WO1996020777A1

WO1996020777A1 - Improved draft tube

Info

Publication number: WO1996020777A1
Application number: PCT/AU1995/000882
Authority: WO
Inventors: Philip Scott Arthur; Gregory Patrick Brown
Original assignee: Comalco Aluminium Limited
Priority date: 1994-12-30
Filing date: 1995-12-28
Publication date: 1996-07-11
Also published as: AUPN034694A0

Abstract

A draft tube (21) for agitating a liquor or slurry in a process vessel (20) includes a draft section (22) and a cylindrical section (23). The lower end of the draft tube is provided with a shaped projection (31), such as a tear drop shaped projection or an outward flaring, to minimise vortex formation in the liquor or slurry near the lower end of the draft tube. In other embodiments, flow directing vanes or an encircling wall are located around the bottom of the draft tube.

Description

TITLE: IMPROVED DRAFT TUBE

The present invention relates to an improved draft tube for agitating the fluid contents of a process vessel.

Vessels which contain liquors and/or slurries are frequently agitated to promote reactions. If the vessel contains a slurry, agitation is used to maintain the solids in suspension.

Draft tube agitators are used in a number of industries and are widely used in the production of alumina by the Bayer process. In the Bayer process, alumina is extracted from bauxite by dissolution in hot caustic liquor. Alumina is subsequently recovered from the caustic liquor by precipitation. The precipitation process is carried out in large tanks which are filled with slurry of caustic liquor and precipitated alumina, with the alumina being maintained in suspension. The tanks may have flat bottoms or conical bottoms.

Draft tube agitators used in the tanks for the precipitation of alumina generally consist of a long cylindrical tube installed vertically in the process vessel. Circulation of the slurry is achieved by a motor driven impeller which is located below the level of the fluid in the upper end of the tube. The impeller forces the fluid downwardly through the draft tube and this causes the fluid to circulate in an upwardly direction in the space between the outside of the draft tube and the wall of the vessel. The circulation of the fluid through the draft tube and in the vessel is controlled to ensure that the solid particles in the slurry remain suspended. Fluid generally flows in a downwardly direction through the draft tube, but it is also possible that the fluid could flow upwardly through the draft tube.

It is an object of the present invention to provide an improved draft tube.

In a first aspect, the present invention provides a draft tube for agitating a liquor or slurry in a process vessel, the draft tube including a cylindrical section having a shaped projection extending around the periphery thereof, the shaped projection having a generally tear-drop shaped cross-section in a radial direction. Preferably, the shaped projection is located at or adjacent to the lower end of the cylindrical section. Especially preferably, the lower part of the cylindrical section includes an opening through which the liquor or slurry can flow and the shaped projection is located around the opening.

In a second aspect, the present invention provides a draft tube for agitating a liquor or slurry in a vessel, the draft tube including a cylindrical section having a side wall and a bottom edge providing an opening through which the liquor or slurry can flow, the draft tube further including a shaped projection comprising a generally convex annular first portion extending from the bottom edge of the cylindrical section and merging with a second portion that extends upwardly and inwardly towards the side wall of the cylindrical section.

Preferably the second portion has an upper end that joins with or abuts the side wall of the cylindrical section.

The generally convex annular first portion may be of substantially semi-circular cross-section and may extend from the bottom edge of the cylindrical section and merge with the second portion near a line that intersects an imaginary horizontal plane extending outwardly from the bottom edge.

The power required for circulating the fluid contents of a vessel that is stirred by a conventional draft tube agitator is usually determined by the ratio of the diameters of the draft tube to the vessel, the circulation rate required and the efficiency of the impeller design. The present inventors have found that if the geometry of the draft tube is changed in accordance with the present invention, then the power required for circulation can be substantially reduced. Alternatively, the flow rate through the draft tube can be increased for a given power input.

