It seems desirable to remove zinc more completely than can be done by acid-treatment, and also to obtain it in a form at once available for precipitation again. I feel safe in saying that from 40 to 50 per cent, of the zinc originally used for precipitation could be thus recovered.
Philip Argall says, in speaking of the practice at Cyanide, Colo., upon the Cripple Creek ores:
“Our general practice shows a consumption of 0.92 lb. of zinc for each ounce of fine bullion produced. Of this amount not more than 40 per cent, is dissolved, [by the cyanide solution], the remaining 60 per cent, being removed from the [precipitation]-boxes with the precipitate at each weekly clean-up, and owing to its fine state of division and richness in gold we find it preferable to destroy the zinc and recover the gold.”
In some cases the amount of fine zinc removed with the precipitates is less than that stated by Mr. Argall; in many others, more.
It has seemed to me that this material could be retorted, like the zinc crusts from the Parkes process; a certain percentage of lead and charcoal being mixed with it—the lead, to form an alloy with the gold and silver present, and prevent loss; the charcoal, to reduce zinc oxide to metallic zinc, and also carbon dioxide to monoxide, thus keeping the interior of the retort and condenser reducing. At a certain temperature below the melting-point of zinc, the zinc vapor would condense as dust; and if the zinc vapor be diluted with some inert gas, the formation of dust is greatly aided. Air would have to be completely excluded from the condenser, since the zinc dust, when hot, is very easily oxidized; and, for use in precipitation, it should contain as little as possible of the oxide, which not only does not precipitate gold and silver but actually consumes cyanide. With proper precaution, practically all the zinc vapor driven off could be recovered as zinc dust, at once available as a precipitant.
When ores containing mercury or tailings from amalgamation-processes are treated by the cyanide process, the precipitates contain mercury. In several cases the mercury contained in the precipitates has been profitably recovered, by retorting the precipitates in an ordinary mill-retort. If zinc were removed by distillation any mercury present could be condensed at the outset with little extra trouble; and in this way mercury could be recovered from precipitates which could not be economically retorted for mercury alone.
Sulman and Teed have retorted the precipitates obtained by their bromo-cyanide process. The practice at Deloro, Can., however, as described by Mr. Bosqui, produced very base bullion, containing about 69 per cent, of gold, and from 25 to 29 per cent, of zinc.
An attempt made at the Golden Gate mill, Mercur, Utah, to distil off the zinc in Faber du Faur furnaces, was unsuccessful because of the impossibility of reducing the large amount of zinc oxide present, and “ the persistent retention ” in the previously muffle-dried charge, of “ combined water, which caused explosions in the furnace.” This test seems far from conclusive.
The trouble seems to have been insufficient heat. The explosions attributed to combined water were probably due to the formation of a crust on top of the charge, due to lack of heat. It has been found in distilling the zinc crusts from the Parkes process that it is necessary to maintain the temperature well above the boiling-point of zinc until distillation is complete ; otherwise crusts form and explosions occur.
Experimental Zinc Distillations
Twelve hundred grams of cyanide precipitates were taken for this experimental work.
A complete analysis of the precipitates gave the results shown in Table IV.
The distillations were made in a small graphite retort, the counterpart, on a small scale, of those used in practice for retorting the zinc crusts from the Parkes process. The retort was heated in the cylindrical gas-furnace belonging to the equipment of the metallurgical laboratory of Columbia University.
The retort was connected by means of the section of a small graphite crucible (which acted as an allonge), with the condenser, which was a large graphite crucible. Figs. 1 and 2 show the whole apparatus and a section of the furnace.
The purpose of the experiments was to determine:
- Whether the zinc could be removed by distillation.
- Whether the losses of gold and silver by volatilization would be prohibitive.
- Whether the zinc could be recovered in a form available for use as a precipitant.
- What temperature was necessary to remove the zinc completely.
- What time was necessary to remove the zinc; and also minor factors of the problem.
Refining Experiment I.—A mixture of 300 g. of precipitates with 500 g. of test-lead and 50 g. of charcoal was placed in a small graphite retort, and this in the gas-furnace. The gas was then turned on, and the retort allowed to heat up slowly for one hour until all moisture was driven out, then the heat was raised and the heating continued for five hours.
The weight of zinc recovered from the condenser was 74 g., or 85 per cent, of the total zinc present. The zinc contained by assay 7.2 oz. of silver and 0.8 oz. of gold, per ton. This experiment seemed to establish the fact that the zinc could be driven off satisfactorily by distillation, in fact more completely than by acid-treatment. Also that losses by volatilization were not high, and in view of the fact that the zinc was to be used again for precipitation, they were of no moment.
Refining Experiment II.—A mixture of 400 g. of precipitates with 100 g. of charcoal and 1,200 g. of granulated lead was treated as in the first experiment; except that a thermo-couple was introduced into the retort as shown in Fig. 2, and temperatures were taken every 15 minutes, as shown in Table V.
This experiment showed a smaller loss by volatilization than No. 1, due probably to the greater percentage of lead in the charge. The assay of the metallic zinc was rather high; but this is due to the fact that it collected in the allonge, and was probably enriched by particles of precipitates which lodged there while charging the retort.
Refining Experiment III.—This was undertaken to ascertain whether zinc could not be completely driven over at a lower temperature than that used in I. and II., and what function time and temperature played in the expulsion of the zinc.
