a comparative study of leaching kinetics of limonitic laterite and
TRANSCRIPT
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Hydrometallurgy 72 (2004) 59–72
A comparative study of leaching kinetics of limonitic laterite and
synthetic iron oxides in sulfuric acid containing sulfur dioxide
G. Senanayake*, G.K. Das
A.J. Parker Cooperative Research Center for Hydrometallurgy, Department of Mineral Science and Extractive Metallurgy,
Murdoch University, Perth, WA 6150, Australia
Received 4 January 2002; received in revised form 2 September 2002; accepted 7 June 2003
Abstract
Limonitic laterite ore of particle size 90–125 Am containing goethite, magnetite and hematite was leached for 6 h at a pulp
density of 10% (wt/vol) in sulfuric acid in the absence or presence of sulfur dioxide at atmospheric pressure and 90 jC in a glass
reactor vessel. The sulfur dioxide flow rate was kept at 0.6 L min� 1 L� 1 of slurry to maintain a constant SO2 concentration of
c 0.3 mol L� 1 in solution, and the sulfuric acid concentration was varied between 0 and 0.72 mol L� 1. The relative percentage
extractions of Fe, Ni, Co and Mn indicate that the Fe and Ni extractions are inter-related at a ratio of Ni/Fe = 0.7–0.9 and
suggest the possibility of catalysis of manganese dissolution by solubilized iron(II). This leads to a Mn extraction of over 90%
in less than 30 min compared with 20–40% Fe extraction in the same period, depending on the acid concentration. The initial
rate of leaching of iron shows first-order dependence with respect to H+. Whilst the synthetic iron oxides leach according to the
shrinking particle/sphere kinetic model, the results obtained in the first 4 h of laterite leaching can be described by a shrinking
particle model with an insoluble product layer that retards the diffusion of H+ to the reaction sites at the interface. The
heterogeneous rate constants for both models increase with the increase in H+ concentration. The effective diffusion coefficient
of H+ (DH+ ) through the product layer (0.5� 10� 9 to 4� 10� 9 cm2 s� 1), determined in the present study, is in the magnitude
range of the reported data for DH+ in polycrystalline Fe3O4 and MnO2, but lower than DH
+ in aqueous media, 9� 10� 5 cm2 s� 1.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Limonitic laterite; Iron; Nickel; Cobalt; Manganese; Leaching; Heterogeneous kinetic models; H+ diffusion
1. Introduction
Sulfur dioxide is an efficient leaching agent for
minerals containing oxides of iron, nickel, cobalt and
manganese (Byerley et al., 1979; Miller and Wan,
1983; Abbruzzese, 1990; Grimanelis et al., 1992;
Kumar et al., 1993; Das et al., 1997). It offers the
0304-386X/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0304-386X(03)00132-4
* Corresponding author. Fax: +61-8-9360-6343.
E-mail address: [email protected]
(G. Senanayake).
choice of acidic leaching of these metal oxides in
nickeliferous limonite under atmospheric conditions
compared to the acid pressure leaching commercially
practiced in Cuba and Western Australia (Chou et al.,
1977; Kyle, 1996). The kinetics of the acid dissolution
(Majima et al., 1985; Cornell et al., 1976) and
reductive dissolution of natural or synthetic iron
oxides in the presence or absence of sulfur dioxide
(Warren and Hay, 1975; Kumar et al., 1993; Byerley
et al., 1979; Chiarizia and Horwitz, 1991) as well as
the reductive dissolution of manganese dioxide by
Table 1
Equilibrium constants (logK) for protonation and sulfate complex-
ation at 25 jCa
Reaction Ionic strength logK
H++ SO42� =HSO4
� 0 1.99 (3.07at 100 jC)1 1.07
H+ +HSO3� =H2O+SO2 0 1.77
1 1.37
Fe2 + + SO42� = FeSO4
0 0 2.3
1 1.0
G. Senanayake, G.K. Das / Hydrometallurgy 72 (2004) 59–7260
sulfur dioxide or by Fe(II) sulfate (Miller and Wan,
1983; Tekin and Byramoglu, 1993) have been studied
in detail. The relevant kinetic data have been corre-
lated with one or more of the physico-chemical factors
that include the ionic activity of H+, complexation
with OH� or SO32�, electrochemical surface reaction
and changes in surface area.
Along with manganese, which may also be present
as pyrolusite, MnO2, nickel and cobalt in laterite
deposits are enriched in weathering products such as
goethite and limonite by ion replacement (Kyle,
1996). A proper understanding of the kinetics and
mechanism of reductive leaching of oxide minerals by
acid and sulfur dioxide is useful in developing meth-
ods for selective leaching of valuable metals from
nickeliferous ores. The reaction mechanisms for pure
components proposed by previous researchers are
important in the discussion of the reductive leaching
of limonitic laterite ore containing multi-valent oxides
of important metals. However, the interpretation of
kinetic behavior of such ores is complicated due to the
fact that they contain more than one component that
can react with reductive leaching agents such as sulfur
dioxide and/or iron(II) sulfate. For example, the
limonitic laterite used in one of the previous inves-
tigations (Das et al., 1997) and the subject of this
study consists of goethite a-FeOOH and magnetite
Fe3O4 as major components and hematite a-Fe2O3 as
the minor component along with quartz. Whilst the
Fe(III) oxides are dissolved by H+ and/or by SO2 to
produce Fe(III) and Fe(II), the SO2 as well as Fe(II)
produced during reduction can subsequently act as
reductive leachants for Mn(IV), Fe(III) and Co(III).
