Regular Article
Influence of Al2O3 and MgO on the Viscosity and Stability of CaO–MgO–SiO2–Al2O3 Slags with CaO/SiO2=1.0
2017 Volume 57 Issue 6 Pages 978-982
Details
2017 Volume 57 Issue 6 Pages 978-982
The viscosity of CaO–SiO2–MgO–Al2O3 slags (CaO/SiO2=1.0, 14–17 mass% Al2O3, 5–15 mass% MgO) was measured to analyze the effect of MgO and Al2O3. The infrared spectra was employed to analyze the ionic structure of the slag. The viscosity of CaO-SiO2-MgO-17% Al2O3 slags decreases with the increase of MgO content which is in the range of 5%–10 mass%. However, the dependence relationship is reversed when MgO content is over 10 mass%. The viscosity of CaO-SiO2-10 mass% MgO–Al2O3 slags and CaO-SiO2-11% MgO–Al2O3 slags exhibits the minimum value at 16 mass% Al2O3, which may contribute to the amphiprotic properties of Al2O3. The above phenomenon can also be verified by the infrared spectra. Simultaneously, in order to make the blast furnace working smoothly, the viscosity value of the slag is below 1.0 Pa·s commonly. Thus, we define the temperature of the slag with a viscosity value of 1.0 Pa·s as “Critical Temperature (CT)”. And, the effect of MgO and Al2O3 content on CT is also studied by the phase diagram. We can conclude that the fall of the CT can be owe to the simplified network structure at the MgO content from 5 mass% to 10 mass%, while the rise may be because of the appearance of solid phase at the MgO content from 10 mass% to 15 mass%.
Slag viscosity is one of the most important physico-chemical properties governing gas permeability,1,2) slag/metal separation3,4) and desulfuration5) in the blast furnace. So viscous behavior of the slag plays a critical role in determining the stability and productivity in the blast furnace operation. Thus, it is necessary to understand the slag viscosity and the influence factors for improving the stability and productivity.
Recently, iron ore containing high Al2O3 content has been largely used as the raw material in the blast furnace. Thus, the Al2O3 content in the slag will be certainly increased, and then the viscous behavior of the slag will be variation.6,7) It is well known that Al2O3 is an amphoteric oxide.8,9) Al2O3 behave as either a basic oxide or an acidic oxide depending on the slag composition.10,11,12,13,14,15) In order to improve the properties of slag containing high Al2O3 content, some researchers have investigated the effect of MgO addition on the viscosity.16) In CaO–MgO–SiO2–Al2O3 slag system, the viscosity decreases firstly and then increases with increasing MgO content.17) In CaO-MgO-SiO2-20 mass% Al2O3 slag system, the viscosity decreased significantly with the MgO content from 5 to 10 mass%, while it decreased slightly with the MgO content from 10 to 13 mass%.18) Therefore, the effect of MgO content on the viscosity in different slag system does not reach an agreement. Thus,the influencing factor on the viscosity should be researched further.
In addition, the temperature of the slag with a viscosity value of 1.0 Pa·s is defined as “Critical Temperature (CT)” in this paper. It is necessary to obtain the CT value of different CaO–MgO–SiO2– Al2O3 slag system for understanding the stability of the slag.
Therefore, the effect of Al2O3 and MgO on the viscosity of CaO–MgO–SiO2–Al2O3 slags (CaO/SiO2=1.0) containing MgO content from 5 to 15 mass% and Al2O3 content from 14 to 17 mass% was investigated in this paper. The CT of the slags containing different Al2O3 and MgO content was analyzed. In addition, fourier transform infrared (FT-IR) spectroscopy of the slag was measured to understand the structural rule of the slags. The relation between the slag viscosity and structure will also be discussed.
