Method for preparing monazite ceramic solidified body through reduction iron removal

Abstract

The invention discloses a method for preparing a monazite ceramic solidified body through reduction iron removal, and relates to the technical field of radioactive waste liquid treatment. The reducing agent is adopted to reduce Fe < 3 + > into Fe < 2 + > in the nitric acid solution, and Fe < 2 + > cannot react with phosphate radicals in the solution with the pH value smaller than 2, so that Fe ions are always kept in the Fe < 2 + > state in the hydrated phosphate precipitation process, generation of iron phosphate can be prevented, and the problem of interference of monazite in treatment of Fe < 3 + > in high-level liquid waste is effectively solved. Therefore, high-level liquid waste with complex components can be treated through monazite ceramic solidification, complete lattice solidification of enriched nuclides in a ceramic solidification body is achieved, and the long-term safety of actinide nuclides in deep geological treatment is improved; in addition, the sintering process is simple in step, secondary pollution of high-level radioactive nuclides in enrichment separation-ceramic solidification is avoided, and the method has a good application prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of radioactive waste liquid treatment, and particularly relates to a method for preparing monazite ceramic solidified body by reduction and iron removal. Background Art

[0002] High-level radioactive waste liquid mainly comes from the acidic extraction liquid generated in the production of nuclear weapons and the PUREX (Plutonium Uranium Redox Extraction) process of spent fuel reprocessing, including more than 200 isotopes of more than 30 elements including actinides and fission nuclides. Among them, actinide nuclides (such as Np, Am, Cm, U, Pu, etc.) have the characteristics of high toxicity, high radioactivity and long half-life. Their treatment and disposal technologies are complex, difficult and costly, which is a difficult point in the research of high-level radioactive waste treatment in countries around the world.

[0003] Glass solidification is the only high-level radioactive waste liquid treatment technology that has achieved engineering application. However, glass is a metastable phase, and the long-term safety of glass solidified body containing long-lived actinide nuclides is controversial. Monazite (LnPO4 = Ln = La - Gd) has been proven to be one of the most promising ceramic solidified body substrates after glass solidification due to its excellent structural flexibility, chemical stability and anti-irradiation stability, and is particularly suitable for solidifying and treating long-lived actinide nuclides. At present, the methods for preparing monazite ceramic solidified body include solid-phase method and hydrothermal synthesis method. Due to the complex components of high-level radioactive waste liquid, the solid-phase method cannot directly solidify and treat high-level radioactive waste liquid. Hydrated phosphate (LnPO4·0.667H2O, Ln = La - Gd) is the precursor for preparing monazite by hydrothermal method. It has good complexing ability for actinides, rare earths and some heavy metal ions in strong nitric acid solution, and can realize the precipitation separation of various radioactive nuclides such as divalent, trivalent and tetravalent; when the temperature is higher than 800 °C, hydrated phosphate will transform into monoclinic monazite. Therefore, the preparation of monazite ceramic solidified body by hydrothermal synthesis method can realize the separation and solidification treatment of target nuclides in high-level radioactive waste liquid, effectively reduce the volume of HLLW, thus making the process of solidifying actinide nuclides in high-level radioactive waste liquid simple and feasible, and improving the long-term safety of actinide nuclides in deep geological disposal.

[0004] Since high-level radioactive waste liquid contains not only radioactive nuclides with complex components and valence states, but also a certain concentration of Fe 3+ ions. In the process of preparing monazite precursor by liquid-phase method, due to the introduction of phosphate radical, Fe 3+ is extremely easy to react with phosphate radical to generate FePO4 (Ksp≈10 -20 ), generating a mixture of LnPO4·0.667H2O and FePO4·2H2O, interfering with the synthesis of target products and reducing the stability of monazite ceramic solidified body. Summary of the Invention

[0005] The object of the present invention is to solve the above technical problems and provide a method for preparing a monazite ceramic solidified body by reduction and iron removal.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is as follows: A method for preparing a monazite ceramic solidified body by reduction and iron removal, comprising the following steps:

