Inorganic treatment method for radioactive tributyl phosphate kerosene waste liquid
The radioactive TBP/kerosene waste liquid is treated through hydrolysis-oxidation coupling technology, which solves the problems of high treatment costs and complex secondary waste liquid treatment in the prior art, and achieves efficient and economical inorganic treatment effects.
Patent Information
- Application Number
- CN202411729151.0
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-11-28
- Publication Date
- 2025-02-18
AI Technical Summary
The prior art is difficult to effectively treat radioactive tributyl phosphate (TBP)/kerosene waste liquid, which has problems such as high treatment costs, difficulty in engineering application and complex secondary waste liquid treatment, and lacks safe and effective treatment methods.
The hydrolysis-oxidation coupling technology is used to perform alkaline hydrolysis of TBP/kerosene waste liquid, kerosene and butanol are separated, and the hydrolysate is treated by ozone catalytic oxidation to reduce the waste organic phase treatment amount of subsequent oxidation processes.
The TBP hydrolysis rate was achieved ≥95%, and the chemical oxygen demand COD reduction rate of the remaining DBP-Na and MBP-Na waste liquids after washing was ≥97%, reducing the scale and processing difficulty of oxidation equipment, with high processing efficiency and capacity-enhancing ratio Advantages of small and cost-effectiveness.
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Figure CN119480198A_ABST
Abstract
Description
Technical Field
[0001] The invention relates to the technical field of radioactive organic waste liquid treatment, and is a method for inorganic treatment of radioactive tributyl phosphate / kerosene waste liquid; in particular, it relates to a hydrolysis-oxidation coupled inorganic treatment method of radioactive tributyl phosphate (TBP) / kerosene waste liquid in which the volume proportion of tributyl phosphate is 15% to 40%. Background Art
[0002] Tributyl phosphate / kerosene is a widely used extractant in the nuclear industry, mainly used in the extraction and purification of uranium, plutonium, thorium and other nuclides in the spent fuel reprocessing process. After repeated use, the TBP / kerosene system is affected by chemical reactions and radiation degradation, resulting in the degradation of TBP performance and becoming a radioactive waste liquid containing a small amount of nuclides, which is a flammable and explosive substance. Due to the long-term operation and discharge of nuclear facilities, the environmental pressure and safety hazards formed by these waste liquids accumulated year by year are becoming increasingly prominent and need to be dealt with urgently.
[0003] At present, the main research technologies for radioactive TBP / kerosene waste liquid include: incineration, wet oxidation, absorption, solidification, distillation, adduct formation phase separation and alkaline hydrolysis. Among the related methods, the incineration method is prone to radioactive ash and tail gas entrainment, the solidification method has a relatively high volume increase, and the concentrated phosphoric acid in the adduct formation phase separation method is seriously corroded. The remaining methods also have problems such as high treatment cost, difficulty in engineering application, and complex secondary waste liquid treatment. The application of waste organic phase treatment technology must not only consider the effectiveness and safety of the actual operation process, but also ensure that the final waste discharge is small and the engineering cost is cost-effective. It is necessary to consider the maturity and economy of the technology. At present, there is still a lack of safe and effective treatment methods for TBP / kerosene radioactive waste liquid. Therefore, seeking a treatment process with mature process technology, small volume increase ratio, and the ability to achieve inorganicization of TBP / kerosene waste liquid is of great significance for enterprises to reduce the risk of radioactive waste liquid storage leakage and solve environmental safety hazards. Summary of the invention
[0004] In view of the problems existing in the current technology, the present invention aims to provide a method for realizing the inorganic treatment of radioactive TBP / kerosene waste liquid. After the TBP / kerosene waste liquid is first subjected to alkaline hydrolysis by hydrolysis-oxidation coupling technology, clean and controlled kerosene is separated, and the kerosene is burned together with butanol produced by TBP hydrolysis. Then, the mixed liquid of TBP hydrolysis products containing radioactive nuclides is subjected to ozone catalytic oxidation. The clean controlled combustion treatment of kerosene can reduce the amount of waste organic phase treated in the subsequent oxidation process by 60% to 85%, and separating the butanol of TBP hydrolysis product from the oxidation system further reduces the pressure of catalytic oxidation inorganic treatment. By coupling the hydrolysis-oxidation technology and developing an efficient ozone oxidation catalyst, the inorganic treatment process of radioactive waste liquid has the advantages of high process efficiency, mature process technology, small volume increase ratio and cost economy.
