Effects of Na2O and B2O3 Addition on Viscosity and ...

ISIJ International, Vol. 58 (2018), No. 10

© 2018 ISIJ1751

ISIJ International, Vol. 58 (2018), No. 10, pp. 1751–1760

* Corresponding author: E-mail: [email protected]: https://doi.org/10.2355/isijinternational.ISIJINT-2018-212

1. Introduction

At present, remelting and modifying of blast furnace dry slag after quenching is the main mode of production of mineral wool in China. This process technology becomes mature, and the operation is simple and can achieve the continuous supply of the melt to ensure stable production. Whereas, the process does not make full use of the sensible heat of the molten blast furnace slag, and the remelting causes a lot of energy consumption and increases the pro-duction cost. Consumption of coke in 250 kg per ton or more, plus other consumption in blast furnace slag remelt-ing will make a great increase in the cost of production. Therefore, it will make great reduction in the production cost of mineral wool if we put the molten blast furnace slag directly into the production of mineral wool and the utiliza-tion of sensible heat will reach higher than 80%, which is consistent with the national energy-saving emission reduc-tion policies, while showing the rational use of resources.1) Due to technical bottlenecks and industry interface and other factors, the use of blast furnace slag production of mineral wool technology is only applied in JFE and other three com-panies of Japan throughout the world. Four production lines using molten blast furnace slag through the direct electrical smelting of wool, these output accounts for 40% of the total rock-mineral wool production in Japan.

Mineral wool production has the following requirements on the melt. Ingredients must be within the formation area and melting in a certain temperature range (1 400–1 500°C) in which have a lower viscosity. The viscosity of melt changes little around the temperature range. It is difficult

Effects of Na2O and B2O3 Addition on Viscosity and Electrical Conductivity of CaO–Al2O3–MgO–SiO2 System

Wanli LI1) and Xiangxin XUE1,2)*

1) School of Metallurgy, Northeastern University, Shenyang, 110819 China.2) Liaoning Key Laboratory of Metallurgical Resources Recycling Science; Shenyang, 110819 China.

(Received on March 26, 2018; accepted on June 18, 2018; J-STAGE Advance published date: August 23, 2018)

The present study was aimed at investigating the effect of Na2O and B2O3 addition on the viscosity and electrical conductivity of slag which was used for mineral wool production. Industrial slag samples of the CaO–Al2O3–MgO–SiO2 system were used to modify its composition with different Na2O and B2O3 addition content and soaked at a certain temperature range (1 400–1 500°C). Raman spectrum was used to repre-sent the structure of the slag system. Results showed that the Na2O addition modified process had a larger effect on the two parameters (macro aspect) and the relative fraction of the structural units (micro aspect) compared with the results of B2O3 addition.

KEY WORDS: viscosity; electrical conductivity; acidity coefficient; mineral wool; structural units.

to be crystalline in the mineral wool procedure temperature range. The performance of produced mineral wool meets the requirements. The composition of the melt is uniform and make ensure stable product quality. Minimize the consump-tion and emission, reduce production cost which facilitates to protect the environment.

There are numerous factors that affect the slag production of mineral wool. Generally, the composition of raw materi-als of slag is reasonable and acidity coefficient Mk (Mk, the ratio of the sum of acidic oxides to the sum of basic oxides) is usually an evaluated empirical parameter. In addition, the flowability of slag will affect the process of drawing slag into the product and its mobility mainly features in the slag viscosity. The general mineral wool melting temperature (1 400°C or so), viscosity controlled within the range of 1–3 Pa∙s is enough to meet the requirements of drawing slag into mineral wool.2)

Considering energy conservation and environmental pro-tection, modifying and compensation heat of blast furnace slag should use electric heating, so we need to understand the electrical conductivity of the slag. As a poor conductiv-ity of blast furnace slag, we need to add additives to increase its conductivity, commonly used additives are Na2O, CaF2, K2O, and B2O3. It is necessary to investigate the effect of Na2O and B2O3 addition on the electrical conductivity and viscosity of the slag. As two important parameters, trying to meet the requirements of mineral wool production is meaningful.

2. Experimental

CaO–SiO2–Al2O3–MgO system has been investigated in the experimental work. The blast furnace slag in the experiment is obtained from Baosteel Group Corporation,


© 2018 ISIJ 1752

China. Blast furnace slag should be modified as its acidity coefficient is low that cannot be directly used to manufac-ture mineral wool. Based on the design of different acidity coefficient, the amount of CaO and SiO2 was calculated and added. Chemical composition and ingredient of different acidity coefficient of BF slag were given in Tables 1 and 2.

