WO1996006670A1

WO1996006670A1 - An isotopic separation process

Info

Publication number: WO1996006670A1
Application number: PCT/US1995/010675
Authority: WO
Inventors: John Hugh Prior
Original assignee: Peltek, Inc.
Priority date: 1994-08-26
Filing date: 1995-08-22
Publication date: 1996-03-07
Also published as: ZA956726B; AU3412195A

Abstract

The invention provides a process for the separation of zinc isotopes from one another to enrich or deplete a zinc-containing starting material with regard to one or more zinc isotopes. A dialkyl zinc compound (34) R1-Zn-R2 in which R1 and R2 are selected from ethyl, propyl and butyl groups, optionally substituted and straight-chain or branched, is subjected to molecular laser isotope separation. Infrared laser irradiation (24) is used to decompose said compound R1-Zn-R2 to obtain elemental zinc.

Description

AN ISOTOPIC SEPARATION PROCESS THIS INVENTION relates, broadly, to an isotopic separation process for the separation of at least one isotope of zinc from other isotopes of zinc. More particularly, the invention relates to such process suitable for the separation of eg ⁶⁴Zn or ⁶⁸Zn from other isotopes of zinc, thereby to obtain a zinc- containing product which is enriched or depleted with regard to any separated isotope of zinc and depleted or enriched with regard to other isotopes of zinc.

According to one aspect of the invention there is provided a process for the separation of at least one isotope of zinc from other isotopes of zinc, to obtain, from a zinc-containing starting material, a zinc-containing product which is enriched or depleted with regard to any said separated isotope of zinc, the method comprising subjecting a zinc-containing starting material comprising at least one dialkyl zinc compound of the formula:

R₁-Zn-R₂ in which R-, and R₂ are the same or different and are selected from ethyl, propyl and butyl groups which may be straight-chain or branched, and which are optionally substituted with substituents selected from deuterium, tritium, chlorine, bromine, iodine and fluorine, to a molecular laser isotope separation step using infra-red laser irradiation, preferentially to decompose compounds of said formula R₁-Zn-R₂ containing at least one isotope of zinc, to form elemental zinc.

Naturally occurring zinc has the following approximate isotopic composition: Zinc Isotope % bv Mass

100 . , 00

For reasons of cost and feasibility, it is expected that the process of the invention will usually be used to separate the 68°Zn isotope or, particularly, the 6°4Z,n isotope, from the other isotopes of zinc. In particular, the process may comprise separating ⁶⁴Zn from other isotopes of zinc to obtain a zinc- containing product which is depleted with regard to ⁶⁴Zn.

Zinc oxide is used in water-cooled nuclear reactors to inhibit the deposition of radioactive cobalt on the interior surfaces thereof which are in contact with cooling water. The zinc used for such zinc oxide preferably comprises no more than 1% by mass ⁶⁴Zn, and ideally contains no ⁶⁴Zn whatsoever. It is accordingly expected that an important application of the process of the present invention will be in the production of zinc suitable for use as a constituent of the zinc oxide used in such water-cooled nuclear reactors, from a starting material comprising zinc having a naturally occurring isotopic composition.

While the starting material may, in principle, comprise a plurality of compounds of said formula R₁-Zn-R₂ in which R.^ and R₂ are different, it is expected that, typically, R._j_ and R₂ will be the same, being selected from ethyl, n-propyl, iso-propyl, n- butyl, iso-butyl, sec-butyl and tert-butyl, i.e. all the isomers of ethyl, propyl, and butyl, Rη_ and R₂ being optionally at least partially substituted with substituents selected from deuterium, tritium, chlorine, bromine, iodine and fluorine. For reasons of cost and availability, unsubstituted diethyl zinc, i.e. CH₃-CH₂- Zn-CH₂-CH₃ is presently preferred, with the zinc therein usually having a substantially naturally occurring isotopic composition. Thus, the starting material may comprise a single compound of the formula R₁-Zn-R₂ in which R-^ and R₂ are the same, being unsubstituted; and, more particularly, the R₁-Zn-R₂ may be unsubstituted diethyl zinc of formula CH₃-CH₂-Zn-CH₃-CH₃ , the zinc in the starting material having the following isotopic composition:

