GB2484333A

GB2484333A - Method of converting the body of a grinding mill

Info

Publication number: GB2484333A
Application number: GB1016921.7A
Authority: GB
Inventors: William Graham Bell
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-10-07
Filing date: 2010-10-07
Publication date: 2012-04-11
Also published as: GB201016921D0

Abstract

A method of converting a grinding mill having a polygonal-section mill body (2, Fig 1) 20 into a round bodied mill 20, wherein grinding means (4), 10 are located within the mill body (2), 20, and optionally a classifier section (6) is located on top of the mill body (2), 20. The method comprising the steps of (i) removing the classifier section (6) when present, (ii) removing the mill body (20) to expose the grinding means( 4), 10, (iii) optionally, reconditioning or replacing some or all of the grinding means (4), 10, (iv) providing a round mill body 20 around the grinding means 10 and (v) optionally, locating a classifier section (6) on the round mill body 20. The polygonal section mill body (2) may be hexagonal and the round mill body 20 may be cylindrical. A rotatable port ring 22 or other suitable auxiliary part may be present to assist in the conversion and in association with throat 24 may itself comprise a number of openings to permit air to flow from beneath grinding ring 10. An access door 40 may be located in the region of the grinding means. Other elements include (see Fig 1) spin initiator vanes (34), downwardly tapering classifier (6), grinding elements (12), compression loading device (14), vertical delivery pipe (8), classifier housing (6B) and classifier section (6)

Description

Industrial apparatus This invention relates to industrial apparatus, namely a pulveriser or grinding mill, in which pieces of a material are pulverised into a finer particulate form. The invention relates particularly, but not exclusively, to a mill in which coal is pulverised into a powder form which is conveyed to combustion apparatus e.g. of a power station.

In particular the invention concerns a mill having a lower grinding ring, which may be a part formed with an annular depression. Grinding elements are sandwiched between the lower grinding ring and a top member, which may have an annular depression facing an annular depression in the grinding ring. The grinding ring and the top member are moveable relative to one another. The grinding ring and the top member are typically ring-shaped; the terms "grinding ring" and "top ring" may hereinafter be used.

Typically the required relative movement between the grinding elements and the lower grinding ring is achieved by driving the grinding ring, while the top ring is held against rotation.

The grinding elements, which are typically steel balls or rollers, are not driven. They may be fixed in position, or free to precess.

The mill with which the invention is concerned has a multi-sided (typically hexagonal) mill body. Usually there is an extension member called a classifier on top of the mill body. The function of the classifier is to allow acceptance of suitable particles for onward passage to the combustion chamber, and to reject other particles, which are recycled to the grinding region of the mill.

Some mill bodies are "round", when constructed. That is to say, they are always generally circular in cross-section. They may, for example, by circularly cylindrical, or may taper. For example they may be frusto-conical.

There are some problems or limitations with mills having mill bodies which are multi-sided. A first is that the internal volume of the mill body is reduced, compared with a mill with a round body, for the same "footprint". A second is that the internal wall of the mill is discontinuous, leading to undesirable discontinuities in air I particle flows. A third is that certain equipment which can significantly promote the milling and classifying process, and which in other mills can be located between the grinding equipment and the wall of the mill, cannot easily be fitted to the inside of a multi-sided mill, due to the discontinuous nature of the surface and I or to the lack of space available.

In accordance with a first aspect of the present invention there is provided a method of converting a grinding mill having a polygonal-section mill body into a round-bodied mill, wherein grinding means are located within the mill body, and optionally a classifier section is located on top of the mill body, the method comprising the steps of -removing the classifier section, when present -removing the mill body to expose the grinding means -optionally, reconditioning or replacing some or all of the grinding means -providing a round mill body around the grinding means 1 0 -optionally, locating a classifier section on the round mill body.

Preferably the mill body has from 5-8 sides, and most preferably 6 sides.

Preferably the grinding mill does have a classifier section, which comprises an internal classifier and an external classifier housing. Ground material rising from the grinding means flows between the classifier housing and the classifier, before entering the classifier. From the classifier fine ground material is conveyed to a pipe which conveys the material downstream. Coarse material is rejected, and conveyed back to the grinding means; Preferably the step of locating a classifier section on the new, round, mill body employs the same classifier as was previously removed. This is not a requirement of the invention, however. The opportunity could be taken to replace or upgrade the classifier.

