Kinetics of Reductive Leaching of Low-grade Pyrolusite with Molasses Alcohol Wastewater in H2SO4

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CATALYSIS, KINETICS AND REACTORS 
Chinese Journal of Chemical Engineering, 18(5) 730 735 (2010) 
Kinetics of Reductive Leaching of Low-grade Pyrolusite with 
Molasses Alcohol Wastewater in H2SO4* 
SU Haifeng ( )1, LIU Huaikun ( )1, WANG Fan ( )1, LÜ Xiaoyan ( )2
and WEN Yanxuan ( )1,**
1 School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China 
2 Educational Administration Department, Guangxi University, Nanning 530004, China 
Abstract The kinetics of reductive leaching of manganese from low grade pyrolusite in dilute sulfuric acid in the 
presence of molasses alcohol wastewater was investigated. The shrinking core model was applied to quantify the 
effects of reaction parameters on leaching rate. The leaching rate increases with reaction temperature, concentra-
tions of H2SO4 and organic matter in molasses alcohol wastewater increase and ore particle size decreases. The 
leaching process follows the kinetics of a shrinking core model and the apparent activation energy is 57.5 kJ·mol–1. 
The experimental results indicate a reaction order of 0.52 for H2SO4 concentration and 0.90 for chemical oxygen 
demand (COD) of molasses alcohol wastewater. It is concluded that the reductive leaching of pyrolusite with molasses 
alcohol wastewater is controlled by the diffusion through the ash/inert layer composed of the associated minerals. 
Keywords pyrolusite, molasses alcohol wastewater, reductive leaching, kinetics 
1 INTRODUCTION 
With an ever increasing demand for manganese 
and gradual depletion of high grade manganese ore, 
many efforts have been made to recover manganese 
from low-grade pyrolusite. Generally, pyrolusite can 
be treated by roasting reduction-acid leaching [1] and 
direct reductive leaching in H2SO4 or HCl solution. 
The reductive agents in H2SO4 solution include H2O2 
[2], H2C2O4 [1], FeSO4 [3] and SO2 [4 6], while the 
reductive agents in HCl solution include alcoholic [6], 
nickel matte [7], pyrite [8] and H2O2 [9]. 
 Recently, carbohydrates from agriculture wastes 
were treated as the potential reducing agents for the 
leaching of manganese ore in mild acidic solution 
[10 14]. Molasses alcohol wastewater (MSW), which 
is generated from the fermentation of cane sugar mo-
lasses for production of alcohol [15], has a high re-
ducing capacity in acidic medium for its content of 
melanoidins, caramel and phenolics. In our previous 
work, MSW was used as reducing agent to leach py-
rolusite, and the optimum condition in dilute sulfuric 
acid solution was reported [16]. In addition, color re-
moval is a main problem in the treatment of such 
wastewater [15]. As indicated in our research [17], 
leaching is an effective method to remove the organics 
in MSW by using pyrolusite and its slurry as reducing 
agent and adsorbent. 
Meanwhile, a comprehensible understanding of 
the leaching kinetics is needed for an efficient design 
of leaching manganese ore. Precious research [18] on 
the leaching kinetics of MnO2 by SO2 showed that the 
key step during the leaching process is chemical reac-
tion and its apparent activation energy is 35.9 kJ·mol–1. 
Similarly, the leaching rate of MnO2 with Fe2+ in 
H2SO4 solution is also controlled by chemical reaction 
with the apparent activation energy of 28 kJ·mol 1 [19]. 
Momade and Momade [20] reported that the leaching 
after 30 min in aqueous methanol-sulfuric acid system 
can be described by the diffusion controlled shrinking 
core model. According the investigation of Veglio et 
al. [12], the leaching in lactose-sulphuric acid follows 
a shrinking core model with variable activation energy. 
Lasheen et al. [14] studied the leaching kinetics of 
manganese oxide ore with molasses in nitric acid so-
lution and found that the leaching is controlled by the 
diffusion through the “product” layer composed of the 
associated minerals and the apparent activation energy 
is 27.5 kJ·mol–1. 
Most works [12, 14, 18 20] on the leaching kinet-
ics of pyrolusite mainly focused on the reductive sys-
tems of small molecule. However, the reductive agents 
in MSW are macromolecule with the molecular 
weight of 2000 9000 [21]. To date, the leaching kinet-
ics of pyrolusite by macromolecular reductive agents 
has not been reported. 
In this work, we investigated the kinetics of the 
reductive leaching manganese from pyrolusite by the 
macromolecular reductive agents of MSW and the main 
system variables on the leaching rate. The kinetic model 
and the apparent activation energy were determined. 
2 MATERIALS AND METHODS 
2.1 Materials 
Manganese ore was obtained from Mugui Man-
ganese Mine, Guangxi, China. The ore samples were 
crushed to 0.147 mm (–100 meshes). The chemical 
composition of the manganese ore used are listed in 
 
