A method for determining high-temperature phase equilibrium of a vacuum-sealed residue containing titanium suboxide oxide

Abstract

The present application belongs to the technical field of metallurgical slag thermodynamics and phase equilibrium measurement, and discloses a high-temperature phase equilibrium determination method for a vacuum sealed slag system containing low-valence titanium oxide. The raw materials are roasted or dried; the high-temperature resistant container is filled with the raw materials according to the preset component proportioning, placed in a quartz tube, and deoxidizers are added; after vacuum preheating, argon gas washing, and high-temperature melting and sealing under the control of the vacuum degree in the tube, a thermodynamic closed system is formed; the slag sample is kept at the experimental temperature until it reaches thermodynamic equilibrium, and then it is rapidly quenched to retain the high-temperature phase structure; finally, the phase equilibrium determination is completed through characterization means. The present application uses a closed system, effectively isolates the interference of external gas, relies on the slag sample itself to establish and maintain the ultra-low oxygen potential required for the stable existence of low-valence titanium oxide based on the redox balance, solves the technical problem that the traditional oxygen control method cannot meet the phase equilibrium determination of the slag system containing low-valence titanium oxide, and realizes the accurate and reliable determination of the phase diagram of the slag system containing low-valence titanium oxide at the steelmaking temperature.

Description

Technical Field

[0001] This invention relates to the field of thermodynamics and phase equilibrium measurement technology of metallurgical slag, specifically to a high-temperature phase equilibrium determination method for a vacuum-sealed slag system containing low-valence titanium oxides. Background Technology

[0002] Titanium is a key alloying element for improving the toughness, wear resistance, corrosion resistance, and high-temperature performance of steel. Various titanium-containing alloy steels have been widely used in high-end manufacturing fields such as aerospace, energy equipment, and rail transportation. As titanium-containing alloy steels develop towards higher titanium content and higher purity, the stabilization and control of titanium in molten steel during smelting has become a core technical challenge. Titanium is chemically highly reactive and readily reacts with elements such as oxygen and nitrogen, as well as components in refining slag, in molten steel. On the one hand, this generates high-melting-point titanium inclusions, which can easily cause nodule formation in continuous casting nozzles, affecting the smooth operation of the continuous casting process and product quality. On the other hand, it leads to a decrease in titanium yield, poor alloying effect, and simultaneously deterioration of refining slag properties and reduced cleanliness of molten steel. Slag phase diagrams are one of the important theoretical foundations for guiding the development and optimization of metallurgical processes. Phase diagram studies on the refining slag system of titanium alloy steel can provide a reliable basis for the precise design of refining slag composition and the regulation of the precipitation and dissolution behavior of titanium inclusions. At the same time, they provide a data foundation for the construction of a slag thermodynamic database. From a theoretical perspective, they can help solve key problems in the smelting process of titanium alloy steel, such as low titanium yield, difficulty in controlling inclusions, and fluctuations in the performance of refining slag, and provide necessary theoretical guidance for the stabilization control of titanium.

[0003] In actual smelting conditions, molten steel is in an environment with extremely low oxygen potential. p (O2) < 1 × 10 -15 Atm (atm), refining slag forms a natural oxygen potential gradient in the vertical direction. In the slag near the molten steel side, titanium mainly exists as a stable phase in low-valence forms such as Ti₂O₃ and TiO. Therefore, the determination of the phase diagram of refining slag systems containing low-valence titanium oxides has more practical guiding significance. Currently, the determination of the phase diagram of slag systems containing variable valence elements generally relies on the external atmosphere to control the oxygen potential of the system. Commonly used atmospheres include oxygen, air, nitrogen, inert gases, H₂-H₂O mixtures, CO-CO₂ mixtures, and graphite-CO. Among these, C-CO controls the lowest equilibrium oxygen potential ( p (O2)=10 -15.35 atm). Document I (Park YJ, Kim WY, Kang Y B. Phase equilibria ofAl2O3-TiO xSystem under various oxygen partial pressures: Emphasis on stability of Al2TiO5-Ti3O5 pseudobrookite solid solution [J]. Journal of the European Ceramic Society, 2021, 41(14): 7362-7374) By controlling different types of external atmospheres, the phase diagrams of Al2O3-TiO2, Al2O3-Ti3O5, and Al2O3-Ti2O3 slag systems were determined at 1600℃. Among them, only C-CO atmosphere could meet the oxygen potential requirements for the determination of Al2O3-Ti2O3 slag system phase diagram, but phase equilibrium results showed that some components did not belong to Al2O3-TiO2. x The TiC precipitate in the slag system alters the equilibrium phase composition, leading to distorted experimental results. Reference II (T Mizoguchi, Y Ueshima. Determination of the Ti2O3-CaO-Al2O3 phase diagram at steelmaking Temperature[J]. Tetsu-to-Hagané, 2005, 91(4): 376-382) used a C-CO atmosphere to control the oxygen potential of the system and determined the phase diagram of the CaO-Al2O3-Ti2O3 slag system at 1600℃, also finding a TiC precipitate that does not belong to this slag system. Since TiO is more reducing than Ti2O3, the oxygen potential required to maintain its stability exceeds the controllable range of all conventional external atmospheres; therefore, there are currently no reports on the phase diagram determination of TiO-containing slag systems. In summary, the traditional method of relying on external atmosphere to control the oxygen potential of the system cannot achieve accurate and reliable phase diagram determination of slag systems containing low-valent titanium oxides.

