Effect of chlorine on the viscosities and structures of blast furnace slags

Abstract

The viscosity of CaO–SiO₂–Al₂O₃–MgO–CaCl₂ slags (C/S = 1.12) were investigated to elucidate the effects of chlorine ranging from 0.02 to 0.53 mass% on the blast furnace slags at high temperatures. Moreover, the Raman spectra of the quenched slags and the X-ray diffraction patterns of the slags cooled in air after viscosity measurement were thoroughly analysed to interpret the transformation of the structures of the slags with increasing the content of chlorine. The viscosity was found to decrease slightly with the increase of chlorine at a given temperature higher than 1673 K, and the critical temperature (T _CR) decreased from about 1660 to 1590 K simultaneously which was possibly deriving from the precipitation of Ca₂Al₂SiO₇, Ca₃Al₂(SiO₄)₃–xCl₄ x and SiO₂ in higher chlorine content. The degree of polymerisation for silicon–oxygen tetrahedra was found to decrease estimating from the decrease of the average amount of bridging oxygen calculated from the deconvolution results of the Raman spectra of the quenched slags, which provided the explanation for the decrease in viscosity. And that the apparent activation energy of the slags was commonly reduced by chlorine increasing demonstrated the decrease in the degree of polymerisation of molten slags simultaneously.

Introduction

Dangers of chlorine to blast furnace (BF) have recently drawn more concentrations of metallurgists because more HCl and other acid gases were brought into the gas pipelines with erosions and destructions to the pipelines and accessory equipments in the substitution of dry-dedusting for wet-dedusting technology which should have been absorbed by gas washing water in wet-dedusting technology BF.^1–4

The increase of chlorine in BF system resulting mainly from the injection of low-quality coal and waste plastics,^5,6 the employment of imported ores after seawater beneficiation, sinter sprayed with CaCl₂ ⁷ and coke coked by coal and waste plastics also leads to different degrees of effects on the BF smelting process, metallurgical properties of raw material and fuel and characteristics of molten slag simultaneously,^8–10 and the certain extent of circulation and accumulation of chlorine in BF makes these effects more obvious.^11,12

It has been well acknowledged that BF slag could absorb the chlorine, and hence would affect its performance. However, only few studies on the effect of chlorine on the molten slag were carried out, Myoung et al.⁶ performed the effect of chlorine content on the viscosity of CaO–SiO₂–12 mass%Al₂O₃–10 mass%MgO BF slag by adding CaCl₂ powder and discovered that the viscosity at temperatures high than 1700 K was less than that of slags without chlorine by about 2.5 dPa s as the chlorine content increased up to 3.2 mass%, while the effect mechanism was not analysed further. Ito and Morita¹³ characterised chlorine bearing phases in the synthetic CaO–SiO₂–Al₂O₃–Cl slags, it was found that the chlorine only condensed in the amorphous phase and 9CaO·6Al₂O₃·5SiO₂·CaCl₂. Wang and Morita¹⁴ measured the phase diagram of the SiO₂–CaCl₂ system and the liquid area of the CaO–SiO₂–CaCl₂ ternary system at 1723 K, two eutectics and an intermediate compound SiO₂·CaCl₂ were found in the SiO₂–CaCl₂ binary system, and the liquidus of the CaO–SiO₂–CaCl₂ ternary system at 1723 K was detected. In order to deal with the trend that the raw material and fuel with higher chlorine content were put into use, more investigations are acquired to study the effect mechanism of chlorine on the BF slag.

Therefore, the viscosities of CaO–SiO₂–Al₂O₃–MgO–CaCl₂ slags were investigated in the present study. Furthermore, Raman spectra and apparent activation energy of the slags were quantitatively applied to the mechanism analysis.

Experimental

Slag preparation

The chemical compositions of the studied BF slags which originated from a normal slag produced by a Chinese ironmaking plant are shown in Table 1. In order to evaluate the effects of chlorine on the viscosities and structures of the BF slags, CaCl₂ powder was added into the preset slags despite of the influence of entrained elements. All of the slags were prepared using reagent grade chemicals such as SiO₂, Al₂O₃, MgO, CaCl₂ and CaO calcined from CaCO₃ at 1273 K in a muffle furnace for 10 h. The mixtures weighted to the preinstall compositions and mixed in an agate mortar in advance about 140 g were kept for direct viscosity measurements to avoid the expected evaporation of the chlorine from the slags.

