OA16322A - Apparatus and method for refining a process liquor by gravity settling. - Google Patents

Abstract

An apparatus for refining a process liquor that includes solids, which apparatus includes a vessel having a base and a side wall that define an internal volume for containing the process liquor and for allowing gravity settling of the solids in the liquor, whereby to produce a refined liquor toward a top of the internal volume and a slurry toward a bottom of the internal volume, the apparatus further includes solids displacement elements disposed within the internal volume for directing settled solids and/or settling solids in the vicinity of the side wall or of the base toward a flow path of the slurry being extracted from the slurry outlet. A processing plant including the above refining apparatus and a method for refining a process liquor.

Description

APPARATUS AND METHOD FOR REFINING A PROCESS LIQUOR i BY GRAVITY SETTLING i

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Field of the Invention

This invention relates to an apparatus and method for extracting solids from a process liquor and in particular to an apparatus and a method for extracting gravrty settled solids, including précipitâtes, from a process liquor.

This invention has particular, although not exclusive, application to industrial processes for preparing viscous stresms with solids, including a high volume of precipitated aluminium hydroxide in the course of producing alumina from bauxite by the Bayer process.

Background

In the course of the Bayer process, aluminium hydroxide is precipitated from a process liquor by controlling prccess conditions associated with the liquor The precipitate settles at the bottom of a vessel referred to as a thickener and the process of causing précipitâtes to settle out of a liquor particles having a range of sizes, including some coarser and some finer sizes. The proportion of particles in the coarser and finer ranges of size dépends on the process conditions. I is referred to as thickening. The précipitâtes will be in the form of

I quor toward the bottom of the thickener being high in coarser very viscous, and the liquor near the top of the vessel being very

Thickening results in I precipitate content, and low in finer precipitate content. The process liquor may pass through a sériés of thickeners; some of w précipitâtes and others create a so-called ‘clear liquor”. The clear liquor is removed and subjected to further processing steps before being retumed to the Bayer process as a caustic liquor used in digestion of bauxite.

-rich hâve the effect of separating coarse précipitâtes from fine substantially removing fine précipitâtes from the process liquor to

Thickeners used to separate the coarse precipitate from the fine precipitate are referred to as classifiers. It will be appreciated that the term thickener” as used herein after includes a reference to a classifier

The viscous liquor (als<

thickener by pumping. However, the high viscosity of the slurry may cause a preferentiel flow path in the vicinity outside the flow path settle and build-up within the thickener. The formed preferential flow path is termed a “rat-ho o termed a “slurry”) is typically removed from the bottom of the of the outlet of the thickener for the slurry. As a resuit, the solids e”.

The effect of rat-holing is that the settled precipitate reduces the overall operational volume within the thickener. This means that process liquor has a shorter résidence time in the thickener and, therefore, the viscous slurry extracted from the thickener has a precipitate content that lis lower than desired for subséquent processing. This also causes i an increase in the precipitate content and precipitate size in process liquor extract from near the top of the thickener, thus affecting negatively the performance of the subséquent processing steps. i

Thickeners must, therefore, be shut-down and the settled precipitate cleaned out every two months on average in order to maîntain suitable precipitate content in the extracted slurry.

Occasionally, settled precipitate will dislodge and fall into the preferential flow path so that the extracted slurry will hâve random spikes in precipitate content. This makes downstream processing difficult and nécessitâtes additional control steps to ensure that

I precipitate content of the extracted slurry is reasonably consistent.

There is a need, theefore, to reduce “rat holes’ and build-up of settled solids in thickeners. It will be appreciated that the term “solids' used throughout this spécification includes précipitâtes.

Summary of the Disclosure

The applicant has recognised that the effects of rat-holing can be reduced by reintroducing settled and settling solids into the flow stream of highly viscous slurry exiting from the base of a thickener. In particular, the applicant has recognised that directing settled solids and highly viscous slurry near side walls of a thickener vessel into the vicinity of the flow path extent to which solids build up around the side walls of a thickening vessel.

of slurry exiting the thickener vessel has the effect of reducing the

According to a first aspect, there is provided an apparatus for refining a process liquor that includes solids, which apparatus includes:

i a vessel havini containing the process liquor and for allowing gravity settling of the solids in the liquor, whereby a slurry toward (a) g a base and a side wall that define an internai volume for to produce a refined liquor toward a top of the internai volume and a bottom of the internai volume;

(b) a refined liquor outlet at or near the top of the internai volume for extracting the low solids content clarified liquor;

(c) a slurry outlet at or near the bottom of the internai volume for extracting the slurry unrestrictedly open to said internai volume; and i

(d) solids displacement éléments extending across and fixed relative to the vessel, wherein the solids displacement éléments are disposed within the internai volume, for directing settled solids and/or settling solids in the vicinity of the side wall or of the base toward a flow path of the sluny being extracted from the slurry outlet at or near the bottom of the internai volume.

