Carbothermic reduction of cobalt and nickel tungstates

Article abstract of DOI:10.1134/S0020168506030174

Data are presented on the formation sequence of metallic, carbide, and intermediate oxide phases in the carbon reduction of CoWO₄ and NiWO₄. The surface reaction between CoWO₄ particles and solid carbon yields the mixed carbide Co₆W₆C, while the reaction with the CO originating from carbon vaporization yields the intermetallic phase Co₇W₆. The initial stage of the solid-state and gas-phase reduction of NiWO₄ yields a solid solution of tungsten in nickel (?10 at % W), Ni₄W, and, presumably, the NiWO₄-WO₃ eutectic. The solid solution reacts with carbon to form Ni₂W₄C and with CO to form filamentary tungsten crystals. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S0020168506030174

ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 3, pp. 310–315. © Pleiades Publishing, Inc., 2006.
Original Russian Text © N.V. Lebukhova, N.F. Karpovich, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 3, pp. 357–362.
Carbothermic Reduction of Cobalt and Nickel Tungstates
N. V. Lebukhova and N. F. Karpovich
Institute of Materials Science, Khabarovsk Scientific Center, Far East Division, Russian Academy of Sciences,
Tikhookeanskaya ul. 153, Khabarovsk, 680042 Russia
e-mail: LNV1@yandex.ru
Received September 26, 2005
Abstract—Data are presented on the formation sequence of metallic, carbide, and intermediate oxide phases
in the carbon reduction of CoWO and NiWO . The surface reaction between CoWO particles and solid car-
4
4
4
bon yields the mixed carbide Co W C, while the reaction with the CO originating from carbon vaporization
6
6
yields the intermetallic phase Co W . The initial stage of the solid-state and gas-phase reduction of NiWO
7
6
4
yields a solid solution of tungsten in nickel (~10 at % W), Ni W, and, presumably, the NiWO –WO eutectic.
4
4
3
The solid solution reacts with carbon to form Ni W C and with CO to form filamentary tungsten crystals.
2
4
DOI: 10.1134/S0020168506030174
INTRODUCTION
oxide phases in the carbothermic reduction of cobalt
and nickel tungstates at different temperatures.
Coreduction of mixtures of oxides, oxide solid solu-
tions, and chemical compounds is used in the extraction
of elements from slags and mineral associations, syn-
thesis of intermetallics, processing of slime waste, and
production of cermets [1–3]. In recent years, intense
scientific and technological interest has been centered
on the application of tungstate systems in the prepara-
tion of composite powders of hard tungsten alloys,
which are known to combine nanostructure with a
highly homogeneous distribution of tungsten carbide
and the metallic binder.
EXPERIMENTAL
CoWO and NiWO were prepared via coprecipita-
4
4
tion from equimolar solutions of divalent-metal nitrates
and ammonium paratungstate. The precipitates were
calcined in air, first at 600 and then at 1000°C for 2 h.
The resultant powders were investigated in the as-pre-
pared state or after grinding in a Petsch PM-400 plane-
tary ball mill. The particle size distribution was evalu-
The known processes for the synthesis of such com- ated using a Biolam M optical microscope (1000×) and
posites [4, 5] involve two consecutive steps: hydrogen Analyzette 22 Comfort laser analyzer. The average par-
reduction of mechanically activated oxide mixtures to ticle diameter was determined as
metals and carbonizing of tungsten with carbon or car-
bon-containing gases. One promising way to ensure
d =
d n / ni,
i i
structural homogeneity of such mixed-phase materials
∑
∑
is to use tungsten oxide compounds, e.g., CoWO and
i
i
4
NiWO , as starting reagents. The preparation process
4
can be reduced to one heat-treatment step by using car- where n is the number of particles examined and i is the
bon as both a reductant and carbonizing agent.
number of size ranges. The reductants used were car-
bon black (RF Standard GOST 12222-78) and CO. In
our preparations, we used stoichiometric (1 : 1) mix-
tures to obtain metals and also mixtures containing up
to 50% excess carbon. Reduction was carried out in
flowing argon during heating at a rate of 5°C/min or
under quasi-isothermal conditions in the furnace of a
MOM Q-1000 Paulik–Paulik–Erdey thermoanalytical
system and also in an SNOL 0.2/1250 programmed
tubular furnace in flowing CO. The gas flow rate was
Despite the wide use and practical importance of
reactions between metal oxides and solid carbon, such
reactions have been studied in less detail than reduction
with gases. There is only limited information regarding
the phase formation sequence in the reduction of cobalt
and nickel tungstates with carbon or natural gases [6–8].
Hydrogen reduction processes have not yet been inves-
tigated in detail. Data on the conditions of carbothermic
reactions, the role of CO, and the distribution of metals
over intermediate oxide phases during the reaction are
missing.
1
5 l/h in both cases. The extent of reduction, α, was
evaluated from thermogravimetric (TG) curves as the
ratio of the weight loss from the sample to the maxi-
The purpose of this work was to establish the forma- mum possible gas release, corresponding to the com-
tion sequence of metallic, carbide, and intermediate pletion of the reaction and formation of the metal.
3
10

