Last updated: 15 May, 2021 13:14
Milling Methods at Stratton´s Independence (Nov. 1911)

The Mining Magazine, London, UK
November 1911
(pages 361 to 370)

In addition to the source text images, I added some other known ones.
By Philip Argall
The Stratton's Independence Mill.
View towards the Independence Mill Structures

INTRODUCTORY.—In mining and shipping some $20,000,000 worth of gold ore, from the Stratton's Independence mine, something like one million tons of refuse was sorted out and deposited on the dumps. This material was generally assumed to average about $4 per ton, but as was to be expected, the content is quite variable; some of it has reached $5, much of it has not exceeded half that value, while the average will probably not exceed $3.25 per ton.

The directors of Stratton's Independence, Ltd., as early as 1903 had experiments made looking to the beneficiation of this ore; an experimental mill was later erected, the ore was treated by various methods, and the dumps sampled.

Ore that assayed as high as $75,000 to the ton was brought to light, while little of a value less than $100 has been sent to the smelters. In fact, there is a mountain, one might say, of ore of a milling grade that will run from $18 to $40, which the lessees are passing by as beneath their attention.

In November 1906 I was engaged to examine and report on the treatment of this low-grade sulpho-telluride ore, and naturally looked into the use of the various processes, but soon found that no chemical then available would assure satisfactory treatment; consequently, I went back to my first experiments on Cripple Creek ores made in the autumn of 1893, and in the following spring. In these tests concentration was the dominant feature. These early experiments in part formed the basis for my review of cyanidation published in the Engineering Magazine, New York, in September 1894, from which it is perhaps permissible to quote:

"In other cases concentration is advisable. For example: An ore may contain 15% of pyrite, which may require special treatment in the way of a stronger solution, or long exposure to solution—say, 80 hours —in order to obtain a good extraction; while the remaining 85% of the ore may have its gold dissolved in 24 hours. Here, then, separating the heavier from the lighter particles, and subjecting each to separate treatment, will result in a gain of time in lixiviating, saving of chemicals, and increased capacity of the works. The same line of argument holds good if the concentrate cannot be successfully treated by cyanide, as in this case a deleterious material is removed from contact with the cyanide solution, to be handled by such other methods as the particular circumstances will indicate. The cyanide process, then, can be applied to ores direct, or as a combination process with amalgamation or concentration, or both, as may be found most convenient and profitable."

The concentration method was adopted and my original experiments brought to the fore, after a lapse of 13 years. In the interval, mechanical progress in crushing, in concentrating and in cyaniding machinery, had been marked; but not much advance had been made in the substitution of cyanide solution for water in the entire milling process—an advantage I had seen and tried in the early 'eighties, and still favoured. Experiments on these lines were therefore in order; and on their completion on a laboratory scale I reported to the directors of Stratton's Independence, Ltd., that a mill of 10,000 tons per month could treat the ore for $1.52 per ton, including 10c. per ton for mining; giving an extraction of about 70% of the contained value, then estimated between $3.60 and $3.80 per ton.

A mill with a capacity of 5000 tons per month was planned, and duly erected, tested completely, and later enlarged to its present capacity of 10,000 tons per month.

THE PROCESS is as follows: Coarse crushing in Gates gyratory breakers, medium crushing in 36 in. rolls, fine crushing in Chilean mills. Cyanide solution is added in the Chileans, and thenceforward the process is crushing and concentrating in cyanide solution in a closed circuit. The pulp from the Chilean mills is separated into sand and slime in Ovoca classifiers, the former is treated on 20 Card tables, the latter on 12 No. 3 Deister slime-tables, and 4 suspended vanners. A first and second-class concentrate is produced, the latter being re-treated on Card and Deister tables, producing more first-grade and a middling product that is returned to a tube-mill and, after grinding, joins the pulp issuing from the Chilean mills and circulates in the main circuit of the mill, finding its position in sand and slime.

