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

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:

0.3 ≤D shaft/ D screw<1.

13. The screw shaft of claim 12 , wherein the relationship Dshaft/Dscrew≥0.4, 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.