GB2117667A - Coal-milling plant with grit recirculation and separation of pyrite and mine-waste - Google Patents

Abstract

In a coal-milling plant with a crushing mill (1) and a sifter (25) material discharged from mill 1 and conveyed to the sifter by an adjustable carrier air flow, the fine-crushed material is carried out of the plant, and the coarse material is recirculated to the crushing mill. Only coarsely crushed coal is recycled to mill material leaving the mill (1) is accelerated in a nozzle (15) and directed to an impact device (16) with at least one impact plate (21) which may be a variable speed rotor. A collection port (29) to convey away the material collected is connected downstream of the impact device in the direction of rebound. <IMAGE>

Description

SPECIFICATION Coal - milling plant with grit recirculation and separation of pyrite and mine - waste The invention concerns a coal - milling plant of the kind comprising a crushing mill and a sifter connected at its outlet, in which the material for crushing is carried out of the crushing mill, and conveyed to the sifter, by a carrier airstream adjustable as necessary, and in which after sifting the fine crushed material is carried out of the plant, and the coarse - crushed material is recirculated to the crushing mill as a flow of grit.

Pit-coal milling-plants consist inter alia of a crushing mill, in particular a roller pan crushing mill, and a sifter connected in series, which operates in particular as a gravitational, diversionary, rotary or combinational air-sifter. The crushed material milled out of the delivered coal is carried out of the crushing mill by means of an adjustable carrier airstream fed to the milling plant, and is separated in the adjacent sifter in such a way that the finely milled coal dust is carried, essentially against the force of gravity, out of the milling plant by the carrier airstream, while the coarse particles are conveyed as a recirculating flow of grit back to the crushing mill, and thus to the milling cycle, via a collection port with a pivoted shutter cutoff, disposed underneath the sifter.The finely-milled dust with a residue of 10-30% on a sieve with a mesh of 90 microns is delivered by the carrier airstream directly to a furnace orto intermediate bunkerage. The sifter separates from each other only particles with distinguishable rates of descent.

The constituent parts of the coal to be milled have the following densities, sharply differentiated one from another: Coal = 1.2 -1 1.5 1.2-1.Sgrammespercc Mine-wastes = 2.0 - 2.5 grammes per cc and Pyrites = 5.0 grammes per cc.

Over and above this, these constituent parts are differentiated in their hardness. Conditioned by the greater hardness of the quartz and pyrite components, the recirculating grit becomes richer in these minerals, as these must be milled repeatedly before they are carried out of the adjoining sifter by the carrier airstream. The finer the crushing achieved, the greater becomes the quantity of the recirculating grit. At the finest crushing, the recirculating quantity can reach 1-2 times the quantity of the throughput of the mill. It is understandable that the abrasion of the mill increases as the quality of recirculating grit and of the enrichment in hard quartz and pyrite crystals grows.

For the removal of pyrite and mine - waste from the recirculating grit, it is already known (German Offenlegungsschrift No. 2943556) with the aid of a vibrating trough with a fluidisation installation, separate from the milling plant, how to extract the fluidised coal at one end of the vibrating trough, while pyrite and mini - waste are extracted at the other end of the vibrating trough.

Further, it is known how to remove pyrite from the material for crushing with the aid of electromagnets.

It is the object of the present invention to provide a coal milling plant of the kind described above in which essentially only the coarsely milled coal is delivered to the crushing mill as recirculating grit.

According to the invention, there is provided a coal - milling plant with a crushing mill and a sifter connected at its outlet, in which the material for crushing is carried out of the crushing mill, and conveyed to the sifter, by a carrier airstream adjustable as necessary, and in which after sifting the fine crushed material is carried out of the plant, and the coarse - crushed material is recirculated to the crushing mill as a flow of grit, wherein the flow of material for crushing leaving the crushing mill is accelerated in an acceleration nozzle device and after acceleration is directed to an impact device with at least one impact plate inclined oppositely to the flow of material and collection means comprising at least one collection port, which conveys away the material collected, is disposed downstream of the impact device in the direction of rebound therefrom.

The acceleration jet device and the impact device work together with the collection means as a density separation facility placed ahead of the normally available sifter in the milling plant, which extracts from the flow of material for crushing the particles whose density is clearly greater than coal, i.e. pyrite and mine-waste.

Without any special density separation facility disposed outside the milling plant, the configuration of the milling plant in accordance with the invention makes possible, through the separation of pyrite and mine - waste, a drop in the emission of sulphur otherwise arising from combustion, as well as a reduction of abrasion in the milling plant. In addition, however, there also ensues a reduction of abrasion in the dust lines leading from the milling plant to the furnace and/or to the intermediate bunkerage, and, beyond the furnace, in the flue gas line to the electrostatic filters connected in series with the furnace.

An especially simple configuration ofthe installation is attained if the acceleration jet device is an annular nozzle and the impact device comprises an annular impact plate.

