FTIR, Raman and NMR investigation of CaO–SiO2–P2O5 and CaO–SiO2–TiO2–P2O5 glasses

Journal of Non-Crystalline Solids 420 (2015) 26–33

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FTIR, Raman and NMR investigation of CaO–SiO2–P2O5 and CaO–SiO2–TiO2–P2O5 glasses Yongqi Sun a, Zuotai Zhang a,b,⁎, Lili Liu a, Xidong Wang a,b a b

Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, PR China Beijing Key Laboratory for Solid Waste Utilization and Management, College of Engineering, Peking University, Beijing 100871, PR China

a r t i c l e

i n f o

Article history: Received 9 December 2014 Received in revised form 2 April 2015 Accepted 8 April 2015 Available online xxxx Keywords: Structure; CaO–SiO2–TiO2–P2O5; FTIR; Raman; MAS-NMR

a b s t r a c t The role of P2O5 on the structure of CaO–SiO2–P2O5 and CaO–SiO2–TiO2–P2O5 glasses was identified using Fourier transformation infrared (FTIR), Raman and Magic Angular Spinning Nuclear Magnetic Resonance (MAS NMR) spectra in this study to provide some fundamental clues for titanium and phosphorus extraction. In both systems, the vibration signals of Q0(Si), Q1(Si), Q2(Si), and Q3(Si) were detected and the dominant structural units associated with P–O groups were isolated Q0(P) and terminal Q1(P) (Qi(Si,P), i represents the number of bridging oxygen per Si or P). The added P2O5 resulted in an increase of Q3(Si) at the cost of Q0(Si), Q1(Si), and Q2(Si) and the degree of polymerization (DOP) of the glasses was therefore increased; additionally, the mole ratio of Q0(P) to Q1(P) decreased with increasing P2O5 content, indicating an equilibrium reaction between Q0(P) and Q1(P). Furthermore, the presence of TiO2 resulted in a more complicated structure in the CaO–SiO2–TiO2–P2O5 glasses and monomers were clearly clarified. the structural units related to Q0(P), O–Ti–O deformation and TiO4− 4 © 2015 Elsevier B.V. All rights reserved.

1. Introduction The structural role of phosphorous (P) greatly influences the properties and utilizations of P-containing glasses or slags, such as the viscosity of P-bearing melts [1,2], the crystallization behaviors of P-bearing slags [3], the recovery of P from steelmaking slags [4,5], and even the bioactivity of P-bearing bioglasses [6,7]. Therefore, an understanding of the structural role of P has attracted much attention during the past years, especially in metallurgical resources recycling industry. Substantial solid wastes have accumulated without reasonable treatment with the development of metallurgical industry, which causes serious environmental pollution. Among them, titaniumbearing blast furnace slag (Ti-BFS) [2,3] and P-bearing steelmaking slag [4,5] are two kinds of promising residual wastes due to the containing valuable elements. Ti-BFS, containing around 20 wt.% TiO2, is an important secondary resource for Ti extraction and it has accumulated up to 70 million tons without timely treatment [8], leading to serious environmental pollution and resource waste [9]. Meanwhile, P-containing steelmaking slags are not reasonably utilized nowadays [10]. With the rapid development of economics, the demand of Ti and P is still increasing. There is no doubt that the aforementioned wastes are good secondary resources for Ti or P extraction. During the Ti (or P) recycling processes, selective crystallization and phase separation (SCPS) method

⁎ Corresponding author at: Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, PR China. E-mail address: [email protected] (Z. Zhang).

http://dx.doi.org/10.1016/j.jnoncrysol.2015.04.017 0022-3093/© 2015 Elsevier B.V. All rights reserved.

