GB2058608A - Pulverisers - Google Patents

Abstract

The invention provides a pulveriser having a solids inlet port (70), an air inlet port (71), an outlet port (94), and a rotor carrying swing hammers (30) in planes between the solids inlet and the outlet port. Air circulation means, including an impeller (31, 32), are mounted to the rotor for expelling size classified and reduced solids entrained in air as a stream from the outlet port with a linear velocity exceeding 40 ft./sec. In preferred embodiments the outlet port is in direct communication with a coal burner (98), and when coal is infed to the pulveriser, the stream velocity between the outlet and the burner prevents burn back. <IMAGE>

Description

SPECIFICATION Pulverisers This invention relates to apparatus for pulverising minerals, and more particularly to apparatus for pulverising minerals to particle sizes of below 80 microns at throughput rates of from approximately 1 quarter to 4 tons per hour. The apparatus may be used with particular advantage for pulverising coal and for mixing the pulverised coal with air so that a product stream from the pulveriser may be fed directly to a coal burner which may be integral with the pulveriser.

For use in a number of industries, it is a requirement that minerals, for example, coal, lime, phosphate, talcum and the like be finely pulverised. Such minerals are generally size reduced down to a particle size of 75 microns by use of ball mill pulverisers. The capacity of ball mill pulverisers such as are used industrially for mineral particle reduction commonly varies from approximately 4 tons per hour to approximately 45 tons per hour.

Ball mill pulverisers suffer from the general disadvantage that they do not function efficiently in humid conditions. With many minerals the in-feed material must be pre-dried by heating if the moisture content is as high as 1 percent. In addition the particle size distribution of the product of a ball mill is usually such as to require a separator, for example a cyclone, to separate coarse product from fines and to recycle coarse product to achieve uniformity of product particle size.

The capital cost of a ball mill installation for mineral pulverisation, including feed drying means and particle size classifying means is thus considerable. There is a continuing need for more efficient pulverisers for minerals and particularly for apparatus for producing an average particle size of below 75 microns.

As a result of growing recognition of a need to converse oil resources and because of rising oil prices, there is presently particular interest in the use of coal as a fuel. Various methods for conversion of coal to liquid fuels have been proposed. Most involve complex chemical plant and appear to be marginally economic. In the case of large plants for example in firing cement kilns and in large power plants it is practised to pulverise coal in a ball mill and to feed it in mixture with air to burners. As it is difficult to obtain coal more finely divided than 75 to 80 microns from a ball mill the product air stream has to be pre-heated to obtain proper ignition and combustion and must be fed to the burners at high velocity to prevent burnback.The additional equipment required for pre-heating and forced feed to burners is an expensive addition to the high cost of ball mill, pre-drier, and classifier discussed above.

Such apparatus is not suitable for firing smaller commercial boilers such as are used for commercial driers, foundaries, brick and tile making, and the like, which are commonly oil fired. For these purposes a coal feed rate of from 1 quarter to 4 tons per hour would be appropriate to replace oil as a fuel and ball mill equipment such as is suitable for providing coal for firing in large scale power generating plants would be uneconomic and inefficient.

One object of the present invention is to provide apparatus for reducing mineral particle size, and for achieving average mineral particle size of below 75 microns.

A second objective is to provide apparatus for pulverising coal to below 75 microns, desirably with a production rate of from 1 quarter to 4 tons per hour while avoiding the disadvantages of ball mills enumerated above.

A third objective is to provide apparatus for producing an air stream containing pulverised coal and suitable for direct feed to coal burners.

According to one aspect the invention consists in a pulveriser comprising a chamber having a solids inlet port, air inlet means and an outlet port, a rotor mounted for axial rotation within said chamber and having swing hammers mounted thereto in a plurality of planes perpendicular to the rotor axis, said hammers being disposed intermediate the solids inlet port and the outlet ports, and air circulation means, including first impeller means mounted to said rotor and sweeping a circumference which lies adjacent said outlet port, whereby in use air and entrained particulate matter are expelled from said outlet port as a stream having a linear velocity exceeding 40 ft/sec.

According to a second aspect the invention consists in apparatus according to the first aspect wherein said outlet port is in direct communication with a fuel inlet of a coal burner and wherein said stream velocity serves to prevent burn back in said burner.

