METHOD FOR PURIFYING IMPURE ALUMINIUM OXIDE BY THERMAL TREATMENT

Abstract: The invention relates to a method for preparing impure aluminium oxide. In accordance with the invention, impure, granular or ground aluminium oxide is subjected to thermal treatment at a temperature of 500-900 DEG C. The thermally-treated aluminium oxide is dissolved in concentrated acid at a raised temperature, is diluted with water, and when necessary, is filtered and centrifugated in order to remove undissolved components. 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 thermal treatment The invention relates to a method for purifying impure aluminium oxide and to the use of the aluminium salts produced with this method as a raw material in industrial applications. In the hydrogen peroxide process, alurninium oxide is used to regenerate and remove organic secondary products formed in the working solution. The porosity of active aluminium 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. The spent aluminium oxide contains organic substances which restrict its utilization. At present, residual aluminium oxide is usually disposed of as a waste, or optionally stored in the scope of its eventual utilization. There have been various attempts to regenerate impure aluminium oxide. US patent specification 3 814 701 sets out removal of organic matter from spent aluminium oxide from an anthraquinone working solution by calcinating aluminium oxide at a temperature of 300-400 °C. The organic matter adsorbed into aluminium oxide will be removed by combustion during the calcination. The calcinated aluminium oxide is then subjected to caustic treatment and recalculated. Nevertheless, treated in this way, aluminium oxide is highly dusting if recycled in the H2O2 process. DE patent application 4 0127 159 describes purifying impure alurninium 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 performed. 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 inventive method is to allow controlled, safe and environment-friendly purification of contaminated and impure aluminium oxide, allowing the aluminium salts obtained as a product to be used as a raw material in industrial applications. Another goal of the inventive method is not only to recycle the organic contaminant compounds but also to utilize them as a fuel. These goals are achieved with the method of the invention, which is principally characterized by the features set out in the characterizing 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 alunrinium salt, such as aluminium sulphate or aluminium chloride, which is even purer than commercially available technical products. The alurninium salts prepared with this method are well-suited to be used as raw materials in industrial applications, particularly alurninium salts are suitable to be employed as water purification and retention chemicals. By implementing the method of the invention, organic hydrocarbons present as impurities in aluminium oxide can be utilized as a fuel. In one preferred embodiment, part of the organic hydrocarbons can be dissolved into the solvent in prepurification, and the useful components can be recycled in the hydrogen peroxide process. Thus, the method allows the environmental hazards of hydro¬ carbons to be markedly reduced compared to most other purifying methods. Porous, active alurninium oxide is used to regenerate the working solution in the preparation of hydrogen peroxide with the anthraquinone method. Porous aluminium oxide adsorbs 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 used to dissolve anthraquinone s in the preparation of 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 potential substances used to dissolve anthraquinone. The inactive and impure un inium oxide removed from the process may thus contain different organic matters in varying amounts, depending on the manufacturing process of the hydrogen peroxide. The invention comprises thermal treatment of the impure aluminium oxide at 500- 900 °C, at which organic components present as impurities evaporate or burn. The invention recommends a sufficiently low calcinating temperature for aluminium oxide to remain soluble in acids, but also sufficiently high for the organic components present as impurities to be removed. In fact, it was found that the calcinating conditions affect the acid solubility of aluminium oxide. When aluminium oxide was calcinated at higher temperatures, for instance 1,000 °C, its solubility in acids dropped markedly. Moreover, aluminium salts were more slowly filtered, and unsolved precipitates remained to a larger extent than with aluminium oxide calcinated at lower temperatures. This is due to the fact that the crystal structure of alurninium oxide changes in thermal treatment. It is true that hairnful organic components evaporate and burn into coal at a lower temperature, for instance at 350-450 °C, yet the carbon content will still be too high. This will appear as a grey-coloured aluminium oxide and as problems in separating aluminium salt after acid dissolving. With a raised temperature, e.g. to 500-900 °C, the free carbon produced will also burn into