Vertical grinding mill, screw shaft, and method of designing and/or manufacturing a screw shaft

Abstract

A screw shaft for a vertically stirred grinding mill is arranged so as to be accommodated within a grinding chamber of the grinding mill while extending in a longitudinal direction. The screw shaft comprises a central shaft and at least one screw flight surrounding the central shaft, and the central shaft comprises an outer shaft wall defining a cavity within the interior of the central shaft, which cavity is closed at least at one longitudinal end of the central shaft.

Claims

1. A screw shaft for a vertically stirred grinding mill, wherein the screw shaft is arranged so as to be accommodated within a grinding chamber of the grinding mill while extending in a longitudinal vertical direction, the screw shaft comprising: a central shaft and at least one screw flight surrounding the central shaft; and the central shaft comprising an outer shaft wall defining a cavity within an interior of the central shaft, and the cavity is closed at least at one longitudinal end of the central shaft, wherein an outer diameter (Dshaft) of the central shaft and a mass of the screw shaft are set such that a buoyancy force acting on the screw shaft during use compensates for at least half of the weight thereof.

2. The screw shaft of claim 1, wherein the central shaft has the shape of a cylinder which is at least partially hollow and closed at one end so as to define said cavity.

3. The screw shaft of claim 1, wherein said cavity is at least partially filled with a material which has a density smaller than that of the material of the central shaft.

4. The screw shaft of claim 1, wherein the buoyancy force compensates for between 60 to 100% of the weight of the screw shaft.

5. The screw shaft of claim 1, further comprising a thrust bearing arranged at a vertically upper section of the screw shaft.

6. The screw shaft of claim 1, wherein the specific gravity of the screw shaft is less than 7.

7. The screw shaft of claim 1, wherein the specific gravity of the screw shaft is less than 6.

8. The screw shaft of claim 1, wherein the specific gravity of the screw shaft is less than 5.5.

9. The screw shaft of claim 1, wherein the outer diameter (Dshaft) of the central shaft and the mass of the screw shaft are set such that a buoyancy force acting on the screw shaft during use is substantially equal to the weight thereof.

10. The screw shaft of claim 9, wherein the buoyancy force compensates for 80 to 95% of the weight of the screw shaft.

11. The screw shaft of claim 10, wherein the buoyancy force compensates for 90 to 95% of the weight of the screw shaft.

12. The screw shaft of claim 1, wherein the outer diameter (Dshaft) of the central shaft and an outer diameter (Dscrew) of the screw flight fulfil the following relationship:
0.3Dshaft/Dscrew<1.

13. The screw shaft of claim 12, wherein the relationship Dshaft/Dscrew0.4, 0.5, 0.6, 0.7, 0.8 or 0.9 is fulfilled.

14. A vertically stirred grinding mill for grinding minerals and/or ore, the mill comprising a screw shaft of claim 1 and a grinding chamber in which the screw shaft is accommodated.

15. The vertically stirred grinding mill of claim 14, further comprising a thrust bearing, wherein the screw shaft hangs from the thrust bearing.

16. A method of designing and/or manufacturing a screw shaft for a vertically stirred grinding mill including a grinding chamber, comprising the steps of: choosing a type of material to be ground by the grinding mill, and choosing the type of grinding media to be used; designing and/or providing the screw shaft to be accommodated within the grinding chamber of the grinding mill, the screw shaft comprising a central shaft and at least one screw flight surrounding the central shaft; and setting an outer diameter (Dshaft) of the central shaft and a mass of the screw shaft such that, in a state in which the grinding chamber is filled with a slurry comprising the material to be ground and with the grinding media, and the screw shaft is at least partially immersed therein, a buoyancy force acting on the screw shaft compensates for at least half of the weight thereof, wherein the central shaft comprises an outer shaft wall defining a cavity within an interior of the central shaft, and the cavity is closed at least at one longitudinal end of the central shaft.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above, as well as additional objects, features and advantages of the present invention will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention with reference to the appended drawing FIG. 4, where the same reference numerals will be used for similar elements. In the drawings:

(2) FIG. 1 shows a vertical grinding mill according to the prior art;

(3) FIG. 2 shows a grinding chamber according to the prior art;

(4) FIG. 3 shows a screw shaft for a vertically stirred grinding mill according to the prior art;

(5) FIG. 4 shows a screw shaft for a vertically stirred grinding mill according to the present invention.

DETAILED DESCRIPTION

(6) As explained above, FIGS. 1 to 3 show a prior art vertical grinding mill. The vertical grinding mill according to the present invention differs as follows from this prior art:

(7) FIG. 4 depicts an impeller (auger) or screw shaft 2 according to the present invention, comprising a central shaft 23 and a number of screw flights 24 surrounding the central shaft 23. The screw shaft is accommodated within a grinding chamber of a vertical grinding mill so as to extend in a longitudinal direction. The longitudinal direction of the screw shaft coincides with the vertical direction in the state in which the screw shaft 2 is mounted in the vertical mill. As shown in the partial section of FIG. 4, the central shaft 23 comprises an outer shaft wall 25 defining a cavity 27 within the interior of the central shaft. The cavity 27 is closed at least at one longitudinal end of the central shaft. As a consequence, the central shaft 23 has the shape of a hollow cylinder which is closed at one end, which is the bottom end in the state in which the screw shaft 2 is mounted in the vertical mill. The wall thickness of the outer wall 25 of the central shaft 23 is about 0.1 m.