CARBOTHERMIC REDUCTION OF COBALT AND NICKEL TUNGSTATES
140
311
Phase composition of reaction products was deter-
mined by x-ray diffraction on a DRON-3M powder dif-
(a)
1
1
20
00
fractometer with CuK radiation. The particle size,
1'
α
morphology, and elemental composition of reduction
products were determined by scanning electron micros-
copy (EVO 40 and LEO-1420 instruments, the latter
equipped with a Rontec energy-dispersive x-ray spec-
trometer). The metal composition was also refined
using a JEOL 35-SDS wavelength-dispersive spec-
trometer. Structural parameters of tungsten single crys-
tals were determined by transmission electron micros-
copy (UMV-100K instrument). In thermodynamic cal-
8
0
60
1
4
2
0
0
0
2
0
culations of the Gibbs energy ∆G (T), we used standard
techniques [9, 10].
140
(b)
1
1
20
00
80
60
40
1
'
RESULTS AND DISCUSSION
1
2
According to the DTG curves recorded during the
carbothermic reduction of CoWO and NiWO (Fig. 1)
4
4
at a constant heating rate, there are several steps of
active reaction. The initial portions of the curves are
attributable to surface reaction. The reduction onset
temperatures (~700°ë for NiWO and ~750°ë for
4
CoWO ) agree well with thermodynamic estimates for
4
2
0
0
the reduction of the tungstates with solid carbon to Ni
and Co. In addition, the reduction rate rises notably
with decreasing particle size (Fig. 1, curves 1, 2), and
excess carbon (curves 1') has no effect on the reaction
rate in this step. In contrast, the maximum around
650 700 750 800 850 900 950 1000
t, °C
9
50°C is notably higher in the presence of excess
carbon.
The DTG curves for the reduction of coarse CoWO4
Fig. 1. DTG curves of the carbon reduction of (a) CoWO4
d = (1) 14.7 and (2) 2.7 µm) and (b) NiWO (d = (1) 11.7
(
4
and (2) 1.6 µm); (1') excess of carbon.
(
(
d = 14.7 µm) and NiWO (d = 11.7 µm) particles
Fig. 1, curves 1) are very similar in shape. The process
4
formation of Ni1 – xW is accompanied by an increase in
lattice parameter from that of Ni (2θ = 44.505°) to the
value corresponding to a solid solution containing
seems to involve two steps and to follow mechanisms
typical of oxide systems stable to vaporization and dis-
sociation [11]. The first step is surface reaction between
the tungstate and carbon, which yields reduced phases
x
1
0 at % W (2θ = 44.03°) [12]. Electron micrographs of
the reduction products (Fig. 2b) demonstrate that tung-
sten is formed within parent tungstate particles. Tung-
sten single crystals are anisometric (about 2 µm in
length and ≤0.3 µm in diameter), in contrast to the
rounded particles of an Ni-rich phase (up to 6 µm in
diameter).
and CO . The reaction rate in this step is limited by
2
reactant diffusion through the layer of the reaction
products. Above 750°C, the reaction rate rises signifi-
cantly owing to the reaction between CO and carbon
2
with the formation and constant regeneration of CO, an
active gaseous reductant. The reduced phases resulting
from the reactions between the tungstates and carbon
differ in composition. During heating to 1000°C at a
The DTG curves of ground CoWO (d = 2.7 µm)
4
and NiWO (d = 1.6 µm) samples differ markedly in
4
constant rate, CoWO reacts with carbon to form
4
shape (Fig. 1, curves 2). The decrease in the particle
size of cobalt tungstate produces no significant changes
in the shape of the DTG curve of the phase composition
of the reduction products but increases the rate of the
surface reaction owing to the increase in reaction sur-
face. The decrease in the particle size of nickel tung-
state notably shifts its DTG curve to lower temperatures
Co W C and the intermetallic phase Co W . The
6
6
7
6
reduced phases consist of rounded particles (Fig. 2a) no
greater than 0.5 µm in diameter. Subsequent heat treat-
ment at 1100°C leads to the known sequence of trans-
formations [8]: ëÓ W ë
ëÓ W ë
ëÓ +
6
6
3
3
W ë + WC.
2
The carbon reduction of NiWO leads to the forma- and gives rise to three maxima in reaction rate, at 750,
4
tion of Ni1 – xW (solid solutions of tungsten in nickel), 800, and 850°C. In addition to Ni W, m-W, and
x
4
Ni W, and tungsten single crystals (m-W) (with reflec- Ni1 − xW , the reduction products contain the mixed car-
4
x
tions corresponding to the 〈111〉 growth direction). The bide Ni W C. To establish the formation sequence of
2
4
INORGANIC MATERIALS Vol. 42 No. 3 2006

