US2240269A - Pulverizing mill - Google Patents

Description

Ap rii 29 1941 w! l. SALLEE 2,240,269

PULVERIZ ING HILL 3 Sheets-$heet 1 Original Filed June 23, 1934 JNVENTOQ Wessm? [.SALLE jlT'rORNE-YS April 29, 1941; w. SALLEE 2,240,269

PULVERIZING MILL Original Filed June 25, 1934 3 Sheets-Sheet 2 J" VE H To Z. MBHEE I $622 is 3 Sheets-Sheet I5 JHVE'HTOZ M &

flTTozn EYS April29, 1941. w. l. SALLEE PULVERIZING MILL Original Filed Jline 23, 1934 v grinding.

Patented Apr. 29, 1941 I rULvEmzmo mu.

Webster 1. Sallee, Cleveland, Ohio: E. Ruth Sallee administratrlx ,of said Webster I. Sallee, de-

ceased Application June 23, 1934, Serial No. 732,125 Renewed April 22, 1940 This invention relates to attrition pulverizing mills for grinding and pulverizing coal, silica, sands, pigments and numerous other materials. The principal object of the present invention is to pulverize such materials to predetermined and accurate fineness more efficiently and to maintain the desired degree of fineness at various rates of feed of the material into the mill.

Correlative objects are to stir and aerate the bed of attrition thoroughly and continuously in a manner such that the particles are removed from the bed substantially concurrently with their reduction to the desired fineness whereby the mutual attrition of the coarser particles is unimpeded and rendered more effective and rapid, the power requirements for grinding reduced, and the relative capacity ofthe mill invelocity and direction as to remove coarser material from the mill and further to mechanically retain coarser particles in the grinding chamber until ground to the proper degree of fineness. A more specific but equally important object is to provide an attrition mill in which theair supplied to the mill is admitted separately from the material and the two are disassociated until actually admitted to the grinding chamber whereby the proportions of vair and material, and consequently the uniformity of grind, may be more accurately controlled for different rates of Other specific objects are to provide a mill having a plurality 'of grinding compartments with means for accurately controlling the currents of air through each independently, and having incorporated therein a centrifugal separator for separating the coarser particles from the finer and permitting only the finer to pass through the mill.

Another object is to provide a mill in which the component parts can be readily assembled and disassembled for replacement and repair and which, when assembled, are protected against undue wear. V

Other objects and advantages will become apparent from the following specification wherein reference is made to the drawings in which:

Fig. 1 is a side elevation of a pulvertizing mill and feed mechanism embodying the principles of .8 Claims. (in. 83-11) the present invention, part thereof being shown in section for clearness in illustration;

Fig. 2 is a top plan view of the mill and feed mechanism illustrated in Fig. 1;

Fig. 3 is an end elevation of the mill andieed mechanism;

Figs. 4, 5, and 6 are sectional views of the mill taken on planesgndicated by the lines 4-4, 5-5,

and 6--6 respectively of Fig. 1, parts remote from the section plane being omitted for clearness in illustration.

Referring to Figs. 1. to 3, the mill comprises generally a-cylindrical casing I having at one end an air inlet housing 2 and at the opposite end an exhaust fan housing 3. Within the casing I is a primary grinding or pulverizing chamher 6 and a secondary grinding chamber 5. The

means which are adjustable independently of the vanes in the passage 6 andcontrol the effective size of the passage I.

The housings 2 and 3 are preferably coaxial with the casing I and are provided with suitable sets of dust proof bearings 8 and 9 respectively, in which the main driving shaft II) of themill is rotatably mounted. Carried on the shaft III, within the grinding chamber 4, are a plurality of rotors II and I2, shown in Fig. 1 as two in number for purposes of illustration, these rotors being secured to the shaft for rotation therewith.- A similar rotor I3 is likewise fixedly secured to the shaft I0 and is disposed within the secondary I ments of the mill, the inner end wall of the air inlet housing I is in the form of an integral annular baflie I5, having a central discharge opening I6 through which air introduced into the inlet housing may pass to the primary grinding chamber axially and centrally of the rotor H. The baflle l5 of the inlet housing likewise provides the adjacent end wall of the primary grinding chamber 4. At the opposite end of the primary grinding chamber 4 and forming the end wall thereof is an annular baflle ll having a central opening I8 which is preferably of substantially the same size as and coaxial with the opening l6. Within the chamber 4 is a circumferential rubber liner [9 coextensive longitudinally with the chamber and forming a. closed and continuous circumferential wall thereof. The liner i9 not only extends the full width of the circumferential wall but, also, at the discharge end of the chamber 4, overlies a portion of the annular baille I1 part way from its circumference toward the center, as indicated at 20. The exposed surface of the portion and of the circumferential portion of the liner is merge on a gradual curve, as indicated at 2|, for purposes later to be described.

