I II 
 
 001 
 
 RLF 
 
INDEX 
 
 PAGE. 
 
 Introduction to Krom's Improvements in Ore Crushing and 
 
 Concentrating Machines, etc., etc., etc., - - 3 
 
 Notice to In f ringers, - 4 
 
 Krom's Original Sectional Ore Breaker, - - 5 
 
 " Improved " 7 
 
 Rolling Toggle (enlarged view), - 8 
 
 Krom's Original Steel Crushing Rollers, - 9 
 
 " Second Improved Steel Crushing Rollers, - 11 
 
 Endorsement of Prof. Alex. Trippel, - 12 
 
 Krom's Third Improved Steel Crushing Rollers, - 14 
 
 Housing for Krom's Rollers, 16 
 
 Endorsements of Krom's Belt Rollers, - - 17 
 Krom's Fourth Improved (Swinging Pillow-Blocks) Steel 
 
 Crushing Rollers, - 19 
 
 Rollers and Stamps compared, - 22 
 Stetefeldt's comparison between Krom's Rollers and the Stamp 
 
 Battery, 27 
 Additional Endorsements by R. D. Clark, Esq., 32 
 Closing remarks of the Author, - 33 
 Theoretical Treatise on Krom's System of Pneumatic Concen- 
 tration of Ores, - 35 
 Endorsements of the System, 40 
 Description of the Pneumatic Jig, - - 46 
 Laboratory Crusher and Rollers, 54 
 Krom's Dry Kiln, 56 
 " Revolving Screen, 60 
 
COLLEGE of MINING 
 
 DEPARTMENTAL 
 LIBRARY 
 
 BEQUEST OF 
 
 SAMUELBENEDICTCHRISTY 
 
 PROFESSOR OF 
 
 MINING AND METALLURGY 
 1885-1914 
 
KROM'S ORE CRUSHING AND CONCENTRATING MACHINES, 
 
 S. R. KROM, ENGINEER, 
 
 n 
 
 AND MANUFACTURER OF 
 
 ORE BREAKERS (STEEL SECTIONAL), 
 STEEL CRUSHING ROLLS, 
 
 PNEUMATIC CONCENTRATORS, 
 REVOLVING SCREENS, 
 GRAVITY DRY KILNS AND ORE FEEDERS. 
 
 LABORATORY CRUSHERS, 
 LABORATORY ROLLS, 
 
 LABORATORY CONCENTRATORS, 
 LABORATORY SCREENS. 
 
 Plans for Lixiriation and Concentration Works, 
 
 Received the Highest Award from the U. S. Centennial 
 Commission for the Pneumatic Concentrator. 
 
 YOPtKl. 
 
 1885. 
 
K-i.1 
 
 ~YY\ 
 
 ' 
 
KROM'S IMPROVEMENTS 
 
 IN 
 
 ORE CRUSHING AND CONCENTRATING MACHINES, 
 
 Etc., Etc., Etc. 
 
 In connection with perfecting a system of pneumatic concen- 
 centration, I had in view the improvement of machines for crushing 
 and pulverizing ores. A study of the whole subject convinced 
 me, that the principle upon which the construction of the Blake 
 Crusher and that of the rollers are based, possessed the mechanical 
 elements, which, if perfected and simplified, would make these 
 machines the successful rivals of every other device. It is well 
 known, that until very recently, the stamp battery held undisputed 
 possession of the field, and mill men as well as mill builders, almost 
 universally, believed that stamps would not be superseded by any 
 other machine for fine crushing. 
 
 The first step to drive stamps from the field, was the introduction 
 of rollers in the Bertrand and the Mt. Cory Mills. In both of 
 these mills, the great superiority and economy of a system of 
 crushing with rollers instead of stamps has been fully demonstrated. 
 
 While perfecting the rollers I had the fact confronting me, that 
 they had frequently been tried without success. In fact, the 
 universal opinion prevailed that rollers could be used only for coarse, 
 and not for fine crushing. The secret of the difficulty, I discovered 
 to be in the imperfect and weak character of the rollers heretofore 
 used, due to a misconception of the requirements for a machine for 
 such work, and a failure to appreciate the advantages of continuous 
 rolling crushing surfaces, operating on the toggle lever principle, 
 which gives the greatest crushing pressure with the least power 
 expended, and the minimum of wear. 
 
But no sooner is the fact established that rollers are destined to 
 supersede stamps, than some persons jump to the conclusion that 
 cheap rollers will also prove successful, and manufacturers of cheap 
 machines are making statements that they sell rollers at one-half 
 the price of my improved steel rollers, which will do the same 
 work. Those who believe such statements and buy such cheap 
 rollers, will pay dear for their experience. 
 
 It is the great strength of my rollers, superior material used, perfect 
 workmanship, completeness and simplicity of design, which makes 
 them a success and so economical ; and no one understanding the 
 subject will consider the price too high for the quality of the 
 machines. As an aid to legitimate mining enterprise, it has been 
 my aim to perfect a system of machinery for milling and con- 
 centrating ores, and to make such machinery a standard of 
 excellence. The advantage and economy of employing the best 
 which can be produced, is becoming slowly but better understood. 
 
 S. R. KBOM. 
 
 NOTICE. 
 
 Some manufacturers are making ore breakers and crushing 
 rollers which infringe on my patents. Purchasers of such machines 
 are notified that they will be held equally liable for damages as 
 the manufacturers. 
 
 All the machines, either ore breakers or crushing rollers which 
 have tie bolts to take the strain due to crushing, are infringements 
 
 on my patents. 
 
 S. R. KROM. 
 
ORE BREAKERS. 
 
 Every one who has any knowledge of ore crushing 
 machinery is so familiar with the Blake Crusher and its 
 construction, that no cuts or description of it is neces- 
 sary here. But during the life of the patent (21 years) 
 of this celebrated and valuable machine, the frame was 
 cast in one piece, and no improvement of any im- 
 portance was made in the machine until after the patent 
 had expired. 
 
 Fig. 1. 
 
 KROM'S ORIGINAL PATENTED SECTIONAL ORE BREAKER. 
 
 Re-issued April llth, 1882. 
 
The first practical plan to improve the construction 
 of the Blake Crusher is shown in cut, Fig. 1, and was 
 patented in 1875. The principal features covered by 
 the patent granted for this machine (Fig. 1) are the tie 
 bolts to take the strain due to crushing, and breaking- 
 cups to relieve the machine from excessive strain. The 
 breaking-cups are placed on the tie-bolts and also on the 
 pitman, but it is intended that the small cups on the 
 pitman should be the weakest and only these give 
 aw r ay. A lug underneath the pitman holds it from 
 dropping too low. This method of constructing a 
 crusher with bolts not only makes a stronger machine, 
 but one of less weight, and easier to transport, and allows 
 the employment of means to prevent the machine from 
 being subjected to undue strain. 
 
 Fig. 2. 
 
 KROM'S IMPROVED PATENTED STEEL SECTIONAL ORE BREAKER. 
 
 Covered by Re-issued Patent, April llth, 1882, and Patent allowed for improvements. 
 
The next steps in improvements are shown in the cut, 
 Fig. 2. In this machine the upper tie-bolt is shortened 
 so as to give a better and more convenient form to the 
 side frame, and the lower tie-bolt is so placed that it 
 receives all the strain due to crushing the ore. In the 
 toggle abutment, through which the main tie-bolt 
 passes, are recesses around the bolt holes, and these 
 recesses are covered with wrought iron washers of 
 sufficient strength so they will not bend under the 
 ordinary strain in crushing the ore, but yield to ex- 
 
 Fig, 3. 
 
 KROM'S IMPROVED PATENTED STEEL SECTIONAL ORE BREAKER. 
 
 Covered by Ke-issuecl Patent April 11, 1882, and Patent allowed for improvements. 
 
8 
 
 cessive strain. These washers take the place of the 
 breaking cups in the first machine, but do not fly to 
 pieces, and therefore both ends of the toggle block yield 
 evenly together, and the frame of the machine is not 
 liable to be twisted or broken, or the bolts bent. 
 
 The third improvement is in forming the crushing 
 faces of bars of steel, and in the means of clamping 
 them securely in place. These bars are of good 
 steel, rolled to standard sizes, and cut to proper lengths. 
 The lower bars are hardened to increase their durability. 
 Provision is made for bringing the jaws closer together 
 to compensate for wear. Thin strips of metal are also 
 provided to put behind the bars to keep the wearing 
 faces in line. The bars or crushing faces can easily be 
 got at by taking the nuts from the bolts, and sliding 
 the stationary jaw forward. The fourth improvement 
 consists in hanging the jaws on an axis below the crushing 
 faces instead of at the top as in the Blake Crusher. 
 This manner of hanging the movable jaw gives a more 
 uniform product, and the principle is correct, as the 
 strain on the jaw is greater at the bottom than at the 
 top, and consequently the motion should be the least 
 where the strain is the greatest. 
 
 The fifth improvement consists inj the^employment of 
 toggles with rolling ends which work without friction or 
 oil. In this cut the teeth which hold ^the \ 
 
 toggles in place are omitted on one end so 
 as to better show the rolling surfaces. 
 
 ROLLING TOGGLES. 
 