The process liquor may be a Bayer process liquor containing precipitated aluminium hydroxide. ί

The solids displacement éléments hâve a two-fold effect. Specifically, the solids displacement éléments move the solids doser engage solids in the vicinity of the side walls or of the base and to the flow path of slurry being extracted from the vessel. These slurry that is extracted from the vessel. The thickened slurry, solids are therefore brought back into entrainment in the flow path and contribute to producing a thickened therefore, has a higher solids content than process liquors thickened without the solids displacement éléments i

The solids displacement éléments also hâve the effect of causing solids that hâve settled near a side wall or near the base of the vessel to move, such as by causing avalanches and cascades of the settled solids, from régions of high solids build-up to régions of low solids build-up. Such advancing through the apparatus and exiting the vessel through the slurry outlet at or near the base of the vessel and, also, régions where the solids displacement éléments engage the settled solids and bring them into the flow path of the slurry exiting the vessel.

régions of low-solids build-up are régions where the slurry is t éléments hâve a further effect of agitating liquor in the régions in

The solids displacemer which the solids displacement éléments are disposed so as to prevent settling of solids in those régions. This agitation is important for re-entraining previously settled solids back into die slurry. It is also liquor remain entraînée walls and other régions important for ensuring that solids gravity settling out of the process with solids pulled back into entrainment after settling at the side of solids build-up.

Consequently, the solids displacement éléments are not actually used for transporting or removtng solids out of producing a thickened settled near a side wal the régions in which sai he vessel. Instead, the solids displacement éléments are used for slurry having higher solids content, for causing solids that hâve or near the base of the vessel to move and for agitating liquor in d solids displacement éléments are disposed.

ln accordance to one aspect of the invention, the solids displacement éléments are disposed for directing solids in the vicinity of the side wall or of the base toward a flow path of the slurry. ln a ny cases including the case when the base of the vessel is not horizontal, the solids dïsplacement éléments can be disposed for directing solids in the vicinity of a wall of the vessel that is above the bottom of said vessel, either the side wall or the base, toward a flow path of the slurry.

In exemplary embodiments of the présent invention, a portion of the solids in the vicinity of the side wall, i.e. the settled solids and/or settling solids, is physically engaged by the displacement éléments^ By the term physically engaged we mean that one or more parts of the displacement éléments contact the portion of the solids in the vicinity of the side wall or of the base în such a way that the portion is moved in the vessel towards a flow path of the slurry being extracted from the slurry outlet, this flow path being oriented toward the base of the vessel.

The tops, at least one of the sides, preferably ail sides, and more preferably the bottoms of the displacement éléments are unshielded and exposed to the solid in the internai volume of the vessel. |n other word, dïsplacement éléments are preferably unshielded, unconfined or not blocked by any other member in the internai volume of the vessel.

The displacement éléments hâve preferably an elongated shape. The elongated displacement éléments are preferably exposed, unshielded, unconfined or not blocked by any other member around said éléments, at least over one part of the length of the element, preferably the majority the length, i.e. more than 50% of its length. This allows unconfined and unrestricted access of the sluny to the displacement éléments at least along the majority of its length, within the vessel and preferably along its entire length within the vessel. The access to the displacement éléments in this way should not encounter any constriction or choke points that cause bridgîng or blocking of the slurry flow as it advances into contact with the element. The slurry should therefore not be caused to pass through narrow openings before reaching the element from the interior of the vessel. Slurry of high viscosity will normally flow under the effects of gravity if there are no confining surfaces or articles to restrict the downward flow.

Preferably the slurry outlet is formed in the base of the vessel.

The slurry outlet is unrestrictediy open to the internai volume of the vessel. In other words, the slurry outlet is in open communication with the intemal volume of the vessel and unshielded, unconfined or not blocked by any other member inside the intemal volume of the vessel.

