AU2011100201A4

AU2011100201A4 - Method of Concentrating a Bayer Process Liquor

Info

Publication number: AU2011100201A4
Application number: AU2011100201A
Authority: AU
Inventors: Peter Stewart Hay; Dean Ilievski
Original assignee: Alcoa of Australia Ltd
Current assignee: Alcoa of Australia Ltd
Priority date: 2006-09-22
Filing date: 2011-02-18
Publication date: 2011-03-17
Anticipated expiration: 2015-09-21
Also published as: WO2008034196A1; CN101563137B; CN101563137A; AU2007299598B2; AU2007299598A1

Description

AUSTRALIA ORIGINAL COMPLETE SPECIFICATION INNOVATION PATENT Invention Title: Method of Concentrating a Bayer Process Liquor Name of Applicant: Alcoa of Australia Limited Actual Inventors: Peter Stewart Hay Dean Ilievski Address for service: WRAYS Ground Floor, 56 Ord Street West Perth WA 6005 Attorney code: WR The following statement is a full description of this invention, including the best method of performing it known to me:- -2 Method of concentrating a Bayer process liquor Field of the Invention The present invention relates to a method for concentrating a Bayer process liquor. More specifically, the present invention relates to a method for 5 concentrating a Bayer process liquor utilising heat recovered from a waste Bayer process gas. Background Art The Bayer process is widely used for the production of alumina from aluminium containing ores, such as bauxite. The process involves contacting alumina 10 containing ores with recycled caustic aluminate solutions, at elevated temperatures, in a process commonly referred to as digestion. In some cases, a significant amount of organic material accompanies the bauxite, a portion of which is responsible for the presence of a range of organic compounds in the resulting solution. 15 After cooling the solution, aluminium hydroxide is added as seed to induce the precipitation of further aluminium hydroxide therefrom. The precipitated aluminium hydroxide is separated from the caustic aluminate solution, with a portion of the aluminium hydroxide being recycled to be used as seed and the remainder recovered as product. The remaining caustic aluminate solution is 20 recycled for further digestion of alumina containing ore. The recovered aluminium hydroxide is then heated to produce alumina, in a process known as calcination. A by-product of the calcination reaction is water, as aluminium hydroxide produces alumina according to the following reaction: 2Al(OH) 3 -+ A1 2 0 3 +-3H 2 0 25 Different types of calciner designs are used commercially with different fuel types (e.g. oil, gas, coal) and operating conditions such as excess air. Thus, a range of -3 calciner flue gas temperatures and compositions may occur; and it is estimated that approximately 35% to 50% by mass % of the gas leaving calcination ('calciner flue gas') is water, with other components including carbon dioxide and volatile organic carbon compounds. Additionally, the calciner flue gas may 5 contain entrained particulate alumina. A significant quantity of heat is lost from the Bayer process by way of the calciner flue gas. It is estimated that the majority of the available heat is low-grade sensible heat or latent heat released when the water vapour in the flue gas is condensed. However, the latter can only be recovered as low grade heat at 10 atmospheric conditions as the dew point is less than 100 0C, typically 80 to 83 *C. However, although significant in quantity, the low grade heat, is conventionally understood to be of limited utility in the Bayer process. The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be 15 appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application. Disclosure of the Invention General 20 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 or group of integers. In the context of the present specification, the term dehumidified shall be 25 understood to encompass gas streams in which all or a portion of the water vapour has been removed.

-4 Other definitions for selected terms used herein may be found within the description of the invention and apply throughout. Unless otherwise defined, all other scientific and technical terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the invention 5 belongs. Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions 10 and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features. The present invention is not to be limited in scope by the specific embodiments described herein, which are intended for the purpose of exemplification only. 15 Functionally equivalent products, compositions and methods are clearly within the scope of the invention as described. The invention described herein may include one or more ranges of values. A range of values will be understood to include all values within the range, including the values defining the range, and values adjacent to the range which lead to the 20 same or substantially the same outcome as the values immediately adjacent to that value which defines the boundary to the range. The entire disclosures of all publications (including patents, patent applications, journal articles, laboratory manuals, books, or other documents) cited herein are hereby incorporated by reference. Inclusion does not constitute an admission is 25 made that any of the references constitute prior art or are part of the common general knowledge of those working in the field to which this invention relates.

