AU2008229997A1 - Method of Hydrothermal Treatment of Organic Contaminants of Alkaline Process Solutions - Google Patents
Method of Hydrothermal Treatment of Organic Contaminants of Alkaline Process Solutions Download PDFInfo
- Publication number
- AU2008229997A1 AU2008229997A1 AU2008229997A AU2008229997A AU2008229997A1 AU 2008229997 A1 AU2008229997 A1 AU 2008229997A1 AU 2008229997 A AU2008229997 A AU 2008229997A AU 2008229997 A AU2008229997 A AU 2008229997A AU 2008229997 A1 AU2008229997 A1 AU 2008229997A1
- Authority
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- Australia
- Prior art keywords
- solution
- bayer process
- wet oxidation
- process solution
- feed
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
- 238000000034 method Methods 0.000 title claims description 89
- 239000000356 contaminant Substances 0.000 title claims description 59
- 230000008569 process Effects 0.000 title claims description 31
- 238000010335 hydrothermal treatment Methods 0.000 title claims description 8
- 239000000243 solution Substances 0.000 claims description 226
- 238000004131 Bayer process Methods 0.000 claims description 165
- 238000009279 wet oxidation reaction Methods 0.000 claims description 72
- CDBYLPFSWZWCQE-UHFFFAOYSA-L Sodium Carbonate Chemical compound [Na+].[Na+].[O-]C([O-])=O CDBYLPFSWZWCQE-UHFFFAOYSA-L 0.000 claims description 69
- 230000003197 catalytic effect Effects 0.000 claims description 67
- 239000003054 catalyst Substances 0.000 claims description 66
- 239000007800 oxidant agent Substances 0.000 claims description 42
- 230000003028 elevating effect Effects 0.000 claims description 36
- 239000012527 feed solution Substances 0.000 claims description 33
- 229910000029 sodium carbonate Inorganic materials 0.000 claims description 32
- 239000010949 copper Substances 0.000 claims description 30
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 claims description 29
- 229910052802 copper Inorganic materials 0.000 claims description 28
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 26
- 238000001816 cooling Methods 0.000 claims description 24
- 238000007254 oxidation reaction Methods 0.000 claims description 24
- 239000012670 alkaline solution Substances 0.000 claims description 23
- 230000003647 oxidation Effects 0.000 claims description 23
- 239000003518 caustics Substances 0.000 claims description 21
- 150000001875 compounds Chemical class 0.000 claims description 20
- 238000007865 diluting Methods 0.000 claims description 12
- 239000000047 product Substances 0.000 claims description 12
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 10
- 229910052799 carbon Inorganic materials 0.000 claims description 10
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims description 6
- 239000000203 mixture Substances 0.000 claims description 6
- 239000001301 oxygen Substances 0.000 claims description 6
- 229910052760 oxygen Inorganic materials 0.000 claims description 6
- 239000007795 chemical reaction product Substances 0.000 claims description 4
- JJLJMEJHUUYSSY-UHFFFAOYSA-L copper(II) hydroxide Inorganic materials [OH-].[OH-].[Cu+2] JJLJMEJHUUYSSY-UHFFFAOYSA-L 0.000 claims description 4
- AEJIMXVJZFYIHN-UHFFFAOYSA-N copper;dihydrate Chemical compound O.O.[Cu] AEJIMXVJZFYIHN-UHFFFAOYSA-N 0.000 claims description 2
- 235000017550 sodium carbonate Nutrition 0.000 description 30
- 238000001556 precipitation Methods 0.000 description 18
- 239000007789 gas Substances 0.000 description 17
- QPLDLSVMHZLSFG-UHFFFAOYSA-N Copper oxide Chemical compound [Cu]=O QPLDLSVMHZLSFG-UHFFFAOYSA-N 0.000 description 13
- 238000010790 dilution Methods 0.000 description 13
- 239000012895 dilution Substances 0.000 description 13
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 description 9
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 9
- 238000012360 testing method Methods 0.000 description 9
- BVKZGUZCCUSVTD-UHFFFAOYSA-L Carbonate Chemical compound [O-]C([O-])=O BVKZGUZCCUSVTD-UHFFFAOYSA-L 0.000 description 8
- 230000006378 damage Effects 0.000 description 8
- 230000009467 reduction Effects 0.000 description 8
- 239000002244 precipitate Substances 0.000 description 7
- 238000011282 treatment Methods 0.000 description 7
- JPVYNHNXODAKFH-UHFFFAOYSA-N Cu2+ Chemical compound [Cu+2] JPVYNHNXODAKFH-UHFFFAOYSA-N 0.000 description 6
- 230000000694 effects Effects 0.000 description 6
- 230000001590 oxidative effect Effects 0.000 description 6
- 238000004519 manufacturing process Methods 0.000 description 5
- 239000000463 material Substances 0.000 description 5
- 230000002829 reductive effect Effects 0.000 description 5
- 239000011734 sodium Substances 0.000 description 5
- 239000007787 solid Substances 0.000 description 5
- QGZKDVFQNNGYKY-UHFFFAOYSA-N Ammonia Chemical compound N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 description 4
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 4
- UIIMBOGNXHQVGW-UHFFFAOYSA-M Sodium bicarbonate Chemical compound [Na+].OC([O-])=O UIIMBOGNXHQVGW-UHFFFAOYSA-M 0.000 description 4
- 150000004645 aluminates Chemical class 0.000 description 4
- WNROFYMDJYEPJX-UHFFFAOYSA-K aluminium hydroxide Chemical compound [OH-].[OH-].[OH-].[Al+3] WNROFYMDJYEPJX-UHFFFAOYSA-K 0.000 description 4
- 229910021502 aluminium hydroxide Inorganic materials 0.000 description 4
- 238000006243 chemical reaction Methods 0.000 description 4
- 229910001431 copper ion Inorganic materials 0.000 description 4
- 230000029087 digestion Effects 0.000 description 4
- 238000010438 heat treatment Methods 0.000 description 4
- 230000001965 increasing effect Effects 0.000 description 4
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 4
- 150000003839 salts Chemical class 0.000 description 4
- ZNCPFRVNHGOPAG-UHFFFAOYSA-L sodium oxalate Chemical compound [Na+].[Na+].[O-]C(=O)C([O-])=O ZNCPFRVNHGOPAG-UHFFFAOYSA-L 0.000 description 4
- 229940039790 sodium oxalate Drugs 0.000 description 4
- 239000012855 volatile organic compound Substances 0.000 description 4
- 238000009825 accumulation Methods 0.000 description 3
- 239000004411 aluminium Substances 0.000 description 3
- 229910052782 aluminium Inorganic materials 0.000 description 3
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 3
- 229910001570 bauxite Inorganic materials 0.000 description 3
- 229910002092 carbon dioxide Inorganic materials 0.000 description 3
- 230000007423 decrease Effects 0.000 description 3
- 238000001704 evaporation Methods 0.000 description 3
- 230000008020 evaporation Effects 0.000 description 3
- 239000001257 hydrogen Substances 0.000 description 3
- 229910052739 hydrogen Inorganic materials 0.000 description 3
- 125000004435 hydrogen atom Chemical class [H]* 0.000 description 3
- 229910052751 metal Inorganic materials 0.000 description 3
- 239000002184 metal Substances 0.000 description 3
- 238000012986 modification Methods 0.000 description 3
- 230000004048 modification Effects 0.000 description 3
- 238000011020 pilot scale process Methods 0.000 description 3
- 239000007790 solid phase Substances 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- BVKZGUZCCUSVTD-UHFFFAOYSA-M Bicarbonate Chemical compound OC([O-])=O BVKZGUZCCUSVTD-UHFFFAOYSA-M 0.000 description 2
- OTMSDBZUPAUEDD-UHFFFAOYSA-N Ethane Chemical compound CC OTMSDBZUPAUEDD-UHFFFAOYSA-N 0.000 description 2
- DGAQECJNVWCQMB-PUAWFVPOSA-M Ilexoside XXIX Chemical compound C[C@@H]1CC[C@@]2(CC[C@@]3(C(=CC[C@H]4[C@]3(CC[C@@H]5[C@@]4(CC[C@@H](C5(C)C)OS(=O)(=O)[O-])C)C)[C@@H]2[C@]1(C)O)C)C(=O)O[C@H]6[C@@H]([C@H]([C@@H]([C@H](O6)CO)O)O)O.[Na+] DGAQECJNVWCQMB-PUAWFVPOSA-M 0.000 description 2
- MUBZPKHOEPUJKR-UHFFFAOYSA-N Oxalic acid Chemical compound OC(=O)C(O)=O MUBZPKHOEPUJKR-UHFFFAOYSA-N 0.000 description 2
