WO2006081626A1

WO2006081626A1 - Method for the causticisation of bayer process solutions

Info

Publication number: WO2006081626A1
Application number: PCT/AU2006/000140
Authority: WO
Inventors: Gerald Dunstan Roach; Geraldine Charmaine De Witt; Geoffrey William Riley
Original assignee: Alcoa Of Australia Limited
Priority date: 2005-02-03
Filing date: 2006-02-03
Publication date: 2006-08-10
Also published as: RU2007131985A; CA2596897A1; BRPI0606560A2; EP1858808A1

Abstract

A method for the causticisation of a Bayer process solution, the method including the steps of: adding a source of phosphate to the Bayer process solution; adding a causticising agent to the Bayer process solution; and forming a calcium phosphate precipitate, thereby causticising the Bayer process solution.

Description

"Method for the Causticisation of Bayer Process Solutions"

Field of the Invention

The present invention relates to a method for the causticisation of Bayer process solutions.

Background Art

The Bayer process is widely used for the production of alumina from alumina- containing ores such as bauxite. The process involves contacting alumina- containing ores with recycled caustic aluminate solutions at elevated temperatures in a process commonly referred to as digestion. Solids are removed from the resulting slurry and the solution cooled.

Aluminium hydroxide is added to the solution as seed to induce precipitation of further aluminium hydroxide therefrom. The precipitated aluminium hydroxide is separated from the caustic aluminate solution (known as spent liquor), with a portion of aluminium hydroxide being recycled to be used as seed and the remainder recovered as product. The remaining caustic aluminate solution is recycled for further digestion of alumina containing ore.

As aluminium hydroxide is precipitated and bauxite dissolved, the concentrations of sodium hydroxide present in the process solution decrease, whilst concentrations of sodium carbonate increase, reducing the efficacy of the solution for digestion of further aluminium-containing ore. Recycling of spent liquor in further digestion steps leads to build up of bauxite impurities in the liquor. It is known to use causticisation processes (the removal of sodium carbonate from process solutions) via the addition of calcium hydroxide or calcium oxide (which forms calcium hydroxide when added to process solutions) to control sodium carbonate concentration according to Reaction 1 shown below.

Na₂CO₃ + Ca(OH)₂(s)-> CaCO₃(S) + 2NaOH (1) Causticisation of Bayer process solutions via Reaction 1 is limited by the fact that, under certain conditions, an alternate reaction (Reaction 2 below) is thermodynamically favoured. Under these conditions, addition of lime (Ca(OH)2) gives no causticisation benefit, leading instead to production of aluminate- containing solids.

3Ca(OH)₂ (s) + 2NaAI(OH)₄ ^■* Ca₃[AI(OH)₆]₂ (s) + 2NaOH (2)

The resulting Bayer process productivity loss renders conventional causticisation under these conditions unattractive.

The composition of a bauxite ore varies widely depending on its source, with the major components generally existing as a mixture of specific mineral forms each of which has differing physical and chemical characteristics. Bauxites are known to contain soluble phosphorous minerals which can cause difficulties in controlling the phosphorous content of the alumina produced therefrom or in formation of solid phosphate scales on Bayer process equipment.

Phosphates have long been considered to be undesirable components of bauxites, requiring application of ongoing countermeasures to control Bayer liquor phosphate concentrations to practically acceptable levels. Conventionally, this is achieved by adding lime (as either CaO or Ca(OH)₂) to the predesilication or digestion stages of the Bayer process, to induce the precipitation of phosphorous as apatite (a solid mineral phase containing calcium and phosphate as major components).

In its full generality, the term apatite refers to a broad family of minerals including fluorapatite (Ca₁₀(PO₄^F₂), chlorapatite (Caio(PO₄)₆CI₂), hydroxylapatite (Caio(PO₄)₆OH₂), carbonate-apatite, carbonate hydroxylapatite or carbonate fluorapatite. Most forms of apatite can be described by the following formula A₁o(XO₄)₆(OH_>F,CI)2.nH₂O, wherein:

A may be selected from Ba, Ca, Ce, K, Na, Pb, Sr, Y; X may be selected from As, P, Si, V, and

CO₃ can be substituted for PO₄.