' Referring next to the rotors of the primary grinding chamber, each of these comprises generally a hub fixedly secured to the shaft HI and having a plurality of radial spokes 23, four spokes being provided in the form illustrated. To the end portions of these spokes are secured grinding paddles 24, each of which is disposed relative to the liner l9 so as to pass in closely spaced relation thereto as the rotor rotates. As most clearly illustrated in Fig. 4, each of the spokes is provided on its trailing face with a web which extends from the outer end of the spoke to the leading face of the succeeding adjacent spoke, merging into the adjacent spoke close to the inner edge of the paddle thereon. For example, the webs 25 extend a sufficient distance radially of the rotor to form, when the rotor is operating, a substantially continuous wall in axial alignment with and coextensive radially with the openings I8 and I8,

the radii of which passages are substantially equal to the radius from the shaft axis to the inner edges of the paddles 24. Consequently, air passing through the mill cannot flow directly from the opening l6 to the opening II! but is deflected radially outwardly around the central portion of the rotor and webs and thus across the path of the paddles and through the bed of attrition and thence radially inwardly at the oppostte end of the chamber preparatory to entering the opening It. Along this .path the air stream becomes saturated with fine loose material and since its direct flow and velocity are broken up and reduced, the coarse particles tend to settle out, and also are mechanically knocked or deflected outof the air stream and not transported thereby. The webs 25 therefore have a dual function; first, to reinforce the spokes and, secondly, to substantially block the passage of air and pulverized material directly longitudinally of the mill past the central portions of the rotors. a I

it should be noted, thatthe corner portions of these paddles 24 which are adjacent the curved portion 2| of the liner, are correspondingly curved, as indicated at 21, so that a full bed of material is provided therebetween and ehlcient grinding and stirring therealong as well as along the circumferential wall is obtained without undue wear of the paddles or liner. The portion 28 of the baflle [1, between the passage i andr the portion 2. of the rubber liner slopes axially in wardly toward the mid-portion of the chamber 4 and the paddles are beveled complementary.

thereto. As a result any material forced and thrown into the path of the paddles by the resiliency of the rubber, does not escape but, by the slopingwalllB is deflected into overhanging relation to the paddles. Since at this radial position, the centrifugal force is in a direction tending to throw the material radially outwardly, all such material is dropped off of the sloping wall into the path of travel of the paddles and across the air stream. This effects additional stirring of the material and also separation .of the finer particles.

To effect more efliclent grinding of the material, as will later .be described, the paddles 24 of one rotor are staggered with relation to those of the other, for example, the paddles and spokes of the rotor I! are preferably offset through an angle of circu-nferentially from those of the rotor II.

It is apparent that when material to be ground is fed into the chamber 4' and the rotors are revolving at high speed, all such material is thrown to the circumference of the chamber 4 and is forced outwardly against the liner I9 by centrifugal force of the material traveling in front of the working faces of the paddles and by centrifugal force of additional material fed into the mill, and temporarily disposed between the spokes. Due to the resiliency of the liner H, as the coarser particles are thrown out under centrifugal force and forced outwardly, the liner depresses a suflicient amount to pocket partially the particles of material lying directly thereagainst. The greater the speed of the rotors, the more effectively are the particles pocketed. This action retards the circumferential travel of the outer stratum of material. The inner or overlying strata, not being impeded in this manner and more directl nder the influence of the paddles, is moved far more rapidly circumferentially than the outer stratum and thus is rubbed against the outer stratum under the high pressure resulting from centrifugal force, and efficient mutual attrition of the com- I mingling surface particles of the strata results. Again, since the inner stratum is moved circumferentially by the rotors and thrown outwardly in this manner by centrifugal force and since the 7 pressure under the ends of the paddles is somewhat relieved by the rubber liner, the wear on the paddles is greatly reduced as they themselves do not directly rub either stratum of material under high pressure but only propel one strata along the other by a direct push of their leading faces. Thus all material fed into the primary grinding chamber passes outwardly toward the liner II and therealong toward the discharge end of the chamber. Material that becomes finely enough ground to float in the air stream passing through the chamber is carried out and the coarser material remaining in the chamber is subjected to additional grinding.