- 229910021529 ammonia Inorganic materials 0.000 description 2
- 229910001680 bayerite Inorganic materials 0.000 description 2
- 229910001593 boehmite Inorganic materials 0.000 description 2
- 238000009835 boiling Methods 0.000 description 2
- 239000001569 carbon dioxide Substances 0.000 description 2
- 230000008859 change Effects 0.000 description 2
- OMZSGWSJDCOLKM-UHFFFAOYSA-N copper(II) sulfide Chemical compound [S-2].[Cu+2] OMZSGWSJDCOLKM-UHFFFAOYSA-N 0.000 description 2
- LBJNMUFDOHXDFG-UHFFFAOYSA-N copper;hydrate Chemical compound O.[Cu].[Cu] LBJNMUFDOHXDFG-UHFFFAOYSA-N 0.000 description 2
- 230000003247 decreasing effect Effects 0.000 description 2
- 229910001679 gibbsite Inorganic materials 0.000 description 2
- 229930195733 hydrocarbon Natural products 0.000 description 2
- 150000002430 hydrocarbons Chemical class 0.000 description 2
- 238000001027 hydrothermal synthesis Methods 0.000 description 2
- FAHBNUUHRFUEAI-UHFFFAOYSA-M hydroxidooxidoaluminium Chemical compound O[Al]=O FAHBNUUHRFUEAI-UHFFFAOYSA-M 0.000 description 2
- 239000007788 liquid Substances 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 150000002894 organic compounds Chemical class 0.000 description 2
- 229940039748 oxalate Drugs 0.000 description 2
- 230000001172 regenerating effect Effects 0.000 description 2
- 229910052708 sodium Inorganic materials 0.000 description 2
- 235000017557 sodium bicarbonate Nutrition 0.000 description 2
- 229910000030 sodium bicarbonate Inorganic materials 0.000 description 2
- 238000011144 upstream manufacturing Methods 0.000 description 2
- 229910000975 Carbon steel Inorganic materials 0.000 description 1
- 239000005751 Copper oxide Substances 0.000 description 1
- VMQMZMRVKUZKQL-UHFFFAOYSA-N Cu+ Chemical compound [Cu+] VMQMZMRVKUZKQL-UHFFFAOYSA-N 0.000 description 1
- 238000003723 Smelting Methods 0.000 description 1
- 238000002835 absorbance Methods 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 150000007513 acids Chemical class 0.000 description 1
- 238000013019 agitation Methods 0.000 description 1
- 229910052783 alkali metal Inorganic materials 0.000 description 1
- -1 alkali metal aluminate Chemical class 0.000 description 1
- ANBBXQWFNXMHLD-UHFFFAOYSA-N aluminum;sodium;oxygen(2-) Chemical compound [O-2].[O-2].[Na+].[Al+3] ANBBXQWFNXMHLD-UHFFFAOYSA-N 0.000 description 1
- 239000002585 base Substances 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 239000006227 byproduct Substances 0.000 description 1
- VNWKTOKETHGBQD-AKLPVKDBSA-N carbane Chemical compound [15CH4] VNWKTOKETHGBQD-AKLPVKDBSA-N 0.000 description 1
- 239000010962 carbon steel Substances 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 229910000431 copper oxide Inorganic materials 0.000 description 1
- BERDEBHAJNAUOM-UHFFFAOYSA-N copper(I) oxide Inorganic materials [Cu]O[Cu] BERDEBHAJNAUOM-UHFFFAOYSA-N 0.000 description 1
- PTVDYARBVCBHSL-UHFFFAOYSA-N copper;hydrate Chemical compound O.[Cu] PTVDYARBVCBHSL-UHFFFAOYSA-N 0.000 description 1
- 238000002425 crystallisation Methods 0.000 description 1
- 230000008025 crystallization Effects 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 238000007323 disproportionation reaction Methods 0.000 description 1
- 239000003623 enhancer Substances 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 230000007717 exclusion Effects 0.000 description 1
- 239000012530 fluid Substances 0.000 description 1
- 238000005187 foaming Methods 0.000 description 1
- 239000013529 heat transfer fluid Substances 0.000 description 1
- 239000002638 heterogeneous catalyst Substances 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-M hydroxide Chemical compound [OH-] XLYOFNOQVPJJNP-UHFFFAOYSA-M 0.000 description 1
- 229910001026 inconel Inorganic materials 0.000 description 1
- 230000002401 inhibitory effect Effects 0.000 description 1
- 150000002500 ions Chemical class 0.000 description 1
- 238000002955 isolation Methods 0.000 description 1
- 238000012804 iterative process Methods 0.000 description 1
- 230000000670 limiting effect Effects 0.000 description 1
- 239000007791 liquid phase Substances 0.000 description 1
- 238000012423 maintenance Methods 0.000 description 1
- 150000002739 metals Chemical class 0.000 description 1
- 239000011368 organic material Substances 0.000 description 1
- 239000005416 organic matter Substances 0.000 description 1
- 231100000572 poisoning Toxicity 0.000 description 1
- 230000000607 poisoning effect Effects 0.000 description 1
- 239000010970 precious metal Substances 0.000 description 1
- 238000011165 process development Methods 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 238000010926 purge Methods 0.000 description 1
- 238000011084 recovery Methods 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 238000000926 separation method Methods 0.000 description 1
- 239000002002 slurry Substances 0.000 description 1
- 229910001388 sodium aluminate Inorganic materials 0.000 description 1
- 229910052979 sodium sulfide Inorganic materials 0.000 description 1
- GRVFOGOEDUUMBP-UHFFFAOYSA-N sodium sulfide (anhydrous) Chemical compound [Na+].[Na+].[S-2] GRVFOGOEDUUMBP-UHFFFAOYSA-N 0.000 description 1
- 239000000725 suspension Substances 0.000 description 1
- 238000012546 transfer Methods 0.000 description 1
- 239000002351 wastewater Substances 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
- C22B21/00—Obtaining aluminium
- C22B21/0015—Obtaining aluminium by wet processes
-
- 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
- C01F7/46—Purification of aluminium oxide, aluminium hydroxide or aluminates
- C01F7/47—Purification of aluminium oxide, aluminium hydroxide or aluminates of aluminates, e.g. removal of compounds of Si, Fe, Ga or of organic compounds from Bayer process liquors
- C01F7/473—Removal of organic compounds, e.g. sodium oxalate
- C01F7/476—Removal of organic compounds, e.g. sodium oxalate by oxidation
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
- C22B3/00—Extraction of metal compounds from ores or concentrates by wet processes
- C22B3/04—Extraction of metal compounds from ores or concentrates by wet processes by leaching
- C22B3/12—Extraction of metal compounds from ores or concentrates by wet processes by leaching in inorganic alkaline solutions
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
Description
Method of Hydrothermal Treatment of Organic Contaminants of Alkaline Process Solutions Field of the Invention The present invention relates to a method of hydrothermal treatment of organic 5 contaminants of alkaline solutions, including Bayer process solutions. Background Art In a hydrothermal system, chemical reactions or a change in process conditions may result in exceeding the solubility of compounds in the system, causing precipitation. Hydrothermal systems are used to treat process liquors and 10 wastewaters. Operating conditions are above the normal boiling point of water, and an overpressure is maintained to retain water in the liquid phase. Gasses such as air or oxygen are injected into the system, which cause some water evaporation and the formation of reaction products. Sometimes a catalyst or other oxidation enhancer is used to enhance the reaction. These factors can 15 result in the precipitation of salts, dissolved catalysts, and other compounds and plugging in the system. The Bayer process is one process of interest to applying the hydrothermal process of wet oxidation. The background to the present invention will now be discussed in the context of catalytic wet oxidation of organic contaminants of 20 Bayer process solutions. However, as will be apparent from the following description of the invention, the application of the method of the present invention is not limited to