Method for preparing exempt calcium fluoride slag from uranium-containing fluorine-containing solution

Abstract

The invention relates to the technical field of nuclear fuel circulation uranium conversion, in particular to a method for preparing exempt calcium fluoride slag from a uranium-containing and fluorine-containing solution. The method comprises the following steps: 1, adding calcium carbonate into the uranium peroxide precipitation mother liquor, reacting, and carrying out solid-liquid separation to obtain a solid mixture of calcium fluoride and calcium carbonate and calcium fluoride precipitation mother liquor; 2, deeply purifying one part of the calcium fluoride precipitation mother liquor to remove uranium and fluorine, then discharging, and using the other part as an absorbent; a calcium carbonate precipitation replacement method is used for replacing a lime milk direct precipitation method, the precipitation environment of calcium fluoride is changed from strong basicity to a carbonate buffer system, the characteristic that carbonate and uranyl ions generate a stable complex is utilized, uranium is prevented from being attached to the precipitate, the concentration of uranium in the mother liquor is kept basically stable, and the method is suitable for industrial production. The specific radioactivity intensity of the calcium fluoride slag reaches the exempt level, and compared with the mode that the calcium fluoride slag serves as the emissive slag to be stored, the storage cost can be greatly reduced, and the influence on the environment is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of uranium conversion in nuclear fuel cycle, and in particular to a method for preparing exempted calcium fluoride slag from a uranium-containing and fluorine-containing solution. Background Art

[0002] In the process of natural uranium conversion, it has to go through two steps, hydrofluorination and fluorination. HF, UF6, and F2 escape during the reaction, and need to be absorbed by sodium carbonate or potassium carbonate. The uranium concentration in the obtained absorption liquid is 0.2-0.5 g / L, the sum of carbonate and bicarbonate concentrations is 15-20 g / L, the fluoride ion concentration is 5-15 g / L, and the sodium concentration is 20-25 g / L. In addition to the uranium in the solution must be recovered, the fluoride ions must also be removed so that the solution can be reused or discharged. The method of removing fluorine from the solution generally adopts the classic calcium fluoride precipitation process. The method of removing fluorine from the uranium conversion absorption liquid is generally carried out in the following way.

[0003] The first is the direct fluorine precipitation process of the absorption liquid. Lime milk is directly added to the uranium-containing fluorine solution to make the fluoride ions generate calcium fluoride precipitation. During this process, nitric acid needs to be added to inhibit the excessive increase of alkalinity and the formation of sodium diuranate. After solid-liquid separation, calcium fluoride solid and uranium-containing solution are obtained. The uranium-containing solution enters the next uranium recovery process; the calcium fluoride solid has a high specific radioactivity intensity due to the high uranium concentration in the precipitation mother liquor, so it can only be stored as radioactive waste.

[0004] 2NaF+Ca(OH)2==CaF2↓+NaOH

[0005] NaOH+HNO3==Na NO3+H2O

[0006] The second is the process of extracting uranium first with the absorption liquid and then precipitating fluorine. There are several processes for extracting uranium with uranium-containing and fluorine-containing carbonate solutions, but they are mainly divided into two categories: acidification and alkalization.

[0007] The acidification process is to add nitric acid to the absorption liquid to drive out carbonate ions, then add sodium hydroxide to precipitate uranium, add lime water to precipitate fluorine in the form of calcium fluoride, and after solid-liquid separation, solid calcium fluoride and mother liquor with low concentrations of uranium and fluorine are obtained, which can be reused or discharged after treatment. When calcium fluoride is precipitated, the uranium remaining in the solution will be adsorbed by calcium fluoride due to the absence of carbonate ions, resulting in a high uranium content in the calcium fluoride slag.

[0008] Acidification to remove carbonate:

[0009] Na4UO2(CO3)3+HNO3===NaNO3+UO2(NO3)2+H2O+CO2↑

[0010] Alkalinization precipitation of sodium diuranate

[0011] UO2(NO3)2+2NaOH==NaNO3+Na2U2O7↓+H2O

[0012] Precipitation of uranium peroxide in acidic solution

[0013] UO2(NO3)2+H2O2===UO4↓+2HNO3

[0014] 2NaF+Ca(OH)2==CaF2↓+NaOH

[0015] The alkalization process is to add a small amount of sodium carbonate to the absorption liquid. After resin adsorption, the uranium concentration of the solution can be reduced to below 5 mg / L, and then lime water or calcium carbonate can be added to obtain calcium fluoride precipitation. The presence of fluoride ions will have an adverse effect on the adsorption effect of anion resins. Therefore, the absorption liquid needs to be diluted with defluorinated water to reduce the fluoride ion concentration of the adsorption liquid. In addition, the presence of nitrate ions will also reduce the adsorption capacity of anion resins for uranium, which will eventually lead to a uranium concentration of ~10 mg / L in the adsorption tail liquid. When fluoride ions are precipitated with calcium hydroxide, calcium carbonate and calcium fluoride are precipitated together, which will increase the amount of calcium fluoride slag produced and more waste needs to be treated.