Flow-Sheet Stratton's Independence, Ltd. Cyanide-Concentration Mill, September 1911
Flow-Sheet Stratton's Independence, Ltd.
Cyanide-Concentration Mill, September 1911.

For details consult flow-sheet and the following legend:

  • 1. Double-track inclined plane.
  • 2. No. 7½ Gates gyratory breaker.
  • 3. Picking-belt 3 ft. wide.
  • 4. No. 5 Gates breaker.
  • 5. 18 in. conveyor-belt.
  • 6. 16 by 36 in. rolls.
  • 7. Stock-bins above Chilean mills.
  • 8. Thickening-cones.
  • 9. No. 3 Deister slime-table.
  • 10. Suspended vanners.
  • 11. Vacuum-filters.
  • 12. Precipitators.
  • 13. Tube-mill.
  • F. Elevator lifting slime 'seconds' for re-concentration.
  • G. Centrifugal pump lifting tailing from second concentrate for tube-milling.

After concentration, the sand and solution in which it was concentrated are pumped to the cyanide department, the solution being separated and the sand automatically distributed in leaching-vats, O 1 to O 6. After treatment the sand is sluiced to the tailing-reservoir.

The middling from the finishing tables not furnishing sufficient supply for a 5 by 14 ft. tube-mill, two spigots are drawn off from the sand on its way to the cyanide department; this is a partly concentrated product, being the heavier sand that tends to follow the bottom of the slightly inclined pipe.

The slime passes from the Ovoca classifier to 16 thickening-cones; the thickened product is automatically distributed over the Deister and vanner tables in a similar manner to the sand, the second-grade re-concentrated and the middling ground in the tube-mill before again joining the main pulp-stream circulating through the mill.

The tailing from the slime-tables is pumped to an overflow-cone in the cyanide department and distributed by launder L to four continuous settling-vats, S 1 to S 4, the product being a thickened slime and a clear - solution overflow. The thickened slime is pumped to S 6 for final treatment by cyanide solution, and when required, to S 7 for bromo-cyanide treatment.

From either of these vats the slime is piped to S 5, which supplies the vacuum-filters. From here the slime-cakes pass to the dump and the pregnant solution to the clarifying press (p), thence through the precipitators, the barren solution dropping into an underground sump, from which it is circulated for distribution over the concentrating-tables; in other words, the barren solution is used as table-water.

Following the solution, it will be noted from the flow-sheet that the cyanide solution in storage-vat B is fed, together with the ore, to the Chilean mills, that 75% of it separated with the slime passes out of the Ovoca classifier to the thickening-cones. The slime-tailing with the solution added to it in the concentration process is pumped to a cone, the overflow passing to launder L in the cyanide department, while the clear-solution overflow from vat S 1 to S 4 returns to the storage-vat B for another circuit.

Returning now to the sand emerging from the head of the Ovoca classifiers C 1 and C 2, it is immediately diluted with clear solution direct from storage B, which carries the sand to the central distributor, which in turn provides a direct supply for each of the 20 Card tables; on these tables barren cyanide solution is used for dressing the ore, passing with the tailing to classifiers C 4 and C 5.

The slime-overflow from these latter enters launder L and becomes the main slime-circuit, giving a clear-solution overflow from the vats S 1 to S 4, which passes direct to storage B for another round in the closed circuit of the mill-solution.

This solution is constantly dissolving gold in its circuit through the mill, and as the aim is to not allow its gold content to exceed $1 per ton, part of the clear overflow from vats S 1—S 4 is continuously passed through the clarifying presses (p, p) and the precipitators (12), finally reaching the sump as barren solution, from which it circulates to the tables, as previously described, joining the main pulp-stream, and thus reducing the gold in the circulating solution.

The barren solution is also used for washing the sand charges in the leaching-vats. The bulk of the gold is, however, usually recovered by the special treatment of the thickened slime in S 6 and S 7, where, after thorough agitation, it passes through S 5 to the vacuum-filters, clarifying presses (p) and precipitators (12), the barren solution reaching the sump preparatory to making another circuit through the mill.

