Method and apparatus for mixing

1. A method of mixing a liquid having particulate, comprising the steps of:

in a vessel for containing the liquid, the vessel including a sidewall, a bottom, and a baffle extending longitudinally along the sidewall approximately from the liquid surface, such that a lower end of the baffle is located in a zone defined as below the liquid surface by a distance that is approximately one-quarter to one-half of the height of the liquid;

rotating an axial impeller about a substantially vertical axis, said axial impeller:

being submerged below the liquid surface in the zone defined by the distance that is approximately one-quarter to one-half of the height of the liquid, and there is no impeller outside the zone;

oriented upwardly to produce (a) an inner, upward flow region located along said vertical axis, (b) a transition flow region located above the impeller in which liquid moves radially outwardly toward the vessel sidewall, and (c) an outer, downward flow region located along the sidewall; and

being variable speed.

2. The method of claim 1 , wherein the ratio of the vessel sidewall height to the vessel diameter is at least 3.

3. The method of claim 1 , wherein the vessel bottom is conical and has a slope of at least 45 degrees.

4. The method of claim 1 , wherein said axial impeller and the lower end of the baffle are adapted for being located approximately the same distance below the liquid surface.

5. The method of claim 1 , wherein the flow is continuous.

6. The method of claim 1 , wherein the baffle extends longitudinally along the vessel sidewall approximately from the liquid surface to the axial impeller.

7. A method of mixing a liquid comprising the steps of:

in a liquid in a vessel having an upper end, a lower end, and a substantially cylindrical containing wall extending between the upper and lower ends, and having a baffle having an upper end and a lower end;

rotating an axial impeller about a substantially vertical axis, said axial impeller having a means for adjusting the rotational speed and being submerged in said liquid to a position that is located approximately one-quarter to one-half of the distance from said upper end to said lower end;

thereby producing a central vortex flow in the liquid with the axial impeller, said flow comprising (a) an inner flow along said vertical axis, moving from the lower end toward the upper end, (b) an outward flow from the axial impeller toward the containing wall, and (c) an outer flow along the containing wall, moving from the upper end toward the lower end, the baffle defining a region of restricted rotational flow region above the impeller and an unbaffled, unrestricted rotational flow region below the lower end of the baffle.

8. The method of claim 7 , wherein said axial impeller is submerged in said liquid to a position that is located approximately one-third of the distance from said upper end to said lower end.

9. The method of claim 8 wherein the baffle extends longitudinally along the vessel sidewall such that a lower end of the baffle and the axial impeller are located below the liquid surface by approximately the same distance.

10. The method of claim 9 , wherein the ratio of the vessel sidewall height to the vessel diameter is at least 3.

11. The method of claim 9 , wherein the vessel bottom is conical and has a slope of at least 45 degrees.

12. The method of claim 9 , wherein the flow is continuous.

13. The method of claim 1 , wherein the flow is capable of entraining solid particles having a settling velocity of up to approximately 1 foot per minute in the liquid and the speed of the impeller is chosen to enable particles having a desired settling velocity to settle to the vessel bottom.

14. The method of claim 9 , wherein the baffle extends approximately from the liquid surface such that the lower end of the baffle is located below the liquid surface by a distance that is approximately one-quarter to one-half of the height of the liquid.

15. The method of claim 14 , wherein the entire vessel wall below the lower end of the baffle is unbaffled.

16. The method of claim 14 , wherein the flow is capable of entraining solid particles having a settling velocity of up to approximately 1 foot per minute in the liquid and the speed of the impeller is chosen to enable particles having a desired settling velocity to settle to the vessel bottom.

17. A method of mixing a liquid comprising the steps of:

in a liquid in a vessel having an upper end, a lower end, and a substantially cylindrical containing wall extending between the upper and lower ends, and having a baffle having an upper end and a lower end;

rotating an axial impeller about a substantially vertical axis, said axial impeller having a means for adjusting the rotational speed and being submerged in said liquid to a position that is located in a zone defined by approximately one-quarter to one-half of the distance from said upper end to said lower end, and there is no impeller outside the zone;

thereby producing a flow in the liquid with the axial impeller, said flow comprising (a) an inner flow along said vertical axis, moving from the lower end toward the upper end, (b) an outward flow from the axial impeller toward the containing wall, and (c) an outer flow along the containing wall, moving from the upper end toward the lower end, the baffle defining a region of restricted rotational flow region above the impeller and an unbaffled, unrestricted rotational flow region below the lower end of the baffle.

18. The method of claim 17 , wherein said axial impeller and the lower end of the baffle are adapted for being located approximately the same distance below the liquid surface.

19. The method of claim 17 , wherein the flow is continuous.

20. The method of claim 17 , wherein said axial impeller is submerged in said liquid to a position that is located approximately one-third of the distance from said upper end to said lower end.

21. The method of claim 17 , wherein the flow is capable of entraining solid particles having a settling velocity of up to approximately 1 foot per minute in the liquid and the speed of the impeller is chosen to enable particles having a desired settling velocity to settle to the vessel bottom.

22. A method of mixing a liquid having particulate, comprising the steps of:

in a vessel for containing the liquid, the vessel including a sidewall, a bottom, and a baffle extending longitudinally along the sidewall approximately from the liquid surface, such that a lower end of the baffle is located below the liquid surface by a distance that is approximately one-quarter to one-half of the height of the liquid;

rotating an axial impeller about a substantially vertical axis, said axial impeller:

being submerged below the liquid surface by a distance that is approximately one-quarter to one-half of the height of the liquid;

oriented upwardly to produce (a) an inner, upward flow region located along said vertical axis, (b) a transition flow region located above the impeller in which liquid moves radially outwardly toward the vessel sidewall, and (c) an outer, downward flow region located along the sidewall, such that the flow includes a central vortex; and

being variable speed.

23. The method of claim 22 , wherein said axial impeller and the lower end of the baffle are located approximately the same distance below the liquid surface.

24. The method of claim 22 , wherein the flow is continuous.

25. The method of claim 22 , wherein the flow is capable of entraining solid particles having a settling velocity of up to approximately 1 foot per minute in the liquid and the speed of the impeller is chosen to enable particles having a desired settling velocity to settle to the vessel bottom.