Modelling studies carried out by the present inventors have shown that standard draft tubes having a straight cylindrical section and arranged for the fluid to flow downwardly through the draft tube result in a large volume of fluid recirculating around the lower end of the draft tube. This recirculating fluid forms a recirculating vortex near the lower end of the draft tube. This consumes a significant amount of energy and acts as an impediment to circulation of fluid around the vessel and through the draft tube. The inventors have found that using a draft tube in accordance with the present invention substantially reduces the size of the vortex and the flow of slurry through the draft tube is increased significantly for a given energy input. As a corollary, using a draft tube in accordance with the present invention results in significantly lower energy consumption to maintain a desired recirculation rate in the vessel, when compared to standard draft tubes.

Accordingly, in a third aspect, the present invention provides a draft tube for agitating a liquor or slurry in a process vessel, the draft tube including a cylindrical section having a shaped projection extending around the periphery thereof, the shaped projection being effective in use to minimise vortex formation near a lower portion of the cylindrical section in the liquor or slurry circulating within the process vessel.

In one embodiment, the shaped projection has a tear-drop shaped profile, when viewed in cross-section. In another embodiment, the shaped projection comprises an outward flaring of the bottom of the draft tube. Preferably, the outward flaring includes an arcuate projection that is downwardly convex. Indeed, the shaped projection may be of any shape that minimises or reduces the vortex in the liquor or slurry that would form near the lower portion of the draft tube in the absence of the shaped projection.

In another aspect, the present invention provides apparatus for agitating a liquor or slurry in a process vessel including a process vessel for holding the liquor or slurry, a draft tube within the tank and mounted in a generally vertical orientation and means to cause the liquor or slurry to flow through the draft tube wherein the draft tube and/or process vessel have a geometry or geometries to minimise vortex formation near a lower portion of the draft tube in the liquor or slurry circulating within the process vessel.

In one embodiment of this aspect of the invention, the geometries of the draft tube and the process vessel may be chosen and the spacing between the draft tube and the walls of the process vessel arranged such that vortex formation is minimised.

The apparatus may be provided with one or more flow directing vanes located exteriorly to the draft tube. Preferably the vane(s) extend around the draft tube and are located near the level of the lower portion of the draft tube. The vanes may be suspended from the draft tube, for example, by suspension rods extending from the draft tube. Alternatively, the vane(s) may be suspended from the process vessel, for example, by suspension rods extending from the sides and/or bottom of the process vessel.

In another embodiment of this aspect of the present invention, the process vessel is fitted with an interior wall that extends upwardly from the floor of the process vessel to a level at or above the lower edge of the draft tube. Preferably the interior wall substantially encircles the lower edge of the draft tube.

The draft tube may be as described with reference to any one of the first, second or third embodiment of the invention, or it may be a conventional draft tube.

The shape of the tank may also be designed to minimise vortex formation. For example, the tank may have a flat bottom with upturned corners between the bottom and side walls. Alternatively, the tank bottom may have a conical projection positioned below the draft tube and extending upwardly towards the draft tube. The conical bottom may form part of a false bottom in the tank.

The present invention will now be described in greater detail with reference to the accompanying drawings. In the drawings, which show preferred embodiments of the invention: FIGURE 1 is a schematic diagram showing a standard

(prior art) draft tube agitator; FIGURE 2 is a schematic diagram showing a draft tube agitator in accordance with one embodiment of the invention; FIGURE 2a is a three-dimensional view of the draft tube shown in FIGURE 2,* FIGURE 3 is a schematic diagram showing a draft tube agitator in accordance with another embodiment of the invention; FIGURE 4 is a schematic diagram showing a draft tube agitator in accordance with a further embodiment of the invention; FIGURE 5 is a schematic diagram showing a draft tube agitator in accordance with yet another embodiment of the invention; FIGURE 6 is a plot of velocity vectors and axial velocity contour at 0.0 m/s for the arrangement shown in FIGURE 1; FIGURE 7 is a plot of velocity vectors and axial velocity contour at 0.0 m/s for the arrangement shown in FIGURE 2; FIGURE 8 is a magnification of the velocity vectors of

FIGURE 7 near the draft tube exit; FIGURE 9 is a plot of velocity vectors and axial velocity contour at 0.0 m/s for the arrangement shown in FIGURE 3; FIGURE 10 is a magnification of the velocity vectors of

