AU2018271243A1 - Process for removing sulphate ions from a bayer liquor - Google Patents

Abstract

Abstract The invention provides a process for removing sulphate ions from a Bayer liquor in a Bayer refinery, the process comprising: providing fluoride ions in the Bayer liquor at a concentration sufficient to induce precipitation of a precipitate comprising fluoride and sulfate ions; separating the precipitate from a sulphate-depleted Bayer liquor; and recycling the sulphate-depleted Bayer liquor in the Bayer refinery.

Description

[1] The invention relates to a process for removing sulphate ions from a Bayer liquor in a Bayer refinery, and in particular to a process wherein a precipitate comprising fluoride and sulfate ions is induced to precipitate from the Bayer liquor by providing a sufficient concentration of fluoride ions in the liquor.

Background of Invention [2] In the Bayer process for alumina production, a concentrated sodium aluminate solution is produced by digesting milled bauxite in a caustic solution, usually under conditions of elevated temperature and pressure. After clarification of the slurry, the concentrated sodium aluminate solution is cooled and seeded with gibbsite crystals, causing gibbsite to crystallise from solution. The gibbsite is calcined to produce alumina, while the depleted (or spent) liquor is recycled to digest more bauxite.

[3] During the milling and digestion steps, a variety of species other than alumina are also extracted and enter the liquor stream. These species are generally considered to be undesirable impurities, and due to the cyclic nature of the Bayer process, will accumulate in the refinery's liquor streams. Each of these species will eventually reach a steady state concentration that is a function of the input with bauxite and other sources and the output with the refinery's red mud residue, sidestream removal processes and with the product alumina.

[4] Efficient removal processes for many of the Bayer process impurities have been developed. Carbonate, one of the most abundant anionic impurities in the Bayer process, is generally removed by causticizing the Bayer liquor with lime according to equation (1). Lime may be added to either or both of the primary Bayer circuit (inside causticisation) or a side causticisation circuit (outside causticisation).

Ca(OH)₂ + Na₂CO₃ ~ Ca(CO)₃ + 2NaOH (1)

2018271243 26 Nov 2018 [5] Sulphate is another anionic impurity present in the bauxite that is highly soluble in caustic aluminate Bayer liquors. Sulphate ions are typically precipitated from the liquor in the refinery in sodium aluminate silicate species (known as desilicated product or DSP). However, the removal of sulphate ions by this mechanism is frequently inadequate, particularly when bauxite with low levels of reactive silica is processed. Sulphate levels can thus build up to very high steady state concentrations without further intervention.

[6] A number of processes have thus been proposed for removing sulphate ions from Bayer liquors. Most of these processes rely upon increasing the supersaturation of sodium sulphate in a spent Bayer liquor, forcing it to crystallise from solution. This can be done by concentrating the liquor by evaporation, by extractive crystallisation using organic solvents (Kaiser process), by chilling the liquor, or a combination of these. Such processes suffer from a number of drawbacks, including the relatively high solubility of sodium sulphate, co-precipitation of other liquor components such as carbonate-containing species, resulting in the loss of the soda values thereof, and - in the case of evaporation at least - the challenges of separating the precipitates from the viscous liquor concentrate. Alternatively, sulphate can be removed by addition of a heavy metal, such as barium, which forms insoluble sulphates (Pechiney process). Such approaches suffer from the disadvantage of introducing expensive, highly toxic and environmentally undesirable metals to the process.

[7] In more recent developments, a deeper appreciation of the reaction chemistry of lime and aluminate in Bayer liquors during causticisation has been exploited to develop alternative methods for removing anionic impurities, including sulphate. It has been recognised that the causticisation reaction proceeds via an isolatable intermediate species: a lamellar calcium aluminate, the interlayer regions of which are filled by charge balancing anions and water (hereafter referred to as “hydrocalumite”, in view of its structural similarity with this naturally occurring mineral). The overall causticisation reaction given by equation (1) is thus achieved by a twostep sequence of reactions, believed to be according to equations (2) and (3):

4Ca(OH)₂ + 2AI(OH)₄“ + ¹/2CO3²“ + nH2O ~ [Ca₂AI(OH)₆]₂*¹/2CO₃*OH*nH₂O + 3OH (2)

2018271243 26 Nov 2018 [Ca₂AI(OH)₆]₂*¹/2CO3*OH*nH2O + 3¹/2CO₃ ²“ ~ 4CaCO₃ + 2AI(OH)₄“ + 50ΗΓ + nH₂O (3) [8] A competing reaction during causticisation is the formation of the thermodynamically stable tricalcium aluminate (hereafter “TCA”), believed to take place via reaction of hydrocalumite with aluminate according to equation (4):

3[Ca₂AI(OH)₆]₂*¹ACO3*OH*nH2O + 2AI(OH)4“ + ΟΚθ 4Ca3[AI(OH)6]2 + 1¹/2CO₃ ²· + 3nH₂O (4) [9] As described in PCT/AU00/0208, sulphate may be removed from a Bayer liquor by causticizing the liquor according to equations (2) and/or (3), separating the resultant hydrocalumite and/or calcium carbonate precipitate from the liquor, and treating the clarified liquor with additional lime. With the carbonate ion substantially depleted, the sulphate ion is the preferred anion for incorporation into the hydrocalumite structure, purportedly according to equation (5):

4Ca(OH)₂ + 2AI(OH)₄- + SO₄ ² + nH₂O ~ [Ca₂AI(OH)₆]₂*SO₄*nH₂O + 4ΟΗΓ (5) [10] The sulphate-containing hydrocalumite precipitate may then be separated from the liquor and disposed of. While providing a means to remove sulphate from the Bayer process, this approach suffers from the disadvantage that both lime and aluminate are consumed in large stoichiometric excess relative to the sulphate.

[11] There is therefore an ongoing need for new methods of removing sulphate ions from Bayer process liquors which at least partially address one or more of the above-mentioned short-comings.

[12] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Summary of Invention [13] The inventors have now discovered that sulphate ions may be removed from a process liquor in a Bayer refinery by providing fluoride in the liquor at a sufficiently high concentration whereby a precipitate rich in fluoride and sulfate ions is induced to precipitate. The precipitate, which is believed to comprise kogarkoite, a

2018271243 26 Nov 2018 double salt of fluoride and sulfate with the formula NaF.Na2SO4, may then be separated from the liquor and disposed of.

[14] It has previously been recognised that kogarkoite may undesirably form at various locations in a Bayer refinery, typically as a fouling scale on lower temperature surfaces of process equipment or as a co-precipitated contaminant in the gibbsite product. Historically, some refineries have therefore been carefully operated to control the fluoride and sulphate concentrations below the levels at which kogarkoite might precipitate, despite the selectivity and operational penalties associated with these control measures. Surprisingly, it has now been found that kogarkoite formation, previously considered a problem to avoid, can in fact be usefully exploited to remove both sulfate and fluoride ions from the Bayer process, without undue operational challenges and without substantial consumption of lime or loss of aluminate. Among the potential advantages provided by this approach is the reduced steady state concentration of sulphate that can be achieved in the refinery liquors compared with prior approaches. Moreover, the precipitation of sulphate by the process of the invention is selective relative to carbonate and other anions in the liquor, such that a relatively lower loss of soda (S) concentration may result compared with prior precipitation-based approaches for removing sulphate ions from Bayer liquor.

[15] In accordance with one aspect the invention provides a process for removing sulphate ions from a Bayer liquor in a Bayer refinery, the process comprising: providing fluoride ions in the Bayer liquor at a concentration sufficient to induce precipitation of a precipitate comprising fluoride and sulfate ions; separating the precipitate from a sulphate-depleted Bayer liquor; and recycling the sulphatedepleted Bayer liquor in the Bayer refinery.

