Effect of Ethanedioic Acid Additives on the Dissolution of Manganese Oxides in Sulfuric Acid Solution

Article abstract of DOI:10.1134/S0036023618030087

The effect of ethanedioic acid additives on the rate of manganese(IV) oxide dissolution in sulfuric acid solutions was studied by kinetic and electrochemical methods. Interaction modes were established, and some details of the studied process mechanism we

Full text of DOI:10.1134/S0036023618030087

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2018, Vol. 63, No. 3, pp. 314–318. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © E.B. Godunov, A.D. Izotov, I.G. Gorichev, 2018, published in Zhurnal Neorganicheskoi Khimii, 2018, Vol. 63, No. 3, pp. 296–300.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Effect of Ethanedioic Acid Additives on the Dissolution
1
of Manganese Oxides in Sulfuric Acid Solutions
E. B. Godunova, b, *, A. D. Izotova, and I. G. Gorichevb, c
aKurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
bMoscow State University of Mechanical Engineering, Moscow, 107023 Russia
cMoscow State Pedagogical University, Moscow, 119992 Russia
*e-mail: gen225@mail.ru
Received April 20, 2017
Abstract⎯The effect of ethanedioic acid additives on the rate of manganese(IV) oxide dissolution in sulfuric
acid solutions was studied by kinetic and electrochemical methods. Interaction modes were established, and
some details of the studied process mechanism were elucidated. Interaction schemes corresponding to the
observed kinetic dependences were proposed.
DOI: 10.1134/S0036023618030087
The dissolution of manganese(IV) oxide was stud-
ied in sulfuric acid solutions of ethanedioic acid within
the range of 0.01 N ≤ c(H2С2O4) ≤ 0.8 N and 293 K ≤
Т ≤ 353 K at ω ≈ 700 rpm. The sulfuric acid concen-
tration of 0.2 N was constant in all experiments.
Electrochemical methods of study. Electrochemical
experiment was performed in an electrochemical cell
on a Pt electrode in a temperature-controlled reactor
with sulfuric acid solutions (at specified pH values) on
an IPC PRO MF potentiostat at different sweep rates
in the potentiodynamic mode.
The development of hydrochemical processes for
the extraction of metals from oxide and sulfide ores
requires the detailed study of mineral dissolution pro-
cesses [1–11]. In this context, the search for reagents
and the determination of kinetic parameters for MnO2
dissolution processes and details for the mechanism of
MnO2 interaction with reagents are topical [10, 11]. It
is of interest to study the kinetic processes of manga-
nese(IV) oxide dissolution in sulfuric acid media with
addition of ethanedioic acid as a reducing agent.
The literature data on the kinetic MnO2 dissolution
processes with the use of organic solvents is almost
unavailable [6–8, 10]. This work is devoted to studying
the MnO2 dissolution kinetics in sulfuric acid media
with addition of ethanedioic acid as a reducing agent
by kinetic and electrochemical methods.
CALCULATIONS
The obtained kinetic data were presented in the
dissolved oxide content (α)–time (τ, min) coordi-
nates, where α = Dτ/D∞ (Dτ and D∞ are the absor-
bances of a filtrate solution sample at a time moment
τ and after the complete dissolution of an oxide portion
(τ∞), respectively) was determined by the spectropho-
tometric method [7, 13] with the use of formaldoxime.
The kinetic parameters of dissolution were calculated
by analyzing the obtained data in MathCad 11.0 to
determine the dissolution rate (W, min–1) by the equa-
tion of heterogeneous reaction kinetics [5, 14] (revers-
ible first-order reaction equation) as
EXPERIMENTAL
Sulfuric acid (extra pure grade), ethanedioic acid
(H2С2O4) (extra pure grade), and MnO2 synthesized
via the calcination of Mn(NO3)2 · 6H2O in air at 400°C
by the method [12] were used as reagents.
Kinetic methods. An experiment was performed in
a temperature-controlled reactor at an agitator rota-
tion speed of 700 rpm. The total concentration of
manganese(II) ions that had passed into a solution was
determined spectrophotometrically with the use of
formaldoxime by the method [7, 13].
α = α∞(1 – exp(–Wτ),
(1)
where α is the dissolved oxide content, unit fractions,
and α∞ is the equilibrium content of Mn2+ ions in a
solution after the maximal dissolution of oxide.
The kinetic models reflecting the effect of the acid
concentration (c, N), the temperature (T, K), and the
electrochemical potential (E, V) on the dissolution
1
The material was presented at the VII Conference of Young Sci-
entists on General and Inorganic Chemistry (Institute of Gen-
eral and Inorganic Chemistry, RAS, April 11–14, 2017).
314

