Effects of MgO and Al 2 O 3 Addition on Redox State of Chromium in CaO-SiO 2 -CrO x Slag System by XPS Method

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1 Effects of MgO and Al 2 O 3 Addition on Redox State of Chromium in CaO-SiO 2 -CrO x Slag System by XPS Method LI-JUN WANG, JI-PENG YU, KUO-CHIH CHOU, and SESHADRI SEETHARAMAN The effects of MgO and Al 2 O 3 on the redox state of chromium in CaO-SiO 2 -CrO x system have been investigated at 1873 K (1600 C) under Ar-CO-CO 2 atmosphere and analyzed by means of X-ray photoelectron spectroscopy. From the analysis of the Cr 2p core level spectra, it was found that both Cr(II) and Cr(III) exist simultaneously in CaO-MgO/Al 2 O 3 -SiO 2 -CrO x, and the quantitative ratio Cr(II)/Cr(III) has been obtained by deducing from the area under the computer-resolved peaks. Substitutions of CaO by MgO, SiO 2 by Al 2 O 3 favored the Cr(II) state existing in the system in the composition ranges of 3 to 10 wt pct MgO and 5 to 20 pct Al 2 O 3. Meanwhile, from the analysis of the O1s spectra in CaO-MgO-SiO 2 -CrO x, it was found that the ratio of the non-bridging oxygen content increased first due to the CrO contribution to the electron distribution uniformly as O at MgO low content. Afterward, it went to decreasing with continuing addition of MgO because ionic contribution of MgO is less than that of CaO and the influence of the CrO clustering on the non-bridging oxygen is limited due to only 5 wt pct CrO x. In CaO-Al 2 O 3 -SiO 2 -CrO x system, Cr(II) acts as a network modifier to compensate Al 3+ charge balance to make the structure stable, so the non-bridge oxygen in this system continues decreasing. DOI: /s Ó The Minerals, Metals & Materials Society and ASM International 2015 I. INTRODUCTION WITH the increasing emphasis on environment and energy issues, serious attention is drawn to achieve effective use of energy in iron and steel industry. The processes are also modified systematically so that the rest of the products are environment friendly. As it is known, chromium is one of the most important alloy elements in both stainless steel and alloy steel. Significant amount of Cr gets lost during the processing in view of the susceptibility of Cr to get oxidized forming CrO x, which would dissolve in the slag phase. Further, when tapped in air, the liquid slag is likely to emit CrO 3 which has high toxicity, causing serious environmental problems. To accomplish both the sustainable development of the stainless steel industry and environment protection issues, further optimizations of steelmaking process are imperative. The optimizations should be based on in-depth knowledge of the thermophysical and LI-JUN WANG, Associate Professor, JI-PENG YU, Master Student, and KUO-CHIH CHOU, Professor, are with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, No. 30, Xueyuan Road, Haidian District, Beijing , P.R. China and also with the School of Metallurgy and Ecological Engineering, University of Science and Technology Beijing, Beijing , P.R. China Contact lijunwang@ustb.edu.cn SESHADRI SEETHARAMAN, Professor, is with the Royal Institute of Technology Sweden, 10044, Stockholm, Sweden. Manuscript submitted November 16, thermochemical properties of stainless steel slags. One of the important basal issues is to understand the behavior of chromium oxides in slags. In the past few decades, a number of studies have been carried out regarding the chemistry of chromium slags, [1 10] such as thermodynamic activities of chromium oxides, solubility of chromium spinel phase as well as the distribution of chromium between Cr(II) and Cr(III). In the last topic, most of the authors have been equilibrated the slag samples under well-controlled experimental conditions. Considering the effects of temperature and oxygen partial pressure, similar conclusion was drawn that the ratio Cr 2+ /Cr 3+ in these slags increased with increasing temperature and decreasing oxygen potential. However, it reveals that some of the results show a wide scatter in the earlier reports regarding the effect of slag chemistry on valence state of chromium in the slag melts based on a critical review. Some values showed a tendency of increase of the divalent chromium in the slag with increasing basicity, which was different from the results of the earlier study. In view of this, the present authors [11] adopted two techniques to determine the relationship between the ratio of Cr 2+ to Cr 3+ with respect to temperature, oxygen partial pressure as well as the slag chemistry; one is classical gas/slag equilibrium with XANES analysis, and the other one is high-temperature mass spectrometry. As reported by Irmann, Cr(III) would exist as [CrO 2 ] in basic slag phase. [12] Considering the redox equilibrium reaction between Cr(II) and Cr(III), (Cr 2+ ) (O 2 ) + 1.5(O 2 ) = [CrO 2 ], we also developed a mathematical expression of X CrO /X CrO1.5