[16] In some embodiments, the precipitate comprises a fluoride-sulfate double salt, preferably kogarkoite. The precipitate may comprise at least 30 wt% kogarkoite, such as at least 50 wt% kogarkoite.

[17] In some embodiments the concentration sufficient to induce precipitation is at least 2.0 g/litre (NaF), at least 2.5 g/litre (NaF), or at least 3.0 g/litre (NaF), such as at least 3.5 g/litre (NaF).

2018271243 26 Nov 2018 [18] In some embodiments the molar ratio of fluoride ions to sulphate ions in the Bayer liquor is less than about 1.5:1, preferably less than about 1.1:1.

[19] In some embodiments, the sulphate concentration is below 15 g/litre (Na2SO₄), preferably below 12 g/litre (Na2SO₄) in the sulphate-depleted Bayer liquor.

[20] In some embodiments, the Bayer liquor is a spent Bayer liquor, typically a spent Bayer liquor circulating in the primary Bayer refinery circuit.

[21] In some embodiments, providing the fluoride ions at the concentration sufficient to induce precipitation comprises increasing a concentration of the fluoride ions in the Bayer liquor from an initial concentration to the concentration sufficient to induce precipitation, generally without adding an extraneous source of dissolved fluoride ions. The initial concentration is typically the concentration of fluoride ions in the spent Bayer liquor circulating in the primary Bayer refinery circuit. The concentration of the fluoride ions may be increased by concentrating the Bayer liquor. The Bayer liquor may be concentrated in an evaporator, preferably a multi-effect salting-out evaporator.

[22] In some embodiments, the sulphate-depleted Bayer liquor has a caustic concentration (C) of less than 600 g/litre, preferably less than 500 g/litre, such as less than 450 g/litre, or between 270 g/litre and 420 g/litre.

[23] In some embodiments, the process further comprises seeding the Bayer liquor to promote the precipitation, preferably with seed particles of the precipitate. Where a multi-effect salting-out evaporator is used to increase a concentration of the fluoride ions in the Bayer liquor to the concentration sufficient to induce precipitation, the liquor may be seeded prior to the first effect where the fluoride concentration is sufficient to induce precipitation. In some embodiments, the seed is added to the liquor feed to the first effect where the fluoride concentration is sufficient to induce precipitation.

[24] In some embodiments, the process further comprises cooling the Bayer liquor to promote the precipitation. In some embodiments, a Bayer liquor concentrated to increase the fluoride concentration is cooled to promote or induce the precipitation.

2018271243 26 Nov 2018 [25] In some embodiments, the Bayer liquor further comprises dissolved carbonate ions, and the precipitation selectively removes the sulphate ions relative to the carbonate ions. In some such embodiments, the Bayer liquor has a C/S ratio of above about 0.85, such as between about 0.85 and 0.90. In some embodiments, the Bayer liquor has a carbonate concentration of below about 60g/litre (Na2CC>3). In some embodiments, the selective removal of sulphate ions relative to the carbonate ions may be such that the precipitate is substantially free of the carbonate ions, or such that the C/S ratio of the sulphate-depleted Bayer liquor after precipitation is no more than 0.05 units higher than that of the Bayer liquor prior to the precipitation.

[26] In some embodiments, the fluoride ions provided in the Bayer liquor are extracted from an aluminous ore in the refinery. In some embodiments, providing the fluoride ions in the Bayer liquor at the concentration sufficient to induce precipitation comprises adding an extraneous source of fluoride ions to a process stream in the Bayer refinery.

[27] In some embodiments, providing the fluoride ions at the concentration sufficient to induce precipitation comprises minimising or controlling an amount of fluoride ions mineralised in tricalcium aluminate in the Bayer refinery, for example by minimising or controlling a quantity of tricalcium aluminate produced in the Bayer refinery. In some such embodiments, the amount of fluoride ions mineralised in tricalcium aluminate may be controlled to maintain a fluoride to sulphate molar ratio or a fluoride ion concentration below a predetermined maximum value in a process stream in the Bayer refinery.

[28] Minimising or controlling the amount of fluoride ions mineralised in tricalcium aluminate may comprise adding a tricalcium aluminate inhibitor to at least one process stream in the Bayer refinery, such as a dilute Bayer liquor process stream circulating in a causticiser side circuit of the Bayer refinery. The tricalcium aluminate inhibitor may be an anionic organic surfactant, and is preferably selected from the group consisting of an anionic homopolymer, an anionic copolymer, a polyacrylic acid, a polymer bearing hydroxamate functional groups, a hydroxamic acid, a humic acid, a tannic acid, a lignosulphonate, a fatty acid, a sulphonated carboxylic acid, a carboxylic acid, and a polyhydroxy carboxylic acid.

2018271243 26 Nov 2018 [29] Minimising or controlling the amount of fluoride ions mineralised in tricalcium aluminate may comprise refraining from adding, or controlling the addition of, a calcium-based reagent to a concentrated Bayer liquor process stream of the primary Bayer refinery circuit. For example, the amount of fluoride ions mineralised in tricalcium aluminate may be minimised by refraining from adding a calcium-based reagent to a concentrated Bayer liquor process stream circulating in the primary Bayer refinery circuit, such as in digestion.

[30] In some embodiments, separating the precipitate from the sulphatedepleted Bayer liquor comprises filtering or centrifuging the precipitate. Separating the precipitate from the sulphate-depleted Bayer liquor may further comprise thickening the precipitate, for example prior to the filtering or the centrifuging. The process may further comprise disposing the precipitate together with red mud residue from the Bayer refinery, for example in a dam. The sulphate-depleted Bayer liquor may be recycled to the primary Bayer refinery circuit of the refinery.

[31] Where the terms “comprise”, “comprises” and “comprising” are used in the specification (including the claims) they are to be interpreted as specifying the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[32] Further aspects of the invention appear below in the detailed description of the invention.

Brief Description of Drawings [33] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

[34] Figure 1 is a schematic block flow diagram of a Bayer refinery in which embodiments of the invention may be implemented.

[35] Figure 2 is a schematic block flow diagram of the evaporation unit in the Bayer refinery depicted in Figure 1.

[36] Figure 3 is a process flow diagram of a multi-effect salting out evaporator in the evaporation unit of the Bayer refinery depicted in Figure 1.

2018271243 26 Nov 2018 [37] Figure 4 is a schematic process flow diagram of an evaporation unit in a Bayer refinery in which an embodiment of the invention was implemented, as described in Example 1.

[38] Figure 5 is an XRD diffraction pattern of a precipitate comprising fluoride and sulphate ions, produced according to the invention as described in Example 2.

[39] Figure 6 is a Scanning Electron Microscopy (SEM) image of a precipitate comprising fluoride and sulphate ions, produced while seeding the evaporator as described in Example 3.

[40] Figure 7 is a Scanning Electron Microscopy (SEM) image of a precipitate comprising fluoride and sulphate ions, produced without seeding the evaporator as described in Example 3.

[41] Figure 8 is a graph depicting the correlation between fluoride concentration and sulphate concentration in the spent liquor processed according to the process of the invention, as described in Example 4.

[42] Figure 9 is a graph depicting the fluoride to sulphate molar ratio in the precipitate as a function of the fluoride to sulphate molar ratio of the spent liquor processed in the evaporator, as described in Example 5.

