The Thomas A. Edison Papers Digital Edition

Transcription

T.A.E Marginalia: Wrote + [illegible] that our Schenectady work have the identical motors on hand for prices E ands Feb 18/92
T.A. Edison Esq 
Menlo Park N.J. 
Dear Sir:- 
I have been using the Sturtevant Mill for granulating our ore ( after roasting) for the past two years 
These machines have given me entire satisfaction, but as I desire to reduce the detail of a concentrating plant as much as possible. 
I have prepared plans which simplify the method of driving these mills. My plan is to drive direct by attaching the motor of whatever type it may be to the main spindle of the mill. 
Naturally I am looking for an electric motor, and if you can make these of say seventy (70) horsepower that will do continuous work economically, I am sure that I can put you in the way of some business before long. 
My stem of arranging the mill will protect the motor entirely from dust. The motors must run from 750 to 1300 revolutions per minute according to the size of the mill, and should be made in sizes of 30,45, and 70 horsepower. Enclosed please find papers that may interest you, and shall be pleased to hear from you at your convenience.  
Yours truly, 
Signed: Hoffman , W.H [Enclosure] 
 
[Transaction of the American Institute of Mining Engineers.] 
 
Practical results in the magnetic concentration of Iron Ore 
NY. W. H. Hoffman, M.K. Croton Magnetic Iron Mines, N.Y 
 
The writer does not claim aa right to discuss this subject as a furnace-man or user of iron-ore in this new form. His efforts have been confined to mining, preparing , and separating the magnetic ore from the gangue. Two years’ experience in experimenting and another two years in producing concentrates on commercial scale, have led me to believe that the new trade of making concentrates must be well learned in all of its details. If there have been any partial failure in this most useful branch of the iron business, the failures have been caused chiefly by inexperience or lack of patience. 
Although I have, within the past five years, constructed machinery for water-jiggling and wet magnetic separation, I shall only speak of the dry magnetic process here. 
Three years ago I was engaged by Mr. J.D. Cheever, the lesson of the Croton Magnetic Iron-mines, at Brewater, N.Y., to adapt an old water-jiggling mill to the magnetic process. The ore to be treated is described by Mesers. John Birkinbine and W.B. Kunhardt in a late report on the Croton Magnetic Iron- mines , as consisting of compact, moderately fine-grained magnetic in a gangue composed mainly of quartz and hornblende. Besides feldspar, aperitive, and union, and more or less pyrite and pyrrhotite, and they say “it is highly satisfactory that the grain of the ore permits a fair disintegration of its [constinents?] by crushing it to pass a 12-inch screen, in such as a finer mineralization would increased the cost of concertation , as here after described.”  
Similar descriptions have been given by Mr. Emerson McMillan, Mr. F. W. Gordon, and Prof, N. S. Shaler. 
By these gentleman the average. Amount of metallic iron in the ore-beds is states to range from 37 to 42 per cent., the average Sulphur from 1.7 to 2.2, and the phosphorus from 0.070 and 0.426. Practical work has shown the average amount of phosphorus to be (2)  Magnetic Concentration of Iron- Ore 
 
0.232. Since May 1, 1891, we have been roasting and concentrating this material to 08 per cent in metallic iron, 0.44 in Sulphur, and 0.036 in phosphorus. Previous to this the concentrate ran about 66 per cent, in iron. 
Up to the 1st of September 1891, opening by drifting and prospecting with the diamond drill has exposed at least 8,000,000 tons of ore, while it can be seen by a fair examination of the property than there is not less than three times this amount of ore in these mines, and this estimate does not include the mining-rights owned personally by Mr. J.D. Cheever. 
Prof. Shaler says: 
“ The considerations previously given concerning the horizontal extension of this deposit beyond the limits of the present workings lead me to feel that it is reasonably to safe to call the length of this bed 4000 feet, the depth within workable limits 1500 feet, and the width 75 feet. These estimation can be more fairly made than in the \case of an ordinary rain, and, though not positively certain, have all the probability they would have if the deposit were a bed of coal. On this basis we have a total of about 50,000,000 tons of ore in the deposit. Of this mass something like three-fourths may be regarded as mineable, the  remainder being the leaner ores that cannot be profitably extracted at great depths, or ore which must be left in place to sustain the walls and roofs of the values.” 
 
