Effect of P₂O₅ Addition on the Viscosity and Structure of Titanium Bearing Blast Furnace Slags

Abstract

This study provided a fundamental analysis of the viscosity and structure of titanium bearing blast furnace slags modified by different levels of P₂O₅ addition. The viscosity of slag melts was obtained by rotating cylinder method, which is a significant factor for blast furnace process and utilization of slags. The results showed that the slag viscosity was greatly influenced by basicity. Slag viscosity decreased and the calculated apparent activation energy for viscous flow also decreased with increasing basicity, which indicated the breakdown of melt structure. The addition of P₂O₅ substantially increased the slag viscosity at each basicity, and the increasing trend was most pronounced at basicity 0.5. To connect the viscosity changes of slags to the slag melt structure, Fourier transformation infrared (FTIR) and Raman spectral spectroscopy analysis were performed in this study. FTIR results clearly indicated that the added P₂O₅ increased the degree of polymerization of slags, corresponding to the increase of slag viscosity. Based on Raman and FTIR curves, it can be concluded that P₂O₅ acted as a characteristic network former in the melts.

1. Introduction

The titanium resource is abundant in China, and more than 91% exists in the form of vanadium titanium-magnetite ore in Sichuan province, South China.¹⁾ Blast furnace process is the main process to utilize the ore and extract valuable elements (e.g. iron, vanadium) nowadays. In this process, hot metal containing vanadium is separated by reduction, whereas titanium components are left into blast furnace slags, forming the titanium bearing blast furnace slags (Ti-BF slags). These Ti-BF slags generally contain about 22 wt% TiO₂ ,²⁾ which are expected as good secondary resources for titanium extraction. However, the Ti-BF slags have not been utilized and treated reasonably. More than 70 million tons of Ti-BF slags have been accumulated and the annual increase is still more than 3 million tons.³⁾ These untreated slags cause a series of problems, including air and water pollution, land occupation and resource wasting. To utilize the Ti-BF slags, a promising method, selective crystallization and phase separation (SCPS) method was proposed and studied.^4,5,6,7,8,9) The whole process of this method can be divided into several steps. Firstly, the Ti-BF slags are modified by additives, such as CaO, CaF₂, MnO, and SiO₂,^9,10,11,12) which are likely to promote the precipitation of selected titanium bearing phases. Secondly, slag temperature is controlled to enrich the titanium element into rutile, anosovite or perovskite through a reasonable cooling path. Thirdly, the titanium bearing phase is separated from slags using gravity separation, flotation separation or other separation methods. Then the obtained titanium bearing phase was treated for further utilization, such as titania production and titanium metallurgy. The efficiency of SCPS method is determined by many variables, among which viscosity has a decisive effect because of its influence on mass transfer. SiO₂ could be used to modify the basicity of the slags in SCPS method, and it has been found that the primary crystalline phase transforms from perovskite to anosovite by adding 10–15 wt% SiO₂.¹²⁾ P₂O₅ is a typical acidic oxide which shows a strong acidity and it is expected that a small amount of P₂O₅ addition can substantially reduce the SiO₂ addition during basicity modification. Furthermore, viscosity is a significant property that influences the Blast furnace process and few studies have reported the viscosity of Ti-BF slags containing P₂O₅. To clarify the influence of P₂O₅ addition on the viscosity of Ti-BF slags, samples with different P₂O₅ contents at varying basicity were designed in this study. It is a consensus that the viscosity was determined by structure of slag melts and the effects of P₂O₅ addition on the structure of Ti-BF slags were also discussed using Fourier transformation infrared (FTIR) and Raman spectra techniques.

2. Experimental

2.1. Sample Preparation

In this study, analytical reagent (AR) pure CaO, MgO, SiO₂, TiO₂, Al₂O₃ and P₂O₅ were used as raw materials to prepare the modified slags. Three slag series were designed with the range of basicity 0.5–0.9 and P₂O₅ content 0, 1 wt%, 3 wt% and 5 wt%, respectively. The mixture was premelted in a molybdenum (Mo) crucible (Φ40×45×H40 mm) under argon atmosphere for 1 h at 1500°C to homogenize the slags. Then the liquid slags were rapidly poured into cold water to obtain the glassy phases of slags. The quenched slags were then analyzed by X-Ray fluoroscopy (XRF), as shown in Table 1. It can be seen that there was a small deviation between designed chemical compositions and XRF results because of the melting process, which showed a quite small effect on viscosity results. To confirm the glassy phases of quenched slags, X-Ray Diffraction (XRD) analysis was carried out and the results of slag series A are depicted in Fig. 1 as an example. Then the glassy slags were crushed and grinded to 300 meshes for subsequent viscosity measurements.

