Recovery of bi-metallic oxalates from low grade Mn ore for energy storage application

Hydrometallurgy 189 (2019) 105139

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Recovery of bi-metallic oxalates from low grade Mn ore for energy storage application Sushree Pattnaika, Priyanka Mukherjeea, Rasmita Barikb,c, Mamata Mohapatraa,

T

⁎

a

Hydro& Electrometallurgy Department, Institute of Minerals and Materials Technology, Bhubaneswar 751013, India University of Witwatersrand, School of Chemistry, South Africa c Indian Institute of Technology Delhi, Department of Chemistry, Hauz Khas, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Low grade manganese ore Leaching Fe-Mn oxalate Photolysis Pseudo capacitance

The depletion of primary resources has led to exploitation of secondaries which are a promising source for the future if explored wisely. The authors report a novel approach of leaching low grade manganese ore and evaluation of the energy storage properties of the precipitated oxalates. The reaction rates were around 3.57 and 2.05 approximately in presence of oxalic acid and 1.92 and 0.43 respectively in presence of acid mix. The energy storage capacity of the precipitated metal oxalates has been thoroughly investigated. Oxalate sample with Fe:Mn ratio of 0.67 showed the highest energy storage behaviour with 86.9 Fg−1 specific capacitance values with 94.3% retention after 1000 cycles. Energy density of 15.4 WhKg−1 and power density of 80 WKg−1 was obtained at 0.1 Ag−1 current density.

1. Introduction Manganese has been the pivotal component for many industrial activities (Pagnanelli et al., 2004a, 2004b; Gharabaghi et al., 2010; Sahoo and Rao, 1989) and the growing demand has led to rapid depletion of its deposits. Thus to suffice its growing need, manganese containing secondaries have become significant, which can be explored for better economical prospects as well as reduce the amount of sludge generation which would otherwise affect the environment (Hilson, 2003). Structurally half of the chemical composition of the low-grade manganese ores comprise of a good percentage of iron manganese oxide (Mn–Fe oxide ores) (Zhang et al., 2017; Baioumy et al., 2013) and the literature provides strong evidence that the separation of the inherent iron oxide using conventional processes is very difficult (Cheng et al., 2009; Hariprasad et al., 2007; Abbruzzese, 1990). Low grade manganese ores have been reported to be leached efficiently by the use of many reductants (Xue et al., 2016a, b) such as hydrogen peroxide (Sahoo et al., 2001), oxalic acid (Acharya, 1991), biomass (Xue et al., 2016a,b) under mild temperature conditions (< 90 °C) (Senanayake, 2004), thiosulfate (Bafghi et al., 2008), iron (De Michelis et al., 2009) etc. Besides that, many other reagents like tartaric acid, phosphoric acid (Chen et al., 2018a,b) have recently been proved to be efficent leachant in multimetallic system. However, these leaching processes appear complicated for industrial application because of convoluted downstream purification process and serious water pollution (Hill, 2009). ⁎

Many direct reductive acid leaching technologies have been developed to treat low-grade manganese ore using agents like H2SO4 and oxalic acid (Azizi et al., 2012; Sahoo et al., 2001; Pagnanelli et al., 2004a,b; Lee et al., 2001), nitric acid (Sayilgan et al., 2010), hydrochloric acid and citric acid (Barik et al., 2016) and surfactants (Hariprasad et al., 2009). These resulted in good leaching efficiencies, but the limitations were high temperature, residual acid generation, high cost and environmental pollution. A process for Mn2+recovery from manganese ores by leaching with H2SO4 has been reported (De Michelis et al., 2009) where the leachate bears low pH and usually high concentrations of impurities, such as Al, Co, Cu, Fe, Ni, Zn, etc., which get dissolute along with Mn (Laus et al., 2007). The common methods found in literature to deal with the removal of impurities and recovery of manganese from solutions and leachates include solvent extraction (Resende et al., 1999), precipitation by hydroxides and carbonates (Sharma, 1992), quicklime, sulphides and jarosite and adsorption methods in which chitosan micro pellets are used to reduce the acidity and remove metals from the wastewater of coal mines. Evaluation of the best conditions for improving the yield in manganese extraction has also been the focus of some investigations (Sharma, 1992; Deguillaume et al., 2005). Therefore, a simple and environmentally acceptable process must be designed to avoid adverse environmental impact and the mechanical complexity of the ores. Organic acid leaching process may be a viable option in this context to environment issue. In this study, a novel one-pot recovery process is designed using H2SO4 and oxalic acid

Corresponding author. E-mail address: [email protected] (M. Mohapatra).

https://doi.org/10.1016/j.hydromet.2019.105139 Received 13 September 2018; Received in revised form 6 August 2019; Accepted 25 August 2019 Available online 26 August 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved.

