Formation of Ti or TiC nanopowder from TiO2 and carbon powders by electrolysis in molten NaCl-KCl

Article abstract of DOI:10.1039/c5ra23063b

A new route to produce pure Ti powder or TiC nanopowder with diameters of ～50 nm by electrolysis in molten KCl-NaCl using TiO₂ and carbon powder was reported in this paper. This electrochemical experiment was carried out with an innovative equipment unitizing the chlorination and electrolyzation. A fine titanium powder was obtained after electrolysis at 4.0 V for 5 h at 850 °C. TiC nanopowder could be prepared in the anode chamber with the cell voltage up to 4.5 V. Furthermore, the product was analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that pure Ti or TiC nanopowder can be prepared after electrolysis. The TiC nanopowder exhibited a polymorphic structure, and it had good thermal stability and oxidation resistance below 345 °C in air investigated by TGA and DSC. Cyclic voltammograms were carried out and the electrode reaction mechanisms during the electrolysis process were discussed in the paper.

Full text of DOI:10.1039/c5ra23063b

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Formation of Ti or TiC nanopowder from TiO2 and
carbon powders by electrolysis in molten NaCl–KCl
Cite this: RSC Adv., 2016, 6, 8644
*
Kun Zhao, Yaowu Wang, Jianping Peng, Yuezhong Di, Kejia Liu and Naixiang Feng
A new route to produce pure Ti powder or TiC nanopowder with diameters of ꢀ50 nm by electrolysis in
molten KCl–NaCl using TiO2 and carbon powder was reported in this paper. This electrochemical
experiment was carried out with an innovative equipment unitizing the chlorination and electrolyzation.
A fine titanium powder was obtained after electrolysis at 4.0 V for 5 h at 850 ꢁC. TiC nanopowder could
be prepared in the anode chamber with the cell voltage up to 4.5 V. Furthermore, the product was
analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). The results indicate that pure Ti or TiC nanopowder can be prepared after
electrolysis. The TiC nanopowder exhibited a polymorphic structure, and it had good thermal stability
and oxidation resistance below 345 ꢁC in air investigated by TGA and DSC. Cyclic voltammograms were
carried out and the electrode reaction mechanisms during the electrolysis process were discussed in the
paper.
Received 4th November 2015
Accepted 12th January 2016
DOI: 10.1039/c5ra23063b
www.rsc.org/advances
developed and make encouraging progress. But up until now,
there are no commercial applications among those methods.
1. Introduction
Recently, it has been reported that TiC powder could be
synthesized by electrolysis in molten salt using TiO2 and C.25
Large and agglomerated TiC particles can be prepared by this
method. However, the conventionally produced TiC is oen
contaminated with free carbon, particularly as nanopowder. TiC
is easily oxidized, as a result, the residual carbons cannot be
removed by oxidation and other methods.
Due to the Kroll process is composed of two step process,
and the equipment is divided into independent reduction and
electrolytic part, to simplify its process and equipment, a new
route is developed to produce Ti or nanocrystalline TiC at
a relatively low temperature with an innovative equipment
unitizing the chlorination and electrolyzation in this study.
Using TiO2 and carbon powder as the raw materials, metallic Ti
or polymorphic TiC powders can be prepared by electrolysis in
NaCl–KCl melts at different cell voltage.
