The Influence of Fluoride Anion on the Equilibrium between Titanium Ionsand Electrodeposition of Titanium in Molten Fluoride­Chloride Salt

Jianxun Song+1, Qiuyu Wang, Xiaobo Zhu, Jungang Hou,Shuqiang Jiao+2 and Hongmin Zhu+2

State Key Lab of Advanced Metallurgy, University of Science and Technology Beijing,No. 30 Xueyuan Road, Beijing 100083, P. R. China

NaCl­KCl­TiClx was prepared via using titanium sponge to reduce titanium tetrachloride in a NaCl­KCl melt under a negative pressure at1023K. The relationship between titanium valence states and [F]/[Ti] molar ratios was investigated with successive adding potassium fluoride inthe pre-prepared NaCl­KCl­TiClx. It was found that the average valence of titanium ions tended to be stable around 3.0 when [F]/[Ti] molarratio was greater than 1.80. The equilibrium redox potentials, ETi3þ=Ti2þ , ETi4þ=Ti3þ , ETi3þ=Ti and ETi2þ=Ti, were also calculated through theobtained concentration of equilibrium titanium ions with different molar ratios of [F]/[Ti]. Meanwhile, the influence of the fluoride anion onover-potential and characteristics of titanium electrodeposition were investigated through changing the molar ratio of [F]/[Ti]. The resultsshowed that, with the increasing of [F]/[Ti] molar ratios, the grain size of electrodeposition products became smaller, while the over-potentialwas higher. [doi:10.2320/matertrans.M2014071]

(Received February 28, 2014; Accepted May 2, 2014; Published June 20, 2014)

Keywords: [F]/[Ti] molar ratio, average valence, equilibrium constant, redox potential, electrodeposition

1. Introduction

The electrodeposition of titanium from molten salts hasbeen extensively investigated over past decade.1­3) Normally,there are two kinds of soluble titanium ions (Ti3+, Ti2+)in molten salts which can result in the disproportionationreactions to lowering current efficiency, or forming ofmetallic mud on the cathode. So far, the electrodepostion oftitanium in molten salts has been mostly focused on moltenchlorides electrolytes. Some of literatures have reported thatthe electrodepostion of Ti in chloride melts could be changedwith adding fluoride.4­11) For the chloride-fluoride system,most of works employed tetravalent titanium as solute(K2TiF6 in LiCl­KCl, NaC1­KC1, or NaCl). The literatureresults indicated that the cathodic process of titanium ions insuch melts mainly consisted of two steps: TiF62¹ + e¹ =TiF63¹ and TiF63¹ + 3e¹ = Ti + 6F¹.

The decisive role played by the parameter F/Cl incontrolling the electro-refining process was confirmed byN. Ene etc. in fluoride­chloride melt.6) They present addi-tional finding regarding its influence on the electrochemicalparameters, Ed and Ep, the current efficiency, the morphologyas well as the chemical purity of the electrodeposited titanium.The morphology of the cathodic deposit is mainly determinedby the concentration of free fluoride anions in the melt. Thefactors including electrodeposition temperature, electrodepo-sition time and the current density on the effect of micrographwere investigated in NaCl­KCl­LiCl­K2TiF6 melt,7) LiF­NaF­KF­K2TiF6 melt,12) KCl­NaCl­KTiF6 melt.13)

However, it is necessary to know the influence of thefluoride anion on electrodeposition characteristics in theelectrolyte beginning with low valence states of titanium.This work is going to demonstrate the behavior of theoxidation states of titanium, Ti2+ and Ti3+, with an addition

of potassium fluoride in the melt. The work is also presentingthe results related to the electrorefining of titanium in a meltof chloride­fluoride.

2. Experiment Details

2.1 Preparation and purification of the meltThe mixture of NaCl­KCl (analytical grade) was dried

under vacuum for more than 10 h at 573K to removemoisture. Then, it was heated up to 1023K under high purityargon atmosphere. The temperature controlled within «2Kwas achieved with a digital temperature controller (CHINODZ3000; CHINO Corporation, Tokyo, Japan) and measuredusing a K-type thermocouple (Omega Engineering, Inc.,Stamford, CT). Schematic diagram of experimental setup canbe seen in Fig. 1.

