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78
UDC 541.135
ELECTROCHEMICAL BEHAVIOR AND KINETICS OF THE INTERVALENCE CHARGE TRANSFER FOR THE SM(III)/SM(II) REDOX COUPLE IN LiF-CaF2 MELT
Yu.V. Stulov1, M. Korenko2, B. Kubikova2, S.A. Kuznetsov1
11. V. Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials of the Kola Science Centre of the RAS, Apatity, Russia
2Institute of Inorganic Chemistry of the Slovak Academy of Sciences, Bratislava, Slovakia Absrtact
This article is focused on the electrochemical investigation (cyclic voltammetry) of the redox couple Sm(III)/Sm(II) in an eutectic LiF-CaF2 melt containing SmF3. The first step of reduction for Sm(III) ions, involving one electron exchange in soluble/soluble Sm(III)/Sm(II) system, was found on a tungsten electrode. The study of the Sm(II)/Sm(0) electrode reaction was not feasible, since it's the redox potential is in the same range of the solvent decomposition. The first step was found reversible at temperatures 1075 and 1125 K up to polarization rate 1 V/s and at temperature 1175 K the process was reversible at all applied in this study sweep rates. The diffusion coefficients (D) of Sm(II) and Sm(III) ions were determined by cyclic voltammetry, showing that D decreases when oxidation state increase, while the activation energy of diffusion (Ea) increases. The standard rate constants of charge transfer were calculated for the redox couple Sm(III)/Sm(II) at 1075 and 1125 K based on the data of cyclic voltammetry.
Keywords:
redox couple, samarium, standard rate constant of charge transfer, diffusion coefficients, cyclic voltammetry.
Today there are two scenarios in the spent nuclear fuel management. The first scenario represents the “unclosed” fuel cycle, which is based on the slow long-term cooling of spent fuel in the intermediate repository and then on the idea of the thousands year ultimate storage in underground repository. The second scenario is based on hydrometallurgical reprocessing of spent fuel by PUREX processes (use aqueous solutions and organic extractants) to obtain strengthened new fuel material. Some task is that even the present day PUREX technology is not capable to separate the tri-valent transuranium elements like americium and curium from tri-valent fission products represented by lanthanides (Ln). The existence of long-lived transuranium elements in the spent fuel results to the environmental problems based on radiotoxicity. However, these long-lived transuranium elements could be in principle incinerated in so-called transmutation reactor systems into short lived or even stabile isotopes.
In the present study, electrochemistry of the redox couple Sm(III)/Sm(II) in the LiF-CaF2 eutectic melt was examined on a tungsten working electrode by cyclic voltammetry. The eutectic mixture LiF-CaF2 has a wide electrochemical window and suitable melting point (1035 K) and appears to be the more appropriate molten system for separation of actinides and lanthanides.
The specific objectives of this study are the determination of Sm(II) diffusion coefficients and standard rate constants of charge transfer for the redox couple Sm(III)/Sm(II). To our knowledge, these data have not previously been reported.
Electrochemical tests were performed in the glassy carbon crucible (conical, top ID: 45 mm, bottom ID: 30 mm) laid in a retort made of stainless steel, closed by a detachable flange with built-in holders for the electrodes, thermocouple and inlet and outlet of inert gas. An argon inert atmosphere (99.998%) was used inside the electrochemical cell within the all measurements. An argon gas was previously dehydrated and deoxygenated. The cell was heated using a programmable furnace and the temperature was measured using a Pt/PTRh10 thermocouple. A resistance furnace heats the retort and allowing uniform thermal field in glassy carbon (GC) crucible up to 1373 K. The inner part of the walls of the retort was protected against fluoride vapors by a glassy carbon liner [1].