Zinc IsotoDe % bv Mass

⁶ Zn 48,6 - 49,2

⁶⁶Zn 27,5 - 28,1

⁶⁷Zn 3,8 - 4,4

⁶⁸Zn 18,3 - 18,9

⁷⁰Zn 0,3 - 0,9

Thus, three grades of diethyl zinc, namely semi-conductor grade, purified grade and technical grade, are commercially available from Akzo NV in the Netherlands on a large scale. Furthermore, the synthesis of dialkyl zinc compounds such as diethyl zinc can readily be accomplished by the reaction of the appropriate alkyl iodide with powdered zinc or a powdered zinc/copper alloy, wherein about half the zinc is reacted to dialkyl zinc, with the remainder being reacted to zinc iodide. Other synthesis routes with higher yields are possible, whereby alkyl magnesium compounds or alkyl aluminium compounds are reacted with anhydrous zinc salts. Accordingly, the process may include the step, prior to the molecular laser isotope separation step, of synthesizing the compound of the formula R₁-Zn-R ; and synthesizing the compound of the formula R₁-Zn-R₂ may be by reacting a compound selected from alkyl magnesium compounds, alkyl aluminium compounds and mixtures thereof with at least one anhydrous zinc salt.

The infra-red laser irradiation used in the process of the present invention will be selected to be of an appropriate wavelength, and, if desired, infra-red irradiation of two or more wavelengths may be employed. The infra-red laser irradiation may be that produced by a Raman laser, i.e. that produced by a C0₂ laser which, after amplification, if necessary, has had its wavelength shifted by a Raman cell to a suitable value.

In the process of the present invention the laser irradiation is preferably selected so that it preferentially decomposes or dissociates molecules of the compound of the formula R₁-Zn-R₂ which contain zinc atoms selected from ⁶⁶Zn, ⁶⁷Zn, ⁶⁸Zn and ⁷⁰Zn atoms, i.e. compounds of the formula R-^-Zn- R₂ in which the Zn atom is not ⁶⁴Zn, with the production, from said compounds, of alkyl radicals and alkyl-zinc radicals, the alkyl-zinc radicals further decomposing spontaneously in the reaction environment to form elemental zinc metal and further alkyl radicals, the elemental zinc being collected as a powder, e.g. by electrostatic precipitation or filtration as a product which is depleted with regard to ⁶ Zn compared with the starting material, and the remaining R₁-Zn-R₂ dialkyl zinc compounds, comprising mainly ⁶⁴Zn, forming a by-product. The zinc product can be reconverted to dialkyl zinc and subjected to further depletion until it is sufficiently depleted with regard to ⁶⁴Zn, whereupon it can be stored or used as a reagent, e.g. to produce zinc oxide for use in water-cooled nuclear reactors. In other words, the laser irradiation may be selected preferentially to decompose compounds of the formula R₁-Zn-R₂ in which the Zn is an isotope of zinc other than ⁶⁴Zn, so that the zinc-containing product is elemental zinc which is depleted, relative to the starting material, with regard to ⁶⁴Zn. In this case the irradiation step may be followed by a synthesis step in which the elemental zinc formed by the irradiation is converted to at least one dialkyl zinc compound of said formula R-_^-Zn-R_j, each of which compounds is subjected to a further molecular laser isotope separation step using infra-red laser irradiation selected preferentially to decompose compounds of the formula R₁-Zn-R₂ in which the Zn is an isotope of zinc other than ⁶⁴Zn. Thus, in general, the irradiation steps and dialkyl zinc synthesis/reconversion steps can be alternated in this fashion, preferably ending with an irradiation step, until a final product containing an acceptably low proportion of ⁶⁴Zn is obtained.

