CARBOTHERMIC REDUCTION OF COPPER, NICKEL
891
–4
release, corresponding to the completion of the reaction
(d /d ) × 10
α
τ
and formation of the metal. Phase composition of reac-
tion products was determined by x-ray diffraction
(XRD) on a DRON-7 diffractometer with CuKα radia-
tion.
4
100
2
80
60
40
20
0
3
RESULTS AND DISCUSSION
The kinetics of reactions between oxides and carbon
during the reduction process can be limited by various
factors [6, 7]. In the absence of phase transitions and
dissociation, the reduction of most metal oxides can be
represented by a two-step scheme: in the first, slow
step, the surface of particles reacts with solid carbon; in
the second, faster step, the particle surface reacts with
the CO resulting from the oxidation of carbon at tem-
peratures above 750°ë. Solid-state reactions between
carbon and compounds having low vapor pressure
(NiO, CuO, Cu2O) are only active in the first step of the
reduction process. The forming barrier layer, consisting
of the lower oxide or metal, prevents direct contact
between the solid reactants. High reaction rates even in
the first step of the reduction process can be insured by
pairs of metal oxides that form via sublimation (MoO3,
WO3, V2O5) or dissociation (Cu2O, Ag2O, MnO2, ZnO).
1
4
400
500
600
700
800
900
1000
t,°C
Fig. 1. DTG curves for the carbon (50 wt % excess) reduc-
tion of oxides: (1) CuO (d = 3.7 µm), (2) (NiO (d = 5 µm),
(3) CoO (d = 4.6 µm); (4) MoO (d = 2.6 µm).
3
oxides. The accumulation of intermediate oxides leads
to a gradual decrease in reaction rate, and the DTG
curve shows a linear portion, corresponding to a negli-
gible reaction rate. Above 775°ë, diffusion limitations
are partially eliminated owing to the formation of a liq-
uid phase in the reaction zone through the melting of
åÓ4é11 and åÓ8é23. The second step of reduction
(maximum in reaction rate at 820°ë) is more active, and
its products include Mo and åÓ2ë. Since this tempera-
ture range corresponds to active CO evolution, the DTG
curve shows another maximum in reaction rate, at
910°ë, which is due to the excess carbon, like in the
case of NiO and CoO.
Figure 1 presents the TG data obtained at a constant
heating rate of the furnace of the thermoanalytical sys-
tem for reactions of carbon with CuO, CoO, NiO, and
åÓé3 powders preground in a planetary mill to roughly
the same particle size, d ꢀ 2–4 µm. The increase in
reaction rate starting at 380°ë (with a maximum at
525°ë) in the initial stages of the reduction of CuO to
the more stable oxide Cu2O (Fig. 1, curve 1) is essen-
tially insensitive to excess carbon and seems to be due
to the solid-state reaction on the surface of the oxide
particles. Raising the temperature increases Cu2O dis-
The onset of the reduction of molybdates in the
DTG curves in Fig. 2 (340°ë for CuMoO4, ꢀ520°ë for
NiMoO4, and ꢀ620°ë for CoMoO4) and the weight loss
rate correlate with the reactivity of the constituent
oxides, which decreases in the order CuO > NiO > CoO
(Fig. 1). In the initial stages of the processes under con-
sideration, the reduction rate notably decreases with
decreasing particle size, which seems to be due to the
reaction of the oxide surface with carbon. In the DTG
curves of NiMoO4 and CoMoO4, excess carbon
increases the peak at 950°ë, where the CO : CO2 ratio
is sufficient to displace the reaction equilibrium toward
CO formation. The formation sequence of reduced and
intermediate oxide phases in different temperature
ranges was studied in isothermal experiments (table).
sociation: according to reference data [8], log pO
=
2
−46.5 at 200°C and log pO = –8.8 at 500°C, which
insures the reduction of Cu22O to copper metal (with a
maximum in reaction rate at 625°ë).
The DTG curves for the reduction of NiO and CoO
(Fig. 1, curves 2, 3) show two weight loss regions. Over
the entire temperature range studied, the reduction
products are metals. The temperatures of the first step,
ꢀ630°ë for NiO and ꢀ730°ë for CoO, correlate well
with the thermodynamic data for the reactions of these
oxide with solid carbon and are attributable to contact
reduction on the surface of NiO and CoO. At higher
temperatures, the DTG curves of the processes under
consideration are indicative of more active reaction
owing to the presence of excess carbon. This is associ-
ated with the formation of CO gas at temperatures
above 750°ë, which takes part in the reduction process.
The reduction of ëuåÓO4 involves three consecu-
tive steps, evidenced by peaks in its DTG curve (Fig. 2,
curve 2). The reaction of ëuåÓO4 with carbon (peak at
405°ë) seems to be accompanied by detachment of
oxygens from divalent copper atoms, which are thus
partially reduced to univalent copper. This is due to the
The carbon reduction of åÓO3 (Fig. 1, curve 4)
involves two steps. In the range 460–620°ë (maximum
in reaction rate at 550°ë), we observe the formation of
åÓé2, which then reacts with åÓé3 to form MoO3 – ı
INORGANIC MATERIALS Vol. 44 No. 8 2008