WO2009114910A1

WO2009114910A1 - Method of concentrating a bayer process liquor

Info

Publication number: WO2009114910A1
Application number: PCT/AU2009/000328
Authority: WO
Inventors: Dean Ilievski; Peter Stewart Hay
Original assignee: Alcoa Of Australia Limited
Priority date: 2008-03-18
Filing date: 2009-03-19
Publication date: 2009-09-24
Also published as: AU2009225953A1; AU2009225953B2; CN102015048A

Abstract

A method for concentrating spent Bayer process liquor, the method comprising the steps of: Passing at least a portion of a calciner flue gas to a separator to provide a dehydrated gas stream and a water vapour enriched stream Contacting the spent Bayer process liquor with the water vapour enriched stream; and Evaporating water from the spent Bayer process liquor, thereby concentrating the spent Bayer process liquor.

Description

Method for 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 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- 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.

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 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:

2AI(OH)₃ → AI₂O₃ +3H₂O

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 calciner flue gas temperatures and compositions may occur; and it is estimated that approximately 35% to 45% 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 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 atmospheric conditions as the dew point is less than 100 ⁰C, typically 80 to 83 ⁰C. 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 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

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 understood to encompass gas streams in which all or a portion of the water vapour has been removed. 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 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 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. 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 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 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.

Specific - A -

In accordance with one aspect of the present invention, there is provided a method for concentrating spent Bayer process liquors, the method comprising the steps of:

passing at least a portion of a calciner flue gas to a separator to provide a dehydrated gas stream and a water vapour enriched stream;

contacting the spent Bayer process liquor with the water vapour enriched stream; and

evaporating water from the spent Bayer process liquor, thereby concentrating the spent Bayer process liquor.

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 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 process, and not yet been recycled back to the digestion phase. Spent liquors may also include certain caustic wash liquors. It will be appreciated that the invention may also be applied to other Bayer liquor streams, and not only spent liquor. It will also be appreciated that the method of the invention may be used, in certain embodiments, not to evaporate water from the liquor streams, but to solely heat such liquor streams as desired or required.

Preferably, the water vapour enriched stream comprises at least 60 % water vapour. More preferably, the water vapour enriched stream comprises at least 80 % water vapour. More preferably still, the water vapour enriched stream comprises at least 90 % water vapour. More preferably still, the water vapour enriched stream comprises at least 95 % water vapour. More preferably still, the water vapour enriched stream comprises at least 99 % water vapour. More preferably still, the water vapour enriched stream comprises 100 % water vapour.

In one form of the invention, the method comprises the further step of:

heating the spent Bayer process liquor with a further heat source.

It will be appreciated that the step of contacting the spent Bayer process liquor with the water vapour enriched stream heats the spent Bayer process liquor, while evaporation may then be accomplished using other sources of heat, such as plant steam or other external heat sources. Alternatively, the spent Bayer process liquor may be contacted with the other sources of heat prior to contact with the water vapour enriched stream. Alternatively still, the spent Bayer process liquor may be contacted with the other sources of heat at the same time as contact with the water vapour enriched stream. Accordingly, an additional step between contacting the process liquor and evaporating water therefrom may include heating the water vapour enriched stream using heating means the water vapour enriched stream to accomplish or assist with evaporation.

Preferably, the step of:

passing at least a portion of a calciner flue gas to a separator to provide a dehydrated gas stream and a water vapour enriched stream,

comprises the step of:

passing at least a portion of the calciner flue gas through a membrane to provide a dehydrated gas stream retentate and a water vapour enriched permeate.

It will be appreciated that the membrane may be a membrane selective for water vapour over other components of the flue gas such as nitrogen, oxygen, carbon dioxide, carbon monoxide, nitrogen oxides (NOx), sulphur oxides (SOx), and volatile organic compounds (VOCs).

Given the magnitude of calciner flue gas flows, economic considerations require that the membrane may also be permeable with respect to water vapour, permitting high water vapour fluxes.

In addition, the membrane should be able to function at the temperature at which the flue gas is added. It will be appreciated that whilst different temperatures may be encountered in different Bayer circuits, calciner flue gases are generally in the range of 120 ⁰C to 195 ⁰C. Further, it will be appreciated that the calciner flue gases may be compressed to increase pressure or heated to increase temperature.