Instead, the laser irradiation can be selected so that it preferentially decomposes or dissociates molecules of the compound R₁-Zn-R₂ containing ⁶⁴Zn atoms and forms a by-product of the compound R₁-Zn-R₂ containing mainly ⁶⁶Zn, ⁶⁷Zn, ⁶⁸Zn and ⁷⁰Zn atoms. As an optional further step, by-product can further be irradiated with laser irradiation selected so that it preferentially decomposes or dissociates molecules of compounds of the formula R₁-Zn-R₂ which contain ⁶⁶Zn, ⁶⁷Zn, ⁶⁸Zn and ⁷⁰Zn atoms, to provide the product in elemental zinc metal form.

A convenient way of carrying out the process of the present invention is to generate a laser beam using a C0₂ laser (oscillator) which may be tunable to produce an infra-red laser beam, the beam being passed through one or more amplifiers and then through a Raman cell, whereby its wavelength is red- shifted, and from which it is projected via a suitable window into a photochemical reactor where photochemical dissociation of the dialkyl zinc compound R -Zn-R₂ takes place, the beam leaving the reactor via a further window, and the starting material being passed through the beam in the reactor at ambient temperature, or being cooled, e.g. by adiabatic expansion or flow cooling, before it passes through the beam in the reactor, to a temperature below ambient temperature.

Cooling of the starting material, which will typically be diethyl zinc vapour, optionally diluted with a diluent gas which is inert in the reaction environment in the reactor, such as nitrogen, argon, helium or the like, and which may contain an otherwise inert scavenger gas such as chlorine, bromine or hydrogen chloride for scavenging alkyl-zinc radicals, may be to a vibrational temperature of 50 - 220 K, preferably 80 - 200 K, e.g. 150 K. Such cooling may be by adiabatic expansion or flow cooling of the starting material, optionally in the reactor, upstream of the beam. The partial pressure of the dialkyl zinc vapour, or its pressure, if used alone, may be 1 - 5000 Pa, preferably 10 - 500 Pa, e.g. 100 Pa.

The C0₂ laser conveniently has a pulsed output, with the pulse frequency of 1 - 30000 Hz, preferably 500 - 2000 Hz, e.g. 1000 Hz. The C0₂ laser, while it in principle can employ ¹³C, or ¹⁸0 for one or both of its oxygen atoms, will typically employ ¹²C and ¹⁶0, i.e. the common isotopes of carbon and oxygen. The Raman cell in turn will usually be a hydrogen Raman cell or a deuterium or tritium Raman cell, selected to shift the wavelength of the irradiation produced by the C0₂ laser into the region of 16 - 19 μm, where the dialkyl zinc molecules containing atoms of zinc isotopes other than ⁶⁴Zn, ie containing ⁶⁶Zn, ⁶⁷Zn, ⁶⁸Zn and ⁷⁰Zn, absorb photons effectively.

In a particular embodiment, the process may comprise generating the laser irradiation by means of a C0₂ laser, red- shifting the irradiation by means of a Raman cell, to provide the irradiation with a wavelength of 16 -19 μm, and projecting a beam of the irradiation through a photochemical reactor through which the starting material is passed, with each compound of formula R₁-Zn-R₂ being in vapour form, at a temperature which is at most ambient temperature and at a pressure of 10 - 500 Pa. More particularly, the starting material may comprise, in addition to molecules of said dialkyl zinc compound of formula R-^Zn-R_j , an otherwise inert scavenger gas for scavenging alkyl-zinc radicals, the starting material passing through the reactor at a temperature of 50 - 220K and a pressure of 50 - 200 Pa, the laser irradiation being in the form of a pulsed output from the laser with a pulse frequency of 500 - 2000 Hz and pulse length of 70 - 500 ns.