Mr. F. W. Gordon says: 

Very extensive workings and explorations show the deposit of ore to be one of the greatest in magnitude that has even been developed in America. The explorations and developments have been intelligently made and warrant an estimate of the quantity of ore in easy reach to be [out?] less than 30,000,000 tons. As the name of the mine implies, the ore is magnetic or FeaOa in composition and dense in structure. Entries 15 to 20 feet wide driven into he ore require no timbering to support the roof. The average of eleven analyses taken from the workings between June 6th and August 13, 1890, gives the following composition:
Pre cent.  
“Metallic Iron ………………… 40.64  
“Phosphorus …………………   .302 
“Sulphur………………………    1.50 
 
Before the old water-jigging mill was erected, some eight years ago, the product of these mines was sorted to remove the more Sulphur ore, and was shipped directly to furnace making foundry irons. In order to meet the requirements of pursuers, the mining company was obliged to reject, in cobbing, 2 tons to get 1 ton of shipping-ore containing 51 per cent. of metallic iron and not more than 1 per cent in Sulphur. About 50,000 tons of this class of ore 
 

                                       Magnetic Concentration of Iron-Ore 

(3) 
was shipped from those mines. The old dumps from this sorting are now being crushed and separated by the new process. 
Mr. John Birkinbine, in his paper on “ Progress in Magnetic Concentration of Iron-Ore” (Trans., xix., 656), quotes from the private letter of a member of the Institute an expression of emphatic doubt whether, at any American mine, it would pay to mine and concentrate a lean magnetic. The write says: “ So far as I can see at this stage of our practice, waste-dumps only, or the rejected portion of an output, other portions of which have been shipped at a profit covering the whole cost of mining , can be used as the raw material of concentration.” 
Without repeating here the argument of this correspondent, or criticizing his knowledge of the situation, and without asserting that magnetic concentration was a commercial success three years ago, I can assert and prove beyond question that the Croton Mines have produced and sold at a fair profit from 50 to 220 tons of concentrates per day during the past year and a half; and the mill has been actually running but twenty months. 
Commercial success in concentrating any ore includes economical mining, preparation and separation; but in the Croton ore the presence of Sulphur calls for very economical roasting as well as economy in all the other processes. The cheap roasting of the ore of the Croton mine, which contains about 2 per cent, of sulphur, was really one of the first problems encountered in reclaiming that property, and was really as important a factor as the concentration. 
A series of experiments was made to determine the best size for economical roasting, and at the end of three months a size that would pass through a 24-inch ring was adopted, as giving the most rapid work for the quantity of fuel consumed. Crude Lima oil is used for roasting. Through experiments conducted by our general foreman, Mr. T. Blass, we found the average consumption of fuel-oil to be 3.75 gallons; but by enlarging the combustion-chambers we have reduced this amount to a little over 3.6 gallons per ton of raw ore. The cost of the oil is 21 cents per gallon, making a fuel-cost of 8 ½ cents per ton of raw ore. The labor is filling and discharging amounts to only 3 cents per ton, as this work is largely automatic. The average temperature is 12500 Fah. Davis-Colby roasters, remodeled to burn fuel-oil, are used for a portion of this work, the remaining portion being done in a roaster of new type designed by the writer. The Davis-Colby roasters have been in operation nearly three years and have done excellent service. The ore is conveyed automatically from the roasters to the Sturtevant mills, where it is ground to 12-mesh wise, all coarser material from the screens being returned to these mills by elevators. Barring the numerous experiments with various types of magnetic separators, the experiments in crushing have been the most elaborate. Nearly all the best-known methods of grinding ores have been tried at the Croton mines during the past three years. Some of these machines have been tested for a year or more, but about fifteen months ago we became thoroughly satisfied that the Sturtevant mill was far superior to any other machine for grinding iron-ores. I consider it necessary to mention this machine thus somewhat prominently, as in economical and uniform granulation plays an important part in answering the question in Mr. Birkinbine’s paper: “ Does magnetic concentrations pay?” If the ore is not properly granulated and [serecued?] no known method of separation or concentration can make it a commercial success.   
The screen-block openings in the Sturtevant mills are ¼ inch wide, and the coarsest material passing through then in less than 7/22 of inch thick, while the finest material would be rejected by a 60-inch screen. 
The ore enters Sturtevant mills at a temperature of about 350, being cooled from about 1200, by a water-bath on its way up the conveyer. Under these conditions the ore is quite friable, and we have no difficulty in grinding 22 tons per hour with the 20-inch mill, and 16 tons in the same time with 15-inch mill. One set of Sturtevant-mill brushings will grind from 4000 to 6000 tons of ore, according to the depth of the chill in the bushing, the cost of each set being $16.00. The screen-blocks for this amount of ore cost $9.00/ This is less than one-half the cost of renewal on any other machine formerly used at our mill. At 22 tons per hour the 20-inch mill requires 94 horse-power to drive it, but it will be remembered that the product is finished on these mills. The 15-inch mill requires 70 horse-power. 
The ground ore is elevated from the discharging-nozzles of the Sturtevant mills to the several screens, covered with slotted steel plates made by the Harrington & King Perforating Co. The slots are 1/12 by ½ inch in some plates , and 1/12 by [legible fraction] in others. 
The slotted plates are easily removed, and when the requirements are exacting as to phosphorus, we substitute plates of 1/14 inch on two of our five screens. WE have demonstrated by exhalative experiments that two sizes of screen plates, three sets course and two  
 
