The ore consisted of quartz, in which, above the 250-ft. level, the iron-minerals were largely oxidized and some free gold was visible; below that level few traces of oxidation occurred, and pyrite constituted the principal mineralizer in the quartz, together with occasional pockets of galena and a few eccentric specks of covellite. The 20-stamp mill was equipped with plates for amalgamation, and three Standard tables for concentration. The tails from the tables were elevated by a centrifugal pump to a launder, by which they were conveyed to a 60-ton cyanide-mill for further treatment. The cyanide-mill was erected at so great a distance from the stamp-mill that a separate crew of workers and a separate power-plant were necessary. No arrangement had been made for the treatment of the concentrates as produced; although later, an adobe roasting-furnace had been erected near the stamp-mill, as well as a small cyanide-plant, for the treatment of the roasted product.
The treatment of the mill-tails in the remote cyanide-plant was unprofitable. The concentrates could not be shipped for treatment, on account of the high freight-cost and high smelter-charges ; and, as the law of Arizona prohibited the use of wood as a fuel for roasting ore, the adobe furnace was useless. As a natural conclusion, it seemed evident that, if possible, an entire change of programme was necessary, involving two requisites : 1. A very clean concentration of the mill-tails, producing a final material containing very low values; 2. A local treatment of the raw concentrates.
The concentrating-plant was unfitted to attain the first requisite ; the Standard tables affording a very fair rough concentration, but making a very poor separation of middlings and slimes. Two Frue vanners were installed, the inefficient middling-pumps from the tables were thrown on the scrap- heap, and the plant started operation as follows: The tails from 10 stamps passed from the plates to one Standard table, and from this upon a second table. The tails from the other 10 stamps were similarly conducted upon a Standard table, but from this into a small spitzkasten, situated above the distributor of Frue vanner No. 1, from which the thickened pulp passed to the vanner for concentration, while the overflow passed to the slime-sump, mentioned later. The slimes from each of the tables were conducted by launder into a sump, from which they were elevated by a 2-in. Byron Jackson centrifugal pump into a set of 3 large spitzkasten arranged in a series, the overflow from the first passing into the second, etc., the final overflow, consisting of fairly clean water, passing into a supply-tank for use as feed-water for the batteries. The more or less de-watered slimes from the large spitzkasten were conducted by launders to a small spitzkasten, which discharged a product containing still less water upon Frue vanner No. 2 for concentration, the overflow from this last spitzkasten also passing back to the slime-sump. In the case of the first 10 stamps, the middlings from the first table passed with the tails to the second table, from which the final middling-product was caught in a separate receptacle and returned to the battery. The middlings from the second 10 stamps passed with the tails from the table to the vanner. By this arrangement, only that water was wasted which was necessary to carry off the final tails and that which was necessary for the vanner-supply. It was found that the final tails were of very low grade; the result being somewhat surprising, considering the smallness of the concentration-plant for an output of 70 tons per day, but being doubtless referable to the comparatively simple character of the ore.
A futile attempt had formerly been made to extract the values of the table-concentrates by cyanidation without roasting, and about 60 tons remained in one of the cyanide tanks, from which a portion of the values had been extracted, but which yet contained about 6 oz. of silver and 2.25 oz. of gold per ton. An analysis of these concentrates suggested the following approximate constitution: SiO2, 17.64; CaO, 4.95; FeS2, 52.73; Fe2O3, 25.31; Pb, not det.; Cu, none; As, none; total, 100.63 per cent.
Continued leaching of these concentrates, for about 21 days, with solutions of various strengths, removed about 75 per cent, of the remaining values, but with a large consumption of cyanide, when the extraction practically ceased. The material was thoroughly aerated by turning over with shovels, and one-half placed in an adjoining tank; both portions were again subjected to leaching with cyanide, but with no compensatory extraction.
In the above treatment of the concentrates, the first solutions drawn off were of a brilliant claret color; and, as no copper had been found in the analysis, the reason for the phenomenon was somewhat obscure. A repeated test of the concentrates again showed no copper, but an analysis of the material, precipitated from the colored solution by addition of acid, proved it to consist of cupric ferro-cyanide. Further consideration discovered the source of the copper in the residue left by evaporation in one of the solution-tanks; this had been taken up by the new solution and produced the color mentioned. Analysis of samples of freshly-made concentrates showed them to contain about 0.2 per cent, of copper; enough to assist nicely in the zinc-precipitation.