2 ratio described as function of temperature, oxygen partial pressure, and slag basicity as shown in Eq. [1]: X CrO log ¼ X CrO1:5 T 0:25 log p O 2 ½1Š 0:203 logðbþþ5:74: Slag basicity is a comprehensive index, which would neglect the various contributions of oxide components. The present investigation is focused on the effects of MgO and Al 2 O 3 in the CaO-SiO 2 -CrO x system, which is part of an extensive study of chromium oxide-containing slags with a view to optimize the alloy steelmaking processes, with special reference to chromium loss to the slags. X-ray photoelectron spectroscopy (XPS) has been applied to a study of the local structure of silicate-, germinate-, and phosphate-based slags as well as Fe, Co, and Cu redox state in these glasses. [13 16] The investigation on redox state of Cr in CaO-MgO/Al 2 O 3 -SiO 2 system by XPS was scarce. [17,18] And the effects of MgO and Al 2 O 3 on redox state of Cr(II)and Cr(III) are also not sufficient. Thus, in the current study, XPS method was used to study the oxidation states of chromium in CaO-MgO/Al 2 O 3 -SiO 2 -CrO x slags with substitution of CaO by MgO and SiO 2 by Al 2 O 3. II. EXPERIMENTAL Fine powders of CaO [decomposed from CaCO 3 at 1273 K (1000 C) for 12 hours], MgO [dried at 1073 K (800 C) for 6 hours], Al 2 O 3, SiO 2 [dried at 1173 K (900 C) for 6 hours], and Cr 2 O 3 [dried at 1073 K (800 C) for 6 hours) were carefully weighed and mixed together in an agate mortar. Table I shows the starting compositions for each sample. The samples were then pressed into pellets (diameter = 10 mm), and then placed into Mo crucible. Gas mixtures of Ar-CO-CO 2 were employed in order to obtain the targeted oxygen potential. The gas composition at equilibrium states was obtained by FACTSAGE. In view of the sensitivity of chromium to oxygen, it was necessary to purify the gases before mixing. The oxygen impurity in the argon gas was removed by passing through copper turnings at 773 K (500 C) and magnesium chips at the same temperature. Table I. Slag Compositions (Weight Percent) No. CaO MgO Al 2 O 3 SiO 2 CrO x M M M M A A A A The moisture in CO and CO 2 gas was removed by passing through the silica gel. In the present study, the flow rates of CO 2, CO, and Ar were fixed at 75, 15, and 100 ml/min, respectively. The partial pressure of oxygen in this mix gas is Pa at 1873 K (1600 C) as calculated by Factsage 6.2. The Mo crucibles containing the samples were positioned inside an alumina holder. The alumina sample holder was pushed into the furnace and placed in the even-temperature zone. The gas mixture was delivered close to the sample by means of a narrow alumina tube in order to minimize the effect of any thermal segregation of the gases. In the case of slag/gas equilibrium measurements, the duration time was set at 20 hours. The experiment was started by evacuating and filling the furnace chamber with purified argon twice before the Ar-CO-CO 2 gas mixture was introduced. The temperature of the furnace was well controlled by FP93. After equilibration, the crucible was quickly pulled to the end of the reaction tube and quenched in Ar gas. The slag samples were separated from the crucible and subjected to XRD and XPS analyses. The slag sample after the equilibrium with Ar-CO- CO 2 was first examined by XRD to ensure the sample maintaining the homogenous state at high temperatures. Then it was subjected to the XPS measurement. The XPS measurements on the glass samples prepared at various conditions were done by means of a K-Alpha spectrometer (AXIS Ultra DLD). Since chromium presents the strong affinity to oxygen, XPS spectrum must be collected with a fresh surface of the equilibrium sample, which is etched twice before the measurement. The measurements were conducted with a spot size of 400 lm at a voltage of 1.5 kv. Since the peaks for bivalent and trivalent chromium are overlapping in the spectra, they were deconvoluted into the Lorentzian- Gaussian subpeaks of Cr 2+ and Cr 3+ ions. III. RESULTS In order to ensure the equilibrium sample obtained, each slag sample was detected through X-ray diffraction examination. The typical result is shown in Figure 1. It is seen that only amorphous pattern could be detected, which indicates that the slag was in a glassy state. The patterns for other samples are similar to that shown in Figure 1 and are not presented for the sake of brevity. Survey-scan X-ray photoelectron spectra for each sample were performed from the newly fractured surfaces. Figure 2 shows a typical one, which is similar to the spectra obtained for other slag compositions. The XPS and Auger peaks from the constituent elements in the slag are easily identified and are marked on the spectra. The C1s peaks arise due to hydrocarbon contamination, and its binding energy is generally accepted as remaining constant, irrespective of the chemical state of the sample. For the sake of consistency, all energies are reported with respect to the C1s transition at ev.