Titanium has attracted considerable attention as one of the most
typical transition-metals.1,2 It has many excellent properties and
the high strength, low density, and good corrosion resistance of
titanium make it a desirable alternative to Al and steel for appli-
cations in aerospace and the marine vehicle industry.3–5 Titanium
carbide as a transition-metal carbide is becoming a promising
material due to its unique properties, such as high melting point,
high hardness, high thermal and electrical conductivities, high
mechanical stiffness, superior chemical and thermal stabilities,
excellent wear resistance and catalytic activity.6–10 This advanced
non-oxide ceramic material is widely used in industry, particularly
as an abrasive material in the manufacture of sintered carbide
plates for cutting tools, an enhancement agent for composite
materials, and in the aerospace materials industry.7,11–13
Due to the labor-intensive and cumbersome process of
production of titanium by Kroll process (magnesium reduction),
electrolysis in a molten salt system as a popular direction has
been considered as more promising to replace Kroll process for
producing titanium.14,15 Many novel methods based on electrol-
ysis have been founded and developed, such as the electrolysis
reduction of TiCl4 (ref. 16 and 17) and the electro-deoxidation
(FFC) process.15,18 However, the small solubility of TiCl4 in
molten chlorides and the low conductivity of TiO2 cathode limit
the developing of above two processes respectively. As a result,
the calciothermic reduction (OS) process,19,20 and the composite
anode electrolysis process (MER21,22 and USTB23,24), etc., have been
2. Experimental
The experimental apparatus for electrolysis is shown in Fig. 1. A
corundum crucible (110 mm in diameter) with a hole (15 mm in
diameter) in the bottom and placed on a molybdenum crucible
(115 mm in diameter) was used as the cell. A cone-shaped
molybdenum cathode was placed in the corundum crucible
and connected to the molybdenum crucible. The molybdenum
crucible was placed in a stainless steel crucible, which was
located at the bottom of a stainless steel reactor. The hollow
anode was made of graphite with a funnel-shape structure and
many holes (3 mm in diameter) on its side face. The anode was
School of Material and Metallurgy, Northeastern University, Shenyang 110819, PR
China. E-mail: wangyw113@163.com; zkngwt@126.com
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Fig. 1 The schematic configuration of the equipment: (1) – electric furnace, (2) – reactor, (3) – stainless steel crucible, (4) – molybdenum
cathode, (5) – corundum crucible, (6) – TiO2 + C pellets, (7) – molybdenum crucible, (8) – graphite anode chamber, (9) – aluminum silicate, (10)
– insulating washer, (11) – stainless steel bar, (12) – DC power source, (13) – vacuum system.
connected to a stainless steel tube stretched out of the sealed
stainless steel reactor.
The phase compositions of the samples were identied using
X-ray diffraction (XRD) measurements with a Cu-Ka character-
Titanium dioxide ($99.5%, Sinopharm Chemical Reagent istic ray. The morphologies and structure of the samples were
Co. Ltd., China) and carbon powder ($99%, Sinopharm observed using scanning electron microscopy (SEM) and trans-
Chemical Reagent Co. Ltd., China) were used in the present mission electron microscopy (TEM). The thermal oxidation
work to prepare the pellets placed in the graphite anode behavior of the powders was investigated by simultaneous TGA/
chamber. Titanium dioxide and carbon powders were weighed DSC measurements in ꢂ1air atmosphere at a temperature-
and mixed with a xed molar ratio. The mixture was ball-milled increase rate of 5 ꢁC min
for 8 h and made into pellets of 5–8 mm in diameter. Then, the
pellets were dried at 120 C for 48 h. Aer drying, the pellets
.
ꢁ
were placed in the graphite anode chamber. NaCl ($99.5%,
3. Results and discussion
3.1 Stability investigation of TiO2/C mixture
Sinopharm Chemical Reagent Co. Ltd., China) and KCl
($99.5%, Sinopharm Chemical Reagent Co. Ltd., China) were
mixed in a composition of 75 mol% NaCl and 25 mol% KCl aer
vacuum drying completely. The mixture was packed in the
corundum crucible and used as the electrolyte.
The effect of submerging the pellets in molten salt was inves-
tigated to conrm the starting material for electrolysis. Fig. 2
shows the XRD pattern of the pellet aer immersion in NaCl–
ꢁ
KCl molten salt at 850 C for 4 h in argon atmosphere.
