US3044714A

US3044714A - Ball race pulverizer

Info

Publication number: US3044714A
Application number: US776559A
Authority: US
Inventors: John M Ballantine
Original assignee: Babcock and Wilcox Ltd
Current assignee: Babcock International Ltd
Priority date: 1958-11-26
Filing date: 1958-11-26
Publication date: 1962-07-17
Anticipated expiration: 1979-07-17

Description

J ly 17, 1 2 J. M. BALLANTINE 3,044,714

BALL RACE PULVERIZER Filed Nov. 26, 1958 3 Sheets-Sheet 1 INVENTOR. John M. Bal lanrine ATTORNEY July 17, 1962 J. M. BALLANTINE 3,044,714

BALL RACE PULVERIZER Filed Nov. 26, 1958 3 Sheets-Sheet 2 3 D/REC 7/0/v OPE/ILL MOVEMENT 26 27 2a INVENTOR.

John M. Bal lanfine ATTORNEY July 17, 1962 I J. M. BALLANTINE 3, V

BALL RACE PULVERIZER Filed Nov. 26, 1958 3 Sheets-Sheet 3 INVENTOR. John M. Ba l Ianrine United States Patent 3,044,714 BALL RACE PULVERIZER John M. Ballantine, Glasgow, Scotland, assignor to Babcock & Wilcox Limited, London, England, a company of Great Britain Filed Nov. 26, 1958, Ser. No. 776,559

4 Claims. (Cl. 241--58) This invention relates to pulverizing mills and particularly to ball mills. There are in common use ball mills in which an upper track member and an intermediate row of balls are arranged immediately above a lower track member provided on a rotor which in operation throws the material to be ground radially outwardly into the grinding zone while elastic fluid such as air directed upwardly around the rotor carries material away from the grinding zone. By the invention there is provided an arrangement entailing a grinding track at the radially outer side of a circular row of balls whereby an improved mill performance both as regards effectiveness in pulverization and as regards power consumption may be obtained.

A ball mill in accordance with the present invention has a radially outer track member with an annular surface curved to afford a grinding track at the radially outer side of a circular row of balls, a rotor, including a radially inner track member rotatable in relation to the radially outer track member and providing a radially inner track for the row of balls, the two tracks including respective zones of ball contact at opposite sides of the medial plane of the row of balls, a biassing force serving to press the balls between the two track members at the said zones, means for feeding material to be ground onto the rotor which is adapted to feed the material to the grinding track by centrifugally throwing the material radially outwardly and means for directing upwardly a stream of elastic fluid adapted to entrain material ground on the track.

The invention will now be described by way of example with reference to the accompanying drawings, in which:

FIGURE 1 is an elevation, in section on the axis thereof, of a ball mill for pulverizing coal, omitting certain mill parts of known construction;

FIGURE 2 is a plan view of a radially outer track member of the mill, removed from the mill;

FIGURE 3 is a plan view, in section on the line III-III of FIGURE 1, of part of the outer track member in place-in the mill;

FIGURE 4 is an elevation of a part of the mill in a section (see the section line IV-IV in FIGURE 2) which would include the mill axis but differs from the section of FIGURE 1;

FIGURE 5 is an enlarged representation of a part of FIGURE 1, and

FIGURE 6 is a plan view of a coal distribution plate, removed from the mill.

Referring to the drawings, within the cylindrical upper section 1 of the housing of a ball mill for pulverizing coal a horizontal circular row of grinding balls 2 is positioned between a non-rotary radially outer track member 3 and a rotary radially inner track member 4.

The inner track member is carried by a drive yoke 5 of generally conical form, the inner track member resting on and being keyed to the lower outer portions of the drive yoke. The drive yoke is keyed to the upper end of a vertical drive shaft 6 which is suitably mounted in journal and foot-step bearings (not shown), and is arranged to bedriven by a pinion drive shaft (not shown) entering horizontally the lower part (not shown) of the mill housing. The mentioned bearings, pinion drive shaft and lower section of the mill housing are suitably similar to the bearings for the vertical drive shaft 8 shown in the drawings of British patent specification No. 523,753, and to the pinion drive shaft 5 and lower part of the mill housing shown in the drawings on the same patent specification. 7

As shown particularly in FIGURE 5, the inner track member 4 is formed so'as to provide a radially inner track 7 forthe row of balls, which track extends over about 60% of the inner lower quadrants of the balls and lies at the bottom of a frusto-conical surface 8 sloping steeply radially outwards in a downward direction from an annular upper surface 9, above the level of the center of the balls, of the inner track member 4. The said frustoconical surface 8 is tangential with the track 7. Below the track 7 the inner track member 4 is formed with a lower frusto-conical surface 10 sloping steeply radially outwards in a downward direction which merges into a curved surface 11 which at the bottom of the inner track member is vertical. Both the upper and the lower frustoconical surfaces 8 and 10 make an angle of with the horizontal.