FIGURE 9 near the draft tube exit; FIGURE 11 is a plot of velocity vectors and axial velocity contour at 0.0 m/s for the arrangement shown in FIGURE ; FIGURE 12 is a magnification of velocity vectors in the draft tube inlet region for the geometry shown in FIGURE 2; FIGURE 13 is a magnification of the velocity vectors near the draft tube inlet for the geometry shown in

FIGURE 4; and FIGURE 14 is a plot of velocity vectors and axial velocity contour at 0.0 m/s for the geometry shown in FIGURE 5. FIGURE 15 is a cross-sectional view of an apparatus of another embodiment of the invention; FIGURE 16 is a three-dimensional perspective view of the lower part of the draft tube and the exterior wall of FIGURE 15, with details of the tank being omitted; FIGURE 17 is a cross-sectional view of another embodiment of the invention; and FIGURE 18 is a perspective view of the lower part of the draft tube and the vanes of FIGURE 17 with details of the tank being omitted.

Referring to FIGURE 1, a slurry, such as a slurry comprising alumina particles in a caustic liquor as is used in the Bayer process, is circulated within a process vessel 10. A draft tube 12 includes a draft section 13 and a cylindrical section 14. An impeller 15 is located in an upper part of draft tube 12. Impeller 15 is driven by a motor (not shown) connected to shaft 16. Impeller 15 causes the slurry to pass into the top of draft tube 12 and flow downwardly through the draft tube, as shown by large arrow 17. The slurry exits the draft tube via bottom opening 18 and then flows in a generally upwardly direction outside the draft tube, as shown by the smaller arrow 19.

In a typical Bayer process precipitation vessel, the velocity of the slurry through the draft tube is typically between 1 m/s and 2 m/s. In the event of blockage of the draft tube, due to for example power failure, a typical draft tube is provided with re-suspension slots (not shown) which enable resuspension of the solids. The operation of such slots is well known to the skilled person and will not be described further.

FIGURE 2 shows a draft tube in accordance with the present invention. In FIGURE 2, a slurry is circulated within a process vessel 20. Vessel 20 is provided with a generally vertically oriented draft tube 21 having a draft section 22 and a cylindrical section 23. An opening 24 is provided in the bottom part of cylindrical section 23. An impeller 25, driven by a motor (not shown) connected to drive shaft 26, causes slurry to circulate within the vessel 20. Impeller 25 causes the slurry to flow downwardly through the draft tube 21, as shown by large arrow 27. Slurry exiting the draft tube 21 through opening 24 flows in a generally upwardly direction (as shown by arrows 28,29) in the annular space between the walls of the vessel 20 and the outside of the draft tube. The slurry enters draft section 22 to complete the circulation. The cylindrical section 23 of the draft tube 21 also has a shaped projection 30 extending around the periphery thereof. The shaped section 30 is of a generally tear-drop shape, when viewed in a radial cross-section as shown in FIGURE 2. The shaped projection 30 includes a first convex annular portion 31 extending from the bottom edge of the cylindrical section 23 that defines opening 24 in the lower end of the draft tube. The first convex annular portion 31 merges with a second portion 32 at a line designated in FIGURE 2 by reference numeral 33. Second portion 32 extends upwardly and inwardly towards the side wall of cylindrical section 23 and the upper end of second portion 32 joins or abuts the side wall of cylindrical section 23.

Although not shown in Figure 2, the draft tube will include resuspension slots. The shaped projection 30 should be arranged such that the shaped projection does not extend over the resuspension slots. This may be achieved, for example, by forming the shaped projection 30 as a series of segments extending along the periphery of the draft tube between the resuspension slots.

Although FIGURE 2 shows the shaped projection 30 as being a solid body, it will be appreciated that the shaped projection 30 may be formed as a hollow body. Projection 30 extends around the outlet of the draft tube and acts to improve the flow patterns near the bottom of the draft tube. A three- dimensional view, partly in cross-section of the draft tube of FIGURE 2 is shown in FIGURE 2a.

.Another embodiment is shown in Figure 3. Figure 3 is generally similar to Figure 2 and like features have been given the same reference numerals with the addition of a prime ("1") . Figure 3 differs from Figure 2 in that the tear-drop shaped projection 30 of Figure 2 in that the tear-drop shaped projection 30 of Figure 2 has been replaced with an arcuate projection 40. Arcuate projection 40 is positioned to extend from the lower periphery of the draft tube and results in the draft tube having a flare at its lower end.