[0016] The third method is to use potassium carbonate as an absorbent to increase the absorption strength of the absorption liquid. Using high-concentration potassium carbonate as an absorbent increases the concentration of each component of the absorption liquid, improves the absorption strength, and reduces the amount of absorbent circulation and treatment. However, the uranium content of calcium fluoride is not paid attention to.

[0017] In summary, in the current uranium conversion waste gas absorption liquid disposal technology, no matter what process is used, the specific radioactivity of the calcium fluoride slag obtained is greater than 10 mg / L, which does not meet the exemption standard. It can only be stored as radioactive waste, and the disposal cost is high. In the long run, radioactive solid waste will pile up like a mountain, posing a great environmental hazard. Summary of the invention

[0018] The technical problem to be solved by the present invention is to provide a method for preparing exempted calcium fluoride slag from a uranium-containing and fluorine-containing solution, wherein the treated calcium fluoride slag has low specific radioactivity and reduces the waste disposal cost.

[0019] The present invention provides a method for preparing exempted calcium fluoride slag from a uranium-containing and fluorine-containing solution, comprising the following steps:

[0020] Step 1: adding calcium carbonate to the uranium peroxide precipitation mother liquor to react, and performing solid-liquid separation to obtain a solid mixture of calcium fluoride and calcium carbonate, and a calcium fluoride precipitation mother liquor;

[0021] Step 2: A portion of the calcium fluoride precipitation mother liquor is deeply purified to remove uranium and fluorine and then discharged, and a portion is used as an absorbent.

[0022] Preferably, the amount of calcium carbonate added is greater than 100% of the theoretical amount.

[0023] Preferably, in step 1, the reaction time is 2 to 8 hours.

[0024] Preferably, in the step 2, the calcium fluoride precipitation mother liquor is used as an absorbent without adding sodium carbonate or potassium carbonate.

[0025] Preferably, the absorbent is added in step one.

[0026] Preferably, the uranium peroxide precipitation mother solution is obtained by removing uranium from the original solution.

[0027] Compared with the prior art, the method of preparing exempted calcium fluoride slag from uranium-containing and fluorine-containing solutions of the present invention has the following beneficial effects:

[0028] (1) Taking full advantage of the low uranium concentration in the uranium precipitation mother liquor, calcium carbonate is used to convert calcium fluoride into precipitation, so that calcium fluoride is precipitated in a sodium carbonate system with low uranium concentration, and CO3 2- With UO2 2+ Can form stable complex ion UO2(CO3)3 4- The properties of the solution reduce the free UO2 2+ The concentration of calcium fluoride is reduced to reduce the adsorption of uranium by calcium fluoride precipitation, so that the uranium content in the precipitate is reduced to below 40ppm, that is, the specific radioactivity intensity is less than 1bq / g;

[0029] (2) The specific strength of the mixed slag of calcium fluoride and calcium carbonate is further reduced;

[0030] (3) The composition of the mother liquor of calcium fluoride precipitation is not changed. Except for the excess water which needs to be discharged, the remaining water can be used as an absorbent after adding sodium carbonate;

[0031] (4) The process route using potassium carbonate as absorbent is also suitable for the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A flow chart showing the method of preparing exempt calcium fluoride slag from a uranium-containing and fluorine-containing solution of the present invention. DETAILED DESCRIPTION

[0033] In order to further understand the present invention, the embodiments of the present invention are described below in conjunction with examples, but it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the present invention.

[0034] The present invention takes uranium peroxide precipitation mother solution as the research object, at this time, the uranium concentration in the solution is less than 5 mg / L, the fluoride ion is about 5-30 g / L, the sodium concentration is 10-40 g / L, the nitrate ion is 5-50 g / L, and the solution acidity pH is 2-5.

[0035] The uranium peroxide precipitation mother solution is obtained by removing uranium from the original solution.

[0036] The embodiment of the present invention discloses a method for preparing exempted calcium fluoride slag from a uranium-containing fluorine-containing solution, such as Figure 1 As shown, the following steps are included:

[0037] Step 1: adding calcium carbonate to the uranium peroxide precipitation mother liquor to react, and performing solid-liquid separation to obtain a solid mixture of calcium fluoride and calcium carbonate, and a calcium fluoride precipitation mother liquor;

[0038] Step 2: A portion of the calcium fluoride precipitation mother liquor is deeply purified to remove uranium and fluorine and then discharged, and a portion is used as an absorbent.

[0039] In the prior art, calcium carbonate is directly added to the stock solution to transform into calcium fluoride without removing uranium. The uranium content in the solid is greater than 40ppm when a trace amount of the mother liquor is carried along. In the present invention, calcium carbonate is added after removing uranium. At this time, the uranium concentration of the solution has been reduced from 2g / L to 5mg / L. The amount of uranium carried along by the mother liquor in the precipitate and accumulated in the solid is greatly reduced.