The bearings are self-adjusting, large and long, and 
 the machine is constructed for high speed, hard work and 
 large crushing capacity. 
 
10 
 
 CRUSHING ROLLERS. 
 
 Fig. 4, represents the first step in the improvement 
 of crashing rollers constructed with bolts to take the 
 strain. It is also the first crushing machine with forged 
 steel tires. The system of gearing is also new, and an 
 improvement on anything before or since it was intro- 
 duced. It will be observed also that the pillow blocks 
 are well adapted to take the strain. It is the first 
 machine of the kind carried on a bed plate in one piece. 
 Breaking cups are also introduced to relieve the machine 
 from undue strain. This is the first time that such a 
 device was introduced in crushing machines. The 
 patent granted for these rollers covers the bolts to take 
 the strain, the system of gearing, the manner of putting 
 on the steel tires and the breaking cups. Some of the 
 details in this cut are omitted as not being essential 
 here, but will appear in the next machine. 
 
 Fig. 5. 
 
 .SIDE-VIEW 
 
 KROM'S SECOND IMPROVED STEEL CRUSHING ROLLERS. 
 
 Covered by patent July 16th, 1872, and patents allowed for improvements. 
 
11 
 
 The next step in the improvement of crushing rollers 
 is shown in cuts, Figs. 5 and 6. It consists of a recon- 
 struction of the pillow blocks so as to adapt them to 
 one bolt on each side instead of two, as in the previous 
 cut, Fig. 4. This at once simplifies the machine, and in 
 other respects makes a better pillow block. The bolts 
 being placed close to the bearings serve the purpose 
 
 Fig, 6. 
 
 -END-VIEW- 
 
 KROM'S SECOND IMPROVED STEEL CRUSHING ROLLERS. 
 
 Covered by patent July 16th, 15T2, and patent allowed for improvements. 
 
 better than four bolts, two bolts being much more con- 
 venient for adjusting the rollers. 
 
 This arrangement also allows the shafts to be lifted 
 out of their bearings for repairs without removing any 
 of the tie-bolts, and permits the use of springs on the 
 tie-bolts, as shown at S, Fig. 5. It is not practicable to 
 
12 
 
 use springs with four bolts. These springs were intro- 
 duced to supersede the breaking cups in Fig. 4. The 
 tie-bolts have solid collars near the middle, and a nut on 
 each end. The springs and pillow blocks are placed 
 between the collar and nut, so that the pressure is obtained 
 without forcing the rollers together when the springs 
 are set up to give the necessary resistance to crush the 
 ore. At the date of this machine it was considered 
 necessary to employ some means to relieve the machine 
 from undue strain; and the springs as here shown are 
 the best arrangement for the purpose yet devised. The 
 arrangement of the gearing is the same as in Fig. 4, 
 except that the intermediate gear D 1 is increased in size, 
 and so placed as to require no change of position when 
 the pillow blocks are moved up to adjust the distance 
 of the rollers, and to compensate for wear. The wheel 
 D 1 is placed nearly on a line drawn perpendicular 
 through wheel B 2 , but two inches in front of it; and 
 when the tires are worn out the wheel D 1 will be two 
 inches in the rear of the perpendicular line through 
 wheel B 2 . This movement of four inches, when so 
 divided, only slightly affects the meshing of the teeth of 
 the wheels D 1 and B 2 , the variation being only -^ of an 
 inch from a true pitch. It will be seen that one 
 roller is driven through the intermediate wheel D 1 , and 
 the other roller by a pinion, C 1 , directly engaging with 
 wheel B 1 . By this arrangement and location of wheel 
 D 1 the rollers can be adjusted without change of 
 position of any of the gear. In the machine shown in 
 Fig. 4, it was necessary to adjust gear-wheel D 1 , as the 
 gear-wheel B 2 was moved forward toward gear-wheel B 1 . 
 It was of these rollers that Prof. Alex. Trippel spoke 
 when he said u The steel rollers of Mr. Krom's are 
 exceedingly well constructed. They are undoubtedly 
 the finest ever made, well proportioned, and very 
 
13 
 
 powerful. * '" An important fact which was 
 
 developed on this trial, was the apparent practicability 
 and advantage in substituting a system of crushing by 
 steel rollers for stamp batteries." 
 
 (The above is from U. S. Commissioner Raymond's 
 report of 1876.) 
 
 It was the success of these geared steel rollers at the 
 Galena Mills, Nevada, that led to the adoption of the 
 improved form at the Bertrand Mill at Geddes, Nevada, 
 as described below. 
 
14 
 
 Fig. 7. 
 
 SIDE VIEW 
 
 KROM'S THIRD IMPROVED STEEL CRUSHING ROLLERS. 
 
 Covered by patent July 16th, 1872, and patent allowed for improvements. 
 
 Figs. 7 and 8 illustrate very important additional 
 improvements in crushing rollers. The 1st is the sub- 
 stitution of band wheels for toothed gearing. 2d. The 
 substantial casing for inclosing the rollers, and a hopper 
 to insure the spreading of the ore evenly across the face 
 of the rolls. These improvements complete the adap- 
 
15 
 
 tation of rollers for pulverizing ores to any degree of 
 fineness required. With pulleys we can run at any 
 speed desired, and thereby increase the capacity of the 
 rollers. The wear is reduced nearly to the crushing 
 faces alone. The danger of breaking the machine is 
 avoided, and the rollers perform their work with but 
 little noise and shock. 
 
 Fig. 8. 
 
 END VIEW 
 
 KROM'S THIRD IMPROVED STEEL CRUSHING ROLLERS. 
 
 Covered by patent July 16th, 1872, and patent allowed for improvements. 
 
HOUSING FOR KROM'S PATENTED STEEL CRUSHING ROLLERS. 
 
 The housing (Fig. 9) so incloses the rollers that the 
 dust made in crushing is easily prevented from escaping 
 into the building by means of a small draught of air 
 produced by an exhaust fan. The housing also forms a 
 very substantial frame for supporting the cheek pieces 
 and the feeding hopper. I prefer to drive with one 
 large pulley, and use the small one to insure the 
 bite of the rollers upon the ore. Both rollers of course 
 when crushing travel at the same surface speed, but I 
 prefer to speed them so that the roller driven by the 
 small pulley, when the machine is not crushing, will 
 revolve one or two revolutions faster per minute. The 
 
17 
 
 reasons for preferring this system of driving with only 
 one large pulley, are : 
 
 1st. It would not be a good arrangement to hang 
 on the movable pillow block, so large and heavy a 
 pulley as would be required. 
 
 2d. The driving of one roller will cause the other 
 to revolve at the same surface speed, where hard ore is 
 fed between them, and therefore it is only necessary 
 to 'drive one roller with a strong and steady power, 
 and the other with sufficient force to insure the rollers 
 always taking hold of the ore, and to keep the same in 
 motion during the time when no ore is between them. 
 
 In reference to these rollers Mr. Stetefeldt said, be- 
 fore their success had been practically demonstrated : 
 
 " I am firmly convinced that Krom's Improved Steel 
 Rollers, if they are given a fair trial, will make stamps 
 a thing of the past. * * * * * 
 
 It is Mr. Krom's merit to have constructed rollers 
 which reduce ores to any degree of fineness. The 
 splendid results at the Geddes Mill, Nevada, have 
 established the fact beyond any question. It is not too 
 much to say that rollers produce the same results as 
 the stamp batteries, with one-half the power and 
 one-half the expense in wear, leaving their much lower 
 original cost out of the question." 
 
 Mr. Albert Arents, Superintendent of the Mt. Gary 
 Mill, says : "Now as to the economical question, the 
 only one of practical interest, I will state that the cost 
 of treating a ton of ore at the Bertrand Mill is thirty 
 per cent. (30$) cheaper than at the Northern Bell Mill 
 (a stamp-mill), although the latter is crowded to its 
 utmost capacity, and treats more ore than any other mill 
 of its size I know of. * * * * * I once advised 
 having a side motion to one of the rollers, but I find 
 if they are fed even the rollers wear even. I am 
 
18 
 
 astonished at the even degree of wear of the steel tires. 
 I would not use stamps now for our purpose under any 
 considerations." 
 
 Mr. R. D. Clark, Superintendent of the Bertrand 
 Mining Co., says : " One great reason of our cheap 
 milling at the Bertrand is that we use rollers instead of 
 stamps. We crushed about 15,000 tons of ore before 
 putting new tires on the finishing rollers, and have 
 crushed in all about 20,000 tons, and we do not expect 
 to put new tires on the roughing rollers for two or 
 three months yet. From my experience the past four- 
 teen (14) months, it is safe to calculate that each set of 
 tires and each set of phosphor bronze linings will 
 last to crush 20,000 tons of ore. There has been no 
 other expense for repairs upon our rollers except the 
 cheek pieces. The faces of the worn-out tires are not 
 grooved at all, and they would be still good for wear 
 had they not expanded by becoming worn thin. They 
 have certainly stood the test of time, and are to-day 
 more in favor with us than ever before. We would 
 not have stamps if furnished without cost, and kept in 
 repair for nothing also." 
 
 Additional testimony from an experienced mill man : 
 " I have been an advocate of stamps, but after seeing 
 the quantity of ore your rollers have crushed at the 
 Bertrand Mill (30,000 tons), I am convinced of their 
 superiority over stamps, and have decided to use them 
 in my new mill. 
 