Other factors affecting the choice of membrane include the mechanical, dimensional and chemical stability, resistance to fouling and poisoning and membrane lifetime. It will be appreciated that the membrane should be substantially chemically and physically stable in the presence of caustic materials.

It will be appreciated that the degree to which water vapour is partitioned between the permeate and the retentate will depend on many factors, including the type of membrane, the size of the membrane device, the composition of the feed, the temperature and the pressure.

The skilled addressee will appreciate that a wide variety of membranes may be used with the present invention. These include microporous membranes, where separation of the gas components is based on the physical size of the gas component molecules or their mean free paths, and nonporous membranes, where separation arises due to differences in the molecules' affinities with the membrane surface and the diffusion rate through the membrane.

In a specific form of the invention, hydrophilic nonporous membranes are used for the separation of the water vapour from the calciner flue gas. Non-limiting examples of nonporous membranes are those provided by Du Pont under the Nafion name. This class of ionomer materials comprise an inert and strong backbone of a tetrafluoroethylene polymer, providing good mechanical and chemical resistance, interdispersed with sulphonic acid group chains that are hydrophilic, polar and ionic in character.

The high selectivity and permeability of Nafion membranes with respect to water vapour is due to the efficiency with which sulfonic acid groups at the membrane surface bind water and then transfer it to adjoining sulphonic acid groups through the membrane. There is no physical hole and separation is by an adsorption- diffusion-evaporation mechanism rather than by separating compounds due to their molecular size. Only compounds that associate strongly with sulphonic acid groups will readily permeate through the membrane. Thus, the other main components of a calciner flue gas, viz. nitrogen, oxygen, carbon dioxide, carbon monoxide, nitrogen oxide, sulphur oxides will not permeate.

Advantageously, Nafion membranes are stable at high temperatures (up to 190 ⁰C) and the initial rate of water absorption increases with an increase in temperature.

In highly specific forms of the invention, the membrane is a Nafion 112 or a Nafion 117 membrane.

Polyvinyl alcohol (PVA) membranes represent another class of non-limiting examples of nonporous membranes that exhibit hydrophobicity, good selectivity for water and good chemical stability. The properties of PVA membranes may be controlled by cross-linking, blending, additive addition and other treatments, e.g. heat treatment. Such treatments may include dialdehydes, such as glutaraldehyde, or other cross-linking agents.

Other non-limitiήg examples of hydrophilic membrane materials include polyacrylonitryle, cellulose acetate, and polyvinylpyrrolidone. Aliphatic polyamides, aromatic polyamides and polyimides membranes also find application in this invention. Other examples of applicable types of membranes include silicon rubbers, e.g. polydimethylsiloxane, and ion-exchange polymers, e.g. polystyrene sulphonates.

In an alternate form of the invention, the step of:

comprises the step of:

passing at least a portion of the calciner flue gas through a porous membrane to provide a dehydrated gas stream retentate and a water vapour enriched permeate.

Such materials operate on a different separation principle to non-porous membranes and examples include molecular sieves and porous forms of polytetrafluoroethylene (e.g. Gortex^®).

Apparatus using membranes to separate specific components of a gaseous feed stream into retentate and permeate streams are known to persons skilled in the art and include spiral wound modules, plate and frame modules, hollow fibre modules, and tubular membrane modules. It will be appreciated that the membrane modules may be arranged in different combinations and have different flow patterns, for example countercurrent and crossflow.

Preferably, the step of:

contacting the spent Bayer process liquor with the water vapour enriched stream;

comprises indirectly contacting the spent Bayer process liquor with the water vapour enriched stream. Apparatus allowing the spent Bayer process liquor to be contacted by indirect contact with the water vapour enriched stream are known to persons skilled in the art, and include vertical tube falling film, horizontal tube falling film, vertical rising film, kettle boiler and forced circulation apparatus.

