Note: Descriptions are shown in the official language in which they were submitted.
llZ7978
This invention relates to a method of chromato-
graphic isotope separation which maintain a platform (in
isotopic equilibrium with the originally fed solution) in
the interior of the chromatogram, thus makinq possible a
more efficient production of the enriched isotope which is
desired.
Generally, the present invention provides in a
process for isotope separation by liquid or gaseous chromato-
graphy in which a platform is maintained in isotopic equilib-
rium with the originally introduced influent in the interior
of the chromatograph, the improvement consisting in that the
isotopic platform is maintained by forming in a chromatographic
column a very wide absorption band in which enriched and
depleted zones which are formed at the head and the tail of
the chromatograph always remain separated from one another.
In accordance with the present invention there may
be supplied to a central portion of the chromatograph an
additional amount of influent without discontinuing the process.
With the method of the present invention, it becomes
possible to separate lithium-6 from lithium-7, boron-10 from
boron-ll, and also other isotopes, such as uranium-235 from
uranium 238.
This invention relates to a method which makes all
the operations simpler and improves the efficiency of the
total separation in liquid or gaseous chromatography processes
to produce enriched isotopes.
In particular, chromatographic processes of this
invention for producinq enriched isotopes may involve the
following tWG stages:
(1) a preliminary enrichment stage (start up)
(2) a production stage.
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The preliminary enrichment stage is that in which
the chromatographic apparatus is started and run without
drawing any product stream until the atomic fraction of the
expected isotope has attained the requested value at the head
or the tail portion of the chromatogram.
The subsequent production stage is that in which a
product of the desired atomic fraction is drawn in a semi-
continuous way, or a continuous way, from the head, or the
tail of the chromatogram.
The efficiency of the separation of the chromato-
graphic isotopic separation of the prior art shows a tendency
to worsen, in fact, because the enriched and the depleted
areas overlap and cause an isotope intermingling.
In the present method, the chromatography is carried
out in such a way that the fronts of displacements 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 made wide enough so that
the enriched and the depleted areas remain always separated
from one another, no mutual overlapping being experienced
therebetween.
The prominent features of the method according to
the present invention can be summarized as follows:
(1) prevention of the isotope intermingling while
maintaining a platform (in isotopic equilibrium with the fed
in solution) between the enriched and the depleted area.
(2) prevention of the widening of the enriched and
the depleted areas as caused by changes in the total con-
centration.
(3) the central portion of the chromatogram can besupplied with additional amounts of feeding solution without
discontinuing the process.
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The result is a more efficient production of the
expected enriched isotope.
In drawings which illustrate embodiments of the
invention:
Figure 1 shows, in a diagrammatical fashion a set
of resin-containing columns for carrying out the process of
this invention;
Figure 2 shows the chromatogram of lithium, with
the maintenance of the isotopic platform, the Li-acetate con-
centrations being plotted on the ordinate axis in mol/litre
as-a function of the position of the chromatogram, expressed
in metres, as measured from the head of the band and plotted
on the abscissa axis;
Figure 3 is a chromatogram like that of Figure 2,
but without maintaining the isotopic platform;
Figure 4 shown a single column for the enrichment
of boron-10;
Figure S represents the chromatogram of boron with
the maintenance of the isotopic platform;
Figure 6 shows a chromatogram similar to that of
Figure 5, but without maintaining the isotopic platform;
Figure 7 diagrammatically shows the arrangement of
the set of columns for the production of boric acid enriched
with boron 10, and
Figure 8 represents the chromatogram of boron with
the maintenance of the isotopic platform.
I More particularly, figure 1 diagrammatically shows
; the arrangement of columns set for the production of lithium-6.
The reference numeral 1 indicates the solution of
lithium acetate employed for the formation of the lithium
absorpton band. The numeral 2 indicates the solution of eluent
sodium acetate, whereas 3 indicates the solution of acetic
acid used for regenerating the resin. The hatched portion 4
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of the columns indicates the lithium-absorption band. The
speckled portion indicates the resin which has been eluted
in the sodium-form R-Na, 5, or in the acidic form, R-H, at 6.