[0005] A method for hydrolysis-oxidation coupled treatment of radioactive TBP / kerosene waste liquid, characterized by comprising the following steps: (1) Take TBP / kerosene waste liquid with a TBP volume ratio of 15-40%, add alkaline solution at 90-130 °C for hydrolysis; the concentration range of the alkaline solution is between 8-15 mol / L; (2) TBP / kerosene waste liquid: After the alkali solution is hydrolyzed, stand and separate the layers, remove the upper layer of the alkali solution, add water to wash the upper layer of the alkali solution; and add water to wash three times and stand and separate the layers; (3) washing and standing to separate into an upper kerosene phase and a lower aqueous phase; mixing the lower aqueous phase with the lower aqueous phase from which the alkali solution upper layer is removed in step (1), removing the upper kerosene phase, and separating the mixed lower aqueous phase by distillation at a temperature of 100-160°C; (4) The remaining aqueous phase after distillation is transferred to the oxidation unit for ozone catalytic oxidation, and finally the residual organic matter in the aqueous phase is completely oxidized.
[0006] In the step 1, the TBP / kerosene waste liquid contains alkane hydrocarbons such as n-dodecane, isododecane, hydrogenated kerosene, sulfonated kerosene, and odorless kerosene; the alkaline solution is a NaOH or KOH solution, the volume ratio of the alkaline solution to the TBP / kerosene waste liquid is 0.7-1.3: 1, and the hydrolysis reaction time is between 1 and 5 hours. In the step 2, the TBP / kerosene waste liquid is hydrolyzed under alkaline conditions to generate dibutyl sodium phosphate DBP-Na, monobutyl sodium phosphate MBP-Na and butanol; the upper layer of the alkaline solution is washed with water three times, so that DBP-Na, MBP-Na and butanol are washed into the lower aqueous phase, and the volume ratio of the washing water to the TBP / kerosene waste liquid is 1-2:1.
[0007] In the step 3, DBP-Na, MBP-Na and butanol in the upper layer of the alkali solution will be brought into the water phase by the washing water. After the upper layer of the alkali solution is washed, it is allowed to stand for stratification. After the kerosene phase and the water phase are separated, the kerosene phase is removed; the water phase is heated at 100-160°C to distill out butanol; the separated kerosene phase and butanol are burned.
[0008] The DBP-Na and MBP-Na in the aqueous phase remaining after distillation in step 4 are catalytically oxidized under ozone conditions, the oxidation reaction temperature is 70-90° C., the pH of the aqueous phase solution is controlled between 8 and 10, and the catalyst is a Mn-Fe or Mn-Cu type catalyst.
[0009] The invention has the advantages of treating TBP / kerosene waste liquid in stages through the hydrolysis and oxidation coupling technology. The hydrolysis and oxidation processes are skillfully combined by combining the properties and distribution characteristics of TBP hydrolysis products and kerosene waste liquid in the process of each level. The butanol produced by the hydrolysis of kerosene and TBP is separated through the hydrolysis unit. Compared with the traditional treatment process, the treatment amount of the subsequent oxidation unit is reduced by about 75%, which provides technical support for reducing the scale of oxidation equipment. The research on the amount of washing water added after hydrolysis and the washing method significantly reduces the volume increase ratio and reduces the difficulty of treatment. In the whole process, the TBP hydrolysis rate is ≥95%, and the chemical oxygen demand COD reduction rate of the residual DBP-Na and MBP-Na waste liquid after washing is ≥97%. The method reduces the problems of large secondary waste liquid volume, high cost, and difficult residue treatment in the traditional research method, and has the advantages of high treatment efficiency, small volume increase ratio, cost economy, mature process technology and strong operability. BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Figure 1 It is a schematic diagram of the process of the present invention. DETAILED DESCRIPTION
[0011] A method for hydrolysis-oxidation coupled treatment of radioactive TBP / kerosene waste liquid comprises the following steps: (1) Take TBP / kerosene waste liquid with a TBP volume ratio of 15-40%, add alkaline solution at 90-130 °C for hydrolysis; the concentration range of the alkaline solution is between 8-15 mol / L; (2) TBP / kerosene waste liquid: After the alkali solution is hydrolyzed, stand and separate the layers, remove the upper layer of the alkali solution, add water to wash the upper layer of the alkali solution; and add water to wash three times and stand and separate the layers; (3) washing and standing to separate into an upper kerosene phase and a lower aqueous phase; mixing the lower aqueous phase with the lower aqueous phase from which the alkali solution upper layer is removed in step (1), removing the upper kerosene phase, and separating the mixed lower aqueous phase by distillation at a temperature of 100-160°C; (4) The remaining aqueous phase after distillation is transferred to the oxidation unit for ozone catalytic oxidation, and finally the residual organic matter in the aqueous phase is completely oxidized.
