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Dissolution of magnetite in acidic oxalate solutions
M.Taxiarchou, D.Panias, I.Douni, I.Paspaliaris and A. Kontopoulos
Synopsis
The dissolution of magnetite in acidic oxalate solutions
has been studied under various experimental condi-
tions. The effects of temperature, pH, oxalate and
ferrous ion concentration in solution were extensively
examined. The dissolution of the magnetite is highly
dependent on temperature and the pH of the solution,
but is practically independent of the oxalate concentra-
tion. The addition of ferrous ions in the solution has a
positive effect on the dissolution rate, especially at
lower temperatures and higher pH values.
Magnetite, along with hematite, is a common impurity in var-
ious industrial minerals and has to be partially removed to
meet market specifications. Its removal can be effected by
leaching in aqueous solutions of mineral or organic acids.
The dissolution of iron oxides, such as magnetite and
hematite, with organic acids comprises three steps:5 (1) acti-
vation of the solid surface, which includes the generation of
active centres and formation of surface complexes; (2) reduc-
tive dissolution of active centres, which is characterized by a
quite well-defined induction period-this step is accelerated
by ferrous ions, which are transferred from the solid to
the solution when bivalent iron occurs in the lattice; and
(3) autocatalytic reductive dissolution of active centres. Of
the various organic acids, oxalic acid has been found particu-
larly effective and is widely used for dissolution ofiron.2
The aim of the work presented here was to study the effect
of the most important operating parameters, such as tempera-
ture, pH of the solution, initial ferrous ion concentration ih
solution and oxalate concentration, on the dissolution of
magnetite in acidic oxalate solutions so as to provide the basis
for an industrial process suitable for the removal of iron from
industrial minerals.
Experimental apparatus
All experiments were carried out in a 500-ml glass reactor
that was equipped with a thermostatically controlled heating
mantle connected to a mercury contact thermometer and a
mechanical stirrer incorporating a speed controller and fitted
with a glass impeller. A pH-meter with combined electrode
and a glass condenser were fitted to the glass reactor.
Since magnetite dissolution is a photochemical process,6-8
the experiments had to be performed under controlled light
conditions. Therefore, an isolated box was constructed to
contain two 15-W white-light sources. The experimental
apparatus was placed in the box and all the experiments were
conducted under similar visible-light conditions.
Manuscript first received by the Institution of Mining and
Metallurgy on 9 October, 1996; revised manuscript received on 29
January, 1998. Paper published in Trans. Instn Min. Metall. (Sect. C:
Mineral Process. Extr. Metall.), 107, January-April 1998. ~ The
Institution of Mining and Metallurgy 1998.
Experimental procedure
Initially, 400 ml of buffer H2C204-K2C204 solution with a
constant total oxalate concentration and preadjusted pH
value was heated in the glass reactor at a preselected tempera-
ture. The solution was agitated at a speed of 600 rev/min.
During the heating period the buffer solution was purged
with argon to avoid the oxidation of ferrous ions by the
dissolved oxygen. Ferrous ions were added to the solution in
the form of pure ammonium iron(n)-sulphate-6-hydrate
(Merck, 99%). A preweighed amount of dry, chemically pure
(Heraeus, 99.5%) magnetite powder was then added to the
solution to create a suspension with pulp density of 0.022%
(wtlvol).
In each test the total and bivalent iron concentrations in
solution were measured as a function of time. Total iron
chemical analysis was carried out by flame atomic absorption
spectroscopy with the use of a Perkin Elmer 2100 atomic
absorption spectrophotometer and bivalent iron chemical
analysis was carried out using a Hitachi UIIOO spectro-
photometer with 1,10-phenanthroline as a complexing
agent.9
Experimental results
Effect of temperature
Tests were conducted at 30, 50, 60, 70 and 80°C at a con-
stant pH of 3 and a total oxalate concentration of 0.3 M. The
iron dissolution is plotted as a function of time for these
temperatures in Fig. 1; for purposes of comparison iron dis-
solution from hematite I0 at 80°C has also been included. The
rate of dissolution is seen to be highly dependent on tempera-
ture. At 80°C dissolution is complete in less than 2 h,
whereas at 30°C only 57% of the iron has been dissolved after
5 h. At 30°C, in contrast to its form for the higher tempera-
tures, the dissolution curve is slightly concave, indicating that
the reaction rate increases with time. This is typical behaviour
of an autocatalytic reaction.
Plots of ferrous ion concentration in solution versus time at
different temperatures are presented in Fig. 2, along with the
results for hematite dissolution. The curves of ferrous ion
50
100 150 200
Time, min
2SO
300
Fig. I Total iron dissolution as function of time for different tem-
peratures (pH 3; oxalate concentration, 0.3 M)
C37
100
90
80
.30'C
..
