The history of the cyanide process in the United States may never be written in its entirety. That it would prove both "amusing" and "instructive" is fairly certain.
Amusing, for it would include opinions and practices diametrically opposed. This feature would, however, verge into the tragic at times. The record would show attempts so fundamentally futile that there would be more tears than laughter in many parts of the narrative.
Instructive, not only in view of the many useful principles and facts discovered, but hardly less because of the light thrown, for the benefit of whoso can walk in the light, on pathways in the regions of "how not to do it."
At first, "viewed with alarm," or, rather, incredulity, it was referred to as something which somehow succeeded only on the other side of the world. When Australian, and more especially African, experience, became too statistical for disbelief there followed a reaction which, like all reactions, went too far.
"Cyanide" was the long-looked for panacea, the salvation of the abandoned low grade mine, whether in stopes or from the dump. It was the simplest, the cheapest to install, the most economic in the running.
Anything would "cyanide" from brickbats to tellurides. All that was needed was a few washtubs. Pour in your solution, and tap it off; there was the long-fabled "oil of gold," the dream alike of the ancient alchemist and the modern fakir.
The process then had already passed some fifteen years ago, through the three stages of "ridicule, argument and adoption."
It would be hardly fair to cite the numerous cases in which it was tried for the working of hypothetical ore, which never materialized. These attempts, which have a generic resemblance to the erection of flour mills in the Sahara desert, are common to all regions and to all processes. Nor will they cease until Carlyle's classification of mankind as "mostly fools" has become an ancient myth.
The failures of more technical importance have been of a different class, viz., the cases where there was plenty of ore but not the right kind. Another class of cases has still greater interest for the practical inquirer, those namely in which ignorant application of the process failed, while a rational treatment would have resulted in pronounced success.
Coming from the country at large to a single district, Cripple Creek, we have the three stages of the evolution of the cyanide process beautifully illustrated. Beginning with the second of the three above outlined, we have here been shown the first enthusiasm, the later backward reaction, and now the successful outcome of painstaking research and intelligent application.
In speaking, however, as many have done, of the first rise, the subsequent fall and the second rise of the process in the treatment of Cripple Creek ores, the words must not be taken too literally. The process that fell was not the same process that rose.
"Pigs is pigs"—but cyanide is not always cyanide. Perhaps after all, twenty years was a short enough time for the development of that fact. It took a much longer time to evolve rational smelting schemes, and possibly the end is not yet.
No one familiar with the early history of Cripple Creek will forget the first attempts at cyaniding. Nor were they entirely unsuccessful, for at that time thousands of tons of oxidized surface ores were still procurable. Even amalgamation was in use in at least two mills. Later the works at Florence marked the development of confidence in the process.
The Arequa mill, long run as a combination of chlorination and cyanide, was abandoned, and the Union Extraction Works went up at Florence. During the earlier part of the time in which the scheme was developed, the idea of a large cyaniding capacity in the new mill was entertained. Then came a reduction in the ratio, and finally the extraction mill was erected at Florence entirely on the chlorination plan.
A large cyanide mill in the region, erected for the sole object of treating Cripple Creek ores, went out of commission entirely.
There were others; but it is not the intent of this article to go into historical details concerning the abandonment of the process in the district. It is a familiar fact that a few years ago it was at its minimum, while it is equally a matter of current knowledge that two important mills have now gone into it wholly or in part, with serious plans for other extensions in the near future in favor of the formerly decadent system.
In short, the first rise of the process was very largely due to the enthusiasm of ignorance. Its relative fall was due to the non-realization of hopes which should never have been entertained.
It remains to speak of its new birth.
The Portland mine long since brought mill treatment of its ores to a commercial basis. The erection of its 300 ton chlorination mill at Colorado Springs marked about the summit of the chlorination movement in the district.
The old cyanide practice was unsatisfactory. Chlorination held out alluring inducements. So the district announced to the world that it was off with cyanide and on with chlorination.
The capacity of the Portland mill was over 300 tons a day. The tailings from the barrels were not immediately rejected, but were concentrated on tables, the concentrates consisting largely of ore which had escaped oxidation in the roasters, being sent to the smelting company.