Preferably the step of locating a classifier housing on the new, round, mill body does involve a new classifier housing, in order to obtain a shape match at the junction, between the lower end of the classifier housing and the upper end of the new mill body. It will be appreciated that in this respect the invention offers simplification of the classifier housing: previously the classifier housing had a polygonal base, matching the shape of the upper end of the old mill body; and a wall shape which transitioned to an annular upper end. With the invention the new classifier housing can be round at its top and bottom ends. Thus it can be conical or circularly cylindrical.

Preferably the mill is a vertical spindle mill.

Preferably the grinding mill is for grinding minerals, for example cementitious materials, or most preferably coal.

It is not a requirement of the invention to replace the grinding means, or any part of it.

However the opportunity could be taken to do so, or to inspect and recondition the existing grinding means, when the walls of the original mill body are removed. It will be appreciated that, at this point, there is an excellent opportunity to inspect, and if necessary work on, the grinding means, which are normally very difficult to access.

Preferably the original, polygonal-section, mill body is substantially unvarying in its horizontal cross-sections, e.g. preferably it does not taper, and does not have a barrelled or waisted shape. Preferably, it defines a polygonal horizontal cross-section, throughout the height of the mill body. Preferably, therefore, it is prismatic in shape.

1 0 Preferably the new, round, mill body is substantially unvarying in its horizontal cross-sections, e.g. preferably it does not taper, and does not have a barrelled or waisted shape. Preferably, it defines a circular horizontal cross-section, throughout the height of the mill body. Preferably, therefore, it is of circularly cylindrical shape.

The volume of the new mill body may be greater than the volume of the old mill body. For example when the old mill body has vertices which are substantially coincident with the circular cross-section of the new mill body at any given height of the mill, the volume of the new mill body may be a few per cent greater than the volume of the old mill body. However in some embodiments of the invention the volume of the new mill body may be substantially the same as the volume of the old mill body. In other embodiments the volume of the new mill body may be less than the volume of the old mill body. Advantage may still arise, because of the improved efficiency of the mill which has the new mill body, in the place of the old mill body.

The result of the method of the invention is a mill with a new mill body with an internal wall without the internal discontinuities of the old mill body.

The absence of discontinuities in the new mill body leads to excellent flow conditions. This is a major advantage of the present invention. It means that a new mill body which has the same volume or even a smaller volume than the old mill body which is replaced, may nevertheless offer better performance.

However an advantage of the present invention is that the new mill body may, if wished, be of larger volume than the old mill body being replaced, and fit onto substantially the same footprint; for example when the radius of the new mill body is substantially the same as the largest radius of the old mill body.

The smooth shape of the inside of the new mill body is a significant advantage in its own right, giving functional benefits even if nothing else is done. However a further benefit can be extracted by providing one or more auxiliary parts around the grinding means.

One such optional auxiliary part is a mill liner, suitably carried on the inside wall of the mill, as an annulus. The liner is typically a downwardly slanted metal skirt or gusset. The fitting of a mill liner to the inside of the mill body, in the region of the grinding means, is an optional further step in the method of the present invention.

Another optional auxiliary part is a rotatable port ring around the circumference of the grinding means. The provision of such a port ring is a further, preferred, step which can be carried out as part of the method of the invention. The use of a port ring already carried by the grinding means is not excluded. Preferably, however, a port ring is fitted to the grinding means as part of the method of the invention.

Preferably the port ring is co-rotatable with the grinding ring. Preferably it is secured to the grinding ring for rotation therewith and includes a plurality of spaced-apart vanes. The vanes have upper and lower ends, and are preferably oriented at an angle in the range of 20° to 40° relative to a vertical axis of the mill, in a manner such that the lower ends lead, in the direction of rotation of the grinding ring, and the upper ends trail. Preferably adjacent vanes are spanned by respective lands or are left open. Preferably the openings and lands alternate, around the port ring.

2 0 There is typically a running clearance outside the port ring and this is a further opening available for air flow. In one embodiment the area available for air flow is the summation of the port ring openings and the running clearance; there are no further openings. When there is a mill liner the running clearance is suitably between the mill liner and the port ring.