Overall, there is extensive literature on apatites (which are important for example in bones and teeth). See for example "Structure and Chemistry of the Apatites and Other Calcium Orthophosphates", J.C.Elliott, Elsevier, 1994. In contrast, there is little published literature on apatites formed under Bayer process or related conditions, and little available knowledge regarding their compositional details. Although not specifically examining Bayer process apatites, an important exception to this is "Contribution a L'Etude Structurale des Apatites Carbonatees de Type B", G.Bonel et al., Colloques Internationaux C.N.R.S. 230 (1975), pp117-125. This infers the following structural formula for one of the families of apatites it studied (synthesised under alkaline conditions):

Ca₁O-XNa₂XZ₃(PO₄)^x(CO₃)X(H₂O)_xOH_2-XZ₃, with O < x < 3.

The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia, or anywhere else, as at the priority date of the application.

Disclosure of the Invention

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features. - A -

The present invention is not to be limited in scope by the specific embodiments described herein, which are intended for the purpose of exemplification only. Functionally equivalent products, compositions and methods are clearly within the scope of the invention as described herein.

The entire disclosures of all publications (including patents, patent applications, journal articles, laboratory manuals, books, or other documents) cited herein are hereby incorporated by reference.

In accordance with the present invention, there is provided a method for the causticisation of a Bayer process solution, the method including the steps of:

adding a source of phosphate to the Bayer process solution;

adding a causticising agent to the Bayer process solution; and

forming a calcium phosphate precipitate;

thereby causticising the Bayer process solution.

As used herein the term "source of phosphate" shall be taken to include, without limitation, any form of phosphorus that provides an orthophosphate anion in Bayer process solutions.

As used herein the term "a calcium phosphate precipitate" shall be taken to include, without limitation, any precipitate containing calcium and phosphate ions.

Advantageously, the method of the present invention permits the causticisation of a Bayer process solution and the simultaneous control of phosphate and calcium concentrations in the solution.

In a specific form of the invention, the calcium phosphate precipitate includes carbonate. In a highly specific form of the invention, the calcium phosphate precipitate is provided in the form of an apatite. Even more specifically, the apatite is a carbonate hydroxyl apatite.

Without being limited by theory, apatite formation is believed to be a function of solution conditions and is influenced by factors such as carbonate, phosphate and calcium concentrations. It is believed that the following non-limiting equation best describes the reaction occurring in Bayer process solutions in accordance with the present invention, although the apatite formula is expected to be influenced by concentration of various reactants present in the Bayer process solution.

7Ca(OH)₂ (s) + 3Na₃PO₄ + 3Na₂CO₃ + 3H₂O -»

Ca₇Na₂(PO₄)_S(COs)₃(H₂O)₃OH (s) + 13NaOH (3)

The formation of apatite under these conditions results in removal of sodium carbonate from the solution, thereby causticising such. Furthermore, whilst the exact stoichiometry of the apatite is influenced by the factors discussed above, its formation is not bound by the same thermodynamic restrictions limiting conventional causticisation. Hence it is possible to causticise Bayer process solutions, via the formation of the apatite, under conditions which do not favour conventional causticisation, attaining greater carbonate removal and thus greater causticity (i.e. higher TC/TA) for this type of liquor than is currently possible using conventional causticisation techniques.

The quantity of causticising agent added to the Bayer process solution is dependent upon the level of carbonate and phosphate in solution and the total alkali concentration of the Bayer process solution. The level of phosphate in the solution is a function of the source of phosphate added to the Bayer process solution and bauxite derived phosphate.

The quantity of the source of phosphate added to the Bayer process solution is dependent upon the level of carbonate and phosphate in solution. Without being limited by theory, it is believed that by adding phosphate and causticising agent in such amounts such that the ratio of the concentrations of calcium to phosphate, respectively, in solution is increased compared to the ratio of calcium to phosphate in the Bayer process solution prior to addition of the phosphate and causticising agent, the formation of apatite reduces phosphate concentrations in the Bayer process solution.

Without being limited by theory, it is believed that by adding phosphate and causticising agent in such amounts such that the ratio of the concentrations of phosphate to calcium, respectively, in solution is increased compared to the ratio of phosphate to calcium in the Bayer process solution prior to addition of the phosphate and causticising agent, the formation of apatite reduces calcium concentrations in the Bayer process solution.

The precipitate formed by the method of the present invention may be removed from the Bayer process solution by any method known in the art including, but not limited to, settling, filtration and cyclonic separation.

The precipitate may be used as a filter aid in the removal of further precipitate or other solids formed in the Bayer circuit.