Hydrogen chloride gas (99.999 pct.) was bubbled into themelt to remove oxide ions. And then, high-purity Argon gaswas bubbled into the melt to remove remanent HCl, oxygenand water. After melting the salts, pumped the system tovacuum of 0.5 Pa, then, liquid state of TiCl4 (AR) was slowlytaken into the crucible to react with sponge titanium:8)

Tiþ TiCl4 ! 2TiCl2

�G ¼ �135:9 kJ�mol�1 at 1023K ð1ÞThe influence of fluoride anion on the equilibrium amongtitanium ions was investigated by addition of potassiumfluoride into the supporting electrolyte. It was dehydratedand pre-melted at 573 and 1173K, respectively. After eachaddition, HCl gas was bubbled into the melt to remove oxideions, then, high-purity argon gas was used to stir the moltensalts for making the reaction of titanium ions and titaniummetal fast.

2.2 Concentration determination of titanium ionsTo determinate the concentration of Ti2+ and Ti3+, a

special quartz sampler was used which consisted of aninjector and a quartz tube. Four parallel samples were taken

+1Graduate Student, University of Science and Technology Beijing+2Corresponding author, E-mail: hzhu@metall.ustb.edu.cn; sjiao@ustb.edu.cn

Materials Transactions, Vol. 55, No. 8 (2014) pp. 1299 to 1303©2014 The Japan Institute of Metals and Materials

out from molten salts by the quartz sampler for analysis ineach experiment, and the average value of the concentrationsof titanium ions is considered as the result. The concentrationof Ti2+ and Ti3+ ions in the sample was determined by H2

volumetric analysis, titration, respectively. The concentrationof Ti2+ was quantified by H2 volumetric analysis as thefollowing reaction (2):14­16)

2Ti2þ þ 2H� ! 2Ti3þ þ H2 ðgÞ ð2ÞIt should be mentioned that the oxygen dissolved in an

aqueous solution could oxidize titanium ions from Ti2+ intoTi3+, which may cause an underestimate of the concentrationof Ti2+ ion. Therefore, the deionized water was treated bya vacuum degassing and fully cleaned by the high purity Ar.A certain amount of concentrated hydrochloric acid wasinjected into the deoxygenized water to prepare dilutedhydrochloric acid (1mol/L). The deoxygenized hydrochloricacid solution was saturated by bubbling high purity H2 for30min in order to prevent the H2 evolved by the reaction 2from dissolving in the hydrochloric acid solution.

It should also be pointed out that the concentration of Ti3+

in the solution consisted of two parts, that were, the initialtrivalent titanium ions in the sample and the oxidized Ti3+

ion from reaction 2. The total concentration of Ti3+ in thesolution was determined by the titration using 0.05MNH4Fe(SO4)2 aqueous solution. The Ti3+ ion in the solutionreacted with Fe3+ by the reaction (3):17)

Ti3þ þ Fe3þ ! Ti4þ þ Fe2þ ð3ÞDAPM­spectrophotometer method was used to determine

total concentration of Ti4+ (Canal:Ti4þ

). The concentration of Ti4+

in the solution should consist of two parts after the sampleswere dissolved in hydrochloric acid then oxidized byconcentrated nitric acid, which were the parts of oxidizedfrom Ti3+ and the initially existed actually. At this point, theconcentration of Ti4+ existing in electrolyte is calculated witheq. (4):

CTi4þ ¼ Canal:Ti4þ � 3CTi3þ=4 ð4Þ

2.3 Electrochemical depositionTitanium deposition experiments were carried out using

constant current technique and the supporting electrolyte waspre-prepared based on above conditions at 1023K. Twotitanium plates (50mm © 25mm © 5mm) and a high puritytitanium plate (35mm © 15mm © 5mm) were employedas anodes and cathode, respectively.8) A series of electro-depositing tests were performed to investigate the influenceof the concentration of fluoride anion on the quality of thecathodic product with keeping the anodic and cathodiccurrent density ( ja and jc) as 0.1A/cm¹2 and 0.3A/cm2,respectively. The cleaned cathodic products were chemicallyanalyzed after leaching with 1mol/L hydrochloric acidsolution. The morphology was observed by using scanningelectron microscope (Japan, JSM-6360).

