在LiCl-KCl-AlCl3熔鹽中直接電化學還原Sm2O3及Al-Sm合金的形成

薛 云 王 倩 顏永得＊,, 陳 浪 張密林 張志儉

(1哈爾濱工程大學核安全與仿真技術國防重點學科實驗室，哈爾濱 150001)

(2哈爾濱工程大學超輕材料與表面技術教育部重點實驗室，哈爾濱 150001)

Nowadays,the treatment of spent nuclear fuels has been a big challenge with the rapid development of nuclear energy.Several methods have been developed to deal with the spent nuclear fuels.Among them, pyro-processing has been successfully developed for the separation of long-lived elements due to its advantages of compactness,nuclear proliferation resistance,and reduction of secondary waste generation[1-2].Molten salt has been proved to be an alternative route of pyrochemical processes for the separation of actinides(Ans)from lanthanides(Lns)[3-5].The separations of Ans from Lns have been attracting more attention because Lns are particularly undesirable in nuclear fuel due to strong neutron absorption cross sections and the existence of low melting temperature eutectics with iron-based fuel cladding[6].Moreover,Lns and Ans have similar chemical and electrochemical behaviors,making it difficult to separate Lns from Ans.Among Lns,we are interested in samarium because samarium is a kind of variant valence element in fission products.The electrolytic extraction of samarium at inert electrodes is impossible for recycling in molten salt.

The electrochemical properties of samarium have been studied extensively.For instance,Cordoba and Caravaca[7]investigated the electrochemical properties of samarium in LiCl-KCl eutectic melt at different temperatures.They found that Sm（Ⅱ）/Sm reaction was not feasible under these conditions,because the reduction of Sm（Ⅱ）is in a more negative potential than the solvent cations.Tlenkopachev et al.[8]investigated the electroreduction of samarium ions at various electrodes in KCl-NaCl-CsCl melt at 823 K,and calculated diffusion coefficients of samarium ions and heterogeneous rate constant for the reduction of Smバ/Sm（Ⅱ）.Chamelot et al.[9]studied the electrochemical behaviour of SmF3in molten LiF-CaF2medium on molybdenum and nickel electrodes.The results showed that it was not possible to produce samarium metal in molten fluorides on an inert cathode.However,SmF3can be reduced on a nickel electrode to formsamariumnickel alloys.It is commonly accepted that the only way to achieve a successful elimination of samarium ions from the molten salt mixture is to make the reduction potential shift to positive direction,i.e.under potential deposition (or by depolarisation effect)[4,10].Gibilaro et al.[4]and Castrillejo et al.[11]subsequently developed a co-reduction method with aluminium and samarium ions together on tungsten electrode for samarium extraction by forming Al-Sm alloys in LiFCaF2(79 ∶21,mol%)and LiCl-KCl(59 ∶41,mol%)media.The extraction efficiency was reported as 99.4%for samarium in fluoride melt.Nohira et al.[12-14]investi-gated the electrochemical formation of Sm-Co alloys at a Co cathode in LiCl-KCl-SmCl3melts and a Cu substrate in LiCl-KCl-SmCl3-CoCl2melts,and electro-chemical formation of Sm-Ni alloy films by Li codeposition method on Ni electrode from corresponding chloride melts.Zhang et al.[15-16]investigated electrochemistry of Smバand prepared Mg-Li-Sm alloyswith different phasesby galvanostatic electrolysis in LiCl-KCl-MgCl2-SmCl3melts at 670℃.The lithium and samarium contents of Mg-Li-Sm alloys depend on electrolytic time and concentration of SmCl3.It should be noted that all these literatures are related to the electrochemistry of Smバions on inert electrode and electroreduction of Smバions on different reactive substrates.Also,the sources of Smバions are either SmCl3or SmF3.

In general,both metal and oxide spent nuclear fuels need to be chloridized and changed into an ionic form before deposition and separation.In addition,toxic and corrosive HCl and HF have to be bubbled into the melts to avoid the formation of Ln2O3precipitates,because Lnバis very sensitive to O2-ions.In order to overcome the drawbacks of the route using chlorides and fluorides as precursors,we directly choose the Ln2O3as raw material since Ln2O3can be effectively chloridized by Al2Cl6[17].In addition,Al(originating from AlCl3)and Lns have strong interaction,and they can form intermetallic compounds.Our group[18]has studied the extraction of europium and deposition of Al-Li-Pr alloys on a tungsten electrode in molten LiCl-KCl-AlCl3-Pr6O11system.The results show that AlCl3can chloridize Pr6O11,and Pr can be extracted on an Al film pre-deposited on a W electrode.In this work,we attempt to extract samarium element from Sm2O3by co-reduction of Al and Sm from LiCl-KCl-AlCl3melts on Mo electrodes,and compare the electrochemical behaviors between Sm2O3and SmCl3．The extraction of Sm from Sm2O3in LiCl-KCl-AlCl3melts is proven to be feasible．

