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Journal of the Southern African Institute of Mining and Metallurgy

On-line version ISSN 2411-9717
Print version ISSN 2225-6253

J. S. Afr. Inst. Min. Metall. vol.119 n.5 Johannesburg May. 2019

http://dx.doi.org/10.17159/2411-9717/153/2019 

PAPERS OF GENERAL INTEREST

 

Reduction rates of MnO and SiO2 in SiMn slags between 1500 and 1650°C

 

 

P.P. Kim; T.A. Larssen; M. Tangstad

Department of Materials Science and Engineering, Norwegian University of Science and Technology, N-7591 Trondheim, Norway

Correspondence

 

 


SYNOPSIS

The kinetics of MnO and SiO2 reduction in SiMn slags based on Assmang/Comilog ore and HCFeMn (high-carbon ferromanganese) slag were investigated between 1500 and 1650°C under a CO atmosphere. The results showed that charges containing HCFeMn slag had relatively faster reduction rates than those without. The difference in the driving force for MnO reduction was insignificant among the SiMn slags at 1500°C, which implies a low contribution of the driving force for reduction rate. The slag viscosities were also rather similar, around 1 poise at 1500°C, which could not explain the different reduction rates. Instead, the different charges containing various sulphur contents are believed to give rise to the varying reduction rates. The estimated activation energies for MnO reduction were around 500-920 kJ/mol for charges containing HCFeMn slag, and between 250-300 kJ/mol for those without. Based on the estimated kinetic parameters, the considered rate models were able to describe the reduction of MnO and SiO2 in SiMn slags between 1500 and 1650°C.

Keywords: SiMn, MnO, SiO2, reduction, kinetics.


 

 

Introduction

Manganese ferroalloys such as ferromanganese (FeMn) and silicomanganese (SiMn) are important ingredients in steel production due to the beneficial effects of manganese on the physical properties of steel. Mn enhances the strength, toughness, and hardness of steel products, and both Mn and Si are used as deoxidizers to prevent the development of porous structures (International Manganese Institute, 2014; Olsen, Tangstad, and Lindstad, 2007; Subramanyam, Swansiger, and Avery, 1990; Tomota et al., 1987).

The thermodynamic background of manganese ferroalloys is well established (Olsen, Tangstad and Lindstad, 2007), but kinetic information is rather scarce, especially for the SiMn process (Tranell et al., 2007; Tore-Andre Skjervheim, 1994). The absence of kinetic information makes it more difficult to understand the reduction mechanisms of MnO and SiO2. It is not clear how different raw materials affect the SiMn process. The main metal-producing reactions in the SiMn process are described by Equations [1] and [2]:

Recent studies have shown that MnO and SiO2 are reduced to Mn and Si significantly above 1500°C (Kim, Holtan, and Tangstad, 2016; Kim et al., 2017). The mass change observed in thermogravimetric experiments was low until 1500°C, but increased significantly at higher temperatures. It was perceived that most of the MnO and SiO2 reduction occurred between 1500 and 1650°C, but the reasons were not fully understood. Therefore, the present study focuses on the kinetic information on MnO and SiO2 reduction in SiMn slags (MnO-SiO2-CaO-MgO-Al2O3) between 1500 and 1650°C using different Mn sources.

 

Theoretical considerations

The reduction rate of MnO was studied previously and was described by Equation [1] (Olsen, Tangstad, and Lindstad, 2007; Ostrovski et al., 2002). This equation is based on the FeMn process and implies that the chemical reaction is the rate-determining step. Since SiMn slags are essentially similar to FeMn slags, Equation [1] can also be used for MnO reduction from SiMn slags. For SiO2, a recent study showed that the dissolution of SiO2 into slag was the rate-determining step (Maroufi etal., 2015). However, contrary results have also been observed where the dissolution of both MnO and SiO2 into slag was completed before significant reduction takes place above 1500°C (Kim and Tangstad, 2018a, 2018b, 2018c; Holtan, 2015). In this study, a similar rate equation for SiO2 reduction was presumed using Equation [2], assuming SiO2 reduction is also mainly controlled by chemical reaction. Both Equations [1] and [2] were considered in this work to estimate the kinetic parameters.

For Equation [1]:

For Equation [2]:

where r is the reduction rate (g/min), k is the rate constant (g/ min·cm2), ko is the frequency factor, A is the reaction area (cm2), E is the activation energy (kJ/mol), R is the gas constant, T is the temperature, aMnO, aSiO2 are the activity values of MnO and SiO2 in the slag phase, aMnO,Eqm., aSiO2, Eqm. are the activity values of MnO and SiO2 in equilibrium, and KT is the equilibrium constant at temperature T.