carbon dioxide, appearing as a white aluminium oxide. In contrast, at higher temperatures, such as 1,000 °C, alurninium oxide tendns to transform into a crystal form which is less soluble in acids than aluminium oxide calcinated at lower temperatures. This is considered to reduce the dissolving yield as the calcinating temperature is raised. The changes in the crystal form can be observed by X-ray diffraction. The calcinating time depends on the amount of organic component to be burnt, on the temperature, and on the calcinating equipment. Calcination may be carried out in any furnace suitable for calcination. Rotary and tubular furnaces were found to be particularly suitable. It is recommended to equip the furnace with a catalyst or a thermal afterburner. During calcination, evaporating or combustible organic exhaust gases are burnt catalytically or with a thermal afterburner. The gases may also be conducted to be burnt in the combustion chamber of a combustion plant. In this way, the method enables the caloric value of the impurities to be utilized. In one embodiment of the invention, impure alurninium oxide can be prewashed before thermal treatment using an organic hydrocarbon solvent. The washing solvent may consist of any organic hydrocarbon capable of dissolving the organic impurity component from aluminium oxide. In one preferred embodiment, the solvent is Shellsol AB, which is a commercially available product containing mainly C10-C11 aromatic hydrocarbons. Preliminary washing allows such organic useful components, for instance quinoneε, to be extracted and subsequently recycled directly into the hydrogen peroxide process. Prewashing also allows the amount of organic matter for combustion to be reduced, while organic compounds are recycled into the hydrogen peroxide process. The invention recommends pulverization of the thermally-treated aluminium oxide. In fact, it was found that the finer the aluminium oxide, the more complete and rapid was acid dissolving. It is advisable to grind aluminium oxide to a particle size 100% under 0.5 mm, preferably 100% under 0.4 mm. In a second embodiment of the invention, impure, granular aluminium oxide may be finely ground before the thermal treatment. The choice of the grinding moment depends on the amount of organic component in the aluminium oxide, and the fact whether the alurninium oxide has been prewashed before the thermal treatment or not. It is a fact that grinding is complicated by considerable amounts of organic impurities. On the other hand, thermal treatment of ground, dusting aluminium oxide is awkward, but organic components burn more effectively in ground oxide than in granular oxide. Acid dissolving comprises dissolving ground alurriinium 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% 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 100-150 °C, while stirring for several hours. After the reaction, the reaction mixture is diluted with water to prevent aluminiiim salts from crystallizing, if a liquid product is desired, which can be filtered to separate the insoluble portion. The reaction mixture may also be directly crystallized by cooling in order to obtain a solid aluminium salt. Whenever necessary, the undissolved aluminium oxide precipitates in the acid can be removed by filtering or centrifugating, and the undissolved portion can be returned to the dissolving step. The filtrate, which contains aluminium salt, can be used as such as an aqueous solution, or then can be solidified by crystallizing. In one embodiment of the invention, the aqueous phase is purified by 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. The use of a flocculating agent allows a finely-divided dissolving residue, which contains undissolved oxide and possibly uncombusted carbon, to be bound in a form which makes it easier to separate from the solution. The perfectly pure aqueous solution of aluminium salt derived from the filtration with active carbon can be used as such, or supplied in a solid crystallized form for use say, as a water purification or retention chemical. Figure 1 shows a block diagram of the various steps of the invention. Figure 2 illustrates the solubility of aluminium oxide in sulphuric acid as a function of the calcinating temperature. In figure 1, impure aluminium oxide is subjected to thermal treatment at 500-900 ° C. Another option is to prewash aluminium oxide before the thermal treatment with an organic hydrocarbon solvent, and in that case the useful organic components extracted into the solvent can be returned to the hydrogen peroxide process. The thermally-treated aluminium oxide is ground before being dissolved in mineral acid, unless it has already been ground before the thermal treatment. Whenever necessary, uminium oxide precipitates undissolved in the acid can be removed by filtering or centrifugating. The purified alurninium salt can be industrially used as an aqueous solution or in solid form say, as a water purification or retention agent. 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 Example 1 Dissolution directly into sulphuric acid without thermal treatment, not in conformity with the invention. 