Production of fine tungsten powder by electrolytic reduction of solid CaWO4 in molten salt

Article abstract of DOI:10.1149/2.113206jes

Direct electrochemical reduction of solid CaWO₄ to fine W powder in molten CaCl₂-NaCl was studied by cyclic voltammetric measurement and potentiostaticconstant-voltage electrolysis. The effect of electrolysis conditions such as electrolysis time, temperature and voltage on the quality of the product (the residual amount of CaWO₄) was systematically investigated. Based on the solubility tests of CaWO₄ in the melt in a wide temperature range, an unprecedented strategy on electrolytic extraction of pure tungsten powder was proposed and confirmed, which involves low-temperature and short-duration electrolysis of solid CaWO₄ in molten CaCl ₂-NaCl eutectic salt (1023 K, 10-hour electrolysis) followed by a simple washing process in the same melt at a higher temperature (1123 K, 1-hour washing). The particle size of the as-prepared W powder is less than 200 nm. This process exhibits a high yield and increased energy efficiency for production of fine W powder from CaWO₄.

Full text of DOI:10.1149/2.113206jes

Journal of The Electrochemical Society, 159 (6) E139-E143 (2012)
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0013-4651/2012/159(6)/E139/5/$28.00 © The Electrochemical Society
Production of Fine Tungsten Powder by Electrolytic Reduction of
Solid CaWO4 in Molten Salt
Dingding Tang,a Wei Xiao,a Huayi Yin,a Longfei Tian,a and Dihua Wanga,b,∗,z
aSchool of Resource and Environmental Sciences, Wuhan University, Wuhan 430072, China
bState Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences,
Shenyang 110016, China
Direct electrochemical reduction of solid CaWO4 to fine W powder in molten CaCl2-NaCl was studied by cyclic voltammetric mea-
surement and potentiostatic/constant-voltage electrolysis. The effect of electrolysis conditions such as electrolysis time, temperature
and voltage on the quality of the product (the residual amount of CaWO4) was systematically investigated. Based on the solubility
tests of CaWO4 in the melt in a wide temperature range, an unprecedented strategy on electrolytic extraction of pure tungsten powder
was proposed and confirmed, which involves low-temperature and short-duration electrolysis of solid CaWO4 in molten CaCl2-NaCl
eutectic salt (1023 K, 10-hour electrolysis) followed by a simple washing process in the same melt at a higher temperature (1123 K,
1-hour washing). The particle size of the as-prepared W powder is less than 200 nm. This process exhibits a high yield and increased
energy efficiency for production of fine W powder from CaWO4.
© 2012 The Electrochemical Society. [DOI: 10.1149/2.113206jes] All rights reserved.
Manuscript submitted December 13, 2011; revised manuscript received April 13, 2012. Published May 1, 2012.
Tungsten, which possesses a high melting point (3695 K) and ex-
cellent mechanical properties, has been widely applied in mining, met-
allurgy, machinery, construction, armament industry and etc.1 Gener-
ally, there are two kinds of tungsten ores consist in the earth crust,
i.e. wolframite ((Fe/Mn)WO4) and scheelite (CaWO4), with a rough
ratio of 30:70. At present, a great majority of industrial tungsten is
produced from feedstock of wolframite despite its relatively low abun-
dance. Wolframite can be easily converted into tungsten trioxide via an
alkaline dissolution process. Metallization is then fulfilled by heating
the resulted tungsten trioxide with hydrogen. Although the reserve
of scheelite is much higher than that of wolframite, the difficulties
encountered in the alkaline dissolution process to transfer scheelite
into tungsten trioxide severely restrict the widespread use of scheelite
in tungsten extraction.2 On the other hand, China accounts for about
half of the tungsten resource and 83% of the worldwide production
of tungsten metal.3 In response to the rapid exhaustion of wolframite
reserves in recent years and to fully exploit the natural W-based re-
sources, a sustainable tungsten extraction route is highly desired. In
this sense, the development of new technology for producing tungsten
from scheelite is needed.