[0004] To overcome the shortcomings of traditional methods of external atmosphere control, such as insufficient lower limit of oxygen potential and easy generation of impurity phases, a quartz tube is used to vacuum seal the slag sample to construct a closed system, which theoretically can overcome the technical limitations of traditional atmosphere control. This closed system does not rely on external atmosphere control; it can autonomously establish and maintain the extremely low oxygen potential required for the stable existence of low-valence titanium oxides based on the redox equilibrium of the slag sample itself. Simultaneously, through physical shielding, it can effectively isolate interference from external gases such as O2, N2, and H2O, fundamentally preventing secondary oxidation and impurity phase generation of the sample. Currently, quartz tube vacuum sealing technology is only applied to low-temperature (… T In fields such as alloy phase diagram determination (<1100℃) and solid-state synthesis of low-valent oxides, there are no precedents for related applications in the high-temperature range of 1400℃~1600℃ corresponding to steelmaking refining. The reason for this is that, on the one hand, the experimental temperature in steelmaking is already close to the softening point of quartz tubes (<1100℃).T (≈1670℃), the huge pressure difference between the inside and outside of the tube under vacuum will cause the quartz tube to collapse and deform, resulting in vacuum leakage and experimental failure. On the other hand, in the high temperature and low pressure environment, the main component of the quartz tube, SiO2, will undergo gasification and decomposition reactions: SiO2(s)=SiO2(g), 2SiO2(g)=2SiO(g)+O2(g). The oxidizing gas produced by the reaction will not only cause oxidation and denaturation of the reducing slag sample, but also introduce silicon impurities into the system, interfering with the phase equilibrium measurement results. At the same time, the tube wall will gradually thin due to continuous gasification, which will further deteriorate the strength of the quartz tube.

[0005] In summary, traditional methods of external atmosphere control are insufficient to meet the extremely low oxygen potential requirements for phase diagram determination of slag systems containing low-valent titanium oxides. The industry urgently needs a solution that can provide the extremely low oxygen potential necessary for the stable existence of low-valent titanium oxides and avoid interference from side reactions for accurate phase diagram determination. While quartz tube vacuum sealing technology theoretically holds promise for solving these problems, overcoming the challenges of tube collapse and SiO2 vaporization and decomposition under experimental conditions close to the softening temperature of quartz tubes is the core breakthrough for achieving high-temperature applications of this technology. Summary of the Invention

[0006] To address the technical challenges of traditional methods that rely on external atmosphere control for oxygen potential, such as insufficient lower oxygen potential limits and the easy introduction of impurities like TiC, which prevent the determination of phase diagrams for slag systems containing low-valent titanium oxides, this invention provides a high-temperature phase equilibrium determination method for vacuum-sealed slag systems containing low-valent titanium oxides. This invention utilizes a quartz tube to construct a closed system, leveraging the redox equilibrium of the components within the slag sample to spontaneously maintain the extremely low oxygen potential required for the stability of low-valent titanium oxides, while effectively isolating external impurity gases (O2, N2, H2O, etc.). Addressing the technical bottlenecks of existing vacuum sealing technologies at high temperatures in steelmaking and refining, such as container collapse leading to vacuum leakage, SiO2 vaporization and decomposition causing material oxidation and the introduction of silicon impurities, this invention, based on the ideal gas law and high-temperature thermodynamic data of SiO2, pre-calculates the total pressure range at the target temperature that simultaneously maintains the structural stability of the quartz tube and suppresses SiO2 vaporization and decomposition. Based on this, a fixed amount of high-purity inert gas is introduced into the quartz tube, achieving precise control of the total system pressure. Its mechanism of action is as follows: The thermal expansion effect of the gas during the heating process counteracts the pressure difference inside and outside the quartz tube, preventing the tube from collapsing and deforming due to softening at high temperatures, thus preventing vacuum leakage caused by container breakage. When the total pressure of the system at the experimental temperature is higher than the sum of the equilibrium partial pressures (i.e., decomposition pressures) of each gas phase, according to Le Chatelier's equilibrium shift principle, increasing the total pressure of the system shifts the equilibrium of the high-temperature gasification and decomposition reaction of SiO2 towards the direction of decreasing gas molecule number. This thermodynamically inhibits the gasification and decomposition reaction of SiO2, fundamentally preventing the generation of gas phases. This enables the stable application of vacuum sealing technology in the steelmaking refining temperature range, ultimately achieving accurate determination of the phase diagram of slag containing low-valence titanium oxides.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for determining the high-temperature phase equilibrium of a vacuum-sealed slag system containing low-valence titanium oxides, comprising the following steps: Step 1: Classify and pretreat the required raw materials according to their chemical stability; Step 2: Weigh and mix the pretreated raw materials according to the preset composition ratio of the target slag system, weigh a uniform initial slag sample, press the uniform initial slag sample into shape and transfer it into a high-temperature resistant container, then place the entire high-temperature resistant container in a quartz tube and add a deoxidizer into the quartz tube. Step 3: Determine the preset inflation pressure range at room temperature based on the volume of the quartz tube used in Step 2 and the vaporization and decomposition of SiO2. Step 4: Place the quartz tube obtained in Step 2 under a vacuum environment for overall preheating; use high-purity inert gas to purge the inside of the quartz tube multiple times; after purging, evacuate to a vacuum, then fill with high-purity inert gas, adjust the pressure inside the quartz tube to the preset pressure range at room temperature calculated in Step 3, and perform high-temperature sealing on the quartz tube. Step 5: Place the quartz tube obtained in Step 4 into a high-temperature furnace, raise it to the experimental temperature at a preset heating rate and keep it at that temperature to allow the components of the target slag system to diffuse fully and reach a thermodynamic equilibrium state. After the holding period, remove the quartz tube from the high-temperature furnace and immerse it in a quenching medium to quench it, thus preserving the phase equilibrium state of the target slag system at high temperature. Step 6: Take a portion of the quenched samples obtained in Step 5 and crush and grind them to analyze the equilibrium phase type and mole fraction of each element in the quenched samples; take another portion of the quenched samples for metallographic preparation, and after mounting, grinding and polishing, analyze the type and composition of each equilibrium phase to finally determine the phase equilibrium relationship of the target slag system.