Received 2010-02-24, accepted 2010-05-02. 
* Supported by the National Natural Science Foundation of China (20866001) and the Natural Science Foundation of Guangxi 
Province (0832035). 
** To whom correspondence should be addressed. E-mail: wenyanxuan@vip.163.com 
Chin. J. Chem. Eng., Vol. 18, No. 5, October 2010 731
Tables 1 and 2. MSW (Nanning Sugarcane Refinery, 
Guangxi, China), containing 7.3% organic compounds, 
2.9% reducing sugars, CODCr 1.3×105 mg·L–1 and 
BOD5 7.0×104 mg·L–1, was used in this work. All 
other chemicals were of analytical grade and used 
without further purification. 
Table 1 Chemical composition of the pyrolusite sample 
Component Mass content/% 
Mn 22.25 
MnO2 31.78 
Fe 12.35 
SiO2 24.17 
Al2O3 11.36 
CaO 0.12 
MnO 0.12 
S 0.005 
P 0.078 
Table 2 Manganese content of different 
particle size of pyrolusite 
Particle size d0/mm Manganese mass content/% 
0.134 25.72 
0.078 24.10 
0.068 9.94 
0.046 19.00 
2.2 Leaching procedure 
To investigate the effects of reaction parameters, 
including stirring speed, particle size, acid concentra-
tion, MSW concentration and leaching temperature, 
the comparative leaching experiments were carried out. 
The leaching experiments were carried out in a 1000 
ml three-neck flask immersed in thermostat water bath 
with mechanical stirring. In a typical experiment, 5.0 
g of ore was added to 500 ml sulfuric acid-MSW solu-
tion under stirring (700 r·min–1), and leaching started 
after manganese ore was added to the solution. After 
reaction, the slurry was filtered and the residue was 
washed with distilled water. The filtrate was then di-
luted in HNO3 solution (pH 2) for analysis. The Mn 
concentration during the leaching process was meas-
ured by an inductively coupled plasma spectropho-
tometer (ICP, Optima 5300 DV, Perkin Elmer). The 
leaching efficiency was calculated by referring the 
amount of leached metal in the liquor to its original 
input quantity. Organic MSW concentration (the mass 
of the all organic compounds in the dilute sulfate acid 
solution) is expressed by the COD values. The ratio of 
liquor to solid was kept as 100 1. The changes of 
H2SO4 concentration and COD can be neglected dur-
ing each leaching test. Unless otherwise specified, the 
values of temperature, H2SO4 concentration, COD and 
particle size of manganese ore were selected as 80 °C, 
1.0 mol·L–1, 37.7 g·L–1 and 0.0676 mm, respectively. 
At the end of leaching experiment, the slurry was fil-
tered and the filtrate was analyzed for leached man-
ganese concentration. All the experiments were re-
peated twice, and their relative deviation is always 
below 1.0 %. 
3 RESULTS AND DISCUSSION 
3.1 Chemical reaction 
The reduction and dissolution mechanism of py-
rolusite in H2SO4 solution with MSW still remains 
unclear. The chemical reactions taking place during 
dissolution of the manganese dioxide in the presence 
of sucrose [6] or glucose [13] can be described by the 
following reactions: 
2 12 22 11 2 424MnO C H O 24H SO 
4 2 224MnSO 12CO 35H O (1) 
2 6 12 6 2 412MnO C H O 12H SO 
4 2 212MnSO 6CO 18H O (2) 
In this study, XRD was used to identify the min-
eralogical composition of ore samples before and after 
leachingtreatment. The corresponding XRD patterns 
are shown in Fig. 1. The raw ore comprises of py-
rolusite (MnO2), hematite (Fe2O3), silicon oxide (SiO2) 
and kaolinite (Al2Si2O5(OH)4), while the leach resi-
dues are mainly silicon oxide, kaolinite and some 
hematite. These results suggested that no insoluble 
product forms during the leaching processes. So the 
reaction between pyrolusite and MSW in H2SO4 solu-
tion can be written as follows: 
2 2 4
4 2 2
(C - H O ) 2 MnO 2 H SO
2 MnSO CO H O
x y m m m
m m z (3) 
Figure 1 XRD pattern of pyrolusite and its residue after 
leaching 
1 pyrolusite; 2 residue; MnO2; SiO2; Al2Si2O5(OH)4;
Fe2O3 
3.2 Effect of parameters 
Figure 2 shows the effect of stirring speed in the 
range of 100 1100 r·min–1 on the reductive leaching 
Chin. J. Chem. Eng., Vol. 18, No. 5, October 2010 732
of pyrolusite. The conversion of manganese first in-
creased when stirring speed increased from 100 to 700 
r·min–1 due to enhanced diffusion of liquid reactants. 
However, a slight decrease in the conversion of man-
ganese was detected when stirring speed was increased 
from 700 to 1000 r·min–1, because violent agitation 
may cause some pyrolusite particles adhered onto the 
inner wall of three-neck flask and worse contact be-
tween particles and liquid reactants, which reduce the 
leaching efficiency. The most leaching rate is achieved 
at 700 r·min–1 stirring speed, which was used in the 
subsequent experiments. 
The effects of initial particle size of manganese 
ore, H2SO4 concentration, COD and leaching tem-
perature on the leaching of ore are illustrated in Fig. 3, 
respectively. The results show the conversion of man-
ganese increases gradually with the increase of H2SO4 
concentration, MSW concentration and leaching tem-
perature and with the decrease of the average sizes 
of particle. 
3.3 Kinetic analysis 
 Kinetic modeling yields comprehensive informa-
tion regarding leaching mechanisms. In fact, reactions 
involved in this process are heterogeneous in nature 
involving mass transport of reactant and product ions. 
In acidic leaching, the heterogeneous non-catalytic 
reaction for most manganese ore might be kinetically 
interpreted by using the shrinking core model (SCM) 
[18 20]. 
In the SCM model, the solid reactant is consid-
ered as non-porous particle and is initially surrounded 
by a fluid film through which mass transfer occurs 
between the solid particle and the bulk of the fluid. As 
the reaction proceeding, an ash/inert layer forms 
around the unreacted core. Detailed derivation of the 
shrinking core model can be found in Refs. [22 24]. 
The dissolution of pyrolusite can be expressed as 
follows: 
A (fluid) bB (solid) fluid products (4) 
The rate-limiting step, which decides the form of 
the rate equation, may be one of the following three 
steps: (1) diffusion through the liquid film surround-
ing a solid particle, (2) diffusion through the ash/inert 
solid layer and (3) chemical reaction on the surface of 
the unreacted core. The simplified equations of the 
shrinking core model when liquid film diffusion, 
ash/inert solid layer diffusion or the surface chemical 
Figure 2 Manganese extraction vs. time at various stirring 
speeds
100 r·min–1; 200 r·min–1; 500 r·min–1; 700 r·min–1;
1100 r·min–1 
 