In one form of the invention, the step of indirectly contacting the spent Bayer process liquor with the water vapour enriched stream utilizes a falling film evaporator having a shell-side and a tube-side, and the method comprises the steps of:

introducing the water vapour enriched stream into the shell-side of the falling film evaporator;

introducing the spent Bayer process liquor into the tube side of the falling film evaporator;

thereby applying heat to the spent Bayer process liquor and condensing the water vapour in the water vapour enriched stream.

It is desirable that the water vapour enriched stream be delivered at the highest possible temperature and pressure. In one form of the invention, the water vapour enriched stream from the separator is compressed before being introduced to the evaporator. The compression may be by mechanical compression or thermal compression, e.g. steam ejectors. In another form of the invention, the water vapour enriched stream from the separator is heated, either directly or indirectly. In a further form of the invention, the water vapour enriched stream from the separator is both heated and compressed.

The water vapour enriched stream may be heated by utilising sources of heat available within the Bayer circuit or external sources available from, for example, power stations or other industrial facilities. It will be appreciated that the heat may be added directly or indirectly to the water vapour enriched stream. It will be appreciated by those skilled in the art that a variety of heat sources in a Bayer circuit may be utilised. 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.

The plant steam may be combined directly with the water vapour enriched stream. The plant steam may be combined with the water vapour enriched stream before or after compression of the water vapour enriched stream to increase the pressure or after heating to increase the steam temperature.

In one form of the invention, water vapours from other sources in the plant or power plant may be combined directly with water vapour enriched stream. The water vapours may be derived from sources such as digestion, blow off tanks and flash tanks. The water vapours from other sources may be added before or after compression of the water vapour enriched stream, to increase the pressure; or before or after heating, to increase the steam temperature.

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 such apparatus may be arranged in series or parallel. It will be appreciated that where the apparatus for applying heat to the spent Bayer process liquor are arranged in series, the water vapour enriched stream from each unit may be compressed to increase the pressure, or heated directly or indirectly to increase the temperature, or both heated and compressed.

In one form of the invention, the method comprises the additional step of:

passing the calciner flue gas through a gas cleaning unit,

prior to the step of:

passing at least a portion of a calciner flue gas to a separator to provide a dehydrated gas stream and a water vapour enriched stream

The gas cleaning unit may comprise dust removal means. Calciner flue gas may contain entrained particulate alumina and it will be appreciated that it may be advantageous to remove said entrained particulate alumina from the calciner flue gas prior to passing calciner flue gas to the separator as the entrained particulate alumina may adversely affect the separation process.

Calciner flue gas may also contain volatile organic carbon compounds, the release of which into the atmosphere is highly environmentally undesirable. It will be appreciated that it may be advantageous to remove said volatile organic carbon compounds from the calciner flue gas prior to passing the calciner flue gas to the separator.

In another embodiment of the invention, the gas cleaning unit is a two-phase contacting tower, or any other two phase contacting arrangement, in which the calciner flue gas is directly contacted by water. This has the additional advantage of increasing the water vapour content of the feed to the membrane unit, hence increasing the driving force for separation. A further advantage is that cooling of the calciner flue gas prior to the compressor is effected, which reduces compressor power requirements and consumption.

It will be appreciated that, depending on the nature of the separator, some compounds, such as volatile organic carbon compounds, may be passed to the water vapour enriched stream. For example, ammonium hydroxide and some aldehydes and ketones after acid catalysis to alcohols may be transferred by Nafion membranes.

The substantial dehumidification of the calciner flue gas in terms of the present invention 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 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 reduced or eliminated. Further, the method of the present invention allows recovery of water that would otherwise be lost to the atmosphere.

Advantageously, the dehydrated gas stream should have sufficient pressure and temperature to be conveyed to a stack without a fan and to disperse effectively without the need for further heating.

Advantageously, the dehydrated gas stream is sufficiently pressurised for energy to be recovered by turbomachinery, such as a turbine or other means known in the art, and such that the stream may be conveyed to a stack without need for additional pressurisation means or turbines. This also allows the stream to disperse effectively without the need for further heating.

In one form of the invention, the method comprises the additional step of:

increasing the pressure of the calciner flue gas,

prior to the step of:

Increasing the pressure of the calciner flue gas can be achieved by any means known in the art, including turbomachinery.