[0012] In step 1, the TBP / kerosene waste liquid contains alkane hydrocarbons such as n-dodecane, isododecane, hydrogenated kerosene, sulfonated kerosene, and odorless kerosene; the alkaline solution is NaOH or KOH solution, the volume ratio of the alkaline solution to the TBP / kerosene waste liquid is 0.7~1.3: 1, and the hydrolysis reaction time is between 1 and 5 hours. The TBP / kerosene waste liquid in step 1 simulates the spent fuel extractant waste liquid containing TBP / kerosene in the nuclear energy field. TBP and its hydrolysis products, dibutyl phosphate (DBP) and monobutyl phosphate (MBP), have strong complexing ability for nuclides such as uranium and plutonium. Therefore, the distribution of a small amount of nuclides in the radioactive waste liquid is consistent with the transfer path of alkyl phosphates. And PO4 3- The complexing ability is stronger, and a small amount of nuclides eventually react with PO4 generated in the subsequent oxidation process. 3- Therefore, in the whole hydrolysis-oxidation treatment process, the nuclides are accompanied by the conversion process of TBP, and it is reasonable to use TBP / kerosene as a simulation solution. Kerosene contains alkane hydrocarbons such as n-dodecane, isododecane, hydrogenated kerosene, sulfonated kerosene, and odorless kerosene. The alkaline solution is NaOH or KOH solution, and the volume ratio of alkaline hydrolysis solution to TBP / kerosene waste liquid is 0.7~1.3: 1. The hydrolysis reaction time is 1~5 hours. In the step 2, the TBP / kerosene waste liquid is hydrolyzed under alkaline conditions to generate dibutyl sodium phosphate DBP-Na, monobutyl sodium phosphate MBP-Na and butanol; the upper layer of the alkaline solution is washed with water three times, so that DBP-Na, MBP-Na and butanol are washed into the lower aqueous phase, and the volume ratio of the washing water to the TBP / kerosene waste liquid is 1-2:1.
[0013] In the step 3, DBP-Na, MBP-Na and butanol in the upper layer of the alkali solution will be brought into the water phase by the washing water. After the upper layer of the alkali solution is washed, it is allowed to stand for stratification. After the kerosene phase and the water phase are separated, the kerosene phase is removed; the water phase is heated at 100-160°C to distill out butanol; the separated kerosene phase and butanol are burned.
[0014] The DBP-Na and MBP-Na in the aqueous phase remaining after distillation in step 4 are catalytically oxidized under ozone conditions, the oxidation reaction temperature is 70-90° C., the pH of the aqueous phase solution is controlled between 8 and 10, and the catalyst is a Mn-Fe or Mn-Cu type catalyst.
[0015] The TBP hydrolysis rate during the experiment was detected by gas chromatography, and the chemical oxygen demand (COD) was determined according to the HJ828-2017 standard. The calculation formula is as follows: TBP hydrolysis rate = (1-TBP content in the product / TBP content in the raw material), COD reduction rate = (1-COD value in the product / COD value in the raw material) × 100%.
[0016] The TBP / kerosene waste liquid hydrolysis-oxidation coupling treatment method of the present invention is further described in detail below in conjunction with specific implementation cases.
[0017] Example 1 (1) Take 1 L of 30% radioactive TBP / kerosene waste liquid as raw material, add 10 mol / L NaOH solution, the volume ratio of NaOH solution to TBP / kerosene waste liquid is 0.8:1, and react at 100℃ for 3 h.
[0018] (2) After the hydrolysis is completed, the upper layer of the alkali solution is removed, and then the upper layer of the alkali solution is washed with water three times to remove the removed kerosene phase. The volume ratio of the total amount of water added to the TBP / kerosene waste liquid raw material is 1.2: 1. After washing with water, the kerosene phase and the water phase are separated. After each washing, the lower water phase is removed, and finally the kerosene phase is separated.