Table 1. Chemical compositions of samples (wt%).

	Initial Composition (wt%)							Final Composition By XRF (wt%)
	Basicity	CaO	SiO2	Al2O3	MgO	TiO2	P2O5	Basicity	CaO	SiO2	Al2O3	MgO	TiO2	P2O5
A1	0.5	18.70	37.32	11.97	7.05	24.96	0	0.56	19.39	34.44	13.29	7.90	24.98	0
A2	0.5	18.53	37.00	11.86	6.93	24.72	0.97	0.57	19.31	34.06	12.91	7.75	24.93	1.04
A3	0.5	18.13	36.25	11.63	6.79	24.27	2.93	0.57	18.93	33.49	13.12	7.61	24.08	2.77
A4	0.5	17.75	35.55	11.42	6.68	23.80	4.80	0.56	18.50	33.28	12.84	7.63	23.20	4.56
B1	0.7	23.08	32.91	12.02	7.00	25.00	0	0.76	23.52	30.79	13.06	7.69	24.93	0
B2	0.7	22.83	32.61	11.94	6.93	24.71	0.97	0.74	22.78	30.97	13.35	7.84	24.10	0.95
B3	0.7	22.34	32.01	11.62	6.80	24.32	2.91	0.77	23.01	30.01	12.57	7.47	24.24	2.69
B4	0.7	22.00	31.37	11.39	6.69	23.77	4.78	0.76	22.56	29.61	12.85	7.60	22.65	4.73
C1	0.9	26.55	29.47	11.98	6.98	25.03	0	0.96	26.53	27.69	13.12	7.74	24.92	0
C2	0.9	26.23	29.18	11.93	6.91	24.74	1.01	0.92	25.80	27.99	13.32	7.87	23.96	1.07
C3	0.9	25.75	28.63	11.61	6.76	24.32	2.93	0.93	25.60	27.65	12.78	7.55	23.56	2.85
C4	0.9	25.25	28.04	11.45	6.68	23.78	4.79	0.97	25.65	26.36	12.47	7.36	23.45	4.71

Fig. 1.

XRD results of the pre-melted slags (basicity=0.5).

2.2. Apparatus and Procedure

The viscosity measurements were carried out using a Brookfield digital viscometer (RTW-10) and experimental apparatus is graphically sketched in Fig. 2. As can be seen, this instrument mainly comprises temperature control part and viscosity measurement part. The crucible and spindle used in the present experiments were made of molybdenum to prevent slag contamination. The dimensions of the main components of the equipment are listed in Table 2. The samples were heated and melted using a resistance furnace with MoSi₂ heating elements and the furnace temperature was controlled by a computer program and monitored by the Pt-10Rh/Pt thermocouple. To confirm the prevention of the oxidation of molybdenum component, the concentration of molybdenum in slags after experiment was analyzed by XRF and the results showed that the concentration of molybdenum in slags was less than 0.5 wt.%. The viscosity was obtained by measuring the torque of the Molybdenum spindle in the Molybdenum crucible filled with liquid slags after a calibration experiment using standard oil of known viscosity of 0.10 Pa.s to 5.00 Pa.s at room temperature to confirm the measurement accuracy.

Fig. 2.

Experimental apparatus for measurement of slag viscosity.

Table 2. Dimensions of components adapted in the present experiments.

Shaft	Material	Alumina
	Shaft diameter/mm	4.5
	Shaft length/cm	45
Spindle	Material	Molybdenum
	Shaft diameter/mm	5
	Shaft length/mm	15
	Bob diameter/mm	10
	Bob length/mm	20
	Degree of tapper/°	120
Crucible	Material	Molybdenum
	Inner diameter/mm	38
	Inner depth/mm	64

The viscosity measurements of the slags were carried out in the following steps. Firstly, Molybdenum crucible containing 140 g slag powder was placed in the heating chamber of the furnace and argon gas was flowed into the heating chamber at the rate of 0.3 NL/min to prevent the oxidation of the Molybdenum crucible and spindle. Secondly, the furnace was heated to 1550°C at a heating rate of 5°C/min and held for 2 h to homogenize the slag temperature and chemical compositions. Thirdly, Molybdenum spindle was carefully immersed into the liquid slags and then rotated to measure the viscosity. Three rotation rates (150, 175 and 200 r/min) were used for the viscosity measurement at a given temperature with an equilibration time of 30 min to stabilize the slag temperature in this study and the average value of these three measured viscosities was taken as the viscosity at the corresponding temperature. The cooling rate used between different temperatures was 5°C/min.