In one form of the invention, the method comprises the additional step of:

passing only a portion of the calciner flue gas prior to the step of compression to the permeate side of the membrane module

followed by the step of:

passing the resultant permeate product to a second membrane separation unit for further separation. In a preferred embodiment, the steps of the method are repeated to provide a cyclical and/or continuous process.

Brief Description of the Drawings

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 a first embodiment of the present invention;

Figure 2 is a schematic flow sheet showing a method in accordance with a second embodiment of the present invention;

Figure 3 is a schematic flow sheet showing a method in accordance with a third embodiment of the present invention;

Figure 4 is a schematic flow sheet showing a method in accordance with a fourth embodiment of the present invention; and

Figure 5 is a schematic flow sheet showing a method in accordance with a fifth 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 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 features.

In Figure 1 , there is provided a generic flow sheet showing how a method in accordance with the present invention is utilized in one embodiment by: passing at least a portion of a calciner flue gas to a separator to provide a dehydrated gas stream and a water vapour enriched stream;

An additional step between contacting the process liquor and evaporating water therefrom includes heating the water vapour enriched stream to increase evaporation. In accordance with the present invention, humidified calciner flue gas 12 from a calciner 36 is passed to a gas cleaner 13 and a membrane section 14 to provide a dehydrated gas stream 16 and a water vapour enriched stream 18. The membrane section 14 comprises an arrangement of individual membrane modules, in series or parallel, with subsequent membrane modules connected to either the permeate, retentate, or feed streams of previous membrane modules. The streams connecting to the individual membrane modules are heated, cooled or compressed by means known in the art. The individual membrane modules provide a suitable flow configuration, such as those known in the art, e.g. counter- current . The dehydrated gas stream 16 goes to a stack 38. In a preferred form, energy is recovered from the dehydrated gas stream 16.

The water vapour enriched stream 18 exits the membrane 14 and is passed to an evaporation system 24. The evaporation system 24, in certain embodiments, is a single evaporator or any combination of evaporator units. In preferred forms of the invention, the evaporator 24 is a multiple effect arrangement of falling film evaporators. Furthermore, it is advantageous in certain embodiments to compress the water vapour enriched stream 18 prior to introducing it into the evaporation system 24. Additionally, it can be advantageous to add heat, either directly or indirectly, to the water vapour enriched stream 18 prior to introducing it into the evaporation system 24. The direct heat addition to the water vapour enriched stream 18 may be in the form of plant steam, and may be added as part of thermocompression of the water vapour stream 18. The heating sections of the individual evaporators comprising the evaporation system 24, are comprised of a shell portion (not shown) and tube portion (not shown). The water vapour 18 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 spent liquor 28 entering as a liquor film via the tube portion, and thereby concentrate the Bayer spent liquor 28. Concentrated Bayer spent liquor 30 exits the falling film evaporator 24. The condensed water vapour enriched stream 33, after leaving the falling film evaporator 24, may be recycled as a cooled water recycled stream. The water vapour evaporated from the spent liquor 43 may be condensed and may be recycled, or it may be channelled to provide other heating duties within the Bayer process.

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 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.

The heat transfer equipment, (the falling film evaporator and vapour condenser), can be designed by a person skilled in the art, by using any standard heat exchanger design text, or references from the technical literature, or by a supplier of evaporation equipment (GEA Kestner, France, Bertrams Salt Plants, Winterthur, Switzerland).

Gas membrane modules are known to persons skilled in the art and may be obtained from membrane equipment suppliers such as, for example. Sulzer ChemTech, Switzerland; BORSIG, Germany, PALL Corporation, USA; Koch Membrane Systems, USA; Membrane Technology & Research, USA. The membrane modules can be spiral wound, plate and frame, hollow fibre or tubular membrane modules, depending on the application thereof. Membranes are produced commercially (e.g. by Du Pont, Dow Chemicals, UBE, and GE). In Figure 2, there is shown a method for concentrating spent Bayer process liquor in accordance with a second embodiment of the invention. The methods of Figures 1 and 2 are substantially similar and like numerals denote like steps and features. In this embodiment there is provided a plurality of evaporators 24, 40 and 42 operating in series. The vapour stream from one evaporator 43 is sent to the shell side of the heat exchanger comprising the next evaporator 40, 42, in a, so-called, multiple effect evaporation arrangement to economize on energy usage in evaporation. The flow configuration may be any of backward-feed, forward- feed, mixed-feed, parallel-feed, or any other configuration. Furthermore, the vapour stream to each evaporator, either as the vapour stream 43 from a preceding evaporator or as the vapour rich stream 18 from the membrane separator, may be compressed to increase the pressure or heated to increase temperature. Heating of vapour streams 18 and 43 may be direct or indirect. The direct heat addition to vapour streams 43 may be in the form of plant steam or water vapours from other sources in the plant or power plant. The water vapours may come from sources such as digestion, blow off tanks and flash tanks. Some of the evaporators in the arrangement may be solely heated by plant steam. The spent Bayer process liquor streams 28 and 45 may be heated or cooled prior to addition to an evaporator.