[0007] S1. Prepare a waste liquid containing cations with Fe(NO3)3·9H2O, La(NO3)3·6H2O, Sr(NO3)2, and Th(NO3)4·xH2O; add a reducing agent to the waste liquid to carry out a reduction reaction;

[0008] S2. Use an ultraviolet spectrophotometer with sulfosalicylic acid as a color developing agent to detect the concentration of ferric iron and then judge the reduction rate of ferric iron. When the reduction rate reaches 100%, add [PO4] 3- solution. After fully mixing, put the mixed solution into a hydrothermal reaction kettle and place it in an oven at 25°C to 90°C for static placement for 12 hours to 3 days for precipitation;

[0009] S3. After the precipitation is completed, use a HERMLE centrifuge to further separate the solid and liquid at a rotation speed of 13,500 revolutions per minute to obtain a hydrated phosphate precipitation product, and then dry it in a constant temperature oven at 60°C for 24 hours to obtain a powder;

[0010] S4. Make the powder in S3 into a sheet sample through steps of grinding, sieving, granulating, aging, and pressing. Place the sample in a corundum boat, degum it in a ceramic fiber furnace, and then heat it to 1200°C to 1400°C at a heating rate of 5°C / min and hold for 2 hours to obtain a monazite solidified body.

[0011] Preferably, in the waste liquid containing cations in S1, the initial concentration of Fe 3+ is 0.01 mol / L, the initial concentration of La 3+ is 0.05 mol / L, the initial concentration ratio of Th 4+ to Sr 2+ is 1:2 to 1:5, and the total volume of the waste liquid is 200 ml.

[0012] Preferably, the reducing agent in S1 is one or a mixture of two of ascorbic acid and hydrazine nitrate; when the reducing agent is a mixture of ascorbic acid and hydrazine nitrate, the ratio of ascorbic acid to hydrazine nitrate is 1:1.

[0013] Preferably, the conditions for the reduction reaction in S1 are a temperature of 25°C to 90°C and a pH of 0.5 to 2.

[0014] Preferably, in S2, [PO4] 3-The preparation method of the solution is as follows: According to [PO4] 3- :(La 3+ +2Th 4+ ) = 1.03:1.03 to 2:1 in molar dosage ratio, weigh out phosphate and dissolve it in deionized water, and adjust the pH of the solution to 0.5 - 2 with nitric acid and ammonia water to obtain it.

[0015] Further preferably, the phosphate is one or a mixture of three of NH4H2PO4, H3PO4, and (NH4)2HPO4; when the phosphate is a mixture of three of NH4H2PO4, H3PO4, and (NH4)2HPO4, the ratio of NH4H2PO4, H3PO4, and (NH4)2HPO4 is 1:1:1.

[0016] The present invention also provides a monazite ceramic solidified body prepared by the above method.

[0017] Hydrated phosphate has an extremely low solubility product (Ksp≈10 -24 ), and has advantages in precipitating and enriching divalent, trivalent, and tetravalent radionuclides. However, during the process of its precipitating and enriching radionuclides, due to the introduction of phosphate ions, Fe 3+ easily reacts with phosphate ions to form iron phosphate (Ksp≈10 -20 ). Since the precipitate is a two-phase of monazite and iron phosphate, when the precipitate is subjected to ceramic solidification treatment, it will cause a decrease in the chemical stability of the solidified body, and iron phosphate will melt and corrode at high temperature during the sintering process of the monazite ceramic solidified body, which is not conducive to ceramic solidification treatment. However, in a solution with pH < 2, Fe 2+ does not react with phosphate ions. Therefore, in the present invention, a reducing agent is used to reduce Fe 3+ to Fe 2+ in a nitric acid solution, so that Fe ions are maintained in the Fe 2+ state during the precipitation process of hydrated phosphate, which can prevent the generation of iron phosphate, thereby solving the problem that Fe 3+ interferes with the synthesis of the target product and reduces the stability of the monazite ceramic solidified body.

