentrating apparatus-vanners, bumping tables, or similar machines-is put on the floor below the battery floor. If hydraulic classifiers are used, they can be suspended from the roof timbers or set on frames, usually parallel to the battery

H

FIG. 7

discharge, and receive the pulp directly from the plates, discharging the sized ore through pipes into the distributing boxes of the concentrators. Slime-saving apparatus below the vanners or other concentrators is seldom used, but the tailings water may be run into large settling vats and the slimes settled out. This is particularly applicable in dry countries, where the water supply is limited, as the water from the settled tailings may be pumped back to the tanks and used over again, with a loss of perhaps 20 or

25 per cent. The tailings, if they contain much value, may be treated by the chlorination or by the cyanide process. In the very dry regions of Australia, the water is sometimes removed from the tailings by means of filter presses. The design of silver amalgamating mills is still different from that of gold amalgamating and concentrating mills. Fig. 7 shows a Boss continuous-process mill in

section.

52. Concentrating Mills.-The object of concentrating works is merely to get the values in an ore into smaller bulk, in order to diminish the trouble and expense of shipping and further treatment, and not for the immediate actual extraction of the metals in the ore.

The operation is purely mechanical, the ore and gangue being separated by crushing, and the gangue, owing to its lower specific gravity, being washed away. This being the case, crushing and careful sizing become highly important.

53. No definite scheme can be laid down for the arrangement of concentrating mills. This depends largely upon the nature of the ore and still more upon the ideas of the designer. Several methods, each requiring different apparatus and arrangement, may be equally well adapted to the concentration of an ore, and the selection of any one method lies with the designer, who is supposed to take into consideration local conditions as far as possible. Thus, local factories, if there are such, are usually given the preference, if their machines can compete on anything like equal terms with those of outside manufacturers. The personal preferences and prejudices of designers are frequently important factors in the designing of mills.

usu

The concentrating gold mill is for several reasons ally much simpler in design than concentrators for copper, lead, and zinc ores. In the first place, the out-of-the-way location of the average gold mill makes the freight on apparatus an important consideration in the first cost and running expenses; and, again, such mills are usually only temporary structures, doomed to abandonment as soon as

are

the ore body is exhausted. As a rule, gold mines a exhausted after being worked continuously for a few years, and the ore body may play out unexpectedly at any time, so that it is desirable to put as little extra expense into the mill as possible. It is seldom worth while to dismantle an old mill. Nevertheless, the mistake of putting too little apparatus in a mill is much more common than that of putting in too much. Additional machines, if of good design and within reasonable limits, will usually pay for themselves.

With immense low-grade deposits, like those of Dakota, Idaho, and Douglas Island, Alaska, it will usually pay to put in more elaborate concentrating plants, as a very small increase in the saving per ton counts up rapidly where several hundred tons of low-grade material are being treated daily and the ore bodies are practically inexhaustible. Fig. 8 is a cross-section of an Idaho concentrating mill, showing jigsj, hydraulic classifiers, buddles b, and vanners v. In the Butte (Montana) copper region, concentration has reached its highest development in America. The ores of this region, though containing small quantities of gold and silver, are essentially copper ores, the gold and silver being. obtained merely as by-products.

54. Roll Crushing Concentrating Mills.-Roll crushing is almost invariably adopted in concentrators, though steam stamps have replaced rolls to a considerable extent in the Montana and Lake Superior copper regions. The usual arrangement of concentrating mills is somewhat as follows: The grizzlies, rock breakers, and bins are practically the same as for amalgamating mills. From the bins the ore goes to the coarse or roughing rolls or to a second rock breaker, set closer than the first, which fills the place of the roughing rolls. In many mills a second rock breaker is placed between the main rock breaker and the rolls to lighten the duty of the latter. In such a case, the product of the first rock breaker usually goes to a trommel whose meshes correspond to the maximum size of the product of

N. M. III.-19