The Applicant has estimated that the band centre for photon absorption by dipropyl zinc compounds is in the region of 549 cm^"1, the band centre for dibutyl zinc being estimated to be about 539 cm^"1, and the band centre for diethyl zinc being in the region 563 -569 cm^"1. The C0₂ laser lines 10P (6) -10P (22) obtained from a ¹²C ¹⁶0₂ laser, shifted by a deuterium S(2) Raman cell with a shift of 414 - 415 cm^-1 are expected to be suitable for dissociating dipropyl zinc compounds; and the laser lines 10P (10) -10P (32) , similarly shifted by said deuterium S(2) Raman cell are expected to be suitable for dissociating dibutyl zinc compounds. Instead, a ¹²C¹⁸0₂ or ¹²C¹⁶0¹⁸0 laser can be used with said deuterium Raman cell, or ¹³C¹⁶0₂ with a S(0) para-hydrogen Raman cell with a shift of 354 - 355 cm^"1 can be used for dipropyl zinc.

In the preferred case of a diethyl zinc starting material a ¹²C¹⁶0₂ laser may be used with said deuterium S(2) Raman cell with a shift of about 414 cm^"1, the laser lines 10R(10) -10R(32) being suitable for diethyl zinc dissociation. Instead, a ¹²C¹⁶0¹⁸0 laser (10R(8) - 10R(25) laser lines) or a ¹²C¹⁸0₂ laser (10R(8) -10R(26) laser lines) may be used with said deuterium S(2) Raman cell. Instead, a ¹³C¹⁶0₂ laser (10R(6) - ΙOR(IO) laser lines) or a ¹³C¹⁸0₂ laser (10P(8) - 10P(30) laser lines) may be used with a S(0) para-hydrogen Raman cell.

The energy/area or fluence suitable for dissociation of R-^ Zn-R₂ is expected to be 10 - 3000 mJ/cm², typically 100 - 1500 mJ/cm², e.g. 200 mJ/cm². Preferably the R₁-Zn-R₂ starting material of the process is subject to said fluence for a period of up to 100 milliseconds (ms) , preferably 1 - 10 ms, e.g. 2 ms, a suitable irradiation time being obtained, for example, by passing the gas at a speed of about 5 m/s through a beam having a diameter of 8 - 12 mm. During such radiation time the radiation is, as indicated above, provided in pulses, each pulse having a pulse length (duration) of 30 ns (nanoseconds) - lOμs (microseconds) , preferably about 100 ns.

In one embodiment the beam, where it passes through the cell, may have a fluence of 100 - 1500 mJ/cm², material flow through the reactor being in a direction transverse to the direction in which the laser beam is projected, so that molecules of said dialkyl zinc compound of formula R_τ-Zn-R₉ pass through the beam and are subjected to the irradiation for an average period of 1 - 10 ms.

If desired, infra-red laser irradiation of two wavelengths may be employed, to improve the level of depletion of the starting material with regard to ⁶⁴Zn. This can be achieved using a second C0₂ laser with one or more amplifiers, and four- wave mixing in the Raman cell.

Preferably, as the starting material is passed through the photochemical reactor, a barrier gas is passed simultaneously through the reactor, between the starting material and each window of the reactor, to resist the deposition of reaction products on said windows. Examples of suitable barrier gases, inert in the reaction environment, are those mentioned above for the diluent gas, the layer thickness of the barrier gas preferably being at least 5 mm.

Calculations have indicated that, using diethyl zinc as the starting material and using the deuterium Raman cell and C0₂ laser described above, a separation factor (α) , i.e.:

(% of ⁶⁴Zn in the zinc in the product)

(100^• - % of ⁶⁴ Zn in the zinc in the product)

Q! (% of ⁶⁴Zn in the zinc in the feed)

!100 - % of ⁶⁴Zn in the zinc in the feed)

with an α value of 10 is obtainable. Thus, to obtain a product comprising < 1% ⁶⁴Zn, a two-step process shows promise. In the fist step a feed comprising 48,9% by mass ⁶⁴Zn can be converted to an intermediate product stream comprising 8,7% by mass ⁶⁴Zn and a waste stream comprising 64,5% by mass ⁶⁴Zn; and in the second step the intermediate product stream comprising 8,7% ⁶⁴Zn can be converted into a final product comprising 0,9% ⁶ Zn and a waste stream comprising 15,3% ⁶⁴Zn which can be recycled as part of the feed to the first step.

