US2507917A - Two-stage air swept ball mill - Google Patents

Description

y 1950 H. E. LONNGREN 2,507,917

TWO-STAGE AIR SWEPT BALL MILL Filed May 12, 1948 INVENTOR.

Patented May 16, 1950 NHTED STATES orrica My invention relates to pulverization of coal and like material aid has particular reference to the art comminution with the aid of loosely arranged metal balls in a horizontal rotating drum One object of my invention is to carry out the comminution of coal and like material in two distinctly separate steps in one and the same rotating drum with the aid of loosely arranged metal balls the first step of which comminution process is devoted to impact action by the metal balls and the second step to attrition. This is accomplished by partitioning of the interior of the drum into two distinct compartments in which separate ball charges are used and actuated in such a manner that impact will result in one compartment and attrition in the other. I have found that the reduction in size by means of metal balls is greatly expedited by imparting a great inertia to each ball. It is well known that the impact is a function oi the inertia. Crushing of the incoming coal requires impact whereas grinding to a powder depends upon friction between the balls. It is therefore advantageous to separate the two comminution steps entirely so as to obtain the best results from each action by the balls. In this manner the metal balls will last much longer because attrition does not take place in the entrance zone where the incoming coal particle size is such that attrition is of no avail.

Another object of my invention is to effect acceleration of the metal balls and the conversion of the imparted kinetic energy into useful comminution work in a controllable manner in each comminution step. For this purpose I provide means within the rotating drum whereby every ball is forced to participate in the process in like manner, that is, to pound the coal in the first comminution step and to grind it in the second step. In both steps the balls are accelerated in like manner by being caused to travel momentarily with the peripheral velocity of the rotating drum. In the first step every ball is free to continue with the given velocity until it strikes the'coal mass from above whereas in the second step its kinetic energy is given up to a mass of balls in its path which is thereby stirred.

Still another object of my invention is to eiiect reduction in size of coal and like material in such a manner that an eventual wear of the metal balls or of the drum lining can not hamper the comminution steps. This is accomplished by providing a perfectly smooth lining against .such ribs are used in my apparatus.

erator member which is pivoted so as to adjust itself as wear sets in.

A further object of my invention is to provide a pulverizing apparatus of the horizontal rotating drum type in which a high coal level (above the centerline of the drum) can not hamper the ability of the apparatus to deliver an acceptable product. This is accomplished by providing a discharge opening which is positioned high above the centerline of the drum so as to prevent the coal mass from clogging it.

A still further object of my invention is to expend the least possible amount of energy -for the comminution of the material at hand. This is accomplished by actuating the balls in such a manner that their resistance to acceleration is the least, that is, by permitting them to roll between two smooth surfaces while kinetic energy is imparted to them.

A specific object of my invention is to eifect withdrawal of ground material from the pulverizing apparatus by means of a mass-flow of air so directed and disposed within the drum proper that a high velocity may be used without withdrawing particles of an undesirable size. For this purpose I introduce carrying air in a zone high above the coal level in the first comminution zone thru which it is permitted to pass in the longitudinal direction at a high velocity whereupon it is caused to enter the second comminution zone in the centripetal direction and to leave this zone in the centrifugal direction. This reversal of the flow direction insures a proper precipitation of coarse particles in the second comminution zone. Only finely ground particles can be accelerated in the centrifugal direction because the gravitational acceleration must first be overcome.

Insofar as I am aware no prior disclosure of horizontal rotating ball mills anticipates the real substance of my invention which, as stated, 1'89 lates primarily to a method of effecting comminution of coal and like material in two distinctly separate comminution zones within the realm of one and the same rotating ball mill by imparting kinetic energy to each ball in like manner in the two zones and by converting this energy into full impact in one zone and to attrition in the other zone. Present types of horizontal rotating ball mills require longitudinal ribs or ridges which actuate the ball charge. No Present types of machines cannot function properly when the coal level is above the center line of the ma- Whieh the balls are pressed by a smooth accelchine because the withdrawal opening is thereby aco-1,01:

reduced and mass-ilowcarries too coarse particles out from the machine. This limitation is not present in my apparatus because thedischarge passageway is positioned high above the center line. The starting torque is also high in present machines of the'rotating drum type because the entire ball charge must be actuated at.

to the acceleration. In present machines of the type in question it is in addition necessary to introduce carrying or floating air together with the coal feed because a large flow area is required in order to keep the velocity down. This in turn necessitates the use of large and costly support bearings. In my machine this condition does not exist and the air and coal are introduced sepflately with the result that smaller bearings serve the purpose. These bearings do not support the entire drum and internal charge load because I transmit a considerable portion of the charge load to an internal structure-hereinafter fully describedwhich -is separately supported bymeans of stationary plategirders.