EFFECT OF ETHANEDIOIC ACID ADDITIVES
315
6
5
logW
α
4
–0.5
–1.0
3
0.8
2
0.6
0.4
0.2
–1.5
1
–2.0
–2.5
–2.0
–1.5 logc (H2C2O4)
Fig. 2. logW versus logc(H С O ) for the dissolution of
2
2 4
MnO .
2
0
50
100 τ, min
Effect of solution pH. The effect of solution pH
(c(H2SO4) = 0.2 N, c(H2C2O4) = 0.01 N) on the
MnO2 dissolution process was also studied to deter-
mine the species that limits the dissolution rate. The
results were shown in Fig. 3 and compared with the
distribution diagrams (plotted by the method [15]) for
various H2C2O4 species at different solution pH val-
ues. The maximum dissolution rate (W) was found to
occur at pH 1.6 0.2 corresponding to the species
Fig. 1. Dissolved MnO content (α) versus time (τ) in a 0.2 N
2
H SO solution (353 K, рН 1.5) at c(H C O ) of (1) 0.01,
2
4
2 2 4
(2) 0.015, (3) 0.02, (4) 0.04, (5) 0.06, and (6) 0.08 N.
Points are experimental data, and lines are Eq. (1).
rate (W, min–1) were constructed by means of orthog-
onal factorial design of experiment: lnW was taken as
a response function, and lnc and 1/T were used as
control factors.
−, which determined the controlling oxide dis-
HC2O4
solution rate (W).
In this case, the dissolution rate W is described by
RESULTS AND DISCUSSION
the kinetic model
The effect of the ethanedioic acid concentration on
the MnO2 dissolution rate is illustrated in Fig. 1 as an
experimental dissolved oxide amount versus time plot.
The experimental data were used to calculate the
MnO2 dissolution rates (W) depending on the ethane-
dioic acid concentration by reversible first-order reac-
tion equation (1) (Table 1).
The plot of the data from Table 1 in the logarithmic
coordinates logW–logc(H2С2O4) (Fig. 2) has pro-
vided the possibility to describe the dissolution pro-
cess by kinetic model (2), in which the reaction order
for oxalate ions was 0.7, namely,
[HC2O4−]
[H +] + K1 [HC2O4−] + K2
[H +]
W = W 0
,
(3)
where W0 is the reaction rate constant, min–1, K1 and
K2 are the adsorption equilibrium constants (for the
adsorption of hydrogen ions H+ and hydrooxalate ions
HC2O4−
on the surface of MnO2, K1 = 0.02 and K2 =
0.003), and [H+] and [
−] are the concentrations
HC2O4
of hydrogen and hydrooxalate ions, respectively.
Effect of the temperature. To estimate the character
of kinetic or diffusion hindrances occurring in the
MnO2 dissolution process, the study was performed
within a temperature range of 293–353 K. It has been
established that the oxide dissolution rate (W) grows
with increasing temperature. It is unreasonable to per-
form the studies at higher temperatures, as ethanedi-
oic acid decomposes at 373–403 K. The activation
energy was calculated by the Arrhenius equation [5,
10, 14, 16]. Based on the obtained experimental data
and the analysis of the logW–1/T dependence, Ea =
80 2 kJ/mol was calculated.
logW = 0.56 + 0.7logc(C2H2O4).
(2)
Table 1. Kinetic characteristics of the dissolution process
(c(H2SO4) = 0.2 mol/L, pH 1.50, 353 K)
W,
min–1
W,
min–1
c(H2С2O4), τ0.5
,
c(H2С2O4), τ0.5,
N
min
N
min
0.01
0.015
0.02
108
30
18
0.03
0.04
0.05
0.04
0.06
0.08
12
8
0.09
0.14
0.22
5
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 63 No. 3 2018

