I tested an ore, which was peculiarly difficult to treat, on several concentrators then in general use, including the Frue-vanner, the Luhrig table and the Cornish buddle.
Owing to the character of the ore it had to be crushed- to a very fine state of division, and the results obtained were far from satisfactory; a large proportion of the values was lost in the slimes, which indicated the necessity of a very careful sizing before dressing. Even after reworking some of the slime-tailings, the results, in my opinion, were not as good as those which might be obtained on a table designed more especially for the handling of slime.
Although the subject of slime-concentration has occupied the attention of many engineers who have threshed the field quite thoroughly, the results obtained in my work led to a careful investigation of the forms and principles of the various concentrators, with a view of devising a table which would be efficient in this class of work in general practice.
A comparison of the principles of concentration and the adoption of the best type in each case led to the conclusion that if a vanning motion of some kind could be given to a table of the buddle type, better results could be obtained. For obvious reasons the buddle form of concentrator is the best for slime work, and with this fact as a basis, the problem of motion and mechanical construction presented itself, a study of which resulted in the adoption of a revolving table with permanent points of feed and discharge.
All vanning motions are good, but some are better than others, an axiom which is especially true of circular motion compared with reciprocating motion, be it side or end-motion; consequently the circular motion was selected as the proper one to use.
To be the most effective, the circular vanning motion should have an adjustment of the size of throw as well as of speed, as then the value of the rotating motion of the table would be greatly increased.
Cases exist in which an adjustment of the slope of the cone-shaped top is essential to good results, more especially if this adjustment can be made while the table was in motion.
A summary of the chief points to be incorporated in the design of a concentrator, in order to obtain the desired results, is as follows:
- Buddle form of table.
- A rotating table with fixed points of feed and delivery.
- A circular vanning-motion.
- An adjustment of the size and of the speed of the vanning motion.
- An adjustment of the speed of the rotating motion.
- An adjustment of the slope of the top which can be made while the table is in motion.
Taking up these points in order, the Buddle table, as stated above, was assumed as a basis.
The second point, in connection with third, demanded considerable study, as it seemed that both motions would be best applied at the center of the table and beneath the top; also, one motion would probably amount to about 200 per minute, while the other would probably not exceed one rotation in the same time; a combination which allowed of the running of the vanning motion either in a forward or backward direction. The backward motion was found to be more effective, as it seemed to hold the metallics up against the action of the wash-water better. The term “ rotation ” is used in this paper with reference to the revolving of the table on its center, and “ revolution ” for the small circular vanning-motion.
The form finally adopted consisted of a hollow shaft at the center which could be actuated independently for the slower motion, and a solid shaft extended up through this hollow shaft to carry the rapid vanning-motion.
At the upper end of this solid shaft, a section 4.5 in. in length was made having an eccentricity of 0.25 in., and to this section was fitted an eccentric sleeve having the same degree of eccentricity, which by turning on the shaft allowed of an adjustment of total eccentricity of from 0 to 0.5 in. Furthermore, by placing the eccentricities against, or with, each other, any circle of motion from 0 to 1 in. was obtainable. This arrangement covered the fourth point, so far as the size of throw was concerned. The speed-adjustment was made by the use of expanding pulleys or similar devices. Both the size of the throw and the speed of the rotating motion were found to be quite constant in the treatment of any one ore, and having once been determined by test they could be adopted as permanent. The slight fluctuations liable to occur in general practice were regulated by another adjustment described later in this paper.
Of the several forms of mechanism considered for the purpose of adjusting the slope of the top, the one shown in Figs. 1 and 2 was finally adopted.
It was found by experiment that the surface best adapted to hold the slimes was smooth pure rubber, which gave the necessary elasticity for the slight increase of size when the top was raised to its extreme slope, and, if well stretched on the top while flat, the slight stretching on the increase of the slope made it very true.
In general construction, the first machine was built as shown in Fig. 1, but after running it was found that there was a tendency toward an uneven motion on the lower wheel bearings, so the form shown in Fig. 2 was adopted. This latter form gave a steadier and smoother action, and is the one now used. In the following description, all references made relate to Fig. 2.