Hydrometallurgy 189 (2019) 105139

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−0.40 V at a sweep rate of 5–100 mVs−1. 0.5 M KOH was used as electrolyte for all electrochemical study. The metal oxalates were dispersed with carbon black and Nafion. The electrode material prepared by (8:1) ratio of material: carbon black dispersed in required amount 10% Nafion and pasted over glassy carbon electrode. It was dried properly prior to the electrochemical experiments. Charge–discharge tests were performed using chrono-potentiometric method at different current density. The EIS (Electrochemical Impedance) study was done at the frequency range of 1 Hz to 1 MHz.For the half-cells (3-electrode configurations), the specific capacitance (Csp), maximum specific power density (Pmax) and specific energy density (Esp) was evaluated using the conventional equations:

as a leaching agent and a precipitant, respectively, which is more efficient than the previous process wherein the precipitation of the material could be feasible in the leached solution without addition of further reagent. The manganese and iron ions were efficiently leached and simultaneously precipitated by controlling mixture of H2SO4 and oxalic acid in a single process. In-situ precipitation of oxalates has been carried out under sunlight. The precipitated pure or mix oxalate compound was used to study electrochemical behaviour, in a single-step reaction without purification process. With this objective in mind, the present study focuses on extraction of manganese and iron from low grade manganese ores with oxalic acid as the reducing agent and precipitant in sulphuric acid medium. The influences of different relevant factors, such as temperature and oxalic acid, and sulphuric acid concentrations, on the leaching rate of manganese and iron were investigated. Further in-situ precipitation of oxalate in presence of sunlight has been reported.

Csp (Fg−1) =

i Δt mΔV

(1)

Energy density (WhKg−1) =

2. Materials and methods

Power density (Wkg−1) = 2.1. Materials

1 CspV2 2

(2)

E Δt

(3) −1

where: Csp is the specific capacitance (Fg ), i is the specific current (A), Δt is the discharging time, m is the mass loading (g), and ΔV is the applied potential window.

The ore used in this study was collected from M/s Gujarat Mineral Development Corporation (GMDC), Ahmadabad, Gujarat. The ores were crushed, grinded and screened to obtain 100% −150 mesh B.S·S fractions (100 μm).

3. Results and discussion Chemical analysis, XRD pattern and SEM analysis of the ore samples provided in Fig. S1 confirmed the ore is in pyrolusite form. Spot analysis through SEM-EDS as given in Table S1 shows that the particles of different morphology have different composition.

2.2. Leaching The leaching experiments were carried out in 250 ml conical flask maintaining the pulp density at 10% and size fraction of the ore at −100 μ. Leaching was carried out by varying the parameters like H2SO4concentration, oxalic acid concentration, and temperature. The leached slurry was filtered and the residue washed and dried. The contents of Mn and other metal in the liquor were analyzed quantitatively by Atomic absorption spectrophotometer (Perkin Elmer Model AA200).

3.1. Leaching studies 3.1.1. Effect of different acid on leaching of Fe and Mn The effect of H2SO4 concentration on the leaching efficiency of manganese, iron and aluminium from the studied ore sample was investigated. The sulphuric acid concentrations were varied from 2.5% to 15% (v/v), temperature of solution was maintained upon the level of 80 °C with a pulp density of 10% (wt/v) and particle size of -150mesh BSS. The results are shown in Fig.S2. However, increasing acid concentration from 2.5% to 15% (v/v), manganese extraction increased from 0.8928% to 6.932% whereas the % leaching of iron and aluminium increased from 8.8% to 98% and from 14.55% to 99.83% respectively. In general, the rate of recovery of these metals depends on the type of acid used and the initial temperature of the process as well as their interaction (Sobianowska-Turek, 2018). In order to study the effect of oxalic acid concentration leaching experiments were performed by varying the oxalic acid concentration from 0.5 M to 1.5 M under 10% pulp density at 80 °C for 4 h using sample sieved at −150 mesh BSS. It is observed that by increasing the oxalic acid concentration from 0.5 to 1.5 M, Fe recovery (Fig. 1a) after 4 h leaching increased from 1.533% to 65.93%. However, manganese recovery is increased very small from 1 to 10.6% (Fig. 1b). From the data it can be observed that effect of increasing oxalic acid concentration in leaching of manganese is not much as compared to iron. Again, leaching experiments were carried out using the combination of acid keeping 5% H2SO4 as common with varying the amount of oxalic acid from 0.5 M to 1.5 M. The other leaching conditions were fixed at a pulp density of 10% at 80 °C for 4 h and average particle size −150 mesh BSS. It is observed that by increasing the oxalic acid concentration from 0.5 to 1.5 M, Fe recovery after 4 h leaching increased from 22. % to 98.6% (Fig. 1c). Though it is reported however, manganese recovery is increased from 85.24% to 99.6% (Fig. 1d). From the data it can be observed that leaching of manganese in combination of H2SO4 and oxalic acid is more favourable than only H2SO4 or oxalic acid. The greater the oxalic acid concentration the higher the leaching efficiency of manganese and