-5 Specific In accordance with the present invention, there is provided a method for concentrating spent Bayer process liquor, the method comprising the steps of: recovering heat from a Bayer process calciner flue gas by directly contacting 5 the calciner flue gas with a body of fluid and heating the body of fluid; applying at least a portion of the recovered heat to the spent Bayer process liquor; and evaporating water from the spent Bayer process liquor, thereby concentrating the spent Bayer process liquor. 10 As discussed in the preceding Background Art section, although a large quantity of heat is lost from the Bayer process through calciner flue gas, the heat is only recoverable as low-grade heat and thus traditionally understood to be of limited utility in the Bayer process. However, it has been found that such heat is useful in the facilitation of evaporation of water from Bayer process liquors. The Bayer 15 process requires careful management of the concentrations and ratios of a number of components, and it is frequently desirable to remove water from the process to concentrate the liquor. As would be understood by a person skilled in the art, a spent Bayer process liquor is liquor that has been subjected to the precipitation phase of the Bayer 20 process, and not yet been recycled back to the digestion phase. Preferably, the step of applying at least a portion of the recovered heat to the spent Bayer process liquor more specifically comprises contacting the spent Bayer process liquor with the heated body of fluid, thereby applying heat to the spent Bayer process liquor and cooling the body of fluid. 25 -6 Thus, in a preferred embodiment, the method comprises the steps of: recovering heat from a Bayer process calciner flue gas by contacting the calciner flue gas with a body of fluid, thereby heating the body of fluid; applying at least a portion of the recovered heat to the spent Bayer process 5 liquor by contacting the spent Bayer process liquor with the heated body of fluid, thereby applying heat to the spent Bayer process liquor and cooling the body of fluid; and evaporating water from the Bayer spent process liquor, thereby concentrating the spent Bayer process liquor. 10 In a highly preferred embodiment of the invention, the method comprises the step of: contacting the cooled body of fluid with Bayer process calciner flue gas, thereby recovering heat from the calciner flue gas and heating the body of fluid; 15 applying at least a portion of the recovered heat to further spent Bayer process liquor by contacting the further spent Bayer process liquor with the heated body of fluid, thereby applying heat to the further spent Bayer process liquor and cooling the body of fluid; and evaporating water from the further Bayer spent process liquor, thereby 20 concentrating the further spent Bayer process liquor. In a preferred embodiment, the steps of the present invention may be repeated to provide a cyclical and/or continuous process. In a preferred form of the invention, the fluid is a liquid. In a highly preferred form of the invention the liquid is, or is substantially, water.

