US 2376747 A
Description (OCR text may contain errors)
May 22, 1945. NH. ANDREWS 2,376,747
PULVERIZER Filed Sept. 7, 1942 Z-Sheets-Sheet 1 Wm. 1/ INVENTOR. A/orwaod ll. Andrews A TTOILA/EY May 22, 1945. N. H. ANDREWS 2,376,747
PULVERIZER Filed Sept 7, 1942 2 Sheets-Sheet 2 Fig.2
7 INVENTOR. A/orwaoo/ H Andrews Alum.
Armu/z 5 Patented M 2, 1945 PULVERIZER Norwood H. Andrews, Moorestown, N. J., assignor to International Pulverizing Corporation, Moorestown, N. .L, a corporation of New Jersey Application September 7, 1942, Serial No. 457,536
This invention relates to apparatus for the particle size reduction of pulverulent substances. It is more particularly concerned with an improvement in apparatus of the type described in United States Letters Patent, 2,032,827 issued to me March 3, 1936.
The apparatus described in my prior patent includes an annular pulverizing chamber, jet means to supply a gas under superatmospheric pressure into said pulverizing chamber, a centrally disposed outlet from said .chamber, and means for introducing material to be ground into said chamber. The gas is introduced under pressure into said chamber in such manner as to create a re-entrant circulatory path forsolid particles larger than predetermined sized ranges. As hereinafter used, the term re-entrant circulatory path isemployed to describe the flow of solid particles travelling in an inclosed zone or chamber in which solid particles above predetermined size ranges are retained in said zone by tion of the charge within the chamber, the larger centrifugal'force and are subjected to the action of high velocity jets of gas and in which the solid particles below predetermined size ranges may be continuously exhausted from the zone by entrainment in gas. leaving the same. Reference is made tothe apparatus and method described in the above-identified patent, and more particularly to the Figures 1 and 2 thereof for further understanding of the 'type of apparatus on which the present invention is an improvement.
The present invention is concerned with apparatus for pulverizing solid materials by centri'i ugal fluid jet action, and more particularly to pulverizing apparatus wherein jets of gaseous fluid are introduced at high velocity into a moreor-less annular grinding zone in an outer portion of a treatment chamber, preferably of circular cross-section, said, grinding zone containing material to be pulverized in the form of discrete small pieces. The high velocity fluid jets, or at least most of them, are introduced in directions inclined forwardly in the direction of rotation along said grinding zone so as to maintain the fluid in rapid circulation in the direction of such forward inclinations, either clockwise or counterclockwise. The grinding zone is confined by peripheral portions of the chamber walls encloslng said zone and at both axial ends, in order to concentrate the discrete solid particles of material in the vicinity of the high velocity fluid jets, as such discrete particles of materials are centrifugated by the rapid fluid circulation in the grinding chamber, so as to promote efiective grinding.
particles are caused to concentrate in the grinding Zone where they come repeatedly under the influence of the introduced high velocity jets and are thus repeatedly subjected to the size reducing action produced thereby, while material which is sufllciently reduced in size is entrained by the gaseous fluid as it spirals inwardly toward the inwardly disposed fluid outlet. 4
It is a particular object of the present invention to increase the efliciency 0f centrifugal fluid jet pulverizing apparatus of the type hereinbefore described, thereby to increase its capacity. Another object of the invention is to provide apparatus capable-of yielding a product of more hereinbefore mentioned whereby the effective loading ofthe fluid jets therein is increased while decreasing the total quantity of discrete solid particlesin the apparatus as a circulating load.
It is also an object of the present invention to permit increased loading of jets while supplying a given amount of energy in the grinding fluid, in comparison with prior centrifugal fluid.
jet pulverizing means.
A particular object of the invention is to provide an improvement in centrifugal fluid jet pul- I verizers of the type hereinbefore mentioned whereby relatively we materials can be fed to the apparatus and concurrently pulverized anddried.- V
A principal object of'the invention is to provide means for utilizing the energy of high velocity fluid jets in a very effective manner in ac-v complishing size reduction of pulverulent materials by particle impaot and attrition; to provide improved means for subjecting the material to be reduced to the influence of high velocity jets of gaseous fluid; and, to provide means for classifying and controlling the particle size of the finished product within narrower limits in centriiugal fluid let pulverizers.