3 Fig. 1 XRD pattern for CaO-3 pctmgo-sio 2 -CrO x slag after equilibrium at 1873 K (1600 C). A. Chromium Spectra The XPS technique is based on measurement of the bonding energy (cohesive force) of the ions in the slag. The bonding energy of an ion depends on both the species of interest and its local environment. The Cr 3+ has a higher charge and a smaller ionic radius than those of the Cr 2+, which leads to a larger effective charge of the Cr 3+ ions. As a consequence of the attraction, the bonding energy between Cr 3+ and O 2 is higher than that between Cr 2+ and O 2. Figures 3 and 4 show the high-resolution Cr 2p spin orbit doublet spectra for the analyzed slag compositions. The Cr2p3/2 binding energies have been reported to be ev for Cr metal, for Cr(II), for Cr(III) in Cr 2 O 3 (111)/Cr(110), [19,20] or for Cr(III) in Cr 2 O 3 /NiAl 2 O 4. [21] For the samples in which wt pct MgO = 3 and 5, the Cr2p3/2 peak is broad with a maximum at ~576 ev corresponding to the contribution from Cr 3+ and a shoulder at ~574 ev due to electrons from the Cr 2+ ions in the samples. Upon increasing the content of MgO substituting for CaO in samples, a slight broader was observed in the shoulder of the peak ascribed to the enhancement of Cr 2+. As can be seen in Figure 4, the Cr2p3/2 peak is broad and shifts to the low binding energy (BE) firstly with the increasing Al 2 O 3 content in slag samples from 5 to 15 pct; afterward, it presents a shift to high BE in 20 pct Al 2 O 3 sample. Generally, the shift to low BE is ascribed to the increase of Cr 2+ content. In order to quantify the chromium redox ratio in the samples, the broad peak covering both Cr 2+ and Cr 3+ peaks in Cr2p1/2 and Cr2p3/2 was deconvoluted into two separate peaks to determine the individual area of Cr 2+ and Cr 3+ peaks. The spectrum shown in Figure 5 exhibits two peaks at a binding energy of ~577 ev (Cr2p3/2) and ~586 ev(cr2p1/2) with a shoulder at a BE of ~575 and ~585 ev, respectively. The formal charge difference between Cr 2+ and Cr 3+ will be reflected in an energy shift of the Cr 2+ transition to Fig. 2 Survey scan XPS spectra of (a) CaO-3 pctmgo-sio 2 -CrO x slag (b) CaO-5 pctal 2 O 3 -SiO 2 -CrO x slag. Fig. 3 A series of high-resolution core level spectra from the Cr 2p transition for 4 slag compositions with various MgO contents from 3 to 10 pct.

