over the surface and is at once absorbed, tending to act as a wash for displacing the value-bearing solution. This filter is moderate in price, its running expenses are low, and it is continuous in operation, and while it cannot yet be considered a perfect solution of the filtration problem it seems to have the elements of more permanent usefulness than any other device at present in vogue. FIG. 24.-Disc Filter. ("American Continuous Suction Filter.") Its principal defects are its small washing capacity, necessitating one or more decantation washes before filtration, and the mechanical difficulties involved in keeping the pool of pulp properly mixed and agitated in cases where there is a larger proportion than usual of granular or sandy material present. It shows itself at its best when employed as a dewaterer after several decantations have been given to remove pregnant solution, and when the material fed to it approaches most nearly to the definition of a true slime. With this, as with all other vacuum filters, a thick pulp is essential to good work, both to ensure the homogeneity of the pulp in the container and also to obtain a high capacity per unit. The Disc Filter is comparatively new, but it appears to be doing satisfactory work where it is in use. The principle of it is sufficiently indicated in the accompanying sketch. The third type of filter is represented by the Kelly, the Sweetland, and the first Burt filter. The fundamental principle of these, as already stated, is the enclosing of a number of vertical filter leaves in a closed receptacle and forcing the pulp by mechanical pressure into such receptacle, with the result of building up cakes of solid on the leaves and expelling the clear liquor into a header pipe outside. The methods of discharge vary. In the Kelly the leaves are withdrawn on a roller carriage before dumping. In the Sweetland the leaves remain stationary and the lower half of the containing shell or receptacle is removed, while in the Burt the cake is first blown off and then the end of the shell is removed: the latter being erected at a steep angle, the residue cake slides out as soon as the door is removed. The later Burt filter is rather different from any of the foregoing. It consists of a revolving horizontal tube somewhat like a tube mill, and has a complete lining of filter material. The pulp is fed to it under mechanical pressure and a cake is built up in the form of a uniform lining inside the tube from 1 to 4 or 5 inches in thickness. When the supply of pulp is cut off, the pressure is maintained with compressed air so that filtration can be carried to a finish and no removal of excess pulp is necessary. For dumping, one end of the cylinder is removed and the revolving motion being restarted, the cake automatically falls off and discharges itself into the tailing launder. This filter is said to be especially applicable in the case of exceptionally granular and porous pulps, and is in successful use at one or more mills in the El Oro district of Mexico, though it has not obtained much vogue elsewhere. The idea, however, seems capable of wider application. The Dorr Continuous Countercurrent Decantation.—This is an interesting development of the old decantation process, devised by John V. N. Dorr, and while it contains the same principle, of displacement by successive dilutions, the action is much more rapid, the same result is attained by the use of considerably less barren solution and it is continuous in action needing little power and practically no attention. For its application a succession of Dorr thickeners is used, the series usually consisting of four. Each succeeding thickener is placed at a slightly higher elevation than the preceding one so that the clear overflow may be able to flow back by gravity. There are thus two separate currents of fluid running in opposite directions, the pulp after the dissolving treatment flows toward the residue discharge end of the series while the clear solution overflow runs backward from the discharge toward the treatment and milling end of the plant. The system is usually, though not necessarily, supplementary to some system of continuous agitation dissolving treatment. The slime pulp from the milling department goes first to a thickener, the clear overflow from which being the highest in value of any in the plant forms the pregnant solution for precipitation. The thick underflow is raised by a diaphragm pump to the series. of continuous agitators and, as it enters, is diluted to the desired degree with solution overflowing from No. 1 washing thickener. The pulp, when the dissolving treatment is finished, passes continuously into No. 1 washing thickener, being diluted on its way by the overflow solution from No. 2 washing thickener. The underflow from No. 1 washing thickener is raised by a diaphragm pump to the level of No. 2 where it meets the stream of solution overflowing from No. 3. The thick underflow from No. 2 is raised to the level of No. 3 where it is diluted with barren solution and also with the overflow solution from No. 4. The underflow from No. 3 is raised to No. 4 and diluted with water. The underflow from No. 4 goes to the dump but if necessary may be first passed through a dewatering filter. The following typical flow sheet and calculations for dissolved value loss are taken from a pamphlet issued by the Dorr Co. Regarding the assumption in these calculations that 75% of the whole metal dissolved goes into solution during milling, this is probably about correct for a gold ore, but for a silver ore it would in the writer's experience be nearer the facts to assume a dissolution during milling of only 15% to 25% of the total silver dissolved. (a) 100 tons of ore per day crushed in cyanide solution. (d) 75% in mill and 25% in Agitators. (e) 400 tons of solution from Thickener V precipitated to $0.02. (f) Agitation with a dilution of 2 of solution to 1 of solids. (g) Let V, W, X, Y and Z represent value in dollars per ton of solution discharged from the respective Thickeners. Equating out of and into each Thickener: (1) 100V 400 V = 500W + (0.75 × $10.00 × 100). (2) 100W +600W = 500X + 100W+(0.25×$10.00×100)+100V. (3) 100X + 500X = = 100W 500 Y. 100Z + 100X + (400 ×0.02). 100Y+ 100 tons of water value $0.00. The amount precipitated from 400 tons @ ($2.51332 · The amount due to neglected decimals 0.02) The amount dissolved 100 tons @ $10.00 = $1,000.00. From the foregoing the following results are deduced: Assay value of the pregnant solution, i.e., value of V = $2.51332 Dissolved value saved 99.7 %. Calculation for Mechanical Loss of Cyanide Conditions Assumed: = 0.02666 (a) Neglect the Cyanide consumption throughout the system. (c) Let V, W, X, Y and Z represent the strength in pounds of Cyanide in Solution discharged from the respective Thickeners. Equating out of and into each Thickener: |