 SIMEON WENBAN, 
 
 COETEZ, NEVADA." 
 
 Much more could be added in the way of favorable 
 comments in relation to steel rollers of the kind shown 
 in the cut and just described in Figs. 7, 8 and 9, but 
 further quotations would be unnecessary here, since the 
 
19 
 
 testimonials above are quite full, and come from persons 
 widely and favorably known as the most skillful in the 
 profession of mill-men and metallurgists. 
 
 Fig. 10. 
 
 SIDE VIEW OF KROM'S FOURTH IMPROVED STEEL CRUSHING ROLLERS. 
 
 Patented July 16th, 1872, and patent for improvements pending. 
 
20 
 
 The next and latest step in the improvement of 
 crushing-rolls is the swinging pillow-blocks, shown in 
 Figs. 10 and 11. This, by reason of its uniqueness and 
 importance, deserves to rank first in the order of merit, 
 In all other machines one pair of the pillow-blocks are 
 arranged to slide on the bed-plate, and each one of the 
 sliding pillow-blocks has to be adjusted separately. It 
 requires care to bring up the two movable pillow-blocks 
 evenly and parallel with the stationary ones ; and any 
 looseness between the faces of the movable pillow-blocks 
 and the bed-plate results in damage to the faces and 
 pounding on the bed. In this machine all the bearings 
 are securely and firmly fixed to the bed-plate, and no 
 chance whatever exists for looseness, causing wear or 
 damage to any part of the machine. (In fact, the 
 movable bearings are now better secured to the bed- 
 plate than the stationary ones, although the latter are 
 very securely bolted on the bed.) The two swinging 
 bearings are united together by a very heavy shaft 
 (???, Fig. 11) 11 inches in diameter, so that the bearings 
 must swing together, and consequently the rollers and 
 shafts are always parallel. The shaft m, connecting the 
 swinging bearings, is so strong that one bolt on one side 
 is sufficient to hold the rolls together when crushing ore. 
 It will be observed that springs have been dispensed 
 with. The machine is constructed so strong that they 
 are no longer required ; and besides I now employ 
 magnets to take out from the ore all pieces of steel 
 which might dent the faces of the rollers. 
 
 The rolls work better and more economically without 
 springs. Contrary to what might at first be expected, 
 the machine is more simple than any other and more 
 durable. The tires are held by two heads slightly cone- 
 shaped. One of these heads is securely fixed to the 
 shaft by shrinking it on. The other is split on one side, 
 
END VIEW OF KROM'S FOURTH IMPROVED STEEL CRUSHING ROLLERS. 
 
 Patented July 16th, 1872, and patent for improvements pending. 
 
 so that when the heads are drawn together by the bolts, 
 within the tires, the split head will close tightly upon 
 the shaft. 
 
22 
 
 Recapitulating the several improvements described 
 we have : 
 
 1. Steel tires and the method of securing them. 
 
 2. Pulley-gearing. 
 
 3. Housing to inclose the rollers. 
 
 4. Swinging pillow-blocks, 
 
 5. Bolts to take the crushing strain. 
 
 6. The hopper for spreading the ore. 
 
 This paper would not be complete without a com- 
 parison between rolls and stamps (other pulverizing 
 machines not being worthy of attention here). On this 
 head I submit the following observations : 
 
 1. The capacity of any machine for crushing ore is 
 dependent on the amount of crushing surface brought 
 in contact with the ore in a given time, and the effi- 
 ciency with which the crushing surface is applied. 
 
 2. The cost of crushing depends on the power con- 
 sumed and the expense of maintaining the machine in a 
 working condition. 
 
 It will be manifest from these axioms that the 
 employment of correct mechanical principles with sim- 
 plicity of construction and fewness of working parts 
 will play an important part in the settlement of the 
 question. For this calculation we will begin by com- 
 puting the capacity of two sets of 26-inch rolls with 
 15-inch face, the average diameter of which by reason of 
 wear will be say 24 inches. One pair of the rolls is to 
 receive the coarsely crushed ore from the breakers, and 
 the other set is for finishing or pulverizing. Such rolls, 
 running 100 revolutions per minute, will bring into 
 contact with the ore 113,100 square inches, and the two 
 sets of rollers are therefore equal to 226,200 square 
 inches of effective crushing surface per minute. . 
 
 The usual diameter of each stamp-shoe and die is 8 
 inches, and the area is therefore 50 square inches to 
 
23 
 
 each stamp. Counting all this as effective crushing surface, 
 we have for each stamp (falling say 90 drops per minute) 
 4,500 square inches of crushing surface. It will there- 
 fore require 50 stamps to give nearly the same surface 
 (namely 225,000 square inches) as can be obtained with 
 two sets of 26-inch rollers. 
 
 Let us now consider the two machines from a purely 
 mechanical point of view. Stamps crush by the percus- 
 sive effects of a falling weight, and the power consumed 
 is in excess of the work performed, because it takes the 
 same power to operate stamps, whether the blows are 
 effective or not, and it is safe to assume that one-half 
 the power is lost. With rollers the crushing force is 
 applied continuously in one direction, and, other things 
 being equal, it must be evident that the power is far 
 more economically utilized. Fig. 12 illustrates the 
 difference, representing the faces of the rolls as composed 
 of the shoes of a stamp battery, and the stamp-heads, 
 stems and tappets as fastened to or composing the fly- 
 wheel, to give weight and steadiness of motion and 
 crushing force sufficient to overcome irregularites of 
 resistance. It must be evident that if the rolls (Fig 12) 
 are held sufficiently rigid together, the pieces of rock 
 falling between them will be crushed and the power 
 consumed will be in proportion to the work done, while 
 the surplus power will be stored up in the fly-wheel. 
 If the same amount of metal were put in stamps on the 
 other hand, and the stamps did not (as of course they 
 would not) give in falling a full equivalent of useful 
 work for the power consumed in raising them, the ex- 
 cess of power is wasted with no possibility of recovery. 
 Moreover, the blow of a stamp has no regular efficiency, 
 and is always only partially effective by reason of crush- 
 ing on too much ore or on ore already fine enough. 
 
 With rolls the crushing effect is positive, and ore 
 
Fig. 12. IDEAL FIGURE TO ILLUSTRATE RELATIVE ECONOMY OF POWER IN STAMPS AND ROLLERS 
 
 Fig.* 13. THE TOGGLE-LEVER PRINCIPLE COMPARED WITH FALLING WEIGHTS. 
 
25 
 
 cannot pass between them without being crushed if 
 they are properly held together. Besides, no ore which 
 is fine enough is acted upon a second time. Again, the 
 toggle-lever principle acting in rolls, as illustrated in 
 Fig. 13, serves to give the necessary crushing pressure 
 with the smallest expenditure of power, while with 
 stamps the contrary is the case. 
 
 1 have met persons who have claimed that the percus- 
 sive effect of a blow has some quality more effective 
 than pressure. Now the velocity which the stamps 
 attain at the moment of striking the ore, dropping 9 
 inches, is 7 feet per second, while the surface of rolls 
 24 inches in diameter, running 100 revolutions per min- 
 ute, travels 628 feet per minute, or about 10^ feet per 
 second. This gives already more percussive effect than 
 stamps ; so that, if percussion is any aid to pulverizing, 
 we have it in the rolls also by giving them velocity. 
 But I do not adopt this reasoning. The whole matter 
 is summed up in the statement, that stamps must attain 
 a certain velocity to crush, while the crushing effect of 
 rolls is produced at either high or low velocity, without 
 material change in quality of product ; speed, however, 
 giving increased capacity. 
 
 Simplicity of construction has also been claimed for 
 the stamp-battery. Let us see if this claim can be 
 sustained. 
 
 The working or wearing parts of each set of two rolls, 
 as above illustrated, are as follows : 
 
 4 journals on two shafts. 
 
 4 pillow-blocks for journals. 
 
 2 crushing-rollers. 
 
 2 side wearing-plates. 
 
 Total: 12 wearing-parts to each set of rolls or 24 in 
 two machines. None of these parts can break, or are 
 subject to rapid wear. 
 
26 
 
 On the other hand, the working or wearing parts in a 
 50-stamp battery are : 
 
 15 journals on 5 shafts (viz., 10 stamps on each shaft). 
 
 5 cam-shafts. 
 
 15 bearing-boxes for journals. 
 
 50 stems. 
 
 100 guide-boxes for stems. 
 
 50 stamp-heads (bosses). 
 
 50 shoes. 
 
 50 dies. 
 
 50 cams with keys. 
 
 50 tappets with keys. 
 
 Total: 435 parts, all of which are subject to severe 
 wear or liable to break. 
 
 Kecapitulating our comparison : 
 
 1. The average crushing surface of 26-inch rolls is 
 226,200 square inches, while the crushing surface of 50 
 stamps is only 225,000 square inches per minute. 
 
 2. Two sets of rolls have only 24 wearing parts, while 
 a 50-stamp battery has 435 wearing parts (to say noth- 
 ing about the screens which are frequently damaged, 
 and the meshes of which become clogged with ore). 
 
 3. The breakage and wear of stamps are much greater 
 than that of rolls. 
 
 4. The rotary crushing motion of rolls, mechanically 
 considered, is superior and more economical than crush- 
 ing under falling weight, or with any other machine or 
 system. 
 