Such a port ring, which is described in EP 507983A, exhibits significant advantages over earlier pulveriser mill designs. Most importantly, it provides for air flow upwardly through the port ring in a manner such that the air flow is essentially vertical (as opposed to predominantly spinning or swirling movement obtained with some other apparatus). With such apparatus the air flow provides excellent upward transport of pulverised material (e.g., coal dust) with minimum required air velocity, and with low tendency to lift large particles.

However it is a limitation that unground pieces of a certain size are not able to fall through the port ring. Rather they may rest on the port ring and block the flow of air.

Preferably the openings in the port ring are fixed. However the provision of the variable openings in the port ring is not excluded.

A preferred port ring for use in the present invention defines, around its 360 degree extent, a plurality of openings which are separated by lands, the openings permitting air to flow from beneath the grinding ring to above the grinding ring and the lands serving as obstructions to the flow of air from beneath the grinding ring to above the grinding ring, wherein the aspect ratio of the openings (length divided by radial width) is in the range from 1:1 to 3:1.

Preferably the lands in total occupy at least 90 degrees of the 360 degree extent of the port ring, preferably at least 120 degrees, preferably at least 180 degrees, most preferably at least 220 degrees.

Preferably the lands in total occupy up to 280 degrees of the 360 degree extent of the port ring, preferably up to 260 degrees.

Preferably the openings in total occupy up to 270 degrees of the 360 degree extent of the port ring, preferably up to 240 degrees, preferably up to 180 degrees, and most preferably up to degrees.

Preferably the openings in total occupy at least 80 degrees of the 360 degree extent of the port ring, and preferably at least 100 degrees.

The aspect ratio of the openings may be defined herein as the (mean) length divided by the (mean) width in the radial direction. Preferably the aspect ratio is in the range from 1:1 up to 2.5:1, most preferably from 1.2:1 up to 2.1:1.

All measurements and definitions based thereon given in this specification are made with reference to the horizontal plane and/or as viewed from above in plan view.

WO 2009/115828 provides more information about this optional auxiliary part.

Another optional auxiliary part which may be installed within the new mill body is an array of additional, variable openings. These are in additional to the openings in a port ring, when provided. For more information WO 2009/016395 may be consulted.

Preferably each variable opening is closable. Preferably each variable opening has a fully open condition and a fully closed condition. Preferably each variable opening has at least one condition in between, and preferably a plurality, more preferably a continuum, of conditions in between.

Preferably each variable opening is associated with a closure or blanking part which may be moved so as to change the condition of the variable opening. Preferably each closure part is slid over or under its opening, to change the effective area of the opening. Preferably the variable openings are provided in an annular part which is U-shaped in cross-section, and the closure part is an annular part which is U-shaped in cross-section, nested against (preferably nested beneath), and supported in rotation by, the annular part containing the variable openings. There may be one such closure part or more than one, defining segments of the periphery of the grinding ring.

The or each closure part may be controlled from outside the mill. Suitably this may be done as a pulveriser operation is under way. The or each closure part may be moved by mechanical, electrical, pneumatic or hydraulic means.

Preferably a plurality of variable openings, when provided, is under the control of a common control member.

Preferably each variable opening, when provided, is provided on a wider radius than the openings in the port ring. Preferably there is present a mill liner outside the openings in the port ring, and the or each variable opening is provided in the mill liner. As mentioned above the mill liner is typically a downwardly slanted metal annulus carried on the inside wall of the mill.

Preferably each variable opening is rectangular, or is arcuate, and follows the curvature of the mill.

Preferably the variable openings are in an array in the hoop direction; each opening preferably being an arc of a circle, centred on the axis of the mill.

Preferably the area of the variable openings, when fully open, is at least 10% of the area of the port ring openings (with the latter fully open, when they themselves are variable); preferably at least 20%, preferably at least 30% and most preferably at least 40%.

Preferably the area of the variable openings, when fully open, is up to 200% of the area of the port ring openings (with the latter fully open, when they themselves are variable), preferably up to 100%, more preferably up to 75%, most preferably up to 60%.

Thus, preferably when variable openings are present they provide, when fully open, from 40 to 60% of the area of the openings in the port ring (with the latter fully open, when they themselves are variable).