Finally, the pregnant solution from the sand-leaching vats O 1 to O 6 is piped back to the precipitators, reaching the sump as barren solution, while the final wash-water passes to waste through a special precipitator.

The process, it will be seen, is crushing and concentrating in cyanide solution in a closed circuit, in combination with incomplete precipitation, so that the circulating solution shall never exceed $1 per ton, and shall average about 50 c. per ton. It will be noted that complete precipitation is unnecessary inasmuch as both sand and slime are subjected to further cyanide treatment and water-washes before passing to the dump.

Having now described our milling method and flow-sheet, I shall proceed to describe some of the more important operations in detail.

Power-Shovel Loading 4-Ton Car. [at Stratton's Independence Mill]
View Power-Shovel Loading 4-Ton Car.

MINING.—The dump is situated on a hillside traversed by deep depressions, giving an irregular and uncertain bottom; the ore is usually frozen near the bottom all the year round and in winter requires considerable blasting to prepare it for the shovel. The ore is loaded into 4-ton cars by a power-shovel with dipper of 1 cu. yd. capacity, the machine being operated by one 40 hp. variable speed electric motor, from which the digging, crowding, swinging, and advancing motion are all derived by clutch-gearing controlled by levers operated by one man.

By official test the machine easily handled 120 tons per hour. In daily work, however, loading small cars where the dipper has to be spotted each time before dumping, we find 60 tons per hour about the average capacity, though we often reach 75 tons per hour with an expert operator.

The ore-cars are hoisted up an incline plane and automatically dumped into a No. 7½ Gates breaker; this ends the mining and delivery at the mill, the cost of which is $0.0891 per ton, divided as follows:

Per ton
Power $610.00 $0.0217
Operating labour 1335.04 0.0476
Operating supplies 13.60 0.0005
Repairs labour 353.25 0.0125
Repairs Supplies 189.74 0.0068
28,078 tons $2501.63 0.0891
Breaker Plat During Construction. [at Stratton's Independence Mill]
View Breaker Plat During Construction.

ORE-BREAKING.—The ore is broken in the 7½ Gates machine to about 4 in. cube, the fine removed by grizzly, the coarse conveyed on a picking-belt to a No. 5 Gates breaker, in which it is reduced to about 1½ in. cube, and conveyed direct to the 300-ton storage-bin, together with the fine removed by the grizzly.

The ore cannot be enriched much by sorting, the waste picked out seldom reaching 3%. Of more importance is the steel and wood removed at this point, for these would otherwise seriously interfere with subsequent milling operations.

This breaker-plant is operated with the power-shovel and inclined plane about 6 hours per day. The total cost is $0.1633 per ton, of which the sorting or picking amounts to, say, $0.04.

COARSE CRUSHING is accomplished in two sets of 36 by 16 in. rolls, operated in series, having a capacity of 30 tons per hour from 1½ to ¼ inch. The storage-bins in front of the Chilean mills not having capacity for a 16 hours run, the roll-plant is operated during part of two shifts each day, and in order to have more men in the mill on the night shifts, the rolls are operated on the second and night shifts by one man per shift, who also attends to general repairs.

Steel Storage-Bin [at Stratton's Independence Mill]
View Figure of Steel Storage-Bin

Probably one of the best things in this department is the 300-ton self-supporting steel storage-bin (Fig. 1) the simplest and most satisfactory ore-bin I ever used. It is thoroughly automatic, the last pound will come out without the use of crow-bar or shovel.

The feeder is of the revolving-table type, subject to two variations: thickness of feed by adjusting the height of the annular sleeve at bottom of the feed-cone, and width of annulus cut off by the adjustable plough; the feed is also steady and continuous, an important matter in feeding rolls. It will be noted that the opening in the bottom of the ore-bin is large (4 ft. diam.), hence it is ample to prevent any semblance of a choke, and small enough to relieve the feed-cone from excessive pressure.