With this arrangement, the degree of separation effect of the impact plate can be made to conform particularly appropriately to the operational requirements in variable crushing and/or to changing carrier-air conditions, if the annular impact plate comprises a variable speed rotor.

In this configuration it is further preferred if carrier-air deflectors extending essentially radially are provided on the impact plate.

A conformity with the conditions of variable operation is however also attained if radially disposed deflectors are provided in the acceleration device in the milling flow, which are at any time adjustable about axes extending essentially radially.

In this way, in the annular impact plate arranged as a rotor, centrifugal force is imparted to the specifically heavier pyrite and mill - waste particles. In both configurations the separate functions for pyrite and mine - waste can be optimised, having regard to the quality of the coal and/or the crushing.

Under certain circumstances the kind of pyrite and mine - waste separation described above can be used in a carrier airstream in a number of milling plants adjoining in series.

The material carried away by the collection means, which consists essentially of pyrite and mine - waste, can - in order to improve the degree of separation effectiveness - be conveyed to a density separation facility in accordance with German Offenlegungsschrift No. 2943556, if this is helpful for the further economic utilisation of the separated material.

Through the high enrichment of the material carried away from the collection means in pyrite and mine waste, as opposed to the enrichment of the recirculating grit hitherto arising in conventional milling plants, it becomes possible for a density separation facility connected in series, in accordance with German Offenlegungsschrift No. 2943556, to be laid out specifically smaller, and therefore more economically, because of the lesser throughput for the same overall separation performance.

Two embodiments of the invention will now be described in more detail, with reference to the accompanying drawings, in which: Figure lisa schematic longitudinal section through an embodiment of a coal - milling plant in which the impact device comprises an annular impact plate in the form of a rotor, and Figure 2 is a schematic longitudinal section through a further embodiment in which radially arranged deflectors are provided in the ring nozzle, which guide the material for crushing onto a stationary annular impact plate.

Other directions of flow of the material for crushing are possible. It is only a matter of directing the flow of material, after acceleration, into an impact device and onto one or several appropriately arranged inclined impact plates, and of allotting collection ports to the impact plate or plates in a corresponding manner.

In the coal-milling plant shown in Figure 1,the crushing mill 1 consists of a rotatable crushing pan 2 and several crushing rollers 3 running on it. The crushing roller carrier 4 is urged towards the crushing pan 2 by means of compression springs 6 bearing against the crushing mill frame 5.

The crushing pressure is adjustable, e.g. hydraulically or pneumatically. Crushing mills of this kind are know in themselves and do not need more detailed description. The delivery of the crude coal is effected via a delivery pipe 7.

A casing 8 surrounding the crushing mill is joined to a housing section 9 extending upwardly. This leads to a cylindrical section 10 which is closed by a cover 11. The lower end of the delivery pipe 7 is surrounded by a grit recirculation funnel 12, at the end of which pivoted flaps 13 are arranged. Together with the housing section 9, a fitting 14 tapering conically upwards forms a ring nozzle 15, in which the crushed material flow MS leaving the crushing mill 1 is accelerated.

Above the ring nozzle 15 is disposed an impact rotor device 16, which is rotatable around the delivery pipe by means of a bearing 17. As is schematically depicted in Figure 1, the impact rotor device has a support section 18 the bracing of which is effected by several spokes 19. Between the delivery pipe 7 and the support section 18 or the spokes 19, is mounted an hydraulic rotating device, which is operated before the starting-up and after the switching-off of a turboset to turn the shafting at a sufficiently high rate of revolution; it usually comprises an impeller supplied with oil under pressure and rigidly connected to the structural part to be rotated. Nozzles are directed onto the impeller, and feed oil jets onto the blading of the impeller.

Reference is made for example to the hydraulic rotating device of the firm KWU, Order No. 402/3195.

Protection of the drive and the bearing against the entry of fine particles can be achieved by means of an air barrier unit. Other rotary drives may be used.

To the supporting section 18 is connected an annular impact plate 21 which is inclined in such a way that the impact or rebound angle of the flow of material for crushing MS, relative to the impact plate, amounts to about 65 at any time. Deflector vanes 22 are arranged on the surface 21, as can be seen from Figure 1, e.g. they are welded to the surface.

At the outside edge of the annular impact plate, air deflectors 23 and 24 are welded on, of which one is inclined outwards and the other inwards.

Above thr rotor device 16, in the embodiment shown, a deflection air sifter 25 is provided, with a multiplicity of deflection vanes 26 extending vertically. Each vane is mounted in the mill housing by means of a pivot shaft 27 protruding throgh the cover 11, and is adjustable. Deflection air sifters and suitable adjusting mechanisms of this kind are known, so that they do not require further description here.

Into the space between the deflection vanes 26 protrudes a coa - dust feed pipe 28, surrounding the delivery pipe 7, which extends right into the interior of the impact rotor device.

