Summarizing some of the principal points brought out in this laboratory Jigging of Minerals’ investigation, I believe the following may safely be accepted:
- The pulsion-reaction is by far the most important one in the process of jigging. During this period, with sized grains of different specific gravities, with proper pulsion-velocity, the separation between them will be complete. The size-limit is indicated by the hindered-settling ratio. If the minerals are not sized, or above these ratios, the separation cannot be complete, but a definite arrangement will result. The positions of equilibrium will be attained when the above ratios of diameters arc attained, after which further separation by pulsion is impossible.
- Suction due to the movement of water-columns supplements gravity. Resisting the sum of these two forces is the resistance of the walls of the tube through which the grain must pass. The reaction, as a whole, must therefore be a resultant. The chief component is the force of the water-columns, which are purely non-selective, but act with equal intensity upon all particles of the same shape and size, regardless of their specific gravity or weight. Any advantage that the small heavy grain would have over a large light one would, of course, appear in the resultant tending to carry it to the hutch. The effect of the forces opposing the movement of the grain depends upon the character of the grain, and the conduit through which it is supposed to pass. Under any condition, the diameter of the grain cannot be greater than that of the conduit. If the channels are inclined, or crooked and zigzag (the condition obtaining on a jig-bed), the particles will more easily lodge against the sides of a tube large enough to pass through if the tube were vertical, but under the force of gravity they remain at rest. The rapidly descending water-currents passing through these channels easily carry the grains along. ‘Thus suction, due only to the moving columns of water, constitutes a powerful impelling force to carry through the interstitial spaces those particles which under the force of gravity alone cannot move. Suction is, therefore, a necessary complement to pulsion in the jigging of all unsized material, and generally valuable in jigging under all conditions.
- From the observations under (2) it is clear what effect the bedding will have upon the result. Any part of the bedding or ore-column remaining fixed during the pulsion-cycle must be looked upon merely as a mass of very irregular tubes, of length somewhat greater than the thickness of such part, owing to their inclination, since they are mostly inclined. To that extent they are only an extension of the jig-sieve. The result of thickening or thinning the bed, or of increasing or decreasing the size-ratio between bedding and feed, is evident. This assumes, of course, that the largest particle of feed is smaller than the sieve-aperture, and always the bedding-grain must be larger than the sieve-aperture. It is evident, too, that the shape of the bedding-grain will have a marked effect. Grains that are more or less equidimensional, as galena, etc., will form a more open bed than one of antimony, which breaks into long pencil-shaped grains. Finally, of course, if the bedding is in use long enough all grains become worn and spheroidal. Any part of the bedding free to pulsate is to be considered as part of the ore-column, and is amenable to all the conditions applying to this reaction.
- The effect of very rapid acceleration, amounting to a shock or blow to the bottom of the jig-bed, is an important factor. Its effect is to accelerate the work done by suction, and render a larger catch possible with a low mean piston-velocity. The pulsation of the jig-bed due to this force and that taking place under the regular interstitial velocity should be distinguished. One sifts, the other separates.
- The results of the many experiments, in which the piston-speeds during the pulsion and suction were not the same, seemed to show that only by properly balancing the two are the best results attained. It has been generally noted that the eccentric, giving equal mean velocities, yields about as good results as any of the accelerated strokes. This observation applies only for the size-ratio used in the tests, and it is not safe to speculate what the results would be for other sizes.
- While the use of the jig for the treatment of material sized between wide limits is possible and practicable, still the advantages that are bound to follow where a more or less perfect sizing has preceded cannot be denied. It must be observed, that in the English system itself, when the hutch-products of one jig are treated on another we are using sizing.
- The more general application of the English system, or the use of the jig in the treatment of unsized material instead of the hydraulic classifier, seems to be clearly indicated. This has been recognized in some quarters, but a wider use than has hitherto been accorded it appears to hold out favorable inducements. This seems to be a field eminently suited for the English methods of jigging—one that is not and cannot be filled by the Continental system.
- The arguments that have been advanced for the adoption of the English system on the ground that equal-settling ratios, many times larger than those obtainable under free-settling conditions, exist on the jig-bed, are not tenable. These hypothetical ratios cannot possibly exist on a jig-bed.
In conclusion, I must acknowledge the great help and many suggestions derived from the works of Professor Richards and Professor Munroe; to the latter personally, I owe cordial thanks for numerous timely suggestions. I have also had the benefit of his valuable criticism in the construction of the laboratory-jig, with which many of the experiments were made.
It is evident that in a machine so simple as the jig there are a number of variables, and a series of tests may therefore be classified according to some one of them.
For purposes of discussion, however, the tests that have been conducted are grouped according to the velocity of the rising current of water, or pulsion-currents, as measured by the mean piston-speed. Figs. 6
to 13, inclusive, show graphically the results given in row C, under each of the experiments, calculated for the mean diameter of the material. Since all material treated on the jig passed through a screen having a square hole, the mean length of the
sides of which was equal to 2.136 mm., and from that as a maximum to the very finest dust, it has been assumed that the mean diameter of the grain caught on the 1.66-mm. screen is (2.136 + 1.66) ÷ 2 = 1.90 mm.; those passing the 1.66-mm. screen and caught on the 0.97-mm. screen, have a mean diameter of (1.66 + 0.97) ÷ 2 = 1.31 mm., and so on for all the sizes; and, finally, that the material passing the 0.16-mm. screen (the finest used in these tests) had a mean diameter of 0.08 mm. In the curves shown in Figs. 6 to 13 the diameter of the grain has been plotted along the X axis, and the weight of pure mineral (galena or sphalerite) on each screen-size, as given in the record of the tests, laid off on the Y axis. The points thus located have been joined by three classes of lines : the solid lines in all cases represent the results obtained in the tests made with the eccentric cam; the dotted lines, tests with the involute cam; and, finally, the broken lines, tests with the circular-arc cams.
