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Sulfation Mechanism of CoO and NiO
Chapter · January 1977
DOI: 10.1007/978-1-4684-2340-2_92
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From:
REACTIVITY OF SOLIDS
Edited by John Wood, O liver Lindgvist, Claes Helgessoi(
and Niis-Gosta Vannerberg
(Plenum Press, 1977)
SULFATION MECHANISM OF CoO AND NiO
L.E.K. Holappa and M.H. Tikkanen
Dr. Tech., Research Center,Ovako Group,Imatra, and
Prof.,Dept. of Metallurgy,Helsinki University of
Technology,Otaniemi,Finland
INTRODUGriON
Sulfatizing roasting is used when separating small contents
of valuable metals from sulfide and composite ores. In sulfatizing roasting the desired constituents are selectively converted
to water soluble sulfates, whilst other constituents such as iron
remain as insoluble oxides. In Finland the sulfatizing roasting
process has been used at the Outokumpu Oy Kokkola Works for the
extraction of cobalt and nickel from roasted calcines and pyritepyrrhotite concentrates since 1967 (1).
The thermodynamics of the sulfation reactions of cobalt and
are well known (3,4,5,6). There is a difference
=tween the sulfation of CoO and NiO since CoO is in equilibrium
with the sulfate only in a specific range of temperature and gas
composition. At normal sulfatizing temperatures the stable oxide
is Co304 and owing to this an intermediate layer of Co 0 is formed
during the sulfation of CoO. In the case of NiO the fotrn~tion of
such an intermediate phase is not possible and NiS04 is formed
directly on the NiO surface.
~ickel oxides
Both laboratory and industrial sulfation has shown that the
sulfation of NiO is much slower than that of CoO and the recovery of Ni also remains low. In this paper the mechanism of
sulfation of cobalt and nickel oxides has been discussed based
on earlier work on the sulfation kinetics and mechanism (2).
635
636
L.E.K. HOLAPPA AND M.H. TIKKANEN
EXPERTIVIENT AL
Dense cobalt oxide samples were prepared by oxidising 0.10 em
thick Sherritt Gordon cobalt sheet (99.9 pet Co, 0.05 Ni, 0.003 Cu,
0.014 Fe, 0.003 S, 0.008 C) at l2000C in air for 14 days. The
specimens were then cooled, ground and polished. Dense nickel oxide
samples were prepared by oxidising 0.02 em thick Outokumpu cathode
nickel at 1250oc in air for 14 days. NiO powder samples were made
by oxidising Mond nickel Grade A (0.07-0.10 pet C, 0.15-0.20 0,
0.044-0.01 Fe, residue Ni) in air at 4oooc for a day followed by
seven days at lOoooc. The powder was then ground and the NiOcompacts were made in a 3 cm2 steel form and then sintered at
1250oc in air for a week. The density obtained was 4.85 g/cm3,
i.e. a relative density of 65 pet.
The sulfation apparatus and gas system have been described
~
elsewhere (2). The growth mechanisms of CoS04 and NiS04 as well as
of Co304 on CoO were studied using gold marker experiments (2).
RESULTS AND DISCUSSION
Sulfation experiments with dense CoO blocks in the temperature range 68o-8850C showed parabolic kinetics. Trials with dense
NiO blocks showed that the sulfation rate was very low. At 750oc
and with a so3 partial pressure of 0.255 the parabolic rate
constant was of the order of 2-4·10-3 mg2/cm4·h, this being about
a hundred times lower than that for CoO (0.4 mg2/cm4·h). When
sintered NiO canpacts were sulfatized the simple parabolic form
was not valid during the initial period since the surface became
covered with a thin sulfate layer. Later when the layer grew dense
and uniform, parabolic kinetics were again found.
The temperature and S03 partial pressure dependence of the
sulfation reactions showed that the sulfation rate is proportional
to the sulfating potential gradient across the sulfate layer. The
activation energies were calculated using a modified Arrhenius
~
equation the values being 28.1 and 19.3 kcal/mole for CoS04 and
NiS04, respectively.
The results from the marker experiments are shown in Fig. l,
2a and 2b. In Fig. l the CoO specimen was oxidised to form a Co304
layer. The gold markers initially situated at the surface of the
CoO were located 15-20 pet inside the Co304 layer. The recorder
traces in Figs. 2a and 2b show that the gold markers, which were
initially located at the surfaces of the CoO and NiO specimens,
can be detected after the sulfation at the CoS04/Co304 and NiS04/
NiO interfaces, respectively.
The sulfation of NiO occurs without any oxidation process
SULFATION MECHANISM OF CoO and NiO
637
Au
Fig. l. Position of gold markers in Co304 layer formed on CoO
at 86ooc in air after 14 days.
since the higher oxide is not stable in the considered conditions.
The S03 potential gradient across the sulfate layer causes sulfation to proceed. The marker experiments showed the sulfate
growth outwards; the proposed mechani~ is the outward diffusion
of Ni2+ and o2- ions through the sulfate layer. The same mechanism
hi
I
I
l . . . ,. . . ,. ., . ,. . ,. ,.-..·-"".· "'·' , ,. . . ~._.-.,' . .~
Fig. 2. Micro-analysis showing the position of gold markers in
the sulfation experiments a) after CoO sulfation 3 days at 68ooc,
b) after NiO sulfation 7 days at 68ooc.
