CH643751A5 - PROCEDURE TO INCREASE PRODUCTION EFFICIENCY IN ISOTOPIC SEPARATION PROCESSES. - Google Patents

Description

The present invention relates to a chromatographic isotopic separation process which maintains a platform (in isotopic equilibrium with the original feeding solution) within the chromatogram, thus allowing a more efficient production of the desired enriched isotope.

With the process of the present invention it is possible to separate lithium-6 from lithium-7, boron-10 from boron-11 and other isotopes, among which uranium-235 from uranium-238.

The present invention relates to a process which simplifies operations and increases the efficiency of total separation in liquid or gas chromatography processes to produce enriched isotopes.

The chromatographic processes for producing enriched isotopes according to the invention involve the following two phases:

(1) a preliminary enrichment phase («goodwill»)

(2) a production phase.

The preliminary enrichment phase is that in which the chromatographic equipment is started and conducted without taking a product stream until the atomic fraction of the desired isotope has reached the required value at the top or bottom of the chromatogram.

The subsequent production step is that in which a product of the desired atomic fraction is taken semi-continuously or continuously from the head or tail of the chromatogram.

The separation efficiency of the chromatographic isotopic separation systems of the prior art tends to decrease, because the enriched and depleted areas overlap, causing the isotopes to remix.

In the present process, the chromatography is carried out in such a way that the displacement fronts are formed at the head and / or at the tail of the chromatogram and the sum of the concentrations of the two isotopic species remains constant.

The chromatographic band is created wide enough so that the enriched and depleted areas always remain separate from each other, never having overlapping them.

The salient features of the process object of the present invention can be summarized as follows:

(1) isotopic mixing is avoided by maintaining a platform (in isotopic equilibrium with the feeding solution) between the enriched and depleted areas;

(2) enlargement of the enriched and depleted areas due to changes in total concentration is prevented;

(3) additional supply solution can be supplied to the central part of the chromatogram without interrupting the process.

This results in a more efficient production of the desired enriched isotope.

By way of non-limiting example, examples of application of this process are described in greater detail, by means of liquid phase chromatography:

Example 1 - DILITIO-6 PRODUCTION

Natural lithium is composed of two isotopes: lithium-6 (7.2%) and lithium-7 (92.8%).

A series of columns as shown in fig. 1 (each column of 22 mm diameter and 1300 mm length) was filled with Dowex 50 W cation exchange resin up to a height of 1200 mm and said resin converted to the acid form R-H.

An absorption band was formed by feeding to the columns a 0.3 M aqueous solution of lithium acetate CH3COOLÌ and then a chromatography eluting the absorption band with a 0.3 M aqueous solution of sodium acetate CH3COONa.

In this way it was possible to enrich lithium-7 at the head and lithium-6 at the tail of the absorption band.

I) With the maintenance of an isotopic platform:

A 6 m wide lithium absorption band was formed and moved across the columns.

After migration over a total distance of 202 m, the atomic fraction of lithium-6 at the end of the tail of the band was increased from 7.2% to 15.0%.

It was therefore possible to produce lithium-6 enriched at 15.0% at a rate of 2.9 mmoles.cnr2. day-1 supplying natural lithium in the central section of the chromatogram.

At the head of the band, the atomic fraction of lithium-6 was reduced from 7.2% to 2.9% after migration over a distance of 202 m, as shown in fig. 2.

II) Without maintaining an isotopic platform:

A 3 m wide lithium band was formed and moved through the columns.

To enrich the atomic fraction of lithium-6 at the head of the band from 7.2% to 15.0% it was necessary to move the band over a total distance of 450 m.

At this point, 15% enriched lithium-6 could have been produced, at a rate of 1.4 mmol.cnr2. day-1.

However, it was difficult to provide new feeding solution in the center of the band without breaking the chromatogram and consequently the production speed declined, as shown in fig. 3.

Example 2 - BORO-IO PRODUCTION

Natural boron is composed of two isotopes: boron-10 (19.8%) and boron-11 (80.2%).

A column as shown in fig. 4 (diameter 22 mm, length 4100 mm), was filled with anion exchange resin, weakly basic, Diaion WA 21 in the form of a free base up to a height of 4000 mm, said resin converted to the basic form R-OH and washed with pure water.

An aqueous solution of

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643 751

boric acid to form an absorption band and chromatography by eluting the boron band with water.

In this way it was possible to enrich boron-11 at the head and boron-10 at the tail of the band.

I) With the maintenance of an isotopic platform:

A 4 m ion exchange column was first balanced with 5.0 liters of 0.1 M aqueous solution of boric acid.

The front part of the absorption band was discharged and therefore the enriched boron-11 was not recovered.

Chromatography was performed by eluting the absorption band with water and boron-10 was enriched from 19.8% to 33.3% at the end of the tail of the band.

The width of the enriched area was 34 cm, as shown in fig. 5.

II) Without maintaining an isotopic platform:

After 2.0 liters of 0.1 M boric acid solution were originally loaded into a column, it was impossible to perform displacement chromatography or maintain an isotopic platform within the chromatogram.

After migration over a distance of 4 m, the atomic fraction of boron-10 at the end of the tail of the band reached only 25.8% and the width of the enriched area grew to 96 cm, as shown in fig. 6.

Example 3 - 90% BORO-IO PRODUCTION

The production of boron-10 at 90% was carried out with the same procedure described in example 2 above.