The outer track member 3 is formed so as to provide a grinding track 12 at the radially outer side of the balls. The grinding track 12 extends from a position 13 near the summits of the balls in the outer upper quadrants thereof to a position 14 in the lower outer quadrants of the balls halfway between the outermost parts of the balls and the lowermost parts of the balls. The grinding balls in the construction shown have a radius of 4%, and the grinding track stops short at the position 13 at a distance of about 1 /2", measured horizontally, from the summits of the balls. The outer track member extends from the top 13 of the grinding track in a vertical annular surface 15, and extends in a frusto-conical surface 21 outwardly and downwardly at an angle of 45 to the horizontal from the lower end 14 of the grinding track. The outer surface of the outer track member 3 includes a horizontal annular surface 22 at the lower end of the member connected by a radiused part with the frusto-conical surface 21 and by another radiused part with a portion 23 of the outer surface concentric with the grinding track 12. Above the said portion 23' an upper part 24 of the outer surface slopes at an angle of 45 to the horizontal and is connected by an uppermost radiused part of the surface of the outer track member with the vertical annular surface 15.

The outer track member 3, shown particularly in FIG-

URES

2, 3 and 4, is formed with a pair of projections 25 one diametrically opposite the other; each projection extends upwardly and out-wardly from the upper outer part of the track member outer surface and is arranged for vertical movement between a pair of guides 26 secured to the vertical wall 27 of the upper section of the ball mill housing. The projections are formed with respective tapped bores 28 for the attachment when necessary of eye bolts by means of which the outer track member may be lifted for removal and replacement. The outer track member 3 is also formed with four brackets 29 at locations equi-angularly spaced around the circumference of the track member and arranged symmetrically with respect to the projections 25; each bracket 29 extends upwardly and radially outwardly from the upper outer part of the outer surface of the outer track member and is adapted to receive downward force from a helical spring 30 (see FIGURE 4) of vertical axis associated with which is provision, including a bolt 31 penetrating a circular plate 32 closing the top of the mill housing, for adjusting the downward force exerted by the spring. In practice,

the downward force exerted by any spring is adjusted to horizontal angular surface 33 (see FIGURE which carries a horizontal annular plate forming an inner throat ring 34 which extends below the grinding track 12 and outwardly to a radial distance from the shaft axis slightly greater than that of the radiused surface which connects the annular surface 22 at the lower end of the outer track member with the portion 23 of the outer track member outer surface concentric with the grinding track. The outer surface of the inner throat ring 34 is frustoconical, sloping upwardly and outwardly at an angle of 65 to the horizontal. Secured to the vertical wall 27 of the upper mill housing section adjacent the inner throat ring 34 and the lower part of the outer track member 3 is an annular member forming an outer throat ring 41 and providing and inwardly facing frusto-conical surface 42 which slopes downwardly :and inwardly at an angle of 75 to the horizontal from a position near the wall 27 to a position adjacent the frusto-conical surface 35 of the inner throat ring 34 and halfway between the top and the bottom of the latter.

Round about the lower part of the upper mill housing section 1 is provided an annular windbox 43 (see FIG. 1) for the delivery of air to an annular zone 44 in the mill below the outer throat ring 41 in order that an upwardly directed stream of air may flow upwardly through the throat or gap 45 between the inner and

outer throat rings

34 and 41. The air enters the said zone 44 through a plurality of air-ports 46 in the lowermost part of the wall 27, which ports are distributed around the circumference thereof.

The annular zone 44 to which the air is delivered through the ports from the windbox is bounded upwardly, except for the throat itself, by a downwardly and outwardly facing frusto-conical surface 47 of the outer throat ring 41, downwardly by a frusto-conical floor surface 48 sloping inwardly and downwardly, and radially inwardly by removable wear segments 49 secured to the outermost rim of the yoke 5, at least one of which segments is in the form of a plough 4% adapted to promote the movement angularly around the floor of particles which drop through the throat, whereby they may be brought to a removal hopper (not shown), which may be as shown in British patent specification No. 523,769, provided for the removal of such particles from the mill.