In d .iet, pulverizers. the grinding enerzv utilized ro'r reduction is usually supplied to the pulverizer by expansion 01' a gaseous fluid under pressure through nozzles or oriflces of restricted cross-section. when air is utilized as the grinding fluid, a fluid pressure of the order or 100 pounds per square inch is commercially readily obtainable: while for installations utilizing steam as the fluid grinding medium, convenient pressuresmayrangeiromlOOtoSOOpoundsper square inch with varying degrees of superheat. Of course, particular grinding problems may utilize operating conditions outside the range 1" specifled, but a large proportion of commercial installations are included within the range of conditions outlined.
While the exact mechanism through which energy is transferred from the fluid medium supplying the grinding energy to the material to be reduced is not known, and may be exceedingly complex, this transfer probably takes place chiefly by impact between high velocity fluid jets issuing from the nozzles and particles of material moving at relatively low velocity within the grinding chamber. During such impact momentum is conserved, while some of the available kinetic energy is dissipated in friction and eddies and may account ior some or the size reduction obtained. The momentum imparted to the larger particles in various directions will result in shattering impacts between larger particles, between large particles and the walls of the grinding chamber and will result in attrition between the particles of material circulating in the grinding chamber.
In a gaseous fluid pulverizer. it is usually highly desirable to control the maximum particle size in the finished product. This can conveniently be accomplished by providing a classification zone radially inwardly oi. the grinding zone and by withdrawing the fluid at an inward point or the classification zone. In this manner the gaseous fluid circulating within the mill is caused to spiral inwardly and to exert a frictional drag upon particles suspended in the fluid in a radially inward direction opposed to the centrifugal force acting on the particles resulting i'rom the radial acceleratlon of their paths oi travel. the centrifugal action tending to return the particles outwardly to the grinding zone. By controlling the rate of whirling oi thegaseous fluid within the classifying zone with respect to the component of inward flow towards the centrally disposed gaseous outlet, it is possible to control the maximum particle size in the product for which the viscous drag of the fluid is just sumcient to overcome radial acceleration.
In commercial centrifugal fluid-let pulverizers, it must be realized that the volumetric extent of the grinding chamber is quite small with relation to the rate at which fluid energy is being dissipated, chiefly for grinding purposes. Furthermore, the density oi most solid materials subjected toreduction within the grinding chamber is oi the order of one thousand times the density or the gaseous fluid supplying the grinding energy. Hence, even though only a small portion or the volumetric contents of the mill may be occupied vthecirculatingloadoi discretesolid particles being reduced, such mrticles constitute a considerable load upon the whirling gaseous fluid. subtracting momentum from the whirling fluid, and dissipating the withdrawn energy in walls oi the grinding chamber and in impacts between particles,
Ordinarily, the gaseous fluid content of the classification zone will be less than one-tenth a pound, and for accurate control of classiflcation, the momentum of the gaseous fluid present in the classification zone should remain substantially constant. As oversize material reaches the classification zone and is slowed down due to triction, impact or otherwise, such material must be accelerated by the whirling fluid, and hence will reduce the momentum oi the fluid. As the oversize material will not reach the classification zone frictional contact with the stationary conflning uniformly distributed in time. the momentum oi the whirling gas will also vary in time. In order to control classification. it has heretofore been necessary to decrease the circulating load in the grinding chamber by decreasing the rate oi feed with respect to the fluid energy input, to such an extent that the minimum rate or whirling still corresponds to the desired classification. Under these conditions, it is round that the available fluid energy is not efllclently utilized in transferring momentum to particles or material in the circulating load for purposes of size reduction by impact and attrition, as suillcient material is not supplied to the high velocity fluid Jets, so that a portion or the available energy is lost in transierring momentum to the fluid circulating in the grinding chamber with consequent degrading oi the available kinetic energy into heat energy.
Figure l is a plan view of apparatus wherein the invention may be practiced.
Figure 2 is a side elevation, partly in section. of the apparatus shown in Figure l.
l lgure3isapartplansectiontakenonline lII--III of Figure 2.
