removal Al-MMA
Electrochemical Removal of AlCl3from LiCl-KCl Melts M.SHEN,B.LI,S.Z.LI,and J.G.YU
In order to remove impurity AlCl3from LiCl-KCl melts before Li electrolysis,the Al3+
reduction potential on a tungsten electrode and the relation between Al3+reduction peak
current and AlCl3concentration in LiCl-KCl-AlCl3melts were determined by cyclic voltam-
metry(CV).Constant potential electrolysis at–1.6V vs Cl2/Cl–on both solid Fe and liquid Zn
cathodes was performed to remove AlCl3impurity from the LiCl-KCl-AlCl3melts.The removal
rate of Al3+from the melts was analyzed by both electrochemical methods and inductively
coupled plasma–atomic emission spectrometry(ICP-AES)analysis.The results showed that
96.11wt pct of Al were removed on a Fe cathode and99.90wt pct on a Zn cathode through
10hours electrolysis,respectively.While stirring the melts by argon gas,99.21wt pct of Al3+
was separated from the melts by4hours of electrolysis at723K(450°C),which e?ectively
expedited the Al3+electrochemical reduction rate and shortened the electrolysis time.
DOI:10.1007/s11661-011-0982-7
óThe Minerals,Metals&Materials Society and ASM International2011
I.INTRODUCTION
C URRENTLY,the only way to produce primary lithium in industry is by molten salt electrolysis from KCl-LiCl melts at693to703K(420to430°C).[1–6] Usually,the KCl and LiCl used in industry have a purity of98to99wt pct containing an amount of impurities, such as NaCl,AlCl3,MgCl2,and CaCl2.These impu-rities can be deposited as the corresponding metals during the Li electrolytic process due to a lower theoretical decomposition voltage than that of LiCl; therefore,the primary Li contains a purity of98to 99wt pct with0.1to0.8wt pct Na,1wt pct K, 0.03wt pct Al,and0.01to0.05wt pct Ca.[7]These impurities have to be further removed by vacuum distillation in a stainless steel reactor at temperatures from873to1073K(600to800°C),and sometimes followed by zone melting,[8–10]to satisfy the purity requirement for use in batteries,alloys,and nuclear power generation.About52kWh/kg Li has to be used to purify the primary lithium from98.5to99.9wt pct accompanied with a serious corrosion of the reactor. Hence,it is necessary to explore an economically feasible method to remove these impurities instead of the traditional one.
Lou et al.[11]and Gulens et al.[12]reported that a kind of lithium ion conductive material Li1.3Ti1.7Al0.3(PO4)3 was used to remove Na+ions from LiCl solution to purify LiCl salt.Jin et al.[13]suggested an alcohol extraction method to prepare a highly puri?ed LiCl salt with lower Na and K content.In our laboratory, electrochemical methods are proposed to separate those impurities that have lower theoretical decomposition voltages than LiCl from the LiCl-KCl melts prior to Li electrolysis.As one of the impurities in LiCl-KCl melts, AlCl3presents a much lower theoretical decomposition voltage than LiCl;it is supposed to be deposited and separated from the AlCl3-KCl-LiCl melts by potentio-static electrolysis prior to Li electrochemical reduction. Usually,the Al3+ion concentration in the KCl-LiCl melts is low and gradually gets lower as the electrolysis proceeds.Since the kinetics of aluminum deposition in chloride melts by electrochemical techniques is con-trolled by ion di?usion according to Yan et al.,[14] Bouteillon and Marguier,[15]Gabc o et al.,[16]and Zhang et al.,[17]the Al3+electrochemical reaction rate will become lower and lower.The main challenges during the removal of Al3+ions from the KCl-LiCl melt are if Al3+ions can be completely removed,how long the process will take,and how to improve the electrochem-ical reaction kinetics of Al deposition in the melts.
In this article,cyclic voltammetry(CV)is employed to determine Al3+reduction potentials on a tungsten electrode in the LiCl-KCl-AlCl3melt at723K (450°C)and the relation of Al3+reduction peak current with AlCl3concentration.Then,a solid Fe and a liquid Zn electrode are used as cathodes,respectively, to remove Al3+ions from the melts by constant potential electrolysis.The removal rate of Al3+from the melts is studied by both electrochemical methods and ICP-AES analysis.Then,stirring the melts by argon gas is performed during potentiostatic electrolysis,and the e?ect of stirring on the removal rate of Al3+from the melts is also examined.