The structure of the slags was detected by FTIR and Raman spectroscopy. The mixture of about 2.0 mg slags and 200 mg KBr was grinded in an agate motor and then pressed to a disc with 13.0 mm in diameter. Then FTIR spectroscopy absorption spectra were recorded in the range of 4000–400 cm^–1 with a resolution of 2 cm^–1 using Nicolet, Magna-IR750 equipped with a KBr detector. The Raman analysis was performed by a laser confocal Raman spectrometer, JY-HR800, manufactured by Jobin Yvon Company. In the present experiments, the excitation wavelength was 532 nm and the light source was a 1 mW semiconductor. The measurement frequency band was in the range of 100 to 2000 cm^–1 with a 1 cm^–1 precision of wavenumber.

3. Results and Discussion

3.1. Viscosity of the Ti-BF Slags

To ensure the reproducibility of the viscosity measurements, repeated experiments were performed for sample A1, and the comparative results are shown in Fig. 3. As is evident from Fig. 3, these two measurements matched quite well with the deviation less than 3%. Considering that the experimental uncertainties were mainly caused by viscosity measurements, this method was credible.

Fig. 3.

Comparison of viscosity changes between original experiment and repeated results of sample A1.

The results of the viscosity in different temperatures are shown in Fig. 4. It was apparent that the viscosity increased with the decrease of temperature. Conventionally, the viscosity of the slag melts was mainly determined by the interaction between the flow units when it was higher than break temperature.¹³⁾ The linkage between flow units became weaker in virtue of the increasing distance caused by volume expansion with increasing temperature, resulting in the viscosity decrease.

Fig. 4.

Viscosity of the slag system (a) basicity=0.5, (b) basicity= 0.7 and (c) basicity=0.9.

3.1.1. Effect of P₂O₅ on the Viscosity of Ti-BF Slags

Figure 4 illustrates the effect of P₂O₅ on the viscosity of the designed Ti-BF slags with different basicities and the range of P₂O₅ content is 0–5 wt%. As can be seen, the viscosity for each slag sample increased with increasing P₂O₅ content, which agreed with several previous studies,^14,15,16,17) although the slag compositions were different. Kozakevitch et al.¹⁴⁾ investigated the effect of P₂O₅ on the viscosity of a binary system of CaO–P₂O₅ and they found that the viscosity of these slags increased with increasing P₂O₅ content. The viscosity of a ternary slag system of CaO–P₂O₅–SiO₂ with the CaO/SiO₂ ratio of 1 was measured by Nikolina et al.¹⁵⁾ and Belikova et al.,¹⁶⁾ and the results showed that the viscosity of these slags slightly decreases with the decrease of P₂O₅ content. A quaternary slag system of Na₂O–Al₂O₃–SiO₂–P₂O₅ has been studied by M. J. Toplis et al.¹⁷⁾ with differing alkali/alumia ratios. It was found that, in a peralkaline melt, an addition of phosphorus results in an increase of viscosity. A more complicated slag system was designed in this study and it was found that P₂O₅ addition increased the slag viscosity, which agreed with the aforementioned studies. According to a previous study, P₂O₅ was a typical acidic oxide, which was a strong network former and showed the strongest acidity (ability to form networks) of acidic oxides with the ion-oxygen parameter of 3.3.¹⁸⁾ The increasing viscosity with the increase of P₂O₅ content suggested that the polymerization degree of the slag melts became higher because of the effect of P₂O₅ on network formation in this study. As a whole, the variation trend of viscosity was obviously influenced by P₂O₅. However, it should be pointed out that some intersection of viscosity curves appeared at high temperature region, which could be caused by the high superheat and the relative weak effect of P₂O₅ and was in the error range.

It is now clear that the added P₂O₅ caused the increase of viscosity to some extent and there is an important point to note about these results. As shown in Fig. 4, it can be observed that the degree of viscosity increase with increasing P₂O₅ content was changed with different basicities. To further verify the mechanism of effect of P₂O₅ content on the viscosity of slags, the viscosity varying with P₂O₅ content at 1364°C and 1465°C was presented in Fig. 5. The viscosity at the basicity of 0.5 increased from 0.75 Pa.S, 0.27 Pa.S at 1364°C to 1.26 Pa.S, 0.58 Pa.S at 1465°C, respectively, i.e., the viscosity increment was 0.51 Pa.S and 0.31 Pa.S, whereas the viscosity increment at the basicity of 0.9 was 0.32 Pa.S and 0.16 Pa.S at 1364°C and 1465°C, respectively. The results indicated that the degree of viscosity increase with increasing P₂O₅ content was greater at lower basicity. It is consistent with the previous study,^19,20) which reported that the activity coefficient of P₂O₅ in slag melts was smaller at a higher basicity, and this resulted in a weak influence on the viscosity.