 5. By the toggle-lever principle in rolls, advantage is 
 taken of the most economical and powerful means of 
 obtaining the necessary crushing-pressure on the ore, 
 which cannot be obtained in any other way, and is not 
 obtained in any other machine. 
 
 Mr. C. A. Stetefeldt, in an appendix to his able paper 
 on RusseWs Improved Process for the Lixiviation of Silver 
 
27 
 
 Ores* has made a comparison between steel rolls and 
 stamps, which I here reproduce. 
 
 "COMPARISON BETWEEN KROM'S ROLLS AND THE STAMP- 
 BATTERY. 
 
 The successful introduction of Krom's rolls in the Bertrand 
 lixiviation mill, Nevada, is of such importance that it deserves 
 notice here. In comparing stamps and rolls we have to consider : 
 1st. The physical difference of the pulp produced. 2d. The ques- 
 tion of economy, namely : a. In regard to original cost of 
 constructing the plant, b. The wear and tear of each machine, 
 and consumption of fuel. 
 
 1. The Physical Difference of the Pulp Produced. 
 
 I refer here to the well known fact, that if pulp produced by 
 rolls or by stamps is sifted through the same size of screen, the 
 ore-particles from the former are more uniform in size, and contain 
 much less impalpable dust than those from the latter. It has been 
 found that for chloridizing-roasting great fineness of the ore is en- 
 tirely unnecessary provided the silver minerals are not too finely 
 impregnated in the gangue. In the lixiviation process great fineness 
 of the ore interferes with rapid filtration. From this it follows 
 that ore pulverized by rolls is in the most favorable condition as 
 far as the mechanical part of lixiviation is concerned. 
 
 2. The Question of Economy. 
 
 A discussion of this subject, which is complete and thorough, 
 and compares the efficiency of rolls and stamps under varying 
 conditions, is not possible at present, because the available statistics 
 concerning Krom's rolls are confined to those from the Bertrand 
 Mill. Sufficient evidence, however, has accumulated to prove the 
 superiority of the rolls beyond any doubt. Their introduction in 
 the Mt. Cory Mill, Nevada, will soon bring additional proof. It 
 seems to me that the application of rolls is most favorable in cases 
 where silver is extracted by lixiviation, and the character of the 
 ore permits comparatively coarse crushing without interfering 
 with good roasting. 
 
 A comparison between rolls and stamps will be made from the 
 following premises: I assume that the crushing capacity of two 
 
 * Transactions of the American Institute of Mining Engineers, vol. xiii., p. 114. 
 
28 
 
 :sets of Krom's 26-inch rolls is equal to that of a 30-stamp battery 
 with stamps of 850 pounds, dropping from 7 inches to 8 inches 
 ninety-four times per minute. Mr. Clark, superintendent of the 
 Bertrand Mill, states that he can crush, with two sets of rolls, 100 
 tons of ore in twenty-four hours to such a fineness that all will pass 
 through a No. 16 wire screen, consuming not over 4 cords of wood 
 for power. The ore has a quartz gangue, and is by no means an 
 easy crushing ore. The fuel required for running 30 stamps is 
 about 6 cords of wood in twenty-four hours. For some remote 
 locality in the West the following prices are assumed, namely: 
 Freight at 3 cents per pound; lumber at $50 per thousand feet; 
 wood at $6 per cord; wages of carpenters at $4.50, and of mill- 
 wrights at $6. Certain items of construction will be about equal, 
 namely: conveyors, elevators, revolving screens, and dust-cham- 
 bers. Revolving screens are also used in connection with a well- 
 appointed battery in order to separate coarse material resulting 
 from the breakage of battery-screens. The building, however, for 
 rolls will be much smaller than that for the battery, and a saving 
 of not less than $1,500 will be effected in its construction. Finally, 
 the rolls requiring less power, a saving of at least $1,250 will be 
 made in providing and setting up engine and boilers in a mill with 
 rolls 
 
 Cost of Erecting a 30-stamp Battery. The plant, including hard 
 wood screen-frames and guides, wooden pulleys on cam-shafts, 
 Tullock's feeders with iron hoppers, and all necessary bolts, weights 
 90,600 pounds, and costs in Chicago *5,850, according to a state- 
 ment of Messrs. Fraser and Chalmers. The framework takes 
 about 36,000 feet of lumber, and the expense of setting up the 
 battery is estimated at $4,000. Hence, total' cost of constructing a 
 30-stamp battery is: 
 
 Plant at foundry, ...... 
 Freight, 
 Lumber, .... . 
 Cost of setting up, . 
 
 . $5,850 00 
 2,718 00 
 1,800 00 
 4,000 00 
 
 To this has to be added in order to compare with rolls : 
 Extra cost of building, 
 Extra cost of engine and boilers, 
 
 $14,368 00 
 
 . 1,500 00 
 . 1,250 00 
 
 Cost of Erecting two sets of Krom's r -6-inch Rolls. The amount 
 of lumber required for setting up the rolls alone is merely nominal. 
 
29 
 
 From this it follows that also the labor of placing the rolls must- 
 be trifling. The weight of one set of 26-inch rolls is 12,000 pounds,, 
 and the cost in New York is $2,250. There is only one self-feeder 
 required, and its weight is estimated at 2,000 pounds, cost $200. 
 From these figures we deduce the following: 
 
 Plant at foundry, $4,700 00 
 
 Freight, 780 00 
 
 Cost of setting up, including lumber, . . . 700 00 
 
 Total, . $6,180 00 
 
 Difference in favor of rolls : 
 
 $10,938 00* 
 
 Wear and Tear of Stamps and Krorti's Rolls. In comparing 
 the wear and tear of stamps and rolls we cannot very well express 
 it per ton of ore crushed, because the capacity of the pulverizing 
 machinery is a function of the hardness of the ore, and of the fine- 
 ness of the pulp produced. A much correcter method will be to 
 take the figures per running time of twenty -four hours. Making 
 estimates from this standpoint it is supposed that the wear and 
 tear in running the machinery at full capacity is a nearly constant 
 quantity, while the capacity is variable as stated above. That this 
 assumption is correct for practical purposes has become evident 
 to me by a comparison of the battery statistics from the Manhattan 
 and Ontario mills. The conditions in these mills are by no means 
 the same. There is a difference in the character of the ore; the 
 Manhattan uses No. 50, and the Ontario No. 30 screens; the Man- 
 hattan stamps weigh 1,000 pounds, those at the Ontario 850 
 pounds; Ontario stamps drop 92 times per minute, those at the 
 Manhattan about 100 times; still, in comparing the wear and tear 
 per actual horse-power expended, the figures are very nearly the 
 same. Hence, my argument does not lack the support of practical 
 experience. The wear of rolls is principally confined to the steel 
 tires, that of the battery to a great number of parts. With rolls 
 the steel tires can be consumed to within less than one-half inch of 
 their thickness, while with stamps, the shoes and dies have to be 
 exchanged after only two-thirds, or less, of their weight has been 
 worn, leaving other parts out of consideration. Another point 
 should not be overlooked. The complicated construction of the 
 battery causes considerable expense in skilled labor for repairs, 
 which, in the case of rolls, is merely nominal. Advocates of the 
 
 *In both estimates elevators, conveyors, and revolving screens are not included, as stated previously. 
 
30 
 
 battery have argued that its great advantage is the continuance of 
 its operation if one battery of five stamps gets out of order, while 
 both sets, or three sets of rolls, as the case may be, have to be 
 stopped if repairs are needed for one set. But it is just the solid 
 construction of Krom's rolls which reduces stoppages from this 
 cause to a minimum. The system of elevators, screens, hoppers 
 and conveyors, if properly constructed, will get out of order very 
 rarely. How often it is necessary to hang up stamps for repairs is 
 too well known to require any statistical proof. 
 
 Wear and Tear of Ifrom's Rolls. As to statistics of wear and 
 tear of Krom's rolls I am confined, at present, to those from the 
 Bertrand Mill. Mr. R. D. Clark states that two sets of steel tires 
 crushed in round figures 20,000 tons of ore. 
 
 As stated previously, the full capacity of the rolls is, in twenty- 
 four hours, 100 tons, the ore being sifted through a No. 16 screen. 
 In the beginning, however, the ore was crushed much finer, name- 
 ly, so as to pass a No. 20 screen, and the daily capacity of the rolls 
 was much less. Taking this into consideration, the actual wear- 
 ing capacity of the tires cannot be estimated at less than 250 work- 
 ing days. The cost of this wear is as follows: 
 
 Two sets of steel tires at New York, . . . $764 00 
 Freight on 3,264 pounds, at 3 cents, ... 98 00 
 
 Total, $862 00 
 
 Wear and tear per twenty-four hours : 
 
 In steel tires, ....... 3 45 
 
 In other parts, screens, supplies and lubricants, . 1 75* 
 
 Wages for repairs, 1 25* 
 
 Total, $6 45 
 
 Wear and Tear of Stamps. I have been favored with correct 
 .statistics from three of the most prominent mills in the West, 
 namely: the Manhattan, Nevada; the Ontario, Utah; the Lexing- 
 ton, Montana. Taking into consideration the somewhat abnormal 
 conditions at the Manhattan Mill, in so far as the weight of 
 stamps there is 1,000 pounds, and the number of drops is greater 
 than in either of the other mills, causing a more frequent breakage 
 of stems and cam-shafts; further, that the statistics from the 
 Lexington Mill are those from the first year's run, where certain 
 breakages are reduced to a minimum; finally, that freight in these 
 
 *These figures will, no doubt, be considered too high by Mr. Krom and Mr. Clark. I consider it 
 ; safe, however, to provide a limit for accidents. 
 