(a) Influence of particle size of manganese ore 
d0/mm: 0.0134; 0.078; 0.068; 0.046 
 
(b) Influence of sulfuric acid concentrations 
cH/mol·L 1: 0.4; 0.6; 0.8; 1.0; 1.2 
 
(c) Influence of molasses alcohol wastewater concentration
cW/g·L 1: 10.0; 20.0; 30.0; 37.7; 50.0 
 
(d) Influence of leaching temperature 
T/K: 313; 323; 333; 343; 353; 363 
Figure 3 Manganese extraction vs. time at different con-
ditions 
Chin. J. Chem. Eng., Vol. 18, No. 5, October 2010 733
reactions is the slowest step can be expressed as fol-
lows, respectively [22]: 
G S
A0 f
S 0
3k M
x c t k t
a r
 (5) 
2
S Aeff A03 d2
S 0
221 1
3
M D cx x t k t
a r
 (6) 
1
S c3 A0 r
S 0
1 1
M kx c t k t
a r
 (7) 
where kf, kd and kr are calculated from Eqs. (5), (6) 
and (7), respectively. 
To obtain the leaching kinetic equation, the ex-
perimental data in Fig. 3 were transformed and fitted 
to Eqs. (5), (6) and (7), respectively. As shown in Fig. 4, 
Eq. (6) fitted the data best for the range of experiments 
and the correlation coefficients (R2) were greater than 
0.98. It is suggested that the reductive leaching of py-
rolusite was controlled by the diffusion through the 
ash/inert solid layer. Hence, the insoluble oxide min-
erals (hematite, quartz, kaolinite, etc.) associated with 
pyrolusite may act as the ash/inert layer [2, 14]. 
Leaching kinetic constant varies with the inverse 
square of initial particle size for diffusion-controlled 
reactions in Eq. (6), or the inverse of the size or the 
surface chemical reaction in Eq. (7). Fig. 5 shows the 
quantitative relationship between the apparent rate 
constant (kd) and particle size ( 20r ), indicating the 
linear relationship between kd and 20r with R
2 of 0.96. 
It is affirmed that the leaching of pyrolusite with 
MSW is controlled by the diffusion of reactant 
through a solid product layer. 
The apparent activation energy was determined 
based on the Arrhenius equation: 
a
d
1ln ln
E
k A
R T
 (8) 
The plot of lnkd versus 1/T data for the six tempera-
tures is linear (Fig. 6). The apparent activation energy 
(Ea) was, hence, determined to be 57.5 kJ·mol–1, 
which is about twice of that reported by Lasheen et al. 
[14]. The reason may be that the diffusion of the mac-
romolecular reductive agents in MSW is slower than 
that of small molecular reductive agents in molasses. 
To decide the apparent reaction order with re-
spect to reagent concentration, the kd values for each 
H2SO4 concentration and COD were determined from 
Figs. 3 (b) and 3 (c); and the plots of lnkd versus lncH 
or lncW were obtained. As shown in Fig. 7, the order 
of reaction was found be 0.52 with respect to H2SO4 
concentration and 0.90 with respect to COD. It seems 
that the leaching rate of manganese depends more on 
the concentration of reductive organics. 
According Eq. (6), the apparent rate constant kd 
can be expressed as follows: 
0.52 0.900
d H W2
0
57500exp
kk c c
RTr
 (9) 
 