The solids displacement éléments may be arranged to direct solids radially inwardly toward a flow path of slurry being extracted from the sluny outlet toward the base of the vessel. Preferably the slurry outlet at or near the base of the vessel is located centrally of the vessel.

The solids displacement éléments may be rotatable screws. It is thought that the rotatable screws are efficient foi' directing solids without causing agitation at upper levels in the internai volume which might disturb gravity settling of solids from this région.

Accordingly, the solids displaœment éléments are preferably disposed in a lower région of the internai volume ofj the thickener vessel where solids build-up from settling solids typically occurs. | i

There may be at least one rotatable screw for directing solids toward a flow path of slurry being extracted from the slurry outlet. Altematively, there may be at least two rotatable screws. In one particuler form, there may be 4 rotatable screws, with each being arranged at about 90° to an adjacent rotatable screw. In another from, there may be sets of two rotatable screws parallel to each other, and optionally there may be two or more sets.

In either case, the rotatable screws are arranged such that solids are directed by the rotatable screws to a central région that coïncides with the position of the flow path of the slurry leaving the vessel via the slurry outlet at or near the bottom of the vessel.

The rotatable screws may be formed with two or more sections and each section may be a different length. The screw in each section may hâve a different pitch and/or a different diameter. i

Each rotatable screw may hâve a first section with a first diameter and/or a first pitch length and a second section with a second diameter and/or a second pitch length and wherein the first diameter and/or the first pitch length is smaller than the second diameter and/or second pitch length.

The first section may be adjacent the side wall of the apparatus.

The rotatable screws may be formed on respective cross-aligned shafts, each shaft may pass through the side wall and be coupled to a driver. The screw on a first end of the shaft may be opposite handed to the screw on the other end of the shaft. This allows two screws on a single shaft and rotating in the same direction to each transport material toward the centre of the vessel and away from the side wall of the vessel.

The driver may comprise an motor and a gear box for controlling rotation of the rotatable shafts. The motor may be electric or hydraulic.

The rotatable screws may be formed to provide a nominal throughput in the range of 50 to

200 m3/h each under normal operating conditions, but preferably 80 to 120 m3/h.

Preferably, the rotatable screws rotate at a speed and are sized so that they are able to transport toward the centre of the vessel solids in the range of 50% to 100% of the total solids exiting the vessel through the outlet at the base of the vessel. The rotatable screws may transport at least 80% or even up to at least 90% of the total solids exiting the vessel.

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The refined liquor may hâve a solids content in the range of 0 to 25 vol%.

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The thickened slurry may hâve a soîids content in the range of 10 to 60 vol%.

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The apparatus may be [a thickener vessel for thickening or classifying a process liquor in a Bayer process. The process liquor may be the product of a bauxite digestion step in the Bayer process. Accordingly, the thickener vessel may be for thickening or classifÿing a Bayer process liquor cc ntaining precipitated aluminium hydroxîde. Refined liquor removed from the apparatus may be recycîed to the bauxite digestion step.

70% of total solids exiting the vessel, but may transport at least

In a second aspect, there is provided a processing plant including a refining apparatus according to the first aspect located in-situ in the plant, the apparatus (a) receiving an input stream of process the refined liquor is extracted from a refined liquor outlet at or near the top of the apparatus and the siurry is extracted from a slurry outlet at or near the bottom of the apparatus.

liquor and (b) containing a refined liquor and a slurry, and wherein

The plant may further include:

(a) (b) (c) a reactor for contacting a feed material with a solution under conditions to produce a process liquor containing a valuable component and residual solids;

a solid/liquid separator for removing the residual solids from the process liquor; and a recovery apparatus for recovering the valuable component from the slurry.

Bayer process plant in which the valuable component is an

The plant may be a aluminium-bearing compound and in which the reactor digests bauxite to produce the process liquor which contains aluminium hydroxîde in solution, the process liquor is subject to conditions to cause précipitation of aluminium hydroxîde and the recovery apparatus treats the aluminium hydroxîde to produce alumina.

