RU2517651C1 - Method for solvent refining of nitrate solutions containing rare-earth metals - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method involves multi-step counter-current extraction of impurities from an aqueous nitrate solution with tributyl phosphate. Purification is carried out using a solution with rare-earth metal concentration of 100-150 g/l, while using 5 extraction steps with the ratio organic phase: aqueous phase=1:1.1, 3-5 washing steps with the ratio organic phase: aqueous phase=10:1 and 5 re-extraction steps in a system of 100% tributyl phosphate and aqueous nitrate solution. A solution with content of ballast impurities of less than 4% of the sum of rare-earth metals and total specific alpha activity of less than 1·10-8 Ku/(kg rare-earth metals) is obtained.

EFFECT: rare-earth metal solution satisfies consumer requirements for concentration and impurity content and sanitary requirements for content of radionuclides.

3 dwg, 9 tbl

Description

The invention relates to hydrometallurgy and can be used to purify impurities from nitrate solutions containing rare-earth metals (REM). The invention can be used, in particular, in the technology of producing rare-earth metals from orthite at the stage of purification of a nitrate solution containing rare-earth metals from ballast (Fe, Al, Ca, Mg) and radioactive impurities, including thorium.

There is a method for the extraction of thorium from aqueous solutions containing rare earth metals [RU 2188157, IPC ⁷ C01F 15/00. Publ. 08/27/2002], in the form of an insoluble salt, moreover, an alkali is added to the aqueous solution to neutralize free nitric acid to a concentration of 0.005-0.1 mol / L in the solution, sodium benzenesulfinate and collector polyfluorinated organic compounds: acids, or alcohols, or aldehydes, or ketones of the general formula C _x F _y O _z H _s , where x = 5-13, y = 5-19, z = 1-2, s = 1-4, in the ratio of aqueous solution: collector (5-10 ): 1, stirred, the collector is separated, followed by extraction of thorium benzenesulfinate from it.

The disadvantage of this method is the need for an operation to neutralize nitric acid and the use of an additional several inorganic and organic compounds.

A known method of extraction extraction and separation of TPE and REE from nitric acid solutions [RU 2106030, IPC ⁶ G21C 19/46. Publ. 02/27/1998]. The extraction and separation of TPE (transplutonium elements) and REE (rare earth elements) from liquid nitric wastes is carried out by extraction with a solution of zirconium salt of organophosphoric acid, mainly dibutylphosphoric acid (DBPC) in a neutral organophosphorus reagent. The organic reagent used is mainly tributyl phosphate at a concentration of 5-45 vol.% In an inert diluent. During the extraction process, the molar ratio Zr / DBPC = 1 / 50-1 / 4. The extractant is compatible with the Purex Process extraction technology. TPE and REE are separated by washing the extractant with a solution of nitric acid at a concentration of 3-12 mol / L, after which REE is reextracted. The reverse extractant is regenerated by removal of the zirconium salt of the organophosphorus acid by carbonate-alkaline washing.

The disadvantage of this method is the need for continuous updating of the composition of the circulating extractant.

Closest to the proposed method is a method of purification of regenerated uranium [RU 2425804, IPC C01G 43/00, B01D 11/04. Publ. 08/10/2011], adopted as a prototype. The method involves extraction of uranyl nitrate from an aqueous nitric acid solution with tributyl phosphate in an organic diluent. In the process of multi-stage countercurrent extraction, the degree of saturation of the extractant with uranium of more than 92.5% of the maximum saturation of the extractant with uranium and the equilibrium concentration of nitric acid in the aqueous phase of not more than 0.7 mol / l at a concentration of nitric acid in the first stream of feed aqueous nitric acid are maintained at the extract delivery stage. a solution of uranyl nitrate supplied to the stage of extracting the extract, not more than 0.6 mol / l and the amount of saturation of the extractant with uranium more than 92.5% of the maximum saturation of the extractant with uranium on the third or last blowing the aqueous phase during the steps when it is applied to a second feed stream of an aqueous solution of uranyl nitrate. Uranyl nitrate solutions are fed for extraction with a uranium concentration of 400-500 g / l. The concentration of nitric acid in the first stream is 0.03-0.04 mol / L, and in the second stream 0.7-0.8 mol / L. Hydrazine stabilized uranium (IV) is introduced into the uranyl nitrate solution.