2.3. Characterization The X-ray diffraction (XRD) of the powder samples were taken using PAnalytical Empyrean Powder Diffractometer series 2 with Cu Kα (λ = 1.54 Å) radiation at a scan speed of 1.2° min−1. SEM (Scanning electron microscope) was used for the determination or identification of morphology as well as quantitative analysis of the sample in a ZEISS EVO 18. The sample was initially dispersed with ethanol followed by sonication for 0.5 h, and was drop casted on an Al foil, of 1 × 1 cm2 was used. The EDX was used for the quantitative analysis of different elements for a particular area. The acceleration voltage was 5 keV, while an in-lens detector was employed with a working distance of 5 mm. The sample morphologies were investigated after the photo electrochemical measurements and sample positions coincided with the illuminated area. Cyclic-voltammograms were recorded using a computer-controlled CHI 660E electrochemical analyzer (CHI, USA). 2.4. Crystallization of oxalates Leached solution was exposed to sunlight for 4 h with an average temperature of 37 °C. Precipitate so obtained was filtered and dried and quantitatively analyzed. 2.5. Electrochemical tests Cyclic voltammetry studies were performed using a two compartment, three-electrode cell having a glassy carbon working electrode (area ¼ 0.07 cm2), a platinum auxiliary electrode and Ag/AgCl (0.5 M KOH) as the reference electrode in the potential window of −1.2 V to 2

Hydrometallurgy 189 (2019) 105139

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80

20

% Fe Extraction

% Mn Extraction

a

Oxalic acid [M] 0.5M 60 1M 1.5M

15

40

0 0

1

0.5M 1M 1.5M

10

20

2

3

5 0

4

0

1

2

Time (h)

80 Oxalic acid [M]

c

100 Oxalic acid [M]

0.5M 1M 1.5M

60 40 20 0 0

1

2

3

4

3

4

0.5M 1M 1.5M

80 60

20 0

0

1

2

3

4

Time(h)

Fig. 1. Effect of oxalic acid on (a) % Fe and (b) % Mn extraction. Effect of oxalic acid in combination with 5% [H2SO4] (c) % Fe and (d) % Mn extraction. Conditions: P.D. 10% (wt/v), temp 80 °C (353 K) and particle size −150 mesh BSS.

2MnOOH + 2H+ → MnO2 + Mn+2 + 2H2 O

(13)

3.1.3. Effect of particle size on leaching efficiency The effect of particle size of LMn was investigated under the conditions of 10% pulp density, 5% of sulphuric acid, 1 M of oxalic acid and 80 °C. Results are reported in Fig. 2. The results show that smaller the particle size of ore, faster was the leaching of manganese. This was due to the effect of increase in the reaction surface area of manganese minerals (Xue et al., 2016a,b). As 100 μm gave better results of leaching efficiency of manganese therefore this initial particle size was used in remaining experiments. Fig. 3. shows the quantitative relationship between the apparent rate constant (kd) and particle size (r0−2) indicates a linear relationship between kd and r0−2 with R2 of 0.984 for Fe and 0.989 for Mn.

other studied metal values. Thus, the reduction of manganese in its minerals to its bivalent state is a pre-requisite for its dissolution as mentioned above. The reduction of tetravalent manganese by oxalic acid follows the reaction:

MnO2 + H2 C2 O4 + 2H+ → Mn+2 + 2CO2 + 2H2 O

(12)