70
c50.C
.3
"
80 .60.C
50
.70.C
:;
Jt
40
A80.C
30
.80.C, hematite
20
10
250 3IJO
50
100 150 200
Time. min
Fig,2 Ferrous ion concentration in solution as function of time for
different temperatures (pH 3; oxalate concentration, 0.3 M)
generation as a function of time are similar to those of iron
dissolution, indicating the dependence of the dissolution rate
on the rate of ferrous ion generation in solution. At 30°C the
ferrous ion dissolution rate is quite low, whereas at tempera-
tures between 50 and 80°C it is high. This correlation
between the ferrous ion concentration in solution and the
magnetite dissolution rate reveals the catalytic effect of fer-
rous ions on the magnetite dissolution process.
The same correlation between ferrous ion concentration in
solution and dissolution rate has been observed in the case of
hematite dissolution in acidic oxalate solutions under similar
conditions (80°C; pH 3; oxalate concentration, 0.3 M; pulp
density, 0.022%).10 The hematite dissolution curve in Fig. 1
has a typical sigmoidal form and is characterized by an induc-
tion period of approximately 3 h, during which ferrous ions
are slowly generated in the solution.
In the case of magnetite the sigmoidal shape of the dissolu-
tion curves is absent, but at 30°C the dissolution curve is
slighdy concave. Magnetite is a spinel that contains ferrous
and ferric ions in its lattice. These ferrous ions are easily
transferred from the lattice to the solution, as recorded in Fig.
2, to eliminate the induction period and the sigmoidal shape
of the dissolution curve.
Effect of pH
To evaluate the effect of pH a series of tests was carried out at
60°C, a total oxalate concentration of 0.3 Mand at pH values
between 1 and 5 (Fig. 3). The dissolution rate of magnetite
was affected significandy by pH. It was highest at pH 3 and
decreased in solutions with higher or lower acidity. The iron
dissolution curves at the other pH values have the same form
with the exception of pH 5. At pH 5 the dissolution curve is
characterized by an induction period. This indicates that the
acidity of the solution plays an important role in the dissolu-
350
50
100 150 200
Time, min
3IJO250
Fig.3 Total iron dissolution as function of time for different values
of pH (60°C; oxalate concentration, 0.3 M)
C38
c:: =
3IJO
350
50 100
150 200
Time, min
250
Fig. 4 Ferrous ion concentration in solution as function of time for
different values of pH (60°C; oxalate concentration, 0.3 M)
tion process and reduction of the hydrogen ion concentration
in solution drastically inhibits this process.
The level of bivalent iron in solution in the same tests is
plotted against time in Fig. 4. At pH 2 and 3 the rate of fer-
rous ion generation is approximately the same. In less acidic
solutions (pH >4) the ferrous ion concentration is signifi-
candy lower and it decreases as the pH increases. At these
DO
:1
I
60
- I
.30'C
50
t.3IJ
1
a 50-<:
1
A
A60'C
.pHI
If
.70'C
e 3IJ
.pH 2
20
t..
x80'C
If
ApH 3
loti/A'
a 80'C, hematite
20
ApH4
I
apH5
10
1:1
A
A
.
g :\
.pH I
.. 60
.pH2
"0
1:i 50
ApH3
:;;
" 40
ApH4
""
3IJ
apH5
20
10
t
Fe2+
'"
60
"
'(j
tft.
40
20
0
0
1 2
3
pH
4 5 6
7
(a)
003;
003
002;
'"
"
002
'(j
"
""
'"
005
tft.
001
0005
0
0 1
2 3 4
5 6 7
(b)
pH
00003;
00003
00002;
'"
"
00002
1
00005
tft.
00001
000005
or'
I
'--,
4
(c)
pH
Fig. 5 Speciation of ferrous ions in oxalatesolution as function of
pH (25°C; oxalateconcentration, 0.1 M)ll
high pH values the ferrous ion generation curves register an
induction period, which lengthens as the pH increases.
The behaviour of magnetite dissolution at different pH
values is typical of tWo pH-dependent and competing
phenomena that occur simultaneously during the dissolution
process.
The pH of the solution is a determining parameter for the
speciation of ferrous ions in an oxalate solution, as seen in
Fig. 5. In the ferrous ions-oxalic acid system the species
Fez+, Fe(CZ04)~- and Fe(CZ04)j are present. In strongly
acidic solutions the ferrous ions remain uncomplex. As the
pH increases the concentration of the Fe(CZ04)~- complex
ion also increases and at pH >2.5 all ferrous ions are in the
form ofFe(CZ04)~-. The presence of Fe(CZ04)j and all the
hydroxo complexes of bivalent iron can be ignored in view of
their very low concentration at all pH values. II Of the prin-
cipal ions present, only Fe(CZ04)~- can be adsorbed on the
solid surface.