The disadvantage of this method is the need for additional consumption of valuable finished product - concentrated (purified and stripped up to 400-500 g / l of uranyl nitrate solution) for washing the extract.

The aim of the invention is to provide a method for the extraction purification of a nitrate solution, using an extractant with only one predetermined concentration, without washing the extract with ready-made concentrated solutions, with an REM extraction degree of at least 95% with ballast impurities (Fe, Al, Ca, Mg) less than 4% the amount of rare-earth metals and radioactive impurities of thorium, the relevant standards for radiation safety and sanitary rules.

The problem is achieved in that the method of extraction purification of nitrate solutions containing rare-earth metals, including multi-stage countercurrent extraction of impurities from an aqueous nitric acid solution with tributyl phosphate (TBP), is used to clean solutions with a concentration of rare-earth metals 100-150 g / l and consists of 5 stages of extraction with a ratio O: B = 1: 1.1; 3-5 washing steps with a ratio of O: B = 10: 1 and 5 stages of stripping in a system of 100% TBP - an aqueous solution of nitric acid salts (O is the organic phase, B is the aqueous phase).

The invention consists in the following.

A method is proposed for the extraction purification of a nitrate solution containing REM from ballast (Fe, Al, Ca, Mg) and radioactive impurities, including thorium, to standards that comply with radiation safety standards and sanitary rules. A schematic diagram of the purification of nitrate solutions containing rare-earth metals is shown in figure 1. For this scheme, the optimal conditions for the extraction purification of the rare-earth nitrate solution from ballast and radioactive impurities were experimentally determined.

1. Extraction cleaning of rare-earth metals from ballast impurities

1.1. Effect of salting out concentration

Determined the minimum concentration of salting out agent at which rare-earth metals are extracted. The concentration of the salting-out agent was set by the concentration of nitric acid used to open the mineral raw materials, for example, orthite, and by diluting the resulting solutions with washing solutions. We performed experiments on countercurrent extraction of rare-earth metals at three stages from solutions with different degrees of dilution with water. The concentration of salting out agent was judged by the nitrate content. The results of the experiments are shown in table 1.

Table 1 The composition of the products obtained in the process of countercurrent extraction of rare-earth metals, depending on the concentration of nitrates in the initial solution of rare-earth metals The composition of the initial solution [REM], g / l [HNO ₃ ], N [REM], g / l [NO ₃ ] ^- , N raffinate extract extract 84.0 8.3 1.3 85.0 0.87 Diluted with water 1.4 times the initial solution of rare-earth metals 61.0 6.0 3.6 81.3 0.73 Note. N is the normality of the solution, g-equiv / L.

From the data shown in table 1, it follows that when the concentration of nitrates in diluted with water the initial solution of rare-earth metals equal to 6.0 N, the extraction of rare-earth metals is 96.8%. Therefore, when obtaining the initial solution of rare-earth metals in the process of opening ore 8-12 N HNO _3, you can do without additional evaporation of the initial solution of rare-earth metals obtained from mixing the mother liquor and promrastnoy if the concentration of the salting out agent in it is higher than 6.0 N.