3. Results and Discussion

3.1 The equilibrium between titanium ions in moltensalt

There are mainly two reactions happening in solventmixing salt with successive adding potassium fluoride. Theequilibrium exists among Ti2+, Ti3+, Ti4+ and metallictitanium in chloride melts which can be expressed by thefollowing reaction:

3Ti2þ , 2Ti3þ þ Ti ð5Þ4Ti3þ , 3Ti4þ þ Ti ð6Þ

The determinate concentration of titanium ions areexpressed as xi, where xi is the molar fraction of a speciesi, which is a cationic molar fraction defined by the eq. (7):

xi ¼100ni

nNaþ þ nKþ þ nTi2þ þ nTi3þ þ nTi4þð7Þ

The results shown in Table 1 reveal the concentration oftitanium ions changing with variation of [F]/[Ti] molar ratio,and the relative standard deviation of titanium ion concen-trations are 3.67 pct.

It is noticed from Table 1 that, the accurate concentrationsof titanium ions were determined. The concentration of Ti3+

ion increases and Ti2+ decreases after adding potassiumfluoride with the molar ratio of [F]/[Ti] less than 1.8.Furthermore, no hydrogen was emitted in diluted hydrochlo-ric acid while chemical analysis, and it confirms that therehas no much Ti2+ when the molar ratio of [F]/[Ti] higherthan 1.80. It demonstrates that the concentration of Ti2+ ioncould be ignored in this case. Meanwhile, the tetra-titaniumwas detected with the measurement of spectrophotometermethod when the molar ratio of [F]/[Ti] higher than 1.80,and also it raised with the increasing of [F]/[Ti] molar ratio.

As we know, titanium ions can form a variety of complexcompounds in the molten salt with anions, and the complex-ation power of fluoride anion with titanium is more force thanchloride anion. In such melts, the complexes containingfluoride anion and titanium ions are formed:13) such as:TiFxCl

ðxþy�3Þ�y þ yF� ! TiF

ðxþy�3Þ�xþy þ yCl�. The concen-

tration of Ti2+ is drastically lessened with the addition ofpotassium fluoride, which mainly causes of the migrationof the balance to the direction of generating Ti3+ ion indisproportionation reaction (5).

Figure 2 shows the average valence of titanium in the melt.It can be found that the average valence is 2.21 before adding

Fig. 1 Schematic diagram of experimental setup for preparation of titaniumsubchloride.

J. Song et al.1300

potassium fluoride. However, the average valence can bechanged as 2.86 when [F]/[Ti] molar ratios higher than 1.80.With the adding of more fluoride anion, the average valencebecomes as high as 3.1 when [F]/[Ti] molar ratio is 12.

To further clear the effect of adding of fluoride anionon titanium ions transformation in molten salt, the detailinvestigation has been done with the equilibrium constant.The equilibrium constant of the reactions (5) and (6), Kci, isgiven by the eqs. (8) and (9).

Keq:c1 ¼ aTia

2Ti3þ

a3Ti2þ

¼ £2Ti3þðx

eq:

Ti3þÞ2£3Ti2þðx

eq:

Ti2þÞ3ð8Þ

Keq:c2 ¼ aTia

3Ti4þ

a4Ti3þ

¼ £3Ti4þðx

eq:

Ti4þÞ3£4Ti2þðx

eq:

Ti3þÞ4ð9Þ

Where ai and £ i are the activity and the activity coefficientof a species, i, respectively. £ i can be regarded as a constantwhen xi is low because it obeys the Henry’s law.14,17)

Therefore, the equilibrium constant can be modified as:

Keq:c1 ¼ ðxeq:

Ti3þÞ2ðxeq:

Ti2þÞ3ð10Þ

Keq:c2 ¼ ðxeq:

Ti4þÞ3ðxeq:

Ti3þÞ4ð11Þ

Figure 3 shows the relationship between [F]/[Ti] molarratio and Kci after it was taken logarithm (lnKci).