The total weight of the mixed slag for each experiment is 130 g. All oxides used in preparing the synthetic slag are analytical reagents. Before making up different series of slag, analytical reagents CaO and SiO2 were roasted at 1 173 K in a muffle furnace for 2 hours to decompose any carbonate and hydroxide. Weighting some analytical reagents precisely which are shown in Table 2 and mixing with BF slag into a milling equipment for 10 hours. Adding mixtures to a graphite crucible which has Mo plate covering both inside wall and bottom of the crucible. The crucible and slag were put into the hot zone of a MoSi2 furnace. Argon gas flowed through the Al2O3 tube and cooling water was circled around the hearth and they were kept flowing during the whole experimental period. The heating rate was 10 K/min. The temperature of the slag was kept at 1 823 K for 30 minutes at first for the slag to be melted completely. Schematic illustration of experimental apparatus (System A and System B) is shown in Fig. 1.

Results of viscosity and electrical conductivity in this study were measured using the method referred in the past research.3) The time used for holding the constant tem-perature at each point is 30 minutes and then measurements will be started. Temperature range of the experiment was 1 823 K to 1 623 K. Mo spindle kept rotating during every constant temperature stage and cooling stage to uniform molten slag. During the test, high-temperature rheometer kept recording along the constant temperature stage until the records hardly changed in 30 minutes at each tempera-ture point. Mo electrodes were inserted at a certain depth

in molten slag and Mo spindle was raised over the molten slag through the furnace body moving up and down pre-cisely after the constant temperature stage finished and then electrochemical workstation began to measure the electrical conductivity. When the electrochemical workstation began to work, data of electrical conductivity can be obtained and the viscosity was considered as a constant at the same time. X-ray imaging device has been used to ensure the right position of Mo spindle and Mo electrodes which is shown in Fig. 1(b). The experiment will end after finishing the third temperature point measurement.

Viscosity and electrical conductivity of different acid-ity coefficient of slag were measured. Results can serve as a basis for the selection of melt composition for mineral wool production. As a poor conductivity of blast furnace slag, we need to add additives to increase its conductivity, commonly used additives are Na2O, CaF2, K2O, and B2O3. Considering the cost and effect, the comparison between the effects of Na2O and B2O3 addition on the electrical conductivity and viscosity of slag was investigated in the subsequent work. The addition of Na2O and B2O3 content in the present study is 1 wt.%, 3 wt.%, 5 wt.%, 7 wt.%, 9 wt.%, respectively. The ingredients plan was shown in Tables 3 and 4. Effect of Na2O and B2O3 addition on the above two parameters was investigated based on the proper acidity coefficient (Mk=1.4, 1.6) at the proper temperature range (1 400–1 500°C).

After the measurement of viscosity and electrical conduc-

Fig. 1. Schematic illustration of experimental apparatus: (a) Measurement diagram, (b) Size and position of Mo elec-trode and Mo spindle in the model and actual measurement.

Table 1. Chemical composition of BF slag.

Composition TFe FeO SiO2 MgO Al2O3 CaO K2O Mk*

Mass% 1.49 1.14 34.09 7.50 16.13 39.47 0.32 1.0691

*M

(SiO ) (Al O )

(CaO) (MgO)k �

��

� ��

2 2 3

Table 2. Ingredient of different acidity coefficient of BF slag.

Mk Slag (g) SiO2 (g) CaO (g) Total (g)

1.0 96.85 0 3.15 100

1.1 98.57 1.43 0 100

1.2 94.21 5.79 0 100

1.3 90.22 9.78 0 100

1.4 86.55 13.45 0 100

1.6 80.04 19.96 0 100

1.8 74.45 25.55 0 100

2.0 69.58 30.42 0 100


© 2018 ISIJ1753

tivity, the slags was cooled down to the room temperature. All the slag samples were crushed, batched and reheated to 1 773 K in the box furnace for 2 h and then quenched in water. The samples were then dried, crushed and tested using a Raman spectrometer (Renishaw inVia, England) with an excitation wavelength of 532 nm and using a 1 mW semicon-ductor laser as a light source (Exposure time is 10 seconds, 5 times stack). Raman spectra were recorded in the range 200–1 600 cm−1 and the wavenumber precision was 1 cm−1.

3. Results and Discussions

3.1. Viscosity and Electrical Conductivity of Samples in Different Series

Viscosity and electrical conductivity of slag have been measured by high temperature rheometer and electrochemi-cal workstation (1 260 impedance/gain-phase analyzer and 1 287 electrochemical interface, Solartron Metrology) in the same temperature schedule at the same time. Viscosity and electrical conductivity of samples in the different series range from 1 823 K–1 623 K were measured in Fig. 2.

