Method and apparatus for mixing

1. An apparatus for mixing a liquid having particulate, the apparatus comprising:

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, the vessel having an impeller zone defined as a distance between approximately one-quarter to one-half of the distance from the surface of the liquid to the bottom of the vessel, wherein a lower end of the baffle is located in the impeller zone, and wherein the vessel bottom is conical; and

an axial impeller configured to rotate about a substantially vertical axis, said axial impeller located in the impeller zone, and there is no impeller outside the impeller zone;

said axial impeller 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 configured to operate at variable speed.

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

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

4. The apparatus of claim 1 , wherein the vessel bottom has a slope of at least 45 degrees.

5. The apparatus 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.

6. The apparatus of claim 2 , wherein the flow produced by the axial impeller is continuous.

7. The apparatus of claim 1 , wherein the vessel further includes a baffle extending longitudinally along the vessel sidewall approximately from the liquid surface to the axial impeller.

8. An apparatus for mixing a liquid comprising:

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

an axial impeller that is configured for operation at a rotational speed that is adjustable, said axial impeller:

adapted for rotating about a substantially vertical axis, such that the vessel and axial impeller are configured to produce a central vortex flow in the liquid when the axial impeller is positioned at a location that is approximately one-quarter to one-half of the distance from the surface of the liquid to the lower end of the vessel, said central vortex 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,

wherein the baffle defines a restricted rotational flow region above the axial impeller and an unbaffled, unrestricted rotational flow region below the lower end of the baffle.

9. The apparatus of claim 8 , 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.

10. The apparatus of claim 9 , 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.

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

12. The apparatus of claim 10 , wherein the vessel bottom is conical and has a slope of at least 45 degrees.

13. The apparatus of claim 10 , wherein the central vortex flow produced by the vessel and axial impeller is continuous.

14. The apparatus of claim 10 , 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 apparatus of claim 14 , wherein the entire vessel wall below the lower end of the baffle is unbaffled.

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

17. An apparatus for mixing a liquid comprising:

a vessel for containing the liquid, the vessel having an upper end, a lower end, an impeller zone, and a substantially cylindrical containing wall extending between the upper and lower ends, the impeller zone being defined by approximately one-quarter to one-half the distance from the surface of the liquid to the lower end of the vessel, the vessel including a baffle having an upper end and a lower end, the lower end of the baffle being located in the impeller zone; and

an axial impeller that is configured for operation at a rotational speed that is adjustable, the axial impeller capable of being located in the impeller zone and having no impeller outside the impeller zone, said axial impeller:

adapted for rotating about a substantially vertical axis, such that the vessel and axial impeller are configured to produce a central vortex flow in the liquid, 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 restricted rotational flow region above the impeller and an unbaffled, unrestricted rotational flow region below the lower end of the baffle.

18. The apparatus 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 apparatus of claim 17 , wherein the flow comprises a continuous flow.

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

22. An apparatus for mixing a liquid having particulate, the apparatus comprising:

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; and

an axial impeller configured to rotate about a substantially vertical axis, said axial impeller:

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

oriented upwardly such that the vessel and axial impeller are configured 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 configured to operate at variable speed,

wherein the baffle defines a restricted rotational flow region above the axial impeller and an unbaffled, unrestricted rotational flow region below the lower end of the baffle.

23. The apparatus 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 apparatus of claim 22 , wherein the axial impeller is further configured to produce a flow that is continuous.

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

26. The apparatus of claim 8 , wherein there is no impeller outside an impeller zone defined as approximately one-quarter to one-half of the distance from the surface of the liquid to the lower end of the vessel.

27. The apparatus of claim 22 , wherein there is no impeller outside an impeller zone defined as approximately one-quarter to one-half of the distance from the surface of the liquid to the lower end of the vessel.

28. The apparatus of claim 22 , wherein the ratio of the vessel sidewall height to the vessel diameter is at least 3.

29. The apparatus of claim 22 , wherein the vessel bottom is conical and has a slope of at least 45 degrees.