The stainless steel reactor was closed and sealed with insu-
lating washer placed on the ange plane. Cooling water was
poured into the jacket, aer that, the apparatus was slowly heated
to 350 ꢁC under vacuum. Argon gas was ushed into the reactor
until saturating it, and the reactor temperature was then ramped
to 850 ꢁC byꢁ6 ꢁC minꢂ1. Once the temperature was achieved and
kept at 850 C, the salt was melt for at least 20 min. Then, the
graphite anode was lowered into the molten salt, and all the holes
distributed on its side face were immersed under the level of
electrolytes. An electric circuit was set up, and the electrolysis was
conducted under a constant voltage of 3.5, 4.0 and 4.5 V,
respectively. Aer electrolysis, the graphite anode was pulled out
of the salt and cooled gradually to room temperature in the
furnace. Finally, the graphite anode and the stainless steel
crucible were removed from the reactor. The crude products were
lied from the molten salt and washed with distilled water to
remove Na+, K+, and Clꢂ, and then the products were ultrasoni-
cally vibrated to remove the solidied salt. The samples were then
dried in a vacuum oven at 50 ꢁC for 24 h.
Fig. 2 XRD pattern of the pellet after immersion in NaCl–KCl molten
salt.
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Table 1 The thermodynamic data of the possible reactions involved
the pellets and molten salts during the electrolysis process at 850 ꢁC
DGq1123 K
/
DEq1123 K
/
V
Number Reaction
kJ molꢂ1
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
TiO2 + 3C ¼ TiC + 2CO
149.443
99.483
51.168
49.781
TiO2 + C ¼ TiO + CO
3TiO2 + C ¼ Ti3O5 + CO
2TiO2 + C ¼ Ti2O3 + CO
NaCl / Na+ + Clꢂ
3.25
3.38
1.63
1.65
1.84
KCl / K+ + Clꢂ
TiCl4 / Ti4+ + 4Clꢂ
TiCl3 / Ti3+ + 3Clꢂ
(9)
TiCl2 / Ti2+ + 2Clꢂ
(10)
(11)
(12)
(13)
(14)
(15)
TiO2 + 2C + 2Cl2 ¼ TiCl4 + 2CO
2TiO2 + 4C + 3Cl2 ¼ 2TiCl3 + 4CO
TiO2 + 2C + Cl2 ¼ TiCl2 + 2CO
TiO2 + C + 2Cl2 ¼ TiCl4 + CO2
ꢂ281.93
ꢂ277.105
37.918
ꢂ312.733
2TiO2 + 2C + 3Cl2 ¼ 2TiCl3 + 2CO2 ꢂ327.531
TiO2 + C + Cl2 ¼ TiCl2 + CO2
7.125
Fig. 3 XRD patterns of the samples obtained on the cathode after
washing in 850 ꢁC NaCl–KCl melts after electrolysis at different cell
voltage for 5 h.
As evidenced in the Fig. 2, the phase composition of the
sample pellets aer the immersion experiment had no change
except adherent NaCl and KCl salts. This result indicates that
no reaction could occur only between TiO2 and carbon during
the process of electrolysis, which is consistent to the results of
the positive DGq1123 K value of reaction (1)–(4) shown in Table 1.
chamber, and only peaks associated with pure TiC are observed
in its XRD pattern as shown in Fig. 4.
Fig. 5 shows the SEM images and EDS spectra of the samples
aer electrolysis at different cell voltages. From the Fig. 5(a),
combined with the result of the XRD data, the morphology of Ti
and NaTi2O2 obtained at 3.5 V were of micron size. With the
voltage increasing to 4.0 V, ne Ti powder was prepared. The
3.2 Inuence of cell voltage
In order to determine the applied potential for the electrolysis, spherical particles were dominant in the sample and the
it is necessary to calculate the decomposition voltage. It can be average grain size of Ti was about 2 mm (Fig. 5(b)). As soon as the
seen from Table 1 that the decomposition voltages of NaCl and production of TiC started, spongy aggregate could be observed
KCl are 3.25 and 3.38 V at 850 ꢁC respectively, which are higher from Fig. 5(c). Fig. 5(d) shows the morphology of TiC obtained
than the one of TiCl4 (1.63 V), TiCl3 (1.65 V) and TiCl2 (1.84 V). It near the anode chamber aer electrolysis at 4.5 V for 5 h in 850
also can be concluded that the chloride of titanium can be ꢁC NaCl–KCl melts. Interestingly, the rod-like and spongy
reduced with decomposing NaCl and KCl. In order to determine aggregate morphology could be found in the gure. The EDS
the inuence of the cell voltage on the electrolysis process, spectra (Fig. 5(e) and (f)) show that the product obtained at 4.0 V
a constant voltage of 3.5, 4.0 and 4.5 V was applied respectively
between the molybdenum cathode and the graphite anode full
ꢁ
of the pellets with TiO2/C molar ratio of 1 : 1 for 5 h at 850 C.
