The art of crushing ores and other materials by means of rolls is a comparatively recent one. While the first record of rolls using iron crushing-surfaces dates back to the year 1806, when they were employed in Cornwall, their principal development has taken place during the past 130 years.
To Stephen R. Krom belongs the credit of the pioneer in introducing the belted high-speed roll, which has had its origin and a marked development in this country. His notable contribution to the art was in the use of a single bed-plate or frame supporting the roll-shafts, and to which levers holding the movable roll-bearings were pivoted. He also made use of steel tension-rods to support the crushing-strains, and of hammered- steel tires for the crushing-surfaces. These changes brought the design of crushing-rolls to a high level.
Following closely thereon, W. R, Eckart conceived the idea of the swivel or ball-and-socket support for the roll-shaft bearings. This is an excellent mechanical conception, especially for bearings held against a spring pressure, and while it may not be necessary for all types of rolls, yet it has been quite generally adopted by other roll-designers, and illustrates the refinement which roll-construction has now reached.
Other engineers, as, for example, Argall, Vezin, Roger, and Sturtevant, to mention only a few among many, have also given the closest attention to the various details of rolls, such as the frame, springs, bearings, and shafts, and have developed many novel and original designs. It has remained, however, for the boldness and originality of Mr. Edison to extend the field of crushing-rolls in two new directions, and to cause them to exercise new functions. In developing crushing-machinery for his Portland cement works, Mr. Edison constructed giant rolls, having a diameter of 6 ft. and a length of face of 7 ft. With these he was not only able to challenge the long-established position of the jaw- and the gyratory crushers as primary crushers, but even to leave them well in the rear. By means of projecting knobs on the roll-surfaces he utilized the stored energy of the revolving rolls, and was able to shatter masses of rock of so huge a size that they could not otherwise be made to enter the rolls, thus saving the expense of block-holing and sledging, which is usually charged to quarrying. This work has had successful applications in crushing limestone rock, and there remain also possibilities of the extension of this new use of rolls to rocks of a still harder nature.
In going to the other extreme of size-reduction, Mr. Edison has also utilized rolls in pulverizing cement-rock in his work. For this purpose, rolls with sectional, corrugated, chilled-iron shells 30 in. in diameter and 8-in. face, are provided with shafts 18 in. in diameter, and are forced together with a spring pressure of 100 tons. The feed-material, which has passed a 0.75-in. screen-opening, is thus reduced in a single operation to a size at which 94 per cent, passes 100-mesh screen, at the rate of 60 tons per hour. This use of rolls carries the principle of choke-crushing to so extreme a limit as to involve practically a new function. It would appear at first sight, however, that there is less profit-margin for rolls in pulverizing than in mass-reduction, and only a close comparison of the final products obtained and the respective costs per ton can determine the relative economy of rolls for pulverizing when compared with the tube-mill and other types of grinding-apparatus.
Irrespective of the part which rolls may play in the future in their relation to the two extreme limits of size-reduction, there is no doubt that they have achieved for themselves a secure position in crushing products of intermediate sizes. This is partly due to their large capacity and low cost of operation. It is also due to the fact of their mechanical simplicity, which involves the principle of the toggle-lever in overcoming crushing-strains exerted by particles brought within the angle of nip of their surfaces. Since their revolving masses also serve to absorb their own peak-loads when properly fed, a moderate and fairly uniform application of driving-power is able to accomplish a considerable amount of effective work in splitting and shattering rock-fragments.
Perhaps the most distinctive advantage of rolls is that their construction permits them to apply the principle of arrested crushing to a greater range of sizes than is possible with any other type of crushing-apparatus. The crushing-pressure exerted by the opposing roll-surfaces during the angle of nip is instantly released and ceases when the rock-fragments reach the horizontal diameter of the rolls, where the open space between them permits the material to be discharged. Roll-crushing thus permits most careful and accurate stage-reduction within a wide range of sizes, and avoids pulverizing and sliming an undue amount of the softer minerals of an ore, in crushing it to the size at which they will unlock sufficiently from the surrounding gangue to permit their concentration to take place. For those ores, therefore, which require concentration, the use of rolls in preparing them for jigs, shaking-tables, or magnetic separators has become almost the universal practice. This applies to many iron-, copper-, lead-, and zinc-ores. Gold- and certain silver-ores, both those which require concentration and those which do not, are in a class by themselves, since usually their values can be extracted without essential relation to the granular condition of the crushed product.
The modern tendency to reduce milling-costs by increasing the mill-capacity has demanded a greater duty from rolls than ever before, and in the larger mills rolls are now employed from 36 up to 54 in. in diameter, and from 15 to 28 in. width of face. Such rolls are used mainly for coarse crushing ; that is, they take the product from the jaw- or the gyratory crusher, from 1.5 to 2.5 in. in size, and reduce it to about 0.5 in. These coarse or No. 1 rolls are then followed by other rolls set closer together for finer crushing, and possibly by others which re-crush certain middlings products obtained in the process of ore-treatment, or even tailings, dependent upon the nature of the ore and its association. Rolls of this general character require massive construction and excellent workmanship. Rolled-steel tires can now be obtained up to 54 in. in diameter. Special hard steels, such as chrome- and manganese-steels, are also used for certain ores, either in the form of tires or of plates bolted to a central mandrel. In this way the life of the crushing-surfaces has been much prolonged.