based materials.5
lg F = 3.54 + 0.33 lg ρ − 1.9 lg T
+ 0.28 lg h + 0.002T + 0.028D
[2]
(F: particle size of tungsten powder, μm; ρ: apparent density of WO3,
g cm−3; T: reduction temperature, K; h: thickness of material layer,
cm; D: particle size of WO3, μm)
Based on the findings that WO3 is unstable in molten CaCl2 and
CaWO4 is insoluble in molten CaCl2-NaCl at lower temperature,
Karakaya et al. recently obtained fine tungsten powder via electrolytic
reduction of solid CaWO4 in CaCl2-NaCl salt at 873 K.6 Compared
with other methods, this method directly utilizes solid CaWO4 as
raw material, which is capable of simplifying the production process
and improving the utilization ratio of scheelite. However, electrolytic
extraction of tungsten from CaWO4 suffers from relatively sluggish
electrode kinetics. Since the process is a direct solid-to-solid reaction,7
the speed of this reaction is slower than that of solid-to-liquid, solid-
to-gas and others. To accelerate the reaction speed, high-temperature
is required to conduct the electrolysis, such as extraction of Ti at
1123–1223 K.8,9 In addition, a high contact resistance exists in the
electrolytic product consisting of less-interconnected fine tungsten
particles (note that the extremely high melting point of tungsten makes
its particles hardly to be sintered), which in turn causes a large iR drop
across the cathodic material. The retarded effect of iR drop becomes
severe when electro-reduction pathway reaches inner part of precur-
sory pellets, causing the presence of unreduced CaWO4 inside of pel-
lets. In order to obtain pure product, a long reaction time is required
to overcome the barrier originated from iR drop, resulting in a low
current efficiency and high energy consumption.10,11 Normally, the
electrode kinetics can be enhanced by elevating the temperature of the
process for direct electro-reduction of solid compounds,7 but exces-
sively elevated temperature will inevitably lead to severe dissolution
of CaWO4 in the melt and eventually cause a low yield. Therefore,
the electrolytic extraction of W was reported to be more effective at
a relative lower temperature of 873K.6 However, such a low reac-
tion temperature is not optimized, yet. In this context, the electrolysis
temperature and duration should be compromised to ensure a balance
between fast electrode kinetics and an acceptable yield.
Besides the forenamed problem of low utilization ratio of scheel-
ite in industry, the main tungsten production method based on
hydrogen reduction (as depicted in reaction 1)1 has its inherent
demerits.
W O3(s) + 3H2(g) = W(s) + 3H2 O(g)
(ꢀG = −1.901 kJ mol−1, ꢀH = 81.37 KJ mol−1, 1123 K) [1]
As a result of the endothermic nature of the reaction at 1123 K,
extra large amounts of energy must be applied to sustain the reaction,
which causes the high energy consumption and low energy efficiency
of traditional tungsten extraction process. The above inconvenience
then results in a market price of pure tungsten as high as $20,075 per
ton as of October 2008.4
Due to its high melting point, fabrication of tungsten products
(rods, wires, tubes, plates, etc) and tungsten alloys is always through
powder metallurgy (PM) process. The particle size and purity of tung-
sten powders are crucial to the PM process. According to the particle
size equation (equation 2), the particle size of tungsten powder pro-
duced from hydrogen reduction method is 4.5∼8.5 μm.2 It is too big
to meet the requirements for preparing high-performance tungsten-
In present study, we propose an unprecedented strategy on elec-
trolytic extraction of tungsten by utilizing low-temperature and short-
duration electrolysis of solid CaWO4 followed by a simple washing
process in the same melt at a higher temperature. Along with solubil-
ity tests of CaWO4 in molten salts, the correlation between electrol-
ysis parameters (such as electrolysis time, temperature and voltage)
and the residual CaWO4 in the products have been systematically
∗Electrochemical Society Active Member.
zE-mail: wangdh@whu.edu.cn
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Journal of The Electrochemical Society, 159 (6) E139-E143 (2012)
investigated, which proves the effectiveness of the proposed strategy
on obtaining pure and fine tungsten powder via direct electrochemical
reduction process in a more energy-saving manner.
Where s = 1, 2, 3, 4, 5, which is related to the temperature of 873,
923, 973, 1023 and 1073 K, respectively.
Materials characterization.— The dried products were character-
ized by powder X-ray diffraction (XRD) and scanning electron mi-
croscopy (SEM). XRD tests were performed at room temperature
(298 K) on a Shimadzu XRD-7000 X-ray diffractmeter (40 kV, 20 mA)
with a graphite monochromator, and nickel-filtered Cu Kα radiation (λ
= 1.5406 Å). The scan rate was 4 degree min−1. SEM analysis was
performed on an FEI sirion field emission gun scanning electron mi-
croscope equipped with a CCD camera.
