Perhaps no branch of metallurgy presents a more attractive field for operation and investment than that of the treatment of mill tailings for the recovery of their metal values. To the economist it is of interest, as it involves the further recovery of wealth from a discarded heap of waste matter.
To the business man seeking an investment the element of chance may be more nearly reduced to a minimum by a careful system of measuring tonnages and deriving a close valuation. Lastly, to the practical operator is presented a mass of metal-bearing material that has been mined, milled and usually deposited conveniently near the works to make handling and treatment comparatively a simple matter; ideal conditions for high extractions at low cost.
A condition that has caused our esteemed friend, Mr. Edison, to protest against a monetary system so arbitrarily based upon a metal (gold) and thereby placing our national finances "at the mercy of the Steam shovel and a chemical works."
With the above points in view the writer became interested in the treatment of the residues of the Isabella cyanide mill, Cripple Creek district, Colorado. These amounted to approximately 120,000 tons of dry, crushed material that had been discharged from the leaching wats upon a conveyor belt and deposited adjacent to the mill in four large mounds.
These mounds were triangular in profile and cross-section, the ridges making an inclination of 10 degrees from horizontal and the bases sloping about 18 degrees; shown as "A." (Fig. 1). In plan the dumps were shaped as at "B."
Careful measurements were taken and the outline of each mound plotted to scale, each figure being divided into a semi-cone and two triangular prisms. Their volumes were computed geometrically and from these their tonnages easily deducted.
Three methods of Sampling were resorted to: (1) Tunneling, (2) auger, (3) pipe.
The tunneling method was at first favored, as it gave a larger area sampled and also a larger and more representative sample. These were small tunnels, about 18 by 36 in., and one shovel in every 10 removed was reserved for a sample. They were driven into the embankments for about 6 or 8 ft. at points midway up the slopes and at 20-ft. centers.
This method was abandoned after a thorough trial, as the depth attained was not sufficient for a representative sample unless at considerable expense. The tunnel was enlarged to working size and the sides and top prevented from caving.
Auger Method.—For this purpose a 2-in. auger bit was placed in a pipe 10 ft. long. A piece of canvas 3 ft. square, with a 2-in. hole at its center, was spread out and securely fastened upon the slope of the embankment, the auger bringing the borings up through this hole and depositing them upon the canvas. These holes were placed at 10-ft. centers, the borings collectively mixed and a sample taken. The holes were at right angles to the slopes.
This method was also abandoned, as the holes had a tendency to cave from the manipulation of the auger, thus vitiating the sample.
Pipe Method.—This gave excellent results and is very satisfactory for the sampling of the slope of an embankment. The pipe used was 16 ft. in length and 1¼ in. in diameter. One end was tapered for a distance of 2 in. and carried to a sharp edge, the opening being reduced from 1¼ in. to ⅞ in. in diameter.
For best results in drilling the holes the back, or driving end, is kept at about 6 in. lower than the front end. Repeated blows of a 4-lb. hammer not only cause the pipe to penetrate the sand but to become filled with the sample. Without the tapered end and reduced opening the pipe will soon become plugged and act as a blunt instrument.
On account of the very low assay value of this material (under $2 per ton) and the long, severe winter season, it was not deemed advisable to install an up-to-date, fine grinding, continuous system of agitation, decantation and filtration, but to rely upon a large tonnage passed through the leaching vats as cheaply and as expeditiously as possible, which would amount to a re-leaching treatment similar to the well-known South African practices.
The six leaching vats used in the mill were side by side and 30 ft. 3 in, in diameter and 5 ft. in depth, having a capacity of 150 tons approximately. These were supported upon 6-ft. posts, leaving plenty of room for the operation of the 14-in. belt conveyor that passed directly under the discharge gates of all the vats.
To eliminate any spilling and to have a greater carrying capacity the width of this belt was increased to 28 in. by simply lap splicing two 15-in, by 4-ply belts and riveting the same with ¼ by ¾-in. rivets spaced every 3 in. A flat roller was made for this conveyor as follows:
Into the ends of a 4-in, pipe were driven wooden plugs 6 in long. Into these wooden plugs were inserted tightly fitting, soft steel gudgeons of ⅝-in diameter, and leaving a projection of 3 in. A ½ by 4-in. pipe nipple, with a very little reaming, fits snugly over this ⅝-in steel and makes a cheap, highly efficient, dirt-proof bearing for conveyor work. Common axle grease served as a lubricant and was inserted at the outside end of the nipple.
A tightly fitting staple, large enough to pass over the nipple and long enough to penetrate the supporting frame of the rollers, held everything in place.