[0018] The beneficial effects of the present invention:

[0019] 1) The present invention proposes a method for precipitating and enriching divalent to tetravalent radionuclides with monazite while avoiding the precipitation of iron ions. This method solves the problem of the formation of iron phosphate precipitation due to the interference of Fe 3+ in iron-containing waste liquid, and the selected sintering process steps are simple, avoiding secondary pollution of high-level radionuclides in enrichment separation - ceramic solidification, and having good application prospects.

[0020] 2) The present invention is conducive to realizing the solidification treatment of highly radioactive waste liquid with complex components by monazite ceramics, achieving the complete lattice solidification of enriched radionuclides in the ceramic solidification body, and improving the long-term safety of actinide nuclides in deep geological disposal. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 For Figure 1 The variation pattern of the SI value of FePO4 in solutions with different pH values.

[0022] Figure 2 For the variation pattern of the SI value of Fe3(PO4)2 in solutions with different pH values.

[0023] Figure 3 For the reduction rate of Fe by mixed reducing agents with different concentrations 3+ .

[0024] Figure 4 For the influence of mixed reducing agents with different concentrations on the stable time of Fe 3+ after reduction.

[0025] Figure 5 For the XRD of the product after reduction precipitation for 12 h.

[0026] Figure 6 For the XRD of the product after reduction precipitation for 24 h.

[0027] Figure 7 For the XRD of the product after adding ascorbic acid for precipitation for 24 h for the second time.

[0028] Figure 8 For the XRD pattern of the monazite sample obtained after iron removal by reduction.

[0029] Figure 9 For the cross-sectional SEM image of the sample at 1300 °C. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0031] Example 1

[0032] Prepare a Fe(NO3)3 solution with a concentration of 0.01 mol / L, use ascorbic acid and hydrazine nitrate as reducing agents, and study the efficiency of reducing ferric iron to ferrous iron under different pH conditions.

[0033] Take 100 ml of 0.01 mol / L Fe(NO3)3 solution, adjust the pH to 0.5, 1, and 2 with nitric acid respectively, and then add 0.5 mol / L hydrazine nitrate to the solution. Detect the concentration of ferric iron through an ultraviolet spectrophotometer every 10 minutes to judge its reduction efficiency for ferric iron. When pH = 0.5, the reduction rate of hydrazine nitrate is 70% in 10 minutes, and the reduction rate reaches 100% in 30 minutes; while when pH is 1 - 2, the reduction rate is 0, which may be caused by the reduction mechanism of hydrazine nitrate. When the pH increases, the H + in the solution decreases, and the degree of protonation decreases, making it impossible for hydrazine nitrate to reduce ferric iron; when pH = 0.3, hydrazine nitrate can be reduced in only 10 minutes. Therefore, hydrazine nitrate can only be used in a highly acidic environment.

[0034] Using 0.5 mol / L ascorbic acid as the reducing agent, when pH = 1 - 2, the reduction rate reaches 100% in 10 minutes; when pH < 1, the reduction effect decreases and cannot be reduced by 100%; when pH < 0.3, it can hardly be reduced.

[0035] Based on the reduction effects of the above two reducing agents, by preparing mixed solutions of hydrazine nitrate and ascorbic acid in different proportions, ferric iron can be reduced at different pH values.

[0036] Example 2

[0037] Prepare a 0.01 mol / L Fe(NO3)3 solution, and use ascorbic acid and hydrazine nitrate to prepare a 1:1 mixed reducing agent. Take 100 ml of 0.01 mol / L Fe(NO3)3 solution, set the solution pH = 0.8, add mixed reducing agents with different concentrations respectively, and detect the concentration of ferric iron through an ultraviolet spectrophotometer to judge the reduction effect of the reducing agent. As Figure 3 shown, when the concentration of the mixed reducing agent is 0.007 mol / L, the reduction rate reaches 100%, and at this time, ferric iron in the solution is completely converted into ferrous iron. However, the preferred precipitation temperature of monazite is 90 °C. When the temperature increases, the mixed reducing agent will be oxidized and lose its reducibility. Therefore, an excessive amount of the mixed reducing agent needs to be added to maintain the stability of ferrous iron.