125 g of aluminium oxide used in the hydrogen peroxide process was dissolved in a 59% aqueous solution of sulphuric acid. The temperature rose to 107 °C. A tar-like layer was formed on the walls of the reaction flask, indicating that the reaction product was inappropriate as a water chemical. Example 2 Dissolution of aluminium oxide thermally treated at 200 °C in sulphuric acid, not in comformity with the invention. Impure aluminium oxide was subjected to thermal treatment at 200 °C over a period of two hours. 100 g of treated aluminium oxide was dissolved in a mixture of sulphuric acid (264 g) and water (278 g). 93% of the aluminium oxide was dissolved in the acid solution. A tar-like layer of impurities was formed on the walls of the reaction vessel. Example 3 Spent aliαminium oxide was prewashed before thermal treatment with an organic hydrocarbon solvent in order to recover the anthraquinones. A laboratory column (h=600 mm, d=40 mm) was filled with spent aluminium oxide withdrawn from the anthraquinone process. The alurninium oxide had been normally evaporated to recover the solvent and the quinones. The aluminium oxide to be washed still had a 2.0% quinone content and a 2.2% solvent content (trialkyl phosphate). The column, which contained 510 g of aluminium oxide, was filled with pure Shellsol AB solvent (200 ml.). The temperature was maintained at about 40 to 50 °C. The mixture was stored over a period of 7 hours. The solvent was evacuated. The thermal treatment was repeated two more times. In the first wash, the quinone content of the aluminium oxide dropped by about 80%. The second treatment yielded a 98-100% reduction. The solvent residues (trialkyl phosphate) decreased at a corresponding rate. Example 4 The procedure of example 3 was repeated, except that alurriinium oxide was washed by circulating the solvent (200 ml, 400 ml, 600 ml) through an aluminium oxide bed (540 g) over a period of seven hours at a rate of 6.5 1/h and at a temperature of 40-50 °C. In each of the three tests, the circulated solution was analysed at intervals of one hour. With this procedure, the quinones were recovered more rapidly than in example 3. Equilibrium was reached in less than two hours. The amount of circulating solvent had a notable impact on the quinone yield. In the last test, using the greatest solvent amount (600 ml=3 x free volume of the bed) a 95% recovery yield was achieved for quinones and a 92% recovery yield for the solvent (trialkyl phosphate). Example 5 Thermal treatment at higher temperatures. The analytical results of untreated and thermally-treated aluminium oxide are set out in table 1. The thermal treatment was performed in a furnace with the aluminium oxide in the crucible (a static bed), subjecting it to calcinating for four hours. The impure aluminium oxide used in the test had a reddish brown colour and contained 75.6% of aluminium oxide and 4.9% of impurities in the form of carbon (7.4% hydrocarbon). The tests indicate that organic components are removed from aluminium oxide at temperatures > 500 °C. The results in table 1 also confirm that with a calcinating temperature of 400 °C, the hydrocarbon content is still too high, causing problems in the filtering step. With a calcinating temperature > 500 °C, hydrocarbons have burnt to a minimum level, at which hydrocarbons, regarded as harmful, no longer occur. Example 6 Dissolution of thermally-treated aluminium oxide in acids. Ground alurninium oxide (135.5 g), thermally-treated at 500 °C for four hours, was dissolved in a mixture of 33% hydrochloric acid (579.4 g) and water (136.5 g). Dissolving was continued for about five hours at a temperature of 150 °C. The alurninium oxide dissolving yield was 95%. Analysing results of the polyaluminium chloride thus formed indicate that the salt contained 14% of AI2O3, 0.019% of Fe and 21% of Cl. Example 7 Spent aluminium oxide from a hydrogen peroxide plant was calcinated in a continuously operating circular furnace at 550 °C so as to provide a 1.5 hour delay. The product was nearly white and was analysed as follows: AI2O3 91%, Fe 0.11% and C 0.2%. The calcinated oxide (150 g) was admixed with water (307 g) in a 2 1 decanter, was heated to 60 °C, and after this 96% sulphuric acid was cautiously introduced into the vessel during about 15 minutes. The temperature then rose from 115 to 130 °C. When the exothermic reaction was over and the main portion of oxide was dissolved, the temperature of the reaction mixture dropped to 113 °C (boiling point), at which the reaction mixture was retained until the total period of three hours had passed. At this point, a small sample was taken for analysis by pouring it onto a sheet to be crystallized. The sample was dissolved in water to form a 10% solution, whose pH was determined: 2.8 (commercial Al sulphate 3.2). Diluting water (810 g) was added to the mixture and the dissolved Al sulphate was analysed: AI2O3 7.8%, free AI2O3 0.02%, SO4 23%, water-insoluble 0.06%, phenolic compounds <0.001%, TOC (= total organic carbon) 2.9 mg/1 of the clarified solution. The dissolving yield was above 99%. The solid substance and turbidity (>200 NTU) of the product solution were reduced by adding 5 mg of polyelectrolyte (polyacrylic amide)/l of solution. A perfectly clear solution was obtained, its turbidity being 1.4 NTU and TOC 3.6 mg/1 (the commercial product in the corresponding concentration 7.6 mg/1). Example 8 Spent aluminium oxide (137 g) containing organic matter was ground 20 s with a Schwing mill and was calcinated in a quarz basin in a 6 mm layer thickness for 1.5 h. The calcination loss was 21.3%. 