The vent gas was bubbled into water for 20 min since the
beginning of the electrolysis. The pH of the water detected using
PH indicator paper showed acidic. The solution was titrated
with silver nitrate, as a result, white precipitate appeared. It
shows that the vent gas contains Cl2 certainly. The XRD patterns
of the obtained sample on the cathode aer washing are shown
in Fig. 3.
From the XRD patterns as shown in Fig. 3, when the voltage
of 3.5 V that was higher than the decomposition voltages of
NaCl and KCl was applied, the Ti phase had been the main
phase accompanied by NaTi2O2 in the product. The pattern of
the sample aer electrolysis at 4.0 V showed that the XRD peaks
associated with the lower oxides, NaTi2O2 had completely dis-
appeared, and only peaks associated with pure Ti were identi-
ed. With the voltage increasing to 4.5 V, the sample exhibited
the diffraction peaks of the mixture of Ti and TiC. It was notable
that there was anodic product appeared near the anode ꢁC NaCl–KCl melts.
Fig. 4 XRD pattern of the product prepared near the anode chamber
with a TiO2/C molar ratio of 1 : 1 after electrolysis at 4.5 V for 5 h in 850
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Fig. 5 SEM images of the cathodic products in 850 ꢁC NaCl–KCl melts after electrolysis at different cell voltages for 5 h ((a) 3.5 V, (b) 4.0 V, (c) 4.5
V) and the product prepared near the anode chamber at 4.5 V (d), EDS spectra (e) of the cathodic product obtained at 4.0 V (specture 1 in (b)) and
EDS spectra (f) of the product prepared near the anode chamber at 4.5 V (specture 2 in (d)).
and 4.5 V was Ti and TiC respectively. The TEM images provide
a direct view of the TiC sample as shown in Fig. 6.
The TiC powder shows an initial mass loss below 200 ꢁC due
to the loss of water. The weight loss remains almost constant
ꢁ
ꢁ
Fig. 6(a) shows that the diameter of TiC particles is around between 200 C and 345 C, indicating that the TiC powder is
50 nm. Fig. 6(b) shows that the rod-like TiC has a spiral pattern, stable in this temperature range. Then, a signicant mass gain
lattice fringes corresponding to the TiC (200) plane (dTiC (200)
¼
corresponding to a broad exothermic peak in the DSC curve
ꢁ
0.20 nm) can be observed in the rod-like sample. Those results occurred at 345 C, which mainly originated from the oxidation
indicate that the TiC powders obtained aer electrolysis at 4.5 V of TiC at a low exothermic rate of reaction. When the temperature
are the polymorphic mixture made up of nanoparticles and reached 550 ꢁC, the slope of the TGA curve became smaller,
nanorods.
probably because TiO2 or TiN formed on the surface of TiC
The stability of the TiC sample was evaluated by TGA–DSC particles by the new phase prevented the diffusion of O2 or N2
analysis under air atmosphere, and the mass loss of the TiC into the interface of TiC and decreased the reaction rate. When
ꢁ
sample vs. temperature during the heating is shown in Fig. 7.
the temperature exceeded 725 C, the molecules moved rapidly,
resulting in further oxidation or nitridation occurring at the
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through the graphite tube at the central position of the funnel-
shape anode. Then, the pellets made from TiO2 and C can react
with Cl2 as reaction (10)–(15) in Table 1.