has been proven as an effective method [2–5,11,12]. The key factor of SCPS method is the modification of slags and the formation of Ti (or P)-enriched phase. The present authors have found that P2O5 was successfully used as the additive to promote the formation of Ti-enriched phase, i.e., rutile [2,3]. As for the P recovery from steelmaking slags, the slag structure is greatly influenced by the content of P and finally determines the recycling efficiency. Furthermore, TiO2 was successfully used as the modifier during the P recovery process [4]. Consequently, it is of crucial importance to acquire information of the effect of P2O5 or TiO2 on structure of slag melts, especially the role and coordination condition of P or Ti, since the macroscopic properties are primarily determined by the microscopic structure. Many previous studies investigated the effect of P2O5 content on the structure of slags or glasses with low content of P2O5 and the main vibration modes of PO4 tetrahedron have been discussed in detail [6,7, 13,14]. According to Mysen et al. [13,14], the main existing species associated with P were PO4 tetrahedron with zero and one bridging oxygen. Recently, many studies have reported the structures of P-bearing bioglasses and discussed the structural P-coordinated units [6,7]. As summarized in Fig. 1(a), four possible structural units associated with PO4 tetrahedron may exist in the melt with different numbers of bridging oxygen per P, namely Q3 (P2O5), Q2 (PO23 −), Q1 (P2O47 −) and Q0 i (PO3− 4 ), denoted as Q (P) (i = 0, 1, 2, 3). In addition, the five structural units related to SiO4 tetrahedron were portrayed in Fig. 1(b), which was denoted as Qi(Si) (i = 0, 1, 2, 3, 4). The structural units in the silicate network were commonly described by the nomenclatures of Qi(Si) and Qi(P), where i represents the number of bridging oxygen per Si and P.

Y. Sun et al. / Journal of Non-Crystalline Solids 420 (2015) 26–33

27

Fig. 1. Schematic of structural units in the silicate network: (a) PO4 tetrahedron and (b) SiO4 tetrahedron.

However, few studies [15,16] were carried out to study the influence of P2O5 content on the structure of Ti-bearing silicate melts, especially the slag melt with relative high contents of P2O5. Therefore, this study was motivated to quantitatively explore the effect of P2O5 content on the structure of CaO–SiO2–P2O5 and CaO–SiO2–TiO2–P2O5 glasses. The results deduced from Fourier transformation infrared (FTIR), Raman and Magic Angular Spinning Nuclear Magnetic Resonance (MAS NMR) spectra will surely facilitate the study and application of Ti-bearing and P-bearing slags which resemble our samples in compositions.

300 meshes for the subsequent tests. Table 1 shows the chemical compositions of samples by X-ray fluorescence (XRF) technique. In order to confirm the amorphous phase of these samples, X-ray diffraction (XRD) measurements were performed and the results are presented in Fig. 2, indicating that no crystal was formed in these quenched samples. The XRD tests were performed using an X-ray powder diffractometer (D/Max PC 2500, Rigaku) with a Cu-target tube, graphite monochromator and Ni filter. The Cu Kα radiation was generated at a voltage of 40 kV and a current of 100 mA and the scan speed of 4° min−1.

2. Experimental 2.1. Sample preparation In this study, the reagent purity grade chemicals of CaO (99.5%), SiO2 (99.5%), TiO2 (99.5%), and P2O5 (98%) (Produced by Alfa Aesar company) were used to prepare the samples. First, these oxides were thoroughly mixed and placed in a Pt crucible (Φ40 × 45 × H40 mm). Then the mixture was pre-melted in a tube furnace at 1773 K (1500 °C) under Ar atmosphere for 2 h to obtain a homogeneous sample. After pre-melting, the liquid slags were quickly poured into cold water and rapidly quenched to room temperature in order to obtain an amorphous phase. Then the slags were dried in air at 105 °C, crushed and ground to

Table 1 Chemical composition of pre-melted samples by XRF. Sample

CaO

SiO2

P2O5

TiO2

CaO/SiO2

CSP-1 CSP-2 CSP-3 CSP-4 CSPT-1 CSPT-2 CSPT-3 CSPT-4

54.95% 52.25% 49.87% 48.04% 43.70% 41.47% 38.74% 36.03%

45.05% 43.29% 41.12% 38.16% 36.21% 33.60% 31.97% 30.25%

0.00 4.46% 9.01% 13.80% 0.00 4.36% 8.87% 13.02%

0.00 0.00 0.00 0.00 20.10% 20.56% 20.42% 20.70%

1.22 1.21 1.22 1.26 1.22 1.21 1.22 1.20

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Y. Sun et al. / Journal of Non-Crystalline Solids 420 (2015) 26–33

Fig. 2. XRD results of the pre-melted samples.