Figure 2 shows the chromatogram of lithium with
the maintenance of the isotopic platform having a length of
6 metres and obtained as a result of the migration of the
absoption band over a length of 202 metres.
The elution curve, 1, indicates the concentration
of lithium expressed in miles per litre and plotted on the
ordinates axis on the left as a ~unction of the position on
the chromatogram expressed in metres and reported on the
abscissae axis.
The plot of the atomic fraction (ratio of the
number of atoms of an isotope present in the element, to
the number of atoms in total), 2, indicates the percentage
value of the atomic fraction of lithium-6 as reported on the
ordinates axis on the left as a function of the position on
the chromatogram in metres, which is plotted on the abscissae
axis.
The dotted line, 3, indicates the original atomic
fraction of lithium-6 which is 7.2%.
Figure 3 shows a chromatoyram like that of Figure 2
but without maintaining the isotopic platform: the symbols
are the same as for Figure 2.
Figure 4 shows a single column for the enrichment
of boron-10.
The reference numeral 1 indicates the solution of
boric acid used for the absorption, while 2 is the eluent
water.
The hatched portion 3 of the column indicates the
boron-absorption band.
The speckled portion, 4, indicates the already
eluted resin.
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Figure 5 shows the chromatogram of boron with the
maintenance of the isotopic platform in correspondence with
the original atomic fraction of boron-10, which is 19.8%.
The elution curve, 1, indicates the concentration
of boric acid expressed in miles per litre and reported on
the axis of the ordinates, on the left, as a function of the
position on the chromatogram, which is expressed in metres and
is reported on the abscissae axis.
Figure 6 shows a chromatogram similar to that of
Figure 5 but without maintaining the isotopic platform, the
symbols being the same as in Figure 5.
! Figure 7 diagrammatically shows the arrangement of
the set of columns for the production of the boric acid
enriched with boron-10, the symbols being the same as in
Figure 4.
Figure 8 shows the chromatogram of boron with the
maintenance of the isotopic platform in the case of the
production of the 90%-boron-10, the symbols being the same
as in Figure 5.
By way of example only and without any limitation,
there will be described in more detail a few examples of
practice of the method in question by liquid-phase chromato-
graphy.
EXAMPLE 1 - PRODUCTION OF LITHIUM-6
Naturally occurring lithium is composed by two
. isotopes, viz.: lithium-6 (7.2%) and lithium-7 (92.8%).
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A set of columns such as shown in Figure 1
(each column has a diameter of 22 mm and a length of
B 1300 mm) were filled with an ion exchange resin Dowex
50 W up to a height of 1200 mm andsaid resin was con-
verted to the acidic form R-H. An absorption band was
formed by feeding the column set with a 0.3 M aqueous
solution of lithium acetate~ CH3COOLi~ and then a chro-
matography run was carried out by eluting the absorp-
tion band with a 0.3 M aqueous solution of sodium a-
cetate, CH3COONa.
By so doing~ it has been possible to enrich
the head with lithium-7 and the tail of the absorption
band with lithium-6.
I) BY maintainin~ an isotopic platform:
A lithium-absorption band 6-metre wide was form-
ed and shifted through the column6. After migration o-
~er an overall distance of 202 metres, the atomic frac-
tion of lithium-6 at the tail end of the band was in-
creased from 7~2% to 1~0%~
It has thus been possible to produce 15~O-en-
riched lithium-6 at the velocity Of 2~9 millimols per
square cm and per day by supplying naturally occurring
lithium to the central shank of` the chromatogramO On
the band head, the atomic fraction of lithium-6 was re-
duced from 7~2% to 2~9% only~ upon migration over a di-
stance of 202 metres~ as shown by Figure 2 of the ac-
companying drawings.
~I) ~
A 3-metre wide lithium colu~m was formcd and
shifted through the column set.
To enrich the atomic fraction of lithium-6 on
the head of the band from 7~2% to 15.0C~o the band had
to be shifted over an overall distance of 450 metres.
At this point, lithi~m-6 ~ehcA b~ ~ could
3~ have been produced at a velocity of 1.4 millimol pcr
6quare cm daily.