Marked progress has thus been made within recent years in the field of coarse crushing by means of rolls, in response to the greater demands of modern mill-practice, and this progress has been largely brought about by increasing the dimensions of the rolls and adopting a more massive construction, as well as a better design, combined with a wider choice of steel adapted to different ore-requirements than has heretofore been available.
On the other hand, it must be admitted that up to the present time rolls designed for fine crushing, where the ore-requirements demand a maximum granulating effect and a minimum pulverizing or sliming effect upon the crushed product, have made little progress compared with rolls designed for coarse crushing. In fact, rolls, as heretofore designed, can hardly be said to have held their own; and since little assurance of their satisfactory operation can be had in connection with an ore which must be reduced to pass a 20- or 30-mesh screen while retaining the crushed material in a granular condition, rolls have been assailed on all sides by various types of ball-mills and other pulverizing-apparatus which claim to accomplish the function of granulating an ore successfully, but usually by means of some reduction in the time during which the pulverizing effect takes place. While there may be an overlapping territory at the limit of fine granulation where pulverizing-apparatus may be so adjusted as to perform the function of approximate granulation with sufficient success to make their use advisable, yet it is clear that a crushing-force exerted upon material placed between walls which do not touch when at their minimum distance apart, must produce a distinctly better granulated product than when it is crushed between walls which are brought into physical contact with a grinding-pressure.
With the presumption of advantage thus on the side of rolls, even down to the finest sizes, the fact remains that heretofore rolls have proved unsatisfactory and inefficient, from lack of control over the granulating action as the roll-faces wear, and also from their small capacity.
In looking more closely into the cause of this inefficiency, it is evident that the effect of the irregular wear of the roll-faces becomes a more serious matter in fine crushing than in coarse crushing, for the reason that in the former, since the faces must be brought quite close together, the ratio of the sectional area due to irregular wear to the total areal opening between the rolls is greater than in the latter case. Hence any ridging, grooving, or corrugating of the roll-faces permits a considerable proportion of the particles in the feed-stream which enter the depressions to pass between the rolls without being crushed. This reduces the capacity of the rolls, measured by the amount of under-size or finished product obtained. Another difficulty arising from irregular wear of the roll-faces is due to the fact that when out of parallelism rolls tend to exert a certain component of the crushing-pressure at right angles to the diameter of the rolls, or in the direction of the axes of their shafts. This produces an end-thrust upon the roll-shafts, which, transmitted by means of collars to their bearings, causes them to heat and the shafts to cut, thus absorbing power wastefully, and still further reducing the crushing-efficiency as measured by the power consumed to operate the rolls in relation to the amount of finished product obtained.
Rolls used for fine crushing thus show a decreasing efficiency in proportion to the wear of their roll-faces until a point is reached where it becomes necessary to stop the crushing-operation, and to restore the faces by chipping, grinding, or machining them until their surfaces are parallel. This involves loss of time and of mill-capacity, besides expense and a wasteful use of the roll-shells.
While a certain amount of choke-crushing is usually advisable in fine crushing, in order to offset some loss of capacity, the best results can only be obtained, where the prime object is to granulate an ore, when the roll-faces are maintained parallel, and when the feed consists of sized material in order to avoid packing and pulverizing it in passing it between the rolls. It seems clear from the above considerations that further advance in the art of fine granulation by means of rolls can only be expected by means of certain refinements of function whereby the roll-faces can be maintained parallel while they undergo wear.
Having had the problem of fine granulation in mind for some time past in its relation to the treatment of certain classes of ores, I have been gratified to find in a recent design of rolls by J. S. Frazee, of Brooklyn, N. Y., that he has completely overcome the obstacles which have heretofore prevented the satisfactory operation of rolls when used for fine crushing, and when a granulated product is required.
Mr. Frazee has given me the privilege of presenting a brief account of his rolls to the Institute; and as I believe they are not generally known, I take pleasure in doing this, in the confident belief that they mark an important step in the advance of the art of roll-crushing.
