2019 Volume 59 Issue 10 Pages 1806-1810
To solve the problem of slow heating rate of traditional surface and interfacial tension test method of molten slag, a fast testing method of interface property is proposed in this paper, which bases on the single hot thermocouple technique (SHTT). The results are compared with those of the sessile drop method and literature. The results show that: 1) Using the capillary action of molten slag on the B-type thermocouple, the contact angle can be obtained quickly. The relative error between the contact angle of the slag and that of the sessile drop method is 0.10%, and the maximum fluctuation range of the repeated experiment is ±2.00°. 2) Substituting the contact angle and surface tension of the slag into Young’s equation, the interfacial tension between the slag and the B-type thermocouple can be obtained. 3) For the commonly used CaO–SiO2–Al2O3–MgO metallurgical slag system, without flux (CaF2, Na2O), the interfacial tension between slag and B-type thermocouple is less affected by the change of composition. The surface tension of the slag can be obtained by the SHTT method, and the relative error with the literature value is less than 2.94%.
In the steel industry, the interface tension of slag-metal and the surface tension of slag are important parameters in the gas-slag-metal interface behavior.1,2,3,4) They affect the interfacial reaction, the separation of molten slag and molten metal, the discharge of inclusions in molten iron and molten steel, the formation of new phase nuclei in the reaction, and the behavior of bubbles in slags. In addition, the study of slag-metal interfacial tension and slag surface tension are also of great significance for the study of interfacial reaction mechanism and phase interface structure.5,6,7)
At present, the common testing methods of slag interface property include the maximum bubble method,8) the sessile drop method,9) and the pendant drop method.10) Boni and Derge11) introduced the maximum bubble method into the surface tension test of high temperature melts in 1917. The maximum bubble method needs not to measure the contact angle and has high accuracy. So it is often used for dynamic measurement of surface tension of high temperature melts. The sessile drop method was invented by Quincke.12) This method obtains the surface tension and the interfacial tension according to the droplet shape on the horizontal gasket. Fujii et al.13) pointed out that due to the small size of sample and the unrestricted sample, the sessile drop method is widely used in melt surface tension testing in many fields. The pendant drop method was proposed by Bashforth and Adams.14) This method is also a simple and widely applicable testing methods of surface tension.
Although the above methods are some accepted method of the interfacial tension test, the methods have the disadvantage of long time consuming. The maximum bubble method needs to get the molten pool of the sample while the process of heating to 1500°C and melting the sample in the silicon-molybdenum furnace needs about 3 hours. The slags melt quickly in sessile drop method and the pendant drop method, but the environment temperature around the slags needs to raise to the required temperature (e.g. 1500°C), which takes approximately 3 hours. Therefore, the traditional test method cannot obtain the surface tension of the slags quickly.
The SHTT is a kind of high temperature measurement method which uses intermediate frequency chopper technology to realize both heating and temperature measurement. The SHTT method has a small sample volume, fast heating rate (0–30°C/s),15,16) high temperature rise temperature (1800°C),17) fast cooling rate (about 150°C/s)18) and other advantages. So the SHTT is widely used in glass, ceramics and other fields of melting, continuous cooling crystallization and constant temperature crystallization.18,19,20) Fred Ordway19) pointed out that the molten slag will form droplets on the thermocouple of SHTT due to capillary action. Wang, Zhou et al. proposed that the contact angle of the slags droplets with thermocouple can be obtained by SHTT21) and combining the double hot thermocouple technique (DHTT) and Young’s equation can get the slag-metal interfacial tension.22) But they lack the comparison of the reliability of the above methods and the study of the practicality of specific metallurgical slags.
Therefore, based on the characteristics of rapid heating of the SHTT method, the capillary action between the molten slag and the B-type thermocouple and previous research,23) this study proposes a rapid interface properties test method by SHTT. The results of SHTT are compared with the test result of the sessile drop method and the datum from literatures. Finally, the suitable slag systems for measuring the surface tension by SHTT is discussed.
Most of the ironmaking and steelmaking slags contain CaO–SiO2–Al2O3–MgO. In order to meet the metallurgical process of ironmaking and steelmaking, CaF2, Na2O, and other fluxes were added in the slags according to the requirements of different production processes. This study selected two kinds of slag systems with known surface tension according to previous research. Among them, the sample of A1–A5 represents CaO–SiO2–Al2O3 (CSA) and CaO–SiO2–Al2O3–MgO (CSAM) slag samples; the sample of B1–B7 represents CSA and CSAM slag samples which contains CaF2 and Na2O. The compositions of the slags and the references are listed in Table 1.