Bayer process solutions but has general application to alkaline process solutions. The Bayer process is widely used for the production of alumina from aluminium 25 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 -2 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 5 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 presence of organic contaminants in Bayer process solutions reduces 10 productivity largely through three effects. Firstly, organic contaminants reduce the amount of soda available to dissolve gibbsite and form sodium aluminate in solution. Secondly, the presence of organic contaminants increases gibbsite solubility, thus reducing hydrate yield in precipitation. Thirdly, organic contaminants reduce the hydrate precipitation rate, due to crystallization 15 poisoning. Other benefits associated with removal of organic contaminants from Bayer process solutions include a reduction in the amount of soda in the alumina product and reduced liquor viscosity. Subsidiary disadvantages associated with organic contaminants of Bayer process solutions include increased boiling point, foaming, liquor and hydrate absorbance and liquor density. 20 One method of reducing the levels of organic contaminants in alkaline solutions is to oxidise the contaminants. Depending on the degree of oxidation achieved, complex organic compounds may be oxidised to simpler compounds and, in turn, to carbon dioxide. One technique for oxidizing organic contaminants is catalytic wet oxidation, where the alkaline solution is exposed to an oxidizing agent and a 25 catalyst, typically under conditions of elevated temperature and pressure. 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 30 as at the priority date of the application.
-3 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. 5 Throughout the specification, unless the context requires otherwise, the word "solution" or variations such as "solutions", will be understood to encompass slurries, suspensions and other mixtures containing solids, dissolved or undissolved. Other definitions for selected terms used herein may be found within the 10 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. Throughout the specification, an alkaline solution is defined as a solution having 15 the capacity to neutralize acids. Such solutions often exhibit a room temperature pH greater than 7, and most often contain bicarbonate, carbonate, or free hydroxide content. Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. 20 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. 25 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 30 range of values will be understood to include all values within the range, including -4 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, 5 journal articles, laboratory manuals, books, or other documents) cited herein are hereby incorporated by reference. Inclusion does not constitute an admission 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. Disclosure of the Invention 10 As discussed above, a by-product of the oxidation of organic contaminants of alkaline process solutions, such as Bayer process solutions, may be carbon dioxide, which will further react with sodium hydroxide (present in alkaline solution, and present in excess in typical Bayer process solutions) to form sodium carbonate and water. Wet oxidation of organic contaminants in such solutions 15 may therefore lead to considerable carbonate concentrations, in addition to the carbonate already present in liquor. This results in the potential to cause undesirable precipitation of sodium carbonate under certain conditions. Further, some methods for the catalytic wet oxidation of organic contaminants of alkaline process solutions, such as Bayer process solutions, employ conditions 20 that heighten the risk of undesirable sodium carbonate precipitation. For example, international patent application PCT/AU2005/001114 discloses methods for the catalytic wet oxidation of organic contaminants of alkaline solutions, including Bayer process solutions, wherein the alkaline solution is exposed to an oxidising agent and a catalyst, under conditions suitable for the catalytic wet 25 oxidation of organic contaminants, while the ratio of the concentration of free caustic in the alkaline solution (expressed in grams per litre equivalent of sodium carbonate) to the concentration of organic contaminants in the alkaline solution (expressed as grams per litre equivalent of carbon) is at least approximately 4. However, increasing the concentration of total sodium in a Bayer process solution 30 typically decreases the solubility of sodium carbonate.
-5 Further still, the solubility of sodium carbonate in some alkaline process solutions, including Bayer process solutions, in the temperature range typically employed for catalytic wet oxidation of organic contaminants of alkaline solutions, such as Bayer process solutions, has been found to be inversely proportional to 5 temperature. Generally, more effective wet oxidation occurs at elevated temperatures (and pressures), so the effect of sodium carbonate precipitation is exacerbated. In accordance with the present invention, there is provided a method for the hydrothermal treatment of an alkaline feed solution, the method comprising the 10 steps of: diluting the feed solution to adjust the concentration of compounds to ensure that compounds and reaction products do not exceed their solubility in the process; and elevating the temperature and pressure of the feed solution. 15 A method for the hydrothermal treatment of an alkaline feed solution, the method comprises the steps of: diluting the feed solution to adjust the concentration of oxidizable compounds to ensure that compounds and oxidation products do not exceed their solubility in the process; 20 exposing the solution to an oxidizing agent to produce an oxidized solution; and elevating the temperature and pressure of the feed solution. Preferably, after the steps of: diluting the feed solution to adjust the concentration of oxidisable 25 compounds to ensure that compounds and oxidation products do not exceed their solubility in the process; and -6 exposing the solution to an oxidizing agent to produce an oxidised solution; the method of the invention comprises the step of flash-cooling the oxidised Bayer process solution, generating water vapour and thereby concentrating the oxidised Bayer process solution. 5 Preferably, the step of flash-cooling the oxidised solution, thereby concentrating the oxidised solution more specifically comprises the step of depressurising the oxidised solution to atmospheric pressure, thereby concentrating the oxidised solution. In one form of the invention, the step of depressurising the oxidised solution to 10 atmospheric pressure occurs in multiple stages. Preferably, the method of the present invention comprises the step of: utilising the water vapour generated by the step of flash-cooling the oxidised solution in the step of elevating the temperature and pressure of the alkaline feed solution. 15 Preferably still, the step of utilising the water vapour generated by the step of flash-cooling the oxidised solution in the step of elevating the temperature and pressure of the feed alkaline solution at least in part comprises the step of: condensing the water vapour generated by the step of flash-cooling the oxidised solution and adding the condensate to the feed alkaline solution. 20 Preferably still, the step of diluting the feed alkaline solution at least in part comprises the step of: condensing the water vapour generated by the step of flash-cooling the oxidised solution and adding the condensate to the feed alkaline solution.