In a third aspect, there is provided a method for refining a process liquor that includes solids by gravity settling the solids in a vessel having a base and side wall that define an internai volume, the me thod including the steps of:

in the process liquor to settle under gravity toward the base, (a) allowing solids thereby forming (i) a high solids-content slurry and a build-up of settled solids in a lower région of the internai volume and (n) a refined liquor in an upper région of the internai volume;

(b) extracting the high solids-content slurry and the refined liquor from separate extraction points in the vessel; and (c) operating solids displacement éléments extending across and fixed relative to the vessel, for directing settled solids and/or settling solids in the vicinity of the side wall or the base toward a flow path of the high solids-content slurry being extracted from the vessel.

The refining method may be a method for thickening or classifying process liquor in a Bayer process. The process liquor may be the product of a bauxite digestion step in the Bayer process. Accordingly, the method may be for thickening or classifying a Bayer process liquor containing precipitated aluminium hydroxide. Clarifled liquor removed from the vessel may be recycled to the bauxite digestion step.

The solids displacement éléments may be in the form described above in respect of the first aspect.

The solids displacement éléments may be operated to provide a nominal throughput of 50 to 100% of the underflow capacity of the vessel under normal operating conditions. The solids displacement éléments may be operated to provide a nominal throughput in the range of 50 to 450 m3/h, preferably of 50 to 200 m3/h, each under normal operating conditions, but preferably 80 to 120 m3/h each.

Step (c) may comprise controlling the solids displacement éléments to enable the method to operate continuously.

Step (c) may further comprise controlling operation of the solids displacement éléments to operate the method under normal operating conditions continuously for at least two months, and more preferably at least three months.

The method may further comprise supplying the process liquor to the vessel. The process liquor may be a refined process liquor.

The method may further comprise a step of conditioning the process liquor to cause précipitation of the solids either before and/or after the liquor is supplied to the vessel.

Brief Description of the Drawings

Embodiments will now be described by reference to an example of an apparatus shown in the accompanying drawings, in which:

Figure 1 is a schematic flow chart of the Bayer process.

Figure 2 is a schematic cross-section of an embodiment of a thickener vessel.

Figure 3A is a photograph of a partially drained thickener vessel of the form shown in Figure 2 after operation of the apparatus to thicken process liquor. Figure 3B is a drawing corresponding to the photograph of figure 3A.

Figure 4 is also a photograph showing a partially drained thickener vessel of the form shown in Figure 2 after operation of the apparatus to thicken process liquor.

Figure 5 is a schematic cross-section of another embodiment of a thickener vessel.

Detailed Description

Gravity classifiera and gravity settling equipment are typically used in hydrometallurgical processes to separate solids from process liquors. An example is the Bayer process which typically uses such equipment to separate aluminium hydroxide from process liquor. Although the invention has a range of applications, including thickening and classifying process liquors, the following description relates to thickening of a Bayer process liquor and producing a tow-solids content clarified liquor. This should not be taken as limiting application of the invention to thickening.

A Bayer process for producing alumina from bauxite is shown generally in Figure 1.

The Bayer process, generally speaking, involves a reactor 1 being supplied with crushed bauxite 3 and a solvent 2, typically caustic solution at elevated température. The solvent 2 selectively dissolves aluminîum-bearing compounds to produce a process liquor loaded with aluminium in the form of aluminium hydroxide. The remainder of the bauxite does not dissolve and instead forms a fine “red mud” 5 which is separated from the process liquor at a solids/Iiquid séparation step 4, typically a multi-stage gravity settling apparatus.

The process liquor is then passed to a precipitator vessel 6, where it is conditioned to cause aluminium hydroxide to precipitate as a solid. The process liquor or slurry 11 is then passed to a thickener apparatus 7 and clarified liquor 13 said thickener apparatus is recycled to the crushed bauxite 3. The thickener apparatus 7 allows the aluminium hydroxide to settle under gravity to form a slurry 11 having a relatively high solids-content. The slurry 11 is extracted from the thickener apparatus 7. Settling of the solids leaves a clarified liquor near the top of the thickener apparatus 7 which is extracted and processed so that it can be recycled to the reactor 1 as at least part of the solvent 2.

The slurry from the thickener apparatus 7 is subject to further processing to separate solid aluminium hydroxide from the slurry. The solids are then calcined in a kiln 8 at around g

1050°C to cause aluminium hydroxide to décomposé to alumina 9 (solids) and water vapour. The alumina may then be used as a feedstock in a separate process for smelting alumina to produce aluminium métal.