In preferred embodiments the impeller has surfaces which are perpendicular to the end walls of the chamber and which are curved in the radial direction so as to be narrow at the tip. The impeller is believed to perform the function of generating an air vortex within the chamber as well as to function as a centrifugal pump impelling the product from the outlet orifice. In a highly preferred embodiment, a number of rails are mounted circumferentially internally of the chamber at levels corresponding to certain hammer planes and having fine clearances with the hammers thereof. The rails may be of rectangular cross-section but desirably each rail is curved on its upper and lower surface so as to cause turbulence in a vertical direction relative to the air vortex.For preference striker bars are cantilevered radially from the chamber wall at levels in staggered relationship with various hammer planes, the striker bars being shaped so as to enhance turbulence both in a circumferential direction of the air vortex and to an extent in the vortex direction. Preferably also secondary air impellers are mounted to the rotor intermediate certain of the hammer planes and contribute to air turbulence within the apparatus. Preferred embodiments of the invention accept a coal feed stock without necessity to predry it. Drying and also classification take place along with size reduction in one unit operation in the pulveriser which produces coal particles having an average size of below 75 microns and down to 35 microns in a relatively uniform size distribution.Coal particles are mixed with air in the apparatus, the output having an air flow velocity which is sufficiently high, typically greater than 95 feet per second, to be fed direct to a burner without burnback. The apparatus generally does not require pressurised air intake but at high throughputs air can be mixed with the output stream prior to burning if required.

Additionally the output stream typically has a temperature six times greater than ambient thus making unnecessary any additional heating of the product stream prior to burning. It should be noted that coal of a particle size below about 40 microns burns adequately at ambient temperature and heating prior to burning is not necessary although not disadvantageous.

The average particle size, size distribution, coal air ratio, output velocity and output temperature are such, or may be so adjusted, that the output may be fed direct to a burner which may therefore be integral with the pulveriser, the pulveriser also functioning as a carburettor. Preferred embodiments for coal feed to burners have a throughput capacity of from 1 quarter to 4 tons per hour and combine in one relatively compact and inexpensive apparatus the functions formally requiring massive and costly equipment comprising coal driers, oil mill, cyclone, pre-heater and burner.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is an elevation of an embodiment according to the invention shown partly in crosssection.

Figure 2 is a plan view of the apparatus of Figure 1, shown partly in section.

Figure 3 is a diagram useful for explaining the effect on air flow of parts of the apparatus of Figures 1 and 2.

Figure 4 shows in plan an impeller member being a part of the apparatus shown in Figures 1 and 2.

Figure 5 shows the impeller of Figure 4 in elevation.

Figure 6 shows in plan a hammer being a part of the apparatus shown in Figures 1 and 2.

Figure 7 shows the hammer of Figure 6 in elevation.

Figure 8 shows in plan a striker bar of the embodiment of Figures 1 and 2.

Figure 9 shows the striker bar of Figure 8 in elevation.

Figure 10 shows an end elevation of the striker bar of Figures 8 and 9.

Figure 11 shows in plan the means for mounting pivot pins being a part of the apparatus in Figures 1 and 2.

Figure 12 shows an elevation in cross-section of the means of Figure 11.

Figure 13 shows in plan a rotary seal being a part of the apparatus.

Figure 14 shows in elevation the seal of Figure 13.

Figures 1 5 and 1 6 show in more detail the mounting of rails, a part of the embodiment shown in Figures 1 and 2.

With reference to Figures 1 and 2 there is shown an apparatus comprising a chamber indicated generally at 11 and defined by an upper end wall 12 a lower end wall 13 and a side wall 14. Side wall 14 has a lower cylindrical portion 15 and an upper portion 1 6 diverging upwardly and outwardly from lower portion 1 5 and of truncated conical shape. Side wall 14 is mounted within an outer cylindrical shell 1 7 of slightly greater diameter than the major diameter of upper wall portion 1 6 and consequently lower inner wall 1 5 is spaced apart from outer shell 17. Inner wall portion 1 5 and outer shell 17 are flanged and bolted together with upper end wall 12 by bolts 9 whereby end wall 12 is removable.

Inner wall 14 is strengthened by an outer flange 1 9 and is reinforced internally by a lining 1 8 bolted in segments thereto.

An elastomeric layer (not shown) may be laminated between wall 1 5 and lining 1 8 if desired. The floor of chamber 1 is likewise reinforced by an inner liner 8.