(5)                                   Magnetic Concentration of Iron-Ore 
sets fine, will prepare the ore containing 0.426 in phosphorus ( the greatest amount we have in the mine) for a separation having 0.036 with two passes on the magnetic separators. Ordinarily, the phosphorus in the Croton ore runs from 1/10 to 3/10. When the phosphorus runs higher than 0/10 three sizes of screen-plates should be used, delivering to three receiving hins, and each size should be treated separately on the magnetic separators; and I am positive that this treatment will insure a Bessemer product running not over 0.050 in phosphorus, using nearly any of the New York State magnetizes that are free from titanium. 
 
In some experiments we have used 18-[inch or mesh?] screens, and with ore prepared for this grade we were enabled to product continuously, with two passes, concentrates showing 70.60 metallic iron, 0.018 phosphorus and 0.220 Sulphur. Of course the silicon was extremely low. With the latest Hoffman separator, using 12-mesh screen, and making two passes, we have produced concentration showing 70.93 metallic iron, 0.017 phosphorus and 0.231 Sulphur; and by using 18-mesh screens we can depend on 71 per cent concentration with one pass on this machine.  
Examination for the purpose of detecting screen-, shute- and hin-leaks must be made at least twice a week. 
The screens deliver their finished product tot wo bins placed on the floor above the separating department, each having a capacity of 80 tons. Eight shuts deliver the ore to the separators, nine in number. The separators were designed and constructed by W. D. Hoffman and T- Blast, at our mines. One eight of the machines the ore is passed before the magnets twice to bring the loss in tailings to 8 per cent. Our Superintendent, Mr. W. D Hoffman, lately designed two new separators, one of which is giving with one. Pass concentrates of 68 to 70 per cent, with a loss of only 6 ½ per cent. Of iron in the tailings. As patents are now pending, these machines cannot be described here. People frequently ask how much it costs to separate the iron-ore from the gangue. We always reply, as little as any other portion of the process. Seven cents per gross ton of concentrates is a liberal allowance, and this includes all repairs to separators. We shall reduce this to less than six cents soon. Repairs and supplies throughout our milling-plant amount to one and eight tenths cent per ton of raw ore ground. Hence, it will be readily seen that the feature of the problem of magnetic separation is the initial mill -grinding or granulating. The very cheapest process is absolutely necessary, as most of the ores to be reclaimed will analyze less than 40 per cent. Metallic iron, and that means from two to Three tons of ore to produce one ton of concentrates. Using Sturtevant mills, and 22 per cent. ore, we can pay a small profit at our plant. Crushers and rolls require 28 per cent. ore to pay at the present price of concentrates. We contract our mining and initial crushing ready for, and delivered to roasters, to Mr. Charles Vivian, of Brewster, N.Y., at an average price of $1.28 per cubic yard. The ore weights