Experiment showed that the blast furnace slag can get a mixture of uniform melt under the sufficient conditions of compensation heat and stirring after silica and lime were added. Viscosity of different acidity coefficient of slag was presented in Fig. 2(a). The viscosity of slag decreased with the increase of temperature, but made a difference in the change of slope. When the acidity coefficient was small, the viscosity curve was smooth, especially when the tempera-ture was higher than 1 400°C. The change of melt viscosity within the temperature range was very small which meant the viscosity of slag was not sensitive to the temperature change and had a wide temperature range. However, for the slag with Mk=1.0 or 1.1, the viscosity did not show smooth change at low temperatures between 1 350–1 400°C, which was probably due to non-uniform state (Generally, alkaline slag has short slag characteristics that it has a narrow tem-perature range at the same viscosity change and a significant turning point on the viscosity-temperature curve. When the acidity coefficient is low, the short slag changes from a free-flowing state to a non-free-flowing state in a narrow temperature range when the temperature decreases near the melting temperature, that is why the viscosity changes rap-idly). With the increase of the acidity coefficient, the slope of the melt viscosity curve increased and a slight fluctuation of the temperature will lead to a big change in the melt vis-cosity, then curves became steeper. At the same temperature conditions, the greater the acidity coefficient, the higher the viscosity. When the acidity coefficient was less than 1.4, the temperature can be controlled at 1 400°C to obtain the melt with viscosity below 2.0 Pa∙s, which met the needs of the mineral wool making process, and the higher acidity coefficient melt must be heated to a higher temperature to get better mobility.

In general, the higher the acidity coefficient, the better

Table 3. Different addition content of additive in 1.4 series (Mk=1.4).

No. Slag (g) SiO2 (g) Na2O (g) B2O3 (g) Total (g)

1.402 85.68 13.32 1 0 100

1.406 83.95 13.05 3 0 100

1.410 82.22 12.78 5 0 100

1.412 80.49 12.51 7 0 100

1.414 78.76 12.24 9 0 100

1.417 85.68 13.32 0 1 100

1.421 83.95 13.05 0 3 100

1.425 82.22 12.78 0 5 100

1.427 80.49 12.51 0 7 100

1.429 78.76 12.24 0 9 100

Table 4. Different addition content of additive in 1.6 series (Mk=1.6).

No. Slag (g) SiO2 (g) Na2O (g) B2O3 (g) Total (g)

1.602 79.24 19.76 1 0 100

1.606 77.64 19.36 3 0 100

1.610 76.04 18.96 5 0 100

1.612 74.44 18.56 7 0 100

1.614 72.84 18.16 9 0 100

1.617 79.24 19.76 0 1 100

1.621 77.64 19.36 0 3 100

1.625 76.04 18.96 0 5 100

1.627 74.44 18.56 0 7 100

1.629 72.84 18.16 0 9 100

Fig. 2. Viscosity and electrical conductivity of samples with different acidity coefficient at different temperature, (a) Viscosity; (b) Electrical conductivity. (Online version in color.)


© 2018 ISIJ 1754

the chemical durability of mineral wool. But high acidity coefficient makes the charge difficult to melt. In order to be given full play to the cost advantage of blast furnace slag production of mineral wool, the acidity coefficient of mineral wool is controlled within 1.4. If we produce higher acidity coefficient of mineral wool, slag modifying process is required to add more accessories, energy consumption of the melting system is higher, so is the cost of production. Under the energy-saving and environmental consideration, blast furnace slag modifying and compensation heat pro-cess should use electric heating, so we need to acquaint the electrical conductivity of slag. The influence of acidity coefficient on electrical conductivity (Fig. 2(b)) of slag was measured using different acidity coefficient of the slag obtained by modifying process above.

With the increase of acidity coefficient, the electrical conductivity in the entire temperature range decreased at a different temperature. As the amount of SiO2 in the slag increased, that is the acidity increased, the conductivity decreased. As the SiO2 content increased, the anion will continue to polymerize to form a more complex network, thereby increasing the degree of polymerization of the melt, reducing the ability of free ions to migrate, and the conductivity of the melt decreased accordingly.3) At the same acidity, the electrical conductivity of the slag was increasing as the temperature increased. The increase of mobility of charge-transferring cation by temperature should be taken into account as a reason of increase of electrical conductivity.

3.2. Effect of Na2O and B2O3 on the Viscosity of 1.4 Series and 1.6 Series

Effect of Na2O and B2O3 on the viscosity of 1.4 series and 1.6 series range from 1 773 K to 1 673 K was shown in Fig. 3, respectively. As can be illustrated in Fig. 3, the effect of Na2O and B2O3 on the viscosity showed the differ-ent trend in two series. The viscosity of samples in 1.4 series increased with the increased addition of Na2O and B2O3 but decreased in 1.6 series.

