Kinetics and Mechanism of the Simultaneous Carbothermic Reduction of FeO and MnO from High-Carbon Ferromanganese Slag
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- Safarian, J., Kolbeinsen, L., Tangstad, M. et al. Metall and Materi Trans B (2009) 40: 929. doi:10.1007/s11663-009-9294-3
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The carbothermic reduction of 38.7 pct MnO-12.1 pct CaO-5.4 pct MgO-9.3 pct Al2O3-24.1 pct SiO2-10.4 pct FeO slag in Ar at 1600 °C was studied using the sessile drop wettability technique. Pure graphite, coke, and charcoal were used as the carbon material substrates. The reduction rates were evaluated by sampling at different reduction times and by analyzing the chemical compositions of the reduced slag and the produced metal. The carbothermic FeO reduction from slag is initially fast followed by a much slower reduction rate. However, the rate of the MnO reduction is slow in the fast FeO reduction stage, and it starts to increase significantly during the slow FeO reduction stage. The kinetics of FeO and MnO reduction are affected by the type of carbonaceous materials. Moreover, the rate of the carbon dissolution/transfer into the produced metal phase and the amount of the transferred manganese to the metal phase depend on the type of carbon. Based on the experimental observations and the thermodynamic calculations, a mechanism for MnO reduction was proposed. According to this mechanism, MnO is mainly reduced through a metallothermic reduction by Fe and the rate of MnO reduction is controlled by the rate of the consumption of FeO from the slag, which takes place simultaneously. In contrast, the rate of FeO reduction in the fast initial reduction stage is controlled by the rate of the carbon dissolution/transfer into the metal phase. However, at the second slow FeO reduction stage, it is reduced mainly by the solid carbon.
A few studies have been carried out to discover the effects of the carbonaceous material properties on the kinetics of MnO reduction. It has been observed that the reductant type influences the SiO2 reduction from a MnO containing silicate slag significantly, although it does not have a large effect on the MnO reduction.[13,14] In contrast, it has been observed recently that the rate of MnO reduction from silicate slags is affected by the type of carbon material.[15,16] It has been observed that the existence of small Fe-Mn particles in the industrial ferromanganese slags increases the rate of the MnO reduction extensively. Furthermore, a higher carbothermic FeO reduction rate from silicate slags that contain initial metal also has been observed.
The objective of the current study is to investigate the MnO reduction from HCFeMn slag with different carbon materials and under the conditions of the existence of a metal phase in the system. In this case, the interaction of a synthetic slag that contains MnO and FeO with different carbon materials is studied using the sessile drop wettability technique. This helps us to simulate the situation of the real ferromanganese furnace, because usually some iron oxide is found in the furnace charge materials. It is also expected to have FeO reduction before MnO reduction. Therefore, MnO will be mainly reduced by carbon while a metal phase is present in the system.
This study focuses on the interaction of a slag with carbon materials using the sessile drop wettability technique. The characterization and preparation of carbon substrates, the slag preparation, the sessile drop equipment, and the experimental conditions are described in the next subsections.
Three different carbonaceous materials consisting of pure graphite, a single coke produced from a single coal, and eucalyptus charcoal were used as the carbon material substrates. Powders of these materials were prepared and then pressed to make pellet disks with flat surfaces. To prepare the carbon powder substrates, they were crushed in a jaw crusher to less than 10 mm, ground in a tungsten carbide disc mill to a fine powder, and sieved to a size fraction of 44 to 105 μm. The charcoal powder was heat treated in CO gas at 1600 °C for 10 minutes to remove its large volatile materials content. These powders were then dried at 100 °C for 24 hours and were subsequently mixed with 3 wt pct stearic acid (binder) to achieve some green strength in the pressed substrates. The powder-binder mix was pressed into a small graphite crucible (10 mm outer diameter, 8 mm inner diameter, 3 mm height, and 1 mm deep) using 63.7 kg/cm2 pressure.
