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It is proposed in these articles to lay before those interested in the treatment of gold ores, an accurate description of three gold mills recently erected in Colorado, dealing with the subject, more especially from a mechanical point of view.
Accurate figures of labor required, and the cost of operating these plants, will be given, also a brief statement of the metallurgical results.
The mills described embrace a number of interesting features, and as each plant has been found to work well, both mechanically and metallurgically, from the time it was put into operation, it is hoped that those interested in the construction of similar mills may be able to derive some helpful information from these descriptions.
In considering the subject of mill construction from a mechanical standpoint, it may be well to state that owing to economic conditions, the question of low first cost is often one of great importance; and for this reason the design and general construction of such plants may seem to lack in the features of permanency and ultimate economy.
And it is no doubt true that a greater expenditure on the first installation for better buildings, and more liberal proportions to all parts of machinery, would be an ultimate economy, provided the plant as first designed fully meets the requirements.
This is seldom the case with gold mills, and most companies prefer to test their ores at as small an outlay as possible; but where the process, ore supply, and economic conditions have been fully exploited, the question of permanency should receive full consideration.
This mill was constructed more particularly as an experimental plant for the purpose of testing the value of the "Page" solution in connection with the treatment of gold ores from the Cripple Creek mining district. The mill was located south, and east, of the Depot area of the F. & C. C. and the Denver & Rio Grande, as a branch east, then north, off its Coal Creek Branch.
On the completion of the mill the company ran a test on 1,087 tons of oxidized ore, after which a complete clean-up was made, and the gold recovered was found to exceed in quantity what would ordinarily have been expected from a new mill under a new process.
The operation of the plant throughout was satisfactory, the test lot being run through without interruption. A statement of the cost of operation, etc., will be given later on.
The capacity of the mill was found to be 60 tons per day, the various departments being in fairly good proportion for handling this amount of ore, running all the mill 24 hours except the stamping department, which had sufficient capacity to handle 60 tons in 10 hours.
By reference to the plans of the mill shown, it will be observed that the plant proper is under one roof, and is constructed on practically a level site, the main floors of the crushing and leaching rooms being on the same level. The power plant is contained in the wing on the south side of the main building.
The ore for treatment is brought in on railroad cars at the east end of the mill, where it is delivered on to a platform of the same height as the car floors. Much of the ore is taken direct from the cars into the mill, being handled in wheelbarrows, the platform being provided for the reception of such ores as it is desirable to unload, but which cannot be taken directly for treatment.
The ores, on being taken into the mill, go first to the coarse crushing and sampling plant which consists of the following:
On a level with the unloading platform a 9 inch by 15 inch breaker of the "Blake" type is set, belted to run 240 revolutions per minute. From this machine the ore passes to a bucket elevator, which raises it high enough to feed directly to the coarse crushing rolls.
This elevator is constructed with a special heavy rubber belt, 7-ply, with 6½ inch by 13 inch cast iron elevator buckets bolted to it, spaced about 15 inch centers.
The coarse crushing rolls are of a special type, which will be illustrated and described later. These rolls are 36 inches in diameter, with a face of 16 inches, driven by means of spur gears, the pinion shaft being fitted with tight and loose pulleys. The rolls are speeded to run 18 revolutions per minute.
From the rolls the material drops into a chute in which is arranged an automatic sampling device consisting of a moving slide 14 inches wide in the bottom of the chute, with a hole 1½ inch wide cast into it. The slide is given a back-and-forth motion by means of a worm-wheel and crank to equalize the flow, and makes about four passes per minute.
The sample taken is about one-tenth of the whole, the nine-tenths going directly to a bucket elevator which elevates it to the storage bins. The elevator from the rolls is similarly constructed to that from the breaker, except that the buckets are of pressed steel 14 inches wide, spaced 18 inch centers on the belt.
The sample delivered from the rolls falls directly to the floor, where it is cut down by hand to the required quantity. This is further ground in a pair of sample rolls 20 inches in diameter, 12 inch face; the pulp resulting goes directly to the testing department for moisture determinations and preparation for the assay office and laboratory.
In the sampling department of the mill all the machinery has been set sufficiently high to admit of the elevator boots being above the floor line.
The fine crushing plant is as follows:
The crushed and sample ore is delivered from the storage bins into an elevator which delivers it to the revolving dryer. The ore bins are constructed of light sheet steel, and have a capacity of about 25 tons each, the bottom of each bin being fitted with cast-iron gates, by means of which the flow of material to the elevator is regulated.