The final tailings were relegated to a vast "dump" whose present tonnage is something over half million tons, for the retention of which dams have been erected across the creek which flows close to the works.
Many acres in extent, something over thirty in all, are occupied by this great spread of low grade stuff, which has awaited for a series of years the arrival of the process which should unlock its hidden treasures.
And the process is the once-neglected cyanide.
It was in 1905 that the first experiments were started at the Portland mill, looking toward the treatment of this mass by the cyanide process. The lessons learned were favorable to the method, especially to the idea which has been so largely developed of late, viz., crushing in cyanide.
It was early ascertained that the tailings as dumped were not fine enough. The progress was soon sufficiently great to demonstrate that the major part of the extraction could be accomplished in the crushing, and the tube mill was indicated as the best means for this "re-grinding," being indeed the method finally adopted.
A small experimental plant was run at the mill for some time, the development being largely the work of Mr. John M. Tippet, long the chemist at the works, now in charge of the cyanide plant as metallurgist.
It was further determined to look abroad for more light, and Mr. Tippet accompanied Mr. George M. Taylor, assistant superintendent of the Portland, in a trip through Utah, Montana and Dakota.
Mr. J. B. Daniels, general superintendent, accompanied by Mr. S. J. Connolly, chief electrician of the works, also made trips to California and Nevada on the same mission.
Nevertheless, it appears that more light was obtained on the process in the home experiments than in all these travels in search of wisdom.
The drawings for the mill and much of the construction were executed by Mr. Connolly. It will be noted, therefore, that both in chemical and mechanical schemes, the new cyanide mill is a home product. The company is to be congratulated on having in its employ the gentlemen whose names are given above and who have so amply developed the process.
The points most to be emphasized in present practice here are:
From this it may at once be inferred that the time element has been minimized. It follows, of course, that the space necessary for treatment of large tonnage on the percolation method is also dispensed with.
The saving in original installation expense and in running costs will suggest themselves at once to any manufacturer.
The main building, which is erected partly on the "terrace" plan, is 176X144 feet, having also some additions or annexes which contain apparatus of various kinds, chiefly, however, pumps and scales.
The upper level holds the main or "standard solution" tanks, whose dimensions are 35 feet diameter by 16 feet high. These, like all the other tanks in the mill, are built of steel sheets. Bins for the reception of the tailings from the dump, and apparatus for their proper feeding to the tube mills are above. The tube mills are on this level, as are also the four main tanks for treatment, although, as will appear, their chief function is to provide means for recovery of the solutions by settling and decantation.
Most of the "treatment" is over before the pulp ever reaches these tanks. Their dimensions are 40 feet diameter by 20 feet high.
The next level is chiefly devoted to the filters which treat the decanted solution. The great bulk of the pulp is at present not filtered at all, as will be later seen, but it is by no means improbable that the filter press will soon be used to extract the last of the solution.
The lowest level holds the sump tanks, two in number, usually designated as the "strong" and "weak" tanks, dimensions 22 feet diameter and 8 feet high. The demand on solution from these tanks being active and constant, very great capacity is not needed. Their supply is (precipitated) solution from the zinc boxes.
From this lowest level is built up a platform holding the zinc boxes, which are thus on a level intermediate between the second and the ground level (i. e., lowest ground level).
Let us first look at the great dump, which does not resemble a common "dump," but is rather like a 25-acre section of the desert—minus the sagebrush. The depth of the material over this area varies; its probable total tonnage has already been stated at half a million.
Scrapers drawn by two horses, six teams to the shift, are used to bring this loose material to a dumping point. It drops to a feeding hopper, thence on a 14-inch endless belt which conveys it to the mill. Stringers are laid directly on the bed of the tailings, carrying rolls for the support and guidance of the belt.
At present the journey is about 800 feet long. The hopper is readily moved as required, the belt being periodically lengthened as the convenience of handling dictates. The mechanism for driving the belt and finally dropping the stuff into the mill, demands no special description.
The teams (twelve in all) work in two eight-hour shifts.