The slags are prepared by mixing pure oxides (MgO, SiO2, Al2O3), and pure CaCO3 as source of CaO respectively. The chemical compositions of slags are shown in Table 1. The 200 g powders are pre-melted for 1 h at 1773 K in order to obtain the quenched slag as the sample for the experiments. The slag sample of 10 g is used to measure the FT-IR spectra of the slags and the other slag sample is used to measure the viscosity.
| Sample Number | Chemical composition/mass% | CaO/SiO2 (Mass/Mass) | |||
|---|---|---|---|---|---|
| CaO | SiO2 | MgO | Al2O3 | ||
| 01 | 39.0 | 39.0 | 5.0 | 17.0 | 1.0 |
| 02 | 38.0 | 38.0 | 7.0 | 17.0 | |
| 03 | 37.0 | 37.0 | 9.0 | 17.0 | |
| 04 | 38.0 | 38.0 | 10.0 | 14.0 | |
| 05 | 37.5 | 37.5 | 15.0 | ||
| 06 | 37.0 | 37.0 | 16.0 | ||
| 07 | 36.5 | 36.5 | 17.0 | ||
| 08 | 37.5 | 37.5 | 11.0 | 14.0 | |
| 09 | 37.0 | 37.0 | 15.0 | ||
| 10 | 36.5 | 36.5 | 16.0 | ||
| 11 | 36.0 | 36.0 | 17.0 | ||
| 12 | 35.5 | 35.5 | 12.0 | 17.0 | |
| 13 | 35.0 | 35.0 | 13.0 | 17.0 | |
| 14 | 34.5 | 34.5 | 14.0 | 17.0 | |
| 15 | 34.0 | 34.0 | 15.0 | 17.0 | |
The rotating-cylinder method9,19) is employed to measure the slag viscosity in this work. The schematic diagram of the experiment apparatus is shown in Fig. 1. An electric resistance furnace with U-shape MoSi2 heating elements is used for system heating. 140 g sample is placed in the graphite crucible (height, 80 mm; inner diameter, 40 mm), and then the sample is heated up to 1773 K at a rate of 5 K/min with constant Ar gas (500 mL/min). The viscosity measurement is carried out at every 10 K interval on cooling. The equilibration time is 20 min at each temperature.
Apparatus for viscosity measurements.
The dependence of viscosity of the CaO-SiO2-17 mass% Al2O3–MgO slag (CaO/SiO2=1.0) on temperature at different MgO content is shown in Fig. 2(a). Viscosity of slag with MgO content more than 12 mass% increases rapidly with decrease of the temperature. Conversely, the viscosity curve of slag with MgO content less than 11 mass% is relatively smooth. The steep rise phenomenon may be due to generate solid phase with high melting point in the slag.
The viscosity in the CaO-SiO2-17 mass% Al2O3–MgO (CaO/SiO2 = 1.0) slag with different MgO content: (a) The change of viscosity as a function of temperature; (b) The change of viscosity as a function of mass% MgO.
Figure 2(b) shows the effect of MgO content on viscosity of the CaO-SiO2-17 mass% Al2O3–MgO slag at 1673 K, 1723 K and 1773 K. In present studies, the viscosity decreases with increasing MgO content from 5 to 10 mass%. This is because of that MgO could provide oxygen ions for the slag and then depolymerized network structure in the slag into simple polymer type units. When the MgO content is about 10 mass%, the network structure could has become the relatively simple units in the slag. The further depolymerization is rarely with increasing the MgO content.8) But the MgO in excess of 10 mass% could be embedded into the simple units, which will result in the increase of the viscosity. At the same time, the increase of the viscosity is more obvious at 1673 K is contribute to the appearance of the solid phase in the slag.17)
3.2. The Effect of Al2O3 on the ViscosityThe dependence of viscosity of the CaO-SiO2-Al2O3-10, 11 mass% MgO slag (CaO/SiO2=1.0) on temperature at different Al2O3 content is shown in Fig. 3(a). The minimum viscosity of the slag containing 10 mass% MgO appear at 16 mass% Al2O3, which has the similar regularity with the slag containing 11 mass% MgO. Figure 3(b) shows the effect of Al2O3 content on viscosity of the CaO-SiO2-Al2O3-10 mass% MgO slag at 1673 K, 1723 K and 1773 K. The viscosity is approximate agreement with the previous work,20,21) but the viscosity is fluctuant with increasing Al2O3 content in present work. It is well known that Al2O3 is an amphoteric oxide. When Al2O3 behave as a basic oxide, it can depolymerize the slag structure and decrease the viscosity. However, when Al2O3 behave as an acid oxide, it can increase the viscosity. Therefore, Al2O3 should exist in two forms that are Al3+ and [AlO4]5−. When the Al2O3 content ranged from 14 to 15 mass%, more [AlO4]5− structure will been generated in the slag, and the viscosity of the slag will be increased. The amount of Al3+ increases more dominant than [AlO4]5− with increasing the Al2O3 content to 16 mass%, and the viscosity is decreased.