Table 1

Chemical compositions of the slags investigated in the present study

Sample number	Initial composition (mass%)					Basicity	Final composition (mass%)					Basicity
Sample number	CaO	SiO₂	Al₂O₃	MgO	CaCl₂	C/S	CaO	SiO₂	Al₂O₃	MgO	CaCl₂	C/S
No. 1	38.95	34.74	15.79	10.53	0.00	1.12	39.38	35.15	14.83	10.41	0.04	1.12
No. 2	38.56	34.39	15.63	10.42	1.00	1.12	39.68	34.35	14.52	10.17	0.80	1.16
No. 3	38.17	34.04	15.47	10.32	2.00	1.12	39.51	34.02	14.53	10.14	1.45	1.16
No. 4	37.00	33.00	15.00	10.00	5.00	1.12	38.14	33.48	14.20	9.89	3.91	1.14
No. 5	35.05	31.26	14.21	9.47	10.00	1.12	36.58	32.01	13.61	9.14	8.42	1.14

Procedure of viscosity measurement

The viscosity measurements were carried out using the rotating cylinder method with a RTW-10 viscometer, and the schematic diagram of the experimental apparatus is shown in Fig. 1. The molybdenum crucible (inner diameter = 40 mm and height = 80 mm) containing about 140 g mixed slag was placed inside the furnace temperature zone, then the furnace was heated to the preset 1823 K from room temperature by the MoSi₂ heating element and held for 0.5 h to homogenise the melts. The measurement of viscosity in the present study was started by lowering the molybdenum spindle into the slag about 7 mm above the bottom of the crucible carefully, and the slag viscosity was measured continuously during the cooling cycle until the viscosity reached up to about 3.5 Pa s. The slag was subsequently reheated to 1793 K and also held for 0.5 h to stabilise the temperature, the slag was partially quenched by pouring the melts into water and partially cooled in air afterwards. In the whole measurement process, the purified Ar gas at a flow rate of 0.3 NL min⁻¹ was inlet to avoid the oxidation of the molybdenum crucible and spindle. The quenched slags after dried and ground less than 0.074 mm were sent for X-ray fluorescence to confirm the compositions, as also shown in Table 1, the final chlorine compositions of the investigated No. 1 to No. 5 BF slags were 0.02, 0.51, 0.93, 2.50 and 5.39 mass%, respectively. And the quenched slags No. 1 and No. 4 were subject to X-ray diffraction (XRD) to verify the amorphous state of all the slags, as shown in Fig. 2.

Experimental apparatus for the measurement of slag viscosity

XRD analysis of the quenched slags

Raman spectroscopy analysis

Raman spectroscopy was applied to the quantitative analysis of the structures of the quenched slags. Raman spectra of the present grassy slags were acquired at room temperature in the frequency range of 200–1600 cm⁻¹ using a semiconductor laser source having excitation wavelength of 532 nm coupled with Jobin-Y’ von Horiba 800 (LabRam HR, France) micro-Raman spectrometer. The PeakFit V4.0 software was employed to deconvolute the spectra, and then the relative abundances of the Q ⁿ units (Q ⁰, Q ¹, Q ² and Q ³, the 0, 1, 2 and 3 represent the number of bridging oxygen in silicon–oxygen tetrahedra, respectively) were calculated based on the area fractions of the best fitted Gaussian curves.

Results and discussion

Effect of chlorine on the viscosity and critical temperature of BF slags

Figure 3 graphically represents the viscosities of the CaO–SiO₂–Al₂O₃–MgO–CaCl₂ slags (CaO/SiO₂ = 1.12) as a function of the temperature. It is found that the viscosity gradually increases with decreasing temperature and this tendency is less significant as the chlorine content increases. In addition, the viscosity of BF slags decreases as the chlorine content increases at a given temperature higher than 1673 K and the chlorine effect is more noticeable at lower temperatures as shown in Fig. 4. Certainly, it is commonly known that the Ca²⁺ could increase the polymerisation of slags for the Ca²⁺ with lower ionisation potential is charge balanced with two open O⁻ ions due to its large [CaO₆] cage,¹⁵ thus the actual effect of chlorine on the viscosity decreasing is greater.

