The Influence of Fluoride Anion on the Equilibrium between Titanium Ions and Electrodeposition of Titanium in Molten Fluoride Chloride Salt

The In

fl

uence of Fluoride Anion on the Equilibrium between Titanium Ions

and Electrodeposition of Titanium in Molten Fluoride

­

Chloride 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

NaCl­KCl­TiClxwas prepared via using titanium sponge to reduce titanium tetrachloride in a NaCl­KCl melt under a negative pressure at

1023 K. The relationship between titanium valence states and [F]/[Ti] molar ratios was investigated with successive adding potassiumfluoride in the pre-prepared NaCl­KCl­TiClx. It was found that the average valence of titanium ions tended to be stable around 3.0 when [F]/[Ti] molar

ratio was greater than 1.80. The equilibrium redox potentials,ETi3þ_=Ti2þ,ETi4þ_=Ti3þ,ETi3þ_=Ti andETi2þ_=Ti, were also calculated through the obtained concentration of equilibrium titanium ions with different molar ratios of [F]/[Ti]. Meanwhile, the influence of thefluoride anion on over-potential and characteristics of titanium electrodeposition were investigated through changing the molar ratio of [F]/[Ti]. The results showed that, with the increasing of [F]/[Ti] molar ratios, the grain size of electrodeposition products became smaller, while the over-potential was 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 has

been extensively investigated over past decade.1­3)Normally,

there are two kinds of soluble titanium ions (Ti3+_{, Ti}2+₎

in molten salts which can result in the disproportionation

reactions to lowering current efficiency, or forming of

metallic mud on the cathode. So far, the electrodepostion of titanium in molten salts has been mostly focused on molten chlorides electrolytes. Some of literatures have reported that the electrodepostion of Ti in chloride melts could be changed

with adding fluoride.4­11) For the chloride-fluoride system,

most of works employed tetravalent titanium as solute

(K2TiF6 in LiCl­KCl, NaC1­KC1, or NaCl). The literature

results indicated that the cathodic process of titanium ions in

such melts mainly consisted of two steps: TiF62¹+e¹=

TiF63¹and TiF63¹+3e¹=Ti+6F¹.

The decisive role played by the parameter F/Cl in

controlling the electro-refining process was confirmed by

N. Ene etc. in fluoride­chloride melt.6) _{They present}

addi-tionalfinding regarding its influence on the electrochemical

parameters,EdandEp, the current efficiency, the morphology

as well as the chemical purity of the electrodeposited titanium. The morphology of the cathodic deposit is mainly determined

by the concentration of free fluoride anions in the melt. The

factors including electrodeposition temperature, electrodepo-sition time and the current density on the effect of micrograph

were investigated in NaCl­KCl­LiCl­K2TiF6 melt,7) LiF­

NaF­KF­K2TiF6melt,12)KCl­NaCl­KTiF6melt.13)

However, it is necessary to know the influence of the

fluoride anion on electrodeposition characteristics in the

electrolyte beginning with low valence states of titanium. This work is going to demonstrate the behavior of the

oxidation states of titanium, Ti2+ _{and Ti}3+_{, with an addition}

of potassiumfluoride in the melt. The work is also presenting

the results related to the electrorefining of titanium in a melt

of chloride­fluoride.

2. Experiment Details

2.1 Preparation and purification of the melt

The mixture of NaCl­KCl (analytical grade) was dried

under vacuum for more than 10 h at 573 K to remove moisture. Then, it was heated up to 1023 K under high purity

argon atmosphere. The temperature controlled within «2 K

was achieved with a digital temperature controller (CHINO DZ3000; CHINO Corporation, Tokyo, Japan) and measured using a K-type thermocouple (Omega Engineering, Inc., Stamford, CT). Schematic diagram of experimental setup can be seen in Fig. 1.

Hydrogen chloride gas (99.999 pct.) was bubbled into the melt to remove oxide ions. And then, high-purity Argon gas was bubbled into the melt to remove remanent HCl, oxygen and water. After melting the salts, pumped the system to

vacuum of 0.5 Pa, then, liquid state of TiCl4(AR) was slowly

taken into the crucible to react with sponge titanium:8)

TiþTiCl4!2TiCl2

G¼ 135:9 kJmol1 at 1023 K ð1Þ

The influence of fluoride anion on the equilibrium among

titanium ions was investigated by addition of potassium

fluoride into the supporting electrolyte. It was dehydrated

and pre-melted at 573 and 1173 K, respectively. After each addition, HCl gas was bubbled into the melt to remove oxide ions, then, high-purity argon gas was used to stir the molten salts for making the reaction of titanium ions and titanium metal fast.