The velocities of the pulsion- or rising-currents, as measured above, have been divided, on purely arbitrary grounds, into four groups: (1) velocities of pulsion 2 in. (50.8 mm.) per sec. and less; (2) velocities of pulsion from 2 to 4 in. (50.8 to 101.6 mm.) per sec. ; (3) velocities of pulsion from 4 to 6 in. (101.6 to 152.4 mm.) per sec.; (4) velocities of pulsion from 6 to 10.66 in. (152.4 to 270.7 mm.) per sec. With this arrangement it has so happened that in nearly every case the tests with the involute cam are plotted in two groups. The experiments with the eccentric and circular-arc cams occur once only in each group. Since each pair of minerals has been run with 10 and with 20 per cent, of either galena or blende, two classes are to be distinguished. In all cases under discussion, Classes I. and III. will refer to mixtures containing 10 per cent., and Classes II. and IV. to mixtures with 20 per cent, of the heavy mineral.
In order to facilitate reference, the following classification is given:
Class I. Galena and Quartz. Galena 10, Quartz 90 per cent.
Group 1. Velocities of pulsion from 0 to 2 in. (0 to 50.8 mm.) per sec.
Tests 8, 11, 12, 14, 15, 17, 18.
Group 2. Velocities of pulsion from 2 to 4 in. (50.8 to 101.6 mm.) per sec.
Tests 2, 4, 6, 9, 10, 13, 16.
Group 3. Velocities of pulsion from 4 to 6 in. (101.6 to 152.4 mm.) per sec.
Tests 4, 5, 7, 9.
Group 4. Velocities of pulsion from 6 to 10.66 in. (152.4 to 270.7 mm.) per sec.
Tests 1, 3.
Class II. Galena and Quartz. Galena 20, Quartz 80 per cent.
Group 1. Velocities of pulsion from 0 to 2 in. (0 to 50.8 mm.) per sec.
Tests 28, 30, 31, 32, 34, 35, 37.
Group 2. Velocities of pulsion from 2 to 4 in. (50.8 to 101.6 mm.) per sec.
Tests 22, 24, 26, 29, 30, 31, 33.
Group 3. Velocities of pulsion from 4 to 6 in. (101.6 to 152.4 mm.) per sec.
Tests 24, 25, 27, 29, 38.
Group 4. Velocities of pulsion from 6 to 10.66 in. (152.4 to 270.7 mm.) per sec.
Tests 21, 23.
Class III. Sphalerite and Quartz. Sphalerite 10, Quartz 90 per cent.
Group 1. Velocities of pulsion from 0 to 2 in. (0 to 50.8 mm.) per sec.
Tests 48, 50, 52, 53, 54, 55.
Group 2. Velocities of pulsion from 2 to 4 in. (50.8 to 101.6 mm.) per sec.
Tests 42, 44, 46, 49, 50, 51, 53.
Group 3. Velocities of pulsion from 4 to 6 in. (101.6 to 152.4 mm.) per sec.
Tests 44, 45, 47, 49.
Group 4. Velocities of pulsion from 6 to 10.66 in. (152.4 to 270.7 mm.) per sec.
Tests 41, 43.
Class IV. Sphalerite and Quartz. Sphalerite 20, Quartz 80 per cent.
Group 1. Velocities of pulsion from 0 to 2 in. (0 to 50.8 mm.) per sec.
Tests 68, 70, 72, 73, 74, 75.
Group 2. Velocities of pulsion from 2 to 4 in. (50.8 to 101.6 mm.) per sec.
Tests 62, 64, 66, 69, 70, 71, 73.
Group 3. Velocities of pulsion from 4 to 6 in. (101.6 to 152.4 mm.) per sec.
Tests 64, 65, 67, 69.
Group 4. Velocities of pulsion from 6 to 10.66 in. (152.4 to 270.7 mm.) per sec.
Tests 61, 63.
Class I., Group 1. Galena, 10 per cent. The lowest two ratios of concentration were obtained with two tests with eccentric cam, using a short stroke and a high frequency in Tests 17 and 18; and of these two, Test 18, with only a 1/16-in. stroke and 400 strokes per min., yields the lowest ratio of the series. An examination of the screen-analysis shows a marked difference between Tests 18 and 15; the longer and slower stroke has caused a larger percentage of the finest size to pass into the hutch; but the shorter and more rapid stroke has increased the percentage of material between 1.31 and 0.69 mm. in the concentrate. In this case, at least, piston-speed does not determine whether the jig-bed will be pulsated, or the proportions of coarse and fine material carried into the hutch.
The highest ratio of concentration is clearly with the involute cam, Test 11, with a pulsion-velocity of 2 in. (50.8 mm.) and suction-velocity of 1 in. (25.4 mm.) per sec. A good catch of fine material is made, and the three largest sizes are of good proportions as to weight and mineral-content; the sudden drop in value for particles of 0.34 mm. diameter should be noted.
Tests 8, 12, 14, and 15 represent the three types of strokes. The weak pulsion and strong suction of Tests 8 and 14 have produced results very similar to those of the eccentric. This style of cam, therefore, between the limits of this group, is not more efficient than the eccentric. In none of the experiments of this group has any material of mean diameter 1.90 mm. been carried through the sieve and into the hutch. It cannot be said that strong suction is superior to moderate suction in saving the fines.