The rotor of the pulveriser comprises a main shaft 20 extending from an upper bearing assembly 21 to a lower bearing assembly 22, a plurality of annular support elements designated generally by numeral 23 sleeve mounted upon and welded to shaft 20. Shaft 20 is preferably stepped in diameter as illustrated having its greatest diameter near the centre of its length for improved strength. Each support element is drilled at six equiangularly spaced positions, indicated by numerals 1 to 6 in Figure 2, and at equal radii from the rotor axis, the drilled holes of successive support elements 23 being in axial alignment. Pivot pins 24 pass through the holes and are retained at the lowest support element 23 by socket means and at the upper support element 23 by means of counter-sunk washers 25 held by studs 26 in recessed holes offset with respect to holes drilled at positions 1 to 6, the washers partially overlying and countersunk in the pivot pins to retain them and prevent rotation as shown in Figures 11 and 12.

Desirably the upper and lowermost support elements 23 are of a greater thickness than the remainder to provide adequate support for the ends of pivot pins 24.

Pivotally mounted by means of pivot pins 24 are hammer members 30, main impeller members 31 and subsidiary impeller members 32, each of which will be described in more detail hereinafter and each of each is sleeve mounted on a pivot pin 24 between a pair of adjacent support elements 23, spacers 33 being provided to ensure that the mounted component is pivotally movable in a horizontal plane but supported against vertical movement.

Hammers 30 are each similar one to another and are members of the shape shown in Figures 6 and 7 having a rectangular cross-section and pierced at adjacent one end by a mounting hole 34 by means of which they may be pivotally mounted on pivot pins 24.

Main impellers members 31 are of the shape shown in Figures 4 and 5 having a mounting hole 34 adjacent one end and curved inwardly towards a narrow flattened tip 35 at the other end. The impellers do not have an angle or set with respect to the plane of rotation.

Subsidiary impeller members 32 are generally of the same shape as main impeller members 31 but are of different lengths in radial direction and of smaller width in the vertical direction. While the main impeller members 31 extend to adjacent the inner lower wall 15 the subsidiary impellers 32 extend to a lesser radius than hammers 30.

Hammers 30 and impellers 31 or 32 are arranged in eleven rows or layers of components pivotally mounted to rotor 20 and at three or more of the six mounting positions provided at each row by support members 23. A preferred arrangement of hammers and impellers is as shown in detail in Table 1.

Table 1 Hammers and Impeller Arrangement Hammers Impeller Row Oty. Position Type Oty. Position 1 3 1-2-3 2 3 4-5-6 3 3 1-2-3 4 - 4-5-6 Subsidiary 3 1-2-3 5 3 1-2-3 6 3 4-5-6 Subsidiary 3 1-2-3 7 3 1-2-3 8 3 4-5-6 Subsidiary 3 1-2-3 9 3 1-2-3 10 3 4-5-6 11 - - - - Main 3 1-2-3 The four uppermost rows lie substantially within the conical wall section 1 6 of chamber 11 , the lower seven rows are substantially within the lower cylindrical portion of chamber 11.

The upper bearing assembly 21 comprises a case 41 enclosing an outer bearing race 42, having angled rollers 43 bearing against inner bearing race 44 mounted to axle 45 which in turn is mounted to rotor 20 by means of axle flange 46 which fits snugly in a recess of the upper end surface of rotor 20 and is mounted thereto by studs 47. Bearing case 41 has a removable cover 48 held in place by studs 49. The lower wall 50 of case 41 extends beyond the case wall as a flange and is bolted to the correspondingly flanged upper wall of an open sided rotary seal housing 52 which is welded to upper end wall 12, the seal housing containing a rotary seal 53 of the shape shown in Figures 13 and 14 and which is mounted to axle 45 serving to prevent dust from entering bearing case 41.An annular collar 54 surrounds the upper support element 23 of rotor 20 and serves to prevent particles from jamming between the upper surface of the rotor and upper end wall 12.

Lower bearing assembly 22 is of substantially similar construction and assembly but utilises ball bearings 55 rather than roller bearings.

Upper end wall 12 is provided with two coal feed ducts 70 mounted by means of flanges and studs and communicating with chamber 11 through apertures. Upper end wall 12 is also provided with one air intake duct 71 similarly mounted. Both coal ducts 70 and air intake duct 71 are of similar diameter and overlie the circumference of rotation of the uppermost row of hammers 30.