4 lower BE. Thus, the spectrum was curve fitted with two components in each peak. The proportions of Cr 2+ and Cr 3+ ions in the slag sample were deduced from the area under the computer-resolved peaks shown in Figure 5. The parameters and results for each fitting are presented in Table II. B. Oxygen 1s Spectra Figures 6 and 7 illustrate the O 1s spectrum of the CaO-SiO 2 -CrO x samples with the addition of MgO and Al 2 O 3, respectively. An asymmetric peak is observed in Figure 6, which consists of two overlapping peaks. Considering from the charge density, the non-bridging atom peak will be shifted toward a lower binding energy with respect to the bridging oxygen atom peak [22]. Thus after splitting Fig. 4 A series of high-resolution core level spectra from the Cr 2p transition for 4 slag compositions with various Al 2 O 3 contents from 5 to 20 pct. Fig. 6 The fitted O1s spectrum for the MgO = 3 pct. The points are experimental data, while the fitted components and their sums are represented by the continuous lines. Fig. 5 The fitted Cr2p spectrum for the MgO = 3 pct sample. Fig. 7 The fitted O1s spectrum for the Al 2 O 3 = 5 pct. The points are experimental data, while the fitted components and their sums are represented by the continuous lines. Table II. Peak Position for the Core Levels Cr2p of the Slag Samples in ev Samples Cr(II)2p3/2 Cr(III)2p3/2 Cr(II)2p1/2 Cr(III)2p1/2 Cr 2+ /Cr 3+ M M M M A A A A

5 Fig. 8 Effect of MgO replacement of CaO on the oxygen state of Cr in slag samples. Fig. 9 Effect of Al 2 O 3 replacement of SiO 2 on the oxygen state of Cr in slag samples. the O1s spectra into two peaks, the lower bonding peak at ~531 ev is associated with the non-bridging oxygen, and the higher energy peak at ~532 ev is related to the bridging oxygen. IV. DISCUSSION A. Oxygen State of Cr The influence of MgO on the valence state of chromium in the slag phase can be seen in Figure 8. The ratio of Cr 2+ /Cr 3+ presents an increasing trend firstly with the substitution of CaO by MgO, then either keeps constant or shows a slightly lower value when MgO is fixed at 10 wt pct. Since the iconicity of MgO is lower than that of CaO, in the procedure of addition with MgO, the apparent basicity of this system is lowered, which is expected to result in increase of Cr 2+ / Cr 3+ ratio. However, the diluting effect of MgO on basicity is limited, and Cr 2+ is also basic as compared with Cr 3+. Hence, the reduction of apparent basicity is likely to be balanced by the addition of MgO = 10 pct. As for addition of Al 2 O 3 to replace the SiO 2, the situation appears to be somewhat more complicated, as shown in Figure 9. It is generally thought that the substitution of Al 2 O 3 for SiO 2 should increase the basicity for the weak Al-O bond relative to Si-O bond, [23] when there are enough metal cations to participate into the charge compensation. In the present case, Cr 2+ is still increasing with substitution of SiO 2 with Al 2 O 3. This is because Al 3+ ions have strong preferences to form tetrahedrons and incorporate into the network, and this process consumes CaO as the network modifier to generate non-bridging oxygen, which cause a depletion of free oxygen ions in the system. Thus, the trend shown by the variation of the ratio of Cr 2+ /Cr 3+ with the addition of Al 2 O 3 would be somewhat similar to the effect of MgO addition. The variation of the ratio of Cr 2+ /Cr 3+ with basicity is presented in Figure 10. The data deduced from XPS Fig. 10 Comparison with the literature data. Cr2p spectrum show a reasonable agreement with literature values. [1,5,10,11] B. Structure of Slag Comparing Figures 6 and 7, O1s spectra in MgO series and Al 2 O 3 series present different shapes. In the former one, the NBO is the dominate form of oxygen presented, while BO in the latter one. 1. Substitution of CaO by MgO From the O1s spectrum, the NBO and BO peaks were deconvoluted, which give the variation of NBO and BO with the substitution of CaO by MgO, as shown in Figure 11. With the increase of MgO, the amount of NBO should be decreased due to the weak iconicity of MgO comparing with CaO. However, it presented as an increasing trend firstly. In silicates, according to the definition of different forms of oxygen in silicates, [24,25] the fundamental reaction can be expressed as in Eq. [2], which is a result