The loading of crushed ore into the vats had formerly been effected by spouting the material from an overhead conveyor fitted with stationary trippers at an elevation of 10 ft. above the vats. These trippers were found to be great consumers of power and the writer originated a simple device to take their place.
This consisted of mounting vertically in a frame two of the regular flat rollers spaced ¾ in. and directly in the center of the path of the belt. For discharging any material, the belt was simply turned up on edge and held in place by the rollers. These rollers could be adjusted at any point while the belt was in motion and the power consumption was nil.
Filling of Vats.—This was formerly accomplished by spouting the crushed ore to different parts of the vats, thus forming a series of blended cones. In coning dry crushed ore the results are well known to mill men.
The fines will build up at the center while the coarse will roll down the sides. Now, if a vat be filled with a series of these cones it is obvious that when the solutions are percolating through they will follow the lines of least resistance, namely, through the coarse ore, leaving the fines, which are oftener the richer part, imperfectly leached and washed. Later these facts were proved conclusively.
To improve upon this method of filling and, further, to give a cheaper system of excavation than that of shoveling, which had formerly been used, the writer inaugurated a simple mechanical equipment described as follows:
At the Center of each wat were mounted vertical shafts, which rested in step bearings. A water and grit-proof protection was secured by inclosing the shaft in a pipe of 2 in. greater diameter, and this pipe extended 3 in. above the top of the vat to the bottom.
These vertical shafts were driven by bevel gearing at a distance of 8 ft. above the top of the vat and motion imparted through a main line shaft, which extended the full length of the line of vats (about 190 ft.)
In lieu of clutches for throwing in and out of use these vertical shafts the driving pinion on line shaft was "feather keyed" and could be easily thrown in or out of mesh. To each vertical shaft and 1 ft. above the top of the vat were fastened at equal distances three iron arms 12 ft. long. To the under side of these arms were bolted a 2 by 6 plank and to the under side of this plank were spiked on edge 2 by 6 blocks 16 in. long at distances of 12 in.
These blocks were parallel to each other, one-half of these making a 45-degree angle with the longitudinal axis of the arm outwardly, and the other half of the blocks in the reverse direction.
When these arms, with their scrapers or plows, are revolved they will distribute the dirt falling into the vat in a very uniform manner and require practically no attention. When the vat had become filled it required but 20 minutes for one man to detach them and place in the adjacent vat.
For the excavating operation the scrapers or plows, with their frame, are not bolted to the revolving arms, but rest freely upon the sand. These frames are supported upon trucks and are adjusted so that the scrapers will dig a small quantity only. They are fastened to the revolving arms by chains and drag the dirt they plow up to the discharge openings. They are self feeding by their own weight, all construction of the excavators being of steel instead of wood.
The excavating apparatus had a capacity of about 25 tons per hour and required less than 2 h. p. for operation and the attention of one man. The apparatus was removed by chain blocks and overhead trolley.
The apparatus for excavating and conveying the dump material to the mill had some novel features and was highly efficient as regards operation and economy. This consisted of a Bagley grader, which is similar to a box with only sides and a curved back fitted with a cutting edge at its base. The grader was operated by a cableway system consisting of a pulling-in line and return tail rope which passed through a sheave mounted in a portable frame on the dump.
This frame was an "A" frame and pivotal at its base, the sheave and guy ropes being fastened at the apex. To the guys were fastened heavy weights to keep tension in the entire system. The hoist was a single drum, 30 h. p. electric, with car controller for forward and reverse motion.
A partition was placed in the center of the drum on the hoist and the pulling-in and return cables wound in reverse directions. The tension weight on the "A" frame compensated for any difference in the diameter of the two windings. The grader was self-filling and self-dumping and held a little less than a ton. The hoist had a drum speed of 425 ft. per minute and, as our working distance was from 25 to 300 ft., this gave us a capacity of nearly 50 tons per hour.
The entire system was operated by one man at hoist. (Fig. 2.)
The grader delivered the sand into a V-shaped hopper, which had a conveyor belt running beneath it. This served as a feeder to a bucket elevator, which delivered the material upon the loading conveyor running over the vats before mentioned, the material being dumped into the vat and leveled automatically.
The excavating of the dump, filling and leveling of vat of 150 tons was accomplished by one man in as short a time as six hours. A careful, comparative test was made to determine the difference in extraction obtained from vats filled by spouting and mechanically leveled.
In each case the latter showed a greater extraction, averaging about 50 per cent, or as $1.50 is to $1.
The entire plant was operated with five men, the writer acting as manager. The following low costs were attained, based upon the last 5,000-ton run:
|Power and light||$0.030|
|Rent of mill||$0.030|
|Total cost per ton||$0.241|
This cost will compare favorably, I think, with those at other re-treatment plants.