The fitting 14, together with the external housing section 9, defines a collection port 29.

The lower end of the collection port 29 is connected to several discharge pipes 30 leading out of the housing section 9, which lead to cell wheel feeders 31. Delivery systems 32, known in themselves and not more precisely described, are connected in series to the feeders; for this purpose, air conveyor systems vibrating conveyors orthe like are suitable.

In operation of the coal - milling plant described, the accelerated flow of the material for crushing MS impinges on the rotating impact rotor device 16. The rate of revolution of the rotor is regulated in dependence on the grinding and/or the quantity of air used forthroughput, i.e. through closing or opening of a stop valve located between the oil and the drive.

The relatively heavy particles of the flow of material for crushing, such as pyrite and mine waste P + B, with higher density and a smaller surface area, in contrast with coal particles of the same dimensions, are driven outwards as a result of the impact, under the additional effect of centrifugal force, more forcefully than lighter coal - dust parti cles, into an area which is no longer subject to the carrier air flow. There they descend under the influence of gravity into the collection port 29 and can be brought to another use.The remaining flow of material for crushing MS is conveyed by the carrier air into the deflection air sifter 25 and there it is separated in such a way that the finely - crushed coal - dust FK is removed via the coal - dust feed 28 with the carrier air, while predominantly the coarse coal particles GK, under the influence of gravity inside the rotor device, leave the area of influence of the carrier air flow and descend through the spokes 19, under the influence of gravity, into the grit recirculation funnel 12.

Via the pivoted flaps 13, they are led back to the crushing cycle, The deflectors 23 and 24 above the annular impact plate 21, and the supporting section 18 operating likewise as a deflector, in this process control the direction of the carrier airflow.

It is expedient that, in the installation described above, the areas of the compounds forming the density separation installation which are susceptible to abrasion, especially the components 12 and 14 constituting the nozzle 15, the impact plate 21 and the deflector vanes 22, should be manufactured out of an abrasion - resistant material or armoured with such a material, e.g. basalt. To the extent that it brings about ease in fitting and a structure convenientfor repair, a circular cross - section configuration for components 9, 12 and 14 should not be chosen, but rather these components should be built up from polygonally arranged panels, which are easier to manipulate when being fitted and repaired.

The annular impact plate 21 can also be approximately polygonal, being made up of individual panels.

In the embodiment according to Figure 2, a stationary impact device 32 is provided, whose guide section 33 corresponding to the support section 18 is secured to the grit recirculation funnel.

An impact ring 34 and air deflectors 35 and 36 are the equivalent of components 21 and 23/24 of the first embodiment.

In the nozzle 15, radially extending vanes 37 are provided, which at anytime are adjustable around a horizontal axis 38, by means of a pivot drive 39. As pivot drives, various know adjustment facilities, such as hydraulic drives, lever systems and the like are suitable.

Instead of the single port 29, in certain circumstances more than one port can be provided, in the radial direction from inside to outside, when for example coarse pyrite and mine - waste particles have to be separated from medium and finer ones.

Instead of a circular ring nozzle 16, several jets separated in the peripheral direction can also be used.

Claims (9)

1. Acoal-milling plant with a crushing mill and sifter connected at its outlet, in which the material for crushing is carried out of the crushing mill, and conveyed to the sifter, by a carrier airstream adjustable as necessary, and in which after sifting the fine crushed material is carried out of the plant, and the coarse - crushed material is recirculated to the crushing mill as a flow of grit, wherein the flow of material for crushing leaving the crushing mill is accelerated in an acceleration nozzle device with at least one impact plate inclined oppositely to the flow of material and collection means comprising at least one collection port, which conveys away the material collected, is disposed downstream of the impact device in the direction of rebound therefrom.

2. A coal-milling plant according to Claim 1, wherein the acceleration nozzle device comprises an annular nozzle and the impact device comprises an annular impact plate.

3. A coal-milling plant according to Claim 2, wherein the annular impact plate is in the form of a variable speed rotor.

4. A coal-milling plant according to Claim 2 or Claim 3, comprising carrier - air deflectors extending essentially radially on the impact plate.

5. A coal - milling plant according to Claim 1 or Claim 2, wherein the acceleration nozzle device comprises, in the flowpath of material for crushing, radially disposed guide vanes, which are adjustable at any given time about axes which extend essentially radially.

6. A coal - milling plant according to any of Claims 1 to 5, wherein the areas of the acceleration nozzle device and of the impact device exposed to abrasion are manufactured from abrasion - resistant material or are armoured with such a material.

7. A coal-milling plant according to any of Claims 1 to 6, wherein at least two collection ports are connected downstream of the impact device.

8. A coal-milling plant according to any of Claims 1 to 7, wherein at least a portion of the internal components of the plant are of polygonal construction in cross-section.

9. A coal-milling plant substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.