Effect of Manganese on the Thermodynamic Property of Tellurium in Molten Iron

Abstract

The effect of manganese on the thermodynamic property of tellurium in molten iron was experimentally evaluated as an interaction parameter using a vapor-liquid equilibration technique, where the vapor pressure of tellurium was controlled using the transpiration method. Manganese was found to stabilize tellurium in molten iron and its effect was relatively small compared to other alloying elements.

1. Introduction

In the steelmaking field, tellurium is used as an alloying element for resulfurized free-machining steel, because it can globularize MnS inclusions and improve both machinability and mechanical properties of such steel. Tellurium is also known as a strong surface-active element in molten iron, and it induces some interesting phenomena when added into molten steel. Studies on these aspects of tellurium were briefly reviewed in our previous report.¹⁾ Since these effects of tellurium addition are sensitive to its concentration in molten steel, the concentration should be precisely controlled during the steelmaking process, taking into consideration the loss of tellurium either by evaporation into the atmosphere or by elution into slags. To analyze and estimate such behavior, information about thermodynamic properties of tellurium in molten steel is necessary. Recently, we have reported measurement of such properties, namely, the standard Gibbs energy for the dissolution of tellurium gas into molten iron¹⁾ and the interaction parameters of tellurium on various alloying elements in molten iron.^2,3) In our measurement, a vapor-liquid equilibration technique was used, in which the partial pressure of tellurium was controlled using the transpiration method. In this study, the effect of manganese, which is one of the most important alloying element in tellurium-containing steel as mentioned above, on the thermodynamic property of tellurium in molten iron was investigated using a consistent methodology as in our previous studies and quantitatively evaluated as a Wagner’s interaction parameter.

2. Experimental

2.1. Principle of Determination of Interaction Parameters

In the limit of a dilute solution, the relation between the partial pressure of tellurium and the solubility of tellurium in molten iron may be expressed by the following equation:   

$$a_{\text{Te}}=f_{\text{Te}}\cdot \left[\text{mass\%}\mathrm{\hspace{0.17em} }\text{Te}\right]=K\cdot P_{\text{Te}}$$(1)

Here, a_Te and f_Te are the activity and activity coefficient of tellurium in molten iron, relative to 1 mass% standard state, respectively. P_Te is partial pressure of tellurium. The term K is the equilibrium constant for the following reaction:   

$$\text{Te(g)}=\underset{\_}{\text{Te}}(1\mathrm{\hspace{0.17em} }\text{mass\%}\mathrm{\hspace{0.17em} }\hspace{0.17em}\text{in}\mathrm{\hspace{0.17em} }\hspace{0.17em}\text{Fe)}$$(2)

Furthermore, the effect of manganese may be expressed as   

$$log{f}_{\text{Te}}=e_{\text{Te}}^{\text{Mn}}\cdot \left[\text{mass\%}\mathrm{\hspace{0.17em} }\text{Mn}\right]$$(3)

Here, $e_{\text{Te}}^{\text{Mn}}$ is Wagner’s first-order interaction parameter of manganese on tellurium. From Eqs. (1) and (3), the following relation can be obtained:   

$$log\left[\text{mass\%}\mathrm{\hspace{0.17em} }\text{Te}\right]=log{a}_{\text{Te}}-e_{\text{Te}}^{\text{Mn}}\cdot \left[\text{mass\%}\mathrm{\hspace{0.17em} }\text{Mn}\right]$$(4)

Therefore, as long as the activity of tellurium, or the partial pressure of tellurium, is kept constant in a series of experiments, a linear relation between log [mass% Te] and [mass% Mn] with a slope of $-e_{\text{Te}}^{\text{Mn}}$ is expected.

2.2. Experimental Method

The experimental apparatus was described in our previous report, along with a detailed discussion on the validity of our method.¹⁾ Experiments were performed in a two-temperature zone furnace. In the upper zone, granular tellurium (99.99%, 30 μm pass, 4 g) was heated to a constant temperature (683 K) within a graphite container (20 mm OD, 11 mm ID, and 50 mm H). A carrier gas (Ar + 3% H₂) was passed over it at a constant rate (200 mL/min) so that the tellurium vapor was generated at a constant partial pressure (~10⁻⁴ atm). In the lower zone, pure iron (99.9%, powder) and manganese (99.97%, grain) were mixed and charged in an alumina crucible (99.7%, 15 mm OD, 12 mm ID and 30 mm H) and heated to melt at a constant temperature (1873 K). The carrier gas containing the tellurium vapor was carried through a thin alumina tube (6 mm OD, 4 mm ID) to the surface of the molten iron-based alloy. After equilibration, the granular tellurium and molten iron were pulled up simultaneously to the highest position inside the reaction tube so that the sample could be solidified quickly without any leakage of tellurium vapor to the outside.