-7 Preferably, the calciner flue gas is directly contacted with the body of liquid in a counter-current gas-liquid contactor. Direct contact of the body of liquid with the calciner flue gas affords several additional advantages beyond heat-recovery. 5 As discussed, calciner flue gas may contain volatile organic carbon compounds, the release of which into the atmosphere is highly environmentally undesirable. To the extent that such compounds are soluble in the body of liquid, their release by calciner flue gas in reduced. A significant range of volatile organic carbon compounds are soluble in water. Accordingly, where the fluid is water, the 10 reduction in volatile organic carbon compounds released by way of the calciner flue gas will be appreciable. Also as discussed, calciner flue gas may contain entrained particulate alumina, the release of which into the atmosphere is also environmentally undesirable. Direct contact of the calciner flue gas with the fluid causes at least a portion of the 15 particulate alumina to remain in the fluid, thereby reducing the quantity exhausted. Preferably, contacting the calciner flue gas with a body of fluid, thereby heating the body of fluid comprises the step of: contacting hot calciner flue gas containing water vapour with a cooled body of fluid, thereby cooling the calciner flue gas to such an extent that at least a 20 portion of the water vapour condenses to liquid water, so producing a cooled substantially dehumidified calciner flue gas and a warm body of water. The substantial dehumidification of the flue gas affords yet another advantage. Water contributes substantially to the visibility of the calciner flue gas plume. A visible plume is undesirable from an environmental perspective. The 25 dehumidification of the flue gas by the method of the present invention means that, at any given temperature, the visible component of the flue gas plume is -8 reduced. Further, the method of the present invention allows recovery of water that would otherwise be lost to the atmosphere. In one form of the invention, the method comprises the further step of: heating the cooled dehumidified flue gas to increase its buoyancy to aid 5 dispersion. Preferably, the step of: heating the cooled dehumidified flue gas to increase its buoyancy to aid dispersion, comprises heating the cooled dehumidified flue gas by direct or indirect contact 10 with a heat source. It will be appreciated by those skilled in the art that a variety of heat sources in Bayer circuit may be utilized to heat the cooled dehumidified flue gas. In one form of the invention, the heat may be sourced from the alumina coolers in the Bayer calcination circuit. In an alternate form of the invention the heat may be sourced 15 from low pressure plant steam. In a further alternate form of the invention the heat may be sourced from a fuel fired burner. It will be appreciated that there may be provided more than one apparatus for applying heat to the cooled dehumidified flue gas. Where there is provided more than one apparatus for applying heat to the cooled dehumidified flue gas they may 20 be arranged in series or parallel. In one form of the invention, prior to the step of contacting the hot calciner flue gas with the cooled body of fluid, the method comprises the step of: contacting the hot calciner flue gas with cooled dehumidified calciner flue gas, thereby heating the cooled dehumidified calciner flue gas.

-9 It has been found that heating the cooled dehumidified flue gas before discharging to the atmosphere is desirable to assist the flue gas buoyancy and enable the flue gas plume to properly disperse. Thus, in accordance with a highly preferred embodiment of the invention, the 5 method comprises the steps of: recovering heat from Bayer process calciner flue gas by directly contacting hot calciner flue gas containing water vapour with a cooled body of water, thereby cooling the calciner flue gas to such an extent that at least a portion of the water vapour condenses to liquid water, so producing a cooled 10 substantially dehumidified calciner flue gas and a warm body of water; applying at least a portion of the recovered heat to the spent Bayer process liquor by indirectly contacting the warm body of water with the spent Bayer process liquor; thereby heating the Bayer process liquor and cooling the body of water; 15 evaporating water from the Bayer spent process liquor, thereby concentrating the spent Bayer process liquor; recontacting at least a portion of the cooled body of water for further contact with hot calciner flue gas containing water; and contacting the cooled dehumidified calciner flue gas with a heat source, 20 thereby heating the cooled dehumidified calciner flue gas. Preferably, the step of recovering heat from a Bayer process calciner flue gas by contacting the calciner flue gas with a body of fluid, thereby heating the body of fluid comprises the step of; contacting the calciner flue gas with a body of fluid at or near atmospheric 25 pressure.

