Sustainable Direct Production of Low-Oxygen Titanium via Magnesiothermic Reduction of Titanium Dioxide = 마그네슘 열환원법을 이용한 친환경 저산소 타이타늄 직접 제조 공정 개발

다국어 초록 (Multilingual Abstract)

Titanium (Ti) has excellent physical and chemical properties, making it an essential material in the aerospace, energy, and biomedical industries. However, the Kroll process, a conventional method for producing high-purity Ti metal, has drawbacks such as low productivity, high energy consumption, and toxic chlorine gas usage. In recent years, reduction followed by deoxidation using magnesium (Mg) in a hydrogen gas (H2) atmosphere has been suggested as a method to produce Ti metal with a low oxygen (O) concentration from titanium dioxide (TiO2). However, this approach still requires a two-step process involving reduction and deoxidation. In addition, it requires excess Mg and molten salts and generates a significant amount of HCl solution as waste due to repeated leaching steps.

In this thesis, a single-step reduction process using Mg metal in an H2 gas atmosphere is proposed to obtain low-O titanium hydride (TiH2) powder directly from TiO2, thereby achieving environmental and economic sustainability. The reduction was conducted via a magnesiothermic reaction of TiO2 in molten magnesium chloride (MgCl2) – potassium chloride (KCl) at 973 K under a 10 % H2 mixed gas atmosphere.

A comprehensive thermodynamic analysis was conducted to establish a sustainable direct reduction process for low-O Ti and elucidate how the simultaneous control of oxygen chemical potential (pO2) and hydrogen chemical potential (pH2) governs the equilibrium O concentration in Ti. In addition, the influence of the sieving methods of TiO2 feedstock, TiO2 particle size, amount of molten salt, amount of Mg metal, reaction time, and microstructure of the TiO2 particle on the O concentration of the Ti product obtained after reduction were systematically evaluated. Under certain conditions, TiH2 with 0.209 mass%O was directly obtained through the magnesiothermic reduction of TiO2 at 973 K in an Ar – 10 % H2 mixed gas atmosphere. Therefore, this study demonstrates the feasibility of sustainable direct production of low-O Ti by magnesiothermic reduction in a single step.

국문 초록 (Abstract)

타이타늄 (Ti)은 우수한 물리·화학적 특성으로 인해 항공우주, 에너지 및 의료 산업 등 다양한 분야에서 널리 사용되고 있다. 그러나 순도 높은 Ti 금속을 제조하는 기존의 Kroll 공정은 낮은 생산성, 높은 에너지 소비, 염소 가스 (Cl2) 사용으로 인한 환경 부하 등의 한계를 가지고 있다. 이러한 문제를 해결하기 위해 최근에는 이산화타이타늄 (TiO2)을 마그네슘 (Mg) 금속을 이용해 수소 (H2) 분위기에서 환원 및 탈산하여 저산소 Ti 금속을 제조하는 공정이 제안되었다. 그러나 해당 공정은 환원과 탈산의 두 단계를 필요로 하며, 이에 따라 과량의 Mg 및 용융염을 사용하고, 반복적인 산 처리 과정에서 상당한 양의 폐수 발생한다는 한계가 있다.
본 연구에서는 TiO2으로부터 직접 저산소 타이타늄 수소화물 (TiH2) 분말을 제조하기 위하여 Mg 금속과 H2 가스를 이용한 단일 단계 환원 공정을 제안한다. 환원 반응은 973 K의 아르곤 (Ar) – 10 % H2 혼합가스 분위기에서 염화 마그네슘 (MgCl2) – 염화 포타슘 (KCl) 용융염을 이용하여 수행하였다.
직접 환원 기반의 친환경 저산소 Ti 제조 공정을 확립하기 위해, 산소 화학퍼텐셜(pO2)과 수소 화학퍼텐셜(pH2)의 동시 제어가 Ti 내 고용 산소 농도에 미치는 영향을 열역학적으로 분석하였다. 또한 TiO2 원료의 분급 방식, 원료 입도, 용융염 사용량, Mg 첨가량, 반응 시간, 그리고 TiO2의 미세구조 등이 환원 후 Ti의 산소 농도에 미치는 영향을 평가하였다. 그 결과, 특정 조건에서 973 K의 Ar–10 % H₂ 혼합가스 분위기에서 Mg을 이용한 단일 단계 환원을 통해 0.209 mass%O의 TiH2 분말을 직접 제조할 수 있음을 확인하였다