[0019] (3) The washed aqueous phase is distilled at 150°C for 2 h to distill out the dissolved butanol. The separated butanol is burned together with the kerosene separated in step (2).
[0020] (4) The residual organic phase such as DBP-Na and MBP-Na dissolved in the remaining aqueous phase was degraded by ozone oxidation in the presence of a Mn-Co catalyst. The temperature during the oxidation process was set at 90 °C, the pH was controlled at 9, and the reaction was carried out for 8 h.
[0021] The hydrolysis rate of TBP was determined by gas chromatography to be 95.6%, and the degradation rate of COD during oxidation was 98.6%.
[0022] Example 2 (1) Take 1 L of 30% radioactive TBP / kerosene waste liquid as raw material, add 12 mol / L NaOH solution, the volume ratio of NaOH solution to TBP / kerosene waste liquid is 1:1, and react at 120℃ for 3 h.
[0023] (2) After the hydrolysis is completed, the upper layer of the alkali solution is removed, and then the upper layer of the alkali solution is washed with water three times to remove the removed kerosene phase. The volume ratio of the total amount of water added to the TBP / kerosene waste liquid raw material is 1.2: 1. After washing with water, the kerosene phase and the water phase are separated, and the lower water phase is removed after each washing, and finally the kerosene phase is separated.
[0024] (3) The washed aqueous phase is distilled at 150°C for 2 h to distill out the dissolved butanol. The separated butanol is burned together with the kerosene separated in step (2).
[0025] (4) The residual organic phases such as DBP-Na and MBP-Na dissolved in the remaining aqueous phase were degraded by ozone oxidation in the presence of a Mn-Fe catalyst. The temperature during the oxidation process was set at 90 °C, the pH was controlled at 9, and the reaction was carried out for 8 h.
[0026] The hydrolysis rate of TBP was determined by gas chromatography to be 98.5%, and the degradation rate of COD during oxidation was 97.9%.
[0027] Example 3 (1) Take 1 L of 15% radioactive TBP / kerosene waste liquid as raw material, add 15 mol / L NaOH solution, the volume ratio of NaOH solution to TBP / kerosene waste liquid is 1:1, and react at 100°C for 4 h.
[0028] (2) After the hydrolysis is completed, the upper layer of the alkali solution is removed, and then the upper layer of the alkali solution is washed with water three times to remove the removed kerosene phase. The volume ratio of the total amount of water added to the TBP / kerosene waste liquid raw material is 1.2: 1. After washing with water, the kerosene phase and the water phase are separated, and the lower water phase is removed after each washing, and finally the kerosene phase is separated.
[0029] (3) The washed aqueous phase is distilled at 130°C for 2 h to distill out the dissolved butanol. The separated butanol is burned together with the kerosene separated in step (2).
[0030] (4) The residual organic phases such as DBP-Na and MBP-Na dissolved in the remaining aqueous phase were degraded by ozone oxidation in the presence of a Mn-Fe catalyst. The temperature during the oxidation process was set at 90 °C, the pH was controlled at 9, and the reaction was carried out for 8 h.
[0031] The hydrolysis rate of TBP was determined by gas chromatography to be 98.9%, and the degradation rate of COD during oxidation was 97.1%.
[0032] Example 4 (1) Take 1 L of 30% radioactive TBP / kerosene waste liquid as raw material, add 15 mol / L NaOH solution, the volume ratio of NaOH solution to TBP / kerosene waste liquid is 1.2:1, and react at 130℃ for 2 h.
[0033] (2) After the hydrolysis is completed, the upper layer of the alkali solution is removed, and then the upper layer of the alkali solution is washed with water three times to remove the removed kerosene phase. The volume ratio of the total amount of water added to the TBP / kerosene waste liquid raw material is 1.5:1. After washing with water, the kerosene phase and the water phase are separated, and the lower water phase is removed after each washing, and finally the kerosene phase is separated.
[0034] (3) The washed aqueous phase is distilled at 150°C for 2 h to distill out the dissolved butanol. The separated butanol is burned together with the kerosene separated in step (2).