The results of thermodynamic calculation seen from Table 1
suggest that all the reactions might occur except reaction (12)
and reaction (15) at 850 ꢁC. The chlorination in Kroll process is
according to reaction (12).14 However, in this paper, because of
the chlorination taking place in molten KCl–NaCl, KCl or NaCl
can form complex with TiCl2 and TiCl3 such as TiCl3$KCl,
TiCl2$KCl, TiCl3$NaCl and TiCl2$NaCl.26 As a result, the Gibbs
free energies of eqn (14) and (15) are driving down. Further-
more, CO2 is easier to be produced in molten chlorides.27
Therefore, the chlorination reaction in the anode chamber can
be described as following:
Fig.
6 Analytical TEM and HRTEM images of the TiC powders
prepared at a TiO2/C ratio of 1 : 1 after electrolysis at 4.5 V in 850 ꢁC
NaCl–KCl melts ((a) low-resolution TEM image, (b) high-resolution
TEM image).
2TiO2 + 2C + 3Cl2 + 2KCl ¼ 2TiCl3$KCl + 2CO2
TiO2 + C + Cl2 + KCl ¼ TiCl2$KCl + CO2
(16)
(17)
(18)
(19)
2TiO2 + 2C + 3Cl2 + 2NaCl ¼ 2TiCl3$NaCl + 2CO2
TiO2 + C + Cl2 + NaCl ¼ TiCl2$NaCl + CO2
TiCl3$KCl, TiCl2$KCl, TiCl3$NaCl and TiCl2$NaCl also can be
described as KTiCl4, KTiCl3, NaTiCl4, NaTiCl3, respectively.
Consequently, those complex particles can be dissociated in
melts:
ꢂ
KTiCl4 ¼ K+ + TiCl4
KTiCl3 ¼ K+ + TiCl3
(20)
(21)
(22)
(23)
ꢂ
ꢂ
ꢂ
NaTiCl4 ¼ Na+ + TiCl4
NaTiCl3 ¼ Na+ + TiCl3
Fig. 7 TGA–DSC curves of the TiC sample.
internal structure of the TiC particles. In this process, CO2 was
formed and adsorbed on the surface of the nanopowder. When
the temperature exceeded 850 C, those CO2 was released, that
Then, titanium ions will enter into the melt and obtain
electron at the cathode to produce metallic titanium.
ꢁ
To investigate the reaction mechanisms, cyclic voltammog-
rans were carried out on a graphite working electrode with
a groove full of the powders with TiO2/C molar ratio of 1 : 1 in
NaCl–KCl melts at 850 ꢁC. The mass of the powders were 0.044
g. The cyclic voltammograms under different scanning rates of
25, 50, 75, 100 and 150 mV sꢂ1 are shown in Fig. 8(a). Three
anodic waves A, B and C and three cathodic waves A0, B0 and C0
are clearly visible in the curves. It can be conrmed that the
cathodic wave A corresponds to chlorine evolution, while wave C
corresponds to the deposition of sodium. Therefore, titanium
deposition should be considered as occurs from 0.65 V to 0.30 V
(wave B, vs. spectrum pure graphite). Small mass of the powders
in working electrode leads to reduction wave B with low inten-
sity. According to ref. 28 and 29, the number of exchanged
electrons can be deduced from the cyclic voltammograms. For
the titanium reduction peak, the plot of potential (V) vs. ln[I/(Ip
ꢂ I)] is linear and shown in Fig. 8(b). From the slope (RT/nF) of
linear equation in Fig. 8(b), the exchanged electron number of
titanium ions reduction is approximately equal to 3 (n z 3). So,
it can be concluded that three electrons are exchanged in this
resulted in weight loss. Finally, the sample was oxidized or
nitrided completely at about 900 ꢁC, and the weight gain
remained almost constant in the TGA curve when the tempera-
ture was above 900 ꢁC, indicating almost no weight change.
As described earlier, TiC is a potential candidate for high-
temperature structural applications. However, in this study,
the size of the TiC particles prepared by electrolysis is very
small. Nanoparticles have large specic surface areas. There-
fore, these TiC particles have a large surface energy, thus
resulting in agglomeration with higher stability. In this study,
the prepared sample was oxidized completely at around 900 ꢁC.