Class I., Group 2. The minimum ratio of concentration is that with the involute cam, Test 4 representing the highest limit of velocity of pulsion for this group and very strong suction. The strong suction, however, has not resulted in increasing the proportion of fines, probably owing to the fact that there is also a rather high pulsion-velocity. The conditions in this test have been favorable for the recovery of grains of mean diameter of 0.69 mm. (on 0.42-mm. screen). The highest ratio of concentration is obtained in Test 6, an eccentric, with velocity of pulsion and suction 2.66 in. (67.7 mm.) per sec. Test 10, an involute cam, with slow pulsion and rapid suction, gives results similar to Test 6. Test 9 gives very good results, with high pulsion and slow suction, just the reverse of Test 10. In this latter case the strong pulsion is clearly an advantage, resulting in almost as good a saving of fines, and a much larger and cleaner product on the large sizes. Test 2, with circular-arc cam, and Test 16, with eccentric cam, give results quite close. Why Test 4 should differ so materially from Test 2 is not easy to explain. Tests 10 and 4 represent the velocity-limits of the group and show marked differences in results; and in Tests 4 and 9, with the same velocity of pulsion, but different suction- velocity, the strong suction has produced a much smaller percentage of the finest size and contains less galena, although the strong suction has been very effective in drawing material having a mean diameter of 0.69 mm. into the hutch. The eccentric, with short stroke and high frequency, Test 16, gives a low ratio of concentration. In all cases only a small catch is made with sizes larger than about 1 mm., and from 75 to 95 per cent, of the mineral saved in the hutch is of a diameter of 0.69 mm. or less.
In this group the same pulsion-velocity but variable suction velocity give different results; the high suction-velocity is not of any distinct advantage in increasing the catch of fines or in enriching any of the sizes. The eccentric at proper rotative and pulsion-speeds yields results equal and in most cases superior to an accelerated and retarded stroke.
Class I., Group 3. In this group of four experiments, Test 4, which was also placed with group 2 of this class, and occupied the lowest position, is also the lowest in this group. The highest ratio of concentration is found in Test 9, an involute cam with the same pulsion-velocity as Test 4, but only one- fourth the suction-velocity. As noted under group 2, the strong suction has resulted in producing a smaller amount of the finest size, and in a decreased percentage of galena in all the sizes. The intermediate positions are marked by an eccentric cam, Test 5, and a circular-arc cam, Test 7, and with the same velocity of pulsion. The strong pulsion and weak suction have resulted in a larger saving of the fine material than in the case of the eccentric, and a somewhat higher ratio of concentration.
It appears, therefore, that in this group the involute cam with strong pulsion and weak suction is the most efficient in producing a high concentrate, and the reverse of these conditions the least efficient; that the circular-arc cam, Test 7, with strong pulsion and weak suction, is somewhat more efficient than the eccentric; and that the same velocities of pulsion yield different results.
It will be noted, also, that all the tests in this group produced some of the coarsest size, and those of strong pulsion and weak suction the largest amount. In comparing this group with group 2 of this series, we find that the maximum ratio has been passed, and that velocities of pulsion more than 4 in. (101.6 mm.) per sec., with the size of material jigged, should not be exceeded.
Class I., Group 4. Only two experiments occur in this group, Tests 1 and 3. Length of stroke in each case 1 in. It appears that the circular-arc cam with the highest velocity of pulsion and least velocity of suction gives a little higher ratio of concentration, but that the results are very much the same. The proper limit for pulsion-velocity has long since been exceeded. Comparing Tests 3 and 4 in the same way, it is found that strong pulsion and weak suction produced practically the same percentages of sieve-sizes, but with strong suction the percentage of heavy material is much reduced. Even with these high velocities of pulsion, a strong suction is not an advantage, in increasing either the amount of fine material drawn into the hutch or the percentage of heavy mineral.
Class II., Group 1. Galena, 20 per cent. Of the seven tests in the group, the involute cam, Test 31, with velocity of pulsion at the maximum limit of the group, gives the highest ratio of concentration. The same was true under Class I. The final minimum ratio is indicated by the reciprocal of Test 31, with weak pulsion and strong suction. With material up to 0.69 mm. in diameter, the eccentric with short and rapid stroke gives the lowest ratio in Test 37, while at the same piston- speed at twice the length of stroke and half the number of rev. per min., the values are very close to the maximum in Test 35.
The involute cam in Test 30, with weak pulsion and strong suction, produces similar results but at different velocities, but at the same ratio to Test 32. In this case the strongest suction has drawn a larger percentage of the fine stuff into the hutch, but has not enriched it.
The two circular-arc cams, Tests 28 and 34, with weak pulsion and strong suction, give similar results, in which about 30 per cent, of the hutch-product passes through a 100-mesh (0.16 mm.) sieve. But again, in Test 31, with the involute cam, strong pulsion and weak suction, a larger percentage of fine material is drawn into the hutch. The eccentric gives about the same percentage of fines as in Test 31.
It may be said for this group that the eccentric at the proper length and rotative velocity gives excellent results, and is generally superior to an accelerated or retarded stroke. The same pulsion-velocities give different results.