The relation between the 4 ft. opening in the bottom of the main bin, the diameter of feed-cone top and its 1 ft. diam. bottom opening just above the revolving table of the ore-feeder, are important points to observe.

The ore stored in this bin is reduced to less than 2 in. cube and is fed to the rolls at the rate of 30 tons per hour. The ore is moistened with cyanide solution on the table of the feeder to lay the dust and slack the lime that is added dry at this point by another automatic feeder.

The lime is thus well incorporated with the ore in passing through the rolls, and at once begins its work of neutralization in the damp ore, which is then elevated to the storage-bins above each Chilean mill, supposedly reduced to ¼ in., but more nearly ⅜ inch. It should be stated, however, that the ore is mostly phonolite and breaks in thin flakes, that most of the coarse caught on ½ in. aperture screens would be less than ¼ in. thick, hence easily crushed by the rollers in the Chilean mills.

As originally designed the ore from the second roll was passed through screens with ¼ in. aperture; but this was found to be an unnecessary refinement, so the screens were removed and the cost of the screening operation eliminated.

Section of Akron Chilian Mill [at Stratton's Independence Mill]
View Figure of Section of Akron Chilian Mill

FINE CRUSHING.—This plant consists of four Akron Chilean mills of 6 ft. diam. (Fig. 3), three of which easily give the capacity required: 10,000 tons per month. The fourth is held in reserve until needed to take the place of either of the others. These mills crush from 100 to 130 tons per day, depending on the feed, the character of the ore and the screens used; with a screen of 0.046 in. aperture 0.054 wire and speed of 33 r.p.m., the capacity is almost 5 tons per hour for an expenditure of 55 hp.

The power consumed by Chilean mills depends chiefly on the amount and size of the ore fed to them; secondly, on its crushing qualities. Using relative terms, with a supply of finely comminuted ore the power will be much less than that required for a coarse product, which offers greater resistance to the rollers, even causing them to rise from the die and occasionally ride over coarse tough particles; then again, the resistance is less with a shallow thin feed in the mill, less power is required to operate, and the crushing capacity is also less.

For a 6 ft. Akron mill operating on average ore crushed to about ¼ in., I would provide a 50 hp. motor guaranteed for 20% overload. The following tests are instructive:

The line-shaft driving two 6 ft. Akron mills is operated by a 100 hp. motor; one of these mills was thrown off, and the tests made on the other.

Input to Motor
Motor and line-shaft 4
Motor line-shaft and Chilean mill, representing the friction load 9
Light feed ¼ to ⅜ in. product to the mill 28
Heavy feed, mill a little crowded but in good running condition 63
The mill was next fed to the limit, crowded, in fact, almost to the stalling point 70
The foregoing results from average of reading every few minutes, for one half-hour.
A watt-meter was next placed in the motor-circuit and gave;
on a 23-hours test 52
on 317¼ hours test (April 5 to 18, 1911) 53

The mill was in average condition, and during the wattmeter tests was fed and operated under our usual milling conditions, giving an average output of 115 tons per day, of the following composition:

Mesh Aperture
Inch %
Plus 50 0.0096 21
Plus 100 0.0056 12
Plus 150 0.0030 5
Minus 150  0.0030 62

The Chilean mill is an admitted slimer, yet it is not by any means as bad as painted. I would call it an efficient crusher that is capable of adjustment, within certain limits, to give a fine, yet granular, product. The comminution of ore in a Chilean mill may be said to be the resultant of two forces, the direct crushing effect, due to the weight and speed of the rollers, the grinding or abrading effect, due to the rollers being constrained to travel in a circular path; hence, they partake of two motions; rotary, around their individual axles, and annular, around the die, concentric to the central axis of the mill.

Let us look into the latter. A circle drawn on the face of the die-ring 4 in. from the side (shown in cross-section at c), and a line drawn around the face of the roller tire 4 in. from the side (similarly shown at d), represents the lines of maximum wear on tires and die in a Chilean mill. The corrugations in the 8 in. crushing-faces reach their maximum approximately along these lines.