This paper describes the analysis of leaching results of
limonitic laterite in sulfuric acid and in perchloric acid
in the absence or presence of sulfur dioxide on the
basis of surface chemical reaction with H+ and SO2 as
well as Fe(II) and the heterogeneous mechanisms
described by the shrinking particle models with or
without insoluble product layer.
Fe2 + +HSO4� = FeHSO4
� 1.2 0.78
Fe3 + + SO42� = FeSO4
+ 1.2 2.23
Fe3 + + 2SO42� = Fe(SO4)2
� 1.2 4.23
Ni2 + + SO42� =NiSO4
0 0 2.40
1 0.57
Mn2 ++ SO42� =MnSO4
0 0 2.28 (3.0 at 45 jC)Co2 ++ SO4
2� =CoSO40 0 2.36
1 1.06
a Data from Sillen and Martell, 1964.
2. Iron chemistry and kinetic models
2.1. Acid dissolution of iron oxides
The initial rate of dissolution of hematite or mag-
netite (Eqs. (1) and (2)) follows first-order kinetics
with respect to the concentration of H+ (Cornell et al.,
1976) due to the slow desorption of FeOH2 + from the
surface, followed by aqueous reaction as shown by
Eqs. (3)–(5) (Warren and Hay, 1975):
FeOOHðsÞ or 0:5Fe2O3H2OðsÞþ3Hþ ¼ Fe3þþ 2H2O
ð1Þ
Fe3O4ðsÞ or Fe2O3 � FeOðsÞ þ 8Hþ
¼ 2Fe3þ þ Fe2þ þ 4H2O ð2Þ
pOFe� OHðsÞ þ Hþ
¼ pOFeþðsÞ þ H2O ðfast equilibrationÞ ð3Þ
pOFeþðsÞ þ Hþ ¼ FeOH2þðaqÞ ðslowÞ ð4Þ
FeOH2þðaqÞ þ Hþ ¼ Fe3þ þ H2O ðfastÞ ð5Þ
Despite the fact that the rate-determining step (4) is
first order with respect to H+, the formal reaction order
of H+ can show different values in the range 0.5–2
depending on the hydration, complexation, redox
reactions and other factors (Azuma and Kametani,
1964; Gorichev and Kipriyanow, 1984; Majima et
al., 1985). For example, Table 1 lists some of the
relevant complexes and their equilibrium constants (K)
showing that K increases with increasing temperature,
but decreases with increasing ionic strength. Potenti-
Fig. 1. Eh–pH diagram for Fe(III)/Fe(II)/SO2/SO42� system at 25
jC, based on thermodynamic data in Table 1 at ionic strength 1 and
equimolar species in solution.
G. Senanayake, G.K. Das / Hydrometallurgy 72 (2004) 59–72 61
ometric measurements and predicted Eh–pH and spe-
cies distribution diagrams have shown that FeSO40 and
Fe(SO4)2�, respectively, are the predominant Fe(II) and
Fe(III)–sulfate complex species in acidic sulfate sol-
utions (Senanayake and Muir, 1988). The Eh–pH
diagram shown in Fig. 1 highlights the need to rewrite
the stoichiometry of acid dissolution shown in Eqs. (1)
and (2) to accommodate actual complex species such
as FeSO40 and Fe(SO4)2
� in solution:
FeOOHðsÞ or 0:5Fe2O3H2OðsÞ þ 2H2SO4
¼ FeðSO4Þ�2 þ Hþ þ 2H2O ð6Þ
Fe3O4ðsÞ or Fe2O3FeOðsÞ þ 5H2SO4
¼ 2FeðSO4Þ�2 þ 2Hþ þ FeSO04 þ 4H2O ð7Þ
2.2. Effect of SO2
The catalytic effect of a reducing agent is the result
of a requirement that the surface of the mineral be
composed of a stoichiometric compound that will
dissolve into the solution (Nicol, 1983). For example,
the proposed mechanism for the dissolution of mag-
netite and goethite in SO2 involves slow desorption of
FeHSO3+ from the oxide surface (Byerley et al., 1979).
The presence of reducing agents capable of lowering
the [Fe(III)]/[Fe(II)] concentration ratio will accelerate
iron oxide dissolution (Gorichev and Kipriyanow,
1984). This shows the importance of considering the
reaction between Fe(III) and SO2 in solution.
Solution chemistry studies of Fe(III) and SO2 have
shown a stoichiometry of 1:1.2 for the reversible
reaction between Fe(III) and SO2 to form Fe(II) and
the monomer HSO3 free radical or the dimer H2S2O6
of dithionic acid at pH 0.5 (Higginson and Marshall,
1957). Despite the experimental evidence for the
formation of both S2O62� and SO4
2� at pH 0–3 (Sato
et al., 1978), IR spectroscopic studies have indicated
that SO42� is the major species formed during the
leaching of goethite with SO2 (Kumar et al., 1993).