2.2. Spectral measurements The structures of the obtained amorphous samples were characterized using FTIR, Raman and NMR spectra. The various vibration modes in a molecule could be Raman-active, IR-active or both and therefore these two techniques were complementarily employed in this study. The FTIR measures were conducted by a spectrophotometer (Tensor 27, Bruker), equipped with a KBr detector. To prepare the samples, ~2.0 mg samples and ~200 mg pure KBr was thoroughly mixed, grinded in an agate motor and pressed into a disc with 13.0 mm in diameter. Besides, the FTIR spectra were recorded in the range of 4000–400 cm−1 with a resolution of 2 cm− 1. As for the Raman tests, a laser confocal Raman spectrometer (JY-HR800, Jobin Yvon Company) was applied at room temperature with the excitation wavelength of 532 nm and the light source of a 1 mW semiconductor. To further identify the structural roles of P in the glasses, solid state 31P MAS-NMR measures were recorded using a 400 M FT-NMR spectrometer (Avance III 400 M, Bruker) with a MAS probe of a 4 mm ZrO2 rotor and two pairs of Dupont Vespel caps.

3. Results 3.1. FTIR spectra of the samples To analyze the structural variations with varying P2O5 contents, the samples were first characterized using FTIR spectra. Fig. 3(a) displays the FTIR spectra of CaO–SiO2–P2O5 glasses and several remarkable changes could be observed. Overall, the effective spectral range of 400–1200 cm−1 was composed of three regions, 800–1200 cm− 1 with strong intensity, 600–800 cm− 1 with weak intensity and 400–600 cm−1 with middle intensity. First, the 800–1200 cm−1 band, generally assigned to the stretching vibrations of SiO4 and PO4 tetrahedron [17–19], shifted to a higher wavenumber region with increasing P2O5 content, which suggested that the degree of polymerization (DOP) of the slags increased. In fact, P2O5 is a typical acid oxide and generally acts as a network former in the glass structures, which generally increases the DOP of the glasses. Specially, a peak near 935 cm−1, attributed to the Q0(P) [17], became more pronounced with increasing P2O5 content because of the increasing formation of PO4 tetrahedron in the glasses. This phenomenon also indicated that the dominant group related to P–O structures was Q0(P), which was in agreement with the previous studies related to different P-bearing systems [13]. Second, the 600–800 cm−1 band, assigned to the Si–O stretching [17,20], got less profound because of the less concentration of SiO4 tetrahedron with increasing P2O5 content. Furthermore, it can be noted that a shoulder associated with P–O–P bending vibrations [17] gradually appeared at

Fig. 3. FTIR spectra of the glasses: (a) CaO–SiO2–P2O5 system and (b) CaO–SiO2–TiO2–P2O5 system.

~600 cm−1 with increasing P2O5 content. The increasingly pronounced P–O–P linkage also indicated the existence of PO4 tetrahedron with more than one bridging oxygen such as Q1(P), Q2(P) and Q3(P), which would be further confirmed by Raman and MAS-NMR measures. Fig. 3(b) displays the FTIR spectra of CaO–SiO2–TiO2–P2O5 glasses. Similar to that of CaO–SiO2–P2O5 glasses, it can be seen that the 800–1200 cm−1 band related to the SiO4 tetrahedron shifted to a higher wavenumber region with increasing P2O5 content, which indicated a higher DOP of the glasses. In addition, an increasingly pronounced signal of stretching vibrations of Q0(P) (~935 cm) and a less profound of Si–O stretching vibrations (600–800 cm−1) were clearly detected. In particular, the bending vibrations of P–O–P, located at ~600 cm−1, became more pronounced with increasing P2O5 content, even comparable with that of Si–O–Si linkages located at ~450 cm−1 [17,20], which suggested that more P atoms were linked into the network structures. 3.2. Raman spectra of the samples 3.2.1. Raman spectra of CaO–SiO2–P2O5 glasses The Raman spectra for the CaO–SiO2–P2O5 system were depicted in Fig. 4(a). As can be noted, these Raman curves showed a remarkable variation trend. Similar to that of FTIR spectra, the whole Raman spectral range between 200 cm−1 and 1200 cm−1 was composed of three regions, i.e., 800–1200 cm−1, 600–800 cm−1 and 200–600 cm−1. In the region of 800–1200 cm− 1, a shoulder at ~ 1000 cm− 1 and a peak at ~ 940 cm−1 became more pronounced with increasing P2O5 content, which can be attributed to the Q1(P) and Q0(P) units, respectively [13,

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Fig. 4. Raman spectra and the deconvolved results of the CaO–SiO2–P2O5 system: (a) Raman spectra of the CaO–SiO2–P2O5 system, (b) sample CSP-1, (c) sample CSP-2, (d) sample CSP-3, (e) sample CSP-4 and (f) mole fractions of various Qi(Si) (i = 0, 1, 2 and 3) and mole ratios Q0(P)/Q1(P).