A section of new steel is shown at A, while B shows the worn steel. Why should the steel wear in this manner? The answer is twofold.

Section of New Steel of Tires and Die in a Chilean Mill [at Stratton's Independence Mill]
View Figure of New Steel at Chilean Mill
Section of Worn Steel in a Chilean Mill [at Stratton's Independence Mill]
View Figure of Worn Steel at Chilean Mill

First, the coarser ore is dropped from the feed-pipe approximately along the circle c of the die, and forms a ridge; the principal reason is, however, the grinding or abrading action occurring there. A roller, in making the circuit of the die, also completes a revolution on the line d at right angles to its axis of rotation. It is this annular direction of the rollers, deflecting them from a straight course and pure rolling motion that causes the grinding or abrading effect; the smaller the diameter of the die, the greater is the abrading effect or twist on the ore particles held between the die-ring and the tire of the rollers; on the other hand, the greater the diameter of the die the more nearly the rollers approach a pure rolling motion with lessened abrasion and lowered efficiency as a fine crusher, or slimer, if you will.

For this reason the so-called 'slow-speed mills' with large diameter die-rings are not slimers and scarcely come within the fine-crushing class, they approximate more nearly the work of rolls than that of Chilean mills. In these the roller receives its rotary motion through friction with the ore on the die-ring, the line d around the centre of the tire-face theoretically should roll upon the fragments while the outer margins of the tire-face slide over them; as the roller advances, constantly bending inward from a straight path, its outer flange crowds the ore in front, inward on the die, its rear outward on the screen, particularly after corrugation is established. In our mills we have abandoned the use of scrapers depending solely on the wheel-splash for the screening effect.

The capacity of the Chilean mill increases slightly as the steel wears, as does also the power; but these are balanced somewhat by the lessening weight of the rollers due to tire wear; with both die-ring and tires corrugated, as in B, the Chilean mill usually gives its maximum efficiency as a fine grinder inasmuch as both the twisting (abrading) action and the weight of the roller is effective on all ore held in the corrugations, not on a thin film as where smooth (new) tires are in use. Rolls differ from Chilean mills in this respect: when corrugated their capacity lessens; still, by what I have elsewhere described as the "choke-feed," they can be made to treat a thick stream, grinding the dry ore upon itself with increased capacity and finer product.

The Chilean mill, however, will do all this wet, and do it better. This is at once the secret of its success as a fine crusher and the cause of its sliming action. For fine comminution, attrition in some form is necessary; in hand-work (cutting down samples, for example) a hammer can be used effectively for a time, but on the bucking-board every one believes in the grinding action of the muller. The machine that depends on a blow alone cuts but a sorry figure in fine crushing, and should not be classed with such machinery.

For this reason I have long held that the stamp is not a fine crusher because most of its work is done by impact, and but little by attrition, a fact that is gradually being recognized.

Rolled steel tires and dies have proved best in our practice, such for example as the Midvale and Latrobe brands. The wear and loss of steel on last year's campaign of 87,814 tons crushed, works out as follows:

lb. lb. % %
Tire Steel worn 0.367 59.2
Tire steel scrap 0.063 10.1
———> 0.43 ———> 69.3
Die steel worn 0.153 24.8
Die steel scrap 0.037 5.9
———> 0.19 ———> 30.7
Total steel per ton crushed 0.62 lb. 100.0

The crushing pressure in the Akron mill is fairly heavy. Assuming a roller rests in ore, on the die, for one inch of its circumference; or, in other words, crushes one square inch of ore per inch of face; then the crushing pressure on the 6-ft. mill is about 900 lb. per sq. in., the roller with a new steel tire of 8 in. face weighing about 7200 lb.

The crushing pressure is no doubt augmented slightly by centrifugal force, particularly if the rollers are run with a slight inward inclination on top. One great advantage of the Akron mill is the ease with which the rollers can be adjusted and the drive-head raised or lowered while the mill is running. See Fig. 3.