A series of columns as shown in fig. 7 (each column 21 mm in diameter, 1100 mm in length) were filled with resin up to a height of 1000 m, and said pretreated resin as described in example 2.

Each column was first balanced with 2.0 liters of 0.1M boric acid.

An excess of solution was fed into each column so that the depleted zones in bodo-10 were discharged, leaving the resin in both chemical and isotopic equilibrium with the feed solution.

The columns were then connected in series by means of a vinyl chloride tubing with an internal diameter of 0.8 mm and chromatography by feeding water to the first column of the series and moving the boric acid through the other columns.

The amplitude of the absorption band was kept very wide so that a platform (in isotopic equilibrium with the feeding solution) was maintained during the whole operation.

After boric acid had been completely eluted from the first column, it was removed from the series, balanced again with a feeding solution and connected, following the current, to the end of the series.

In this way the absorption band could be easily moved over a long distance without interrupting the process. After the band had migrated over a total distance of 250 m, the atomic fraction of boron-10 at the end of the tail of the band was 90% enriched and 90% enriched boron-10 was produced at a rate of 0.09 mmoli.cnr2. day'1, as shown in fig. 8.

As is clear from these examples, enriched isotopes can be produced more effectively by maintaining an isotopic platform within the separation chromatogram.

Fig. 1 schematically shows the arrangement of the series of columns for the production of lithium-6.

The reference number (1) indicates the lithium acetate solution used for the formation of the lithium absorption band; with (2) it indicates the solution of sodium acetate eluent, while with (3) it indicates the solution of acetic acid used s for the regeneration of the resin.

The dotted part (4) of the columns indicates the lithium absorption band.

The dotted part indicates the resin eluted in sodium form R-Na (5) or in acid form R-H (6).

io In fig. 2 shows the lithium chromatogram with the maintenance of the isotopic platform, having a length of 6 m and resulting from the migration of the absorption band over a 202 m path.

The elution curve (1) indicates the lithium concentration, ■ s expressed in moles / liter and reported on the ordinate axis on the left, according to the position on the chromatogram expressed in meters and reported on the abscissa axis.

The curve of the atomic fraction (ratio between the number of atoms of an isotope present in the element and the number of atoms 20 total) (2) indicates the percentage value of the atomic fraction of lithium-6 reported on the axis of the ordinates on the left in function of the position on the chromatogram in meters, shown on the abscissa axis.

The dotted line (3) indicates the original atomic fraction 25 of lithium-6 (7.2%).

In fig. 3 shows the analogous chromatogram without maintaining the isotopic platform. The notations are the same as in fig. 2.

In fig. 4 a single column for enrichment 30 of boron-10 is shown.

The reference number (1) indicates the boric acid solution used for absorption, while (2) indicates the eluent water.

The dotted part (3) of the column indicates the boron adsorption band.

The dotted part (4) indicates the already eluted resin.

In fig. 5 shows the boron chromatogram with the maintenance of the isotopic platform at the original atomic fraction of boron-10 (19.8%).

4 ° The elution curve (1) indicates the concentration of boric acid, expressed in moles / liter and reported on the ordinate axis on the left, according to the position on the chromatogram, expressed in meters and reported on the abscissa axis.

In fig. 6 shows the analogous chromatogram without the maintenance of the isotopic platform.

The notations are the same as in fig. 5.

In fig. 7 schematically shows the arrangement of the series of columns for the production of boric acid enriched in boron-10.

5 ° The notations are the same as in fig. 4.

In fig. 8 shows the boron chromatogram with the maintenance of the isotopic platform, in the case of the production of boron-10 at 90%.

The notations are the same as in fig. 5.

55 It can be seen that at a chromatogram length of 250 m it corresponds to the tail of the boron-10 band enriched at 90%.

Although the present invention has been described in correlation with certain embodiments, the inventive concept is susceptible of numerous other applications which will be presented to those skilled in the art.

Furthermore, without departing from the spirit of the invention, many variations can be made in the practical embodiment, all however to be considered included in the fundamental concepts 65 above.

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Claims (4)

643 751 1. Chromatographic isotopic separation process (liquid or gaseous), consisting of maintaining a platform in isotopic equilibrium with the original feeding solution within the chromatogram, which allows a more efficient production of the desired enriched isotope, characterized in that the isotopic platform is maintained by forming a very wide absorption band in the column, in which the enriched and depleted areas that form at the top or bottom of the chromatogram always remain separate from each other. 2. Isotopic separation process according to claim 1, characterized in that the maintenance of a platform in isotopic equilibrium with the original feeding solution allows to avoid the mixing of the isotopes, the enlargement of the enriched or depleted areas and to provide in the central part of the chromatogram additional feeding solution without interrupting the process. 2 3. Isotopic separation process according to claim 1, characterized in that at the preliminary start-up phase, carried out maintaining a flat shape without removing the product until the atomic fraction of the desired isotope has reached the required head enrichment value or at the end of the chromatogram, a production phase follows in which the product of the desired atomic fraction is withdrawn semi-continuously or continuously from the head or tail of the chromatogram. 4. Isotopic separation process according to claim 1, characterized in that with the above mentioned process, in addition to the exemplified lithium-6 and boron-10 isotopes, other isotopes can also be produced, among which uranium-235.