Considering the formation of both dithionate and
sulfate and the sulfate complex of Fe(II), the reductive
dissolution of goethite by SO2 may be represented as:
FeOOHðsÞ or 0:5Fe2O3H2OðsÞ þ SO2 þ H2SO4
¼ FeSO04 þ 0:5H2S2O6 þ H2O ð8Þ
Fe3O4ðsÞ or Fe2O3 � FeOðsÞ þ 3H2SO4 þ 2SO2
¼ 3FeSO04 þ H2S2O6 þ 2H2O ð9Þ
H2S2O6 ¼ H2SO4 þ SO2 ð10Þ
FeOOHðsÞ or 0:5Fe2O3�H2OðsÞþ0:5SO2þ0:5H2SO4
¼ FeSO04 þ H2O ð11Þ
Fe3O4ðsÞ or Fe2O3 � FeOðsÞ þ 2H2SO4 þ SO2
¼ 3FeSO04 þ 2H2O ð12Þ
2.3. Kinetic models
The initial rates of leaching have been used to
model the kinetics of the dissolution of hematite on
Table 2
Experimental conditions, apparent rate constants (kss and kpl) and
proton diffusion coefficients (DH+) for iron dissolution from
synthetic iron oxides and limonitic laterite ore
Material Particle
size
(�m
mean)
Pulp
density
(%,
wt/vol)
T
(jC)SO2
(M)
H2SO4
(M)
106
kss(s� 1)
106
kpla
(s� 1)
109
DH+ b
(cm2
s� 1)
FeOOHc 17 2.5 80 nil 3 111
0.1 3 157
Fe3O4d 75 0.4 50 0.54 nil 1.7
Lateritee
Test 1 (T1) 106 10 90 0.3 nilf 0.06 0.12
Test 2 (T2) nil 0.72 3.3 0.51
Test 3 (T3) 0.3 0.18 1.5 0.87
Test 4 (T4) 0.3 0.36 3.3 1.0
Test 5 (T5) 0.3 0.54 30g 10 2.0
Test 6 (T6) 0.3 0.72 43g 27 4.2
Test 7 (T7) 0.3 nilh 18 2.2
a Rate constant for shrinking particle model with solid product
layer for 4 h (Fig. 10).b Proton diffusion coefficient based on Eqs. (14) and (15):
r = 5.3� 10� 3 cm, qm= 3.8 g cm� 3 (Berkman, 1995) and %Fe = 39.c Synthetic goethite (Chiarizia and Horwitz, 1991).d Synthetic magnetite,1 g solid in 250 mL liquid (Byerley et al.,
1979).e This work.f No H2SO4 added, natural [H+] = 0.062 M, based on mass
balance for Eq. (16).g Rate constant for shrinking particle model for the first 60 min
(Das et al., 1997).h No H2SO4 added, 1 M HClO4.
G. Senanayake, G.K. Das / Hydrometallurgy 72 (2004) 59–7262
the basis of surface chemical reaction with H+ in the
absence of SO2 (Cornell et al., 1976; Majima et al.,
1985) or surface electrochemical reaction in the pres-
ence of SO2 (Kumar et al., 1993). For example, the
initial rate of dissolution of a-FeOOH, h-MnO2 and
g-MnO2 has been found to be proportional to [SO2]0.5
indicating a half order reaction with respect to SO2
(Miller and Wan, 1983; Kumar et al., 1993). In the
case of relatively fast surface reactions, it is essential
to consider the decrease in surface area in a shrinking
sphere model with no product layer formation (Leven-
spiel, 1972; Ray, 1993). For the first-order 1:1 molar
heterogeneous rate-controlling step for the dissolution
of FeOOH (Eq. (4)), this model can be represented by
the mathematical relationship:
1� ð1� X Þ1=3 ¼ ½Hþbulkkr�1q�1t ¼ ksst ð13Þ
where, [H+]bulk (mol cm� 3) = concentration in the
bulk solution, k (cm s� 1) = intrinsic rate constant of
the surface chemical reaction having the units of a
mass transfer coefficient, r (cm) = initial particle radi-
us of solid, q (mol cm� 3) =molar concentration of Fe
in the solid, kss (s� 1) = apparent rate constant in the
shrinking sphere kinetic model and X = fraction of Fe
reacted in time t. The relationship between q and wt.%
Fe in the material and its density (qm, g cm� 3) is:
q¼ qm�%Fe in material=ð100� molar mass of FeÞð14Þ
If a porous solid product layer is formed on the
surface during the reaction, the slow diffusion of H+
through the product layer becomes the rate-controlling
step. The shrinking core with product layer model in
such cases is represented by the equation:
1� 3ð1� X Þ2=3 þ 2ð1� X Þ
¼ 6½HþbulkDþHr
�2q�1t ¼ kplt ð15Þ
where DH+ (cm2 s� 1) = diffusion coefficient of H+
through the product layer and kpl (s� 1) = apparent rate
constant. Eq. (13) assumes that the decrease in rate
with time is due to the decrease in particle size and
thus the surface area. Eq. (15) assumes that the
diffusion of H+ through a solid product layer is the
rate-determining step and the decrease in rate is due to
the increase in thickness of the solid product layer
with time, whilst the spherical nature and the radius of
the particle remains unchanged.