2.2 Concentration determination of titanium ions

To determinate the concentration of Ti2+ _{and Ti}3+_{, a}

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

+1_{Graduate Student, University of Science and Technology Beijing}

+2_{Corresponding author, E-mail: hzhu@metall.ustb.edu.cn; sjiao@ustb.}

edu.cn

out from molten salts by the quartz sampler for analysis in each experiment, and the average value of the concentrations of titanium ions is considered as the result. The concentration

of Ti2+ and Ti3+ ions in the sample was determined by H2

volumetric analysis, titration, respectively. The concentration

of Ti2+ was quantified by H2 volumetric analysis as the

following reaction (2):14­16)

2Ti2þ_þ2H!2Ti3þ_{þH2 ðgÞ ð2Þ}

It should be mentioned that the oxygen dissolved in an

aqueous solution could oxidize titanium ions from Ti2+_into

Ti3+_{, which may cause an underestimate of the concentration}

of Ti2+ _{ion. Therefore, the deionized water was treated by}

a vacuum degassing and fully cleaned by the high purity Ar. A certain amount of concentrated hydrochloric acid was injected into the deoxygenized water to prepare diluted

hydrochloric acid (1 mol/L). The deoxygenized hydrochloric

acid solution was saturated by bubbling high purity H2 for

30 min in order to prevent the H2 evolved by the reaction 2

from 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 initial

trivalent titanium ions in the sample and the oxidized Ti3+

ion from reaction 2. The total concentration of Ti3+ _{in the}

solution was determined by the titration using 0.05M

NH4Fe(SO4)2aqueous solution. The Ti3+ion in the solution

reacted with Fe3+_{by the reaction (3):}17)

Ti3þ_þFe3þ_!Ti4þ_þFe2þ _ð3Þ

DAPM­spectrophotometer 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 samples were dissolved in hydrochloric acid then oxidized by concentrated nitric acid, which were the parts of oxidized

from Ti3+and the initially existed actually. At this point, the

concentration of Ti4+existing in electrolyte is calculated with

eq. (4):

CTi4þ¼Canal_Ti4þ:3CTi3þ=4 ð4Þ

2.3 Electrochemical deposition

Titanium deposition experiments were carried out using

constant current technique and the supporting electrolyte was pre-prepared based on above conditions at 1023 K. Two

titanium plates (50 mm©25 mm©5 mm) and a high purity

titanium plate (35 mm©15 mm©5 mm) were employed

as anodes and cathode, respectively.8) A series of

electro-depositing tests were performed to investigate the influence

of the concentration of fluoride anion on the quality of the

cathodic product with keeping the anodic and cathodic

current density (ja and jc) as 0.1 A/cm¹2 and 0.3 A/cm2,

respectively. The cleaned cathodic products were chemically

analyzed after leaching with 1 mol/L hydrochloric acid

solution. The morphology was observed by using scanning electron microscope (Japan, JSM-6360).

3. Results and Discussion

3.1 The equilibrium between titanium ions in molten salt

There are mainly two reactions happening in solvent

mixing salt with successive adding potassium fluoride. The

equilibrium exists among Ti2+, Ti3+, Ti4+ and metallic

titanium in chloride melts which can be expressed by the following reaction:

3Ti2þ_,2Ti3þ_{þTi ð5Þ}

4Ti3þ_,3Ti4þ_{þTi ð6Þ}

The determinate concentration of titanium ions are

expressed as xi, where xi is the molar fraction of a species

i, which is a cationic molar fraction defined by the eq. (7):

xi¼ 100ni

nNaþþnKþþn_Ti2þþn_Ti3þþn_Ti4þ ð7Þ

The results shown in Table 1 reveal the concentration of

titanium 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 concentrations

of titanium ions were determined. The concentration of Ti3+

ion increases and Ti2+ decreases after adding potassium

fluoride 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 there

has no much Ti2+ _{when the molar ratio of [F]/[Ti] higher}

than 1.80. It demonstrates that the concentration of Ti2+_ion

could be ignored in this case. Meanwhile, the tetra-titanium was detected with the measurement of spectrophotometer

method 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 complex compounds in the molten salt with anions, and the

complex-ation power offluoride anion with titanium is more force than

chloride anion. In such melts, the complexes containing

fluoride anion and titanium ions are formed:13) such as:

TiF_xClðxþy3Þ

y þyF!TiFðxþy

3Þ

xþy þyCl. The

concen-tration of Ti2+ is drastically lessened with the addition of

potassium fluoride, which mainly causes of the migration

of the balance to the direction of generating Ti3+ _{ion in}

disproportionation 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

[image:2.595.53.282.69.239.2]

potassium fluoride. However, the average valence can be

changed as 2.86 when [F]/[Ti] molar ratios higher than 1.80.

