BACKGROUND TO THE INVENTION Seed thickeners are employed in the Bayer process in an alumina refinery to increase the concentration of solid particles (seed crystals) of aluminium hydrate in a Bayer liquor.

The hydrate seed enters the thickener suspended in a concentrated feed slurry following precipitation, and the process of gravity thickening involves facilitating the settling and aggregation of the seed crystals onto the bottom of the thickener. The bottom of the thickener is generally of conical shape and a rotating rake moves the settled hydrate solids into a central underflow discharge outlet. Clarified or supernatant liquor overflows into an overflow launder or an overflow discharge pipe at the periphery of the thickener.

Flocculant may be added to the feed slurry to induce aggregation and settling of the solids.

Most thickeners are provided with a centrally located feedwell through which the concentrated feed slurry is fed into the thickener. The purpose of the feedwell is essentially two fold: (i) to contain the momentum of the feed slurry and allow mixing of the feed slurry with suitable added reagents, such as flocculants; and, (ii) to dissipate the kinetic energy of the feed slurry so as to inhibit degradation of the flocculated aggregates and avoid disturbing the bed of the thickener.

Some de-aeration of the feed slurry in the feedwell may also occur. Feedwells may be either open or closed in design depending on the percentage of solids in suspension in the feed slurry and the type of material in the feed slurry. Open type feedwells are generally used with feed slurries having a high percentage of solids with higher specific gravity such as in Bayer slurries. Closed type feedwells are typically employed with feed slurries having a lower percentage of solids with lower specific gravity, such as in coal washing settlers.

In an open type feedwell it is known to provide one or more shelves within the feedwell to support the feed slurry and prevent it falling through the feedwell before adequate mixing with flocculant has occurred. This is particularly necessary when the feed slurry has a high percentage of solids so that its density is significantly higher than that of the liquor already in the feedwell. However, such shelves tend to act in a similar fashion to an orifice plate or flow restriction in the feedwell and therefore tend to increase turbulence.

This results in variable feedwell discharge flow, uneven settling of solids and disturbance of the settling zone in the thickener. This in turn can contribute to an increase in the percentage of solids in the thickener overflow and hence loss of mineral production. Some open type feed wells do not require a shelf due to the flocculant being added prior to the feedwell and/or due to the low specific gravity of the solids in the feed slurry.

SUMMARY OF THE INVENTION The present invention was developed with a view to providing an improved feedwell design that produces less turbulence and provides more uniform feedwell discharge flow.

Throughout this specification the term"comprising"is used inclusively, in the sense that there may be other features and/or steps included in the invention not expressly defined or comprehended in the features or steps subsequently defined or described. What such other features and/or steps may include will be apparent from the specification read as a whole.

According to the present invention there is provided an improved feedwell for introducing slurry to a thickener or similar treatment vessel, the feedwell comprising:

a substantially cylindrical surface defining an inner wall of the feedwell and having an upper edge defining a mouth of the feedwell into which the slurry is fed; and, a substantially annular shelf provided at the mouth of the feedwell, and having an outer diameter larger than the diameter of the mouth of the feedwell whereby, in use, slurry discharged onto said shelf flows over the shelf and into the mouth of the feedwell.

Preferably said shelf is provided with a substantially cylindrical retaining wall having an inside diameter which is larger than the diameter of the mouth of the feedwell and substantially equal to the outer diameter of the shelf. In one form of the invention the improved feedwell is of the open type wherein the flow of slurry down through the feedwell once it enters the mouth of the feedwell is substantially unimpeded and streamlined.

Preferably said shelf has an inside diameter substantially equal to the diameter of the mouth of the feedwell. Alternatively, said shelf has an inside diameter slightly smaller than the diameter of the mouth of the feedwell.