Viscosities of the BF slags as a function of temperature at different chlorine concentrations

Effect of chlorine on the viscosities of the BF slags at temperatures from 1673 to 1873 K

The critical temperature (T _CR) which has been defined as the temperature at which the viscosity increases suddenly and the slag becomes non-Newtonian in behaviour exhibits an abrupt change in the activation energy when viscosity measurements are carried out during the cooling cycle.^16–18 As shown in Fig. 5, the critical temperature of the slags decreases from about 1660 to 1590 K with increasing chlorine content from 0.02 to 0.539 mass%, this trend can easily be expected from the XRD analysis of No. 4 and No. 5 slags cooled in air after viscosity measurement. As Fig. 6 shows, the primary crystallisation region of the studied quaternary BF slag (CaCl₂ is excluded) is located in either Ca₂Al₂SiO₇ or Ca₃MgSi₂O₈, while it is certified to be the Ca₂Al₂SiO₇ precipitation by the XRD patterns of present No. 4 and No. 5 slags as shown in Fig. 7. Meanwhile, the tremendous decrease in T _CR may be caused by the precipitation of Ca₃Al₂(SiO₄)₃–xCl₄ x and SiO₂ detected in No. 5 slag, it can be concluded that the CaCl₂ significantly reduces the T _CR of the CaO–SiO₂–Al₂O₃–MgO–CaCl₂ slags consequently. The slight enhancement in T _CR of No. 2 and No. 3 slags may be possibly resulting from the improvement of slag basicity, which can increase the solid suspended particles, despite of the small additive chlorine content therein.

Effect of chlorine on the critical temperature of the BF slags

Phase diagram of the CaO–SiO₂–Al₂O₃–(10.53 mass%)MgO slag system

XRD analysis of No. 4 and No. 5 slags cooled in air after viscosity measurement

Raman spectra analysis of the structures of BF slags by the chlorine addition

The room-temperature Raman spectra of the five quenched slags as a function of wavenumbers ranging from 700 to 1300 cm⁻¹ are shown in Fig. 8. The major bands at 850–880 cm⁻¹, 900–930 cm⁻¹, 950–980 cm⁻¹ and 1040–1060 cm⁻¹ are related to the occurrence of zero, one, two and three bridging oxygen per tetrahedrally coordinated silicon (BO/Si), respectively.^15,19–22 It is found that the Raman spectra for silicon–oxygen tetrahedra gradually shift to lower wavenumber with increasing the chlorine content of the investigated BF slags, and the intensity of the high frequency Q ³ band is very weak. Consequently, it is undoubtedly that the chlorine acts as a role in the depolymerisation of the BF slags.

Raman spectra of the quenched BF slags with different chlorine contents

The measured Raman spectra of the quenched slags with different chlorine contents are deconvolved on the basis of Gaussian functions with the minimum correction coefficient equalling 0.9989, and the results are shown in Fig. 9. The relative area fractions of Q ⁰ (SiO₄-monomer), Q ¹ (Si₂O₇-dimer), Q ² (SiO₃-chain) and Q ³ (Si₂O₅-sheet) units obtained from the Gaussian deconvolution of Raman spectra are listed in Table 2. The fraction of Q ⁰ unit increases and the fractions of Q ¹ and Q ² units decrease continuously as the chlorine content increases, which indicates the Si₂O₇-dimer and SiO₃-chain structures are depolymerised into simpler SiO₃Cl-monomer and Si₂O₆Cl-dimer structures as follows²³: (1) (2) In the present study, the average amount of the bridging oxygen of each slag sample is applied to evaluating the variation of the silicates structure network using the area fraction of Q ⁰, Q ¹, Q ² and Q ³ units multiplied by the number of its bridging oxygen.²⁴ It can be observed from Fig. 10 that the average amount of bridging oxygen reduces with the increase of chlorine content from 0.02 to 5.39 mass%, resulting from the role of network modifier chlorine played in the silicon–oxygen tetrahedra to decrease the degree of polymerisation.