A 200-g. sample of the precipitates, mixed with 200 g. of test-lead and 50 g. of charcoal, was retorted, the temperature being taken as before at intervals of fifteen minutes. A sample of the material in the retort was removed every half-hour and assayed for zinc. The results of these tests are given in Table VI.
NOTE.—Much trouble was experienced in introducing the thermo-couple into the retort in such a way that it is not injured by the zinc vapor. In the first experiment the couple was introduced as shown in Fig. 2, but this was found unsatisfactory, as upon strongly heating the retort the short clay tube became loose in the retort and zinc vapors escaped. The plan finally adopted, which has given no trouble, is to enclose the wires of the couple in a porcelain tube having a high fusing-point, and long enough to reach outside of the furnace. The closed end is introduced into the retort about two inches. The space between the porcelain tube and the walls of the retort is packed with asbestos, which is found to do the tube less harm than any other substance which would withstand the high temperature. This joint of course is not impervious to zinc vapor, but as the collecting of the zinc in this experiment was of secondary importance that disadvantage was of no moment.
Refining Experiment IV.—A 150-g. sample, mixed with 40 g. of charcoal and no lead, was retorted in this experiment; the temperature and a sample of zinc being taken every fifteen minutes. Table VII. shows the results.
In this case no assays were made of the condensed zinc, as they would be of no value; for even when the utmost care was exercised in removing the samples for the zinc-determination, small particles would fall off into the condenser, thus causing the condensed zinc to show much higher values in gold and silver than could occur by volatilization.
Experiment V.—A mixture of 95 g. of precipitates with 95 g. of borax-glass, 400 g. of test-lead, 25 g. of soda, 10 g. of silica and 35 g. of charcoal, was retorted as in the previous experiments. The thermocouple was not used in this experiment, as it was feared that the fluxes used would attack the porcelain tube which protected it.
The object of this experiment was to determine whether a fusible slag could not be formed which could be easily poured from the retort and from which the lead would separate. But although there was a large excess of fluxes used, the material came out as a sintered mass, probably by reason of the excess of charcoal in the charge.
The residues after distillation showed 0.14 per cent, of condensed zinc, 3.86 oz. of gold, and 35.8 oz. of silver, per ton.
The total time of the experiment was five hours, and about the same temperature was maintained as in Experiment IV.
Zinc Distillation Experiments
The retorting of the gold-silver-zinc precipitates, while resembling that of the silver-gold-zinc alloys obtained in the Parkes process, requires a greater proportion of charcoal by reason of the large percentage of zinc oxide, and also the carbon dioxide present. In both cases the best results are obtained by raising the heat to the proper degree at once and maintaining it at that point until distillation is completed.
The time required on a small scale has been 4 or 5 hours. About 90 per cent, of the zinc is removed in the first 2.5 hours and at a temperature not much exceeding 1,200° C. To remove the remaining zinc requires from 1.5 to 2.5 hours, and a somewhat higher temperature. It seems desirable to heat the material finally to about 1,300° C. in order to remove the last of the zinc.
The same thing is noted in retorting gold- and silver- amalgams. Most of the mercury is removed at a temperature not much above the boiling-point of mercury, but to remove the remainder requires a much higher temperature.
The volatilization of gold and silver is not excessive, and in view of the fact that the zinc can be used again for precipitation, it is of no importance. Indeed it is an advantage, because the zinc containing a small amount of gold and silver would be a very active precipitant.
I have not been able to determine exactly the conditions of condensation required to form a zinc dust suitable for use as a precipitant. It was noticed that the first zinc that came over, which was largely diluted with carbon monoxide, condensed as a dust fairly free from oxide. The condenser was cool at this time. A sample of this zinc dust showed 97.1 per cent, of metallic zinc. But later, as the zinc vapor became less diluted with carbon monoxide and the condenser became hotter, a portion of the zinc condensed as a liquid bath and the remainder as zinc dust, which in its highly heated condition rapidly oxidized to zinc oxide. There was always a chance for air to enter the condenser, as it was necessary to keep the orifice (d, Fig. 2) open a part of the time to permit the removal of accretions from the allonge. The result was that most of the product from these experiments was not fitted for precipitation.
The formation of the zinc dust seems to require that the zinc vapor be diluted with some inert gas (carbon monoxide is probably the best and most available) and condensed at a temperature below the melting-point of zinc, without access of air. This could be accomplished by condensing in a water- jacketed condenser, in which there was a stuffing-box through which a rod could be inserted, to keep the allonge free from accretions. Provision should also be made for introducing gas into the condenser, if it were found that the carbon monoxide formed in the retort was not sufficient.
The material in the retort at the end of the distillation is in appearance the same as at the start; but upon closer examination there will be found small shots of bullion all through it, and usually one or two large masses, especially if lead is used which very much increases the bulk of metal.
This mass can be poured from the retort like so much sand; no trouble is experienced from its sticking. The retort may be easily washed or cleaned by throwing in a little metallic lead while it is yet hot, and shaking the molten lead around. This seizes upon any particles of bullion sticking to the retort, and is subsequently poured and added to the main mass before it is run down.
The residues from the five experiments were united, mixed with four or five parts of litharge, and melted down on a test. Enough iron filings and silica was added to make a fluid slag, which, after thorough fusion, was skimmed off, and the lead was cupelled. The amount of bullion recovered in this way was close to that shown to be present by assays on the original precipitates, although a little deficiency, due to the large number of samples taken for analysis, could not be avoided.