In a further embodiment shown in Figure 4, the draft tube is provided with another tear drop shaped projection 42 around the upper periphery of the draft tube. Projection 42 acts to smooth the flow of liquor or slurry into the top of the draft tube.

The embodiment shown in Figure 5 is similar to that shown in Figure 2, with the exception that the bottom of tank 10 includes a flat portion 44, upturned corners 46, 48 between the side walls and bottom of the tank and a conical projection 50 located below the draft tube and extending upwardly towards the draft tube.

In order to assess the designs shown in Figures 2 to 5, computational fluid dynamic simulations were conducted. The following parameters were used in the simulations: Solids content 210 g/1 Solids density 2420 kg/m³ Liquor density 1260 kg/m³ Liquor viscosity 1.4 cp

Slurry viscosity 15 cp A particle size of 90 microns was used and the flow rate through the draft tube was set at 40000m³/hr. These conditions are based upon typical operating parameters for a precipitation tank in the Bayer process. For comparison purposes, the standard design (prior art) shown in Figure 1 was also modelled in the simulations.

Figure 6 shows a plot of velocity vectors in the process vessel obtained from the simulation of the prior art arrangement shown in Figure 1. Figure 6 also shows the axial velocity contour at 0.0 m/s for the circulating fluid. This axial velocity contour indicates the areas in the tank where the upflowing fluid changes direction and begins to flow downwardly. As can be seen from Figure 6, a large vortex of recirculating fluid is formed in the annular space between the draft tube and the process vessel walls near the lower part of the draft tube. This vortex is generally indicated by reference numeral 40. It will be appreciated that the energy consumed by this vortex is wasted energy that results in an increase in the energy input to the impeller required to maintain the circulation of slurry throughout the vessel .

Figure 7 shows a plot of velocity vectors and the axial velocity contour at 0.0 m/s in the process vessel for the simulation of the arrangement shown in Figure 2. As can clearly be seen, the presence of the tear drop projector around the lower periphery of the draft tube markedly reduces the volume of fluid involved in the recirculation region at the bottom of the tank between the draft tube and the tank wall. The reduction in the size of this vortex caused by the presence of the tear drop lowers the power requirement for circulation in the arrangement shown in Figure 2 by 27%, when compared to the power requirements for■circulation in the standard design shown in Figure 1. Despite the tear drop shaped projections shown in Figure 2 reducing vortex size and power requirements, further modelling studies have shown that the design is not optimum for the particular operating parameters used in the simulation. Figure 8 shows a magnification of the velocity vectors near the draft tube exit for the arrangement of Figure 2. The figure shows that, on leaving the draft tube, the flow briefly attaches to the baffle, but then quickly detaches from it and follows the floor of the tank before reattaching to the draft tube just above the top of the baffle. A recirculation is then set up, with fluid moving down the outside wall of the baffle before recombining with the flow leaving the draft tube. This is not a desirable flow pattern, but the presence of the teardrop baffle still results in a reduction, compared to the standard tank design (Figure 1) , of the volume of fluid involved in the recirculation region.

One drawback of applying the teardrop baffle to the tank design in tanks that have a conical bottom (such as that shown in FIGURE 7) is that it results in a greater build-up of solid particles at the tank bottom. The baffle helps to draw the fluid leaving the draft tube toward the outer wall of the tank, which results in a larger zone of stagnant fluid forming at the tank bottom. This is clearly shown by the vector plots in figure 7. This change to the flow results in a larger degree of solids segregation at the tank bottom. The maximum solids concentration at the tank bottom in the standard case is 591 g/1. In the case of Figure 2, this value is 829 g/1. With flat bottom tanks, settling at the bottom of the tank is not believed to be such a problem.