Referring next to the side wall 20, since the material is fed into the chamber at the opposite end therefrom, the material entering tends to progress the material already in the chamber toward the wall 20 and force it bodily along the wall toward the axis of the mill and thus allow it of material is greatly reduced, the material at traveling along the liner portion II" and not tend to pack at this portion and creep up the liner toward the axis of the mill they are continuously forced by .the liner back out into the path the paddles. For example, they are pressed firmly ainst the liner and compress it during the passage of a paddle past any portion but immediately the paddle has passed such compressed portion of the liner, this portion again assumes a normal uncompressed condition and forces the material back out into the path oi the paddles. preventing it from packing and lying along the wall 20. If sumcient material, by any chance, remains against the wall 20, it is ground by the paddles 26. Due to the rounded portion 2! of the liner and the rounded portion 21 of the paddles, and due to the fact that the paddles are spaced a somewhat greater distance from the liner l9 than from the wall portidns 20 and 2|, the material may travel across the mill up-the wall 20 a sumcient amount to be again thrown out into the path of .the rotors. Thus the material is not only prevented from entering the opening before it is fully ground but it is also continuously stirred up and dropped across the air stream so that any line particles which have become entrapped are again released and can be caught in the air stream and carried out, while the coarser particles are again passed to the grinding strata.

This same stirring effect. but in a lesser degree, is provided along the circumferential wall by the liner is so that a packed bed of line ma-- terials'such as would become packed against a rigid back wall never results. Instead, the outer stratais always sufliciently loose to permit the finer material to be carried'out by the air stream passing throughthe mill. As stated, the air must pass along and over the material strata on the liner IS'and wall 20, and the stirring effects along both of these walls causes the fine particles oi material to be exposed more freely to the air stream at all times so that they may enter and be transported by this stream substantially concurrently upon'their reduction toa 'su mclent fineness to float therein. -Consequently, ground material-which has already been reduced to the required fineness is not additionally subjected to the eilect of the rotors nor additionally pushed around the interior of the Several effects result from this, especially in combination with the control'vanes and arrangement of paddles, latert be described, namely; a saving in power inasmuch as already ground material is notad' dition'ally stirred or moved about} the mutual attrition of the larger particles is not reduced due to a protective covering of already fine materials; the power requirements are less; and the relative capacity of themill increased,

Next'it should be noted that the paddles 24 are disposed'on a bias to a plane through the axis of their rotor, and, of course, to their paths of travel, both sets of paddles having their outermost edges disposed in slightly leading relation to their adjacent or inner edges. For example, both may be disposed at an angle of- 5 to a plane .throughthe axis of the shaft ill. Consequently, any material'entering the grinding chamber dis forced by" the paddles of'the rotor ll toward the center of the chamber and into the path of the paddles of the rotor 12, and, liketwise, any ma terial near the discharge end of the chamber, which passes into the path of the paddles of rotor I2 is forced back into the path of the paddles of rotor l I. Thus the material is moved along a sinuous path of travel, being additionally stirred and moved to and fro transversely of its circumferential path of travel concurrently with its circumferential travel. The adjacent edges of the paddles of the two sets preferably-overlap a set toward the other set and partially into the path theerof. When the paddles are offset circumferentially, ample time is permitted for unimpeded movement of the material to and fro in this manner. This results in additional stirringoi the material so that the fines may readily be freed and caught in the air stream and carried out.

In order to feed materials into the chamber 4, a chute 3| is provided. The chute extends from the outside of the mill through the outer and inner end walls of the air inlethousing 2 on a bias across the interior thereof and is uncommunlcated therewith. The inner or discharge end 33 of the chute opens into the primary grinding chamber below and offset from the axis of the chamber and spaced from the opening l6, as better illustrated in Figs. 2 and 3. Thus the material from the chute passes into the chamber 4 and is'discharged thereinto inwardly of the rotors from the paddles so as to be caught and thrown radially outwardly by the paddles at high velocity toward the circumference of the chamber. The batlle wall i 5 of the inlet housing, forming also the wall of the grinding chamber 4, is provided with an inturned flange 34 about the opening 16 protruding into the chamber 4 to prevent fractured materials from bounding out. through the opening i6 and into the air,

stream entering the opening id.

