This cloth is tacked on the stave ends of the slats. The covering for this cloth filter is 8-ounce canvas and is made 12 inches greater in diameter than the false bottom. The edges of the filter covering are fastened in place by pressing a 1-inch rope into the space between the side of the vat and the ring piece, as shown in Fig. 12 (a). This arrangement is very efficient in preventing the sand from washing under the filter.

76. Wrought-Iron Solution Pipes. Each leaching vat has separate drain pipes for strong and weak solutions. The pipe a, shown in Fig. 13, is arranged to lead to the

HHOH

a

FIG. 13

strong-solution gold tank and the pipe b is arranged to lead to the weak-solution gold tank. These pipes are connected with the exhaust cylinder of a vacuum pump that is used to produce a partial vacuum under the filter bottom, in order to increase the rate of percolation.

77. Launders. The wrought-iron solution pipes are arranged in Fig. 14, with valves a and b, so that the vacuum pipe c may drain through the pipe leading to the launder. The launder is divided into two sections d and e, each 4 inches wide and 4 inches deep. The section e is for the weak gold solution and the section d is for the strong gold solution. The valve a has a short rubber hose h attached,

N. M. III.-28

so that the solution can be discharged into either section of the launder as desired.

78. Discharging the Vat.-There are several methods of removing tailings from a vat, some of which depend on

d

a

FIG. 15

о

the cost of labor and others on the supply of water at command. At some plants the tailings are shoveled through gates in the bottom of the tank into dump cars. Where

there is good water pressure, sluicing out the residue is found to be cheap and quick. When the tailings contain coarse gold, the residues are sluiced slowly over amalgamated copper plates placed below the discharge hole.

When large brick leaching tanks are used, they are sometimes unloaded with the aid of traveling cranes, which lower empty car bodies into the tanks, where men shovel tailings into them. When the car bodies are filled, they are raised, placed on their trucks, and wheeled away to the dump.

The

79. Discharge Doors. - In the case of bottom discharging, there may be from 2 to 8 openings, depending on the size of the vat. In Fig. 15, Butter's bottom discharge is illustrated by a vertical cross-section. In the figure, a represents the tank floor, b a cast-iron ring that acts as a washer for the cast-iron cylinder c inside the tank, and a seat for the valve h. ring band the cylinder c are bolted together SO as to draw them tight against the tank bottom a. There is a lug d inside the cylinder for the hanger e to rest upon. The hanger at its lower end is provided with a screw threadƒ, upon which the butterfly nut g works in order. to tighten and hold in position the cast-iron valve h. The faces of the cover h and ring b should be planed perfectly smooth so as to make a tight joint.

80. Bottom Discharge.-A simple form of bottom-discharge valve is that shown in Fig. 16. It consists of a deep-flanged iron collar a, which is placed in a hole in the bottom of the vat and fastened to the vat floor by bolts passing through the flange and floor. The discharge valve b is operated from the top of the tank. The valve seats on a rubber ring and is lifted by a screw at c, to leave a clear opening for flushing out the tank. should be about 10 inches in diameter.

FIG. 16

The opening

81. The side-discharge door is shown in Fig. 17. It

82. Gold-Solution Tanks.-There are two gold-solution tanks, which should be placed low enough to receive the solution from the percolating tanks above and high enough to discharge by gravitation into the zinc boxes. The tanks act as settlers for solutions containing impurities in suspension that would interfere with the precipitation of the gold, if allowed to pass into the zinc boxes.

83. Construction of Zinc Boxes.-Zinc precipitating boxes are usually divided into compartments having double partitions. In Fig. 18 (a) is shown an elevation; in Fig. 18 (6) a plan; and in Fig. 18 (c) a cross-section of a zinc precipitation box. The first partition a does not reach to the bottom of the box b; the next partition c does not reach quite to the top of the box, and so on, to permit the solution to enter each chamber from below and pass upwards through the perforated bottom d, which supports the zinc shavings. The overflow from one box to the next occurs at the upper part of partition c.

This arrangement for upward flow permits a free flow of solution, and the gold being deposited largely at the bottom of the box falls off, leaving the passage clear for the solution. The gold and zinc slimes as they form pass through the false bottom d into the compartment below.