The Properties of the Carbonaceous Materials
Fix C (wt pct)
Ash Content (wt pct)
Surface Area, SS (cm2/g)
Sulphur Content (ppm)
Crystallite Size, LC (Å)
Amorph. Fraction (pct)
CO2 Reactivity (10−3/min)
High purity (+99.99 pct) fine powders of CaO, MgO, SiO2, and Al2O3 were mixed in a ratio of CaO/MgO = 2, CaO/Al2O3 = 1.34, and Al2O3/SiO2 = 0.4. The mixture was then melted in a graphite crucible in air and higher than 1600 °C using an induction furnace. The obtained slag was crushed in a tungsten carbide disc mill to a fine powder and was then mixed with appropriate amount of high purity MnO (+99.95 pct) fine powder by hand in plastic containers. The obtained mixture was melted in graphite crucible in air at 1550 °C. The produced slag was again crushed in the tungsten carbide disc mill to a fine powder and was then mixed with pure 99.5 pct FeO powder (CAS: 1345-25-1 from Alfa Aesar GmbH & Co KG, Karlsruhe, Germany). The mixture was melted in platinum crucible in air at 1550 °C. The slag composition was measured by electron probe microanalyzer (EPMA) and the composition in wt pct was 38.7 pct MnO-12.1 pct CaO-24.1 pct SiO2-9.3 pct Al2O3-5.4 pct MgO-10.4 pct FeO. It is worth noting that a portion of Fe2+ and Mn2+ are oxidized to Fe3+ and Mn3+ during the melting in air. These oxides will be reduced rapidly again to Fe2+ and Mn2+ in the slag reduction experiments through reduction by carbon. In the present study the measured elemental Fe and Mn in the slags by EPMA were converted to their oxides in the form of FeO and MnO.
The experiments were carried out through the stop of reduction within the above-mentioned reduction times to obtain the kinetic data points. After the experiments were completed, the samples were quenched, mounted in epoxy, and prepared for EPMA supported by wavelength dispersive spectroscopy (WDS). The slag and metal chemical compositions were measured in ten different points of each phase, and the averages were considered as the total slag and metal compositions. It is worth noting that the proper reproducibility of the experimental technique has been already observed by the authors for MnO reduction from a silicate slag by graphite substrates at different temperatures.[15,19]
Results and Discussion
The thermodynamics of the slag reduction and the results of the reduction experiments are presented and discussed in this section. Moreover, based on the observed reduction kinetics and the chemical reactions in the system, the mechanism of slag reduction is explained.
Slag Reduction Thermodynamics
The Rates of FeO and MnO Reduction
Although the wettability technique was applied in the current study, the wettability parameters such as the contact angle, the rate of contact angle changes, and the contact area between the slag and carbon substrate were not used to discuss the reduction kinetics. Because considerable gas bubble formation and coexistence of the gas phase in the slag droplet make it difficult to measure the contact angle and the slag drop volume properly. Hence, the changes in the chemical compositions of the slag and the produced metal during reduction were considered to study the reduction kinetics.
The Produced Metal
The Measured Concentrations in the Produced Metal, and the Calculated Carbon Solubilities and Manganese Activities in the Metal Phase by the Proposed Formulas in Literature. Pure Liquid Fe and Mn Were Used as the Standard States for Activity Calculations
Reduction Time (min)
Measured Metal Concentrations (wt pct)
Calculated wt pct Csat*
Regarding the importance of carbon in the metal phase, the carbon solubility in metal produced by the carbonaceous materials was calculated by the formula suggested by Fenstad. In this case, the measured Fe and Mn contents of the produced metal at various reduction times and the corresponding sample temperatures were considered. The calculated carbon solubilities in each alloy are listed and compared with the measured carbon concentrations in Table II. Moreover, these calculated carbon solubilities are illustrated on the carbon graph in Figure 7 by the dashed lines. Obviously, the measured carbon concentrations are higher than saturation, except for the metal produced by graphite in 4 minutes. This finding is expected with regard to the sample preparation for microprobe, where a thin layer of carbon was coated on the samples, which influences the resulting carbon concentrations higher than the actual carbon. It is worth noting that the reported measured carbon concentrations by EPMA in this article are around 10 random analyzed points of the metal drops, on average. For instance, the average carbon concentration in the metal produced through slag reduction by coke after 4 minutes is 6.7 wt pct with a standard deviation of 0.4 wt pct. The carbon graph in Figure 7 shows that the concentration of carbon is not significantly changed after exceeding around 6 wt pct, which indicates that the Fe-Mn alloys are approximately saturated above this level. Therefore, the metal is approximately carbon saturated in all coke and charcoal samples, except for the metal produced by graphite within a reduction time of 4 minutes. It is worth mentioning that the lower carbon concentration in the produced metal by graphite than by charcoal and coke in the first 4 minutes was confirmed by the microstructure studies. In particular, relatively lesser amounts of carbides were observed in the metal produced by graphite than that produced by charcoal, whereas they have similar concentrations of Mn. Although the measured carbon concentrations by WDS are not absolute and they always show higher concentrations than reality (because of the carbon coating), they can be used to indicate how much the metal produced by graphite in 4 minutes is under saturation. Considering the reduction by coke as an outlier, the measured carbon concentrations in the metal phase are on average 1.2 ± 0.5 wt pct higher than the calculated concentrations for the carbon saturation in the alloys (Table II). This may indicate that the real carbon concentration in the metal produced by graphite within 4 minutes can be in the range 3.45 to 3.95 wt pct, which is less than the expected saturation of 5 wt pct (Table II).
