METHOD FOR PURIFYING IMPURE ALUMINIUM OXIDE BY EXTRACTION

Abstract: The invention relates to a method for purifying impure aluminium oxide. In accordance with the invention, impure aluminium oxide is finely ground, dissolved in a concentrated acid at a raised temperature, diluted with water, and an organic solvent is added to extract organic impurities. The phases are separated, the aqueous phase containing aluminium salts settled on the bottom is filtered or centrifugated to remove undissolved components, and is used as such or further purified. Part of the organic phase is taken to combustion and part is recycled for use as a solvent in the extraction step. Aluminium salts purified with the method of the invention are suitable to be used as water purifying and retention agents.

Description: Method for purifying impure alurninium oxide by extraction The invention relates to a method for purifying impure aluminium oxide and to the use of aluminium salts produced with this method as a raw material in industrial applications. In the hydrogen peroxide process, aluminium oxide is used to regenerate and remove organic secondary products formed in the working solution. The porosity of active aliiminium oxide allows it to be used particularly in the anthraquinone process with a view to adsorb spent organic compounds. The impurities are mostly aromatic hydrocarbons. Having lost its activity, aluminium oxide is removed from the process and replaced with a new oxide. Spent aluminium oxide contains organic substances which restrict its utilisations. At present, residual aluminium oxide is usually disposed of as a waste, or optionally stored for the eventuality of ultimate utilisation. There have been various attempts to purify impure aluminium oxide. US patent specification 3 814 701 sets out removal of orgamc matter from spent alumimum oxide deriving from an anthraquinone working solution by calcinating sduminium oxide at a temperature of 300-400 °C. The organic matter adsorbed into aluminium oxide will be removed by combustion during calcination. The calcinated alumimum oxide is then subjected to caustic treatment and recalculation. Nonetheless, treated in this way, aluminium oxide is highly dusting if recycled in the H2O2 process. DE patent application 4 027 159 describes purifying impure aluminium oxide with a method comprising dissolving aluminium oxide into mineral acids or lye. The component insoluble in acids or lye is separated, the purified aluminium oxide is filtered, dried and eventually calcinated. The patent application does not specify how the various steps of the invention can be perforaied. All the same, the aluminium oxide dissolved in acids or lye and the accompanying impurities constitute a diphase solution which is particularly hard to treat, containing a tar-like, sticky organic phase. Technically speaking, it is nearly impossible to perform additional treatments of great volumes of such a solution. What is more, the health hazards caused by the hydrocarbons in the organic phase entail an additional problem, and for this reason the treatment of the solution should be controlled and safe. Previously known techniques aim to regenerate aluminium oxide and to recycle it in the hydrogen peroxide process. However, on the basis of experience, the structure and mechanical strength of oxide grains always deteriorate in regeneration to such an extent that they cannot be recycled in the process. Moreover, the grains will still contain some amount of harmful organic compounds, which are not wanted in the process, since the regenerating circumstances must be fairly cautious. The purpose of the present invention is to provide a method for removing organic substances from aluminium oxide waste deriving from the hydrogen peroxide process. The goal of the method of the invention is to allow controlled, safe and environment-friendly purification of contaminated and impure alumimum oxide, allowing the alumimum salts obtained as a product to be used as a raw material in industrial applications. Another goal of the method of the invention is to avoid the calcination step, which is most expensive. A further goal is not only to recycle the organic compounds forming impurities but also to utilise them as a fuel. These goals are achieved with the method of the invention, which is principally characterised by the features set out in the characterising clause of claim 1. The invention relates to a new method for purifying impure aluminium oxide, which has been used in hydrogen peroxide preparation for the adsorption of organic components. The method of the invention makes it possible to obtain a perfectly pure aluminium salt, such as aluminium sulphate or aluminium chloride, which is even purer than commercially available products. The aluminium salts prepared with this method are well suited to be used as