17]. With increasing P2O5 addition, more P atoms were introduced into the slag structure, resulting in the presence of more PO4 tetrahedrons, and this made the signals of PO4 tetrahedron, Q0(P) and Q1(P), more prominent. These results were well in agreement with the study of Mysen [13], although the present study adopted the glasses with a higher CaO/SiO2 ratio, which was, indeed, in favor of the existence of non-bridging oxygen and the formation of PO4 tetrahedron with less bridging oxygen, i.e., Q0(P) and Q1(P). Conversely, the peak centered at around 870 cm− 1, assigned to Q0(Si) [20,21], got less prominent with P2O5 addition. Because P2O5 is a typical acidic oxide with a stronger ionicity than SiO2 [22], the added P atom could preferentially link with O atoms, which caused a higher DOP of SiO4 tetrahedron and therefore a smaller abundance of Q0(Si).

The 600–800 cm− 1 band, associated with the presence of Si–O stretching linkages [20,21,23,24], became less pronounced with increasing P2O5 content due to the decreasing content of SiO4 tetrahedron. Meanwhile, the center of this envelope curve shifted to a lower spectral frequency, which generally indicated a higher DOP of silicate structures, which was in agreement with the apparent higher wavenumber shift of 800–1200 cm−1 in the FTIR spectra. Additionally, the region between 200 and 600 cm−1 was mainly composed of two peaks with the central frequencies of 350 cm−1 and 450 cm−1, respectively. As can be observed, the peak at around 450 cm−1, assigned to the bending mode P–O–P bending [17] became more pronounced; whereas that at 350 cm−1, associated with the bending motion of Si–O–Si [24,25] gradually faded away with P2O5 addition, which could be explained by the

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increasing content of PO4 tetrahedron and the decreasing content of SiO4 tetrahedron with more P2O5 addition. In summary, based on the Raman spectra of CaO–SiO2–P2O5 system, it can be seen that, with increasing P2O5 content, the signal of the P-related structures became more profound, accompanied with a less pronounced Si-related structure; consequently, a higher DOP of the glass structure was achieved. To quantitatively investigate the silicate structure, deconvolution of the Raman curves was performed in the range of 800–1150 cm−1, according to the rules proposed by Mysen et al. [25] It is assumed that the Raman curves are abided by Gaussian functions and the bands are fitted only at the regions where obvious shoulders or peaks were observed or strictly proven by previous studies. Many researches [13,18, 19] have investigated the Raman spectra of silicate glasses and melts and assigned the peaks at ~ 870, ~ 960, ~ 990, and ~ 1050 cm−1 to the stretching vibrations of Q0(Si), Q1(Si), Q2(Si), and Q3(Si), respectively. In addition with the peak (Q0(P)) at around 940 cm−1 and the shoulder (Q1(P)) at around 1000 cm−1, six Gaussian functions were used to fit the Raman curves of the CaO–SiO2–P2O5 system. In particular, the sample CSP-1, without P2O5 addition, could be used as a reference sample for the Raman deconvolution because of its simple compositions. Furthermore, based on the fitting results of Raman curves, the abundance of the coexisting structural units could be calculated. According to the studies of Mysen and Frantz [26,27], the calculation of the mole fractions of different structural units, i.e., Qi(Si) (i = 0, 1, 2, and 3), should be calibrated by Raman scattering coefficient, which is only related to Qi(Si) species. Therefore the following expression could be used to calculate the mole fractions: X i ¼ θi Ai ;

i ¼ 0; 1; 2; and 3

ð1Þ

where Xi, θi, and Ai denote the mole fraction of Qi(Si), Raman scattering coefficient, and band area of Qi(Si), respectively. The reciprocal of θi, Si, has been obtained by careful analysis of various silicate systems [28]. With the assumption that S0 equaled 1, S1, S2, and S3 could be calculated as 0.514, 0.242, and 0.09. These constants, in agreement with those proposed by Frantz [26], were adopted in this study and then the mole fractions of different Qi(Si) species could be calculated by the following equation: X i ¼ ðAi =Si Þ=

3 X

! Ai =Si :