TELLURIDES are friable, and while the fine is richer than the coarse ore in most cases, yet the ores of Cripple Creek afford the greatest contrast in this respect. From the lumps that will pass through a 2 in. grizzly to the dust caught in the rafters of the mill the finer pieces and particles are invariably the richer. Advantage has been taken of this fact, and extensive experiments conducted to determine the best method of crushing and the most economical size of reduction.

It was recognized from the first that Cripple Creek sulpho-tellurides were most difficult to cyanide by any method known to me, apart from roasting, consequently concentration was chosen as the sheet anchor of the process, and every effort was put forth to remove by concentration, in advance of the final cyanide treatment, all the sulpho-tellurides possible in both slime and sand.

The ore of Stratton's Independence appears at first glance to consist of a rather sparse scattering of sulphides in phonolite and phonolitic breccia; on closer examination it is found that the gold occurs chiefly as films along fracture-planes, or in cavities, or in small veinlets; the sulphide in the body of the rock is invariably low-grade, and occasionally worthless. To crush the rock fine enough to liberate all the sulphides was out of the question on account of the cost, the dressing loss in concentrating slime, and the low-grade product obtained from the concentrators.

Experiments were made with rolls, stamps, pans, and Chilean mills, the latter giving the best results. Then followed experiments with size-crushing on a full working scale, made through screens varying from 0.02 to 0,10 in. aperture.

The best average results were, however, obtained around 0.046 screen aperture, which on $3.25 ore give us a sand-tailing averaging, after concentrating, about 80 c. per ton, and a slime running about $2 per ton after concentration, both products being subjected to further cyanide treatment. We have always failed to get any of the slime-tailings as low as the sands, either in the concentrating or in the cyaniding department. Another argument against further reduction of the sand might be predicated on this fact.

The guide, chosen after months of experiment, is not to allow the sand-tailing from the table to exceed $1 per ton; when this limit is approached a finer screen is used on the mills. Comparatively coarse crushing, and eliminating 40 to 50% of the ore as sand, with a poorer tailing than we can obtain from the slime, is one of the nice points in our practice, not generally seen or at least understood by either the casual observer or the critic.

Here the physical character of the ore is utilized to obtain:

(1) a coarse sand from which a high-grade concentrate (carrying 5 to 7 oz. gold) and an almost worthless tailing is produced in one operation;

(2) An enriched slime from which a high-grade concentrate can also be obtained, leaving but 50 to 60% of the original ore for special treatment, such as air agitation, oxidizing chemicals, and filtration.

CONCENTRATION.—The sand from the Ovoca classifiers is automatically distributed to 20 Card tables or any lesser number, the machine being so arranged that one or more tables can be eliminated by merely inserting plugs in the divisions of the classifiers supplying them, while the supply of the idle tables is automatically distributed over the tables in operation. This distributor, as improved by the mill superintendent, is about the best I have seen.

A similar one distributes the slime, and largely through their use one man on shift attends to 22 sand-tables, 4 vanners, and 13 Deisters. The tables once adjusted require but little attention so long as the feed and table-water are regular. Originally the sand was divided into plus and minus 40 mesh product, but the screens proved troublesome and expensive to maintain; later we found that just as good concentration could be done without them; our present practice is to make but one sand product and one thickened slime.

From careful trials made in 1907 I decided that the Card table gave the best result on our sand, and I have been quite satisfied with its performance in the mill. The No. 3 Deister has done equally as good work on our slime as the modern vanner, and at less than one-tenth the maintenance. Vanner-belts do not last in our solution over 9 months; they cost about $120 delivered; this item alone would maintain a Deister table for several years. Then the rollers, gears, etc., on the vanners, require careful attention, and are a source of constant expense.

I have shown how the residual value of the sand is controlled by the size of the aperture in the screens of the Chilean mills; we have, however, another controller in the second concentrate produced on the general tables, inasmuch as by drawing off a greater or smaller amount of 'seconds' and middlings for re-concentration and re-grinding all the tail in a tube-mill, the tailing discharged from the general tables is largely modified.