-7 In accordance with the present invention, there is provided a method for the catalytic wet oxidation of organic contaminants of a feed Bayer process solution, the method including the steps of: diluting the feed Bayer process solution to adjust the concentration of 5 organic contaminants to ensure that the total sodium carbonate in the oxidised Bayer process liquor does not exceed its solubility; then exposing the feed Bayer process solution to an oxidising agent and a catalyst, suitable for the catalytic wet oxidation of organic contaminants, to produce an oxidised Bayer process solution; and 10 elevating the temperature and pressure of the feed Bayer process solution. The steps of exposing the feed Bayer process solution to an oxidising agent and a catalyst, suitable for the catalytic wet oxidation of organic contaminants, to produce an oxidised Bayer process solution and elevating the temperature and pressure of the feed Bayer process solution may be performed in either order, or 15 simultaneously. In a preferred form of the invention, the step of elevating the temperature and pressure of the feed Bayer process solution is performed after the step of exposing the feed Bayer process solution to an oxidising agent and a catalyst, suitable for the catalytic wet oxidation of organic contaminants, to produce an 20 oxidised Bayer process solution. Preferably, after the steps of: exposing the feed Bayer process solution to an oxidising agent and a catalyst, suitable for the catalytic wet oxidation of organic contaminants, to produce an oxidised Bayer process solution; and 25 elevating the temperature and pressure of the feed Bayer process solution; the method of the invention comprises the step of: -8 flash-cooling the oxidised Bayer process solution, generating water vapour and thereby concentrating the oxidised Bayer process solution. It is generally understood that dilution of process solutions generally, and Bayer process solutions in particular, is fundamentally highly undesirable. Dilution of 5 Bayer process solutions results in a digestion production loss: lower caustic concentrations result in decreased digestion yield and thus decreased alumina production. Further, dilution results in soda loss. If a Bayer process plant is operating at maximum evaporation capacity, dilution results in less water being available to wash the bauxite residue for recovery of soda. If evaporation capacity 10 is available, then additional energy is required to remove the excess dilution. In a preferred form of the invention, the Bayer process solution is a spent Bayer process solution. Green Bayer process solutions are undesirable for catalytic wet oxidation because green Bayer process solutions have a lower free caustic/total organic carbon ratio than spent Bayer process solutions, reducing the extent of 15 oxidation; and green Bayer process solutions have a higher alumina concentration and a higher alumina/caustic ratio than spent Bayer process solutions. When a Bayer process solution is oxidised, caustic is consumed by reaction with C02, causing the alumina/caustic ratio to rise. If a green Bayer process solution were oxidised, hydrate would precipitate from solution and cause scaling problems. The 20 issues associated with using a green Bayer process solution (reduced oxidation, high scaling) can be overcome by addition of caustic, and the scope of the present inventions should not be construed as being limited to spent Bayer process solutions. Caustic addition increases the free caustic/total organic carbon ratio and reduces the alumina/caustic ratio. 25 Thus, the present invention enables the use of the elevated temperatures and pressures desirable for catalytic wet oxidation of organic contaminants of alkaline process solutions, including Bayer process solutions, while reducing the possibility of undesirable sodium carbonate precipitation. Unlike alternate methods of managing sodium carbonate precipitation, such as the use of reactors capable of 30 managing or purging solids, the method of the present invention is easily implemented with relatively little capital cost.
-9 The extent to which the alkaline feed solution is diluted according to the method of the present invention depends on a number of factors. Prior to the present invention, and subject to the use of alternate methods of managing sodium carbonate precipitation, catalytic wet oxidation of organic contaminants of alkaline 5 solutions, such as Bayer process solutions, would necessitate a direct balancing of competing requirements, with lower reaction temperatures being advantageous from the perspective of increasing sodium carbonate solubility, while higher temperatures increase the level of oxidation of organic contaminants, for a given residence time. 10 The method of the present invention affords a greater degree of control, enabling a balance to be struck between temperature, residence time and dilution that maximises the rate of oxidation of organic contaminants, while still achieving the minimum degree of destruction required to avoid issues with exceeding the solubility of refractory organics such as sodium oxalate. For example, in the 15 context of Bayer process solutions, if lower degrees of organic contaminant destruction are achieved, it is common to have high concentrations of sodium oxalate present in the oxidised liquor that will crystallise as the liquor is cooled, presenting scaling problems. Other factors affecting the desirable extent of dilution include the initial carbonate concentration of the Bayer process solution, 20 the extent of oxidation of organic contaminants achieved and the concentration of components that affect the carbonate solubility such as the free caustic and total soda concentrations of the Bayer process solution. Once the sodium carbonate solubility under particular operating conditions has been determined, the feed Bayer process solution may be diluted to adjust the 25 concentration of organic contaminants to ensure that the total sodium carbonate in the oxidised Bayer process liquor does not exceed its solubility, where; Sodium carbonate concentration in oxidised Bayer process solution (g/L) = g/L sodium carbonate in feed Bayer process liquor + (g/L organic contaminants destroyed *106/12) (normalised to a tie component to allow for 30 volume changes in the solution).
-10 When the feed Bayer process solution is diluted, it causes small changes to the sodium carbonate solubility and the extent of destruction of the organic contaminants, so determining the precise extent of dilution required is typically an iterative process. 5 Conditions appropriate for catalytic wet oxidation of Bayer process solutions are known to those skilled in the art. For example, US patent 4,215,094, July 29, 1980, "Method for the removal of organic substances from alkali metal aluminate solution", Inao et al., Sumitomo Aluminium Smelting Company Ltd, describes a process whereby liquor is contacted with an oxygen-containing gas under 10 elevated pressure in the presence of copper ions. Sodium sulfide is added to form an insoluble copper sulfide precipitate. The precipitate is filtered and reused as the copper ion source in the wet oxidation step. Incomplete organics destruction is targeted, leaving some solid phase oxalate present to act as a filter aid for the copper sulfide precipitate. 15 Further, US patent 4,668,486, May 26, 1987, "Method for removing organic substances from caustic aluminate liquors", Brown et al., Vereinigte Aluminium Werke Atkiengesellschaft, describes a process whereby liquor is contacted with an oxygen-containing gas under elevated pressure in the presence of copper ions. The copper ions are co-precipitated with boehmite/bayerite. The crystalline 20 co-precipitate (Cu/boehmite or Cu/bayerite) is filtered and recycled, thereby acting as a carrier for the copper catalyst. Particularly advantageous conditions are described in international patent application PCT/AU2005/001114 (Alcoa of Australia Inc), where the highly significant effect of the ratio of the concentration of free caustic in the alkaline 25 solution to the concentration of organic contaminants in the alkaline solution is recognised. More generally, Brown, N., "Kinetics of copper catalysed oxidation of Bayer liquor organics", Light Metals (1989), pp. 121-130, provides an overview of the effects of temperature, catalyst concentration, oxygen charge and level of agitation on the 30 degree of destruction of organic contaminants.
