" Formerly the dimensions of lixiviation-tanks were taken quite small: ore-tanks not larger than 12 ft. diameter and 3 to 4 ft. deep; precipitating-tanks, solution-sumps, and storage tanks of corresponding dimensions. In recent works, however, ore-tanks of 16 to 20 ft. diam eter and 8 to 9 ft. depth; precipitating-tanks, solution-sumps, and storage-tanks of 12 ft. diameter and 8 to 9 ft. depth are put up. As can readily be seen, the care and attention re quired to finish a charge in an ore-tank, or to precipitate a solution in a precipitating-tank, arc independent of the size of the vessel ; • hence, the great advantages of large sizes.
"The capacity of an ore-tank for 24 hours depends upon the specific gravity of the ore, the quantity of first and second wash-water, and of stock-solutions required for treatment, but principally upon the rate of lixiviation. Capacity increases in proportion to diameter, but remains nearly stationary so far as depth- is concerned; that is, the same number of ore-tanks will be required whether their depth is 9 ft. or only 4 or 5 ft., in order to treat a stipulated quantity of ore per day. In fact, should the rate of lixiviation increase With reduced depth, the same number of shallow tanks would put through in 24 hours more ore than deep ones. The principal advantage of increased depth consists, therefore, only in reducing the number of charges treated.
" Where water is abundant, tailings are removed by sluicing, and great depth of the charge is no disadvantage. Even where water is scarce, and tailings have to be removed by hand, deep tanks should be used. It is only necessary to provide mechanical means for moving above the tanks large buckets into which the tailings are shoveled.
" The false bottoms for the filter, and the latter itself, are prepared as follows : Wooden slats, in. high and 1 in. wide, and separated 1 in. from each other, are fastened to the tom. This has so far been done with iron screws bedded in white lead ; I would suggest
pins of hard wood. The inside of the slats, next to the bottom, is cut out in many places in. deep and 3 in. wide, so that a free passage of the solution below the filter is established. Between the ends of the slats and the staves a clear space 14- in. wide is left. A strip of wood 11 in. high and 1 in. wide, previously cut with a saw in many places, and well soaked in water so that it will bend easily, is now fastened round the slats, leaving an annular space in, wide between the strip and the staves. One thickness of stiff matting, covering the slats and the circular strip, but not the annular space, forms the foundation of the filter-cloth proper. The latter, No. 10 canvas-duck, is cut to a diameter 6 in. greater than the inside of the tank, so that the ends can be pressed into the annular space described above, and kept in position by forcing down a i-in. rope." The precipitating-vats are provided with a machine-stirrer of the construction indicated in Fig. 7, or else with a stirrer similar to a screw-propeller. The stirrer has to make about 30 revolutions per min. if the diameter of the tank is not more than 8 or 9 ft. It is set in motion Dr stopped by working the friction-clutch f. Fixed to the inner side of the vat are wings, which reach near to the bottom, are about 3 in. wide. and are kept in position near triangular pieces of board. They break the violent current around the periphery and throw the solution toward the center, thus causing a strong Nvhirling motion. The calcium sulphide is fed into the solution in this vat, agitated by the stir rer, and the silver thus precipi tated. At the bottom of the vat are suitable valves for drawing off the precipitate, which is al lowed to settle, and the super natant liquid.