- 10 In the context of the present invention, the term near atmospheric pressure shall be understood to indicate a pressure such that the vessel does not require certification as a pressure vessel. In a preferred embodiment, the steps of the method are repeated to provide a 5 continuous process. The step of directly contacting the calciner flue gas with a body of fluid may be effected in any of a range of gas-liquid contactor apparatus known to persons skilled in the art. For example tray tower, structured packed tower, random packed tower, fluidized packed tower, spray tower and cascade tower gas-liquid 10 contactor apparatus are appropriate for use in the method of the present invention. Desirably, the gas-liquid contactor apparatus used to effect the method of the present invention contains means designed to increase the heat and mass transfer efficiency of the gas-liquid contact. Said means may include static packing, fluidized packing and the use of trays and baffles. Some convenient 15 forms of packing, such as polypropylene packing, may be heat sensitive and it may be necessary to reduce the temperature of the hot calciner flue gas before introducing such into any apparatus containing such packing. A convenient way of achieving such in accordance with the method of the present invention is to directly contact the hot calciner flue gas with a portion of the body 20 of fluid prior to introducing such into any contacting vessel. In a preferred form of the invention, the hot calciner flue gas is directly contacted with a portion of the heated body of fluid prior to entering any contacting vessel. Preferably, the step of contacting the spent Bayer process liquor with the heated body of fluid, thereby applying heat to the spent Bayer process liquor, more 25 specifically comprises indirectly contacting the spent Bayer process liquor with the heated body of fluid, thereby applying heat to the spent Bayer process liquor. Apparatus allowing the spent Bayer process liquor to be heated by indirect contact with the heated body of fluid are known to persons skilled in the art, and include vertical tube falling film, horizontal tube falling film, vertical rising film, - 11 kettle boiler and forced circulation apparatus. In one form of the invention, the step of indirectly contacting the spent Bayer process liquor with the heated body of fluid, whereby heating of the spent Bayer process liquor utilizes a falling film evaporator having a shell-side and a tube-side, and the method comprises the 5 steps of: introducing the heated body of fluid into the shell-side of the falling film evaporator; introducing the spent Bayer process liquor into the tube side of the falling film evaporator; 10 thereby applying heat to the spent Bayer process liquor and cooling the body of fluid. It will be appreciated that there may be provided more than one apparatus for applying heat to the spent Bayer process liquor. Where there is provided more than one apparatus for applying heat to the spent Bayer process liquor they may 15 be arranged in series or parallel. In one form of the invention, the method comprises the further step of: further heating of the heated body of fluid. The heated body of fluid may be further heated by utilising sources of heat available within the Bayer circuit or external sources available from, for example, 20 power stations or other industrial facilities, It will be appreciated by those skilled in the art that a variety of heat sources in Bayer circuit may be utilized. In one form of the invention the heat may be sourced from the alumina coolers in the Bayer calcination circuit. In an alternate form of the invention the heat may be sourced from low pressure plant steam. 25 In one form of the invention, the method comprises the further step of: - 12 increasing the water vapour content of the calciner flue gas prior to the step of recovering heat from the Bayer process calciner flue gas. Preferably, the step of: increasing the water vapour content of the calciner flue gas prior to the 5 step of recovering heat from the Bayer process calciner flue gas. comprises the step of: passing at least a portion of the calciner flue gas to a separator to provide a dehydrated gas stream and a vapour enriched stream. Brief Description of the Drawings 10 The present invention will now be described, with reference to the embodiment thereof, and the accompanying drawings, in which: Figure 1 is a schematic flow sheet showing a method in accordance with the present invention; Figure 2 is a schematic flow sheet showing a method in accordance with a 15 first embodiment of the present invention; Figure 3 is a schematic flow sheet showing a method in accordance with a second embodiment of the present invention; Figure 4 is a schematic flow sheet showing a method in accordance with a third embodiment of the present invention; 20 Figure 5 is a schematic flow sheet showing a method in accordance with a fourth embodiment of the present invention; Figure 6 is a schematic flow sheet showing a method in accordance with a fifth embodiment of the present invention; and -13 Figure 7 is a schematic flow sheet showing a method in accordance with a sixth embodiment of the present invention. Best Mode(s) for Carrying Out the Invention Those skilled in the art will appreciate that the invention described herein is 5 amenable to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or 10 features. In Figure 1, there is provided a generic flow sheet showing how a method in accordance with the present invention may be utilised. In accordance with the present invention, humidified calciner flue gas 12 is fed to a gas-liquid contactor 14 where it is directly contacted with a cooled water stream 15 16. Latent heat recovered from the warm humidified flue gas 12 is transferred to the cooled water stream 16, thereby heating the cooled water stream 16, at least a portion of the water vapour of the warm humidified calciner flue gas 12 condenses to liquid water, so producing a cooled substantially dehumidified flue gas 18 and a warm water stream 20. 20 The gas-liquid contactor 14 is designed to increase the heat and mass transfer efficiency of the gas-liquid contact and is operated with counter-flow of the cooled water stream 16 and the warm humidified calciner flue gas 12. As the means 22 to increase the heat and mass transfer efficiency of the gas-liquid contact may be heat sensitive, it may be necessary to cool the warm humidified calciner flue gas 25 12 prior to feeding it to the gas-liquid contactor 14. A large portion of the warm water stream 20 exits the gas-liquid contactor 14 and is passed to a falling film evaporator 24 by way of a water pump 26.