Experimental
Experimental set-up, electrolyte and electrodes.— CaWO4 powder
(with purity higher than 99.9 wt%; purchased from Tianjin chemical
reagent company and used as received) was die-pressed using a pres-
sure of ∼7 MPa, and then the preformed pellet (weight: 2.0∼2.5 g;
diameter: 20 mm; height: 1.8∼2 mm) was annealed in air for 2 h at
923 K. The annealed pellet, which was first wrapped by foamed Ni
film and was tightened using Mo wire, was used as cathode assem-
bly in the electrolysis process. A eutectic CaCl2-NaCl (1:1 in molar
ratio) was contained in an alumina crucible (inner diameter: 75 mm;
height: 130 mm). The salt mixture was baked for 48 h at 523 K in
an open electric furnace (Wuhan electric furnace company) to remove
residual moisture. Following the drying process, the alumina crucible
contained with the salt was then placed on the bottom of a stainless
steel reactor which was placed in an electric furnace. The reactor was
then heated to 1123 K at a ramp of 4 K min−1 under the protection of
internal argon flow. The same ramp (4 K min−1) was also used to de-
crease temperature to desired reaction temperatures. A two-electrode
cell configuration was applied to both pre-electrolysis and electrolysis
processes. A graphite rod (15 mm in diameter and 200 mm in length)
connected to an iron rod (diameter: 5 mm) was employed as anode
(the iron rod was kept away from the melt). The pre-electrolysis was
performed for 4 h at a constant voltage of 2.7 V using foamed Ni film
(about 35mm × 10mm) which was attached to a Mo wire (diameter: 2
mm) as cathode. A schematic representation of similar electrochem-
ical cell was previously illustrated in Reference 11 and 12.
Cyclic voltammograms (CVs) of CaWO4 powder were recorded
using a computer-controlled electrochemical system (CHI440A,
Shanghai Chenhua, China). A CaWO4 powder modified Mo
electrode,12 a graphite rod, and a home-made mullite-sheathed high
temperature Ag/AgCl electrode were used as the working, counter,
and reference electrode, respectively. Such a three-electrode configu-
ration was also employed for the potentiostatic electrolysis of CaWO4
pellets. All presented potentials in current study are reported with
respect to the Ag/AgCl reference electrode.
Duration-dependent (0∼24 h), temperature-dependent (873
∼1023 K) and voltage-dependent (2.2∼3.1 V) electrolysis of CaWO4
pellets were carried out in parallel. In some selected experimental
runs, samples were cleaned by a molten salt washing process after
electrolysis. The detailed washing steps were: (1) stopping electroly-
sis after it being carried out for 10 h, (2) rising molten-salt temperature
to 1123 K at a rate of 5 K min−1, (3) holding this temperature for 1 h,
(4) lifting the sample above the molten salt, (5) after cooling in Ar
atmosphere for 10 min, taking the sample out of the reactor. All sam-
ples were first washed using distilled water and then collected after
centrifugal filtration and vacuum-dry at 323 K.
Results and Discussion
The solubility of CaWO4 in molten CaCl2–NaCl eutectic salt.—
Based on the phase diagram,13 the eutectic point of CaCl2-CaWO4 is
1008 K with a composition of 95 mol% CaCl2 and 5 mol% CaWO4.
Considering the melting point of CaCl2 (1057 K), it is impossible to
extract tungsten through the solid-to-solid electrochemical reduction
in pure molten CaCl2. The melting point of CaCl2-NaCl with a roughly
equal molar ratio of CaCl2 to NaCl is 780 K, making it a suitable
electrolyte compatible with solid CaWO4. However, the solubility of
CaWO4 in the CaCl2-NaCl eutectic is unclear so far. In this work, the
solubility of CaWO4 in molten CaCl2–NaCl eutectic salt at different
temperature was measured using the method described in section 2.2.