raw materials in industrial applications, particularly the alruninium salts are suitable to be employed as water purification and retention chemicals. In the implementation of the method of the invention, organic hydrocarbons present as impurities in Eduminium oxide are recovered into the solvent and can be recycled to the extraction step, and part of them can be utilised as a fuel. In one preferred embodiment, part of the organic hydrocarbons can be dissolved into the solvent in prepurifϊcation, and its useful components can be recycled in the hydrogen peroxide process. Thus, the method allows the environmental hazards of organic hydrocarbons to be markedly reduced compared to most other purifying methods. Porous, active aluminium oxide is used to regenerate the working solution in the preparation of hydrogen peroxide with the anthraquinone method. Porous aluminium oxide adsorbs mainly the organic hydrocarbons used as an anthraquinone solvent, working solution components and other degradation and oxidation products from the working solution. Among previously known solvents for anthraquinone working solutions we cite among others secondary alcohols, trialkyl phosphates, alkyl benzenes, triacetyl benzene, alkyl cyclohexanones, naphtalenes, xylenes, anilines and quinones. The working solution may further contain many other substances used to dissolve anthraquinone. The inactive and impure aluminium oxide removed from the process may thus contain different organic matters in varying amounts, depending on the hydrogen peroxide manufacturing process. In one embodiment of the invention, the impure aluminium oxide can be prewashed with an organic hydrocarbon solvent before thermal treatment. The washing solvent may consist of any organic hydrocarbon capable of dissolving the organic component forming an impurity in aluminium oxide. A preferred embodiment uses Shellsol AB as a solvent, which is a commercially available product containing mainly CJO-CI J aromatic hydrocarbons. Prewashing allows those organic useful components to be extracted from aliiminium oxide which are recyclable directly to the hydrogen peroxide process. Moreover, prewashing allows the amount of burnt organic matter to be reduced and simultaneously the organic compounds to be recirculated in the hydrogen peroxide process. The invention recommends pulverisation of the impure aluminium oxide. In fact, it was found that the finer the a___minium oxide, the more complete was the acid dissolving. It is advisable to grind aluminium oxide to a particle size 100% under 0.5 mm, preferably 100% under 0.4 mm. The acid dissolving-extraction step comprises dissolving ground _d__minium oxide in a concentrated acid solution. Any concentrated mineral acid is appropriate for dissolving, however, sulphuric and hydrochloric acid are recommended. Sulphuric acid used in a 50-60% by weight concentration is considered particularly suitable. The dissolving is appropriately enhanced by heating. To ensure nearly complete dissolving of aluminium oxide it is recommended to heat the mixture to a temperature of about 100-160 °C, while stirring for several hours. In the course of dissolving, the organic component contained in aluminium oxide is separated into a separate phase on the surface of the aqueous phase. The reaction mixture is diluted with water to prevent aluminium salts from crystallising. Separation of organic components is enhanced by extracting by means of either entirely pure or entirely recycled, or partly recycled and partly pure organic solvent. The extraction solvent may consist of any organic hydrocarbon capable of dissolving organic components from aluminium oxide. A preferred embodiment uses Shellsol AB as a solvent, which is a commercially available product containing mainly Cjo-Cn aromatic hydrocarbons. Toluene is a particularly advantageous solvent, which after the phase separation can be easily distillated from the organic phase and returned to the step following dissolution. The recycled organic solvent to be returned to the extraction step can, if desired, be purified after the phase separation, and if necessary, enhanced with a pure solvent. In accordance with the invention the phases are allowed to separate. Impurities in the organic phase settled on the surface mainly contain organic components deriving from the aluniinium oxide and possibly also organic solvents used in extraction. The organic phase is partly returned to the extraction step and partly conducted to a combustion plant. If desired, the portion returned to the extraction step can be purified, for instance by distillation. The aqueous phase settled on the bottom is filtered to remove undissolved components. The precipitate is returned to the dissolving step, burnt along with organic components, or collected as waste. The filtrate, containing aluminium salt, is usable