The heavy cast-iron base is designed to admit the hollow shaft, to which is attached a worm gear imparting the rotating motion to the table and giving the required reduction of speed. The upper end of the hollow shaft is in the form of a hub-flange, having attached thereto a six-armed spider-frame through which the rotating motion is transmitted. Within
the hollow shaft is placed the solid shaft, the eccentric section of which extends above the hollow shaft. On this eccentric section is placed the eccentric sleeve, which is secured to the solid shaft by means of a heavy set-screw. Over this sleeve is placed a hub-flange, to which is attached another six-armed spider-frame, which in turn is connected to the lower spider-frame by means of spiral springs. When the hollow shaft is revolved, and with it the lower spider-frame, it transmits the motion through these springs to the upper frame. The solid shaft is free to revolve in everything except the eccentric sleeve. By revolving it imparts no motion to the lower frame, but gives a small circular vanning-motion to the upper frame, the throw of which is dependent upon the position of the
sleeve. This motion is allowed by the elasticity of the connecting springs. In this manner the two motions are imparted to the upper frame, and by this construction, it matters not in which direction the vanning motion is made to run.
On the upper frame is mounted a rim to support the top, which is composed of wedge-shaped boards placed radially, the points resting on a plate supported by the device used for adjusting the pitch of the top. On this top is stretched the rubber cover which gives the required surface for working the slime.
As shown in Fig. 3, the feed-box is semicircular in shape and is placed very nearly concentric with the table, the openings being on the circular side. The diameter of the feed-box is 18 in., which gives a peripheral length of feeding-space of 28 in.
Around the table is placed an annular trough which slopes to one point of outflow, the high point of the trough being located approximately in line with the back, and opposite the initial end of the feed-box. The outflow is located diametrically opposite the high point and opposite the final end of the feed-box. At the final end of the feed-box is placed a wash-water box which delivers water to the table in a gentle “ washing ” flow. This box is placed diagonally across the radial lines, the lower end reaching about half across the radius to a point almost directly behind the feed-box. From this point to the high point of the trough is placed a spray-pipe to deliver water under pressure to the surface, which it strikes with some force.
The general action of the table is practically divided into four stages: 1. The distribution of the pulp. 2. The stratification. 3. The washing of the “ silica.” 4. The washing of the metallics.
During the distribution of the pulp the flow increases from a width of 28 in. at the feed-box to a width of nearly one-half of the periphery of the table, or about 15 ft. on a 10-ft. table, which is the regular size in use at the present time.
There is a certain amount of stratification accomplished during the distribution. From the point where the table ceases to take the feed to the point where the wash-water first strikes the pulp is a small intervening space in which the stratification is finished and the pulp is prepared for the action of the wash-water, said action being carried on until the spray-pipe is reached where the washing is quite complete. The action of the spray-pipe is interesting, as it commences at once to wash the metallics of lighter specific gravity to the edge, in some cases making a very good quality of separated products. This feature has not been thoroughly investigated as yet, but from results that have been incidentally obtained there is quite sufficient ground for the belief that under certain conditions a practical separation can be made.
In general practice the pitch of the table varied from 0.75 in. to 1.25 in. per foot, depending on the character of the work. The speed of rotation was in most cases three-quarters of a revolution a minute, but in some special cases was as much as one rotation. The vanning motion was, of course, the most variable, being in different ores as low as 160 and as high as 210 per minute. The higher motions were used in cases where the specific gravities of the materials separated were widely different, and the lower, where they were more nearly the same. The size of the vanning motion varied from 0.25 in. to 0.75 in.
The capacity was found to depend largely upon the condition of the pulp as it was fed to the table, being inversely proportional to its fluidity. In one case, 1,800 lb. per hour, of a peculiarly difficult ore, were successfully treated. Under ordinary circumstances, however, it is probably safe to calculate on 12 tons in 24 hours.
When the point of cutting between the headings and tailings has been determined, or if a middlings-product is desired, partitions are placed in the annular trough approximately at the points of cutting, and discharge pipes are fitted in the one or more compartments as the case may be. At these points, saddles are placed, as shown in Fig. 4, which by sliding backward or forward locate accurately the cutting point. This adjustment serves also to correct the slight fluctuations in the working of the table mentioned above, which are common to all concentrating practice. The cutting space being from 4 to 10 ft. in length renders possible a very close separation.
A statement of some of the results obtained shows some very interesting facts.