Detailed Description [43] The present invention relates to a process for removing sulphate ions from a Bayer liquor in a Bayer refinery. In its most general form, the process comprises providing fluoride ions in the Bayer liquor at a concentration sufficient to induce precipitation of a precipitate comprising fluoride and sulfate ions; separating the precipitate from a sulphate-depleted Bayer liquor; and recycling the sulphate-depleted Bayer liquor in the Bayer refinery.

[44] Throughout this specification various terms commonly used in the alumina industry are used. In the interests of clarity, such terms are now defined.

[45] The term “Bayer liquor” refers to a sodium caustic aluminate liquor generated by digesting (dissolving) bauxite in a caustic soda solution at elevated

2018271243 26 Nov 2018 temperatures and pressures. In Bayer liquors, the principal constituents are sodium aluminate and sodium hydroxide. Other impurities in the liquor stream are present as soluble sodium salts.

[46] The term “A” refers to the alumina concentration of the liquor and more specifically to the concentration of sodium aluminate in the liquor, expressed as equivalent g/litre of alumina.

[47] The term “C” refers to the caustic concentration of the liquor, this being the sum of the sodium aluminate and sodium hydroxide content of the liquor expressed as equivalent g/litre concentration of sodium carbonate.

[48] The term “A/C” is thus the ratio of alumina concentration to caustic concentration.

[49] The term “S” refers to the soda concentration or more specifically to the sum of C and the actual sodium carbonate concentration, this sum once again being expressed as the equivalent g/litre concentration of sodium carbonate.

[50] A Bayer liquor's carbonate impurity level is expressed in terms of the caustic to soda ratio, or “C/S”. A fully causticised (carbonate-free) Bayer process liquor will possess a C/S ratio of 1.00.

[51] The term “pregnant Bayer liquor” refers to a Bayer liquor stream after digestion and before the gibbsite precipitation stage. A pregnant Bayer liquor typically has a high A/C ratio, such as above 0.5.

[52] The term “spent Bayer liquor refers to a Bayer liquor stream after the gibbsite precipitation stage and prior to digestion. A spent Bayer liquor typically has a low A/C ratio, such as below 0.5.

[53] The term “dilute Bayer liquor” refers to a Bayer liquor stream with a low S, typically less than 220 g/litre. A “concentrated Bayer liquor” has a high S, such as above 220 g/litre.

[54] The term “lime” as used throughout this specification is a generic term used to refer to calcium oxide (CaO or quicklime) in dry form, or calcium hydroxide

2018271243 26 Nov 2018 (Ca(OH)₂) either in the form of a slaked lime slurry or the dry form of Ca(OH)₂ also referred to as 'hydrated lime'. Thus, a “slaked lime slurry” is produced when lime is mixed with a slaking solution which can be any aqueous solution, typically water. In an alumina refinery, a dilute Bayer liquor can be used as a slaking solution due to the presence of water in such dilute Bayer liquors.

[55] “Causticisation” is the term used by persons skilled in the art of the Bayer process to describe the process whereby carbonate is removed from a Bayer liquor and replaced with hydroxide through the addition of lime and precipitation of insoluble calcium carbonate.

[56] The term “hydrocalumite” is used throughout this specification to refer to aluminium-based layered double hydroxide of the form [Ca₂AI(OH)6]2*X2*nH₂O, where “X₂” represents a charge-balancing anion or anions. By way of example, when X is carbonate, hydrocalumite may have the formula of [Ca₂AI(OH)6]2*CO3*nH₂O or [Ca2AI(OH)₆]2*¹XCO3*OH»5¹XH₂O depending on a number of factors which govern the preferential intercalation of species. The interlayer regions are filled with charge balancing ions and water molecules.

[57] The term “TCA” refers to tricalcium aluminate having the chemical formula of Ca₃[AI(OH)6]₂, including where one or more of the hydroxide ions is substituted by another anion. TCA is also commonly written using the formula 3CaO.AI₂O3.6H₂O (TCA6) or C3AH6 in cement industry notation.

[58] The term “calcium-based reagent” refers to calcium-containing reagents which are generally suitable for causticizing Bayer liquors. Commonly, lime is used as the calcium-based reagent in Bayer refineries, however it has also been reported to add hydrocalumite or TCA to causticize Bayer liquors. In principle, other calcium compounds such as calcium chloride may also be suitable, though at the expense of introducing extraneous anionic impurities.

[59] The term “lime efficiency” is defined as the percentage of available lime that is converted to calcium carbonate during causticisation. A number of processes may be used to calculate lime efficiency, including Total Inorganic Carbon (TIC) analysis, x-ray fluorescence (XRF) analysis, Thermo Gravimetric Analysis (TGA) analysis, liquor or mass balance.

2018271243 26 Nov 2018 [60] Figure 1 depicts a schematic process flow diagram of Bayer refinery 1, in which the process of the invention may be implemented. Bauxite ore 10 is ground in comminution unit 11 and digested in digestion unit 12 in a caustic process stream comprising spent Bayer liquor 13 and make-up caustic 14. Steam 15 is added to digestion unit 12 to increase the temperature, thereby increasing the rate of digestion. Digestion product 16 is clarified in decantation unit 17, with clarified pregnant liquor 18 then passing through filter unit 19 for further removal of solid contaminants. Optionally, lime or other suitable calcium-based reagent is added to pregnant liquor 18 prior to filtration to causticize the liquor and/or to produce tricalcium aluminate.

[61] Filtered pregnant liquor 18 is then cooled in counter-current heat exchanger 21, using the heat to increase the temperature of spent Bayer liquor 13 sent to digestion. The cooled pregnant liquor 18 is seeded with gibbsite seed 22 to induce bulk precipitation of gibbsite from the liquor in precipitation unit 23. Gibbsite precipitate 24 is then separated from spent liquor 13 in classification unit 25, and sent to calcination unit 26 to produce product alumina 27. Spent liquor 13 is recycled via heat exchanger 21 to comminution unit 11 and digestion unit 12.

[62] Bayer refinery 1 thus comprises at its core a caustic process stream circulating in a loop, passing repeatedly through grinding/digestion, decantation, filtration, precipitation, and classification steps. This loop is hereafter referred to as the primary Bayer refinery circuit, and will thus be distinguished from a number of side circuits in Bayer refinery 1 which will now be described.

[63] Solid residue 26 (known as “red mud”), concentrated in decantation unit 17 into a separate process stream also comprising a fraction of the pregnant liquor, is washed with wash water 27 in residue wash circuit 28 to recover most of the caustic content of the liquor. The washed red mud is then sent on to mix tank 29, and thereafter disposed in the refinery’s red mud residue dams. Washer overflow stream 30, a dilute Bayer liquor comprising wash water and the pregnant liquor recovered from residue 26, is then sent to causticiser circuit 31, where it is contacted with lime 20 (or other suitable calcium-based reagent such as hydrocalumite) in an amount sufficient to causticize the liquor. Causticisation precipitate 32, generally comprising calcium carbonate, hydrocalumite and tricalcium aluminate in varying proportions, is

2018271243 26 Nov 2018 separated from causticized liquor 33 in causticiser circuit 31 and recycled to wash vessel 28. One or more clarified causticized liquor streams 33 is sent to decantation unit 17, thereby rejoining the primary Bayer refinery circuit.

[64] Bayer refinery 1 thus also includes a caustic process stream circulating in a side loop to the primary Bayer liquor circuit. This process stream is withdrawn from decantation unit 17, diluted and clarified in residue wash circuit 28, causticized in causticiser circuit 31 and returned to decantation unit 17.