ꢁ
However, the sample has good thermal stability below 345 C.
3.3 Discussing of the principle of the electrolysis process
The process using TiO2 and carbon in NaCl–KCl melts to
produce Ti can be divided into two main stages actually: elec-
trolytic process and chlorination process. First, the electrolytic
reaction of NaCl and KCl occur as reaction (5) and (6), and Cl2 is
prepared at the anode. Cl2 can enter into the anode chamber
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Fig. 8 Cyclic voltammograns in NaCl–KCl melts at 850 ꢁC (a) and ln[I/(Ip ꢂ I)] as function of potential (b).
reduction. That is to say, the electrode reactions are according
to the following equation,
Q0 in this process is 4F, which is higher 1/4 than that in this
paper. It indicates that the titanium production has signicant
improvement by the route in this paper compared to using TiCl4
to prepare Ti. Furthermore, this new process can greatly reduce
energy consumption when the electrolysis under the same
conditions. The principle of the formation of metallic Ti from
TiO2 and carbon particles by electrolysis is shown in Fig. 9.
Anodic reaction: 2Clꢂ ꢂ 2e ¼ Cl2
(24)
ꢂ
Cathodic reaction: Ti3+ + 3e ¼ Ti or TiCl4 + 3e ¼ Ti + 4Clꢂ
(25)
3.4 Formation of TiC
The theoretical charge (Q0) is dened as the charge required
in order to obtain 1 mol metallic Ti, and F is the Faraday's With the cell voltage up to 4.5 V, O2ꢂ may be present in NaCl–
constant (F ¼ 96 485 C molꢂ1).
KCl melts from the reduction of some TiO2 placed in the anode
As shown in eqn (25), Q0 can be calculated as 3F. However, chamber. Consequently, CO2 is produced at the graphite anode
the cathodic reaction of the study of the electrolysis process combines with O2ꢂ in the melt to form CO32ꢂ, and then the
2ꢂ
using TiCl4 to prepare Ti can be described as the following CO3
equation:
enters into the anode chamber and reacts with the
pellets. As a result, TiC is prepared in the chamber as the
following equation:
Ti4+ + 4e ¼ Ti
(26)
2ꢂ
CO2 + O2ꢂ ¼ CO3
(27)
Fig. 9 Principle of the formation of Ti from TiO2 + C pellet by electrolysis.
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2ꢂ
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2CO3 + 2TiO2 + 5C ꢂ 4e ¼ 2TiC + 5CO2
(28)
A part of TiC or carbon might drop to the cathode. Carbon
powder also can react with the Ti prepared at the cathode as this:
Ti + C ¼ TiC
(29)
Consequently, the powder obtained at cathode is the mixture
of Ti and TiC at the cell voltage of 4.5 V.
11 M. S. Song, B. Huang, M. X. Zhang and J. G. Li, Int. J. Refract.
Met. Hard Mater., 2009, 27, 584.
12 S. M. Hong, J. P. Jin, K. P. Eun, Y. K. Kyeong and G. L. Jung,
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4. Conclusions
In summary, metallic Ti or TiC nanopowder could be prepared
by electrolysis in molten NaCl–KCl using TiO2 and C powder
with a new cell at different cell voltage. Fine Ti powder could be
obtained at the cathode by electrolysis at 4.0 V in 850 ꢁC NaCl–
KCl melts for 5 h. Pure TiC nanopowder could be obtained at
the anode chamber at 4.5 V. This improved design provides
a new route to prepare Ti or TiC with good specic properties,
and thus provides a new method to prepare other transition-
metal and its carbides.
16 A. D. Hartman, S. J. Gerdemann and J. S. Hansen, JOM, 1998,
50, 16.
17 M. V. Ginatta, JOM, 2000, 52, 18.
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We are grateful for the nancial support from the State Key
Development Program for Basic Research of China (973
Program, Grant No. 2013CB632606-1).
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