The leaching of pure MnO2 has been studied in
SO2/Na2SO3/pH 1–2 (Miller and Wan, 1983) and in
FeSO4/H2SO4 (Tekin and Byramoglu, 1993) at tem-
peratures up to 50 jC under controlled hydrodynam-
ic conditions. The stirring speed was sufficiently
large to confirm that the rate of leaching was not
governed by the transport process. The decrease in
the rate of leaching with time has been related to the
shrinking particle/sphere kinetic model with a value
of kss = 4.4� 10� 3 s� 1 and k = 1.2� 10� 3 cm s� 1
for the leaching of MnO2 in 0.5 M SO2 at pH 2 at a
stirring speed of 650 rpm. The value of k was less
than the predicted mass transfer coefficient for
suspended particles, 2.4� 10� 2 cm s� 1 in the leach-
ing reactor, showing that the leaching rate was con-
G. Senanayake, G.K. Das / Hydrometallurgy 72 (2004) 59–72 63
trolled by the surface reaction rate (Miller and Wan,
1983).
3. Experimental
Awater-jacketed 1-L Pyrex glass vessel fitted with
a Teflon-coated stirrer, baffles, gas inlet tube, sampling
tube, thermometer and water condenser was used as
the reactor for leaching; other details of dimensions
were similar to those reported by Tekin and Byramoglu
(1993). The limonitic nickel laterite of particle size
range 90–125 Am from Bulong, Western Australia,
was used in all experiments. The standard leaching
conditions were: pulp density = 10% (wt/vol), temper-
ature = 90 jC, agitation speed = 650 rpm, leaching time
6 h. The sulfur dioxide flow rate was maintained at 0.6
L min� 1 L� 1 of slurry and the acid concentration was
varied between 0 and 0.72 M H2SO4 or 1 M HClO4 in
seven tests T1–T7 (Table 2). These conditions were
consistent with the previous investigation (Das et al.,
1997). Samples were collected at 0.5, 1, 2, 4 and 6 h,
and the solutions were analysed for Ni, Co, Mn and Fe
by atomic absorption spectrophotometry.
Fig. 2. Kinetic plots for iron extraction from limonitic laterite. T1, T2 and T
2 for conditions.
4. Results and discussion
4.1. Time and acid dependence of Fe and Ni
extraction
Fig. 2 plots %Fe extractions from limonitic laterite
at time intervals 0.5, 1, 2, 4 and 6 h and compares
with some of the data reported previously (Das et al.,
1997). The leaching with SO2 in the absence of added
acid (Test 1) shows the lowest iron extraction but a
linear increase in % extraction with time. The %Ni
extraction presented in Fig. 3 also shows the lowest in
Test 1, but %Ni remains at 20% and independent of
time. The highest Fe and Ni extraction at 6 h is shown
by SO2 + 0.72 M H2SO4 (Test 6) but the rate decreases
with time. In Test 3, Ni extraction reaches only 25%
but Co and Mn reach 100% in 30 min.
The %Fe and %Ni extraction after a given
leaching time generally increases in the order of
acid concentration: Test 1 < Test 3 < Test 4 < Test
5 < Test 6. This can be mainly attributed to the
initial rate of leaching with SO2, represented by
the initial slopes in Figs. 2 and 3, which also
increase in the same order. However, the rate
6 from Das et al. (1997); other data from the present study, see Table
Fig. 3. Kinetic plots for nickel, cobalt and manganese extraction from limonitic laterite. Solid lines represent Ni extraction in T1 to T6 (Table 2),
dashed line represents Co and Mn extraction in T3.
G. Senanayake, G.K. Das / Hydrometallurgy 72 (2004) 59–7264
decreases with time, except in Tests 2 and 3 carried
out with no SO2 or low acid concentrations (0.18
M), respectively, where the rate after 2 h remains
fairly constant. In general, the decrease in rate with
increasing leaching time can be related to one or
more of the following reasons: (i) the decrease in
acid concentration with the consumption of H+ in
Eqs. (6)–(12), (ii) the change in equilibrium SO2
concentration in Eq. (16) caused by the decreasing
acid concentration and changing ionic strength (Ta-
ble 1), (iii) the decrease in surface area of the
particles or (iv) the increase in thickness of a solid
layer, which retards the diffusion of reactants or
products.
HSO�3 þ Hþ X SO2 þ H2O ð16Þ
Unlike in the case of Fe and Ni, the % extraction of
Co and Mn rapidly increases and reaches 85–100%
during the first 30 min in the presence of SO2 due to
the reactivity of SO2 and the occurrence of Co with
Mn (Das et al., 1997). Consequently, despite the
high acid concentration of 0.72 M in Test 2, the
extraction of these metals remains low at 12–28%
Co and 20–40% Mn in 0.5–6 h of extraction time
due to the absence of SO2 (see later in Fig. 5). This
behavior can be further examined by considering the
relative extractions of the four metals.