from 5500 to 6800 pounds per yard. We are selling concentrates at present to six furnaces, which use from 35 to 53 per cent. of them in their regular mixture. The furnacemen tell us that their flux and fuel are reduced, but most of them decline to give the customers as to quality or fineness. I have personally witnessed the use of several hundred tons of our concentrates at the experimental Ramel Conley steel works, near our mines. They were used in various ways; sometimes loosely thrown in the bath and at other times made into briquettes and charged in the open-hearth furnaces along with the piles of scrap or African ore.  
Every one of the practical open-hearth melters employed there at different times has informed me that he was surprised at the rapidity with which he could handle the furnaces when using concentrates, and I understand this has been the experience of others. There is no difficulty found with the blast at Scranton, where concentrates have been used by Mr. E. Moffat for several years. 
An accurate, though condensed, statements of the cost of mining, crushing, roasting, preparing, and separating one gross ton of 68 per cent. concentrates, from two and one-fifth tons of 38 per cent. ore, according to the present daily practice at the Croton mines, is given below. The tailing run, in iron, from 7 to 8 per cent. About one-third of the ore is taken from the old dumps. On the basis given below, 580 tons is crushed every 20 hours, with a production of about 255 tons of concentrates in the same time: 
Statement of Coal. 
Mining, crushing, and delivering to roaster 2 ½ tons of raw ore, at 2 ½ gross tons per yard, ……………………………..….. $1.13 
Roasting, including top-flling………. $.23 
Handling at roasters, ………………..$.03 
Preparation and screening…………...$.22 
Daily renewals, supplies, and repairs of all machinery and roasters. 
………………………………………………………………………$.0051 
Separating, including labor and power, …………………………$.07 
Delivery to Harlem R.R. switch, including R.R. repairs,………..$.04 
Office and Laboratory expenses………………………………….$.041 
Insurance, interest, and taxes on plant,…………………………$.13 
$1.95 
(7)                                         Magnetic Concentration of Iron-Ore 
 
During the last winter and spring, a continuous run of five months was made of twenty hours each day, and the average cost, of a gross ton of concentrates for the whole term was $2.10. 
 
Improvements have reduced this amount to the figure of $1.95, given above.  
 
Analyses are made at our laboratory daily of ore at mines, ore after leaving roasters, concentrates and tailings, by Mr. G. K. Volckening, Jr., our chemist. 
 
The following analyses cover an average two weeks shipments in July, August, and September of the present year. All concentrates were from screens with slots 1/12 X ½ inch, and all samples were form car-loads: 
 

Chart showing samples of car loads from the months July , August and September. The chart lists metallic iron, phosphorus, Sulphur, and silica.
 
The average Bessemer concentrates for a period of eight months contained: 
 
Metallic Iron,……………………………………….  . 66.78 
Phosphorus,………………………………………… .054 
Sulphur,……………………………………………… .468 
 