[0038] Take 100 ml of 0.01 mol / L Fe(NO3)3 solution, set the solution pH = 0.8, add 0.01, 0.02, and 0.03 mol / L of the mixed reducing agent respectively. After detecting that the reduction rate reaches 100% through an ultraviolet spectrophotometer, put the solution into a 90 °C water bath, and then detect the maintenance time of ferrous iron with an ultraviolet spectrophotometer. As Figure 4 shown, at 90 °C, the 0.01 mol / L mixed reducing agent can only be maintained for 5 h, while the 0.02 and 0.03 mol / L mixed reducing agents can both be maintained for more than 12 h.

[0039] Example 3

[0040] Using Fe(NO3)3·9H2O, La(NO3)3·6H2O, Sr(NO3)2, Th(NO3)4·xH2O, NH4H2PO4, (NH4)2HPO4, H3PO4, ascorbic acid, and hydrazine nitrate as raw materials, prepare Fe 3+ with an initial concentration of 0.01 mol / L, La 3+ with an initial concentration of 0.05 mol / L, Th 4+ and Sr 2+ with an initial concentration ratio of 1:5. The solution volume is 200 ml, and the pH of the solution is 1. The waste liquid contains cations. Ascorbic acid and hydrazine nitrate are prepared as a mixed reducing agent in a ratio of 1:1. According to the molar dosage ratio of [PO4] 3- :(La 3+ +2Th 4+ ) = 1.03:1, weigh NH4H2PO4, H3PO4, and (NH4)2HPO4 in a ratio of 1:1:1 to provide phosphate radicals, dissolve them in a certain amount of deionized water, and adjust the pH of the solution to 1 with nitric acid and ammonia water to obtain the [PO4] 3- solution.

[0041] First, add 0.03 mol / L of the mixed reducing agent to the waste liquid containing cations and stir well at a temperature of 25°C to 90°C. Detect the divalent iron content by an ultraviolet spectrophotometer. After the divalent iron reduction rate reaches 100%, add the [PO4] 3- solution. After thoroughly mixing with a magnetic stirrer, load the solution into a hydrothermal reaction kettle and place it in an oven at 90°C and let it stand still for 12 hours to 3 days.

[0042] After precipitation, use a HERMLE centrifuge to further separate the solid and liquid at a rotation speed of 13,500 revolutions per minute to obtain the hydrated phosphate precipitation product. Then, dry it in a constant-temperature oven at 60°C for 24 hours to obtain the powder.

[0043] Make the powder into a sheet sample through steps of grinding, sieving, granulating, aging, and tabletting. Place the sample in a corundum boat, degrease it in a ceramic fiber furnace, and then raise the temperature to 1200°C to 1400°C at a heating rate of 5°C / min and hold for 2 hours to obtain the monazite solidified body.

[0044] Figure 5 This is the XRD pattern of the precipitate after 12 hours. At 12 hours, the precipitate is xenotime. After calcining the precipitate at 1000°C, the phase is lanthanum phosphate, and no iron phosphate appears; while Figure 6The XRD pattern of the precipitate after 24 h of precipitation is shown. It can be seen that although the precipitate is in the form of rhabdophane phase, after calcination at 1000 °C, it is composed of lanthanum phosphate and iron phosphate phases, indicating that between 12 h and 24 h, the mixed reducing agent has been completely oxidized and lost its reducing effect, unable to maintain the stability of divalent iron, resulting in the formation of iron phosphate. Figure 7 The XRD pattern of the precipitate obtained by adding 0.03 mol / L of the mixed reducing agent to the solution when the precipitation time is 12 h and then reacting for another 12 h. After calcination at 1200 °C, only the lanthanum phosphate phase is present.