[65] Bayer refinery 1 also comprises evaporation unit 34. A portion of spent liquor 13 is withdrawn from the primary Bayer refinery circuit after classification and sent to evaporation unit 34, where it is heated with steam 35 and evaporated to remove evaporated water 36. Evaporation-concentrated spent liquor 37 is then returned to the primary Bayer refinery circuit. The primary function of evaporation unit 34 is to remove water from the Bayer process liquor such that water added to the process, mainly via wash water 27 and the water content of the bauxite, does not build up.

[66] Figure 2 depicts a schematic process flow diagram of an embodiment of evaporation unit 34. Spent liquor 13 from the primary Bayer refinery circuit is sent to salting out evaporator 40, which concentrates the liquor by removing evaporated water 36. Evaporator 40 is seeded with seed precipitate 41 to promote precipitation of a salt precipitate, as will be described in greater detail hereafter. Evaporated stream 42, comprising the precipitate slurried in the evaporation-concentrated Bayer liquor, is sent to crystalliser tank 43, where further precipitation may occur. Resultant precipitate-liquor slurry 44 is then separated in thickener 45, to produce clarified overflow 46 and thickened precipitate 47, taken as the underflow of thickener 45. A flocculent may be added to thickener 45 to improve the separation. A portion of thickened precipitate 47 is then de-liquored in filter unit 48, while another portion is diverted to evaporator 40 for use as seed precipitate 41. Filtrate 49 from filter unit 48 is combined with clarified thickener overflow 46 and returned to the primary Bayer refinery circuit as concentrated spent liquor 37. Filtered precipitate 50 is added to mix tank 29, optionally via a dissolver tank (not shown), where it is combined with the red mud residue and disposed in the refinery’s residue dams.

2018271243 26 Nov 2018 [67] An embodiment of the invention will now be described with reference to Figures 1 and 2. Spent liquor 13 circulating in the primary Bayer refinery circuit comprises sulphate ions to be removed according to the process of the invention. In some embodiments, the caustic concentration (C) of spent liquor 13 is between about 240 and 300 g/litre. In some embodiments, the concentration of sulphate ions in spent liquor 13 is above about 6 g/litre, or about 8 g/litre. However, it will be appreciated that the concentration of sulphate ions in spent liquor 13 prior to removal, is itself reduced by the process of the invention, since the steady state concentrations of ion impurities circulating in the primary Bayer refinery circuit are influenced by the effectiveness of their removal from the process as a whole. Accordingly, the concentration of sulphate ions in spent liquor 13 may be below 25 g/litre, preferably below 20 g/litre, more preferably below 15 g/litre and most preferably below 12 g/litre.

[68] Fluoride ions, originating from the bauxite ore digested in digestion unit 12, are also present in spent liquor 13. The concentration of fluoride ions in spent liquor 13 is insufficient to induce precipitation of a precipitate comprising fluoride and sulfate ions prior to evaporation of the liquor in evaporator 40, since this would result in undesirable co-precipitation with the gibbsite and/or fouling of process equipment in the refinery. Nevertheless, as will be described in greater detail hereafter, it is typically important that a sufficient initial concentration of fluoride ions is provided in spent liquor 13 sent to evaporator 40, to ensure that a concentration of fluoride ions sufficient to induce such precipitation is provided during and/or after evaporation.

[69] Evaporation of spent liquor 13 in evaporator 40 removes evaporated water 36 and increases the caustic concentration (C) of the liquor, as well as the concentrations of all impurity anions including sulphate, fluoride and carbonate. As a consequence, fluoride ions are provided in spent liquor 13 at a concentration sufficient to induce precipitation of a precipitate comprising fluoride and sulfate ions, thereby producing a sulphate-depleted Bayer liquor which may be separated from the precipitate.

[70] It is believed that the precipitate comprising the fluoride and sulphate ions is at least partially in the form of kogarkoite, a fluoride-sulfate double salt having the formula NaF.Na2SC>4.

2018271243 26 Nov 2018 [71] The inventors have found that an initial fluoride concentration of approximately 3.4 g/litre NaF (81 mmol/litre) is sufficient for precipitation to be induced when a spent Bayer liquor with initial C of 298 g/litre and initial sulphate concentration of approximately 10.8 g/litre Na₂SO₄ (76 mmol/litre) is concentrated to a C of approximately 404 g/litre, while seeding the evaporator with seed precipitate. The minimum concentration of fluoride sufficient to induce precipitation was thus, in this case, greater than 3.4 g/litre NaF and less than 4.6 g/litre NaF. The actual concentration of fluoride at which precipitation was induced is thought to be about 4 g/litre.

[72] However, it will be apparent to the skilled person that the minimum concentration of fluoride ions sufficient to induce precipitation of a fluoride- and sulphate-containing precipitate will depend on a number of factors which are expected to vary in implementations of the invention. These factors include the concentration in the liquor of other ionic components that crystallise with fluoride in the precipitate, such as sulphate and sodium ions, the temperature of the liquor during evaporation and the addition of seed crystals (since it is believed that seeding lowers the kinetic barriers to crystallisation). In some embodiments, the concentration sufficient to induce precipitation is at least 2.0 g/litre (NaF), or at least 2.5 g/litre (NaF), or at least 3.0 g/litre (NaF), such as at least 3.5 g/litre (NaF).

[73] The inventors have found that up to about 30% of the sulphate ions in a spent Bayer liquor having initial sulphate concentration of approximately 10.8 g/litre Na₂SO₄ (76 mmol/litre) may be removed according to the process of the invention. Again, it should be appreciated that the extent of sulphate ion removal will depend on a number of factors, including the concentrations of sodium, fluoride and sulphate present in the liquor. Notably, as the sulphate removal process of the invention is implemented in a Bayer refinery, the concentration of sulphate ions in the spent Bayer liquor decreases, so that the efficiency of removal decreases until a steady state sulphate concentration is obtained. In some embodiments of the invention, therefore, at least 3%, and preferably at least 5%, such as at least 10%, of the sulphate ions are removed from the Bayer liquor. The concentration of dissolved sulphate ions in the sulphate-depleted evaporated liquor after removal of the sulphate ions may be below 15 g/litre (Na₂SO₄), preferably below 12 g/litre (Na₂SO₄). Depending on the

2018271243 26 Nov 2018 concentrations of fluoride and sulphate ions in the Bayer liquor, and the process conditions during precipitation, it is believed that sulphate ion concentrations of 5 g/litre (Na2SO4) or even lower can be produced in the sulphate-depleted evaporated liquor.

[74] Providing a concentration of fluoride ions in excess of the minimum concentration required to induce precipitation will generally enhance the amount of precipitate formed, thereby advantageously increasing the sulphate ion removal. However, in at least some embodiments, an upper threshold limit of fluoride concentration, or fluoride to sulphate molar ratio, in the spent liquor to evaporation may be encountered, beyond which additional fluoride provides no further improvement and may indeed produce an adverse result. Without wishing to be bound by any theory, it is believed that precipitation of villiaumite (NaF) together with, or instead of, kogarkoite may be induced if excessive amounts of fluoride are present in the liquor. Villiaumite precipitation does not remove sulphate ions from the liquor. Indeed, if the fluoride and sulphate concentrations are such that villiaumite becomes the most insoluble salt during evaporation, the resultant loss of soluble fluoride ions due to villiaumite precipitation may reduce or even terminate the precipitation of kogarkoite, and consequently also the removal of sulphate ions. The inventors have found that fluoride to sulphate molar ratios of greater than about 1.1:1 may lead to villiaumite precipitation during evaporation of spent Bayer liquors having a sulphate concentration of below about 12 g/litre (Na2SC>4). Once again, it will be appreciated that the threshold fluoride concentration, or fluoride to sulphate molar ratio, beyond which kogarkoite is no longer the most insoluble salt will depend on a variety of implementation-specific factors, including the sodium and sulphate ion concentrations.