Na2O and B2O3 had the same promotion in viscosity of 1.4 series but had the same reduction in 1.6 series. The viscosity in 1.4 series increased from 0.05 to 0.15 Pa∙s and from 0.3 to 0.7 Pa∙s. Viscosity changed little within the temperature range (1 400–1 500°C) that meant it was not sensitive to the temperature change and had a wide range of temperature adaptation. Taking into account the stability of production, 1.4 series was more suitable for production then. The viscosity in 1.6 series decreased from 1.0 to 0.3 Pa∙s and from 1.1 to 0.5 Pa∙s. It was evident that the variation range in viscosity increased. This series is aimed to make a comparison with 1.4 series, as it is a proper composition to produce mineral wool when the acidity coefficient is 1.4.

The temperature-viscosity relationship can be described by the Arrhenius equation, Eq. (1).

ln lnA RT� �� E / ........................ (1)

where η, A, R, T, and Eη denote the slag viscosity, Pa∙s; preexponential factor; gas constant, 8.314 J/(mol∙K); abso-lute temperature, K; and the apparent activation energy for viscous flow, kJ/mol, respectively. It is generally considered that Eη represents the energy barrier that fluid units have to

overcome for movement to occur. In addition, the variation of Eη suggests a structural change in the melts. The value of Eη evaluated from the slope of linear fitting of lnη with 10 000/T was shown in Figs. 4 and 5 in 1.4 series and 1.6 series and the results obtained were displayed in Tables 5 and 6.

Fig. 3. Effect of Na2O and B2O3 on the viscosity of 1.4 series and 1.6 series, (a) Na2O addition on 1.4 series; (b) B2O3 addi-tion on 1.4 series; (c) Na2O addition on 1.6 series; (d) B2O3 addition on 1.6 series. (Online version in color.)

Fig. 4. Plots of lnη as a function of (10 000/T) in 1.4 series. (Online version in color.)

Fig. 5. Plots of lnη as a function of (10 000/T) in 1.6 series. (Online version in color.)


© 2018 ISIJ1755

The viscosity of slags (Mk=1.4) with 3, 5, 7, 9 wt.% Na2O showed very steep changes at low temperature which should be excluded from the linear relationship between logarithm of viscosity and inverse temperature. It can be seen in Tables 5 and 6 that the addition of Na2O resulted in a different trend of Eη in samples of 1.4 and 1.6 series. It might be related to the amount of divalent cations in the slag. In the 1.4 series, the sum of the molar fraction of Ca2+ and Mg2+ was more than that of the 1.6 series. The addition of basic oxide (Na2O) generally resulted in a decrease in melt polymerization and an increase in the amount of bridging oxygen in the melt. However, divalent cations generally connected two bridging oxygen atoms, making their local structure complex with respect to the anion group to which the monovalent cation was attached. As a result, the local structure of the melt became tight, so that the resistance to ion diffusion was relatively increased. Similar Eη values were observed having not change much in the case of B2O3 with the same addition levels in two series. Whereas Eη values changed more obviously with Na2O addition, which suggested that Na2O was more effec-tive than B2O3 at improving the viscous flow. This could be due to their different structural modification behaviors on the CaO–Al2O3–MgO–SiO2 slag.

3.3. Effect of Na2O and B2O3 on the Electrical Conduc-tivity of 1.4 Series and 1.6 Series

Effect of Na2O and B2O3 on the electrical conductivity of 1.4 series and 1.6 series range from 1 773 K to 1 673 K was shown in Fig. 6, respectively. As can be illustrated in Fig. 6, the effect of Na2O and B2O3 on the electrical conductivity showed the same trend in two series. The electrical conduc-tivity of samples in two series increased with the increased addition of Na2O and B2O3 content.

Na2O and B2O3 had the same promotion in electrical conductivity of two series. The electrical conductivity in 1.4 series increased from 0.05 to 0.55 S∙cm −1 by Na2O addition and from 0.1 to 0.25 S∙cm −1 by B2O3 addition. The electrical conductivity in 1.6 series increased from 0.05 to 0.35 S∙cm −1 and from 0.09 to 0.25 S∙cm −1. Electrical conductivity changed little within the temperature range (1 400–1 500°C) with the addition of B2O3. It was evident that the variation range in electrical conductivity has not changed much with the addition of B2O3 compared with that of Na2O.

