UNDERSTANDING SULFIDE CAPACITY OF MOLTEN ALUMINOSILICATES VIA STRUCTURAL INFORMATION FROM
‘RAMAN’ AND ‘NMR’ SPECTROSCOPIC METHODOLOGIES Joo Hyun PARK*
Department of Materials Engineering, Hanyang University, Ansan 426-791, Korea
* Contact ; [email protected]
Keywords: Sulfide capacity, Aluminosilicate melts, Structure, Raman, NMR spectroscopy Abstract
The effect of Ca-Mn substitution on the sulfide capacity of the MnO-CaO-SiO2 (-Al2O3-MgO) melts were explained from the Raman scattering data, from which the structure information for the network modifying role and sulfur stabilizing role of Ca2+ and Mn2+ ions were obtained. The effect of Ce2O3 on the sulfide capacity of the MnO-SiO2-Al2O3-Ce2O3 melts were understood based on the structure data, from which the charge compensating role of Ce3+ and the amphoteric behavior of alumina were obtained. Employing the structure analysis, the thermochemical properties such as capacity of the oxide melts with no thermodynamic data can be understood in terms of ‘composition-structure-property’ relationship.
Introduction
Desulfurization (de-S) has been emphasized over several decades in iron- and steelmaking processes, because sulfur is harmful to the mechanical properties of steel products. Additionally, the de-S of Mn (ferro-)alloys and high Mn steels is an important issue, due to the introduction of AHSS such as TRIP and TWIP aided steels that were recently developed to contain Mn up to about 30% [1]. Thus, Even though many researchers have investigated the sulfide capacities of molten slags, there are a few experimental data regarding high MnO-containing aluminosilicate melts. Moreover, there is still room for further understanding in regard of the influence of Ca-Mn substitution on the sulfur dissolution mechanism in (alumino-)silicate melts.
On the other hand, the addition of rare earth elements such as Ce, etc. in molten steel has been known to improve the high temperature oxidation resistance of stainless steels [2]. We recently found that the CeOx-containing aluminosilicate inclusions are formed in melting and refining processes of Ce-containing steels [3,4], during which the sulfur in molten steel can be dissolved into the CeOx-containing oxides. Thus, the thermodynamic information of the sulfide absorption ability, i.e. sulfide capacity of the CeOx-containing aluminosilicate system is also necessitated. Unfortunately, however, there is no experimental and thermodynamic data for this oxide system.
Therefore, in the present paper, the influence of molten oxide composition on the sulfide capacity of the MnO- and CeOx-containing aluminosilicate melts will be discussed in terms of composition-structure-property relationship by employing the micro-Raman and NMR spectroscopic methodologies for structure analysis.
Experimental procedure
A super-kanthal vertical electric resistance furnace was used for the equilibration between the MnO-CaO-SiO2 (-Al2O3-MgO) slags and gas phase at 1873 K. The temperature
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
was controlled within ±2 K using an installed B-type thermocouple and a PID controller. The slag samples were prepared using reagent-grade chemicals. The slag sample of 1.2 g was maintained in a Pt crucible which was held in the porous alumina holder under the CO-CO2- SO2-Ar gas mixture for 8 hours. A constant flow rate of 400 ml/min was maintained during the equilibration of the slag with gas mixture at the experimental temperature. Each gas was passed through the purification system to remove the impurities. The oxygen and sulfur partial pressures were p(O2)=2.810-7 and p(S2)=4.710-3 atm, respectively. A schematic diagram of the experimental apparatus is given elsewhere [5-7].
After equilibration, the sample was quickly drawn from the furnace and then quenched by dipping it into brine. The quenched samples were crushed to <100 m using stainless and agate mortars for chemical analysis. The content of sulfur and each component in the slag were determined by combustion analyzer and XRF spectroscopy, respectively. The activity of each component in slag phase was calculated by commercial thermochemical computing program, FactSageTM7.0 with ‘‘FToxid’’ database.