Figs. 1 and 2 show a side and an end view of a pair of 18- by 12-in. rolls. It will be seen from Figs. 3 and 4, which show the same rolls in part sectional elevation and plan, that the bearings of the roll shafts are supported in side-frames. These are of cast-iron, without tension-rods, and are held together by stay-bolts and lock-nuts to permit the width between the frames to be slightly adjusted, if necessary, when the rolls are in place. This construction has certain advantages for fine crushing over the single bed-plate or frame. It is lighter, less expensive, and permits the rolls to be completely boxed in by means of a wooden housing. This is hoppered at the bottom, and is connected with an exhaust air-pipe, through which the fine dust is drawn by means of a fan, thus keeping the mill-space free from dust. The bearings are of the solid type, lined with babbitt, and each provided with dust-caps at each end. All four are movable in the guides provided for them in the frames. Two of the bearings are held against a spring pressure, and the other two against screw adjusting-bolts, for the purpose of regulating and controlling the space between the roll-faces. These adjusting-bolts are provided with sprocket-wheels and an endless link chain, which is kept taut by means of a small idler between them. By inserting a long-handled spanner in openings in either of the sprocket-wheels, the adjusting-screws can be moved in exact unison in varying the position of the roll-space. This feature
than the other, so as to permit flanges to form on it, between which the shorter roll revolves, with a slight clearance. The rolls are fed from a hopper, which is supported independently of the roll-frame, and in such a position as not to interfere with the removal of the rolls. The lower end of the hopper is made of cast-iron, and is provided with a gate, which is operated by means of a shaft and levers, so that it can be controlled from the forward end of the roll-frame. A feed-tray is placed below the hopper so as to convey the material to be crushed from the hopper-gate to the space between the rolls. This feed-tray is supported at the back of the hopper by a pivot-bolt, which allows it to rotate slightly, and at the front end by means of a wooden connecting-rod to a horizontal arm flexibly connected to a support at the front of the hopper. When the front end of the arm is raised by the teeth of the revolving cam below it,
the discharge end of the feed-tray is caused to rise and fall with a jarring or humping motion. This causes the material in the feed-tray to be carried forward and to be discharged in a steady stream over the angle-iron which forms a lip or dam at the extreme edge of the tray. The bottom of the feed-tray is made of sheet-steel to permit the material to slide freely. The amount of motion imparted to the end of the feed-tray can be varied by means of an adjusting screw-bolt, which regulates the height of the drop of the horizontal arm, and thereby also the effect of the bumping action upon the feed. A small lever at one side of the adjusting-bolt permits the horizontal arm to be raised above the level of the cam, and in this way the feed can be instantly stopped and started.
It has been a common experience that as rolls are usually fed, even when great care is taken to secure a feed-sheet of uniform thickness, the roll-shells will groove and wear more rapidly at their centers than at their ends. This is frequently caused by a difference in the rate of flow of the feed-stream, owing to friction of the sides of the chute or spout leading from the hopper to the rolls. Another cause of greater wear of the central portion of the roll-shells is the greater mobility of the feed-stream in certain directions than in others, when subjected to a crushing pressure. This difference in mobility causes a difference in the amount of abrasion, and it shows itself by the cupping, grooving, or corrugating of the roll-faces along the center, and by ridging at the edges of the roll-shell. Mr. Frazee has found that the difficulty of irregular wear of the roll-shells can be entirely overcome by a close control over the sectional shape and area of the feed-stream, and by feeding a greater amount of material along the sides of the roll-faces than at their centers.
As shown more clearly on a larger scale in Figs. 5 and 6, the projecting side of the angle-iron which forms the lip of the feed-tray is cut down at its two ends, so as to slope gradually towards the ends, leaving its full height only at the center. In this way the feed-stream passing over the lip or dam is made thicker at the ends than at its center. Furthermore, to regulate the flow and upper contour of the feed-stream, as well as the lower, a guide-plate, which is usually hinged to the sides of the feed-tray, is allowed to rest upon the surface of the feed-stream. This is provided with a wearing-plate at its lower edge, and is so beveled at its two ends as to permit a somewhat greater freedom of flow of the feed-stream at its edges than at its center. The shaded area in Fig. 6 shows the approximate cross-section of the feed-stream, which is adjusted to secure an equal abrading effect across the entire width of the roll-faces, and thus to maintain them in perfect parallelism while they undergo wear. The exact cross-section of the feed-stream will vary somewhat with the hardness of the different materials to be crushed. This can be easily adjusted by varying the amount of bevel given to the edges of the angle-iron and of the guide-plate in the feed-tray until the feed-stream exerts the desired uniform abrading action.
The flanges upon the longer roll-shell perform two functions. They not only confine the feed-stream at its ends, and prevent ridges from forming at the ends of the unflanged roll-shell, but by carrying the feed-stream a slight distance beyond the end of the shorter roll-shell, a small amount of crushing is performed between the sides of the flanges and the ends of the roll-shell. This flange-crushing, being in opposite directions, balances, and all tendency to exert end-thrust of the shafts against their bearings is thus neutralized and overcome. By the simple means thus provided it becomes possible for the roll-tender to maintain the roll-faces parallel while crushing even the hardest material, and to keep the rolls in continuous operation until the roll-shells are completely worn out. The only assistance required of a machinist is occasionally to turn off the edges of the flanges when they become so long that they strike the bolts of the opposite roll which hold its roll-core in place.
Figs. 7 and 8 illustrate the effect of Mr. Frazees invention in connection with 12-in. and 24-in. roll-shells, in keeping their surfaces cylindrical until the shells are completely worn out. The 12-in. roll-shell shown in Fig. 7 has been reduced to 6.75 in. in diameter.
