Journal of NUCLEAR SCIENCE and TECHNOLOGY, 30(11), pp. 1195~1197 (November 1993). 1195 In this note, the proper deposition potential, SHORT NOTE nitric acid concentration, temperature, and Electrolytic Extraction of Platinum electrode materials were investigated, and the effect of coexisting ions was examined. Group Metals from Dissolver 1. Experimental Solution of Purex Process The experimental apparatus is shown in 1. The electrolytic cell (500 ml) consistedFig. KenjiKOIZUMI, Masaki OZAWA of a counter electrode (1 mmo, Pt wire), a re- and TornioKAWATA ference electrode (SCE), and a working elec- Power Reactor & Nuclear Fuel Development Corp.* trode. Ti and Ta were used as the elec- trode material. The electrolyte was agitated Receired May 19, 1993 to maintain a constant temperature. Elec- Revised July 30, 1993 trode potential were controlled by the po- KEYWORDS: electrolytic extraction, tentiostat. Pt group metals concentration deposits, palladium, rhodium, ruthe- was adjusted to about 1.0E-4 mol/l. Urani- nium, purex process, solvent extrac- um (VI) ion was tested as a main coexisting tion, deposition potential, recovery, actinide ion in this experiments. Potential- electrodeposition, dissolver solution time sequences for metal deposition and potential-current curves corresponding to Pd- In the commercial light water reactor fuel dissolution are shown in Fig.2. First the at a burn-up of 33 MWd/kg, approximately working electrode was cathodically polarized 4 kg of Pt group metals are included per ton in nitric acid solution containing Pt group of heavy metals(1). Elimination of the ionic metal. The polarization potential (05) was Pt group metals (Pd, Rh, Ru) from dissolver changed from +0.5 to -0.5 V. Then, the solution seems to be an effective method electrode was moved to pure nitric acid solu- to improve the Purex extraction process for spent nuclear fuel reprocessing. A method of electrolytic extraction from dissolver solution was investigated. Currently there are two electrochemical recovery techniques ; one is the electrolytic oxidation to volatilize Ru(2)(3), the other is an electrolytic reduction technique which deposits Pt group metals(4). They are likely to be recovered by the electrolytic extraction method, Fig.1 Electrolytic extraction system because their theoretical reduction potentials are relatively high. Standard redox potentials of Pt group metals are given by 0.685 V (vs. SCE), 0.528V (vs. SCE), 0.230V (vs. SCE) for Pd, Rh and Ru, respectively(5). Actual reduc- tion potential will be given by Nernst equation. Each metal's calculated reduction potential is 0.57, 0.47 and 0.13 V for Pd, Rh and Ru, re- spectively. Because high concentration of nitric acid might disturb the deposition rate due to the Polarization potentials was changed from 0.5 to -0.5 V. competitive reduction reaction of HNO3, the Fig.2 Anodic dissolution of Pd effects of nitricacid concentration, polarization deposited on Ta electrode * Tokai-mura potential, and coexisting ions are significant. , 1baraki-ken 319-11. 109- 1196 SHORT NOTE (K. Koizumi et al.) J. Nucl. Sci. Technol., tion and scanned from 0 to 2.0 V to obtain the factor =1). it means that until Ta and Ti potential-current curve by scanning speed 50 electrode was covered by Pd ion, Pd ion mV/s. When the metal ion was deposited, deposited by a pair with Ta and Ti atom. the anodically dissolution current (CPfchssolve) After this initial depositing step, Pd ion con- was observed. QMdissolve was disappeared by tinued depositing on the initially deposited Pd. scanning from 0 to 2.0 V and holding a poten- This deposition rate differs from that on the tial 2.0 V. Moreover, this peak potential of original electrode surface. In the following dissolution current made a difference for each tests, the deposition time was controlled within metal. Pd, Rh and Ru showed each potential the initial depositing step. of 1.75, 1.82 and 1.25 V. Then, depositing The dependency of metal deposition rate on metal quantity was determined by calculating the polarization potential is shown in Fig.4. the current quantity that corresponded to this Deposition rates were apparently dependent on peak area. The depositing quantity is given as the polarization potential, and the max. de- Datom=QM dissolve/Fxn ,(1) position rates were observed on Pd and