[75] In some preferred embodiments of the invention, the precipitate contains only minor or insignificant quantities of carbonate, hydroxide or aluminate ions, thereby minimising the loss of soda concentration (S) from the liquor. If a suitable amount of fluoride is present, kogarkoite has a lower solubility than other potentially crystallisable salts present in spent Bayer liquors so that highly selective removal of sulphate (and fluoride) ions can be achieved. The inventors have found that sulphate ions can be removed by producing the precipitate while increasing the C/S ratio of the

2018271243 26 Nov 2018 liquor by less than 0.05 units. In some embodiments, the precipitate contains at least 50 wt% kogarkoite. Depending on the fluoride and sulphate concentrations, it is believed that a precipitate with close to 100% kogarkoite content can be produced.

[76] As already noted, evaporation-induced precipitation has previously been employed to remove sulphate from spent Bayer liquors. However, in such approaches, fluoride ions are not provided in the liquor at a sufficient concentration to induce kogarkoite formation. As a result, sulphate ions are precipitated from the evaporated liquor predominantly in the form of sodium sulphate (Na₂SO₄), together with carbonate containing salts such as sodium carbonate (Na₂CO3) and the double salt burkeite (2Na₂SO₄.Na₂CO₃).

[77] The concentration of carbonate ions in spent Bayer liquors circulating in the primary Bayer refinery circuit can be significant. In refineries processing bauxites with high carbonate levels and/or having a silica pre-wash step before digestion, C/S values of below 0.90 are typical, despite improvements in causticisation technology. In such cases, it has previously been considered that carbonate precipitation, with associated loss of soda values, is an inevitable consequence of removing sulphate ions via evaporation-induced precipitation from spent Bayer liquors. The process of the invention advantageously allows sulphate to be removed from Bayer liquors, such as spent Bayer liquors, without substantially reducing the carbonate content of the liquor, or with reduced losses of carbonate content compared with alternative approaches. For example, the inventors have found that negligible carbonate precipitation occurs at C/S values between about 0.86 and 0.90.

[78] As a result of the evaporation in evaporator 40, the caustic concentration (C) of the sulphur-depleted Bayer liquor is higher than that of the spent Bayer liquor 13. Increasing the caustic concentration of the sulphate-depleted liquor via deeper evaporation is expected to improve the removal of sulphate, since the concentrations of each of the kogarkoite constituent ions (i.e. sodium, fluoride and sulphate ions) are thereby increased. However, deep evaporation is an energy and capital intensive process and the viscosity and corrosivity of the sulphate-depleted liquor increases at elevated C values, complicating the separation of the precipitate from the liquor. In

2018271243 26 Nov 2018 some embodiments of the invention, therefore, the sulphate-depleted Bayer liquor has a caustic concentration (C) of less than 600 g/litre, or less than 500 g/litre.

[79] In some refineries, cost savings and operational efficiencies may be achieved if sufficient sulphate removal can be produced with an even lower final degree of evaporation (and thus lower C values) in the sulphur-depleted liquor product of evaporator 40. Due to the low solubility of the precipitate comprising fluoride and sulphate ions, a reduced extent of evaporation is required to remove sulphate according to the process of the invention. Moreover, co-precipitation of carbonate-containing salts may advantageously be avoided or minimised in evaporated liquors with lower final concentrations. In some embodiments, the sulphate-depleted Bayer liquor therefore has a caustic concentration (C) of less than 450 g/litre, such as between 270 g/litre and 420 g/litre. By contrast, higher evaporated liquor caustic concentrations may be required when removing sulphate ions as precipitated sodium sulphate and/or burkeite.

[80] With continued reference to Figure 2, evaporator 40 is seeded with seed precipitate 41 to induce precipitation of the precipitate comprising fluoride and sulfate ions. Thickened precipitate 47 sourced from the underflow of thickener 45, which is believed to comprise kogarkoite crystals, may be introduced to evaporator 40 to seed the bulk crystallisation. It is believed that seeding of the evaporation-concentrated liquor allows precipitation to be effectively induced at lower concentrations of the ionic components in the liquor, since kinetic barriers to crystallisation which inhibit precipitation even when a solution is supersaturated may be overcome thereby. As an alternative approach, spent liquor 43 may be concentrated to supersaturation in evaporator 40 without seeding, with seed precipitate 41 optionally being added to crystalliser tank 43. Although the inventors have verified this approach, a favourable precipitate morphology has been obtained when the evaporator is seeded, and the rate of fouling the evaporator surfaces is also reduced.

[81] As a further alternative, filtered precipitate 50 may be used as seed precipitate 41, for example by introducing it to the evaporator as a slurry in the feed liquor. Compared with the use of thickened precipitate 47, this advantageously reduces the amount of liquor recycled to the evaporator, thereby increasing the

2018271243 26 Nov 2018 capacity of the evaporator. Although seed precipitate 41 is most conveniently sourced from a precipitate-containing stream in Bayer refinery 1, it is not excluded that another suitable seed material, for example synthetic kogarkoite, is employed instead. Furthermore, although seeding of precipitation is considered particularly advantageous, it is with the scope of the invention that precipitation is induced by providing the fluoride ions at a concentration in the evaporated spent liquor sufficient to induce precipitation without seeding. The Bayer liquor may optionally be cooled to promote precipitation, for example after evaporation.

[82] Figure 3 depicts a salting out evaporator 40, suitable for use in the process of the invention, in greater detail. Evaporator 40 comprises five evaporation effects 60a, 60b, 60c, 60d and 60e, each effect comprising a recirculation pump 61, a heat exchanger 62, an evaporation vessel 63 and a forward feed pump 64 (only recirculation pump 61a, exchanger 62a, evaporation vessel 63a and forward feed pump 64a are indicated in Figure 3 for clarity). Each evaporation effect produces evaporated water overhead stream 65 and concentrated liquor bottoms stream 66 (only overhead stream 65a and bottoms stream 66a are indicated in Figure 3 for clarity).

[83] Spent liquor 13 is fed to the suction of recirculation pump 61a in first evaporation effect 60a, and then heated and combined with the overhead stream from second effect evaporator 60b in heat exchanger 62a. The combined stream is fed to evaporation vessel 63a where overhead stream 65a is removed by evaporation. Overhead stream 65a is then condensed in condenser 67; condensate 68 may then be recycled to a suitable location in Bayer refinery 1, such as residue wash circuit 28. Bottoms stream 66a is then fed to second evaporation effect 60b via forward feed pump 64a; the recirculation ratio in first effect 60a being governed by the relative flow rates of recirculation pump 61a and forward feed pump 64a. Bottoms stream 66a is then further concentrated in second evaporation effect 60b in an analogous fashion. In this manner, spent liquor 13 is progressively concentrated from its initial caustic concentration of about 240-300 g/litre to a final caustic concentration of about 380420 g/litre as it passes through each successive evaporation effect. Steam 35, added to the final effect 60e, provides the heat input for evaporation.

2018271243 26 Nov 2018 [84] Evaporator 40, as depicted in Figure 3, includes five evaporation effects in series. It will be appreciated, however, that evaporator 40 may suitably have more or fewer effects, and that evaporator effects may also be arranged in parallel. Other evaporator configurations available to the skilled person may also be used.