The material to be ground is fed into the chute 31- from a suitable hopper 35"which is detachably connected to the inlet chute 3| through the me-. dium of companion flanges.

In present attrition mills, the material to be ground and the air are introduced through the same passage. Consequently, the air entering the mill is varied in inverse relation to the feed of material by virtue of the material blocking the air inlet passage to a greater or less extent. as theamount of material fed is varied. If the mill is to be effective and the fineness of grind accurately controlled throughout a wide range of capaclty and fineness, some means must be pro vided to control the'rate of feed of material and the air independently, For example, if it is dethrough, there is provided in the present in-.

stance, at the point of connection between the hopper and inlet chute, a throat 36 having an annular flange 35a adapted to be received between the companion flanges of the hopper and I chute so that it may be readily installed. The

throat 36 is in the form of a frustro-conical shell having its smaller base disposed downwardly, the smaller base being of less diameter than the chute 3! so that as the material passes the throat and enters the lower portion of the chute it is loose and will pass readily into the grinding I chamber without danger of becoming packed in the chute. By inserting difierent sizes of throats,

36, the passage of material from the hopper into the mill can be effected without admission of any substantial amount of air with the material withinthe range at which the mill is to operate,

as the lower base of the throat is proportioned so as to be substantially filled by the amount of material normally being fed. Air is accordingly admitted through'a separate inlet, uncommonlcated with the chute 3| and adjustable independently thereof, as will later be described.

Referring next to the secondary grinding chamber 5, this chamber corresponds to the primary chamber 4. In the form shown, however, only one set of paddles is provided. These paddles 40 are in more closely spaced relation to the circumferential wall 4| of the grinding chamber inasmuch as only a small portion of the material fed into the mill is ground in the secondary chamber, a large portion of the material passing out of the primary chamber as "fines" of suflicient fineness to require no further grinding in the secondary chamber. If desired, however, the secondary chamber can be so arranged so that a still-finer grind of all the material is effected.

In order to maintain a flow of air through the mill for carrying out the finely ground and comminuted particles, the fan I4 is provided at the discharge end of the mill. Thus all the material passing through the grinding chamber is drawn into the fan chamber and by the fan is discharged at relatively high velocity to the points of application; for example, in the-case of fuel, to a suitable burner, or in the case of pigments and the like to a suitable air flotation separator.

In order to more accurately control the fineness of grind throughout a wide range of rates of feed, there is provided intermediate the primary and secondary chambers, preferably within and adjacent the inlet passage 5 of the secondary chamber, a centrifugal separator 43 which is better illustrated in Fig. 6. This separator comprises a plurality of circumferentially spaced radial baflles 44 secured onto the shaft in and rotatable therewith at shaft speed. If extreme fineness is required, the space between the baflles maybe provided with screening of the desired mesh, as indicated at 45. Obviously, all materials carried in the air stream from the primary chamber are discharged into the centrifugal separator 43.

The baflles of the separator are comparatively wide axially of the mill and are arranged so closely together, that, at the usual operating speed of the mill, the speed of travel of the coarser material axially of the mill is not sumcient to carry it the width of the baffle 44 during the passage of two successive adjacent baiiles. Consequently, such material is struck by the bailles and thrown at high velocity outwardly to the circumference of the secondary grinding chamber where any coarser particles are ground before passing through the mill. Thebaflles 44 terminate radially close to the inner end of the paddles 40 so that the fine materials and air may pass across the secondary chamber within the radial limits of the paddles 40. Further, the separator acts as a fan for forcing the air from the passage 5 over the circumferential bed of attrition.

As previously described, at the discharge ends of each of the primary and, secondary grinding chambers-in the passages 5 and I, are mounted the directional air control vanes or damper means for the respective chambers. Referring particularly to the control vanes between the primary and secondary chambers, these are shown more clearly in Fig. 5. The passage 5 between .the primary and secondary grinding chambers is defined by an annular housing or frame 41 coaxial with the shaft and provided with inwardly extending radial spider arms 48. Carried on the inner ends of the arms 48 is a collar 48 coaxial with the shaft Ill and in slightly spaced relation thereto. Mounted at their inner ends in the collar 49 and extending radially through the housing 41 are a plurality of rock shafts 50a on each of which is carried one of the vanes 50. The

I outer ends of theshaft 50a are bent over and received in suitable notches in an operating ring 5| rotatably mounted on the outside of the housing 41. Thus upon rotation of the ring 5|, the vanes 50 are swung to different angles relative to an axial path of travel of the air through the housing 41.