The source of phosphate may be selected from laboratory or analytical grade (ortho)phosphoric acid, laboratory or analytical grade sodium phosphate with varying degrees of hydration, phosphate rock minerals (for example, strengite, variscite, barrandite, crandallite), fertilisers (for example super phosphate, TSP - triple super-phosphate, ammonium phosphates, e.g. mono-, di-) or phosphate- containing complexes. A preferred source is one containing alumino phosphates such as crandallite, variscite or barrandite.

Preferably, the causticising agent comprises a source of calcium. The causticising agent may be selected from dry lime, slaked lime, slaked lime putty, tricalcium aluminate such as filter aid used in the Bayer process or various calcium aluminate species known to form in Bayer circuits under certain conditions, such as hydrocalumite. Where the causticising agent is provided in the form of slaked lime putty, the slaked lime putty preferably has a low particle size and a high solids density.

The Bayer process solution may be provided in the form of a digestion solution, a pre-digestion solution, a post-digestion solution or a process side stream.

Where the source of phosphate and the causticising agent are added to a digestion solution, a pre-digestion solution or a post-digestion solution, the causticisation reaction advantageously removes carbonate originating from the bauxite.

In one form of the invention, the source of phosphate and the causticising agent are added to a pre-digestion solution.

In an alternate form of the invention, the source of phosphate is added to a pre- digestion solution and the causticising agent is added to a digestion solution.

The source of phosphate may be added directly to the Bayer process solution in a Bayer process circuit or mixed in a process side stream with a Bayer process solution prior to adding it to the Bayer process circuit.

The causticising agent may be added directly to the Bayer process solution in a Bayer process circuit or mixed in a process side stream with a Bayer process solution prior to adding it to the Bayer process circuit.

Where the Bayer process solution is provided in the form of a process side stream, the Bayer process solution may be provided in the form of a washer overflow solution, a washer feed solution, a spent liquor or a green liquor.

Where the source of phosphate and the causticising agent are added to a spent liquor, the causticisation reaction advantageously increases TC/TA before digestion thereby improving aluminium extraction. In accordance with the present invention, there is provided a method for the causticisation of a Bayer process solution, the method including the steps of:

adding a source of phosphate to the Bayer process solution;

adding a causticising agent selected from the group dry lime, slaked lime, slaked lime putty tricalcium aluminate or various calcium aluminate species known to form in Bayer circuits under certain conditions, such as hydrocalumite to the Bayer process solution; and

forming an apatite precipitate;

thereby causticising the Bayer process solution.

In accordance with the present invention, there is provided a method for the control of calcium and/or phosphate concentrations in Bayer process solutions, the method including the steps of:

adding a source of phosphate to the Bayer process liquor;

adding a causticising agent to the Bayer process liquor; and

forming a calcium phosphate precipitate.

obtaining the Bayer process solution from a Bayer circuit;

adding a source of phosphate to the Bayer process solution;

adding a causticising agent to the Bayer process solution; and

returning the causticised Bayer process solution to the Bayer circuit. In a specific form of the invention, the calcium phosphate precipitate includes carbonate. In a highly specific form of the invention, the calcium phosphate precipitate is provided in the form of an apatite. Even more specifically, the apatite is a carbonate hydroxyl apatite.

Preferably, the method comprises the further step of:

forming a precipitate in the Bayer process solution.

In one form of the invention, the causticised Bayer process solution is returned to the Bayer circuit prior to a solid separation stage within the circuit to allow the removal of the precipitate. The solid separation stage may be provided in the form of a settling stage, a filtration stage or a cyclonic stage.

In an alternate form of the invention, the method for causticisation of a Bayer process solution of the present invention may, before returning the causticised Bayer process solution to the Bayer liquor circuit, comprise the additional step of:

directing the causticised Bayer process solution to a dedicated solid separation stage to remove the precipitate.

The solid separation stage may be provided in the form of a settling stage or, filtration stage or a cyclonic stage.

Where the method for causticisation of a Bayer process solution of the present invention includes the step of directing the causticised Bayer process solution to a dedicated solid separation stage to remove the precipitate, the method may further comprise the step of:

collecting the precipitate for use as an aid to the filtration of the main liquor stream. The source of phosphate may be selected from laboratory or analytical grade (ortho)phosphoric acid, laboratory or analytical grade sodium phosphate with varying degrees of hydration, phosphate rock minerals (for example, strengite, variscite, barrandite, crandallite), fertilisers (for example super phosphate, TSP - triple super-phosphate, ammonium phosphates, e.g. mono-, di-) or phosphate- containing complexes. A preferred source is one containing alumino phosphates such as crandallite, variscite or barrandite.