While it is in principle possible to project the laser beam through the reactor in the direction of flow of the starting material (co-current or counter-current) through the reactor, it is preferred to use a cross-flow reactor, in which the laser beam is projected through the reactor in a direction normal to the direction of flow of the starting material. In this case the reactor can be used having a cross-section, normal to the direction of flow of the starting material, at the zone occupied by the laser beam projected into the reactor, which is profiled to have an outline which corresponds to the outline of the laser beam in side elevation where it passes through the reactor. In this way, the laser beam, where it passes through the reactor, fully occupies the cross-section of the reactor so that any flow of starting material through the reactor and past the laser beam is reduced, if not eliminated, all the starting material being encouraged to pass through the beam.

The laser beam may be focused so that it tapers or converges to a focus of maximum fluence, after which it diverges with increasing diameter away from the focus. While the focus may be located outside the reactor before the beam enters the reactor or after the beam has left the reactor, so that the beam either diverges or converges in diameter as it passes through the reactor, it is expected that it will be preferred to locate the focus in or on the side of the reactor remote from the Raman cell so that the beam converges or tapers to a waist in or on said remote side of the reactor, after which it diverges, the focusing being selected to provide a beam of desired average fluence in the reaction zone through which the starting material passes .

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 shows a schematic flow diagram of the process in accordance with the invention; and

Figures 2 - 4 show respectively schematic three-dimensional partial views of reactors in accordance with the invention for use in accordance with the process of the invention.

In Figure 1 of the drawings reference numeral 10 generally designates a schematic flow diagram of an installation for carrying out the process of the present invention.

In the drawings reference numeral 12 designates a tunable ¹²C¹⁶0₂ laser oscillator shown projecting a pulsed laser beam 14 at a pulse frequency of 1000 Hz into a ¹²C¹⁶0₂ laser amplifier 16, the amplifier in turn projecting an amplified laser beam 18 at a mirror 20. The mirror 20 deflects the beam 18 into a deuterium S(2) Raman cell 22 with a shift of about 414 cm^-1.

The Raman cell 22 is shown projecting a frequency-shifted laser beam 24 into an inlet window 26 of a photochemical reactor 28 having an outlet window 30, from which the beam 24 emerges.

The installation 10 is shown having a feed gas make-up stage 32 receiving a flow of starting material along flow line 34, and receiving a flow of barrier gas along flow lines 36. The make-up stage 32 feeds starting material along flow line 38 into the reactor 28, and feeds barrier gas along flow lines 40 into the reactor 28. The reactor 28 in turn feeds barrier gas and reaction products along flow line 46 into the product collection stage 44.

The product collection stage 44 feeds reaction products and barrier gas along flow line 48 to a waste collection stage 50, the waste collection stage 50 feeding barrier gas along flow line 52 to a vacuum system 54.

The vacuum system 54 is shown feeding barrier gas along the flow line 56 which recirculates it to the flow lines 36. The waste collection stage 50 has respective outlet flow lines 60 and 62, respectively for dialkyl zinc waste and for hydrocarbon by-products.

In accordance with the process of the invention the laser 12 produces a beam 14 which is pulsed at a pulse frequency of 1000 Hz and has a wavenumber of about 975 cm^"1.

In the amplifier 16 the beam 14 is amplified to a pulse energy of about 2000 mJ.

In the Raman cell 22 the wavelength of the beam 18 is shifted, so that the beam 24 has a wavenumber of 560 cm^"1.

Diethyl zinc, whose zinc content has the natural isotopic composition, is fed along flow line 34 into the gas make-up stage 32, together with barrier gas which is argon and is fed to the make-up stage 32 along flow lines 36.