3
12
LEBUKHOVA, KARPOVICH
1
µm
(
a)
2
µm
(
b)
Fig. 2. Scanning electron micrographs of powders obtained by the carbothermic reduction of (a) CoWO and (b) NiWO in argon.
4
4
reduced and intermediate oxide phases in different tem- highest rate at 750°C). The release of nickel from the
perature ranges, we carried out a number of isothermal tungstate occurs probably more rapidly than that of
reduction experiments (table).
cobalt, which leads to a selective character of the pro-
cess and the formation of a tungsten oxide and NiWO4–
Just above 700°C, the thermodynamically plausible
WO solid solution. In the temperature range under
3
reactions of CoWO and NiWO with carbon are those
4
4
consideration, WO may react with W C and WC to
3
2
leading to the formation of Co, Ni, W, W C, WC,
2
form the intermediate oxides WO , WO2.72, and WO2,
2
.9
WO , WO , and WO . The reduction rates of
2
.9
2.72
2
as evidenced by the fact that no carbide phases were
detected at 700°C and by the formation of the interme-
diate oxide WO2.72 (table). The Ni : W ratio in Ni W C,
CoWO and NiWO differ markedly at these tempera-
4
4
tures (Fig. 1). The gradual rise in the rate of reaction
2
4
between CoWO and carbon in the initial stage of the
4
which appears near 780°C (second maximum in reac-
process and the absence of metal oxides among the
reduction products (table) indicate that cobalt and tung-
sten are released from the tungstate with comparable
rates. It seems likely that the forming tungsten carbides
dissolve cobalt atoms. The Co : W atomic ratio in
tion rate), is close to that in the NiWO –WO eutectic
4
3
(
73 mol % WO ) [13].
3
The carbon reduction of the Ni-deficient solid solu-
tion may also be an isostoichiometric process, similar
to the reduction of cobalt tungstate. To verify this
Co W C is the same as in the parent tungstate, which is
6
6
also favorable for the isostoichiometric reduction of assumption, we prepared an NiWO –Wo solid solution
4
3
CoWO . The formation of the mixed carbide Ni W C
4
2
4
via coprecipitation from nickel nitrate and ammonium
through the surface reaction between NiWO and car- paratungstate solutions (in the ratio 0.3 : 0.7), followed
bon is preceded by the formation of Ni1 – xW (with the by heat treatment at 600 and 1000°C. Indeed, the car-
4
x
INORGANIC MATERIALS Vol. 42 No. 3 2006

CARBOTHERMIC REDUCTION OF COBALT AND NICKEL TUNGSTATES
313
(
a)
2
µm
(
b)
Fig. 3. Transmission electron micrographs of filamentary tungsten crystals grown through the reduction of NiWO in flowing CO;
4
(a, b) different magnifications. Inset: selected area electron diffraction pattern.
bon reduction of that sample at 740°C for 2 h yielded tungsten oxide in the reaction zone. The Gibbs energy
Ni W C.
of the reaction between WO and CO is close to zero
2
4
3
and, below 900°C, only intermediate tungsten oxides
can form [14]. Nevertheless, in the temperature range
of the maximum in question, the x-ray diffraction pat-
terns of samples heated to 800°C show well-defined
reflections from single-crystal tungsten (table).
Increasing the temperature leads to the formation of
Co W , Ni W, and WO (table), presumably owing to
7
6
4
3
carbon vaporization and CO formation. Nickel and
cobalt tungstates differ significantly in reactivity with
CO. At the temperatures under consideration, the reac-
tion between CoWO and CO yielding both tungsten
4
The reduction of NiWO with flowing CO was
4
0
0
(
∆G (T) > 0) and cobalt (∆G (T) ~ 0) is thermodynam- found to be accompanied by active growth of filamen-
ically hindered. Above 950°C, carbon monoxide is the tary tungsten crystals. This process took place starting
main equilibrium vapor species, which correlates with
the maximum in the rate curves (Fig. 1a) in this temper-
ature range. With increasing carbon content, the maxi-
at 850°C. The reaction products were m-W, Ni,
Ni1 − xW , Ni W, WO , WO , and WO2.72. A selected
x
4
3
2.9
area electron diffraction pattern of tungsten crystals
showed a bcc structure with a lattice parameter of
3.16 Å (Fig. 3a, inset). The crystals had a hexagonal
mum grows. In contrast to that of CoWO , the reduc-
4
tion of NiWO with CO gas is thermodynamically plau-
4
0
sible (∆G (T) < 0), and a maximum in reaction rate cross section, with the 〈111〉 growth direction, and were
appears at a lower temperature of 850°C (Fig. 1b). As 10 to 50 µm in length, with a constant diameter of up to
mentioned above, selective nickel liberation from the 0.2–0.3 µm. The tungsten content of the crystals was
crystal lattice of NiWO may lead to the formation of 99.75–99.89 wt %. Nickel (about 0.3 wt %) was pre-
4
INORGANIC MATERIALS Vol. 42 No. 3 2006