The openings in the port ring preferably together provide the major air flow area in the present invention. Additionally there is air flow through the running clearance. The variable openings, when present, are suitably intended for "trimming" the performance.

It is a limitation of the existing mill designs described herein that when there is a need to change coal throughput, air speed must be changed in order to maintain the correct air-coal ratio, and hence the optimal velocity in the mill. When the air velocity is simply increased, as may happen in existing mills, there is an increased tendency to lift large pieces of mineral, and to advance them to the combustion apparatus. On the other hand when the air velocity is too low there is an adverse effect on the coal particle size distribution in the ground material advanced to the combustion apparatus, and consequently poor combustion. The provision of variable openings as a preferred aspect of the present invention substantially improves mill operation by permitting air velocity to be held within suitable limits, even when there are large changes in throughput.

The variable openings may be adjusted to vary the air flow rate (i.e. to allow more, or less, air to flow in a given time), but still at a desired air speed.

Operating the mill with the variable opening(s) partly open or open to the maximum extent reduces the requirement to increase the air speed.

Preferably the air speed is kept substantially constant (e.g. ± 20% of the mid-value, preferably ± 10%) during the operation of the mill.

In accordance with a second aspect of the present invention there is provided a method of grinding minerals using a converted grinding mill provided by a method of the first aspect.

In accordance with a third aspect of the present invention there is provided the use of a round mill body in a mill in which there was previously a polygonal-section body, in order to improve the performance of the converted mill compared with the unconverted mill.

Preferred features of the first aspect of the present invention are preferred aspects of the second and third aspects. In particular, in any aspect of the invention the converted mill preferably has a mill liner carried on the inside wall of the mill body and a port ring carried on the periphery of the grinding ring for rotation therewith.

The present invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Fig. I is a generally cross-sectional view of an existing coal grinding mill of the vertical spindle type, which is to be adapted by the method of the invention; and Fig. 2 shows a modified mill, produced by modification of the mill of Fig. 1.

The mill shown in Fig. I has a non-tapering body 2 of hexagonal cross-section (i.e. a hexagonal prism), containing a grinding means 4. Fitted onto the top of the mill body is a classifier section 6 which comprises a downwardly tapering classifier 6A and a more gradually downwardly tapering classifier housing 6B. The classifier 6A is conical, being circular at its top and bottom ends. The classifier housing 6B is hexagonal at its bottom end and annular at its top end. It has a complex shape, transitioning from hexagonal to annular. More information about the classifier section may be found, if needed, in EP 0761309A. Coal to be crushed is delivered to the grinding means through a vertical delivery pipe 8.

The grinding means comprises a grinding ring 10 which rotates in the horizontal plane. Above it are grinding elements 12. The grinding elements 12 are urged against the grinding ring 10 by a compression loading device generally indicated as 14; the construction of which will be well known to the skilled person.

Around the grinding ring there is a fixed port ring 16. This is a non-movable ring of openings through which air is driven, to carry suitably ground particles of coal upwards, into the classifier, and onward to the combustion chamber.

The arrangement described is common and has been reasonably successful. However we have realised that there are some limitations. In particular we have realised that the flow of entrained particulates in air is not optimal due to the hexagonal cross-section. This design has vertices or corners between wall regions. The wall regions in effect offer a smaller diameter than the vertices. In other words the vertices offer larger spaces for the flow of air I coal than the wall regions. According to our analyses, this design -with less constrained flow pathways adjacent to the vertices and more constrained flow pathways past the wall regions -gives rise to more turbulent flow conditions than is desirable.

A further limitation is that the shape of the old mill body does not have an efficient footprint, and so is not optimally space-efficient. Space is often at a premium where grinding mills are located.

A further limitation is that the hexagonal shape of the old mill body is a hindrance to the utilisation of auxiliary parts within the mill body. One such part described herein is a mill liner, intended to be fitted to the inside wall of the mill. The hexagonal shape of the mill body makes fitting difficult. Another such auxiliary part, a port ring, is fitted to the outside of the grinding ring, closely adjacent to the inside wall of the mill. The fitting itself is not difficult, as it is not to the wall, but there is an uneven space between the port ring and the wall: wide space (near vortex) -narrow space (near wall) -wide space -narrow space.... This is highly undesirable in achieving controlled flow. This problem of uneven spacing can be overcome by using a mill liner but fitting thereof is, as explained above, difficult.

