CS215867B1 - Method of solidifying uranium from non-aqueous liquid phases having limited water intermiscibility - Google Patents

Abstract

The invention relates to those parts of a nuclear fuel cycle in which extraction is applied uranium by trialkyl phosphates or tertiary ones amines. It's one-stage solidification of uranium from said liquid phases, miscible with water, namely by contacting with a precipitating solution as its basic component contains different chemical forms of ammonia and carbon dioxide, preferably at a molar ratio of about 1.6. A precipitate of ammonium carbonate is formed it is still macrocrystalline, though its composition is clearly different from the maximum NH 4 / 4.0 and CO 3 3.0 and corresponds to the formula (NH 4 - / UO 2 CO 2 7 s) x é 4.0 and y 3.0. It means lower consumption chemicals in preparation and lower the content of the neuronal components in the mother liquor or waste processing waters.

Description

The invention relates to a process for solidifying uranium from a non-aqueous liquid phase limited by miscible and water contacting and aqueous precipitation solution, and can be applied, for example, to the extraction of aqueous uranium solutions.

So far, solidification of uranium has been carried out in two stages. First, the uranium from the extractant solution in an organic diluent is re-extracted into an aqueous solution, from which uranium is then precipitated by an agent of generally alkaline nature. It is required that the residual uranium content in the organic phase be kept to a minimum, that the resulting uranium precipitate is well separated from the mother liquor and that its properties are favorable for further processing during which the precipitate is generally converted into uranium dioxide powder in high temperature operations. production of dense tablets.

The disadvantages of the prior art processes are often the low efficiency of reextraction and sometimes poor filterability of the uranium precipitate formed. If an ammonium carbonate solution is used for precipitation, a very good filterability of the precipitate can be achieved, but its solubility in the mother liquor is relatively large, unlike the ammonia precipitation product. The solubility of this carbonate precipitate can be reduced by applying the salting-out effect of various salts, even with an excess of ammonium carbonate alone, but this increases the demand for wastewater disposal.

According to what. No. 174,298 uses a single-stage solidification of uranium from organic solutions in which uranium is present as a solvate by preparing a ceramic nuclear fuel by contacting the aqueous uranium containing solution directly with a saturated aqueous solution of ammoniacalized ammonium carbonate to form macroscopically crystalline salts of tetraammonium tri carbonateouranylate (NH4J4FuOgO4) (hereinafter referred to as TKUTA). Also British Patent Nos. 1,474,529 and 1,487,223 and U.S. Pat. each one node as a complicated technology for the cleaning or separation of the uranium, wherein, inter alia, claimed as solutions by di-alkylphosphates, the concentration of ammonium and carbonate ions in the range of 1.5 mol.l Ar for 2.5 ^- ^ · and a precipitate is formed exclusively eložení TKUTA.

Disadvantages of the prior art uranium solidification processes are eliminated by the method of solidifying uranium from a non-aqueous, water-miscible liquid phase according to the invention, which comprises contacting a non-aqueous liquid phase containing trialkylphosphates or tertiary amines with an aqueous precipitation solution containing components of the various chemical forms of ammonia and carbon dioxide, wherein the precipitation solution is not saturated with any of these constituents under given physicochemical conditions, the molar ratio of ammonia to carbon dioxide before contacting is greater than 1, preferably 1.6, and the volume ratio of the non-aqueous phase; The aqueous phase on contact is 0.1 to 250, whereupon the resulting uranium precipitate with a molar ratio of carbonate component to uranium equal to or less than 3 and a molar ratio of ammonia component to uranium equal to or less than 4 is separated from the mother liquor and subjected to further treatment. Racing.

Preferably, the ammonium salt of the acid, preferably ammonium nitrate, is added from the outside to the aqueous precipitation solution.

The aqueous precipitation solution used for contacting is preferably re-contacted with new portions of the non-aqueous liquid phase after the uranium precipitation has been separated and, if necessary, adjusted to the desired composition by increasing the concentration of carbon dioxide and / or ammonia.

