EMD Deposition on Mn2O3/Ti Anode for Manganese Recovery from Zinc Electrowinning Solutions

Abstract

Electrolytic manganese dioxide, EMD, is the positive electrode material of primary batteries and the raw material of LMC used as the positive mass of lithium ion batteries and is currently produced by anodic deposition on and harvesting from a titanium anode in acidic manganese sulfate solutions prepared by dissolving high purity of manganese sulfate into sulfuric acid solutions. This paper presents an alternative method of EMD production using a novel anode, Mn₂O₃ coated titanium, at lower operating temperature than the current process to explore a possibility of manganese recovery from zinc electrowinning solutions. The Mn₂O₃/Ti anode prepared by thermal decomposition of a precursor solution containing Mn(II) showed the anodic deposition of MnO₂ by constant current electrolysis and the current efficiency was 95% in maximum, while the current efficiency and the crystallographic structure of deposited MnO₂ depended on the current density, the electrolysis time, the bath temperature, and the concentration of Mn(II). The deposited MnO₂ was successfully harvested from the anode and the recovery rate was 98%.

Introduction

Electrolytic manganese dioxide (EMD) is one of the important components for batteries and capacitors and is used as the positive electrode of alkaline batteries. EMD is also the raw material to produce NMC (lithium nickel manganese cobalt oxide), that is the positive mass of lithium ion secondary batteries. The current production process of EMD is anodic deposition of MnO₂ on a titanium electrode from acidic aqueous solutions containing Mn(II) ions followed by harvesting, in which the deposition reaction is described as follows,

${\text{Mn}}^{{ 2 { + }}} {\text{ + 2H}}_{ 2} {\text{O}} \to {\text{MnO}}_{ 2} {\text{ + 4H}}^{ + } {\text{ + 2e}}^{ - } \,\,\,\,\,\,\,\,\,\,{\text{E}}^{\text{o}} { = 1} . 2 3 {\text{V vs}} . {\text{ NHE}}$

(1)

Titanium is well known to be easily passivated in acidic media, so that EDM production needs the specific operating conditions that the current density is as low as 5 mA/cm² to 30 mA/cm² and the bath temperature is about 80 to 98 °C, even though the electrolytic bath is strongly acidic [1]. While this condition is hazardous because such a high temperature easily induces acid mist from the bath, there has been no development on a novel anode for EMD production which enables to replace titanium anodes.

On the other hand, zinc electrowinning solutions typically contain Mn(II) ions coming from zinc ore and such Mn(II) ions are oxidized to MnO₂ on lead alloy anodes during electrowinning of zinc. The deposited MnO₂ finally drops off from the anode surface and makes a sludge that also contains some other elements, e.g., lead, strontium, and others, and no recycle of manganese from the sludge has been done. However, our previous works have revealed that amorphous oxide coated titanium anode can suppress MnO₂ deposition in zinc electrowinning [2–9], so that the electrowinning exhausted solution is expected to contain a high concentration of Mn(II) ions, from which EMD is possible to be produced, if there is an alternative anode that can work at lower operating temperature than titanium anodes.

In this work, we tried to develop a novel anode for EMD production and investigated the effects of the preparation conditions of the anodes and the electrolysis conditions on EMD production.

Experimental

Anode Preparation and Characterization

The anode for EMD production was prepared by thermal decomposition of a precursor solution painted on a titanium plate (1 cm × 5 cm × 1 mm) that had been degreased in acetone and etched in oxalic acid solution (10 wt%) at 90 °C for 1 h. The precursor solution was made by dissolving commercially available Mn(NO₃)₂ into distilled water. Then the precursor solution was painted, dried at 120 °C, and finally calcined at a temperature ranging from 350 °C to 600 °C. The painting to calcination process was repeated to make Mn₂O₃ layers on the titanium plate. The obtained electrode was analyzed by XRD (Rigaku, Ultima IV).

Electrochemical Measurements

Electrochemical measurements were performed with two-electrode or three-electrode cells, in which the Mn₂O₃/Ti electrode was the anode or the working electrode, of which the surface area was 1 cm² (1 cm × 1 cm). For comparison, a titanium plate was also utilized as the anode or the working electrode. A platinum plate was used as the cathode or the counter electrode, and the reference electrode in the three-electrode cell was KCl saturated Ag/AgCl electrode. The electrolytes were 2.0 mol/L H₂SO₄ solutions with and without 0.50 mol/L or 0.20 mol/L MnSO₄. The electrolyte’s temperature was controlled to be at 40 °C or 75 °C. The measurements were done with commercially available electrochemical measurement system (Hokuto Denko, HZ7000) and multi-channel electrolyzer (Hokuto Denko, HJB1001SM8A).