The difference in the carbon content of the metals produced in the first sampling indicates that the rate of iron production with coke and charcoal is small compared with the rate of carbon transfer/dissolution into the metal, so that the metal phase is always saturated by carbon. In contrast, the rate of metal production with graphite is relatively high compared with the rate of carbon dissolution/transfer in metal, and therefore, the carbon concentration of metal is below the saturation. Considering the carbon content differences and the rates of FeO reduction (Figure 4) with two carbon materials, we can say that the FeO reduction in the system is dependent on the carbon content of the metal phase, and FeO is mainly reduced by the dissolved carbon in the liquid metal. In this case, charcoal is a better source of carbon than graphite, because carbon dissolution/transfer in iron is not a rate-limiting step, although it can be rate limiting for the graphite.
The Initial Slag Mass and the Measured Slag Chemical Compositions of the Reduced Slags by Carbonaceous Materials
Reduction Time (min)
Slag Initial Mass (mg)
Measured Slag Concentrations (wt pct)
Considering the Mn content of the metal phase in Figure 7 and the manganese production rate (Figure 8(b)), it is found that the concentration of Mn in the metal is mostly higher for the carbon samples with higher MnO reduction rates. The maximum Mn evaporation rate is about 0.5 mg/min for the coke sample. This evaporation rate is much lower than the Mn evaporation rate from the pure Mn at 1600 °C, which is about 5 mg/min as measured previously in Ar atmosphere by sessile drop method. It is worth noting that when a similar FeO-free slag is reduced, no metallic Mn have been found at the slag/carbon interface, which indicates a faster Mn evaporation in Fe-free system.[19,21] This finding indicates that Fe plays an important role to stabilize the liquid state of the produced Mn with maintaining low Mn activity in the metal phase. This may show how Fe benefits the industrial process by decreasing the amount of Mn, which is evaporated in the coke bed area and condensed in the upper zones of the furnace.
The Chemistry of Slag Reduction
Regarding the above discussions, a possible explanation of the reduction mechanism pertaining to a slag containing MnO and FeO as well as SiO2 is given in the next section in an effort to combine general knowledge about the possible reactions in the system and the experimental observations.
Reaction  is dominating initially for FeO reduction, where Fe metal is first formed. From a thermodynamics point of view, reaction  is the first possible reaction in the system and it takes place at least for a small extent with an unknown mechanism. Because this reaction involves the coexistence of four phases and they can contact only at a point, it is probably limited to some active sites at the slag/solid carbon contact area. This reaction leads to the formation of new reaction areas in the system as slag/gas, metal/gas, slag/metal, and metal/carbon interfaces, and reaction  will consequently proceed through combination of subreactions , , and , as well as subreaction . The kinetics and mechanism of FeO reduction by the dissolved carbon in liquid iron through these subreactions has been studied already.[3,5,6,10,17] According to Pomfret and Grieveson, reaction  takes place through subreactions  and  at the slag/gas and at the gas/metal interfaces, where a gas layer exist at the slag/metal contact area. The dissolution/transfer rate of carbon to the metal can affect reaction  regarding the different carbon materials. For instance, reaction  might be slower for graphite substrate than for coke substrate because of the slower carbon transfer rate from graphite into the liquid iron, because the FeO reduction kinetics is dependent on the carbon concentration in the liquid Fe-Mn alloys.
The reduction of MnO by carbon starts parallel with, and it continues along with, the FeO reduction. Because reactions  and  involve four and three phases, respectively, they are limited to reaction points and lines in the system. However, MnO reduction can potentially take place at the slag/metal contact area and between only two phases through reactions  and  as discussed below.
MnO reduction by Fe
The metallothermic MnO reduction by Fe is also confirmed by examining Figure 4. For slag reduction by coke, the FeO content decreases faster than the other samples, and the MnO reduction at a high rate starts earlier than others. In contrast, FeO reduction by graphite substrate is slower and getting low FeO concentrations requires longer times, so the fast reduction by Fe is postponed. Considering Figures 4 and 9, and using the activity data in Figure 3, it is observed that reaction  proceeds considerably below about 1 pct FeO in the slag. This can prove why the starting point of rapid MnO reduction occurred before the first sampling for the coke, around the first sampling for the charcoal, and before the second sampling for the graphite. The slower MnO reduction rate by charcoal than by coke may indicate that the MnO reduction by Fe is linked to another reaction, such as reactions  and .