The feed gas make-up stage 32 feeds diethyl zinc vapour at a pressure of about 100 Pa along the flow line 38 into the reactor 28, and similarly feeds barrier gas along flow lines 40 at a pressure of about 100 Pa into the reactor 28.

The reactor 28 has a passage therethrough, described in more detail hereunder with reference to Figures 2 - 4, into which the windows 26, 30 open, the windows 26, 30 being about 1000 mm apart. The barrier gas is fed into this passage via the flow lines 40 respectively adjacent the windows 26, 30, so that it flows as a suitably laminar layer over the windows 26, 30 to separate the diethyl zinc vapour from said windows. The diethyl zinc vapour flows along said passage between the layers of barrier gas from the inlet to the passage to the outlet from the passage. The barrier gas acts to resist deposition of reaction products formed during the reaction in the reactor 28 on the windows 26, 30. The laser beam 24 passes through the reactor 28 as a focused beam, whose focus is centrally located in the reactor 28 in said passage, midway between the windows 26, 30, so that it tapers from the window 26 to the focus and then diverges or increases in diameter from the focus to the window 30. The average fluence in the beam in the reactor 28, between the layers of barrier gas on the windows 26, 30 is about 200 mJ/cm².

In the passage of the reactor 28, in the volume occupied by the beam 24, photochemical dissociation of the diethyl zinc takes places, with those molecules comprising the ⁶⁶Zn, ⁶⁷Zn, ⁶⁸Zn and ⁷⁰Zn isotopes of zinc being preferentially dissociated. The products of the dissociation are ethyl zinc radicals and ethyl radicals, the ethyl zinc radicals dissociating spontaneously into zinc particles and ethyl radicals.

Reaction products issue from the reactor 28 along the flow line 46 to the product collection stage 44, together with barrier gas. In the product collection stage 44 zinc particles which are depleted with regard to the ⁶⁴Zn isotope are filtered from the gases in question, which comprise barrier gas, hydrocarbon vapour formed by recombination of the ethyl radicals and diethyl zinc which is enriched with regard to the ⁶⁴Zn zinc isotope.

In the reactor 28 a separation factor (a) of 10 is achieved, so that the zinc has its ⁶ Zn proportion reduced from the naturally occurring value of about 48,89% down to about 8,7%.

In the product collection stage 44 the ⁶⁴Z-depleted particles, filtered from the gases and vapours, are periodically removed, batchwise, as the principal product.

In the waste collection stage 50 the ⁶⁴Zn-enriched diethyl zinc is separated from the hydrocarbon vapour and barrier gas, the hydrocarbon vapour and barrier gas also being separated from each other. The ⁶⁴Zn-enriched diethyl zinc issues from stage 50 along flow line 60 as a zinc-containing by-product, while the hydrocarbon vapour issues along flow line 62, the barrier gas issuing along flow line 52 to the vacuum system 54.

It will be appreciated that the vacuum system 54 causes gas and vapour flow through the system from the feed lines 34, 36 to the line 52, and the vacuum system 54 then recycles barrier gas along the flow line 56 to the flow lines 36. The reactor 28 is operated with the gas and vapour feed from the make-up stage 32 at ambient temperature (15 - 25°C) . The feed rate of the diethyl zinc is such that it passes through the reactor at about 5 m/s.

In accordance with the process of the invention the

Zn-depleted product can be reacted to reconvert it to diethyl zinc, which can then be fed into a further substantially similar installation (not shown) through which it is passed in substantially the same fashion, again with a separation factor

(a) of about 10, so that in this further installation an eventual zinc product is obtained whose zinc has a proportion of about 0,9% ⁶⁴Zn zinc isotope.

The diethyl zinc waste obtained from the further installation will comprise about 15,3% by mass ⁶⁴Zn zinc isotope. This diethyl zinc waste has a Zn content which is substantially depleted compared with naturally occurring zinc, as regards the ⁶⁴Zn zinc isotope. This diethyl zinc can then be fed, together with diethyl zinc formed from naturally occurring zinc, along the flow line 34 into the first installation 10 shown in the drawings.