Preferably, the causticising agent comprises a source of calcium. The causticising agent may be selected from dry lime, slaked lime, slaked lime putty, tricalcium aluminate such as filter aid used in the Bayer process or various calcium aluminate species known to form in Bayer circuits under certain conditions such as hydrocalumite. Where the causticising agent is provided in the form of slaked lime putty, the slaked lime putty preferably has a low particle size and a high solids density.

The Bayer process solution may be provided in the form of a washer overflow solution, a washer feed solution, a spent liquor or a green liquor.

In an embodiment of the invention where the Bayer process comprises the steps:

digestion of bauxite with caustic solution;

liquid-solid separation to provide a residue and a liquor;

precipitation of aluminium hydroxide from the liquor; and

calcination of the aluminium hydroxide to provide alumina,

the source of phosphate is added prior to the step of liquid-solid separation to provide a residue and a liquor. Where the method comprises the step of addition of the source of phosphate prior to the step of digestion of bauxite with caustic solution, the method may further comprise the step of:

adding a causticising agent to the process after the step of digestion of bauxite with caustic solution and before the step of liquid-solid separation.

Where the method comprises the step of addition of the source of phosphate prior to the step of digestion of bauxite with caustic solution, the method may further comprise the step of:

adding a causticising agent to the process prior to the step of digestion of bauxite with caustic solution.

adding a causticising agent to the process at the step of digestion of bauxite with caustic solution.

Without being limited by theory it is believed that the rate of reaction between carbonate, phosphate and lime increases with temperature. Preferably, the source of phosphate and the causticising agent are added to an alkaline stream at a temperature between 30 ⁰C and 320 ⁰C. More preferably, the source of phosphate and the causticising agent at added to an alkaline stream at a temperature between about 60 ⁰C and about 260 ⁰C.

Preferably, the total alkalinity of the Bayer process solution is between about 15 gL^"1 to about 400 gL^"1, expressed as gL^"1 sodium carbonate. More preferably the total alkalinity of the Bayer process solution is between about 100 gL^"1 to about 36O gL^"1. Where the source of phosphate and the causticising agent are added to a Bayer process solution at lower temperatures within the Bayer process circuit, the' residence time of the solution in the circuit may be increased to allow further causticisation of the solution. In particular, where the source of phosphate and the causticising agent are added to a washer overflow solution, the washer overflow solution may be directed to a large tank and the residence time of the solution within the tank increased compared to conventional washer overflow solutions.

In accordance with the present invention, there is provided alumina produced by any one of the Bayer processes described hereinabove.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to four embodiments thereof, and the accompanying drawings, in which:-

Figure 1a is a schematic flow sheet showing how a method in accordance with a first embodiment of the present invention may be utilised in a Bayer Process circuit;

Figure 1b is a schematic flow sheet showing how a method in accordance with a second embodiment of the present invention may be utilised in a Bayer Process circuit;

Figure 1c is a schematic flow sheet showing how a method in accordance with a third embodiment of the present invention may be utilised in a Bayer Process circuit;

Figure 1d is a schematic flow sheet showing how a method in accordance with a fourth embodiment of the present invention may be utilised in a

Bayer Process circuit; Figure 2 is a plot showing the effect of adding varying amounts of causticising agent and phosphate addition to a Bayer process solution;

Figure 3 is a plot showing the effect of adding varying amounts of causticising agent and phosphate addition to a Bayer process solution using Bauxite as the Phosphate Source;

Figure 4 is a plot showing the effect of adding varying amounts of causticising agent and phosphate addition to a Bayer process solution using Tricalcium Aluminate as the causticising agent;

Figure 5 is a plot showing the effect of adding varying amounts of causticising agent and phosphate addition to a Bayer process solution on calcium and phosphate concentrations in the process solution; and

Figure 6 is a plot showing the effect of causticising a Bayer process solution at low temperature over time.

Best Mode(s) for Carrying Out the Invention

By way of example, the method of the present invention is described in the context of the causticisation and phosphate reduction of a Bayer process solution, although such should not be seen as limiting the generality of the foregoing description.

Figures 1a to 1d show a schematic flow sheet of the Bayer process circuit 10 comprising the steps of:

digestion 12 of bauxite 14 in a caustic solution;

liquid-solid separation 16 of the mixture to residue 18 and liquor 20;

filtration 22 of the liquor 20;

cooling of the liquor 20 to cause aluminium hydroxide precipitation 24; separating aluminium hydroxide 24 and liquor 26;

recycling spent liquor 26 to digestion 12; and

calcining 28 the aluminium hydroxide 24 to alumina.