The 24-in. roll-shells shown in Fig. 8 have been reduced to 19.25 in. in diameter, leaving a thickness of only 5/8 in. of metal at their edges. At their centers they are somewhat thicker, owing to their beveled inner surfaces, which are required in order to mount them upon their coned centers. A pair of the 24-in. crucible-steel roll-shells, 14.5-in. face, when machined and ready for use, weigh approximately 1,776 lb. When worn down to the size shown in Fig. 8, their weight is 438 lb. The differenceviz., 1,338 lb., or 75.3 per cent, of the original weighthas been entirely expended in useful and effective work. Such a novel and valuable result, whereby the efficiency of the crushing-rolls is maintained uniform while the roll-surfaces undergo wear, clearly marks a step in advance in the art of roll-crushing. The only machine-work required, as already explained, consists in a partial removal of the flanges on the longer roll-shell as the wear of the shells per-
While the method of feeding the rolls which Mr. Frazee has adopted accomplishes admirably the purpose for which it was designed, and secures a close control over the feed-stream at its point of discharge, it is evident that the same principle might also be adapted to other types of feeders, such as the roller- feeder, the shaking-feeder, the plunger-feeder, the scraping- feeder, etc.
This new roll-design and manner of operating rolls has been developed by Mr. Frazee in connection with the dry crushing of very hard materials used for abrasives, such as quartz, garnet, etc. It is equally applicable to ores, and by the settling and removal of water from the mill-feed, it is possible to apply it to wet crushing as well as dry. The field which is thus opened up in connection with ores which require careful granulation, in order to prepare them for concentration, is a very large one.
Perhaps the most interesting and obvious application of Mr. Frazees new roll-design is in connection with the treatment of complex ores. Innumerable processes have been proposed for the recovery of the values contained in these perplexing ore- mixtures, but they may be divided broadly into two classes, viz., that which includes mechanical methods, and that which includes all the others, such as chemical processes, leaching, smelting, etc. While the former are the least expensive, the results heretofore obtained have been proximate only, and the recoveries have shown a low efficiency. All of these mechanical processes, whether they employ gravity, magnetic, electrostatic, or flotation methods (with the single exception of the flotation process adopted by the Minerals Separation Co., Ltd.), require an ore to be crushed to a point where the separate minerals it contains are sufficiently unlocked to permit them to be mechanically separated from each other, so that they may be collected in different groups. The point does not seem to have received sufficient attention, that any one of the above mechanical processes of treating complex ores can be seriously interfered with, if not completely upset, by the means employed to crush such ores in preparing them for treatment.
The mineral association of these mixed sulphide ores is usually a very intimate one, so much so that in many cases it involves crushing them to pass a 30- to 40-mesh screen, or even finer, in order to sufficiently unlock the associated minerals.
By the ordinary methods of crushing there is great danger of producing a large amount of fine powder, or slime, in crushing to such sizes. Particles of a very minute size, usually placed at about 200-mesh, then become, in a measure, a law unto themselves, and the losses in treating such material are very great. The obvious way to solve the dust, or slime, problem is not to make any dust, or slime, or as little as possible, in crushing these ores for mechanical separation.
The future of the mechanical processes of ore-treatment, in competition with others which promise larger returns, will thus depend, in many cases, upon successful granulation. In this direction I believe that Mr. Frazees invention, as outlined above, has accomplished a great step in advance in the art of roll-crushing, and it holds out great hopes for the future in the more successful preparation of many ores for mechanical concentration, by reason of the control he has secured over the wear of his roll-shells, whereby a better granulating action can be obtained.
The flanges are formed by the wearing of the shorter roll into the other, which is about an inch longer. When the roll-shells are new, the longer one has machined at each end a ridge about 1/8 in. or 0.25 in. higher than the face. These slight projections help to center and steady the rolls, by locating the flanges in advance.
Possibly they might be left off at the start; but Mr. Frazee has found it better, in machining the new roll-shells, to turn such small flanges upon the longer one. In his practice the feed-stream is extended a short distance beyond the two ends of the shorter roll-shell. This allows a small amount of crushing to be done between the flange-faces and the vertical ends of the shorter roll-shell, which prevents any ridges from forming at the ends of the shorter roll-shell.
In common practice, where two roll-shells of the same width are used, it is impossible to prevent ridging of the diagonally-opposite ends of the roll-shells. These ridges prevent accurate work in fine crushing. They also exert end-thrust upon the shaft, and strains upon the roll-frame. In Mr. Frazees design the flanges not only act as movable cheek-plates, but they overcome all end-thrust upon the roll-shafts, and by preventing ridges from forming at the ends of the shorter roll-shell, they help to maintain the roll-faces in perfect parallelism while they undergo wear. The flanges become, therefore, of great importance in fine crushing. When they extend so far as to strike the draw-bolts of the shorter roll-centers, they are cut off.
One of the great difficulties in ordinary rolls is to get an even feed. How is the feed brought to this roll (indicating) ? If the feed came more in one place than in another, that place would wear more.
That raises a very interesting question. Mr. Frazee has found that an even, or uniform, feed is not desirable. In fact, a uniform feed produces irregular wear. This is certainly true in the case of fine crushing, in spite of the general opinion of roll-manufacturers to the contrary.
The reaction of a feed-stream upon the crushing-surfaces of roll-shells is a subject to which little attention seems to have been directed heretofore. Mr. Frazee has found that in order to maintain the roll-faces parallel, it is essential to feed a larger amount of material along the sides of the faces than at the centers of the roll-shells, as explained in my paper. This seems to be partly due to the unequal mobility of different portions of the feed-stream as it is subjected to the crushing-pressure.