-14 The falling film evaporator 24 comprising a shell portion (not shown) and tube portion (not shown) is provided to concentrate Bayer spent liquor 28. The warm water stream 20 entering the shell portion of the falling film evaporator 24 is used as a heat source to indirectly apply heat and thereby evaporate water from Bayer 5 spent liquor 28 entering as a liquor film via the tube portion. Concentrated Bayer spent liquor 30 exits the falling film evaporator 28. The warm water stream 20, after leaving the falling film evaporator 24, is recycled as a cooled water recycled stream 32. A portion of the recycled cooled water stream 32, after leaving the falling film 10 evaporator 24, is recycled back to the cooled water stream 16 whilst the remaining portion of the recycled cooled water stream 32 is collected 34. The Bayer spent liquor feed 28 is heated, and a vapour 36 generated is transferred to a condenser 38. A portion of the concentrated Bayer spent liquor 30 is returned to the liquor circuit 15 40, whilst a second portion of the concentrated Bayer spent liquor 30 is recycled back 42 and mixed with the Bayer spent liquor feed 28 prior to entering the falling film evaporator 24. It is believed that the installation of the various units required to construct a plant to perform the present invention may accomplished by the skilled addressee with 20 little input other than the provision of various parameters such as flow rates, fluid types and temperatures to the necessary suppliers. To construct the components necessary to perform the invention certain process data is required by the appropriate equipment manufacturer. An example of the process data from one of the applicants natural gas fired calciners in their refineries in Western Australia 25 is provided below. The approximate flue gas flows are: nitrogen 110 t/hr, oxygen 6 t/hr, carbon dioxide 20 t/hr, water 90 t/hr, Total 226 t/hr. Temperature 165 *C, dewpoint 83 *C at atmospheric pressure.