A tree electrode design has been applied for electrochemical investigations. A glassy carbon crucible served as a counter electrode. Tungsten wire (99.95 % Sigma Aldrich) was used as working electrode (OD: ca 0.8 mm). Platinum wire was utilized as quasi-reference electrode. The surface area of the working electrode was determined after each experiment by measuring the depth of the immersion in the bath mixture (usually 5 mm). The electrodes were interconnected with AUTOLAB (PGSTAT30 potentiostat/galvanostat) controlled by PC with original software (GPES 4.9). The potential scan rate was varied between 5 10-3 and 2.8 V/s. The electrolytic bath consisted of an eutectic LiF-CaF2 (77/33 molar ration) salt mixture (LiF - optical grade, LOMO Plc., St-Petersburg, Russia; CaF2 - reagent grade, NevaReactive, St-Petersburg, Russia). Before use, it was dehydrated by heating under vacuum from room temperature up to the 673 K for several hours. Samarium ions were introduced into the mixture as a samarium fluoride SmF3 (reagent grade, NevaReactive, St-Petersburg, Russia). The weight of the carried salt (LiF-CaF2) was usually 70 g. The total concentration of samarium was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES).
The cyclic voltammograms obtained at different scan rates (50, 100, 200, 400, 600, 800, ... , 2800 mV/s) on a W working electrode at 1175 K are shown in Fig. The voltammogram indicates one peak in the cathodic region at -1.136 V (vs. Pt quasi-reference electrode) and the anodic peak at 0.811 V (vs. Pt quasi-reference electrode). As can be also seen in Fig. 1, the peak potentials do not change significantly with increasing of scan rate.
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Fig. Cyclic voltammograms of the LiF-CaF2-SmF3 system at different scan rates (50, 100, 200, 400, 600, 80... , 2800 mV/s) at 1175 K. WE: W; RE: Pt wire; CE: GC crucible. Co(SmF3) = 8.259 x 10-4 mol/cm3;
S = 0.231 cm2
At temperatures of 1075 and 1125 K, the peak current of electroreduction up to at least v = 1.0 V/s is directly proportional to v"12, while the peak potential was independent on the potential sweep rate to the values of v = 1.0 V/s. According to the cyclic voltammetry theory [2, 3], the electrode process at these temperatures is controlled by diffusion up to the polarization rate of v = 1.0 V/s. At v > 1.0 V/s, a deviation of the experimental points was observed from the straight line passing through the origin of coordinates, which evidences a transition from a reversible process to a quasireversible process. At the higher temperature (1175 K) the process is controlled by diffusion at all applied scan rates.
Diffusion coefficients (D) of Sm(III) and Sm(II) ions in LiF-CaF2 melt were determined using cyclic voltammetry by Randels - Sevchik equation:
Ip = 0.446(zF)3/2(RTr1/2SC° (Dv)1/2 (1)
where Ip is the peak current in A: z is a number of electrons: S is the electrode area in cm2: C0 is the concentration of electroactive species in mol/cm3: D is the diffusion coefficient of electroactive species in cm2/ s: v is the potential scan rate in V/s.
Temperature dependencies of diffusion coefficients were following:
lnD(Sm3+) = - 0.96612 - (10442/T), (2)
lnD(Sm2+) = - 0.84885 - (10328/T). (3)
Knowing the diffusion coefficients of Sm(III) and Sm(II) the standard rate constants of charge transfer (ks) for the
redox couple Sm(III)/Sm(II) in LiF-CaF2 melt can be determined. According the analysis described before, the studied
system shows at the concentration of C°(SmF3) = 4.24 10-4 mol/cm3 a quasi-reversible behavior at sweep rates 1 - 2 V/s. In this range of scan rate, the constants of charge transfer could be calculated [4]. Table presents calculated values of the standard rate constants of charge transfer for the Sm(III)/Sm(II) redox couple at 1075 and 1125 K. At the higher temperature (1175 K), the constant of change transfer could not be calculated, since at this temperature the system is reversible at all applied polarization rates.
The standard rate constants of charge transfer for the Sm(III)/Sm(II) redox couple at different temperatures
T, K Ep, V Eva, V AEB, V VT ks (Sm(III)/Sm(II)), cm2/s
1075 -1.335 -1.028 0.307 1.803 0.0557
1125 -1.227 -0.910 0.317 1.937 0.0788
The electrochemical reduction of Sm(II) into samarium metal was not observed. It is evident that a samarium has a more negative discharge potential than the solvent cations.
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Referencies
1. Korenko M. Electrochemical investigation of the redox couple Sm(III)/Sm(II) on a tungsten electrode in molten LiF-CaF2-SmF3 / M. Korenko, Y.V. Stulov, S.A. Kuznetsov, M. Ambrova, B. Kubikova // J. Radioanal. Nucl. Chem. 2015. Vol. 301, Is. 2. Р. 589-595.