[0008] The required raw materials are chemically stable and are pretreated by high-temperature roasting; the required raw materials are easily oxidized and are pretreated by low-temperature drying.

[0009] The high-temperature roasting temperature is 600℃~1200℃; the low-temperature drying temperature is 60℃~120℃; and the high-temperature roasting or low-temperature drying time is 4h~12h.

[0010] Based on the ideal gas law, the increase factor of the experimental temperature pressure compared to the room temperature pressure is calculated to determine the required high-purity inert gas pressure at room temperature, so that the total pressure inside the quartz tube at the experimental temperature is balanced with the atmospheric pressure outside the quartz tube. Combining the high-temperature thermodynamic data of SiO2, the sum of the equilibrium partial pressures of each gas phase generated by the vaporization and decomposition of SiO2 at the experimental temperature is calculated to determine the minimum total pressure required to inhibit the decomposition of SiO2. Based on the increase factor, the minimum charging pressure required to inhibit the decomposition of SiO2 at room temperature is calculated. The required high-purity inert gas pressure at room temperature and the minimum charging pressure required to inhibit the decomposition of SiO2 at room temperature are used as the preset charging pressure range at room temperature.

[0011] The preset inflation pressure range at room temperature is controlled within 90% to 95% of the upper limit of the range between the high-purity inert gas pressure required at room temperature and the minimum inflation pressure required to suppress SiO2 decomposition at room temperature.

[0012] The mass ratio of the target slag system to the volume of the quartz tube is 0.01 g / mL to 1 g / mL; the high-temperature resistant container has a melting point higher than the experimental temperature and does not react with the target slag system and the quartz tube at the experimental temperature; the deoxidizer is one of metallic chromium, vanadium, titanium, and silicon.

[0013] The experimental temperature was 1400℃~1600℃.

[0014] The preheating temperature is 100℃~200℃; the preheating time is not less than 0.5h; the number of gas purgings is not less than 2; the vacuum limit value is 0.1Pa.

[0015] The heat preservation time is 24h~48h to ensure that the slag sample reaches thermodynamic equilibrium; the quenching medium is an ice-water mixture or liquid nitrogen.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention abandons the traditional method of controlling the oxygen potential of the system by external atmosphere. It uses a quartz tube as the slag sample to construct a closed system. Relying on the redox equilibrium reaction between titanium oxides (TiO, Ti2O3, Ti3O5, etc.) of different proportions and valence states in the slag sample, it autonomously constructs and maintains an extremely low oxygen potential of the system. This completely eliminates the dependence on external atmosphere control for phase diagram determination of systems containing variable valence components, and fundamentally solves the technical problem of insufficient lower limit of oxygen potential in traditional methods. At the same time, it effectively blocks the infiltration of impurity elements such as carbon and nitrogen from the external environment through physical isolation, fundamentally preventing the formation of non-target impurity phases such as TiC and TiN, and ensuring that the measured phase equilibrium data is true and reliable.

[0017] 2. This invention overcomes the technical bottleneck of existing vacuum sealing technology, which is prone to structural collapse at high temperatures, and achieves stable application of this technology in the steelmaking and refining temperature range. By pre-filling the quartz tube with an appropriate amount of high-purity inert gas at room temperature, the thermal expansion effect of the high-purity inert gas inside the quartz tube during the heating process offsets the pressure difference inside and outside the quartz tube, preventing the quartz tube from collapsing and deforming when approaching its softening temperature. This effectively prevents vacuum leakage and external atmosphere contamination caused by quartz tube breakage, successfully raising the applicable temperature limit of vacuum sealing phase equilibrium measurement technology from the traditional 1100℃ to the steelmaking and refining temperature range of 1400~1600℃.

[0018] 3. This invention overcomes the technical bottleneck of slag contamination caused by the high-temperature vaporization and decomposition of SiO2 in existing vacuum sealing technologies, ensuring the accuracy and reliability of slag phase diagram determination. By precisely controlling the initial gas volume and the total system pressure at the target temperature within the quartz tube, this invention ensures that the total system pressure at the experimental temperature is strictly higher than the sum of the equilibrium partial pressures of each gas phase during SiO2 vaporization and decomposition. Utilizing the principle of equilibrium shift, it effectively suppresses the high-temperature decomposition of SiO2 from a thermodynamic perspective. This not only avoids the thinning of the quartz tube wall due to vaporization and maintains the high-temperature mechanical stability of the quartz tube, but also prevents the introduction of silicon impurities. Simultaneously, it prevents the secondary oxidation of low-valent titanium oxide raw materials by oxidizing gases generated from SiO2 decomposition, effectively ensuring the chemical stability of the raw materials. Attached Figure Description

[0019] Figure 1 The microstructure and XRD results of CaO-Al2O3-TiO slag sample at 1600℃ (vacuum sealed and argon-filled in this invention); (a) shows the microstructure, and (b) shows the XRD results. Figure 2 The microstructure and XRD results of CaO-Al2O3-Ti2O3 slag sample at 1400℃ (vacuum sealed and argon-filled in this invention); (a) shows the microstructure, and (b) shows the XRD results. Figure 3 The microstructure and XRD results of Al2O3-TiO-Ti2O3 slag samples at 1600℃ (vacuum sealed, argon-filled); (a) shows the microstructure, and (b) shows the XRD results. Figure 4 The microstructure and XRD results of CaO-Al2O3-TiO slag sample at 1600℃ (vacuum sealed, without argon gas); (a) shows the microstructure, and (b) shows the XRD results. Figure 5 The microstructure and XRD results of CaO-Al2O3-Ti2O3 slag samples at 1400℃ (traditional graphite-CO atmosphere control method); (a) shows the microstructure, and (b) shows the XRD results. Detailed Implementation

[0020] The present invention will be further described below with reference to specific embodiments, but this is not intended to limit the scope of protection thereof.