The solubility-temperature curve is shown in Fig. 1. As can be seen,
the solubility increases with temperature in the range between 873
and 1073 K. Below 1023 K, the solubility remains in small values
less than 0.2 wt%. While a sharp increase of solubility appears when
the temperature is increased from 1023 to 1073 K. The solubility
changes from 0.2 wt% at 1023 K to 0.8 wt% at 1073 K. To ensure a
predominant solid-to-solid reduction process, the electrolysis should
be performed at a temperature not higher than 1023 K. To optimize
the electrolysis temperature, electrolysis experiments were carried out
at different temperatures in the range between 873 and 1023 K.
CV tests and electrolytic reduction of CaWO4 pellet.— The SEM
images of CaWO4 pellet before and after electrolysis for 10 h at 3.1 V
at 1023 K are given in Fig. 2. It is revealed that a steep decrease in
particle size of the solid powder from ∼1 micron (in precursor) to
less than 200 nm (in product). Equation 2 indicates that the particle
size of tungsten powder prepared via hydrogen reduction method is
correlated with the particle size of precursory WO3, which means the
employment of fine WO3 feedstock is the prerequisite for the prepa-
ration of fine tungsten powder. In contrast to the traditional hydrogen
reduction method, the direct electrolytic reduction of solid CaWO4 is
capable of producing ultra-fine tungsten powder from relatively larger
tungsten oxide particles.
Given the fact that the molar volume of W (9.6 cm3) is only
∼20% of that of CaWO4 (47.5 cm3), it is conceivable that the
Solubility tests.— The solubility of CaWO4 in molten CaCl2–NaCl
at different temperature was measured by a series of static dissolution
experiments. 873, 923, 973, 1023 and 1073 K were selected as the
test temperature, respectively. During the test, CaWO4 pellets were
weighed and then wrapped by foamed Ni films and Mo wire. The
initial weights of CaWO4 pellets were denoted as As. The wrapped
CaWO4 pellets were immersed in the molten salt (weight: Wm) for
2 h at different temperatures (from low to high) and then taken out
from the melt. After being cleaned by distilled water, the residual
CaWO4 was weighed again, and the weights were denoted as Bs.
Correspondingly, the solubility (Ss) of CaWO4 in the salt at different
temperatures was calculated by following equation:
s
ꢀ
(Bs − As)
Figure 1. Solubility-temperature curve of CaWO4 in molten CaCl2–NaCl
eutectic.
i=1
Ss =
× 100%
[3]
Wm
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Journal of The Electrochemical Society, 159 (6) E139-E143 (2012)
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Figure 3. (a) first-cycle CVs of bare Mo electrode (dotted) and CaWO4 mod-
ified Mo electrode (solid) recorded in molten CaCl2-NaCl at 1023 K; (b) XRD
patterns of samples obtained from potentiostatic electrolysis of CaWO4 pellets
in molten CaCl2-NaCl at 1023 K at indicated conditions.
Figure 2. Typical SEM images of CaWO4 pellet (a) before and (b) after
electrolysis for 10 h at 3.1 V, 1023 K.
At anode : 1.5C + 3O2− − 6e = 1.5CO2(g)
and/or 3C + 3O2− − 6e = 3CO(g)
[5a]
[5b]
electro-reduction process will leave voids in the solid product while
the agglomeration of the produced fine tungsten particles will be sub-
stantially retarded due to the extremely high melting point of tungsten.
This synergetic effect will lead to the formation of ultrafine tungsten
powder with diameter less than 200 nm. In fact, unlike the reduction
of other metal oxides as described in previous investigations,12,14 in
which an oxide pellet was transformed to an integrated metal or alloy
pellet after electrolysis, it was found in this work that the CaWO4
pellet was transformed to a pile of tungsten powder. The genera-
tion of less-interconnected tungsten powder is in agreement with the
electro-generated Ta15 and Nb,16 which is believed to be a general
phenomenon for electro-generated refractory metals or alloys.
To further elucidate the electro-reduction path of solid CaWO4,
CV tests were carried out. Fig. 3a presents the first-cycle CVs of both
bare Mo electrode and CaWO4 modified Mo electrode recorded in
molten CaCl2-NaCl at 1023 K. As can be seen, the onset potential of
reduction appears at around −1.1 V with the formation of a current
shoulder within the potential range between −1.1 and −1.8 V (notated
as C1). At more negative potentials, large cathodic current forms at
−2.0 V, resulting in the appearance of peak C2 located at −2.2 V.