After various experiments upon the raw concentrates, it was found that if the material was ground to 100-mesh and agitated for 32 hr., about 85 per cent, of the gold and 70 per cent, of the silver could be extracted, at an expense of about 6.5 lb. of cyanide per ton. This result promised a very fair return (especially in a case where no other procedure was available); and the system was put in effect in the mill, with the happy, and somewhat unusual, result that the mill-practice has yielded very much better returns than those obtained in the laboratory experiments ; a fact which is probably due to the rise in temperature produced during the grinding of the pulp.
The yield of concentrates from the mill was about 2 tons per day. For grinding so small a product, the purchase of a ball-or tube-mill was out of the question; so also was the use of one of the stamp-batteries for the purpose; and about the least expensive machine which could be thought of, which would have a sufficient capacity and which would probably fulfill the other requirements, was one of the old-style amalgamating pans, such as are used in silver-mills. A second-hand, 5-ft. pan, with wooden sides, was bought in, installed in the mill, and arranged to discharge into each of two leaching-tanks belonging to the small cyanide-plant formerly mentioned. The pan was charged with about one ton of solution carrying 6 lb. of cyanide, 6 lb. of lime and 1.5 tons of raw concentrates; it was set in motion at the rate of about 75 rev. per min., and continued to grind for 8.5 hr.; about 2 lb. more of cyanide being added during the day, as the strength failed. At the end of the period, the material was found to be finely ground, and was discharged into the leaching-tank; it was also found that, at the close of the grinding, the temperature of the mass had risen about 40° F. above the outside temperature. A sample of the ground pulp was taken, filtered, washed and assayed, with the somewhat surprising result that an extraction was shown of 90.7 per cent, of the values.
Since the initial test, the operation has been carried on continuously with similar results; the only variation occurring when the grinding was affected by the clogging of the mullers by foreign matter. Since this occurrence, the concentrates have been passed through an 8-mesh screen, and no further difficulty has been noticed. After the grinder is discharged into the leaching-tank, and after the solution has settled to some extent, it is customary to cover the material in the tank with dry middlings from the table, for the purpose of facilitating percolation; in this manner, filtration goes on with sufficient rapidity. When one leaching-tank is filled, it is continuously leached with cyanide solution, while the other tank is being filled; it is then washed and discharged through a bottom-discharge door. The capacity of each tank is about 15 tons; so that a tank has about 4 days of extra leaching after it is filled; it then has 3 days’ washing before discharge. During a campaign of 6 months, in which time various grades of concentrates have been treated, the average extraction has been about 94 per cent, of the total values. The grinding and cyanide-plant require little attention, except in the matter of charging the grinder and discharging it, and the whole plant is easily operated, without additional cost, by the man who has charge of the tables and vanners. The pan consumes about 8 h.p., and, as stated, the total cyanide consumption is about 8 lb. per ton; adding the cost of 6 lb. of lime, the total cost of the treatment of the concentrates, including the values left in the tails, does not materially exceed $5 per ton.
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The concentrates, as they are taken from the tables and vanners, are given as much chance as possible to aerate and oxidize, since it is found to be the case that partly oxidized material grinds more quickly and gives up its values with a notably less amount of cyanide. In the extraction of the final values from the old concentrates upon which tests were first made, it is found necessary to grind them for 2.5 hr. only. Muller-shoes last about 3 months; while the dies last much longer. The zinc-precipitation is good, and the solutions have not yet become too foul for use; most of the copper is slagged off in remelting the bullion.
In the first attempts at the use of the grinding-pan, it was the intention to make the pan work continuously, by overflowing and allowing the continual discharge of finely-ground ore-particles into the leaching-tank. It was found, however, that the rapid motion of the mullers carried coarse as well as fine to the surface, and the scheme was abandoned. It was next attempted to allow a continuous discharge through pipes set midway up the sides of the pan, sizing with cyanide solution by spitzlutten, and return of the coarse material to the grinder. This also was abandoned, and the discharge-opening at the bottom of the pan was threaded and a valve put in. The charge in the pan is now ground, the valve connected with a discharge-pipe, the valve then opened and the contents of the pan discharged.
To show the grinding-efficiency of the pan, a series of sizing- tests were made upon concentrates as they came from the tables, and upon similar material after having been ground for 8 hr., as follows:
In crushing to the above degree of fineness, the material seemed to be fairly amenable to cyanidation, while not too fine for good filtration; so that 8-hr. grinding is continued.