Many factors influence the performance of the baffle, such as flowrate, particle size and baffle geometry. It appears likely that the efficiency of the baffle at the current flowrate and particle size (40000 m³/hr, 90 microns) could be increased by locating the baffle lower in the tank, and by increasing its width. This would allow the flow exiting the draft tube to remain attached to the baffle longer and hence would eliminate the recirculation region underneath the baffle. However, it is likely that the flow would then separate from the baffle towards the end of the arc section, creating a recirculation on the upper surface on the baffle. Also, as discussed above, increasing the extent to which the fluid is drawn away from the bottom of the tank would increase the degree of segregation of the particle and liquor phases at the tank bottom. Reducing the flowrate would also move the separation point on the baffle radially outward, and there will be a flowrate at which the flow would remain fully attached to the baffle surface, thus eliminating any recirculation in this region. A disadvantage in this case is that, by keeping the flow attached to the draft tube, the flow may separate from the tank floor before reaching the outer wall . This would result in the formation of a recirculation region next to the outer wall of the tank. Reducing the flowrate would again increase the build-up of solid particles at the tank bottom due to reduced penetration of the flow into this area.

In the tank geometry shown in Figure 3, the bottom of the draft tube has been modified through the addition of a quarter- circle shaped flare. A vector plot for this design is given in Figure 9 and when compared to the vector plot for the standard tank geometry (Figure 6) , it can be seen that the addition of the flare significantly alters the flow patterns in the tank. The results show that the flare acts in a similar fashion to the tear-drop shaped projection in that it helps to bend the fluid exiting the draft tube towards the outer wall of the tank.

This effect can be seen more clearly in figure 9, which shows a magnification of velocity vectors in the draft tube exit region of Figure 8. This figure shows that, for the geometry of Figure 3, the flow leaving the draft tube briefly attaches to the flare, but then quickly detaches from it and follows the floor to the tank. However, the geometry of the flare causes a low pressure region to form on its upper surface, which effectively pulls the flow back toward the draft tube and results in very rapid reattachment of the flow to the draft tube surface. A recirculation is then set up, with fluid moving down the top of the flare and recombining with the flow leaving the draft tube. The reattachment of the flow to the draft tube occurs so suddenly, in fact, that a significant secondary recirculation is set up next to the outer wall of the tank.

As a result of reducing the volume of fluid involved in the recirculation region, the geometry of Figure 3 shows a lower power requirement than the standard design (see table 1) . However, the power requirement is higher than that of Figure 2, where a teardrop baffle is utilised at the draft tube exit. This is due to the difference in flow patterns generated in the draft tube exit region by the two designs. Figure 9 shows that, although the teardrop baffle is not totally efficient at this flowrate and particle size, the shape of the baffle prevents any rapid changes in flow direction. The flare, however, causes the flow exiting the draft tube to reattach to the draft tube surface very rapidly, and in doing so the flow is forced to undergo a turn of nearly 270°. This turn requires more work to be carried out on the fluid than the less sudden change in direction brought about by the teardrop baffle in Figure 2, hence the higher power requirement for the Figure 3 geometry. It should also be noted that the open shape of the flare means that under most flow conditions a low pressure region will form on its upper surface, resulting in fluid recirculation. Hence it may not be possible to optimise the flow to the same extent using a flare design, as could be achieved using a teardrop baffle.

.Another effect of the flare design is to increase the amount of solids build-up at the tank bottom with respect to the standard tank design. This occurs because, as with the teardrop baffle design, the flare helps to bend the flow exiting the draft tube toward the outer wall, hence reducing penetration of the flow toward the bottom of the tank. The maximum solids concentration at the tank bottom in Figure 3 is

769 g/1, compared to a value of 591 g/1 in the standard case.

The geometry of Figure 4 is similar to that of Figure 2, but with the addition of a teardrop shaped projection to the entrance to the draft tube. This projection serves as a replacement for the more conventional flared inlet to the draft tube seen in the standard design of Figure 1. The dimensions of this projection are identical to the projection employed at the draft tube exit. Comparison of the vector plots in figures 7 and 11 shows that alteration of the geometry at the top of the tank has not changed the flow patterns in the lower two-thirds of the tank, with respect to Figure 7. Again, this results in a very similar level of solids build-up at the tank bottom compared to the geometry shown in Figure 2.