1 Experimental

The mixture of LiCl-KCl(wLiCl∶wKCl=1∶501,analytical grade)was first dried under vacuum for more than 72 h at 473 K to remove excess water．Metal ion impurities in the melts were removed by preelectrolysis at-2．0 V (vs Ag+/Ag)for 4 h．Aluminum and samarium ions were introduced into the bath in the form of AlCl3and Sm2O3powders．All electrochemical measurements were carried out using an electrochemical workstation (Im6eX,Zahner Co．,Ltd．)with THALES 3．08 software package．The reference electrode was a Ag/AgCl electrode,which consisted of a silver wire(1 mm diameter)dipped into a pyrex tube in AgCl(1．0wt%)-LiCl-KCl molten mixture．A spectrally pure graphite rod with a diameter of 6 mm was served as the counter electrode．The working electrodes were molybdenum wires(diameter 1 mm,depth 1 cm,99．99%),which were polished using SiC paper to remove the surface oxides,then cleaned ultrasonically with ethanol prior to use．The active electrode surface area (0．322 cm2)was determined after each experiment by measuring the immersion depth of the electrode in the molten salts．The deposits were analyzed by XRD(Rigaku D/max-TTR-Ⅲ diffractometer) using a graphite monochromatized Cu Kα radiation(λ=0．154 18 nm)at 40 kV and 150 mA with a 2θ-θscan mode in the 2θ range of 10°～80°,and the step size was 0．02°．

2 Results and discussion

2.1 Cyclic voltammetry

Typical cyclic voltammogram for LiCl-KCl-SmCl3(1．21wt%)at 803 K is shown in Fig．1(a)．Within a potential window range of 0 to-2．5 V,one couple of cathodic/anodic peaks (E/E′)is observed which corresponds to the deposition/dissolution of lithium．Another group of current peaks A/A′prior to the signals E/E′is also observed．The shape of the peaks A/A′is typical for a soluble-soluble system．The votammogram exhibits a cathode peak A at-0．91 V,corresponding to the reduction of Smバ to Sm（Ⅱ）．In the positive scan direction,peak A′is related to the oxidation of Sm（Ⅱ）．The reduction of Sm（Ⅱ） to Sm is not observed in this cyclic voltammogram under our experimental conditions because the reduction potential of Sm（Ⅱ）is more negative than that of Li(I)/Li couple．The result is in agreement with those in the literatures[7-8]．

Fig．1 (a)Cyclic voltammogram of LiCl-KCl-SmCl3(1．21wt%)system on a Mo electrode at 803 K,scan rate 200 mV·s-1,(b)Cyclic votammograms for the LiCl-KCl-AlCl3(2．14wt%)melt after the addition of SmCl3(1．71wt%,dotted line)and Sm2O3(1．71wt%,solid line)on Mo electrodes at 803 K,scan rate 100 mV·s-1

Fig．1(b)exhibits the cyclic voltammograms of Sm2O3(1．71wt%)and SmCl3(1．71wt%)in LiCl-KCl-AlCl3(2．14wt%)melts at 803 K．It can be seen that the peak potential ranges of cyclic voltammograms obtained from the two systems are almost the same．Three pairs of signals B/B′,C/C′and D/D′are observed before the reduction and oxidation of Li in the two melt systems．The peak B at-1．00 V is attributed to the reduction of the Alバ to Al．Compared with the cyclic voltammogram of LiCl-KCl-SmCl3system without the addition of AlCl3(Fig.1a),the peak C and D at-1.68 and-1.94 V should be associated with the formation of Al-Sm alloys.The reoxidation peaks at around-1.7 and-1.5 V(D′and C′)correspond to Sm dissolution from the Al-Sm alloy.The reduction potential of Sm（Ⅱ）is shifted toward a more positive one after the addition of AlCl3because of the decrease of activity of the lanthanide metal deposited by co-reduction with aluminium ions due to the depolarization effect.In comparison with Sm2O3,an extra pair of signals A/A′(redox reaction of Smバ to Sm（Ⅱ）)is only observed in the LiCl-KCl-AlCl3melts with SmCl3even though the oxidation and reduction signals are magnified in the melts with Sm2O3(see inset graph).In addition,the differences between the peak currents of LiCl-KCl-AlCl3-SmCl3and LiCl-KCl-AlCl3-Sm2O3melts are surprising,considering the same concentrations of Sm2O3and SmCl3in melts.In general,the peak currents of LiCl-KCl-AlCl3-SmCl3melts are higher than those of LiCl-KCl-AlCl3-Sm2O3melts.These interesting phenomena are likely caused by the partial chlorination of Sm2O3.In other words,the dissolved samarium concentration of LiCl-KCl-AlCl3-Sm2O3melts is lower than that of LiCl-KCl-AlCl3-SmCl3melts.