4.2. Relative extraction of Fe, Ni, Co and Mn
4.2.1. Ni vs. Fe
Despite the higher iron content compared to nickel
in the starting material, 39.05% Fe, 1.17% Ni, 0.124%
Co, 1.25% Mn (Das et al., 1997), Fig. 4 shows linear
correlations between the %Ni and %Fe extractions at
different time intervals up to 6 h. These linear rela-
tionships of slopes ranging from 0.7 to 0.9 in the
range 20–85% Ni extractions can be related to the
incorporation of nickel in the iron oxide fraction of
laterite and thus the release of nickel(II) into the
solution is associated with the leaching of iron.
However, the nickel and iron extraction by sulfur
dioxide in the absence of added acid (Test 1) was
low, < 20% Ni and < 10% Fe, and the results do not
show a correlation with the other data in Fig. 3. In
contrast, 0.72 M H2SO4 alone with no SO2 (Test 2)
extracts c 45% Fe and Ni; SO2 + 0.72 M H2SO4
(Test 6) extracts c 85% Fe and Ni in 6 h. These
results also show the necessity of breaking up of the
goethite structure by SO2 and/or H+ to release both
iron and nickel into solution.
Fig. 4. Correlation between nickel and iron extraction at different time intervals in Tests T1–T7.
G. Senanayake, G.K. Das / Hydrometallurgy 72 (2004) 59–72 65
4.2.2. Ni, Co, Mn vs. Fe
Fig. 5 plots the % extraction of Ni, Co and Mn
against Fe in selected Tests 2, 6 and 7. The lower
extraction of 45% Fe in Test 2 in 6 h is clearly due to
the fact that the dissolution of iron oxide is caused
only by the acid attack according to Eqs. (6) and (7) in
the absence of SO2. This is further supported by the
measured Eh of the leach liquors in Test 2, which
were cooled to 25 jC. The measured Eh in the range
0.60–0.68 Vat pH c 1.5 in Test 2 corresponds to the
stability region of Fe(SO4)2�, indicating a ratio of
[Fe(III)]/[Fe(II)] greater than unity (Fig. 1). Although
the %Ni vs. %Fe extraction corresponds to the line of
slope 0.9, Co and Mn extraction corresponds to lower
slopes of 0.5–0.8 due to the lower extraction in the
absence of SO2 in Test 2.
Fig. 5 also shows the % extraction of Ni, Co and
Mn against Fe in the three tests to compare the effect of
SO2 in 0.72 M H2SO4 (Tests 2 and 6) and the effect of
changing 0.72 MH2SO4 to 1 MHClO4 (Tests 6 and 7).
The measured Eh of the leach liquors produced in the
presence of SO2 in Tests 4–6, which were cooled to 25
jC, were lower (0.56 V) than in the case of Test 2. ThisEh falls in the stability region of FeSO4
0 in Fig. 1 and
shows the reductive dissolution of ore according to
Eqs. (8)–(12) forming FeSO40, causing the [Fe(III)]/
[Fe(II)] ratio to drop to values below unity. The main
feature in Fig. 5 is that the Co and Mn extractions
remain 85–100% even at lower Fe extractions; this is
much higher than the Co and Mn extractions ( < 40%)
in the absence of SO2 in Test 2. Higher and rapid
extraction of Mn and Co in the presence of SO2 is a
result of the reductive leaching of the type:
MnO2ðsÞ þ SO2 ¼ MnSO04 ð17Þ
The mechanism of this reaction has been well docu-
mented (Miller and Wan, 1983; Abbruzzese, 1990;
Grimanelis et al., 1992). High leaching rates at pH 1–2
have been related to the formation of complexes of
Mn(II/III) of the type Mn(SO3)22� and Mn(SO3)2
�,
respectively.
Fig. 5. Correlation between nickel, cobalt, manganese and iron extraction at different time intervals. Comparison between the effect of SO2 (T2
and T6), 0.72 M H2SO4 and 1 M HClO4 (T6 and T7).