1.2. The effect of REM content in the extract

To determine the effect of the REM content in the extract on the distribution of ballast impurities, the degree of saturation of TBP with rare-earth metals was changed. The experimental results are presented in table 2.

table 2 The effect of the content of rare-earth metals in the extract on the distribution coefficients of ballast impurities between 100% TBP and an aqueous nitric acid solution of rare-earth metals Extract The distribution coefficients of impurities, To _p [REM], g / l [HNO ₃ ], N REM Fe Al Ca Mg Si Mn Cr Ni 36.5 1.23 0.28 0.31 0.68 0.23 1,0 1,5 0,021 - 0.50 47.3 0.24 0.52 0.32 0.34 0.12 0.22 0.64 0.017 0.2 0.50 116 0.34 - 0,047 0,029 0.11 0.20 0.87 - - 0.21

From the data presented in table 2, it follows that with an increase in the saturation of TBP with rare-earth metals, the distribution coefficients, K _p , of ballast impurities decrease, but for calcium, magnesium, nickel, and silicon, even when the REM content in the extract is 116 g / l, they remain quite large

K _p (Ca) = 0.11; K _p (Mg) = 0.20; To _p (Ni) = 0.21; To _p (Si) = 0.87.

It follows that for minerals having a high content of calcium, magnesium and silicon, for example orthite, these elements will limit the extraction purification of rare-earth metals from impurities.

1.3. The effect of washing the extract of REM water

To determine the number of theoretical stages of REM purification from impurities during the operation of washing the extract with water, experiments were carried out to determine the distribution coefficients of REM and related impurities between 100% TBP and washing water. For the experiments, we used a solution obtained by evaporation of solutions from opening an orthite containing REM 145 g / l. Extraction from this solution at three stages yielded an extract of TBP containing 116.3 g / L REM, 0.34 N nitric acid, and impurities. Four portions of TBP extract were obtained from the REM extract by diluting with an extract containing 3.7 N nitric acid and pure TBP, the compositions of which are shown in Table 3.

Table 3 Extract Formulations Extract No. Content [REM], g / l [HNO ₃ ], N one 77.5 0.23 2 77.5 0.54 3 77.5 0.84 four 77.5 1.46

The obtained extracts were brought into equilibrium with water at a ratio of the organic phase (O) and the aqueous phase (B) O: B = 5: 1. The results of the analysis of the obtained solutions are shown in table 4.

Table 4 The effect of phase composition on the distribution coefficients, K _p , REM, Th and HNO ₃ between 100% TBP and an aqueous wash solution Extract number [REM], g / l To _p REM [Th], g / l K _r Th [HNO ₃ ], N To _p HNO ₃ Organic phase Water phase Organic phase Water phase Organic phase Water phase one 47.3 91.3 0.52 8.0 0.35 23 0.24 0.3 0.8 2 43.8 98.7 0.44 8.3 0.28 thirty 0.3 0.4 0.75 3 37.5 103 1.36 8.0 0.2 40 0.76 0.65 1,2 four 36.5 129 0.28 7.8 0.15 52 1.23 1.3 0.95

From the data given in table 4, it follows that the distribution coefficients of thorium, nitric acid and rare-earth metals, implemented at the stages of washing and stripping, are located in the following row:

K _p (Th) >> K _p (HNO ₃ )> K _p (REM).

Therefore, thorium and nitric acid will follow REMs on flushing. On the re-extraction of rare-earth metals, thorium, in which K _{p is} approximately two orders of magnitude higher than K _p REM, for the most part will remain in TBP. Given the high content of calcium, magnesium and silicon in the orthite, it can be assumed that it is these elements that will limit the cleaning of rare-earth metals from impurities.

Experiments were conducted on the redistribution of extraction-washing, which allowed to determine the cleaning coefficients, K _Pts , REM from impurities, the values of which are given in table 5.

Table 5 The content of impurities in products after experiments on the extraction purification of rare-earth metals Product [REM] g / l The content of impurities in nitric acid solutions of rare-earth metals,% wt. (in relation to REM) Fe Al Ca Mg Si Mn Cr Ni REM stock solution 84.0 48 68 35 6.2 6.8 6.7 0.35 0.47 Extract 63.0 0,032 0.12 0,035 0.27 0.72 0.006 0.016 0.016 To _Pts REM from impurities - 1500 570 1000 23 9,4 1030 22 29th

From the data presented in table 5, it follows that the smallest purification of rare-earth metals from silicon: To _Pts = 9.4. Therefore, the silicon content in the washed extract determines the total impurity content in the rare-earth metal stripping. The washed extract turned out to contain a lot of magnesium - 0.27% in terms of rare-earth metals, but their content in the initial solution is small (less than 0.5%) and their contribution to the total content of impurities in the final nitric acid solution of rare-earth metals will also be insignificant. With an increase in the number of washing stages to 5, the total content of impurities determined in the washed REM extract was less than 0.1% due to washing the REM extract from aluminum, calcium, and magnesium.

