Reduction of WO3 to W-metal by mechanochemical reaction

Article abstract of DOI:10.1016/j.jallcom.2009.02.001

A new non-thermal route for reduction of tungsten oxide (WO₃) to metallic tungsten (W) by a milling operation in ammonia (NH₃) gas atmosphere in the presence of lithium nitride (Li₃N) is proposed in this paper. A sample of

Full text of DOI:10.1016/j.jallcom.2009.02.001

Journal of Alloys and Compounds 480 (2009) 666–669
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Reduction of WO3 to W-metal by mechanochemical reaction
Junya Kanoa,∗, Eiko Kobayashia, William Tongampb, Shoko Miyagia, Fumio Saitoa
a Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan
b Faculty of Engineering and Resource Science, Akita University, 1-1 Tegata-Gakuen cho, Akita 010-8502, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A new non-thermal route for reduction of tungsten oxide (WO3) to metallic tungsten (W) by a milling
operation in ammonia (NH3) gas atmosphere in the presence of lithium nitride (Li3N) is proposed in this
paper. A sample of WO3 was milled with Li3N under ammonia gas atmosphere in a planetary ball mill with
ZrO2 balls to induce mechanochemical (MC) reaction between the starting samples. Characterization of
milled product by X-ray diffraction (XRD) analysis confirms that W-metal could be obtained by the new
mechanochemical (MC) process within 1 h.
Received 18 December 2008
Received in revised form 29 January 2009
Accepted 1 February 2009
Available online 10 February 2009
Keywords:
Tungsten
© 2009 Elsevier B.V. All rights reserved.
Tungsten oxide
Lithium nitride
Milling
Mechanochemical reaction
1. Introduction
Application of mechanochemical effect by a milling operation
to induce mechanochemical effects such as phase transformation
Tungsten (W) is a rare metal, and with its unique properties
such as high melting point (3422 ◦C), excellent high-temperature
mechanical properties, high thermal and electrical conductivities,
high hardness, is used in a wide variety of commercial and indus-
trial uses as tungsten carbide, as alloy additive, as pure tungsten,
and as tungsten chemicals [1–3]. The chief economic minerals of
tungsten are wolframite ((Fe,Mn)WO4)) and sheelite (CaWO4) and
extraction of W-metal from those minerals is achieved through high
temperature and pressure processes followed by alkaline solution
leaching [2,3].
The grade of tungsten containing scrap was reported to be in
the range (40–95 wt.% W) as compared high grade raw material
containing (7–60 wt.% W) and about one-third of total demand
for tungsten in the world is supplied from scrap [2,4]. Recovery
of W-metal from scrap containing tungsten by alkaline solu-
tion/leaching process for useful industrial application is discussed
elsewhere [4,5]. Recovery of tungsten as WO3 from tungsten cat-
alysts in a reaction system containing an organic compound and
hydrogen peroxide was reported [6]. These methods require high
temperature/pressure, acid and/or basic solutions involving several
operational steps.
and solid state reaction is widely available in literature [7–13]. A
process involving milling to effect MC reaction between CaWO4 and
magnesium to obtain W-metal is also reported [14,15]. In this work
we have developed a new process involving MC reaction between
WO3 and Li3N under NH3 gas to obtain W-metal.
2. Experimental
2.1. Sample preparation and MC reaction
Tungsten oxide (WO3) and lithium nitride (Li3N), supplied by Wako Pure Chem-
ical Industries, Ltd., Japan, were used as starting materials. A planetary ball mill (P-7,
Fritsch, Germany), having a pair of ZrO2 mill pots charged with 24 × 10 mm balls in
each pot, was used for the milling of WO3 with Li3N in NH3 gas atmosphere. The
inner diameter and length of the mill pots are the same size (40 mm).
A sample mixture of 2.9 g WO3 (2.0 g) and Li3N (0.9 g) for a molar ratio of
(1:3 = WO3:Li3N) was charged into the mill pot and then the mill pot was set in
a container made of stainless steel (overpot) as shown in Fig. 1. The inner air in the
mill pots was degassed with a vacuum pump, and charged with NH3 gas at 0.8 MPa.
The mill pots were set at the mill device to run at 300 rpm for different times. The
milled product was removed from the pots and washed in water for 10 min to remove
soluble by-products and to recover W-metal. Overall schematic illustration of the
experimental procedure is shown in Fig. 2.
2.2. Characterization
X-ray powder diffraction (XRD) analysis using Rigaku, RINT-2200/PC system
with a Cu K␣ irradiation source (ꢀ = 1.5405 Å) at 40 kV and 20 mA in a continuous
scan mode between 10 and 60◦ in 2ꢁ was used to analyze solid products after milling.
Morphology of the milled mixture was observed by Scanning Electron Microscope
(SEM) equipment, S-4100L (Hitachi).
∗
Corresponding author. Tel.: +81 22 217 5137; fax: +81 22 217 5137.
E-mail address: kano@tagen.tohoku.ac.jp (J. Kano).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.02.001