2.2 Square wave voltammograms

The similar distribution of peaks is observed more clearly in the square wave voltammogram than cyclic voltammogram on a Mo electrode in the LiCl-KCl-AlCl3system at 803 K containing SmCl3and Sm2O3,respectively(Fig.2).The numbers and potential regions of the peaks obtained from the two systems are almost the same.The peaks at-0.77,-1.00,-1.69,and-1.97 V correspond to the reaction of Smバ/Sm（Ⅱ）,Alバ/Al and the formation of two Al-Sm intermetallics,respectively.The peak current of square wave voltammetry is higher in LiCl-KCl-AlCl3system containing SmCl3than that of the system containing Sm2O3.This phenomenon also provides the evidence of the partial chlorination of Sm2O3by AlCl3,and the decrease of AlCl3concentration(Sm2O3reacts with AlCl3).

Fig.2 Square wave voltammograms of the LiCl-KCl-AlCl3(1wt%)system after the addition of SmCl3(2wt%,dotted line)and Sm2O3(2wt%,solid line)on Mo electrode at 803 K.Pulse height:25 mV;potential step:1 mV;frequency:15 Hz

2.3 Chronopotentiometry

The above results are also confirmed by chronopotentiometry measured on a Mo electrode in the LiCl-KCl-AlCl3-Sm2O3melt with various applied currents at 803 K as shown in Fig.3.Chronopotentiometric curves show the existence of five potential plateaus at around-0.81,-1.03,-1.72,-1.97 and-2.39 V,respectively.The plateaus 1 and 2 should be related to the reduction of Smバ/Sm（Ⅱ） and Alバ/Al,respectively.At a cathodic current more negative than-139.8 mA·cm-2,the curves exhibit two additional potential plateaus (plateaus 3 and 4),which are associated with the formation of two Al-Sm intermetallic compounds.At this current intensity,coreduction of Al and Sm occurs.When the current reaches-170.8 mA·cm-2,a fifth plateau appears.This plateau is caused by the reduction of lithium ions.

Fig.3 Chronopotentiometric curves obtained on a Mo electrode in LiCl-KCl-AlCl3(1wt%)melt after the addition of Sm2O3(2wt%)at 803 K

2.4 XRD analysis of deposits

Galvanostatic electrolysis was carried out in LiCl-KCl melts containing different concentrations of AlCl3and Sm2O3.However,dendritic alloys were obtained at 803 K.It is difficult to carry out XRD analyses.To obtain bulk alloys,we conducted the galvanostatic electrolysis at 923 K.Fig.4 presents the XRD patterns of deposits obtained by galvanostatic electrolysis on Mo electrodes(S=0.322 cm2)at 2 A for 2 h at 923 K.When the concentration of AlCl3and Sm2O3is 9 and 1wt%,the deposit only contains Al4Li9and Li phases(pattern a).When the concentration of Sm2O3increases from 1 to 2wt%,a new phase Al2Sm appears in the pattern b.Moreover,the diffraction peaks of Al4Li9and Li phases become weak.With increasing AlCl3content from 9 to 15wt%,the deposit is identified as Al2Sm alloy and KCl(pattern c).The diffraction peaks of impurity KCl indicate that the KCl attached on Al2Sm alloys has not been removed during the cleaning process.At the same time,the phases of Al4Li9and Li disappear.Al-Sm alloys can be formed by adjusting the concentration of AlCl3and Sm2O3,indicating a promising way to extract Sm by forming Al-Sm alloys from Sm2O3.

Fig.4 XRD patterns of deposits obtained by galvanostatic electrolysis on Mo electrodes in the LiCl-KCl melts with(a)9wt%AlCl3 and 1wt%Sm2O3;(b)9wt%AlCl3 and 2wt%Sm2O3;(c)15wt%AlCl3 and 1wt%Sm2O3

3 Conclusions

The electrochemical behavior of SmCl3and Sm2O3in molten LiCl-KCl-AlCl3system has been investigated using various electrochemical techniques,such as cyclic voltammetry,square wave voltammetry and chronopotentiometry.These electrochemical measurements show that AlCl3can effectively chloridize Sm2O3,and the formation signals of two Al-Sm intermetallics areobserved.The XRDpatternsof deposits obtained by galvanostatic electrolysis indicate that Al2Sm phase is formed in the electrolysis process.This study proves that the extraction of Sm from Sm2O3in LiCl-KCl melts isfeasible.Thisnewtechniqueovercomesthelimitation of low Ln2O3solubility in molten salts.

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