G. Senanayake, G.K. Das / Hydrometallurgy 72 (2004) 59–7266
Fig. 5 shows that the Ni vs. Fe extraction in Tests 6
and 7 corresponds to a line of slope 0.7, lower than
the slope 0.9 for data in Test 2, irrespective of the
different acids, 0.72 M H2SO4 (in Test 6) and 1 M
HClO4 (in Test 7), but remains above the line of slope
0.9. This indicates a higher Ni extraction, compared to
Fe, in the presence of SO2. However, the Ni extraction
in Tests 6 and 7 starts to drop back to the line of slope
0.9 after the complete extraction of Co and Mn. It is
possible that the decrease in Fe extraction is due to the
involvement of the dissolved Fe(II) in the reductive
dissolution of MnO2, and the cobalt associated with it,
in acidic media causing Fe(II) to re-precipitate as
Fe(III) species
4.3. Catalytic effect of Fe(II)
The catalysis of reductive leaching of MnO2 by
H+/Fe(II) takes place via the intermediate MnOOH
(Tekin and Byramoglu, 1993) leading to the partial
precipitation of iron as the basic sulphate FeSO4OH or
the oxides Fe2O3, Fe3O4 or FeOOH due to the
hydrolysis and/or the increase in Fe(II) concentration
in solution:
MnO2ðsÞ þ 2FeSO04 þ H2O
¼ MnSO04 þ Fe2O3ðsÞ þ H2SO4 ð18Þ
MnO2H2OðsÞþFeSO04 ¼ MnOOHðsÞ þ FeSO4OHðsÞ
ð19Þ
MnOOHðsÞ þ 0:5SO2 þ 0:5H2SO4 ¼ MnSO04 þ H2O
ð20Þ
MnOOHðsÞ þ FeSO04 ¼ MnSO0
4 þ FeOOHðsÞ ð21Þ
FeSO4OHðsÞ þ H2O ¼ FeOOHðsÞ þ H2SO4 ð22Þ
2FeSO4OHðsÞ þ H2O ¼ Fe2O3ðsÞ þ 2H2SO4 ð23Þ
2FeSO4OHðsÞ þ FeSO04 þ 2H2O
¼ Fe3O4ðsÞ þ 3H2SO4 ð24Þ
G. Senanayake, G.K. Das / Hydro
Some evidence for these conclusions and chemical
reactions also comes from the previous studies (Gri-
manelis et al., 1992) with the sulfur dioxide leaching
of a manganese rich pyrolusite ore containing 40.74%
MnO2 and 0.84% Fe2O3 at ambient temperatures. The
extraction of Mn and Fe at 30 jC in the first 2 min
was 75% Mn and 20% Fe, respectively. In 20 min, Mn
extraction increased to 85%, which remained constant
over time, whilst the Fe extraction decreased to about
10% during the same time interval due to the re-
precipitation of hydrated Fe(III) oxide or an insoluble
basic sulfate. Moreover, the ratio of %Mn/%Fe in-
creased with the decrease in pH indicating that high
acidity led to more Mn dissolved relative to Fe, due to
the catalytic effect of Fe(II).
It is also of interest to note that the XRD patterns
showed peaks indicating the presence of FeOOH,
Fe3O4, Fe2O3 and SiO2 in the starting material used
in the present study, but the presence of only Fe3O4 and
SiO2 in the leach residue (Das et al., 1997). This
observation supports the formation of Fe3O4 Eq. (24),
in contrast to the hydrothermal conversion of goethite
to hematite in the acid pressure leaching of limonitic
laterite (Briceno and Osseo-Asare, 1995) where there
were no peaks corresponding to Fe3O4 in the leach
residue. The formation of FeSO4OH(s) as a solid
species in sulfuric acid pressure leaching of laterite
has not been confirmed due to the extremely fast ki-
netics of Eq. (23) (Rubisov and Papangelakis, 2000).
4.4. Effect of changing H2SO4 to HClO4
When the acid was changed from 0.72 M H2SO4 to
1 M HClO4 in the present study (Test 6 to Test 7), the
Mn and Co extraction decreased from 100% to 85–
95% in Fig. 5, although the Ni extraction was unaf-
fected. This shows the influence of background sulfate
on Mn and Co extraction, compared to background
perchlorate. It is possible that the formation of stable
sulfate complexes of MnSO40 and CoSO4
0 (Table 1) as
well as the intermediates such as FeSO4OH(s) and
FeSO40 formed in sulfate solutions as indicated in Eqs.
(18)–(24) would favor the thermodynamics and kinet-
ics of leaching of these metals. Thus, the results
summarized in Figs. 2–5 clearly indicate that the
overall leaching behavior of laterite is largely con-
trolled by the kinetics and mechanism of the dissolu-
tion of iron.
4.5. Kinetic models
4.5.1. Initial rate of iron leaching
Results from the previous studies on initial rate of
dissolution of goethite failed to agree with the rate
equation: � {dnFeOOH/dt} = constant [H+]n[SO2]m
with the proposed values of n =m = 0.5 based on an
electrochemical reaction model (Kumar et al., 1993).
The experimental values for m for leaching in 0.3–
0.5 M SO2 at pH 1.1–2.2 varied between 0.50 and
0.67; but n showed a much larger variation from 0.17
to 0.40. This leads to the suggestion that leaching
may also be taking place via acid attack in addition to
the reductive leaching (Kumar et al., 1993). However,
the literature data for the dissolution of hematite
(Majima et al., 1985) in sulfuric acid in the absence
of sulfur dioxide at 50–55 jC plotted in Fig. 6 show
a slope of 0.67 with respect to acid concentration.
Therefore, it is important to establish the order of the
initial leaching reaction with respect to H+, before the
effect of change in surface area and/or the formation
of insoluble solids at the reaction interface can be
considered.
