GB2108006A - Process for pulverising raw coal with the aid of an impact pulveriser and apparatus for carrying out the process - Google Patents

Abstract

Raw and especially hard coal, is pulverised in an impact pulveriser 3 in the presence of a drying gas discharged after pulverisation with the aid of the drying gas and subjected to sifting. In order to reduce wear and tear on the mill by regrinding coarse and heavy material, and equally to separate the incombustible components from the coal, the pulverised material is withdrawn from the impact pulveriser without pre-sifting and is subsequently subjected to grade sifting by fractions having different speeds of descent, wherein the heavy, coarse fraction is not returned anew to the initial impact pulverisation. <IMAGE>

Description

SPECIFICATION Process for pulverising raw coal with the aid of an impact pulveriser and apparatus for carrying out the process The invention relates to a process for pulverising raw coal, especially hard coal, in which the material to be pulverised is pulverised in an impact pulveriser in the presence of a drying gas and, after pulverisation, is discharged with at least the aid of the drying gas, and in which the pulverised material is sifted.

The hard coal supplied by mines is usually sorted and graded at the coal-field itself, so that users receive a quality of coal suited to the plants operated by them. However, quantities of dirt and pyrite crystals are still aggregated with this coal.

Moreover, the coal contains a certain amount of moisture. This moisture is composed of so-called coarse moisture adhering externally to the individual particles and also of moisture stored in pores within the particles. A certain part is also present in the form of water of crystallization.

For use in hard coal-dust furnaces the coal is usually subjected to a grinding and sifting process in which it is at the same time dried by means of drying gases. Suitable mills for this purpose are illustrated, for instance, in the Babcock Handbook, 4th Edition (1965) by Dr. Eng. Lobscheid, pp. 276-279. Deflecting or centrifugal sifters are usually employed for the sifting. The sifters operate by returning into the pulverisation process the components of the mixture of coal, dirt and pyrite which are difficult to grind until the size of the individual particles is so much reduced that it is extracted from the pulverisation and sifting process by the drying and conveyor gas flow because of its low settlement rate.

The speed of descent of the individual particles depends inter alia on their mass and thus also on their specific weight. According to its gas content, hard coal has a specific weight of 1.3-1.5 g/cm3.

With dirt the specific weight lies between about 2.0 and 2.5 g/cm3 and pyrite has a specific weight of about 5 g/cm3. The particle thresholds of the individual particles of coal, dirt or pyrite which can leave the pulverisation process are thus in approximately inverse proportion to their specific weights. Consequently, for extracting the dirt and especially the pyrite, fine grinding of the incombustible components of the coal is necessary, which leads to considerable attrition of the mill. Extraction of the pulverised material from the grinding space of the mill into the sifter is effected by means of the grinding gas flow.As a result of this usual arrangement, both with rod mills, ball mills and dish grinding mills, as well as with beater mills, a double sifting takes place: on the one hand, the pulverised material is only extracted from the pulverising area when the conveyor gas flow is in a position to transport the material into the sifter; on the other hand, the actual sifting takes place in the sifter, after which the sufficiently ground material leaves the sifter with the conveyor gas flow and the still too coarse material falls back into the mill.

Even with a forced-draught beater mill in a sifterless design for brown coal, as illustrated on p. 279 of the above-mentioned book, there is a preliminary sifting, as all the particles remain in the pulverisation area whose speed of descent is greater than the speed of the conveying air. Only when they are so pulverised that their speed of descent corresponds to the speed of the conveying air are they extracted from the mill.

It is, therefore, the purpose of the invention to provide a process in which the combustible and incombustible parts of the coal can be separated from one another and equally wear and tear during the pulverising is reduced. This problem is solved according to the invention in that the pulverised material is withdrawn from the impact pulveriser without preliminary sifting, and is subsequently subjected to a grade sifting by fractions with different speeds of descent, wherein the heavy, coarse fraction is not returned anew to the initial impact pulverisation.

When carrying out the process the raw coal is thrown by means of the drying gas against, for instance, an impact plate in a sifterless mill working with impact effect, or is introduced into an impact, beater or beating wheel mill. The material is pulverised by impact on to the impact plate or the rotating impact tool. Particles difficult to crush, which consist preponderantly of dirt or pyrite, remain largely unpulverised on impact, whilst the particles of coal tend to split along the crystal boundaries. Aggregated components divide along the boundaries of the materials. The material thus pulverised is conveyed by the grinding gas flow to the grade sifting, without preliminary sifting, which separates the individual particles from one another according to differing speeds of descent.Separation is possible by simple means because the components of the raw coal which are difficult to crush and incombustible are not pulverised as much as the coal, and because the coal has a lower specific weight than the incombustible components. As all the pulverised particles are extracted from the impact pulveriser without the preliminary sifting described above, there is no unnecessary too fine grinding of the incombustible components of the coal, i.e. the wear and tear on the mill is reduced.