Fig. 5.

Viscosity varying with P₂O₅ content (a) T=1364°C, (b) T=1465°C.

3.1.2. Effect of Basicity on the Viscosity of Ti-BF Slags

Figure 6 shows the viscosity variation of the slag melts without P₂O₅ with varying basicity. It can be noted that the viscosity of the melts decreased with increasing basicity, suggesting that degree of polymerization (DOP) of slag structure decreased with increasing basicity. Conventionally, Arrhenius equation (shown as Eq. (1)) was used to relate slag viscosity η to temperature T, through which the apparent activation energy for viscous flow E_w could be calculated.   

$$\eta =Aexp\left(\frac{E_w}{RT}\right)$$(1)

where η is slag viscosity, A is the proportionality constant, E_w is apparent activation energy, T is the absolute temperature, and R is the gas constant. The apparent activation energy E_w represents the frictional resistance for viscous flow in slag melts. The variation in E_w may indicate the change of the structure in the slag melts or more directly a change of the flow units. The calculated apparent activation energy was 217.98 kJ/mol, 177.28 kJ/mol and 158.90 kJ/mol for samples A1, B1 and C1 without P₂O₅, respectively. It was clear that the activation energy decreased with increasing basicity, which indicated a lower polymerization degree and simpler structural units.

Fig. 6.

Viscosity with varying basicity without P₂O₅.

3.2. Structure Analysis

It is necessary to investigate the slag structure to reveal the mechanism of viscosity change with P₂O₅ using FTIR and Raman analysis. Figure 7 presents the FTIR absorption spectra of slags. The FTIR spectra can be separated into several bands, a strong band in the 1200–800 cm^–1 range, a weak band in the 800–600 cm^–1 range and a medium intense band in the 600–400 cm^–1 range. The band in the 1200–800 cm^–1 range has been generally assigned to the symmetric stretching vibration of SiO₄ tetrahedra^21,22) and stretching vibration of PO₄ groups.^23,24,25) As for the stretching vibration of SiO₄ tetrahedra, the bands indicated more polymerized structure with different numbers of bridging oxygen per tetrahedrally coordinated silicon from low wavenumber (~800 cm^–1, Q⁰) to high wavenumber (~1100 cm^–1, Q³) where “n” denotes numbers of bridging oxygen per tetrahedrally coordinated silicon in “Qⁿ”.²¹⁾ And as for the vibration modes of PO₄ groups, two bands at about 1092 cm^–1 and 1040 cm^–1, were related to antisymmetric stretching vibration (ν₃) and the band at 962 cm^–1 was related to symmetric stretching vibration (ν₁).²³⁾ An apparent tendency was observed that the band for SiO₄ tetrahedral vibration in the 1200–800 cm^–1 range shifted successively to a higher wavenumber region with increasing P₂O₅ at each basicity, which indicated that the numbers of Q³ and Q² increased compared with Q¹ and Q⁰ and the DOP became higher. This variation trend agreed well with the viscosity increase. The medium intense band in the 600–400 cm^–1 range generally represented bending vibration of T–O–T and T denotes Si, Al, or P elements.^23,26) As can be seen from Fig. 7, the band in the 600–400 cm^–1 range slightly shifted successively to a lower wavenumber region with increasing P₂O₅ content at each basicity, which indicated a higher polymerization degree in another respect. For slag series A (basicity=0.5), a peak at about 1040 cm^–1 representing the antisymmetric stretching vibration (ν₃) of PO₄ groups,²³⁾ appeared for sample A3 and A4 because of the increasing P₂O₅ content, which was consistent with the greater effect of P₂O₅ on slag viscosity at lower basicity. The weak band in the 800–600 cm^–1 range has been assigned to symmetric stretching vibration of AlO₄ tetrahedra.^27,28) For slag series A, it can be noted that the intensity slightly became lower with increasing P₂O₅ content, which might be in virtue of that the added P₂O₅ captured the oxygen of AlO₄ tetrahedral to form PO₄ groups. As aforementioned, the medium intense band in the 600–400 cm^–1 range generally represented bending vibration of T–O–T and T denoted Si, Al, or P elements. It can be observed that a peak about 540 cm^–1 appeared when P₂O₅ content was 3–5 wt% at each basicity, which can be associated with bending vibration (ν₄) of P–O–P²³⁾ because it only occurred at higher P₂O₅ content. To further analyze the effect of basicity on the structure of slag melts, FTIR spectra of the slags without P₂O₅ is presented in Fig. 8. It was obvious that the band for SiO₄ tetrahedral vibration in the 1200–800 cm^–1 range shifted successively to a lower wavenumber region whereas the band for bending vibration of T–O–T (T denoted Si, Al elements) in the 600–400 cm^–1 range shifted to a higher wavenumber region with increasing basicity, which indicated that the network structure of the melts was broken down, the DOP of slags became lower and the structural unites became simpler, corresponding to the decrease of slag viscosity.

