DE1592240C - Process for the back-extraction of acids from organic solvents - Google Patents

Description

The invention relates to a method for back-extraction of actinides, in particular uranium, neptunium and plutonium, from an organic solvent, into which they are extracted during the extraction of their nitric acid Solutions containing this solvent have entered.

The purification of uranium, neptunium and plutonium is generally carried out using extraction Solvents made. These solvents, for example tributyl phosphate or trilaurylamine, Dissolved in a paraffinic hydrocarbon, such as dodecane, can be obtained by contact with the nitric acid Load solutions of the actinides with the actinides.

The following stage of the process consists in a back extraction of these contained in the solvent Metal nitrates. The back extraction can be done in different ways:

Back extraction by dilution

Since the metals are extracted into the solvent in the form of nitrate complexes, it is theoretically sufficient to reduce the nitrate ion concentration in the aqueous solution so that the complex decomposes and the metal can no longer be extracted by the solvent. This back-extraction method is used, for example, in the back-extraction of uranyl nitrate, which is contained in tributyl phosphate, using dilute HNO ₃ .

Back extraction through change in value

With the help of a suitable oxidizing or reducing agent you can convert an extracted metal from an extractable valence level to a non Transfer extractable valency level. This is the case with nitric acid solutions of iron (II), which reduce the Pu (IV) to Pu (III) and thus drive it out of the solvent in which it is contained are.

The inventive method consists in the actinides and in particular uranium, plutonium and neptunium, from organic solvents, in which they are after the extraction of their nitric acid Solutions with these solvents are contained, to be extracted back using nitrous acid.

The organic solvents preferably consist of tributyl phosphate or trilaurylamine, the are diluted with an inert solvent.

This new back-extraction process is based on the following two findings:

1. The nitrous acid HNO ₂ has a sufficiently high affinity for the common organic solvents tributyl phosphate and trilaurylamine to cause back extractions by displacement.

2. Under defined conditions, nitrous acid can reduce Pu (IV) to Pu (III). The thus obtained The back extraction effect is superimposed on the displacement effect.

The nitrous acid can be introduced into the back-extracting aqueous solutions in two ways: either in the form of sodium nitrite NaNO ₂ in weak nitric acid solutions; it is unstable therein and slowly disproportionates according to the equation:

3HNO ₂ -> 2NO + HNO ₃ + H ₂ O (1)

however, their temporary presence enables the back extractions to be carried out, if you work fast enough.

The nitrous acid is preferably introduced into the aqueous back-extracting solution in the form of nitric oxide, which is brought into contact with a nitric acid solution kept under exclusion of air. The formation of the HNO ₂ takes place according to the inverse of the above reaction equation as follows:

2 NO + NO ₃ - + H ⁺ + H ₂ O <= * 3 HNO ₂ (2)

Back extraction by displacement

Your equilibrium constant is

If an acid has a sufficiently high affinity for a solvent, it occupies all the binding sites of the same and pushes the metals previously retained there back into the aqueous solution. This type of back-extraction is for example in back-extraction of plutonium nitrate contained in trilaurylamine before gen means Uberchlorsäurelösun-.

K =

(ENT ₂ ) ³

0.69, (3)

Back extraction through complex formation

If any chemical compound in aqueous solution forms a non- extractable complex with the extracted metal, their solutions can back-extract that metal. This is the case with the back-extraction of plutonium contained in trilaurylamine using sulfuric acid.

In practice there is often a mixed type of back extractions; that is, a back extraction by displacement at the same time also a back-extraction by dilution or even by Complex formation is.

from which it follows that the HNO ₂ concentration is proportional to the NO pressure raised ^{to the 2} / jP ° ^tenz.

The yield of the back extraction by displacement obviously depends on the ability of the nitrous acid to pass from the aqueous phase into the solvent and to saturate it. This ability differs depending on the solvent and can be expressed in terms of the distribution coefficient:

Kd =

(HNO ₂ ) org
"(HNO ₂ ) aq ~

The value of this coefficient was determined by adding NO to atmospheric pressure in mixtures initiated, which consist of equal volumes of the respective solvent and differently concentrated salt

pitric acid passed. The results obtained are shown in Table I.

table I. Trilaurylamine 1 (TLA) ENT, in aqueous
solution Tributyl phthalate
(TBP) 0.96 Normality (N) 30%. 5% 0.74 20% 0.68 2.4 0.05 0.61 2.3 0.1 2.5 0.31 2.5 0.2 2 0.3 7th 1.6 0.5 7.5 0.94 1 5.4 1.5 5.5

With regard to the trilaurylamine, it follows from the table that the increase in the nitric acid concentration of the aqueous phase due to the mass (competitive) effect, a reduction in the partition coefficient the nitrous acid causes. On the other hand, it turns out that nitrous acid is a special one Has affinity for tributyl phthalate.