The viscosity in the CaO–SiO2–Al2O3–MgO (CaO/SiO2 = 1.0) slag with different Al2O3 content: (a) The change of viscosity as a function of temperature; (b) The change of viscosity as a function of mass% Al2O3.
To understand the effect of the slag structure on the viscosity, FI-TR spectra22) of the quenched slag at 1773 K is measured. The FT-IR spectra of CaO-SiO2-17 mass% Al2O3–MgO slag (CaO/SiO2=1.0) at different MgO content is shown in Fig. 4. The trough of the Si–O symmetric stretching bands are found between 1200 and 760 cm−1 when the slag contains 5 mass% MgO. However, they are found between 1200 and 730 cm−1 when the slag contains 10 mass% MgO. The results indicates that the degree of polymerization of silicate units decreases with increasing MgO content from 5 mass% to 10 mass%. The trough of the bands of 10 mass% MgO is similar with that of 15 mass% MgO. At the same time, the prominent peak that caused by the simple [SiO4]4− tetrahedra appears at about 860 cm−1. It may be cause that the most of the silicate units have already been depolymerized to the relatively simple units at the 10 mass% MgO. The silicate units will seldom change with the MgO content up to 15 mass%. However, the depth of the transmittance bands becomes deeper at 15 mass% MgO than that at 10 mass% MgO. This suggests that the silicate units may behave more distinct in the slag containing 15 mass% MgO. Therefore, the structure of the slag containing 10 mass% MgO may be the most simple, which corresponds well with the result of the viscosity in Fig. 3(b).
Effect of MgO on the FT-IR transmittance spectra of the CaO-SiO2-17 mass% Al2O3–MgO (CaO/SiO2=1.0) at 1773 K.
The FT-IR spectra of CaO-SiO2-Al2O3-10 mass% MgO slag (CaO/SiO2=1.0) with different Al2O3 content is shown in Fig. 5. The trough of the [AlO4]5− symmetric stretching bands are found between about 730 cm−1 and 630 cm−1. The peak that caused by [AlO4]5− tetrahedra is apparent at 15 and 17 mass% Al2O3 but relatively inapparent at 16 mass% Al2O3. In addition, the depth of the transmittance bands for the Si–O–Al rocking near 480 cm−1 becomes shallower at 16 mass% Al2O3. Therefore, the viscosity of the slag containing 16 mass% Al2O3 behave the most lowest.
Effect of Al2O3 on the FT-IR transmittance spectra of the CaO-SiO2-Al2O3-10 mass% MgO (CaO/SiO2=1.0) at 1773 K.