II.EXPERIMENTAL
A.Chemicals and Preparation of Melts
The chemicals used in the experiments were composed of anhydrous LiCl(AR,>99wt pct,Shanghai,China),
M.SHEN,Postdoctoral Student,B.LI and J.G.YU,Professors, and S.Z.LI,M.A.Sc Student,are with the National Engineering Research Center for Integrated Utilization of Salt Lake Resource,East China University of Science of Technology,Shanghai200237,People’s Republic of China.Contact e-mail:bingli@https://www.wendangku.net/doc/9b9628952.html, Manuscript submitted September5,2010.
Article published online December21,2011
anhydrous KCl(AR,>99.5wt pct,Shanghai,China), and AlCl3(AR,>99.5wt pct,Shanghai,China).Before experiments,LiCl,KCl,and AlCl3were dried at473K (200°C)for4hours under vacuum and stored in a glove box to avoid moisture for use.A certain stoichiometric amount of electrolyte consisting of LiCl-KCl(1:1mol) with various concentrations of AlCl3was loaded to an Al2O3crucible located in a stainless-steel vessel with an airtight seal.The electrolyte was heated to473K(200°C) and maintained for4hours under vacuum,and then heated to the required temperature under argon gas. B.Electrodes
In all electrochemical experiments,the working elec-trode used was a1-mm tungsten wire welding on a1-mm nickel-cadmium wire shrouded in an alumina tube with 1to2cm left exposed.The counter electrode used was a 6-mm spectral pure graphite shielded by an alumina tube with3cm left exposed.During electrolysis exper-iments,Fe and Zn plates were used as cathodes, respectively,and spectral pure graphite was employed as an anode.The reference electrode was a Ag/AgCl (4mol pct)in LiCl-KCl(1:1mol)melt contained in a porcelain tube fabricated according to Qi.[18]All the potentials in this article will be measured with respect to this reference electrode and then referred to the Cl2/Cl–standard potential for reporting an accurate potential.
C.Analytical Techniques
All electrochemical measurements were performed using a PARSTAT2273(PAR-Ametek Co.,Ltd.,Oak Ridge,TN)with a powersuite software package.After several hours of potentiostatic electrolysis,the cathode products were examined by energy dispersive spectrom-etry(EDS,FALCON-60S,EDAX Inc.,Mahwah,NJ), and the content of Al in the remaining melts was analyzed by inductively coupled plasma–atomic emis-sion spectrometry(ICP-AES,Varian710-ES,Varian Inc.,Santa Clara,CA).
III.RESULTS AND DISCUSSION
A.Al3+Electrochemical Behavior
Figure1shows typical CVs obtained on a tungsten electrode before and after addition of0.247mol Là1 AlCl3in LiCl-KCl melts at723K(450°C).The dotted curve represents the cyclic voltammogram before addi-tion of AlCl3.Only one couple of cathodic/anodic signals B and B￠is observed,which corresponds to the deposition and dissolution of liquid Li.The solid curve shows the cyclic voltammogram after addition of AlCl3. Since the deposition potential of Al is more positive than that of Li in a chloride system,[19]the peak A at the potential–1.55V vs Cl2/Cl–in the cathodic scan is ascribed to Al3+ion reduction.[14]In the reverse scan,in addition to the anodic peak corresponding to oxidation of liquid Li,the pro?le of peak A’clearly indicates the dissolution of deposited Al.