When further extending potential scan toward the negative potential
limit, the electro-reduction and/or co-electro-reduction of cations in
the melt bring about the formation of peak C3 which also emerges in
the CV of bare Mo electrode.
Overall reaction : CaW O4(s) + 1.5C
= W(s) + CaO(s) + 1.5CO2(g)
[6a]
[6b]
and/or CaW O4(s) + 3C = W(s) + CaO(s) + 3CO(g)
As depicted in the above reactions, the generation of solid tungsten
is accompanied by the formation of CaO. It was reported that the satu-
ration solubility of CaO in molten CaCl2 at 1123 K is ∼18.5 mol%.17
Although its solubility in currently used CaCl2-NaCl melts remains
unspecified, it is rational to speculate that the formed CaO would
dissolve into the melt until it is saturated with CaO under the experi-
mental condition. Such in-situ generated CaO can be electro-reduced
at potential more positive than that for salt decomposition, leading to
the formation of peak C2 in the negative-direction potential scan.
The above results indicate that the electro-reduction of CaWO4
to tungsten is a one-step reaction, implying that accurate control
of electrolysis potentials is not compulsory for tungsten extraction.
Therefore, constant-voltage electrolysis was utilized in the following
experiments.
Temperature-dependent constant-voltage electrolysis.—To elucidate
the influence of electrolysis temperature on the reduction rate, a series
of electrolysis were carried out in parallel at 873, 973 and 1023 K
using the same electrolysis duration (10 h) and voltage (3.1 V). The
XRD patterns of the electrolytic products are displayed in Fig. 4a. It is
obvious that more CaWO4 was reduced to tungsten metal with rising
electrolysis temperature, which means the reaction rate could be im-
proved by increasing electrolysis temperature. This phenomenon well
agreed with the common knowledge of the dependence of chemical
reaction rate on temperature. Therefore, the reaction temperature was
controlled at 1023 K for the following experiments.
Potentiostatic electrolysis of CaWO4 pellet was then performed,
aiming to specify the electro-reduction mechanism. The XRD patterns
of electrolytic products obtained from potentiostatic electrolysis at
−1.2, −1.5 and −1.8 V are displayed in Fig. 3b. It is found that
tungsten can be extracted at the above selected potentials with the
presence of unreduced CaWO4. It is then rational to conclude that the
formation of shoulder C1 is ascribed to the direct electro-reduction
of solid CaWO4 to solid tungsten. The electrolysis of CaWO4 can be
described by the following reactions:
At cathode : CaW O4(s) + 6e = W(s) + Ca2+ + 4O2−
[4]
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Journal of The Electrochemical Society, 159 (6) E139-E143 (2012)
Figure 5. XRD patterns of samples obtained from electrolysis for (a) 0 h;
(b) 2 h; (c) 5 h; (d) 8 h; (e) 10 h; (f) 20 h and (g) 24 h at 3.1 V, 1023 K.
the rate of electro-reduction of solid pellets in the depth direction
(penetration) becomes progressively slower as a result of the ris-
ing ohmic and concentration polarizations. The electro-reduction of
CaWO4 pellet starts at the surface of the pellet at a fast rate,18 resulting
in relatively high energy efficiency in the beginning of electrolysis.
After then, the electro-reduction propagates into the inner parts of the
pellet, where the longer oxygen diffusion path results in an increased
concentration polarization, together with a high iR drop originating
from the less-interconnected structure of generated fine particles. The
increased ohmic and concentration polarizations across the inner part
of pellets cause the barriers toward the complete electro-reduction of
the residual CaWO4 and then account for the low current efficiency
in the later stage of electrolysis. This is also the reason why it needs
24 h to obtain pure W product.
Figure 4. XRD patterns of samples obtained from (a) electrolysis at 3.1 V for
10 h at different temperatures and (b) electrolysis at 1023 K for 10 h under
different cell voltages.