(8) In this embodiment, additionally, the diameter of the central shaft 23 is enlarged as compared to screw shafts of the prior art. Preferably, the diameter of the screw shaft 2 is constant, i.e. not changed along the axis of rotation. The diameter of the screw shaft, i.e. the outer diameter of the flights 24, is not necessarily different from conventional screw shafts. Hence, in a preferred embodiment, only the central shaft 23 relative to the flights 24 of the screw shaft 2 is modified.

(9) The screw shaft comprises means for being supported by way of a thrust bearing, said means being arranged at a vertically upper section of the screw shaft 2. The vertically stirred grinding mill comprises a thrust bearing, wherein the screw shaft 2 hangs from the thrust bearing.

(10) The outer diameter D.sub.shaft of the central shaft 23 and a mass of the screw shaft 2 are set such that, in a state in which the grinding chamber is filled with a slurry comprising material to be ground and with grinding media, and the screw shaft 2 is at least partially immersed therein, a buoyancy force acting on the screw shaft 2 compensates for a predetermined share of the weight thereof, e.g. at least half of the weight thereof.

(11) Hence, the force acting on the thrust bearing is reduced, since the only half of the weight of the screw shaft acts on the bearing.

(12) As indicated in FIG. 4, the diameter of the central shaft may be larger than in the case of conventional shafts. The diameter of the central shaft D.sub.shaft relative to the diameter of the screw D.sub.screw preferably at least fulfills the relationship: D.sub.shaft/D.sub.screw0.3, 0.4, 0.5, preferably 0.6, more preferably 0.7, most preferably 0.8 or 0.9. D.sub.shaft/D.sub.screw is smaller than 1.

(13) Preferably, the outer diameter D.sub.shaft of the central shaft 23 and the mass of the screw shaft 2 are set such that the buoyancy force acting on the screw shaft 2 during use is substantially equal to the weight thereof, preferably the buoyance force compensates for 80 to 95% of the weight, more preferably 90 to 95%.

(14) The central shaft 23 of the screw shaft 2 may at least partially be hollow and closed at the bottom so as to provide a surface for the buoyancy force to act upon. Put differently, the central shaft is closed or sealed against the outside, so as to avoid that slurry and/or grinding media gets into the central shaft.

(15) For example, if D.sub.shaft equals 3 m and the wall thickness is 0.1 m, the buoyancy force is approximately 730 kN, wherein the weight of the shaft is about 666 kN, which is about 75 tons. The buoyancy force exceeds the weight of the shaft and can also counter-act the other downward forces, such as the axial force created upon rotation of the screw shaft.

(16) In a preferred embodiment, the diameter of the central shaft D.sub.shaft is increased from 0.7 m (conventional shaft thickness) to about 4.0 m, wherein the (constant) diameter of the screw is about 4.8 m. The reduction in volume of the grinding media in the grinding chamber is, for this case, about 50%. Thus, the amount of media between the helices is also reduced by increasing the shaft diameter.

(17) In another example, a conventional screw shaft would have a screw flight diameter of 8 inches and a central shaft diameter of 2 inches, whereas a screw shaft according to the present invention has the same screw flight of 8 inches on a central shaft having a 6 inches diameter. This leaves a 1 inch wide helix on either side of the central shaft.

(18) In the embodiment of FIG. 4, the central shaft 23 is substantially hollow, so that the weight thereof is low, while a high buoyancy force acts on the screw shaft 2 due to its large diameter when the screw shaft is installed in the vertical grinding mill and the slurry and grinding media are filled in. It might be necessary to fill the hollow shaft with ballast, so as to prevent the buoyancy force from excessively exceeding the gravity force.

(19) The screw shaft 2 may be designed as follows: Choosing a type of material to be ground by means of the grinding mill, and choosing the type of grinding media to be used, designing and/or providing a screw shaft 2 to be accommodated within a grinding chamber of the grinding mill, the screw shaft 2 comprising a central shaft 23 and at least one screw flight 24 surrounding the central shaft 23, wherein the central shaft comprises an outer shaft wall defining a cavity within the interior of the central shaft, which cavity is closed at least at one longitudinal end of the central shaft, and preferably, setting an outer diameter D.sub.shaft of the central shaft 23 and a mass of the screw shaft 2 such that, in a state in which the grinding chamber is filled with a slurry comprising the material to be ground and with the grinding media, and the screw shaft 2 is at least partially immersed therein, a buoyancy force acting on the screw shaft 2 compensates for compensates for a predetermined part of the weight thereof and/or a predetermined part of the total forces acting on the screw shaft in the direction of gravity.