The results demonstrate that the initial balance of titaniumions has been broken after adding potassium fluoride. Theionic balance between Ti2+ and Ti3+ disappears when thevalue of [F]/[Ti] molar ratio across the red dot line (higherthan 1.8). And then, the disproportionation reaction (6)dominates the concentration converting in the melt.

The results shown in Fig. 3 discloses that the equilibriumconstant of K

eq:c1 increases sharply with adding potassium

fluoride gradually. Keq:c1 is even bigger than five orders of

magnitude through varying [F]/[Ti] molar ratio from 0 to1.80. Moreover, the titanium trichloride and alkali tetra-fluoride are active components in molten salt when [F]/[Ti]molar ratio is higher than 1.80. Therefore, the balancebetween Ti3+ and Ti4+ was observed. The K

eq:c2 turns from

1.2 © 10¹4 to 4.4 © 10¹3 with changing the [F]/[Ti] molarratio at range of 2.1 to 11.4.

To achieve the aim of proclaiming the regular patternamong titanium ions for further, the equilibrium redoxpotential is introduced in present work. The equilibriumpotentials could be calculated with the well-known expres-sions (12) and (13):18)

ETi3þ=Ti2þ ¼ EªTi3þ=Ti2þ � RT

FlnKeq:

c1 ð12Þ

ETi4þ=Ti3þ ¼ EªTi4þ=Ti3þ � RT

FlnKeq:

c2 ð13Þ

Where ETi3þ=Ti2þ and ETi4þ=Ti3þ are equilibrium potential of

Ti3+/Ti2+ and Ti4+/Ti3+ vs. Cl2/Cl¹, respectively; Eª

Ti3þ=Ti2þ

and Eª

Ti4þ=Ti3þare the standard redox potentials of Ti3+/Ti2+

and Ti4+/Ti3+ vs. Cl2/Cl¹, respectively. ETi3þ=Ti and ETi2þ=Ticould also be calculated with the same method.

The equilibrium potentials of various redox couples list onthe curves in Fig. 4. The redox reactions Ti3+/Ti2+, Ti3+/Tiand Ti2+/Ti occur at potentials of ¹1.58, ¹1.73 and ¹1.79Vwhen the melt without fluorides ([F]/[Ti] = 0). The equi-librium potential of ETi4þ=Ti3þ could also be calculated as0.09V. The balance of disproportionate reaction of 3Ti2þ ,2Ti3þ þ Ti and 4Ti3þ , 3Ti4þ þ Ti depends on the con-centration of fluoride anions. In the fluoride free electrolyte,the redox reaction of 3Ti2þ , 2Ti3þ þ Ti is involved in themelt. The addition of fluoride ions induces a negative shift

0 2 4 6 8 10 12

2.2

2.4

2.6

2.8

3.0

3.2

Molar ratio of F to Ti, ri / x

F-/xTin+

Average Valence

Ave

rag

e V

alen

ce, n

Fig. 2 The relationship between average valence of titanium ion and[F]/[Ti] molar ratios (ri) in molten salt, at temperature of 1023K.

0 2 4 6 8 10 12-4

-2

0

2

4

6

8

10

Molar ratio of F to Ti, ri / x

F-/xTin+

lnK

c1

-2

-1

0

1

2

3ln

Kc

2

lnKc1

lnKc2

Fig. 3 The relationship between equilibrium constant and [F]/[Ti] molarratios (ri), at temperature of 1023K.

Table 1 Concentration of titanium ions in molten salt under different[F]/[Ti] molar ratios (ri), at temperature of 1023K.