Results and Discussion

Characterization of Anode

The anodes obtained by thermal decomposition at 350 °C, 450 °C, 550 °C, or 600 °C were analyzed by XRD, and the results are shown in Fig. 1. All electrodes indicated the diffraction patterns of Mn₂O₃ (ICDD No. 01-073-1826) and Ti (ICDD No. 00-044-1294), so that the surface of the titanium substrate was coated with Mn₂O₃. The coating obtained at 450 °C or higher temperature level also presented the diffraction pattern of TiO₂ (ICDD No. 01-089-4202), and the peak intensity increased with increasing thermal decomposition temperature, which suggests that non-conductive TiO₂ is formed between the Mn₂O₃ coating and the substrate, and the TiO₂ interlayer is thicker at higher temperature, although the thickness of the interlayer could not be analyzed. The formation of TiO₂ interlayer is well known to make the ohmic drop large for coated titanium electrodes. This was confirmed by the polarization measurements as described later.

../images/468727_1_En_130_Chapter/468727_1_En_130_Fig1_HTML.gif — Fig. 1
XRD patterns of Mn₂O₃/Ti electrodes prepared at 350 °C, 450 °C, 550 °C, or 600 °C

Polarization of Mn₂O₃/Ti Anode in H₂SO₄ Solutions

Linear sweep voltammetry was carried out with the Mn₂O₃/Ti anodes in 2.0 mol/L H₂SO₄ solutions with no manganese ions, and the results in Fig. 2 (left) showed that the anodic polarization became higher when the anode prepared at higher temperature is used, as suggested in the previous section. The ohmic resistance of the anode measured by current interruption method revealed that the increase in polarization is caused by the increase in ohmic resistance with temperature, as shown in Fig. 2 (right). This is reasonable because non-conductive TiO₂ was created on the titanium substrate, when the thermal decomposition temperature was 450 °C or higher. From the result shown in Figs. 2, the authors decided to choose the anode prepared at 350 °C for further measurements of EMD production, since the other electrodes’ resistances are too high. The results shown in the later sections are those with the anode prepared at 350 °C.

../images/468727_1_En_130_Chapter/468727_1_En_130_Fig2_HTML.gif — Fig. 2
(Left) Linear sweep voltammograms of Mn₂O₃/Ti electrodes in 2.0 mol/L H₂SO₄ solutions at 75 °C. Scan rate: 1 mV/s, Thermal decomposition temperatures: 350 °C, 450 °C, 550 °C, 600 ^oC. (Right) Relationship between thermal decomposition temperature and ohmic resistance of Mn₂O₃/Ti electrodes measured by current interruption method at 75 °C

Polarization of Mn₂O₃/Ti Anode in H₂SO₄ Solution with Mn(II)

The anodic polarization curve of the Mn₂O₃/Ti electrode in 2.0 mol/L H₂SO₄ solution with 0.50 mol/L MnSO₄ at 75 °C was measured by cyclic voltammetry (anodic part only) at 10 mV/s and the result was compared to that obtained in 2.0 mol/L H₂SO₄ solution without MnSO₄. As shown in Fig. 3, the rest potential in the solution with Mn(II) shifted negatively compared to that without Mn(II), and the oxidation current of Mn(II) followed by diffusion-limited current at about 25 mA/cm² was observed. The oxidation current increased again after the diffusion limited current region and both the curves coincide, suggesting that oxygen evolution occurs. The cyclic voltammogram obtained in the solution with Mn(II) clearly demonstrates that the oxidation of Mn(II) occurs on the Mn₂O₃/Ti anode and MnO₂ seems to be the oxidation product. It is noted that the standard potential of the reaction in Eq. (1) is 1.23 V versus NHE and 1.03 V versus KCl saturated Ag/AgCl electrode (the reference electrode in this work).

../images/468727_1_En_130_Chapter/468727_1_En_130_Fig3_HTML.gif — Fig. 3
Cyclic voltammograms (anodic parts only) of Mn₂O₃/Ti electrodes in 2.0 mol/L H₂SO₄ solutions with and without 0.50 mol/L MnSO₄ at 75 °C. Scan rate: 10 mV/s, Thermal decomposition temperatures: 350 °C

This was supported by the XRD result of the Mn₂O₃/Ti anode after constant current electrolysis in the solution with Mn(II), as shown in Fig. 4. In this figure, the result obtained with a titanium anode in the same electrolysis condition is also presented for comparison. The product on the Mn₂O₃/Ti anode seems to be mainly ε-MnO₂ and contains γ–MnO₂, which are appropriate for battery uses, while the deposit on the Ti anode also involves those two phases with α–MnO₂. The current efficiency of MnO₂ deposition was calculated with the increase in anode’s weight by the electrolysis and the charge passed through the deposition.