The solubility of SO2 in water and in 1 mol L� 1
H2SO4 at 1 atmospheric pressure of SO2 is c 0.3 mol
L� 1 and fairly independent of the acid concentration
(Linke and Seidell, 1958). Therefore, it is reasonable
to assume that the SO2 concentration remains constant
in the tests carried out in the present study under a
constant and continuous flow of SO2. Moreover the
Eh–pH diagram predicts that SO2 is the predominant
species (Fig. 1) and that the equilibrium concentration
of HSO3� in Eq. (16) is negligibly small at pH < 2
(Abbruzzese, 1990). Thus, the slope is close to 1 for
the plot of log{d[Fe]/dt} against log[H+] in Fig. 6 for
the initial (t = 0–0.5 h) leaching in Tests 2–7, reflect-
ing first-order kinetics with respect to H+. This is in
agreement with the previous studies for the dissolution
of a-FeOOH in perchloric acid (Cornell et al., 1976).
4.5.2. Shrinking particle (sphere) model for iron
leaching
The kinetic studies for the dissolution of synthetic
iron oxides in H2SO4 in the absence or presence of
SO2 carried out by previous researchers have provided
evidence for a shrinking particle model (Chiarizia and
Horwitz, 1991). Some of the literature data for goe-
thite and magnetite dissolution are summarized in Fig.
metallurgy 72 (2004) 59–72 67
Fig. 6. Effect of acid concentration on initial rate of iron leaching. Laterite: Das et al. (1997) and this work (Table 2). Hematite: Majima et al.
(1985) (74–104 Am 95.4% Fe2O3).
G. Senanayake, G.K. Das / Hydrometallurgy 72 (2004) 59–7268
7 as plots of 1� (1�X)1/3 against time and the rate
constants kss are summarized in Table 2. The value of
kss for the dissolution of synthetic FeOOH in 3 M
H2SO4 at 80 jC increases by a factor of 1.4 with the
addition of 0.1 M SO2. The dissolution of synthetic
Fe3O4 in 0.54 M SO2 at 50 jC corresponds to a kssvalue that is 1/100 times less than that for FeOOH at
80 jC with 3 M H2SO4, due to the lower temperature
and absence of added acid (Table 2).
Fig. 7. Comparison of kinetic plots for iron dissolution from goethite, magn
References and conditions described in Table 2.
On the basis of the dissolution behavior of the
synthetic goethite and magnetite summarized in Fig.
7, it was expected that the dissolution of iron in the
forms of goethite, magnetite and hematite in the
limonitic laterite ore investigated in the present study
would follow the same trend. However, as reported
previously (Das et al., 1997), the iron dissolution
obeys the shrinking particle kinetic model only during
the first hour of leaching and the relevant kss data are
etite and limonitic laterite: shrinking sphere (particle) kinetic model.
G. Senanayake, G.K. Das / Hydrometallurgy 72 (2004) 59–72 69
listed in Table 2. Limited data (Tests 2 and 6) shown
in Fig. 7 confirm this behavior.
As a matter of interest, the value of kss for the dis-
solution of h-MnO2 in 0.5 M SO2 (Tekin and Byra-
moglu, 1993) and g-MnO2 in FeSO4 (Miller and Wan,
1983) at pH 1–2 seem to be 1000–5000 times greater
than the values for iron reported in this work leading
to faster leaching of Mn compared to Fe. This
explains why the %Mn and %Co extraction reached
100% in Tests 3 in 30 min compared to 7% Fe and
25% Ni (Figs. 2 and 3).
4.5.3. Shrinking core model with product layer for
iron leaching
The results from the two tests T2 and T6 in 0.75 M
H2SO4 in the presence or absence of SO2, plotted in
Fig. 7 do not show a linear correlation for the shrinking
particle model over the longer leaching period of 6 h.
Figs. 8 and 9 compare the plots of 1� (1�X)1/3 and
1� 3(1�X)2/3 + 2(1�X) against t, respectively, for
the dissolution of Fe, Ni, Co and Mn in 0.72 M H2SO4
in the absence of SO2 (Test 2) during the first 2 h.
Clearly, the Fe and Ni extraction in Fig. 9 show an
excellent linear relationship with correlation coeffi-
cients of R2c 0.99 and the rate constant kpl = 4.2�10� 6 s� 1 for iron and 6.1�10� 6 s� 1 for nickel. The
fact that manganese and cobalt do not behave in the
same way as iron and nickel supports the view that the
dissolution of manganese (and cobalt) is a result of the
Fig. 8. Two-hour kinetic data: shrinking sphere (particle) kinetic model for
H2SO4 in the absence of SO2 (Test 2).
reactions with SO2 and Fe(II) described previously.
Fig. 10 shows that all the results obtained for the
dissolution of iron during the first 4 h in the present
investigation fit with a shrinking particle model with a
solid product layer, with R2>0.99 for Tests 3 and 6 and
R2c 0.98 for the others. The kpl values obtained from
the slopes are listed in Table 2. The product layer may
consist of basic iron sulfate and/or iron oxides (Eqs.
(18)–(24)) and quartz, in addition to any other hydro-
lysis products from the gangue.