Class II., Group 2. The minimum ratio of concentration is indicated by the circular-arc cam, Test 33, with strong pulsion and weak suction. The maximum is attained with an involute cam, Test 31, with rapid pulsion and weak suction. The involute cam has already been considered under the first group. Test 29, also an involute, under the same conditions, gives good ratios, but the higher pulsion-velocity results in a smaller saving of the very fine material; larger sizes appear more abundantly in the hutch, however. Tests 24 and 30, involute cams with suction in excess of pulsion, give final results that are very close, but the strongest suction, Test 24, yields relatively less fine and more coarse material than the weaker suction.
It will be noted, further, that with the exception of Test 31, some material larger than 1.66 mm. is found in all the products. The circular-arc cam with very high velocity of suction has produced a relatively high percentage of the finest size. The eccentric, Test 26, gives good average results—a large percentage by weight of the finest, and containing at least an average percentage of galena. With the exception of Tests 29 and 31, the five other tests are, in general, much the same. Both of these tests have been classed in other groups. Of the three highest ratios of concentration, two have low suction- velocity and the third has equal pulsion and suction.
Class II., Group 3. Of these five tests the minimum is found in Test 24, repeating the conditions of Test 4. It will be noted, however, that until the size next the largest is reached, the lowest ratio is indicated by the eccentric, Test 38, with short stroke and high rotative speed. Test 29, involute cam, strong pulsion and weak suction, and Test 25, eccentric, at the same piston-speed as Test 38, give almost the same final results.
The circular-arc cam, Test 27, strong pulsion and weak suction, produces results very similar to those of Tests 25 and 29.
In all cases with these high pulsion-velocities, more material having a diameter of 1.31 and 1.90 mm., and correspondingly less of the finer sizes, have been obtained. The advantages of high suction-velocity over those of pulsion are not apparent.
Class II., Group 4. An examination of the two tests in this group indicates at once a close correspondence. The maximum limit for pulsion-velocity has been passed, but it appears that with the richer feed these velocities vary between considerably wider limits than with the poorer material.
In general, it may be said for all the tests, that for each condition under which jigging takes place, certain sizes—20- or 40-mesh (0.97 or 0.42 mm.)—are very rich, and then on smaller sizes a very violent drop in the percentage of galena takes place. This will be noticed for all tests on galena and blende as well. Also, that moderate suction and stronger pulsion give better results than the reverse. The strong pulsion usually results in a larger yield of the coarser sizes of higher percentage in mineral, and the fines are saved almost equally as well. In most cases the eccentric, at the proper length of stroke and rotative speed, is equal and usually superior to accelerated or retarded stroke; but when the stroke becomes too short and the rotative speed high, the ratio falls off. Observations on the behavior of the bed under these conditions showed that the bedding and ore- column pulsated, although at the same piston-speed with the longer stroke no movement in the bedding took place. This indicates that, with very sudden impulses to the ore-column, the water acts more like a solid than a liquid, and that mineral-particles are not subject to the full force of a rising current of water, but that the material is sifted down through the interstitial spaces of the bedding. Possibly another cause is at work, as noted in the behavior of the bed during the long strokes. Here the top of the bed pulsated for a longer time, and had a longer amplitude of vibration, and therefore the grains on the bottom came to rest sooner than those above, which would tend to limit the size of the particles passing into the hutch; and the longer and slower the stroke above the limits which will move the grains, the more pronounced will this differential motion be, and with it the increased perfection of the classification that must take place.
Class III., Group 1. Sphalerite, 10 per cent. Here the lowest ratio is found in Test 50, with weak pulsion and strong suction. An examination of the weights and percentages of Tables IV. and V. shows, however, that only relatively small amounts of the finest sizes are secured; but material of 0.69 mm. (on 40-mesh) is recovered to an amount equal to about 41 per cent., while material larger or smaller than this size is not materially increased. Test 48, under similar conditions, gives similar results. Both of these tests indicate that strong suction, within the limits of this group, is not advantageous. Test 54, with the same ratio of pulsion and suction, but only one-half the intensity, gives a higher ratio of concentration and a slightly better recovery in the finest sizes. Test 52, under analogous conditions, gives somewhat similar results, except material on 20-mesh (1.31 mm. mean diameter). Test 58, involute cam, with strong pulsion and weak suction, gives very good results. Test 55, eccentric, gives the best results of all. In this case, not only a high percentage of the finest sizes of fair mineral-content was obtained, but the coarse sizes also were well represented, containing a high percentage of sphalerite, which accounts chiefly for the high ratio of concentration.
It may be said for this group that the eccentric easily yields the best results; that strong suction and weak pulsion give the lowest, and weak suction and strong pulsion an improved ratio of concentration.
Class III., Group 2. An inspection of the tests in this group shows that the lowest ratio of concentration is indicated by Test 50, an involute cam, with strong suction and weak pulsion, already considered in Class III., Group 1; and very near it is Test 51, a circular-arc cam, with strong pulsion and weak suction, resulting in the production of very small amounts of the finest sizes; but nearly 45 per cent, of material on the 40- mesh (0.69 mm. mean diameter). Tests 42, 44, and 49 give results in the final ratios that are close together, but differing in the details. Test 42, circular-arc cam, with moderate pulsion and strong suction, and Test 49, involute cam, with strong pulsion and weak suction, give practically the same final result; and Test 44, involute cam, with moderate pulsion and strong suction, similar results.
The two higher ratios are those of Test 46, eccentric, and Test 53, involute cam, with strong pulsion, but the lowest for the group, and less suction. An inspection of the records of the experiments shows that, with the relatively low pulsion-velocity used, these two tests yielded relatively less of the coarsest sizes, but increased amounts of the finest sizes.