Referring to Figures 1 and 2, a circular planar grinding zone III is defined by an upper plate li, a peripheral wall It, and a rotatable bottom plate It. It is generally desirable to provide a liner It on" the inner periphery oi the mill and an annular floor ring it is provided to seat the rotatable bottom plate I3. The annular floor ring ll rests upon an annular support II which is in turn carried by a vertical cylindrical shell II. A plurality of clamping members ll, integrally Joined to the annular support I. and shell I! serve tohold the grinding zone elements together. Thus. by adlusting the position of a can screw ll, bearing on anannularringfl attheouteredgeoithetop plate ii, and stud bolt II, which also bears on the annular top plate II, the pressure tending to hold together the top plate I I and other members defining zone is, may be varied.
The top plate may have a racing liner 2i, and the circular plate H and liner 2! are provided with a centrally disposed outlet pipe 23. The pipe 23 is shown ca ying a flange as for connecting the mill assembly to a suitable product collection system (not shown). The all take pipe 28 carries a sealing ring 2|. seating on the top plate ii of the grinding zone II and an annular ring as which may be drawn tightly against the member 21 ot the clamping mechanism II by means of a stud bolt II anchored in the top plate H,
The peripheral wall I: and its liner is are pierced at a plurality of points around the circumierence oi the grinding zone II to admit nozzles II from a circular header ll. Figure 3 shows in particular the arrangement oi the nozzles ll oi the peripheral wall I! or the grinding zone, tangent to a theoretical circle 45 within the zone. The header is supplied with a gas under pressure through an inlet pipe I l.
The 'rotatable bottom plate is carried at its center on a vertical shaft 32.. The upper end of the shaft 32 projects through a centrally disposed opening in the rotating bottom plate 13 and carries a flange 33 integrally joined to the shaft 32. The threaded upper end of the shaft 32 projects above said flange 33 and carries a lock washer 34 and a retaining nut 35. The annular ring 33 is fixed to the bottom plate i3 by a plurality of clrcumferentially disposed shear pins 36 which will cause the plate 13 to rotate with the turning of the shaft 32. The upper end of shaft 32 is tapered below the bottom plate 13 and a supporting hub 31 rests on the taper. The shear pins 38 extend through the bottom plate 13 into the hub 31.
The lower end of the shaft 32 carries a pulley which may be driveh from any suitable power source (not shown). The shaft 32 is journaled in roller bearings 39 and 39' just above the pulley 38.
Inasmuch as the'pressure in the grinding zone l0 may be comparatively high at its periphery, it is desirable to maintain gas pressure on the under side of the bottom plate iii, in the space between said plate l3 and the annular supporting member it. This is accomplished by supplying a gas under pressure through an inlet tube 40, around the shaft 32, providing a passageway 41 upwardly along the shaft and sealing off the shaft 32 from downward flow of gas by a split gland 42 sealed with a suitable high temperature packing 43. The cylindrical shell or casin 11 may be in sections, as shown, and serves as a supporting frame for carrying the working parts of the apparatus. A tube 44 is provided through which material may be fed into the grinding zone Ill.
The operation of the apparatus may be described as follows: A gas under pressure is introduced into a header, causing high velocity fluid jets to issue from the peripheral openings in the circular wall of the grinding chamber. The fluid jets are directed forwardly in one direction of rotation and also have an inward component. Material to be reduced is fed into the grinding chamber and, due to the action of the jets, is entrained in the rotating fluid.- The larger particles of the material are concentrated by centrifugal action in the radially outward grinding chamber. There such material is picked up by the jets and projected inwardly along their respective axes. The material is then reduced in size by impact. When the material is sufficiently reduced the fluid drag resulting from th radially inwardly component of the fluid flow as the fluid spirals inwardly will overcome the centrifugal force acting on the particles and fine particles of the material will vent from the grinding zone in the exit gas stream.
The bottom plate is rotated in the same direction as the direction of rotation of the fluid within the grinding chamber as indicated by the arrow in Figure 3. Some of the benefits of the invention may be still secured if the bottom plate is rotated in a direction opposite to the direction of rotation of the fluid in the grinding chamber, but the frictional drag on the fluid is increased.