ð2Þ

i¼0

For the structural units of PO4 tetrahedron, the mole ratio of Q0(P) and Q1(P) could be deduced by the following equation: X 0 =X 1 ¼ A0 =A1

more than 10 wt.%. This band, assigned to the bending vibration ν4 of PO4 tetrahedron [17,30], suggested that more P atoms were inserted into the silicate network with increasing P2O5 content. From the foregoing analysis, it could be seen that in the Raman spectra of the CaO–SiO2– TiO2–P2O5 system, with increasing P2O5 content, the P-related structure became more prominent because more P atoms were introduced into the glass structures and a higher DOP was therefore resulted in, which were complementarily demonstrated by the FTIR and Raman measures. Furthermore, it is essential to fit the Raman curves to quantitatively or semi-quantitatively explore the influence of P2O5 on the CaO–SiO2– TiO2–P2O5 glasses. The case of the CaO–SiO2–TiO2–P2O5 system, however, got more complex because of the presence of TiO2. By referring to the previous studies on the structures of Ti-bearing glasses or melts and the present fitting results of the CaO–SiO2–P2O5 system, the Raman curves of the CaO–SiO2–TiO2–P2O5 system were tentatively deconvoluted. It should be pointed out that, sample CSPT-1, without P2O5 addition, could also be used as a reference sample for the deconvolution due to its relatively simple chemical compositions. Fig. 5(b) showed the fitting results of sample CSPT-1 using six Gaussian functions. Different from the CaO–SiO2–P2O5 system, two bands in the region of 600– 800 cm−1 were obtained, which were associated with the structural units of Ti–O structures. According to the studies by Mysen et al. [31] and Wang et al. [32], the band at around 710 cm−1 could be assigned to the O–Ti–O deformation and the band at around 790 cm− 1 could be related to the structural units of TiO44 − monomers. These two bands, O–Ti–O deformation and TiO44 − monomers, were clearly observed in the present system, which were also detected in the CaO– SiO2–TiO2 system by Zheng et al. [33,34]. Summary above the Sirelated units and the Ti-related units, in addition with Q0(P) and Q1(P), the Raman spectra of the CaO–SiO2–TiO2–P2O5 system could be deconvoluted. The fitting results of the Raman curves of CaO–SiO2–TiO2–P2O5 glasses were presented in Fig. 5(b)–(e), and nine Gaussian functions were observed in this range, including Q3(Si) (~ 1040 cm− 1), Q2(Si) (~ 970 cm− 1), Q1(Si) (~ 910 cm−1), Q0(Si) (~ 840 cm− 1), Q1(P) (~1010 cm−1), Q0(P) (~940 cm−1), O–Ti–O deformation(~710cm− 1), TiO44 − monomers(~ 790 cm− 1) and ν4 of PO4 tetrahedron (~ 570 cm− 1). First, it can be seen that the peak associated with Q0 (P) became sharper and the area became larger with increasing P2O5 content, which indicated that the dominant type of PO4 tetrahedron was Q0(P). Second, the area of the peak centered at around ~570 cm−1 became larger with P2O5 addition, which suggested an increasing existence of PO4 tetrahedron. 3.3. 31P MAS-NMR spectra of the samples

ð3Þ

where X0, A0, X1, and A1 denote the mole fraction and band area of Q0(P) and Q1(P), respectively, and in the present study only these two structural units were detected and analyzed. The deconvolution results of the Raman curves of Ti-free slags were presented in Fig. 4(b)–(e), and it can be observed that these six Gaussian functions well fitted the Raman envelopes. 3.2.2. Raman spectra of CaO–SiO2–TiO2–P2O5 glasses As for the Raman spectra of the CaO–SiO2–TiO2–P2O5 system, the effective envelope was located in the range of 500–1200 cm− 1, as shown in Fig. 5(a). As can be observed, a peak at around 940 cm−1, originating from the stretching vibration of Q0(P) [13,17], became more pronounced with increasing P2O5 content. This variation trend agreed with that of the CaO–SiO2–P2O5 system, which was supposed that Q0(P) existed as a dominant PO4 unit. Meanwhile, the center of the band of 800–1150 cm−1 gradually shifted to a higher frequency region, and this indicated that the added P2O5 increased the DOP of silicate structure [13,29]. In addition, a peak at around 570 cm−1 slightly became visible with P2O5 addition, especially when P2O5 content was