Figure2shows the deposition and dissolution of Al in
LiCl-KCl-AlCl3(0.247mol Là1)on a tungsten electrode with di?erent scan rates at723K(450°C).The sharp
oxidation peak A￠implies an insoluble reduction prod-
uct since the ratio of i A=i A0is smaller than1.The plot of the cathodic peak current density i A vs v1/2after
subtracting the background current,as shown in
Figure3,gives a good linear relation and passes through the origin.This indicates that the electrode process of Al3+ions is controlled by ion di?usion.For a soluble-insoluble system,[20]the di?usion coe?cient of the Al3+ ions in the melts can be calculated according to the Berzins–Delahay equation in Eq.[1]:
I p?0:61nFSC0
nF
R T
1=2
D1=2v1=2?1
where S is the electrode surface area in cm2,C0is the solute concentration in mol cmà3,D is the di?usion coe?cient in cm2sà1,F is Faraday’s constant
(96,485 Fig.1—Typical CVs of the LiCl-KCl-AlCl3(0.247mol Là1)melts on a tungsten electrode at723K(450°C).Scan rate:0.1V sà1,WE: W(S=0.1963cm2),CE:spectral pure graphite,and RE:
Ag/AgCl.
Fig.2—CVs of the LiCl-KCl-AlCl3(0.247mol Là1)melts on a tung-sten electrode at di?erent scan rates at723K(450°C).WE:W (S=0.1963cm2),CE:spectral pure graphite,and RE:Ag/AgCl.
C mol à1),R is the universal gas constant,n is the number of exchanged electrons,v is the potential sweep rate in V s à1,and T is the absolute temperature in Kelvin.
Equation [1]yields D Al 3t?3:1?10à5cm 2s à1:For a reversible electrode reaction involving the deposition of
an insoluble substance,E p àE p =2 (where E p /2is the
half-peak potential)should have a value of 2.29R T/nF or 0.0475V for a three-electron reaction at 723K (450°C).
[21]In this case,E p àE p =2 ?0:046is found almost equal to the value 0.0475V.These observations suggest the deposition/dissolution reaction of Al 3+ion is reversible,which is in good agreement with the before mentioned references.[14–17]
At a given temperature of 723K (450°C),from the obtained CVs,the current density of the reduction peak for Al 3+ions has a linear relation with AlCl 3concen-tration at the scan rate 0.1V s à1,as shown in Figure 4.This plot also can be used to determine the concentra-tion of Al 3+ions present in the melts after several hours of electrolysis.
B.Feasibility of Electrochemical Removal of Aluminum Al can form solid solutions with Fe at 723K (450°C),but existing in the solid state according to Figure 5.At the same time,Al can form uniform liquid mixtures with liquid Zn at this temperature,as shown in Figure 6.Therefore,according to this principle,it is feasible to remove impurity Al 3+by potentiostatic electrolysis at its reduction potential on these cathode materials to form alloys.In order to compare the e?ects of electrode materials on the removal rate and the electrochemical reaction kinetics of Al 3+,solid Fe and liquid Zn electrodes [22–25]at the temperature of 723K (450°C)are selected as the cathodes,respectively.
Potentiostatic electrolysis is carried out on both solid Fe and liquid Zn electrodes at Al 3+ion reduction potentials,and the responses of the current vs electrol-ysis time are recorded.After electrolysis,the cathode deposits with metallic luster are dissolved into alcohol to remove adherent melts on cathode and then analyzed by EDS.The content of Al in LiCl-KCl (1:1mol)melts before and after several hours electrolysis is analyzed by ICP-AES.The theoretical removal amount of Al can be calculated using Faraday law,shown in Eq.[2],as follows:
m ?A ?
Z t
It ?A ?
Q 3600
?2
where A is the Al electrochemical equivalent of 0.336in g A à1h à1,I is the current in A;t is the electrolysis time in seconds,and Q is the consumed electricity quantity obtained from the chronoamperograms in C.
1.Electrochemical removal of aluminum using a solid Fe electrode
Figure 7(a)gives the current response vs electrolysis time on a solid Fe electrode at –1.6V vs Cl 2/Cl –in LiCl-KCl-AlCl 3(0.247mol L à1)melts.The current decreases from –0.55to –0.15A in the ?rst 2hours of electrolysis,and further decreases to –0.07A for the successive 2hours of electrolysis,and ?nally the current is lower than –0.005A for another 5hours of electrolysis.Accordingly,the consumed electricity quantities for the three stages are equal to 1272C,652C,and 138C,respectively,as shown in Figure 7(b).The theoretical Al removal rates are 61.76,93.42,and 100.12wt pct for 2,4,and 10hours electrolysis,respectively,which are a little di?erent from the results obtained by the ICP analysis (Table I ).