The mill shown in Fig. 2 is the same as that shown in Fig. 1 except that -the hexagonal mill body has been replaced by a circularly cylindrical mill body 20. The old hexagonal mill body can be seen in dotted lines (for reference) 20'. The cross-section taken in a vertical plane at a position between vertices of the hexagonal mill), -the classifier housing has been replaced by one whose bottom end matches the shape of the top end of the new mill body i.e. it is frusto-conical, and in alternative designs may be cylindrical, -different arrangements outside the grinding ring 10 (which will be further described).

A port ring 22 can be fitted to the periphery of the grinding ring. The port ring may be of the type described in EP 0761309A, or, preferably, its improved version described in WO 2009/115828.

In each case, round the grinding ring there is a throat 24, and in the throat there is provided the port ring. This rotates with the grinding ring, to impart a desired movement to the upwardly-directed air, which carries the coal fines to the combustion apparatus. The port ring comprises a plurality of spaced-apart vanes. The vanes are welded between spaced-apart support rings which are inner and outer circumferential walls of the port ring. Preferably the inner and outer support rings are short sections of vertical concentric cylinders. The vanes are inclined. The angle of inclination of the vanes is in the range of 20 degrees to 40 degrees from vertical. Preferably the angle of inclination is 25 degrees to 30 degrees. The upper ends of the vanes are tilted in a direction opposite to the direction of normal rotation of the grinding ring (that is to say, the tilt of the vanes is such that the upper ends trail the lower ends, with reference to the direction of rotation of the grinding ring). The inner support ring may be secured to the periphery of the grinding ring by means of bolts, or by welding, for example.

The variant of WO 2009/115828 is to provide port ring openings that are wider than they otherwise would be, and to keep the overall flow area in the desired zone by providing blanking plates or lands, between the openings. In effect, the openings are made wider and (by means of the lands) shorter, than was described in EP 0761 309A. Preferably the ratio of the mean length to mean width of these openings is 2: 1.

An annular mill liner 30 extends downwardly from the inside wall of the mill body, to which it is secured (the securement being aided by the circularity of the wall) towards the upper and outer edge of the port ring. Then the mill liner extends vertically downwardly to within about 1 cm of the upper edge of the outer member of the port ring. The angle of inclination of the mill liner is typically between 20 degrees and 60 degrees to the wall of the mill body (i.e. to the vertical).

It will be seen in Fig. 2 that there is an access door 40 in the side of the mill, at the level of the grinding means. This is provided in the new cylindrical wall. It is located above the mill liner and port ring. It will be seen that there is reasonable space between the access door and the grinding means, unlike the situation with an access door in the body mill of Fig. 1. Thus a further advantage of the present invention is that it can provide improved access to key working elements of the mill. Ongoing operations may therefore be improved. More than one door may be provided, if wished. Larger door(s) can be provided than could have been provided in the Fig. I mill body, where the facets of the mill body provided a constraint.

In use air I coal mixture flows from the grinding means and into the space between the classifier 6A and the new classifier housing 6B. This space narrows in the upwards direction, as a consequence of the cone angle of the classifier 6A exceeding the cone angle of the classifier 6B. The inside of the new classifier housing is without internal discontinuities, and upward flow of the fluent air Icoal mixture is smoother than it was when the old classifier housing, of the hexagon-to-round design, was in place. The fluent air / coal enters the classifier via openings between spin initiator vanes 34. Heavy particles drop out of the flow, back to the grinding means, whilst light fines flow out of the classifier, along pipe 36, towards a combustion apparatus.

In this embodiment, to upgrade the mill, the classifier section 6 (classifier housing 6B and classifier 6A) is removed. The mill body is detached from the base of the mill, and lifted away.

The grinding parts, now exposed and accessible, may now be replaced or serviced (a unique opportunity). The port ring described above, having openings and lands, is fitted to the periphery of the grinding ring. The replacement cylindrical wall is lowered in. The mill liner is welded in place. A classifier is fitted; typically the same classifier 6A, re-fitted. A replacement classifier housing, cylindrical or (as here) conical in shape, is fitted. The base of the classifier housing 6B is a shape match to the upper end of the mill body.