To further identify the structural roles of P in the networks and verify the forgoing analysis, 31P MAS-NMR spectra were recorded for the Tifree glasses (CSP-2 and CSP-4) and Ti-bearing glasses (CSPT-2 and CSPT-4), as presented in Fig. 6. In all spectra, the peak with the chemical shifts of 1–3 ppm was confirmed, which could be assigned to the Q0(P) in the networks [7,35]. Additionally, a peak appeared to be to the right of Q0(P) with the chemical shifts of −1.5–0 ppm, which was generally attributed to the Q1(P) unit [7,35,36]. Overall, the results of 31 P MAS-NMR spectra are consistent with those of FTIR and Raman spectra, which confirmed that the main structural units related to P–O groups were Q0(P) and Q1(P) in the present systems. 4. Discussion For the CaO–SiO2–P2O5 glasses, the abundance of Qi(Si) and the mole ratio of Q0(P) to Q1(P) varying with P2O5 content are plotted in Fig. 4(f) based on Raman deconvolutions. It can be noted that the mole fractions of Q0(Si), Q1(Si), and Q2(Si) considerably decreased with increasing P2O5 content, while that of Q3(Si) showed an opposite variation tendency. These results indicated that the introduced P2O5

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Fig. 5. Raman spectra and the deconvolved results of the CaO–SiO2–TiO2–P2O5 system: (a) Raman spectra of the CaO–SiO2–TiO2–P2O5 system, (b) sample CSPT-1, (c) sample CSPT-2, (d) sample CSPT-3, (e) sample CSPT-4 and (f) mole fractions of various Qi(Si) (i = 0, 1, 2 and 3) and mole ratios Q0(P)/Q1(P).

captured the oxygen in Q0(Si), Q1(Si), and Q2(Si) tetrahedron, forming the Q3(Si) species while itself existed as Q0(P) and Q1(P) species in the structures. Meanwhile, it can be observed that the mole ratio of Q0(P) to Q1(P) decreased with increasing P2O5 content, and this trend was in agreement with a previous study [37]. This indicated that there was an acid–base reaction between Q0(P) and Q1(P) [7], as described by Eq. (4). With increasing P2O5 content, the oxygen required to form Q0(P) and Q1(P) increased with a fixed mole ratio of Q0(P) and Q1(P), whereas the oxygen supplied by Qi(Si) decreased because of decreasing SiO2 content; thus a smaller mole ratio of Q0(P) and Q1(P) was resulted in. As the pure P2O5 glass exists as Q3(P) (shown in Fig. 1 (a)), it can be assumed that the added P2O5 originally appeared as Q3(P). Then the Q3(P) unit captured the oxygen atoms of Q0(Si), Q1(Si) and Q2(Si); thus more Q3(Si), Q0(P) and Q1(P) were formed. Accordingly Eqs.

(5)–(7) could be applied to describe the variation of structural units in the present system based on the aforementioned analysis: 0

1

2−

2Q ðPÞ→2Q ðPÞ þ O

ð4Þ

3

0

0

1

3

ð5Þ

3

1

0

1

3

ð6Þ

3

2

0

1

3

ð7Þ

Q ðPÞ þ Q ðSiÞ→Q ðPÞ or Q ðPÞ; þ Q ðSiÞ Q ðPÞ þ Q ðSiÞ→Q ðPÞ or Q ðPÞ; þ Q ðSiÞ Q ðPÞ þ Q ðSiÞ→Q ðPÞ or Q ðPÞ; þ Q ðSiÞ:

For the CaO–SiO2–TiO2–P2O5 glasses, the mole fractions of Qi in SiO4 tetrahedron could be deduced based on the fitting results and Eq. (2), as

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Fig. 6. 31P MAS NMR spectra of the selected glass samples (samples CSP-2 and CSP-4 of the CaO–SiO2–P2O5 glasses and samples CSPT-2 and CSPT-4 of the CaO–SiO2–TiO2–P2O5 glasses).

illustrated in Fig. 5(f). It can be seen that the mole fractions of Q0(Si), Q1(Si), and Q2(Si) apparently decreased while Q3(Si) concentration increased with increasing P2O5 addition. A higher DOP of the glasses was therefore achieved, which was in consistent with our previous study [2] that P2O5 addition led to a higher viscosity of Ti-BFS melts.