3.1.2. Effect of temperature on leaching of Fe and Mn in combination of acid The effect of temperature on the dissolution of manganese and iron was investigated in 5%H2SO4 and 1 M oxalic acid solution using ore ground to −150 mesh BSS at temperatures of 60–120 °C. The results obtained are shown in Fig. S3. It is observed that by increasing the temperature from 60 to 120 °C, manganese recovery at 4 h leaching time increased from 80.5% to 98.1% whereas iron recovery at 4 h of leaching time increased from 56% to 96.87%. It seems kinetic manganese extraction is faster than the iron extraction at higher temperature.

d

40

Time (h)

Mnx Oy + 2H+ → zMnOOH + Mn+2

In both cases, low-solubility MnO2 is formed on the oxide surfaces.

Time (h) % Mn Extraction

% Fe Extraction

100

transfer) are higher than the rate constants for its dissolution (via ligand-assisted dissolution). Again, the production of Mn (II) depends on the pH and oxalic acid concentration. MnxOy dissolution mechanisms proposed the negative orders for protons and positive orders for manganese ions (Artamonova et al., 2013):

b

Oxalic acid [M]

(4)

The main H2SO4 leaching reaction for low grade manganese ore are presented below;

2 Fe2 O3 + 3 H2 SO4 → Fe2 (SO4 )3 (aq) + 4 H2 O

(5)

Mn2 O3 + H2 SO4 → MnSO4 (aq) + H2 O

(6)

Al2O3 + H2 SO4 → Al2 (SO4 )3 (aq) + H2 O

(7)

MnO + H2 SO4 → MnSO4 + H2 O

(8)

3.2. Kinetics study It is concluded from the results discussed above that the main parameters which influence the dissolution kinetics are temperature, H2SO4 concentration and oxalic acid concentration. The data so generated was fitted to shrinking core and diffusion-controlled models (Eqs. S1 and S2). According to the plots of two models given in Fig.S4 and S5, the kinetic study indicates that leaching proceeds through shrinking core model and is controlled by diffusion through inner layer as for this model the plots do not pass through origin. The order of reaction with respect to amount of reductant and acid concentration were evaluated from the slopes of the plots of ln kd vs. ln (variable) mentioned in Fig. 4. Orders of reaction were estimated to be 3.57 and 2.05 for iron and manganese with respect to amount of only oxalic acid respectively. Whereas order of reaction were estimated to be 1.92 and 0.43 for iron and manganese respectively with respect to amount of oxalic acid when mixture of H2SO4 and oxalic acid is used for leaching.

As shown in Reactions (5) to (8) leaching of manganese ore consumes H2SO4.SiO2 represents a main compound in a solid product after leaching. In oxalic acid, the dissolution of iron oxide is believed to take place via a photo-electrochemical reduction process where charge transfer took place between the predominant oxalate species, namely ferric oxalate Fe(C2O4)33−, ferrous oxalate Fe(C2O4)22− and the oxalate ligand on the iron oxide surface (Panias et al., 1996). Since in our system the iron in the original matrix is only 3.37% the concentration of total Fe leached is very low and could finally be dissolved by oxalate. Oxidation of oxalate to form carbonic acid or carbon dioxide,

HC2 O4 – = H+ + 2CO2 + 2e−

(9)

Reduction of hematite forming Fe (II) oxalate,

2H+ + Fe2 O3 + 4HC2 O4− + 2e− = 2Fe (C2 O4 )22 − + 3H2 O

(10)

The dissolution reaction is therefore:

H+ + Fe2 O3 + 5HC2 O4− = 2Fe (C2 O4 )22 − + 3H2 O + 2CO2

(11)

The charge transfer mechanism could also be assisted by the presence of Fe (II) as experienced from previous studies. In case of dissolution of manganese the reaction path may involves the adsorption followed by ligand-assisted dissolution yielding Mn (III) complexes in solution, which then undergo intramolecular electron transfer, yielding Mn (II) (aq.) and oxidized chelating agent, if rate constants for the consumption of dissolved Mn (III) (via intra molecular electron

Fig. 2. Plot of leaching efficiency of manganese (%) with leaching time showing the effect of particle size of ore on manganese leaching. 3

Hydrometallurgy 189 (2019) 105139

S. Pattnaik, et al.

energy in the range of 40–100 kJ/mol (Lee et al., 2005; Uçar, 2009; Baba et al., 2015). It proves that diffusion reaction is the rate limiting step. The apparent activation energy so obtained indicates leaching process controlled by inner diffusion model (Xue et al., 2016a,b). Considering parameter concentrations and temperature, the empirical kinetic equation in combination of H2SO4 and oxalic acid for Fe and Mn can be expressed as: For Fe dissolution

1–2/3α − (1 − α)2/3 = {(kFe/r0ρ) {[C2O4 2 −]i1.92 e−1835/T} t Fig. 3. Relationship between the rate constant and average particle size.