-11 Preferably, the step of flash-cooling the oxidised Bayer process solution, thereby concentrating the oxidised Bayer process solution more specifically comprises the step of depressurising the oxidised Bayer process solution to atmospheric pressure, thereby concentrating the oxidised Bayer process solution. 5 In one form of the invention, the step of depressurising the oxidised Bayer process solution to atmospheric pressure occurs in multiple stages. Preferably, the method of the present invention comprises the step of: utilising the water vapour generated by the step of flash-cooling the oxidised Bayer process solution in the step of elevating the temperature and pressure 10 of the feed Bayer process solution. Preferably still, the step of utilising the water vapour generated by the step of flash-cooling the oxidised Bayer process solution in the step of elevating the temperature and pressure of the feed Bayer process solution at least in part comprises the step of: 15 condensing the water vapour generated by the step of flash-cooling the oxidised Bayer process solution and adding the condensate to the feed Bayer process solution. Preferably still, the step of diluting the feed Bayer process solution at least in part comprises the step of: 20 condensing the water vapour generated by the step of flash-cooling the oxidised Bayer process solution and adding the condensate to the feed Bayer process solution. Preferably, the step of exposing the Bayer process solution to an oxidising agent and a catalyst, suitable for the catalytic wet oxidation of organic contaminants 25 more specifically comprises the step of: -12 exposing the Bayer process solution to an oxidising agent and a catalyst, suitable for the catalytic wet oxidation of organic contaminants while the ratio of the concentration of free caustic in the alkaline solution (expressed in grams per litre equivalent of sodium carbonate) to the concentration of 5 organic contaminants in the alkaline solution (expressed as grams per litre equivalent of carbon) is at least approximately 4. In accordance with the present invention, there is further provided a method for the catalytic wet oxidation of organic contaminants of a Bayer process solution comprising the steps of: 10 introducing an oxidising agent suitable for catalytic wet oxidation to the Bayer process solution; introducing a catalyst suitable for catalytic wet oxidation to the Bayer process solution; elevating the temperature of the Bayer process solution; and 15 exposing the Bayer process solution to conditions suitable for catalytic wet oxidation of organic contaminants of a Bayer process solution for an appropriate time; wherein the step of introducing an oxidising agent suitable for catalytic wet oxidation to the Bayer process solution is performed prior to the step of elevating 20 the temperature of the Bayer process solution. Introduction of the oxidising agent to the Bayer process solution prior to elevating the temperature of the Bayer process solution has been found to be highly advantageous. It has been found that elevation of the temperature of Bayer process solutions to 25 the temperatures typically required for catalytic wet oxidation without the presence of an oxidising agent facilitates the generation of flammable gases, such as -13 hydrogen, as well as volatile organic carbon-based compounds and ammonia. All are highly undesirable from a safety perspective, with at least the latter two being equally undesirable for environmental reasons. Additionally, it has been found that addition of catalysts suitable for catalytic wet 5 oxidation to Bayer process solution without the presence of the oxidising agent may cause reduction of the catalyst under elevated temperatures. Depending in part on the catalyst, this may render the catalyst ineffective and/or cause the catalyst to precipitate, both outcomes being potentially highly undesirable. Typically, the temperature of Bayer process solutions is elevated by heat 10 exchange apparatus which, by their nature, comprise a considerable surface area of liquor being contacted indirectly with a heat donor. Precipitation of catalyst over such a surface area may cause considerable maintenance problems. Accordingly, the steps of introducing a catalyst suitable for catalytic wet oxidation to the Bayer process solution; and introducing an oxidising agent suitable for 15 catalytic wet oxidation to the Bayer process solution; are preferably performed prior to the step of elevating the temperature of the Bayer process solution. The steps of introducing an oxidising agent suitable for catalytic wet oxidation to the Bayer process solution and introducing a catalyst suitable for catalytic wet oxidation to the Bayer process solution may be performed in either order, or 20 concurrently, depending on the catalyst. For example, it has been found that copper-based catalysts, such as copper (II) salts, may be chemically reduced to copper (1) and/or copper metal at temperatures appropriate for catalytic wet oxidation. However, other catalysts may reduce to a problematic extent at typical Bayer 25 process solution temperatures. For such catalysts, the step of introducing an oxidising agent suitable for catalytic wet oxidation to the Bayer process solution; is preferably performed prior to the step of introducing a catalyst suitable for catalytic wet oxidation to the Bayer process solution, which is in turn performed prior to the step of elevating the temperature of the Bayer process solution.
-14 In a preferred form of the present invention, the step of elevating the temperature of the Bayer process solution more specifically comprises elevating the temperature of the Bayer process solution at elevated pressures. Where the step of elevating the temperature of the Bayer process solution more 5 specifically comprises elevating the temperature of the Bayer process solution at elevated pressures, the steps of introducing an oxidising agent suitable for catalytic wet oxidation to the Bayer process solution; and introducing a catalyst suitable for catalytic wet oxidation to the Bayer process solution; are preferably performed prior to the step of elevating the temperature of the Bayer process 10 solution at elevated pressures. Introducing the catalyst to the Bayer process solution prior to elevation of the pressure avoids the engineering complications associated with introducing material to the liquor at elevated pressures, Oxidising agents suitable for catalytic wet oxidation are known to those skilled in 15 the art and include oxygen, air, H 2 0 2 and mixtures of such, but is not limited thereto. In a preferred form of the invention, the oxidising agent is air. Catalysts suitable for catalytic wet oxidation of Bayer process solutions are known to those skilled in the art. The catalyst may be a solid or in solution. In a preferred form of the invention, the catalyst is a copper-based catalyst. Copper-based 20 catalysts include, but are not limited to copper (II) hydroxide and copper (II) oxide. Conveniently, the copper-based catalyst is solid copper (11) oxide. It has been found that introducing a copper-based catalyst to an alkaline solution, such as a Bayer process solution, then elevating the temperature of the solution to a level required for catalytic wet oxidation of organic contaminants of alkaline 25 solutions, without first introducing an oxidising agent can cause the copper-based catalyst to be reduced to copper metal.
