AU2012203292C8

AU2012203292C8 - High temperature processs for causticisation of a bayer liquor

Info

Publication number: AU2012203292C8
Application number: AU2012203292A
Authority: AU
Inventors: Steven Philip Rosenberg; Daniel Mark Roworth; Darrel James Wilson
Original assignee: South32 Aluminium Worsley Pty Ltd
Current assignee: South32 Aluminium Worsley Pty Ltd
Priority date: 2012-03-07
Filing date: 2012-03-07
Publication date: 2013-12-19
Anticipated expiration: 2032-03-07
Also published as: AU2013201517A1; AU2012203292C1; AU2015215920A1; AU2013201517B2; AU2012203292B1; AU2018202054A1

Abstract

- 43 A high temperature causticisation process for the causticisation of a Bayer process liquor is disclosed. The process comprises the steps of: a) adding a quantity of a causticising agent 5 to the Bayer process liquor in a reactor vessel operating at a target reaction pressure for a given target reaction temperature, b) allowing the causticising agent to react with the Bayer process liquor for a sufficient residence time to produce a reaction slurry comprising a quantity of reaction solids in a causticised Bayer liquor; and, c) subjecting the reaction slurry to solid/liquid separation at a target separation temperature to produce a separated 10 stream of reaction solids and a product stream of clarified cauticised Bayer liquor. The process is characterised in that the target reaction temperature is not less than 11 5'C and the target separation temperature is not less than 11 5'C. Figure 1.

Description

rk 1/AUZUI/UUU43 HIGH TEMPERATURE PROCESSS FOR CAUSTICISATION OF A BAYER LIQUOR FIELD OF THE INVENTION 5 The present invention relates to a process and system for the causticisation of a Bayer liquor. BACKGROUND TO THE INVENTION In the Bayer process for alumina production, a concentrated sodium aluminate solution is produced by grinding and digesting bauxite in a caustic solution, usually under conditions of 10 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. 15 During digestion, some of the caustic is consumed in undesirable reactions with impurities within the bauxite, reducing the liquor's productivity. One of the most significant of these reactions results in the formation of sodium carbonate, arising from the dissolution of inorganic carbonates within the mineral phases present, or from the thermal and oxidative degradation reactions of organic compounds. Unless controlled, with each cycle of the 20 liquor through the process the sodium carbonate concentration would continue to rise, with a corresponding reduction in the liquor's ability to digest gibbsite or boehmite from the bauxite. Carbonation also occurs due to atmospheric CO 2 reacting with dilute liquor in the bauxite residue disposal area that is returned to the alumina refinery circuit. 25 The most common technique for controlling the sodium carbonate concentration in Bayer process liquors is to causticise using either quicklime or slaked lime. This process can be carried out either within the digestion circuit itself (by introducing lime with the bauxite in a process referred to in the art as 'inside causticisation'), or more commonly, as a side-stream process in a process referred to in the art as 'outside causticisation'. The addition of lime 30 directly with bauxite is not common except where lime is required to control other impurities (such as titanium or phosphorus), because the very concentrated liquors contribute to poor efficiency. Unless the temperature is very high during inside causticisation, most of the lime rk 1/AUZUI/UUU43 -2 undergoes side-reactions with the aluminate in solution to yield calcium aluminate species, particularly tricalcium aluminate (TCA, often also referred to as C3A in the cement industry). In the more prevalent side-stream or 'outside causticisation', a dilute liquor stream (usually taken from one of the mud washing stages) is reacted with a slaked lime slurry, 5 generally at a temperature that is close to but below the atmospheric boiling point of the combined liquor. Alternatively, the lime slurry is sometimes added directly to the mud washer. The amount of sodium carbonate converted and the efficiency of lime utilisation are dependent upon many variables, but in most refineries, the lime efficiency is in the vicinity of 50 to 70%. 10 The causticisation reaction of pure mixed solutions of sodium carbonate and sodium hydroxide with slaked lime is quite simple. The final concentration of hydroxide and carbonate ions is a function of the activities of the various ionic species present, in equilibrium with the solid phases calcium hydroxide and calcium carbonate. The reaction 15 can be described by the following equation: Ca(OH), + NaCO 3 <- CaCO 3 + 2NaOH ...(1) When the reaction of equation (1) occurs in a Bayer process liquor, it is advantageous if 20 the sodium carbonate that is present as an impurity in the Bayer process liquor reacts with calcium hydroxide to form calcium carbonate (usually in the form of calcite). The Bayer process liquor is said to be 'causticised' because reaction (1) above results in the generation of sodium hydroxide (also known in the art as 'caustic'). The actual reaction that takes place when lime is added to a Bayer process liquor is complicated by competing side 25 reactions. More specifically, co-owned International Patent Publication Number W0018684 (PCT/AU1999/00757) by Rosenberg disclosed for the first time that when calcium hydroxide (Ca(OH) 2 ) is added to a Bayer process liquor, it reacts to form an unstable intermediate compound known as 'hydrocalumite' (nominally of the formula [Ca 2 Al(OH) 6 ]2.%CO 3 .OH.5/ 2

H

2 0 ) before forming either (i) calcium carbonate (CaCO 3 ) 30 or (ii) tricalcium aluminate ('TCA'). Of these two available reaction pathways, the formation of TCA is undesirable because it consumes alumina and lime without removing carbonate.

rk 1/AUZUI/UUU43 -3 In the alumina industry it is common to refer to a Bayer liquor's carbonate impurity level in terms of the caustic to soda ratio, or 'C/S'. Typically, the C/S ratio of the concentrated liquor stream in many alumina refineries is in the range 0.8 to 0.85. C/S ratios higher than this are difficult to achieve, because causticisation processes in current use are incapable of fully 5 removing all of the sodium carbonate in the liquor streams fed to them in an economic way. For example, liquor with an S concentration of 135 g/L will typically only causticise to a C/S ratio of about 0.890. This limitation arises because the traditional implementation of the causticisation reaction with slaked lime is controlled by a number of complex equilibria, including a competing reaction involving the aluminate ion in which TCA is formed. 10 Australian patent application number 2004224944 (Roach et al) discloses a process for the causticisation of alkaline solutions at 140'C using slaked lime. The process results in a rapid rise in the liquor TC/TA ratio following which the TC/TA ratio decreases with increasing residence time. The TC/TA ratio referred to by Roach is equivalent to the C/S 15 ratio terminology used in this patent specification. The rate of decrease in C/S ratio from the point of maximum gain increases with increasing lime charge and increasing temperature. The causticised Bayer process solution is cooled before being directed to a dedicated solid separation stage to remove lime residue solids and returned to the Bayer circuit. However, while Roach et al advocate a residence time of less than 15 minutes at 20 140'C, Roach et al provides no information as to how to avoid the degradation in the C/S ratio during the cooling process prior to separation. It is an object of the present invention to at least partially overcome the abovementioned problems associated with the prior art, or provide an alternative thereto. 25 SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided a high temperature causticisation process for the causticisation of a Bayer process liquor, the process comprising the steps of: 30 a) adding a quantity of a causticising agent to the Bayer process liquor in a reactor vessel operating at a target reaction pressure for a given target reaction temperature; b) allowing the causticising agent to react with the Bayer process liquor for a rk 1/AUZUI/UUU43 -4 sufficient residence time to produce a reaction slurry comprising a quantity of reaction solids in a causticised Bayer liquor; and, c) subjecting the reaction slurry to solid/liquid separation at a target separation temperature to produce a separated stream of reaction solids and a product stream of 5 clarified cauticised Bayer liquor; the process characterised in that the target reaction temperature is not less than 11 5'C and the target separation temperature is not less than 11 5'C. In one form, said target separation temperature is at or above the target reaction 10 temperature. In one form, said target separation temperature is not more than 5'C or not less than 10 C or not less than 15'C or not less than 20'C below the target reaction temperature. In one form, the target reaction temperature is between 115'C and 300'C or between 115'C and 180'C or between 120'C and 155'C. In one form, the sufficient residence time of step b) is less than 1 minute, less than 3 minutes, less than 5 minutes, 15 less than 10 minutes, less than 15 minutes or less than 20 minutes. In one form, step c) is conducted in a solid/liquid separator and the process further comprises retaining the reaction slurry in the solid/liquid separator for a period of time not exceeding one hour, not exceeding 45 minutes, or not exceeding 30 minutes. In one form, the solid/liquid separator is a pressure decanter, a pressure filter, a cyclone, or a centrifuge. 20 In one form, the ratio of total caustic concentration to total alkali concentration of the Bayer process liquor, when expressed in grams of sodium carbonate per litre of solution, exceeds 0.9. exceeds 0.92, exceeds 0.94 or exceeds 0.95 after the addition of the quantity of causticising agent. In one form, the causticising agent is one or both of lime or 25 hydrocalumite. In one form, the causticising agent is slaked lime. The slaked lime may be formed by adding quicklime to water or by adding quicklime to a Bayer process liquor. In one form, the Bayer process liquor has an initial total alkali concentration of between 40 and 250 grams or between 80 and 160 grams per litre or between 130 and 170 grams per litre expressed as grams of sodium carbonate per litre of solution. In one form, the Bayer 30 process liquor has an A/C ratio of between 0.2 and 0.95, or between 0.3 and 0.8, or preferably greater than 0.55.

rk 1/AUZUI/UUU43 -5 In one form, the Bayer process liquor is preheated prior to step a). In one form, the Bayer process liquor is preheated to the target reaction temperature in a heating circuit, the heating circuit comprising one or more heating stages. The heating stage may comprise direct or indirect steam injection. In one form, the clarified causticised liquor is cooled 5 after step c) by countercurrent heat exchange with the Bayer process liquor. In one form, the Bayer process liquor is a spent Bayer liquor or a dilute Bayer liquor. The Bayer process liquor may be a washer overflow liquor. In one form, step a) is performed in the presence of a TCA inhibitor. In one form, the 10 TCA inhibitor is an anionic organic surfactant. In one form, the anionic organic surfactant is selected from the group comprising: an anionic homopolymers, 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. 15 According to a second aspect of the present invention there is provided a high temperature causticisation process for the causticisation of a Bayer process liquor, the process comprising the steps of: a) adding a quantity of a causticising agent to the Bayer process liquor in a reactor 20 vessel operating at a target reaction pressure for a given target reaction temperature; b) allowing the causticising agent to react with the Bayer process liquor for a sufficient residence time to produce a reaction slurry comprising a quantity of reaction solids in a causticised Bayer liquor; and, c) subjecting the reaction slurry to solid/liquid separation at a target separation 25 temperature to produce a separated stream of reaction solids and a product stream of clarified cauticised Bayer liquor; the process characterised in that the target separation temperature is at or above the target reaction temperature and the target reaction temperature is greater than the atmospheric boiling point of the Bayer process liquor. 30 In one form, the target reaction temperature is between 115'C and 300'C or between 115'C and 180'C or between 120'C and 155'C.

rk 1/AUZUI/UUU43 -6 In one form, the sufficient residence time of step b) is less than 1 minute, less than 3 minutes, less than 5 minutes, less than 10 minutes, less than 15 minutes or less than 20 minutes. In one form, step c) is conducted in a solid/liquid separator and the process further comprises retaining the reaction slurry in the solid/liquid separator for a period of 5 time not exceeding one hour. not exceeding 45 minutes, or not exceeding 30 minutes. In one form, the solid/liquid separator is a pressure decanter, a pressure filter, a cyclone, or a centrifuge. In one form, the ratio of total caustic concentration to total alkali concentration of the 10 Bayer process liquor, when expressed in grams of sodium carbonate per litre of solution, exceeds 0.9, exceeds 0.92, exceeds 0.94 or exceeds 0.95 after the addition of the quantity of causticising agent. In one form, the causticising agent is one or both of lime or hydrocalumite. In one form, the causticising agent is slaked lime. The slaked lime may be formed by adding quicklime to water or by adding quicklime to a Bayer process liquor. In 15 one form, the Bayer process liquor has an initial total alkali concentration of between 40 and 250 grams or between 80 and 160 grams per litre or between 130 and 170 grams per litre expressed as grams of sodium carbonate per litre of solution. In one form, the Bayer process liquor has an A/C ratio of between 0.2 and 0.95, or between 0.3 and 0.8, or preferably greater than 0.55. 20 In one form, the Bayer process liquor is preheated prior to step a). In one form, the Bayer process liquor is preheated to the target reaction temperature in a heating circuit, the heating circuit comprising one or more heating stages. The heating stage may comprise direct or indirect steam injection. In one form, the clarified causticised liquor is cooled 25 after step c) by countercurrent heat exchange with the Bayer process liquor. In one form, the Bayer process liquor is a spent Bayer liquor or a dilute Bayer liquor. The Bayer process liquor may be a washer overflow liquor. In one form, step a) is performed in the presence of a TCA inhibitor. In one form, the 30 TCA inhibitor is an anionic organic surfactant. In one form, the anionic organic surfactant is selected from the group comprising: an anionic homopolymers, an anionic copolymer, a polyacrylic acid, a polymer bearing hydroxamate functional groups, a hydroxamic acid, a -7 humic acid, a tannic acid, a lignosulphonate, a fatty acid, a sulphonated carboxylic acid, a carboxylic acid, and a polyhydroxy carboxylic acid. 5 10 THIS SECTION IS INTENTIONALLY BLANK -8 According to a third aspect of the present invention there is provided a causticisation process substantially as herein described with reference to and as illustrated in the accompanying figures. 5 10 THIS SECTION IS INTENTIONALLY BLANK 15 rk 1/AUZUI/UUU43 -9 BRIEF DESCRIPTION OF THE DRAWINGS In order to facilitate a more detailed understanding of the nature of the invention several embodiments of the improved causticisation process and system will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: 5 Figure 1 is a conceptual flow diagram of a first embodiment of the present invention showing a basic implementation; Figure 2 is a schematic illustration of one embodiment of a reactor vessel for use with 10 the process of the present invention Figure 3 is a schematic illustrate of one embodiment of a solid/liquid separator for use with the process of the present invention; 15 Figure 4 is a conceptual flow diagram a second embodiment of the present invention including a countercurrent heating/cooling circuit; Figure 5 is a conceptual flow diagram a third embodiment of the present invention wherein the causticising agent is a hydrocalumite slurry; and, 20 Figure 6 is a conceptual flow diagram a third embodiment of the present invention wherein the causticising agent is a mixture of a re-slurried hydrocalumite slurry and a stream of supplement lime. 25 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Throughout this specification various terms commonly used in the alumina industry are used. In the interests of clarity, such terms are now defined. The term 'Bayer process liquor' refers to a sodium caustic aluminate liquor generated by 30 digesting (dissolving) bauxite in a caustic soda solution at elevated 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.

rk 1/AUZUI/UUU43 -10 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/L of alumina (A1 2 0 3 ). 5 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/L concentration of sodium carbonate. The term 'A/C' is thus the ratio of alumina concentration to caustic concentration. 10 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/L concentration of sodium carbonate. Thus, S-C (soda concentration minus caustic concentration) gives the actual concentration of sodium carbonate (Na 2

CO

3 ) in the 15 liquor, in g/L. A Bayer process 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. 20 The term 'spent Bayer process liquor' refers to a liquor stream after the gibbsite precipitation stage and prior to digestion. A spent liquor typically has a low A/C ratio. The term 'dilute Bayer process liquor' refers to a liquor stream with a low S, typically less than 220g/L. 25 The term 'TCA' is used to refer to tricalcium aluminate having the chemical formula of Ca 3 [Al(OH) 6

]

2 which is also commonly written using the formula 3CaO.Al 2 0 3 .6H 2 0 (TCA6) or C3AH6 in cement industry notation. 30 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 (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 rk 1/AUZUI/UUU43 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. 5 '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. 10 The term 'hydrocalumite' is used throughout this specification to refer to aluminium-based layered double hydroxide of the form [Ca 2 Al(OH) 6

]

2

-X

2 -nH 2 0, where 'X' represents a charge-balancing anion or anions. By way of example, when X is carbonate, hydrocalumite may have the formula of [CaAl(OH) 6

]

2 -CO.Q or Ca 2 Al(OH) 6

]

2

./

2

CO

3 .OH.5/ 2

H

2 0 depending on a number of factors which govern the preferential intercalation of species. 15 The interlayer regions are filled with charge balancing ions and water molecules. The term 'Lime efficiency' is defined as the percentage of available lime that is converted to calcium carbonate during causticisation. A number of processes were used to calculate the lime efficiency, including Total Inorganic Carbon (TIC) analysis, x-ray fluorescence (XRF) 20 analysis, Thermo Gravimetric Analysis (TGA) analysis, liquor or mass balance. Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer 25 or group of integers. A first embodiment of the process of the present invention will now be described with reference to the process flowchart illustrated in FIG. I in which the process is generally designated by the reference numeral (10). A quantity of a causticising agent (12) and a 30 preheated Bayer process liquor (14) are mixed in a reactor vessel (16) operating at a target reaction pressure for a given target reaction temperature. The causticising agent is allowed to react with the Bayer liquor for a sufficient residence time to produce a reaction slurry rk 1/AUZUI/UUU43 - 12 comprising a quantity of reaction solids in a causticised Bayer liquor. The reaction slurry to subjected to solid/liquid separation to produce a separated stream of reaction solids and a product stream of clarified cauticised Bayer liquor. With reference to Figure 1, a solid/liquid separator (20) is arranged to receive the reaction slurry (18) and produce an 5 overflow stream of clarified cauticised Bayer liquor (22) and an underflow stream of thickened reaction solids (24). The reaction solids present in the reaction slurry removed from the reactor vessel will include calcium carbonate, hydrocalumite and TCA in varying proportions depending on the operating conditions of the reactor vessel. 10 The process of the present invention is characterised in that best results are achieved when both the target separation temperature and the target reaction temperature are not less than 11 5'C. The target separation temperature may be at or above the target reaction temperature. Alternatively, the target separation temperature is less than the target separation temperature may be lower than that target reaction temperature provided that 15 the target separation temperature is not less than 115'C. Alternatively, the target separation temperature is less than the target separation temperature provided that the temperature differential between the target separator temperature and the target reaction temperature is not greater than not greater than 5'C, not greater than I 0 0 C, or not greater than 20'C. 20 Because the lower limit set for the target reaction temperature is greater than the atmospheric boiling point of the Bayer process liquor (typically 105 to 108'C), the target reaction pressure is greater than atmospheric pressure. The reactor vessel (16) is a pressure vessel that is capable of operating at the target reaction temperature. The target 25 reaction temperature may be in the range of 115'C and 300'C, more preferably in the range of 115'C and 170'C or more preferably in the range of 120'C and 155'C. Using the process and system of the present invention, the reaction products are removed from the reaction slurry under pressure and temperature conditions whereby flashing of the reaction slurry is avoided. Flashing would result in a sharp drop in temperature and an increase in 30 S which encourages the undesirable formation of TCA. Solid/liquid separation may be conducted using a pressurised settler, a pressurized decanter, a pressurised thickener, a filter or any other suitable solid/liquid separator that is capable of operating under pressure rk 1/AUZUI/UUU43 - 13 at an elevated temperature. In this way, the reaction slurry is maintained at or above the target reaction temperature until after separation. As set out above in the 'Background to the Invention', co-owned International Patent 5 Publication Number WOOO18684 (PCT/AU1999/00757) by Rosenberg described the two competing reaction pathways that may be followed when calcium hydroxide (Ca(OH) 2 ) is added to a Bayer process liquor. The first reaction pathway, which is the desirable one, leads to the formation of calcium carbonate. The second reaction pathway, which is undesirable because it consumes alumina, is the formation of TCA. For both the first 10 reaction pathway and the second reaction pathway, the first compound that forms very rapidly is the unstable intermediate compound known as hydrocalumite'. Because there are two competing reactions occurring, there are also two competing pseudo-equilibria the first being the pseudo-equilibrium between calcite and hydrocalumite and the second being the pseudo- equilibrium between TCA and hydrocalumite. The present invention is 15 based in part, on the realization that, for a given 'S' concentration, the reaction pathway that favours the formation of calcite over TCA is favoured at higher target reaction temperatures, but, in order to capitalise on this, the reaction solids must be separated as quickly as possible from the reaction slurry without any significant drop in temperature to avoid reversion of calcite to hydrocalumite. Without wishing to be bound by theory, the 20 inventors have found that if the reaction slurry is allowed to cool whilst the reaction solids are still in contact with the causticised Bayer liquor, the calcite present in the reaction solids reverts back to unstable hydrocalumite. This unstable hydrocalumite may then follow the second reaction pathway that results in the formation of stable but undesirable TCA at lower temperatures. Using the process and system of the present invention, the 25 reaction products are separated out of the reaction slurry as quickly as possible and without any significant drop in temperature. The Bayer process liquor (14) that is fed to the reactor vessel (16) is preheated to a temperature that is at or above the target reaction temperature immediately prior to being 30 introduced into the reactor vessel (16). For best results, there is no mixing of the preheated Bayer process liquor (14) and the causticising agent (12) outside of the reactor vessel (16). The causticising agent may be heated in an analogous manner prior to being rk 1/AUZUI/UUU3 - 14 charged into the reactor vessel. The ratio of the Bayer process liquor to the quantity of the causticising agent added will depend on the concentration of carbonate present in the Bayer process liquor. By way of example, the ratio may be greater than or equal to 10:1, 15:1 or 20:1, with higher ratios required for causticising dilute Bayer process liquors and 5 lower ratios required if the Bayer process liquor has a high carbonate concentration. Thus, preheating of the causticising agent is less critical than preheating of the Bayer process liquor and can be compensated for by heating the Bayer process liquor to a temperature above the target reaction temperature so that the target reaction temperature is reached after mixing of the causticising agent with the Bayer process liquor. 10 The Bayer process liquor will have an 'S' concentration of between 40 and 250 g/L (preferably between 130 and 170 g/L or between 80 to 160g/L), and an A/C ratio of between 0.2 and 0.95, between 0.3 and 0.8, and preferably greater than 0.55. One example of a suitable causticising agent is lime whilst one example of a suitable Bayer process 15 liquor is a dilute Bayer liquor such as a first washer overflow or a second washer overflow from a Bayer process circuit. The lime may be quick lime, hydrated lime or, for best results, slaked lime. When quicklime is used as the causticising agent, an exothermic slaking reaction will occur very rapidly when the quicklime comes into contact with the water present in the Bayer process liquor, resulting in the formation of calcium hydroxide. 20 The present invention is further based in part on the realisation that, for a given 'S' concentration, increasing the target reaction temperature increases the equilibrium C/S ratio that can be achieved. By way of example, for an S of 150g/L, the equilibrium C/S ratio when operating at a temperature of 140'C, is ~0.94 whereas at 103'C, the equilibrium 25 C/S ratio is only 0.90. The further advantage of operating at higher reaction temperatures is the residence time to achieve this higher equilibrium C/S ratio is lower. Using the process and system of the present invention, the formation of TCA is minimised to achieve high causticisation efficiency. The side reaction to TCA may be further suppressed using an additive as discussed in greater detail below. 30 One embodiment of a suitable reactor vessel (16) for high temperature causticisation is illustrated schematically in Figure 2 in the form of a vertical pressure vessel. A pipe rk 1/AUZUI/UUU43 - 15 reactor may be used as an alternative. The reactor vessel may be one of a plurality of reactor vessel arranged in parallel. With reference to Figure 2, the reactor vessel (16) stands generally upright and includes an elongated cylindrical pressure vessel wall (36) of sufficient strength and thickness to withstand the pressures, temperatures and 5 corrosiveness of the reaction slurry. The reactor vessel is provided with a dish-shaped closed upper end (38) and a gas inlet (42) for introducing a gas into the closed upper end (38) of the reactor vessel (16). The closed upper end of the reactor vessel is further provided with an internal distributor (44), in the form of an internal distributor plate or an internal distributor cone, to promote rapid mixing of the causticising agent and the Bayer 10 process liquor. In the embodiment illustrated in Figure 2, the quantity of causticising agent (12) is introduced to the reactor vessel (16) via a causticising agent inlet (48) while the preheated Bayer process liquor (14) is introduced to the reactor vessel via a separate Bayer process 15 liquor inlet (46). Using the process and system of the present invention, the causticising agent and the Bayer process liquor are not allowed to mix with each other until after they have been both been introduced into the reactor vessel. When a quantity of lime is added to a preheated Bayer process liquor under the operating conditions of the reactor vessel, the formation of hydrocalumite occurs so rapidly at the target reaction pressure and target 20 reaction temperature that reaction solids can deposit like scale at the site where mixing first occurs. In the embodiment illustrated in Figure 2, the reactor vessel has been specifically designed to overcome this scaling problem. In this embodiment, the causticising agent inlet (48) and the Bayer process liquor inlet (46) are provided at the closed upper end (38) of the reactor vessel (16). The causticising agent inlet (48) has a 25 first end (50) terminating outside of the reactor vessel (16) and a second end (52) terminating inside of the reactor vessel (16) at a height that is vertically offset at a preset distance from the maximum height of the distributor (44). The Bayer process liquor inlet (46) has a first end (56) terminating outside of the reactor vessel and a second end (58) terminating inside of the reactor vessel. The second end (52) of the causticising agent inlet 30 (48) is coaxially aligned with the second end (58) of the Bayer process liquor inlet (46) and arranged such that the second end (58) of the Bayer process liquor inlet (46) terminates at a height within the reactor vessel that is above the height of the second end rk 1/AUZUI/UUU43 -16 (52) of the causticising agent inlet (48). In this way, the preheated Bayer process liquor discharged from the second end of the Bayer process liquor inlet forms a curtain around the quantity of causticising agent being charged into the reactor vessel via the second end of the causticising agent inlet. This arrangement is used to minimize scaling which can 5 otherwise occur when the causticising agent first comes into contact with the Bayer process liquor. In use, a stream of non-reactive gas (23) is fed through the gas inlet (42) into the closed upper end (38) of the reactor vessel (16) to create a gas cap (60) above the reaction slurry 10 bed level (62). The flow rate of the non-reactive gas (23) introduced into the closed upper end (38) of the reactor vessel (16) is controlled using a first control valve (68). The second end (52) of the causticising agent inlet (50) and the second end (58) of the Bayer process liquor inlet (48) both terminate within the gas cap (60) to minimize scaling of the causticising agent inlet (48) and the Bayer process liquor inlet (46). The gas cap (60) is 15 used to avoid direct contact between the causticising agent and the preheated Bayer process liquor within their respective inlets. Any suitable non-reactive gas, such as nitrogen or air, may be used to provide the gas cap in the reactor vessel. Suitable non reactive gases include air or nitrogen. The gas cap is maintained by increasing or decreasing the pressure of the non-reactive gas using the first control valve (82) to ensure 20 that the reaction slurry bed level (62) does not rise above the maximum height of the internal distributor (44) which would result in poor mixing of the causticising agent and alkaline liquor. Whilst the embodiment illustrated in Figure 2 shows one particular arrangement that 25 reduces scaling, other arrangements that encourage mixing of the causticising agent and preheated Bayer process liquor within the gas cap of the reactor vessel would be suitable, provided that the second end of the causticising agent inlet is never immersed in preheated Bayer process liquor to ensure that no mixing of the causticising agent and the preheated Bayer process liquor can occur within the second end of the causticising agent inlet. 30 The reactor vessel (16) is further provided with a closed lower end (64) for accumulation of the reaction slurry. The lower end may be flat, hemispherical or conical, although rk 1/AUZUI/UUU43 -17 hemispherical is preferred as being the most efficient design in terms of strength versus material thickness for a pressure vessel. The lower end (64) includes a reaction slurry outlet (66) for removing the stream of reaction slurry (18) from the reactor vessel (16). A pipeline (70) leads from the reaction slurry outlet (66) of the reactor vessel (16) to the 5 reaction slurry inlet (72) of the downstream solid/liquid separator (20). The flow rate of the reaction slurry out of the reactor vessel may be controlled using the first control valve (68) which controls the flow of non-reactive gas (23) into the closed upper end (38) of the reactor vessel (16) to increase or decrease the pressure in the reactor vessel (16). Alternatively, a variable speed takeoff pump (not shown) may be used instead of or in 10 combination with a control valve (not shown) positioned in the pipeline (70) in the event that there is a pressure drop between the reactor vessel (16) and the solid/liquid separator (20). By way of example, when the causticising agent is slaked lime, the Bayer process liquor is 15 a washer overflow liquor, and the target reaction temperature is 140'C, the first control valve (68) may be regulated to ensure that residence time of the reaction slurry in the reactor vessel (16) is between 30 seconds and 5 minutes, preferably 2 to 4 minutes. If the causticising agent is hydrocalumite and the Bayer process liquor is a dilute Bayer process liquor, a longer residence time of at least 10 minutes and preferably 15 minutes in the 20 reactor vessel should be used for best results. When the causticising agent is hydrocalumite, the hydrocalumite can be added alone or mixed with a quantity of lime. Longer residence times in the reactor vessel are required at lower target reaction temperatures. Advantageously, using the process and system of the present invention, shorter residence times are used for higher target reaction temperatures so as to minimise 25 TCA formation and decrease capital costs. Using the process of the present invention, the pressurized reaction slurry from the reactor vessel (16) is directed via the pipeline (70) to the downstream solid/liquid separator (20) with no significant reduction in temperature. For best results, the target separation 30 temperature is at or above the target reaction temperature. A pressure drop between the reactor vessel (16) and the solid/liquid separator (20) may be tolerated provided that the pressurized slurry does not flash, as this would result in an unacceptably high drop in rk 1/AUZUI/UUU43 - 18 temperature. The use of non-reactive gas to form the gas cap in the closed end of the reactor vessel is used to control this pressure drop. The solid/liquid separator (20) may be any device that is capable of separating the reaction solids from the pressurized reaction slurry at the target separation temperature and target separation pressure to produce the 5 stream of clarified causticised liquor (22). By way of example, the solid/liquid separator may be a pressurized filter, a pressurized settler, a pressurized thickener, a centrifuge or a pressure decanter. The separator may be one of a plurality of separators. One embodiment of a suitable solid/liquid separator (20) in the form of a pressure decanter 10 is illustrated schematically in Figure 3. In this embodiment, the separator (20) stands generally upright and includes an elongated cylindrical pressure vessel wall (73) of sufficient strength and thickness to withstand the pressures, temperatures and corrosiveness of the reaction slurry. The separator is provided with a dish-shaped closed upper end (74) and a vent (75) which can be opened if required to permit release of gases 15 that may accumulate in the vessel during operation. An overflow outlet (77) is provided at the upper end (74) of the separator to facilitate removal of the overflow stream of clarified causticised liquor (22). The separator is further provided with a bottom section (76) for accumulation and subsequent removal of the underflow stream of thickened reaction solids (24). The bottom section (76) of the separator (20) is provided with an underflow outlet 20 (80) through which the underflow stream of thickened reaction solids (24) is removed from the separator (20). The bottom section may be either hemispherical or conical, although conical is preferred to encourage discharge of thickened reaction solids from the separator. Preferably, the bottom section (76) has inclined sides (78) as shown in FIG. 1, and these inclined sides should make an angle (labeled as 'a' in Figure 3) between 30 and 25 60 degrees from the horizontal. An angle of about 45 degrees from the horizontal is preferred, because such a conical shape is easiest to fabricate and poses an acceptable height penalty, while providing for a good flow of solids out of the vessel. A portion of the underflow stream of thickened reaction solids (24) may be recirculated via 30 a recirculation port (82) arranged in the bottom section (76) of the separator (20) if desired to assist in controlling the viscosity of the bed of thickened solids (92). A second control valve (84) is provided in a pipeline (86) leading away from the underflow outlet (80) to rk 1/AUZUI/UUU43 -19 regulate the flow rate of the underflow stream of thickened reaction solids (24) without loss of pressure in the separator (20). A variable speed takeoff pump (not shown) may be used instead of or in combination with the second control valve (84). By way of example, when the causticising agent is lime and the Bayer process liquor is a spent or dilute Bayer 5 process liquor, the bed of thickened reaction solids (92) is maintained to ensure a residence time of less than one hour in the separator (20). When the Bayer process liquor is a spent or dilute Bayer process liquor and the causticising agent is hydrocalumite (either on its own or in combination with lime), longer residence times of the reaction slurry in the separator may promote the undesirable chemical reactions that result in the formation of 10 TCA. However longer residence times allow for the buildup of a larger bed of thickened solids (92), resulting in an increase in an increase in the thickness of the underflow stream of thickened solids (24) and improved recovery of the overflow stream of clarified causticised liquor (22) from the overflow outlet (77). 15 The separator (20) is provided with a slurry inlet (72) through which pressurized solid liquid slurry from the reactor vessel (16) is fed into a feedwell (90) located within the top section (74) of the separator (20). The feedwell is used to reduce the velocity and kinetic energy of the pressurized reaction slurry such before it impinges on the bed (92) of thickened reaction solids already accumulated in the separator (20) in use. The outlet end 20 (94) of the feedwell (90) is positioned at a predetermined distance above the slurry bed level (96). To aid in the settling of the reaction solids and encourage solid/liquid separation, a flocculant is added to the pressurized reaction slurry via one or more flocculant injector(s) 25 (88). The flocculant is injected into the pipeline (70) upstream of or adjacent to the slurry inlet (72) at a point of maximum turbulence. Alternatively or additionally, flocculant may be added via a sparge line (98) directly into the feedwell (90). The flocculant may be diluted using a dilute Bayer process liquor such as process water, prior to addition. The flocculant dose is determined as a function of the mass flow rate of solids present in the 30 reaction slurry that enters the separator (which in turn is a function of the flow rate of the Bayer process liquor (14) and the quantity of causticising agent (12) into the reactor vessel (16) and the clarity of the overflow stream of clarified causticised liquor (22). Suitable rk 1/AUZUI/UUU43 -20 flocculants include but are not limited to an anionic sodium polyacrylate or similar flocculant, such as ALCAR 600 available from Allied Colloids, Limited, diluted to a concentration of less than about 1.0% by weight. 5 The bottom section (76) of the separator (20) is further provided with a stirrer (100) extending from a drive shaft (102). The stirrer (100) is provided with one or more arms (104) correspondingly positioned to follow the internal profile of the bottom section (76) to leave as little unswept area as possible. In use, the pressurized reaction slurry (18) from the reactor vessel (16) is directed via pipeline (70) into the slurry inlet (72) of the separator 10 (20) wherein it receives an injection of flocculant via the one or more flocculant injector(s) (88). The pressurized reaction slurry enters the feedwell (90) and flows downwardly towards the outlet end (94) of the feedwell (90). The reaction solids form 'flocs' which deposit themselves at the top of the bed of thickened solids (92). Over time, the causticised liquor rises to the top of the separator (20) where it is removed via the 15 overflow outlet (77) as the overflow stream of clarified causticised liquor (22). The stirrer (100) is used to eliminate dead spots in the bed (92) which could otherwise lead to the deposition of scale. The stirrer (100) encourages the flow of thickened reaction solids to the underflow outlet (80), and provides some assistance in further thickening the reaction slurry to increase the rate of separation. 20 In one embodiment of the present invention, the target reaction pressure may be set higher than the target separation pressure to avoid the need to pump the reaction slurry from the reactor vessel (16) to the separator (20). By way of example, when the target reactor pressure is around 540kPag (as measured at the upper end of the reactor vessel), the target 25 separation pressure may be around 51 OkPag (as measured at the upper end of the separator). The pressure within the separator is maintained at the target separation pressure by controlling the rate of removal of the overflow stream of clarified causticised liquor or the rate of removal of the underflow stream of thickened reaction solids or both, from the separator. The target separator pressure and temperature are kept high enough at 30 all times during operation to prevent flashing of the causticised liquor or depressurization of the reaction slurry.

rk 1/AUZUI/UUU43 -21 The primary variables in the process of the present invention are the feed rate of the Bayer process liquor to the reactor vessel, the rate of addition of the causticising agent to the reactor vessel, the target reaction temperature, the target separation temperature, and the residence time in the reactor vessel. In the case when the causticising agent is lime, the 5 quantity of lime charged to the reactor vessel is the most important control parameter in the process. Excess lime charge to the reactor vessel may result in in TCA formation and a drop in the C/S and A/S of the stream of clarified causticised liquor produced by the separator. Insufficient lime charge may also result in a deficit in the C/S of clarified causticised liquor produced by the separator. Accurate measurement of the initial C/S and 10 S and flow rate of the preheated Bayer process liquor added to the reactor vessel is used to calculate the required lime charge rate required to achieve a target C/S to minimize the risk of formation of undesirable TCA. In any event, using the process and system of the present invention, the overall time span between the point at which the quantity of causticising agent (12) is first mixed with the Bayer process liquor (14) in the reactor 15 vessel (16) and the point at which the overflow stream of clarified causticised liquor (22) and the underflow stream of thickened reaction solids (24) is removed from the separator is greater than 15 minutes. A second embodiment of the present invention is now described with reference to the 20 schematic flowsheet of Figure 4 for which like reference numerals refer to like parts. In this embodiment, the reactor vessel (16) and separator (20) operate in the manner described above in relation to the first embodiment. Downstream of the separator (20), the underflow stream of thickened reaction solids (24) is cooled prior to being discharged to one or more atmospheric tank(s) (200) to avoid flashing. By way of example, cooling 25 of the underflow stream of thickened reaction solids may be achieved by mixing with a stream of spent Bayer liquor (202) such as a second washer overflow stream to avoid flashing upon discharge to atmospheric tanks. Cooling may equally be achieved using any other liquid such as water or process water. As an alternative, a depressurizing chamber (not shown) operating at a lower pressure than the separator may be used. Alternatively, if 30 direct mixing of the thickened reaction solids with another stream is not desired, the underflow stream of thickened reaction solids may be cooled using a suitable heat exchanger such as a shell and tube heat exchanger.

rk 1/AUZUI/UUU43 - 22 Upstream of the reactor vessel (16), a feed stream of Bayer process liquor (201) is heated to form the preheated Bayer process liquor (14) a heating circuit (202). The heating circuit (202) is made up of one or more heating stages (204). An example of a suitable heating stage is a plate heat exchanger, a shell and tube heat exchanger or direct steam injection. 5 Three heating stages (204) are shown in the heating circuit (202) of Figure 4. Downstream of the separator (20) the overflow stream of clarified causticised liquor (22) is cooled using a cooling circuit (206) comprising one or more cooling stages (208) to form a cooled stream of clarified causticised liquor (210). An example of a suitable cooling stage is a plate heat exchanger. The cooled clarified causticised liquor may be 10 directed to one or more product liquor tank(s) (212) for storage or the cooled clarified causticised liquor (210) may be returned directly to a Bayer process circuit. In the embodiment illustrated in Figure 4, the heating circuit (202) and the cooling circuit (206) are countercurrent for heat recover with three heating/cooling stages (204 and 208, 15 respectively). Using the countercurrent arrangement of Figure 4. at least some of the heat of the overflow stream of clarified causticised liquor (22) is used to preheat the feed stream of Bayer process liquor (201). In the first stage (214) of the heating circuit (202), the feed stream of Bayer process liquor 20 (201) is heated by exchanging heat with the overflow stream of clarified causticised liquor (22) to form the cooled clarified causticised liquor (210) and a first partially heated stream of Bayer process liquor (216). In the second stage (218) of the heating circuit (202), the first partially heated stream of Bayer process liquor (216) is further heated by exchanging heat with a warm condensate stream (220) discharged from the third stage (222) of the 25 heating circuit (202) to form a second partially heated stream of Bayer process liquor (224) and a cool condensate stream (226). In the third stage (222) of the heating circuit (202), the second partially heated stream of Bayer process liquor (224) is heated to the target reaction temperature by exchanging heat with a sufficient quantity of steam (228) to form the preheated Bayer process liquor (14) and the warm condensate stream (220). By way of 30 example, sufficient steam at a pressure of 1300kPag may be added to the shell side of a shell and tube heat exchanger to produce a preheated Bayer process liquor at a target reaction temperature of 140"C. The stream of cooled clarified causticised liquor (210) is rk 1/AUZUI/UUU43 - 23 discharged from the heating circuit (202) at a temperature below the atmospheric boiling point of the Bayer process liquor. Alternatively, the feed stream of Bayer process liquor (201) can bypass the first and second stages of the heating circuit and be heated to the target reaction temperature in the third stage of the heating circuit using steam alone. 5 However, this requires the use of an alternative cooling stream to decrease the temperature of the clarified causticised liquor to below the atmospheric boiling point. It is to be clearly understood that the heating circuit and cooling circuit may remain separate and independent of each other and any number of heating/cooling stages can be 10 used depending on such relevant factors as the level of heating/cooling to be achieved and the size and efficiency of heating/cooling apparatus used. It is to be further understood that the use of the heating circuit (202) and the cooling circuit (206) is entirely optional to the working of the present invention. The cooled clarified causticised liquor (210) may alternatively be flash cooled prior to its introduction back in a Bayer process circuit. 15 A third embodiment of the present is now described with reference to the schematic flowchart of Figure 5 for which like reference numerals refer to like parts. In this embodiment the causticising agent is a hydrocalumite slurry (300). One suitable way of producing a hydrocalumite slurry is by adding a quantity of lime (302) to a bypass stream of 20 Bayer process liquor (304) in a mixing tank (306). The bypass stream of Bayer process liquor is cooled to between 60 and 80'C upstream of the mixture tank (306) to ensure that the hydrocalumite present in the hydrocalumite slurry (300) is stable. The residence time in the mixing tank can range between 20 minutes and 2 hours with best results obtained with a residence time of around 60 minutes. The causticising reaction that occurs when the 25 hydrocalumite slurry (300) is mixed with the preheated Bayer process liquor (14) in the reactor vessel (16) to form calcite is endothermic. To counteract this, the Bayer process liquor (14) should be preheated to a temperature greater than the target reaction temperature prior to its introduction to the reactor vessel (16). The mixing tank (306) is provided with a low shear agitator (308) to minimize the unwanted side reaction that 30 produces TCA. Best results for this third embodiment of the present invention are achieved when a quantity of TCA inhibitor (described in greater detail below) is added to the mixing tank (306).

rk 1/AUZUI/UUU43 -24 The lime that is added to the mixing tank (306) is preferably slaked lime with an S concentration of 15-20gpL to achieve high conversion rates to hydrocalumite and high conversion of hydrocalumite to calcite in the reactor vessel. Vigorous mixing either by direct injection of lime into the liquor stream entering the tank or a short residence time 5 pre-mix tank (not shown) is required to ensure good conversion of lime to hydrocalumite. Conversion rates to hydrocalumite when the lime is slaked at ~10gpL are only 50-70%. This increases to 90% when the lime is slaked at an S concentration of 15-20gpL. There is some residual (un reacted) lime in the hydrocalumite slurry (300). This residual lime is available to react with calcium carbonate when the hydrocalumite slurry (300) is 10 subsequently added to the reactor vessel (16). Calcite and other impurities that may be present in the quicklime used to produce slaked lime may also be present may also be present. When the hydrocalumite slurry produced in the mixing tank is then used as the causticising agent (12) that is mixed with the preheated Bayer process liquor (14) in the reactor vessel (16), the residence time in the reactor vessel is in the range of 5 to 30 15 minutes, preferably around 10 to 15 minutes. A fourth embodiment of the present is now described with reference to the schematic flowchart of Figure 6 for which like reference numerals refer to like parts. In this embodiment the causticising agent is a mixture of a re-slurried hydrocalumite slurry (400) 20 and a supplemental lime slurry (402) to provide an increase in the C/S achieved. The re slurried hydrocalumite slurry (400) is generated by directing the hydrocalumite slurry (300) from the mixing tank (306) to a filter (308) to produce a stream of highly causticised liquor (310) and a hydrocalumite filter cake (312) which is retained by the filter (308). The hydrocalumite filter cake is mixed with a re-slurry liquor (314) in a slurry tank (316) 25 to produce the re-slurried hydrocalumite slurry (400). In one example of this embodiment, a portion of the cooled clarified causticised liquor (210) is used as the bypass stream of Bayer liquor (304) feed to the mixing tank (306). As the reaction that occurs to form hydrocalumite when slaked lime is mixed with a Bayer process liquor is exothermic, the bypass stream of Bayer process liquor may be sub-cooled using a cooling stage (316) to 30 ensure that the final mixture (including heat of reaction) is at the desired reaction temperature in the mixing tank (306).

rk 1/AUZUI/UUU43 - 25 A fifth embodiment of the present invention is now described in which a quantity of TCA inhibitor, such as sucrose or sodium gluconate, is added to the process at one or more TCA inhibitor dosing points (150) to suppress the kinetics of the TCA reaction. Dosing with the TCA inhibitor allows a higher C/S to be achieved with higher lime efficiency, higher 5 alumina concentration in the causticised liquor. The TCA inhibitor also provides greater process control. The TCA inhibitor stabilises the hydrocalumite as it forms, preventing the usual simultaneous side-reaction that leads to the formation of TCA. The TCA inhibitor may be added at any stream upstream of the reactor vessel or within the 10 reactor vessel itself for any of the previously described embodiments of the present invention and for any of the schematic flowcharts of Figures 1, 4, 5, or 6. By way of example, the TCA inhibitor can be added prior to, during or after preheating of the Bayer process liquor (14). The TCA inhibitor may be added with the quantity of causticising agent (12) being added to the reactor vessel (16) or dosed directly into the reactor vessel 15 (16) itself. It is also possible to dose the TCA inhibitor into other locations within a Bayer process circuit, provided that a significant proportion of the TCA inhibitor reports to the reactor vessel (16). Best results with minimum consumption of inhibitor are achieved when the TCA additive dosing point is the reactor vessel or added to the Bayer process liquor immediately upstream of the reactor vessel. 20 Suitable TCA inhibitors described in co-owned International Patent Publication Number WOOO 18684 (PCT/AU1999/00757) reduce the undesirable reaction of the hydrocalumite to form TCA, without appreciably influencing the reaction of hydrocalumite with carbonate to form calcium carbonate. Virtually any class of surfactant can be used as the TCA inhibitor, 25 providing it adsorbs to the hydrocalumite structure. For example, sugars such as sucrose and glucose, and polysaccharides such as starch can be used. However, anionic organic surfactants are most effective. A non-exclusive list of examples TCA inhibitors includes the following materials, their salts and derivatives: any anionic homopolymers or copolymers (e.g. polyacrylic acid and its co-polymers with acrylamide, or polymers bearing hydroxamate 30 functional groups), hydroxamic acids, humic and tannic acids, lignosulphonates, fatty acids, sulphonated carboxylic acids, carboxylic acids, and polyhydroxy carboxylic acids.

rk 1/AUZUI/UUU3 - 26 The amount of the TCA inhibitor to be added is dependent upon a number of relevant factors including the type of TCA inhibitor selected and the location of the TCA addition point. Thus, the dose rate for a particular inhibitor must be determined by experiment. 5 Advantages of various aspects of the present invention are further described and illustrated by the following examples and experimental test results. These examples and experimental test results are illustrative of a variety of possible implementations and are not to be construed as limiting the invention in any way. It can be seen from the experimental data presented below that the causticisation process of the present invention has demonstrated 10 the ability to consistently achieve a C/S of 0.940 at 140 0 C with a lime efficiency of greater than 90% or greater than 95% over an S concentration in the range of 125-170 g/L as measured in the overflow stream taken from the separator when the causticising agent is slaked lime. Operation at higher S is possible but at a penalty in the highest C/S that can be achieved. The minimum residence time in the reactor vessel is somewhere between 40 15 seconds and 3-4 minutes when using slaked quicklime as the causticising agent and 15 minutes when using recycled hydrocalumite as the causticising agent. Example 1: Lime as Causticising Agent in presence of TCA inhibitor In this example, the causticising agent is hydrated lime slurried in deionised water and the 20 Bayer process liquor is first washer overflow. The TCA inhibitor used was sodium gluconate, added to the first washer liquor prior to lime addition such that the final concentration after lime slurry addition was 0.075 g/L. The initial A, C and S of the first washer overflow is shown at 0 minutes in Table I below. 25 First washer overflow liquor was collected from the refinery and filtered to remove suspended solids. The filtered liquor (1.999 litres) was then added to a 3.75 litre stirred Parr reactor along with the TCA inhibitor. The reactor was sealed and the liquor was heated to 140 0 C. 30 A slurry of industrial grade hydrated lime (available Ca(OH) 2 84.7%) was prepared by weighing 31.65g of the hydrated lime into a polypropylene bottle and adding 150ml of hot (80'C) deionised water. This hydrated lime charge was calculated to increase the C/S of the rk 1/AUZUI/UUU3 -27 first washer overflow to 0.945 assuming 90% lime efficiency. The hydrated lime slurry was quantitatively transferred with the assistance of a small volume of deionised wash water to a 300ml stainless steel injection vessel attached to the 3.75 litre 5 reactor through a series of valves. The injection vessel was sealed, pressurised with nitrogen gas, and the hydrated lime slurry injected into the reactor. The reactor was held at 140 0 C for a total time of 120 minutes. Samples of the reaction slurry were taken at the reaction times (representative of residence times) shown in Table 1 below. 10 Each sample of the reaction slurry was filtered through a 0.45 ptm Supor filter membrane. The filtrate was analysed for A, C and S and the solids were washed with deionised water and damp cake analysed by X-Ray diffraction. 15 Table 1: Liquor analyses for Example I Reaction Time A C S A/C C/S (mins) 0 93.3 137.8 164.0 0.677 0.840 5 82.2 136.8 145.8 0.600 0.939 15 82.2 137.3 146.0 0.599 0.940 30 82.1 137.3 146.0 0.598 0.940 60 82.0 137.2 145.9 0.597 0.941 90 82.1 137.5 146.1 0.597 0.941 120 82.4 138.0 146.8 0.597 0.940 The liquor analyses displayed in Table 1 above demonstrates that the C/S ratio rises rapidly in less than 5 minutes and remains stable for the full 120 minutes period over which the reaction slurry was retained in the reactor vessel. XRD analysis of the reaction solids 20 sampled at the end of 120 minutes shows that the reaction solids are predominantly calcite with very little TCA present and very little residual hydrocalumite remains.

rk 1/AUZUI/UUU43 - 28 It is apparent from Table 1, that the TCA inhibitor has allowed the improvement in C/S by way of the addition of lime as the causticising agent to a preheated Bayer liquor at 140'C to be retained for up to two hours, allowing ample time to transfer the reaction slurry from the reactor vessel to a solid/liquid separator for removal of the reaction solids. 5 Example 2: Lime as Causticising Agent - No Rapid Separation of Reaction Solids from Reaction Slurry This example demonstrates the effect on the C/S ratio when lime is used as a causticising agent (without the addition of a TCA inhibitor) and rapid separation of the reaction solids is 10 not conducted. In this example, a first washer overflow liquor was collected from the refinery and filtered to remove suspended solids. The filtered liquor (2.630 litres) was then added to a 3.75 litre stirred Parr reactor, the reactor sealed, and then the liquor heated to 144'C. The initial A, C and S of the first washer overflow is shown at 0 minutes in Table 2 below. 15 A sample of slaked lime slurry was taken from the refinery and analysed for %solids (23.0%) and the filtered and washed solids were dried and analysed by XRF to give dry Ca(OH) 2 content (62.64% as CaO). The lime slurry charge was calculated to increase the C/S of the first washer overflow to 0.945 assuming 90% lime efficiency. 20 The slaked lime slurry (172.6 g) was quantitatively transferred with the assistance of a small volume of deionised wash water to a 500ml stainless steel injection vessel attached to the 3.75 Litre reactor through a series of valves. The injection vessel was then sealed, pressurised with nitrogen gas, and the hydrated lime slurry injected into the reactor. 25 The reactor was then held at 140'C for 30 minutes. Samples of the reaction slurry were taken at the reaction times (representative of residence times) shown in Table 2 below. Each sample of the reaction slurry was filtered through a 0.45 pm Supor filter membrane. The filtrate was analysed for A, C and S and the solids were washed with deionised water and 30 damp cake analysed by X-Ray diffraction.

rk 1/AUZUI/UUU3 - 29 Table 2: Liquor analyses for Example 2 Reaction A C S A/C C/S time (mins) 0 95.2 136.4 164.0 0.698 0.832 0.6 86.3 137.4 150.3 0.628 0.914 5 85.7 136.2 150.5 0.629 0.905 10 85.4 135.3 150.6 0.632 0.898 20 85.2 134.5 150.7 0.634 0.892 23 85.2 134.5 151.1 0.634 0.890 25 85.5 134.7 151.3 0.634 0.890 28 85.6 134.9 151.7 0.634 0.889 30 86.0 135.5 152.3 0.634 0.890 From the liquor analyses set out in Table 2 above, it is apparent that the C/S did not reach the 5 same value as that achieved in Example 1 and that there is a rapid degradation in the C/S ratio over time with a drop starting to occur after five minutes. XRD analysis of the solids after 5 minutes shows that substantial TCA has already formed in this system, indicating an unstable system. From the behaviour of the liquor C/S after 5 minutes it is clear that further TCA continues to form until a liquor equilibrium C/S (without inhibitor) was reached at -23 10 minutes (~0.890). Example 3: Lime as Causticising Agent - Effect of Cooling prior to Separation of Reaction Solids from Reaction Slurry In this example, the causticising agent is hydrated lime slurried in deionised water and the 15 Bayer process liquor is first washer overflow. The TCA inhibitor used was sodium gluconate, added to the first washer liquor prior to lime addition such that the final concentration after lime slurry addition was 0.075 g/L. The initial A, C and S of the first washer overflow is shown at 0 minutes in Table 3 below. 20 First washer overflow liquor was collected from the refinery and filtered to remove suspended solids. The filtered liquor (2.000 litres) was then added to a 3.75 litre stirred Parr reactor with the inhibitor, the reactor sealed, and then the liquor heated to 140'C.

rk 1/AUZUI/UUU43 -30 A slurry of industrial grade hydrated lime (available Ca(OH) 2 84.8%) was prepared by weighing 34.72g of the hydrated lime into a polypropylene bottle and adding 180m] of hot (80'C) deionised water. The hydrated lime charge was calculated to increase the C/S of the first washer overflow to 0.935 at 90% lime efficiency. The hydrated lime slurry was 5 quantitatively transferred with the assistance of a small volume of deionised wash water to a 500ml stainless steel injection vessel attached to the 3.75 litre reactor through a series of valves. The injection vessel was then sealed, pressurised with nitrogen gas, and the hydrated lime slurry injected into the reactor. 10 The reactor was then held at 140'C for 40 minutes. At 40 minutes, an internal cooling coil was then used to cool the reactor contents to 90'C over the subsequent 30 minute period of time (taking the total reaction time to 70 minutes). Samples of the reaction slurry were taken at the reaction times (representative of residence times) shown in Table 3 below. Each sample of the reaction slurry was filtered through a 0.45 im Supor filter membrane. The 15 filtrate was analysed for A, C and S and the solids were washed with deionised water and damp cake analysed by X-Ray diffraction. Table 3: Liquor analyses for Example 3 Reaction Reaction Temp A C S A/C C/S Time (mins) (Deg C) 0 101.9 147.1 177.9 0.692 0.827 5 140 89.7 146.4 156.3 0.613 0.937 10 140 89.7 146.4 155.7 0.613 0.940 20 140 89.6 146.6 156.4 0.611 0.938 30 140 89.5 146.4 156.0 0.611 0.938 40 140 89.5 146.4 156.3 0.611 0.937 70 90 88.6 143.1 156.5 0.619 0.914 20 Table 3 shows that the C/S is stable for the first 40 minutes at 140 0 C and that the C/S ratio has fallen substantially when the reactor slurry was cooled to 90'C. This result demonstrates that when the reaction slurry is allowed to cool without first separating the causticised liquor from the reaction solids results in a decrease in the liquor C/S. This effect was observed at shorter reaction times as well.

rk 1/AUZUI/UUU43 -31 Laboratory results have confirmed that this drop in liquor C/S that occurs when the reaction slurry is cooled (without first separating the causticised liquor from the reaction solids present in the reaction slurry) is even worse when the reaction occurs without the addition of 5 a TCA inhibitor. XRD scans confirmed a change in the proportion of TCA in the solids after 40 minutes and 70 minutes. Only a trace of TCA was present at 40 minutes when the system is stable. Substantial TCA was present after 70 minutes after the C/S fell to 0.914 upon cooling of the 10 reaction slurry without first separating out the reaction solids from the causticised liquor. Example 4: Hydrocalumite Slurry as Causticising Agent in presence of TCA inhibitor In this example, the causticising agent is hydrocalumite slurried in Is' washer overflow. First washer overflow liquor was collected from the refinery and filtered to remove 15 suspended solids. The filtered liquor (1.2 litres) was then added to in a 2.0 litre stirred Parr reactor with the inhibitor, the reactor sealed, and then the liquor heated to 145'C. A slurry of industrial grade hydrated lime (available Ca(OH) 2 85.0%) was prepared by weighing 26.46g of the hydrated lime into a polypropylene bottle and adding 124ml of hot (80'C) deionised water. The hydrated lime slurry was added, with stirring, to 537 mL of causticised Is' washer 20 overflow (C/S ~0.940) preheated to 80'C to form a hydrocalumite slurry. This hydrocalumite slurry was reacted in a polypropylene bottle for 30 minutes at 80'C in a rolling waterbath. Through the mildly causticising reaction of Ca(OH) 2 to hydrocalumite the C/S of this liquor reached 0.984, producing the highly causticised liquor stream designated as (310) in the embodiment illustrated in Figure 7. 25 The hydrocalumite slurry was then vacuum filtered to produce a deliquored hydrocalumite cake (the solids are not washed). The hydrocalumite cake was then reslurried in 92 mL of 1s' washer overflow at 80'C and returned to a rolling waterbath for a further 15 minutes at 80'C to completely disperse the cake. This is the reslurried hydrocalumite slurry is then used as 30 the causticising agent in the embodiment illustrated in Figure 7. In this example, the Bayer process liquor is first washer overflow. The TCA inhibitor used rk 1/AUZUI/UUU43 - 32 was sodium gluconate, added to the first washer liquor prior to the addition of the reslurried hydrocalumite slurry. The initial A, C and S of the first washer overflow is shown at 0 minutes in Table 4 below. 5 First washer overflow liquor was collected from the refinery and filtered to remove suspended solids. The filtered liquor (2.000 litres) was then added to a 3.75 litre stirred Parr reactor with the inhibitor, the reactor sealed, and then the liquor heated to 140'C. The reslurried hydrocalumite slurry was quantitatively transferred with the assistance of a small volume of deionised wash water to a 300ml stainless steel injection vessel attached to the 2.0 10 litre reactor through a series of valves. The injection vessel was then sealed, pressurised with nitrogen gas, and the hydrocalumite slurry injected into the reactor. The reactor was then held at 140'C for 120 minutes. The reactor contents were sampled with time, with the samples of slurry filtered through a 0.45 ptm Supor filter membrane. The 15 filtrate was analysed for A, C and S and the solids were washed with deionised water and damp cake analysed by X-Ray diffraction. Table 4: Liquor analyses for Example 4 Reaction Time A C S A/C C/S (minutes) 0 89.8 131.3 158.2 0.684 0.830 5 91 144.5 154.1 0.630 0.937 15 91.2 145.2 154.4 0.628 0.941 30 91.2 145.5 154.4 0.627 0.942 60 91.2 145.7 154.4 0.626 0.944 90 91.1 145.7 154.4 0.625 0.943 120 91.1 145.7 154.3 0.625 0.944 20 rk 1/AUZUI/UUU3 -33 The liquor analyses show that the C/S is stable for the full 120 minutes of reaction, with little to no change in liquor composition. This is directly comparable to Example 1, and shows that in the presence of the TCA inhibitor is equally effective when the causticising agent is hydrocalumite as it is with lime. 5 The effect shown in Table 3 whereby the C/S falls substantially when the reactor slurry is cooled prior to removal of the reaction solids from the causticised liquor was also observed when hydrocalumite slurry is used instead of lime as the causticising agent. 10 Now that several embodiments of the invention have been described in detail, it will be apparent to persons skilled in the chemical engineering arts that numerous variations and modifications can be made without departing from the basic inventive concepts. For example, the rate of discharge from the reactor vessel (16) may be by controlling the rate of removal of the overflow stream of clarified causticised Bayer liquor (22) from the 15 overflow outlet (77). All such modifications and variations are considered to be within the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims. Furthermore, the preceding examples are provided to illustrate specific embodiments of the invention and are not intended to limit the scope of the process of the invention. 20

Claims

1. A high temperature causticisation process for the causticisation of a Bayer process liquor, the process comprising the steps of: 5 a) adding a quantity of a causticising agent to the Bayer process liquor in a reactor vessel operating at a target reaction pressure for a given target reaction temperature; b) allowing the causticising agent to react with the Bayer process liquor for a sufficient residence time to produce a reaction slurry comprising a quantity of reaction solids in a causticised Bayer liquor; and, 10 c) subjecting the reaction slurry to solid/liquid separation at a target separation temperature to produce a separated stream of reaction solids and a product stream of clarified cauticised Bayer liquor; the process characterised in that the target reaction temperature is not less than 115'C and the target separation temperature is not less than 115'C. 15

2. The causticisation process of claim 1 wherein said target separation temperature is at or above the target reaction temperature.

3. The causticisation process of claim 1 or 2 wherein said target separation 20 temperature is not more than 5'C or not less than 10'C or not less than 15'C or not less than 20'C below the target reaction temperature. 3. The causticisation process of claim 1 or 2 wherein the target reaction temperature is between 115'C and 300'C. 25

4. The causticisation process of claim 1 or 2 wherein the target reaction temperature is between 115'C and 180'C.

5. The causticisation process of claim 1 or 2 wherein the target reaction temperature 30 is between 120'C and 155'C. -35

6. The causticisation process of any one of the preceding claims wherein the sufficient residence time of step b) is less than 1 minute, less than 3 minutes, less than 5 minutes, less than 10 minutes, less than 15 minutes or less than 20 minutes. 5

7. The causticisation process of any one of the preceding claims wherein step c) is conducted in a solid/liquid separator and the process further comprises retaining the reaction slurry in the solid/liquid separator for a period of time not exceeding one hour, not exceeding 45 minutes, or not exceeding 30 minutes. 10

8. The causticisation process of any one of the preceding claims wherein the solid/liquid separator is a pressure decanter, a pressure filter, a cyclone, or a centrifuge.

9. The causticisation process of any one of the preceding claims wherein the ratio of total caustic concentration to total alkali concentration of the Bayer process liquor, when 15 expressed in grams of sodium carbonate per litre of solution, exceeds 0.9, exceeds 0.92, exceeds 0.94 or exceeds 0.95 after the addition of the quantity of causticising agent.

10. The causticisation process of any one of the preceding claims wherein the causticising agent is one or both of lime or hydrocalumite. 20

11. The causticisation process of any one of the preceding claims wherein the causticising agent is slaked lime

12. The causticisation process of claim 11 wherein the slaked lime is formed by adding 25 quicklime to water or by adding quicklime to a Bayer process liquor.

13. The causticisation process of any one of the preceding claims wherein the Bayer process liquor has an initial total alkali concentration of between 40 and 250 grams or between 80 and 160 grams per litre or between 130 and 170 grams per litre expressed as 3 0 grams of sodium carbonate per litre of solution. -36

14. The causticisation process of any one of the preceding claims wherein the Bayer process liquor has an A/C ratio of between 0.2 and 0.95, or between 0.3 and 0.8, or preferably greater than 0.55. 5

15. The causticisation process of any one of the preceding claims wherein the Bayer process liquor is preheated prior to step a).

16. The causticisation process of claim 15 wherein the Bayer process liquor is 10 preheated to the target reaction temperature in a heating circuit, the heating circuit comprising one or more heating stages.

17. The causticisation process of claim 16 wherein a heating stage comprises direct or indirect steam injection. 15

18. The causticisation process of any one of the preceding claims wherein the clarified causticised liquor is cooled after step c) by countercurrent heat exchange with the Bayer process liquor. 20

19. The causticisation process of any one of the preceding claims wherein the Bayer process liquor is a spent Bayer liquor or a dilute Bayer liquor.

20. The causticisation process of any one of the preceding claims wherein the Bayer process liquor is a washer overflow liquor. 25

21. The causticisation process of any one of the preceding claims wherein step a) is performed in the presence of a TCA inhibitor.

22. The causticisation process of claim 21 wherein the TCA inhibitor is an anionic 30 organic surfactant. -37

23. The causticisation process of claim 22 wherein the anionic organic surfactant is selected from the group comprising: an anionic homopolymers, 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 5 carboxylic acid, and a polyhydroxy carboxylic acid.

24. A high temperature causticisation process for the causticisation of a Bayer process liquor, the process comprising the steps of: a) adding a quantity of a causticising agent to the Bayer process liquor in a reactor 10 vessel operating at a target reaction pressure for a given target reaction temperature; b) allowing the causticising agent to react with the Bayer process liquor for a sufficient residence time to produce a reaction slurry comprising a quantity of reaction solids in a causticised Bayer liquor; and, c) subjecting the reaction slurry to solid/liquid separation at a target separation 15 temperature to produce a separated stream of reaction solids and a product stream of clarified cauticised Bayer liquor; the process characterised in that the target separation temperature is at or above the target reaction temperature and the target reaction temperature is greater than the atmospheric boiling point of the Bayer process liquor. 20

25. The causticisation process of claim 24 wherein the target reaction temperature is between 115'C and 300'C or between 115'C and 180'C or between 120'C and 155'C.

26. The causticisation process of any one of claims 24 or 25 wherein the sufficient 25 residence time of step b) is less than 1 minute, less than 3 minutes, less than 5 minutes, less than 10 minutes, less than 15 minutes or less than 20 minutes.

27. The causticisation process of any one of claim 24 to 26 wherein step c) is conducted in a solid/liquid separator and the process further comprises retaining the 30 reaction slurry in the solid/liquid separator for a period of time not exceeding one hour, not exceeding 45 minutes, or not exceeding 30 minutes. - 38

28. The causticisation process of any one of claims 24 to 27 wherein the solid/liquid separator is a pressure decanter, a pressure filter, a cyclone, or a centrifuge.

29. The causticisation process of any one of claims 24 to 28 wherein the ratio of total 5 caustic concentration to total alkali concentration of the Bayer process liquor, when expressed in grams of sodium carbonate per litre of solution, exceeds 0.9, exceeds 0.92, exceeds 0.94 or exceeds 0.95 after the addition of the quantity of causticising agent.

30. The causticisation process of any one of claims 24 to 29 wherein the causticising 10 agent is one or both of lime or hydrocalumite.

31. The causticisation process of any one of claims 24 to 30 wherein the causticising agent is slaked lime 15

32. The causticisation process of claim 31 wherein the slaked lime is formed by adding quicklime to water or by adding quicklime to a Bayer process liquor.

33. The causticisation process of any one of claims 24 to 32 wherein the Bayer process liquor has an initial total alkali concentration of between 40 and 250 grams or between 80 20 and 160 grams per litre or between 130 and 170 grams per litre expressed as grams of sodium carbonate per litre of solution.

34. The causticisation process of any one of claims 24 to 33 wherein the Bayer process liquor has an A/C ratio of between 0.2 and 0.95, or between 0.3 and 0.8, or preferably 25 greater than 0.55.

35. The causticisation process of any one of claims 24 to 34 wherein the Bayer process liquor is preheated prior to step a). 30

36. The causticisation process of claim 35 wherein the Bayer process liquor is preheated to the target reaction temperature in a heating circuit, the heating circuit comprising one or more heating stages. -39

37. The causticisation process of claim 36 wherein a heating stage comprises direct or indirect steam injection.

38. The causticisation process of any one of claims 24 to 37 wherein the clarified 5 causticised liquor is cooled after step c) by countercurrent heat exchange with the Bayer process liquor.

39. The causticisation process of any one of claims 24 to 38 wherein the Bayer process liquor is a spent Bayer liquor or a dilute Bayer liquor. 10

40. The causticisation process of any one of claims 24 to 39 wherein the Bayer process liquor is a washer overflow liquor.

41. The causticisation process of any one of claims 24 to 40 wherein step a) is 15 performed in the presence of a TCA inhibitor.

42. The causticisation process of claim 41 wherein the TCA inhibitor is an anionic organic surfactant. 20

43. The causticisation process of claim 42 wherein the anionic organic surfactant is selected from the group comprising: an anionic homopolymers, 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. 25

44. A causticisation process substantially as herein described with reference to and as illustrated in the accompanying figures. 30