The tWo competing phenomena are the adsorption of
hydrogen ions and that of complex Fe(CZ04)~- ions on the
surface of magnetite particles. The first creates surface active
centres and the second accelerates dissolution through these
active centres. These tWo phenomena are intensified under
extreme experimental conditions.
In strongly acidic solutions the concentration of the
Fe(CZ04)~- ions in the solution is very low and the amount of
the complex adsorbed is very small. As the pH increases the
number of surface active centres decreases, since the number
depends on the hydrogen ion concentration in the leaching
solution, and most of the Fe(CZ04)~- ions are adsorbed on
non-activated magnetite sites. As a result, the dissolution of
magnetite is inhibited.IZ Dissolution is optimized when the
solution is a compromise betWeen those favourable for the
competing phenomena; this has been established experimen-
tally at around pH 3.
Effect ofFe2+ addition in solution
The foregoing tests have shown that iron dissolution follows
the rate of ferrous ion generation in solution. To study the
behaviour of magnetite dissolution when ferrous ions are ini-
tially added in the oxalate solution one test was performed
with the addition of 10 mgll Fez+ at 70°C, constant pH 3 and
a total oxalate concentration of 0.3 M. The results were com-
pared with those of a similar test without the addition of
ferrous ions. Iron dissolution as a function of time for the tWo
50
100
Time, min
150
200
Fig. 7 Ferrous ion concentration in solution as function of time with
and without addition of bivalent iron in solution (70°C; oxalate
concentration, 0.3 M): diamonds, without Fe2+; blank squares, with
10 ppm Fe2+
tests is shown in Fig. 6. At the temperature studied the addi-
tion of ferrous ions increases the dissolution rate only in the
initial stages of the process. As the reaction proceeds the dis-
solution rate with and without the addition of ferrous ions
becomes the same. Similar behaviour is observed for the gen-
eration offerrous ions in the tWo tests (Fig. 7). Generation of
ferrous ions in the solution takes place in the first stage of the
reaction, so the existence of ferrous ions in the initial solution
eliminates this stage and accelerates the dissolution process.
As a result the time required for the complete dissolution of
magnetite is decreased from approximately 3 to 2 h.
As seen in Figs. 1 and 2, at low temperatures the genera-
tion of ferrous ions and, consequently, the magnetite
dissolution are characterized by a prolonged induction
period. When all these data are compared it can be deduced
that the effect of ferrous ion addition in solution is probably
more significant at lower temperatures, at which the dissolu-
tion of the ferrous ions in the magnetite lattice is more
difficult.
Effect of total oxalate concentration
The effect oftotal oxalate concentration on the dissolution of
magnetite was studied through a series of tests carried out at
60°C, constant pH 3 and total oxalate concentrations
betWeen 0.1 and 0.3 M. In the system studied a buffer solu-
tion was used; the total oxalate concentration is therefore
the sum of the oxalate concentrations of all oxalate species
300
Fig. 6 Total iron dissolution as function of time with and without
addition of bivalent iron in solution (70°C; pH 3; oxalate concentra-
tion, 0.3 M): diamonds, without Fe2+; blank squares, with 10 ppm
Fe2+
50 100 150 200
Time, min
250
Fig.8 Total iron dissolution as function of time for different oxalate
concentrations (60°C; pH 3)
C39
100
90
80
=
70
0
.....
....
..e
60
0
'"
'" 50
.....
'0
40
30
20
10
0
0
60
50
40
-
t;o
e
30
+
"'G)
20
10
100
90
eo
0:1
70
.S:
.a
:1 17/ I 1.0.lM
.
aO.2M
..,
.. 0.3M
30
50 100
150
200
20
10
Time, min
60
50
40
~
C 30
+
.
""
~ 20
.O.lM
aO.2M
A O.3M
10
50 100 150
Time, min
200 250
Fig. 9 Ferrous ion concentration as function of time for different
total oxalate concentrations (60°C; pH 3)
existing in the system. Iron dissolution and ferrous ion con-
centration in the solution are plotted against time in Figs. 8
and 9, respectively. It is clear that this variable has practically
no effect on the dissolution of magnetite or the generation of
bivalent iron in the solution. At a concentration of 0.1 M the
dissolution rate is slightly lower, without significant effect on
the system. This can be attributed to the slightly decreased
rate of ferrous ion generation in solution at this temperature.
Discussion
The present stUdy has shown that the rate of magnetite disso-
lution depends on the dissolution rate of ferrous ions
contained in the magnetite lattice. The ferrous ion dissolution
is affected significantly by temperature and the pH of the
solution. At low temperatures (30°C) and high pH values
(>3) the dissolution rate increases with time, indicating that
the reaction proceeds via an autocatalytic mechanism.