[0035] (4) The residual organic phases such as DBP-Na and MBP-Na dissolved in the remaining aqueous phase were degraded by ozone oxidation in the presence of a Mn-Fe catalyst. The temperature during the oxidation process was set at 90 °C, the pH was controlled at 9, and the reaction was carried out for 8 h.
[0036] The hydrolysis rate of TBP was determined by gas chromatography to be 99.6%, and the degradation rate of COD during oxidation was 97.6%.
[0037] Example 5 (1) Take 1 L of 40% radioactive TBP / kerosene waste liquid as raw material, add 8 mol / L NaOH solution, the volume ratio of NaOH solution to TBP / kerosene waste liquid is 1.2:1, and react at 110℃ for 3 h.
[0038] (2) After the hydrolysis is completed, the upper layer of the alkali solution is removed, and then the upper layer of the alkali solution is washed with water three times to remove the removed kerosene phase. The volume ratio of the total amount of water added to the TBP / kerosene waste liquid raw material is 1:1. After washing with water, the kerosene phase and the water phase are separated, and the lower water phase is removed after each washing, and finally the kerosene phase is separated.
[0039] (3) The washed aqueous phase is distilled at 150°C for 2 h to distill out the dissolved butanol. The separated butanol is burned together with the kerosene separated in step (2).
[0040] (4) The residual organic phases such as DBP-Na and MBP-Na dissolved in the remaining aqueous phase were degraded by ozone oxidation in the presence of a Mn-Cu catalyst. The temperature during the oxidation process was set at 90 °C, the pH was controlled at 9, and the reaction was carried out for 8 h.
[0041] The hydrolysis rate of TBP was determined by gas chromatography to be 96.1%, and the degradation rate of COD during oxidation was 97.2%.
[0042] Example 6 (1) Take 1 L of 25% radioactive TBP / kerosene waste liquid as raw material, add 8 mol / L NaOH solution, the volume ratio of NaOH solution to TBP / kerosene waste liquid is 1.2:1, and react at 110℃ for 3 h.
[0043] (2) After the hydrolysis is completed, the upper layer of the alkali solution is removed, and then the upper layer of the alkali solution is washed with water three times to remove the removed kerosene phase. The volume ratio of the total amount of water added to the TBP / kerosene waste liquid raw material is 1:1. After washing with water, the kerosene phase and the water phase are separated, and the lower water phase is removed after each washing, and finally the kerosene phase is separated.
[0044] (3) The washed aqueous phase is distilled at 120°C for 2 h to distill out the dissolved butanol. The separated butanol is burned together with the kerosene separated in step (2).
[0045] (4) The residual organic phases such as DBP-Na and MBP-Na dissolved in the remaining aqueous phase were degraded by ozone oxidation in the presence of a Mn-Fe catalyst. The temperature during the oxidation process was set at 90 °C, the pH was controlled at 9, and the reaction was carried out for 8 h.
[0046] The hydrolysis rate of TBP was determined by gas chromatography to be 96.7%, and the degradation rate of COD during oxidation was 98.8%.
[0047] Example 7 (1) Take 1 L of 35% radioactive TBP / kerosene waste liquid as raw material, add 12 mol / L NaOH solution, the volume ratio of NaOH solution to TBP / kerosene waste liquid is 1.2:1, and react at 110℃ for 3 h.
[0048] (2) After the hydrolysis is completed, the upper layer of the alkali solution is removed, and then the upper layer of the alkali solution is washed with water three times to remove the removed kerosene phase. The volume ratio of the total amount of water added to the TBP / kerosene waste liquid raw material is 1:1. After washing with water, the kerosene phase and the water phase are separated, and the lower water phase is removed after each washing, and finally the kerosene phase is separated.
[0049] (3) The washed aqueous phase is distilled at 110°C for 2 h to distill out the dissolved butanol. The separated butanol is burned together with the kerosene separated in step (2).
[0050] (4) The residual organic phase such as DBP-Na and MBP-Na dissolved in the remaining aqueous phase was degraded by ozone oxidation in the presence of a Mn-Co catalyst. The temperature during the oxidation process was set at 90 °C, the pH was controlled at 9, and the reaction was carried out for 8 h.
[0051] The hydrolysis rate of TBP was determined by gas chromatography to be 98.3%, and the degradation rate of COD during oxidation was 97.3%.