L.E.K. HOLAPPA AND M.H. TIKKANEN
638
has been proposed earlier ( 4) but the inward growth of the
sulfate layer has also been suggested (8).
In the case of CoO the intermediate phase Co304 is formed.
The results of the marker experiments in Fig. l can be-explained
by the simultaneous insi0e and outside growth of Co304 caused by
the outward diffusion of Co-ions. The proposed mechanism is
shown in Fig. 3. The following reaction can be presumed at the CoO/
Co304 interface:
l2Co0 = [3co2+ + 6e-J + 6co3+ + 3Co2+ + 1202- + 3 Dco2+
(l)
The species shown in the brackets diffuse through the Co304 scale
to form new co 04 at the oxide/gas interface II as follows:
3
(2) ~
{3Co2+ + 6e-}+ 202 = Co304
The remaining species at the interface I precipitate as Co304:
(3)
6co3+ + 3Co2+ + 1202- + 3 o c 0 2+ = 3Co304
This is the same reaction as for the precipitation of defects in
nonstoichiometric Co1-xO. Owing to the reactions (1) .. (3), when one
Co304 is formed at the outer surface, three Co304 is formed at the
CoO/Co304 interface. The calculation is for stoichiometric CoO . ftrl
analogous mechanism has been proposed for the growth of Fe304 (9,
10). On the other hand, inward diffusion of oxygen has been suggested as the growth mechanism for Co304 (7).
Marker experiments showed CoS04 to grow outwards. By combining
these two mechanisms a model for oxidation-sulfation reactions has
been suggested in Fig. 4. Without any sulfating agent the pure
oxidation reaction occurs. When small contents of S03 are present,
!.
I
TI
!.
!.
coa
Co 1 a4 1=Cost~ot•
te-1 .. ,
te'•
Sl,+
tlttltlll
h-
1/ZI,
co,a, ~coso,
...
JCt'•
u'-
Fig. 3. Proposed mechanism for
the growth of co 3o4 on CoO.
Fig. 4. Proposed mechanism for
the growth of CoS04.
SULFATION MECHANISM OF CoO and NiO
639
the growth of CoS04 starts. When the sulfation potential is low
the sulfation rate is slow compared to the oxidation rate and some
inward migration of oxygen occurs through the sulfate layer resulting in the Co304 growth. The growth of CoS04 occurs by the outward diffusion of Co-ions balanced with electrons (Fig. 4a).
When the S03 and oxygen potential are both high, the conditions
for sulfation and oxidation are roughly equal and the reaction rates can also be assumed equal (Fig. 4b). Sulfate formation, likewise,
consumes Co-ions liberated in the CoO--Co304 transformation. Oxidation can be understood as the decrease of cations in the CoO core
which thus gradually converts to Co304.
When pure Co304 is sulfated, Co3+ ions must be reduced and
oxygen liberated (Fig. 4c). The sulfation rate, however, is in~ dependent of the base oxide and the oxidation reaction (2).
According to this and the discussion above it is suggested that
the oxygen ion diffusion in the sulfate layer occurs freely, the
removal of cobalt ions from the outer surface of Co304 is relatively easy and the rate-determining factor in sulfation is the
diffusion of cobalt ions through the sulfate layer. In the case of
NiO the removal of Ni-ions from the NiO/NiS04 interface seems to be
difficult because of the great stability and few defects in the Niolattice. This is assumed to be the explanation for the slow sulfation
of dense NiO.
SUMMARY
Kinetic measurements and gold marker experiments showed both
coba~t sulfate and nickel sulfate grow outwards on the initial oxi-
de Slll'faces. The much higher sulfation rate of CoO compared to NiO
is explained by the formation of an intermediate Co304 layer and
different oxide properties. Oxidation of CoO to Co304 is proposed
to occur by cobalt ion diffusion outwards this causing simultaneous
inward and outward growth of the Co304 layer.
~
REFERENCES
1 Palperi,M., Aaltonen,O., J.Metals, Febr.(l971), p. 34-38.
2 Hola~pa,L.E.K., Dissertation, Helsinki Univ. Tech. (1970).
3 Evans,W.H., Wagman,D.D., J.Res.Natn.Bur.Stand. 49(1952), p. 141.
4 Ingraham,T.R., Marier,P., Trans.A.I.M.E. 236(1966), p.l064-1067.
5 Ingraham,T.R., Can.Met.Quart. 3(1964), p. 221-234.
6 Kellogg,H.H., Trans.A.I.M.E. 230(1964), p. 1622-1634.
7 Hocking,M.G., Diss.Univ. London, (1962).
8 Wright,J., Pigott,J.R., Corros.Sci.9(1969),p.l21-122,Appendix 2.
9 Davies,M.H. et al, Trans.A.I.M.E. 191(1951), p. 889-896.
10 Hauffe,K., Reaktionen in und an festen Stoffen, Berlin (1955).
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