Typically, the inside diameter of the inner wall of the feedwell is between 4.0m to 8.0m, for a thickener having an inside diameter of between 22.0m to 50.0m. Preferably, a feedpipe for discharging slurry onto the shelf is mounted tangential to the mouth of the feedwell. In the case where the feedwell rotates, the feedpipe remains stationary and is mounted so that it passes over the retaining wall of the shelf. In the case of a stationary feedwell, the feedpipe may pass through or end at the retaining wall of the shelf.

BRIEF DESCRIPTION OF THE DRAWINGS In order to facilitate a more detailed understanding of the nature of the invention, a preferred embodiment of the improved feedwell will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 illustrates a prior art open type feedwell; Figure 2 illustrates a preferred embodiment of the improved feedwell according to the present invention; Figure 3 illustrates the flow pattern and velocity vectors for a prior art feedwell using a Computational Fluid Dynamics (CFD) model; and, Figure 4 illustrates the flow pattern and velocity vectors for a preferred embodiment of the feedwell according to the invention using a CFD model.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT The open type feedwell 10 illustrated in Figure 1 is a prior art feedwell known as a Fitch feedwell after its inventor, E B Fitch. The Fitch feedwell 10 has three annular shelves 12 and a bifurcated feedpipe 14. The feedwell 10 remains stationary and a rotating rake drive shaft 16 passes down coaxially through the centre of the feedwell. Two streams of feed slurry enter tangentially and circulate in opposite directions in the two compartments or races created by the shelves 12. The two streams of slurry are displaced inward and forced out of the two races created by the shelves 12 by fresh incoming feed. The counter- rotation of the two streams of slurry as they descend down through the feedwell is intended to dissipate the kinetic energy of the feed slurry.

However, as noted above, the location of the shelves 12 within the feedwell tends to introduce a flow restriction and produce turbulence. Computational Fluid Dynamics (CFD) modelling (carried out by the CSIRO Division of Minerals) of a feedwell employing a single shelf confirms this increase in turbulence and the resulting variable feedwell discharge flow. Figure 3 illustrates the flow pattern and velocity vectors for a feedwell having a single shelf 18, using a CFD model. On the right hand side of Figure 3, the flow pattern down through the feedwell at the point of entry of the feed slurry onto the shelf 18 is illustrated, whereas in the left hand side of Figure 3 the flow pattern down through the feedwell at 1800 from the discharge entry of the feed slurry is shown, where

most of the feed slurry starts to be pushed off the shelf 18. It will be appreciated that as the feed slurry flows down through the feedwell, less dense liquor must flow upwards into the feedwell to replace the displaced fluid. As can be clearly seen in Figure 3, significant turbulent flow is created below the shelf 18 resulting in a variable feedwell discharge flow into the volume of the thickener.

In order to overcome this problem the present inventor has found that simply by moving the shelf away from the internal wall of the feedwell produces a substantial improvement in the feedwell discharge flow. Figure 2 illustrates a preferred embodiment of an improved feedwell 20 in accordance with the present invention. In the improved feedwell 20 a substantially cylindrical surface 22 defines an inner wall of the feedwell. There are no shelves or other projections on the inner wall 22 to impede or otherwise disturb the downward flow of feed slurry through the feedwell. An upper edge of the wall 22 defines a mouth 24 of the feedwell into which the slurry is fed via a feedpipe 26.

A substantially annular shelf 28 is provided at the mouth 24 of the feedwell, and has an inside diameter equal to the diameter of the mouth 24. The shelf 28 is provided with a substantially cylindrical retaining wall 30 having an inside diameter which is larger than the mouth of the feedwell and substantially equal to an outer diameter of the shelf 28.

Feed slurry discharged onto the shelf 28 from feedpipe 26 flows over the shelf and is contained by the retaining wall 30 until it flows into the mouth 24 of the feedwell by fresh incoming feed slurry.

The feedwell 20 of Figure 2 is of the open type and once the feed slurry enters the mouth 24 of the feedwell its flow down through the feedwell is substantially unimpeded and streamlined. The shelf 28 still performs the same function as a conventional shelf, namely, facilitating mixing of the feed slurry with suitable reagents such as flocculants, allowing floc attachment and dissipating the kinetic energy of the feed slurry. However, by relocating the shelf 28 above the mouth 24 of the feedwell, it does not restrict or impede the flow of slurry down through the feedwell as in prior art feedwells.