[0045] Example 4

[0046] Using Fe(NO3)3·9H2O, La(NO3)3·6H2O, Sr(NO3)2, Th(NO3)4·xH2O, NH4H2PO4, (NH4)2HPO4, H3PO4, ascorbic acid, and hydrazine nitrate as raw materials, Fe 3+ with an initial concentration of 0.01 mol / L, La 3+ with an initial concentration of 0.05 mol / L, Th 4+ and Sr 2+ with an initial concentration ratio of 1:2, the solution volume is 200 ml, the solution pH = 2. According to the molar dosage ratio of [PO4] 3- :(La 3+ +2Th 4+ ) = 2:1, weigh a certain proportion of NH4H2PO4, H3PO4, (NH4)2HPO4 to provide phosphate radicals. First, add 0.03 mol / L of the mixed reducing agent to the solution containing iron nitrate and stir well. Detect the content of divalent iron by ultraviolet spectrophotometer. After the reduction rate of divalent iron reaches 100%, add a certain proportion of NH4H2PO4, H3PO4, (NH4)2HPO4. Stir the solution well and then transfer it to a hydrothermal reaction kettle and place it in a constant temperature oven at 90 °C. Take out the reaction kettle every 12 h, add 0.03 mol / L of the mixed reducing agent and then put it back into the 90 °C oven. After 7 d, take out the reaction kettle and use a HERMLE centrifuge to further separate the solid and liquid at a rotation speed of 13500 rpm to obtain the hydrated phosphate precipitate product, and then dry it in a constant temperature oven at 60 °C for 24 hours.

[0047] Take a certain amount of the precipitate and put it into a beaker. Add PVA at 1% of the precipitate mass, add an appropriate amount of alcohol and stir for 5 h under a magnetic stirrer to make the precipitate and PVA evenly mixed. Then pass through 40-mesh and 100-mesh sieves, and take the precipitate between 40 - 100 meshes and let it age for 12 h. Weigh 1 g of the aged powder, use a 13 mm mold to press it into a disc, and then use cold isostatic pressing to keep the pressure at 200 Mpa for 3 min. The XRD pattern of the sample after normal pressure sintering at 1200 - 1400 °C is as Figure 8, all are single-phase monazite and do not contain iron phosphate phase. When the sintering temperature is 1300 °C, the diffraction peak intensity is the largest and the crystallinity of the sample is the best. The SEM image of the cross-section of the 1300 °C sample is as shown in Figure 9 . The grains of the sample are well-developed, the connection between grains is tight, and no second phase is generated. A single-phase monazite ceramic solidified body is successfully prepared in the iron-containing solution.

[0048] Comparative Example 1

[0049] The influence of different pH conditions on the precipitation of FePO4·2H2O and Fe3(PO4)2·8H2O was studied by phreeqc software. Figure 1 is the change diagram of the SI value (mineral saturation index) of FePO4·2H2O in solutions with different pH values at 90 °C. When pH = 0.25, SI = -0.06, and the system is in an undersaturated state, and FePO4·2H2O cannot be generated; when pH = 0.3, SI = 0.06, and the system is in a supersaturated state, and FePO4·2H2O precipitation can occur. Therefore, when pH ≥ 0.3, SI > 0, and FePO4·2H2O precipitation will occur, while the preferred precipitation pH of lithiophilite is above 1, and the generation of FePO4·2H2O cannot be avoided.

[0050] Figure 2 is the change diagram of the SI value of Fe3(PO4)2·8H2O in solutions with different pH values at 90 °C. When the iron ion changes from trivalent to divalent, when pH ≤ 3.6, SI < 0, and PO4 3- will not react with Fe 2+ , and Fe3(PO4)2·8H2O will only precipitate when pH ≥ 3.7. When there is no trivalent iron in the solution, lithiophilite can precipitate at pH = 1 - 3.6 while Fe 2+ cannot react with PO4 3- to form precipitation. Therefore, a suitable reducing agent can be added to reduce trivalent iron to divalent iron, and by controlling the pH and the valence state of iron ions, the generation of iron phosphate can be avoided during the precipitation process of lithiophilite.