(14)

For Mn dissolution

1–2/3α − (1 − α)2/3 = {(kMn/r0ρ) {[C2O4 2 −]i 0.43 e−1203/T} t

(15)

3.3. In-situ crystallization of metal oxalate from leach liquor Precipitates were obtained from the leached solutions obtained from only oxalic acid concentration 1.0 M (H1) and 1.5 M (H2) and combination of 5% H2SO4and oxalic acid 1.0 M (H3) and 1.5 M (H4). These precipitate are obtained due to photochemical reaction of stable complex of [Fe/Mn (C2O4) n](3-2n)+. Depending on the pH, ionic strength, and concentration of Fe/ Mn ion and oxalate, three complexes are formed: Fe/Mn (C2O4)+, which has low photochemical reactivity, and Fe/Mn (C2O4)2− and Fe/Mn (C2O4)33−, both of which undergo photochemical reduction of iron with high quantum yield. The expected chemical reactions are shown in Equation S3-S8. Manganese is also behaving in the same way as iron under similar condition. However, in mixed solution depending on the concentration of species for each metal ion crystallization of mono or bimetallic oxalates took place through photolysis reaction. Depending on the concentration of oxalic acid and presence of H2SO4, the color of the collected crystallized powder changes from yellow to yellowish white. Chemical analysis of both the filtrate and crystallized powder has shown that there was no aluminium in the filtrate, neither was any aluminium content found in any of the powder. Therefore, under experimental condition aluminium impurity can be discarded in leached solution. Fig. 5(a) showed the % precipitation of iron and manganese from various leaching conditions in only oxalic acid medium and amount of iron and manganese in crystallized product. Similar study for the crystallization of the leached solution obtained in presence of combination of H2SO4and oxalic acid has been shown in the Fig. 5(b). From the figure it is observed that, the % precipitation of manganese is initially increased up to 1 M oxalic acid then decreased with further increase in oxalic acid when only one acid is used for leaching study whereas the % precipitation is drastically low when the crystallization occurred in the leached solution obtained by using combination of acid. However, the trend of increase in the % precipitation is consistent with the increase in oxalic acid concentration; % precipitation of iron is increased from 12 to 85% with increase

Fig. 4. Plot of lnk vs. ln concentration of oxalic acid for (a) Fe and (b) Mn obtained from variation of amount of only oxalic acid and for(c) Fe and (d) Mn obtained from variation of amount of oxalic acid combination with 5% [H2SO4].Conditions: P.D. 10% (wt/v), temp 80 °C (353 K) and particle size −150 mesh BSS.

Fractional order of the reaction for the dissolution rate of MnO2 with respect to oxalic acid is also reported in few literature (Godunova et al., 2012). The Arrhenius plot is given in Fig. S6. Plot between ln k versus 1/T gives a linear relationship, where –Ea/RT is slope and ln A is the intercept. According to –Ea/RT activation energy was calculated to be 18.35 and 12.03 kJ/mol for Fe and Mn extraction respectively. The diffusion controlled processes are slightly dependent on the temperature with an activation energy < 20 kJ/mol. The chemically controlled processes are strongly dependent on temperature with activation

a

30

40

20

20

10

0.6

0.8

1.0

1.2

Oxalic acid (M)

1.4

0 1.6

50

% Precipitation

% Precipitation

40

60

0 0.4

100

50

Mn Fe % Fe Content % Mn Content

80

b

60

Mn Fe % Fe Content % Mn Content

40 30

40

20

20

10

0 0.4

0.6

0.8

1.0

1.2

1.4

0 1.6

5% H2SO4+ Oxalic acid (M)

% Metal ion in ppt product

80

% Metal ion in ppt product

100

Fig. 5. (a) % precipitation of Fe and Mn and % Fe and Mn content in the crystallized product obtained from oxalic acid leached solution (b) % precipitation of Fe and Mn and % Fe and Mn content in the crystallized product obtained from [H2SO4 + oxalic acid]leached solution. 4

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Fig. 6. (a) XRD patterns of the crystallized product and the standard ICDD pattern C2MnO4.2H2O (ICDD file no 00-025-0544), [Fe2Mn (C2O4)2(OH) 3(H2O) 3]. H2O. (ICDD File no 00-052-07) and FeC2O4 (H2O)2(ICDD file no-01-072-1305). (b)SEM of the crystallized product obtained under different leached liquor.