MnO reduction by Si
Reaction  is thermodynamically possible in the system and even for low SiO2 activities. However, this reaction is dependent on the supply of Si through reaction , where Si is produced and transferred to the metal phase. The silicon reduction takes place at the slag/metal interface, because the formation of Si metal requires a low Si activity. Thus, MnO reduction with silicon is controlled by reaction . However, reaction  mostly takes place at the slag/metal/carbon line, where CO gas desorption, which is the rate limiting step of reaction , preferably takes place. Thus, regarding the small slag/metal/carbon interfacial area, it is reasonable to assume that slow Si transfer to the melt occurs, so that the reduction of MnO by silicon is much less than the reduction by Fe. It is worth mentioning that at low MnO contents of the slag and high Si contents in the metal phase, the effect of the silicothermic reaction is not negligible.
The Gas Bubble Formation
Summary of the Possible Mechanism of Slag Reduction
Two possible mechanisms can be studied for the MnO reduction in the main MnO reduction stage. The first mechanism is the MnO reduction by the solid carbon according to reaction  and without Fe contribution. If such a reaction takes place in the system, then the order of sequence of the MnO reduction rates by carbonaceous materials in Figure 4 has to be similar to the MnO reduction rates from FeO-free slag (Figure 5). But, as we can observe, the order of the MnO reduction rates from theses two type of slags is different. Moreover, reaction  leads to the formation of CO gas at the slag/carbon interface with no bubble bursting from the slag. However, gas bubble bursting is always observed from the slag drop top, which means that reaction  cannot be dominant in the system. As described in section E, this reaction involves four phases, and it is limited to a few points in the system. In contrast, if this reaction proceeds at slag/carbon contact area without Fe contribution, it leads to the formation of a metal rich of Mn with high manganese activity. However, some metal in the system that contains Mn with low activity (Table II) already exists, which maintains better thermodynamic conditions for the reaction. Thus, MnO reduction does not take place in the system at the slag/carbon interface without the contribution of the metal droplets that contain iron.
A possible mechanism for manganese oxide reduction in the main MnO reduction stage can be MnO reduction by Fe through reaction  and then the reduction of the produced FeO by the solid carbon. This can be confirmed with obtaining similar MnO and FeO reduction rate orders with regard to the carbonaceous materials in both reduction stages (Figure 4). It was also observed that in the main MnO reduction stage, the metal phase is almost saturated of carbon. Therefore, if the FeO reduction by the dissolved carbon in the metal is dominant, then the corresponding MnO reduction rates for different substrates must not be significantly different, which is not observed in the experimental results (Figure 4). Hence, the FeO reduction in the main MnO reduction stage may occur mainly by the solid carbon through reaction . This reaction involves four phases (slag, metal, carbon, and gas), and it seems to be impossible in the system. However, this reaction is the first reaction in the system after interacting the slag with carbon, and it may be reasonable to consider it as a proceeding reaction with an unknown mechanism in the main MnO reduction stage. This result can indicate that the kinetics of FeO reduction, and consequently the MnO reduction, in the second stage are dependent on the reactivity of the solid carbon material. Subsequent work would be beneficial to increase our knowledge about the mechanism details and to clarify the remaining uncertanties.
The carbothermic reduction of FeO and MnO from the slag takes place simultaneously. However, the initial rate of the FeO reduction is fast, and it is then followed by a slow reduction rate. But the MnO reduction is slow in the rapid FeO reduction stage and then the speed increases significantly.
The kinetics of FeO and MnO reduction is affected by the type of carbon. Slag is reduced by coke faster than by charcoal and much faster than by graphite.
When FeO is present in the slag, a metal phase at slag/carbon interface is maintained and it increases the rate of MnO reduction through reduction by Fe and the dissolved carbon.
The carbon dissolution/transfer to the metal phase can be the rate-limiting step for the FeO and MnO reduction in the rapid FeO reduction stage.
The MnO reduction in the main MnO reduction stage is dependent on the rate of simultaneous FeO reduction, which is dependent on the reactivity of solid carbon material.
The authors acknowledge the project funds provided by the Norwegian Research Council and the Norwegian Ferroalloy Producers Research Association, through the CarboMat and ROMA projects.
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