-15 Thus, in a highly preferred form of the invention, the method comprises the steps of: introducing air to the Bayer process solution; and introducing a copper-based catalyst to the Bayer process solution; then 5 elevating the temperature of the Bayer process solution. A treatment time appropriate for the catalytic wet oxidation of organic contaminants of a Bayer process solution will depend at least on the conditions selected and the extent of oxidation desired, and will be determined readily by a person skilled in the art. 10 In accordance with the present invention, there is provided a method for the catalytic wet oxidation of organic contaminants of a feed Bayer process solution comprising the steps of: introducing an oxidising agent suitable for catalytic wet oxidation to the feed Bayer process solution; 15 introducing a catalyst suitable for catalytic wet oxidation to the Bayer process solution; diluting the feed Bayer process solution to adjust the concentration of organic contaminants to ensure that the total sodium carbonate in the oxidised Bayer process liquor does not exceed its solubility; 20 elevating the temperature of the feed Bayer process solution; exposing the feed Bayer process solution to conditions suitable for catalytic wet oxidation of organic contaminants of a Bayer process solution for an appropriate time; -16 wherein the step of introducing an oxidising agent suitable for catalytic wet oxidation to the Bayer process solution is performed prior to the step of elevating the temperature of the Bayer process solution. As would be understood by a person skilled in the art, a small amount of copper 5 precipitation may be advantageous, since this builds a reservoir of catalyst in the base of the reactor. Brief Description of the Drawings Figure 1 is a schematic representation of an apparatus for the implementation of a method of an embodiment of the present invention directed to Bayer liquor; 10 Figure 2 is a cross sectional view of plugged heater coil; Figure 3 plots off-gases concentrations after anaerobic conditions at 290 0 C (total hydrocarbons (THC) concentrations are ppm (v/v) as ethane equivalent); Figure 4 plots sodium bicarbonate solubility as a function of temperature; and Figure 5 plots sodium carbonate solubility as a function of temperature. 15 Figure 6 is a schematic representation of a general wet air oxidation system for the treatment of an alkaline process solution. Best Mode(s) for Carrying Out the Invention The following discussion should not be understood as in any way limiting the breadth or generality of the preceding description of the invention. The present 20 invention relates in general to inhibiting the precipitation of sparingly soluble salts or compounds in a hydrothermal process such as wet air oxidation. The current invention also relates to the hydrothermal treatment of an alkaline feed solution where the alkaline feed solution has a pH greater than or equal to 7 including an alkaline feed solution such as a Bayer process solution. As will be apparent from 25 that description, the precise operating conditions, including the precise extent of -17 dilution of the feed Bayer process liquor, are dependent on the properties of the feed Bayer liquor. The properties of the feed Bayer liquor will vary considerably from application to application, and thus the following description of the best method makes no reference to operating conditions. 5 Generally the invention disclosed herein is applicable to all alkaline process treatments including those that are not necessarily at risk of carbonate precipitation. For example, it will be appreciated by a person of ordinary skill in the art that the inventive process is applicable, for example, to prevent other types of precipitation during hydrothermal oxidation or in the cooling of solutions to near 10 ambient conditions, such as bicarbonate or oxalate precipitation in the coolers of a hydrothermal system. The disclosed invention may preferably include a catalyst. However, as is exemplified below, although catalysts may be preferably included as part of certain embodiments, in certain alternate embodiments, use of a catalyst may neither be necessary nor even preferred. 15 Referring to Figure 1, a schematic representation of an apparatus for the implementation of a method of an embodiment of the present invention directed to Bayer liquor is shown. A feed Bayer process solution 10 is introduced into a feed tank 12, whereupon the ratio of free caustic to total organic carbon is adjusted by the addition of caustic 14, and a catalyst 16 suitable for catalytic wet 20 oxidation of organic contaminants, such as copper (II) oxide, is added. The concentration of the feed Bayer process solution 18 may be lowered by dilution 20 prior to the solution being heated. Air 22 is added to the diluted feed Bayer process solution 24 prior to heating, thereby minimising the possibility of reduction of the catalyst to metallic copper, and minimising the production of 25 undesirable volatile organic carbon-based compounds. In an alternate embodiment of the invention (not illustrated), the feed is partially heated in a pretreatment step prior to the addition of air, thereby allowing the use of lower cost carbon steel in the construction of the pre-heater. The pressure is then raised to the target operating pressure. 30 The temperature of the aerated, diluted feed Bayer process solution 26 is then elevated by way of a multi-stage heating process. First, the solution 26 is -18 contacted via a heat exchanger 28 with a gas stream product 30 (the generation of which will be explained subsequently). Cooling of the gas stream product 30 occurs on the shell side of the heat exchanger 28, whereby heat is recovered to the aerated, diluted Bayer process solution 26 by condensing part of the water 5 vapour present in the gas stream product 30. The partially heated feed solution 32 is then further heated through one or more regenerative pre-heaters 34, with heat provided by one or more flash-vapour streams 36 (the production of which will be discussed subsequently). The water vapour of the flash vapour streams 36 condenses on the shell side of each pre 10 heater 34 to heat the feed solution 32 passing through the tube side. Condensate recovered from the shell sides of the regenerative pre-heaters 34 can be injected back into the feed stream 32 to provide further dilution. Further elevation of the temperature of the further heated feed solution 38 can be achieved by way of an additional heat exchanger 40 using a heat transfer fluid such as oil or high 15 pressure steam to transfer heat into the feed solution. When the further heated feed solution 42 is at target temperature, it is introduced into a reactor 44 suitable for catalytic wet oxidation of organic contaminants of Bayer process solutions, such as a bubble-column type reactor, at elevated pressure. In the reactor 44, the heated feed solution 42 is exposed to conditions suitable for 20 the catalytic wet oxidation of the organic contaminants of Bayer process solutions for an appropriate time, after which, the reactor contents are subjected to a gas liquid separation, to form an oxidised Bayer process solution 46 at elevated temperature and pressure, and the gas stream product 30 which is used to heat the feed Bayer process solution 26 as described above. 25 The oxidised Bayer process solution 46 can be subjected to a multi-stage flash cooling process, whereby it is cooled by flashing down to lower pressures, thereby producing the flash vapour streams 36 used to heat the feed Bayer process solution 32 as referred to above, and a cooled, oxidised Bayer process solution 48. 30 The cooled, oxidised solution 48 can be directed to an atmospheric flash tank, 50 -19 with the gas 52 being further processed by way of a scrubber/condenser, and the oxidised Bayer process solution 54 returned to the process proper. A WAO system for the treatment of an alkaline process solution is shown in Figure 6. The alkaline feed is compressed in a high pressure pump and 5 combined with compressed oxidation gas (air in this description). The fluid passes through a heat exchanger and then into a WAO reactor. The reacted material exits the reactor and passes through a heat exchanger where it is cooled. The pressurized material is depressurized and passes to a separator. The off-gas exits the separator vent and the treated liquor exits the bottom. In some cases a 10 catalyst is also added to the feed. The catalyst may be a dissolved ion, a heterogeneous catalyst, or some other compound. Common examples include various forms of Cu, Fe, Mn/Ce, and precious metal catalysts. Oxidation products such as sodium carbonate may precipitate at the elevated temperature in the reactor. Other oxidation products such as sodium oxalate or 15 sodium bicarbonate may precipitate in the cooler portions of the system. To prevent this precipitation, dilution water may be added to the feed. Examples The following examples are intended to illustrate the principles discussed in the preceding description of the invention. They should not be understood to limit the 20 breadth or generality of the preceding description of the invention. Sodium carbonate management and water balance Pilot scale wet oxidation tests (continuous, bubble column configuration) showed that treatment of undiluted Bayer liquor led to a limited operating time before a significant decline in TOC ('Total Organic Carbon') destruction efficiency, 25 ultimately leading to system shutdown. The cause of the decline in TOC destruction was shown by chemical analysis to be a build up of solid phase sodium carbonate within the reactor causing loss of liquid volume (residence time) and eventual blockage of the process coolers by solid phase sodium oxalate.