3.5. Electrochemical study

in oxalic acid from 0.5 to 1.5 M. In case of leached solution obtained using combination of oxalic and H2SO4, % precipitation of iron increased from 80 to 95%. Content of the iron and manganese in the oxalate product was dependant on the leaching condition. Iron content in the oxalate product is more when only oxalic acid is used for leaching.

Electrochemical properties of synthesized oxalate materials were studied. In the present study cyclic voltammetry studies were done with 0.5 M KOH at scan rate from 5 to 100 mVs−1 for all the samples. The charge-discharge study was done by chronoamperometric method. Cyclic Voltammograms of Fe–Mn oxalate from scan rate 5 mVs−1 to 100 mVs−1 in 0.5 M KOH electrolyte (b) the charge discharge cycle at different current densities, (c) Specific capacitance and current density relation and (d) Ragone plot showing relation between energy density and power density at various current densities were given in Fig.S7 and Fig. 7. CV curves proposed that pseudo-capacitive behaviour of all the materials. However the Sample H3 is showing most effective results among all. Simultaneous oxidation and reduction peaks were observed for the sample H3 at the −0.7 and −1 V with rectangular shape curve, and approximately symmetrical in anodic and cathodic directions, suggesting good capacitance behaviour. Both the peaks were corresponded to the Faradic pseudo-capacitive characteristics of the surface metal ion (Liu et al., 2014; Zhang et al., 2015). The current density of the same sample was varied between 0.1 Ag−1 to 0.4 Ag−1. The specific capacitance was calculated the Eq. (1). The specific capacitance values obtained from charge discharge curves for all the material were 86.9 Fg−1 at 0.1 Ag−1current density (CD) shown in Fig. 7b inset. At higher scan are the diffusion limits the movement of OH– ions are restricted only in the outer active surface whereas at low scan rates all the active surface areas were utilized for charge storage. Therefore, at low current density high specific capacitance value was obtained. The charge-discharge curves are linear and symmetric which proves the good capacitive performance of material with reversible Faradic reaction. The retention in cyclic stability was 94.3% after 1000 cycle's numbers shown in Fig. 7c. The stable charge-discharge cycle number with high conductivity indicates the fast-Faradic charge and discharge of Fe–Mn oxalate particles. The energy density and power density as a function of current density of the material was calculated by following the Eqs. (2) and (3). Current density directly varies with power density and inversely with energy density. It is evident from the graph that energy and power density are inversely related to each other. The Ragone plot with respect to current density was shown in Fig. 7c (inset). The high energy density 15.4 WhKg−1 and power density 80 WKg−1 was obtained at 0.1 Ag−1 current density. The data from electrochemical study of other samples were given in Table 1. The sample H3 under study is showing comparative electrochemical capacitance

3.4. Characterization of oxalate product The powder X-ray diffraction pattern of oxalate products obtained under different experimental condition was characterized. Precipitates were obtained from oxalic acid variation of 1.0 M (H1 with Fe:Mn = 1.48) and 1.5 M (H2 with Fe:Mn = 0.83) leached at a temperature of 80 °C for 4 h and for H2SO4 (5%) with oxalic acid variation 1.0 M (H3 with Fe:Mn = 0.67) and 1.5 M (H4 with Fe:Mn = 0.36) after being exposed in sunlight for 4 h at 37 °C.is shown in Fig. 6a. All the reflections in the pattern could be indexed based on three type of oxalate (a) C2MnO4.2H2O (ICDD file no 00-025-0544), (b) [Fe2Mn (C2O4)2(OH) 3(H2O)3]. H2O. (ICDD File no 00-052-0710) and (c) FeC2O4 (H2O)2(ICDD file no-01-072-1305). However, percentage of each phase is depended on leaching condition and concentration of metal ion in leached solution. Table S2 shows the d values with intensities of peaks obtained from XRD patterns for the precipitated products. Table S3 the ratio of main peaks of three phases is compared with the samples obtained under experimental condition. It can be observed that iron oxalate phases are formed more in the absence of H2SO4 and when low concentration of oxalic acid is used. Pure or bimetallic oxalates are formed depending on the concentration of oxalic acid without or with H2SO4. The scanning electron microscope image of the samples H1 (Fig. 6b) shows pseudo cubic particles with rough and cracked surface. However, SEM of the sample (obtained from leached solution with only 1.5 M oxalic acid) is more or less homogeneous rodshape of the crystals. The rods of oxalate are fragmented and with uneven walls. In contrast the samples obtained from the leached solution in presence of H2SO4 are lamellar and platy shaped. The plates are more distinguished with sharpen edge and uniformly self-assemble in higher oxalic acid concentration. The elemental mapping of sample H3 is shown in Fig. S7 gives the distribution of the elements in the sample and Table S4 provides the weight % and atomic % of the elements.