[0040] The present invention adopts an operation method different from that of adding lime milk to the uranium peroxide mother liquor. Lime milk is added to the solution to produce calcium fluoride precipitation. At this time, no carbonate is produced in the solution, and all or most of the uranium in the solution will be adsorbed by the calcium fluoride precipitation, resulting in a high uranium content in the calcium fluoride, which is difficult to meet the exemption standard. The fluoride ion concentration in the uranium peroxide mother liquor is 15-20g / L. Calculated based on the specific radiation intensity of uranium of 25.4bq / g, and based on the assumption that all uranium in the solution is wrapped in calcium fluoride crystals, the uranium concentration of the solution is required to be 1.2-1.7mg / L. Such a low uranium concentration is unattainable.

[0041] The present invention adds calcium carbonate to the uranium peroxide precipitation mother solution to carry out the reaction. Calcium carbonate is used to transform calcium fluoride precipitation, and a large amount of carbonate and bicarbonate are generated in the solution. Due to the strong complexing ability of carbonate to uranyl ions, the existence of uranium in the solution changes from UO2 2+ Converted to UO2(CO3)3 4- , the uranium adsorbed on the calcium fluoride precipitate will be greatly reduced, so that the uranium content in the precipitate can be reduced to below 40ppm.

[0042] The time for the reaction of uranium peroxide with calcium carbonate in the precipitated mother liquor is preferably 2 to 8 hours.

[0043] The present invention is to prepare exempted calcium fluoride, which is different from the preparation of calcium fluoride with chemical purity requirements. The preparation of calcium fluoride with chemical purity requirements is to control the amount of calcium carbonate within 100% in the first step, and try to reduce the amount of unconverted calcium carbonate in calcium fluoride to save the amount of nitric acid used during pickling; the present invention is to control the amount of calcium carbonate to more than 100% of the theoretical addition amount, try to make the fluorine conversion complete, and the presence of a certain amount of calcium carbonate in the calcium fluoride does not affect the storage of this mixture as non-slag.

[0044] In the step 2, the calcium fluoride precipitation mother liquor is used as an absorbent without adding sodium carbonate or potassium carbonate.

[0045] The absorbent is added into step 1, thereby making full use of each product and reducing waste discharge.

[0046] The invention replaces the lime milk direct precipitation method with the calcium carbonate precipitation replacement method, so that the precipitation environment of calcium fluoride is changed from strong alkalinity to a carbonate buffer system, and the characteristic of carbonate and uranyl ion forming a stable complex is utilized to prevent uranium from adhering to the precipitate, and the uranium concentration of the mother liquor is kept basically stable, so that the specific radioactivity intensity of the calcium fluoride slag reaches the exemption level. Compared with the radioactive slag as a storage material, the storage cost and the impact on the environment can be greatly reduced.

[0047] In order to further understand the present invention, the method for preparing exempted calcium fluoride slag from uranium-containing and fluorine-containing solutions provided by the present invention is described in detail below in conjunction with examples, and the protection scope of the present invention is not limited by the following examples.

[0048] Examples 1 to 7

[0049] Taking the uranium precipitation mother liquor as the starting point, pay attention to the uranium and fluorine concentrations, add calcium carbonate powder according to the amount of fluoride ions, stir for 2 hours, the temperature is not required, filter, and do not wash to reduce the amount of washing water. The filtrate is collected and used for two purposes. One part is added with sodium carbonate as an absorbent, and the other part enters the wastewater treatment and is discharged.

[0050] Table 1 Experimental results of the embodiment

[0051]

[0052] The above embodiments are only used to help understand the method and core idea of ​​the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

[0053] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing exempted calcium fluoride slag from a uranium-containing and fluorine-containing solution, characterized in that: The following steps are involved: Step 1: adding calcium carbonate to the uranium peroxide precipitation mother liquor to react, and performing solid-liquid separation to obtain a solid mixture of calcium fluoride and calcium carbonate, and a calcium fluoride precipitation mother liquor; Step 2: A portion of the calcium fluoride precipitation mother liquor is deeply purified to remove uranium and fluorine and then discharged, and a portion is used as an absorbent.

2. The method for preparing exempted calcium fluoride slag from uranium-containing and fluorine-containing solutions according to claim 1, characterized in that: The added amount of calcium carbonate is greater than 100% of the theoretical added amount.

3. The method for preparing exempted calcium fluoride slag from uranium-containing and fluorine-containing solutions according to claim 1, characterized in that: In the step 1, the reaction time is 2 to 8 hours.

4. The method for preparing exempted calcium fluoride slag from uranium-containing and fluorine-containing solutions according to claim 1, characterized in that: In the step 2, the calcium fluoride precipitation mother liquor is used as an absorbent without adding sodium carbonate or potassium carbonate.

5. The method for preparing exempted calcium fluoride slag from uranium-containing and fluorine-containing solutions according to claim 4, characterized in that: The absorbent is added into step one.

6. The method for preparing exempted calcium fluoride slag from uranium-containing and fluorine-containing solutions according to claim 1, characterized in that: The uranium peroxide precipitation mother solution is obtained by removing uranium from the original solution.

Images