Ru at specific potentials. As for Pd deposition, where Damn,: Depositing metal quantity (mol) the peak potential corresponded to max. de- F: Faraday constant (=96,485 C/mol) position rate, i, e. max. deposition potential, n: Number of electrons participating is apparently effected by hydrogen generation. in the reaction. Deposition of Rh and Ru, however, seems to This procedure was repeated by changing the be independent of hydrogen generation. The polarization potential. In experiments, poten- same characteristics of polarization potential tiostatic procedure were effective for investi- effect were observed under coexisting Uranium gation of electrochemical behavior of Pt (VI) ion (U/Pd, Rh, Ru =10). Compared with cal- group metals, because tests were over in culated deposition potential, each deposition short time and it was not necessary to analyze potential was shifted from positive to negative deposited metal concentration by various anal- direction. ysis equipments. 2. Results and Discussion The deposition process was investigated by using Ta and Ti electrode. Result is shown in Fig.3. An inflection point was observed for both electrodes. This point corresponds to the saturated number of Pd atom on Ta and Ti electrode surface (calculated by roughness Nitric acid concentration was 3.0 mol/l. Temperature was 40dc. Fig.4 Potential dependency of deposition rate of Pd, Rh and Ru on Ta electrode The effect of nitric acid concentration for deposition rates was investigated by changing the acidity from 0.5 to 5.0 mol/l. Results are shown in Fig.5. The deposition rate de- creased linearly with increase of the nitric Polarization potentials are given by 0.18 V, -0.1 V acid concentration. for Ti and Ta. Nitric acid concentration was 3.0 mol/l. The dependency of Pd deposition rate on Temperature was 40dc. temperature is shown in Fig.6. In general, Fig.3 Relation between Pd deposition rate and electrode material the deposition rate increases with rise of tern- 110 Vol. 30, No. 11 (Nov . 1993) SHORT NOTE (K. Koizumi et al.) 1197 effect of coexisting ions. Uranium (VI) ion was selected as a coexisting ion in this experiment. The dependency of the deposition rate of Pd on Uranium(VI) concentration is shown in 7. The deposition rate significantly de-Fig. creased even under the presence of a small amount of Uranium (VI) (U/Pd=1). Uranium (VI) ion effect is decreased above U/Pd>10. However, since the concentration ratio of U/Pd is much more and another ions exists in real dissolver solution, the coexisting ion effect in real dissolver solution should be in- Fig.5 Effect of nitric acid concentration for vestigated still more. Pd deposition rate on Ta electrode Nitric acid concentration was 3.0 mol/l. Polarization potential was -0.1 V. Temperature was 40dc. Fig.7 Effect of Uranium(VI) ion for deposi- Polarization potentials are given by 0.18 V, -0.1 V for Ti and Ta. Nitric acid concentration was 3.0 mol// tion rate of Pd on Ta electrode Fig.6 Temperature dependency of Pd deposi- A long term deposition test with synthetic tion rate on Ta and Ti electrode dissolver solution showed that ca. 90% re- perature. In this experiment, the temperature covery of Pd reached the steady state after affected Ta differently. This phenomenon 8 h, but the recovery of Rh and Ru is still seemed to be related to the hydrogen genera- 23% and 10%, respectively. Those depositions tion potential. This potential moved toward were still continuing to increase linearly. the positive side as the temperature increased. REFERENCES For both Ta and Ti, a max. deposition poten- tial exists in the more positive side than hy- (1) BUSH, R. P. : Platinum Metals Rev., 35(4), 202~208 (1991). drogen generation potential (pH)), but Ta's max. (2) TUNER,A. D., HtROSE,Y., et al.: RECOD'91, deposition potential is closer to pH than Ti's. Vol. 2, 735 (1991). It is supposed that the Pd deposition on Ta is (3) MOTOJtMA,K.: J. Nucl. Sci. Technol., 27(3), inhibited by hydrogen generation with increase 262 (1989). of temperature. (4) GRIESS,J. C., Jr. : J. Electrochem. Soc., 100, 429 (1953). In dissolver solution, TRU elements and (5) EBIIIARA, K. : "Denkikagaku Binran", (4th fission products exist in high concentration ; ed.), (in Japanese), (1985), Denkikagaku Kyo- therefore, it is necessary to investigate the kai, Tokyo. 111—.