[85] Seed precipitate 41 is added to the liquor feed of at least one of the evaporation effects of evaporator 40, thereby seeding precipitate formation in that (or a subsequent) evaporation effect. Generally, it is preferred to seed the first evaporation effect where supersaturation of kogarkoite in the liquor is expected; precipitation is thus induced in the seeded evaporator effect. As depicted in Figure 3, the third evaporation effect 60c is seeded; however it will be appreciated that other evaporator effects may be suitably seeded as determined by the expected ionic concentrations in each successive effect.

[86] Bottoms stream 66e of the final evaporation effect 60e is then fed via a series of knockout vessels 69a, 69b and 69c, with final concentrated liquor stream 42 being sent to crystalliser tank 43.

[87] Referring again to Figure 2, further precipitation of crystallised salts may be allowed to occur in crystalliser tank 43. Precipitate-liquor slurry 44 is then separated in raked thickener 45. Thickened precipitate 47 is then further de-liquored in filter unit 48, which comprises plate and frame filters. Evaporation-concentrated spent liquor 37, the combination of filtrate 49 and clarified thickener overflow 46, is returned to the primary Bayer refinery circuit. This stream is a sulphate-depleted Bayer liquor, as a result of the precipitation and separation of the fluoride- and sulphate-containing precipitate from the liquor.

[88] Although Figure 2 as described herein depicts separation of the precipitate occurring via a raked thickener and plate and frame filters, it will be appreciated that other solid/liquid separation device configurations may be applied. Embodiments of the invention may suitably include direct filtration without pre-thickening, the use of other filter technologies such as vertical pressure filtration, candle filtration, hyperbaric filtration, or screw filtration, or the use of other solid separation technologies such as centrifugation.

2018271243 26 Nov 2018 [89] As described herein, the fluoride ions are typically provided at the concentration sufficient to induce precipitation by evaporating the Bayer liquor, thereby increasing the concentration of the fluoride ions from an initial concentration to the concentration sufficient to induce precipitation, preferably in the presence of seed crystals. However, it is also contemplated that the required fluoride concentration could be provided by adding a source of fluoride ions directly to a Bayer liquor, optionally in the presence of seed crystals. For example, a concentrated NaF solution could be added. As such, precipitation of the precipitate comprising fluoride and sulfate ions may in principle be induced without concentrating the Bayer liquor. Regardless of how the sufficient concentration of fluoride ions is produced in the Bayer liquor, precipitation may be promoted by cooling the Bayer liquor to increase the super-saturation of the fluoride- and sulphate-containing salt.

[90] In some embodiments, the primary or only source of the fluoride ions provided in the Bayer liquor will be the bauxite, or other aluminous ore, digested in the primary Bayer refinery circuit. However, it is envisaged that an extraneous source of fluoride ions may be used to supplement the fluoride extracted from the bauxite, added either directly to the Bayer liquor for processing according to the invention or to another process stream in the refinery. This may be required, for example, if the aluminous ore contains elevated levels of native sulphate and/or low levels of native fluoride. Bayer refineries are commonly co-located with aluminium smelters, in which fluoride emissions are captured by scrubbing with refinery lake water. This lake water may be introduced into the Bayer refinery via the wash water, and will thus supplement the fluoride ions directly extracted from the bauxite.

[91] In at least some Bayer refineries, the bauxite ore contains a theoretically sufficient quantity of fluoride ions to implement the process of the invention without introducing an extraneous source of fluoride. However, fluoride ions extracted during digestion have conventionally been irreversibly consumed elsewhere in the refinery, either co-incidentally or by design, and have thus generally not been present in sufficient quantities in spent Bayer process streams to permit selective precipitation of a fluoride- and sulfate-containing precipitate. Therefore, an important secondary aspect of the present invention may involve the suppression or control of fluorideconsuming reactions in the Bayer refinery, such that a concentration of fluoride ions

2018271243 26 Nov 2018 sufficient to induce precipitation of the precipitate comprising fluoride and sulphate ions is provided in the Bayer liquor.

[92] Without wishing to be bound by any theory, the inventors believe that fluoride ions are consumed by mineralisation in solid tricalcium aluminate (TCA) produced (either as an unwanted by-product or by design) in many Bayer refineries. As described in PCT/AU02/00459, it is believed that fluoride ions reversibly interchange with hydroxyl ions in the caustic aluminate liquor, according to equation (6):

AI(OH)₄“ + F“~ AI(OH)₃F“ + OH” (6) [93] The fluoride-containing aluminate anion is believed to be preferentially incorporated into TCA, possibly as Ca3[AI(OH)₅F][AI(OH)₆], Ca₃[AI(OH)₅F]2 or similar species, such that fluoride is mineralised into the TCA precipitate to a greater degree than would be expected based on its abundance relative to aluminate. As such, some Bayer refineries have been intentionally operated to produce TCA in an amount sufficient to reduce fluoride levels below a target concentration in the Bayer liquor, despite the loss of aluminate that results.

[94] The present invention may therefore be facilitated by any intervention in the Bayer refinery which minimises or controls the formation of TCA, or which minimises or controls the fluoride content of the TCA that is formed, so as to provide a sufficient concentration of fluoride ions in the Bayer liquor.

[95] In some embodiments, the amount of fluoride ions mineralised in tricalcium aluminate is controlled to maintain a fluoride to sulphate molar ratio, or a fluoride ion concentration, below a predetermined maximum value in a process stream in the Bayer refinery. For example, one or more operations which affect TCA formation in the refinery may be regulated responsively to a fluoride content analysis in the process stream. The process stream subject to the analysis may optionally be the Bayer liquor to be processed according to the invention, or another liquor in the refinery. Such active control of fluoride mineralisation may be required to ensure that a precipitate comprising fluoride and sulphate ions (such as kogarkoite) is formed

2018271243 26 Nov 2018 preferentially to a precipitate containing only fluoride ions (such as villiaumite), as described herein.

[96] In other embodiments, the fluoride to sulphate molar ratio, or the fluoride ion concentration, in the Bayer liquor to be processed may be safely within a suitable range for precipitating a fluoride- and sulphate-containing precipitate according to the invention, such that additional fluoride ion content in the liquor may ordinarily be tolerated or even preferred for increased sulphate removal. In such embodiments, the amount of fluoride ions mineralised in TCA may be minimised, for example by minimising the amount of TCA formed in the refinery (or by producing no more than the minimum amount of TCA otherwise required in the refinery, e.g as filter aid).

[97] It will be appreciated that the necessity to either actively control or minimise fluoride mineralisation in the refinery will depend on various factors, and in particular on the relative amounts of sulphate and fluoride extracted from the bauxite ore.

[98] In some embodiments, controlling or minimising the amount of fluoride ions mineralised in TCA comprises adding a tricalcium aluminate inhibitor to a process stream in the Bayer refinery. TCA is commonly formed as a by-product of outside causticisation; therefore in some embodiments the TCA inhibitor may be added to a process stream circulating in the causticisation section of the refinery. With reference again to Figure 1, the TCA inhibitor may be added to dilute washer overflow stream 30 to control TCA formation in causticiser circuit 31. Suitable TCA inhibitors may include a wide range of surfactants, preferably anionic organic surfactants such as those selected from the group consisting of an anionic homopolymer, an anionic copolymer, a polyacrylic acid, a polymer bearing hydroxamate functional groups, a hydroxamic acid, a humic acid, a tannic acid, a lignosulphonate, a fatty acid, a sulphonated carboxylic acid, a carboxylic acid, and a polyhydroxy carboxylic acid. A polyhydroxy carboxylic acid may be particularly preferred.

