Elsevier

Powder Technology

Volume 330, 1 May 2018, Pages 310-316
Powder Technology

The mechanism on reducing manganese oxide ore with elemental sulfur

https://doi.org/10.1016/j.powtec.2018.02.035Get rights and content

Highlights

  • The mechanism on reducing manganese oxide ore with elemental sulfur is revealed.

  • MnO2 is reduced stepwise in the order of MnO2 → Mn2O3 → Mn3O4 → MnS/MnO, together with MnSO4 or SO2.

  • Higher temperature is beneficial to the formation of MnO.

  • The phase composition of the roasted product under different conditions is confirmed.

Abstract

Extraction of Mn from manganese oxide ore via sulfur-based reduction followed by acid leaching has received much attention. In this study, the mechanism on reducing manganese oxide ore with elemental sulfur was investigated. The reduction of manganese dioxide by sulfur is feasible thermodynamically, and the experimental validation proved that manganese dioxide was reduced stepwise to low-valence state in the order of MnO2 → Mn2O3 → Mn3O4 (together with MnSO4 or SO2). Subsequently, Mn3O4 would be reduced to form MnS or MnO, and the tendency of forming MnO was enhanced with increase in the temperature. Manganese oxide (MnO) will further react with excessive sulfur forming MnS and MnSO4. However, the formed MnS on the surface of the particles would inhibit further reduction or sulfidation. During the reductive roasting process, MnSO4 as well as gaseous SO2 were simultaneously generated. The reaction between MnS and Mn3O4 occurred at temperatures over 500 °C, which facilitated producing MnO and SO2. The roasted product contained MnS, MnSO4, MnO and unreacted Mn3O4 at lower temperature (<500 °C), whereas the main products were MnO and MnSO4 at higher temperature (≥500 °C). The content of MnO increased gradually while that of MnS was decreased by increasing the roasting temperature.

Introduction

Manganese is an important fundamental metal that has been widely applied in many fields, such as steel production, battery, non-ferrous metallurgy, catalyst [1], electrode materials [2], etc. Manganese oxide ore is the most important Mn-bearing resource in the world, which accounts for approximately 70% of the total manganese ore deposits [3]. To meet the ever-increasing demand for manganese, particularly in China, considerable effort has been made to recover manganese from low-grade pyrolusite. However, manganese dioxide must be primarily reduced into a soluble form before it can be extracted in dilute sulfuric acid or alkaline medium [4].

There are various reductants reported to have been employed for reducing manganese oxide ore in the processes of direct reductive leaching or pre-reduction followed by leaching. Manganese dioxide is likely reduced in acid solutions in the presence of reductants such as metallic iron or iron(II) sulfate [5,6], pyrite [7], hydrogen peroxide [8], biomass [9] or oxalic acid [10], etc. Manganese is also capable of being extracted by SO2 leaching [11], electrolysis as well as bioleaching [12]. In recent years, the reductant of renewable biomass has been widely studied. A common problem with biomass is, however, the relatively low leaching efficiency and the decomposition product of biomass is adverse to the subsequent electrolysis [9]. On the other hand, pre-reduction of manganese oxide ore by roasting in the presence of reductants was expected to be employed prior to acid leaching. Carbothermal treatment of manganese oxide ore using coal as reducing agent is the most widely used approach [13]. Although this traditional method is characterized by high efficient, it produces a great deal of dust and SOx/NOx which pollute environment seriously. Worse still, roasting is generally performed at temperatures over 850 °C which consumes huge amounts of energy [14,15]. Other reductants reported in the literatures include pyrite [16], SO2 [17,18], ammonium salts ((NH4)2SO4 or NH4Cl) [19], CO/H2, CH4 [[20], [21], [22]] and carbohydrate [23].

Recently, sulfur-based reduction or sulfidation using elemental sulfur as reductant has received increasing attention. The sulfidation of a nickeliferous lateritic ore by sulfur indicated that nickel oxide can be selectively sulfidized to a nickel–iron sulfide at 400–550 °C [24]. Similarly, studies on sulfidation roasting of cervantite, chalcopyrite, lead–zinc oxide ore, as well as zinc and lead carbonate were also reported [[25], [26], [27], [28]]. The oxides or carbonates were likely converted to sulfides which can be recovered by flotation. A major problem of these technologies is the submicron particle size of sulfides obtained after low temperature sulfidation roasting which usually causes low metal recovery in flotation.

An alternative process of reductive roasting manganese oxide ore with elemental sulfur prior to acid leaching has also been successfully developed and industrialized in China [29,30]. This process employed a relatively lower roasting temperature which was considered as a cost-effective method. Fluid-bed furnace (boiling furnace) was the main roasting device due to the low boiling point of elemental sulfur. Sulfuric acid leaching was proved to be appropriate in extracting manganese as manganese sulfate solution was usually used for subsequent electrolysis. The generated SO2 gas during reductive roasting can be absorbed to prepare sulfuric acid and then used for acid leaching.

In our previous work, desirable extraction ratio of manganese has been obtained via sulfur-based reduction followed by acid leaching [17,30,31]. However, the industrialization production indicated that the components of roasted product were very complicated, and the formation of manganese sulfide (MnS) was unfavorable to acid leaching due to the release of poisonous H2S. Thus, to better understand the behavior of sulfur-based reduction, in this study, the effects of S/Mn molar ratio and roasting temperature on the phase transformation during reductive roasting was first investigated. After that, an analysis of gas composition in the outlet gas, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) are also used to characterize the reduced solid samples so as to identify the reduction mechanism.

Section snippets

Materials

The manganese ore sample was obtained from a mine located in China. The as-received ore was crushed by hammer, and high purity pyrolusite was manually separated under an optical microscope. The pyrolusite sample was used as the raw material after being ground to 100 wt% of the particles passing 0.074 mm. The main chemical composition of the sample is shown in Table 1. The Mn content was 59.6 wt% which is equivalent to 94.3 wt% MnO2. The XRD pattern in Fig. 1 indicates manganese mainly existed

Thermodynamic analysis of reductive roasting

The main possible chemical reactions between manganese oxides and sulfur are listed in Table 2. Their logKθP values based on free energies calculated (by Factsage 7.0) under a total pressure of 1 atm at specified temperatures are also presented in Table 2. It can be observed that the reactions of Eqs. (1)–(6) are predicted to occur spontaneously at temperatures <900 K. Manganese oxides are likely reduced by sulfur forming low-valence compounds due to their negative free energies. It is

Effect of S/Mn molar ratio

To identify the phase transformation of manganese dioxide during reductive roasting, XRD analysis was performed for determining the phase composition of roasted products. The effect of S/Mn molar ratio was investigated by keeping roasting temperature at 350 °C and roasting time of 30 min, and the results are compared in Fig. 3.

It can be observed that the products were very complicated. The patterns of roasted products are different from that of the pyrolusite sample. Manganese dioxide was

Conclusions

In this study, the mechanism on reducing manganese oxide ore with elemental sulfur was investigated. The conclusions can be summarized as follows:

  • (1)

    The reduction of manganese dioxide by sulfur is feasible. Manganese dioxide was reduced stepwise to low-valence state in the order of MnO2 → Mn2O3 → Mn3O4. Subsequently, Mn3O4 would be reduced to form MnS or MnO (together with MnSO4 or SO2), and the tendency of forming MnO was enhanced with increase in the roasting temperature. Manganese oxide (MnO)

Acknowledgements

The authors wish to express their thanks to the National Natural Science Foundation of China (Grant No. 51234010) and the Fundamental Research Funds for the Central Universities (Project No. 106112017CDJXY130001) for the financial support of this research.

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