An essential difference between our previous and current experimental methods was that the partial pressure of tellurium was not determined explicitly from the loss in the weight of granular tellurium but just fixed at a constant value. It enabled us to reduce the experimental duration significantly, which was necessary to minimize the changes in concentrations of alloying elements in molten iron.

The samples obtained from these experiments were dissolved in acid solutions and their tellurium and manganese contents were analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-AES; SPS7700, SII NanoTechnology, Japan). For the analysis of tellurium, iron ions were removed from the aqueous solutions using the palladium co-precipitation method⁴⁾ beforehand, to avoid serious spectral interference.

3. Results

3.1. Time Dependence of Contents of Tellurium and an Alloying Element in Molten Iron

To determine an appropriate experimental duration for equilibration experiments, time dependence of contents of tellurium and manganese in molten iron was investigated. Several samples were prepared whose initial contents of manganese were the same (5 mass%), and the experimental durations were varied. Figure 1 shows the results of this experiment. The content of manganese tended to decrease with time, which was attributed to the evaporation of manganese from molten iron. The content of tellurium increased rapidly and saturated before 60 min of experimental duration. After that, it tends to decrease in accordance with the change in the manganese content. Therefore, the dissolution of tellurium gas into molten iron was sufficiently fast compared with the change in the manganese content. Thus, it can be assumed that tellurium in molten iron was always in equilibrium with that in the atmosphere after 60 min or so. Therefore, we set the experimental durations to 60 min for the following equilibration experiments.

Fig. 1.

Time dependence of (a) manganese and (b) tellurium contents in molten iron.

3.2. Partial Pressure Control of Tellurium Gas

To ensure that the partial pressure of tellurium was the same across multiple experiments, equilibration experiments between tellurium gas and pure molten iron were repeated several times, for exactly the same experimental conditions. The results of this experiment are shown in Fig. 2. The solubility of tellurium was constant within an error, which was calculated only from the error associated with the composition analysis of tellurium, over four experiments. This makes sure that the partial pressure of tellurium can be kept at constant over several experiments.

Fig. 2.

Solubility of tellurium in pure molten iron.

3.3. The Effects of Manganese on the Solubility of Tellurium

The effect of manganese on the solubility of tellurium in molten iron was investigated. The errors of tellurium contents were calculated based only on the errors of composition analysis of tellurium by ICP-AES. The relationship between the content of manganese and that of tellurium is shown in Fig. 3 based on Eq. (4). Manganese was found to increase the solubility of tellurium and the following Wagner’s interaction parameter was obtained:   

$$e_{\text{Te}}^{\text{Mn}}=-0.0132\pm 0.0002,$$

Fig. 3.

Relationship between manganese content and tellurium content in molten iron.

The error with the above value corresponds to the standard error of the slope of the regression line. Figure 4 graphically compares the obtained parameter and the reported ones for other elements^2,3) in terms of the change in the activity coefficient of tellurium according to the content of an alloying element. Note that this figure does not indicate the composition range of experimental data from which those interaction parameters are derived.

Fig. 4.

Graphical comparison of the interaction parameters of various elements on tellurium in molten iron.

4. Conclusion

Wagner’s interaction parameters of manganese on tellurium in molten iron have been determined experimentally using a vapor-liquid equilibration technique, where the transpiration method was used to control the chemical potential of tellurium in the atmosphere:   

$$e_{\text{Te}}^{\text{Mn}}=-0.0132\pm \mathrm{0.0002.}$$

References

• 1)   S.  Ueda,  S.  Suzuki,  T.  Yoshikawa and  K.  Morita: ISIJ Int., 57 (2017), 397.
• 2)   S.  Ueda and  K.  Morita: Asia Steel Int. Conf. 2015 Proc., The Iron and Steel Institute of Japan, Tokyo, (2015), 42.
• 3)   S.  Ueda,  Y.  Matsuki and  K.  Morita: Applications of Process Engineering Principles in Materials Processing, Energy and Environmental Technologies, The Minerals, Metals & Materials Series, Springer, Berlin, (2017), 485.
• 4)   T.  Ashino,  K.  Takada and  K.  Hirokawa: Anal. Chim. Acta, 297 (1994), 443.