In order to receive the critical temperature (CT), An Arrhenius-type relationship was adopted. The Arrhenius-type is expressed as
Where η is viscosity, A is Arrhenius constant, R is gas constant, T is temperature, Eη is activation energy. The viscosity value less than 1.0 Pa·s in Figs. 2 and 4 is used to calculate A and E to ensure the linear relation. The results of the calculation are provided in Table 2 and Fig. 6. The Figs. 6(a) and 6(b) shows the dependence of natural logarithm of viscosity of the CaO–SiO2–Al2O3–MgO slag (CaO/SiO2=1.0) on reciprocal of temperature at different MgO and Al2O3, respectively. The slag containing 10 mass% MgO has the lowest activation energy in the CaO-SiO2-17 mass% Al2O3–MgO slag system. Meanwhile, in the CaO-SiO2-Al2O3-10 mass% and 11 mass% MgO slag system, both the activation energy of the slag containing 16 mass% Al2O3 has the lowest value.
| CaO/SiO2 (Mass/Mass) | MgO (Mass%) | Al2O3 (Mass%) | Eη (kJ/mol) |
|---|---|---|---|
| 1.0 | 5 | 17 | 216 |
| 7 | 17 | 212 | |
| 9 | 17 | 224 | |
| 10 | 14 | 223 | |
| 15 | 222 | ||
| 16 | 215 | ||
| 17 | 200 | ||
| 11 | 14 | 208 | |
| 15 | 229 | ||
| 16 | 204 | ||
| 17 | 206 | ||
| 12 | 17 | 206 | |
| 13 | 17 | 210 | |
| 14 | 17 | 188 | |
| 15 | 17 | 161 |
The dependence of natural logarithm of viscosity of the CaO–SiO2–Al2O3–MgO slag (CaO/SiO2=1.0) on reciprocal of temperature. (a) different MgO content; (b) different Al2O3 content.
The CT can be calculated according to Table 2 and Fig. 6. Sometimes, The CT will be also revised by Figs. 2(a) and 3(a) for reducing errors. The CT in CaO–SiO2–Al2O3–MgO (C/S = 1.0) slag containing Al2O3 content from 14 mass% to 17 mass% as a function of MgO content is showed in Fig. 7. The CT firstly decreased and then increased with increasing MgO content when the Al2O3 content is 17 mass%. Figures 8(a) and 8(b) shows the phase diagrams of CaO–SiO2–Al2O3–MgO slag containing 15 mass% and 20 mass% Al2O3.23) It can be seen that the liquidus temperature is close to 1673 K at MgO content from 5 mass% to 10 mass%. However, the liquidus temperature rises obviously when the MgO content is 15 mass%. This indicates that the fall of the CT can be owe to the simplified network structure at the MgO content less than 10 mass% while the rise is because of the appearance of solid phase in the slag containing more than 10 mass% MgO. The CT has a minimum value at 16 mass% Al2O3 and a maximum value at 15 mass% Al2O3 when the slag contains 10 mass% and 11 mass% MgO. The existence form of Al2O3 in the slag play a major role in the CT.
The temperature in the CaO–SiO2–Al2O3–MgO quaternary slag system as a function of mass% MgO.
Phase diagram of CaO–SiO2–Al2O3–MgO slag, K: (a) 15 mass% Al2O3; (b) 20 mass% Al2O3.
In this study, the viscosity of CaO–SiO2–Al2O3–MgO quaternary slag (CaO/SiO=1.0) containing different Al2O3 and MgO content is measured. The viscosity of the slag containing 17 mass% Al2O3 decreases with the increase of MgO content from 5 to 10 mass%, and then the viscosity increases with MgO concent from 10 up to 15 mass%. The FI-TR spectra also shows that the silicate units have already been depolymerized to the most simple units at the 10 mass% MgO. The viscosity of the slag containing 10 and 11 mass% MgO is fluctuant with the increase of Al2O3 content from 14 to 17 mass%. It shows a minimum when the slag contains 16 mass% Al2O3, which can be confirmed by the FI-TR spectra. The fall of the CT can be owe to the simplified network structure at the MgO content less than 10 mass% while the rise is because of the appearance of solid phase in the slag containing more than 10 mass% MgO.
The authors gratefully express their appreciation to the National Natural Science Foundation of China (51274120) for supporting this work.