The sulfide capacity of the MnO-SiO2-Al2O3-CeOx system at 1873 K was measured by similar procedure under the same gas potentials as mentioned above. In order to confirm the predominant oxidation state of Ce under the present experimental conditions, the XPS analyzer was employed with a reference to the binding energy of Ce3+ and Ce4+. The details of the preparation of glass samples and the experimental procedure of Raman and NMR spectra analysis are given elsewhere [8,9].
Results and Discussion
Composition-Structure-Capacity relationship of the MnO-CaO-SiO2 (-Al2O3-MgO) melt The iso-sulfide capacity of the MnO-CaO-SiO2 and the MnO-CaO-SiO2-20%Al2O3- 5%MgO slags at 1873 K is shown in Figure 1. The capacity contours commonly seem to rotate clock-wisely from the MnO-free side to the MnO-rich corner in both systems [5-7].
Thus, the sulfide capacity increases by increasing the MnO/CaO ratio at a fixed silica content which is greater than about 30%, whereas it decreases by increasing the MnO/CaO ratio at silica content lower than about 30%.
Figure 1. Sulfide capacity contours in the MnO-CaO-SiO2 (-Al2O3-MgO) slags at 1873 K.
The CaO which has more ionic bond character, i.e. 79% based on the Pauling’s equation
[10], dominantly contributes to the depolymerization of silicates than the MnO (ionic bond character 63%) does. Hence, the large amount of Ca2+ is electrically balanced with two non- bringing oxygen (NBO) ions, indicating that the Mn2+ is relatively free from the role of network modifier and mainly participate into the de-S reaction in high silica region.
However, in the relatively low silica region, viz. less than about 30% SiO2, the amount of Ca2+ balancing with NBO is reduced and thus free Ca2+ and Mn2+ competitively react with the S2- ions, resulting in the dominant contribution of Ca2+S2- attraction which is greater than Mn2+S2- attraction in terms of the stability of each sulfide determined from the Gibbs free energy of the formation of CaS and MnS at 1873 K [11]. This indicates that the contribution of Ca2+ to the stabilization of S2- ions would be larger than that of Mn2+ in the relatively low silica region.
The Raman spectra of the MnO-CaO-50%SiO2 system is shown in Figure 2(a) in order to understand the structural change as MnO substitutes for CaO at a fixed silica content. The main silicate envelope is definitely resolved to Q0 (860(5) cm-1 for SiO4-monomer) and Q2 (960(5) cm-1 for SiO3-chain) bands in the calcium silicate binary system, while there is a broad asymmetric band between 800 and 1150 cm-1 at Mn/(Mn+Ca)~0.8 [5-7]. Even though it is not easy to find a conclusive variation of Raman spectra with one’s eye as MnO substitutes for CaO, a broadening of Raman band qualitatively suggests that the substitution of CaO by MnO made a stronger perturbation and hence a more depolymerized configuration of local environment of Q-species in high MnO melts at a fixed silica content.
(a) (b)
Figure 2. (a) Raman spectra and (b) fraction of structural unit of MnO-CaO-50%SiO2 system.
The relative fractions of the silicate anionic units obtained from Gaussian deconvolution of Raman bands shown in Figure 2(a) are also plotted against the MnO/(MnO+CaO) ratio in Figure 2(b). The fraction of Q2 (SiO3-chain) unit increases and that of Q3 (Si2O5-sheet) unit decreases with increasing MnO/(MnO+CaO) ratio [5-7]. The fractions of Q0 (SiO4-monomer) and Q1 (Si2O7-dimer) units slightly decrease but not significant. Therefore, the Q3/Q2 ratio, viz. degree of polymerization decreases by increasing the MnO/(MnO+CaO) ratio based on the following equation:
[Si2O5]2- (Q3) + (O2-) = 2[SiO3]2- (Q2) [1]
The network-modifying role of Ca2+ and Mn2+ in this composition range can be discussed as follows. From an analysis of McMillan [12], doubly-charged M2+ of large ionic radius, i.e. small ionization potential (=Z/r2) should preferentially occupy the more open, coupled Q3 (Si-O-)2 sites, while smaller M2+ with larger ionization potential will favor the
higher charge concentration offered by the Q2 (=Si-2O-) sites. This is schematically expressed in Figure 3. Because the ionization potential of Ca2+ (Z/r2=2) is lower than that of Mn2+
(Z/r2=2.4~3.0 according to electron spin) [13], the Ca2+ is charge balanced with two open O- ions due to its large size of [CaO6] cage, whereas the Mn2+ is balanced with two adjacent corner-shared O- ions due to its small size of [MnO6] cage as shown in Figure 3 [5-7].