Besides, this tendency could be clearly described by Eqs. (5)–(7). Furthermore, based on the fitting results the mole ratio of Q0(P) to Q 1 (P) could be calculated by Eq. (3), which were 2.78 ± 0.12, 2.22 ± 0.22 and 1.91 ± 0.20 for samples CSPT-2, CSPT-3, and CSPT-4, respectively. A decreasing mole ratio of Q0(P) to Q1(P) was identified, which also indicated the presence of the equilibrium reaction between Q0(P) and Q1(P) (Eq. (4)). To further distinguish Q0(P) and Q1(P) and deduce the relative content of these peaks, the MAS NMR spectra were deconvoluted using Gaussian functions. The results after deconvolution were presented in Fig. 7 and it was found that using two Gaussian functions could well fit the spectra. Based on the peak area and Eq. (3), the mole ratios of Q0(P) to Q1(P) (X0/X1) were 2.23 ± 0.23, 1.26 ± 0.12, 2.06 ± 0.26 and 1.43 ± 0.08 for samples CSP-2, CSP-4, CSPT-2 and CSPT-4, respectively. It can be seen that the X0/X1 value decreased with increasing P2O5 content, which was exactly in consistence with the Raman results and further demonstrated the acid–base reaction between Q0(P) and Q1(P). According to the aforementioned analysis, two kinds of PO4 tetrahedral species, Q0(P) and Q1(P), were observed in the CaO–SiO2–P2O5 and CaO–SiO2–TiO2–P2O5 glasses. These two structural units can form three types of linkages, the linkage between two Q1(P) units (P2O47 −), the linkage between a SiO4 tetrahedron and a terminal Q1(P), and the isolated Q0(P) (PO3− 4 ). These three types of linkages made up the main forms of P existence in the present glasses. Most importantly, during the treatment of the Ti-bearing melts using SCPS method, the variation of the structural groups and DOP with varying P2O5 content could cause great influence on the properties of the slag melts, such as viscosity and crystallization ability, which would determine the crystalline

Fig. 7. Deconvolved results of 31P MAS NMR spectra of the selected glass samples: (a) sample CSP-2, (c) sample CSP-4, (d) sample CSPT-2 and (e) sample CSPT-4.

Y. Sun et al. / Journal of Non-Crystalline Solids 420 (2015) 26–33

phase precipitated in the melts, the growth rate and the particle size of the crystals. This could finally bring a great effect on the efficiency of SCPS method. 5. Conclusions In this study, a remarkable variation trend of the structures of CaO– SiO2–P2O5 and CaO–SiO2–TiO2–P2O5 glasses with varying P2O5 contents and a fixed CaO/SiO2 ratio of 1.2 was observed complementarily using FTIR, Raman and MAS NMR spectra. The main conclusions were summarized as follows: 1) The results of FTIR and Raman spectra undoubtedly demonstrated the existence of Si–O–Si linkages in the CaO–SiO2–P2O5 and CaO– SiO2–TiO2–P2O5 glasses and an increasingly pronounced P–O–P linkage in the structures with increasing P2O5 content. 2) The results of Raman deconvolutions showed that the added P2O5 increased the mole fraction of Q3(Si) at the cost of Q0(Si), Q1(Si), and Q2(Si) in both glass systems, and therefore a higher DOP was achieved, which was in consistent with the FTIR results. 3) Based on the Raman spectra analysis and the results of Raman deconvolutions, the structural units of O–Ti–O deformation −1 ) were clearly iden(~710 cm−1) and TiO4− 4 monomers (~790 cm tified in the glass structures for the CaO–SiO2–TiO2–P2O5 system. 4) The Raman spectra and 31P MAS-NMR spectra complementarily proved that the main P-related structures were Q0(P) and Q1(P) in the glasses. Through the deconvolutions of these two spectra, it was found that the mole ratio of Q0(P) to Q1(P) decreased with increasing P2O5 content and this indicated that there existed an acid–base equilibrium reaction between Q0(P) and Q1(P). Acknowledgments The authors acknowledge financial support by the Common Development Fund of Beijing, the National Natural Science Foundation of China (51172001, 51074009 and 51172003), the National High Technology Research and Development Program of China (863 Program, 2012AA06A114) and Key Projects in the National Science & Technology Pillar Program (2011BAB03B02 and 2011BAB02B05).