The revolving drying furnace is constructed with a tapered revolving cylinder, 5 feet diameter at one end, 4 feet at the other, and 20 feet 10½ inches long, made of ½ inch sheet steel. This cylinder is divided into four compartments by means of four ⅜ inch plates, running the full length.
The cylinder is supported on rollers with its axis horizontal, and is revolved at a speed of 2½ revolutions per minute. At the large end a brick firebox is constructed, the gases of combustion passing through the cylinder.
There is also a chute at this end which delivers the dried material into an elevator. At the small end of the cylinder a brick dust-chamber is formed with a chimney leading from it through the roof.
The fuel used for drying, as for all other purposes around the mill, is residuum from the distillation of petroleum burned as a liquid.
The dry ore, on being delivered to the elevator under the fine crushing rolls, is elevated and delivered to the revolving screens shown in Figs. 1 and 2. These screens are hexagonal in form, tapering from 4 feet 10 inches at the large end, to 2 feet 11 inches in diameter at the small end, are covered with 24-mesh steel wire cloth made of No. 28 wire, and are revolved at a speed of 12 revolutions per minute.
Each screen has an effective surface of about 70 square feet, and is provided with a bin under it which holds about 30 tons, fitted with sliding gates opening into the tank room.
The material which will not pass the screens is delivered into the hoppers of the three automatic roll feeders, from which it is fed into three sets of rolls, 26 inches in diameter, 14 inch face, belt-driven, speed 45 revolutions per minute. On passing the rolls the material goes again to the elevators, and thence to the screens, the coarse returning to the rolls, as explained.
The fine crushing rolls are of the same general construction as the large ones. The crushing shells are of forged steel, and the machines are belt-driven.
In connection with the fine crushing plant, an apparatus for collecting the dust has been provided, and it is found to be very serviceable, in that it keeps the mill free from dust, collects a large quantity of fine material which would otherwise be lost, and by removing the very fine dust from the crushed ore, facilitates the leaching of the material in the tanks.
The dust-collecting apparatus consists of a large exhaust fan, collected by means of galvanized pipes to each of the screen casings, the air with the dust being delivered by the fan into a centrifugal dust collector. The exhaust fan is run at sufficient speed to cause a flow of air inwardly at all points in the housings of the rolls, elevators, screens, etc., thereby avoiding the escape of dust into the building.
The amount of dust collected by this means is about 4½ per cent, of the ore handled. There is an escape of very fine material, from the outlet of the dust collector, but the quantity is not large.
The power plant for the operation of the above machinery consists of one horizontal return-tubular boiler, 60 inches in diameter, with 16 foot flues, heating surface 1,050 square feet. The engine has a cylinder 14 inches in diameter, 20 inch stroke, and makes 130 revolutions per minute.
All shafts and machinery in the mill are arranged to run at very moderate speeds, the result being that the whole operates with very little power.
No diagrams have been taken to show what the power required is, but everything seems to work perfectly with 20 pounds of steam, the regular working pressure being 90 pounds per square inch. The ores after being crushed as above described, next go to the leaching department which adjoins the crushing mill on the west.
The ore is drawn from the screen bins into a dump car holding about one ton, which by means of 18 gauge tracks over the leaching tanks, may be discharged into any one of these tanks. The leaching tanks are 36 feet in diameter, 42 inches deep, made with the bottoms inclining towards the center, where a 16 inch discharge gate is provided for emptying them after washing.
These tanks are constructed throughout of 3-16 inch steel and are supported on 4 inch by 16 inch joist spaced 18 inch centers, these being carried on a series of brick walls 6 feet 6 inches high running lengthwise of the building; the space is provided underneath for the accommodation of tailing chutes, sump tanks, etc.
The bottoms of the leaching tanks are fitted with filters constructed with 1 inch by 1 inch pine slats, laid with a space of 1 inch between them. On this is laid a heavy rattan matting, covered with specially woven jute cloth and an upper covering of open muslin. The muslin is calked in tightly around the edges of the tank with 1 inch rope.
Each tank is provided with a 1½ inch solution pipe tapped into the bottom at its lowest point, which has connections to each of the sump tanks. Each tank is also provided with pipes connecting to the solution tanks and to the water supply system. The sump tanks are three in number, 16 feet in diameter by 5 feet deep, set low enough so that the solutions from the leaching will flow to them by gravity.
These tanks are constructed of 3-16 inch steel.