The temperature-electrical conductivity relationship can also be described by the Arrhenius equation, Eq. (2).

ln lnA RT� �� ( )/E ........................ (2)

where κ, A, R, T, and Eκ denote the electrical conductiv-ity, S∙cm −1; preexponential factor; gas constant, 8.314 J/(mol∙K); absolute temperature, K; and the apparent activa-tion energy for electric current, kJ/mol, respectively. It is generally considered that Eκ represents the internal friction

Table 5. Calculated activation energy for samples in 1.4 series.

Item/No. 1.402 1.406 1.410 1.412 1.414 1.417 1.421 1.425 1.427 1.429

Eη (kJ/mol) 101.4 126.3 133.8 139.6 142.0 148.9 150.4 150.3 148.6 147.1


Item/No. 1.602 1.606 1.610 1.612 1.614 1.617 1.621 1.625 1.627 1.629

Eη (kJ/mol) 164.8 163.8 155.0 156.1 155.8 151.4 163.7 159.1 155.6 163.4

Fig. 6. Effect of Na2O and B2O3 on the electrical conductivity of 1.4 series and 1.6 series, (a) Na2O addition on 1.4 series; (b) B2O3 addition on 1.4 series; (c) Na2O addition on 1.6 series; (d) B2O3 addition on 1.6 series. (Online version in color.)

Fig. 7. Plots of lnκ as a function of (10 000/T) in 1.4 series. (Online version in color.)

Fig. 8. Plots of lnκ as a function of (10 000/T) in 1.6 series. (Online version in color.)


© 2018 ISIJ 1756

between ions that fluid units have to overcome when an external electric field was joined. The value of Eκ evaluated from the slope of linear fitting of lnκ with 10 000/T was shown in Figs. 7 and 8 in 1.4 series and 1.6 series and the results obtained were displayed in Tables 7 and 8.

It can be seen that the addition of Na2O and B2O3 resulted in a decrease of Eκ in both 1.4 and 1.6 series. Similar Eκ values were observed changing more obviously in 1.4 series with the same addition levels of Na2O and B2O3. While Eκ values of Na2O-added slag was noticeably higher, which suggested that Na2O was more effective than B2O3 at decreasing the internal friction between anion and cation. This could also be due to their different structural modifica-tion behaviors on the CaO–Al2O3–MgO–SiO2 slag that we need to investigate by Raman spectra in the subsequent work.

3.4. Effect of Na2O on the Structure of Samples in 1.4 Series

Slags are constituted of ions, and slag reactions are electrochemical in nature, concerning the exchange of ions. Slags contain two forms of bonds:4) (i) covalent Si–O bonds that form into chains, rings, and (ii) ionic bonds involving cations such as Na + or Ca2+ that break the silicate chains to form Na +−O − bonds or Ca2+−O − bonds. The structure of a slag can be customary to divide various constituents into either network formers (e.g. SiO2, B2O3) or network break-ers (e.g. CaO, Na2O).4–6) Na2O and B2O3 play different roles in the structure of slag which make them have an unusual influence on the degree of polymerization of slag.

Raman spectra were used to characterize the structure of

samples in two series. When the measurement of conduc-tivity and viscosity was finished, the sample was cooled in the furnace to room temperature. After the sample was taken out from the crucible and broken up to −200 mesh, it was placed in a crucible and reheated to 1 500°C. After the heat for 2 hours, samples were taken out directly into the water. After the water cooling, the sample was glassy and had a small particle size. After slag crushing, Raman test was performed. The test results were analyzed to obtain Raman spectra and the percentage of each QSi

n . Morphology of samples before reheating, the process of water cooling and morphology of samples after quenching were shown in Figs. 9, 10 and 11, respectively.

In previous study,4,7–13) the relative intensity of the stretching vibration QSi

n (n is the number of bridging oxygen per tetrahedral unit, n=0 to 4) can be structurally defined as 3-dim (QSi

4 , 1 190 cm −1, 1 060 cm −1), sheet (QSi

3 , 1 060 cm −1 to 1 040 cm −1), chain (QSi2 , 980 cm −1 to

950 cm −1), polyhedra (QSi1 , 920 cm −1 to 900 cm −1), and

monomers (QSi0 , 880 cm −1 to 850 cm −1), respectively. This

decrease in the depth of the transmission trough indicates a depolymerization of the slag structure as discussed in the previously published literature.14,15) The Raman spectra for Na2O content in the range of 0 to 9 wt.% in 1.4 series were shown in Fig. 12.

Fig. 9. Morphology of samples before reheating. (Online version in color.)

Fig. 10. The process of water cooling. (Online version in color.)