[0021] A method for determining the high-temperature phase equilibrium of a vacuum-sealed slag system containing low-valent titanium oxides, specifically including the following steps: Step 1: Pre-treat the required raw materials according to their chemical stability. Specifically, the required raw materials must be of analytical grade or superior grade purity; to fully remove moisture and volatile impurities, the high-temperature roasting temperature is 600℃~1200℃; to remove surface adsorbed water while avoiding oxidation, the low-temperature drying temperature is 60℃~120℃; the roasting and drying time is 2h~10h.

[0022] Step 2: Weigh and mix the raw materials obtained in Step 1 according to the preset composition ratio of the target slag system. Weigh a uniform initial slag sample, press the uniform initial slag sample into shape, and transfer it into a high-temperature resistant container. Then, place the entire high-temperature resistant container inside a quartz tube and add a deoxidizer to the quartz tube. Specifically, the mass ratio of the target slag system to the volume of the quartz tube is 0.01 g / mL to 1 g / mL; the melting point of the high-temperature resistant container is higher than the experimental temperature, and it does not react with the target slag system and the quartz tube at the experimental temperature; the deoxidizer is one of metallic chromium, vanadium, titanium, and silicon.

[0023] Step 3: Based on the volume of the quartz tube used in Step 2 and the vaporization and decomposition of SiO2, determine the preset charging pressure range at room temperature. Specifically, calculate the increase factor of the experimental temperature pressure compared to the room temperature pressure based on the ideal gas law, determine the pressure of the high-purity inert gas required to be charged at room temperature, so that the total pressure inside the quartz tube at the experimental temperature is balanced with the atmospheric pressure outside the quartz tube; combine the high-temperature thermodynamic data of SiO2, calculate the sum of the equilibrium partial pressures of each gas phase generated by the vaporization and decomposition of SiO2 at the experimental temperature, determine the minimum total pressure required to suppress the decomposition of SiO2, and back-calculate the minimum charging pressure required to suppress the decomposition of SiO2 at room temperature based on the increase factor; finally, the actual charging pressure at room temperature is determined within the range formed by the charging pressures obtained from the two calculations, and preferably controlled within 90% to 95% of the upper limit of this range; the experimental temperature is 1400℃~1600℃.

[0024] Step 4: Preheat the quartz tube obtained in Step 2 under vacuum; perform multiple gas purgings on the inside of the quartz tube using high-purity inert gas; after purging, evacuate to a vacuum, then fill with high-purity inert gas again, adjusting the pressure inside the quartz tube to the preset pressure range at room temperature calculated in Step 3, and perform high-temperature sealing on the quartz tube. Specifically, the preheating temperature is 100℃~200℃ to remove slag, deoxidizer, moisture adsorbed on the high-temperature container and tube wall surface; the preheating time is not less than 0.5h; the number of gas purgings is not less than 2; and the ultimate vacuum of the mechanical pump is 0.1Pa.

[0025] Step 5: Place the quartz tube obtained in Step 4 into a high-temperature furnace, raise the temperature to the experimental temperature at a preset heating rate, and hold it at that temperature for a sufficient duration to allow the components of the target slag system to fully diffuse and reach thermodynamic equilibrium. After the holding period, quickly remove the quartz tube from the high-temperature furnace and immerse it in a quenching medium to quench it, preserving the phase equilibrium state of the target slag system at high temperature. Specifically, the holding time is 24h~48h to ensure that the slag sample reaches thermodynamic equilibrium; the quenching medium is an ice-water mixture or liquid nitrogen.

[0026] Step 6: Take a portion of the quenched sample obtained in Step 5 and crush and grind it to analyze the equilibrium phase type and mole fraction of each element in the quenched slag sample; take another portion of the slag sample for metallographic preparation, and after mounting, grinding and polishing, analyze the type and composition of each equilibrium phase to finally determine the phase equilibrium relationship of the target slag system.

[0027] Any aspects not described in the following embodiments are the same as those described in the specific embodiments above.

[0028] Example 1; A method for determining the phase equilibrium of a vacuum-sealed CaO-Al2O3-TiO system slag sample at 1600℃ includes the following steps: Step 1: Calcine the superior pure CaO and Al2O3 raw materials at 1200℃ for 4 hours; dry the superior pure TiO raw materials at 80℃ for 2 hours. Step 2: Weigh the CaO, Al2O3 and TiO raw materials obtained in Step 1 precisely at a mass ratio of 26:27:47, mix them thoroughly, weigh 1.5g of uniform initial slag sample, press the uniform initial slag sample into shape and transfer it into a molybdenum crucible; to prevent the slag sample from overflowing after melting and directly contacting the quartz tube, causing vacuum leakage, nest the molybdenum crucible in a corundum crucible for protection, and then place the entire crucible in a quartz tube with a volume of 30mL, and add metallic chromium into the quartz tube as a deoxidizer; Step 3: Based on the volume of the quartz tube used in Step 2, the pressure increase factor at 1600℃ compared to room temperature (25℃) is calculated to be 6.28 using the ideal gas law. Therefore, it is determined that high-purity argon gas needs to be introduced into the quartz tube at room temperature until the total pressure reaches 16.1 kPa. This ensures that the total pressure inside the quartz tube at 1600℃ is roughly balanced with the external atmospheric pressure, offsetting the pressure difference and preventing the quartz tube from collapsing. Combining this with the high-temperature thermodynamic data of SiO2, the FactSage thermodynamic software is used to calculate the pressure at 1600℃. The sum of the equilibrium partial pressures of each gas phase generated by the vaporization and decomposition of SiO2 is 8.29 Pa, which is the minimum total pressure required to suppress the decomposition of SiO2. Based on the aforementioned increase factor, the minimum charging pressure required to suppress the decomposition of SiO2 at room temperature is calculated to be 1.32 Pa. Finally, the actual charging pressure at room temperature is determined within the range of [1.32 Pa, 16.1 kPa], and preferably controlled within 90% to 95% of the upper limit of this range, i.e., 14.49 kPa to 15.30 kPa.