The vanes 50 are arranged so that they may swing from a substantially closed position to the fully open position illustrated. It should be noted that the vanes 50 extend from their shafts 50a at an angle to a plane through the axis of the passage 5 and are sloped from their shafts toward the rotor in the direction of travel of the rotor. For example, referring to Figs. 4 and 5 and assuming the; rotor to be rotating in the direction indicated by the arrow A, the vanes 5 are disposed at an angle such that the air stream passing them must turn abruptly and reverse its direction as indicated by the arrows B. Thus any material passing from the grinding chamber cannot pass directly through the passage 6 but must make an abrupt turn in direction to pass around the vanes. Consequently, any particles traveling due only to mechanically imparted velocity or impact received within the grinding chamber are stopped and deflected back by striking of such particles against the vanes 50, whereupon such particles drop back into the grinding bed. However, any material sufllciently fine to float in the air readily turns abruptly and passes around the vanes.

It is desirable that the vanes 50 be adjusted -from the exterior of the mill and for this purpose an operating lever 54 is provided, this lever being connected to the operating ring 5| for rotating the same consequent upon axial movement of the lever. The lever extends through a suitable opening in the casing and is mounted in a suitable housing 55 which is in sealed relation to the housing of the mill about the opening therein. The outer end of the lever 54 is threaded as indicated at 55. Secured to the outer end of the housing 55 and rotatable relative thereto is a hand wheel 51 internally threaded complementary to thethreads 55 so that upon rotation of the hand wheel the lever 54 is moved axially inwardly and outwardly of the mill. The inner end of the lever is pivotally connected to the ring 5| in a manner such that, in intermediate position of the vanes 50, the lever is to the radius of the ring through the point of connection. Thus the greatest possible movement of the ring for a given movement of the lever 54 is eifected. The vanes 10 inthe passage 1 between the secondary grinding chamber 5 and the exhaust fan housing are similarly arranged and are separately controlled independently of the vanes 50 by a correspondinghand wheel. a

In the grinding of materials it is quite often desirable to use preheated air so as to more effectively dry the materials and at the same time eliminate undue moisture in the mill which would deleteriously effect the grinding operation and reduce the efllciencythereof. For this purpose the inlet end 55 of the air inlet housing may be purposes of illustration, this damper is shown as operated to the desired-positions by a lever 60 which, in turn, is operated by suitable gear and spring device. While the lever 60 can be set directly by hand, it is shown as driven by a cam Sl/through a gear train from a rotatable shaft 6 so that the amount-of air admitted can be proportioned automatically in accordance with the rate of feed of the materials to the mill as is more fully set forth hereinafter and in my copending application, Ser. No. 44,466, filed October 10, 1935, which became Patent No. 2,176,824.

Thus invoperation, the amount of air fed into the primary grinding chamber and also its resultant velocity, direction, and flow characteristics through both primary and secondary chambers can be accurately controlled and the fineness of grind regulated.

The mill is preferably operated at constant speed,'all'regulation of fineness of grinding being effected by control of :the air passing through the various chambers by the damper 59, and the directional vanes.

The setting of the vanes 50 relative to the vanes It is preferably such that the efiective size of the passage 6 is less than the efiective size of the passage. l and is equal to or greater than the greatest passage past the inlet valve 59. Consequently, bnce ,the vanes 58 and 16 are set in the desired adjusted position, the fiowof air through the mill is directly under the control in a more nearly closed position than the vanes it, the velocity of air passing the vanes 56 is increased, due to the Venturl effect and a high velocity discharge stream and reduced atmospheric of the damper 59. By maintaining the vanesfidpressure existing in the passage 8 and at its inlet opening. Thusany material that is actually floating in-ithe air at this opening is quickly transported from the primary grinding chamher. Since this passage discharges into the chamber 5 which is comparatively large, the velocity of the stream is again reduced. However, the particles which are of such fineness as to floalt in air will pass on with the stream regardless ofthis reduction in velocity. Due to the slightly higher velocity past the vanes 50, coarse particles maybe drawn into the chamber 5 but these are separated by the centrifugal separator 63 or settle out due .to the decrease in the velocity of the air stream in the chamber 5. As

stated the vanes Til are kept at a more open po-.

sition and their capacity is equal to or greater than that of the inlet valve. Consequently, only a slow fiow of air therethrough results. Thus in the-chamber t, the fines are drawn out immediately upon separation from the attrition bed even at the expense of drawing through some slightly coarser material. This greatly increases the emciency of the chamber t, and, in the chamher 5, such fines as'are floating in the air may pass on through the chamber 5 at slower velocity without additional grinding or interference with the grinding therein, but any such coarser particles carried into .the chamber are separated and additionally pulverized.