J. Kano et al. / Journal of Alloys and Compounds 480 (2009) 666–669
667
Fig. 1. Illustration of ZrO2 pot covered with stainless steel pot.
3. Results and discussion
3.1. Mechanochemical (MC) reaction
A sample mixture of WO3/Li3N (1:3) milled in NH3 gas environ-
ment for 1 h to effect MC reaction between the starting materials to
reduce WO3 to W-metal is shown in Fig. 3 (XRD patterns). It could
be seen from the figure that, peaks of WO3 sample before milling (a)
were completely removed as a result of MC reaction and new dom-
inant peaks of W-metal appear in the milled product (b). Washing
milled product in water for 10 min allowed removal of water soluble
compounds such as LiOH·H2O formed during MC reaction leading
to recovery of pure W-metal (Fig. 3(c)). The overall mechanochem-
ically induced solid state reaction when WO3 is milled with Li3N
under NH3 gas environment could be represented by the reaction,
Eq. (1):
Fig. 3. XRD patterns of (a) WO3 sample before milling, (b) WO3/Li3N (1:3, mol/mol)
sample mixture after milling in NH3 gas environment for 1 h and (c) after washing
milled sample in (b) with water.
WO3 + Li3N + NH3 → W + 3LiOH + N2
WO3 + 2Li3N → W + 3Li2O + N2
(1)
(2)
From our previous experience [11–13], gaseous reagents such
as NH3 and N2 do not clearly enter in reaction; however, they
could create a reducing or non-oxidative environment which assists
the rate of reductive reaction. In order to investigate the reaction
mechanism between WO3 and Li3N under NH3 gas environment, a
Fig. 4. X-ray diffraction patterns of (a) WO3 sample before milling, (b) WO3 sample
milled in NH3 gas environment for 1 h and (c) WO3/Li3N (1:3) mixture milled in N2
gas environment for 1 h.
Fig. 2. Schematic illustration of the experimental procedure used to effect
mechanochemical reduction of WO3.

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J. Kano et al. / Journal of Alloys and Compounds 480 (2009) 666–669
Fig. 7. SEM image of WO3/Li3N (1:3) sample mixture milled in NH3 gas atmosphere
for 1 h at 300 rpm.
for 1 h. The figure clearly shows that milling of WO3 without Li3N
only shows a reduction of peak intensity as sample is transformed
to amorphous phase due to the milling impact, even in the pres-
ence of NH3. Milling of WO3 with Li3N under N2 gas environment
gives results similar to NH3 and the likely reaction between WO3
and Li3N can be represented by the reaction Eq. (2). All preceding
tests were conducted under NH3 gas environment.
Reaction under NH3 gas environment as given in Eq. (1) suggests
that 1 mol of each of the starting materials (WO3, Li3N and NH3)
is required to obtain W-metal. However, to increase reaction rate
for WO3 reduction to W-metal, excess amount of Li3N would be
required to induce MC reaction between starting materials during
milling. Fig. 5 shows effect of Li3N addition from 1 to 3 mol (1:1, 1:2
and 1:3 = WO3:Li3N) and NH3 moles constant at 1.9 mol (0.8 MPa).
The peak intensity of W-metal is similar at (1:1) and (1:2), however,
the peak intensity is twice stronger when Li3N addition is increased
to (1:3).
Fig. 5. XRD patterns of WO3/Li3N sample mixture of varying molar ratios milled in
NH3 gas environment for 1 h at 300 rpm and washed in water for 10 min.
sample of WO3 was milled first without Li3N but under NH3 gas
environment and another sample of WO3 was milled with Li3N
under N2 gas environment. Fig. 4 shows X-ray diffraction patterns of
(a) WO3 before milling, (b) WO3 milled in NH3 gas environment for
1 h, and (c) WO3/Li3N (1:3) mixture milled in N2 gas environment
A sample mixture of WO3/Li3N (1:3) was milled for 1 and
2 h respectively to investigate effect of reaction time and results
obtained is shown in Fig. 6. The peak intensity of sample mixture
milled for 1 h is twice stronger (Fig. 6(a)), than sample mixture
milled for 2 h (Fig. 6(b)). This suggests that MC reaction between
the starting materials is completed within 1 h of milling, and further
milling changes W-metal formed to amorphous phase.
Fig. 7 shows the SEM image of WO3/Li3N (1:3) sample milled
for 1 h under NH3 gas atmosphere and washed in water for 10 min.
W-metal of sub-micron size resulting from milling operation and
MC reaction could be seen from the SEM image.
4. Conclusion
A new non-thermal process for the reduction of WO3 to W-
metal was developed in this work. In the new process, a mixture
of WO3 and Li3N under NH3 or N2 atmosphere was subject to
milling in a planetary ball mill to effect MC reaction between
the starting materials. Characterization of the milled products by
XRD showed W-metal as the single dominant phase and subse-
quent washing of milled products after milling results in removal
of soluble LiOH·H2O leading to recovery of high purity W-metal.
The results showed that W-metal could be obtained by this pro-
cess within 1 h of milling WO3/Li3N (1:3) mixture at a moderate
mill speed of 300 rpm, followed by washing in water for 10 min.
The result also shows that a non-oxidative environment, as in this
case provided by NH3 or N2 gases allows the reductive reaction
between WO3 and Li3N to proceed fast to completion at faster
rate.
Fig. 6. XRD patterns of WO3/Li3N (1:3) sample mixtures milled in NH3 gas environ-
ment for different milling times and washed in water.

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669
This process could be applied to recovery of W-metal from tung-
sten bearing wastes, as it avoids high temperature, acidic solution,
and many operational steps.
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Acknowledgements
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The authors sincerely thank Mr. K. Yagi and Ms. A. Saito for their
assistance for conducting additional experiments.
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