Separation of lithium isotopes by chemical exchange chromatography

1. National Institute for Research and Development of Isotopic and Molecular Technologies 65-103 Donath Str., P.O.Box 700 RO-400293 Cluj-Napoca, Romania; E-mail: axente@itim-cj.ro Separation of lithium isotopes by chemical exchange chromatography D. Axente, Ancuţa Mureşan

2. H 3 1 6Li (7.4%) 6LiD 7Li (92.6%) ELEX process, electric driven chemical exchange (1951) Operated 1953-1956 COLEX process (Column Exchange) Li+ + Li(Hg) Alpha 4: operated 1955-1963; dismantled 1980. Alpha 5: operated 1955-1959; dismantled 1965-1966. Employed ~ 10.9x106 Kgs mercury.

3. The mercury-nitric acid purification system, utilized in COLEX process, was the source of the major mercury-bearing waste stream at Y-12 plant. This system discharged a diluted, neutralized acid waste, containing mercuric nitrate. ~ 910,000 Kgs of mercury have still not been accounted for. ~ 310,000 Kgs of this material have been lost in waste streams, evaporation and spills. It was estimated that 23,269 Kgs of mercury were released in the air by venting of mercury vapors and smelting of mercury contaminated scrap. The COLEX process discharged 108,408 Kgs of mercury to East Fork Poplar Creek, being contaminated Watts Barr Lake, Poplar Creek and Clinch River.

4. 6Li (Li/Al alloy) 6Li(OH) 6LiH 6LiD 6Li is utilized to manufacture of the secondaries of so called “dry thermonuclear devices”, as 6LiD (lithium deuteride) 6Li + Neutron →3H + 3He + Energy (1) D + 3H →4He + n + 17.6 MeV (2) The neutrons, from a fission of “primary” device, bombarded 6Li, liberating tritium, which quickly fuses with the nearly deuterium of 6LiD. In France COGÉMA enriched 6Li by chemical exchange in columns, using Li(Hg) and a solution of LiOH.

5. In 1995 the releases contained 12.1 Kg mercury, representing a major reduction since 1984, when they rose to 240 Kg/yr. Lithium-6 will be used to produce tritium in magnetically confined nuclear fusion reactors using deuterium and tritium as fuel. Tritium will be produced by surrounding the reacting plasma with a “blanket” containing 6Li, where neutrons, from the deuterium-tritium reaction in the plasma, will react with lithium-6 to produce more tritium (1). ITER will be designed to produce ~ 500 MW of fusion power, sustained for up to 400 sec., by burning of about 0.5 g of deuterium/tritium mixture in its 840 m3 reactor. Fusion power offers potential of “environmentally benign, widely applicable and essentially inexhaustible” electricity. These properties will be needed as world energy increase, while simultaneously greenhouse gas emissions must be reduced. ITER: European Union; India; Japan; China; Russia; South Korea; USA.

6. Separation of Lithium Isotopes Chemical exchange methods have been considered as useful isotope separation techniques. Enriching coefficient: ε = α -1 (3) Column chromatography with: » [2.2.1]-cryptand resin: ε = 0.014 at 40°C » [2B.2.1]-cryptand resin: ε = 0.047 at 20°C » cation exchange resin: ε = 0.00089-0.00171 at 25°C » ion exchange using hydrous manganese (IV) oxide, as ion exchanger, and elution chromatography: The adsorption capacity of MnO2 was: 0.5 meq/g. (4)

7. R = local enrichment factor = log R Δm/m x 100 (5) (6Li+/7Li+)0 = the isotopic ratio of the lithium feed solution, of natural isotopic abundance. (6Li+/7Li+) = the isotopic ratio of an individual fraction, extracted from the chromatographic column. Δm/m is the proportion of lithium in the individual fraction Fig.1. Separation of lithium isotopes by cationic exchange chromatography

8. Table 1. Separation of lithium isotopes by ion exchange chromatography [Dong Won Kim, Bull. Korean Chem. Soc., 2001, Vol.22, No.5, 503-506]

9. Cryptand (2B,2,1) polymer Criptands are macrobicyclic polydentate ligands that Lehn has termed, e.g. R – CH2O – C6H3 – N2O5[J.M. Lehn, Nobel Lecture, Angew. Chem. Int. Edn. Engl., 27, 89 (1988)] A cryptand (crypt) has a rigid molecular cavity and can form a complex, e.g. [NH4 (crypt)]Cl, in which the ligand encapsulates the cation with a bicapped trigonal prismatic coordination polyhedron. Polymers with a cryptand, as an anchoring group, are called cryptand polymers. Cryptand (2B,2,1) polymer is able to bind to alkali and alkaline cations and stability constant of potassium is highest among alkali cations. Therefore stability constant of amonium ion is estimated to be high.