The superiority of the eccentric over the other forms of stroke is at once evident. In the case of all styles of stroke, the same pulsion-velocity gives final results much the same.
Class III., Group 3. Of the four tests grouped here, three are almost identical—namely, Tests 44, 47, and 49; and of these three, two have already been considered in Class III., Group 2. Test 47, circular-arc cam, with strong pulsion and weak suction, and Test 49, also strong pulsion and weak suction, produce about the same results as very strong suction and weaker pulsion, but in which, however, the pulsion-velocity is about the same. This indicates that the velocity of pulsion is the principal determining factor.
Class III., Group 4. The two tests in this group are very closely related. It is evident that the proper velocity of pulsion has been passed. The records of the experiments show that, at these high velocities, the coarse sizes readily pass into the hutch, but at the same time the percentage of mineral is much decreased, and much of the fine material is lost.
An examination of the four groups indicates that in the fourth the maximum velocity of pulsion for good work has been exceeded, but in the other three groups the best velocity is not so clearly indicated. With the three eccentrics good ratios have been secured in each of the groups, and this is also the most efficient of the three types of stroke. A high pulsion-velocity is very efficient in saving material that rests on 20- and 12-mesh (1.81 and 1.90 mm. mean diameter of grain), but, on sizes smaller than these, less so than decreased velocities. A high suction-velocity is not generally more efficient in recovering the finest sizes than a more moderate one.
Class IV., Group 1. Sphalerite, 20 per cent. A comparison with the corresponding group of Class II. shows many features in common. The lowest ratio of concentration is found in Test 70, and next to it Test 68, both weak pulsion and strong suction. Test 72, the reciprocal of Test 68, gives better results. Tests 73 and 74, reciprocals of each other, indicate that between these velocities the involute cam is very efficient. Test 74, with strong suction, gives the highest ratio of the group.
Class IV., Group 2. Some differences as compared with the corresponding group of Class I. are found here. The minimum ratio of concentration is marked by Test 64, involute cam, with moderate pulsion and strong suction. Tests 62, 70 and 71— 62 and 70, circular-arc cams and involute, respectively, with weak pulsion and strong suction, and Test 71, with strong pulsion and weak suction—give results that do not differ materially, indicating once more that even though the suction- velocity differs widely, the final results will not differ widely if the pulsion-velocities are close together. The eccentric, Test 66, shows a good ratio. Test 73, an involute cam, with stronger pulsion than suction, gives the maximum ratio for the group.
Class IV., Group 3. The four tests in this group give results agreeing very closely with the corresponding group of Class I., and the observations made under that group apply here.
Class IV., Group 4. A glance shows at once that this group agrees exactly with the corresponding group under Class I.
An examination of the four groups of Class II. indicates that in the first, with a pulsion-velocity not exceeding 2 in. (50.8 mm.) per sec., the highest ratios are obtained, and that at these velocities by far the largest percentage of the mineral recovered has a mean diameter of 0.69 mm. (through 20-mesh.). As the velocity is increased, more of the coarse sizes appear and less fine material.
For both Classes III. and IV., with sphalerite and quartz, it appears that generally a stronger pulsion-velocity than suction is more efficient in producing a better concentrate, and effects an equally good saving of the fines. The eccentric, between wide velocity-limits, is an efficient type of stroke. Of the two types of cams, the involute is generally the best. An examination of the tests will show that certain types and velocities of strokes are especially suited to the recovery of particles of fixed diameters.
Mineral Jig Pulsion & Suction
Since the pulsion- and suction-velocity, as measured by the piston-speed, have been the chief variables in this investigation, the question naturally arises: can the exact role of each be definitely defined ?
The accepted meaning of the terms “pulsion” and “suction” is doubtless familiar to all. A pulsion-current is one acting opposite to gravity, and tending to raise the grain off the jig- sieve; and a suction-current is one acting in the direction of gravity and supplementing it. In both cases, therefore, are reactions caused by the movement of a column of water or other liquid relative to some solid.
When the results of a series of tests are arranged according to the pulsion-velocity in the free part of the jig-column, or, in other words, the piston-speed, even though the suction-velocity differed widely, the final results are quite close together, in-dicating that the reactions occurring during this cycle determine the final result. With a perfect-fitting piston, given the areas of piston and jig-sieve, length and number of strokes per unit of time, the mean pulsion-velocity in the free or unoccupied section of the jig-column may be accurately determined ; and similarly for the suction-velocity.
It has been demonstrated that under the reaction of pulsion with mixed sizes of grains of different specific gravities certain definite positions are established according to diameters. Thus, in the case of quartz and galena, the grain of quartz in equilibrium with a particle of galena was 5.8 times the diameter of the galena grain.
Stated in other words, the results of the pulsion-jig experiments indicate that in order to effect a perfect separation by pulsion alone, the grains should be sized between the limits of these ratios, which may be distinguished from those of “ free- settling ratios ” by “ interstitial equilibrium factors,” or “ hindered-settling ratios or factors.” It is important to note that they are larger than those obtained by Rittinger’s well-known formula. This formula states that in the case of a sphere the uniform velocity under “ free-falling ” conditions is :
v =5.11√d ( x – 1.0)
in which:
v = Velocity of fall in meters per second.
d = Diameter of sphere in meters.
x = Specific gravity of sphere.
1 = Specific gravity of liquid (unity in case of water).