Obviously,since Al 3+electrochemical reduction is controlled by ion di?usion rate,as explained previously,the electrochemical reaction current for the third stage is decreased with the lower Al 3+ions di?usion rate,which results in a very low removal rate.Therefore,enhancing the Al 3+ions di?usion rate is required to increase the reduction current and further increase the removal rate of Al 3+ions.If stirring is applied in the melts
during
Fig.3—Variation of cathodic peak current density as a function of square root of the potential scan rate on a tungsten electrode in the LiCl-KCl-AlCl 3(0.247mol L à1)at 723K (450°C).WE:W (S =0.1963cm 2),CE:spectral pure graphite,and RE:
Ag/AgCl.
Fig.4—Linear relationship of Al 3+reduction peak current density vs Al 3+concentration in the LiCl-KCl-AlCl 3melts at 723K (450°C).Scan rate:0.1V s à1,WE:W (S =0.1963cm 2),CE:spec-tral pure graphite,and RE:Ag/AgCl.
electrolysis,it will accelerate the Al 3+ions di?usion in the melt.This will be used to remove Al 3+ions in Section II .
As mentioned previously,AlCl 3concentration in the melts is proportional to the reduction peak current density in the CVs.Therefore,the Al 3+ions concentra-tion remaining in the melts after several hours electrolysis
can be gained from the reduction peak current density in the corresponding CV curves.Figure 8shows CVs of LiCl-KCl-AlCl 3(0.247mol L à1)melts before and after 2,4,and 10hours electrolysis at –1.6V vs Cl 2/Cl –(curves 1through 4),respectively.Obviously,the reduction peak current density of Al 3+decreases to 0.0129A cm à2after 4hours electrolysis,even no distinct reduction peak
can
Fig.5—Al-Fe binary phase
diagram.
Fig.6—Al-Zn binary phase diagram.
be seen in curve 4after 10hours electrolysis,which indicates that most of the Al 3+impurity was removed by electrolysis on the Fe cathode.According to the linear relationship of the Al 3+ions reduction current density with the AlCl 3concentration shown in Figure 4,the Al 3+contents remaining in the melts are 0.62910à1mol L à1and 0.97910à2mol L à1,respectively,for 4and 10hours electrolysis,while ICP-AES analysis gives 0.20910à1mol L à1and 0.96910à2mol L à1Al 3+contents in the melts,respectively,which correspond to 91.77and 96.11wt pct removal rates after 4and 10hours
electrolysis,as shown in Table I .Obviously,the electro-chemical method gives a higher value than the ICP-AES method.The cathodic product contains 7.00wt pct Al and 93.00wt pct Fe analyzed by EDS
2.Electrochemical removal of aluminium using a liquid Zn electrode
Another choice is to use liquid Zn as a cathode;the advantage is maintaining the cathode in the liquid state during the entire electrolysis process at 723K (450°C).The amount of the Zn used is 4.03g,as listed in Table II ,placed in the bottom hole of the Al 2O 3crucible (6.5cm 2),which is connected to the current collector.Figure 9shows the relations between current (a)and electricity quantity variation (b)with time during electrolysis on a liquid Zn electrode at –1.6V vs Cl 2/Cl –for the ?rst 2hours,and another 2and 5hours.The Al theoretical removal rates are calculated to be 74.81,97.61,and 105.83wt pct,respectively,for 2,4,and 10hours https://www.wendangku.net/doc/9b9628952.html,pared with the solid Fe cathode,the liquid Zn cathode theoretically accelerates the Al 3+electrochemical reduction reaction in the ?rst 2hours electrolysis,though the Zn cathode area is smaller than the Fe cathode.This may be the reason a fresh Zn surface is always maintained during the electrolysis process and can easily form solid solution with the deposited Al,which results in a successive under-potential deposition of Al 3+ions on the liquid Zn electrode.Due to a larger amount of Al 3+ions
being
Fig.7—Relation of (a )current with time and (b )consumed total charges with time (1:electrolysis at ?rst stage,2:at second stage,and 3:at third stage)in LiCl-KCl-AlCl 3(0.247mol L à1)on a solid Fe cathode at 723K (450°C).WE:Fe (S =15cm 2),CE:spectral pure graphite,and RE:Ag/AgCl.