With extremely heavy feed rates, oversize material may reach the classification zone and be slowed down by friction or impact. It then has a tendency to separate out of the fiuid due to the effect of gravity. As it comes into contact with the rotating plate, due to friction between the material and the rotating plate, the material is caused to move outwardly into the grinding zone before it can accumulate in the classifiestion zone in sufilcient quantity to cause fluctuations in the rate of whirling in the classification zone. Due to the action of rotating bottom plate in returning oversize material to the grinding zone, sufllcient material to eillciently load the high velocity jets introduced into the grinding chamber is obtained with a decreased amount of circulating load in the treatment chamber, as the circulating load iskept concentrated along the peripheral wall where it can be acted upon by the introduced high velocity jets. At the same time the classification zone is kept free from possible accumulations of oversize material which would be likely to withdraw momentum from the fluid in the classification zone at a widely varying rate, wlth a resulting variation in the classifying action.
The following example illustrates the practice of my invention but is not to be-construed as limiting the same: ANorth Carolina clay, in ap proximately 8 mesh particle size, was introduced number two drill jets were employed on the periphery of the mill set tangential to a theoretical 9 inch diameter circle within the mill. The feed was injected with steam supplied at .a pressure of 120 pounds per square inch and was fed into the grinding chamber at a substantially uniform rate at 360 pounds per hour. Steam at a temperature of approximately 660 F. was supplied to the jets under a pressure of 120 pounds per square inch, at a rate of approximately 1800 pounds per hour. The product obtained gave a particle size analysis of less than 0.35% on a 325 mesh screen and was predominantly below 15 microns in size.
By way of illustrating the increase in output obtained according to the present invention the following illustration is given: In a mill constructed substantially as described in the preceding example it was determined that 325 pounds per hour of ball clay used in the porcelain enamel industry could be processed with 340 cubic feet per minute of free air supplied at a pressure of pounds per square inch. The products had a particle size analysis of less than 0.3 percent on a 325 mesh screen and was 10-15 micron average size. In a mill similar in all other respects save the use of a rotatable bottom plate, it was found that the maximum feed rate which could be maintained with the same quantity of air at the same pressure was only 250 pounds per hour and the product showed'2.1 percent on a 325 mesh screen with a 15-18 micron average particle size.
In some instances a portion of the circulating load in the grinding chamber will be composed of fairly large particles, for instance, from 10 to 15 mesh material. With some materials, such particles may circulate for a time without being actively engaged by the high velocity fluid jets. Such particles circulate by bouncing along the outer peripheral wall, withdrawing energy from the grinding fluid and dissipating the energy with the stationary walls of the grinding chamber. Under such circumstances, it is of advantage to attach projections, such as rods. plates and other protuberances, to the rotating bottom plate. Thus, the rotation of the plate .causes the projection to engage or impact particles bouncing along the peripheralwall into the grinding chamber and return the particles outwardly more rapidly. .In 5
this manner the circulatin load. is further concentrated along the peripheral wall where it can be acted upon by the high velocity jets. Thereby the jets can be efficiently loaded with a further reduction of circulating ioadin the treatment 10 chamber.
While the rotation of the floor or bottom plate of the grinding zone yields the greater advantage. it is of benefit, when certain materials are to be pulverized, to rotate the top confining member of the pulverizing chamber casing. In some instances, it is unnecessary that the rotatable member forming a portion of the casing for confining the flowing gas stream be driven. The said member may be free to rotate, thus taking less energy 20 from the grinding fluid merely to keep the rotatable member spinning than is taken from the gas by a stationary casing member.
An advantage of the apparatus herein described arises out of the fact that a feed containing a 25 sarily be planar, but the top and bottom portions may be somewhat convex or slightly concave.
1. In a centrifugal fiuid let pulverizer, the combination of: a generally circular planar pulverizing chamber for confining a flowing gas stream; means to introduce. a gas under superatmospheric pressure into said casing at a plurality of points from the periphery thereof tangential to a theoretical circle within the casing to create a gaseous Jet pulverizing zone in the form of a re-entrant circulatory path for solid particles larger than predetermined size ranges; a rotatable member forming at least a part of the bottom of said pulmospheric pressure into said casing to create a gaseous jet pulverizing zone in the form of a reentrant circulatory path for solid particles larger than predetermined size ranges; means to introduce solid particles to be reduced in size into said gas stream; a rotatable member forming at least a portion of the confining wail of the pulverizing zone and rotating in the direction of fiow of said gas stream to assist in returning solid particles larger than predetermined size ranges to the action of the gaseous Jets; and, an ofitake from said pulverizing zone inwardly disposed with respect to said re-entrant circulatory path of solid particles.
NORWOOD H. ANDREWS.