31 
 
 localities, on account of direct railroad communications, is slightly 
 less than I have assumed in my premises, I arrive, by making such 
 allowances, at the following figures for wear and tear of a 30-stamp 
 battery per twenty-four hours running time: 
 
 In all parts subjected to wear and breakage, supplies, 
 
 screens, and lubricants,* $11 50 
 
 Wages for repairs, 5 50 
 
 Total, $17 00 
 
 Wear and tear of rolls, 6 45 
 
 Difference in favor of rolls, . . . . . $10 55 
 
 Interest and Amortisation. In comparing the expense of run- 
 ning rolls and stamps, interest and amortisation on the excess of 
 capital required in the original construction of the plant for stamps 
 cannot be neglected. Considering the short life of most silver 
 mines in this country, this item should not be taken at a lower rate 
 than 15 per cent, per annum. If we take the running time of a 
 mill at three hundred and fifty days in the year, and consider that 
 a mill with stamps will cost $10,938 more than one with rolls, the 
 interest and amortisation amount to $4.68 per day. 
 
 Summary. From the above we find the following daily saving 
 in a mill with two sets of Krom's 26-inch rolls as compared with 
 30 stamps: 
 
 Wear and tear, and repairs, $10 55 
 
 Interest and amortisation, 4 68 
 
 Fuel, 2 cords of wood, at $6, 12 00 
 
 Total $27 23 
 
 If no great accuracy can be claimed for this estimate, it is the 
 best which can be given at present. 
 
 Mr. Krom, supported by Mr. Clark, claims that two sets of rolls 
 will crush more ore than 30 stamps. Others will consider my 
 estimate too much in favor of the rolls. Time will establish the 
 correctness or fallacy of these views." 
 
 *0f this amount, the wear and tear of shoes and dies represents only 40 per cent.; tappets, bosses, 
 cams, stems, cam-shafts, flanges, and boxes, 33 per cent.; and screens, lubricants, screen-frames, bat- 
 tery guides, and carpenters' and machinists' supplies, 22 per cent. 
 
32 
 
 To the comparison of Mr. Stetefeldt I need add 
 nothing except to corroborate and emphasize it from 
 later data of experience. 
 
 Since Mr. Stetefeldt's paper was written, the capacity 
 of the rolls in the Bertrand mill has been rated at 150 
 tons per twenty-four hours crushing to pass a 16-mesh 
 screen. This is 50 per cent, more than Mr. Stetefeldt 
 felt justified in assuming. The rolls have also been put 
 in successful operation in the Mt. Cory mill, Nevada, 
 and the Haile mill, South Carolina. 
 
 What the full capacity of the improved rolls is, has 
 not yet been demonstrated by actual test, as either the 
 screening or elevator capacity has not been sufficient to 
 allow a complete demonstration of this point. We only 
 know that at the Bertrand mill the rolls crushed 50 tons 
 of hard ore in twelve hours, and in the Mt. Cory mill, 
 50 tons to 30-mesh fineness in the same time. If we 
 rate the capacity of stamps of 850 pounds at 2 tons per 
 stamp in twenty-four hours, doing the same kind of 
 work, this would show that two sets of 26-inch rolls are 
 equal to 50 stamps. The calculation given in the pre- 
 ceding pages, and results in practice justify, I think, this 
 rating. 
 
 Mr. Clark, of the Bertrand Company, says : " We have 
 demonstrated the following facts : That we can pulp 
 more ore with the rollers than can be done with a 
 40-stamp battery; that we have crushed 9,000 tons of 
 ore without a dollar's expense in repairs; that we have 
 done it with less than one-half the power that would 
 have been required had we used stamps ; that the cost 
 of repairs, wear and tear, will not exceed one-quarter of 
 that of crushing with stamps ; that we make less dust, 
 jar and noise." 
 
 At a later date Mr. Clark says : " We crushed 15,000 
 tons of ore before putting new tires on the fine-crushing 
 
33 
 
 rollers, and have crushed now 20,000 tons ; and it will 
 be two or three months yet before we will put new tires 
 on the coarse-crushing rollers. It will be safe to calcu- 
 late that each set of tires with a set of composition 
 liners for journals will last to crush 20,000 tons of ore. 
 There has been no other expense for repairs upon our 
 rollers except the check-pieces." 
 
 On the above basis (crushing 20,000 tons with a 
 wear of two sets of steel tires and one set of composi- 
 tion liners) the cost of renewal at present prices of steel 
 will be as follows : 
 
 Two sets of steel tires, $530 00 
 
 Freight on 3,264 pounds, at 3 cents, . . . 98 00 
 
 Composition liners for journals and check-piece, 100 00 
 
 Total $728 00 
 
 which amounts to 3 T 6 cents per ton of ore. 
 
 But after all that has been proved for rolls it is still 
 sometimes asserted that they are only suitable for coarse 
 crushing; that ore* cannot be pulverized with them 
 sufficiently fine for amalgamation and other purposes 
 where pulverizing is needed, such as cement, phosphate- 
 rock, etc. This is true, as before pointed out, of ordin- 
 ary cheap rollers only. The same assertion was made 
 when it was proposed to adopt rolls in the Bertrand 
 mill for pulverizing ore for lixiviation ; and almost 
 everybody who knew of the affair predicted that the 
 rolls would prove a failure. At the same time, such 
 critics admit that stamp-batteries, centrifugal roller-mills, 
 attrition-mills, ball-pulverizers, mill-stones, and various 
 impracticable machines will pulverize. It would be as 
 good reasoning to argue that a heavy load can be drawn 
 on a sleigh on bare ground, but that the same heavy 
 load cannot be carried on a strong and well-constructed 
 wagon or railroad car. Crushing with rolls is carrying 
 
34 
 
 the load on wheels while all the other devices are similar to 
 dragging the load with great loss of energy. 
 
 Rolls as a pulverizing machine, if properly con- 
 structed, cannot be improved upon either in principle 
 of operation or in economy of results. All the parts 
 are of such a character as to contribute to their dura- 
 bility, and there is no limit to proportioning the journals 
 so as to fit them to carry any load that may be put upon 
 them. The capacity of rolls for quantity and hard work 
 is far greater than that of any other existing device. 
 
 Rolls are at last receiving the recognition they de- 
 serve as a pulverizing machine. It is now proposed to 
 employ rolls to crush fine after the ore has passed the 
 stamp-battery, so as to increase the capacity of an exist- 
 ing plant which is a complete reversal of all former 
 ideas on the subject. Yet I have sold rolls for this 
 purpose. Considering the subject from a mechanical 
 standpoint it is astonishing that rolls were not brought 
 to perfection and widely adopted long ago. 
 
35 
 
 KROM'S SYSTEM OF PNEUMATIC CONCEN- 
 TRATION OF ORES. 
 
 Concentration of ores by means of air is a problem 
 which has received much study, and has been the sub- 
 ject of many experiments, both in this country and in 
 Europe. The employment of air for the separation of 
 ores was generally admitted to be a matter of consider- 
 able importance, but soon after I had shown that air 
 could be employed successfully as the concentrating 
 agent, and, in fact, had demonstrated its superiority, 
 then some critics claiming to be conversant with the 
 theory and art of ore dressing began to write treatises 
 to prove the impracticability of employing air for this 
 purpose ; they said air is " specifically too light, and 
 could move only small particles." These critics evi- 
 dently had not heard of a cyclone, or the various appli- 
 cations of air as a motive power. In my experience I 
 found that air applied in intermittent impulses, similar 
 to water in the wet-jig, exhibited phenomena favorable 
 to its employment as a concentrating medium. The 
 theory found in literature, and relied upon by the sev- 
 eral writers to sustain their criticisms, was that for 
 separating two particles of solids of different density, 
 such liquid medium is the most effective that possesses 
 a density lower than the density of the specifically 
 heavier particles, and higher than the specifically lighter 
 ones. For example (they said), let us suppose that an 
 ore consisting of galena, specific gravity 7.5 and quartz, 
 specific gravity 2.5, so finely crushed that each particle 
 will consist of either galena or gangue to be placed in a 
 liquid of a specific gravity of 5, then it is evident that 
 the quartz would remain floating on the surface, while 
 the galena would sink to the bottom." 
 
36 
 
 A medium of intermediate density is not only imprac- 
 ticable to obtain, but would not serve as the concentrat- 
 ing medium if we had it. To separate minerals of different 
 specific gravity, a medium of lower density is required 
 with sufficient motion imparted to it to give the neces- 
 sary resistance. 
 
 The less dense between certain limits that medium is 
 the better it will serve the purpose. In the wet and 
 dry jig it is the resistance caused by the motion of the 
 fluid that effects the separation. In practice we can 
 only use one of two mediums, air or water, all others 
 being chimerical. 
 