- 20 The data in Table 1 illustrate the effect of feed liquor TOC concentration upon the accumulation of sodium carbonate within the reactor. At higher feed liquor TOC concentrations a substantial proportion of the sodium carbonate produced during treatment is retained in the reactor. Once sufficient feed liquor dilution has been 5 achieved, the accumulation is negated and the operational problems are overcome. Parameter Test 1 Test 2 Test 3 Feed TOC (g/L C) 22.1 14.7 11.4 %TOC destruction 84 86 74 carbonate expected in oxidised liquor (g/L Na2CO3) 201 135 104 carbonate measured in oxidised liquor (g/L Na2CO3) 103 108 106 Total sodium (TS) change (g/L Na2CO3) -86 -19 2 Analytical results normalised to account for volume changes. Table 1: Effect of feed liquor concentration on sodium carbonate accumulation Copper catalyst reduction under anaerobic conditions 10 For catalytic wet oxidation treatment of Bayer liquor, appropriate locations for addition of both the oxidant and the catalyst must be selected during process development to ensure an efficient design. A variety of combinations of copper catalyst and air addition points were trialled during pilot scale test work. It was discovered that when copper catalyst in the 15 form of a copper (II) salt was added to the liquor upstream of heat exchangers under anaerobic conditions i.e. air was added downstream of heat exchangers, a plug formed within the heater coils after approximately 2 hours' operation. The heater coil was isolated and dissected to reveal the nature of the blockage. The plug viewed on a cross section is shown in Figure 2. The material was 20 determined by AAS to be elemental copper. When the point of air addition was moved upstream of the heat exchangers and the copper catalyst addition remained in the feed liquor, the problem of the copper reduction and associated plugging was no longer observed. It is believed that the -21 aerobic conditions under this scenario allow the catalyst to remain in copper Il form, rather than undergoing reduction and possible disproportionation to yield elemental copper. The cause of the reduction under anaerobic conditions is theorised to be predominantly the presence of organic matter in the liquor, 5 coupled with the high temperatures of the process. Further evidence to support the reduction of copper under anaerobic conditions in Bayer liquor comes from laboratory autoclave test work. In this case, 2L of Bayer liquor was dosed with low concentrations of copper oxide catalyst (<5g/L) and heated to 1450C in an Inconel autoclave under anaerobic conditions. Samples of 10 the solids were taken before and after heating for analysis by XRD to determine the mineralogy. The results showed that the copper (II) oxide (tenorite) was substantially reduced to copper (1) oxide (cuprite) after the heating period. However, when conditions were made aerobic at the elevated temperature by addition of an oxidant, copper (II) oxide was again found to be the dominant 15 component. Minimisation of VOCs and flammable qases Catalytic wet oxidation of Bayer liquor with the oxidant added in a manner to ensure that the system is maintained aerobic leads to low and acceptable levels of flammable gases and volatile organic compounds (VOCs) from the findings of 20 this work. However if the Bayer liquor is allowed to become anaerobic under the elevated temperatures of wet oxidation, significant quantities of undesirable gases such as hydrogen, methane, VOCs and ammonia will be formed. Evidence of this was first obtained from measurement of catalytic wet oxidation off gas from laboratory 25 scale tests. A pilot scale test (continuous operation, bubble column configuration) was performed in which the oxidant supply to the reactor was isolated, leaving only Bayer liquor pre-heated to a temperature of 2900C. Figure 3 displays the off-gas concentrations measured with time after the air was turned off.
-22 It is clear that around 30 minutes after air isolation the concentrations of hydrogen, total hydrocarbons (THC) and methane increased significantly. Full interpretation of the data is complex due to the system not being at equilibrium during the measurements, but the data does demonstrate the importance of 5 oxidant presence under wet oxidation conditions. The equivalent test performed under aerobic conditions led to off-gas concentrations of <0.05% H2 and <25ppm THC (as ethane equivalent), similar to those shown in Figure 3 during the first 30 minutes. Oxidation of TOC in an alkaline solution without catalyst 10 A wet oxidation reactor is fed with an alkaline solution containing 40 g TOC/kg solution and sufficient sodium hydroxide to maintain alkaline conditions. This solution is oxidized and 75% of the TOC is destroyed at 2000C. Upon cooling the effluent pH is between 8 and 10. This indicates that the oxidized carbon is predominantly present as NaHCO 3 . The oxidation reaction converted the carbon 15 to a concentration of approximately 200 g NaHCO 3 / kg solution (20 g NaHCO 3 / kg 100 g solution). As can be seen from Figure 4, the NaHCO 3 remains soluble at reactor conditions, but begins to precipitate upon cooling. This causes plugging in the coolers of the WAO system. However, if the feed or hot effluent is diluted to 25% strength with water, then the resultant concentration is only 5 g NaHCO 3 /100 20 g solution, and the salt is soluble even at room temperature. Hydrothermal processing of a sodium carbonate solution without catalyst A gas-free hydrothermal reactor is fed with an alkaline solution containing 150 g Na 2
CO
3 / kg solution and sufficient sodium hydroxide to maintain caustic conditions. The feed solution is at 400C. This solution is heated in the reactor to 25 3000C. As is apparent from Figure 5, the Na 2
CO
3 begins to precipitate. This causes plugging in the heaters and reactor. However, if the feed or hot oxidized effluent is diluted to 50% strength with water, then the resultant concentration is only 7.5 g Na 2
CO
3 /100 g solution, and the salt will be soluble at 300*C.
- 23 Modifications and variations such as would be apparent to a skilled addressee are considered to fall within the scope of the present invention.
Claims (21)
1. A method for the hydrothermal treatment of an alkaline feed solution, characterised in that the method comprises the steps of: diluting the feed solution to adjust the concentration of compounds to ensure 5 that compounds and reaction products do not exceed their solubility in the process; and elevating the temperature and pressure of the feed solution.
2. A method for the hydrothermal treatment of an alkaline feed solution, according to claim 1, characterised in that the method comprises the step of diluting the 10 feed solution to adjust the concentration of compounds to ensure that compounds and reaction products do not exceed their solubility in the process comprises the steps of: diluting the feed solution to adjust the concentration of oxidizable compounds to ensure that compounds and oxidation products do not exceed 15 their solubility in the process; and exposing the solution to an oxidizing agent to produce an oxidized solution.
