FILTER- PRESSES, ETC. —Thc pressure-tank used in connection with the filter-press for filtering the precipitated sulphides in the lixiviatitin process consists of a cylindrical tank of boiler-iron with a funnel-shaped bottom. Through the top of the tank a vertical pipe extends almost to the bottom. In the cylindrical portion of the tank is a woollen diaphragm (the vertical pipe passing through its center) which floats on the liquid within the tank. The solution containing sulphides is introduced through the vertical pipe, and rises under the floating diaphragm. A pipe at the bottom of the funnel leads to the filter-press. When the solution no longer runs free ly through the latter, steam or compressed air at a pressure of 150 lbs. per sq. in. is forced into the upper part of the pressure-tank, above the floating diaphragm, and the solution is thus forced through the filter-press.
Johnson's _Hydraulic (Fig. 8) consists of a series of round or square plates, either cast iron, bronze, or other suitable metal, having project ing lugs cast on each side for the purpose of supporting them in a press-frame in juxtaposi tion face to face, and are capa ble of being screwed up tightly between the head and follower of the press. The plates are concave on each side: the pro jecting outer edge or rim, being truly surfaced, maintains the plate-surfaces at distances cor responding to the depth of such rims. These rims are some times made separate from the plates, and varying in depth, to suit the requirements of the purchaser. The plates are cov ered with suitable filtering cloth, and are also provided with ribs or channels on the plate-surface under the cloth, to allow the filtrate to flow away to the outlet formed in the• bottom of the filter-plate at the back of the cloth. The spaces between the cloth-lined plates form chambers or cells, into which the liquid or semi liquid material to be filtered is forced under pressure. A passage or opening, also lined with cloth, is formed through each plate, so that there is a free communication between the several filtering-cells. When the liquid or semi-liquid material to be filtered is forced into this battery of cloth-lined chambers or cells, the liquid is forced through the filtering-cloths which cover the plates, and flows away to the outlet of the plate by following the channels or grooves in the plates, which have free communication with the outside of the filter. The solid matter is stopped back on the surface of the cloth, and by a continuance of the operation ultimately fills the cells. It is then removed from between the two cloth-eovered concave
plates, forming any one of the chambers, in a state of almost perfect dryness, by unscrewing the press and separating the plates, without removing the cloths.
The Roessler Converter is an apparatus for the condensation of sulphurous acid and its conversion into sulphuric acid, which is used in silver-refineries where dor bullion is parted by sulphuric acid, and in lixiviation-mills where the precipitated sulphides are refined by roasting. It consists of a cylindrical leaden tank, 8 or 9 ft. high and 5 ft. in diameter, through the top of which is introduced a 6-in. leaden pipe, extending nearly to the bottom of the tank, where it branches into two arms. These arms connect with a hollow, leaden ring, supported horizontally a few inches above the bottom of the tank, with numerous holes drilled in its lower side. The bottom of the tank is provided with a valve for discharging precipi tates. The tank is filled rather more than half full with a not too acid solution of cupric sulphate. The operation of the converter is as follows : Sulphurous acid from the muffle furnaces in which the sulphides are roasted is forced into the tank by a Korting injector through the pipe in the top, escaping through the holes in the leaden ring at the bottom. The sulphurous acid reduces the cupric sulphate in solution to cuprous sulphate, as is shown by the change in color of the solution from blue to dirty green. If, now, air is blown into the solu tion, the cuprous sulphate is oxidized, forming cupric sulphate again, and setting free sulphuric acid. In practice the sulphurous acid and air are forced into the tank toge.ther by the in jector and the reactions go on simultaneously and indefinitely. If the liquor reaches a certain concentration in free sulphuric acid, however, the reaction is weakened ; hence, either cement copper, scrap copper, or copper oxide, are put into the tank to neutralize the free acid. If the sulphuric acid from the muffles is not too much diluted with other gases, from 80 to 90 per cent of it, it is claimed, will be converted into sulphuric acid ; but late investigations at the Marsac mill, Park City, Utah, have shown the worthlessness of this apparatus for acid-mak although it saves quite an amount of silver volatilized and carried over mechanically from the roasting of argentiferous precipitates.