Coal has a percentage of non-combustible material in it as well as mixed with it.

Frequently this material is comparatively hard and takes longer to reduce to an acceptable size for transfer to the next stage of processing (normally transfer to a furnace) thus effectively reducing mill capacity. Additionally, this hard material is normally a significant contributor to erosion of the mill and subsequent process equipment. Pyritic sulphur common in coal, is a hard dense material responsible for a significant proportion of the SO2 content of combustion gases and contributes to the fouling of heating surfaces.

Furthermore, hard, dense materials inhibit mill capacity, limiting the performance of low Nox equipment.

OBJECT OF THE INVENTION It is the object of the present invention to overcome or substantially ameliorate at least one of the above disadvantages and/or more generally to provide an improved device for the removal of non-combustibles from mills.

DISCLOSURE OF THE INVENTION There is disclosed herein a vertical spindle mill for the treatment of coal, the mill comprising a hollow body, a mill table supported in the hollow body and rotatable about a generally vertical axis, the mill table being generally radially extending relative to its rotational axis, at least one grinding roller engageable with the table to crush coal delivered to the table, means to create an upward travelling air stream within the body to carry coal from the table to an outlet in the body, and means to remove at least a portion of non-combustibles delivered to the interior of the body with the coal, the means to remove the non-combustibles comprising a chamber attached to the hollow body and communicating with its interior at a position upstream of the table in the direction of flow of said air stream, means to deliver a controlled source of air to the chamber, and outlet means from the chamber through which the non-combustibles can pass.

Preferably, the chamber comprises a deflector liner extending inwardly from the hollow body and a selective screen thereabove delimiting the chamber from the interior of the hollow body.

Preferably, the outlet means from the chamber comprises a hopper extending to an outlet.

Preferably, the mill further comprises a central feed pipe extending to the interior of the hollow body and an inverted conical deflector therebeneath, the feed pipe passing through a classifier cone, the inverted conical deflector and the classifier cone having situated therebetween a non-return flap valve.

Preferably, two or more chambers are attached to the hollow body with at least one chamber being attached to the hollow body at a position higher or lower than at least one other chamber.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred forms of the present invention will now be described by way of example with reference to the accompanying drawings, wherein: Figure 1 is a schematic part sectioned side elevation of a vertical spindle mill with a vane wheel, with the portion to the left of the vertical centre line representing a first embodiment and the portion to the right hand side representing a second embodiment, Figure 2 is a schematic part sectioned perspective view of a portion of either of the mills of Figure 1, Figure 3 is a schematic part sectioned side elevation of a vertical spindle mill 10 showing a crushing roller 26 and spring 30, Figure 4 is a schematic partial plan view of a hopper showing a deflector line and a selective screen employed in the mills of Figures 1 and 3, Figure 5 is a schematic front elevation of the features of Figure 4, Figure 6 is a schematic side elevation of the features of Figure 4, and Figure 7 is a schematic plan view of a portion of the screen of Figure 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In Figure 1 there is schematically depicted a vertical spindle mill 10 for the grinding of coal. To the left of the vertical centre line in Figure 1, a first embodiment is depicted.

To the right of the vertical centre line a second embodiment is depicted. In both embodiments, the mill 10 comprises a hollow body 11 with a central feed pipe 12 passing vertically through a classifier cone 15 to deliver coal to the interior of the mill 10 and onto a mill table 16. Attached to the lower extremity of the feed pipe 12 is a conical deflector or cone 13 which, in combination with a non-return flap valve 14 forms the lower part of the classifier cone 15. Classifier cone 15 has side apertures 17 which communicate with outlet pipes 18 through which processed coal can be delivered to a furnace for example.

Table 16 is rotatably supported by means of a shaft 19 which in turn is supported by bearings 20. Fixed to the shaft 19 is a gear 21 engaged by a worm gear 22 to rotate the shaft 19. The worm gear 22 rotates the shaft 19 at an approximate speed of 50 rpm. The gear 21 is contained in a gear box 23 provided with oil and an oil cooler 24. The table 16 has a slightly inclined grinding surface 25 or hot surface or is equipped with a grinding groove and is engaged by a crushing roller 26. The crushing roller 26 is rotatably supported so as to be rotatable about an axis 27. The body 28 supporting the roller 26 is pivotally supported by means of a trunnion shaft 29 so that the roller 26 can also be pivotally moved about an axis extending generally transverse of the axis 27 but spaced therefrom. Extending between the body 11 and the roller body 28 is a spring 30 biasing the roller 26 to a crushing position engaged with the crushing surface 25.

The table 16 is provided with a vane wheel 31 as best depicted in Figure 2. Vane wheel 31 causes air to circulate upwardly through the interior space 32 enclosed by the body 11. Hot primary air is delivered to the air inlet 33. A baffle 42 ensures an even air distribution as the air then delivered to the vane wheel 31.