Item/No. 1.402 1.406 1.410 1.412 1.414 1.417 1.421 1.425 1.427 1.429

Eκ (kJ/mol) 234.1 178.1 169.8 151.8 114.7 205.8 164.9 171.7 161.3 151.1


Item/No. 1.602 1.606 1.610 1.612 1.614 1.617 1.621 1.625 1.627 1.629

Eκ (kJ/mol) 195.8 170.9 132.3 136.8 173.7 190.2 193.4 174.3 168.8 160.1


© 2018 ISIJ1757

It can be seen in Fig. 12(a) that the Raman peak in the high-frequency region from 800 to 1 200 cm −1 decreased and narrowed with the increase in Na2O addition content. This implied that the degree of polymerization of the silicate network structure decreased at higher Na2O content. All the spectra in the high-frequency region were fitted properly in Fig. 13. The deconvolution results as a function of Na2O content were shown in Fig. 12(b).

The fraction of fully polymerized QSi4 units decreased as

Na2O content increased from 0 to 9 wt.%. The predominant QSi

2 units were also found to significantly decreased with the increasing Na2O content. The relative fraction of QSi

0 units varied slightly with the increasing Na2O content. The relative fractions of QSi

1 and QSi3 units both decreased. When

the Na2O content reached 9 wt.%, QSi0 became the pre-

dominant unit present in the slag systems, and the relative fractions of QSi

1 , QSi3 , and QSi

4 were equal. The basic oxide Na2O (a so-called network modifier) dissociated into “free oxygen” O2− anions in silicate melts. With the addition of Na2O increased, the O2− can react with “bridging oxygen”

Fig. 12. Effect of varying Na2O content on the (a) Raman spectra and (b) the structural units of the samples in 1.4 series. (Online version in color.)

Fig. 13. Deconvoluted Raman spectra of samples with different Na2O addition content in 1.4 series, (a) Na2O = 0 wt.%; (b) Na2O =3 wt.%; (c) Na2O =5 wt.%; (d) Na2O = 9 wt.%. (Online version in color.)

Fig. 11. Morphology of samples after quenching. (Online version in color.)


© 2018 ISIJ 1758

O0 (connected with two Si atoms) and break the existing bonds. The silicate network structure polymerized by O0 can be modified into discrete structural polymeric units by the formation of “non-bridging oxygen” O − (connected with one Si atom).

The most commonly used parameter to represent the structure of the slag is the ratio of NBO to tetragonal ions (NBO/T). Some workers prefer using the parameter Q (4-NBO/T) since this provides a measure of the polymer-ization of the slag that is easy to visualize. Na2O acted as a network breaker in the present study, theoretical value of Q was calculated using the mentioned equation.4) It can be calculated that the value of NBO/Si increased as Na2O content increased from 0 to 9 wt.% which can be seen in Table 9. This can be explained by the Na2O playing the role of network modifier and causing depolymerization of the network structure, which decreased the degree of polymer-ization of silicate network and resulted in almost no increase in slag viscosity and an increase in electrical conductivity in 1.4 series.

3.5. Effect of B2O3 on the Structure of Samples in 1.4 Series

The Raman spectra for B2O3 content in the range of 0 to 9 wt.% in 1.4 series were shown in Fig. 14. It can be seen in Fig. 14(a) that the Raman peak in the high-frequency region from 800 to 1 200 cm −1 varied slightly with the increase in B2O3 content. This implied that the degree of polymerization of the silicate network structure changed less at higher or lower B2O3 content. All the spectra in the high-frequency region were fitted properly in Fig. 15. The deconvolution results as a function of B2O3 content were shown in Fig. 14(b).

The predominant QSi2 units were also found to signifi-

cantly decreased when the B2O3 addition content reached to 3 wt.%, but remained the same in the next two addition. The relative fraction of QSi

3 units increased slightly with the increase in B2O3 content. The relative fractions of QSi

0 , QSi

1 , and QSi4 units stayed almost unchanged. When the B2O3

content reached 9 wt.%, the three values reached the same height. As an amphoteric modifier in the melt, boron oxide (B2O3) acted as both network former and breaker. Depend-ing on the previous studies,16) the modifier in the slag melts acted in two different ways as the modifier content was lower than 30% or not. However, generally, B2O3 acts as a network former due to the covalent bond between B and O ions. Besides, B2O3 addition sometimes decreases slag viscosity, which may be related to the fact that B2O3 has a very low melting point compared with other pure oxides. In a word, the addition of B2O3 content had little impact on the degree of polymerization which made the inconspicu-ous variation in the electrical conductivity and viscosity of samples in 1.4 series.