2. Nicholson R.S., Shain I. Theory of stationary electrode polarography. Single scan and cyclic methods to reversible, irreversible, and kinetic systems // J. Anal. Chem. 1964. Vol. 36, Is. 4. Р. 706-723.
3. Galyus Z. Theoretical foundations of electrochemical analysis. M.: Mir, 1974. 224 p.
4. Nicholson R.S. Theory and application of cyclic voltammetry for measurement of electrode reaction kinetics // Anal. Chem. 1965. Vol. 37, № 11. P. 1351-1355.
Information about the authors Stulov Yuriy Vyacheslavovich,
PhD (Chemistry), I.V.Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials of the KSC of the RAS, Apatity, Russia, stulov@chemy.kolasc.net.ru Korenko Michal,
PhD, Institute of Inorganic Chemistry of the Slovak Academy of Sciences, Bratislava, Slovakia, Michal.Korenko@savba.sk Kubikova Blanca,
PhD, Institute of Inorganic Chemistry of the Slovak Academy of Sciences, Bratislava, Slovakia Kuznetsov Sergey Aleksandrovich,
Dr.Sc. (Chemistry), I.V.Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials of the KSC of the RAS, Apatity, Russia, kuznet@chemy.kolasc.net.ru
УДК 669.713.1; 669.713.7
СИНТЕЗ АЛЮМО-СКАНДИЕВЫХ СПЛАВОВ И ЛИГАТУР В ОКСИДНО-ФТОРИДНЫХ РАСПЛАВАХ
А.В. Суздальцев1, А.Ю. Николаев12, Ю.П. Зайков12, А.А. Панкратов1, Н.Г. Молчанова1
Институт высокотемпературной электрохимии Урральского отделения РАН, Екатеринбург, Россия 2Уральский федеральный университет, Екатеринбург, Россия
Аннотация
Химическим и электрохимическим способами синтезированы алюмо-скандиевые сплавы (до 0.5 мас. % скандия) и лигатуры (до 2 мас. % скандия) в расплавах KF-AIF3, NaF-AlF3 и KF-NaF-AlF3 с добавками Sc2O3 и AI2O3 в интервале температур от 750 до 980°С. Электролиз проводили в лабораторном электролизере на силу тока 20 А с графитовым анодом и жидким алюминиевым катодом. Исследовано влияние катодной плотности тока (0-1 А/см2), заданного содержания Sc2O3 (1, 2, 4 и 6 мас. %) в расплаве KF-NaF-AIF3, перемешивания алюминия (0, 100 об/мин) и длительности синтеза (30-180 мин) на содержание, форму и распределение скандия в алюминии.
Ключевые слова:
красный шлам, оксидно-фторидный расплав, алюмотермия, электролиз, сплав, Al-Sc.
SYNTHESIS OF ALUMINUM-SCANDIUM ALLOYS AND MASTER ALLOYS IN THE OXIDE-FLUORIDE MELTS
A.V. Suzdaltsev1, A.Yu. Nikolaev12, Yu.P. Zaikov12, A.A. Pankratov1, N.G. Molchanova2
1Institute of High-Temperature Electrochemistry of the Ural Branch of the RAS, Yekaterinburg, Russia 2Ural Federal University, Yekaterinburg, Russia
Abstract
Aluminium-scandium alloys (up to 0.5 wt % Sc) and master alloys (up to 2 wt % Sc) has been synthesized via chemical and electrochemical method in the KF-AF3, NaF-AlF3 and KF-NaF-AlF3 melts with additions of Sc2O3 and AhO3 in the temperature range 750-980°C. The electrolytic process was carried out in the lab-scale electrolyser with graphite anode and liquid aluminium cathode at 20 A current. The effect of cathodic current density (0-1 A/cm2), Sc2O3 content in the KF-NaF-AlF3 melt, aluminium stirring rate (0, 100 rpm) and the synthesis duration (30-180 min) on scandium content and distribution in the aluminium matrix, has been studied.
Keywords:
red mud, oxide-fluoride melt, aluminothermy, electrolysis, alloy, Al-Sc.
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