The average moisture of the dump gives 8%.
Tonnage moved daily, slightly over 400 tons. Cost is given in general summation, later.
The tailings are of a nominal 14-mesh fineness. But 15% to 20% of them will pass a 200-mesh, and various large proportions of them will naturally be found at all intermediate stages.
Arrived in the mill the material first seeks the tube mills, a brief account of which appliances is here given. It may be well to indicate the rate at which these mills are catching up with the main (chlorination) output of tailings. The latter is close to 350 tons a day.
Thus we have 50 tons daily as the gain of the rabbit on the tortoise. The chlorination tortoise, however, has a half million ton start of the cyanide rabbit. The gain is considerably under 20,000 tons a year. However, as we are dealing in very rough "futures," we may allow a gain of 20,000 tons a year as an estimate, at which rate, as some very elementary mathematics will demonstrate, 25 years will be required for the cyanide to "catch up."
But a great many things may happen before 1932.
The tube mills are two in number and are twins except as to lining. Dimensions 22 feet long by 5 feet internal diameter. One is lined with "silex" blocks 4 inches thick, thus diminishing diameter by 8 inches; the other with white-iron plates, 1 inch thick, diminution of diameter, 2 inches.
They are charged with "Danish pebbles," whose average diameter is about 3 inches. Nine tons of these go to the more capacious iron-lined tube and seven tons to the silex-lined. These weights fill the tubes to rather more than one-half of their capacity.
The comparative merits of the two linings cannot yet be stated.
The "pebbles" cost $20 a ton, laid down at the mill.
The tubes, and all the other "moving parts" at the mill, are driven by electric power derived from the lighting and power plant of the city of Colorado Springs. The total power developed approximates 105 horse-power.
The tubes are set horizontally. At 28 revolutions per minute they can easily handle 10 tons per hour each, making a total capacity of 480 tons a day. They are just now limited to 400 tons a day for total service.
In the delivery of pulp to mill, it meets, just before it drops into the final feeding hopper, with a carefully graduated stream of solution from the standard tank. Strength of this solution is 1.2 lbs. per ton and its weight is from 40% to 50% of the total; ordinarily the proportion is: solution 40, tailings 60.
The tube is fed through the trunnion, and the delivery is accomplished by a small, detachable "spiral" pump. At the same time lime is fed into the tube, at present by hand, although this is to be soon supplanted by mechanical feed.
The present lime addition is 5.2 lbs. of lime to a ton of tailings.
The lime addition is a matter of chemical observation and is, of course, subject to variation. The nature of the material, however, and the uniformity of the conditions to which it has been subjected, makes this variation small. Careful regulation of alkalinity is too familiar a topic to be dwelt upon here.
The pulp issues from the tubes with a fineness of 60-mesh. This is quite as nominal as the "14-mesh" at which it entered. In fact 80% of the product from the tubes goes through an 80-mesh screen, and as a matter of course a very large proportion of this is much finer.
This reduction in size is of fundamental importance in the treatment. While it is hardly true that the proposition is reduced entirely to a "sliming" one, it is evident that the approach to that stage has brought about the very satisfactory yields already recorded. From the first the proportion of gold dissolved was far greater in the mill than in all the subsequent operations, but the proportion taken from the pulp now, in the settling tanks, is smaller than thought at first.
Since the mechanical separation to be presently noted, its proportion has become still less.
As the pulp issues from the mill it is met by a stream of weak cyanide solution, pumped from one of the sump tanks on the lowest level. This solution has already been passed through the zinc boxes, and is regulated in flow so as to deliver nearly three times the volume of the solution from the tube mill.
The solution as it comes from the "mill" has strength of from 0.9 to 1.0 lbs., while the solution which dilutes it has strength of from 0.4 to 0.5 lbs.
The solution which accompanies the material to the decantation tanks runs from 0.5 to 0.6 lb. per ton. (At this point we continue description of process as it was seen during the last week in August.)
The capacity of the settling tanks being greater than a 12-hour run, they are never filled. Pumping is continued for the period mentioned, then they are allowed to settle. However, decantation begins within 14 hours of the time pumping ceases.