Furthermore, it is obvious that the production of this second-class material for re-concentration and tube-milling, renders possible the comparatively coarse crushing in the Chilean mills, through which a high saving of granular sulpho-telluride is made, that if slimed in the first instance would entail a great decrease in the gold recovered by concentration and incidentally a higher final tailing from the cyanide plant.

To summarize, crushing through 10 mesh 0.046 aperture screens is made possible by returning a concentrated middling product for re-grinding in a tube-mill preparatory to again entering the main circuit of the mill.

CYANIDATION.—After concentration the various sands are pumped to the Ovoca classifiers C 4 and C 5 in the cyanide department, to be separated from the final traces of slime. These classifiers give a sand practically free from slime and carrying from 15 to 25% moisture, as desired; they consequently perform a double service, slime separation, and sand dewatering.

The clean sand delivered at the head of the classifiers is mixed with as much barren solution as will pass through the vat-filters during the filling process and sluiced through highly inclined launders to fixed points over the empty leaching-vats, or is moved there by reciprocating conveyors when sufficient fall cannot be obtained for launders; in either case the maximum solution that will pass the filters is brought with the sand to aid the distribution in the vats.

Meanwhile, leaching continues throughout the filling process. We find the leaching rate uniform over the entire charge, notwithstanding that no mechanical filling device is used. This filling method was not invented, it grew. Elaborate filling precautions and devices found necessary (with slimy sand) to prevent the slime from choking the charge wholly or in part, are absolutely unnecessary when the slime is removed before filling; the Ovoca classifier does this thoroughly, so, in a word, our practice is to sluice the ore into the vats and sluice it out again after treatment.

Recently some over-enthusiastic advocates of the all-sliming method called attention to the expense of handling sand as compared with slime; even this argument falls to the ground, when sand-vats are filled and emptied by sluicing.

Pneumatic Agitator [at Stratton's Independence Mill]
View Figure of Pneumatic Agitator

The slime is collected and agitated in steel vats of the form shown in Fig. 2. Air agitation is used after the method illustrated. The discharge from the central air-lift being dispersed, about 10 ft. below the surface of the charge, by a small reverse cone; in this way the upper portion of the charge is kept in brisk agitation by a surprisingly small amount of air.

Sand, if present, sinks and is deflected toward the side of the vat by the lower cone; rapidly settling to the suction of the lift it passes up again and in this way is kept in active agitation. With us the granular portion of the slime requires all the agitation we can give it. Others have described our method of agitation in the technical Press, but I regret to say have drawn largely on their imagination.

In the pneumatic type of agitator where the air-lift discharges above the surface of the charge, the compressed air escapes into the atmosphere before it is fully expanded, while in the method shown in Fig. 4 the expanding air is dispersed and keeps the whole charge, above the reverse cone, in brisk agitation, instead of that small portion of the charge within the central tube, as in the old style of pneumatic agitator.

The slime is worked in charges of about 70 tons, agitated in cyanide solution of varying strength, and, as required, given a final treatment with bromo-cyanogen.

The solutions are precipitated on zinc-shaving, the dried precipitate being ground to impalpable powder in a special tube-mill and sold to the refineries. At first we had much trouble in sampling the precipitate, and many disagreements with the refineries; since it has been ground to powder, however, we have had no trouble in determining the value of the product.

COST OF MILLING.—The mill reached the 10,000 tons mark in March 1911, and averaged a little better than 10,000 tons per month for the following three months, closing our fiscal year. I take the expenses for these months in analysing the cost:

Month Tons Cost
April 9,899 $11,892.55
May 10,277 12,840.91
June 9,945 12,587.21
30,121 $37.320.67
Average: $1.239 per ton.

The mill receives one-tenth of its supply from a small breaker-plant where the low-grade mine-ore is crushed to about ¼ in. and conveyed direct to the Chilean mills. The dump-breaker cost is based on the ore actually mined from the dump, while the crushing and concentrating plant treats the entire tonnage from both mine and dump, less the waste picked out.