Preferably, the non-aqueous liquid phase contains tri-n-butylphosphate as trialkyl phosphate or tri-n-octylamine as a tertiary amine.

The advantage of solidifying uranium by the process of the present invention is that it consumes less chemicals and makes waste mother liquors easier to process which contain fewer neurane components. The advantages achieved by the process of the author's certificate no. 174 298 as well as the British patents 2. 1,474,529 and 1,487,223 and the US patent No. 3,966,873 compared to the two-stage solidification can be further increased by the solidification method according to the invention so that A dilute ammoniacal carbonate solution having a NH / / COg molar ratio of less than 2.0 is used for contacting. In particular, the ammonium content of the mother liquor is reduced, thereby reducing the wastewater treatment requirements. Also, the total concentration of carbonates, expressed as CO ₂ , may be less than 1.5 mol.l -1. And the composition of the resulting uranium precipitate, while fully maintaining excellent filtration properties, can differ significantly from the composition of the TKUTA tricarbonate complex, for which the ratio of NH 4}, COg and U is 4 s 3 i 1.

The uranium solidification process of the present invention can be advantageously applied at multiple sites in the nuclear fuel cycle, for example in the preparation of nuclear-pure uranium by tributyl phosphate extraction, where besides technological advantages such as releasing the reextraction operation into the aqueous phase. the purification effect is also a consequence of the formation of very strong complexes between carbonate and uranyl ions. Another possibility to apply the uranium solidification process of the present invention is to convert uranium hexafluoride, for example after isotopic enrichment to uranium dioxide, where after hydrolysis of hexafluoride in a modified aqueous solution and extraction of uranium from the solution with tributyl phosphate - cf. Certificate No. 193 808 - the procedure can be continued by precipitation reextraction. While maintaining the above technological advantages, ee here specifies the cleaning effect to purify uranium from undesirable fluorine. The uranium solidification process according to the invention can also be advantageously used in the reprocessing of spent fuel by the volatile fluoride method, where in the final phase the isolated uranium hexafluoride must be converted to an oxide product. The cleaning effect results in a reduction in the content of fission products.

Further, the uranium solidification according to the invention can be used wherever a higher precipitation reextraction efficiency is applied compared to reextraction into water or acidified aqueous solutions, for example in analytical procedures for the determination of uranium in the organic phase or by using very efficient extraction agents from which uranium is transferred. by normal reextraction into low efficiency water. In some cases, it is appropriate to use a uranium-saturated precipitation carbonate solution to isolate uranium from the organic phase. In this case, the proportions of uranium transferred from the organic to the aqueous phase form a precipitate so that the uranium content in the aqueous phase is maintained or the precipitation reextraction efficiency is not reduced; it is only necessary to maintain sufficient concentrations of the ammoniacal and carbonate components in the precipitation solution, since they both pass together with the re-extracted uranium into a precipitate. This can be achieved, for example, by recycling the mother liquor after separation of the precipitate and saturating it with ammonia and carbon dioxide.

The procedure is further elucidated by means of exemplary embodiments, but do not exhaust all the possibilities of a new procedure which can be further adapted or varied according to the conditions.

A solution of 0.40 mol.l ^{-1 of} uranium in the non-aqueous liquid phase, consisting of solutions of 30% by volume of tri-n-butylphosphate in kerosene, was contacted with various volumes of an aqueous carbonate solution prepared from 95 g of HH 2 HCO 3 and 95 ml. aqueous ammonia having a concentration of 7.12 mol.l ™ ", i.e. a carbonate concentration (expressed as carbon dioxide) of CO2 > 1.2 mol.l ™ " and a molar ratio of ammonia to carbonate component NH4 / GOg = 1.56. The molar precipitation ratio of carbonates to uranium (C / U) used for the preparation, together with the x-ray structure of the prep precipitate and its molar ratios of NH4 / U and COg / U, is shown in Table 1.