A number of breaker plates 80 are mounted by bolts 81 around the interior of upper wall portion 1 6. Around the interior of lower wall portion 1 5 rails segments 82 are mounted circumferentially at three levels. Rails 82 provide a close clearance with the hammers of the sixth, eighth and tenth rows and are curved from their innermost surface upwardly and outwardly on the top surface and downwardly and outwardly on the lower surface of each as is more clearly visible in Figures 1 5, 1 6 and 4.

In other embodiments (not illustrated) rails 82 may be rectangular cross-section. The number of rails employed will depend upon the grindability of the coal. Below 50 grade hardness (as measured by Australian Standard method for determining the hard groove grindability index for hard coal, K1 641967) and arrangement of 6 rows of rails is preferred. At above 50 grade, 6 rows each extending of 1 800 of arc of circumference and arranged in staggered relationship with respect both to the circumferential direction and the rotor axial direction are preferred.

Three rows of striker bars 84 are cantilevered to project radially into chamber 11 between the fifth and sixth rows, between the seventh and eighth rows, and between the ninth and tenth rows of hammers. Striker bars 84 are located at four equiangularly spaced positions and pass from outside the apparatus into inner chamber 11 being rigidly mounted by passing through support apertures in both outer shells 17 and inner walls 1 5 and 18 and by means of brackets 85 and bolts 86. The striker bars may have a rectangular cross-section but it has been found desirable to shape striker bars 84 so that in addition to the usual function, they act also as air foils causing turbulence in the air vortex generated in the apparatus within operation. For this purpose it is advantageous that striker bars have the shape shown in Figures 8 and 9.Bars 84 are chamfered having a trapezoidal cross-section along the distance projecting into chamber 11.

The apparatus is provided with access ports 90 in the side walls which are held in place by studs 91 and with access ports 92 in the upper end wall 12.

Adjacent the main impeller members 31 the apparatus is provided with an exit port 94 which communicates via chambers 95 and 96 with pipe 97 feeding direct to a coal burner 98. Chambers 95 and 96 are formed from cowls 99 and 100 respectively welded to pipe 97 which in the space enclosed by the cowls is open along the top in the manner of a trough communicating fully with chambers 95 and 96. The product is thus discharged from chamber 11 by main impeller members 31 to exit port 94 and deflected by cowl 100 into pipe 97 and thence to burner 98. The burner may be of a type conventional for coal. An additional air inlet pipe 101 is provided so that the mixture may be air enriched prior to burning if desired.

It is desirable that exit port 94 have an area of opening which is small in relation to the circumferential area swept by main impellers 31 and preferably port 94 has a dimension in the rotor axial direction equal to or smaller than the dimension in that direction of impellers 31. Furthermore it is highly desirable that the area of opening of exit port 94 be equal to the sum of the areas of the two coal inlet ports 70 and air inlet port 71.

It has been found that the clearance dimension between the chamber barrel and the impeller influences the fineness of the product. At a 25 mm gap and a rotor speed of 1900 RPM the same fineness is obtained as at a 45 mm gap and a rotor speed of 2500 RPM.

In operation the rotor is driven at speeds preferably of 1,900 to 2,500 rpm and coal is fed into infeed port 70. It is noteworthy that the coal need not be pre-dried prior to infeed. The infed coal is fractured by action of swing hammers and also by striking walls, striker bars and by friction. Finer particles move downwards primarily near the walls where they are brought into close proximity with hammers by rails 82. An air vortex is believed to be caused by the action of the main impellers 31 which tends to suspend fines thus having a classifying affect as well as exposing the fines to further grinding action for a longer period than in a conventionai hammer mill.This affect is apparently accentuated by turbulence created by the shape of rails 82, by striker bars 84 and by subsidiary impellers 32 as indicated by the direction of arrows in Figure 3 which indicates air flow direction believed to be taken. The turbulence presumably contributes to further abrasion of the particles. The product stream emanating from the exit port has a velocity typically of 96 feet per second which is in excess of the burnback velocity of coal and thus the stream can beefed direct to burners.

Particle sizes down to 37 microns can be obtained at which size coal dust is spontaneously combustible and a size below 45 microns has been found to be desirable for producing a steady flame in a coal burner. The air stream from the exit port is typically at six times ambient temperature so that even when the apparatus is adjusted for particle sizes of between 40 and 80 microns no pre-heating of the coal/air stream is required to achieve direct burning.