Depending on the degree of aggregation between the coal and the other components and on the grindability of the individual components, it is practical if the coal is pulverised with a controllable degree of impact.

If the fineness of the separated coal obtained with the impact pulverisation is still not sufficient for the purpose in view, for instance coal-dust firing, the separated coal may be subjected to yet another pulverisation process.

It is also possible to divide the coal separated from the incombustible components into at least two fractions so as to obtain coal largely purified from dirt of different granulation size for various uses.

In particular, cross-flow sifting or jet sifting, both known per se, are recommended for grade sifting.

In order not to be compelled to use additional inert gas or extra air for grade sifting, it is practical to use at least part of the grinder drying gas as sifting gas.

Especially excellent processing is available if, besides pulverising the coal with a controllable degree of impact, a controllable quantity of sifting gas is used on the pulverised material.

The invention also provides apparatus for carrying out the process according to the invention, and comprising a pulveriser including a coal feed, an impact pulverisation element, a conveyor gas supply and an outlet for the pulverised material, and having a sifter, wherein the outlet for the pulverised material is so disposed with respect to the impact pulverisation element that the direction of settlement of the individual particles and the direction of flow of the conveyor gas are substantially the same in the region of the outlet, and in that the outlet is connected with a grading sifter which grades the pulverised material into fractions according to different rates of settlement.

The invention will now be described in greater detail with reference to the accompanying drawings, in which: Figure 1 shows diagrammatically apparatus including a beater mill as an impact pulveriser, and Figure 2 shows apparatus including a pneumatic impact pulveriser.

In the embodiment according to Figure 1 the raw coal is introduced from a raw coal bunker 1, via a distributing belt 2, into the drop shaft 3a of a sifterless beater mill 3. Hot grinding gas for drying and transporting the coal is fed through a duct 4 connected with the drop shaft 3a. Either hot flue gas or, when crushing coal which is not very ignitable, hot air may be used as the hot grinding gas. The outlet 3b for the pulverised material is located in the housing 3c in such a way that the material pulverised by the beater tool 3d can be extracted in such a manner that the direction of descent and the conveyor gas flow in the region of the outlet 3b are substantially unidirectional, i.e. a pre-sifting with the aid of the conveyor gas flow is not possible.The mixture of grinding gas, coal, dirt and pyrite, passing through a duct 5 connected with the outlet 3b, reaches a cross-flow sifter 6, which is provided, for exampie, with four funnels 7, 8, 9 and 10. In the sifting space 6a of the crossflow sifter a current of sifting gas flows in the direction of the arrow, i.e. from left to right in Figure 1. The heavy, coarse components of the grinding stock, especially pyrite and dirt, fall into the funnel 7, whilst the coal components, substantially separated from dirt and pyrite, fall into funnels 8 to 10, according to their granulation size. The residual portion of dirt in funnels 8 to 10 can, of course, vary. Funnels 7 to 10 are equipped at the lower end with drawing off devices 11, so as to be able to feed the individual fractions to different applications.

Surplus grinding gas is conveyed from the sifting space 6a via a duct 12 into a filter 13 so as to separate out the fine dust contained in the grinding gas, before discharging the surplus grinding gas into the atmosphere. The find dust is collected in a funnel 14 located beneath the filter and can be extracted via a drawing off device 1 5.

A compressor 1 6 draws off the purified grinding gas. Since the fine dust is particularly ignitable, it can be usefully employed for ignition and support firings.

If the equipment is directly linked with a furnace, the grinding gas mixed with fine dust may also be fed directly into the furnace via duct 12.

The flow of sifting gas required for the grade sifting is extracted from the sifting space 6a through a duct 1 7 and is forced into the sifting space 6a via a fan 18. A regulating valve 19 is inserted after the fan 1 8 so that the quantity of sifting gas conveyed to the sifting space 6a can be controlled, depending on the grindability and the particle fractions of the grinding stock. In place of the regulating valve 19, rotational speed regulation of the fan 1 8 may also be considered.