Differences of mobility would naturally cause differences of abrasion. A certain analogy may, perhaps, be found in the different rate of flow of metal in different portions of a hot ingot while passing between reducing-rolls. Whatever the reason may be, it has been found by actual practice that irregular abrasion of the roll-shells can be prevented by varying the thickness of the feed-stream in such a way that a larger amount of material is fed towards the ends than at the center of the roll-shells.
A further advantage of Mr. Frazees roll-design, resulting from the high crushing-efficiency attained by control over the wear of the roll-shells and balanced end-thrust, is, that the roll-frame can be made considerably lighter than would otherwise be possible. By reason of the reduced strains, single side- frame castings, bolted together, can be used to support the roll-bearings in place of the usual massive continuous bed-plate casting. In actual operation these new rolls run almost as smoothly as a lathe. The 24-in. rolls are run at 100 rev. per min.; the 18-in. at 150, and the 12-in. at 250.
This system of crushing has been developed gradually. There are eight of these rolls now in actual operation, all exhibiting the same uniform control over the wear of the roll-shells. It is therefore not a happy hit, but the result of close study of actual conditions on the part of the millman rather than of novel design on the part of the mechanical engineer. For this reason I think the new roll-design of Mr. Frazees is of especial value and interest.
There is a comic side to this: When we were preparing the first two volumes of our book on Ore-Dressing we had all our data in; and then the Wilfley table was developed, and that knocked the underpinning from under our plans. Now, we have our third and fourth volumes out, and we have taken great pains to show that flanges should be prevented, and now it turns out that the flanges are the best things to have. It reminds one of the old darkeys phrase, The world sure do move.
It seems to me that the principal novelty in the Frazee rolls is the feeder. Flanging one roll was common in Joplin a few years ago; while this helped to keep the roll-faces true, it was generally abandoned because the flanges, being made of cast-iron, frequently broke and caused much trouble when the pieces got into the jigs. Connecting the two adjusting-screws by a chain and sprockets is practically what has been done with other rolls. The feeder, however, is a new arrangement, and I believe a very good one. Even without the testimony of Mr. Paynes photographs, I should expect, from my own experience, that it would tend greatly to keep the faces true.
A few years ago we had a mill consisting of a 10 by 20 roll crusher, two 6 by 20 breakers in parallel, and a pair of 20 by 28 Krom rolls. When in good condition this just about kept the works in operation. We found, as Mr. Payne says, that the tendency of the rolls to wear hollow in the middle greatly reduced the capacity of the mill and necessitated much loss of time in shutting down to turn off the rolls. We first tried a pair of hard-steel rolls, but found that they greatly reduced the capacity of the mill, as the hard surfaces would not bite, but slipped, and the rolls packed full and threw off the belts, making it necessary to shut down and shovel out. Our next trial was with a feeder on the principle of Mr. Frazees. While not as efficient as his, it decreased the trouble from uneven wear enough to answer our purpose. This device was a series of V-shaped riffles in the feed-chute. They were half round, about 0.5 in. in diameter, and placed with the points upward, so as to throw more feed to the sides than the center of the rolls. By a little experimenting we found what spacing and angle of these riffles gave us approximately even wear of the rolls. This very simple arrangement greatly increased the time the rolls could be run without requiring to be turned off, and enabled the work to be done without increasing the size of the mill.
Mr. Paynes statement that S. R. Krom was the pioneer in introducing into the United States crushing-rolls driven by belts, is, to the best of my knowledge, correct; but Mr. Paynes statement that S. R. Krom introduced high-speed rolls is not correct. As we understand the term high-speed namely, a peripheral tire-velocity far greater than the falling velocity of the feedsuch machines have never been built by any Krom manufacturer. Krom rolls have always been driven only slightly in excess of the feed-velocity. A high-speed roll, introduced a few years ago, naturally failed. Such a roll, revolving ahead of its feed, requires additional power, and is subject to additional wear on its bearings, without accomplishing any more work.
The hall-and-socket bearing, which Mr. Payne mentions, has only been adopted by those makers who were unable to design and build a roll with a back-bone stiff enough to stand up and do its work like a real roll. Such a bearing is a poor substitute, involving, as it does, extra parts in a machine that, owing to the nature of its work, should have as few parts as possible.
I think the use, for reducing large sizes, of the giant rolls mentioned by Mr. Payne, is, both mechanically and economically, a mistake. Rolls used for such work encroach upon the field of machines far better adapted to it. The cost of the jaw- and gyratory crushers for installation, power, and repairs is, in proportion to the work done, so much smaller than that of giant rolls as to counterbalance any advantage claimed for the latter in other respects.
We found that in order to keep the rolls in fairly-good shape, we had to put on two new tire-plates each Sunday, at a labor-cost of from $30 to $50. Two plates with freight cost about $185, making a total of about $225 per week.
We operated the rolls 4 months and averaged 400 tons per day ; but this small output cannot be charged against the rolls, as we had but one shovel and a great deal of trouble with the electric power.