3.6. Effect of Na2O on the Structure of Samples in 1.6 Series

The Raman spectra for Na2O content in the range of 0 to 9 wt.% were shown in Fig. 16. It can be seen in Fig. 16(a) that the Raman peak in the high-frequency region from 800 to 1 200 cm −1 decreased and narrowed with the increase in Na2O content. This implied that the degree of polymeriza-tion of the silicate network structure decreased at higher Na2O content. All the spectra in the high-frequency region were fitted properly in Fig. 17. The deconvolution results as a function of Na2O content were shown in Fig. 16(b).

With the increase in Na2O content, the predominant QSi

2 units were found to increase, the relative fraction of QSi

0 units varied slightly, the relative fraction of QSi3 units

Fig. 16. Effect of varying Na2O content on the (a) Raman spectra and (b) the structural units of the samples in 1.6 series. (Online version in color.)

Fig. 14. Effect of varying B2O3 content on the (a) Raman spectra and (b) the structural units of the samples in 1.4 series. (Online version in color.)

Fig. 15. Deconvoluted Raman spectra of samples with different B2O3 addition content in 1.4 series, (a) B2O3= 0 wt.%; (b) B2O3=3 wt.%; (c) B2O3=5 wt.%; (d) B2O3= 9 wt.%. (Online version in color.)

Table 9. NBO/Si value of the system in 1.4 series with the increased Na2O content.

Content/wt.% 0 1 3 5 7 9

Value 1.28 1.32 1.38 1.46 1.53 1.61


© 2018 ISIJ1759

Fig. 17. Deconvoluted Raman spectra of samples with different Na2O addition content in 1.6 series, (a) Na2O = 0 wt.%; (b) Na2O =3 wt.%; (c) Na2O =5 wt.%; (d) Na2O = 9 wt.%. (Online version in color.)

Fig. 18. Effect of varying B2O3 content on the (a) Raman spectra and (b) the structural units of the samples in 1.6 series. (Online version in color.)

Fig. 19. Deconvoluted Raman spectra of samples with different B2O3 addition content in 1.6 series, (a) B2O3= 0 wt.%; (b) B2O3=3 wt.%; (c) B2O3=5 wt.%; (d) B2O3= 9 wt.%. (Online version in color.)

decreased slightly. The relative fractions of QSi1 and QSi

4 units both changed irregularly and the decline of QSi

1 value accompanied with raise of QSi

4 value. As a network breaker in the slag system, Na2O addition can easily cause the depolymerization of the network structure, which results in a decrease in slag viscosity and an increase in electrical conductivity in 1.6 series.

3.7. Effect of B2O3 on the Structure of Samples in 1.6 Series

The Raman spectra for B2O3 content in the range of 0 to 9 wt.% were shown in Fig. 18. It can be seen in Fig. 18(a) that the Raman peak in the high-frequency region from 800 to 1 200 cm −1 varied slightly with the increase in B2O3 content. This implied that the degree of polymerization of the silicate network structure changed less at higher or lower B2O3 content. All the spectra in the high-frequency region were fitted properly in Fig. 19. The deconvolution results as a function of B2O3 content were shown in Fig. 18(b).

The predominant QSi2 units were found to slightly

decreased with the increasing B2O3 content. The relative fraction of QSi

1 units increased slightly with the increas-ing B2O3 content. The relative fractions of QSi

0 , QSi3 , and

QSi4 units stayed almost unchanged. The addition of B2O3

content in 1.6 series should result in the little change in the degree of polymerization which makes the slight increase in conductivity and decrease in viscosity.

3.8. Summary of the Effect of Na2O and B2O3 on the Structure of Samples in Two Series

As we mentioned before, the relative intensity of the stretching vibration QSi

n can be structurally defined as 3-dim (QSi

4 ), sheet (QSi3 ), chain (QSi

2 ), polyhedra (QSi1 ), and mono-

mers (QSi0 ), respectively. In previous study,4) the equivalent

of chain (QSi2 ) is CaO∙SiO2 (Si2O6

4−) and the example is blast furnace slag which is also the basic slag system (CaO–SiO2–Al2O3–MgO) we studied. The relative fraction of the structural unit (QSi

2 ) of the samples in two series

stayed steady and was drawn in Fig. 20 which can also be seen in Figs. 12(b), 14(b), 16(b), and 18(b). The relative fraction of the structural units (QSi

1 and QSi4 ) of the samples

in two series was reciprocal or level off which can be seen in Fig. 21. The relative fraction of the structural units (QSi

0 and QSi

3 ) of the samples in two series was also reciprocal or level off which can be seen in Fig. 22.

As we can see in Figs. 20–22, the addition of Na2O and B2O3 did not change the QSi

2 value of the base slag. The

Fig. 20. The relative fraction of the structural unit (QSi2 ) of the

samples in two series. (Online version in color.)