6 Fig. 11 Effect of MgO substitution of CaO on the slag structure. of the charge balance required by the tetrahedral coordination of oxygen with silicon, and when any silicate anions associate to form higher polymers plus oxygen ions. Based on the results by Raman and NMR, generally, the free oxygen is less than O and O 0, especially in high-sio 2 systems like A1 to A4. Thus, the amount of free oxygen (O 2 ) was ignored here. 2O ¼ O 2 þ O 0 : In our previous study, [26] the magnitudes of the basicities of four metallic oxide silicates (CaO-SiO 2, MgO-SiO 2, FeO-SiO 2, and MnO-SiO 2 ) were elucidated according to the enthalpy change during the desulphurization process. The reaction of (2) reflects the change in distribution of electrons in silicates. In the left side, the electron distribution was more even than that in the right side. Comparing CaO-SiO 2 with FeO-SiO 2, CaO in silicates contributes to the electron distribution unevenly, while in FeO, due to the presence of FeO clustering, the electron distributed even more uniformly, since FeO tends to be more covalent. The same situation also can be found in CrO x -bearing silicates. It is known that the clustering of metallic oxides in silicates corresponds to the clustering of non-bridging oxygen ions, O -. Mekki et al. [27] studied the redox state of transition metal oxides in sodium silicate glasses with the composition of 0.3Na 2 O-0.7SiO 2. They also found that the NBO contribution has been increased in iron-doped glass, and explained that FeO x also played the similar role as Na 2 O in the glass structure. In the present case, with the substitution of CaO with MgO, the ionic character is decreased and covalent character is increased, leading to a corresponding lowering of the stretching corresponded to the Si-O-Mg bond. As the electron associated with oxygen can be considered to be distributed uniformly, there is a likelihood of chromium to get stabilized in the divalent state. [26] The bonding energy of Cr-O in Cr-O- Si is reduced, which, in turn, may lead to an enhancement of NBO value. When the addition of MgO is equal to 8 pct, which is higher than the content of CrO x in slag phase, the function of Cr 2+ ion to the enhancement of ½2Š Fig. 12 Effect of Al 2 O 3 substitution of SiO 2 on the slag structure. NBO would be limited because the total amount of CrO x is only 5 pct. In such case, the electron distribution becomes more uneven, which leads to a decrease in the amount of NBO. 2. Substitution of SiO 2 by Al 2 O 3 In this case, the degree of polymerization is higher than the above system since the total mass fraction of SiO 2 and Al 2 O 3 is 63 pct, which also can be noticed from the O1s spectrum, as shown in Figure 12. With the substitution of SiO 2 with Al 2 O 3, the bonding between Si-O-Si is modified to the Si-O-Al gradually, Si being replaced by Al in the silicate structure. This will cause the deformation of Si-O-Al bonding due to the asymmetry. Meanwhile, it is known that Al 3+ can fit into the Si polymeric chain but need to maintain charge balance e.g., if an Al 3+ is incorporated into a Si 4+ chain, it must have a one-half of a positive divalent ion near the Al 3+ to maintain local charge balance, which would lead to the number of Si-O-M binding getting reduced for reasons stated earlier. Therefore, in the O1s spectrum, the amount of NBO would be