To investigate the extent of the re-extraction of metals by displacement by means of nitrous acid mixtures were, which consisted of equal volumes of loaded with uranium (VI) or neptunium (IV), solvent and dilute nitric acid, treated _^3/4 hour by introduction of NO with atmospheric pressure , whereby one could determine the values of the distribution coefficients Ej of UO ₂ ⁺⁺ and Np ⁴⁺ for each type of mixture by means of the «-count rate of the separated phases and compare them with the values obtained without the introduction of NO.

Table II

Watery
Normality (N)

0.1

0.2

0.3

0.5

1.5

With NO

U (VI) in TBP 30%

0.083

0.23

0.50

Np (IV) in TLA 20% 0.65
0.24
0.42
0.46
0.86

Without NO

U (VI) in TBP 30%

0.25
0.7
1.3
2.5
7th
10

Np (IV) in TLA 20%

23
30th
70
95

The nitrous acid also enables a back-extraction of plutonium from an organic one Phase through change in value.

The nitrous acid reduces the Pu (IV) according to the following reaction equations and potentials:

HNO ₂ + H _; υ * = » 0.058 NOJ + 3H ⁺ + 2 Pu ⁴⁺ + 2 e * = * Pu ^{3 +} , E ₄ - 0.94 + 0.058 - log - (NOJ) (H ⁺ ) ³ E ₅ - 0.968 + log (ENT ₂ ) Pu ^{4 +}
r> .. 3+

The potential E ₄ can also be written:

E ₄ = 0.94 + 0.029 log - -0.087 pH

on the ratio NO / NO ₃ . The same applies to the oxide reduction potential.

By getting HNO ₂ from the following two reaction equations

and shows that the NOj / HNO ₂ pair has a more reducing effect as the acidity of the solution decreases.

Restricted to avoid the dangers of polymerisation and hydrolysis of plutonium however, set the working range to a pH below 1.4.

If nitrous acid is formed in the solution by the introduction of NO, its concentration for a given H monene concentration depends on ⁺ + emitEo = 0.99K (8)

(4)

(5) and

50 (4)
eliminated, one obtains

H ₂ O <= ± NO ₃ + 4H ⁺ + 3e with E «= 0.957 K,

whose oxydo-reduction potential can be written as follows:

E ₇ = 0.957+ 0.019 log

-0.077 pH

and is a linearly decreasing function of pH.

This function is shown graphically in the figure for (NO) = 1 atmosphere and (NOj ") = 0.1N. In this figure the area where one can reasonably work is hatched around the plutonium to reduce.

According to the process according to the invention, the plutonium-laden solvent is _{stirred with dilute HNO 3} under an NO atmosphere, the nitrous acid displacing part of the plutonium into the aqueous phase, where it is then reduced

will. The shift in equilibrium continues until the oxide reduction potentials of the pairs Pu ⁴⁺ / Pu ³⁺ and NO ₃ ~ / NO equalize in the aqueous phase. In the final state of these equilibria there is a distribution of the plutonium between the organic and the aqueous phase, which can be expressed by the distribution coefficient B ₁ 'of the plutonium.

A number of values of this coefficient was determined for each case after _^3/4 hour equilibration obtained and shown in Table III.

1
£ i! Pu Table III TBJP 30%
aqueous HNO ₂
in balance
Molarity (M) E ¹ J Pu TLA 20%
aqueous HNO ₂
in balance
. Molarity (M) £ 11 pu TLA 5%
aqueous HNO ₂
in balance
Molarity (M) Aqueous ENT ₃
Normality (N) 0.028 0.036 0.018 0.037 0.05 '0.006 0.1 0.031 0.047 0.021 0.046 0.1 0.11 0.045 0.036 0.063 0.2 0.015 0.1 0.38 0.067 0.022 0.075 0.3 0.013 0.09 0.86 0.095 0.067 0.105 0.5 0.080 0.15 7.3 0.180 1.6 0.19 1 0.12 0.16 1.5

In the following two embodiments of the method according to the invention are described, which Back extraction of Pu from a trilaurylamine in 5% (volume content) solution in dodecane as organic solvent and the back-extraction of 6-valent uranium from a 30% Solution of tributyl phosphate in dodecane concern existing organic solvents.