F/Ti molar ratio,ri/xF¹/xTin+

Molar fraction, x/mol%

xTi2+ (©102) xTi3+ (©102) xTi4+ (©103)

0 1.448 0.501 ®

0.52 0.792 0.894 ®

1.20 0.279 1.192 ®

1.50 0.141 1.266 ®

1.83 0.068 1.294 ®

2.14 ® 1.226 0.064

2.52 ® 1.154 0.101

2.91 ® 1.088 0.134

3.50 ® 1.057 0.133

5.10 ® 0.976 0.133

7.91 ® 0.873 0.120

11.37 ® 0.764 0.114

The Influence of Fluoride Anion on the Equilibrium between Titanium Ions and Electrodeposition of Titanium 1301

of ETi3þ=Ti2þ and ETi2þ=Ti. When more fluoride is added, the[F]/[Ti] molar ratio greater than 1.8, ETi3þ=Ti2þ becomes morenegative than ETi2þ=Ti, and the direct three electron reductionis obtained. Fluoride anion results in a negative shift in thestandard equilibrium potential, ETi4þ=Ti3þ , for the Ti4+/Ti3+

couple with [F]/[Ti] molar ratios higher than 1.8. Thepotential shifts more negative with [F]/[Ti] molar ratiosincreasing mainly because of a common ligand for titaniumions formed.

3.2 Electrochemical depositionThe influence of the fluoride anion on electrodeposition

characteristics of titanium was investigated. The averagediameter, dm, the metallic powder grains may be described byeq. (14),19) where m1, m2 + mn denote the sieve mesh and p1,p2 + pn the weights of various grain size fractions.

dm ¼ m1p1 þm2p2 þ � � � þmnpn

p1 þ p2 þ � � � þ pnð14Þ

Figure 5 shows the relationship between dm values of thedeposits recovered from molten salts varying [F]/[Ti] molarratios. It discloses that the size of metallic grains is controlledby the [F]/[Ti] molar ratios under a constant current density:the higher content of free fluoride anion is, the finer thetitanium powder. Thus, it is possible to control the electro-deposition through changing the parameter [F]/[Ti] ratios.

Figure 6 from (a) to (d) shows the SEM images of titaniumproducts regarding with varying [F]/[Ti] molar ratios rangefrom 0 to 6.00. The micrographs of the titanium depositexhibit regular granular structures. The formation of dendritecan be observed when there is no fluoride anion in theelectrolyte, while the crystalline grain decreases withincreasing of [F]/[Ti] molar ratio. These results are similarwith what the previous literature reported.20)

The morphology is affected by the nucleation and thecrystal growth. There are two competitive factors governingthe growth of the deposit responding the nucleation current(In) and the growth current (Ig). The relationship between

0 2 4 6 8 10 12-2.4

-2.0

-1.6

-1.2

-0.8

-0.4

0.0

Molar ratio of F to Ti, ri / x

F-/xTin+

c

db

a

cd

b

a-E Ti4+/Ti3+

b-E Ti3+/Ti2+

c-E Ti2+/Tid-E Ti3+/Ti

Po

ten

tial

,E /

V

a

Fig. 4 Equilibrium redox potentials vs. Cl2/Cl¹ for the systems, Ti4+/Ti3+,Ti3+/Ti2+, Ti3+/Ti, and Ti2+/Ti under various [F]/[Ti] molar ratios (ri), attemperature of 1023K.

0 1 2 3 4 5 6

0

20

40

60

80

100

120

140

Molar ratio of F to Ti, ri / x

F-/xTin+

Grain Size

Gra

in S

ize,

dm

/µm

Fig. 5 The relationship between the average diameter of products and[F]/[Ti] molar ratios.

Fig. 6 SEM image of titanium electrochemical deposit under different [F]/[Ti] molar ratios. Ti mass% = 2.75: (a) ri = 0; (b) ri = 0.50;(c) ri = 1.50; (d) ri = 6.00.

J. Song et al.1302

nucleation density N0 and over-potential © showed ineq. (15):21)

N0 ¼ A exp�B

©2

� �ð15Þ

It is easy to find that the crystal diameter lessens with theover-potential increasing. Depending on the solvent nature,the weaker Ti­Cl bond is replaced by stronger Ti­F bondafter adding fluoride anion, which results in over-potential oftitanium ion electrodeposition increasing. Figure 7 indicatesthe relative results.