as such as an aqueous solution or can be solidified by crystallising. In one embodiment of the invention, the aqueous phase is repurified after filtering in order to remove organic residues that may remain in the aqueous phase. Purification can be performed with an active carbon treatment. Other chemicals may also be used, for instance flocculating agents, instead of or along with active carbon. After the active carbon treatment, the perfectly pure aqueous solution of aluminium salt can be used as such, or supplied in a solid crystallised form for use say, as a water purification or retention chemical. Figure 1 shows a block diagram of the various steps of the invention. In figure 1, impure aluminium oxide is ground and subsequently dissolved in an acid. Organic impurities are separated as an organic phase. The mixture is diluted with water, and the separation of organic impurities is enhanced with extraction by using recycled and/or pure organic solvents. After the phase separation a portion of the organic phase is conducted to a combustion plant and another portion is recycled to the extraction step. The aqueous phase is filtered or centrifugated. Undissolved components from the aquoeus phase are returned to the dissolving step and the filtrate is used as such or is further purified. The invention will be described in further detail below with reference to examples and tests. The invention is by no means restricted to the enclosed examples, but may vary within the scope of the accompanying claims. Examples Grinding and dissolving aluminium oxide Impure aluminium oxide supplied by a hydrogen peroxide plant was ground to a particle size under 0.35 mm. A Schwing mill was used for grinding and a screen was used to obtain the particle size desired. Table 1 sets out the dissolution yield in sulphuric acid obtained in three tests. Aluminium oxide was dissolved in the same way in all of the three tests. Sulphuric acid was introduced in water (164 g) in a flask, causing the temperature to rise to approx. 70 °C. Aluminium oxide (100 g) was gradually added to the mixture of water and sulphuric acid. During the adding no visible reaction occured, nor did the temperature rise owing to the added alumimum oxide, but the flask had to be continuously heated to prevent a drop of temperature. Eventually the temperature was raised to 90-110 °C. The mixture was maintained at the boiling point for 3 h. Its colour was dark brown. The mixture obtained was diluted with water. The goal was to obtain an 8% solution with regard to aluminium oxide. Table 1. Extraction of organic components and separation of phases The organic components were extracted from the diluted solution obtained above either with Shellsol (test 1) or with toluene (tests 2 and 3). The extraction solution was introduced in a 2 1. flask containing the diluted A_2 (Sθ4)3 solution (table 2), and when using toluene the temperature was allowed to drop below 50 °C before adding the solution in order to prevent evaporation. Stirring was continued for 1 h. The extraction solution in test 3 consisted of distillated toluene from test 2 and "fresh" toluene to compensate for the solution loss in test 2 (table 2). A brown "ring" of organic component sticked to the flask sides, which did not dissolve directly under stirring in the extraction solution, but had to be "scaled off 1 , and thus it was indeed dissolved. At the end of 1 h of stirring the mixture was poured into a separating funnel, into which the extraction solution used for rinsing the flask was also introduced. The two solutions were mixed in a separating funnel, and after this the phases were allowed to separate. With Shellsol the settling time was 1 h, whereas with toluene, settling had to be continued overnight. The difference of time required may also be due to the amount of extraction solution used, the amount of toluene being but one fourth of the amount of Shellsol. After settling, the phases were separated and weighed (table 2). Table 2 Test Extraction of organic components Separation of phase No Extraction Amount, g T, Σ mass of Aqueous Organic Total g solution °C the phase g mixture g g Extraction Rince Σ amount 1 Shellsol AB 356.5 100 456.5 70 1392 935.5 456.5 1392 2 Toluene 87 8.7 95.7 45 1009 882.8 104.0 986.8 3 Toluene: -from test 2 77.4 77.4 45 - 881.3 109.8 991.1 -fresh 9.6 8.7 18.3 Distillation of the organic phase Test 1 (table 2) Distillation of Shellsol (Cjo- n aromatic hydrocarbons) was conducted in a 1 1. round-bottomed flask, which was heated in an oil bath to 1 10-120 °C. Table 3 shows how the distillation proceeded, the times, temperatures and pre^ures applied, and the fractions recovered. At the outset, there was 450 g of organi. matter to be distillated. 