Fig. 7.

FTIR absorption spectra of the slags (a) basicity=0.5, (b) basicity=0.7 and (c) basicity=0.9.

Fig. 8.

FTIR absorption spectra of the slags without P₂O₅.

As for Raman spectra, many researches have reported the vibration modes of different structural units in the 400–1200 cm^–1 range. Amongst these vibration modes, the stretching vibration of SiO₄ tetrahedral, AlO₄ tetrahedral, PO₄ groups, and TiO₄, Ti₂O₆ groups have been reported in the relatively high wavenumber region,^{23,29,30,31,32,33,34)} whereas the bending vibrations of T–O–T where T denotes Ti, Al, Si or P elements could appear in the lower wavenumber region (400–600 cm^–1 range), as reported by some previous studies.^23,26,33) Raman spectra for slag melts are presented in Fig. 9. Considering the overlapping of the signals of different vibration modes, it is difficult to totally deconvolute the Raman spectral curves. However, Fig. 9 presents some trend of melt structure with varying P₂O₅ content as a whole. It can be observed that a shoulder (~970 cm^–1) appeared for each sample, which can be assigned to the stretching vibration of SiO₄ tetrahedral (Q²) and PO₄ groups.^23,33) For slag series A (basicity=0.5), the shoulder became slightly stronger with increasing P₂O₅ content and sample A4 showed the strongest intensity. Whereas for slag series B and C, shown in Figs. 9(b) and 9(c), the intensity of the shoulder band (~970 cm^–1) substantially increased with increasing P₂O₅ content, which reflected that P₂O₅ was inserted into melt structure and a characteristic network was built up.

Fig. 9.

Raman spectra of the slags (a) basicity=0.5, (b) basicity=0.7 and (c) basicity=0.9.

With the help of FTIR and Raman analysis, the slag structure was investigated and the results showed that the polymerization degree of slag melts became higher with increasing P₂O₅ content at each basicity, which was consistent with the trend of slag viscosity increase. The increasing basicity of slag melts showed an opposite effect and broke down the network structure of the melts, making the flow units simpler and smaller and resulting in the decrease of slag viscosity correspondingly. It is a consensus that mass transfer in slag melts was greatly influenced by viscosity, which affected the blast furnace process and the utilization of slags.

4. Conclusions

Viscosity of slag melts is a significant property that influences the blast furnace process and the further utilization of slags. In this study, viscosity of Ti-BF slags was obtained and the effect of basicity and P₂O₅ content on viscosity of Ti-BF slags was investigated by rotating cylinder method, which matched well with the analysis of slag structure carried out by FTIR and Raman analysis. The main conclusions were summarized as follows:

(1) The viscosity of the Ti-bearing blast furnace slags was greatly influenced by P₂O₅ content and basicity. The viscosity decreased with increasing basicity. With the addition of P₂O₅, slag viscosity increased at each basicity and the increase increment was more remarkable at lower basicity because of the smaller activity coefficient.

(2) FTIR spectra showed that the polymerization degree of the network structure in slag melts increased with increasing P₂O₅ content, corresponding to the increase of slag viscosity.

(3) FTIR spectra and Raman spectra showed that phosphorus interconnected into melt structure and built up a characteristic network.

Acknowledgement

The authors gratefully acknowledge financial support by the Common Development Fund of Beijing and the National Natural Science Foundation of China (51074009, and 51172001). Supports by the National High Technology Research and Development Program of China (863 Program, 2012AA06A114) and Key Projects in the National Science & Technology Pillar Program (2011BAB03B02) are also acknowledged.

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