© 2018 ISIJ 1760

addition B2O3 of can hardly change the relative fraction of the structural units (QSi

0 , QSi1 , QSi

3 and QSi4 ) of the samples

in two series. Effect of Na2O addition on the relative frac-tion of the structural units (QSi

1 and QSi4 /QSi

0 and QSi3 ) was

reciprocal. On the whole, the addition of Na2O increased the relative fraction of QSi

0 (monomer, SiO44−) in two series that

made it an effective modifier compared with the addition of B2O3. The Na2O addition modified process had a larger effect on the two parameters (macro aspect) and the relative fraction of the structural units (micro aspect) compared with the results of B2O3 addition from the experimental results.

4. Conclusions

In the present study, viscosity and electrical conductivity of different acidity coefficient of slag were measured. As a poor conductivity of blast furnace slag, Na2O and B2O3 were added to increase its conductivity. Considering the cost and effect, the comparison between the effects of Na2O and B2O3 addition on the electrical conductivity and viscosity

Fig. 22. The relative fraction of the structural units (QSi0 and QSi

3 ) of the samples in two series. (Online version in color.)

of two different acidity coefficient (1.4 and 1.6) of slag was investigated. Raman spectrum was used to represent the structure of the slag system. Experimental results can serve as a basis for the selection of melt composition for mineral wool production:

(1) At the same temperature conditions, the greater the acidity coefficient, the higher the viscosity. When the acidity coefficient was less than 1.4, the temperature can be controlled at 1 400°C to obtain the melt with viscosity below 2.0 Pa∙s;

(2) With the increase of acidity coefficient, the electrical conductivity in the entire temperature range decreased at a different temperature;

(3) Eη and Eκ values changed more obviously with Na2O addition, which suggested that Na2O was more effec-tive than B2O3 at improving the viscous flow and decreasing the internal friction between anion and cation;

(4) The effect of optimization in viscosity and electrical conductivity parameters was not obvious with the addition of B2O3 compared with that of Na2O addition;

(5) Different effects of Na2O and B2O3 addition on the electrical conductivity and viscosity of CaO–Al2O3–MgO–SiO2 system could be due to their different structural modification behaviors which can be explained by Raman spectrum;

(6) Experimental results showed that Na2O was more suitable as a modifier than B2O3 in the modified process and 1.4 series was more suitable for producing mineral wool than 1.6 series.

AcknowledgmentsThe financial support from Major Program of the National

Natural Science Foundation of China (No. U1360204) is gratefully acknowledged. We are grateful to anonymous reviewers for helpful comments on original manuscript.

REFERENCES

1) L. Zhao, H. Wang, S. Qing and H. Liu: J. Nat. Gas Chem., 19 (2010), 403.

2) K. C. Mills and M. Suza: Slag Atlas, 2nd ed., Verlag Stahleisen GmbH, Düsseldorf, (1995), 157.

3) W. Li, X. Cao, T. Jiang, H. Yang and X. Xue: ISIJ Int., 56 (2016), 205.

4) K. C. Mills, Y. Lang and R. T. Jones: J. South. Afr. Inst. Min. Metall., 111 (2011), 649.

5) K. C. Mills: ISIJ Int., 33 (1993), 148.6) B. O. Mysen: Structure and Properties of Silicate Melts, Elsevier

Science, Amsterdam, (1988), 281.7) D. W. Matson, S. K. Sharma and J. A. Philpotts: J. Non-Cryst. Solids,

58 (1983), 323.8) P. Mcmillan: Am. Mineral., 69 (1984), 622.9) K. Fukumi, J. Hayakawa and T. Komiyama: J. Non-Cryst. Solids, 119

(1990), 297.10) B. O. Mysen and J. D. Frantz: Am. Mineral., 78 (1993), 699.11) B. O. Mysen and J. D. Frantz: Contrib. Mineral. Petrol., 117 (1994),

1.12) J. You, G. Jiang and K. Xu: J. Non-Cryst. Solids, 282 (2001), 125.13) B. O. Mysen, D. Virgo and C. M. Scarfe: Am. Mineral., 65 (1980),

690.14) H. Kim, W. H. Kim, I. Sohn and D. J. Min: Steel Res. Int., 81 (2010),

261.15) H. Kim and I. Sohn: ISIJ Int., 51 (2011), 1.16) E. Kamitsos, M. Karakassides and G. Chryssikos: J. Phys. Chem., 91

(1987), 1067.

Fig. 21. The relative fraction of the structural units (QSi1 and QSi

4 ) of the samples in two series. (Online version in color.)

Effects of Na2O and B2O3 Addition on Viscosity and ...

Documents

Transcript of Effects of Na2O and B2O3 Addition on Viscosity and ...