- Again, by means of the control vanes, the cur- 5 rents of air through the chambers 4 and 5 can be so controlled that the concurrent. increase of air and material supplied to the mill does not cause undue variation in fineness. Since the mill is originally designed for a given fineness while working at capacity, depending on the particu-- lar use to which it is to be put, the same fineness for lesser rates of feed of material can readily be attained.

In order that the mill may be assembled and disassembled readily for the installation or removal of any parts, the form illustrated has been found highly practicable. In' this form the air inlet housing 2 with its inner wall l5 preferably comprises an integral casting, which is tapped at its outer end to receive the bearing assembly for the shaft Ill. The body 1 comprises a cylinder of constant diameter within andhaving suitable external flanges 651st each end. adapted to engage complementary flanges on' the air inlet housing and exhaust fan housing respectively, these housings being secured to the housing I by suitable bolts in cooperation with the flanges 85. The exhaust fan housing 3 is likewise a sinthe inner end wall of the exhaust fan housing.

Assuming the mill is disassembled except that the exhaust fan housing is secured in place and the exhaust'fan and shaft ill installed, the plate 67 can readilybe inserted from the opposite end of the mill into place.

Next the secondary grinding chamber is installed. This comprises an annular plate 69 of L-shaped cross section, the circumferential portion'of'which snugly engages the inner -wall of the chamber i and the radial wall extends inwardly from the circumference and fits snugly against the inner wall of the frame 88, complementary shoulders being provided on the frame 68 and the plate as. Obviously, this can readily be inserted from the inlet end of the mill. Next the secondary grinding rotor l3 and centrifugal separator 63 are inserted and secured firmly on the shaft. The plate N forms the innermost end wall of the secondary grinding chamber and carries the frame 41. The primary grinding cham- I her is formed in a similar manner to the second,- ary and likewise correspondingly arranged with relation to the vane assembly 50. -With these two in place, the primary rotors l l and 12 may be inserted and secured firmly on the shaft by suitable keys or set screws. The air inlet housing is next secured to the flanges 65, its inner wall l5 holding in place the bafile l! and plate H. The bearing assembly 8 is next inserted in the air inlet housing and on the shaft and se-. cured in place by suitable bolts thus securing the shaft in place. "Thus each of the elements defining the compartments, chambers and housings are held in fixed position by mutual abutment and all may be readily removed from so arranged that they may be removed bodily and are likewise protected against the infiltration of dust or any foreign matter thereinto. Referring next to the correlation between the rate of feed of material and admission of air, the material to be ground may be fed to the mill hopper 35 by means of a magnetic feed chain and separator more fully set forth in my Patent No. 2,092,025 granted September 'I, 1937, which is operated in definite relation to the air inlet damper 59, through the cam and gearing above mentioned.

Briefly, the magnetic feed chain and separator 'is arranged to discharge continuously into a hopper 35 which, in turn, communicates with the feed chute 3| of the mill. In general, this mechanism comprises a pairof transversely spaced parallel chains arranged to travel horizontally above the open inlet end of the hopper 35. As more clearly illustrated in Fig. 1, each chain has laterally extending prongs 11 which extend toward the opposite chain but terminate in spaced relation thereto, the prongs of one chain preferably being alternated along the path of travel with respect to those of the opposite chain. These chains are driven at the same speed so as to maintain a fixed relation between the prongs 11.