Thus, in the case of quartz and galena, if for x the specific gravities of the two minerals are substituted, equating and solving for the respective diameters, a ratio of about 4 to 1 is obtained. It is evident that the reactions occurring during pulsion have resulted in increasing materially the ratios possible under “ free-settling ” conditions. It seems to me that part of this increase may be accounted for according to Professor Munroe’s grain of maximum falling-velocity. He has shown that in a tube the grain of maximum falling-velocity is one having a diameter 0.4 that of the tube. Under the force of pulsion the interstitial channels are constantly undergoing a change in their diameters. A small grain of heavy mineral surrounded by the larger grains of lighter mineral will have frequent opportunities for occupying a channel about 2.5 times its own diameter. No doubt the greater acceleration of the small particle over that of the large one will always aid the separation, as pointed out by Rittinger.
It is evident that the experimental interstitial-factors or ratios obtained in the pulsion-jig are much smaller than called for by Munroe’s theory, where, in the case of above minerals, large grains closely surrounded by smaller ones, he obtains a ratio of about 31 to 1 for equal-falling grains.
Whatever may be the theoretical diameter-ratios between two minerals under pulsion, it is an easy matter, as pointed out by Professor Richards, to determine what it is under practical conditions, and the ratios that exist under these conditions on a jig-bed are the ones that most closely concern the mill-man.
As a resultant of all the forces acting upon the grains during the pulsion-cycle, a certain definite and distinct separation takes place according to the diameter-ratios of the two minerals. When this point has been reached, further separation, or an increase in the diameter-ratios, is not possible. In order now to remove the small grain of heavy mineral from the large grains of light mineral associated with it, the application of some other reaction is necessary. This force is suction, or, perhaps more properly, the reactions that occur during the suction-cycle.
Under the conditions that exist in a jig-bed, we are dealing with a number of columns of water moving with some velocity relative to the grain. The forces acting upon the grain will be those of the water-currents, of gravity, and of the resistance opposed by the walls of the channel or other grains. The effect of the water-current alone upon the grains may be considered a purely non-selective force. For grains of the same size and shape a given current will exert as much effort upon a particle of galena as upon one of quartz. Any advantage that the small heavy grain has over its larger companion, due to acceleration, will always be a positive force. The resistance offered to the passage of the grain by other surrounding grains will depend upon the relative diameter of the channel and the grain, and the length, shape, and inclination of the channel. If the grains are all the same size and shape, then the mean diameter of the channels will be less than the diameter of any of the grains, and none of them could be carried through the interstitial spaces. Take as an extreme case a column of shot, steel balls, or marbles of the same diameter, they are all absolutely fixed as regards any possible suction-velocity. The same is true, through to a less extent, in rounded particles not all the same size, as well-worn sand, gravel, etc. Again, the possibility of the mass becoming packed is small. Of course, the reason for this is well understood, and is owing to the feet that in these cases the surfaces of the particles are curved, and therefore the points in contact are reduced to a minimum. Under any practical conditions existing in the jig-bed, the par¬ticles are not all the same shape or size, and instead of being bounded by curved surfaces they are angular and bounded by planes. This results in neighboring grains having not few but many points in contact, accompanied always by a more or less wedging action, and therefore jigging under excessive suction- velocity results in a tight bed. The wider the size-ratio the greater the effect, and vice versa. The possibility of applying suction depends upon the ability to maintain within the jig-bed interstitial channels somewhat larger than the maximum grain to be saved. Under the conditions existing on a jig-bed, the effect of increasing the diameter-ratio of bedding and feed, the number of bedding-grains in a vertical column, or thickness of bed, and the character of the bedding-grains themselves, whether they are rough and angular, cubical, or well worn and spherical, is at once evident.
The increased catch secured on a jig-bed over that obtainable by rising current alone, under either free or hindered settling conditions, is due to the reaction occurring during suction. In order that suction may become effective, it is necessary that the reaction of pulsion precede. During pulsion a selection and arrangement takes place; and during suction a destruction of the conditions of equilibrium set up under pulsion, by the removal of the small heavy grain through the interstitial channels into the hutch, results. Suction always supplements gravity, but in a way in which gravity cannot act efficiently—that is, in the movement of grains in channels more or less inclined or crooked, where a particle could easily lodge, although large enough for the grain to move in if vertical. The current moving with high velocity in these spaces serves to move the particle. Pulsion may be said to be the master reaction, while suction is its necessary complement, completing what has been initiated by pulsion. Suction is therefore necessary in jigging all unsized material. Excessive suction with sized material, under practical conditions, would be disadvantageous. With very close sizing on coarse jigs it would not be particularly harmful, but it would be useless. In jigging under any conditions, more or less suction will be of advantage, as helping to save the smaller particles of heavy mineral that otherwise might be carried off with the tailings.
Discussion of Acceleration
It has been pointed out (Tests 16, 17, 18, 37) that with a very short and quick stroke, but relatively low piston-speed, the ratio of concentration obtained was low. Moreover, the jig-bed pulsated under the influence of the short, rapid stroke, and did not with the longer one of less frequency, but having the same mean piston-speed in inches or millimeters per second. This was a movement of the grains en masse, the bottom pulsating quite as much as the top, and was altogether different from that gentle, selective action observed with proper speeds and frequencies. The jig-bed moved as it would if acted on by a solid piston from below. Thus, by giving many quick sharp blows to the jig-bed, the water-columns have not time to adjust themselves to the increased pressures, except by raising the grain which happens to be in the direction of impulse. In addition to the mean piston-speeds, as derived from Professor Munroe’s formula, the element of time during which the impulse lasts should be included. This solid or piston-effect of a water-column can, perhaps, never be entirely eliminated, nor does it seem desirable that it should be. The results show that increased quantities of hutch-work are produced, supplementing suction by keeping the interstitial channels cleared. Since the grains on the bottom are the first to feel the impulse and be raised, it has been shown that true pulsion is diminished, and the important reactions dependent on it diminished. Sharp, rapid strokes, by increasing the piston-effect, promote sifting, and therefore aid suction, but decrease the reaction of pulsion.