[0029] Step 4: Place the quartz tube obtained in Step 2 in a vacuum environment at 100°C for overall preheating; use high-purity argon to purge the inside of the quartz tube three times; after purging, evacuate to a vacuum, then fill with high-purity argon to adjust the pressure inside the quartz tube to the range of 14.49 kPa to 15.30 kPa, and perform high-temperature sealing on the quartz tube. Step 5: Place the quartz tube obtained in Step 4 into a high-temperature furnace, raise the temperature to 1600℃ at the preset heating rate and keep it at that temperature for 24 hours to allow the components of the target slag sample to fully diffuse and reach a thermodynamic equilibrium state. After the holding period, quickly remove the quartz tube from the high-temperature furnace and immerse it in a 0℃ ice-water mixture for rapid quenching to preserve the phase equilibrium state of the target slag sample at high temperature. Step 6: Take a portion of the quenched sample obtained in Step 5 and crush and grind it to analyze the equilibrium phase type and mole fraction of each element in the quenched slag sample; take another portion of the slag sample for metallographic preparation, and after mounting, grinding and polishing, analyze the type and composition of each equilibrium phase to finally determine the phase equilibrium relationship of the target slag system.

[0030] Example 1: The final microstructure and XRD results of the quenched sample are as follows. Figure 1 As shown in the figure. According to the test results, the slag sample is in equilibrium between a liquid phase and TiO, with no oxidation products or other impurities formed. EPMA analysis shows that the mass fractions of each component in the equilibrium liquid phase are: 34.59% CaO, 35.62% Al2O3, and 29.79% TiO; the mass fractions of each component in the TiO equilibrium phase are: 0.86% CaO, 1.96% Al2O3, and 97.18% TiO. Calculations show that the equilibrium composition of the liquid phase and the TiO precipitated phase conforms to the lever law. In conclusion, conducting high-temperature phase equilibrium experiments using vacuum-sealed slag samples is effective for phase equilibrium studies and phase diagram determination of the CaO-Al2O3-TiO slag system.

[0031] Example 2; A method for determining the phase equilibrium of a vacuum-sealed CaO-Al2O3-Ti2O3 system slag sample at 1400℃ includes the following steps: Step 1: Calcine the superior purity CaO and Al2O3 raw materials at 1200℃ for 4 hours; dry the superior purity Ti2O3 raw materials at 80℃ for 2 hours. Step 2: Weigh the CaO, Al2O3 and Ti2O3 raw materials obtained in Step 1 precisely at a mass ratio of 27:17:56, mix them thoroughly, weigh 1.5g of uniform initial slag sample, press the uniform initial slag sample into shape and transfer it into a molybdenum crucible; to prevent the slag sample from overflowing after melting and directly contacting the quartz tube, causing vacuum leakage, nest the molybdenum crucible in a corundum crucible for protection, and then place the entire crucible in a quartz tube with a volume of 30mL, and add metallic chromium into the quartz tube as a deoxidizer; Step 3: Based on the volume of the quartz tube used in Step 2, the pressure increase factor at 1400℃ compared to room temperature (25℃) is calculated to be 5.61 using the ideal gas law. Therefore, it is determined that high-purity argon gas needs to be introduced into the quartz tube at room temperature until the total pressure reaches 18.06 kPa. This ensures that the total pressure inside the quartz tube at 1400℃ is roughly balanced with the external atmospheric pressure, offsetting the pressure difference and preventing the quartz tube from collapsing. Combining this with the high-temperature thermodynamic data of SiO2, the FactSage thermodynamic software is used to calculate the pressure at 1400℃. The sum of the equilibrium partial pressures of each gas phase generated by the vaporization and decomposition of SiO2 is 1.06 Pa, which is the minimum total pressure required to suppress the decomposition of SiO2. Based on the aforementioned increase factor, the minimum charging pressure required to suppress the decomposition of SiO2 at room temperature is calculated to be 0.19 Pa. Finally, the actual charging pressure at room temperature is determined within the range of [0.19 Pa, 18.06 kPa], and preferably controlled within 90% to 95% of the upper limit of this range, i.e., 16.25 kPa to 17.16 kPa.