100 g of the product (screened with a 250 μm screen, 99.3% passing through) was elutriated in 205 g of water, the reaction mixture was heated to 60 °C and sulphuric acid (96%, 273 g) was introduced into the reaction mixture during about half an hour. The temperature rose to 120 °C. The mixture was allowed to react for 3 hours at the boiling point (112 °C at the end), and was subsequently diluted with water (1078 g). 100 ml of the reaction mixture was filtered with a Millipore filter having a 12 μm membrane, the precipitate was washed, dried, weighed and analysed. The filtrate (pH 1.85) was admixed with the precipitate wash water, and was diluted to the fixed volume and analysed. The filtrate (200 ml) contained 21 g of Al/1 and 5.6 mg of Fe/1, the precipitate (0.315 g) contained 52% of Al. The aluminium dissolving yield was 96.2% and the solution contained 0.014% of Fe in AI2O3, this rate being 0.022% in a commercial pure alurninium sulphate, in other words, the product was purer than commercial sulphate. Figure 2 illustrates the dissolving yield as a function of the calcinating temperature. All the tests were conducted in the same way as the above example, and thus they are mutually comparable. Figure 2 shows graphically the solubility of aluminium oxide in sulphuric acid, calcinated at various temperatures, as a function of the calcinating temperature. The results show that when aluminium oxide is calcinated at > 900 °C, the aluminium oxide dissolving yield drops below 90%. A dissolving yield below 90% is not considered acceptable. In this situation, the slow filtering of the undissolved substance is a further inconvenience. Table 1 Analytic data for untreated and thermally treated alurr nium oxide

Claims: Claims 1. A method for purifying impure aluminium oxide, which has been used for adsorbing organic component in the preparation of hydrogen peroxide, characterized in that the impure, granular or ground aluminium oxide a) is subjected to thermal treatment at a temperature of 500-900 °C b) is dissolved in concentrated acid at a raised temperature c) the acid solution is diluted with water, filtered or centrifugated when necessary in order to remove undissolved components d) the aqueous solution containing aluminium salts is used as such or is further purified. 2. A method as claimed in claim 1, characterized in that the impure aluminium oxide is optionally prewashed with an organic hydrocarbon solvent before the thermal treatment. 3. A method as claimed in claim 2, characterized in that the organic hydrocarbon used in prewashing is preferably a mixture of Cjo-Cl l aromatic hydrocarbons. 4. A method as claimed in any of the preceding claims 1-3, characterized in that the impure aluminium oxide used in the preparation of hydrogen peroxide with the anthraquinone process contains organic hydrocarbons adsorbed mostly from the working solution. 5. A method as claimed in claim 1, characterized in that the thermal treatment is preferably performed at a temperature of 500-800 °C, most preferably at a temperature of 500-650 °C. 6. A method as claimed in claim 1, characterized in that the concentrated acid used for dissolving is sulphuric acid or hydrochloric acid. 7. A method as claimed in claim 5, characterized in that the concentrated acid used for dissolving is preferably a 50-60% sulphuric acid. 8. A method as claimed in claim 1, characterized in that the impure granular alurninium oxide is ground before thermal treatment or after it. 9. A method as claimed in claim 1. characterized in that dissolving in acid is performed at a temperature of 100-160 °C. 10. A method as claimed in claim 1, characterized in that the exhaust gases released in thermal treatment are burnt catalytically, thermally with a reheater, or are conducted directly to be burnt in the combustion chamber of a combustion plant. 11. A method as claimed in claim 1, characterized in that the aqueous solution containing alunrinium salts is filtered whenever necessary in order to remove undissolved aluminium oxide, which can be returned to the dissolving step, and the filtrate is treated with active carbon, when necessary, in order to remove organic residues, and is used either as an aqueous solution or as a crystallized solid. 12. A method as claimed in claim 1 or 11, characterized in that a flocculating agent is used to enhance filtration and centrifugation of the aqueous solution containing aluminium salts. 13. Use of alunrinium salts prepared with the method of any of the preceding claims 1-12, such as aluπiinium sulphate or chloride, as a water purification or retention chemical.

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