Resume and Conclusions
Referring to the 13 conclusions of Professor Munroe, quoted in the early part of this paper, it may be said that no experiments have been carried out with the idea of duplicating the work covered by the first six of his conclusions. In the absence of positive experimental data, it may be considered quite out of place to enter into a discussion of them. However, in the light of results of the present investigation, a few observations concerning these first six conclusions may be given. The careful record of so many tests, under the conditions observed by Professor Munroe, seems to cover the field thoroughly.
Conclusions 1, 2, and 3 are undoubtedly fundamental propositions in any system of jigging. To Professor Munroe is due the credit of having first clearly pointed these out and applying them to jigging. Following, as corollaries, are the formulas given for the velocity of fall of grains en masse. The formulas for the falling-velocities of grains en masse under the assumed conditions, when applied to piston-speed, have been demonstrated by experiments with the pulsion-jig, the Vezin jig, and the Harz jig to yield satisfactory results.
Conclusion 4 has been noted elsewhere. A grain 0.4 the diameter of the channel will have a maximum falling-velocity, which therefore increases its chance of being saved, and of increasing the interstitial settling-ratio.
Conclusion 5, in the first part, follows, also, from the first three conclusions, and its application is fully demonstrated by Professor Munroe. It seems to me that there is a reasonable doubt about accepting the second part of this conclusion. There is no doubt about this part of it: “ The falling-velocity [of a mass of grains] increases or diminishes with the distance apart of the grains,” since this is merely a re-statement of Conclusions 1, 2,3, and the first part of 5. When, however, the balance of this statement is examined—that is, “the velocity of the current necessary to support or raise the mass of grains increases or diminishes with the distance apart of the grains,” I believe we are entitled to withhold judgment until it has been shown what these velocities, under the conditions of jigging, actually are. This statement is true if we assume that the velocities supporting or raising the grain are equal to the observed velocities in the free or unobstructed part of the tube; or in practice the piston-speed. But are these the velocities acting upon the grains ? Under the conditions obtaining on a jig-bed, the grains occupy a considerable area, and therefore constrict the passage. It is a matter of actual observation that the velocity in the interstitial spaces is much higher than that of the jig-piston. It is the same principle of conducting a given volume of water through a pipeline made up of, say, a 12-in. and a 6-in. pipe. In the 12-in. pipe the column of water will have a mean velocity of x feet per sec., and in the 6-in. section the velocity has been increased to 4 x. Thus, we must be careful not to confuse the falling-velocity of grains en masse with the velocity of the water-column actually supporting them during pulsion.
It has been noted by Professor Munroe that spheres falling in tubes have a maximum falling-velocity when the diameter of the sphere is 0.4 that of the tube; and spheres either smaller or larger than this size fall with less velocity. If the column of water in the tube has a velocity of 0, or is at rest, a solid falling through this water-column will displace a volume of water equal to its own volume as often as it traverses a distance equal to one of its three dimensions. This displaced volume must escape within the interstitial space of tube and body with some velocity, depending on the velocity of the falling body and the ratio of the diameters of the falling body and the tube. If the falling body has a diameter nearly equal to that of the tube, the area of the interstitial space is small, and a low falling-velocity of the body may correspond to a high interstitial velocity of the water-current. Thus, while the velocity of fall decreases as the diameter of the solid approaches that of the tube, at the same time the velocity of the current tending to support it increases. If the body has a diameter equal to that of the tube, any motion of the solid would mean an infinite velocity to the interstitial current, and the body stops. On the other hand, as the diameter-ratio between the solid and the tube increases, the area of the interstitial space increases, and the volume of displaced water decreases, and with it the interstitial velocity, and the body would tend to fall with a high velocity; but the force causing it to fall, its weight, is also smaller, and therefore its ability to overcome the inertia of the liquid, and other resistances, is less, so that its falling-velocity is less. The possibility of interstitial currents depends upon a solid of any diameter less than the tube, and having a specific gravity greater than that of the liquid, and which is free to fall, or resists the motion of a column of the liquid in which it is immersed.
It is evident that if a velocity be given to the water-column in the free part of the tube equal to the observed velocity of fall of the body in the stationary column, then the body will be supported or remain at rest. This velocity of the water in the column is the apparent velocity necessary to support the grain, and some function of the actual velocities supporting it. In jigging, it is not so much the velocity of fall of a mass of grains that concerns us, as the velocity of the current necessary to raise or support them. In jigging, the grains are not free to fall, since they are firmly supported on a sieve, but they are quite free to move when the interstitial currents are acting in pulsion. When the force due to the velocity of the rising currents is greater than all other forces holding the body at rest, then the body moves in the direction of the greatest forces, and continues its motion so long as the forces are unbalanced. Thus, it has been observed that the particles will be raised to positions higher than at rest during the action of the pulsion-current. The grains in the bed are being raised because each one in motion is seeking a position higher up in the column where the distance between grains is greater, or, in other words, where the interstitial velocity is lower.
Conclusion 6 admits of no doubt.