Located below the table 16 are scrapers 34 which deliver reject material to a rejects chute 35. It has been found that heavier material will often fall past the vane wheel 31, which heavier material is gathered by the scrapers 34 and are delivered to the rejects chute 35.

In operation of the above described mill 10, the table 16 is caused to rotate and coal is delivered via the feed pipe 12 to the grinding surface 26. The coal delivered to the surface 25 is engaged by roller 26 and thereby is reduced in size. The coal of reduced

particle size is picked up by the upward travelling air stream and is delivered to the slots 17. The coal is then delivered to the outlet 18. Oversized particles are collected by the cone 15 and are again delivered to the crushing surface 25. Additionally, oversized particles too heavy for the air stream travelling upward to the cone 15 are again circulated and delivered to the crushing surface 25.

It should be appreciated that normally the mill 10 would be provided with two or three of the rollers 26 equally spaced around the vertical axis of the mill 10.

Respectively, two or three equally spaced deflector liners 36 or sets of liners 36 would also be provided. As shown in Figure 1, sets of liners 36 can comprise one liner situated at a position above another.

Located on the internal surface of the body 11 are at least two deflector liners 36.

Liners 36 are associated with respective chambers which extend to respective hoppers 39.

The chambers and associated deflector liners 36 are adapted to collect at least a percentage of the recirculating heavier material. This recirculating heavy material will often be silica, iron oxides, pyritic sulphur and other minerals which are too heavy for the air stream and tend to circulate around the lower portion of space 32. During this recirculation of the heavier material, a significant portion will be collected by the hoppers 39 via the respective chambers 43 and the selective screens 38.

As shown in Figures 4 to 7, each of the respective chambers 43 comprise components at either side of an aperture in the wall of body 11. The selective screens 38 which comprise a number of spaced rods 42 are positioned above an inclined wall which comprises deflector liners 36. That is, the liners 36 depend from the inclined wall.

The selective screens 38 allow precipitating coarse material to pass into the collection hopper 39 and through the pipe 40 to a D. M. Transport System. Deflector liner 36 is fed with air 37 to control the selectivity of dense material. That is, a controlled air flow is delivered through lines 41 for select particle capture within the chambers 43.

The mill 10 is intended to remove a percentage of the non-combustible material before it has been reduced significantly in size. By doing this, the recirculating load in the mill 10 is reduced, the percentage of highly erosive material recirculating in the mill

is reduced and the percentage of erosive material passed on to subsequent equipment is reduced. The net result is lower mill load (reduced recirculating load), subsequent increased mill grinding capacity, lower mill erosion, less mill maintenance and lower erosion of subsequent plant and lower stack emissions.

Due to the removal of pyritic sulphur, emissions of SO2 from a downstream combustion process are significantly reduced. Dust burdened loadings on flue gas cleaning plant is significantly reduced and there is a resultant increase in mill grinding capacity. Emissions of NOX are reduced. Thermal efficiency of the steam generating plant is also improved.

Referring again to Figures 4,5 and 6, the various parameters in respect of the size and location of the chamber 43 are listed in the table below.

The selective screen is more fully depicted in Figure 7 and comprises a plurality of spaced bars 42. Again, the various parameters governing the size of the selective screen 38 are listed in the table below. PARAMETER PREFERRED RANGE OPTIMUM 30° to 90° 60° note (1) H 500mmtollOOmm 900mm note (1) A0 to 600 mm 520mm note (1) E 300 mmto 600 mm 500 mm note (1) R 600 mm to 2400 mm note (1) C 100 mm to 400 mm 230 mm note (1) L 400 mm to 800 mm 630 mm note (1) D 30 to 150 mm note (1) F 300 mm to 500 mm 420 mm note (1) G 300 mm to 500 mm 450 mm note (1) al 30° to50° note (I) a2 30° to50° note (1) a3 30° to50° note (I) cl 1.0 mm to 2. 4 mm note (1) P 1 mm to 5 mm note (1) M balance air note (1)

Note (1) These Values Are Dependent On Actual Mill Size.

The parameters in the above table are identified as follows: ~ Horizontal reference angle from datum Vertical reference height from datum X l A Height geometry of collection hopper E Width of selective hopper R Wall radius C Depth of selective screen L Width of collection hopper D, F & G Outlet Geometry (separator body) Angle of incline bottom of collection hopper a2 Particle separation in acceleration face a3 Angle of incline of selective screen (0 Width of selective screen rods PPitch of selective screen rods M Balance air

Note: 1.6, H, C, E are critical design parameters for operation of collector.

2. a2 is a critical design parameter for preferentially offering non-combustibles to the screen.

3. a3 is a critical design parameter for efficiency of screening.

4. m, P, are critical design parameters for screening selectivity of unwanted materials.

5. M is critical design parameters for maximising non-combustibles capture rate.