At usual throughputs the apparatus draws sufficient air from the air intake to produce a product stream having a desirable coal air ratio for burning. At high coal throughputs the addition of the air via air inlet 101 is desirable.

Embodiments of the present invention when used to fire boilers have a number of advantages in comparison with oil, gas and coal stoker fired systems.

In gas fired systems, control of the boiler is by means of on/off firing which introduces undesirable stresses in the structure and piping of the boiler.

In the case of coal stoker boilers it is impossible to cut off heat radiation and immediate shut down therefore requires flooding of the fuel.

In preferred embodiments of the present invention firing operates automatically and the heat may be modulated by control of the apparatus.

If immediate shut down of the boiler is required for some reason, feed to the pulveriser can be stopped immediately. The mass of the spinning parts is such that the machine is evacuated of coal before the pulverizer speed and air flow falls to an unsafe value for example to below 45 ft/sec.

The run down time of the rotor of the pulveriser may be for example 6 minutes permitting remaining coal in the chamber to be pulverized and producing sufficient volume and velocity of air/coal to avoid burn back in the firing pipe. Also, in the event for example of an electrical failure, coal feed to the inlet port stops, the chamber is evacuated of remaining coal, and orderly shut down of the boiler may be achieved with safety.

It will be understood that while the apparatus has been herein described with reference to supply of pulverised coal direct to a burner, the apparatus also has application in fine grinding of other minerals and in that event the burner assembly is of course not required.

Claims (14)

Claims

1. A pulveriser comprising a chamber having a solids inlet port, air inlet means and an outlet port, a rotor mounted for axial rotation within said chamber and having swing hammers mounted thereto in a plurality of planes perpendicular to the rotor axis, said hammers being disposed intermediate the solids inlet port and the outlet port, and air circulation means, including first impeller means mounted to said rotor and sweeping a circumference which lies adjacent said outlet port, whereby in use air and entrained particulate matter are expelled from said outlet port as a stream having a linear velocity exceeding 40 ft/sec.

2. A pulveriser according to claim 1 wherein the linear velocity of said air, expelled exceeds 90 ft./sec.

3. A pulveriser according to claim 1 or claims 2 further including a plurality of rails mounted to an internal wall of said chamber and extending in a circumferential direction coaxial with said rotor.

4. A pulveriser according to claim 3 wherein at least one of said rails is substantially in the plane of a swing hammer, said hammer during rotation passing said rail with fine clearances.

5. A pulveriser according to claim 3 or claim 4 wherein said rails extend along an arc of said circumferential direction and wherein alternate rails are in staggered relationship with respect to the circumferential direction and to the rotor axial direction.

6. A pulveriser according to any one of claims 3 to 5 wherein at least one rail is curved on its upper and/or lower surface.

7. A pulveriser according to any one of the preceding claims wherein said air circulation means includes at least one second impeller means mounted to said rotor.

8. A pulveriser according to claim 7 wherein said second impeller means comprises at least one blade mounted to said rotor in a plane intermediate hammer planes.

9. A pulveriser according to any one of the preceding claims further comprising at least one striker bar extending radially inwards from the chamber wall.

1 0. A pulveriser according to claim 9 wherein said striker bar has a generally trapezoidal crosssection along a distance thereof projecting into said chamber.

11. A pulveriser according to claim 9 or claim 10 wherein such striker bar extends radially inwards from the chamber wall in a plane disposed between the plane of a swing hammer and the plane of a second impeller means.

12. A pulveriser according to any one of the preceding claims wherein said first impeller means rotates in a plane adjacent the plane of an end wall of said chamber.

1 3. A pulveriser according to any one of the preceding claims wherein said first impeller means comprises a blade having a face extending in a rectalinearly in a direction parallel the rotor axis and curvilinearly in the rotor radial direction.

14. A pulveriser according to claim t3 wherein said blade is narrower at the radial tip than at the rotor.

1 5. A pulveriser according to any one of the preceding claims wherein said oulet port is in direct communication with a fuel inlet of a coal burner and wherein said stream velocity serves to prevent burn back in said burner.

1 6. A pulveriser according to any one of claims 2 to 1 5 wherein, upon subsantially simultaneous failure of solids feed to the solids inlet port and motive power to the rotor, the rotational inertia of the rotor and parts mounted thereto is sufficient to evacuate said chamber of solids prior to said stream velocity falling below 45 ft./sec.

1 7. A pulveriser constructed and arranged substantially as herein described with reference to the accompanying drawings.