[99] In some refineries, TCA is also formed in the primary Bayer refinery circuit, either as by-product of inside causticisation or when producing TCA filter aid in the clarified pregnant liquor sent to filtration. Indeed, TCA may be produced in this

2018271243 26 Nov 2018 manner to intentionally reduce fluoride levels, as described in PCT/AU02/00459. In some embodiments, therefore, controlling or minimising the amount of fluoride ions mineralised in TCA comprises refraining from adding, or controlling the addition of, lime or other calcium-based reagent to the concentrated Bayer liquor process stream circulating in the primary Bayer refinery circuit.

[100] Various principles and methods for controlling the amount of TCA formed, or the mineralised fluoride content thereof, have been disclosed in PCT/AU99/00757, PCT/AU02/00459 and PCT/AU2012/00235. The skilled person is able to apply these, and other strategies that suggest themselves, to provide a suitable fluoride concentration in the Bayer liquor to be processed according to the invention.

EXAMPLES [101] The present invention is described with reference to the following examples. It is to be understood that the examples are illustrative of and not limiting to the invention described herein.

Example 1 [102] A schematic process flow diagram of a spent liquor evaporation unit of a Bayer refinery (c.f. unit 34 in Figure 1) is depicted in Figure 4. Spent liquor 113, obtained from the primary Bayer refinery circuit, was processed in the evaporation unit. Detailed analytical data of spent liquor 113 are shown in Table 1 below. Notably, the C concentration was 298 g/litre, fluoride concentration was 81 mmol/litre (3.4 g/litre NaF) and the sulphate concentration was 76 mmol/litre (10.8 g/litre Na₂SO₄). The [F] I [SO₄ ²'] molar ratio in the feed to evaporation was thus 1.03.

[103] Spent liquor 113 was evaporated in five-effect evaporator 140, similar to that schematically depicted in Figure 3, and transferred to crystalliser tank 143 having a residence time of approximately 30 minutes. The product of crystalliser tank 143, comprising a precipitate in the evaporation-concentrated Bayer liquor, was thickened in raked thickener 145. Flocculent 144 was added to the thickener to improve the separation. Seed crystals 141, sourced from underflow 147 of thickener 145, were recycled to the third evaporation effect of evaporator 140. Underflow 147 was filtered in press filters 148. The clarified overflow of thickener 145 and the filtrate of press

2018271243 26 Nov 2018 filters 148 were combined in tank 151 and returned to the primary Bayer refinery circuit as concentrated liquor 137. Filtered precipitate 150 from press filters 148 was dissolved with condensate in dissolver tank 152. Dissolved precipitate 153 was then directed to the refinery’s mix tanks, and thus ultimately disposed of in the red mud residue dams.

[104] Analytical data of evaporated Bayer liquor 154, obtained by manual filtration of a sample of underflow 147, are also displayed in Table 1. The C concentration was 404 g/litre. Since sodium aluminate and sodium hydroxide do not precipitate in evaporation of spent liquors, the increase in C across the evaporator may be used as a proxy for the extent of evaporation. It is thus estimated that the fluoride concentration of the liquor when precipitation was induced in the third effect was about 4 g/litre (as NaF), and the sulphate concentration was about 13 g/litre (as Na2SC>4).

[105] The calculated percentage of each dissolved component removed from the liquor as a result of precipitation is also shown in Table 1. Notably, approximately 30% of both the sodium fluoride content and the sodium sulphate content was precipitated from the liquor in evaporation. It may be calculated from this analysis that the F and SO₄ ²' ions were removed from the liquor in a molar ratio of 1.03 (i.e. approximately 1:1). By contrast, the loss of all other anionic impurity components of the liquor, including carbonate, chloride and oxalate, was negligible (i.e. within analytical error of zero).

Table 1

Parameter	Units	Spent liquor 113	Evaporated liquor 154	Loss via precipitation a
A	g/litre	138	187	0.2 %
C	g/litre	298	404	0
S	g/litre	346	473	-0.4 %
A/C		0.46	0.46	-
C/S		0.86	0.86	-
Na2CC>3	g/litre	48.9	68.3	-2.8 %
NaF	g/litre	3.4	2.4	30.3 %
Na2SC>4	g/litre	10.8	10.1	31.2 %
NaCI	g/litre	6.6	9.0	-0.7 %
N32C2O4	g/litre	2.9	3.0	-3.1 %
[F]	mmol/litre	81	77

2018271243 26 Nov 2018

[SO42-]	mmol/litre	76	71
[F ] / [SO, ]		1.06	1.08

^a calculated with the assumption that the caustic content of the liquor (i.e. mass of NaAI(OH)₄ + NaOH) remains constant over evaporation.

^b calculated [F'] / [SO4²'] in precipitate based on molar consumption of F and SO4²' from the liquor across evaporation.

Example 2 [106] Referring again to Figure 4 as described in Example 1, a sample of filtered precipitate 150 was recovered from press filters 148, dried, and analysed by X-Ray powder diffraction (XRD). At the time the sample was taken, the steady state [F] I [SO₄ ²'] molar ratio in the spent liquor feed 113 to evaporation was 0.73:1. The XRD diffraction pattern of the precipitate is shown in Figure 5.

[107] The diffraction pattern was compared against library standards of a number of different pure crystalline phases, including kogarkoite (NaF.Na₂SO₄); burkeite (2Na2SO4.Na2CC>3), sodium carbonate, sodium sulphate, sodium aluminate and sodium oxalate. The main phase of the precipitate was identified as kogarkoite (reflections K shown in Figure 5). It is estimated that kogarkoite formed at least 50% of the precipitate. The amount of carbonate-containing phases in the precipitate was negligible.

Example 3 [108] With continued reference to Figure 4 as described in Example 1, the effect of seeding the evaporator with seed crystals 141 from underflow 147 was further investigated, while evaporating spent liquor 113 as described in Example 1. At a first experimental condition, the seed crystals were directed to the suction of the first evaporator effect, i.e. before evaporation. The seed crystals were carried through the evaporator in the liquor, and were thus present when precipitation was induced.

[109] At a second experimental condition, the seed crystals were directed only to crystalliser tank 143, i.e to the concentrated post-evaporation liquor stream (depicted by the dotted arrow in Figure 4). It is believed that precipitation was still induced in the evaporator, despite the lack of seeding. Evidence for this included a faster rate of heat exchanger fouling in the evaporator compared with the first experimental

2018271243 26 Nov 2018 condition. Moreover, the morphology of the precipitate was very different: see SEM images in Figure 6 (first condition) and Figure 7 (second condition). It is believed that seeding in evaporation with kogarkoite seed crystals (the seed stream also comprising some of the flocculent) resulted in the well-defined, spherical particles of precipitate, whereas unseeded precipitation produced an amorphous mass of solids. Seeding thus leads to at least some of the following operational advantages: reduced fouling of the evaporator, reduced precipitate carry-over in the overflow of thickener 145, improved bed density in the underflow of thickener 145 and improved performance (e.g. lower soda losses) of filters 148.

Example 4 [110] The spent liquor evaporation unit described in Example 1 was designed with the assumption that sulphate ions would be precipitated as sodium sulphate and/or sulphate-carbonate double salts such as burkeite. As such, it was expected that sulphate concentrations of greater than 20 g/litre must be accepted in the liquor circulating in the primary Bayer refinery circuit. Therefore, in order to avoid kogarkoite precipitation in the gibbsite or as fouling scale on process equipment, the fluoride concentration in the liquor circulating in the primary Bayer refinery circuit was suppressed, typically to below 2.0 g/litre, by deliberate TCA formation in the refinery. This was achieved primarily by adding lime to digestion (i.e. to unit 12, Figure 1), as insufficient fluoride was mineralised in the external causticisation circuit (i.e. in unit 31, Figure 1).