5

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Fig. 7. (a) Cyclic Voltammograms of Fe–Mn oxalate from scan rate 5 mV/s to 100 mV/s in 0.5 M KOH electrolyte (b) shows the charge discharge cycle at different current densities and specific capacitance values in Fg−1 with variation in CD (Inset), (c) Cyclic stability of Fe–Mn oxalate at CD 0.4 Ag−1 and calculated Energy and power densities at different CD and (d). EIS study in 0.5 M KOH electrolyte with frequency ranging from 1 Hz to 1 MHz with the fitted circuit diagram (inset).

facilities the formation reaction and prevents the agglomeration and protects the porous structure of the material. The high surface area of the Fe–Mn oxalate also provides significant and worth plat form for the development of energy storage material. Further EIS measurements of Fe–Mn oxalate was carried out at a frequency range from 1 MHz to 1 Hz in open circuit potential (OCP). Moreover, 0.5 M KOH aqueous solutions were used as the electrolyte to study the resistance behaviour of supercapacitors, and the comparative Nyquist plots are explained in Fig. 7d. The Nyquist plot obtained showed semicircular arc in high frequency region, whereas it displayed a straight line at a low frequency region. The discrete semicircle can be attributed to the surface properties of the electrodes and electrolyte. A very low internal resistance was observed from Fig. 7d (inset). It is believed that the indirect diffusion of OH– ions into the pores of oxalate material and showed faster reaction at surface which was the cause of semicircular arc (Wang et al., 2011; Yan et al., 2012). The sample shows a semicircle but a vertical line appears at low frequency region which proves that sample has ideal capacitive behaviour. The equivalent circuit was given in Fig. 7d (inset). Where R1 is the solution resistance with a value of 12.04 Ω, C is the capacitance i.e. 2.02e−7, R2 is the charge-transfer resistance with a value of 12.08 Ω and M2 represents the Warburg resistance which is related to the diffusion of the ions with a value of 0.002328. The total impedance Z was found to be 12.041 Ω using Eq. (16).

Table 1 Data obtained from electrochemical study. Current density (A/g)

Specific capacitance (F/g)

Energy density (Wh/kg)

Power density (W/kg)

H1 2.00E-01 3.00E-01 4.00E-01 5.00E-01

6.78E+01 5.45E+01 4.79E+01 4.48E+01

1.21E+01 9.69E+00 8.52E+00 7.97E+00

1.60E+02 2.40E+02 3.20E+02 4.00E+02

H2 8.00E-02 1.20E-01 1.60E-01

2.56E+01 2.24E+01 2.09E+01

4.56E+00 3.98E+00 3.72E+00

6.40E+01 9.60E+01 1.28E+02

H4 4.00E-02 8.00E-02 1.20E-01

2.78E+01 2.23E+01 1.95E+01

4.93E+00 3.96E+00 3.47E+00

3.20E+01 6.40E+01 9.60E+01

behaviour when compared to the other reported oxalate material (Table 2) (Wang et al., 2011; Yan et al., 2012; Ang et al., 2012; Aragón et al., 2008). The results confirmed that the synthesized Fe–Mn oxalate (H3 with Fe/Mn 0.67) is a promising super capacitor material for energy storage application. Specifically, high porosity of the material with high surface area value plays the vital role for the high electrochemical activity. The presence of oxalic acid and H2SO4 in the reaction medium

Total impedance Z = R1 + {R2. C/R2 + C}

(16)

where: R1 is the solution resistance, R2 is the charge-transfer resistance 6

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Table 2 Electrochemical study of various metal oxalates. Sample