In accordance with a first embodiment of the present invention best seen in Figure 1a, a source of phosphate 32 as a slurry of sodium phosphate, and a causticising agent 34 as a slurry of calcium hydroxide are added to the spent liquor 26 prior to digestion 12 of the bauxite 14.

In accordance with a second embodiment of the present invention best seen in Figure 1b, a source of phosphate 32 as a slurry of sodium phosphate, and a causticising agent 34 as a slurry of calcium hydroxide are added to the spent liquor 26 prior to a settling stage 36. The precipitate 38 from the settling stage 36 is directed to the filtration step 22 to act as a filter aid and the spent liquor 38 directed to digestion 12 in the normal manner.

In accordance with a third embodiment of the present invention best seen in Figure 1c, the washer overflow 40 is redirected to a reactor 42 and a source of phosphate 32 as a slurry of sodium phosphate, and a causticising agent 34 as a slurry of calcium hydroxide added to the reactor 42. The solution 44 from the reactor 42 is returned to the Bayer circuit 10 after the step of digestion 12 and prior to the step of liquid-solid separation 16.

In accordance with a fourth embodiment of the present invention best seen in Figure 1d, a source of phosphate 32 as a slurry of sodium phosphate, and a causticising agent 34 as a slurry of calcium hydroxide are added to the bauxite 14 prior to digestion 12.

The following examples, each comprising several experiments, are intended to assist in the understanding of the reaction parameters of the present invention. It must be appreciated that the following description of the examples is not to limit the generality of the above description of the invention. Example 1

Table 1 and Figure 2 display the results from a series of experiments using digestion tests in the laboratory, with slaked lime and AR grade sodium phosphate, to demonstrate the effectiveness of reaction (3) as a causticisation reaction of Bayer process solutions.

Aluminium hydroxide was added to a solution of spent liquor to raise the A/TC ratio to a target ratio of 0.7. Analytical Reagent grade Na₃PO^ 12H₂O and slaked lime (Ca(OH)₂) were added to the mixture in stoichiometric amounts calculated to remove 10 - 50% of the total carbonate in the original liquor as carbonate hydroxy! apatite, Ca₇Na₂(PO₄)_S(COs)₃(H₂O)₃OH (see reaction (3)) and the mixture digested at 145⁰C for 20 min.

The TC/TA (= 1 - Na₂CO₃/TA) increases shown in Table 1 were accompanied by XRD analyses confirming the formation of carbonate hydroxyl apatite. They show that reaction (3) can be used to causticise significant amounts of dissolved Na₂CO₃, and lift TC/TA ratios to high levels.

Table 1 - Digest test results with varying amounts of causticising agent and phosphate addition to a Bayer process solution. Phosphate doses (added as Na₃PO₄.12H₂O) are expressed as Na₃PO₄ concentrations. Example 2

Table 2 and Figure 3 display the results from a series of experiments designed to test the effectiveness of causticisation by carbonate hydroxyl apatite formation (reaction (3)), when the phosphate in bauxite is used as the phosphate source.

High temperature digestions of bauxite in spent liquor were performed for 20 min at 145⁰C, to a target A/TC ratio of 0.68. Bauxite charges with increasing phosphate content were obtained by blending appropriate proportions of bauxites containing 0.2% & 6.1 % phosphate (calculated as PaO₅), respectively. Slaked lime was added to the mixture prior to digestion in stoichiometric quantities calculated to precipitate the phosphate entirely as apatite, according to reaction (3).

Table 2 - Digest test results with varying amounts of causticising agent and phosphate addition to a Bayer process solution using Bauxite as the Phosphate Source. Extractable phosphate concentrations in bauxite are expressed as w/w % mole equivalent P₂O₅. Digest conditions were such that 1% w/w P₂O₅ in bauxite was equivalent to adding 2.7 g/L Na₃PO₄ to the test solution.

The TC/TA increases show that causticisation via reaction (3) works effectively when bauxite is used as the phosphate source. They also show that, when sufficient phosphate is present, causticisation by carbonate hydroxyl apatite formation can overcome the usual increase in dissolved carbonate (manifested by a drop in TC/TA) associated with extraction of carbonate from bauxite during digestion.