My invention is fully disclosed in this specification and claims reference being had to the accompanying drawing.

In the accompanying drawing Fig. I represents a sectional elevation of an apparatus constructed in accordance with my invention and comprises essentially an elongated horizontal drum or cylinder mounted with trunnions in support bearings in a rotatable manner, driving mechanism for the drum, a specially shaped internal structure which divides the interior of the drum into two compartments or comminution zones, support girders for the internal structure, loosely arranged metal ball charges in each compartment in the drum, specially shaped accelerator members pivotaliy mounted on the internal structure, means to feed coal into the apparatus, means to introduce carrying air into the apparatusand means to withdraw a final product from the apparatus.

'- l'ig. II represents a cross-sectional view taken along line II-JI in Fig. I.

Fig. III represents a cross-sectional view taken along line III-III in Fig. I.

These features and others will more fully and at large appear from the following detailed description.

"Referring in detail to Fig. I, a feed of precrushed coal is supplied thru hopper I at the bottom of which a belt conveyor 4 picks it up and transports it into the apparatus proper wherein it first is subjected to a crushing to minute particles in a crushing zone 6 and to pulverization to afine powder in a pulverization zone I. Amotor I with conventional gears and drive shafts operates the belt conveyor. The comminution is effected by means of a multiplicity of round metal balls I in each zone as each ball therein is caused to travel with a velocity equal to the peripheral velocity of the rotating drum 9 and the imparted kinetic energy is converted into useful comminution work. In the crushing zone the imparted kinetic energy of each ball is converted into imiii balls in its path. From that in the crushing zone continues to travel freely with the imparted velocityuntilitstrikesthecoalbedinitspath. whereas in the pulverization zone as shown in I'll. III each accelerated ball loses its inertia upon c0ntactingtheballmassinitspath. Thetwolones I and I are formed by a stationary partitioning plate II which is held in place by an internal stationary structure which. comprises an air inlet portion ll, shafts l2, II, a withdrawal portion l3 and tie bars It. The entire structure is supported independently at the end sleeves II and It by support plate girders i1, I! in such a manner that the drum I can freely rotate around said structure. Sleeves I I and II are mounted concentrically'with drum trunnions It and it so as to permit free rotation in bearings II. A conventional motor driven gear drive 2| rotates the drum. Leakage is prevented by packing glands 22, 22. The inner envelope of the drum shell is provided with a renewable wear resistant lining 23 the inner surface of which is perfectly smooth. Acceleration of the metal balls is effected by means of special accelerator members 24, 24 pivotally mounted on shafts i2, I: in such a manner that their dead weight furnishes sui'iicient grip or friction to cause each ball to travel with the rotating drum. Minute particles of coal are conveyed thru the apparatus in entrainment with air which is introduced thru a multiplicity of apertures 28 in end portion ll of the stationary internal structure. A segmental passageway It connects the crushing zone with the pulveriza tion zone. Baiile plate 21 in the pulverization zone directs the mass-flow in the centripetal direction which causes minute particles to precipitate readily. The final ground product is carried away by the air mass-flow which sweeps around the baiiie plate 21 in the centrifugal direc-' tion in the pulverization zone. Withdrawal from the pulverization zone takes place thru a segmental passageway 28 which communicates with conduit 29 in the withdrawal portion II. Sleeve ll serves also as outlet conduit and is preferably equipped with an upward turned elbow piece onto which the discharge pipe II is bolted. Sleeve It is provided with a partition plate ll so as to form a separate carrying air passageway 32 which communicates with conduit 33 in the air inlet portion i I. Carrying air is supplied from a plenum box M.