Labour includes superintendents, and all salaried men employed in the mill.

Independence Mine, Victor, Colorado.
Postcard View Independence Mine & Mill, Victor.
Photo by Julia Skolas
Independence Mine and Mill, Cripple Creek District.
Another View Independence Mine & Mill, Victor.
Per ton treated
Power $1225.00 0.0436
Operating labour 1314.50 0.0468
Operating supplies 133.16 0.0048
———> 0.0516
Repairs labour 659.94 0.0235
Repairs supplies 1253.12 0.0446
———> 0.0681
28.078 tons    $4585.72 $0.1633
Power $5862.75 $0.1946
Operating, labour 3238.11 0.1075
Operating supplies 354.38 0.0117
———> 0.1192
Repairs, labour 2441.18 0.0810
Repairs supplies 3530.36 0.1172
———> 0.1982
Loading concentrate. 289.41 0.0097
30,121 tons    $15716.19 $0.5217
Power $1459.00 $0.0484
Operating, labour 4039.83 0.1341
Operating supplies 6264.03 0.2079
———> 0.3420
Repairs, labour 1010.40 0.0336
Repairs supplies 642.05 0.0213
———> 0.0549
30,121 tons    $13415.31 $0.4453
Heating $151.43 $0.0050
Water service 144.62 0.0048
Liability insurance 169.21 0.0057
Fire insurance 898.40 0.0298
Taxes 1400.87 0.0465
30,121 tons    $2764.53 $0.0918
Dump-breaker $4585.72 $0.1633
Crushing and concentrating 5716.19 0.5217
Cyaniding 13415.31 0.4453
Miscellaneous 2764.53 0.0918
Mine-breaker 838.92 0.0169
30,121 tons    $37320.67 $1.2390

We are now in position to compare the original estimate made in March 1907, based on milling 10,000 tons per month, with the average actual cost per ton milled, in April, May, and June 1911.

1907 estimate Actual cost
Power $0.30 $0.2943
Breaking, sorting, fine-crushing,
concentrating, and mine-breaker
(without sorting)
(sorting included)
Cyaniding 0.47 0.3969
Miscellaneous 0.09 0.0918
Total milling cost $1.24 $1.2390
Mining and dump-loading 0.10 0.0891
Total mining and milling cost $1.34

The original estimate contained an item of 18 cents per ton milled, for treating the concentrate on the ground, making the total cost $1.52 per ton. This estimate covered everything.

The two cents caused some amusement, as few believed it possible to figure such a process within 25 cents per ton; while many were unkind enough to say it would never be done under $4 per ton. The figures, however, speak for themselves.

The concentrate was not worked-up on the premises, as better arrangements were made with the smelters, so this part of the estimate cuts no figure, inasmuch as the cost is lower than my estimate of 1907. The concentrate varies from 4 to 7 oz. gold per ton, and averages nearly 5 oz. It contains about 30% iron excess over silica, and is otherwise an excellent and desirable smelting product, since it forms the ideal binder required in the Huntington - Heberlein or pot-roasting process.

Hand-sorting was not included in my original estimate, nor was the extra cost of operating a breaker at the mine, partly to supply the fine-crushing and concentrating plant. These items increase the cost by 5 c. per ton; deducting this, the breaking, crushing, and concentrating cost becomes 40 c. as against an estimated 38c.

Cyaniding is 7⅓ c. below the estimate, due to improvements in agitating, in the general mechanism, and to cheaper chemicals than were available four years ago.

EXTRACTION.—During the fiscal year ended June 30, 1911, the mill treated:

Oz. Oz.
102,364 of dump ore assaying 0.152 15,564.91
7,436 of mine ore assaying 0.231 1,724.05
109,800 tons averaging 0.157 17,288.96
Oz. %
The concentrate contained 7546.80 43.65
The bullion contained 4814.80 27.85
Total recovery 12361.60 71.50

From the foregoing results it is apparent that the estimates of working cost and extraction have been translated into fact.


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