3
14
LEBUKHOVA, KARPOVICH
Phase composition of powders obtained by the carbothermic reduction of CoWO (d = 2.7 µm) and NiWO (d = 1.6 µm)
4
4
t, °C
τ, h
α
Oxide phases
Reduced phases
NiWO + C (1 : 1)
4
7
7
7
8
8
00
30
50
20
80
3
3
3
2
2
2
0.14
0.39
0.66
0.76
0.77
0.98
NiWO , WO
Ni1 – xWx
Ni W C, Ni1 – xWx
4
2.72
2.72
NiWO , WO
4
2
4
WO2.72
Ni W C, Ni1 – xWx
2 4
WO2.72, WO2
WO2
Ni W C, Ni W , Ni W
2 4 1 – x x 4
Ni W C, Ni W, m-W, Ni1 – xWx
2
4
4
1
000
Ni W C, Ni W, m-W, Ni1 – xWx
2 4 4
CoWO + C (1 : 1)
4
7
7
8
8
50
80
20
80
4
3
2
2
2
0.27
0.31
0.55
0.89
0.99
CoWO4
CoWO4
CoWO4
Co W C
6 6
Co W C
6
6
Co W C
6
6
CoWO , WO
Co W C, Co W
6 6 7
4
3
6
1
000
WO2
Co W C, Co W
6 6 7
6
sumably concentrated on the faces of the crystals. The solution reacts with carbon to form Ni W C and with
2
4
nickel content of the rounded particles was 89.82 wt %, CO to form tungsten single crystals. The reduction of
and the tungsten content was 10.18 wt %. We observed NiWO with flowing CO leads to the growth of filamen-
4
both isolated crystals and intergrowths of five to ten
crystals. It seems likely that tungsten crystals nucleated
within tungstate particles. At the back end of the crys-
tals, one can see melt film (Fig. 3b). The composition
of the low-melting-point phase is not yet clear. The
tary tungsten crystals 10 to 50 µm in length, with a con-
stant diameter of up to 0.2–0.3 µm.
The reduction kinetics of NiWO and the phase
4
composition of the reaction products are shown to
depend on the particle size of the tungstate, which can
be understood in terms of a selective reduction mecha-
melting point of NiWO is 1420°C, and that of the
4
NiWO –WO eutectic is 1245°C [13]. It seems likely
4
3
nism. In the case of fine NiWO particles (d = 1.6 µm),
owing to the large reaction surface Ni1 – xW forms rap-
idly at temperatures below 750°C, insufficient for car-
bon vaporization. In the absence of CO, the resulting
oxide solid solution reacts with carbon. In the case of
that the formation of lower melting point carbonate sys-
4
tems in the presence of CO reduces the melting point
x
2
of the eutectic. Tungsten may then form through melt
saturation with CO gas. This reduction process ceases
as a result of changes in the composition of the tung-
state system and melt solidification.
coarser particles (d = 11.7 µm), the conversion neces-
sary for the formation of the eutectic is only achieved
above ~900°ë, where CO is formed actively. It seems
likely that, under such conditions, the reaction with the
gaseous reductant prevails.
CONCLUSIONS
The present results demonstrate that the reduction of
NiWO and CoWO with carbon begins as a surface
4
4
reaction at 700 and ~750°ë, respectively. At higher
temperatures, the process is accompanied by carbon
vaporization and formation of CO gas, which then par-
ticipates in the reduction of the tungstate.
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