With the adding of more fluoride anion, the average valence

becomes as high as 3.1 when [F]/[Ti] molar ratio is 12.

To further clear the effect of adding of fluoride anion

on titanium ions transformation in molten salt, the detail investigation has been done with the equilibrium constant.

The equilibrium constant of the reactions (5) and (6),Kci, is

given by the eqs. (8) and (9).

Keqc1:¼ aTia2_Ti3þ

a3_Ti2þ

¼£ 2

Ti3þðxeq_Ti3:þÞ2 £3

Ti2þðxeq_Ti2:þÞ3

ð8Þ

Keqc2:¼ aTia3_Ti4þ

a4_Ti3þ

¼£ 3

Ti4þðxeq_Ti4:þÞ3 £4

Ti2þðx eq:

Ti3þÞ4

ð9Þ

Whereaiand£iare the activity and the activity coefficient

of a species, i, respectively.£i can be regarded as a constant

when xi is low because it obeys the Henry’s law.14,17)

Therefore, the equilibrium constant can be modified as:

Keqc1:¼ ðxeq_Ti:3þÞ2

ðxeq_Ti:2þÞ3

ð10Þ

Keqc2:¼ ðxeq_Ti:4þÞ3

ðxeq_Ti:3þÞ4

[image:3.595.324.530.73.211.2]

ð11Þ

Figure 3 shows the relationship between [F]/[Ti] molar

ratio and Kciafter it was taken logarithm (lnKci).

The results demonstrate that the initial balance of titanium

ions has been broken after adding potassium fluoride. The

ionic balance between Ti2+ and Ti3+ disappears when the

value of [F]/[Ti] molar ratio across the red dot line (higher

than 1.8). And then, the disproportionation reaction (6) dominates the concentration converting in the melt.

The results shown in Fig. 3 discloses that the equilibrium

constant of Keqc1: 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 to

1.80. 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 balance

between Ti3+ _{and Ti}4+ _{was observed. The K}eq:

c2 turns from

1.2©10¹4 to 4.4©10¹3 with changing the [F]/[Ti] molar

ratio at range of 2.1 to 11.4.

To achieve the aim of proclaiming the regular pattern among titanium ions for further, the equilibrium redox potential is introduced in present work. The equilibrium potentials could be calculated with the well-known

expres-sions (12) and (13):18)

ETi3þ_=Ti2þ¼Eª_Ti3þ₌Ti2þRT

F lnK eq:

c1 ð12Þ

ETi4þ_=Ti3þ¼Eª_Ti4þ_=Ti3þRT

F lnK eq:

c2 ð13Þ

WhereETi3þ_=Ti2þandE_Ti4þ_=Ti3þare equilibrium potential of

Ti3+_/Ti2+_{and Ti}4+_/Ti3+_vs.Cl

2/Cl¹, respectively;Eª_Ti3þ_=Ti2þ

and Eª_Ti4þ_=Ti3þ are the standard redox potentials of Ti3+/Ti2+

and Ti4+_/Ti3+_vs.Cl

2/Cl¹, respectively.ETi3þ_=TiandE_Ti2þ_=Ti

could also be calculated with the same method.

The equilibrium potentials of various redox couples list on

the curves in Fig. 4. The redox reactions Ti3+/Ti2+, Ti3+/Ti

and Ti2+/Ti occur at potentials of¹1.58,¹1.73 and¹1.79 V

when the melt without fluorides ([F]/[Ti]=0). The

equi-librium potential of ETi4þ_=Ti3þ could also be calculated as

0.09 V. The balance of disproportionate reaction of3Ti2þ,

2Ti3þ_{þTi and 4Ti}3þ_,3Ti4þ_{þTi depends on the}

con-centration of fluoride anions. In thefluoride free electrolyte,

the redox reaction of3Ti2þ,2Ti3þþTiis involved in the

melt. 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, r_i / x_F-/x_Tin+

Average Valence

A

verage V

alence,

n

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

0 2 4 6 8 10 12

-4 -2 0 2 4 6 8 10

Molar ratio of F to Ti, r_i / x_F-/x

Tin+

ln

Kc

1

-2 -1 0 1 2 3

ln

K

c

2

lnK_c1

lnK_c2

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

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

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

[image:3.595.51.287.84.438.2] [image:3.595.48.290.94.270.2] [image:3.595.109.232.602.663.2]