Figures 12 and 13 show a magnification of velocity vectors in the draft tube inlet region for Figures 2 and 5. These Figures show that the standard inlet geometry (Figure 2) forces the flow on the outside of the tank to turn very sharply in order to enter the draft tube. The addition of the teardrop projection in Figure 4 results in a far smoother transition in the flow. The lower momentum of the flow at the top of the tank allows the flow to attach to the baffle and be drawn smoothly from the outside of the tank into the draft tube . No separation of the flow from the baffle occurs during this process. Figure 4 shows a 10% power reduction, compared to Figure 2, as a result of improving the flow patterns in the draft tube inlet region (see Table 2) .

The tank and draft tube geometry shown in Figure 5 is similar to that shown in Figure 2 in that the draft tube has a tear drop shaped projection at the exit of the draft tube. Figure 5 differs from Figure 2 in that it also includes a false bottom on the tank having a conical centre section.

Figure 14 shows the velocity vector plot for the geometry of Figure 5. Again, the tear drop projection at the bottom of the draft tube reduces the size of the recirculation region between the draft tube and the tank wall at the bottom of the tank. Furthermore, modelling studies have shown that there is no significant segregation of particles from the liquor phase, thereby avoiding settling of particles in the bottom of the tank.

A summary of the modelling studies are presented in Table 1. The modelling studies also calculated the power consumption required for each geometry. The calculated power consumption is an ideal value that ignores mechanical losses. Accordingly, the relative values of power consumption are more meaningful than the actual figures. Therefore, the power consumption results have been normalised by dividing the calculated power consumption for each geometry by the power consumption calculated for the standard draft tubes shown in Figure 1.

TABLE 1: Summary ofSimulation Results

Geometry Particle Average Flowrate Average Maximum Normalised

Size Velocity in (m³/hr) Solids Solids Impeller (microns) Draft Tube Content in Content in Power

(m/s) Tank (g 1) Tank (g/1)

Figure 1 90 1.91 40 250 210.6 591 1.0

Figure 2 90 1.90 40 000 212.7 829 0.69

Figure 3 90 1.90 40 000 211.6 769 0.87

Figure 4 90 1.91 40 250 211.8 806 0.62

Figure 5 90 1.89 39 750 212.3 392 0.69

As can be seen from Table 1, the draft tube of the present invention can provide significant energy savings when compared to standard draft tubes, which can significantly lower the energy costs of the plant.

The change in geometry represented by the present invention can be applied to existing draft tubes (for example, by retrofitting existing draft tubes with the shaped projection) and it can be incorporated into future installations.

In addition there may be other advantages particularly where solids are circulated in a liquor for purposes of precipitation for example, in the alumina hydrate precipitation process of a Bayer Aluminium Refinery. In Bayer plants using draft tube agitators in precipitation there is a tendency in most cases to form large particles commonly called "gravel" which are too big to be suspended by the normal agitation. These particles are formed in the recirculation vortex at the bottom of the draft tube and have to be frequently removed to avoid accumulation.

Changing the geometry by installation of the "tear-drop" configuration will assist in minimising or eliminating the formation of the gravel and hence the requirement for frequent removal of larger material. This will lead to improvement in overall operation of the vessels and result in cost savings.

The invention also extends to an apparatus for agitating a liquor or slurry that includes a process vessel and a draft tube, with the vessel and/or draft tube having geometry to minimise vortex formulation. The shaped projections on the lower edge of the draft tube are one embodiment that achieves this aim. Other embodiments that also achieve this aim are shown in FIGURES 15 to 18.

In FIGURES 15 and 16, the apparatus includes tank 100 having a flat bottom 101. Draft tube 102 is suspended in the tank such that the exit 103 from the draft tube is positioned above tank bottom 101. A cylindrical wall 104 is placed around the exit 103, with the wall 104 extending upwardly from the bottom 101 of the tank. The cylindrical wall 104 is effective to straighten the fluid flow, which minimises vortex formation near the exit of the draft tube.