The samples were prepared using the reagent grade powders. In the process of preparation, the samples were put into the graphite crucible and kept at 1400°C for 30 min in a silicon-molybdenum resistance furnace to ensure a complete melting. Then the melts were poured into a water-cooled copper plate to quench the slag. The slags were subjected to XRD analysis to ensure that the slag sample was uniform and amorphous. The quenched samples were placed in an agate crucible and passed through a 200-mesh sieve to get the slag samples for interfacial property tests.
The schematic diagram of the SHTT device is shown in Fig. 1. Kashiwaya et al.16) has explained the principles of SHTT. Based on the SHTT method, a kind of furnace with controllable atmosphere and auxiliary heat source is designed by our research group. The sealed design of the furnace body can ensure the stability of the test environment, and the atmospheres can be set according to the requirements. The halogen lamp is used for auxiliary heating which has a fast heating effect. Therefore, the SHTT equipment can test the interface properties of molten slag under controlled atmosphere and stable ambient temperature.
The schematic of SHTT. (Online version in color.)
The analyses of the slag-metal interfacial properties and the surface tension of the slags were based on Young’s equation, as shown in Eq. (1). In Eq. (1), γs is the surface free energy of platinum, which is 2370 mN/m,29) γsl is the slag-metal interfacial tension, γl is the surface tension of slag, and θ is the contact angle of slag and B-type thermocouple.
The slag interface property test procedure was: (i) chosen air atmosphere for testing the interfacial properties of molten slag.; (ii) increased the temperature of the thermocouple to 1500°C and keeps the molten slag sample in contact with the thermocouple; (iii) adjusted the size of the droplet on the thermocouple wire (approximately 1 mg) and made the axis of the droplet parallel to the observation surface; (vi) measured the contact angle of the droplet with the thermocouple; (vii) calculated the surface tension of the slag or the interfacial tension of the slag-metal by Young’s equation. The schematic of adding sample is shown in Fig. 2.
The schematic of adding sample. (Online version in color.)
In step (vi), to ensure the accuracy of the contact angle data, the contact angle measurement was calculated according to the Laplace equation, as shown in Eq. (2).30) The droplet profile was first fitted as a circle, then the height “h” of the droplet on the upper surface of the thermocouple was measured. The contact radius “a” between the droplet and the upper surface of the thermocouple was measured. Finally, the contact angle was got by using the Laplace equation, as shown in Fig. 3.
The schematic of contact angle test. (Online version in color.)
In the step (vii), substituting the slag surface tension, the contact angle, and the Pt surface free energy into the Young’s equation could get the slag-metal interfacial tension when the surface tension of the slag is known. If the slag-metal interfacial tension was known, substituting the slag-metal interfacial tension, the contact angle, and the Pt surface free energy into the Young’s equation could get the slag surface tension.
The interfacial property test of molten slags by SHTT is based on the contact angle between the molten slag and the B-type thermocouple. The Young’s equation is used to calculate the slag interface property parameters. Therefore, the contact angle is the key to the interface property test method of molten slag by SHTT, and affects the accuracy of the results. In Young’s equation, the contact angle θ is created by the contact of the droplet with the horizontal plane. But the surface of the B-type thermocouple is a curved surface. To explore the impact of this difference, the sample of B6 was selected for the sessile drop method. In sessile drop test, the temperature was 1500°C, the test atmosphere was air, and the gasket material was platinum. The results are shown in Fig. 4. In Fig. 4 the result of the contact angle tested by the sessile drop method is 61.00°, which is similar to the SHTT test result (61.06°), and the relative error is 0.10%. Therefore, the contact angle measured by SHTT can satisfy the calculation of Young’s equation.
The contact angle of sample B6 tested by the sessile drop method.
To ensure the accuracy of the contact angle test, the test procedure must comply with the following four requirements. (i) Ensure the B-type thermocouple test area is straight, as the curved thermocouple directly affects the accuracy of the contact angle. (ii) Make the size of the droplet suitable (approximately 1 mg). If the droplet is too large, the droplet will flow under the thermocouple due to gravity, and the standard contact angle test picture cannot be obtained, the incorrect picture is shown in Fig. 5(a). (iii) Excluding the bubbles in the droplets, the bubbles in the droplets will affect the contact angle test results, as shown in Fig. 5(a). (iv) Ensure that the central axis of the droplet is parallel to the observation surface, so that the contact angle tested by SHTT is consistent with the sessile drop method. The correct test result diagram is shown in Fig. 5(b).
Contact angle test results by SHTT method: (a) incorrect result; (b) correct result.