Three hundred and twenty 2.5- by 10-in. bolts are used to fasten the plates to the rolls. Occasionally a bolt breaks, and in going through the mill with the tailings is sure to do damage to the machines used for secondary crushing. In this way we broke three shafts in our No. 6 crusher.
A jaw-crusher constructed upon certain lines, having a jaw-opening 10 in. by 20 in., and weighing not over 15,000 lb., will take a steady stream of 10-in. run-of-mine and reduce it without sliming to 1-in. size. To make such a reduction with rolls having equal capacity would require a roll at least 8 ft. in diameter, weighing about 80,000 lb. (see Fig. 1). The cost of the crusher would be about $1,500, and that of the roll, apart from the extra expense for foundations and transportation, about $8,000.
The practice of dropping the feed from a considerable height to the roll, in order to drive it through by force of its falling-velocity, would permit the use of a somewhat smaller roll. But the height required is in most cases not available, hence additional expense for elevating would be involved. Moreover, the violence of this method of feeding and crushing would, in ore-milling, produce an excessive amount of slimes.
Again, it is desirable in such operations to remove the fine enough material as fast as it is produced. The product of the giant rolls, containing a large amount of coarse material of various sizes, would require a very extensive screen-system to remove the fines, the ore passing through a number of graded coarse screens, until an oversize was obtained that would not destroy the screen removing the finished product. It is better practice to crush uniform at each step of the reduction, and screen near the end of the operation, thus making the screen-installation as simple as possible.
In ore-milling, the use of rolls is desirable when it is necessary to keep the slimes down to the smallest possible amount. The ability to do this is their most valuable feature, but this point has often been overlooked, and rolls have been used for purposes unsuited to their construction.
It has been found in general ore-milling practice that a reduction of 4 to 1 should be the limit for rolls, under the most favorable conditions. A reduction of 3 to 1 is the average, and often it should not exceed 2 to 1; so that in actual practice it would require a 30-in. roll working in connection with the 8-ft. roll to produce the 1-in. crusher-delivery size; the first roll reducing from 10 to 3, and the latter from 3 to 1.
A good roll, like a good horse, will stand (and generally receives) an awful lot of abuse; but experience shows that it does not pay to abuse a roll, any more than a horse. I once had a customer who insisted on reducing quartz from 2-in. to fine powder, with one set of 30-in. rolls. I could see that this man was anxious to be a pioneer. Somehow or other he did not break the roll, but he did break his company.
Mr. Payne says that rolls for fine crushing, namely, to pass 30- to 40-mesh, etc., have not given satisfaction, and since little assurance can be given that they will do so and retain the granular feature of the crushed product, they have been assailed by various types of tube-mills, etc. My experience with rolls has taught me that the product of a roll is always granular, no matter how fine the crushing is. The attack on rolls for fine-crushing by tube-mills, etc., is to my mind justifiable, since, below 20-mesh, the capacity of the roll becomes too small for general mill-practice, in proportion to its installation- and operating-cost; and the capacity diminishes very rapidly with the fineness of the crushing.
Mr. Payne says that the loss of capacity is due to the escape of the ore-particles through the rolls, owing to irregular or grooved tires. This is true to a certain extent; but the principal cause for the loss of capacity is the fact that the stream of mineral passing through the roll has, by reason of the fineness of its individual particles, become very thin, hence its tonnage drops off. Choke-feeding does not remedy this to any extent, and is difficult to regulate. The only remedy is a greatly increased surface for action, i e., more mineral surface to act on, and more crushing-surface to perform the work. Here the roll must give up the job to a machine fulfilling the above requirements.
Such a machine is found in the modern pebble-mill of the Hardinge type. The granular nature of the product from this machine is not lost to any damaging extent, and for fine-grinding operations the production of slimes can be kept within reasonable limits, considering the fineness of the work, by proper regulation of the feed and the adjustments of the mill. The mill is no more fool-proof than a roll iswhich is no drawback in the ore-milling business. I have yet to see a milling-proposition that would pay a profit if operated by fools. I do not know where the term fool-proof machinery originated, but I think it must have been invented to fit some local condition in Joplin, Mo.
In regard to the grooving of tires, I have often found it due to uneven texture of the tire-metal. Under such a condition, grooving will take place, no matter how even the feed. A practical mill-man can easily arrange any one of several feeding-devices now on the market to deliver an even feed. In case of unavoidable tire-grooving, various methods of truing up the tires are successfully practiced. John Sargenson, of the Hipissing Reduction Works, Cobalt, has developed a simple apparatus for doing this while the roll is at work.
The Frazee method of dry-crushing, as outlined by Mr. Payne, could not be adapted to general mill-practice, since it would be impracticable or too expensive to dry the ore for crushing, and then wet it again for concentration, cyanidation, etc.
justment, flanged tires, etc. They have all been used from time to time, and are now chiefly epidemic in the Joplin region, and are responsible, together with other local conditions, in making that district notorious for poor milling-practice.
Fig. 2 shows detail-drawing of correct roll-construction. The movable bearings are connected by a solid yoke, RC-2 (see lower left-hand corner of drawing). This yoke holds the movable bearings in perfect alignment and parallel to the fixed bearings.