 A mathematicMliscussion of the subject would no 
 doubt point out a theoretical medium of such density 
 that its effect taking power, adhesion, etc., into consid- 
 eration would be a maximum, and that air, instead of 
 water, would be found to be nearest to that theoretical 
 medium. In technical literature it is stated u That the 
 difference between diameters of spheres obtaining equal 
 speed falling in air is less than between diameters of 
 spheres falling equally fast in water." That is, a 
 -J-" globule of galena and |-" globule of quartz fall with 
 equal speed in still and moving water. But it is asserted 
 that in air the -f-" globule of quartz would fall much 
 faster. This, however, applies to air at rest, which 
 would not comply with the conditions in practice which 
 requires the air to have a motion sufficient to make its 
 resistance equal to water. It is a remarkable circum- 
 stance that none of these theoretical writers have com- 
 prehended the following facts : 
 
 1st. That either air or water can be converted into 
 the exact " theoretical and ideal minimum " by giving 
 motion to it to produce any degree of resistance. 
 
 2d. That the principle of uniform intermediate resist- 
 ance does not succeed in practice, but that our most 
 
37 
 
 successful practice is based on the intermittent resistance 
 of the concentrating agent. 
 
 To prove the incorrectness of the formulas on the 
 subject of the free fall of mineral grains in air and water 
 as applied to ore dressing, I erected two glass tubes (as 
 illustrated) each 2" in diameter and 8' long. One of 
 these tubes I filled with water and through the other 
 I forced a current of air. I found that practically as 
 stated the -J-" globule of galena and |" globule of quartz 
 are equal falling in a column of water, but in a current 
 of air adjusted to retard the galena from falling to the 
 same extent as in water, the |-" globule of quartz was 
 sustained and did not fall, but oscillated in the open 
 end of the tube. 
 
 I also employed bodies of other forms with conical 
 heads (as illustrated in the cut). In experimenting 
 with these I regulated the current of air in the air tube 
 to give the same resistance as water would to the falling 
 bodies of equal weight and size, one in the water, and 
 the other in the current of air, both reaching the bottom 
 in equal time. I then let fall Nos. 1 and 3 in water, 
 and when No. 1 reached the bottom, No. 3 was about 
 10 behind. Next I let fall the same Nos 1 and 3 in 
 the current of air, and No. 1 fell in the same time in 
 the air current as it did in water, but No. 3 did not fall, 
 but, like the quartz ball, it oscillated in the open end of 
 the tube. Philosophical reasoning would have led to 
 no other conclusion than that the galena and quartz 
 globules would fall with equal velocity in the air cur- 
 rent, under the conditions here described. The experi- 
 ment, however, demonstrated, as practice had before, the 
 superiority of air as the concentrating medium. This 
 experiment has been criticised in a paper written by 
 J. C. Bartlett, A. M. Mr. Bartlett claims that the 
 more favorable effect was due to the narrow cross 
 
Wote-r 
 
 ILLUSTRATING EXPERIMENTS WITH EQUAL FALLING GRAINS 
 IN WATER AND AIR. 
 
I/ 
 
 Wle 
 
 ILLUSTRATING EXPERIMENTS WITH EQUAL FALLING GRAINS 
 IN WATER AND AIR. 
 
40 
 
 section of the tubes. To avoid such criticisms I per- 
 formed the second part of the experiment with the 
 conical headed tacks. In this experiment the proper 
 conditions as required by Mr. Bartlett were perfectly 
 complied with, but Mr. Bartlett, however, ignored this 
 part of the experiment. On these criticisms the dis- 
 tinguished Councilor Althaus, one of the Centennial 
 judges, says: "It must not be overlooked that in 
 pneumatic jigging the grains suspended in the stream of 
 air are always in narrow air spaces, and that in cases of 
 the large grains in comparison with the adjacent smaller, 
 but specifically heavy ones, they are similarly situated 
 to those in Krom's experiment in narrow glass tubes. 
 In the opinion of the author, therefore, practical ex- 
 perience alone can decide which process, the hydraulic 
 or pneumatic, can effect the closer separation according 
 to specific gravity and size of grain." These remarks of 
 the councilor are a complete answer to every criticism 
 made against the pneumatic system of concentration. 
 
 ENDORSEMENTS OF PNEUMATIC CONCEN- 
 TRATION. 
 
 Dr. R. W. Raymond in the Engineer and Mining 
 Journal, Dec. 21, 1878, in reviewing a treatise on the 
 subject of ore dressing by E. F. Althaus, member of the 
 Royal Mining Court, Breslau, Germany, and judge of 
 group 1, Centennial Exhibition, says: " Councilor 
 Althaus, fortunately for the profession, represented 
 group 1 at the Centennial Exhibition, a member of the 
 jury of that group, and by another piece of good luck or 
 judgment the subject of ore dressing was assigned to 
 him. The treatise on that subject which he contributes 
 to the Reports and Awards is a model of intelligent 
 
41 
 
 criticism, and even as a manual of art superior to any- 
 thing we possessed in the English language before its 
 appearance." 
 
 The following is taken from Councilor Althaus' 
 Centennial report on ore dressing: 
 
 "THE PKINCIPLES OF KKOM'S PNEUMATIC 
 JIGGING."* 
 
 u The idea of using air as the medium for the sep- 
 aration of mineral grains, rejected in Europe as fruit- 
 less, was again taken up in an entirely new direction in 
 the United States by Stephen R. Krom of New York, 
 in the year 1868, and has since been systematically 
 developed in exceedingly ingenious machines, which 
 were an ornament to the ^Centennial Exhibition. This 
 new treatment, only within a few years successfully 
 introduced into some ore dressing establishments, is yet 
 too little known in its economical results to give a final 
 judgment on its possible advantage over ore dressing in 
 the wet way, but it can even now be designated as a 
 surprising advance in the art of ore dressing. That the 
 spirit of invention even in this department has so decided- 
 ly succeeded in producing unheard of mechanical effects, 
 in contradiction to prevailing theoretical opinions, and 
 furnishing an ocular demonstration so clear that the 
 correctness of the facts cannot be disputed. 
 
 The earlier experiments of air separation were con- 
 fined to a continuous stream of air produced by a ven- 
 tilator, in a high, horizontal and narrow conductor (two or 
 three metres square) through which the dried mineral 
 grains introduced from above were separated in a manner 
 
 * The autht r had an opportunity at the Centennial Exhibition of observing a Krom machine 
 at work. 
 
42 
 
 entirely similar to that of the horizontal stream of 
 water, working in the settling pit according to the 
 principle of free fall. But Krom starts with the funda- 
 mental idea of compressing the air by means of a kind 
 of piston with quick successive strokes, and thus com- 
 pressed to let it work with great rapidity through a low 
 stratum of mineral grains, previously assorted according 
 to size (sized), which rests on fine sieves enclosed lat- 
 erally in a narrow space. Krom's pneumatic jig or dry 
 concentrator is similar to the continuous hydraulic jig in 
 outward appearance, and must be designated as an 
 intermittent working air stream apparatus, in which, 
 therefore, the dynamic effect of the living force produced 
 by an upward jerk through the dense medium has a 
 more energetic effect than by free fall. In the employ- 
 ment of air in puffs as a medium, its inferior density is 
 a decided advantage. With water the vis inertia of the 
 mass of the medium to be moved prevents driving a 
 jigger at more than 60 to 120 strokes per minute. 
 Troublesome setting back of the water takes place, 
 which acts detrimental to the separation on the sieve, 
 to avoid which either the stroke and velocity must be 
 diminished, or a complicated arrangement be made use 
 of. On the contrary, the air escapes at each stroke 
 without set back, and it is therefore possible to drive 
 the pneumatic jigger 420 to 500 strokes per minute, 
 consequently very much more rapidly than the hydraulic 
 one. 
 
 Air is a medium to be obtained every where, while the 
 procuring of the necessary water supply is attended with 
 difficulty as a rule, and in many places is impossible. 
 While the grains to be separated have to pass over a 
 distance of one or two metres on the continuous 
 hydraulic jigger, they have to pass over only thirteen 
 centimetres of sieve length on Krom's machine. This 
 
43 
 
 apparatus can, therefore, for the sake of concentrated 
 efficiency, be more extended laterally without assuming 
 troublesome proportions. As the adhesive effect of air 
 compared with water is extremely small, it is evident 
 that sands can be treated pneumatically which are very 
 much finer than the finest treated on the hydraulic 
 jigger. The dry sand forms a loose mass easily pene- 
 trated by the compressed air, while on the hydraulic 
 jigger it is raised altogether in a closely adhering mass 
 by the water, thrust, and even in falling hinders its own 
 separation through mutual adhesion. While grains 
 from .5 to 1 millimetre can scarcely be prepared on the 
 hydraulic jigger, it is said that grains of .01 millimetre 
 can be separated pneumatically." 
 
 Prof. Alex. Trippel, at the close of an article on ore 
 dressing published in the Mining Record, June 25, '83, 
 says: " We will probably never have dressing works 
 which will save all that is valuable, and work without 
 any disadvantage, but taking all points into con- 
 sideration, I think that the pneumatic principle as 
 applied by Mr. Krom will gradually be more appre- 
 ciated as one which is the least wasteful." 
 