Voltage-dependent constant-voltage electrolysis.—According to the
Gibbs free energy data at 1023 K, the decomposition voltages of
CaCl2, NaCl, and CaWO4 were calculated to be 3.2, 3.3 and 1.1 V, re-
spectively. In consideration of the inevitable iR drop and other kinetic
barriers, the electrolysis voltages were controlled in the range between
2.2 and 3.1 V. In the voltage-dependent electrolysis, the electrolysis
duration and temperature remained the same, i.e. 10 h and 1023 K,
respectively. Fig. 4b presents the XRD patterns of corresponding sam-
ples. As can be seen, with increasing electrolysis voltage from 2.2 V to
3.1V, theresidual CaWO4 after electrolysis decreases remarkably, ren-
dering the trend that the higher the electrolysis cell voltage, the faster
the electro-reduction of the pellet cathode. A reasonably high elec-
trolysis voltage is essential to overcome the iR drop caused by large
resistance of fine particles and loose structure of product (Fig. 2) and
to ensure a practically acceptable production rate. In this regard, the
electrolysis voltage was fixed at 3.1 V for the following experiments.
Duration-dependent constant-voltage electrolysis.—To obtain the de-
tailed information of the electro-reduction process of CaWO4 pel-
lets in molten salt and to optimize the current efficiency, the pellets
were electrolyzed for different durations (from 0 to 24 h) at 3.1 V,
1023 K. The corresponding XRD patterns of samples obtained from
the duration-dependent electrolysis are exhibited in Fig. 5. Accord-
ing to Fig. 5, the electro-reduction of CaWO4 pellet was enhanced
with prolonged reaction time. In detail, the enhancement of electro-
reduction against electrolysis time is much more significant during
the initial 10 h (see Fig. 5a–e), while only minor enhancement occurs
when further increasing the electrolysis duration from 10 to 24 h (see
Fig. 5e–g). In addition, pure W product cannot be obtained until the
electrolysis time lasts for more than 24 h, resulting in quite low cur-
rent and production efficiencies. Especially, the electro-reduction of
residual CaWO4 becomes sluggish after 10 h of electrolysis, which
deteriorates the overall energy efficiency. In addition, the current effi-
ciencies of electrolysis for 10 and 24 h were calculated to be 54.56 and
25.48%, respectively. Based on the three-phase interlines model,10,11
Removing residual CaWO4 by high temperature molten salt
washing.— As mentioned in section 3.2, some unreduced CaWO4
remained after electrolysis for 10 h at 3.1 V, 1023 K. The residual
CaWO4 can be fully reduced by prolonging electrolysis. However,
the further electrolysis will inevitably lead to lower current efficien-
cies. Alternatively, a more energy-saving process was proposed and
tested. After 10 h electrolysis of CaWO4 pellet at 3.1 V, 1023 K,
the electrolysis process was suspended and the temperature of molten
salt was then elevated to 1123 K. The electrochemically reduced
pellet was maintained in the same salt at this elevated temperature
for 1 h. According to the CaWO4 solubility test, solubility of solid
CaWO4 in the melt increases from 0.2 to 0.8 wt% with the increase
of temperature from 1023 to 1073 K. At 1123 K, the residual CaWO4
tends to dissolve into the melt quickly, leaving pure tungsten. The
XRD patterns of the samples produced from 10 h electrolysis and
the aforementioned washing process are exhibited in Fig. 6. As can
Figure 6. XRD patterns of the samples after the electrolysis for 10 h at 3.1 V,
1023 K without (a) or with (b) immersing the product in the same molten salt
at 1123 K for 1 h.
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Journal of The Electrochemical Society, 159 (6) E139-E143 (2012)
E143
Table I. Comparisons between two processes.
Electrolysis for 24 h
Electrolysis for 10 h followed by washing for 1 h
Mass of CaWO4 pellet
Theoretical mass of product
Obtained mass of product
Yield
1.9986 g
1.2768 g
1.2448 g
2.0005 g
1.2781 g
1.2142 g
97.49%
95.00%
Theoretical charge
Practical charge
Current efficiency
Energy consumption
1087.8 mAh
4268.6 mAh
25.48%
1061.1 mAh
1944.7 mAh
54.56%
10.63 Wh (g W)−1
4.965 Wh (g W)−1
be seen, after the simple washing process in the same melt at higher
temperature, residual CaWO4 was removed, resulting in the forma-
tion of pure tungsten powder. On the other hand, the dissolution of
CaWO4 from precursor to the melt will inevitably decrease the pro-
ductivity of tungsten. However, such a downgrade in yield is in-
significant in our proposed washing process, since vast majority of
precursory CaWO4 has already been reduced during the first 10 h
electrolysis.
scheelite (CaWO4) is expected to have a potential application in
industry.