It is accomplished by very simple means; a swinging pipe, which delivers low down on the tank, is kept with its open top at the surface of the liquid, and is intermittently lowered as decantation proceeds. Except, then, the first 1½ hours, settling and decantation are proceeding simultaneously.
The decanted solution is of course not at all clear, and passes to the filter, as later detailed. The settling and decantation take considerably under 12 hours, so that the alternation from tank to tank has the time margin essential to systematic working.
Two of the four tanks receive the diluted tube-mill product alternately, one receiving while the other is being decanted. The other two tanks operate similarly, receiving the pulp from the first pair.
The decantation having been carried as far as practicable (always some of the solution stands above the level of the settled pulp), the tank is sluiced out with weak solution from below, and pumped into one of the second pair of tanks. When compressed air can be spared from the main works, which is not always, it is used to aerate the weak solution at this point.
Treatment in the second pair of tanks is a repetition of that in the first pair, and the decantation having been carried as far as possible, there remains in the tank the "final tails" which, being sluiced out, this time with water, is carried by launders below the dump, to find its way in due time to the Gulf of Mexico.
The filters, which occupy the middle level of the mill, are 57 x 14 feet and 8 feet deep. They are, as usual, packed with gravel and sand. There are three of them, so that during a clean-up two can still be in active service. The solutions from first and second sets of the original tanks are kept separate to the last.
Thus, as we have strong and weak tanks (a pair of each as above), we have also "strong" and "weak" filters, and strong and weak sets of zinc boxes, even a strong and a weak sump tank.
It is contemplated to put the final tails, with the adhering solution into presses, and squeeze out the last of the solution which, of course, still carries gold. This has not yet been carried out, but the principle is excellent. It is obvious that in a very low grade proposition these last "squeezes," both figurative and physical, are the important parts of the whole business.
Flow from the decantations to the filters is nearly constant; from the latter to the zinc boxes almost entirely so.
The zinc boxes are 108 in number, arranged in 18 sets or rows of 6 each. Each set of 6 is a complete system in itself, receiving the fresh zinc in box No. 1 and sending out the final precipitated gold from No. 6.
At the present time 84 of the boxes are in service, 42 taking solution from the strong and the same number from the weak side. The solution used to sluice out the first pair of tanks after decantation is the weakest of all, coming chiefly from the "weak" zinc boxes. But in spite of every care, it is inevitable that a certain constant amount of gold in solution is lost with the final tails, and it is this that the filter-press for tailings is designed to save.
An experimental press is already operating.
The boxes are exact cubes, 4x4x4 feet, hence of a capacity of 64 cubic feet each. The set of 6 stands in a container, with a small space between boxes, so that solution may flow over from one box into the space between, and then, descending in this space, find its way through the bottom perforations of the next box, and ascend again.
Thus, the solution, flowing first into one of the spaces—the end one—goes up through the last in series of the 6 boxes, by means of bottom perforations and wire screen, flows over the top into the space between No. 6 and No. 5 (about 3 inches wide and of course same as side of the box in its other dimensions) and, descending to bottom of No. 5, goes through exactly the same course in each and every box. These boxes are open, and the arrangement is very compact, only space enough being left between rows to allow proper handling.
When the precipitation is judged to be complete enough, box No. 6 is cleaned up. Evidently, it is not a really essential point whether it would still continue to precipitate a little—there are five others back of it, all "fresher."
This clean-up comes at 10 days' interval for any particular set. When, then, the zinc with its adherent gold is removed from No. 6, all the other boxes "move up"—not literally, but their contents do.
No. 5's charge is taken carefully out and repacked in No. 6 box, and so on down the line. Box No. 1 being now empty is charged with fresh zinc. Two hundred pounds of the "shavings" are placed in each box, which fills it about two-thirds full.
The flow from these sets is carried to the sumps, whence it is being taken again, some of it constantly for dilution of the original solution as it issues from the tube mills, some of it intermittently for sluicing out the decantation tanks.