In order to verify the reproducibility of the SHTT contact angle test, the experiment selected the samples of A1, A5, B1, and B7 for testing. Under the above test specifications, each group of slag needs to be tested 3 times and take the average, the test results are shown in Table 2. From Table 2, the maximum fluctuation range of the contact angle in the reproducibility experiment is ±2.00°, which indicates the SHTT experiment can meet the contact angle reproducibility requirement.
The slag-metal interfacial tension can be calculated by the Young’s equation. The contact angles obtained in section 3.1 and the known surface tension values are shown in Table 3. From Fig. 6, the interfacial tension of the sample A1–A5 remains stable, and ranges from 2117.76 mN/m to 2131.89 mN/m. While the interface tension of the sample B1–B7 varies widely, which ranges from 2042.67 mN/m to 2179.3 mN/m. So the interfacial tension of CSA slag and CSAM slag with B-type thermocouple remains stable, while the interfacial tension of CSAM slag which contains Na2O and CaF2 with B-type thermocouple changes obviously.
* The references are listed in Table 1.
Slag-metal interface tension of sample A1–A5 and B1–B7. (Online version in color.)
Further analysis found that the interfacial tension of CSA and CSAM slag containing Na2O and CaF2 with B-type thermocouple increases with the increase of Na2O and CaF2. According to Gouy–Chapman’s electric double layer theoretical model31,32,33) and the structure of the covalent bond and ionic bond described by Mills and Däcker,34) the surface of the metal layer is negatively charged, and the freely moving ions in the slag are Ca2+, Mg2+, Na+, F− and O2−. The radius of Na+ is similar to that of Ca2+, which is larger than that of Mg2+,35) and Na+ is one-valence, which is attracted by the negative charge of the metal surface weakly. Therefore, Na+ migrates in the slag easily, and increases the slag-metal interfacial tension. F− plays a role of dilution in slag,36) and the electronegativity of F− is greater than O2−.37) Combined with the Gouy–Chapman model, it is speculated that the increase of F− increases the attraction of anions to cations at the interface in the slag, and thus weakens the attraction of cations to the negative surface of the metal, i.e. increase the slag-metal Interfacial tension.
The surface tension calculation of molten slag by the SHTT is also based on the Young’s equation. When the slag-metal interfacial tension is known, the slag surface tension can be calculated by the Young’s equation. Kim and Sasaki38) pointed out that the interfacial tension between slag and metal cannot be directly measured and needs to be calculated indirectly by the surface tension. Therefore, it is not feasible to calculate the surface tension of the slag by obtaining the interfacial tension firstly. If there is a slag system in which the slag-metal interfacial tension does not change with the ratio of slag components, the slag-metal interfacial tension can be regarded as a constant, and the surface tension of the slag system can be calculated. From the calculation results of the interfacial tension of Section 3.2, the interfacial tension of A1–A5 slags is less affected by the change of composition, while the interfacial tension of B1–B7 slags changes obviously. Therefore, A1–A5 slags are selected, and the average slag-metal interfacial tension (2124.89 mN/m) is regarded as a constant of interfacial tension in this study. The Young’s equation is used to calculate the surface tension of the slag. Comparing the calculated surface tension values and the literature values to explore the feasibility of calculating the surface tension of CSA & CSAM slag based on the SHTT method.
The surface tension of A1–A5 slags at 1500°C is calculated by Young’s equation, as shown in Table 4. From Table 4, the relative errors between the calculated values and the literature values are 0.41%–2.94%. Therefore, the computational surface tension of CSA & CSAM slag is relatively accurate. The SHTT can be used to test the surface tension of molten CSA & CSAM slag.
In this paper, the CSA & CSAM slag system is selected for testing the interfacial properties. To verify the reliability and applicability of using SHTT to test the interface properties of slag, the contact angle test between slag and B-Type thermocouple, the interfacial tension of slag-metal interface, and the surface tension of the slag are carried out in this study. The main conclusions are as follows:
(1) The contact angle can be obtained by the capillary action of molten slag on the B-type thermocouple quickly and conveniently. The relative error of the contact angles between the SHTT method and the sessile drop method is 0.10%, and the maximum fluctuation range of the contact angle of the repeated experiment is ±2.00°;
(2) The interfacial tension between slag and B-type thermocouple can be calculated by Young’s equation when the contact angle and the surface tension are known;
(3) The interfacial tension between the molten CSA & CSAM slag and the B-type thermocouple is less affected by the composition. The surface tension of the molten CSA & CSAM slag can be obtained by the SHTT, and the relative error between the calculated data and literature data is less than 2.94%.
This work was supported by the Graduate Scientific Research and Innovation Foundation of Chongqing, China [grant number CYB19002], and the National Natural Science Foundation of China [grant number 51574050].