With the shelf 28 relocated above the mouth of the feedwell, it has been found in practice that the unimpeded flow of slurry down through the feedwell can product a quite strong

upwards flow of less dense liquor into the feedwell to replace the displaced fluid. If the upwards flow is too strong, it may be desirable to re-introduce a degree of turbulent flow below the shelf 28 to reduce the velocity of the downward flow of feed slurry through the feedwell. In this case, the annular shelf 28 may extend slightly inwards over the mouth 24 of the feedwell. Typically, the inner diameter of the shelf would not be more than 5% to 10% smaller than the diameter of the mouth 24 of the feedwell. This contrasts with a conventional internal shelf which has an inside diameter typically at least 20% smaller than the mouth of the feedwell. By extending the inner edge of the shelf 28 slightly over the mouth 24 of the feedwell, a controlled degree of turbulence can be created below the shelf to balance the downward flow of feed slurry with the upward flow of less dense liquor.

In the illustrated embodiment, the feedwell 20 is of the rotating type, as it is mounted on the drive mechanism for the rake (not shown). Hence, the feedpipe 26 is mounted independently of the feedwell and passes over the upper edge of the retaining wall 30 in order to feed the slurry tangentially onto the shelf 28. However, it will be understood that the same design philosophy can be applied to a stationary feedwell. In this case, the feedpipe may pass through or end at the retaining well 30 of the shelf 28. In Figure 2, the end of the feedpipe 26 is located along the centre line of the shelf 28, however it may also be located adjacent the edge of the shelf adjacent to the mouth of the feedwell.

The improved design, involving the relocation of the shelf 28, can also be applied to feedwells of the closed type, in order to improve the feedwell discharge flow.

Figure 4 illustrates the flow pattern and velocity vectors for a feedwell similar to that illustrated in Figure 2 using a CFD model. The elevations illustrated in Figure 4 are effectively that of a section view taken vertically through the centre of the feedwell, similar to Figure 3.

As can be seen in the left hand side of Figure 4, the flow of the feed slurry down along the inner wall of the feedwell is substantially streamlined and turbulence free. Hence, as the flow is discharged from the bottom of the feedwell it disperses uniformly throughout the

ambient liquor and settles uniformly towards the bottom of the thickener creating minimum disturbance in the settling zone and bed of the thickener. These flow characteristics have been verified by tracer tests and by actual trial plant results. In a typical Bayer refinery seed thickener, equipped with a conventional feedwell having an internal shelf similar to that illustrated in Figure 3, solids lost to overflow are estimated to be as high as 1 SOOmg/l. This represents a substantial loss of production. It is estimated that the improved design of feedwell can reduce overflow solids in this application by at least 40%. Similar benefits would also be expected in other mineral slurry process applications.

It will be apparent from the above description of a preferred embodiment of the improved feedwell that it provides a number of significant advantages over conventional feedwells, including the following: (a) more streamlined flow down through the feedwell produces a more uniform discharge flow; (b) there is an improvement in energy dissipation down the feedwell due to wall friction losses; (c) the discharge out of the bottom of the feedwell is more even, thus allowing more uniform settling and less disturbance to the bed in the thickener; (d) this in turn results in a decrease in solids entrained in the overflow liquor; and, (e) improved flocculation of the slurry allowing increased feed flow rates to be achieved.

Numerus variations and modifications will suggest themselves to persons skilled in the mineral processing arts, in addition to those already described, without departing from the basic inventive concepts. For example, two or more shelves could be provided above the mouth of the feedwell without in any way impeding the downward flow of slurry through the feedwell. All such variations and modifications are to be considered within the scope of the present invention, the nature which is to be determined from the foregoing description and appended claims.