- 15 The flue gas is to be quenched with water to cool the gas to its dewpoint before entering the tower. Water to tower: Flow approximately 2200 kL/hr at 62 *C 5 Required tower performance: Exit flue gas dew point 64 0 C, exit water temperature 81 OC. The design of the evaporator requires information on the physical properties of the Bayer spent liquor being treated, including, liquor composition, density, viscosity, heat capacity, thermal conductivity, vapour pressure. 10 It is believed that the above information would be sufficient to enable a supplier to design an appropriate tower, e.g. a packed tower, (Rhine Rhur Pty Ltd, Australia, tower diameter, packing type, packed height and pressure drop), a tray tower, (Lisbon Engineering Pty Ltd, Australia, tower diameter, number of trays, tray design, tower height and pressure drop), and a fluidized turboid tower, (Fluid 15 Technologies Ltd, UK, tower diameter, turboid type, static and fluidized packed height and pressure drop). The heat transfer equipment, (the falling film evaporator and vapour condenser), could be designed by any person skilled in the art, by using to any standard heat exchanger design text, or references from the technical literature, or by a supplier 20 of evaporation equipment (GEA Kestner, France, Bertrams Salt Plants, Winterthur, Switzerland). The embodiments of the invention were formulated, evaluated and refined using a combination of inhouse models built on chemical engineering first principles and tuned to existing Bayer unit operations, an extensive database of Bayer properties 25 and thermodynamic data, Bayer operating experience and flowsheet models built within ASPEN Plus T M , ASPEN Technology Inc. software process simulation -16 software with state-of -the-art physical properties packages, including added Bayer process properties and unit operations built inhouse. It will be appreciated that the numbers provided below in relation to flow rates and temperatures are specific to the models and embodiments used and are 5 influenced by parameters put into the models. For example, the flue gas composition and temperature will affect temperatures and flow rates down stream. It will be appreciated that the location of the site where the method of the present invention is to be utilised, and in particular, access to an adequate heat sink, such as a supply of water for process cooling, can affect a number of the steps in the 10 method. In Figure 2, there is shown a method for concentrating spent Bayer process liquor in accordance with a first embodiment of the present invention. The methods of Figures 1 and 2 are substantially similar and like numerals denote like steps and features. It is envisaged that the present embodiment will be utilised where there 15 is provided a suitable heat sink such as a large cooling water supply. In the present embodiment, the calciner flue gas 12 is quenched by direct contact with a water mist spray 50 typically at approximately 50 to 150 kL/hr, to cool the warm humidified calciner flue gas 12 before feeding to the gas-liquid contactor 14. The cooled dehumidified flue gas 18 exiting the gas-liquid contactor 14 is heated 20 by a gas heater 52 to aid its buoyancy. The gas heater 52 may operate by, for example, direct combustion of a small amount of natural gas and air in the flue gas stream 18, or by indirect heating with a steam, hot gases or hot water heat exchanger, requiring about 10 GJ/hr heat. The warm water stream exits the gas-liquid contactor 14 at approximately 83 OC 25 and is pumped to the falling film evaporator 24 where approximately 70 t/hr of vapour 36 are generated from the Bayer spent liquor feed 28 and collected in a water cooled condenser 54.