[0051] In summary, the present invention uses a reducing agent to reduce Fe 3+ to Fe 2+ in a nitric acid solution, and in a solution with pH < 2, Fe 2+ does not react with phosphate. Therefore, during the precipitation process of hydrated phosphate, the Fe ions are always maintained in the Fe 2+ state, and the generation of iron phosphate can be prevented, effectively solving the interference problem of Fe 3+ in the treatment of high-level radioactive waste by monazite.

[0052] The description and drawings of the present invention are considered to be illustrative rather than restrictive. Based on the present invention, those skilled in the art can make some substitutions and modifications to some of the technical features without creative work according to the disclosed technical content, and all of them are within the protection scope of the present invention.

Claims

1. A method for preparing a monazite ceramic solidified body by reduction and iron removal, characterized in that, It includes the following steps: S1. Prepare a waste liquid containing cations with Fe(NO3)3·9H2O, La(NO3)3·6H2O, Sr(NO3)2, and Th(NO3)4·xH2O; add a reducing agent to the waste liquid and conduct a reduction reaction. S2. Use an ultraviolet spectrophotometer with sulfosalicylic acid as the color reagent to detect the ferric iron concentration and then judge the reduction rate of ferric iron. When the reduction rate reaches 100%, add [PO4] 3- solution. After fully mixing, put the mixed solution into a hydrothermal reaction kettle and place it in an oven at 25°C to 90°C for static placement for 12 hours to 3 days for precipitation; S3. After precipitation, use a HERMLE centrifuge to further separate the solid and liquid at a rotation speed of 13,500 revolutions per minute to obtain a hydrated phosphate precipitation product, and then dry it in a constant-temperature oven at 60 °C for 24 hours to obtain a powder. S4. Make the powder in S3 into a flaky sample through steps of grinding, sieving, granulating, aging, and tabletting. Place the sample in a corundum boat, degum it in a ceramic fiber furnace, and then raise the temperature to 1200 °C - 1400 °C at a heating rate of 5 °C / min and keep it warm for 2 hours to obtain a monazite solidified body.

2. The method according to claim 1, wherein In the waste liquid containing cations in S1, Fe 3+ The initial concentration is 0.01 mol / L, La 3+ The initial concentration is 0.05 mol / L, Th 4+ And Sr 2+ The initial concentration ratio is 1:2 to 1:5, and the total volume of the waste liquid is 200 ml.

3. The method according to claim 1, wherein In S1, the reducing agent is one or a mixture of two of ascorbic acid and hydrazine nitrate; when the reducing agent is a mixture of ascorbic acid and hydrazine nitrate, the ratio of ascorbic acid to hydrazine nitrate is 1:

1.

4. The method according to claim 1, wherein The conditions for the reduction reaction in S1 are a temperature of 25 °C - 90 °C and a pH of 0.5 - 2.

5. The method according to claim 1, characterized in that, In the above S2, [PO4] 3- The preparation method of the solution is as follows: According to [PO4] 3- :(La 3+ +2Th 4+ ) = 1.03:1.03 to 2:1 in molar dosage ratio, weigh out phosphate and dissolve it in deionized water, and adjust the pH of the solution to 0.5 - 2 with nitric acid and ammonia water to obtain it.

6. The method according to claim 5, characterized in that, The phosphate is one or a mixture of three of NH4H2PO4, H3PO4, and (NH4)2HPO4; when the phosphate is a mixture of three of NH4H2PO4, H3PO4, and (NH4)2HPO4, the ratio of NH4H2PO4, H3PO4, and (NH4)2HPO4 is 1:1:

1.

7. A monazite ceramic solidified body prepared by the method according to any one of claims 1 - 6.

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