Table 1

( ^{C / U)} pr.p Product structure NH 3 / U WHAT _? /AT After drying, the filter cake was ... 1.5 hydroxocarbonate 1,25 1.35 .... compact 3.0 hydroxocarbonate 2.30 2.41 .... compact 4.0 TKUTA ^and ^ 3.69 2.93 .... loose powder 5.0 TKUTA 3.70 2.99 .... loose powder 6.0 TKUTA 3.71 3.00 .... loose powder

(a) Abbreviation for tetraammonium tricarbonateouranylate of formula (NH4)

For systems producing TKUTA structures, the residual uranium concentration in the organic solution was less than 0.008 mol.l -1, and the carbonate concentration (expressed as COg) in the mother liquor was in the range of 0.15 to 0.18 mol.l -1. 0.44 mol.l '·' ·.

Example 2

The same organic solution as in Example 1, with a uranium concentration of 0.17 2101.1 ^-1 and 1 a charged aqueous carbonate solution of the same composition as in Example 1, but the prepared syngas of water with ammonia and carbon dioxide gas gave the results of brief mixing. in Table 2.

Table 2

(C / U) '' proo Product structure NH 3 / U co ₂ / u After drying, it was filtered; oláč .. 3.0 amorphous 0.47 1.14 .... compact 4.0 TKUTA ^a ) 3.00 2.56 .... loose powder 5.0 TKUTA 3.50 2.72 .... loose powder 6.00 TKUTA 3.71 2.84 .... loose powder

a) For symbolism see Table 1

For systems in which a precipitate formed with the structure TKUTA, the residual kon_Ί oentrace of uranium in the organic phase is less than 0.003 mol.l, carbonate concentration in natural xoztoku ranged from 0.30 to 0.41 mol.l ^"1»

Example 3

The same organic uranium solution as in Example 2 'was continuously fed in a constant volume ratio of 1: 0.555 to a stirred reactor with an overflow to an aqueous precipitation carbonate solution whose concentration of carbonate component was constant and equal to 1.2 mol.l ^{- 1} while the ammonia concentration the components were varied in the NH 4 / CO 3 molar ratio, reported along with the other data in Table 3.

Table 3

NH ₃ / CO ₂ Product structure NH-J / U GOg / U After drying, the filter cake was 1.0 hydroxocarbonate 1.89 2.00 .... compact 1,2 hydroxo carbonate t 3.08 2.61 .... lumpy powder Ι, θ TKUTA _of ^a ) 3.92 2.87 .... loose powder 2.2 TKUTA 3.91 2.89 .... loose powder 3.1 TKUTA 3.96 3, 01 .... loose powder

a) Symbolism see tab. 1

The residual uranium content in the organic phase increased with an increase in the NH4 / COg ratio in the precipitation solution ranging from 0.003 to 0.006 mol.l -1, the carbonate content of the aqueous mother liquor was always close to 0.30 mol.l, whereas the content of ammonia increased and ranged from 1.5 to 3.3 M,

Example 4

The starting solutions of Example 1 were contacted by adding a constant volume of the organic phase to the beaker to which an aqueous phase of the composition of Example 1 was added under stirring, according to the glass electrode data registered by a laboratory pH meter. The data are shown in Table 4.

Table 4

pH meter reading Product structure NH4 / U ' co ₂ / u After drying, the filter was a cake 5.5 hydroxocarbonate 1.05 1.29 ... compact 6.5 Hydrocarbonate 1.83 1.85 ... compact 7.0 TKUTA 2.65 2.34 .... loose powder 7.5 TKUTA 3.13 2.45 ... loose powder θ, 5 TKUTA 3.81 2.74 ... loose powder

a) Symbolism see tab. 1

In this embodiment, the difference between the preparations with TKUTA structure and the others was apparent in that the precipitate formed was largely dissolved in the pH range of 6.5 to 7.0, and a new precipitate began to form thereafter. The difference between these types of precipitates - as in other cases - was that the macrocrystalline precipitate with the TKUTA structure sedimented very rapidly in the aqueous phase and formed a sharp interface. This is also important when separating the two liquid phases, which is very fast and sharp.

