Phase diagram study and the thermodynamic database development for the CaO-MgO-Al2O3-SiO2-VOx system = CaO-MgO-Al2O3-SiO2-VOx 계에 대한 상평형 연구 및 열역학적 데이터베이스 구축

국문 초록 (Abstract)

복잡한 고체, 액체 및 기체 상에 대한 전산 열역학 데이터베이스는 산업 공정 중 발생하는 화학 반응 및 상평형을 예측하는 데 활용될 수 있다. 예를 들어, 제강 공정에서 발생하는 다양한 반응, 슬래그–금속 반응, 개재물 형성, 내화물 부식 등과 같은 복합적인 화학 반응을 예측하는 데 열역학 데이터베이스가 적용될 수 있다.
고품위 철광석이 점점 줄어듦에 따라 여러 불순물이 포함된 저춤위 광석의 유입이 점점 늘어나고 있다. 바나듐은 저품위 철광석에서 볼 수 있는 대표적인 불순물이다. 광석 내의 바나듐은 제선 단계에서 환원되어 용강에 섞여 들어간다. 이렇게 들어간 바나듐은 정련 과정에서 산화되어 슬래그로 전이된 후 별도의 공정을 통해 회수된다. 이 과정의 효율을 높이기 위해서는 바나듐 산화물 계의 상거동과 열역학적 특성에 대한 정량적 이해가 필요하다.
본 연구는 Ca–Mg–Al–Si–V–O 계 산화물 시스템의 상평형 및 열역학적 특성을 규명하고, 이를 기반으로 신뢰성 있는 열역학 데이터베이스를 개발하는 것을 목표로 한다. 정확하고 일관적인 데이터베이스 구축을 위해 기존에 보고된 상평형 및 열역학 데이터를 정밀히 검토하였으며, 특히 환원 조건에서의 바나듐 함유 산화물 계에 대한 연구가 매우 부족함을 확인 하였다.
이에 따라 본 연구에서는 열역학 모델링을 진행하기위해 핵심적인 이원 및 삼원 산화물 계에 대해 평형 열처리 및 급랭(quenching) 실험, DTA, TGA를 이용하여 실험적으로 여러 상평도를 측정하였다. 상평형 실험은 환원 분위기(log(pO2/atm) = ‒10)와 산화 분위기(pO2 = 0.21 atm, 대기조건)에서 진행되었다. 산소 분압은CO–CO2 혼합비를 온도에 따라 변화하여 조절하였다. 또한 액상에서의 바나듐 양이온의 평균 산화수를 TGA를 통해 측정하였다.
이렇게 측정한 여러 실험 데이터를 바탕으로 열역학 모델링을 진행하였다. 열역학 모델링은 액상에 대해 MQM (Modified Quasichemical Model)을 사용하였고, 고상은 결정 구조에 따라 random mixing model 또는 CEF(Compound Energy Formalism)가 각각 적용되었다.
최적화된 모델 파라미터를 활용하여, Ca–Mg–Al–Si–V–O 계 내의 바나듐이 포함된 이원계 및 삼원계에 대한 상평형 및 열역학적 성질을 정밀히 계산할 수 있었다. 본 연구에서 개발된 열역학 데이터베이스는 향후 FactSage™ 데이터베이스에 통합되어 제강 및 기타 고온 공정 중 발생하는 다양한 복합 화학 현상의 열역학적 시뮬레이션에 활용될 수 있을 것이다.

다국어 초록 (Multilingual Abstract)

Computerized thermodynamic database for complex solid, liquid and gas phases can be utilized to calculate chemical reactions and phase equilibria that occur during industrial processes. For example, thermodynamic database can be applied to the prediction of complex chemical reactions in the steelmaking which includes ore-metal-gas reaction, slag-metal reactions, inclusion formation, refractory degradation, etc.
Vanadium is often introduced into the steelmaking process through vanadium-containing ores. During the initial steel refining stage, vanadium is removed from molten iron and oxidized into slag in the converter. Subsequently, vanadium is extracted from the slag in a separate process. Optimizing this vanadium removal and extraction process requires an accurate understanding of phase equilibria and thermodynamic properties of the vanadium-containing oxide system. A reliable thermodynamic database is essential for optimizing these industrial processes and improving efficiency.
The objective of the present study is to develop a thermodynamic database for the V-Ca-Mg-Al-Si-O system with the temperature range from room temperature to above liquidus and oxygen partial pressure range from metal saturation to 1 atm. To establish an accurate and self-consistent thermodynamic database, a critical evaluation of all available phase diagram and thermodynamic data was performed. However, thermodynamic studies on vanadium-containing oxide systems, especially under reducing conditions, are insufficient for accurate thermodynamic database development. Therefore, in this study, the phase equilibria of several key binary and ternary oxide systems were experimentally investigated by classical equilibration and quenching method, differential thermal analysis (DTA) and thermogravimetric analysis (TGA) in order to assist the thermodynamic modeling of the target oxide system.
Equilibration experiments were conducted under reducing condition (log(pO2/atm) = ‒10) and oxidizing condition (pO2 = 0.21atm, air). The oxygen partial pressure was controlled by a CO-CO2 gas mixture, where the CO-CO₂ ratio varied depending on the temperature. Thermogravimetric analysis was performed to measure the mean valence state of vanadium cations in liquid solution. In the thermodynamic optimization of this study, the Modified Quasichemical Model (MQM) was employed for liquid solution, while the random mixing model and Compound Energy Formalism (CEF) were used to describe solid solutions.
Using the thermodynamic models with optimized model parameters, the phase diagrams and thermodynamic properties of binary and ternary systems containing V oxide within the Ca-Mg-Al-Si-V-O oxide system were accurately calculated. The resultant thermodynamic database will be integrated to the FactSage™ thermodynamic database for the simulations of various complex chemical phenomena occurring in steelmaking and other high temperature processes.