Re-standardizing, of course, always proceeds according to the daily tests made of strength of the various solutions. There is some loss of volume by evaporation, some by solution carried away in "final tails," the balance is well maintained.
Whether any complication in this respect will be introduced when the squeezing-out filter comes into play is a point yet unsettled. But it is evident that the question of superfluous volumes has no such importance here as in mills using "percolation" and the washing-out with water (displacement), so that with present capacity it is fairly certain that no demand for extra tanks will arise to plague them.
The treatment of the zinc-box product starts with sulphuric acid, the zinc being wholly dissolved away. After thorough washing, the gold is passed to the filter press. In the subsequent melting a siliceous flux is used. Quartz to the extent of 40% of the "slimes" from the zinc, 15% of soda, 10% of borax, and borax cover.
Some items of interest as to costs are annexed. We are still adhering to results as realized before the recent curious addition to practice in the mechanical removal of free gold, which is treated as a separate topic.
The July totals were as follows:
|Cost per ton
|Tailings to mill
|Melting and potation
* Treatment includes that portion of electric power cost that goes to pumps, water, store house, lights, pay-roll (partial, see below under solution), including watchman, three machinists, one sluicer, one foreman.
The total cost of 57 cents a ton has been very materially reduced since this statement was taken from the books. In another division of items it appears that for this same month of July, the more important items figured as follows:
|Cyanide used per ton of ore
|Cost 4.00 cents.
|Zinc used per ton of ore
|Cost 1.54 cents.
|Pebbles used per ton of ore
|Cost 0.53 cents.
|Lime used per ton of ore
The general statement is now made that cost per ton is certainly down to 50 cents, and probably a shade under that figure.
The statement for July and 20 days of August, as to assay values and yield, is annexed. It has small interest in the light of the great additional saving to be mentioned later.
The total extraction on these averages is 60%. The actual extraction in ounces is 0.0434 per ton, or 90 cents. Of this total the tube mill is to be credited with just 70%. So far the results upto September 1st. A very curious tale remains to be told.
As is seen from the figures above, the total average value of the tailings was $1.49 per ton. Extraction 90 cents. Cost, 57 cents. Net profit per ton, 33 cents.
It has already been stated that costs are now at 50 cents, which alone would make the net profit 40 cents instead of 33 cents.
It is found to be practically impossible to get an accurate estimate of the relative extractions from the first and second pairs of tanks, so in the table above there is no statement between tube-mill extraction and the final.
The four treatment (decantation) tanks discharge at a point from 8 to 10 inches above the bottom. In examining the residues at the bottoms of the tanks after sluicing out, Mr. Tippet assayed these with reference to values, first concentrating them, and obtaining such astonishing results in gold that it was at once apparent that in the rejection of these tank concentrates considerable amounts were being actually thrown away into the "final tails."
The strange part of it seemed to be that this free gold (for it proved to be such) had escaped both the concentration tables of the original chlorination mill and the action of the cyanide afterward. But the latter speedily proved to be no mystery at all, for the gold, although obtainable in a very fine condition by "panning," was hardly attacked by cyanide!
As for the fact that it escaped the "table" concentration, it does not seem to the writer to be very mysterious. The material has been recrushed, and with the reduction of the average mesh to such an enormous extent as already indicated, there has evidently been liberated, mechanically millions of fine particles of gold.
It is, in fact , astonishingly fine, probably made so in part by the tube mills themselves. But very fine gold may easily escape on the concentrating table, especially when the flow of water is too great.
It is certain that during a great part of the time in the chlorination mill, the flow of water was too great, and that owing to a faulty original construction this could not for a long time be remedied.
But, whatever the theory, there was the gold!
Launders placed with blankets in the final discharge caught enough of the precious metal to settle the fact that gold was getting out of the mill entirely too fast. Even with the first imperfect means adopted, the return from blanketings for September alone was $1,300.00!
The plan now adopted for the saving of this hide-and seek product is as follows, as detailed in a letter to the writer from Mr. J. M. Tippet.
Launders are arranged leading from the discharge of the pumps which carry from the tube-mill discharge to the treatment tanks. The whole of the material passes over the blankets which are placed in these launders. Thus the saving of the free gold is at present effected between the tube mills and the first set of tanks.