(20) According to the method, the screw shaft and the vertically stirred grinding mill can be manufactured. For example, the following settings could additionally be implemented: the buoyance force compensates for 80 to 95% of the weight, more preferably 90 to 95%, the outer diameter D.sub.shaft of the central shaft 23 and an outer diameter D.sub.screw of the screw flight (24) fulfil the relationship:
0.3D.sub.shaft/D.sub.screw<1, the density of the screw shaft is less than . . . kg/m.sup.3.

(21) The screw shaft of the invention can include one screw flight or helix or several helices. In a preferred variant, the screw shaft of the invention includes four helices, equally distributed about the diameter of the central shaft.

The Following Items Also Relate to the Invention

(22) Item 1: A screw shaft (2) for a vertically stirred grinding mill, wherein the screw shaft (2) is arranged so as to be accommodated within a grinding chamber of the grinding mill while extending in a longitudinal direction, the screw shaft (2) comprising a central shaft (23) and at least one screw flight (24) surrounding the central shaft (23), and the central shaft (23) comprising an outer shaft wall (25) defining a cavity (27) within the interior of the central shaft (23), which cavity is closed at least at one longitudinal end of the central shaft (23).

(23) Item 2: The screw shaft (2) of item 1, wherein the central shaft (23) has the shape of a cylinder which is at least partially hollow and closed at one end so as to define said cavity.

(24) Item 3: The screw shaft (2) of item 1 or 2, wherein said cavity is at least partially filled with a material which has a density smaller than that of the material of the central shaft (23).

(25) Item 4: The screw shaft of any one of the preceding items, wherein an outer diameter (D.sub.shaft) of the central shaft (23) and a mass of the screw shaft (2) are set such that, in a state in which the grinding chamber is filled with a slurry comprising material to be ground and with grinding media, and the screw shaft (2) is at least partially immersed therein, a buoyancy force acting on the screw shaft (2) compensates for at least half of the weight thereof, preferably 60, 70, 80, 90, or 100% of the weight thereof.

(26) Item 5: The screw shaft of any one of the preceding items, wherein the outer diameter (D.sub.shaft) of the central shaft (23) and the mass of the screw shaft (2) are set such that the buoyancy force acting on the screw shaft (2) during use is substantially equal to the weight thereof, preferably the buoyance force compensates for 80 to 95% of the weight, more preferably 90 to 95%.

(27) Item 6: The screw shaft (2) of any one of items 1 to 5, further comprising means for being supported by way of a thrust bearing arranged at a vertically upper section of the screw shaft.

(28) Item 7: The screw shaft of any one of the preceding items, wherein the outer diameter (D.sub.shaft) of the central shaft (23) and an outer diameter (D.sub.screw) of the screw flight (24) fulfil the following relationship:
0.3D.sub.shaft/D.sub.screw<1.

(29) Item 8: The screw shaft (2) of item 7, wherein the relationship D.sub.shaft/D.sub.screw 0.4, 0.5, 0.6, 0.7, 0.8 or 0.9 is fulfilled.

(30) Item 9: The screw shaft (2) of any of the preceding items, wherein the specific gravity of the screw shaft, i.e. the ratio of the mass of the screw shaft and the volume thereof relative to the density of water, is less than 7, preferably less than 6, more preferably less than 5.5.

(31) Item 10: A vertically stirred grinding mill for grinding minerals and/or ore, the mill comprising a screw shaft (2) of any of the preceding items and a grinding chamber in which the screw shaft (2) is accommodated.

(32) Item 11: The vertically stirred grinding mill of item 10, further comprising a thrust bearing, wherein the screw shaft (2) hangs from the thrust bearing.

(33) Item 12: A method of designing and/or manufacturing a screw shaft (2) for a vertically stirred grinding mill, comprising the steps of: choosing a type of material to be ground by means of the grinding mill, and choosing the type of grinding media to be used, and designing and/or providing a screw shaft (2) to be accommodated within a grinding chamber of the grinding mill, the screw shaft (2) comprising a central shaft (23) and at least one screw flight (24) surrounding the central shaft (23), wherein the central shaft (23) comprises an outer shaft wall defining a cavity within the interior of the central shaft (23), which cavity is closed at least at one longitudinal end of the central shaft (23).

(34) Item 13: The method of designing and/or manufacturing a screw shaft (2) for a vertically stirred grinding mill according to item 12, comprising the further the step of: setting an outer diameter (D.sub.shaft) of the central shaft (23) and a mass of the screw shaft (2) such that, in a state in which the grinding chamber is filled with a slurry comprising the material to be ground and with the grinding media, and the screw shaft (2) is at least partially immersed therein, a buoyancy force acting on the screw shaft (2) compensates for a predetermined part of the weight thereof and/or a predetermined part of the total forces acting on the screw shaft (2) in the direction of gravity.