(a) Influence of particle size of manganese ore 
d0/mm: 0.0134; 0.078; 0.068; 0.046 
 
(b) Influence of sulfuric acid concentrations 
cH/mol·L–1: 0.4; 0.6; 0.8; 1.0; 1.2 
 
(c) Influence of COD of waste water 
cW/g·L–1: 10.0; 20.0; 30.0; 37.7; 50.0 
 
(d) Influence of leaching temperature 
T/K: 313; 323; 333; 343; 353; 363 
Figure 4 Plot of 
2
321 (1 )
3
x x vs. time at different 
conditions 
and 0k 119.09 mm
2·(mol·L–1)–0.52·(g·L–1)–0.90·s–1 as 
calculated from the provided data in Fig. 4. 
Chin. J. Chem. Eng., Vol. 18, No. 5, October 2010 734
Figure 5 Relationship between rate constant and average 
initial particle diameter 
Figure 6 Arrhenius plot of reaction rate against reciprocal 
temperature 
Figure 7 Determination of reaction order for pyrolusite 
dissolution with respect to H2SO4 and COD of molasses 
alcohol wastewater
COD; H2SO4 concentrations 
By the above analysis, the kinetic model of py-
rolusite leaching by MSW is: 
2
0.52 0.903 H W2
0
2 119.09 575001 1 exp
3
x x c c t
RTr
 
(10) 
To determine the adaptability of Eq. (10), the x 
values calculated by Eq. (10) were compared with that 
obtained from the experiments. The calculated con-
versions in Fig. 8 agree well with the experimental 
data with the average relative error of 0.00107. 
4 CONCLUSIONS 
The reductive leaching kinetics of pyrolusite with 
molasses alcohol wastewater in dilute H2SO4 solution 
was investigated. The main parameters influencing 
leaching rate, such as leaching H2SO4 concentration, 
molasses alcohol wastewater concentration, particle 
size and stirring speed, were studied. The results re-
veal that the leaching rate increases gradually with the 
increaseof H2SO4 concentration, COD of molasses 
alcohol wastewater and leaching temperature and with 
the decrease of the size of particles. 
The kinetic analysis shows that the leaching 
process is controlled by diffusion through the insolu-
ble layer of the associated minerals. The leaching 
process follows the kinetic model: 
2
0.52 0.903 H W2
0
2 119.09 575001 1 exp
3
x x c c t
RTr
 
with apparent activation energy of 57.5 kJ·mol–1. The 
experimental results also show a reaction order of 0.52 
with respect to H2SO4 concentration and a reaction 
order of 0.90 with respect to molasses alcohol wastewa-
ter. The effects of particle size and rate constant further 
confirm the mechanism of the leaching kinetic model. 
NOMENCLATURE 
A constant of the Arrhenius equation 
b stoichiometric coefficient 
cA0 concentration of component A in bulk phase, mol·L–1 or g·L–1 
cH H2SO4 concentration, mol·L–1 
cW COD of molasses alcohol wastewater, g·L–1 
DAeff effective diffusion coefficient of component A in the ash/inert 
solid layer, m·s–2 
Ea apparent activation energy, kJ·mol–1 
kc chemical kinetic constant, m·s–1 
kd apparent rate constant when ash/inert solid layer diffusion con-
trols, min–1 
kf apparent rate constant when liquid film diffusion controls, min–1 
kG mass transfer coefficient of component A in the liquid film, m·s–1 
kr apparent rate constant when chemical reaction controls, min–1 
k0 kinetic constant, mm2·(mol·L–1)–0.52·(g·L–1)–0.90·s–1 
MS molecular weight of MnO2 
R gas constant, 8.314 J·mol–1·K–1 
r0 initial radius of pyrolusite, mm 
T reaction temperature, K 
t reaction time, min 
x fraction of reacted pyrolusite 
s pyrolusite density, kg·m–3 
Figure 8 Mn extraction (x) of calculation and experiments
Chin. J. Chem. Eng., Vol. 18, No. 5, October 2010 735
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