FIGURES 17 and 18 show another embodiment in which a tank 110 having a flat bottom 111 has a draft tube 112 suspended therein such that draft tube exit 113 is located above tank bottom 111. A plurality of cylindrical vanes 114 are suspended in the tank near the exit 113 of the draft tube 112. The vanes 114 may be suspended from the draft tube or they may be suspended from the tank. The vanes are effective to straighten the fluid flow and thereby act to minimise vortex formation in the vicinity of the lower end of the draft tube.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically disclosed. It is to be understood that the invention is considered to encompass all such variations and modifications that are all within its spirit and scope.

Claims

CLAIMS :

1. A draft tube for agitating a liquor or slurry in a process vessel, the draft tube including a cylindrical section having a shaped projection extending around the periphery thereof, the shaped projection being effective in use to minimise vortex formation near a lower portion of the cylindrical section in the liquor or slurry circulating within the process vessel.

2. A draft tube as claimed in claim 1 wherein the shaped projection has a generally tear-drop shaped profile, when viewed in cross-section in a radial direction.

3. A draft tube as claimed in claim 1 wherein the shaped projection comprises an outward flaring of the bottom of the draft tube.

4. Apparatus for agitating a liquor or slurry in a process vessel including a process vessel for holding the liquor or slurry, a draft tube within the tank and mounted in a generally vertical orientation and means to cause the liquor or slurry to flow through the draft tube wherein the draft tube and/or process vessel having a geometry or geometries to minimise vortex formation near a lower portion of the draft tube in the liquor or slurry circulating within the process vessel.

5. Apparatus as claimed in claim 4 wherein one or more flow-directing vanes are located exteriorly of the draft tube, said one or more vanes located near the level of the lower edge of the draft tube.

6. Apparatus as claimed in claim 5 wherein the one or more vanes are suspended from the draft tube.

7. Apparatus as claimed in claim 5 wherein the one or more vanes are suspended from the process vessel .

8. Apparatus as claimed in claim 5 wherein the process vessel includes an interior wall extending upwardly from the floor of the process vessel to a level at or above the lower edge of the draft tube.

9. Apparatus as claimed in claim 8 wherein the interior wall substantially encircles the lower edge of the draft tube.

10. Apparatus as claimed in any one of claims 4 to 9, wherein the draft tube is provided with a shaped projection extending around the bottom periphery thereof.

11. Apparatus as claimed in claim 10 wherein the shaped projection has a tear-drop profile, when viewed in cross- section in a radial direction.

12. Apparatus as claimed in claim 10 wherein the shaped projection comprises an outward flaring of the bottom of the draft tube.

13. Apparatus as claimed in claim 10 or claim 11 wherein the process vessel includes a conical projection located below the draft tube and extending upwardly towards the draft tube.

14. Apparatus as claimed in claim 13 wherein the conical projection is part of a false bottom on said process vessel.

15. A draft tube for agitating a liquor or slurry in a process vessel, the draft tube including a cylindrical section having a shaped projection extending around the periphery thereof, the shaped projection having a generally tear-drop shaped cross-section in a radial direction.

16. A draft tube as claimed in claim 15 wherein the shaped projection is located at or adjacent a lower end of the cylindrical section.

17. A draft tube as claimed in claim 16 wherein the lower part of the cylindrical section includes an opening through which the liquor or slurry can flow and the shaped projection extends around the opening.

18. A draft tube for agitating a liquor or slurry in a vessel, the draft tube including a cylindrical section having a side wall and a bottom edge providing an opening through which the liquor or slurry can flow, the draft tube further including a shaped projection comprising a generally convex annular first portion extending from the bottom edge of the cylindrical section and merging with a second portion that extends upwardly and inwardly towards the side wall of the cylindrical section.

19. A draft tube as claimed in claim 18 wherein the second portion has an upper end that joins with or abuts the side wall of the cylindrical section.

20. A draft tube as claimed in any one of claims 1 to 3 or 15 to 19 wherein the draft tube includes a plurality of resuspension slots.

21. Apparatus as claimed in any one of claims 4 to 14 wherein the draft tube includes a plurality of resuspension slots.

22. A draft tube for agitating a liquor or slurry in a vessel, characterised in that the draft tube includes a projection extending around an outlet through which the liquor or slurry exits the draft tube, said projection smoothing the flow of liquor or slurry in the vicinity of the projection.