261.8 g of distillate was yielded. The distillate was a perfectly clear, colourless liquid. The distillation loss was 8.5%. Test 2 (table 2) Toluene (99.1 g) was distillated in a round-bottomed flask, which was heated to 111 °C. The temperature eventually rose to 116 °C, and at that point the distillation was interrupted. There was a yield of 78.1 g of distillate, which was a clear, colourless liquid. There was 17.9 g of brown distillation residue. The distillation loss was 3.1%. Test 3 (table 2) 95.7 g of toluene (110 ml) was added in the extraction step of test 2. The distillate yield was 78.1 g. The distillation loss was 8.3%. Table 3. Distillation of Shellsol in vacuum Time h:min. T. Pressure Distillate Distillation N.B °c mbar ml residue g 0:00 _, - _ 450 2:00 120 450 23 _ 2:40 400 28 _ 1 st distillate fraction: 28 ml = 19.2 g Boiler stones and compensating air was introduced in the flask. Manual air control = boiling more regular 0:00 100 1000 _. 417.9 2:20 106 120 45 _ 3:30 108 79 100 . 2nd distillate fraction: 100 ml = 82.4 g 3:45 _. 85 5 . 4:00 _, _ 14 319.4 3rd distillate fraction: 14 ml = 7.2 g 143.54 g was separated from the distillate residue in the flask. 172.6 g = 54.6 % remained in the flask and was totallv distillated 0:00 101 85 _. 172.6 0:20 104 73 10 0:40 102 110 19 1:00 106 82 22 1:20 108 75 34 2:00 107 71 63 2:40 108 67 88 97.3 4th distillate fraction:88 ml = 72.9 g 3: 10 66 11 3:40 100 72 21 4: 10 106 52 49 66.7 5th distillate fraction: 49 ml = 28.28 g 0:00 102 83 _ 66.7 0:30 75 10 1:00 106 54 12 3:30 116 81 30 5:00 120 75 38 36.5 Addition to the 5th fraction of 38 ml = 28.6 g => 5th fraction total 57.4 g 0:00 108 60 _ 36.5 1:15 112 45 17 25.2 Addition to the 5th fraction of 10.1 g => 5th fraction total 67.5 g 0:00 106 72 _ 25.2 1:10 116 44 17 13.4 The distillation was ended, the distillation residue seemed thick and vapour started to crystallise in the cooler. Addition to the 5th fraction of 11.59 g of distillate => 5th fraction total 79.06 g Treatment of the aqueous phases Separation of insoluble components The insoluble precipitate was separated from the aqueous phase deriving from the extraction step either by filtering or by centrifugating. Test 1 related to separating the precipitate by filtering. To enhance filtering, a 0.1% solution of a flocculating agent, i.e. 12 ml of Fennopol K 211 was added at a ratio of 30 ppm to about half of the aqueous phase obtained, i.e. 400.9 g. Agitation by means of a Heidolph mixer for 15 sec, speed of rotation 450 rpm. The Fennopol addition yielded an easily settling floe. The mixture obtained was filtered with a Buhner device with a diametre of 9 cm. A glass fibre paper, Whatman GF/A, was used. Vacuum was sucked with a vacuum pump, under a pressure of 600 mbar. The filtrate obtained was taken apart and the precipitate was washed with warm water (table 4). The precipitate was dried in a heating chamber at 100 °C overnight. Separating the precipitate by centrifugation proved a practical method. In this case also, a 0.1% solution of a flocculating agent, Fennopol, was added at a ratio of 30 ppm. Centrifugation was carried out with a Heraeus Labofuge T centrifugal apparatus. A 3000 rpm speed of rotation was applied for 20 min. After centrifugation water was removed from the surface. The water was nearly clear, but a few ligth "flakes" floated on the surface. The precipitate was washed with water, filtered through a glass fibre paper and dried at 100 °C (table 4). Removal of organic matter from the filtrates Organic residues were removed from the filtrates by means of an active carbon treatment. 150 ml of filtrate and 7.5 g of active carbon granulates (used for wash water purification at an H2O2 plant) were admixed with a magnetic mixer for 1 h. After the active carbon addition the filtrate was actively bubbling. Filtering with a Buhner apparatus provided with a glass fibre paper. Analysis TOC was analysed in all the filtrates and Fe and Al (AAS flame) were analysed in the filtrates of tests 1 and 3. Fe and Al were analysed in the precipitates from tests 1 and 3. The results are given in table 4. Aluminium and iron solubility and distribution The amount of sulphuric acid used to dissolve aluminium oxide (test 1 : stoichiometric amount and test 3: 95% of the stoichiometric amount) did not affect akrminium and iron solubility. In both the tests 90% of alumimum and 98.4% of iron was dissolved in the aqueous phase (table 5). Treatment of the aqueous phase and separation of insoluble matters Insoluble matters were separated from the aqueous phase either by filtering or by centrifugating. Both are usable means of separation, yielding a clear filtrate, still the filtrate obtained by centrifugating has a tenfold turbidity (22) compared to the turbidity (2, 4) of the filtrate obtained by filtering. In contrast, the means of separating has no impact on the quantity of organic carbon (0.07%) in the filtrate (table 4). The active carbon treatment of the water phase dropped the turbidity to the range of 0.15 to 0.5 and the TOC to the range of 0.004 to 0.006% (table 4). Table 4. Treatment and analysis of the aqueous phase Test Aqueous phase Filtrate Insoluble No Wash treatment amount sample amount P pH pH turbidity TOC Fe % Al water sample g % Fe % Al g No g g/cm 1 :10 % % g No % 3 1 Filtering, glass 450.9 1/1 427.3 1.28 0.46 1.85 2.4 0.07 - - 185 1/1 4.75 9.8 - - fibre paper -active carbon treatment of 1/1 190.5 1/3 178.4 0.49 0.004 _ _ _ _, _. . . Centrifugation 3000 rpm, 20 min 469.3 1/2 455.4 22 0.07 , _ 297.0 1/2 5.0 0.010 38 -active carbon treatment of 1/2 190.5 1/4 177.0 0.28 0.004 0.0069 3.8 _ . . 