316
GODUNOV et al.
Electrochemical study of the oxidation of ethanedi-
–logW
β
H2C2O4
oic acid on a Pt electrode. To confirm the effect of the
potential (E) on the H2C2O4 oxidation rate and clarify
the mechanism of the dissolution of manganese oxides
in sulfuric acid solutions with ethanedioic acid addi-
tives, we studied the electrochemical features of the
behavior of H2C2O4 in the process of polarization on a
Pt electrode.
C2O42–
HC2O–4
0.8
0.6
0.4
0.2
1.4
1.6
1.8
2.0
The results of the experimental study of the anode
voltage–current characteristics for the oxidation of
ethanedioic acid on a Pt electrode (the dependence of
the polarization potential on the H2C2O4 concentration
at a scanning rate of 0.05 V/s in the background electro-
lyte with c(Na2SO4) = 0.2 N) are shown in Fig. 4.
0
2
4
pH
6
The anode current (I) was found to increase with
increasing H2C2O4 concentration. To determine the
reaction orders for the anode process, the obtained
experimental data were replotted in the coordinates
logI–logc(H2C2O4) (Fig. 5).
Fig. 3. logW versus solution pH for the dissolution of
MnO in sulfuric acid solutions of H C O in comparison
2
2 2 4
with the distribution diagram of an aqueous H C O solu-
2
2 4
tion (β is the content of ions in a solution). Points are
experimental data logW–pH.
The analysis of the electrochemical oxidation of
H2C2O4 (Figs. 4 and 5) at solution pH of 6 has allowed
us to establish that the process rate passes through a
maximum at E = 1.4 V for a normal hydrogen elec-
trode and is characterized by the following parameters
(before its maximum):
From our data it follows that MnO2 dissolution
kinetic model (3) taking into account the temperature
has the form
[H +]
[HC2O4−]
E
W = W 0
exp −
⎛
⎞
⎟
a
,
(4)
⎜
[H +] + K1 [HC2O4−] + K2
⎝ RT ⎠
⎛
⎞
∂E
where Ea is the activation energy, kJ/mol, R is the uni-
versal gas constant, and T is the temperature, K.
(5)
= 0.07 V,
⎜
⎟
2−
∂ log I
⎝
⎠
pH,c(C2 O4
)
I × 105, A/cm2
11
7
10
15
10
5
9
8
6
5
4
3
2
1
0
1.0
1.1
1.2
1.3
E, V
Fig. 4. Anodic voltage–current characteristics for the oxidation of H C O at different concentrations of (1) 0.0000, (2) 0.0001,
2
2 4
(3) 0.0002, (5) 0.0004, (6) 0.0005, (7) 0.0006, (8) 0.0007, (9) 0.0008, (10) 0.0009, and (11) 0.0010 mol/L on a Pt electrode
depending on the electrode potential.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
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EFFECT OF ETHANEDIOIC ACID ADDITIVES
317
where βz = 0.5 (β is the transfer coefficient, and z is
the number of electrons participating in the reduction
of manganese oxide).
logI
–3.2
Analyzing the effect of different factors on the
oxide dissolution rate, we have found that an essential
part is played by the potential jump, the concentration
of hydrogen ions and hydrooxalate ions of ethanedioic
acid, as reflected in Eq. (12). Based on the analysis of
the obtained kinetic and electrochemical data on the
dissolution processes, kinetic model (13) of the MnO2
dissolution rate in sulfuric acid solutions of ethanedi-
oic acid has been proposed. It has been revealed that
the cathode MnO2 reduction current will be higher
than the dissolution current, thus indicating that the
dissolution rate is determined by the rate of the transi-
tion of ions into a solution instead of the rate of their
reduction, i.e.,
–3.4
–3.6
–3.8