Figure 3. Structure modification by network-breaking role of Ca2+ and Mn2+ in silicate melts.
Composition-Structure-Capacity relationship of the MnO-SiO2-Al2O3-Ce2O3 melt The effect of Ce2O3 on the sulfide capacity of the MnO-SiO2-Al2O3-Ce2O3 system (XMnO/XSiO2M/S0.28, 0.85, 2.54) at 1873 K is shown in Figure 4. It is very interesting that the influence of Ce2O3 on the sulfide capacity exhibits a different tendency according to the M/S ratio [14]. The sulfide capacity of the oxide melts with highly basic composition, i.e.
M/S=2.54, decreases with increasing content of Ce2O3 to approx. 4 mol%, beyond which the sulfide capacity increases by increasing the Ce2O3 content. The sulfide capacity continuously decreases as the Ce2O3 is added to the Mn-aluminosilicate melts in the less basic system, i.e.
M/S=0.85, whereas it is hardly affected by Ce2O3 in the relatively acidic (high silica) composition, i.e. M/S=0.28.
Figure 4. Effect of Ce2O3 on sulfide capacity of MnO-SiO2-Al2O3-Ce2O3 system at 1873 K.
In order to thermodynamically understand these complicated phenomena, both the activity coefficient of MnS (or Ce2O2S) and the activity of MnO (or Ce2O3) should be evaluated. However, unfortunately, because none of thermodynamic data (activity of each component, phase equilibria, etc.) of the MnO-SiO2-Al2O3-Ce2O3 system is available in the literature, the Raman spectroscopic analysis for the influence of Ce2O3 on the structure of Mn-aluminosilicate system was employed in the present study to reveal the composition- structure-property relationship.
The Raman spectra of the MnO-SiO2-Al2O3-Ce2O3 systems (M/S=2.54 and 0.85) are shown in Figure 5 in order to understand the relationship between the structural change and the variation of sulfide capacity of the oxide melts. It is commonly found that the Raman band for the Si-O asymmetric stretching vibration between 800 and 1150 cm-1 very slightly shifts to the lower wavenumbers by increasing the content of Ce2O3, indicating that there is no considerable change in the environment of silicate units by the incorporation of Ce3+ ions [14].
(a) (b)
Figure 5. Raman scattering of MnO-SiO2-Al2O3-Ce2O3 system at different Ce2O3 contents.
In the Raman spectra of the M/S=2.54 system shown in Figure 5(a), there is a significant increase in the relative intensity of the scattering band at about 600 cm-1 with increasing content of Ce2O3. It is considered that the increase in the relative intensity of Raman band at 600 cm-1 is due to the [AlO6]-unit as mentioned by Okuno et al. [15], because of the strong attraction between Al2O3 and Ce2O3 in the present system. However, this tendency was not observed in the less basic melts, viz. M/S=0.85 system as shown in Figure 5(b).
Lin et al. [16] suggested that the Ce3+ conducts a role of charge compensator or network modifier rather than network former because Ce3+ ion (r=1.01A) has a radius close to that of Ca2+ (r=1.0A), which is too large to be a (tetrahedrally coordinated) network former like [SiO4] or [AlO4] units. Wu and Pelton reported the strong affinity between Ce2O3 and Al2O3
in the Ce2O3-Al2O3 system [17]. Morita et al. [18,19] found that the addition of Ce2O3 to the CaO-Al2O3 system decreases the activity coefficient of Al2O3 in the melts because of the strong attraction between Ce2O3 and Al2O3 based on the following reaction.