Turning to Figure 2 of the drawings, reference numeral 28 generally designates a photochemical reactor in accordance with the present invention. The reactor 28 is a transverse flow reactor, and only a portion thereof is illustrated. The gas flow direction is shown by arrow 64 and the direction of propagation of the laser beam 24 (see Figure 1) is shown by arrow 66. The outlet end of the reactor is shown at 68, the inlet end (not shown) having a similar shape to that of the outlet end 68.

The reactor has a pair of opposed walls 70 having machined inner surfaces which are convexly curved as at 72. The side edges 74 of the walls 70 are joined by windows 26, 30 (see Figure 1) which are transparent to the shifted C0₂ laser radiation in the beam 24 (see Figure 1) .

The curvature of the interior surfaces 72 of the walls 70 in Figure 2 is such that the cross-section of the gas flow passage 76 defined between the walls 70 and inwardly of the windows 26, 30 is of generally hourglass shape, converging or tapering from a broad region at the window 26 to a narrow waist at 78, and then diverging and widening to a broad region at the window 30. The outline of the cross-section of the passage 76 is selected substantially to coincide with the outline in side elevation of a focused laser beam having a focus at 78 and which substantially fully occludes the interior of the passage 76. This means that gas flowing along the passage 76 in the direction of arrow 64 cannot flow past the laser beam, between the beam and the walls 70 of the passage, which promotes an increased level of dissociation.

Turning to Figure 3, the same reference numerals are used for the same parts as in Figure 2. The chief difference between Figure 3 and Figure 2 is that the cross-section of the passage 76 tapers from a broad region at the window 26 to a narrow region at the window 30. The reactor 28 in Figure 3 is intended for use with a laser beam 24 which is focused so that it tapers continuously in a direction from the window 26 to the window 30. The degree of focusing accommodates and compensates for absorption of laser energy across the width of the passage 76 from the window 26 to the window 30, to promote a relatively constant fluence across the width of the reactor, which promotes selectivity and yield, and promotes improved photon use.

Finally, in Figure 4, the same reference numerals again also refer to the same parts as in Figure 2 and 3. The reactor 28 of Figure 4 is similar with regard to the cross-section of its passage to that of Figure 3, except that the inner surfaces 72 of the walls 70 each have a part-cylindrical groove 80, the grooves 80 registering with each other and tapering so that they become of progressively reducing radius of curvature in a direction from the window 26 to the window 30. The radius of curvature R of the groove 80 at the window 26 is half the diameter of the laser beam at the window 26, and the radius of curvature of the groove 80 at the window 30 is half the diameter of the laser beam at the window 30, the laser beam having a diameter D slightly greater than the spacing S, on opposite sides of the groove, between the walls 70. Provision of the grooves 80 further promotes full use of the radiation of the beam 24 in dissociating the diethyl zinc.

It is an advantage of the invention that it provides a practical and advantageous process for the depletion of zinc, particularly naturally occurring zinc, with regard to the ⁶⁴Zn zinc isotope. Zinc depleted with regard to ⁶⁴Zn can easily be obtained, of a quality suitable for use in water-cooled nuclear reactors. The product of the process, namely elemental zinc, whose zinc component comprises < 1% by mass ⁶⁴Zn can easily be converted to zinc oxide with a similarly low ⁶⁴Zn content, whose purity otherwise depends only on the grade of dialkyl zinc employed as the starting material.

Claims

1. A process for the separation of at least one isotope of zinc from other isotopes of zinc to obtain, from a zinc- containing starting material, a zinc-containing product which is enriched or depleted with regard to any said separated isotope of zinc, the method comprising subjecting the zinc-containing starting material comprising at least one dialkyl zinc compound of the formula:

R₁-Zn-R₂ in which R-^ and R₂ are the same or different and are selected from ethyl, propyl and butyl groups, which may be straight- chain or branched, and which are optionally substituted with subtituents selected from deuterium, tritium, chlorine, bromine, iodine and fluorine, to a molecular laser isotope separation step using infra-red laser irradiation, preferentially to decompose compounds of said formula R₁-Zn-R₂ containing at least one istope of zinc, to form elemental zinc.

