New understanding on reduction mechanism and alloying process of rich manganese slag: Phase formation and morphological evolution
Graphical abstract
Introduction
Submerged arc furnace (SAF) technology is widely used in the industrial production of high‑carbon ferromanganese alloys. In SAFs high manganese ores, as an acceptable feed (Mn/Fe mass ratio over 5) for manganese alloy production, are a serious scarce manganese resource [[1], [2], [3], [4], [5]]. Low-grade ferruginous manganese ores (FeMn ores) with Mn/Fe mass ratios of less than 5.0, as an alternative to high-grade manganese ores, have great commercial value and are a significant manganese resource [[6], [7], [8], [9]]. The carbothermic reduction roasting process via magnetic separation has been applied as an effective technological process to enrich manganese from FeMn ores, and the obtained rich manganese product acts as an acceptable feed in the smelting ferromanganese alloy [1,4,[10], [11], [12], [13], [14], [15]]. FeMn ores are highly heterogeneous and contain various minerals and accompanying metals and other impurities with different levels of manganese and iron [16,17]. Nevertheless, it is essential to find a rational understanding of both the phase transformation and phase morphology to understand the reduction reaction mechanism of FeMn ore for smelting ferromanganese alloys.
Currently, ferromanganese alloys in industrial production are predominantly produced from rich manganese ores reduced by solid carbonaceous materials [[18], [19], [20]]. Numerous studies have been conducted to calculate the equilibrium phases and to optimize the critical parameters using FACTSage software for smelting ferromanganese alloys [[21], [22], [23], [24], [25]]. Several researchers [[26], [27], [28], [29]] have studied and calculated the equilibrium phase chemistry in the Al2O3–MgO–SiO2–CaO–MnO slag system to optimize the slag-metal system during the process of manganese smelting. The structure of the slag system was optimized by adding fluxing materials (limestone and dolomite) to obtain the distribution of manganese in the slag and the alloy with the goal of producing a high-quality ferromanganese alloy. In addition, J. Safarian et al. [17,19] reported the carbothermic reduction of ferromanganese slag using the sessile drop wettability technique with different carbon materials and in conditions where there was a metal phase. The reduction behaviors of MnO have been observed to be controlled by metallothermic Fe and the rate of FeO consumption from the slag. Previous researchers [3,30,31] investigated the reduction behavior of manganese ores in the carbonization and alloying process. The results showed that the interactive reaction between iron oxide and manganese oxide could have an obvious effect on the whole carbonization and alloy process. Many studies [[32], [33], [34]] have investigated the high-temperature reduction and smelting behavior with the goal of understanding the decomposition and dissolution behaviors of manganese ore and have also researched the effect of FeO and MnO on the kinetics and mechanism of simultaneous carbothermic reduction of ferromanganese slag. In addition, previous studies [21,22,26,29] have also focused on the optimization of the technological parameters for smelting ferromanganese alloys from FeMn ores. In addition, previous research [17,[21], [22], [23],26,27] results show that ferromanganese alloy and slag cannot melt smoothly and that separation is unavailable when maintaining the original basicity. The separation of the ferromanganese alloy and slag is successfully achieved by adding CaO during the ferromanganese alloy smelting process. The addition of calcium oxide can decrease the viscosity and improve the fluidity, which, together, improves the migration and reduction rate of valuable components in the manganese ore. Many scholars [3,17,22,26,27,31] have theoretically explained the phase transformation and reduction mechanisms of FeMn ores in the process of smelting ferromanganese alloy using thermodynamic calculations and experimental analysis but have failed to explain the relationship of the phase transformation and reconstruction in the ferromanganese alloy smelting process. At the same time, ferromanganese wüstite is an intermediate product, and its formation process has also been ignored. Therefore, a new understanding of the phase transformation and reduction behaviors of FeMn ores was proposed in this paper.
Based on observations of the phase transformation and morphology evolution in smelting ferromanganese alloys, the objectives of the present work were to clarify the smelting process of ferromanganese alloys. To fulfill this objective, the effects of the operating variables on smelting ferromanganese alloys were initially investigated. Then, the work mainly focused on the phase transformation, reduction behaviors and intrinsic morphology evolution of composite oxide phases containing Mn, Fe, Ca, Al and Si elements during the ferromanganese alloy smelting process. To further investigate the phase transformation and phase formation mechanisms, the phase transformation and formation mechanism of olivine (CaxFeyMn2-x-y)SiO4, spinel (CaxFeyMn1-y-x)Al2O4, gehlenite (CaxMn2-x)SiAl2O7 and wüstite MnxFeyCa1-x-yO were clarified in the ferromanganese alloy smelting process. In addition, the stepwise reduction behaviors and phase transformation of these phases were investigated to understand the interfacial reaction behaviors and formation behaviors of ferromanganese alloys at high temperatures. Based on the findings, recommendations were made to provide theoretical support and technical guidance for the efficient production of ferromanganese alloys from FeMn ores.
Section snippets
Raw materials
The manganese-rich ores used in this paper were obtained from low-grade ferromanganese ore (FeMn ore) by carbothermic reduction via magnetic separation in South Africa [11,13,14,35,36]. The carbothermic reduction roasting vs. magnetic separation includes a reduction temperature of 1050 °C, a roasting time of 6 h, a particle size of 8–13 mm, an FC/O of 2.5, and a magnetic field strength of 75 mT. The manganese-rich ores were typical high-alumina, low-silicon manganese ores that contained
Results of smelting ferromanganese production from rich manganese ore
In this study, single-factor control variable methods were applied to study the effects of various smelting parameters on smelting indexes; to do this, a single parameter changed and other parameters were maintained under basic conditions. The basic smelting parameters included a smelting temperature of 1525 °C, a smelting time of 30 min, an FC/O of 1.1 and a CaO/(SiO2 + Al2O3) basicity of 0.7. Fig. 2 shows the effect of the ternary basicity CaO/(SiO2 + Al2O3) (a), FC/O (b), smelting time (c)
Conclusions
In this paper, the phase transformation and reduction mechanisms of FeMn ores were reported to produce excellent ferromanganese alloys. The ferromanganese alloys were successfully obtained and had a Mn recovery of 80.47 and a Mn grade of 76.76% at a smelting temperature of 1550, a smelting time of 60 min, an FC/O of 1.1 and a ternary basicity (mole ratio: CaO/(SiO2 + Al2O3)) of 0.7, which meets the standard (FeMn75C7.5) for a high-quality ferromanganese alloy.
The phase formation mechanisms and
Credit authorship contribution statement
Zhenggen Liu: Writing - review & editing. Lifeng Zhang: Software, Validation. Mingyu Wang: Resources, Investigation. Zichuan Zhao: Methodology, Software. Lihua Gao: Writing - original draft. Mansheng Chu: Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors wish to express their thanks to the National Natural Science Foundation of China (51704061), the China Postdoctoral Science Foundation (2016M601321) and the Fundamental Research Funds of the Central Universities of China (N162503003) for supporting this study.
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