And the average Foundry concentrates: 
Metallic Iron,……………………………………….  . 65.29 
Phosphorus,………………………………………… .069 
Sulphur,……………………………………………… .724 (1) 
Subject to Revision 
[Transaction of the American Institute of Mining Engineer] 
Granulating Magnetic iron ores with the Sturtevant Mill, At Croton Magnetic Iron Mines, N.Y. 
By W.H. Hoffman, Brewster, N.Y. 
Baltimore Meeting, February, 1892. 
At the Glen Summit meeting I described in a general way the grinding-machine known as the Sturtevant mill, built and sold by the Sturtevant Mill Co., of Boston, Mass. My first experiments in granulating. Magnetic iron-ores with this machine were made five years ago in the northern portion of New York State.  
The drawings, Figs 1 to 5, show the arrangements of casing, screen-block-holders, screen-blocks, bushings, spindles, collars, etc. The central casing and feeding-hopper are combined, though sectional in construction. The casing is bolter firmly to a strong cast iron bed-plate, fitted for three-fourths of its length with V-shaped slides, which receive the movable driving head-stocks. To the cast iron hopper connected with the casing is fastened a much larger wrought-steel hopper, covered at the top, which will hold four tons of ore, already crushed to 2 ½ inch cubes by common single-jaw Blake crushers.  
The movable head-stocks carry the revolving spindles, to which are fastened the chucks for holding the bushings. The head-stocks, with all their attachments, can be moved longitudinally about two feet so as to afford ample room for renewing the bushings and screen-blocks. The end-thrust wear is taken up by collars and steps at the extreme outer ends of the spindles.  
The bushings, while in operation , enter the casing about 1 ½ inches, lapping over the edge of the screen-blocks about ½ inch; and this relative position is maintained by setting up the bushings to the position indicated, every five hours. The regular wear of a steel-chilled bushing is ½ inch per day of 20 hours; but irregular chilling bring the wear in practice to [2/8?] on each bushing per 20 hours. 
The moving of the head-stocks is very readily performed by one man, who first slacks the gripping-bolts holding the V-shaped slides And then driven the head forward or back by means of the rack-and-pinion connecting the bed and head-stock. 
The screen-blocks wear, as a rule, a little faster than the bushings, for the reason that there is usually more imperfection in the chilling of these blocks. We have, however, lately superintended the making of these blocks and have found no difficulty in making them as regular in the chill as the bushings usually are.  
The screen-holder is made up of a series of 1 inch round iron bands, reaching on either side to the under-edges of the hopper where they are dove-tailed into the facing-blocks immediately above the screen-blocks. The bands are places about 1 ¼ inches apart, in order to allow a free discharge of the material, as it falls through the blocks. 
The mills at the Croton magnetic iron mines have been in use two years, and thus far the screen-holders show very little wear. They will probably last several years longer. As I remarked at Glen Summit, we finish our granulation with the Sturtevant mill, because we return the 20 per cent. rejected by the 12-mesh screens to the mill to be re-grounded. We have demonstrated that the capacity of the mill is reduced very little by returning the rejections, as these fine particles help to pack the coarse pieces more closely together in the mill casing and bushings.  
The screen-blocks are made in short segments, 4 inches wide and 5 inches long. The bridges between the slots are [fraction] inch wide. The screen block segments do no wear exactly alike; and therefore the renewals are not all made at the same time.  
It is a peculiar feature of granulation in a Sturtevant mill, that the size of the grains can be changed by changing the speed of the mill. I have found by experiments that, running a 20-inch mill 970 revolutions per minute, we can pass 80 per cent. of our product through a 12-mesh screen, while if the speed is increased to 950 revolutions, 80 per cent. of the product will pass through a 14-mesh screen—the size of the slots in the screen-blocks being the same in both instances. 
The bushings of a 20-inch mill are 20 inches in diameter inside, and 23 ½ inches outside, thus leaving the shell of the busing 1 1/3 inches thick. The chill of good bushing should extend inward to a depth of 1 ¼ inches. Chilling charcoal-iron must be used in making the bushing and screen-blocks. 
In operation, the ore is the casing and nearly surrounded by the  
 
(3)                                  Granulating Magnetic iron ores 
 
Screen-blocks, remains stationary, except at intervals, when the feed may be irregular. Then the whole mass will have a slow shifting  