Several tests were made with various tables on the zinc-sludge obtained from the Joplin district. Two of these tests are tabulated below, one made on a table of the Wilfley type and another on the Sperry vanning-buddle. The first results were not analyzed as closely as were the second, complete data being lacking; and on account of the limited cutting space there was no chance to make a series of numbers of concentrates.
In Table II. column A shows the weight of the material on that line-column, B the combined weight of the concentrate on that line with those preceding, C the percentage of zinc in the material on that line, D the percentage of saving in the concentrate on that line, E the combined percentage of saving of the concentrate on that line with all the concentrates preceding, F the percentage of zinc in that and in all preceding concentrates, G the proportion of concentration, and H the proportion of concentration reduced to the same basis of calculation as in the first test on the other type of machine. This last calculation of proportion was made on account of the wide difference in the values of the two samples, one containing 6.6 per cent and the other 14.6 per cent, or 2.21 times as much as the first.
A test was made to determine the fineness of the zinc and the following results were obtained:—
The tests given in Tables I., II. and III. were made by William Russell, chief chemist of the Gold and Silver Extraction Co. of America, Ltd.
There are several very interesting deductions to be drawn from the two tests, one, taking the combined products more nearly equal to the value of zinc in the first test (in this case being the entire amount), a value of 37.9 per cent is obtained in the Sperry test as compared with 34.62 per cent in the Wilfley, with a concentrating proportion of 7.88 into 1 as compared with 9.1 into 1, and a saving of 72.9 per cent of the zinc as compared with 57.57 per cent.
Following out another line, taking the same percentage of saving, or 57.57 per cent, by interpolating between No. 3 and No. 4, which lies about halfway between the two, a value of 52 per cent is obtained as compared with 34 per cent, and a proportion of concentration of 10.84 into 1 as compared with 9.1 into 1. Other comparisons of equal interest may be made.
One trouble experienced was that as the material treated was not sized before dressing, some of the larger particles were caught by the water flow and dislodged, and in rolling along in the current they cut a channel through the slimes which were already bedded and broke them up to some extent. As is clearly shown by the size tests, this difficulty could be easily avoided by sizing.
A trial plant of two machines was installed at one of the mills at Aspen and operated part of two months. The results obtained demonstrated several interesting facts as shown in Table IV., on page 583.
The analyses given in Table IV. were made by the assayer of the mill at which the work was done.
The problem of the concentration of these tailings was confined almost entirely to the lead, the silver being so intimately associated with the gangue of dolomite that it was carried off with the tailings of the slime work.
The efficiency of the saving of slime values in this case especially was completely demonstrated in the following manner:— After several tons of concentrates had collected, hand-samples were taken from a number of places, which were dried and placed on a 200-mesh sieve. The entire quantity passed through the sieve with but two or three shakes. Another fact quite as pertinent was that on catching the overflow-water for a given length of time after it had passed through a series of five settling tanks, allowing it to become settled, decanting and drying, it was found, by comparing the resultant concentrates with the entire amount taken from the table in the same length of time, that about 10 per cent of the quantity of lead saved was floating off in suspension.
Other interesting features were brought out in this work, notably the very perfect stratification on the last quarter of the table, which showed a substratum of lead covered with a stratum of zinc, the latter from its character seeming to “ shingle ” the lead, so much so that it was rather hard to break it without using a pressure of spray considerably heavier than that usually used.
The table presented quite an artistic appearance, especially when the clear yellow of the zinc began to assume the blue tint of the lead, growing bluer until the lead gave its characteristic blue color, almost like a coat of blue paint. This circumstance indicated that the table might be of value in the separation of metallics, and a run on the middlings-product was made on May 15, but the product separated from the lead was not analyzed.
Note.—The capacity of the Sperry Vanning-Buddle was from 10 to 12 tons per 24 hours, and that of the Frue Vanner was about 6 tons per 24 hours.
Another test of peculiar value and interest was made on a telluride ore from Boulder county, Colo. Of the two statements of results given in Tables V. and VI., the first is of a run on the tailings obtained by concentrating the original ore on a table of the Wilfley type, with a view of maximum extraction by concentration, and following by treatment with the cyanide process. This original run showed an extraction of about 28 per cent, which left 0.78 oz. of gold in the tailings.
The length of the cutting-line renders it possible to make the concentrates of any desired value.
Many other results might be presented, but those given are sufficiently representative to convey a general idea of the work of the machine.