The circular plate 32 supports a stationary classifier 50 of the kind comprising a whirl chamber 51 defined by a conical wall 52, the whirl chamber being arranged with its axis vertical and having at its upper end an annular inlet 53 controlled by a row, extending around the inlet, of vanes 54; the vanes may (by suitable means not shown) be simultaneously adjusted on respective vertical pivots to angular positions in which they give to air which entrained coal particles entering the whirl chamber through the annular inlet an appropriate whirling motion around the whirl chamber axis. The whirl chamber is co-axial with the vertical drive shaft 6, the lower half thereof extends downwardly through a discharge aperture 55 in the plate 32 into the space within the mill housing upper section 1, and at the lowermost end of the whirl chamber there leads from the bottom of the conical wall 52 an outlet passage 56 closable by a pivoted flap 57 urged upwardly towards the closed position by a bias weight 58 on an arm.

Secured to a flange 59 on the uppermost part of the yoke 5 and extending outwardly therefrom is a horizontal annular coal distribution plate 60, the plane of the upper surface of which lies at a level just above the summits of the balls, when the latter are in their normal, lowermost positions, and of which the periphery lies at a distance from the summits of the balls, when the latter are in their normal, lowermost positions, slightly less than half the radius of the balls. The coal distribution plate is pierced, in a radially inner zone thereof which is clear of the receptacle and of the yoke flange, with eight arcuate equiangularly spaced apertures 61.

Oversize coal particles discharged from the classifier outlet passage 56 fall therefrom onto a metal shield at downwardly and outwardly sloping frusto-conical surface 63 of which leads to the upper surface of the distribution plate 60 inwardly of the apertures 61 therein.

The access for air and entrained particles from the space within the mill housing upper section 1 to the classifier inlet 53 is by way of an upwardly and outwardly leading frusto-conical passage 64 defined inwardly by the upper half of the conical wall 52, and outwardly by a wall 65 extending from the outer rim of the aperture 55 to a plate 66; the plate 66 closes the top of the whirl chamber 51 except where a collar 67, which is co-axial with the whirl chamber and extends both above and below the plate 66, defines the outlet from the classifier for air and entrained particles.

Raw coal, supplied to a feed-spout 68 of adjustable inclination by a table feeding device (not shown) attached to the mill housing falls from the lower end of the feedspout onto the coal distribution plate 60.

As will be seen, the mill provides one grinding stage only. In the operation of the mill, most of the coal which falls from the feed-spout 68 onto the coal distribution plate 60 is centrifugally thrown outwardly from the radially outer edge of the plate with a distribution of the supplied material around the grinding zone. Some of the coal, however, falls through the apertures 61 in the plate 60, moves outwardly over the upper surface 9 of the inner track member 4 and attains to the inner track 7. Centrifugally thrown material attains to the grinding track 12 in the outer track member 3 and is crushed by the balls 2 rolling thereover; some of the material, however, is crushed on the track 7. The weight of the outer track member 3 is supported through the balls from the inner track member; thus as the ball rolls over a layer of coal on the grinding track 12 the force to crush the coal results from a reaction between the ball and the inner track member 4. In other words, the coal on the track 12 tends to cause the ball to move radially inwards, but such movement is opposed by the radially inner track member 4, and as a result the coal is crushed.

The frusto-conical surface 8 at the top of the track 7 on the radially inner track member 4, in the event that an abnormal layer of coal to be crushed or some uncrushable object gains access to the grinding track 12, enables any large force brought into play substantially horizontally between the ball and the said layer or object, to exert a component upwardly along the surface 8 sufiicient to move the ball upwardly and therefore radially inwards whereby jamming of the mill is avoided.

Material falling from the grinding zone moves radially outwardly over the upper surface of the inner throat ring 34 until it is acted upon by the upwardly flowing air stream in the throat 45, which carries ground particles upwardly through the annular space between the outer track member 3 and the cylindrical wall 27 of the upper section 1 of the mill housing and then radially inwardly and upwardly towards the classifier 50. At the same time a downward flow of air is induced through the space between the coal distribution plate 60 and the vertical annular surface 15 of the outer track member 3 and through the space between the inner and outer track members.