4.6. Effect of H+ concentration on rate constants
Both H+ and SO2 should be available for the
reductive leaching reactions represented by Eqs. (8)–
(12). The value of the heterogeneous reaction rate
constant, kpl increases eightfold from 3.3� 10� 6 s� 1
in 0.72 MH2SO4 to 27� 10� 6 in SO2 + 0.72MH2SO4
(Table 2), mainly due to the change from acid leaching
(Eqs. (6) and (7)) to reductive leaching (Eqs. (8)–
(12)). Since the SO2 concentration was maintained
constant in the present study, it is reasonable to assume
that the increase in kpl with increasing concentration of
H2SO4 is mainly due to the change in H+ concentration
in the bulk (Eq. (15)). The value of pKa for Eq. (26),
H2SO4 ! Hþ þ HSO�4 ð25Þ
HSO�4 X Hþ þ SO2�
4 ð26Þ
dissolution of Fe, Ni, Co and Mn from limonitic laterite with 0.72 M
Fig. 9. Two-hour kinetic data: shrinking core model with a solid product layer for dissolution of Fe, Ni, Co and Mn from limonitic laterite with
0.72 M H2SO4 in the absence of SO2 (Test 2).
G. Senanayake, G.K. Das / Hydrometallurgy 72 (2004) 59–7270
depends on the ionic strength and temperature
(Table 1). The ionic strength of leach liquor changes
with time due to the formation of sulfur species
Fig. 10. Four-hour kinetic data: shrinking core model with a
and dissolved metal ions and their complexes. If it
is assumed that the ionic strength remains close to
1, the mass balance for equilibrium in Eq. (26)
solid product layer for dissolution of Fe in Tests 1–7.
Table 3
Effect of medium and material on proton diffusion coefficient DH+
Medium/material DH+(cm2 s� 1) Reference
Aqueous solutiona 9� 10� 5 Bockris and Reddy, 1977
Single-crystal ironb 8� 10� 5 Bockris and Reddy, 1977
g-MnO2 8� 10� 8 Allen et al., 1979
Product layer in
laterite leaching
0.5� 10� 9 to
4� 10� 9
This work (Table 2)
Fe3O4 8� 10� 10c Allen et al., 1979
NiO 10� 10–10� 12 Allen et al., 1979
a At infinite dilution and 25 jC.b Diffusion of eloctronated H+ (atomic hydrogen) into the metal.c Minimum possible value based on electrochemical studies
with polycrystalline magnetite.
G. Senanayake, G.K. Das / Hydrometallurgy 72 (2004) 59–72 71
based on the pKa value at I = 1 leads to the linear
relationship:
½Hþ ¼ 1:085½H2SO4 þ 0:0178 ð27Þ
This can be used to examine the effect of H2SO4
concentration on the rate constants. A higher con-
centration of [H+]bulk would favor the adsorption
equilibrium: Solid +H+ X Solid�H+(ads) and thus the
rate controlling surface chemical reaction (Eq. (4);
Cornell et al., 1976) occurs to improve the rate of
dissolution. This would increase kss according to
Eq. (13), as reflected in the data listed in Table 2
for Tests 5–6. Likewise, the increase in [H+]bulkwould increase the rate of diffusion of H+ through
the product layer to the reaction interface and hence
increase kpl according to Eq. (15).
The calculated values of DH+ , based on Eq. (15)
listed in Table 2 range from 0.5� 10� 9 to 4.2� 10� 9
cm2 s� 1. The variation of DH+ with the change in acid
concentration largely reflects the non-validity of
the assumptions of constant values of r and [H+]bulkused in the calculation of DH
+ using Eq. (15). Never-
theless, the increasing order of DH+ shown in Table 3
indicates that DH+ calculated in the present study is
c 1/10000 times smaller than DH+ in aqueous solutions
but within the magnitude range reported for the diffu-
sion of H+ through solids such as Fe3O4 and g-MnO2
based on electrochemical methods (Allen et al., 1979).
5. Summary and conclusions
� At constant [SO2], the rate of leaching of iron from
limonitic laterite in the first 30–60 min is
approximately first order with respect H+ but
appears to obey the shrinking particle model due
to the decrease in surface area.� This is consistent with the leaching behavior of
synthetic iron oxides in the absence or presence of
SO2, which also follows first-order kinetics and the
shrinking particle kinetic model.� Subsequently, the rate of leaching appears to be
controlled by the slow diffusion of H+ through an
insoluble solid layer produced during leaching. The
magnitude of the diffusion coefficient of H+
through the insoluble solid product (0.5�10�9 to
4�10�9 cm2 s�1) is much less than that in aqueous
media, but in the same order as that in solids such
as Fe3O4 and g-MnO2.� The dissolution of nickel follows the same trend as
iron with an extraction ratio of Ni/Fe = 0.7–0.9 and
obeys the shrinking core model with a solid product
layer. However, the dissolution of manganese (and
cobalt), which reaches 90–100% in the first 30 min
before the product layer sets in, does not follow the
same trend as iron and nickel due to the direct
reaction with SO2 and the catalytic effect of Fe(II)
produced by the reductive leaching of iron oxide.
Acknowledgements
The authors thank Profs. David Muir and Pritam
Singh for valuable discussion during the Targeted
Institutional Research Links Program.
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