[0032] Step 4: Place the quartz tube obtained in Step 2 in a vacuum environment at 100°C for overall preheating; use high-purity argon to purge the inside of the quartz tube three times; after purging, evacuate to a vacuum, then fill with high-purity argon to adjust the pressure inside the quartz tube to the range of 16.25 kPa to 17.16 kPa, and perform high-temperature sealing on the quartz tube. Step 5: Place the quartz tube obtained in Step 4 into a high-temperature furnace, raise the temperature to 1400℃ at a preset heating rate and hold it for 24 hours to allow the components of the target slag sample to fully diffuse and reach thermodynamic equilibrium. After the holding period, quickly remove the quartz tube from the high-temperature furnace and immerse it in a 0℃ ice-water mixture for rapid quenching to preserve the phase equilibrium state of the target slag sample at high temperature. Step 6: Take a portion of the quenched sample obtained in Step 5 and crush and grind it to analyze the equilibrium phase type and mole fraction of each element in the quenched slag sample; take another portion of the slag sample for metallographic preparation, and after mounting, grinding and polishing, analyze the type and composition of each equilibrium phase to finally determine the phase equilibrium relationship of the target slag system.

[0033] Example 2: The final microstructure and XRD results of the quenched sample are as follows. Figure 2 As shown in the figure. According to the test results, the slag sample is in two-phase equilibrium between the liquid phase and Ti2O3, with no oxidation products or other impurities formed. EPMA analysis results show that the mass fractions of each component in the equilibrium liquid phase are: 39.5% CaO, 25.66% Al2O3, and 34.84% Ti2O3; the mass fractions of each component in the Ti2O3 equilibrium phase are: 0.52% CaO, 0.49% Al2O3, and 98.99% Ti2O3. Calculations show that the two-phase equilibrium system of the liquid phase and the Ti2O3 precipitated phase at 1400℃ satisfies the lever law. In summary, conducting high-temperature phase equilibrium experiments using vacuum-sealed slag samples is effective for phase equilibrium studies and phase diagram determination of the CaO-Al2O3-Ti2O3 slag system.

[0034] Example 3; A method for determining the phase equilibrium of a vacuum-sealed Al2O3-TiO-Ti2O3 system slag sample at 1600℃ includes the following steps: Step 1: Calcine the high-purity Al2O3 raw material at 1200℃ for 4 hours; dry the high-purity TiO and Ti2O3 raw materials at 80℃ for 2 hours. Step 2: Weigh the Al2O3, TiO and Ti2O3 raw materials obtained in Step 1 precisely at a mass ratio of 36:54:10, mix them thoroughly, weigh 1.5g of uniform initial slag sample, press the uniform initial slag sample into shape and transfer it into a molybdenum crucible; to prevent the slag sample from overflowing after melting and directly contacting the quartz tube, causing vacuum leakage, nest the molybdenum crucible in a corundum crucible for protection, and then place the entire crucible in a quartz tube with a volume of 30mL, and add metallic chromium into the quartz tube as a deoxidizer; Step 3: Based on the volume of the quartz tube used in Step 2, the pressure increase factor at 1600℃ compared to room temperature (25℃) is calculated to be 6.28 using the ideal gas law. Therefore, it is determined that high-purity argon gas needs to be introduced into the quartz tube at room temperature until the total pressure reaches 16.1 kPa. This ensures that the total pressure inside the quartz tube at 1600℃ is roughly balanced with the external atmospheric pressure, offsetting the pressure difference and preventing the quartz tube from collapsing. Combining this with the high-temperature thermodynamic data of SiO2, the FactSage thermodynamic software is used to calculate the pressure at 1600℃. The sum of the equilibrium partial pressures of each gas phase generated by the vaporization and decomposition of SiO2 is 8.29 Pa, which is the minimum total pressure required to suppress the decomposition of SiO2. Based on the aforementioned increase factor, the minimum charging pressure required to suppress the decomposition of SiO2 at room temperature is calculated to be 1.32 Pa. Finally, the actual charging pressure at room temperature is determined within the range of [1.32 Pa, 16.1 kPa], and preferably controlled within 90% to 95% of the upper limit of this range, i.e., 14.49 kPa to 15.30 kPa.

[0035] Step 4: Place the quartz tube obtained in Step 2 in a vacuum environment at 100°C for overall preheating; use high-purity argon to purge the inside of the quartz tube three times; after purging, evacuate to a vacuum, then fill with high-purity argon to adjust the pressure inside the quartz tube to the range of 14.49 kPa to 15.30 kPa, and perform high-temperature sealing on the quartz tube. Step 5: Place the quartz tube obtained in Step 4 into a high-temperature furnace, raise the temperature to 1600℃ at the preset heating rate and keep it at that temperature for 24 hours to allow the components of the target slag sample to fully diffuse and reach a thermodynamic equilibrium state. After the holding period, quickly remove the quartz tube from the high-temperature furnace and immerse it in a 0℃ ice-water mixture for rapid quenching to preserve the phase equilibrium state of the target slag sample at high temperature. Step 6: Take a portion of the quenched sample obtained in Step 5 and crush and grind it to analyze the equilibrium phase type and mole fraction of each element in the quenched slag sample; take another portion of the slag sample for metallographic preparation, and after mounting, grinding and polishing, analyze the type and composition of each equilibrium phase to finally determine the phase equilibrium relationship of the target slag system.

[0036] Example 3: The final microstructure and XRD results of the quenched sample are as follows. Figure 3As shown in the figure. According to the test results, the slag sample is in two-phase equilibrium of Al2O3 and TiO, with no oxidation products or other impurity phases formed. EPMA and ICP tests show that the mass fractions of each component in the Al2O3 equilibrium phase are: 99.05% Al2O3, 0.46% TiO, and 0.49% Ti2O3; the mass fractions of each component in the TiO equilibrium phase are: 7.45% Al2O3, 78.12% TiO, and 14.43% Ti2O3. The Al2O3 phase has virtually no solid solution, while the TiO phase can dissolve large amounts of both Al2O3 and Ti2O3. Calculations show that the equilibrium composition of Al2O3 and TiO at 1600℃ conforms to the lever law. In summary, conducting high-temperature phase equilibrium experiments using vacuum-sealed slag samples is effective for phase equilibrium studies and phase diagram determination of the Al2O3-TiO-Ti2O3 slag system.