2. Cleaning the solution of rare-earth metals from thorium

It was shown above that thorium is extracted with 100% TBP better than REM, therefore, during REM extraction with tributyl phosphate and during the washing of the extract, thorium will follow with REM. In the process of re-extraction of rare-earth metals with water, thorium must also be re-extracted. To determine the behavior of thorium in the processes of extraction and re-extraction of rare-earth metals, we conducted two series of experiments (series of experiments P1 and series of experiments P2) on a countercurrent extraction cascade, the scheme of which is shown in Fig. 1.

For research, mother liquors from opening orthite ore with a maximum Th content of 1.32% were used. following compositions:

- for a series of experiments P1: [REM] = 110 g / l; [Th] = 1.29 g / l; [HNO ₃ ] = 1.44 N; total content of ballast impurities - 4.7% in relation to rare-earth metals;

- for a series of experiments P2: [REM] = 84 g / l; [Th] = 0.63 g / l; [HNO ₃ ] = 0.87 N; the total content of ballast impurities is 3.8% with respect to rare-earth metals.

Schemes of extraction and stripping are shown in figure 2 and 3.

The compositions of the products obtained in the series of experiments P1 and P2 are shown in table 6.

Table 6 The composition of the products obtained in experiments on the extraction cleaning of rare-earth metals from thorium No. of reextract (extract), (see figure 2) Experiment Series P1 Test Series P2 [REM] _V , g / l [Th] _B , g / l [Th] _Oh , g / l [REM] _V , g / l [Th] _B , g / l [Th] _Oh , g / l one 36 0,028 1.3 45.6 0.013 0.63 2 44 0 0.84 55.0 0 0.63 3 86 0.012 0.10 61.3 0 0.63 four 93 0.007 0.12 66.3 0,048 0.63 5 97 0.008 0.11 68.8 0,065 0.63 Note. The content of rare-earth metals in all samples of the extract was less than 0.04%.

From the data presented in table 6, it follows that REM and thorium from the initial solution are extracted almost completely (more than 99.99%). The content of rare-earth metals in the combined extract is not less than 84.9 g / l, and the maximum content of rare-earth metals in the reextract is 97 g / l. The thorium content in the rare-earth metal reextract in the process of reaching equilibrium decreased to 65 mg / L, which corresponds to the total specific alpha activity of the solution for thorium-232, thorium-228 and alpha-active decay products of 1.28 × 10 ^-9 Ku / l In the normative documents [Temporary sanitary rules for the organization of production of civilian products in the sanitary protection zone of enterprises of 4 SSTU Minatomenergoprom (VSP-K4-91). - M .: 1991] this parameter should not exceed 1 × 10 ^-8 Ku / l.

The result of spectral analysis of samples of rare-earth metal stripping products for a series of experiments P2 on the content of impurities is presented in table 7.

Table 7 The content of impurities in the products obtained in experiments on the extraction cleaning of rare-earth metals from thorium Product Name [REM], g / l The content of impurities in nitric acid solutions of rare-earth metals, wt.%, In terms of rare-earth metals Fe Al Ca Mg Si Mn Cr Ni REM stock solution 84.0 4.8 6.8 3,5 6.2 6.8 6.7 0.35 0.47 REM reextract 5 68.8 <0.23 0.25 1.7 0.23 1,2 <0.02 <0.05 <0.08 REM purification factors from impurities - 20.9 27.0 2.0 27 5.7 > 35 > 7 > 6

From the data given in table 7, it follows that the total content of impurities in terms of rare-earth metals in the reextract obtained in the P2 series of experiments is 3.8%, and the thorium content in the reextract is up to 65 mg / l (0.43 × 10 ^{- 7} Ku / l).