목차 (Table of Contents)

• Abstract i
• Table of contents iii
• List of Figures ix
• List of Tables xv
• Chapter 1. Introduction 1
• 1.1. Reference 4
• Chapter 2. Overview of previous V oxide database 6
• 2.1. Reference 9
• Chapter 3. Overview and organization 16
• 3.1. Main goal of present study 16
• 3.2 Organization 17
• 3.3. Reference 17
• Chapter 4. Thermodynamic modelling and optimization 18
• 4.1. Critical evaluation/optimization 18
• 4.2. Equilibrium phases and thermodynamic models 20
• 4.2.1 Stoichiometric compounds 20
• 4.2.2 Solution phases 20
• 4.3. Reference 28
• Chapter 5. Key phase diagram experiments and challenges 32
• 5.1. Equipment for quenching experiment 32
• 5.2. Starting materials 33
• 5.3. Sample preparation 34
• 5.4. Quenching experiment 35
• 5.5. Characterization techniques 36
• 5.6. Differential thermal analysis and thermal gravimetric analysis 37
• 5.7. Experimental challenges 38
• 5.8. Reference 40
• Chapter 6. Phase equilibria and thermodynamic modelling of Ca-V-O system 44
• 6.1. Review of previous phase diagram study 44
• 6.1.1. CaO-VO system 44
• 6.1.2. CaO-V2O3 system 44
• 6.1.3. CaO-VO2 system 46
• 6.1.4. CaO-V2O5 system 47
• 6.2. Phase diagram experiment result 48
• 6.3. Thermodynamic modelling 52
• 6.4. Conclusion 56
• 6.5. References 57
• Chapter. 7. Phase equilibria and thermodynamic modelling of Mg-V-O system 83
• 7.1. Review of previous phase diagram study 83
• 7.1.1. Compound in Mg-V-O system 83
• 7.1.2. Phase diagram study for MgO-V2O3 84
• 7.1.3. Phase diagram study for MgO-V2O3-V2O4 85
• 7.1.4. Thermodynamic properties of spinel solid solution 86
• 7.1.5. MgO-V2O5 system 86
• 7.2 Phase diagram experiment result and discussion 87
• 7.3. Thermodynamic modeling 89
• 7.4. Conclusion 92
• 7.5. Reference 93
• Chapter 8. Phase equilibria and thermodynamic modelling of Al-V-O system 104
• 8.1. Review of previous phase diagram study 104
• 8.1.1. Al2O3-V2O3 system 104
• 8.1.2. Al2O3-V2O5 system 104
• 8.2. Phase diagram experiment result 106
• 8.2.1. Al2O3-V2O3 system under log(pO2/atm) = –10 106
• 8.2.2. Al2O3-V2O5 system in air (pO2 = 0.21 atm) 107
• 8.3. Thermodynamic modelling 110
• 8.3.1. Al2O3-V2O3 system under log(pO2/atm) = –10 110
• 8.3.2. Al2O3-V2O5 system in air (pO2 = 0.21 atm) 111
• 8.4. Conclusion 113
• 8.5. Reference 114
• Chapter 9. Phase equilibria and thermodynamic modelling of Si-V-O system 129
• 9.1. Review of previous phase diagram study 129
• 9.1.1. SiO2-V2O3 system 129
• 9.1.2. SiO2-V2O5 system 129
• 9.2. Phase diagram experiment result and discussion 130
• 9.3. Thermodynamic modelling 132
• 9.4. Conclusion 134
• 9.5. Reference 135
• Chapter 10. Phase diagram study and thermodynamic modelling of multi-component system 143
• 10.1. Al2O3-SiO2-V2O5 system in air 143
• 10.1.1. Review of previous phase diagram study 143
• 10.1.2. Phase diagram experiment result 144
• 10.1.3. Thermodynamic modelling 146
• 10.2. MgO-Al2O3-V2O3 system under log(pO2/atm) = ‒10 147
• 10.2.1. Review of previous phase diagram study 147
• 10.2.2. Phase diagram experiment result 148
• 10.2.3. Thermodynamic modelling 150
• 10.3. CaO-Al2O3-VOx system under various pO2 152
• 10.3.1. Review of previous phase diagram study 152
• 10.3.2. Thermodynamic modelling 153
• 10.4. CaO-SiO2-V2O3 system under reducing condition 154
• 10.4.1. Review of previous phase diagram study 154
• 10.4.2. Thermodynamic modelling 155
• 10.5. Conclusion 157
• 10.6. Reference 159
• Chapter 11. Application: Calculation of vanadium distribution ratio (LV) 181
• 11.1. Reference 184
• Chapter 12. Conclusion 186
• 12.1. Summary 186
• 12.2. Original contribution to knowledge 187
• 12.3. Future suggestions 188
• 12.4. Reference 190
• Appendix A. Summary of unsuccessful experiments 191
• A.1. Sample reduction 191
• A.2. Volatility of VO2 192
• A.3. Sample overflow 192
• A.4. Low quenching speed 193
• A.5. Reference 194
• Appendix B. Oxidation state analysis of V cations using thermalgravimetric analysis (TGA) 195
• B.1. Available Methods for Determining Vanadium Oxidation States 195
• B.2. Procedure for Vanadium Valence Determination Using TGA 197
• 요약 (국문 초록) 199
• Acknowledgement 201