../images/468727_1_En_130_Chapter/468727_1_En_130_Fig4_HTML.gif — Fig. 4
XRD patterns of a Mn₂O₃/Ti and b Ti anodes after the electrolysis at 5 mA/cm² for 16 h in 2.0 mol/L H₂SO₄ solutions with 0.50 mol/L MnSO₄ at 75 °C

Effects of Electrolysis Conditions on Current Efficiency

The current efficiency was examined with various electrolysis conditions; for example, the concentration change of MnSO₄ from 0.5 mol/L to 0.2 mol/L resulted in the decrease in current efficiency from 90% to 59% in case of the electrolysis at 5 mA/cm² for 4 h at 75 °C. The bath temperature strongly affected the current efficiency because it was 7.7% with 0.5 mol/L MnSO₄ solution by the electrolysis at 5 mA/cm² for 4 h at 40 °C. Because 0.5 mol/L MnSO₄ and 75 °C were better than the other conditions, the constant current electrolysis under those conditions was examined with changing applied current density and electrolysis time, and the results are summarized in Table 1. The current efficiency decreased with increasing electrolysis time at the same current density and also with increasing current density at the same charge. The above results are reasonable because the diffusion limited current density for MnO₂ deposition changes with the concentration of MnSO₄, and under the situation that the applied current density is beyond the limited current density, the current efficiency decreased with increasing current density and electrolysis time. It is also suggested from the results shown in Table 1 that the surface area of deposited MnO₂ on the Mn₂O₃/Ti anode is smaller than that of the Mn₂O₃ layer, because the current efficiency decreased with increasing electrolysis time even at 2.5 mA/cm². The maximum current efficiency for MnO₂ deposition in this work was 95% at 75 °C, which is quite higher than that in current EMD production with Ti anodes at 90 °C or more. Therefore, this work demonstrated that the EMD production with the developed anode can enhance the current efficiency even at lower operating temperature than the current method, and the low temperature operation is valuable to suppress acid mist generation and also operating costs.

Table 1

Current efficiency of MnO₂ deposition on Mn₂O₃/Ti anode for the electrolysis in 2.0 mol/L H₂SO₄ solutions with 0.50 mol/L MnSO₄ at 75 °C

Current density (mA/cm²)	2.5		5
Electrolysis time (h)	16	32	2	4	8	16
Charge (C)	144	288	36	72	144	288
Current efficiency (%)	95	81	95	90	65	53

Harvesting EMD from Mn₂O₃/Ti Anode

The deposited MnO₂ was harvested by hitting the anode with a plastic hammer. Figure 5 shows the photos of the anodes before and after the electrolysis at 5 mA/cm² for 168 h in 2.0 mol/L H₂SO₄ with 0.50 mol/L MnSO₄ at 75 °C under stirring at 300 rpm, the anode after harvesting, and harvested MnO₂. In this case, the weight of MnO₂ deposited on the anode was 271 mg and the harvested MnO₂ was 264 mg, so that the recovery rate of MnO₂ is about 97%. The XRD analysis and SEM observation of the anode after harvesting indicated that the Mn₂O₃ layer was still present and no damage was observed on the surface morphology. This implies that no TiO₂ contamination occurs in harvested EMD.

../images/468727_1_En_130_Chapter/468727_1_En_130_Fig5_HTML.gif — Fig. 5
Photos of Mn₂O₃/Ti anodes a before and b after the electrolysis at 5 mA/cm² for 168 h in 2.0 mol/L H₂SO₄ solutions with 0.50 mol/L MnSO₄ at 75 °C under stirring at 300 rpm, c the anode after harvesting deposited MnO₂, and d harvested MnO₂

Conclusions

This work developed the Mn₂O₃/Ti electrode by thermal decomposition method as the novel anode for EMD production. The anode prepared at 350 °C showed a low ohmic resistance enough to use as the anode, while those prepared at 450 °C or more were inappropriate because of high resistance due to non-conductive TiO₂ interlayer formation. The anode prepared at 350 °C presented a higher current efficiency for EMD deposition than the titanium anode under the same electrolysis condition, and the maximum current efficiency was 95%. The current efficiency changed with the current density, the electrolysis time, the electrolyte temperature, and the concentration of Mn(II) in H₂SO₄ solutions. Harvesting MnO₂ deposited on the anode was successfully done and the recovery rate of MnO₂ was 97%.

Acknowledgements

This work was supported by “Kyoto Area Super Cluster Program” of Japan Science and Technology Agency (JST).

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EMD Deposition on Mn₂O₃/Ti Anode for Manganese Recovery from Zinc Electrowinning Solutions

Abstract

Keywords

Introduction