Under the experimental conditions studied the addition of
10 mg/l Fe2+ to the initial solution had the same effect on the
magnetite dissolution rate as an increase in the temperature
from 70 to 80°C, reducing the time for complete dissolution
by 30% (Figs. I and 6). When all the above data are taken
into account it is probable that the effect of ferrous ion
addition to the initial solution is more significant at lower
temperatures and in less acidic solutions, where dissolution of
the ferrous ions of the magnetite lattice is more difficult.
The pH of the solution, besides affecting the rate of ferrous
ion generation in solution, governs the activation of the
magnetite surface with adsorbed hydrogen ions and the
adsorption of Fe(C204)~- complex ions on surface active
sites. It is, therefore, a very important parameter for the dis-
solution of magnetite.
Finally, the total oxalate concentration in the solution,
although it has almost no effect on the dissolution process,
affects the speciation of the oxalate species and the concen-
tration of Fe(C204)~- ions in solution, which, in tUrn,
influences the adsorption of Fe(C204)~- ions on surface
active sites. The rate of this dissolution process is decreased
when the total oxalate concentration is decreased below 0.2 M
(Fig. 8).
Conclusions
An investigation was carried out into the effects on the disso-
lution of magnetite in oxalate solutions of temperature, pH of
the solution, initial ferrous ion concentration in solution and
oxalate concentration-these being the most important
parameters.
Magnetite dissolution is accelerated by dissolution of the
ferrous ions of the magnetite lattice. The ferrous ion disso-
lution rate is affected significantly by temperatUre. As the
C40
c:..:
temperatUre increases the dissolution rate also increases. At
30°C the dissolution is characterized by an induction period,
which is eliminated at higher temperatures.
The pH of the solution is very important for magnetite
dissolution because it affects: (1) the rate of ferrous ion disso-
lution, which is optimal at pH 2-3 and decreases in solutions
of lower or higher acidity; (2) the stage of activation of the
magnetite surface with adsorbed hydrogen ions; and (3) the
stage of adsorption of Fe(C204)~- complex ions on surface
active sites. The pH is also a determining parameter for speci-
ation in oxalate solutions that contain ferrous ions. Since the
effect of pH on one leaching stage is opposite to its effect on
another, a compromise has to be reached. The dissolution of
magnetite proceeds most rapidly at pH 3.
In the case of magnetite the addition of bivalent iron in the
oxalate solution has a positive effect on the dissolution rate in
the initial stages of the process, but as the reaction proceeds
the dissolution rates with and without the addition of ferrous
ions become the same. The addition of ferrous ions in the ini-
tial solution eliminates the stage of ferrous ion generation in
solution and, therefore, accelerates the overall dissolution
process. The effect of ferrous ion addition is probably more
significant at lower temperatures or high pH values, at which
the dissolution of lattice ferrous ions is more difficult.
The total oxalate concentration in solution has no serious
effect on the dissolution process, although the interaction
between oxalate speciation, oxalate concentration and pH
has to be taken into consideration for the optimization of
magnetite dissolution.
Acknowledge~ent
The financial support of the European Commission within
the framework of the Brite-Euram II Programme (contract
no. BRE2-CT92-0215) is gratefully acknowledged.
References
I. Baumgartner E. et ai. Heterogeneous electron transfer as a path-
way in the dissolution of magnetite in oxalic acid solutions. Inorg.
Chern., 22, 1983, 2224-6.
2. Blesa M. A. et ai. Mechanism of dissolution of magnetite by
oxalic acid-ferrous ion solutions. Inorg. Chern., 26(22), 1987,
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5. Panias D. et ai. Mechanisms of dissolution of iron oxides in
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7. Parker C. A. and Hatchard C. G. Photodecomposition of com-
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Chern., 77,1973,2437-40.
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Authors
M. Taxiarchou is a mining and metallurgical engineer employed as
a senior researcher in the Laboratory of Metallurgy, National
Technical University of Athens (NTUA), where she gained her
Ph.D.
Address: Laboratory of Metallurgy, Department of Mining and
Metallurgical Engineering, National Technical University of Athens,
P.O. Box 640 56, 157 10 Zografos, Athens, Greece.
D. Panias is also a senior researcher of the Laboratory of
Metallurgy, NTUA, where he gained his Ph.D.
I. Douni is a researcher of the Laboratory of Metallurgy, National
Technical University of Athens, where she is undertaking work
towards a Ph.D.
I. Paspaliaris is a professor of the Laboratory of Metallurgy,
Department of Mining and Metallurgical Engineering, National
Technical University of Athens.
A. KontopouIos was a professor and director of the Laboratory of
Metallurgy, Department of Mining and Metallurgical Engineering,
National Technical University of Athens.
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