Along one portion of the path of travel of the prongs, the prongs pass beneath the outlet of a fuel hopper 18 which is kept filled with fuel which is fed between the prongs of the chain as the chain moves therebeneath. Beneath this portion of the path of the prongs is a dead plate 19, the dead plate extending from beneath the feed hopper I8 at least as far as the near edge of the hopper so that material will be retained in position to be progressed along this portion of the path by the prongs. Along that portion of the path of travel coextensive withthehopper 35 are longitudinally extending tracks 80 engaging the under surface of the prongs 11 so as to define therewith a checker grill, the tracks 80 preferably being of a material which is not conductive of magnetic flux. Thus material passing out of the hopper 18 between the prongs I1 is moved thereby along the dead plate and tracks 80. When over the tracks 80 and hopper 35, .all material of the proper size could normally drop through the resultant grill into the hopper 35.

Along the portion of the path of travel above the hopper 35 the prongs are magnetized so that any tramp or other iron, regardless of size, will be retained on the chain while the fuel may pass into the hopper 35. For accomplishing this purpose, a plurality of electromagnetic coils are provided, these coils being so arranged that one chain and its prongs are responsive only to the north pole of the electromagnetic coils and the other chain and its prongs are responsive only to the south pole. Consequently, alternateones of the prongs 11 are of opposite polarity. The coils 82 are so arranged as to maintain this magnetic field along that portion of the path of travel above the hopper 35 and for a sufficient distance therebeyond to insure that all iron magnetically retained on the prongs will be carried beyond the remote edge of the hopper 35. Since the prongs are no longer magnetized beyond this point, all such iron and material may drop. off.

The magnetic chains may be driven by suitable sprockets 84 carried on a common shaft which, in turn, is driven by a pulley 55. The pulley 55, in turn is driven through the pulleys 85, 81, 85

and it in the order enumerated. Thepul ey 89 may be controlled by the reversible pilot immediately connecting type having a variable speed shaft 90 and a constant speed shaft 9|. The variation in the rate of drive of the shaft 90 is effected by movement of the pulley elements 92 toward and away from each other so that the belt 93 rides on the pulley elements 92 at different distances from the axis, depending upon the distance of the pulley elements from each other.

In order to move the pulley elements to the desired positions for the speed required, a hand wheel 94 may be utilized but, since it is generally desirable to control the perature changes, boiler pressures and the like, the pulley elements 92 are also operable toward and away from each other by a pilot motor 95 which may be responsive to a suitable control switch which operates in accordance with the boiler pressure, room temperature etc., as desired,

in a well known manner.

As previously set forth, it is desirable that the volume of air admitted to the mill be controlled in a direct proportional relation to the rate of feed of the fuel and consequently the damper 59 motor 95. This control is effected by maintaining a desired geared relation between the rotatable shaft 55 of the pilot motor 95 and the shaft 62 which operates the damper, operating lever-50, in turn, through the medium of a cam 91 and toggle links, as illustrated in Fig, 2.

The change speed gearing may be driven from anysuitable source of power, for example, from the motor M which is of sufllcient power todrive the feed mechanism for all mill capacities.

Since the mill is often used in connection with a fuel burningsystem, it is necessary to supply an auxiliary means for utilizing oil in case of the exhaustion of the supply of coal or for some other reason and for this purpose, a hand operated clutch 99 is provided for disassociating the particular chainfeed and driving mechanism. A similar hand operated clutch I00 is utilized for the change speed mechanism to a suitable oil pump lOl so that the changeover can be made without any break in operation. Since it is also desirable to vary the amount of secondary air supplied to thefuel burners in proportion to the variations in the rate of feed of the fuel, anoperating lever I02 may be provided, this being connected through a suitable link arrangement to the lever 50 so that the secondary air damper (not shown) and damper 59 may be operated ina definite relation to each other.

While I have described briefly the feed mechanism for use in connection with the present mill and means for maintaining the desired relation between the rate of feed and grinding, the admission of air to the mill, and of secondary air to burners, in case of pulverized fuel, the specific structure for these purposes herein briefly described is more fully described and claimed in my Patent No. 2,092,025 granted September 7, 1937.

Having thus described my invention, I claim: 1. In an attrition grinding mill comprising a grinding chamber having a passage for admitting speed in response to temthe outer radially extending edge portions of each being in closely spaced relation to the side wall,

the interior faces of said circumferential wall and side wallbeing resilient rubber and joined by a gradually curved rubber wall portion..

2. In an attrition mill comprising a grinding chamber having side walls and an imperforate circumferential wall, means to feed materials into the interior of the chamber, a rotary grinding mean within said chamber, one of said side walls having a passage therethrough, means to maintain a flow of air through said passage and chamber, and djustable directional vanes cooperable with the passage to vary the volume and direction of the air stream passing through said chamber.