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1 Current status of Ti metal 1
• 1.2 Conventional Ti production processes 3
• 1.2.1 Hunter process and iodide process 3
• 1.2.2 Kroll process 4
• 1.2.2.1 Carbo-chlorination and TiCl4 purification 4
• 1.2.2.2 Magnesiothermic reduction of TiCl4 5
• 1.2.2.3 Electrolysis of MgCl2 and reagent recycling 5
• 1.2.2.4 Limitations of the Kroll process 5
• 1.3 Emerging Ti production processes 6
• 1.3.1 TiCl4-based emerging processes 6
• 1.3.2 TiO2-based emerging processes 7
• 1.3.3 Calciothermic reduction processes 7
• 1.3.4 Electrolytic processes using molten salt 8
• 1.4 Hydrogen-assisted magnesiothermic reduction (HAMR) process 9
• 1.5 Objectives 10
• 1.6 Thesis outline 11
• Chapter 2. Mechanism of low-O Ti production via magnesiothermic reduction in a H2 gas atmosphere 29
• 2.1 Introduction 29
• 2.2 Principle for producing low-O Ti metal via magnesiothermic reduction 30
• 2.2.1 Consideration of Sieverts’ law to control O chemical potential 30
• 2.2.2 Strategies for controlling the three thermodynamic factors 32
• 2.2.2.1 Influence of reaction temperature 32
• 2.2.2.2 Reducing the activity of MgO 32
• 2.2.2.3 Increasing the activity coefficient of O in Ti 33
• 2.3 Thermodynamic analysis by chemical potential diagram of Ti – H – O system at 973 K 34
• 2.3.1 Calculation of Ti – O solid solution at 973 K 35
• 2.3.2 Calculation of Ti – H – O solid solution at 973 K 36
• 2.4 Summary of the thermodynamic analysis for the mechanism of low-O Ti production via magnesiothermic reduction in a H2 gas atmosphere 38
• Chapter 3. Influence of sieving method of TiO2 feedstock on the production of low-O TiH2 49
• 3.1 Introduction 49
• 3.2 Experimental 50
• 3.2.1 Classification of TiO2 feedstock 50
• 3.2.2 Pre-melting of the mixutre of salts and TiO2 feedstock 51
• 3.2.3 Magnesiothermic reduction 52
• 3.2.4 HCl leaching of the product obtained after the reduction 53
• 3.2.5 Analysis 54
• 3.3 Results and discussion 54
• 3.3.1 Characteristics of TiO2 feedstock 54
• 3.3.2 Influence of sieving method of TiO2 feedstock on O concentration in Ti product 55
• 3.4 Conclusion 58
• Chapter 4. Optimization of magnesiothermic reduction to produce low-O TiH2 from wet-sieved TiO2 68
• 4.1 Introduction 68
• 4.2 Experimental 70
• 4.2.1 TiO2 feedstock preparation 70
• 4.2.2 Pre-treatment for magnesiothermic reduction 72
• 4.2.3 Magnesiothermic reduction 73
• 3.2.4 HCl leaching of the product obtained after the reduction 73
• 3.2.5 Analysis 74
• 4.3 Results and discussion 74
• 4.3.1 Influence of reaction time on the reduction of various particle sizes of wet-sieved TiO2 74
• 4.3.2 Influence of the amount of wet-sieved TiO2 77
• 4.3.3 Influence of the microstructure of TiO2 79
• 4.4 Conclusion 82
• Chapter 5. