WO2013131119A1 - Système destiné à la caustification d'une liqueur bayer - Google Patents

Système destiné à la caustification d'une liqueur bayer Download PDF

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Publication number
WO2013131119A1
WO2013131119A1 PCT/AU2012/000236 AU2012000236W WO2013131119A1 WO 2013131119 A1 WO2013131119 A1 WO 2013131119A1 AU 2012000236 W AU2012000236 W AU 2012000236W WO 2013131119 A1 WO2013131119 A1 WO 2013131119A1
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WO
WIPO (PCT)
Prior art keywords
liquor
reactor vessel
slurry
reaction
stream
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PCT/AU2012/000236
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English (en)
Inventor
Daniel Mark Roworth
Original Assignee
Bhp Billiton Worsley Alumina Pty Ltd
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Publication date
Application filed by Bhp Billiton Worsley Alumina Pty Ltd filed Critical Bhp Billiton Worsley Alumina Pty Ltd
Priority to CN201280070574.5A priority Critical patent/CN104136374B/zh
Priority to PCT/AU2012/000236 priority patent/WO2013131119A1/fr
Priority to BR112014021591-0A priority patent/BR112014021591B1/pt
Publication of WO2013131119A1 publication Critical patent/WO2013131119A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00—Compounds of aluminium
    • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
    • C01F7/06—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom by treating aluminous minerals or waste-like raw materials with alkali hydroxide, e.g. leaching of bauxite according to the Bayer process
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J8/00—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes
    • B01J8/005—Separating solid material from the gas/liquid stream
    • B01J8/006—Separating solid material from the gas/liquid stream by filtration
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J8/00—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes
    • B01J8/005—Separating solid material from the gas/liquid stream
    • B01J8/007—Separating solid material from the gas/liquid stream by sedimentation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J8/00—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes
    • B01J8/18—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes with fluidised particles
    • B01J8/20—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes with fluidised particles with liquid as a fluidising medium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2208/00—Processes carried out in the presence of solid particles; Reactors therefor
    • B01J2208/00008—Controlling the process
    • B01J2208/0061—Controlling the level

Definitions

  • the present invention relates to a system for the causticisation of a Bayer liquor.
  • a concentrated sodium aluminate solution is produced by grinding and digesting 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.
  • 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 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.
  • aluminate in solution to yield calcium aluminate species, particularly tricalcium aluminate (TCA, often also referred to as C3A in the cement industry).
  • TCA tricalcium aluminate
  • a dilute liquor stream (usually taken from one of the mud washing stages) is reacted with a slaked lime slurry, generally at a temperature that is close to but below the atmospheric boiling point of the combined liquor.
  • 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%.
  • 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 can be described by the following equation:
  • reaction of equation ( 1 ) occurs in a Bayer process liquor
  • 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- reactions.
  • Hydrocalumite forms very quickly and can deposit as a scale at the point where the calcium hydroxide and the Bayer process liquor mix.
  • a Bayer liquor ' s carbonate impurity level in terms of the caustic to soda ratio, or 'C/S'.
  • 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 removing all of the sodium carbonate in the liquor streams fed to them in an economic way.
  • 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.
  • Roach et al advocate a residence time of less than 15 minutes at I 40°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.
  • Roach et al provides no information as to how to deal with the hydrocalumite that forms very quickly and can deposit as a scale at the point where the calcium hydroxide and the Bayer process liquor mix. 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.
  • a system for high temperature causticisation of a Bayer process liquor comprising a reactor vessel and a solid/liquid separator, the reactor vessel comprising:
  • a causticising agent inlet for introducing a quantity of a causticising agent into the reactor vessel to react with the Bayer process liquor and produce a reaction slurry bed within the reactor vessel, the reaction slurry bed having a reaction slurry bed level;
  • a gas inlet for introducing a non-reactive gas into the closed upper end of the reactor vessel to form a gas cap above the reaction slurry bed level
  • the causticising agent inlet has a first end terminating outside of the reactor vessel and a second end terminating inside of the reactor vessel within the gas cap
  • the liquor inlet has a first end terminating outside of the reactor vessel and a second end terminating inside of the reactor vessel within the gas cap.
  • the second end of the liquor inlet terminates at a height within the wall of the reactor vessel that is vertically above the height of the second end of the causticising agent inlet.
  • the second end of the causticising agent inlet is coaxially aligned with the second end of the liquor inlet.
  • the reactor vessel includes an internal distributor arranged above the height of the reaction slurry bed level for mixing of the causticising agent and the Bayer process liquor in the gas cap.
  • the second end of the causticising agent inlet terminates inside of the reactor vessel at a height that is vertically offset at a preset distance above the internal distributor.
  • the internal distributor may be an internal distributor plate or an internal distributor cone.
  • the non-reactive gas is nitrogen or air.
  • the system further comprises a first control valve for controlling the flow rate of the non-reactive gas into the closed upper end of the reactor vessel to control the residence time of the reaction slurry in the reactor vessel.
  • the reactor vessel is one of a plurality of reactor vessels arranged in parallel.
  • the solid/liquid separator is a pressure filter, a pressurized settler, a pressurized thickener, a centrifuge or a pressure decanter.
  • the separator comprises:
  • a closed upper end including a slurry inlet for receiving the reaction slurry from the reactor vessel and an overflow outlet for removing the overflow stream of clarified causticised Bayer liquor;
  • a bottom section including an underflow outlet for removing of an underflow stream of thickened reaction solids
  • the separator further comprises a recirculation port in the pressure vessel wall or the bottom section for recirculating a portion of the underflow stream of thickened reaction solids.
  • the system further comprises a second control valve in a pipeline leading away from the underflow outlet of the separator for controlling the residence time of reaction solids in the slurry bed of the separator by regulating the flow rate of the underflow stream of thickened reaction solids out of the separator.
  • the system further comprises one or more flocculant injectors for adding a flocculant to reaction slurry.
  • the flocculant injector is located upstream of or adjacent to the slurry inlet of the separator.
  • the thickened reaction solids form a slurry bed within the wall of the separator having a slurry bed level, and the upper end of the separator includes a feedwell positioned at a predetermined distance above the slurry bed level.
  • the flocculant injector is a sparge line extending into the feedwell.
  • the separator is one of a plurality of separators.
  • the stream of Bayer process liquor is preheated upstream of the reactor vessel in a heating circuit comprising one or more heating stages.
  • the overflow stream of clarified causticised liquor is cooled downstream of the separator using a cooling circuit comprising one or more cooling stages.
  • the heating circuit and the cooling circuit are countercurrent such that a portion of the heat of the overflow stream of clarified causticised liquor is used to preheat the feed stream of Bayer process liquor.
  • the system further comprises a mixing tank for producing a causticising agent in the form of a hydrocalumite slurry by adding a quantity of lime to a bypass stream of Bayer process liquor.
  • the system further comprises a filter and the hydrocalumite slurry from the mixing tank is filtered to produce a stream of highly causticised liquor and a hydrocalumite filter cake, the system further comprising a slurry tank for producing a re-slurried hydrocalumite slurry by mixing the hydrocalumite filter cake with a re-slurry liquor.
  • a portion of the cooled clarified causticised liquor is used as the bypass stream of Bayer liquor fed to the mixing tank.
  • the system further comprises one or more TCA inhibitor dosing points for introducing a quantity of TCA inhibitor to the causticising agent, the Bayer process liquor or the reaction slurry.
  • a TCA inhibitor dosing point is within the reactor vessel.
  • the TCA inhibitor dosing point is into the Bayer liquor stream entering reactor vessel or into the causticising agent entering the reactor vessel.
  • the TCA inhibitor is one or more of: a sucrose, a glucose, a polysaccharide, a starch, or an anionic organic surfactant.
  • the causticising agent is one or both of lime or hydrocalumite.
  • 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 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
  • Figure 4 is a conceptual flow diagram a second embodiment of the present invention including a countercurrent heating/cooling circuit
  • FIG. 5 is a conceptual flow diagram a third embodiment of the present invention wherein the causticising agent is a hydrocalumite slurry.
  • FIG. 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.
  • 'Bayer process liquor ' refers to a sodium caustic aluminate liquor generated by digesting (dissolving) bauxite in a caustic soda solution at elevated/ temperatures and pressures.
  • the principal constituents are sodium aluminate and sodium hydroxide.
  • Other impurities in the liquor stream are present as soluble sodium salts.
  • '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 ).
  • 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.
  • 'A/C is thus the ratio of alumina concentration to caustic concentration.
  • '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.
  • S-C sala concentration minus caustic concentration gives the actual concentration of sodium carbonate (Na 2 C0 3 ) in the 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.
  • 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.
  • 'dilute Bayer process liquor' refers to a liquor stream with a low S, typically less than 220g/L.
  • 'TCA' is used to refer to tricalciiim aluminate having the chemical formula of Ca 3 [Al(OH) 6 ]2 which is also commonly written using the formula 3CaO.AI 2 O 3 .6H 2 O (TCA6) or C3AH6 in cement industry notation.
  • '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 lime ' .
  • a 'slaked lime slurry' is produced when lime is mixed with a slaking solution which can be any aqueous solution, typically water.
  • a slaking solution which can be any aqueous solution, typically water.
  • a dilute Bayer liquor can be used as a slaking solution due to the presence of water in such dilute Bayer liquors.
  • '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.
  • the term 'hydrocalumite' is used throughout this specification to refer to aluminium-based layered double hydroxide of the form [Ca2Al(OH)6] 2 X 2 nH 2 0, where 'X' represents a charge-balancing anion or anions.
  • hydrocalumite may have the formula of [ € ⁇ ⁇ ⁇ 1( ⁇ ⁇ 2 CO, or Ca 2 AI(OH) 6 ] 2 . l / 2 C03.0H.51 ⁇ 2H 2 0 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.
  • '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) analysis, Thermo Gravimetric Analysis (TGA) analysis, liquor or mass balance.
  • TIC Total Inorganic Carbon
  • XRF x-ray fluorescence
  • TGA Thermo Gravimetric Analysis
  • a first embodiment of the process of the present invention will now be described with reference to the process flowchart illustrated in FIG. 1 in which the process is generally designated by the reference numeral ( 10).
  • a quantity of a causticising agent ( 12) and a 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 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.
  • a solid/liquid separator (20) is arranged to receive the reaction slurry ( 18) and produce an 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.
  • 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 1 1 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 the target separation temperature is not less than 1 15°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 10°C, or not greater than 20°C.
  • the reactor vessel ( 16) is a pressure vessel that is capable of operating at the target reaction temperature.
  • the target reaction temperature may be in the range of 1 15°C and 300°C, more preferably in the range of 1 15°C and 170°C or more preferably in the range of 120°C and 155°C.
  • 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 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 at an elevated temperature. In this way, the reaction slurry is maintained at or above the target reaction temperature until after separation.
  • the present invention is 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.
  • the 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.
  • the 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 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 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.
  • the ratio may be greater than or equal to 10: 1 , 1 5: 1 or 20: 1 , with higher ratios required for causticising dilute Bayer process liquors and lower ratios required if the Bayer process liquor has a high carbonate concentration.
  • 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.
  • the Bayer process liquor will have an 'S' concentration of between 40 and 250 g/L (preferably between 130 and 1 70 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.
  • a suitable causticising agent is lime whilst one example of a suitable Bayer process 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.
  • 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.
  • the equilibrium C/S ratio when operating at a temperature of 140°C, is ⁇ 0.94 whereas at 103°C, the equilibrium 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.
  • 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.
  • FIG. 2 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 reactor may be used as an alternative.
  • the reactor vessel may be one of a plurality of reactor vessel arranged in parallel.
  • 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 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 process liquor.
  • an internal distributor 44
  • 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 liquor inlet (46).
  • 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.
  • 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 reaction temperature that reaction solids can deposit like scale at the site where mixing first occurs.
  • the reactor vessel has been specifically designed to overcome this scaling problem.
  • 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 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 (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 (52) of the causticising agent inlet (48).
  • 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 otherwise occur when the causticising agent first comes into contact with the Bayer process liquor.
  • 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 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 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 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.
  • 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 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 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).
  • a variable speed takeoff pump (not shown) may be used instead of or in 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).
  • the causticising agent is slaked lime
  • the Bayer process liquor is a washer overflow liquor
  • 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 reactor vessel should be used for best results.
  • 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.
  • 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.
  • the target separation 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 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 stream of clarified causticised liquor (22).
  • 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.
  • a suitable solid/liquid separator (20) in the form of a pressure decanter is illustrated schematically in Figure 3.
  • 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 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 (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.
  • 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 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 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 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).
  • the bed of thickened reaction solids (92) is maintained to ensure a residence time of less than one hour in the separator (20).
  • 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 TCA.
  • 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 (94) of the feedwell (90) is positioned at a predetermined distance above the slurry bed level (96).
  • a flocculant is added to the pressurized reaction slurry via one or more flocculant injector(s) (88).
  • the flocculant is injected into the pipeline (70) upstream of or adjacent to the slurry inlet (72) at a point of maximum turbulence.
  • 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 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 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.
  • 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.
  • the pressurized reaction slurry ( 18) from the reactor vessel ( 16) is directed via pipeline (70) into the slurry inlet (72) of the separator (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 'floes' which deposit themselves at the top of the bed of thickened solids (92).
  • the causticised liquor rises to the top of the separator (20) where it is removed via the 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.
  • 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).
  • the target separation pressure may be around 510kPag (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 all times during operation to prevent flashing of the causticised liquor or depressurization of the reaction slurry.
  • 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.
  • the causticising agent is lime
  • the 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 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.
  • 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 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 1 5 minutes.
  • 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.
  • cooling 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.
  • a depressurizing chamber (not shown) operating at a lower pressure than the separator may be used.
  • the underflow stream of thickened reaction solids may be cooled using a suitable heat exchanger such as a shell and tube heat exchanger.
  • 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. 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 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.
  • the heating circuit (202) and the cooling circuit (206) are countercurrent for heat recover with three heating/cooling stages (204 and 208, respectively).
  • 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 ).
  • the feed stream of Bayer process liquor (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).
  • 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 heating circuit (202) to form a second partially heated stream of Bayer process liquor (224) and a cool condensate stream (226).
  • 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).
  • 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 discharged from the heating circuit (202) at a temperature below the atmospheric boiling point of the Bayer process liquor.
  • 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.
  • this requires the use of an alternative cooling stream to decrease the temperature of the clarified causticised liquor to below the atmospheric boiling point.
  • the heating circuit and cooling circuit may remain separate and independent of each other and any number of heating/cooling stages can be 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.
  • 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.
  • 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 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 hydrocalumite slurry (300) is mixed with the preheated Bayer process liquor ( 14) in the reactor vessel ( 16) to form calcite is endothermic.
  • 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 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).
  • 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 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 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.
  • 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 minutes, preferably around 10 to 15 minutes.
  • the causticising agent is a mixture of a re-slurried hydrocalumite slurry (400) 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) to produce the re-slurried hydrocalumite slurry (400).
  • 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).
  • the bypass stream of Bayer process liquor may be sub-cooled using a cooling stage (316) to ensure that the final mixture (including heat of reaction) is at the desired reaction temperature in the mixing tank (306).
  • 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.
  • TCA inhibitor such as sucrose or sodium gluconate
  • Dosing with the TCA inhibitor allows a higher C/S to be achieved with higher lime efficiency, higher 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 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.
  • 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 ( 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).
  • TCA additive dosing point is the reactor vessel or added to the Bayer process liquor immediately upstream of the reactor vessel.
  • Suitable TCA inhibitors described in co-owned International Patent Publication Number WO0018684 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, providing it adsorbs to the hydrocalumite structure.
  • sugars such as sucrose and glucose, and polysaccharides such as starch can be used.
  • anionic organic surfactants are most effective.
  • 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 functional groups), hydroxamic acids, humic and tannic acids, lignosulphonates, fatty acids, sulphonated carboxylic acids, carboxylic acids, and polyhydroxy carboxylic acids.
  • anionic homopolymers or copolymers e.g. polyacrylic acid and its co-polymers with acrylamide, or polymers bearing hydroxamate functional groups
  • hydroxamic acids e.g. polyacrylic acid and its co-polymers with acrylamide, or polymers bearing hydroxamate functional groups
  • hydroxamic acids e.g. polyacrylic acid and its co-polymers with acrylamide, or polymers bearing hydroxamate functional groups
  • humic and tannic acids e.g. polyacrylic acid and its
  • 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. 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.
  • the causticisation process of the present invention has demonstrated the ability to consistently achieve a C/S of 0.940 at 140°C with a lime efficiency of greater than 90% or greater than 95% over an S concentration in the range of 125- 1 70 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 seconds and 3-4 minutes when using slaked quicklime as the causticising agent and 15 minutes when using recycled hydrocalumite as the causticising agent.
  • the causticising agent is hydrated lime slurried in deionised water and the 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 1 below.
  • 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°C.
  • 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 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 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°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. Each sample of the reaction slurry was filtered through a 0.45 ⁇ Supor filter membrane.
  • 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 not conducted.
  • 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.
  • 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.
  • 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.
  • 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 ⁇ 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.
  • the causticising agent is hydrated lime slurried in deionised water and the 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.
  • 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.
  • 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 180ml 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 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.
  • 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 ⁇ 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.
  • Table 3 shows that the C/S is stable for the first 40 minutes at 140°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.
  • Example 4 Hydrocalumite Slurry as Causticising Agent in presence of TCA inhibitor
  • the causticising agent is hydrocalumite slurried in 1 sl washer overflow.
  • First washer overflow liquor was collected from the refinery and filtered to remove 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 1 st washer 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.
  • 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 1 st 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 the causticising agent in the embodiment illustrated in Figure 7.
  • the Bayer process liquor is first washer overflow.
  • the TCA inhibitor used 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.
  • 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 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 ⁇ 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.