If, for instance, the pulverisation fineness of the coal collected in funnel 8 is not good enough, it can be subjected to a further pulverisation process. As indicated by dashes in Figure 1, the too coarse coal can, for instance, be conveyed back into the drop shaft 3a. When a beater mill 3 is employed, the degree of impact of the pulveriser can be controlled by regulating the speed of rotation of the beater tool 3d. The same holds good for impact crushers with rotating impact surfaces and beating wheel mills.

In the embodiment shown in Figure 2 a pneumatic impact pulveriser 20 is used as the pulveriser. The raw coal leaving the drop shaft 20a is accelerated to impact speed in an impact flow duct 20b and emerges from an impact nozzle 20c at high speed into an impact space 20d. Opposite the impact nozzle 20c an impact plate 20e is mounted in such a way that its angle relative to the impact jet and its distance from the impact nozzle are adjustable.

The impact gas flow in the impact duct 20b is built up with the aid of a fan 21, the conveyor flow of which is forced through a duct 22 and a nozzle 23 into the impact flow duct 20b.

The pulverised material enters the sifting space 6a through an outlet 20f at the lower end of the impact space 20d.

With the arrangement shown, the degree of impact can be varied by adjusting the flow (rotational speed regulation of the fan 21), altering the distance between nozzle 20c and impact plate 20e, and altering the angle of inclination of the impact plate 20e with respect to the impact flow.

Distance and inclination of the impact plate 20e from and with respect to the impact nozzle 20c can be determined by shifting the support elements (shown as rods) of the impact plate which is hinged to the ends of the rods lying in the impact space.

Besides the cross-flow grade sifter shown in Figures 1 and 2, other sifters can be employed, such as jet sifters, spiral sifters, bucket wheel sifters, scatter sifters, gravity sifters. Of course, all sifters can be used as grade sifters if several sifting stages with different limiting particle sizes corresponding to the coarse and fine material fractions desired are arranged one after another.

In impact pulverisers the desired separation of raw coal particles into the various components is effected substantially in a single impact on the stationary or rotating impact tool.

Claims (14)

1. A process for pulverising raw coal, especially hard coal, in which the material to be pulverised is pulverised in an impact pulveriser in the presence of a drying gas and, after pulverisation, is discharged with at least the aid of the drying gas, and in which the pulverised material is sifted, wherein the pulverised material is extracted from the impact pulveriser free from preliminary sifting and is afterwards subjected to a grading sifting by fractions with different settling rates whereby the heavy, coarse fraction is not returned to the initial impact pulverisation.

2. A process according to claim 1 , wherein the coal is pulverised with an adjustable degree of impact.

3. A process according to claim 1 or 2, wherein the coal separated from the non-combustible components of the raw coal is subjected to a further pulverisation process.

4. A process according to any one of claims 1 to 3, wherein the coal separated from the noncombustible parts is graded into at least two fractions.

5. A process according to any one of claims 1 to 4, wherein the pulverised material is subjected to cross-flow sifting.

6. A process according to any one of claims 1 to 4, wherein the pulverised material is subjected to jet sifting.

7. A process according to any one of claims 1 to 6, wherein the drying gas is used as the sifting gas.

8. A process according to any one of claims 1 to 7, wherein a controllable quantity of sifting gas is used.

9. Apparatus for carrying out the process according to any one of claims 1 to 8, comprising a pulveriser including a coal feed, an impact pulverisation element, a conveyor gas supply and an outlet for the pulverised material, and having a sifter, wherein the outlet for the pulverised material is so disposed with respect to the impact pulverisation element that the direction of settlement of the individual particles and the direction of flow of the conveyor gas are substantially the same in the region of the outlet, and in that the outlet is connected with a grading sifter which grades the pulverised material into fractions according to different rates of settlement.

10. Apparatus according to claim 9, wherein the impact pulveriser is a pneumatic impact pulveriser in which the raw coal is directed against an impact plate by means of an impact gas flow.

11. Apparatus according to claim 9, wherein the impact pulveriser is a pulveriser having a rotating impact tool.

12. Apparatus according to any one of claims 9 to 11, wherein the grading sifter is a cross-flow sifter.

13. Apparatus according to any one of claims 9 to 12, wherein the grading sifter is a jet sifter.

14. Apparatus according to any one of claims 9 to 13, wherein a closed circuit is provided for the sifting gas flow in the grading sifter.

1 5. Apparatus according to any one of claims 9 to 14, wherein the grading sifter includes a regulating unit for regulating the amount of sifting gas.

1 6. A process for pulverising raw coal substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.

1 7. Apparatus for pulverising raw coal substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.