2. A process as claimed in claim 1, which comprises separating ^⁴Zn from other isotopes of zinc to obtain a zinc-containing product which is depleted with regard to ⁶⁴Zn.

3. A process as claimed in claim 1 or claim 2, in which the starting material comprises a single compound of the formula R-,-Zn-R₂ in which R₁ and R₂ are the same, being unsubstituted.

4. A process as claimed in claim 3, in which the R₁-Zn-R₂ is unsubstituted diethyl zinc of formula CH₃-CH₂-Zn-CH₃-CH₃, the zinc in the starting material having the following isotopic composition:

Zinc Isotope % bv Mass

⁶⁴Zn 48,6 - 49,2

⁶⁶Zn 27,5 - 28,1

⁶⁷Zn 3,8 - 4,4

⁶⁸Zn 18,3 - 18,9

⁷⁰Zn 0,3 - 0,9

5. A process as claimed in any one of the preceding claims, which includes the step, prior to the molecular laser isotope separation step, of synthesizing the compound of the formula R₁-Zn-R₂ .

6. A process as claimed in claim 5, in which synthesizing the compound of the formula R₁-Zn-R₂ is by reacting a compound selected from alkyl magnesium compounds, alkyl aluminium compounds and mixtures thereof with at least one anhydrous zinc salt.

7. A process as claimed in any one of the preceding claims, in which the laser irradiation is selected preferentially to decompose compounds of the formula R₁-Zn-R₂ in which the Zn is an isotope of zinc other than ⁶⁴Zn, so that the zinc-containing product is elemental zinc which is depleted, relative to the starting material, with regard to ⁶⁴Zn.

8. A process as claimed in claim 7, in which the irradiation step is followed by a synthesis step in which the elemental zinc formed by the irradiation is converted to at least one dialkyl zinc compound of said formula R₁-Zn-R₂, each of which compounds is subjected to a further molecular laser isotope separation step using infra-red laser irradiation selected preferentially to decompose compounds of the formula R₁-Zn-R₂ in which the Zn is an isotope of zinc other than ⁶⁴Zn.

9. A process as claimed in any one of the preceding claims, which comprises generating the laser irradiation by means of a

C0₂ laser, red-shifting the irradiation by means of a Raman cell, to provide the irradiation with a wavelength of 16 - 19 μm, and projecting a beam of the irradiation through a photochemical reactor through which the starting material is passed, with each compound of formula R^-Zn-R₂ being in vapour form, at a temperature which is at most ambient temperature and at a pressure of 10 - 500 Pa.

10. A process as claimed in claim 9, in which the starting material comprises, in addition to molecules of said dialkyl zinc compound of formula R₁-Zn-R₂ , an otherwise inert scavenger gas for scavenging alkyl-zinc radicals, the starting material passing through the reactor at a temperature of 50 - 220K and a pressure of 50 - 200 Pa, the laser irradiation being in the form of a pulsed output from the laser with a pulse frequency of 500

- 2000 Hz and with a pulse length of 70 - 500 ns.

11. A process as claimed in claim 9 or claim 10, in which the beam, where it passes through the reactor, has a fluence of 100

- 1500 mJ/cm² , material flow through the reactor being in a direction transverse to the direction in which the laser beam is projected, so that molecules of said dialkyl zinc compound of formula R₁-Zn-R₂ pass through the beam and are subjected to the irradiation for an average period of 1 - 10 ms.

12. A process as claimed in claim 1, substantially as described herein.

13. A zinc-containing products depleted with regard to ⁶⁴Zn compound with the isotope composition of naturally occurring zinc, whenever produced by the process according to any one of claims 1 - 11 inclusive.