According to an embodiment of the invention the thickener apparatus 7 is formed as vessel 10 in Figure 2.

The vessel 10 has a base 12 with an inverted frusto-conical shape from which a sidewall 14 extends upwardly to define collectively an internai volume for receiving an input liquid in the form of a process liquor 22 and for allowing the process liquor 22 to thicken by a gravity settling of suspended solids. The term “process liquor” is used hereinafter to dénoté a liquid that contains solids, which in the case of the Bayer process may be précipitâtes of aluminium hydroxide.

A thickened slurry is removed via slurry outlet, in the form of an extraction port 16, formed in the base 12. Extraction of the thickened slurry causes a flow stream designated by arrows marked F inside the internai volume. The extracted high solids-content slurry is subjected to further processing, for example to further separate the solids from the process liquor remaining in the slurry.

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As the solids settle out of the process liquor, a clarified liquor 30 forms at an upper région of the vessel 10 and is extracted via a liquor outlet, in the form of a clarified liquor port 20, for subséquent processing downstream, for example, to recover and recycle caustic for re-use in the Bayer process or recover and recycle other solvents in alternative hydrometallurgical processes.

The process liquor 22 in the vessel 10 includes solids that are very fine, typically having a médian solid particulate as clay-like materials when settled. As the solids are settling under gravity, the conditions inside the vessel 10 are relatively quiescent. This causes the solids settling on the base 12 to form a layer 50.

size in the range of 95 to 105 pm. These solids therefore, behave

The flow stream F forithe slurry is maintained where the thickened slurry, i.e. the high solids-content slurry, continues to flow through the vessel 10 and out via the slurry extraction port 16.

In operation, should a build-up of a layer 50 of solids on the base 12 occur, it will reduce the operating internai volume ofthe vessel 10 and so reduce the extent to which process liquor can be processed in the vessel 10. In effect, the résidence time of input process liquor 22 in the internai volume is reduced so that the slurry extracted via the slurry extraction port 16 has variable solids content and may typically be much lower than desired.

To counteract this effect, the vessel 10 includes screws 60 which comprise helical vanes 62 on a shaft 64. The shaft 64 extends across the diameter of the vessel 10 and out the side wall 14. The shaft 64 is driven by a gearbox 66 which is coupled to an electric motor 68 to control rotation of the shaft 64 and hence the screws 60. The vanes 62 of the screws extend from close proximity to the sidewall 14 to a central région in the internai volume that coïncides with the flow stream F of high solids-content slurry.

Accordingly, the screws 60 direct solids in the layer 50, that has built up on the base 12 and sidewall 14 of the yessel, into a central région of the vessel 10 which coïncides with the flow stream F. Accordingly, the solids are directed radially inwardly toward the flow stream F so that the flow stream F is supplied with solids to facilitate forming a high solidscontent slurry. ·

In the absence of the screws 60, it is typical for the layer 50 to build up to the extent shown by the dashed lines in Figure 2.

Figure 2 shows in schematic form a démarcation between the clarified liquor 30 and the slurry 40. However, the solids concentration in the vessel typically increases with the depth in the vessel so jthat at the upper surface of the liquid in the vessel 10, the solids hâve almost completely settled out of the process liquor leavtng a clarified liquor 30. Nevertheless, it can be seen that with a build-up of solids in layer 50 to the extent of the dash lines, the usable reduce this build up and ensure that despite continuous use, the usable volume is substantially maintained. This has an important effect in that the campaign Irfe of the vessel 10 is increased beyond two months and generally is at least three months, depending on the size of the vessel 10, before maintenance is required to clear the layer 50.

volume in the vessel is significantly reduced. The screws 60

The réduction in the layer 50 build-up in the vicinity of the screws 60 is best seen in Figures 3 and 4. Speciïcally with reference to Figure 4, the level of solids is clearly higher on the sidewall 14 of tf i

to the level of solids in the vicinity of the screws 60.

e vessel 10 in the région away from the screws 60 in comparison

Altematively, the screws 60 can be instalied through the conical base 12 as shown on Figure 5.

Without wishing to be held to any particular theory, the applicant believes that the screws 60 transport solids in the layer 50 toward a central région of the vessel which coïncides with the flow stream F ci high solids-content slurry. Although the layer 50 may continue to build up in régions away from the screws 60, the layer 50 of solids is destabilized by the différence in build-up and will typically fall as an avalanche or cascade of solids towards the screws 60 ortowards the central région ofthe vessel.