Example 3

Table 3 and Figure 4 display the results from a series of experiments designed to test the effectiveness of causticisation by carbonate hydroxyl apatite formation ■ (reaction (3)), using an alternate causticising agent, commonly used in the Bayer process as a filter aid (tricalcium aluminate, Ca₃[AI(OH)₆]2).

High temperature digestions were performed in spent liquor at 145⁰C with increasing additions of phosphate, added as Analytical Reagent grade Na₃PO₄.12H₂O and tricalcium aluminate. These digests were held at temperature for 20 minutes, using aluminium hydroxide to raise A/TC to a ratio of 0.6. Phosphate charges were calculated to add 7 - 28 g/L phosphate (as Na₃PO₄) to the digest liquor. Filter aid charges were calculated to remove 100% of the added phosphate, on the basis of the carbonate hydroxyl apatite stoichiometry given in reaction (3).

The results show that the causticisation benefits of reaction (3) can be achieved using an alternate source of causticising agent.

Table 3 - Digest test results with varying amounts of causticising agent and phosphate addition to a Bayer process solution using Tricalcium Aluminate as the causticising agent. Phosphate doses (added as Na₃PO₄.12H₂O) are expressed as Na₃PO₄ concentrations. Example 4

Table 4 and Figure 5 display the results from a series of experiments designed to show that by appropriate control of Ca: P ratios on input to reaction (3), calcium and/or phosphate concentrations in reaction (3) exit liquors can be controlled to low levels.

Digests of a fixed quantity of high phosphate bauxite were performed in spent liquor, with increasing additions of slaked lime. As in Examples 1 , 2 and 3, the digest temperature was held at 145⁰C for 20 minutes. Lime charges were calculated to remove 25% - 100% of the total P₂O₅, on the basis of the carbonate apatite stoichiometry given by reaction (3).

Calcium in liquor increased with increasing lime charge and phosphate in liquor decreased correspondingly. It is believed that similar behaviour would be observed if phosphate were varied while lime charge was kept constant. Hence, the technique can be used to control phosphate and/or calcium concentrations in Bayer process liquors.

Table 4 - Digest test results with varying amounts of causticising agent and phosphate addition to a Bayer process solution. Phosphate concentrations, measured by ICP, are expressed as ppm mole equivalent P₂θ₅. 1 ppm P₂O₅ is equivalent to 2.31 mg/L Na₃PO₄ in solution. Example 5

Table 5 and Figure 6 display the results from a laboratory experiment conducted at 90 ⁰C, with slaked lime and AR grade sodium phosphate, to demonstrate the effectiveness of reaction (3) as a low temperature causticisation reaction in spent Bayer liquor.

Analytical Reagent grade Na₃PO₄.12H₂O and slaked lime (Ca(OH)₂) were added to the mixture in stoichiometric amounts calculated to remove 5 - 6 g/L of the total carbonate in the original liquor as carbonate hydroxyl apatite, Ca₇Na₂(PO₄)S(COs)₃(H₂O)₃OH (see reaction (3)) and the mixture reacted at 90°C for up to 60 min.

The results show that the causticisation benefits of reaction (3) can be achieved at low temperatures in spent liquor.

Table 5 - Digest test results of causticising agent and phosphate addition to a Bayer process solution with varying reaction times. Phosphate doses (added as Na₃PO₄.12H₂O) are expressed as Na₃PO₄ concentrations.

Claims

The Claims Defining the Invention are as Follows

1. A method for the causticisation of a Bayer process solution, the method including the steps of:

adding a source of phosphate to the Bayer process solution;

adding a causticising agent to the Bayer process solution; and

forming a calcium phosphate precipitate,

thereby causticising the Bayer process solution.

2. A method for the causticisation of a Bayer process solution, the method including the steps of:

obtaining the Bayer process solution from a Bayer circuit;

adding a source of phosphate to the Bayer process solution;

adding a causticising agent to the Bayer process solution; and

returning the causticised Bayer process solution to the Bayer circuit.

3. A method for the causticisation of a Bayer process solution according to claim 2, wherein the method comprises the further step of:

forming a calcium phosphate precipitate in the Bayer process solution.

4. A method for the causticisation of a Bayer process solution according to claim 2 or 3, wherein the causticised Bayer process solution is returned to the Bayer circuit prior to a solid separation stage within the circuit to allow the removal of the precipitate.

5. A method for the causticisation of a Bayer process solution according to any one of the preceding claims, wherein the calcium phosphate precipitate includes carbonate.