2 Centrifugation 3000 rpm, 20 min 870.9 2/1 861.6 1.30 0.54 2.0 9.8 0.06 _ _ 453.9 2/1 8.07 8.1 . . - filtrating of 2/1 with glass fibre paper 400.0 2/2 375.7 0.2 0.06 _ _ _ _, , - . - active carbon treatment of 2/1 192.0 2/3 179.7 0.16 0.005 _ _ _ _ . . 3 Centrifugation 3000 rpm, 20 min 871.1 3/1 857.1 1.30 0.80 2.27 9.5 0.07 _ _ 355.3 3/1 9.1 9.1 0.0092 40 - active carbon treatment of 3/1 191.0 3/2 179.0 0.15 0.005 0.0060 3.9 _ _. _ _ _ Table 5. Solubility and distribution of aluminium and iron Both Shellsol AB and toluene were used to extract organic impurities from the aqueous phase. Both are suitable solvents as the TOC results of the aqueous phase indicate. Toluene distilled easiere due to its lower boiling point. On the basis of the tests the recycling of toluene has no effect on the turbidity of the aqueous phase nor on the TOC content (table 4, tests 2 and 3).

Claims: Claims 1. A method for purifying impure aluminium oxide, which has been used for adsorbing organic components in the preparation of hydrogen peroxide, characterised in that the impure aluminium oxide a) is finely ground b) is dissolved in a concentrated acid at a raised temperature c) is diluted with water and an organic solvent is added to extract organic impurities d) the phases are separated e) the aqueous phase containing aluminium salts settled on the bottom is filtered or centrifugated to remove undissolved components, and is used as such or further purified f) a portion of the organic phase settled on the surface is taken to combustion and a portion is returned for reuse as a solvent in the extraction step. 2. A method as claimed in claim 1, characterised in that the impure aluminium oxide is optionally prewashed with an organic hydrocarbon solvent before grinding. 3. A method as claimed in claim 2, characterised in that the organic hydrocarbon used in prewashing is preferably a mixture of C i o-Ci j aromatic hydrocarbons. 4. A method as claimed in claim 1, characterised in that the impure alurninium oxide used in the preparation of hydrogen peroxide with the anthraquinone process contains mainly organic hydrocarbons adsorbed from the working solution. 5. A method as claimed in claim 1, characterised in that the concentrated acid used for dissolving is sulphuric acid or hydrochloric acid. 6. A method as claimed in claim 5, characterised in that the concentrated acid used for dissolving is preferably a 50-60% by weight sulphuric acid. 7. A method as claimed in claim 1, characterised in that the impure aluminium oxide is ground to a particle size under 0.5 mm, preferably under 0.4 mm. 8. A method as claimed in claim 1, characterised in that dissolving in acid is performed at a temperature of 100-160 °C. 9. A method as claimed in claim 1, characterised in that the organic solvent used in the extraction step is either a recycled and/or pure organic hydrocarbon. 10. A method as claimed in claim 1 or 9, characterised in that the organic hydrocarbon used in the extraction step is preferably a mixture of C i rj-C \ \ aromatic hydrocarbons or toluene. 11. A method as claimed in claim 1, characterised in that after the phase separation part of the organic phase is returned as a solvent to the extraction step and part is conducted to a combustion plant to be burnt into ashes. 12. A method as claimed in claim 1, characterised in that the aqueous phase containing aluminium salts is filtered or centrifugated in order to remove undissolved aluminium oxide, which can be returned to the dissolving step, and the filtrate is treated with active carbon to remove orgamc residues and is used either as an aqueous solution or as a crystallised solid. 13. A method as claimed in claim 1 or 12, characterised in that a flocculating agent is used to enhance filtration and centrifugation of the aqueous solution containing aluminium salts. 14. Use of aluminium salts, such as aluniinivim sulphate or chloride, prepared with the method of any of the preceding claims 1 to 13, as a water purification or retention chemical.

Inventors: JOKINEN SIMO, JOKINEN, SIMO
Applicants: JOKINEN SIMO, KEMIRA CHEMICALS OY, JOKINEN, SIMO
Application year: 1995
Main classification: C01F7
IPC codes: B01J20, C01B15, C01F7, C02F1