The following examples serve to more fully describe the manner of using the above-described invention. It is understood that these examples in no way serve to limit the true scope of this invention, but rather are presented for illustrative purposes.

The examples presented below of various embodiments of the invention were formulated, evaluated and refined using a combination of in-house models built on chemical engineering first principles and tuned to existing Bayer unit operations, an extensive database of Bayer properties and thermodynamic data, Bayer operating experience and flowsheet models built within ASPEN Plus™, ASPEN Technology Inc. software process simulation software with state-of-the art physical properties packages, including added Bayer process properties and unit operations built inhouse. Quantitative membrane performance parameters were taken primarily from the results of a experimental testing program sponsored by Alcoa of Australia and conducted by the CSIRO, in which commercial gas permeation membranes and CSIRO developed membranes (e.g. SR10120) were tested with an artificial flue gas, composition matching a typical Alcoa calciner flue gas, at different temperatures and pressures. 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 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 method.

Example 1: Single membrane section and three effect evaporation of spent liquor

An example of a second embodiment is shown in Figure 2, comprising a single membrane stage and three effect evaporation. In this example, calciner flue gas 12 (165 ⁰C, 41 % ^w/_w water vapour, 202 t/hr) from a calciner 36 is passed to the membrane at a feed pressure of 2.55 bar and separated into a dehydrated gas stream 16 (171.5 t/hr) and a water vapour enriched stream 18 (30.5 t/hr). The dehydrated stream 16 contains goes to an expander or turbine (not shown) to recover some energy and is then passed to the stack 38; its dewpoint is 78 ⁰C. The 30.5 t/hr water vapour enriched stream 18 is superheated to a temperature of 150⁰C from various available waste heat sources (not shown) and sent to a shell side of the heat exchanger section of the first of a series of three falling film evaporators 24, 40, 42. The evaporated water vapour streams 43 are compressed prior to being sent to the heat exchanger sections of evaporators 40, 42, requiring 1.6 MW. The total evaporation achieved is 115 t/hr. The spent liquor feed 28 enters the first evaporator 24 at 61 ⁰C and at a rate of 2303 t/hr. The concentrated Bayer spent liquor 30 exits at 2188 t/hr. The total alkalinity (TA) and total caustic (TC) concentrations increase, respectively, from 238 g/L, as Na₂CO₃, and 195.2 g/L, as Na₂CO₃, in the feed 28, to 254.2 g/L and 208.5 g/L in the concentrated spent liquor steam 30. If the membrane feed pressure were increased to 3.5 bar then the total evaporation increases to 180 t/h. The TA and TC concentrations increase in the concentrated spent liquor steam 30 to 264.4 g/L, as Na₂CO₃, and 216.8 g/L, as Na₂CO₃, respectively. The cost for this extra evaporation is additional compression power.

Example 2: Single membrane section with wet scrubbing of calciner flue gas and three effect evaporation of spent liquor

Another example of the second embodiment is illustrated in Figure 3. It is substantially similar to the case presented in Example 1 with the difference that the calciner flue gas 12 (165 ⁰C, 41 % ^w/_w water vapour, 202 t/hr) is cleaned by wet scrubbing 13 with water 10 prior to a compression system 50, which is coupled to an expander 52 for energy recovery. Wet scrubbing achieves a number of benefits including dust removal, further humidifying the membrane feed and cooling the feed to the compressor unit. The stream 15 leaving the wet scrubber 13 has a temperature of 85 ⁰C and a moisture content of 43.8 wt%. This stream is compressed to 2.55 bar using a compressor or blower 50. The temperature of the membrane feed stream 17 is reduced prior to addition to the membrane unit 14 by exchanging heat with other streams, including the water vapour enriched stream 18, raising its temperature to 160 ⁰C.