These launders are 70 feet long by 2½ feet wide. The tube-mill product going to the tank flows over one of these blankets for 12 hours, giving a charge of ore in the tank of over 200 tons, then the flow (now diverted to the second tank) is carried over the second launder, while the first one is being "cleaned up."
The cleaning-up is simple, and consists of washing off the blankets in a wooden tub, and concentrating by simple settling. The concentrates weigh an average of 100 lbs. from the 12-hour run, and are worth the amazing amount of 50 cents a pound!
Thus there is a product of $100 a day, a net addition to profits, for this saving costs nothing. The free gold is said to be "off-color," having a white or grey look. A saving of over 20 cents a ton in an operation previously yielding a bare 40 cents means an increase of just 50%!
At times the dump shows very low averages. It is at such times that the free gold saving will be felt to be a boon indeed. It would appear that some of this gold had been caught by cyanide before, since the ratio of tube mill saving now figures higher than ever in the returns.
The new process has not been running long enough for any averages to be made by the month. Here, as elsewhere, nothing but a month's run can make any true exhibit of profit. Calculations founded upon samples taken here and there, no matter how carefully, do not lend themselves to trustworthy estimates, but the total bullion return minus the month's expenses cannot be gainsaid.
Under the new blanket saving device, assay of final tails has dropped to 40 cents.
Now comes a curious piece of observation, as to the residual values in these same "final tails": They show and almost uniform assay of 40 cents and this irrespective of the assay of the original dump material.
Here is an "irreducible minimum" yielding plenty of matter for theorizing. Granting the practicably invariant value of this product, it becomes at once an element for computation, given the assay value of the original dump material. We may calculate "backward."
Let us, for example, assume an assay value of $1 in dump matter.
Then we shall have a yield of 60 cents and a profit of one dime.
We may, in short, figure extraction on the basis of the irreducible minimum.
The new departure at the Portland may bear other fruit than the mere continuation of the present mill. It is already "in the air" that the entire chlorination plant may be converted into "cyanide."
The expense would not be as terrifying as might appear at first glance. In the first place the sampling, crushing and roasting plants remain as they are.
The barrels may readily be converted into tube mills; They are, in fact, about as large as the present mills.
An ample spread of concentration tables is already provided and is now in active use.
Power would not be greater than now being leased from the electric works.
Tanks, of course, and other means of handling the solutions, in short, duplication of all the essential parts of the present cyanide mill, would be the new installation. It should not cost more, nor as much, as the present cyanide mill, and granting that there would be a sacrifice of some old capital, that would in all likelihood not figure as against the great saving in treatment charges.
Meantime, the example of the Portland is not being waited on. The new "Golden Cycle", (much of the first structure was burned early in the year) is entirely on the cyanide basis. Whether the practice here will resemble that of the Portland it is too early to say.
Of this we feel assured. Fine grinding, and grinding in the solution; quick after-treatment, settling rather than "percolation"; saving devices for free gold (and possibly for heavy particles of imperfectly-roasted ore. These will be among the features of the future cyaniding of Cripple Creek ores.
The writer desires to acknowledge courtesies on the part of the management, Messrs. Daniels and Taylor, already mentioned as general superintendent and assistant, having tendered him every possible facility for inspection and reference to books and records.
He wishes to give thanks also to another gentleman, Mr. H. E. Kier, whose work as assistant to Mr. Tippet has been most valuable in many directions.
He cannot close without mentioning again the work of the latter gentleman (Mr. Tippet) who has long and patiently wrought at both the experimental and actual plant.
Any work that helps conserve and give to the world a better fraction of the mineral wealth of our state, has not only a scientific interest and an economic value apart from the profits to owners, but is of still further value as pointing out the way to others, and thus aiding to build the metallurgical edifice which will perhaps never be finished.
For, as Bayard Taylor once finely said, in contrasting the modern with the old Egyptian civilization:
"Ours is the higher ambition, always to aspire—forever seeking and forever unsatisfied."