Specific capacitance

Energy density

Power density

References

MnC2O4/graphene composites

122F/g at a Current Density = 0.5A/g Capacitance retention of 94.3% after 1000 cycles Current density = 0.6 A/g, 990 F/g Current density = 4 A/g, 600F/g Largest specific capacitance it can reach = 86.3 mF·cm−2 at a Current density = 0.5 mA·cm−2 Scan rate = 2 mV/s,383 F/g and scan rate = 50 mV/s, 225 F/g 500 cycles Current density = 1.34 A/g, 179 F/g, retaining ~85% of its initial capacitance. Cocoons = 48.057 F/g Rod = 49.474 F/g 26.118 F/g after 50 cycles Capacities are always above 22.3868 F/g during 75 cycles 86 F/g at a Current Density = 0.1 A/g Capacitance retention of 94.3% after 1000 cycles

Not reported

Not reported

[Liu et al., 2014]

Co0.5 Mn0.4Ni0.1C2O4*nH2Ographene SASC

Mn0.8Co0.2C2O4.nH2O

Mesoporous (FeC2O4) cocoon and rod Mesoporous FeC2O4 nanoribbons Fe-MnC2O4/carbon black composites

and C is the capacitance. The variations in electrochemical properties have been evaluated at different aging periods for sample H3. The result is provided in Fig. S9 and Table S5 which shows at 4 h sample shows maximum energy storage capacity than other aging periods.

0.46 mWh·cm

−3

46 mWh·cm

−3

[Zhang et al., 2015]

Not reported

Not reported

[Yan et al., 2012]

Not reported

Not reported

[Wei et al., 2012]

Not reported

Not reported

[Aragón et al., 2008] [Current work]

−1

15.44 WhKg

80.00 W/Kg

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4. Conclusions The authors thus report a process of leaching followed by photo assisted crystallization of various bimetallic Fe–Mn oxalates in a single step. Kinetics of the leaching reaction for single acids and acid-mix has been discussed along with the order of the reaction. The activation energy for mixed acid leaching was estimated to be 18.35 and 12.03 kJ/ mol for Fe and Mn extraction respectively. The Fe:Mn ratio in the crystallized samples varied depending on the leaching parameters. The oxalate material with Fe:Mn 0.67 showed better electrochemical activity for energy storage with 86.9 Fg−1 specific capacitance and 15.44 WhKg−1 energy density. The total impedance Z was found to be 12.041 Ω. Declaration of Competing Interest The authors declare that they have no conflict of interest. Acknowledgements The financial support provided by Ministry of Mines, Govt of India and instrumental support provided by Ministry of Earth Sciences is thankfully acknowledged. Authors are thankful to the HOD, Hydrometallurgy and Director, CSIR-IMMT for his kind consent to carry out this work. References Abbruzzese, C., 1990. Percolation leaching of manganese ore by aqueous sulfur dioxide. Hydrometallurgy 25 (1), 85–97. https://doi.org/10.1016/0304-386X(90)90066-B. Acharya, S., 1991. Reductive ammonia leaching of manganese nodules by thiosulfate. Metall. Trans. B 22 (2), 259–261. https://doi.org/10.1007/BF02652490. Ang, W.A., Gupta, N., Prasanth, R., Madhavi, S., 2012. High-performing mesoporous iron oxalate anodes for lithium-ion batteries. ACS Appl. Mater. Interfaces 4, 7011–7019. https://doi.org/10.1021/am3022653. Aragón, J., León, B., Vicente, C.P., Tirado, J.L., León, B., Vicente, C.P., Tirado, L., 2008. Synthesis and electrochemical reaction with Lithium of Mesoporous Iron oxalate Nanoribbons synthesis and electrochemical reaction with Lithium of Mesoporous Iron oxalate Nanoribbons. Inorg. Chem. 47, 10366–10371. https://doi.org/10.1021/ ic8008927. Artamonova, I.V., Gorichev, I.G., Godunov, E.B., 2013. Kinetics of manganese oxides dissolution in sulphuric acid solutions containing oxalic acid. Engineering 05, 714–719. https://doi.org/10.4236/eng.2013.59085. Azizi, D., Shafai, S.Z., Noaparast, M., Abdollahi, H., 2012. Modeling and optimization of low-grade Mn bearing ore leaching using response surface methodology and central composite rotatable design. Trans. Nonferrous Metals Soc. China 22, 2295–2305.

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8