Abstract

L'invention concerne un système destiné à la caustification d'une liqueur du procédé Bayer comprenant un récipient de réacteur et un séparateur solide/liquide. Le récipient de réacteur comprend une paroi ; une extrémité supérieure fermée et une extrémité inférieure fermée ; une entrée de liqueur pour l'introduction de la liqueur du procédé Bayer dans le récipient de réacteur ; une entrée d'agent de caustification pour l'introduction d'une quantité d'un agent de caustification dans le récipient de réacteur en vue de produire un lit de suspension de réaction dans le récipient de réacteur présentant un niveau de lit de suspension de réaction ; une entrée de gaz pour l'introduction d'un gaz non réactif dans l'extrémité supérieure fermée en vue de former un bouchon gazeux au-dessus du niveau de lit de suspension de réaction ; l'entrée d'agent de caustification présentant une première extrémité se terminant à l'extérieur du récipient de réacteur et une deuxième extrémité se terminant à l'intérieur du récipient de réacteur dans le bouchon gazeux et l'entrée de liqueur présentant une première extrémité se terminant à l'extérieur du récipient de réacteur et une deuxième extrémité se terminant à l'intérieur du récipient de réacteur dans le bouchon gazeux.
PCT/AU2012/000236 2012-03-07 2012-03-07 Système destiné à la caustification d'une liqueur bayer WO2013131119A1 (fr)

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CN201280070574.5A CN104136374B (zh) 2012-03-07 2012-03-07 用于拜耳液的苛性化的系统
PCT/AU2012/000236 WO2013131119A1 (fr) 2012-03-07 2012-03-07 Système destiné à la caustification d'une liqueur bayer
BR112014021591-0A BR112014021591B1 (pt) 2012-03-07 2012-03-07 sistema para causticização de temperatura alta de um licor de processo bayer

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WO2000018684A1 (fr) * 1998-09-25 2000-04-06 Worsley Alumina Pty Ltd Caustification de bayer amelioree
WO2002083564A1 (fr) * 2001-04-11 2002-10-24 Worsley Alumina Pty Ltd Procede d'evacuation d'impuretes anioniques de solutions d'aluminate caustiques
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Publication number Priority date Publication date Assignee Title
EP2886177A1 (fr) * 2013-12-20 2015-06-24 Rio Tinto Alcan International Limited Décanteur pour décanter des boues minérales et procédé de séparation de liquide décanté à partir de boue épaissie desdites boues minérales
EP3082992A4 (fr) * 2013-12-20 2017-11-01 Rio Tinto Alcan International Limited Décanteur pour décanter des boues minérales et procédé de séparation de liquide clarifié d'une boue épaissie de ces boues minérales

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