logc(H2C2O4)
–1.0 –0.8 –0.6 –0.4 –0.2
Fig. 5. Logarithmic plot of the current logI versus ethane-
dioic acid concentration logc(H C O ).
2
2 4
[HC2O4−]
[H+] + K1 [HC2O4−] + K2
Ea βzFE
[H+]
W = W 0
⎛
⎞
∂ logI
∂ logc(C2 O4 )
.
(6)
= 0.6 0.2 A
⎜
⎜
⎝
⎟
⎟
⎠
2−
(13)
pH,E
⎛
⎞
⎟
⎛
⎜
⎞
st
⎟
× exp −
exp
.
⎜
⎝ RT ⎠
⎝ 2RT ⎠
The anode current order calculated for H2C2O4
from the dependence logI–logc(H2C2O4) (Fig. 5) is
0.6 0.2.
The proposed equation (13) characterizes the
dependence of the dissolution rate on the concentra-
tion of hydrogen and hydrooxalate ions, Est, and T,
and it agrees with experimental data.
∂ logI
∂pH
⎛
⎞
,
(7)
(8)
= 0.65 V (pH < 2.5)
⎜
⎝
⎟
⎠
Clarification of the dissolution mechanism. The
experimental data obtained in this work allow us to
propose the following mechanism of MnO2 dissolu-
tion in sulfuric acid solutions of ethanedioic acid with
consideration for the theory of the structure of a dou-
ble electrical layer [16]. The dissolution process can be
described in the form of several intermediate stages
−
c(C2 O24−),E
∂ logI
∂pH
⎛
⎞
.
= −0.50 V (pH > 2.5)
⎜
⎝
⎟
⎠
c(C2 O24−),E
Based on the results of experimental studies, it is
possible to reveal that the mechanism of the anode
oxidation of
to the following stages:
− ions on Pt electrodes corresponds
with consideration for the adsorption of
on active sites as
ions
HC2O4
HC2O4
≡Mn − OH0s+ An− + H+
Stage 1: H2C2O4 – ē → HC2O4, s + H+,
(9)
(10)
(11)
θ1
k1
+
−
−
Stage 2: HC2O4,s → HCO2, s + CO2,
Stage 3: HCO2, s – ē → CO2 + H+.
⎯⎯⎯→
←⎯⎯⎯
k−1
≡ MnOH2 ...Ans + HC2O4
θ2
k
⎯⎯⎯→
←⎯⎯⎯
+
−
−
2
≡MnOH2 ...HC2O4,s+ An
k
−2
θ3
The stagewise oxidation of H2C2O4 is confirmed by
the data of potentiometric measurements (one-elec-
tron process) and the results of estimating the slope of
the Tafel “overpotential–logI”dependence.
k3
⎯⎯⎯→ dissolution products,
where is the unfilled oxide surface degree, and
θ3
θ1
θ2
the degree of oxide surface filling with complexes
From the obtained data it follows that the rate of
anodic H2C2O4 oxidation on a Pt electrode depending
on the stationary potential (Est) is described by elec-
trochemical model (12) coinciding with empirical
equation (4)
≡MnOH+2 ...An−s
≡MnOH+2 ...HC2O4,s
−
and
, respec-
tively, and k1, k–1, k2, k–2, and k3 are the equilibrium
constants of the corresponding reactions.
CONCLUSIONS
[HC2O4−]
[H+]
I+ = k+
The dependences of the MnO2 dissolution rate on
the ethanedioic acid concentration, pH, temperature,
electrochemical potential, and process time have been
studied. The kinetic model providing the calculation
−
[H+] + K1 [HC O ] + K2
2
4
(12)
Ea
βzFE
⎝ 2RT ⎠
⎛
⎞
⎟
⎛
⎜
⎞
st
⎟
× exp −
exp
,
⎜
⎝ RT ⎠
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 63 No. 3 2018

318
GODUNOV et al.
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of the dissolution rate in the simultaneous presence of
these parameters has been derived. The oxide dissolu-
tion process occurs in the kinetic modemode. The
most probable rate-controlling stage is the diffusion of
ions towards the oxide surface.
The kinetic and electrochemical characteristics
obtained for the behavior of MnO2 in sulfuric acid
solutions of ethanedioic acid can be used for the opti-
mization of existing technologies and the development
of new technologies in hydrometallurgy.
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Vol. 63
No. 3
2018