[111] However, an increase in the fluoride concentration in the spent Bayer liquor above the previous target operating range was surprisingly found to cause a substantial drop in the sulphate concentration of the spent liquor, without causing operational or product quality problems in the refinery. The fluoride levels were thus allowed to rise to approximately 3 g/litre by cutting back, and eventually ceasing, addition of lime to digestion, causing precipitous further reductions in sulphate concentration in the liquor. A steady state sulphate concentration of between 8 and 10 g/litre (Na2SO₄) was eventually reached. The correlation between fluoride concentration and sulphate concentration in spent liquor 113 to evaporation is shown in Figure 8.

2018271243 26 Nov 2018 [112] These results support the proposition that, by producing an elevated initial fluoride concentration in the spent liquor to evaporation, a concentration of fluoride sufficient to precipitate a fluoride- and sulphate-containing precipitate (rather than a carbonate- and sulphate-containing precipitate) was provided during evaporation.

Example 5 [113] Referring again to Figure 4 as described in Example 1, the concentrations of key ionic components in spent liquor 113 (evaporator feed), concentrated liquor 137 (evaporator product) and dissolved precipitate 153 (from dissolver tank 152) were analysed over a period of time in the refinery. Figure 9 depicts the fluoride to sulphate molar ratio of the dissolved precipitate 153 as a function of the fluoride to sulphate molar ratio of spent liquor 113 sent to evaporation. It is evident that a molar ratio of 1:1 in the precipitate, consistent with kogarkoite formation, is obtained at feed molar ratios of below about 1.1:1. During periods of operation above this value, the fluoride to sulphate molar ratio of the precipitate was seen to increase above 1:1. A different precipitate morphology was also evident in these periods. It is believed that the precipitate formed in evaporation at elevated [F']/[SC>4²'] values (i.e. above about 1.1:1) was at least partially villiaumite, i.e. crystalline NaF.

[114] The C and S concentrations of spent liquor 113 and concentrated liquor 137 were also analysed during this period. From the negligible change in C/S ratio across evaporation, it may be inferred that little or no precipitation of carbonatecontaining species was occurring.

[115] 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 understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

Claims (26)

1. A process for removing sulphate ions from a Bayer liquor in a Bayer refinery, the process comprising:

providing fluoride ions in the Bayer liquor at a concentration sufficient to induce precipitation of a precipitate comprising fluoride and sulfate ions;

separating the precipitate from a sulphate-depleted Bayer liquor; and recycling the sulphate-depleted Bayer liquor in the Bayer refinery.

2. The process according to claim 1, wherein the precipitate comprises a fluoridesulfate double salt, preferably kogarkoite.

3. The process according to claim 1 or claim 2, wherein the precipitate comprises at least 30 wt% kogarkoite, preferably at least 50 wt% kogarkoite.

4. The process according to any one of claims 1 to 3, wherein the concentration sufficient to induce precipitation is at least 2.0 g/litre (NaF), preferably a least 2.5 g/litre (NaF), more preferably at least 3.0 g/litre (NaF), and most preferably at least 3.5 g/litre (NaF).

5. The process according to any one of claims 1 to 4, wherein the molar ratio of fluoride ions to sulphate ions in the Bayer liquor is less than about 1.5:1, preferably less than about 1.1:1.

6. The process according to any one of claims 1 to 5, wherein the sulphate concentration is below 15 g/litre (Na2SO₄), preferably below 12 g/litre (Na2SO₄) in the sulphate-depleted Bayer liquor.

7. The process according to any one of claims 1 to 6, wherein the Bayer liquor is a spent Bayer liquor.

8. The process according to any one of claims 1 to 7, wherein providing the fluoride ions at the concentration sufficient to induce precipitation comprises increasing a

2018271243 26 Nov 2018 concentration of the fluoride ions in the Bayer liquor from an initial concentration to the concentration sufficient to induce precipitation.

9. The process according to claim 8, wherein the concentration of the fluoride ions is increased by concentrating the Bayer liquor.

10. The process according to claim 9, wherein the Bayer liquor is concentrated in an evaporator, preferably a multi-effect salting-out evaporator.

11. The process according to any one of claims 1 to 10, wherein the sulphatedepleted Bayer liquor has a caustic concentration (C) of less than 600 g/litre, preferably less than 500 g/litre, more preferably less than 450 g/litre, and most preferably between 270 g/litre and 420 g/litre.

12. The process according to any one of claims 1 to 11, further comprising seeding the Bayer liquor to promote the precipitation, preferably with seed particles of the precipitate.

13. The process according to any one of claims 1 to 12, further comprising cooling the Bayer liquor to promote the precipitation.

14. The process according to any one of claims 1 to 13, wherein the Bayer liquor further comprises dissolved carbonate ions, and wherein the precipitation selectively removes the sulphate ions relative to the carbonate ions.

15. The process according to claim 14, wherein the Bayer liquor has a C/S of above about 0.85.

16. The process according to claim 14 or claim 15, wherein the precipitate is substantially free of the carbonate ions.

17. The process according to any one of claims 1 to 16, wherein providing the fluoride ions in the Bayer liquor at the concentration sufficient to induce precipitation

2018271243 26 Nov 2018 comprises adding an extraneous source of fluoride ions to a process stream in the Bayer refinery.

18. The process according to any one of claims 1 to 17, wherein providing the fluoride ions at the concentration sufficient to induce precipitation comprises minimising or controlling an amount of fluoride ions mineralised in tricalcium aluminate in the Bayer refinery.

19. The process according to claim 18, wherein the amount of fluoride ions mineralised in tricalcium aluminate is controlled to maintain a fluoride to sulphate molar ratio or a fluoride ion concentration below a predetermined maximum value in a process stream in the Bayer refinery.

20. The process according to claim 18 or claim 19, wherein minimising or controlling the amount of fluoride ions mineralised in tricalcium aluminate comprises minimising or controlling a quantity of tricalcium aluminate produced in the Bayer refinery.

21. The process according to any one of claims 18 to 20, wherein minimising or controlling the amount of fluoride ions mineralised in tricalcium aluminate comprises adding a tricalcium aluminate inhibitor to at least one process stream in the Bayer refinery, preferably to a dilute Bayer liquor process stream circulating in a causticiser side circuit of the Bayer refinery.

22. The process according to claim 21, wherein the tricalcium aluminate inhibitor is an anionic organic surfactant, and is preferably selected from the group consisting of an anionic homopolymer, an anionic copolymer, a polyacrylic acid, a polymer bearing hydroxamate functional groups, a hydroxamic acid, a humic acid, a tannic acid, a lignosulphonate, a fatty acid, a sulphonated carboxylic acid, a carboxylic acid, and a polyhydroxy carboxylic acid.

23. The process according to any one of claims 18 to 22, wherein minimising or controlling the amount of fluoride ions mineralised in tricalcium aluminate comprises refraining from adding, or controlling the addition of, a calcium-based

2018271243 26 Nov 2018 reagent to a concentrated Bayer liquor process stream of the primary Bayer refinery circuit.

24. The process according to any one of claims 1 to 23, wherein separating the precipitate from the sulphate-depleted Bayer liquor comprises filtering or centrifuging the precipitate.

25. The process according to claim 24, wherein separating the precipitate from the sulphate-depleted Bayer liquor further comprises thickening the precipitate prior to the filtering or the centrifuging.

26. The process according to any one of claims 1 to 25, further comprising disposing the precipitate together with red mud residue from the Bayer refinery.