[0037] Comparative Example 1; The raw material ratios, experimental parameters, operating procedures, and detection methods in this comparative example are all the same as in Example 1. The difference is that high-purity argon gas was not introduced into the quartz tube; instead, vacuum sealing was performed directly. The specific steps are as follows: Step 1: Calcine the superior-grade CaO and Al2O3 raw materials at 1200℃ for 4 hours; dry the superior-grade TiO raw materials at 80℃ for 2 hours. Step 2: Weigh the CaO, Al2O3 and TiO raw materials obtained in Step 1 precisely at a mass ratio of 26:27:47, mix them thoroughly, weigh 1.5g of uniform initial slag sample, press the uniform initial slag sample into shape and transfer it into a molybdenum crucible; to prevent the slag sample from overflowing after melting and directly contacting the quartz tube, causing vacuum leakage, nest the molybdenum crucible in a corundum crucible for protection, and then place the entire crucible in a quartz tube with a volume of 30mL, and add metallic chromium into the quartz tube as a deoxidizer; Step 3: Place the quartz tube obtained in Step 2 in a vacuum environment at 100°C for overall preheating; use high-purity argon to purge the inside of the quartz tube three times; after purging, evacuate to a vacuum, do not fill the quartz tube with argon, and directly perform high-temperature sealing on the quartz tube. Step 4: Place the quartz tube obtained in Step 3 in a high-temperature furnace, raise the temperature to 1600℃ at the preset heating rate and keep it at that temperature for 24 hours to allow the components of the target slag sample to fully diffuse and reach a thermodynamic equilibrium state. After the holding period, quickly remove the quartz tube from the high-temperature furnace and immerse it in a 0℃ ice-water mixture for rapid quenching to preserve the phase equilibrium state of the target slag sample at high temperature. Step 5: Take a portion of the quenched sample obtained in Step 4 and crush and grind it to analyze the equilibrium phase type and mole fraction of each element in the quenched slag sample; take another portion of the slag sample for metallographic preparation, and after mounting, grinding and polishing, analyze the type and composition of each equilibrium phase to finally determine the phase equilibrium relationship of the target slag system.

[0038] Comparative Example 1: The final microstructure and XRD results of the quenched sample are as follows. Figure 4 As shown in the figure. During sampling, the quartz tube was observed to have collapsed and cracked, indicating that the lack of high-purity argon gas resulted in an excessive pressure difference between the inside and outside of the tube, damaging the container. According to XRD analysis, the slag sample was in equilibrium between a liquid phase and TiO2, indicating that oxidation occurred at high temperatures. EPMA analysis showed that the mass fractions of each component in the equilibrium liquid phase were: 28.13% CaO, 28.97% Al2O3, 30.23% TiO2, and 12.67% SiO2, indicating that SiO2 impurities had entered the liquid phase; the mass fractions of each component in the TiO2 precipitate phase were: 0.26% CaO, 0.47% Al2O3, and 99.27% ​​TiO2. Comparison with the results of Example 1 shows that filling the quartz tube with an appropriate amount of high-purity argon gas in Example 1 not only effectively maintained the structural integrity of the quartz tube at high temperatures but also inhibited the vaporization and decomposition of SiO2 and the oxidation of the slag sample, thus ensuring the stability of the low-valence titanium oxides in the slag sample and the accuracy of the phase diagram determination.

[0039] Comparative Example 2; The raw material ratios, heating rates, holding times, quenching methods, and testing methods in this comparative example are all the same as in Example 2. The difference is that vacuum sealing technology was not used; instead, the traditional method of controlling the oxygen potential of the graphite-CO atmosphere system was employed for phase equilibrium determination. The specific steps are as follows: Step 1: Calcine the superior purity CaO and Al2O3 raw materials at 1200℃ for 4 hours; dry the superior purity Ti2O3 raw materials at 80℃ for 2 hours. Step 2: Weigh the CaO, Al2O3 and Ti2O3 raw materials obtained in Step 1 precisely at a mass ratio of 27:17:56, mix them thoroughly, weigh 1.5g of uniform initial slag sample, press the uniform initial slag sample into shape and transfer it into a molybdenum crucible. Place the whole sample in a graphite crucible, ensuring that the slag sample and the graphite crucible do not come into contact. Step 3: Place the graphite crucible obtained in Step 2 in a high-temperature furnace, raise the temperature to 1400℃ at a preset heating rate and hold for 24 hours to allow the components of the target slag sample to fully diffuse and reach thermodynamic equilibrium. During this period, high-purity CO gas is introduced into the furnace at a flow rate of 1L / min. After the holding period, quickly remove the crucible from the high-temperature furnace and immerse it in a 0℃ ice-water mixture for rapid quenching to preserve the phase equilibrium state of the slag sample at high temperature. Step 4: Take a portion of the quenched sample obtained in Step 3 and crush and grind it to analyze the equilibrium phase type and mole fraction of each element in the quenched slag sample; take another portion of the slag sample for metallographic preparation, and after mounting, grinding and polishing, analyze the type and composition of each equilibrium phase to finally determine the phase equilibrium relationship of the target slag system.