6. A method for the causticisation of a Bayer process solution according to claim 5, wherein the calcium phosphate precipitate is provided in the form of an apatite.

7. A method for the causticisation of a Bayer process solution according to claim 6, wherein the apatite is a carbonate hydroxyl apatite.

8. A method for the causticisation of a Bayer process solution according to any one of claims 4 to 7, wherein the calcium phosphate precipitate is removed from the Bayer process solution by settling, filtration or cyclonic separation.

9. A method for the causticisation of a Bayer process solution according to any one of claims 4 to 8, wherein the calcium phosphate precipitate is used as a filter aid in the removal of further precipitate or other solids formed in the Bayer circuit.

10. A method for the causticisation of a Bayer process solution according to any one of the preceding claims, wherein the source of phosphate is selected from laboratory or analytical grade (ortho)phosphoric acid, laboratory or analytical grade sodium phosphate with varying degrees of hydration, phosphate rock minerals (such as strengite, variscite, barrandite, crandallite), fertilisers (such as super phosphate, TSP - triple super-phosphate, mono- or di- ammonium phosphates) or other phosphate-containing complexes.

11. A method for the causticisation of a Bayer process solution according to any one of the preceding claims, wherein the causticising agent comprises a source of calcium.

12. A method for the causticisation of a Bayer process solution according to any one of the preceding claims, wherein the causticising agent is selected from dry lime, slaked lime, slaked lime putty, tricalcium aluminate or hydrocalumite.

13. A method for the causticisation of a Bayer process solution according to claim 12, wherein the slaked lime putty has a low particle size and a high solids density.

14. A method for the causticisation of a Bayer process solution according to claim 1 or any one of claims 4 to 13, wherein the Bayer process solution is provided in the form of a digestion solution, a pre-digestion solution, a post- digestion solution or a process side stream.

15. A method for the causticisation of a Bayer process solution according to claim 1 or any one of claims 4 to 14, wherein the source of phosphate is added directly to the Bayer process solution in a Bayer process circuit or mixed in a process side stream with a Bayer process solution prior to adding it to the Bayer process circuit.

16. A method for the causticisation of a Bayer process solution according to claim 1 or any one of claims 4 to 15, wherein the causticising agent is added directly to the Bayer process solution in a Bayer process circuit or mixed in a process side stream with a Bayer process solution prior to adding it to the Bayer process circuit.

17. A method for the causticisation of a Bayer process solution according to claim 15 or 16, wherein the Bayer process side stream is provided in the form of a washer overflow solution, a washer feed solution, a spent liquor or a green liquor.

18. A method for the causticisation of a Bayer process solution according to any one of claims 2 to 13, wherein before the step of returning the causticised Bayer process solution to the Bayer liquor circuit, the method comprises the additional step of: directing the causticised Bayer process solution to a dedicated solid separation stage to remove the precipitate.

19. A method for the causticisation of a Bayer process solution according to any one of claims 2 to 13 or claim 18, wherein the solid separation stage is provided in the form of a settling stage, a filtration stage or a cyclonic stage.

20. A method for the causticisation of a Bayer process solution according to any one of claims 2 to 13 or claim 18 or 19, wherein the method comprises the further step of:

collecting the precipitate for use as an aid to the filtration of the main liquor stream.

21. A method for the causticisation of a Bayer process solution according to any one of claims 2 to 13 or claims 18 to 20, wherein the Bayer process side stream is provided in the form of a washer overflow solution, a washer feed solution, a spent liquor or a green liquor.

22. A method for the causticisation of a Bayer process solution according to any one of the preceding claims, wherein the Bayer process comprises the steps:

digestion of bauxite with caustic solution;

liquid-solid separation to provide a residue and a liquor;

precipitation of aluminium hydroxide from the liquor; and

calcination of the aluminium hydroxide to provide alumina,

the source of phosphate is added prior to the step of liquid-solid separation to provide a residue and a liquor.

23. A method for the causticisation of a Bayer process solution according to claim 22, wherein the method comprises the steps of:

adding the source of phosphate prior to the step of digestion of bauxite with caustic solution; and

adding the causticising agent to the process after the step of digestion of bauxite with caustic solution and before the step of liquid-solid separation.

24. A method for the causticisation of a Bayer process solution according to any one of the preceding claims, wherein the source of phosphate and the causticising agent are added to the Bayer process solution at a temperature between 30 ⁰C and 320 ⁰C.