3. A method according to any one of the preceding claims characterised in that after the steps of: exposing the alkaline feed solution to an oxidising agent and a catalyst, 20 suitable for the catalytic wet oxidation of organic contaminants, to produce an oxidised solution; and elevating the temperature and pressure of the alkaline feed solution; the method comprises the step of: -25 flash-cooling the oxidised solution, generating water vapour and thereby concentrating the oxidised solution.
4. A method according to claim 3 characterised in that the step of flash-cooling the oxidised solution, thereby concentrating the oxidised solution, more 5 specifically comprises the step of: depressurising the oxidised solution to atmospheric pressure, thereby concentrating the oxidised solution.
5. A method according to claim 4 characterised in that the step of depressurising the oxidised solution to atmospheric pressure occurs in multiple stages. 10
6. A method according to any one of claims 3 to 5 characterised in that the method comprises the step of: utilising the water vapour generated by the step of flash-cooling the oxidised solution in the step of elevating the temperature and pressure of the alkaline feed solution. 15
7. A method according to claim 6 characterised in that the step of utilising the water vapour generated by the step of flash-cooling the oxidised solution in the step of elevating the temperature and pressure of the alkaline feed solution at least in part comprises the step of: condensing the water vapour generated by the step of flash-cooling the 20 oxidised solution and adding the condensate to the alkaline feed solution.
8. A method according to any one of claims 3 to 7 characterised in that the step of diluting the alkaline feed solution at least in part comprises the step of: condensing the water vapour generated by the step of flash-cooling the oxidised solution and adding the condensate to the alkaline feed solution. -26
9. A method according to any one of the preceding claims characterised in that the alkaline feed solution is a Bayer process solution.
10.A method for the catalytic wet oxidation of organic contaminants of a Bayer process solution comprising the steps of: 5 introducing an oxidising agent suitable for catalytic wet oxidation to the Bayer process solution; introducing a catalyst suitable for catalytic wet oxidation to the Bayer process solution; elevating the temperature of the Bayer process solution; and 10 exposing the Bayer process solution to conditions suitable for catalytic wet oxidation of organic contaminants of a Bayer process solution for an appropriate time; wherein the step of introducing an oxidising agent suitable for catalytic wet oxidation to the Bayer process solution is performed prior to the step of 15 elevating the temperature of the Bayer process solution.
11. A method according to claim 10 characterised in that the step of elevating the temperature of the Bayer process solution more specifically comprises elevating the temperature of the Bayer process solution at elevated pressures.
12. A method according to claim 11 characterised in that the steps of introducing 20 an oxidising agent suitable for catalytic wet oxidation to the Bayer process solution; and introducing a catalyst suitable for catalytic wet oxidation to the Bayer process solution; are preferably performed prior to the step of elevating the temperature of the Bayer process solution at elevated pressures.
13. A method according to any one of claims 9 to 12 characterised in that the 25 oxidising agent suitable for catalytic wet oxidation is selected from the group: oxygen, air, H 2 0 2 and mixtures of such. -27
14. A method according to claim 13 characterised in that the oxidising agent suitable for catalytic wet oxidation is air.
15. A method according to any one of claims 9 to 14 characterised in that the catalyst suitable for catalytic wet oxidation is a copper-based catalyst. 5
16. A method according to claim 15 characterised in that the copper-based catalyst is selected from the group: copper (II) hydroxide and copper (11) oxide and mixtures thereof.
17. A method according to claim 15 or 16 characterised in that the method comprises the steps of: 10 introducing the oxidising agent suitable for catalytic wet oxidation to the Bayer process solution; and introducing the copper-based catalyst to the Bayer process solution; then elevating the temperature of the Bayer process solution.
18. A method according to claim 17 characterised in that the oxidising agent is air. 15
19. A method for the catalytic wet oxidation of organic contaminants of a feed Bayer process solution comprising the steps of: introducing an oxidising agent suitable for catalytic wet oxidation to the feed Bayer process solution; introducing a catalyst suitable for catalytic wet oxidation to the Bayer 20 process solution; diluting the feed Bayer process solution to adjust the concentration of organic contaminants to ensure that the total sodium carbonate in the oxidised Bayer process liquor does not exceed its solubility; -28 elevating the temperature of the feed Bayer process solution; exposing the feed Bayer process solution to conditions suitable for catalytic wet oxidation of organic contaminants of a Bayer process solution for an appropriate time; 5 wherein the step of introducing an oxidising agent suitable for catalytic wet oxidation to the Bayer process solution is performed prior to the step of elevating the temperature of the Bayer process solution.
20.A method according to any one of claims 9 to 19 characterised in that the step of exposing the Bayer process solution to an oxidising agent and a catalyst, 10 suitable for the catalytic wet oxidation of organic contaminants more specifically comprises the step of: exposing the Bayer process solution to an oxidising agent and a catalyst, suitable for the catalytic wet oxidation of organic contaminants while the ratio of the concentration of free caustic in the alkaline solution (expressed in 15 grams per litre equivalent of sodium carbonate) to the concentration of organic contaminants in the alkaline solution (expressed as grams per litre equivalent of carbon) is at least approximately 4.
21.A method according to any one of claims 9 to 20 characterised in that the Bayer process solution is a spent Bayer process solution. 20
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AU2008229997A AU2008229997A1 (en) | 2007-10-18 | 2008-10-17 | Method of Hydrothermal Treatment of Organic Contaminants of Alkaline Process Solutions |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AU2007905711 | 2007-10-18 | ||
| AU2007905711A AU2007905711A0 (en) | 2007-10-18 | Method of Hydrothermal Treatment of Organic Contaminants of Alkaline Process Solutions | |
| AU2008229997A AU2008229997A1 (en) | 2007-10-18 | 2008-10-17 | Method of Hydrothermal Treatment of Organic Contaminants of Alkaline Process Solutions |
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| AU2008229997A1 true AU2008229997A1 (en) | 2009-05-07 |
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| Application Number | Title | Priority Date | Filing Date |
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| AU2008229997A Abandoned AU2008229997A1 (en) | 2007-10-18 | 2008-10-17 | Method of Hydrothermal Treatment of Organic Contaminants of Alkaline Process Solutions |
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| AU (1) | AU2008229997A1 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102442688A (en) * | 2011-09-23 | 2012-05-09 | 中南大学 | Method for processing sodium oxalate crystallized from industrial sodium aluminate solution |
| WO2021175731A1 (en) * | 2020-03-06 | 2021-09-10 | Scfi Limited | A hydrothermal process for oxalate destruction |
-
2008
- 2008-10-17 AU AU2008229997A patent/AU2008229997A1/en not_active Abandoned
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102442688A (en) * | 2011-09-23 | 2012-05-09 | 中南大学 | Method for processing sodium oxalate crystallized from industrial sodium aluminate solution |
| WO2021175731A1 (en) * | 2020-03-06 | 2021-09-10 | Scfi Limited | A hydrothermal process for oxalate destruction |
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