decreasing. With the increasing Al 2 O 3 in system, part of the bondings of Si-O- Cr has become Al-O-Cr. Since the Al-O is weaker than Si-O binding, the binding of Al-O-Cr is stretched, which can be deduced when the ionic character in O-Cr is increasing. It means that the ratio of Cr 2+ /Cr 3+ is increasing, which also can be concluded from Cr2p spectrum in the previous section. V. CONCLUSIONS In the present work, slags of CaO-SiO 2 -CrO x containing different contents of MgO or Al 2 O 3 were synthesized employing a gas-slag equilibrium technique and rapid quenching. XPS has been used to investigate the effects of substituting various amounts of CaO for MgO and SiO 2 for Al 2 O 3 in CaO-CrO x -SiO 2 -based slag. It was found that chromium existed in both divalence and trivalence states in the silicate slags. The proportions of Cr(II) and Cr(III), in each glass sample, have

7 been obtained by resolving the Cr 2p spectra and by measuring the relative ratio of Cr 2+ /Cr 3+ which increases when an increasing amount of MgO and Al 2 O 3 is substituted for CaO and SiO 2, respectively, in the base glass. From the O 1 s XPS spectra, it has been found that Cr(II) behaves as a glass modifier, which is beneficial to electron distribution uniformly in the case of MgO series, and with the presence of Al 2 O 3, Cr(II) will play a role of charge compensation Al 3+ to make the network stable. ACKNOWLEDGMENTS The authors would like to express their appreciation to National Nature Science Foundation of China (No , No ), a Fundamental Research Funds for the Central Universities (FRF-TP A2), Beijing Higher Education Young Elite Teacher Project (YETP0349), and China Postdoctoral Science Foundation (2014M560046). REFERENCES 1. M.G. Frohberg and K. Richter: Arch. Eisenbuttenwes., 1968, vol. 39, pp H.D. Schreiber and L.A. Haskin: Proc. Lunar. Sci. Conf. 7th, 1976, pp W.J. Rankin and A.K. Biswas: Arch. Eisen Huttenwes., 1979, vol. 50, pp E.B. Pretorius, P. Snellgrove, and A. Muan: J. Am. Ceram. Soc., 1992, vol. 75 (6), pp E.B. Pretorius and A. Muan: J. Am. Ceram. Soc., 1992, vol. 75 (6), pp K. Morita, M. Mori, M. Guo, T. Ikagawa, and N. Sano: Steel Res. Int., 1999, vol. 70, pp Y. Xiao and L. Holappa: ISIJ Int., 1993, vol. 33 (1), pp W. Pei and O. Wijk: Scand. J. Metall., 1994, vol. 23, pp A.J. Berry and H.S.C O Neill: Am. Mineral., 2004, vol. 89, pp A.J. Berry, H.S.C. O Neill, D.R. Scott, D.R. Scott, G.J. Foran, and J.M.G. Shellley: Am. Mineral., 2006, vol. 91, pp L.J. Wang and S. Seetharaman: Metall. Mater. Trans. B, 2010, vol. 41B, pp F. Irmann: J. Am. Chem. Soc., 1952, vol. 74, pp A. Mekki, D. Holland, C.F. McConville, and M. Salim: J. Non- Crystalline Solids, 1996, vol. 208, pp M.A. Salim, G.D. Khattak, P.S. Fodor, and L.E. Wenger: J. of Non-Crystalline Solids, 2001, vol. 289, pp H.W. Nesbitt, G.M. Bancroft, G.S. Henderson, R. Ho, K.N. Dalby, Y. Huang, and Z. Yan: J. Non-Crystalline Solids, 2011, vol. 357, pp M. Jensen, L. Zhang, and Y. Yue: Chem. Geol., 2012, vols , pp M.C. Biesinger, C. Brown, J.R. Mycroft, R.D. Davidson, and N.S. McIntyre: Surf. Interface Anal., 2004, vol. 36, pp D. Park, Y.S. Yun, and J.M. Park: J. Colloid Interface Sci., 2008, vol. 317, pp A. Maetaki and K. Kishi: Surf. Sci., 1998, vol. 411, pp C. Xu, M. Hassel, H. Kuhlenbeck, and H.J. Freund: Surf. Sci., 1991, vol. 258, pp A.M. Venezia, C.M. Loxton, and J.A. Horton: Surf. Sci., 1990, vol. 225, pp V.M.H. Smets and D.M. Krol: Phys. Chem. Glasses, 1984, vol. 25, pp B.O. Mysen: Structure and Properties of Silicate Melts, Elsevier, Amsterdam, 1988, pp G.W. Toop and C.S. Samis: Can. Met. Quart., 1962, vol. 1, pp G.W. Toop and C.S. Samis: Trans. Metall. Soc. AIME, 1962, vol. 224, pp L.J. Wang, M. Hayashi, K.C. Chou, and S. Seetharaman: Metall. Mater. Trans. B, 2012, vol. 43B, pp A. Mekki and M. Salim: J. Electron Spectrosc. Related Phenom., 1999, vols , pp