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It became difficult, however~ to supply fresh
feeding solution to the band center without breaking
the chromatogram, so that the production ~elocity de-
cayed~ as shown in Figure 3.
EXAMPLE 2 - PRODUCTION OF BORON-10
Naturally occuring boron consists of two iso-
topes~ viz.: boron-10 ( 19~8%) and boron-11 (80.2~).
A column such as shown on ~igure 4 of the ac-
companying drawings and having a diameter of 22 mm and
a height of 4100 mm was filled with an anionic exchange
resin~ weakly basic, Diaion WA 21 in the form of a
free base to a height of 4000 mm~ said resin being
converted to the basic form R-OH and washed with pure
water.
The column was then charged with an aqueous so-
lution of boric acid to form an absorption band and a
chromatography was carried out by eluting the boron
band with water.
By so doing~ it has been possible to enrich the
head with boron-11 and the band tail with boron-100
I) Bv maintainin~ an isotopic Platform
A 4-metre ionic exchange column was first ba-
lanced with 5.0 litres of a 0.1 M aqueous solution of
boric acid.
The front portion of the absorption band was
dumped so that the enriched boron-11 was not recoveredO
A chromatography was effected by eluting the
absorption band with water and boron-10 was enriched
f`rom 19~8% to 33.35~ at the tail ~nd of the band. The
width of the enriched zone was 34 centimetres as shown
in Figure 5 of the accompany~ng drawings.
II) 1~ithout maintainin~ an isotopic platf~rm
Upon originally charging the column with 2.0
litres of a 0.1 M solution of boric acid, it proved
impossible to carry out any shift chromatography, or
to maintain an isotopic platform in the intorior of the
11*7978'
chromatogram.
Upon migration over a distance of 4 metres, the
atomic fraction of boron-10 at the tail end of the band
attained only 25.8~ and the width of the enriched zone
was 96 centimetres as shown in Figure 6 of the accom-
panying drawings.
EXAMPLE 3 - P~ODUCTION 0~ 90',~-BORON-10
The production of boron-10 at a 90% rating was
carried out with the same procedure as described in
the previous example 2 hereof.
A set of columns such as shown in Figure 7 of
the accompanying drawings~ each column having a diame-
ter of 21 mm and a length of 1100 mm~ were filled with
resin up to a height ~f 1000 mm and the resin was pre-
liminarily processed as described in example 2 hereof.
Each column was balanced~ at the outset~ with 2.0
litres of 0.1 M boric acid solution.
Each ~olumn was fed by an excess of the 901u-
tion so that the zones which had been depleted of bo-
ron-10 were dumped~ leaving the resin both in chemical
and isotopic equilibrium with the fed in solution~
The columns were then serially connected by
vinyl chloride pipes having an inside diameter of 0.8
mm and a chromatography was carried out by feeding wa-
ter to the first column of the set and shifting the bo-
ric acid through the remaining columns. The width of
the absorption band was kept very wide so that a plat-
form (in isotopic equilibrium with the incoming solu-
tion) was maintained during the entire operation.
After that the boric acid had completely been
eluted from the first column, the column was withdrawn
from the column set, balanced once again with the feed-
ing solution and connected, in the streamline direc-
tion~ at thc end of the set.
By so doingS the absorption band was easily
shifted along a long distance without discontinuing
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the procedure. After that the band had migrated over an
overall distance of 250 metres, the atomic fraction of boron-10
at the tail end of the band was enriched to 90% and 90%-
enriched boron-10 was produced at a velocity of 0.09 millimol
per square cm daily, as shown in Figure 8.
As these example clearly show, enriched isotopes
can be produced more efficiently if an isotopic platform is
maintained in the interior of the separation chromatogram.
It can be appreciated that a length of chromatogram
of 250 metres corresponds, at the band tail, to the band
of 90%-enriched boron-10.
While the invention has been described in connection
with a few defininte embodiments thereof, the basic idea of the
invention can be embodied otherwise within the purview of those
skilled in the art.
Moreover, without departing from the scope of the
invention, modifications and changes can be introduced in the
reduction to actual practice within the scope of the invention
as set forth hereinbefore.