Acknowledgments
The authors thank the NSFC (Grant Nos. 20873093, 51071112),
MOE of China (NCET-08-0416) and the Fundamental Research Funds
for Central Universities of China for financial support.
As evidenced in Fig. 5, pure tungsten product could also be ob-
tained after electrolysis for 24 h. Therefore, the two processes of
electrolytic extraction of pure tungsten were compared in terms of
yields, current efficiencies and electrolysis energy consumptions. As
summarized in Table I, when compared with 24 h electrolysis, al-
though a slight decrease in yield from 97.49 to 95.00% can be
observed, the energy efficiency of the proposed extraction process
significantly increases, with an improvement in current efficiency
from 25.48 to 54.56% and a decrease in energy consumption from
10.63 to 4.965 Wh (g W)−1. This analysis exhibits that, by us-
ing the washing process, the energy efficiency of tungsten extrac-
tion process is greatly enhanced without manifest compromise in
yield.
References
1. E. Lassner and W. D. Schubert, Tungsten: Properties, chemistry, technology of the
element, alloys, and chemical compounds, Kluwer Academic/Plenum Publishers,
New York, NY (1999).
2. S. W. H. Yih and C. T. Wang, Tungsten : sources, metallurgy, properties and appli-
cations, Plenum Press, New York, NY (1979).
3. K. B. shedd, Tungsten, in: U. S. G. Survey (Ed.), 2009.
4. “Metal Bulletin”. Retrieved on 2009-05-05 from http://www.mineralprices.com/.
5. S. Eroglu and T. Baykara, J. Mater. Process. Technol., 103, 288 (2000).
˙
6. M. Erdog˘an and I. Karakaya, Metall. Mater. Trans. B, 41, 798 (2010).
7. D. H. Wang, X. B. Jin, and G. Z. Chen, Annual Reports Section C (Physical Chem-
istry), 104, 189 (2008).
8. M. Ma, D. H. Wang, W. G. Wang, X. H. Hu, X. B. Jin, and G. Z. Chen, J. Alloys
Compd., 420, 37 (2006).
9. G. Z. Chen, D. J. Fray, and T. W. Farthing, Nature, 407, 361 (2000).
10. W. Xiao, X. B. Jin, Y. Deng, D. H. Wang, and G. Z. Chen, Chem.-Eur. J., 13, 604
(2007).
Conclusions
11. W. Xiao, X. B. Jin, Y. Deng, D. H. Wang, X. H. Hu, and G. Z. Chen, ChemPhysChem,
7, 1750 (2006).
12. D. H. Wang, G. H. Qiu, X. B. Jin, X. H. Hu, and G. Z. Chen, Angew. Chem.-Int. Edit.,
In summary, the solubility of CaWO4 in molten CaCl2-NaCl eu-
tectic at different temperature was measured. Based on the obser-
vation that the CaWO4 solubility steeply increases at temperature
higher than 1023 K, an unprecedented strategy on electrolytic ex-
traction of fine tungsten powder was proposed and demonstrated
by utilizing low-temperature and short-duration electrolysis of solid
CaWO4 followed by a simple washing process in the same melt but
at a higher temperature. The proposed process shows a remarkable
improvement in current efficiency and productivity and a vast de-
crease in energy consumption. Considering the reserves of tungsten
resources, direct electrochemical production of pure tungsten from
45, 2384 (2006).
13. U. Blechschmidt and R. Mo¨bius, Z. Anorg. Allg. Chem., 368, 181 (1969).
14. J. J. Peng, K. Jiang, W. Xiao, D. H. Wang, X. B. Jin, and G. Z. Chen, Chem. Mater.,
20, 7274 (2008).
15. T. Wu, X. B. Jin, W. Xiao, X. H. Hu, D. H. Wang, and G. Z. Chen, Chem. Mater., 19,
153 (2007).
16. T. Wu, W. Xiao, X. B. Jin, C. Liu, D. H. Wang, and G. Z. Chen, Phys. Chem. Chem.
Phys., 10, 1809 (2008).
17. D. A. Wenz, I. Johnson, and R. D. Wolson, J. Chem. Eng. Data, 14, 252 (1969).
18. Y. Deng, D. H. Wang, W. Xiao, X. B. Jin, X. H. Hu, and G. Z. Chen, J. Phys. Chem.
B, 109, 14043 (2005).
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