- 17 In Figure 3, there is shown a method for concentrating spent Bayer process liquor in accordance with a second embodiment of the present invention. The methods of Figures 1, 2 and 3 are substantially similar and like numerals denote like steps and features. 5 The present embodiment is designed for implementation at sites where it is desired to increase the recovery of water from the flue gas or when a adequate water supply for process cooling is not available. The present embodiment further depicts examples of the use of alumina cooling heat from the calciners. In this embodiment, the warm water stream 20 exiting the gas-liquid contactor 14 10 is heated with waste heat 60 from the calciner to approximately 84 'C before being passed to the first 62 of two falling film evaporators to provide a heated water stream 61. The waste heat 60 is heat obtained during the cooling of alumina in the calciner. It will be appreciated that heat may be obtained from other sources 64 in the Bayer circuit or elsewhere. 15 There are provided two falling film evaporators 62, 66 operating in series, the first stage 62 operating about 10 *C hotter than the second stage 66. The condensing flash vapour 68 from the first stage 62 is at a suitable temperature, (typically above about 60 *C), to use indirect condensing with fin fan air coolers 70 reducing the consumption of water for process cooling, thus increasing the net recovery of 20 water. The condensing flash vapour 72 from the second falling film evaporator 66 is condensed in a water cooled condenser 74 (typically below 60 *C). The heated water stream 61 entering the shell portion of the first falling film evaporator 62 is used as a heat source to apply indirect heat and thereby evaporate water from Bayer spent liquor 28 entering as a liquor film via the tube 25 portion. Concentrated Bayer spent liquor 76 exits the first falling film evaporator 62 and enters the tube portion of the second falling film evaporator 66. The heated water stream 61, after leaving the first falling film evaporator 62, enters the shell portion 78 of the second falling film evaporator 66 and is used as -18 a heat source to apply indirect heat and thereby evaporate water from concentrated Bayer spent liquor 76. Concentrated Bayer spent liquor 80 exits the second falling film evaporator 66. A portion of the recycled cooled water stream 82, after leaving the second falling 5 film evaporator 66, is recycled back to the cool water stream 16 whilst the remaining portion of the recycled cooled water stream 82 is collected 34. In Figure 4, there is shown a method for concentrating spent Bayer process liquor in accordance with a third embodiment of the present invention. The methods of Figures 1, 2, 3 and 4 are substantially similar and like numerals denote like steps 10 and features. The present embodiment is designed for implementation at sites where it is desired to increase the amount of evaporation and further reduce the plant steam needed for evaporation. The present embodiment further depicts examples of the use of alumina cooling heat from the calciners. 15 The third embodiment differs from the second embodiment by the replacement of the finned fan air cooler 70 with mechanical vapour recompression 90 to upgrade the heat entering the second falling film evaporator 66. In Figure 5, there is shown a method for concentrating spent Bayer process liquor in accordance with a fourth embodiment of the present invention. The methods of 20 Figures 1, 2, 3 and 4 are substantially similar and like numerals denote like steps and features. In the present embodiment, a portion 100 of the flue gas 12 is passed through a gas separator 102 to strip off at least a portion of the water vapour and provide a permeate of water vapour 104 and a retentate stream of dehydrated gas 106. 25 The permeate 104 is combined with the portion 107 of the flue gas by-passing the gas separator 102 to provide a stream of 108 providing a stream with increased -19 water vapour, thereby increasing the dewpoint. The temperature of the water stream 20 exiting the gas-liquid contactor 14 is increased. Those skilled in the art will appreciate that the embodiment described herein is susceptible to variations and modifications other than those specifically described. 5 In one example of the embodiment, hot humidified calciner flue gas 100 at 165 *C with a flow rate of approximately 141 t/hr is passed into a membrane gas separator 102, comprising for example, a Dupont Nafion membrane, to produce a permeate of water vapour 104 of 54.5 t/hr and a retentate stream of dehydrated gas 106 of 86.5 t/hr and 160 *C. The permeate of water vapour 104 is combined 10 with the hot humidified calciner flue gas stream 107 with a flow rate of approximately 61 t/hr to provide a stream 108 with a vapour content of approximately 69 %. The combined stream 108 is quenched by direct contact with a mist spray 50 typically at approximately 50 kL/hr before being fed to the gas-liquid contactor 14 at approximately 106 kPa. The warm water stream 20 15 exits the gas-liquid contactor 14 at approximately 88 *C and is passed to the falling film evaporator 24 generating approximately 77 t/hr evaporation 36 on a spent liquor feed 28 of approximately 2303 t/hr. The cooled dehumidified flue gas 18 leaving the gas-liquid contactor is combined with the retentate stream of dehydrated gas 106 producing a flue gas exhaust 113 20 of 129 t/hr with an approximate dew point of 50 *C and a temperature of about 129 *C, sufficient for acceptable dispersion. In Figure 6, there is shown a method for concentrating spent Bayer process liquor in accordance with a fifth embodiment of the present invention. The methods of Figures 1, 2, 3, 4 and 5 are substantially similar and like numerals denote like 25 steps and features. The fifth embodiment differs from the fourth embodiment by the heating of the warm water stream 20 exiting the gas-liquid contactor 14 with waste heat 60 from the calciner, or other available heat sources 64, before being passed to the falling film evaporator 34.