of ETi3þ_=Ti2þ andE_Ti2þ_=Ti. When more fluoride is added, the

[F]/[Ti] molar ratio greater than 1.8,ETi3þ_=Ti2þbecomes more

negative thanETi2þ_=Ti, and the direct three electron reduction

is obtained. Fluoride anion results in a negative shift in the standard equilibrium potential, ETi4þ_=Ti3þ, for the Ti4+/Ti3+

couple with [F]/[Ti] molar ratios higher than 1.8. The

potential shifts more negative with [F]/[Ti] molar ratios

increasing mainly because of a common ligand for titanium ions formed.

3.2 Electrochemical deposition

The influence of the fluoride anion on electrodeposition

characteristics of titanium was investigated. The average

diameter,dm, the metallic powder grains may be described by

eq. (14),19)_wherem

1,m2+mndenote the sieve mesh andp1,

p2+pnthe weights of various grain size fractions.

dm¼m

1p1þm2p2þ þmnpn

p1þp2þ þpn ð

14Þ

Figure 5 shows the relationship betweendmvalues of the

deposits recovered from molten salts varying [F]/[Ti] molar

ratios. It discloses that the size of metallic grains is controlled

by the [F]/[Ti] molar ratios under a constant current density:

the higher content of free fluoride anion is, the finer the

titanium 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 titanium

products regarding with varying [F]/[Ti] molar ratios range

from 0 to 6.00. The micrographs of the titanium deposit exhibit regular granular structures. The formation of dendrite

can be observed when there is no fluoride anion in the

electrolyte, while the crystalline grain decreases with

increasing of [F]/[Ti] molar ratio. These results are similar

with what the previous literature reported.20)

The morphology is affected by the nucleation and the crystal growth. There are two competitive factors governing the 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, r_i / x_F-/x_Tin+

c d b

a

c d

b a-E _Ti4+

/Ti3+ b-E _Ti3+

/Ti2+ c-E _Ti2+

/Ti d-E _Ti3+

/Ti

Potential,

E

/ V

a

Fig. 4 Equilibrium redox potentialsvs.Cl2/Cl¹for the systems, Ti4+/Ti3+, Ti3+_/Ti2+_{, Ti}3+_{/Ti, and Ti}2+_{/Ti under various [F]/[Ti] molar ratios (r}

i), at temperature of 1023 K.

0 1 2 3 4 5 6

0 20 40 60 80 100 120 140

Molar ratio of F to Ti, r_i / x_F-/x_Tin+

Grain Size

Grain Size,

dm

/

µ

m

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

[image:4.595.77.263.69.204.2] [image:4.595.335.519.71.212.2] [image:4.595.112.483.260.512.2]

nucleation density N0 and over-potential © showed in

eq. (15):21)

N0¼Aexp _©B₂

ð15Þ

It is easy tofind that the crystal diameter lessens with the

over-potential increasing. Depending on the solvent nature,

the weaker Ti­Cl bond is replaced by stronger Ti­F bond

after addingfluoride anion, which results in over-potential of

titanium ion electrodeposition increasing. Figure 7 indicates the relative results.

4. Conclusions

The behavior of the oxidation states Ti2+_{and Ti}3+_{has been}

investigated with adding of potassium fluoride into the

chloride melts. It is found that the average valence of

titanium ions tends to be rising up to 3.0 when [F]/[Ti] molar

ratio is greater than 1.80. The equilibrium redox potential has also been calculated with the determinate equilibrium

concentration of titanium. Meanwhile, the influence of the

fluoride anion on over-potential and characteristics of

titanium electrodeposition was investigated. The results discloses that the grain size of electrodeposition products becomes smaller, and the over-potential turns to higher with

the increasing of [F]/[Ti] molar ratio.

Acknowledgments

The authors are grateful to the National Science Founda-tion of China (No. 51322402, 50934001), the NaFounda-tional High Technology 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 Program for New Century Excellent Talents in University (NCET-11-0577), and the Fundamental Research Funds for the Central Universities (No. FRF-AS-11-003A).

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0 600 1200 1800 2400 3000 3600 -0.90

-0.85 -0.80 -0.75 -0.70 -0.65 -0.60 -0.55

d c

b a

Time, t / s

Potential,

E

/ V

Fig. 7 The cell voltage plots during electrolysis, starting current density: 0.3 A/cm¹2_{, Ti mass% =2.75: (a) r}

[image:5.595.65.274.67.220.2]