In the southeast corner of the tank room a second floor is provided, about on a level with the tops of the stock solution tanks. This space is partitioned off to form a gold-recovery room, where the gold is precipitated from the solutions. The solutions are raised from the sump tanks by means of two small belt-driven triplicate pumps, and are delivered to a 5 foot iron distributing tank placed at a sufficient height above the recovery room floor to admit of the solution flowing by gravity through the precipitating tanks and into the stock solution tanks.
The stock solution tanks are two in number, 15 feet in diameter and 10 feet deep, constructed of 3-16 inch steel, and are set on timber foundations of sufficient height to enable the solutions to be drawn from the bottoms of the tanks into the leaching tanks.
The operation of the leaching department is as follows:
The crushed ore is delivered into the leaching tanks by car, and when one tank is completely filled a certain number of tons of the solvent solution of suitable strength is run into it. This is allowed to percolate through the ore and drain off into the sump tanks and settle; it is then ready for precipitation.
When the first solution has drained off another is put on, and another, until all of the soluble gold has been dissolved and washed into the sump tanks, the number of solutions and their strength being determined by the requirements of the ore being treated. When the ore has been thoroughly washed in one tank, the central gate is opened and the waste material is washed out through the tailing chutes by hydraulics.
The precipitates recovered from the solution are roasted in a muffle furnace, the muffle being of cast iron, 3 feet 6 inches long, 30 inches wide, and 10 inches high, set in a brick setting. The roasted material is melted down in a crucible with suitable fluxes yielding a gold bar of comparative purity.
In operating this plant various difficulties are encountered, but none of them of a serious character. The mill worked smoothly from the first, the difficulties being more in restricted capacity rather than in the operation of the plant. It was found that larger screen surface would have increased the capacity of the fine rolls, but as these were fully up to the capacity of the other parts of the plant, no change was made.
The three sets of 14 inch by 26 inch fine crushing rolls, as at first put in, were arranged to work in tandem, that is to say the material was passed through the machines in series, the last roll being called upon to finish all the returns from the other machines, together with the returns also from the screens to which this roll delivered.
The arrangement did not work well. It was found that No. 1 and No. 3 rolls did most of the work. No. 2 rolls getting very much less than their share of the material. A screw conveyor was put in to rectify this difficulty, by means of which the ore could be distributed at will to the machines in the proper proportions, resulting in a largely increased product from the fine rolls.
The tailing chutes under the leaching tanks were found to be too flat, requiring a good deal of care in flushing the tanks to prevent their getting choked up. Charging the tanks by means of a car was not entirely satisfactory, owing to the amount of dust resulting from dumping the ore into the tanks.
No attempt has been made so far to rectify these last two difficulties.
The necessary working force in the mill was 19 men at a cost of $43.25 per day as follows:
|Sampling works, 4 men at $1.75||$7.00|
|Sampling works, 1 man at $2.50||$2.50|
|Fine crushing department, 1 man at $2.00||$2.00|
|Leaching department, 2 men at $2.00||$4.00|
|Recovery department, 2 men at $2.50||$5.00|
|Power department, 1 man at $2.50||$2.50|
|Day Shift Total||$23.00|
|General foremen, 2 men at $3.00||$6.00|
|General laborer, 1 man at $1.75||$1.75|
|Fine crushing department, 1 man at $2.00||$2.00|
|General millwright, 1 man at $3.00||$3.00|
|Recovery department, 2 men at $2.50||$5.00|
|Power department, 1 man at $2.50||$2.50|
|Night Shift Total||$20.25|
|Description||Per Ton of
|Chemicals for all purposes||$0.75|
|Wear and tear||$0.20|
|Lubricating oils and supplies||$0.06|
No salaries or interest on investment are included in the above figures. It should be remembered that no roasting is done in this plant. The percentage of the gold extracted varied with the character of the ore. In the case of thoroughly oxidized mineral 92 per cent, could be obtained.
This 1900 Sanborn Fire Insurance Map for the Page Reduction Works of the London & Cripple Creek Reduction Corporation, Limited is the only one that exist with this type of details! The Mill was properly built in 1898, was closed temporarily when the Sanborn people visited and was later known as The Florence Mill, before it faded out of existence without having seen much work on processing Cripple Creek ores.
I added in some pieces from other places on the larger sheet this is cropped out of, and straighten the map so it aligns along the south wall, but the map is as it was made.
*1 From the Engineering. Magazine/Journal.