- 20 One example of the embodiment uses the heat from the alumina coolers 60 to increase the temperature of the warm water stream 20 leaving the gas-liquid contactor 14 to 90.5 *C in the warm water stream 61 entering the falling film evaporators 24 generating more than 80 t/hr of evaporation 36. 5 In Figure 7, there is shown a method for concentrating spent Bayer process liquor in accordance with a sixth embodiment of the present invention. The methods of Figures 1, 2, 3, 4, 5 and 6 are substantially similar and like numerals denote like steps and features Hot humidified calciner flue gas 12, typically at about 165 *C and atmospheric 10 pressure with a flow rate of approximately 202 t/hr is passed into a gas-gas heat exchanger 110, where it is indirectly contacted with cooled dehumidified flue gas 18. It is envisaged that a flow rate of approximately 139 t/hr of cooled dehumidified flue gas at approximately 64 *C is fed to the gas-gas heat exchanger 110 15 In the gas-gas heat exchanger 110, a portion of the sensible heat is recovered from the hot humidified calciner flue gas 12 thereby cooling the hot humidified calciner flue gas 12 to form a stream of warm humidified calciner flue gas 112, at approximately 125 OC. The cooled dehumidified flue gas 18, after acquiring sensible heat from the hot 20 humidified calciner flue gas 12, is heated to approximately 135 *C before being released to the atmosphere at a rate of 139 t/hr, as a stream of hot dehumidified calciner flue gas 114. Contacting the hot humidified calciner flue gas 12 with the cooled dehumidified flue gas 18, thereby heating the cooled dehumidified flue gas 18, substantially 25 increases the buoyancy of such and enables the flue gas plume to properly disperse when it is being released to the atmosphere. The warm humidified calciner flue gas 112 after exiting from the gas-gas heat exchanger 110 at a flow rate of 202 t/hr is quenched by direct contact with a mist - 21 spray 50 typically at approximately 100 kL/hr before being fed to the gas-liquid contactor 14 where it is directly contacted with the cooled water stream 16 with a feed rate of approximately 1800 t/hr at approximately 62 *C. Latent heat recovered from the warm humidified flue gas 112 is transferred to the cooled 5 water stream 16, thereby heating the cooled water stream 16 to such an extent that at least a portion of the water vapour of the warm humidified calciner flue gas 112 condenses to liquid water, so producing the cooled dehumidified flue gas 18 and the warm water stream 20. A large portion of the warm water stream 20 exits the gas-liquid contactor 14 at a 10 flow rate of approximately 1864 t/hr, and is passed to the falling film evaporator 24 by way of the water pump 26. The warm water stream 20 enters the shell portion of the falling film evaporator 24, at a flow rate of approximately 1864 t/hr and concentrated Bayer spent liquor 30 exits the falling film evaporator 24. The warm water stream 20, after leaving 15 the falling film evaporator 24, is recycled as a cooled water recycled stream 32.

Claims

1. A method for concentrating spent Bayer process liquor, the method comprising the steps of: recovering heat from a Bayer process calciner flue gas by directly 5 contacting the calciner flue gas with a body of fluid and heating the body of fluid; applying at least a portion of the recovered heat to the spent Bayer process liquor; and evaporating water from the spent Bayer process liquor, thereby 10 concentrating the spent Bayer process liquor.

2. A method for concentrating spent Bayer process liquor, the method comprising the steps of: recovering heat from a Bayer process calciner flue gas by contacting the calciner flue gas with a body of fluid, thereby heating the body of fluid; 15 applying at least a portion of the recovered heat to the spent Bayer process liquor by contacting the spent Bayer process liquor with the heated body of fluid, thereby applying heat to the spent Bayer process liquor and cooling the body of fluid; and evaporating water from the Bayer spent process liquor, thereby 20 concentrating the spent Bayer process liquor.

3. A method for concentrating spent Bayer process liquor according to claim 1 or claim 2, wherein the method comprises the steps of: - 23 contacting the cooled body of fluid with Bayer process calciner flue gas, thereby recovering heat from the calciner flue gas and heating the body of fluid; applying at least a portion of the recovered heat to further spent Bayer 5 process liquor by contacting the further spent Bayer process liquor with the heated body of fluid, thereby applying heat to the further spent Bayer process liquor and cooling the body of fluid; and evaporating water from the further Bayer spent process liquor, thereby concentrating the further spent Bayer process liquor. 10

4. A method for concentrating spent Bayer process liquor according to any one of the preceding claims, wherein the calciner flue gas is directly contacted with the body of fluid in a counter-current gas-liquid contactor.

5. A method for concentrating spent Bayer process liquor according to any one of the preceding claims, wherein the calciner flue gas is directly contacted 15 with the body of fluid at or near atmospheric pressure.