3. In an attrition mill comprising a grinding chamber having side walls and an imperforate circumferential wall, means for feeding materials into the interior of the chamber, arotary grinding means within said chamber, one of said side walls having an air inlet passage and the other having an air discharge passage coaxial with the chamber, means to maintain a stream of air through said chamber and out through the discharge passage, and a plurality of sets of radial vanes cooperable with the passages respectively, vanes of each set being rotatable in'unison with others of the same set to different adjusted positions about their respective radially extending axes to vary the direction of the air stream passing through their associated passage, and means disposed exteriorly of the chamber and operatively connect- -ed to the sets of vanes respectively for adjusting the position of the vanes about their respective axes.

4. In an attrition, mill comprising a grinding chamber having side walls and an imperforate circumferential wall therebetween, means for feeding materials into the chamber, a rotary grinding means within the chamber, one of said side walls having an air intake passage'and the other wall having an air discharge passage, one of said passages being coaxial with the chamber, means to maintain a stream of air through said chamber from the inlet passage, a plurality of vanes cooperable'with one of said coaxial passages, means mounting said vanes for rotation to difierent adjusted positions about respective axes extending substantially radially of the passage with which the vanes are associated, and means operatively connected to the vanes for rotating the same about the respective axes to diiferent adjusted positions.

5. In an attrition grinding mill, a cylindrical casing, a coaxial fan housing secured to one end thereof and removable axially therefrom, a coaxial air inlet housing secured to the opposite end of the casing andremovable axially therefrom, said air inlet housing having an annular radially extending inner end wall, coaxial bearings inthe housings, a shaft rotatably mounted in said bearings, a, pulverizing rotor on said shaft, an annular sleeve insertable axially in the casing from one end of the casing and snugly received'coaxially in the casing with one end abutting the annular end wall of the air inlet housing, an annular radial flange at the opposite end of the sleeve extending radially inwardly from-the circumferential wall thereof, said sleeve walls andair inlet wall providing a grinding trough, and means receivable axially in the casing and abutting the flange end of the sleeve at one and and abutting the fan housing at the other end iotholding'the sleeve in said position axially of the casing, whereby all parts of the mill may be removed readily axially from the casing.

6. In an attrition grinding mill comprising a grinding chamber having a circumferentially closed wall and a passage for admission of madirection to urge the material being ground toward and against said resilient side wall.

'7. An attrition grinding mill, comprising circumferentially closed communicating primary and secondary grinding chambers, means for feeding materials to be ground into the primary chamber, grinding rotors mounted within the chambers, respectively, means .to maintain a flow of air progressively through'the chambers, and an adjustable set of circumferentially spaced and radially disposed vanes which are tiltable about radially extending axes, said vanes being disposed intermediate said chambers and operable to vary the air currents in the chambers when adjusted to different predetermined positions.

8. In an attrition grinding mill comprising a grinding chamber having a circumferentially closed wall, means for admitting loose material to be ground into said chamber, a rotor within the chamber, and a plurality of circumferentiaily spaced paddles on said rotor, each paddle having a blade portion which is narrow circumferentially of the chamber and has an outwardly disposed end surface which extends generally longitudinally of the rotor axis and which is narrow in relation to the circumferential spacing of said end surfaces, means overlying said circumferentially closed wall and exposed toward the said end surfaces with uniform radial clearance therefrom continuously around the circumference of the said means, said means being sufficiently resilient to be compressed by centrifugal force of the material pushed by the paddles and thereby to partially pocket temporarily particles of the outer stratum of said material when the rotor is turned at high speed and to return toward normal condition, at the instant after' passage thereover of a paddle and the material being pushed thereby, for loosening the outer stratum of the bed of material disposed on said means outwardly beyond the radial extremities of the paddles and thereby freeing fine particles, said.

end surfaces being spaced closely to the said means to limit the depth of said outer stratum of material so that said stratum can be loosened for itsfull depth by said return of each portion of said means immediately after passage of each paddle thereover, and said paddles being widely spaced circumferentially to such extent that material being pushed by a padlle is spaced to the rear of the immediately preceding paddle so as to provide, at the rear of each paddle, a space of substantial circumferential extent and which is free of pushed material, whereby the return of the said means is unimpeded radially by material being pushed by-the next succeeding paddle.

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