From the air streams in the space above the outer track member 3 and travelling towards the classifier some oversize particles fall directly into the space between the balls or onto the coal distribution plate 60. Other oversize particles are caught by the classifier, being thrown towards the conical wall 52 as the air and entrained coal particles whirl in the whirl chamber 51 and descending into the lowermost end of the whirl chamber, whence, provided there is present an accumulation of oversize particles suflicient in total weight to open the flap 57 against its bias to the closed position, oversize particles fall from the outlet passage 56 onto the shield 62 and attain to the coal distribution plate 60. Oversize particles falling onto the coal distribution plate 60 will normally be small enough to fall through the slots 61, and a proportion of them will do so,

after which they travel outwardly over the upper surface 9 of the inner track member 4, and enter the grinding zone.

Pulverized particles entrained in the air streams and fine enough not to be trapped by the classifier leave the ball mill housing through the outlet provided by the collar 67. Oversize or overdense particles which fall through the throat 45 against the upwardly rising air stream in the throat are moved by the plough or ploughs 49A around the mill axis over the floor 48 below the zone 44 until they attain to the removal hopper (not shown) previously re ferred to.

Experiments with a test installation for gauging the performance of a mill with the shown arrangements and forms of grinding rings, coal distribution plate and throat rings indicate that the mill described and illustrated, when worked with coal having a Hardgrove grindability index of 50 and to yield pulverized fuel such that 70% passes a No. 200 British fine mesh sieve, is capable of delivering an increased output in tons/ hour while requiring reduced power in kwh./ ton, the comparison being made with a nor mal performance of a mill having a similar stationary classifier and balls of the same diameter and on the same diameter pitch circle but running between grinding rings formed according to the drawings of British patent specification No. 523,753; one series of test supplied the figure of an output increased by more than 60% while simultaneously the power consumption was reduced by more than 25%.

What is claimed is:

1. A pulverizer comprising a housing enclosing a rotary grinding ring having an arcuate outwardly and upwardly facing grinding race, means for rotating said grinding ring, a circular row of grinding balls engaging said arcuate grinding race, a stationary grinding ring having an arcuate inwardly and downwardly facing grinding race, means for pressing said balls between said grinding races, means for directing an annular stream of an elastic fluid upwardly past the outer circumference of said rotary grinding ring to entrain pulverized material passing through said row of grinding balls, and rotating means for the distribution of material to be pulverized to the grinding surfaces between each of said grinding rings and said row of grinding balls, the periphery of said rotating means being positioned at a level upwardly adjacent the row of balls and inwardly of the face of said stationary grinding ring.

2. A pulverizer according to claim 1 wherein said rotating means comprises an annular plate having a plurality of apertures therein for the downward passage therethrough of smaller size material toward the grinding race of said rotary grinding ring.

3. A pulverizer comprising a housing enclosing a rotary grinding ring having an arcuate grinding race, means for rotating said grinding ring, a circular row of grinding balls engaging said arcuate grinding race, a stationary grinding ring having an arcuate grinding race, means for pressing said balls between said grinding races, means for directing an annular stream of elastic fluid upwardly past the outer circumference of said rotary grinding ring to entrain pulverized material passing through said row of grinding balls, and a circular plate driven by said rotary grinding ring for the distribution of material to be pulverized to the grinding surfaces between each of said grinding rings and said row of grinding balls, the periphery of said plate being positioned adjacent the row of balls and spaced from the face of said stationary grinding ring.

4. A pulverizer comprising a housing enclosing a rotary member, grinding ring having an arcuate outwardly and upwardly facing grinding race supported on said member, means for rotating said member, a circular row of grinding balls engaging said arcuate grinding race, a stationary grinding ring having an arcuate inwardly and downwardly facing grinding race, means for pressing said balls between said grinding races, an annular extension to said rotary member having its periphery projecting outwardly of saidballs, means on said housing at the level of said annular extension and cooperating therewith to define an annular gap therebetween, means for directing an annular stream of an elastic fluid upwardly through the gap formed between the outer circumference of said rotary member annular extension and said housing means to entrain pulverized material passing through said row of, grinding balls, and a rotary plate for the distribution of material to be pulverized to the grinding surfaces between each of said grinding rings and said row of grinding balls, the periphery of said plate being positioned at a level upwardly adjacent the row of balls and inwardly of the face of said stationary grinding ring.

References Cited in the file of this patent UNITED STATES PATENTS