The membrane section 14 directs 40.3 t/h of steam to a three effect evaporation arrangement comprising evaporators 24, 40 and 42, respectively. Total evaporation of 148 t/h of evaporation is achieved from the spent liquor feed 28, which enters the first evaporator 24 at 61 ⁰C and at a rate of 2303 t/hr. The concentrated Bayer spent liquor 30 exits at 59 ⁰C. The total alkalinity (TA) and total caustic (TC) concentrations increase, respectively, from 238 g/L, as Na₂CO₃, and 195.2 g/L, as Na₂CO₃, in the feed 28 to 259.3 g/L and 212.7 g/L in the concentrated spent liquor steam 30. The dehydrated stream 16 is passed through an expander 52 for energy recovery purposes and is then passed to the stack 38; its dewpoint is 78 ⁰C. If the membrane feed pressure is increased to 3.5 bar, then the total evaporation increases to 219 t/h. The TA and TC concentrations increase in the concentrated spent liquor steam 30 to 271.1 g/L, as Na₂CO₃, and 222.2 g/L, as Na₂CO₃, respectively.

Figure 4 shows a method for concentrating spent Bayer process liquor in accordance with a third embodiment of the invention. The methods of Figures 1 to 3 are substantially similar and like numerals denote like steps and features. In accordance with the present invention, humidified calciner flue gas 12 from a calciner 36 is passed to a wet scrubber 13 and a compressor or blower 50, thereafter to a first membrane section 14. The temperature of the membrane feed stream 17 is reduced prior to addition to the membrane unit 14. In a specific form of the invention, this is done by exchanging heat with the water vapour enriched stream 18 to increase its temperature or with the dehydrated gas stream 32 from the second membrane unit to increase its temperature.

The dehydrated gas stream 21 from the first membrane section is passed to a second wet scrubber 56 and a second compressor 54. The temperature of the membrane feed stream 58 is reduced to the second membrane section 34. In a specific form of the invention this is done by exchanging heat with the water vapour enriched stream 19 to increase its temperature. The dehydrated gas stream 32 from the second membrane unit 34 is passed to an expander 52 for energy recovery and then to the stack 38. The two water vapour enriched steams 18 and 19 are combined 20 and sent to an evaporation section, such as has been described above. Streams 18, 19 and 20, in one embodiment, are compressed and are heated, either directly or indirectly.

Example 3: Two membrane sections with wet scrubbing and three effect evaporation

In an example of the embodiment illustrated in Figure 4, the calciner flue gas 12 (165 ⁰C, 41 % ^w/_w water vapour, 202 t/hr) is cleaned by wet scrubbing 13 with water 10 and compressed by a compressor or blower 50, to 2.55 bar. The temperature of the membrane feed stream 17 is reduced to 180 ⁰C prior to addition to the membrane unit 14, by exchanging heat with the water vapour enriched stream 18 to increase its temperature to 161 ⁰C. The flow of the membrane feed stream 17 is 211.7 t/h.

The water vapour enriched stream 18 from the first membrane section 14 sends 40.3 t/h of vapour steam to a header arrangement (not shown), which also receives 38.8 t/h of steam from the water vapour enriched stream 19 from the second membrane section 34.

The 171.4 t/h of the dehydrated stream 21 from the first membrane section is passed to a water quench 55, which results in an increased flow rate to 178.7 t/h at a temperature of 103.7 ⁰C, which goes to a compressor 54 raising the pressure to 5 bar. It is then passed to the second membrane section. The 139.9 t/h of the dehydrated stream 32 from the second membrane section is passed through an expander 52 to recover some energy and is then passed to the stack 38; its dewpoint is 62.5 ⁰C.