In the Hardinge pebble-mill, the multitudinous crushing-surfaces formed by the contact of the pebbles with the walls of the mill, and between the pebbles themselves, is a mechanical feature with which a roll cannot compete. It will be noted from the results given in the accompanying test-card that with a comparatively coarse feed, 4-mesh, only 35 h-p. is required to give a capacity of 65 tons per 24 hr., reducing to sizes fine enough to meet the requirements of the average tailings-regrind.
In cases where it would be necessary to crush the original ore to 40-mesh or finer, before beginning concentration, it would also be better practice to carry the roll-crushing to about 6-mesh only, classify out the fine enough, and finish the oversize with the pebble-mill. Slime-tables have now reached such a degree of perfection, both as to low cost of operation and high recovery of the values, that it would be much cheaper to install extra tables to take care of the slimes produced by the pebble-mill, crushing 6-mesh feed, than it would be to try to avoid making slimes, and at the same time crush very fine with non-sliming machines of a small capacity, making fine reductions.
In giving Stephen R. Krom the credit of introducing the belted high-speed crushing-roll into the United States, I may say that the difference in speed of the early geared Cornish rolls, which were sometimes driven as slowly as 4 or 5 rev. per min., and that of the belted rolls, which, when run at 100 rev. per min. and over, may be considered high speed, as compared with the former, seemed to me sufficiently distinctive to be noted. The same distinction is made by Philip Argall in his interesting paper, Sampling and Dry Crushing in Colorado. The narrow-tired roll, which was driven at an extremely high speed, was a later development, and proved to have other disadvantages.
I regret to notice that Mr. Krom makes a misleading use of quotation-marks in his comments by inclosing between them statements which do not quote accurately from my paper. I think, however, that this must be due to oversight or haste on his part, for I notice also that he entirely disregards Mr. Edisons remarkable use of rolls in pulverizing cement-rock, when he says that his experience has taught him that the product of a roll is always granular, no matter how fine the crushing is.
He has again overlooked my statement, when he says that the Frazee rolls cannot be applied to general mill-practice owing to the necessity of drying the ore. No such necessity exists. I must also notice that while Mr. Krom classes the Frazee rolls with Joplin practice among things to be condemned, he finds the correct roll-construction in the Krom roll. As nothing is stated about feeding the Krom roll, this conclusion seems to be too general to be of value in this connection, or quite convincing, even if it might not have been inferred.
In regard to Joplin practice, the use of flanges on one of the roll-shells in some of the mills is certainly an interesting development. No feed-mechanism is, however, employed. The ore is carried to the rolls by a stream of water, and the object of the flanges is to direct the ore downward, and to prevent the water from splashing it out sideways while it is being crushed. These rolls are not used for fine crushing in the sense that Mr. Frazees rolls are, and the flanges do not prevent grooving and irregular wear of the shells. The latter are made of cast-iron, and are given a very hard chill. When the chill is worn through they are discarded. As Mr. Stone points out, the distinctively novel feature of the Frazee roll-design is the method of feeding the rolls. Here the feed-stream is extended slightly beyond the ends of the shorter roll, so that by a certain amount of crushing between the ends of the roll and the flanges, the ridging of the ends of the shorter roll, as well as grooving, can be entirely overcome. The Frazee roll-shells can thus be brought quite close together, and in this way used for much finer crushing than the Joplin practice would permit. They have, therefore, little or nothing in common with the latter.
I am aware that it is not unusual for manufacturers of machinery to take a fling at Joplin practice. From a mining engineers point of view, however, there is much to commend it. The Missouri zinc-deposits, for the most part, are irregular and uncertain in extent. Much of the ground has heretofore been leased to operators in comparatively small sections. The expense of simple mill-equipments can be more quickly recovered from operating-profits than more elaborate and expensive machinery. It is always good practice to make money, and rolls which would not be advisable for porphyry copper-ores, for example, under the peculiar conditions surrounding zinc-mining at Joplin have held their own, because they have justified themselves financially.
In my paper I have endeavored to press the point, that in treating those ores which must be kept in a granular condition while crushing them finely for concentration, the Frazee rolls hold out the promise of great usefulness by avoiding losses due to sliming. In Mr. Kroms comment I notice that he gives the screen-analysis of a material which has passed through a Hardinge tube-mill, but which does not have to be kept in a granular condition for concentration. The reference, therefore, does not seem pertinent to anything contained in my paper.
I regret that Mr. Krom should rely for his opinion of Mr. Edisons giant rolls upon a correspondent who seems to have been unfortunate in not having secured the benefit of their enormous capacity, and who does not state the nature of the rock crushed. So indirect an attack may well be disregarded. Mr. Edisons mechanical genius does not need any tribute at my hands. His is the faith which literally removes mountains, and no one can watch the giant rolls in operation at Stewartsville, N. J., without increased respect for the human spirit which can measure itself with the enormous forces which the rolls unlock.
Whether the particular design developed thus far is found to be the best for the purpose or not, there is no doubt that all mining engineers will watch with keen interest the reduction of costs in mining and quarrying by means of coarse crushing, in which Mr. Edison has led the advance. He has certainly made a most notable contribution to the art of roll-crushing.