[0052] Example 8 (1) Take 1 L of 35% radioactive TBP / kerosene waste liquid as raw material, add 8 mol / L NaOH solution, the volume ratio of NaOH solution to TBP / kerosene waste liquid is 1.2:1, and react at 90℃ for 4 h.
[0053] (2) After the hydrolysis is completed, the upper layer of the alkali solution is removed, and then the upper layer of the alkali solution is washed with water three times to remove the removed kerosene phase. The volume ratio of the total amount of water added to the TBP / kerosene waste liquid raw material is 1:1. After washing with water, the kerosene phase and the water phase are separated, and the lower water phase is removed after each washing, and finally the kerosene phase is separated.
[0054] (3) The washed aqueous phase is distilled at 150°C for 2 h to distill out the dissolved butanol. The separated butanol is burned together with the kerosene separated in step (2).
[0055] (4) The DBP-Na and MBP-Na dissolved in the remaining aqueous phase were degraded under ozone oxidation conditions. The temperature during the oxidation process was set at 90 °C, the pH was controlled at 9, and the reaction was carried out for 8 h.
[0056] The hydrolysis rate of TBP was determined to be 95.4% by gas chromatography, and the degradation rate of COD during oxidation was 98.5%.
Claims
1. A method for hydrolysis-oxidation coupled treatment of radioactive TBP / kerosene wastewater, characterized in that The steps include: (1) Take TBP / kerosene waste liquid with a TBP volume ratio of 15-40%, add alkaline solution at 90-130 °C for hydrolysis; the concentration range of the alkaline solution is between 8-15 mol / L; (2) TBP / kerosene waste liquid: After the alkali solution is hydrolyzed, stand and separate the layers, remove the upper layer of the alkali solution, add water to wash the upper layer of the alkali solution; and add water to wash three times and stand and separate the layers; (3) washing and standing to separate into an upper kerosene phase and a lower aqueous phase; mixing the lower aqueous phase with the lower aqueous phase from which the alkali solution upper layer is removed in step (1), removing the upper kerosene phase, and separating the mixed lower aqueous phase by distillation at a temperature of 100-160°C; (4) The remaining aqueous phase after distillation is transferred to the oxidation unit for ozone catalytic oxidation, and finally the residual organic matter in the aqueous phase is completely oxidized.
2. The method of claim 1 wherein the radioactive TBP / kerosene waste liquid is hydrolyzed and oxidized in a coupled manner. In the step 1, the TBP / kerosene waste liquid contains alkane hydrocarbons such as n-dodecane, isododecane, hydrogenated kerosene, sulfonated kerosene, and odorless kerosene; the alkaline solution is a NaOH or KOH solution, the volume ratio of the alkaline solution to the TBP / kerosene waste liquid is 0.7-1.3: 1, and the hydrolysis reaction time is between 1 and 5 hours.
3. The method for the hydrolysis-oxidation coupling treatment of radioactive TBP / kerosene waste liquid according to claim 1, characterized in that In the step 2, the TBP / kerosene waste liquid is hydrolyzed under alkaline conditions to generate dibutyl sodium phosphate DBP-Na, monobutyl sodium phosphate MBP-Na and butanol; the upper layer of the alkaline solution is washed with water three times, so that DBP-Na, MBP-Na and butanol are washed into the lower aqueous phase, and the volume ratio of the washing water to the TBP / kerosene waste liquid is 1-2:
1.
4. The method of claim 3 wherein the radioactive TBP / kerosene waste liquid is hydrolyzed and oxidized and treated in a coupled manner. In the step 3, DBP-Na, MBP-Na and butanol in the upper layer of the alkali solution will be brought into the water phase by the washing water. After the upper layer of the alkali solution is washed, it is allowed to stand for stratification. After the kerosene phase and the water phase are separated, the kerosene phase is removed; the water phase is heated at 100-160°C to distill out butanol; the separated kerosene phase and butanol are burned.
5. The method for hydrolysis-oxidation coupling treatment of radioactive TBP / kerosene waste liquid according to claim 1, characterized in that The DBP-Na and MBP-Na in the aqueous phase remaining after distillation in step 4 are catalytically oxidized under ozone conditions, the oxidation reaction temperature is 70-90° C., the pH of the aqueous phase solution is controlled between 8 and 10, and the catalyst is a Mn-Fe or Mn-Cu type catalyst.