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It is also believed that the screws provide suffirent agitation to keep solids entrained in the flow of solids exiting the vessel in their vidnity and thereby enable high solids-content slurry to be transferred rom the sidewalls 14 into the flow stream F.

The apparatus includes of the vessel 10 and typically where the solids form layer 50. In this configuration, the screws direct solids radially inwardly toward the central région. It is anticipated however that alternative configui screws 60 are utilized in a vessel 10. It is preferred, nevertheless, to space the screws 60 equidistantly about the between maintenance clean out.

four rotatable screws 60 disposed at right angles at a lower région rations may be adopted when more than four or less than four vessel 10 to even out build-up of layer 50 and to prolong time

With reference to Figurie 3, two screws 60 are formed on one shaft 54 and the remaining two screws are formed on another shaft 64. The shafts 64 are disposed orthogonally, but are vertically separated to avoid interférence.

Mounting fixtures on the inside of the sidewall 14 are provided to fix each shaft 64 relative to the vessel 10 and yet enable rotation of the shafts. The screws on either end of the shaft 64 are opposite handed so that each screw moves solids away from the side wall and towards the flow stream F, typically in the centre of the vessel.

In practice in a vessel rotated at 7.5 rpm by diameter of 450 to 550i having an internai diameter of about 7.3m, the screws 60 are the electric motor 66 and the gear box 66. Each screw has a mm in a first section adjacent the side wall 14 and a diameter of 550 to 650 mm in a second section that is inboard of the first section. The first section is about 1.5m long and the pitch of the vanes 62 is 400 to 500 mm. The second section is about 1.1m long and the pitch ofthe vanes 62 is 500 to 650 mm.

Each screw 60 is of a type having a leading edge which cuts into solids in layer 50 causing them to be drawn into and transported along the length of the screw 60. Where the screw 60 changes diameter, there is an additional leading edge which gathers additional solids into the additional volume of the screw transportation path and also directs these additional solidstoward the centre ofthe vessel 10.

Based on the above figures, the screws 60 each have a nominal throughput of 100 m3/h. It will be appreciated, however, that the length, pitch and diameter of the screws may be selected to provide required solids transfer depending on the diameter of the vessel 10 and the nature of the solids in the sluny.

demonstrates that a vessel 10 without screws 60 produces an

Preliminary test work underflow with a solids concentration of 600-850gpl (i.e. 24.8 to 35.1 vol% solids). By contrast, the same ves sel 10 fitted with screws 60 as shown in Figures 3 and 4 produces an underflow with a solids concentration of 1050-1150gpl (i.e. 43.4 to 47.5 vol% solids). It will be appreciated that these results were obtained under laboratory scale test conditions and the actual solids concentration under normal plant conditions may be less. It is expected, however, that use of the screws 60 will provide a significant rmprovement in solids concentration from thickener vessels 10.

Eherwise due to express language or necessary implication, the ations such as “comprises” or “comprising is used in an inclusive

In the daims which follow and in the preceding description of the invention, except where the context requires o word “comprise” or van sense, i.e. to specify the presence ofthe stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Many modifications may be made to the preferred embodiment of the présent invention as described above without departing from the spirit and scope of the présent invention.

Claims (24)

1.

2.

3.

4.

5.

6.

An apparatus (7) for refining a process liquor that includes solids, which apparatus includes:

ving a base (12) and a side wall (14) that define an internai volume (a) a vessel (10) ha for contaîning U te process liquor (22) and for allowing gravity settling of the solids in the liquor, wl'ereby to produce a refined liquor (30) toward a top of the internai volume and a slurry (40) toward a bottom of the internai volume;

(b) a refined liquor the refined liquor;

outlet (20) at or near the top of the internai volume for extracting (c) a slurry outlet (16) at or near the bottom of the interna! volume for extracting the slurry, unrestrictiedly open to said internai volume; and (d) solids displacement éléments extending across and fixed relative to the vessel (10), wherein the solids displacement éléments are disposed within the internat volume for directing settled solids and/or settling solids in the vicinity of the side wall or of the base toward a flow path (F) of the slurry being extracted from the slurry outlet.

An apparatus according to claim 1, wherein the process liquor is a Bayer process liquor contaîning precipitated aluminium hydroxide.