 T. Gr. Negus, Esq., the former superintendent of the 
 Clear Creek Co, Col., says: 
 
 " There is a very great difference in the quality of ore 
 for concentrating. Some ores are susceptible of very 
 close concentration, while others are not ; for instance, 
 some ores contain precious metals in the gangue in such 
 fine particles that disintegration by crushing is impos- 
 sible. In such ores a high percentage cannot always be 
 obtained. I am prepared to say, however, that the 
 Krom system of dry concentration is eminently superior 
 to all others of which I have any knowledge for all 
 grades of ore." 
 
 Mr. Stetefeldt in his recommendations of May 4th, 
 
44 
 
 1883, to the Alta Montana Co., says: "In examining 
 the advantages of the Krom system of dry concentration 
 compared with the wet method, we find, 1st. That 
 Krom's air jig effects a more complete separation of 
 minerals of different specific gravities than the water jig. 
 
 2d. That material of greater fineness can be treated 
 in the air jig than in the water jig. 
 
 3d. That in wet concentration the great losses occur 
 in those machines which treat material too fine for the 
 
 jig- 
 
 4th. That the dust which results in the Krom system 
 
 is of higher value than the original ore, and is a concen- 
 trated product. But besides this other conditions are 
 to be considered. If the products of concentration are 
 to be roasted and a portion of them have previously to 
 be finer pulverized, it becomes necessary to get them 
 perfectly dry. Now it is evident that it must be very 
 much cheaper, and require a much simpler, and less 
 bulky plant to dry the ore after it leaves the crusher 
 than to remove the moisture from concentrations and 
 slimes completely saturated with water. It furnishes 
 dry dust ready for the roasting furnace, concentrations 
 ready for finer pulverization, .a concentrated product 
 ready for smelting, purer and of higher percentage in lead 
 than the water jigs. It also makes possible a better 
 arrangement of the location of the dry kiln in connec- 
 tion with the mill." 
 
 Pneumatic concentration has not so far had an oppor- 
 tunity for a long and fair trial ; when it does you will 
 see as convincing a demonstration favorable to air as the 
 concentrating medium, as has been shown in favor of 
 employing rollers in place of stamps. By this I do not 
 mean to say that pneumatic concentration has not made 
 a good record, on the contrary, it has shown its superi- 
 ority whenever and wherever tested. In the Clear 
 
45 
 
 Creek Company's Mill at Georgetown, Colorado, the 
 pneumatic jigs gave 10 to 15 per cent, better results 
 than tests on the same ore in competition with wet con- 
 centration. This mill was kept in operation as long as 
 ore could be obtained. At Galena, Nevada, in the 
 White & Shiloh Mining Company's Mill it gave results 
 satisfactory to the owners, and above the guaranteed 
 percentage of saving, and on such ore which previous 
 trials had shown could not be successfully separated by 
 the wet process, and this works remained in successful 
 operation until the mine produced'no more ore. 
 
 At Austin, Nevada, the results were decidedly in 
 favor of pneumatic concentration. At Wickes, Montana, 
 the dry concentration works were destroyed by fire 
 before they had been put in proper running order, or 
 placed in charge of persons competent to run them. 
 
 Also the works at Star Canyon, Nevada. The first 
 mill erected on the pneumatic system have always 
 been considered a success. It is purely prejudice and 
 the misinterpretation of the theories applicable to ore 
 dressing and accidental circumstances which has prevented 
 the more rapid introduction of the pneumatic system. 
 
46 
 
 THE PNEUMATIC JIG. 
 
 The first plate, Fig. 1, is a perspective view of the 
 pneumatic jig. Fig. 2 is an end view, and Fig. 3 a 
 transverse sectional view. The machine is composed 
 essentially of the following parts: A receiver, H, to 
 hold the crushed ore ; and ore bed, O, on which the ore 
 is submitted to the actions of the air; the two gates, 
 G, G, one to regulate the flow of ore from the receiver 
 H, the other to determine the depth of ore on the ore bed ; 
 passage C, in which the concentrated ore descends, and 
 roller R, to effect and regulate the discharge of the same ; 
 a fan, B, to give the puffs of air, a trip- wheel T, lever 
 L, and spring S, to operate the fan, and a ratchet-wheel 
 W, and pawl P, to impart revolution to the roller R. 
 
 The mode of operating the machine is as follows: 
 Ore is placed in the receiver H, and the driving pulley 
 set in motion. The cam shaped trip- wheel T, fixed on 
 the opposite end of the pulley-shafts, works against the 
 lever L. By the alternate action of this wheel, forcing 
 the lever in one direction, and the spring which suddenly 
 carries it back again, the fan B is made to swing on the 
 shaft I, sending at each upward movement a quick and 
 sharp puff of air through the ore bed, and lifting slightly 
 the ore lying on it. There are six projections upon the 
 trip- wheel, so that the moderate speed of 80 to 90 
 revolutions per minute will give 480 to 540 upward 
 movements of the fan in the same time, and a correspond- 
 ing number of puffs of air to agitate the ore. This rate 
 is sufficient to secure a steady motion of the heavy pulley, 
 and yet not so fast as to produce any perceptible vibration, 
 the machine working smoothly and easily. The ore bed 
 is composed of wire-gauze tubes, placed at distances 
 from each other of T 3 , 1, f and -J- of an inch, accord- 
 ing to the grade of ore to be concentrated, the finer 
 
47 
 
 Patented Aug. 4, 1SC8, and Sept. 1, 1868 ; Ke-issue Nov. 3, 1868; Re-issue June 20, 1871 ; Patented 
 Dec. 5, 1871, and Patents pending for improvements. 
 
48 
 
 requiring the tubes set nearer together, while the coarser 
 allow the tubes to be placed farther apart. The ore bed 
 (situated in front of the fan, as plainly shown in the 
 sectional view) is composed of these tubes. Their ends 
 next to the fan being open, the air from the bellows 
 enters and escapes through the top and sides of the tubes, 
 agitating the ore that lies on them, and also that between 
 them near the surface. 
 
 The ore between the tubes rests on that immediately 
 underneath in the passage C, and sinks as fast as the 
 roller K effects its discharge. The tubes being open also 
 on the bottom, any fine ore passing through the meshes 
 of the wire gauze descends with the main body C, thus 
 preventing any liability of the tubes to fill up with fine 
 ore. 
 
 The roller R is operated (as above mentioned) by 
 means of the ratchet-wheel W and pawl P, and the latter 
 being carried by a crank on the trip- wheel, it follows 
 that its speed is governed by the speed of this wheel, 
 which also gives motion to the fan B. By this connection 
 the fan, which effects the concentration, and the roller, 
 which discharges the concentrated ore, are made to act 
 in concert with each other. The importance of this 
 feature will be apparent when it is remembered that the 
 amount of ore concentrated in a given time depends on 
 the rapidity of the puffs of air, so that the motion of the 
 discharge roller R should be regulated to correspond 
 with the speed of the fan. 
 
 The crank which carries the pawl can also be varied 
 in length so that the speed of the rollers may be regulated 
 according to the richness of the ore. As already stated, 
 the upper gate, G, governs the flow of ore from the re- 
 ceiver, H, to the ore bed, while the lower gate, G, regulates 
 the thickness of the stratum of ore lying on the latter, 
 as it is necessary to increase or diminish the depth of the 
 
49 
 
 SECTIONAL VIEW OF KROM'S PNEUMATIC JIG. 
 
50 
 
 bed of ore operated upon according to its coarseness or 
 fineness. The finer the crushed ore, the thinner the stra- 
 tum must be. The strap with its screw fastenings, 
 serves to prevent the roller attachment of the lever L 
 from striking the body of the trip-wheel, as it falls from 
 each of the cam-shaped projections, and to regulate the 
 extent of movement of the fan. That is, the strap must 
 in all cases be so adjusted that the small roller working 
 against the trip-wheel shall not strike at the foot of the 
 cam the strap serving in this manner to cushion the 
 blow. Further, by tightening up or slacking off, by 
 means of the screw fastening, the fan is carried in its 
 vibration through a greater or less space, producing a 
 stronger or lighter puff of air. It will be understood 
 that the volume of the puff of air required varies with 
 each grade of ore operated upon. Now, with the strap 
 arrangement alone, the puff of air can be regulated to 
 the requirements of different grades of ore. But as the 
 finer grades demand so much less movement of the 
 fan than do the coarser, it is better to select a trip- 
 wheel from the sizes furnished which gives a movement 
 corresponding most nearly to that required, and then to 
 make the closer adjustments by means of the screw and 
 strap ; but the roller must in no instance strike at the foot of 
 the cam. 
 
 The novel features to be particularly noted in this 
 separator, are as follows : 
 
 1st. The ore-bed. 
 
 2d. The automatic discharge rollers. 
 
 3d. The fan for producing the puffs of air. 
 
 4th. The trip-wheel and springs. 
 
 5th. The strap and adjustable screw. 
 
 6th. the gate on the receiver H. 
 
 1. The ore -bed, formed of wire gauze tubes which are 
 set in a frame at short distances apart, to allow the ore 
 
51 
 
 to sink between them, is a novel device for securing the 
 removal of the concentrated ore as fast as the separation 
 on the bed is completed. 
 