The rate models considered for MnO and SiO2 reduction also imply that the driving force, which is the difference between the activity of slag (MnO, SiO2) and the produced metal (Mn, Si), contribute to the reduction rates. The simplified models for activities of slag and metal have been recently studied (Olsen, 2016). These activities were based on FactSage 7.0, database FTOxid and FactPS (CRCT and GTT, 2015), and thermodynamic data from HSC Chemistry 7 (Outotec, n.d.) was used to calculate the driving forces of MnO and SiO2 reduction at different temperatures.

 

Experimental procedures

The chemical compositions of the raw materials are shown in Table I and the SiMn charges are described in Table II. Note that manganese is present as MnO and MnO2 in manganese ores. Three different Mn sources, Assmang ore, Comilog ore, and HCFeMn slag, were used to study different SiMn charges in this work. The sizes of the raw materials were between 0.6 and 1.6 mm. Each raw material was weighed to aim at approximately 40 wt% SiO2 in slag and 18 wt% Si in the metal phase, which is close to the thermodynamic equilibrium at 1600°C (Olsen, Tangstad, and Lindstad, 2007). The charge materials were added into graphite crucibles (36 mm outer diameter, 30 mm inner diameter, 70 mm height, and 61 mm depth).

A TGA furnace, which is schematically depicted in Figure 1, was used to conduct the experiments. The furnace can reach temperatures up to 1700°C and the maximum heating rate is 25°C/ min. A mass balance is installed at the top and a molybdenum (Mo) wire was used to suspend the graphite crucible inside the chamber. The temperature schedule of the experiment was considered to simulate an industrial furnace operation and is described in Figure 2. Initially, the furnace was rapidly heated up to 1200°C at a rate of 25°C/min and held for 30 minutes to secure complete prereduction (Olsen, Tangstad, and Lindstad, 2007). Then, further heating at a rate of 4.5°C/min was done and stopped at targeted temperatures between 1500 and 1650°C, followed by cooling. All experiments were conducted in CO at atmospheric pressure.

 

 

 

 

The weight loss of each charge sample was recorded and the data was logged every 5 seconds during the experiment. Lastly, a portion from each charge sample was prepared by mounting it in epoxy for electron probe microanalysis (EPMA) using a JEOL JXA-8500F instrument. The average slag composition from more than five analysed points was used to calculate the metal composition (as the metal analyses are more uncertain than the slag analyses).

 

Results and discussion

The mass changes recorded during the TGA runs were the main information used. Figure 3 describes the TGA results from different SiMn charges between 1200 and 1650°C. Note that complete prereduction was assumed at 1200°C and used as a new reference point for further reduction of MnO and SiO2. The mass changes for all SiMn charges were insignificant below 1500°C, which indicated a low degree of MnO and SiO2 reduction. The reduction rate increased above 1500°C, which is in accordance with previous studies (Kim, Holtan, and Tangstad, 2016; Kim et al., 2017), but the degree of reduction differed with different SiMn charges. It was observed that charges containing HCFeMn slag were reduced faster and attained higher degrees of reduction. Although the apparent accelerated reduction can be thought to result from the use of HCFeMn slag, the TGA results do not adequately describes the reduction degrees of MnO and SiO2 separately. Quantitative slag and metal analyses are required to provide further information.

 

 

The average slag and metal compositions of the different charges, with their respective activities (slag: aMn0and , metal: aMn/KT and aSi/KT), between 1500 and 1650°C are shown in Table III. Significant reduction of MnO was clearly observed with charges containing HCFeMn slag. The MnO content in slag between 1500 and 1650°C for charges 'As/HCS' and 'HCS' decreased from approximately 40 and 31 wt% to 5 and 7 wt%, respectively. Similar degrees of reduction were also observed from charge 'Com/HCS', where the decrease of MnO was from approximately 43 to 6 wt% between 1500 and 1650°C. On the other hand, for charges without HCFeMn slag ('As' and 'Com') the MnO contents were still relatively high compared to the charges containing HCFeMn slag. This shows good accordance with the TGA results in Figure 3, which imply that most of the mass change was due to MnO reduction between 1500 and 1650°C.

The aMn0 also represents the reduction of MnO for all charges. Figure 4 compares the aMn0 for all charges between 1500 and 1650°C. Note that each point represents an experimental run. The aMn0 for all the charges was similar at 1500°C: approximately 0.2 (1550°C for 'Com/HCS'). Only for charges containing HCFeMn slag have the aMn0 values dropped down near to zero at 1650°C, which also indicates a higher degree of MnO reduction. This implies that the driving force for MnO reduction (aMn0 - aMn " aMn0) will have insignificant impact on the reduction rate according to Equations [1] and [2] at the start of reduction Hence it is not the driving force, but the rate constant, that differs more between the reactions occurring with different charges.