An apparatus according to any one of claims 1 and 2, wherein the solids displacement éléments are disposed for directing solids in the vicinity of a wall (14; 12) of the vessel (10) above the bottom of said vessel toward the flow path (F) of the slurry being extracted from the slurry outlet (16).

An apparatus according to any one of claims 1 and 3, wherein the solids displacement éléments are arranged to direct settled solids and/or settling solids radially inwardly toward the flow path (F) of the slurry.

12).

An apparatus according to any one of claims 1 to 4, wherein the slurry outlet (16) is formed in the base

An apparatus according to any one of claims 1 to 5, wherein the slurry outlet (16) is located centrally ofthe vessel (10).

An apparatus according to any one of claims 1 to 6, wherein solids displacement éléments are disposed in a lower région ofthe internai volume ofthe vessel (10).

7.

8.

9.

An apparatus accojrding to any one of claims 1 to 7, wherein solids displacement éléments are unshielded at their top and at least one of their sides.

An apparatus according to any one of claims 1 to 8, wherein the solids displacement éléments are unshielded, unconfined or not blocked by any other member in the internai volume ofthe vessel (10).

10. An apparatus according to any one of claims 1 to 9, wherein the solids displacement éléments comprise at least one rotatable screw (60).

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11. An apparatus according to claim 10, wherein the solids displacement éléments comprise at least two rotatable screws (60).

12. An apparatus according to claim 11, wherein the solids displacement éléments comprise four rotatable screws, with each being arranged at about 90° to an adjacent rotatable screw.

13. An apparatus according to claim 11, wherein the solids displacement éléments comprise sets of two rotatable screws parallel to each other.

14. An apparatus according to any one of claims 10 to 13, wherein the at least one rotatable screw (60) is arranged such that solids are directed by the rotatable screw to a central région that coïncides with the position of the flow path (F).

15. An apparatus according to any one of claims 10 to 14, wherein the at least one rotatable screw (60) is formed with two or more sections of different length, different pitch and/or different diameter.

16. An apparatus according to claims 15, wherein the at least one rotatable screw (60) or each rotatable screw (60) hâve a first section with a first diameter and/or a first pitch length and a second section with a second diameter and/or a second pitch length and wherein the first diameter and/or the first pitch length is smaller than the second diameter and/or second pitch length.

17. An apparatus according to claims 16, wherein the first section is adjacent the side wall (14) or the bottom (12) of the apparatus.

18. An apparatus according to any one of claims 10 to 17, wherein at least one rotatable screw (60) or each (64), each shaft being passed through the side wall and being coupled to a driver (66, 68).

rotatable screw (60) is formed on respective cross-aligned shafts

19. An apparatus according to claim 18, wherein the screw on a first end of the shaft (64) is opposite handed to the screw on the other end of the shaft (64).

thickening or classifying a process liquor in a Bayer process.

20. An apparatus according to any one of claims 1 to 19, wherein said apparatus is a thickener vessel for

21. An apparatus according to claim 20, wherein the process liquor is the product of a bauxite digestion step in the Bayer process.

22. A processing plant wherein it comprises a refining apparatus (7) according to any one of claims 1 to 21 located in-situ in the plant, the apparatus (a) receîving an input steam (11) of process liquar and (b) containing a refined liquor and a slurry, and wherein the refined liquor (13) is extracted from a refined liquid outlet at or near the top of the apparatus and the slurry is extracted from a slurry outlet at or near the bottom of the apparatus.

23. A method for refinirig a process liquor that includes solids by gravity settling the solids g the steps of:

in a vessel (10) having a base (12) and side wall (14) that define an internai volume, the method includin (a) allowing solids thereby forming lower région of internai volume:

extracted from t in the process liquor to settle under gravity toward the base, (i) a high solids-content slurry and a build-up of settled solids in a he internai volume and (ii) a refined liquor in an upper région of the ligh solids-content slurry and the refined liquor from separate (b) extracting the extraction points in the vessel; and (c) operating solids vessel (10), for side wall or of displacement éléments extending across and fixed relative to the directing settled solids and/or settling solids in the vicinity of the the base toward a flow path of high solids-content slurry being ie vessel.

ted aluminium hydroxide.

24. A method according to claim 23, wherein the process liquor is a Bayer process liquor containing précipita