10. Ion exchange resin (1,12,15-triaza-3,4: 9,10-dibenzo-5,8-dioxacycloheptadecane), named “NDOE bonded Merrifield peptide resin” [D.W. Kim et. al., J. Radioanal. Nucl. Chem., Vol. 242, No. 1, (1999), 215-218] The absorption capacity: 0.2 meq/g dry resin. The lighter isotope, 6Li, concentrated in the solution and the heavier isotope, 7Li, in the resin phase. The distribution coefficient of Li+, between resin and solution, was determined: Ciand Cf are the concentrations of initial and final electrolyte solution. M is the mass (g) of dry resin. V is the total volume (cm3) of the solution. (6)

11. log Kd NDOE resin was slurried in ammonium chloride solution and then was packed in a water jacketed glass column (32 cm length, 0.1 cm I.D.). 1 ml of 500 ppm solution of Li+ in H2O was loaded on the top of the resin. 2.0 M NH4Cl solution was used as an eluent for lithium isotope separation. The effluent was collected as fractions and isotopically analysed. The separation factor α = 1.0201 was found. α = 1.035 were reported by the same author, using monobenzo-15-crown-5 and reduced dibenzo pyridino diamide azacrown, as anchor groups, respectively. α= 1.068 was obtained on N3O3 azacrown ion exchanger. Fig.2. Distribution coefficient of Li+ on NDOE resin for different concentrations of NH4Cl

12. [Yasutoshi Ban et. al., J. Nucl. Sci. Technol., vol. 39, No 3, 279-281 (2002)] prepared a B15C5resin in porous silica beds, with uniform size, 60 μm, for lithium isotope separation. The volume of silica beads is not changed by the outer solutions; the expansion and shrinking of the packed resin are avoided. Two runs of column experiment were performed with 21 g of B15C5 resin packed in a glass column (0.8 cm i.d., 100 cm long), at 35°C. Dibenzo-15-crown-5 ether

13. pH Concentration of Li, mol/dm3 pH Conc. Li Fig.3. Values of pH and concentration of Li in the sample obtained through effluent fraction Volume of the effluent, cm3 The resin packed in the column was washed with pure water and charged with mixture solution of methanol and 12 M HCl (70 vol. %). Then 0.55 M LiCl, dissolved in the same mixture solution, was fed into the column by a high pressure pump. The effluent was collected in small fractions.

14. The total amount of lithium in the B15C5 resin: (7) Qtot = Co(VFB – Vd) Co is the concentration of lithium in the feed solution VFB is the volume of the breakthrough point (where the concentration is Co/2) Vd is the dead volume of the column. The total Li adsorption capacity of the B15C5 resin was 0.15 mmoles/g. If the column contains 21 g of resin, therefore Qc = 3.15 mmoles.

15. Isotopic ratio 7Li/6Li Initial ratio Fig.4. Isotopic ratios 7Li/6Li vs. volume of effluent Volume of the effluent, cm3 Table 2. Determined isotopic ratios of lithium in the sample fraction

16. qi is the amount of lithium in an effluent sample. Qtot is the total adsorption of lithium on column resin. 7r is the isotopic mole fraction of 7Li and subscript (i) and (o) denote the fraction sample and feed solution, respectively. ε7/6 = 0.0127, smaller than the value reported by Kim et al. who utilized B15C5. The heavier isotope is enriched in the solution phase, in the front of lithium band. ε7/6 = Σqi·(1 + ro)|7ri – 7ro| / {7ro·Qtot·(1 + 7ri)} (8) In Kim’s work Chromatography of lithium was done in an elution manner, where the lithium band showed a bell shaped concentration profile.

17. Fig.5. Comparison between the isotopic concentration profiles at steady state on silica grafted resin and organic resin Molar isotopic ratio of 7Li silica grafted resin organic resin Band length, cm Y. Barré et al., [“Separation des isotopes du lithium par chromatographie ionique avec des ligands greffe’s sur silice”, Conférénce Internationale sur les isotopes stables at les effects isotopique, 20-25 Juin 1999, Carry le Rouet, France] developed a silica grafted resin, that presents greater isotope separation factor and better kinetic than with conventional organic resins. Silica allows the use of silane chemistry, which gives strong, stable bonding of organic ligands to the support, by the reaction of surface silanols with methoxy silanes.

18. Fractions extraction 1 2 3 4 pH HCl pH pH pH 1 2 Selector Valve CH3COOLi 2 Pump CH3COONa 3 1 3 Fig.6. Flow diagram of the plant for lithium isotope separation by continuously displacement chromatography

19. The four columns, each 100 cm length, 1-2 cm i.d., are serially connected. At the bottom of each column the pH-meter measures and records continuously the solution pH. At the both end of the columns there are selector valves, which change the solution flow in order to continue the development of the lithium band as long as is necessary. The adsorption band of lithium is moved continuously through the columns, in order to realize adequate separation of isotopes. 7Li is enriched in the front of the band, in solution, and 6Li at the rear of the band, in adsorbent phase. The degree of enrichment, at each boundary, increases with the distance travelled by the band, until a steady state is reached. After a certain period of operation enriched 6Li and 7Li are recovered from the recycling line, through a selector valve, and equal amounts of feed lithium are fed to the enrichment column.

20. Conclusions The methods for lithium separation, based on lithium isotope exchange 7Li/6Li in the system Li+ - Li(Hg) has been abandoned in USA according to high level of mercury pollution of the environment and big mercury risk of the workshop personnel. In France, Japan, Russia, etc., the research work for a new lithium isotope separation technology, efficient and clean toward environment, are in progress. Lithium enriched in the lighter isotope, 6Li, is used in weapons components, in the form of Lithium deuteride and is necessary for nuclear fusion reactors in the frame of ITER Program. Using the ligands, with a good selectivity towards lithium isotopes, grafted on small silica particles, is very promising because combines a high selectivity with a good isotope exchange kinetic and an important mechanical stability. These characteristics would be considered in order to develop a new technology for lithium isotope separation.