At the present day probably the only mill of importance where the entire treatment of the slime is done in presses is at the Homestake. "The Merrill press is essentially of the ordinary rectangular flushplate and distance frame pattern with internal channels, but equipped with the automatic discharging device which is the distinguishing feature of the press. A standard press, 28 of which are now in service, containing 92 4-inch frames and 91 plates, is 46 ft. long and holds somewhat over 600 cu. ft. or 25 tons of slime; under the usual treatment schedule and with average charges, its capacity is about 70 tons per day."1 The automatic discharging device referred to consists of a nozzle in the interior of each frame capable of being rotated vertically through an arc of 200 degrees. When it is desired to discharge the residue cake water is supplied to these nozzles at a pressure of 60 lb. per square inch and simultaneously the nozzles are rotated backward and forward breaking down the cake and sluicing it into the discharge channel in the lower part of the press through which it finally finds its way to the dump. Homestake Slime Process.-The thickened slime in which the proper amount of lime has already been added is supplied to the press by gravity under 30 lb. pressure, the surplus water flowing off to waste until the frames are filled with firm slime cake, which takes about 70 minutes. Air is then passed through the cake in a horizontal direction, that is, entering from behind the cloth on one side of a cake and passing out through the cloth on the other side of the same cake. This aeration lasts one hour and is followed by a wash of weak precipitated solution for about 30 minutes. The cake is again aerated for an hour and then receives a wash of strong solution (0.1% KCN) for an hour, followed by weak solution and water wash Clark and Sharwood2 state that "The treatment time is varied according to the leaching rate of the charge and is calculated to pass approximately equal amounts of solution 1 Clark and Sharwood, Proceedings I. M. M., London, Nov., 1912. 2 Proceedings I. M. M, London, November, 1912. through the slime cakes, irrespective of the leaching rate. The leaching rate, as described here and referred to later on, is an empirical standard, being taken as the number of seconds required to discharge 5 U. S. gallons from the press under the standard wash water pressure. A charge is treated according to the leaching rate shown by the previous charge, a reasonable system in that the leaching rate is affected chiefly by the accumulation of carbonate of lime on the cioths, there being few sudden variations in the character of the slime. The working schedule now in use is given." Working Schedule Homestake Filter Treatment.-The "Class of press" refers to the rate of leaching of the various presses in the system. Washing out of the Metal-bearing Solution after the Dissolving Treatment. This operation constituted the chief problem in the earliest attempts to work out a slime treatment method. As early as 1893 James Park1 patented a process whereby the slime was agitated in a filter bottomed tank, agitation and filtration proceeding simultaneously, the paddles being arranged in such a way that no solid cake was built up on the filter medium but a 1 The Cyanide Process, page 140 (Fifth Edition). gradual and homogeneous thickening of the whole mass took place. When this thickening had proceeded to a suitable point the mass was diluted with more solution and re-thickened, the operation being repeated as often as necessary. In South Africa it was very soon decided that a series of settlements, decantations, and re-dilutions was the only method devised up to date that would even approximately meet the local conditions, and this system has been the standard and universal practice there ever since, though recently vacuum filtration has been introduced in some mills. While the decantation process was being worked out on the Rand the problem was being solved in Australia by the use of filter presses, the principal reason for the difference probably being the scarcity of water which would have rendered the decantation process almost prohibitive in some localities. The frame and plate type of filter press is so well known that it is unnecessary to give a detailed description of it here. The form most generally used in Australian practice is the Dehne. The advantages of the system are that the metal-bearing solution may be almost completely displaced from association with the solids in a short time and with a much smaller volume of wash liquor than is possible by decantation, and also with a minimum loss of moisture in the residue. The drawbacks consist in the comparatively high cost for labor and repairs, making it unsuitable for large tonnages of low-grade material. To obviate the expense incidental to the use of the standard type of pressure filter, developments have taken place in three directions, the first being represented by the Merrill self-dumping frame-and-plate press already described, the second by various forms of vacuum filter whereby the pressure of the atmosphere is substituted for that of pumps or hydrostatic head, and third a combination of ideas gathered from both types whereby free filtering leaves or units are suspended in a closed receiver filled with slime pulp and mechanical pressure applied to the interior as a whole. Group No. 2 consists of the Butters, the Moore, the rotary drum filters such as the Oliver and Portland, and the disc filter or "American Continuous Suction Filter." The Butters filter consists of a number of filtering units or leaves suspended vertically in a containing box with hopper bottom. The leaves are usually constructed of a frame-work of pipe suspended from a 3′′ × 9" wooden support. The filtering medium consists of a cocoa matting foundation enclosed between sheets of canvas, the edges of which are so adjusted that only solution free from solid matter can enter into the interior of the unit. The pipe composing the lower part of the frame is perforated, so that when the extension of this pipe projecting above the level of the pulp is connected with a vacuum drum, the solution from the external pulp enters the unit through the canvas and passes away through the pipe system, leaving the solid matter on the canvas surface in the form of a cake. The containing box is first filled with pulp by means of a centrifugal pump and the vacuum applied to the leaves. When a cake of sufficient thickness has been built up on the canvas surface, the vacuum is lowered to a point just sufficient to keep the cakes adherent to their supports, the surplus pulp in the containing box is pumped out and replaced with barren wash solution or water, and the vacuum again raised to its maximum. The lowering of the vacuum during the time the cake is exposed to the air is very important and its purpose is to prevent excessive drying and shrinkage of the cake with formation of cracks which reduce the displacing efficiency of the subsequent wash and also tend to cause a premature dropping of the cake. When sufficient wash liquor has been forced through the cakes to displace the pregnant solution originally contained therein, the surplus remaining in the box is pumped back whence it came and the vacuum cut off the leaves. The cake is then dropped to the bottom of the box and discharged to the dump through doors at the apex of the hoppers. The dropping of the cake may be arranged to take place automatically by gravity, or pressure (either air or water) may be applied to the interior of the leaf as may be more convenient. Where possible it is usually better to drop the cake while the |