[0040] Comparative Example 2: The final microstructure and XRD results of the quenched sample are as follows. Figure 5 As shown in the figure. XRD analysis revealed distinct diffraction peaks for Ti3O5 and TiC in the slag sample, indicating that controlling the oxygen potential of the system using a graphite-CO atmosphere not only fails to guarantee the stability of the raw material Ti2O3 but also inevitably introduces the TiC impurity phase. EPMA analysis showed that the molar fractions of each element in the equilibrium liquid phase were: 16.50% Ca, 11.79% Al, 12.61% Ti, 52.18% O, and 6.92% C, indicating that a large amount of carbon in the atmosphere had penetrated into the slag sample and dissolved in the liquid phase. Compared with the vacuum sealing results of Example 2, it is evident that the traditional graphite-CO atmosphere control method cannot guarantee the stability of the raw material Ti2O3, leading to its oxidation to Ti3O5; nor can it prevent the penetration of carbon impurities, generating the TiC impurity phase. This severely interferes with the phase equilibrium results of the target slag system and makes it impossible to accurately obtain the true phase composition and equilibrium composition of the slag system containing low-valent titanium oxides.

Claims

1. A method for determining the high-temperature phase equilibrium of a vacuum-sealed slag system containing low-valent titanium oxides, characterized in that, The steps include the following: Step 1: Classify and pretreat the required raw materials according to their chemical stability; Step 2: Weigh and mix the pretreated raw materials according to the preset composition ratio of the target slag system, weigh a uniform initial slag sample, press the uniform initial slag sample into shape and transfer it into a high-temperature resistant container, then place the entire high-temperature resistant container in a quartz tube and add a deoxidizer into the quartz tube. Step 3: Based on the volume of the quartz tube used in Step 2 and the vaporization and decomposition of SiO2, determine the preset filling pressure range at room temperature; calculate the increase factor of the experimental temperature pressure compared to the room temperature pressure based on the ideal gas law, and determine the pressure of the high-purity inert gas required at room temperature to balance the total pressure inside the quartz tube with the atmospheric pressure outside the quartz tube at the experimental temperature; combine the high-temperature thermodynamic data of SiO2, calculate the sum of the equilibrium partial pressures of each gas phase generated by the vaporization and decomposition of SiO2 at the experimental temperature, determine the minimum total pressure required to suppress SiO2 decomposition, and deduce the minimum filling pressure required to suppress SiO2 decomposition at room temperature based on the increase factor; the pressure of the high-purity inert gas required to be filled at room temperature and the minimum filling pressure required to suppress SiO2 decomposition at room temperature are taken as the preset filling pressure range at room temperature. Step 4: Place the quartz tube obtained in Step 2 under a vacuum environment for overall preheating; use high-purity inert gas to purge the inside of the quartz tube multiple times; after purging, evacuate to a vacuum, then fill with high-purity inert gas, adjust the pressure inside the quartz tube to the preset pressure range at room temperature calculated in Step 3, and perform high-temperature sealing on the quartz tube. Step 5: Place the quartz tube obtained in Step 4 into a high-temperature furnace, raise it to the experimental temperature at a preset heating rate and keep it at that temperature to allow the components of the target slag system to diffuse fully and reach a thermodynamic equilibrium state. After the holding period, remove the quartz tube from the high-temperature furnace and immerse it in a quenching medium to quench it, thus preserving the phase equilibrium state of the target slag system at high temperature. Step 6: Take a portion of the quenched samples obtained in Step 5 and crush and grind them to analyze the equilibrium phase type and mole fraction of each element in the quenched samples; take another portion of the quenched samples for metallographic preparation, and after mounting, grinding and polishing, analyze the type and composition of each equilibrium phase to finally determine the phase equilibrium relationship of the target slag system.

2. The method for determining the high-temperature phase equilibrium of a vacuum-sealed slag system containing low-valent titanium oxides according to claim 1, characterized in that, The required raw materials are chemically stable and are pretreated by high-temperature roasting; the required raw materials are easily oxidized and are pretreated by low-temperature drying.

3. The method for determining the high-temperature phase equilibrium of a vacuum-sealed slag system containing low-valent titanium oxides according to claim 2, characterized in that, The high-temperature roasting temperature is 600℃~1200℃; the low-temperature drying temperature is 60℃~120℃; and the high-temperature roasting or low-temperature drying time is 4h~12h.

4. The method for determining the high-temperature phase equilibrium of a vacuum-sealed slag system containing low-valent titanium oxides according to claim 3, characterized in that, The preset inflation pressure range at room temperature is controlled within 90% to 95% of the upper limit of the range between the high-purity inert gas pressure required at room temperature and the minimum inflation pressure required to suppress SiO2 decomposition at room temperature.

5. The method for determining the high-temperature phase equilibrium of a vacuum-sealed slag system containing low-valent titanium oxides according to claim 1, characterized in that, The mass ratio of the target slag system to the volume of the quartz tube is 0.01 g / mL to 1 g / mL; the high-temperature resistant container has a melting point higher than the experimental temperature and does not react with the target slag system and the quartz tube at the experimental temperature; the deoxidizer is one of metallic chromium, vanadium, titanium, and silicon.

6. The method for determining the high-temperature phase equilibrium of a vacuum-sealed slag system containing low-valent titanium oxides according to claim 1, characterized in that, The experimental temperature was 1400℃~1600℃.

7. The method for determining the high-temperature phase equilibrium of a vacuum-sealed slag system containing low-valent titanium oxides according to claim 1, characterized in that, The preheating temperature is 100℃~200℃; the preheating time is not less than 0.5h; the number of gas purgings is not less than 2; the vacuum limit value is 0.1Pa.

8. The method for determining the high-temperature phase equilibrium of a vacuum-sealed slag system containing low-valent titanium oxides according to claim 1, characterized in that, The heat preservation time is 24h~48h to ensure that the slag sample reaches thermodynamic equilibrium; the quenching medium is an ice-water mixture or liquid nitrogen.

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