Example 5

At low concentrations of uranium in the organic solution, e.g., from 0.01 mol.l ^-! and below, by contacting with an equal volume of the precipitation solution, the composition corresponding to Example 1, no uranium precipitate forms. If, under otherwise identical conditions, the water phase volume several times smaller was used, a crystalline product with the structure of TKUTA, having systems of this type, was collected in Table 5.

Table ^

U concentration in org. phase before precipitation after precipitation mol.I ^-1 Volumetric ratio ^ r V. i V, o? g vod Precipitated U % by weight U in ratchet. solution of masses,% Partition ratio ¹³ 0,083 0.0001 2.4 84.3 15.6 320 0,056 0.0001 3.6 84.7 15.1 290 0,033 0.0001 3.0 74.1 25.5 234 0.010 0.0002 _____ 2 ^ 7 17.9 79.6 93

b) The ratio of uranium concentration in the aqueous phase, the mother liquor and the organic phase after the precipitation re-extraction.

These data illustrate the considerable efficiency of the transition of uranium from the organic to the aqueous phase under the above-described conditions, and the possibility of achieving a uranium concentration by concentrating it into the aqueous phase by using a smaller volume of an aqueous precipitation solution compared to the volume of the starting organic solution.

Example 6

10 µl of a 7.5 vol% trioctylamine solution with a uranium content of 0.04 mol.l ^-1 was 5 minutes

4 ^'5 mixed with 10 ml of the precipitating solution, the composition corresponding to Example 1 or a changed ratio of NH ^ / COG 3.0. In both cases, a crystalline precipitate formed which, after air drying, formed a compact cake. According to X-ray analysis, in the first case it was hydroxocarbonate, in the second the structure was identical with TKUTA.

Example 7

The organic phase was comprised of a solution of 30 vol.% Tri-n-butyl phosphate in kerosene had either precipitant solution composition as in Example 1 (Run A), charge additionally contained ammonium nitrate concentration of 1 mol.l ¹ (experiment B) and 3 mol.l ¹ (experiment C). Table 6 shows the uranium material balance after precipitation reextraction, which illustrates the relationship between decreasing the solubility of uranium in aqueous mother liquor and increasing the amount of solidified uranium.

Try U in ma1 M tangent mg solution % U ve mg precipitate % Total amount of U as analyzed mg% (based on Eastern amount; AND 0.137 163.5 43.5 212.1 56.5 375.6 105.2 (B) 0.024 28 7.7 337.0 92.3 365.0 102.2 C 0.0067 8 2.1 365.8 97.9 368.0 104.7

215 3t-7

Claims (4)

A process for solidifying uranium from a non-aqueous liquid phase, miscible with water, by contacting an aqueous precipitation solution, characterized in that the non-aqueous liquid phase containing trialkylphosphates or tertiary amines is contacted with an aqueous precipitation solution containing various chemical forms of ammonia as its constituents. of carbon dioxide whose concentrations in the aqueous phase are lower than their saturation under given physical and chemical conditions, the molar ratio of ammonia and carbon dioxide before contacting is greater than 1, preferably 1.6, and the volume ratio of the non-aqueous liquid phase and the aqueous phase upon contacting 0.1 to 250, the resulting uranium precipitate, with a molar ratio of carbonate component to uranium equal to or less than 3 and a molar ratio of ammonia to runau equal to or less than 4, is separated from the mother liquor and subjected to further processing. 2. A process according to claim 1, wherein an ammonium salt of a strong acid, preferably ammonium nitrate, is added externally to the aqueous precipitation solution. 3. A method according to claim 1 or 2, characterized in that the aqueous precipitation solution used for contacting is re-contacted with new portions of the non-aqueous liquid phase after the uranium precipitate has been separated and, if necessary, adjusted to the desired composition by increasing the concentration of carbon dioxide and / or ammonia. 4. A process according to claim 1, wherein the non-aqueous liquid phase comprises tri-n-butyl phosphate as trialkyl phosphate or tri-n-octylamine as tertiary amine.