Example I.

The apparatus used consists of a 12-stage, completely closed battery of mischurid decanting vessels. The twelfth stage serves only the introduction of the aqueous back extraction phase and the introduction of NO into the same, the NO excess then fills all the free volumes of the battery and from stage 1 through a intended outlet escapes.

The one entering stage 1, the back extraction to. Subjecting solvent has the following Composition:

HNO ₃ , TLA 5% in dodecane with 0.375 g / l Pu; free 0.05 N HNO ₃ ; Throughput: 4 V / hour

The aqueous back-extraction solution consisting of 0.05 N HNO ₃ saturated with NO is used in stage 11 in an amount of 1 V / hour. introduced.

The analyzes carried out after the battery had reached equilibrium yielded the Table IV shows the values of the concentrations of the various chemicals used Units (species).

Table IV

battery hnöTTm) org. • aq. Aqueous ENT ₁ org. Pu mg / 1 aq. Pooh step 0.046 0.064 (N) 470 1,500 0.31 0.046 0.061 260 1,820 0.14 1 0.047 0.061 0.317 140 1,380 0.10 2 0.048 0.064 0.226 50 960 0.052. 3 0.049 ' 0.070 0.205 13th 438 0.03 4th 0.046 0.066 0.181 6th 219 0.027 · .. 5 0.050 0.070 0.163 2 120 0.017 6th 0.044 0.060 0.131 .0.8 43 0.019 7th 0.047 0.063 0.130 0.6 12th 0.05 8th 0.048 0.058 0.118 0.4 . 4th 0.1 9 0.044 0.040 0.107 0.7 3 0.23 10 0.103 Il 0.048

These figures show that the back extraction yield is 99.8%.

B e i s ρ i e I II

The same apparatus as in Example I was used for the back-extraction of 6-valent uranium.

The organic phase consists of a 30% solution of tributyl phosphate in dodecane. It is loaded with 72 g / l uranium; their throughput is 150cm7h.

The aqueous phase has a concentration of nitric acid of 0.125 N, and introducing

concentration uranium
'(g / l) nitrate
(M) Total-
acidility
, (N) Organic phase .....
Aqueous phase 4.5
135 0.37
0.09 0.145

Nitrous acid is introduced into the battery during operation, nitric oxide. The throughput is 75 cm ³ / hour. · ,,

The nitric oxide is previously used to remove nitric peroxide with a concentrated sulfuric acid solution washed. Its pressure in the battery is adjusted to atmospheric pressure.

After 4 hours of operation, the total acidity of the aqueous phase fed in is 0.125 increased to 0.150N.

The concentrations of the emerging solutions are summarized in the following table:

and the solution obtained can be fed directly to precipitation or a new extraction will.

The back extraction of U (VI), Np (IV), Pu (IV) contained in TBP or TLA is carried out by Displacement achieved at medium acidity (1 to 1.5 N).

Np (IV) can be separated from Pu (IV) in the back extraction.

Under an NO atmosphere and in a 1 N nitric acid environment, the distribution coefficients of Np (IV) are and Pu (IV) in 20% TLA E «Np = 0.86 or £« Pu = 7.3, and they can under these conditions be separated by a countercurrent back extraction. .

Claims (2)

The method according to the invention has a number of interesting features: The back-extraction of the plutonium extracted by TLA or TBP is effected with the aid of a reagent which disappears by itself after use, so that the subsequent treatment process is not disturbed. In addition, if a more rapid decomposition of the nitrous acid remaining in the back-extraction solution is desired, all that is necessary is to acidify. The nitrous acid then reoxidizes the plutonium (III) to plutonium (IV), 1. Process for the back-extraction of actinides, in particular uranium, plutonium and neptunium, from organic solvents in which they are after the extraction of their nitric acid solutions are contained with these solvents, characterized in that the Back-extraction is carried out using nitrous acid. 2. The method according to claim I, characterized in that that the nitrous acid in the aqueous back-extracting solution in the form of nitric oxide, in the absence of air in contact with a dilute aqueous nitric acid solution is brought, is introduced. 1 sheet of drawings 109085/152