The rate constants in this study are expressed as the Arrhenius equation according to Equations [1] and [2]. The Arrhenius plots of MnO and SiO2 reduction for all charges are described in Figure 5, and the estimated activation energies are shown in Table IV.The Arrhenius plot for MnO reduction shows that charges containing HCFeMn slag have higher rate constants with increasing temperature. Also, the temperature dependency (activation energy) seems to be similar with different charges types: charges including HCFeMn slag ('As/HCS', 'Com/HCS', and 'HCS') were approximately between 500 and 920 kJ/mol and charges without HCFeMn slag ('As' and 'Com') were between 250 and 300 kJ/mol. The Arrhenius plot for SiO2 reduction was difficult to estimate due to small amount of Si produced. However, the temperature dependencies for SiO2 reduction were somewhat similar within the range of experimental conditions.

 

 

As the main difference between the different charges is the rate constant, the rate constants for MnO reduction were compared with the viscosity of the slag (Figure 6). If the assumption of chemical reaction being rate-determining is not valid, the viscosity could affect the rate. The viscosities were calculated by using FactSage 7.0 (CRCT and GTT, 2015). No correlation between the viscosity of the slag in the initial phase of the reduction was found at 1500°C, and the rate constant is hence not affected by the viscosity in the slag and probably neither the diffusion, which can be correlated with viscosity.

Sulphur is known to behave as a strong surface-active species for most metals (Stolen and Grande, 2004). The comparison of the initial amount of sulphur in the charge and the rate constants (MnO reduction) between 1500 and 1650°C is shown in Figure 7. Note that the amount of sulphur was calculated from the raw materials (excluding coke). It is seen that the rate constant initially increases with the sulphur content and then decreases again. This is in accordance with previous observations where the sulphur content was studied separately (Kim and Tangstad, 2018a, 2018b, 2018c; Kawamoto, 2016; Larrsen, 2016).

 

 

By applying the estimated kinetic parameters in Figure 5, the changing amounts of MnO and SiO2 in SiMn slags can be described by using the rate models (Equations [1] and [2]). The comparisons between the rate models and the measured amount of MnO and SiO2 between 1500 and 1650°C are described in Figure 8. Note that the parameters which describe the optimal fit were applied to the rate models (approximately 2% error in the raw materials analyses). The comparison showed that the rate models considered in this study are applicable to describe the amounts of MnO and SiO2 for SiMn slags. The symbols, which indicate the measured amounts of MnO and SiO2 from the experiments, show a good match with the calculated amounts (solid and dotted lines) between 1500 and 1650°C. The considered rate models were successfully used to describe the changing amounts of MnO and SiO2 regardless of the charge type and degree of reduction.

 

Conclusions

The objective of this study was to estimate the kinetics of MnO and SiO2 reduction in SiMn slags and to observe the reduction rate between 1500 and 1650°C. The results showed that SiMn charges containing HCFeMn slag as raw material are reduced faster than those without. The measured amount of MnO in slag at 1650°C was low for charges containing HCFeMn slag, and the aMnOshowed good accordance: The aMnOwas around 0.2 at 1500°C but decreased to near zero at 1650°C. Also, the aMnO at 1500°C was approximately 0.2 for all charges, which implies a low contribution to the driving force for reduction rate. From the kinetic estimations, the activation energies differed for the two types of charge: for MnO reduction, the values for charges containing HCFeMn slag were approximately 500-920 kJ/mol, and for those without, approximately 250-300 kJ/mol. The comparison of slag viscosity with rate constants showed that slag viscosity does not significantly influence the reduction rate of MnO. Instead, small amounts of sulphur impurity in the charge showed significant impact on the reduction rates. At more than 0.15 wt% of initial sulphur, the rate constants increased drastically with increasing temperature. In addition, the considered rate models for MnO and SiO2 reduction were able to describe the changing amounts of MnO and SiO2 in SiMn slags. The results are applicable for estimating the production rate during SiMn smelting.

 

Acknowledgments

This publication has been partly funded by the SFI Metal Production (Centre for Research-based Innovation, 237738). The authors gratefully acknowledge the financial support from the Research Council of Norway and the partners of the SFI Metal Production.

 

References

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Correspondence:
P.P. Kim
Email: pyung.h.kim@ntnu.no

Received: 30 Mar. 2018
Revised: 2 Oct. 2018
Accepted: 29 Oct. 2018
Published: May 2019

 

 

ORCiD ID: P.P. Kim. https://orchid.org/0000-0001-5449-5385
T.A. Larssen. https://orchid.org/0000-0003-0224-3201
M. Tangstad. https://orchid.org/0000-0001-9751-7716

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