1) Match the equipment in Column A with the corresponding process in Column BColumn A(P) Centrifugal sifter (Q) Bowl mill (R) Gravity thickener (S) Two-arm kneaderColumn B(I) Mixing (II) Sedimentation (III) Screening (IV) Grinding(A) P-I, Q-IV, R-II, S-III(B) P-III, Q-IV, R-II, S-I(C) P-IV, Q-I, R-II, S-III (D) P-IV, Q-III, R-I, S-II
2) Critical speed of a ball mill depends on(A) the radius of the mill (shell) and the radius of the particles (B) the radius of the mill (shell) and the density of the particles (C) the radius of the balls and the radius of the particles(D) the radius of the balls and the radius of the mill (shell)
3) In a roll crusher, rolls of diameter 1 m each are set in such a manner that minimum clearance between the crushing surfaces is 15 mm. If the angle of nip is 31 degree, the maximum diameter of the particle (in mm) which can be crushed is _____ .
1) In a cyclone separator used for separation of solid particles from a dust laden gas, the separation factor is defined as the ratio of the centrifugal force to the gravitational force acting on the particle. Sr denotes the separation factor at a location (near the wall) that is at a radial distance r from the centre of the cyclone. Which one of the following statements is INCORRECT?(A) Sr depends on mass of the particle(B) Sr depends on the acceleration due to gravity(C) Sr depends on tangential velocity of the particle(D) Sr depends on the radial location (r) of the particle
1) In the Tyler standard screen scale series, when the mesh number increases from 3 mesh to 10 mesh, then(A) the clear opening decreases(B) the clear opening increases(C) the clear opening is unchanged(D) the wire diameter increase
2) 100 ton/h of a rock feed, of which 80% passed through a mesh size of 2.54 mm, were reduced in size such that 80% of the crushed product passed through a mesh size of 1.27 mm. The power consumption was 100 kW. If 100 ton/h of the same material is similarly crushed from a mesh size of 5.08 mm to a mesh size of 2.54 mm, the power consumption (in kW, to the nearest integer) using Bonds law, is _____.
1) A cylindrical vessel with hemispherical ends is filled with water as shown in the figure. The head space is pressurized to a gauge pressure of 40 . The vertical Force F (in kN) tending to lift the top dome and the absolute pressure P(in ) at the bottom of the vessel are
2) The power required for size reduction in crushing is(A) Proportional to (1/Surface energy of the material)(B) Proportional to (1/ square root of Surface energy of the material)(C) Proportional to Surface energy of the material(D) Independent of the Surface energy of the material
1) Match the systems in Group I with equipment used to separate them in Group IIGroup IA. gas solid B. liquid liquidGroup II 1. filter press 2. cyclone 3. decanter 4. thickenera. A-1, 2-Bb. 2-A, 3-B c. 3-A , 4-B d. 4-A, 1-B
2) A sand mixture was screened through a standard 10-mesh screen. The mass fraction of the oversize material in feed, overflow and underflow were found to be 0.38, 0.79 and 0.22, respectively. The screen effectiveness based on the oversize isa. 0.50 b. 0.58 c. 0.68 d. 0.62
3) Arrange the following size reduction equipment in the decreasing order of the average particle size produced by each of them.a. Jaw crushers, Ball mills, Fluid energy millsb. Ball mills, Jaw crushers. Fluid energy millsc. Fluid energy mills, Jaw crushers, Ball millsd. Fluid energy mills, Ball mills, Jaw crushers
1) The energy required per unit mass to grind limestone particles of very large size to 100 m is 12.7 kWh/ton. An estimate (using Bonds Law) of the energy to grind the particles from a very large size to 50 m isa. 6.35 kWh/tonb. 9.0 kWh/tonc. 18 kWh/tond. 25.4 kWh/ton
1) Match the items in the left column with the appropriate items in the right columnLIST I (I) Saltation velocity (II) Compressible cakeLIST II -(A) Filteration (B) Fluidization (C) Pneumatic conveying (D) Screw conveyor
1) A particle A of diameter 10 microns settles in an oil of specific gravity 0.9 and viscosity 10 poise under Stokes Law. A particle B with diameter 20 microns settling in the same oil will have a settling velocity(A) same as that of A(B) one-fourth as that of A(C) twice as that of A(D) four-times as that of A
2) Match the following :(I) Cut diameter (II) Specific cake resistance (III) Size Reduction Ratio (IV) Angle of Internal Friction(A) Filtration (B) Cyclone separators (C) Storage of solids (D) Kicks law
maximum nip angle on gyratory crusher model can accept. (B) Nip angle: Sinter Crusher; Jaw Gyratory Crusher; Gyrasphere Cone Crushers ...Dear All I would like to know what is a nip angle & why it ... Wide nip angles can tend to expel material as the jaw closes as a ... Gyratory Crushers; Impact ...
Ore beneficiation equipment, sand making equipment, crushing equipment and powder grinding equipment, which are widely used in various industries such as metallurgy, mine, chemistry, building material, coal, refractory and ceramics.Get in Touch with Mechanic