The combined water vapour enriched stream 20 has a steam flow of 79.1 t/h at 132 ⁰C, which goes to a three effect evaporation arrangement operating comprising evaporators 24, 40 and 42, respectively. A total evaporation of 263 t/h of evaporation is achieved from the spent liquor feed 28, which enters the first evaporator 24 at 61 ⁰C and at a rate of 2303 t/hr. Approximately 2039 t/h of concentrated Bayer spent liquor 30 exits at 59.6C⁰. The total alkalinity (TA) and total caustic (TC) concentrations of the spent liquor increase, respectively, from 238 g/L, as Na₂CO₃, and 195.2 g/L, as Na₂CO₃, in the feed 28, to 279 g/L and 229 g/L in the concentrated spent liquor steam 30.

Figure 5 shows a method for concentrating spent Bayer process liquor in accordance with a fourth embodiment of the invention.

In accordance with this fourth embodiment of the present invention, humidified calciner flue gas 12 from a calciner 36 is passed to a wet scrubber 13. The humidified gas stream 12 is split with a bypass portion 15 and sent to the permeate side of the first membrane section 14 to act as a sweep gas, with the remainder being sent or diverted to the compressor or blower 50. The compressed gas 17 is then sent to the feed side of the first membrane section 14. The temperature of the membrane feed stream 17 is reduced prior to addition thereof to the first membrane unit 14. In a specific embodiment of the invention, this is done by exchanging heat with the bypass stream 15 or the water vapour enriched stream 19 from the second membrane section 34, or with the dehydrated gas stream 16. The dehydrated gas stream 21 from the first membrane section is sent to the stack 38. In one form of the invention, the dehydrated gas stream 21 is sent to the stack via an expander 52 to recover some energy.

The water vapour enriched stream 18 is sent, via a compressor or blower 54 to the second membrane section 34. The water vapour enriched stream 19 from the permeate side of the second membrane 34 is sent to an evaporation section, such as has been described above. The water vapour enriched stream 19, in certain embodiments, is compressed and is heated, either directly or indirectly. The dehydrated gas stream 32 is sent to the stack 38; in one form of the invention it is sent via an expander to recover some energy.

Example 4: Two membrane sections, one with a sweep gas, wet scrubbing and three effect evaporation

In an example of the embodiment illustrated in Figure 5, the calciner flue gas 12 (165 ⁰C, 41 % ^w/_w water vapour, 202 t/hr) is cleaned by wet scrubbing 13 with water 10. The cleaned and humidified gas is split, with approximately 80 % thereof compressed using the blower 50 to 2.55 bar and sent to the feed side of the first membrane section 14. The remaining 20% is sent directly to the permeate side of the first membrane section, where it acts as a sweep gas. Approximately 84.7 t/h of the water vapour enriched stream 18 leaves the first membrane section with a water vapour content of 80.9 vol%. It is compressed 54 to a pressure of 262 kPa and the compressed gas 58 is cooled by heat exchange (not shown) with the water vapour enriched stream 19 leaving the second membrane section 34. The cooled compressed gas 58 is sent to the second membrane section 34. The water vapour enriched stream 19 flow of 50.6 t/h and at 149 ⁰C is sent to a three effect evaporation section, as described above. A total evaporation of 179.5 t/h of evaporation is achieved from the spent liquor feed 28.

Example 5: One membrane section, sub-atmospheric operation on permeate side, wet scrubbing and three effect evaporation

In a further embodiment based on the system illustrated in Figure 2, the calciner flue gas 12 (165 ⁰C, 41 % ^w/_w water vapour, 202 t/hr) is cleaned by wet scrubbing (not shown), increasing its flow to 211.7 t/h and water vapour content to 43.8 wt%, following which it is sent to the membrane section 14. On the permeate side, 59.3 t/h of water vapour enriched stream 18 is drawn at 0.3 bar through a compressor (not shown) and raised to 1 bar; it is then sent to a three effect evaporation system, as described hereinbefore. A total evaporation of 207 t/h of evaporation is achieved from the spent liquor feed 28, which enters the first evaporator 24 at 61 ⁰C and at a rate of 2303 t/hr. The total alkalinity (TA) and total caustic (TC) concentrations of the spent liquor increase, respectively, from 238 g/L, as Na₂CO₃, and 195.2 g/L, as Na₂CO₃, in the feed 28 to 269 g/L and 220.6 g/L in the concentrated spent liquor steam 30.