Table I.Concentration of Al in LiCl-KCl (1:1Mol)Melts before and after Electrolysis on an Fe Cathode
Concentration of Al in the Melts (Before Electrolysis)
LiCl-KCl-AlCl 3(0.247mol L à1)
Electrolysis time (h)
4.5
10
Concentration of Al in the melts by ICP-AES (after electrolysis)0.20910à1mol L à10.96910à2mol L à1Concentration of Al in the melts by CVs (after electrolysis)
0.62910à1mol L à10.97910à2mol L à1Al removal rate by theoretical electrochemical calculation (Eq.[2])93.42wt pct 100.12wt pct Al removal rate by ICP-AES analysis data
91.77wt pct
96.11wt
pct
Fig.8—Typical CVs of the LiCl-KCl-AlCl 3(0.247mol L à1)melts on a tungsten electrode (0.1963cm 2)before and after electrolysis on a solid Fe cathode at 723K (450°C)with scan rate 0.1V s à1:WE:W (S =0.1963cm 2),CE:spectral pure graphite,and RE:Ag/AgCl.
reduced in the?rst2hours electrolysis,the reduction current and electricity quantity are distinctly decreased in the following electrolysis stages:2and5hours of electrolysis.So,it is necessary to stir the melts to increase the Al3+ions di?usion rate in the melt and enhance the removal rate of Al3+ions.After10hours of electrolysis,the removal rate of Al on a liquid Zn cathode reaches99.90wt pct.The cathode product is composed of Zn67.80wt pct and Al32.20wt pct,as detected by EDS analysis,and a partly solid solution and a liquid phase are formed.[26]Compared with the solid Fe electrode,it is obvious that the liquid Zn electrode e?ectively removes the impurity of Al3+from the KCl-LiCl-AlCl3melt.
C.Stirring Effects
In order to expedite the Al3+ions di?usion rate in the melts,a stirring installation,shown in Figure10,is placed in the bottom of the crucible during electrolysis and0.3L minà1argon gas continuously?ows from the hole(2-mm i.d.)to the electrolyte surface for e?cient mixing of the electrolyte.Under the preceding condi-tions,potentiostatic electrolysis is carried out on a Fe electrode at–1.6V vs Cl2/Cl–in LiCl-KCl-AlCl3 (0.247mol Là1)melts for3and4hours,respectively. The current variation with electrolysis time is given in Figure11(a).
The reduction current observably decreases from –0.88to–0.05A for3hours of electrolysis and gradu-ally decreases to0.01A after4hours of electrolysis,and the electricity quantities consumed for3and4hours of electrolysis are2001C and2151C,respectively,as shown in Figure11(b),corresponding to97.16and 104.44wt pct of theoretical Al removal rates,according to Eq.[2](the104.44wt pct may be caused by the trace of other impurities).The reduction current and the consumed electricity quantities in Figure11(b)are far
Table II.Content of Al in LiCl-KCl(1:1Mol)Melt Before and After10Hours Electrolysis on Di?erent Cathodes Electrolyte Composition LiCl-KCl-AlCl3(0.247mol Là1) Cathode condition Fe Zn
Square area(cm2)15 6.5
Mass(g)22.64 4.03
Al content by ICP-AES analysis(after electrolysis10h)0.96910à2mol Là10.25910à3mol Là1 Concentration of Al in the melts by CVs(after electrolysis)0.97910à2mol Là10.29910à3mol Là1 Al removal rate by theoretical electrochemical calculation100.12wt pct105.83wt pct
Al removal rate by ICP-AES analysis data96.11wt pct99.90wt
pct
Fig.9—Relation of(a)current with time and(b)consumed total electricity quantity with time(1:the?rst stage,2:the second stage, and3:the third stage)in LiCl-KCl-AlCl3(0.247mol Là1)melts on a liquid Zn cathode at723K(450°C).WE:Zn(S=6.5cm2),CE: spectral pure graphite,and RE:
Ag/AgCl.Fig.10—Apparatus for stirring in LiCl-KCl-AlCl3(0.247mol Là1) on a solid Fe electrode at723K(450°C)with?ow rate of 0.3L minà1.
greater than for the Fe electrode without stirring within the short electrolysis time.