Picture of the Sturtevant Mill In Operations*
motion; but this motion ceases the moment the mill is full. As the ore feeds down, the bushings are filled with ore closely packed, and Hold firmly against the inside surface of the bushings by centrifugal force. 
This the ore in the bushings is carried with them, and almost perfect granulation is the result of the attrition between the pieces of ore in the bushings and the stationary pieces of ore in the central casings. 
In mills above the 12-inch size, both spindles revolve in the same direction; but in the smaller mill, it is thought, when the material entering the hoper is larger than the 2 ½ inch cubes, that better results are obtained by crossing one of the driving-bolts. 
A 15-inch mill will handle 3 ¼ inch cubes, and a 20-inch mill 4 ½ inch cubes, without reducing the capacity materially. 
Should one of the spindles need repairs, the other spindle can be used singly by simply slipping a false head under the collars on the side to be required. 
In order to obtain economical separation with any magnetic separator, the prepared ore must be granulated, and not mashed or split into thin flakes. The particles must be nearly regular cubes in order to present them in proper form to the influence of the magnetic field. 
Three types of crushers and two types of rolls, were used at the Croton mines previous to the adoption of the Sturtevant mills, and although many devices were attached to these crushers and rolls, in order to assist in producing fairly even granulation, they all failed to accomplish the desired result. 
In conclusion, I will say, that I know of no other machine that will granulate as perfectly, as rapidly, and with as little power and wear. When our ore is well roasted, we can granulate to 12-mesh 24 gross tons per hour in our 20-inch mill, with a n expenditure of 96 horse-power, and less than [2/3?] of one cent per ton for renewals. In grinding to any grade of fineness, the grains are always in the form of cubes, thus assuring very satisfactory work on any good separator.  
The following descriptions of the illustrations herewith given are taken from the statement of the manufacturers. 
The material to be ground is conveyed through the hopper at the top to the case, A, filling the case and the revolving cylinders or heads, B, B, which, being put in motion, hurl their contents against each other with such power that the rock is at once crushed t atoms. The mill does not grind the materials, but simply furnishes the power that compels the rocks to crush themselves; consequently, 
 
(5)                                             Granulating Magnetic iron ores 
The hardness of the rock does not affect the result, as it acts upon itself. 

picture of Sturtevant Mill*
 
Fig.2 shows the cups or heads-drawn back to give a view of the interior of the mill, showing the cast-iron screen, C, through which the material, as fast as ground, passes and fails into the hopper, marked D. When necessary to reduce the rock to a greater fineness than the screen-outlets allow, the coarser part of what leaves the screen in re-conveyed by an elevator to the mill for re-grinding that 

picture of revolving cups, taken apart* (Fig.3)
which is already sufficiently fine being first removed by the usual apparatus employed in milling. 
A suctions-blower causes the air to draw strongly into the mill, and prevents the escape of dust. 

picture of revolving cups, in place.* (Fig.4)
The cast-iron screen, C, is composed of small sections, and the worn parts are cheaply and easily replaced. The wear upon this screen is very slight, as it is protected from the action of the rocks, thrown from the heads or cups, by a cushion of interposing material 
(7)                                                   Granulating Magnetic iron ores 
formed by the rocks, which always fill the case and cover the screen.  
Fig. 3 shows one of the revolving heads or cups taken apart. It is composed of two pieces, one of which, E, a simple hard iron cylinder (called the bushings) is removable, and when worn is easily taken out and replaced. 
Fig. 4 shows the same put together, and Z, the conical cup-like stone lining, which always forms itself inside the head or cup in the process of grinding, This lining forms itself by the caking within the cup of the material being ground, and completely protects the metal of the cup from wear, except at its edge. 

picture of Sturtevant Mill, longitudinal Section shown* (Fig. 5)
In Fig. 5, B, B represents the two opposite heads or cups of the mill, holding the two bushings, E , E, which slightly project into the case. The hopper is shown filled with rock, which drops into the case of the machine between the two heads. When the mill is started, the two stone hollow cones, Z, Z, form themselves in a few moments, and become as hard as the rock. When these hollow cones have formed, it is plain that the centrifugal force given by their revolution will hurt out, in the general directions indicated by the arrows, all the pieces of rock forced into them. These rocks , thrown violently out of the two hollow cones oppositely, cannot strike the case, for they are thrown from it and against each other, and the force is lost by collision.