Deconvolved results of Raman spectra for samples in different chlorine contents

Effects of chlorine on the average amount of bridging oxygen

Table 2

Relative area fractions of Q ⁿ units of the five Raman spectra

Sample number	Cl (mass%)	Q ⁰	Q ¹	Q ²	Q ³	Correction coefficient
No. 1	0.02	0.364	0.326	0.298	0.011	0.9994
No. 2	0.51	0.366	0.340	0.277	0.017	0.9993
No. 3	0.93	0.371	0.348	0.267	0.014	0.9989
No. 4	2.50	0.386	0.327	0.275	0.012	0.9992
No. 5	5.39	0.402	0.314	0.258	0.026	0.9991

Activation energy for the viscous flow of the BF slags

The activation energy reflects the energy barrier for viscous flow. Thus, the variations in the apparent activation energy for the viscous behaviour imply the change in the structures of molten slag.^23,25 The degree of polymerisation of the slag network structure is the characteristic function of temperature and composition. It is assumed that the composition of the slags during experimental procedures is a constant, the apparent activation energy of slags can be approximated from the following Arrhenius equation: (3) where η is the viscosity, η ₀ is the pre-exponent constant, E_η is the activation energy, R is the gas constant, and T(K) is the absolute temperature. Therefore, the activation energy E_η is acquired by plotting lnη vs. 1/T, and the calculation results of apparent activation energy are listed in Table 3. The viscosities of the partially crystallised melts in a non-Newtonian region are not included as shown in Fig. 11, it is seen that the lnη of the slags linearly increases with increasing 1/T. The activation energy of BF slags ranges from about 182.8 to 203.4 kJ mol⁻¹ and generally decreases with increasing the chlorine content, this consequence is consistent with that of viscosity changes by the depolymerisation of the BF slags. Therefore, the decrease in the apparent activation energy manifests the decrease in the degree of polymerisation of molten slags.

Arrhenius plot for the BF slags in the Newtonian flow region

Table 3

Apparent activation energy of the molten slags

Sample	Cl (mass%)	E_η (kJ mol⁻¹)
No. 1	0.02	203.4
No. 2	0.51	198.7
No. 3	0.93	199.8
No. 4	2.50	193.8
No. 5	5.39	182.8

Conclusions

The viscosities of CaO–SiO₂–Al₂O₃–MgO–CaCl₂ slags were measured to elucidate the influence of chlorine on the BF slags at high temperatures, and Raman spectra of the quenched slags with chlorine content ranging from 0.02 to 0.53 mass% were analysed to illustrate the role of chlorine in modifying the structure of BF slags. The obtained consequences are summarised as follows:

The viscosity of the investigated slags decreases with increasing the chlorine content at a given temperature higher than 1673 K and the effect of chlorine is more noticeable at lower temperatures. Meanwhile, the critical temperature (T _CR) decreases from about 1660 to 1590 K, which possibly results from the precipitation of Ca₂Al₂SiO₇, Ca₃Al₂(SiO₄)₃–xCl₄ x and SiO₂ with more chlorine addition in BF slag.

The effects of chlorine on the viscosity of the present BF slags can be interpreted based on the decrease in the degree of polymerisation by chlorine anion, which is verified by the decrease of the average amount of bridging oxygen calculated from the deconvolution results of the Raman spectra of the quenched slags.

By the addition of chlorine in BF slags, the apparent activation energy generally decreases, which is consistent with that of viscosity changes by the depolymerisation of the slags. Therefore, the decrease in the apparent activation energy demonstrated the decrease in the degree of polymerisation of molten slags.

Footnotes

Acknowledgements

This work was financially supported by the Key Program of the National Natural Science Foundation of China (No. U1260202) and the 111 Project (No. B13004).

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Effect of chlorine on the viscosities and structures of blast furnace slags

Abstract

Introduction

Experimental

Slag preparation

Chemical compositions of the slags investigated in the present study

Procedure of viscosity measurement

Raman spectroscopy analysis

Results and discussion

Effect of chlorine on the viscosity and critical temperature of BF slags

Raman spectra analysis of the structures of BF slags by the chlorine addition

Relative area fractions of Q n units of the five Raman spectra

Activation energy for the viscous flow of the BF slags

Apparent activation energy of the molten slags

Conclusions

Footnotes

Acknowledgements

References

Relative area fractions of Q ⁿ units of the five Raman spectra