Claims

The Claims Defining the Invention are as Follows:

1. A method for concentrating spent Bayer process liquor, the method comprising the steps of:

2. The method of claim 1 , wherein the method comprises the further step of:

heating the spent Bayer process liquor with a further heat source.

3. The method of claim 2, wherein the step of heating the spent Bayer process liquor with a second heat source is conducted either prior to the step of contacting the spent Bayer process liquor with the water vapour enriched stream, after the step of contacting the spent Bayer process liquor with the water vapour enriched stream or during the step of contacting the spent Bayer process liquor with the water vapour enriched stream.

4. The method of any one of claims 1 to 3, wherein the step of:

comprises the step of: passing at least a portion of the calciner flue gas through a membrane to provide a dehydrated gas stream retentate and a water vapour enriched permeate.

5. The method of claim 4, wherein the membrane is selected from microporous membranes and nonporous membranes.

6. The method of claim 5, wherein the membrane is a nonporous membrane made of a material selected from the group consisting of tetrafluorethylene, polyvinyl alcohol (PVA), polyacrylonitrile, cellulose acetate, polyvinylpyrrolidone, aliphatic polyamide, aromatic polyamide, polyimide, silicon rubber, polydimethylsiloxane, polystyrene sulphonate and ion- exchange polymers.

7. The method of claim 6, wherein the nonporous membrane is a membrane prepared with polyvinyl alcohol.

8. The method of claim 6, wherein the nonporous membrane is a membrane provided by Du Pont under the Nation name such as a Nafion 112 or a Nafion 117 membrane.

9. The method of claim 5, wherein the porous membrane is a membrane selected from molecular sieves and porous forms of polytetrafluoroethylene.

10. The method of claim 5, wherein the porous membrane is a membrane made from a material available under the trade name Gortex^®.

11.The method of any one of claims 1 to 10, wherein the step of:

comprises the step of: indirectly contacting the spent Bayer process liquor with the water vapour enriched stream.

12. The method of claim 11 , wherein the step of contacting the spent Bayer process liquor with the water vapour enriched stream is accomplished using apparatus selected from the group consisting of: vertical tube falling film; horizontal tube falling film; vertical rising film; kettle boiler; and forced circulation apparatus.

13. The method of claim 12, wherein the step of indirectly contacting the spent Bayer process liquor with the water vapour enriched stream utilises a falling film evaporator having a shell-side and a tube-side.

14. The method of claim 13, which comprises the steps of:

introducing the water vapour enriched stream into the shell-side of the falling film evaporator; and

15. The method of any one of claims 1 to 14, which includes the step of compressing, heating or compressing and heating the water vapour enriched stream from the separator.

16. The method of any one of claims 10 to 15, wherein more than one apparatus for applying heat to the spent Bayer process liquor is utilised, such apparatus being arranged in series or parallel.

17. The method of claim 16, wherein, when the apparatus for applying heat to the spent Bayer process liquor are arranged in series, the water vapour enriched stream from each unit is compressed to increase the pressure, or heated directly or indirectly to increase the temperature, or both heated and compressed.

18. The method of any one of claims 1 to 17, comprising the additional step of:

passing the calciner flue gas through a gas cleaning unit,

prior to the step of:

19. The method of claim 18, wherein the gas cleaning unit comprises a two phase contactor that directly contacts the calciner flue gas with water.

20. The method of any one of claims 1 to 19 which comprises the additional step of:

increasing the pressure of the calciner flue gas,

prior to the step of:

passing at least a portion of a calciner flue gas to a separator to provide a dehydrated gas stream and a water vapour enriched stream.

21. The method of any one of claims 1 to 20, wherein the steps of the method are repeated to provide a cyclical process.

22. The method of any one of claims 1 to 20, wherein the steps of the method are repeated to provide a continuous process.

23.A method for concentrating spent Bayer process liquor, substantially as herein described with reference to the Figures.

24.A method for concentrating spent Bayer process liquor, substantially as herein described with reference to the Examples.