Conclusion 7 is an axiom as regards the first part. The second part concerns the ratio of equal-falling particles of the pair chosen—namely, quartz and galena. No ratios were obtained in any of the investigations approaching those called for by this theory. Evidently the conditions required by the theory were not present. The conditions assumed were that the fine grains should closely surround the large grain of quartz. It has been observed in all experiments that the large grains quickly settled on the bottom—the smaller and lighter above, whether of bedding or ore. This fact was also pointed out by Professor Munroe in his paper on his experiments with mixed shot. There is but one force that can carry the small, light grain to the top. That force resides in the velocity of the interstitial currents acting during pulsion. Certainly, in the pulsion-jig experiments, where the unsized material was thoroughly mixed, and added practically dry in order to avoid any classification in falling through a water-column, and where very large percentages of the heavy mineral (exceeding 70 per cent.) were used, the above ratios should have been secured. In the case of the above pair it was found that a particle of galena and one of quartz 5.8 times its diameter were in equilibrium. If the conditions called for by the theory were present, and the results not fulfilled, then an examination of the theory is in order. But we have observed above that while the material was thoroughly mixed when added to the tube, the fine, light grains immediately separated from the large, heavy ones during the first few strokes of pulsion. From this we must conclude that the fine, light grains were not in equilibrium with the large neighbors, and sought positions higher up in the column where they were. When this was found they remained fixed, or were in equilibrium. These ratios have been given elsewhere. For quartz and galena the ratio was 5.8 to 1.
The conditions assumed cannot, under any possible conditions, exist on the jig-bed, and therefore the results that would follow cannot possibly be attained in practice. The conditions would be fulfilled if we caged all the light and heavy grains, and prevented any movement among them; but this is the very condition that we do not want on a jig-bed. It is hardly fair to assume that if in one way or other we are able to keep a mass of mixed grains together, under conditions where the smaller ones cannot escape, therefore the small grain is falling with the same velocity as the large grain. It is in equilibrium by force, not choice; and on the jig-bed we try, as far as possible, to encourage the grains to exercise the latter and not the former.
Possibly if formulas had been derived showing the velocity of the interstitial currents (the currents supporting or raising the grain), and from these, equal-settling ratios were derived, the values would be much less than 81 to 1, and probably close to those obtained in the pulsion-jig.
Conclusion 8 follows from the conclusions 1, 2, 3 and the first part of 5. I can bear testimony as to the practical accuracy of this, since I have calculated many a piston-speed and velocity in the free tube in the pulsion-jig. With the formulas given, it has been shown that with pulsion-jigs, Vezin jigs, Harz jigs, etc., the piston-velocity so calculated suffices to move the grains. Given the size or diameter, and the specific gravity of the minerals to be separated, the jig- piston velocity may be calculated with almost a nicety. It has been shown in the experiments in piston-velocity that a considerable variation is permissible in jigging.
Conclusion 9, the first part of Conclusion 11 and all of 13 are corollaries of the last part of Conclusion 7. Since the conditions assumed for Conclusion 7 cannot exist on a jig-bed, therefore no support is left for 11 and 13, and some other explanation must be given to account for the applicability of the English system. This action has been discussed under pulsion and suction.
Conclusion 10 has been abundantly demonstrated. It might be added that if the theory were applicable little or no suction would be necessary.
Conclusion 11. The last part of this conclusion, concerning the presence of more or less coarse material in jigging very fine material, agrees with practice, since, if not in the feed, a bed is used, which fulfills the conditions. The tests do not cover cases in which any large percentage of feed was less than 0.10 millimeter.
Conclusion 12 accords with all results of practice and experi-ment, and is therefore another fundamental proposition in jigging.
Finally, to Professor Munroe must be given the credit for having pointed out the fact that bodies fall with less velocity in tubes than in large bodies of water, and for having demonstrated the applicability of formulas based on this fact to obtain correct jig-piston velocities under the assumed conditions. It is to be always understood throughout this paper, that under records of the experiments, where pulsion-velocity and suction-velocity are given, the value expressed in inches or millimeters per second is that of the piston or the water-column in the free or unobstructed part of the jig only, and clearly not the actual pulsion-velocity acting upon the grains during these reactions. One is a function of the other, but under the very conditions obtaining on a jig they cannot be equal.
Comparing the results given by Professor Richards, a part of whose conclusions are quoted earlier in this paper, it will be found that, so far as the experiments may be compared, there is a very close agreement between us. Since we both started from the same experimental basis, on which we were agreed, it is but natural that our conclusions should be in close harmony. This theorem, which forms the basis in every practice of jigging, is all important, and of course is the establishment of the value of the resultant measured by the diameter of grains differing in specific gravity obtained during pulsion alone. This factor represents all that can possibly be expected from every force acting upon the grains of a jig-bed during the time the pulsion-currents are acting, or while the grains are free to fall. To Professor Richards is due the credit of having demonstrated the value of this resultant as measured by the ratios of diameters. My own researches, carried out differently (see pulsion-experiments), have abundantly confirmed the substantial accuracy of these ratios. When, therefore, Professor Richards says: “ The two chief reactions of jigging are pulsion and suction,” I see no escape from his conclusion. If we go a little further and say: “ The reactions occurring during pulsion and suction are the only reactions of jigging,” we have included every force imaginable that can act upon the grains. As pointed out above, the resultant of all the forces acting upon the grains during pulsion is given by the interstitial or hindered-settling ratios as determined by Richards and myself. The resultant of suction cannot be separately determined, apart from that of pulsion. There is no determinable resultant of suction as measured by a ratio or factor.