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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]

M.J. Toplis, D.B. Dingwell, Geochim. Cosmochim. Acta 60 (1996) 4107. Y.Q. Sun, K. Zheng, J.L. Liao, X.D. Wang, Z.T. Zhang, ISIJ Int. 54 (2014) 1491. Y.Q. Sun, J. Li, X.D. Wang, Z.T. Zhang, Metall. Mater. Trans. B 45 (2014) 1446. J. Diao, B. Xie, Y.H. Wang, X. Guo, ISIJ Int. 52 (2012) 955. M. Ishikawa, ISIJ Int. 46 (2006) 530. R.C. Lucacel, M. Maier, V. Simon, J. Non-Cryst. Solids 356 (2010) 2869. H. Grussaute, L. Montagne, G. Palavitn, J.L. Bernard, J. Non-Cryst. Solids 263 (2000) 312. L.P. Wang, H. Wang, G. Qi, X.J. Li, Z.Q. Chen, Y.Q. Dou, Chin. J. Rare Met. 28 (2004) 265. Y. Peng, Titan. Ind. Prog. 22 (2005) 44. J. Wu, Z.J. Wen, M.J. Cen, Steel Res. Int. 82 (2011) 494. J. Li, Z.T. Zhang, L.L. Liu, W.L. Wang, X.D. Wang, ISIJ Int. 53 (2013) 1696. W. Zhang, L. Zhang, J.H. Zhang, N.X. Feng, Ind. Eng. Chem. Res. 51 (2012) 12294. B.O. Mysen, Am. Mineral. 81 (1996) 1531. B.O. Mysen, F.J. Ryerson, D. Virgo, Am. Mineral. 66 (1981) 106. M.D. O'Donnel, S.J. Watts, R.V. Law, R.G. Hill, J. Non-Cryst. Solids 354 (2008) 3554. M.D. O'Donnel, S.J. Watts, R.V. Law, R.G. Hill, J. Non-Cryst. Solids 354 (2008) 3561. B.O. Fowler, M. Markovic, W.E. Brown, Chem. Mater. 5 (1993) 1417. B.O. Mysen, Am. Mineral. 75 (1990) 120. P.F. McMillan, Am. Mineral. 69 (1984) 622. D.R. Neuville, D.D. Ligny, G.S. Henderson, Rev. Mineral. Geochem. 78 (2014) 509. J.L. You, G.C. Jiang, K.D. Xu, J. Non-Cryst. Solids 282 (2001) 125. Y. Waseda, J.M. Toguri, The Structure and Properties of Oxide Melts, World Scientific Publ., Singapore, 2002. 189. P.F. McMillan, B.T. Poe, P. Gillet, B. Reynard, Geochim. Cosmochim. Acta 58 (1994) 3653. B.O. Mysen, L.W. Finger, D. Virgo, F.A. Seifert, Am. Mineral. 67 (1982) 686. B.O. Mysen, D. Virgo, C.M. Scarfe, Am. Mineral. 65 (1980) 690. J.D. Frantz, B.O. Mysen, Chem. Geol. 121 (1995) 155. B.O. Mysen, J.D. Frantz, Am. Mineral. 78 (1993) 699. Y.Q. Wu, G.C. Jiang, J.L. You, H.Y. Hou, H. Chen, Acta Phys. Sin. 54 (2005) 961. B.O. Mysen, Chem. Geol. 98 (1992) 175. I.J. Hidi, G. Melinite, R. Stefan, M. Bindea, L. Baia, J. Raman Spectrosc. 44 (2013) 1187. B.O. Mysen, D.R. Neuville, Geochim. Cosmochim. Acta 59 (1995) 325. Z. Wang, Q. Shu, K. Chou, ISIJ Int. 51 (2011) 1021. K. Zheng, J.L. Liao, X.D. Wang, Z.T. Zhang, J. Non-Cryst. Solids 376 (2013) 209. K. Zheng, Z.T. Zhang, L.L. Liu, X.D. Wang, Metall. Mater. Trans. B 45 (2014) 1389. M. Sakamoto, Y. Yanaba, K. Morita, J. Non-Cryst. Solids 358 (2012) 615. C. Mercier, C. Follet-Houttemane, A. Pardini, B. Revel, J. Non-Cryst. Solids 357 (2011) 3901. R. Dupree, D. Holland, M.G. Mortuza, J.A. Collins, M.W.G. Lockyer, J. Non-Cryst. Solids 106 (1988) 403.