25. A method for the causticisation of a Bayer process solution according to any one of the preceding claims, wherein the source of phosphate and the causticising agent at added to the Bayer process solution at a temperature between about 60 ⁰C and about 260 ⁰C.

26. A method for the causticisation of a Bayer process solution according to any one of the preceding claims, wherein the total alkalinity of the Bayer process solution is between about 15 gl_^"1 to about 400 gl_^"1.

27. A method for the causticisation of a Bayer process solution according to any one of the preceding claims, wherein the total alkalinity of the Bayer process solution is between about 100 gl_^"1 to about 360 gl_^"1.

28. Alumina produced according to any one of claims 1 to 27.

29. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions, the method including the steps of:

adding a source of phosphate to the Bayer process liquor; adding a causticising agent to the Bayer process liquor; and

forming a calcium phosphate precipitate.

30. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to claim 29, wherein the calcium phosphate precipitate includes carbonate.

31. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to claim 29 or 30, wherein the calcium phosphate precipitate is provided in the form of an apatite.

32. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to claim 31 , wherein the apatite is a carbonate hydroxyl apatite.

33. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of claims 29 to 32, wherein the calcium phosphate precipitate is removed from the Bayer process solution by settling, filtration or cyclonic separation.

34. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of claims 29 to 33, wherein the calcium phosphate precipitate is used as a filter aid in the removal of further precipitate or other solids formed in the Bayer circuit.

35. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of claims 29 to 34, wherein the source of phosphate is selected from laboratory or analytical grade (ortho)phosphoric acid, laboratory or analytical grade sodium phosphate with varying degrees of hydration, phosphate rock minerals (such as strengite, variscite, barrandite, crandallite), fertilisers (such as super phosphate, TSP - triple super-phosphate, mono- or di- ammonium phosphates) or other phosphate- containing complexes.

36. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of claims 29 to 35, wherein the causticising agent comprises a source of calcium.

37. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to claim 36, wherein the causticising agent is selected from dry lime, slaked lime, slaked lime putty, tricalcium aluminate or hydrocalumite.

38. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of claims 29 to 37, wherein the Bayer process solution is provided in the form of a digestion solution, a pre- digestion solution, a post-digestion solution or a process side stream.

39. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of claims 29 to 38, wherein the source of phosphate is added directly to the Bayer process solution in a Bayer process circuit or mixed in a process side stream with a Bayer process solution prior to adding it to the Bayer process circuit.

40. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of claims 29 to 39, wherein the causticising agent is added directly to the Bayer process solution in a Bayer process circuit or mixed in a process side stream with a Bayer process solution prior to adding it to the Bayer process circuit.

41. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to claim 39 or 40, wherein the Bayer process side stream is provided in the form of a washer overflow solution, a washer feed solution, a spent liquor or a green liquor.

42. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of claims 29 to 41 , wherein the Bayer process comprises the steps: digestion of bauxite with caustic solution;

liquid-solid separation to provide a residue and a liquor;

precipitation of aluminium hydroxide from the liquor; and

calcination of the aluminium hydroxide to provide alumina,

43. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of claims 29 to 42, wherein the method comprises the steps of:

44. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of claims 29 to 42, wherein the method comprises the steps of:

adding the causticising agent to the process prior to the step of digestion of bauxite with caustic solution.

45. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of claims 29 to 42, wherein the method comprises the steps of: adding the source of phosphate prior to the step of digestion of bauxite with caustic solution; and

adding the causticising agent to the process at the step of digestion of bauxite with caustic solution.

46. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of the preceding claims, wherein the source of phosphate and the causticising agent are added to the Bayer process solution at a temperature between 30 °C and 320 ⁰C.

47. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of the preceding claims, wherein the source of phosphate and the causticising agent at added to the Bayer process solution at a temperature between about 60 °C and about 260 ⁰C.

48. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of the preceding claims, wherein the total alkalinity of the Bayer process solution is between about 15 gl_^"1 to about 40O gL^"1.

49. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions according to any one of the preceding claims, wherein the total alkalinity of the Bayer process solution is between about 100 gL^"1 to about 36O gL^"1.

50. A method for the causticisation of a Bayer process solution as hereinbefore described with reference to the accompanying Examples.

51. A method for the causticisation of a Bayer process solution as hereinbefore described with reference to the accompanying Figures.

52. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions as hereinbefore described with reference to the accompanying Examples.

53. A method for the control of calcium and/or phosphate concentrations in Bayer process solutions as hereinbefore described with reference to the accompanying Figures.