CeO1.5s + AlO1.5s = CeO1.5·AlO1.5 s , ∆G = -58730 J/mol at 1873 K [2]
Before adding Ce2O3, the concentration of [AlO6]-unit in the less basic (M/S0.85) systems are larger than that in the highly basic (M/S=2.54) system as shown in Figure 5.
200 400 600 800 1000 1200 1400
Relative Intensity
Wavenumber(cm-1)
M/S=2.54
5.6 3.7 2.7 1.2 0.0 mol% Ce2O3
Thus, Ce3+ ions added in the less basic melts instantly interact with large amounts of (pre- existing) [AlO6]-units to form more stable [(Al,Ce)O6]-unit as schematically shown in Figure 6(a). Therefore, the structure of aluminosilicate melts was not significantly disturbed by addition of Ce2O3, resulting in the insensitive dependency of sulfide capacity of the melts on the content of Ce2O3 as shown in Figure 4 [14].
On the other hand, in the highly basic system (M/S=2.54), relatively large amounts of (pre-existing) [(Al,Mn0.5)O4]-units should be converted to the [(Al,Ce)O6]-unit with consumption of free oxygen by addition of Ce3+ ions, resulting in a decreases in sulfide capacity. However, the sulfide capacity increases by addition of Ce2O3 greater than about 4mol% (Figure 4) because the excess Ce3+ and free Mn2+ cations contribute to the stabilization of S2- ions as schematically shown in Figure 6(b). This tendency was not observed in the less basic systems because the cations (Ce3+ and Mn2+) are still not free from a charge compensating role with large amounts of silicate anions within the present experimental compositions [14].
(a) (b)
Figure 6. Structural changes of (a) less basic and (b) highly basic melts by Ce2O3 addition.
Conclusions
The influence of molten oxide composition on the sulfide capacity of the aluminosilicate melts containing MnO and/or Ce2O3 was discussed in terms of ‘composition-structure- property’ relationship by employing the micro-Raman spectroscopy for structure analysis.
For the sulfide capacity of the MnO-CaO-SiO2 (-Al2O3-MgO) melts at 1873 K, because CaO dominantly contributes to the depolymerization of silicates than MnO does, the large amount of Ca2+ is electrically balanced with two NBO, indicating that the Mn2+ is relatively free from the role of network modifier and mainly participate into the de-S reaction in high silica region. However, in the relatively low silica region, the amount of Ca2+ balancing with NBO is reduced and thus free Ca2+ and Mn2+ competitively react with the S2- ions, resulting in the dominant contribution of Ca2+S2- attraction which is greater than that of Mn2+S2- pair. This indicates that the contribution of Ca2+ to the stabilization of S2- ions would be larger than that of Mn2+ in the relatively low silica region.
For the sulfide capacity of the MnO-SiO2-Al2O3-Ce2O3 melts at 1873 K, because the concentration of [AlO6]-unit in the high silica systems are larger than that in the lower silica system, Ce3+ ions added in the former instantly interact with large amounts of [AlO6]-units to form more stable [(Al,Ce)O6]-unit. Hence, the structure of aluminosilicate melts was not significantly disturbed by addition of Ce2O3, resulting in the insensitive dependency of sulfide capacity of the melts on the content of Ce2O3. However, in the lower silica region, the relatively large amounts of [(Al,Mn0.5)O4]-units should be converted to the [(Al,Ce)O6]-unit with consumption of free oxygen by addition of Ce3+ ions, resulting in a decreases in sulfide capacity. The sulfide capacity increases by addition of Ce2O3 greater than about 4mol%
because the excess Ce3+ and free Mn2+ cations contribute to the stabilization of S2- ions. This
tendency was not observed in the high silica systems because the cations (Ce3+ and Mn2+) are still not free from a charge balancing role with large amounts of silicate anions.
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