From the CVs in Figure 12,no prominent reduction peak current for Al 3+is observed after 4hours of electrolysis;meanwhile,the oxidation current is also very small and can be neglected.The AlCl 3concentration in the remaining melts is 1.95910à3mol L à1by ICP-AES analysis,so more than 99.00wt pct Al 3+was removed within 4hours of electrolysis under stirring,which means stirring the melts can accelerate the removal rate of the Al 3+ions and shorten the electrolysis time.
Compared with the results with and without stirring in Table III ,Al 3+ions concentration in the melts can be reduced to 0.0016wt pct by 4hours of electrolysis at –1.6V vs Cl 2/Cl –on a solid Fe electrode under stirring,which excel one distillation e?ect applied in industry.Electrochemical potentiostatic electrolysis is a low cost and convenient operation method to remove impurity AlCl 3manipulation before Li electrolysis.
IV.CONCLUSIONS
The Al 3+reduction potential is –1.55V vs Cl 2/Cl –obtained by CV in LiCl-KCl-AlCl 3melts at 723K (450°C).The electrochemical reduction process of Al 3+is controlled by ion di?usion.The relation between Al 3+reduction current density and the AlCl 3concentration is linear and can be used to determine the Al 3+content remaining in the melts after several hours of electrolysis.By potentiostatic electrolysis at –1.6V vs Cl 2/Cl –in LiCl-KCl-AlCl 3(0.247mol L à1)melts on a solid Fe electrode,the Al 3+theoretical removal rates are 93.42and 100.12wt pct for 4and 10hours electrolysis,respectively,calculated from the Faraday law,while 91.77and 96.11wt pct of removal rates are achieved after 4and 10hours electrolysis analyzed by https://www.wendangku.net/doc/9b9628952.html,pared with the solid Fe electrode,the liquid Zn electrode can e?ectively remove the 99.90wt pct of impurity Al 3+from KCl-LiCl-AlCl 3melts.Al
3+
Fig.11—Relation of (a )current with time and (b )consumed total charges with time in LiCl-KCl-AlCl 3(0.247mol L à1)on a solid Fe cathode at 723K (450°C)with ?ow rate of 0.3L min à1stirring.WE:Fe (S =15cm 2),CE:spectral pure graphite,and RE:Ag/
AgCl.
Fig.12—Typical CVs of the LiCl-KCl-AlCl 3(0.247mol L à1)melts on a tungsten electrode (0.1963cm 2)before and after electrolysis on a solid Fe cathode at 723K (450°C)under stirring.Scan rate:0.1V s à1,stirring with ?ow rate of 0.3L min à1,WE:W (S =0.1963cm 2),CE:spectral pure graphite,and RE:Ag/AgCl.
Table III.Al Impurity in LiCl-KCl by Electrolysis
Content of Al in LiCl-KCl (ICP-AES Analysis)
Electrolysis before
0.247mol/L
0.202wt pct
Electrolysis after 10h (on Fe)
9.6910à3mol/L 0.0078wt pct (removal rate:96.11pct for 10h)Electrolysis after 3h (on Fe with stirring) 1.2910à2mol/L 0.0098wt pct (removal rate:95.14pct for 3.0h)Electrolysis
after 4h (on Fe with stirring)
1.95910à3mol/L
0.0016wt pct (removal rate:99.21pct for 4.0h)
concentration in the melts can be reduced99.21wt pct by4hours electrolysis at–1.6V vs Cl2/Cl–on a Fe cathode under stirring the melts,which means stirring distinctly accelerates the removal rate and shortens the electrolysis time.
ACKNOWLEDGMENTS
This work was?nancially supported by the863pro-jects of the Ministry of Science and Technology of China(Grant No.2009AA06Z102),the Key Program of the National Natural Science Foundation(Grant No.50934001),and the National Natural Science Foundation(Grant No.51054004);we also appreciate the support of the Key Laboratory of Chemical Engi-neering,Ministry of Education.
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