The modified mill shown in Fig. 2 provides improved performance. Particles of coal produced by the crushing or pulverising process are carried upwardly by means of air passing through the rotating port ring. The crushed particles are lifted upwardly in a smooth and efficient manner, and fluid flow within the mill body and classifier housing is not disturbed by discontinuities. Rejected particles which fall back from the classifier 6A to the grinding means and the port ring, and which previously could have rested on the port ring, and disturbed the operation, may fall through the port ring openings due to their width and low aspect ratio.

Mills of the type described in Fig. 1 are extremely high capital items and there is a strong incentive to make them function better, without going to the cost of replacement. We have determined that replacing the mill body is, surprisingly, a technically and economically feasible step. It produces a more efficient flow pattern for the fluent air / particulate mixture and offers the further benefits that we have described. Spaces of varying width in the region of the wall of the mill body are avoided.

The invention permits a mill liner to be fitted more readily than was the case in the prior art hexagonal mill.

The invention means that a port ring can be fitted to a grinding table without there being variations in spacing between the port ring and the internal wall of the new mill body.

Claims

CLAIMS1. A method of converting a grinding mill having a polygonal-section mill body into a round-bodied mill, wherein grinding means are located within the mill body, and optionally a classifier section is located on top of the mill body, the method comprising the steps of -removing the classifier section, when present -removing the mill body to expose the grinding means -optionally, reconditioning or replacing some or all of the grinding means -providing a round mill body around the grinding means -optionally, locating a classifier section on the round mill body.
2. A method as claimed in claim 1 wherein the mill, before conversion, has a hexagonal-section mill body.
3. A method as claimed in claim I or 2 wherein the round mill body is circularly cylindrical.
4. A method as claimed in any preceding claim wherein the grinding mill has a classifier section which has an internal classifier and an external housing, and in the method: the 2 0 classifier is removed and the same or different classifier is fitted; and the classifier housing is removed and replaced by a new classifier housing giving a shape match between the bottom end of the classifier housing and the top end of the round mill body.
5. A method as claimed in any preceding claim, wherein one or more auxiliary parts is I are provided in the mill body around the grinding means, to assist the operation of the converted mill.
6. A method as claimed in claim 5 wherein one auxiliary part is a downwardly slanted annular mill liner which is fitted to the inside of the round mill body, in the region of the grinding means.
7. A method as claimed in claim 5 or 6 wherein one auxiliary part is a rotatable port ring secured to the grinding ring for rotation therewith and including a plurality of spaced-apart vanes separated by openings.
8. A method as claimed in claim 7, wherein the port ring defines, around its 360 degree extent, a plurality of said openings which are separated by lands, the openings permitting air to flow from beneath the grinding ring to above the grinding ring and the lands serving as obstructions to the flow of air from beneath the grinding ring to above the grinding ring, wherein the aspect ratio of the openings (length divided by radial width) is in the range from 1:1 to 3:1.
9. A method as claimed in claim 8, wherein the lands in total occupy at least 90 degrees and up to 280 degrees of the 360 degree extent of the port ring
10. A method as claimed in claim any of claims 5 to 9, wherein one auxiliary part is an array of additional, variable openings around the grinding means of the converted mill.
11. A method as claimed in claim 10, wherein variable openings have fully open conditions 1 0 and fully closed conditions.
12. A method as claimed in claim 9 or 10, wherein each variable opening is associated with a closure or blanking part which may be moved so as to change the condition of the variable opening; and wherein each closure part is controlled from outside the converted mill.
13. A method as claimed in any preceding claim wherein the converted mill is fitted with an access door in the region of the grinding means.
14. A grinding mill having a round mill body when it once had a polygonal-section mill body, the change being achieved by application of a method as claimed in any preceding claim.
15. A method of grinding minerals, using a converted grinding mill as claimed in claim 14.
16. Use of a round mill body in a mill in which there was previously a polygonal-section mill body, in order to improve the performance of the converted mill compared with the unconverted mill.
17. A coal grinding mill having a round mill body when it once had a polygonal-section mill body; and a method of converting a mill from a polygonal-section mill body to a round mill body; in each case substantially as hereinbefore described substantially as hereinbefore described with particular reference to the accompanying drawings.