4. Conclusions

The behavior of the oxidation states Ti2+ and Ti3+ has beeninvestigated with adding of potassium fluoride into thechloride melts. It is found that the average valence oftitanium ions tends to be rising up to 3.0 when [F]/[Ti] molarratio is greater than 1.80. The equilibrium redox potentialhas also been calculated with the determinate equilibriumconcentration of titanium. Meanwhile, the influence of thefluoride anion on over-potential and characteristics oftitanium electrodeposition was investigated. The resultsdiscloses that the grain size of electrodeposition productsbecomes smaller, and the over-potential turns to higher withthe increasing of [F]/[Ti] molar ratio.

Acknowledgments

The authors are grateful to the National Science Founda-tion of China (No. 51322402, 50934001), the National HighTechnology Research and Development Program of China(863 Program, No. 2012AA062302), the Program of the Co-Construction with Beijing Municipal Commission of Educa-tion of China (Nos. 00012047 and 00012085), the Programfor New Century Excellent Talents in University (NCET-11-0577), and the Fundamental Research Funds for the CentralUniversities (No. FRF-AS-11-003A).

REFERENCES

1) M. B. Alpert, F. J. Schultz and W. F. Sullivan: J. Electrochem. Soc. 104(1957) 555­559.

2) F. J. Schultz: US Patent No.:2734856, (1956).3) H. Rosenberg: US Patent No.:6024847, (2000).4) F. Lantelme, K. Kuroda and A. Barhoun: Electrochim. Acta 44 (1998)

421­431.5) F. R. Clayton, G. Mamantov and D. L. Manning: J. Electrochem. Soc.

120 (1973) 1193­1199.6) N. Ene and S. Zuca: J. Appl. Electrochem. 25 (1995) 671­676.7) D. Wei, M. Okido and T. Oki: J. Appl. Electrochem. 24 (1994) 923­

929.8) X. H. Ning, H. Asheim, H. F. Ren, S. Q. Jiao and H. M. Zhu: Metall.

Mater. Trans. B 42 (2011) 1181­1187.9) W. C. Kreye and H. H. Kellogg: J. Electrochem. Soc. 104 (1957) 504­

508.10) G. S. Chen, M. Okido and T. Oki: Electrochim. Acta 32 (1987) 1637­

1642.11) Q. Y. Wang, G. J. Hu, S. Q. Jiao and H. M. Zhu: TMS Orlando FL,

(2012) pp. 103­106.12) A. Robin and R. B. Riberio: J. Appl. Electrochem. 30 (2000) 239­246.13) G. S. Chen, M. Okido and T. Oki: J. Appl. Electrochem. 17 (1987)

849­856.14) Q. Y. Wang, J. X. Song, G. J. Hu, X. B. Zhu, J. G. Hou, S. Q. Jiao and

H. M. Zhu: Metall. Mater. Trans. B 44 (2013) 906­913.15) S. Mellgren and W. Opie: J. Metal. 9 (1957) 266­269.16) S. Mellgren and M. B. Alpert: Anal. Chem. 30 (1958) 2061­2063.17) H. Sekimoto, Y. Nose, T. Uda and H. Sugimura: Mater. Trans. 51

(2010) 2121­2124.18) F. Lantelme and A. Salmi: J. Electrochem. Soc. 142 (1995) 3452­3456.19) A. Calusaru: Electrodeposition of Metal Powders, (Elsevier Scientific

Publishing Company Oxford, 1979).20) G. P. Dovgaya, V. V. Nerubashchenko, S. P. Chernyshova and L. K.

Mineeva: Power Metal. Met. Ceram. 26 (1987) 782­785.21) B. Scharifker and G. Hills: Electrochim. Acta 28 (1983) 879­889.

0 600 1200 1800 2400 3000 3600

-0.90

-0.85

-0.80

-0.75

-0.70

-0.65

-0.60

-0.55

dc

ba

Time, t / s

Po

ten

tial

, E /

V

Fig. 7 The cell voltage plots during electrolysis, starting current density:0.3A/cm¹2, Ti mass% = 2.75: (a) ri = 0; (b) ri = 0.50; (c) ri = 1.50;(d) ri = 6.00.

The Influence of Fluoride Anion on the Equilibrium between Titanium Ions and Electrodeposition of Titanium 1303