 The entire width of the ore-bed is 4 feet, and along 
 this tubes only ^ inch wide are set with intervening 
 spaces of T 3 -g- to J- inch, consequently the total extent of 
 the spaces through which the ore sinks is -^ to ^ of 
 the entire ore-bed. By this arrangement the downward 
 flow is very gradual between the tubes. 
 
 The automatic discharge roller R, This being driven 
 by the same motion that works the fan, it follows that 
 the concentration of the ore and its discharge are effected 
 in concert, so that when the speed of the machine 
 slackens, the concentration being less, the rate of delivery 
 is correspondingly reduced. 
 
 The device for producing the puffs of air, namely 
 A fan directly actuated by means of a single lever, L, 
 is very simple and effective. 
 
 The trip-wheel and springs actuatings the fan or 
 bellows The wheel by the gradual action of its cams 
 throws back the lever L, and consequently the fan or 
 bellows-plate downward with a movement as gradual as 
 possible, and immediately the springs carry the fan 
 quickly upward. 
 
 The superiority of this device over cams, cranks, etc., 
 is, that with it a considerable variation of the speed of 
 the machine does not effect the quality of the concen- 
 tration, but only the quantity. 
 
 If the trip-wheel revolves slowly the number of vibra- 
 tions of the fan is less, but as the spring causes the 
 upward movement of the fan the puff of air acts with 
 uniform force, and we thereby obtain at all speeds what 
 we may term a concentrating stroke of the bellows, as 
 the more sharply the puffs of air are given the more perfect 
 will be the separation. 
 
52 
 
 In reference to the strap and adjustable screw, the 
 explanation already given is sufficient. 
 
 The gate on the hopper, H, compels the ore to flow 
 on the ore-bed as an under- current, and as the puffs are 
 regulated to agitate very slightly the heavy particles, 
 only the lighter portions will rise to the surface and 
 be thrown over the lower gate, Gr, as tailings, while the 
 heavy will sink through the ore-bed to be discharged by 
 the roller R 
 
 This feature enables us to concentrate perfectly with 
 a very short travel of the ore, or in other words, to 
 employ a short ore-bed which is of great advantage. 
 
 By having a short ore-bed we can extend what we 
 properly term the width of the bed, thereby greatly 
 increasing the capacity of a machine of a given size. 
 All other experimenters have caused the discharge of 
 the ore to take place over the narrow side of the 
 machine and the travel of the ore over the greater dis- 
 tance. The reverse of this takes place in this machine, 
 viz. : the distance of travel of the ore over the bed is 
 only 5 inches, while the line of overflow is 4 feet, and 
 can be, at pleasure, made still greater. A short bed 
 enables us to use a small fan, and reduces the vibrations 
 attending rapid movements. A more even and uniform 
 agitation is secured when the ore is confined within nar- 
 row limits, and more satisfactory results are obtained. 
 
 Since I have discovered that a short ore-bed, of only 
 5 to 6 inches in length, is no.t only sufficient, but, in 
 fact, much superior, and that the width of the bed and 
 extent of overflow can accordingly be increased, I am 
 able so to place the fan, and to group in compact form 
 all the working parts, and to very considerably reduce 
 the size of the machine. 
 
 The puffs of air are regulated to agitate sufficiently 
 the ore on the bed, but should the richness of the ore 
 
53 
 
 increase during working and too large an amount collect 
 on the bed, the air ceases to lift or agitate the material 
 as much, and so a check is furnished to prevent loss in 
 the tailings. No such check is possible in water concen- 
 tration, because water moves practically as a solid and 
 carries all before it. 
 
 All parts in this machine liable to wear are manufac- 
 tured in duplicate and can be cheaply replaced. The 
 machine measures 6 feet in length over all, 3 feet in 
 width, and 3 feet 10 inches in height. Its weight, com- 
 plete, is 1,200 pounds, and it is capable of concentrating 
 ^ ton per hour with -J- horse power. 
 
KROM'S PATENTED LABORATORY CRUSHER. 
 
 The foregoing cut represents a crusher for labor- 
 atory use. 
 
 In this machine both jaws oscillate on centers fixed 
 some distance from the crushing faces. The principal 
 feature is the employment of segments of circles be- 
 tween which the ore is crushed on the same principle as 
 rollers act. 
 
 The lower ends of the crushing plates are true seg- 
 ments of circles, and throughout all the movements of 
 the jaws they remain at fixed distances from each other, 
 but the top parts of the plates recede from each other 
 with straight lines. The crusher can be adjusted, by 
 means of bolts, so as to produce either fine or coarse 
 material. The crushing faces are made of steel, and the 
 lower half of the steel plates are hardened. It is the 
 only machine in the market adapted for laboratory use. 
 
55 
 
 LABORATORY JAW CRUSHER. 
 
 LABORATORY ROLLERS. 
 
J 
 
 56 
 
 KROM'S DRY KILN. 
 
 Patent for this Dry Kiln pending. 
 
57 
 
 KROM'S DRY KILN. 
 
 The cut, Fig. 2, illustrates my improved dry kiln for 
 drying ores after it is broken by an ore breaker. The 
 cast-iron plates, b, on which the ore rests while drying, 
 are arranged in steps, with spaces between each step of 
 3 or 4 inches. These spaces allow the hot air and gases 
 from the fire underneath to pass up through the strata 
 of coarsely crushed ore, as plainly indicated by the 
 arrows. 
 
 The waste heat and evaporated moisture pass out 
 through the chimney. The plates, b, or steps, are placed 
 at an angle of 45 p , but it will be observed that the 
 furnace assumes an angle of 58. Therefore to maintain 
 the strata of ore of a uniform thickness it is necessary 
 to have the check plates, a. 
 
 The distance between the lower edge of these plates, 
 a, and the plates b, determines the thickness of ore 
 strata. This thickness can range from 6 to 9 inches. 
 The check plates, a, can be readily varied in height by 
 means of holes in the flanges which support them. 
 
 If the location of the mill site will admit of so much 
 
 
 elevation, the plan shown in Fig. 1 will save much labor 
 and expense in handling the ore. It will be seen that 
 the plan here suggested is for continuous and automatic 
 operation. 
 
 Feed roller c is to control and regulate the flow of 
 ore to the crushing rollers. It has become necessary to 
 employ some means for collecting the pieces of iron and 
 steel before they pass to the pulverizing machines. 
 
 A system of magnets is placed in the shute a, z, d. 
 
 The arrangement of the magnets are more plainly 
 shown in Figs. 4 and 5. 
 
58 
 
 This dry kiln has decided advantages over the 
 revolving drier. The kiln here illustrated requires no 
 power to operate it, and the gentle flow of ore by 
 gravity over the plates does not tend to stir up dust or 
 create more from abrasion, and consequently no dust 
 chamber is required as with cylinder drier. 
 
 The fuel required to dry ore in this kiln is the mini- 
 mum amount. It is 20 feet long, 5 feet w r ide, and holds 
 a strata of ore 6 to 8 inches thick. The capacity of this 
 kiln may be estimated between 2J to 5 tons per hour. 
 
V 
 
 Patent for this Dry Kiln pending. 
 
60 
 
 KROM'S REVOLVING SCREEN. 
 
 The cut on the following page represents my improved 
 screen frame. The frame is composed of light wrought 
 iron angle-bars, rolled expressly for the manufacture of 
 this screen. The form of this bar is shown in the 
 sectional cut of the rail marked k. The spiders which 
 carry these angle-bars are composed of a hub and 
 wrought iron spokes. The spokes are screwed into the 
 hub to a shoulder formed on the spoke. On the outer 
 end of each spoke is another shoulder on which the 
 angle-bars rest. All the parts are made of uniform 
 dimensions, and are therefore interchangeable. The 
 angle-bars hold the screen-head in the small end by 
 means of small bolts. 
 
 The iron framework of the screen, with its clamps, 
 are independent of the wooden frame on which the 
 wire cloth is nailed. To fasten the screen-frame in 
 place it is simply slid under the clamps, and screwed 
 down to the angle-bars. 
 
 The clamps which hold the screen-frames to the angle- 
 bars are all held up with springs, so that when they are 
 released the clamps are carried clear from the screen- 
 frame, and the same is taken out and put in without 
 difficulty by one person. 
 
 The iron angle-bars carry bolts on which are sliding 
 weights to jar the screen. On the bolt, next to the 
 screen, I put a rubber cushion, so that a sharp blow is 
 given when the weight falls from the top of the screen, 
 and a soft blow when the weight falls on the lower side 
 of the screen. The object of the jar is to keep the 
 meshes of the wire-cloth free from particles of ore. I 
 also sometimes provide a casing, 0, for the sliding weight, 
 to protect fine screens from damage, which may result 
 from the breaking of the bolts on which the weight 
 
KROM'S REVOLVING SCREEN. 
 
 Patent for this Screen allowed September llth, 1885. 
 
62 
 
 slides. These casings are made of thin iron tubes 
 screwed in a hub, similar to the hubs in which the 
 spokes of the screen are secured. The tubes are large 
 enough to clear the sliding or falling weight, and long 
 enough to enclose the bolt nearly its whole length. In 
 case the bolt breaks the tubes act as a pocket to hold it. 
 This screen is more complete and perfect in all its 
 arrangements and details than any other in the market. 
 
YD 07539