It is pointed out that the amount and size of the balls in the comminution zones is determined on the basis of the duty to be performed therein by each ball. In the first zone-the crushing zone-the magnitude of the impact by each ball is the governing factor. In this zone I therefore use large balls-from about 3 to 4 inches in diameter-because the kinetic energy is a fimction of the mass. The amount of balls is about 50 to percent in excess of the amount which it takes to cover the projected area of the accelerator member in this zone. In the second sanepulverization zone-the comminution work is based upon attrition. A large surface area of the ball mass is therefore a prerequisite. For this reason I use smaller balls in this zone-from about 1 to 2.5 inches in diameterin an amount equal to about 5 to 20 times the amount which is required to cover the projected area of the accelerator member. a

A readily crushable material-such as soft coal-requires less expended energy for comminution than a harder material, as for instance anthracite coal. With present types of machines t is necessary to employ a large amount of balls. '.hat is, larger machines are required when the material is harder. This is so because their funcdoning depends upon the weight of the ball :harge for crushing purpose. With my apparatus it is feasible to pulverize materials of varyng hardness simply by varying the rotational weed of the drum. As this speed is increased additional kinetic energy is imparted to the balls with the result that more energy can be con- ;erted into comminution work. While the roaational speed of present types of machines :annot be much higher than about 25 R. P. M. [f excessive wear is to be avoided-my machine in the other hand may well be operated at speeds 1p to about 100 R. P. M. without encountering Jrohibitive wear. This is readily appreciated from the fact that an increased speed does not :ause a material increase in wear when each ball :an roll freely while it is being accelerated. The 5 ncrease in kinetic energy varies as the square if the change in velocity.

Moreover, in present types of horizontal rotat- .ng ball mills it is necessary to create and main- :ain a definite eccentricity of the ball charge in .he machine because only when the ball charge .evel is at a certain slope with the horizontal plane is it possible to eifect a proper absorption if the coal feed by the ball charge. In other words, a considerable portion of the power input 5 consumed tocause this eccentricity. This re- ;uirement results inevitably in rapid wear of the actuating drum lining and the balls themselves and it is not uncommon that the degree of lineiess actually becomes greater than desirable iimply because the required number of balls is iiltated by the necessary dead weight to accomallsh crushing of the coal feed down to a size when actual grinding can take place rather than w the attrition requirements for the grinding alone. In my apparatus these conditions and re- :uirements are obviated. The accelerator memaers in my machine are mounted in a sloping )OSltlOll which causes the balls and the coal feed to mix properly and to always find the accelera- :ion zone in the machine.

' It will be understood that numerous modifications of the invention may be made without departing from the spirit thereof, and the said invention is not limited to the precise details herelnbefore enumerated but is to be construed as broadly as the appended claims will permit.

What I claim is:

A two stage air swept ball mill comprising an elongated rotatably mounted horizontal drum having a renewable smooth shell liner, an externally supported stationary structure within said 6 drum adapted to provide stationary conduits tor the material and air at its end portions and to partition the internal drum space into a crushing zone and a grinding zone the partitioning member of which structure is of less height than the space within the drum so as to provide a segmental passageway between said zones thereabove, metal balls of identical size loosely arranged in said crushing zone so as to travel therein freely, metal balls of identical size loosely arranged in said grinding zone so as to be stirred therein, an accelerator member having a convex lower face pivotally supported on said internal structure at the base thereof in each zone so as to provide an annular space between the shell liner through which space only one layer of metal balls may pass at one and the same time said accelerator member being adapted to furnish a pressure upon said metal balls in said annular space whereby the resulting flriction causes the rotary motion of the drum to accelerate said balls and to rotate them about their own axis, means in the feed end conduit of said stationary structure to independently feed material into the crushing zone at the axis of the drum, means in the feed and conduit of said stationary structure to independently conduct air into the crushing zone at an elevation high above the axis of the drum in the form of a plurality of horizontally directed Jets, 9. segmental baiile in the grinding zone adjacent to the partitioning member with an upper arcuate edge closely adjacent the liner and a lower edge below the top of the partitioning member and means to withdraw a final product in entrainment with air from the grinding zone at an elevation high above the axis of the drum.

HARALD E. LONNGREN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 15,930 Sherban Oct. 7, 1924 926,441 Shafter June 29, 1909 1,491,841 Bell Apr. 29, 1924 1,628,609 Newhouse May 10, 1927 1,690,712 Bonnot Nov. 6, 1928 1,788,825 Danks Jan. 13. 1931 FOREIGN PATENTS Number Country Date 4,899 Great Britain Mar. 7, 1895 296,697 Great Britain Sept. 3, 1928

Images