For a more complete purification of the rare-earth metal reextract from thorium obtained in the P1 series experiment, thorium tributyl phosphate was extracted twice from the combined re-extract samples Nos. 4 and 5 at O: B = 3: 6. The raffinate was evaporated to [REM] = 228 g / L. The thorium content in one stripped off extract was 2.5 mg / L or, in terms of the total specific alpha activity of rare-earth metals in thorium-232, thorium-228 and alpha-active decay products, 7.4 × 10 ^-9 Ku / kg REM. Thus, the coefficient of purification of rare-earth metals from thorium increased to 10700. Thorium according to the scheme in figure 1 leaves the block of re-extraction of rare-earth metals with tributyl phosphate in the block of carbonate re-extraction. The maximum thorium content in the extract is 1.3 g / l (see table 6). The thorium carbonate solution in accordance with the proposed scheme is neutralized with nitric acid, and the precipitated thorium hydroxocarbonate precipitates (see table 8) are filtered off and sent for disposal as solid radioactive waste of the middle activity group. The mother liquor, containing up to 2 mg / l of thorium, is directed to washing the insoluble orthite residue.

Table 8 The thorium content in the mother liquor after precipitation of thorium hydroxocarbonate stock solution pH 6 5 four 3 [Th], mg / l 260 1,6 1,2 240

To compare the results of the analytical determination of thorium in orthite with the results of gamma spectrometric determination, a gamma spectrum was made. The research results showed that the total specific alpha activity of orthite, calculated analytically (8.71 × 10 ^-6 Ku / kg), practically coincided with the result of gamma-spectrometric determination: 8.94 × 10 ^-6 Ku / kg. The obtained values of the total specific alpha activity of orthite are well correlated, which confirms the reliability of the results.

The results of the experiments show that the proposed scheme for the extraction purification of a rare-earth nitrate solution of REM from thorium impurities allows one to obtain a commodity solution of rare-earth nitrates, which meets the requirements of regulatory documents on the content of alpha-active products.

The proposed technological scheme allows you to clean REM from related impurities with the cleaning factors listed in table 9.

Table 9 Purification factors for impurities extracted from nitrate solution of rare-earth metals Thorium-232, -228 Fe Al Ca Mg Si Mn Cr Ni > 10,000 1500 570 1000 23 9,4 1030 22 29th

Studies have shown that for the extraction purification of nitrate solutions with a rare-earth metal concentration of 110-150 g / l from ballast and radioactive impurities, 5 extraction steps, 3-5 washing steps, 5 re-extraction steps in a system of 100% TBP - an aqueous solution of nitric acid salts are required .

The proposed method of extraction purification of nitrate solutions allows to obtain a solution containing REM 100-150 g / l, with a content of ballast impurities (Fe, Al, Ca, Mg) of less than 4% to the sum of REM and the total specific alpha activity of less than 1 · 10 ^-8 Ku / (kg REM) using only one extractant - 100% TBP. The resulting solution of rare-earth metals satisfies the requirements of the consumer for the concentration and content of impurities and the requirements of sanitary rules for the content of radionuclides.

Claims (1)

The method of extraction purification of nitrate solutions containing rare-earth metals (REM), including multi-stage countercurrent extraction of impurities from an aqueous nitric acid solution with tributyl phosphate (TBP), characterized in that the solution is purified with a concentration of REM 100-150 g / l, with 5 stages of extraction with a ratio of organic and aqueous phases O: B = 1: 1.1, after which 3-5 washing steps are carried out with a ratio of O: B = 10: 1 and 5 stages of stripping in a system of 100% TBP - an aqueous solution of nitric acid salts.

Images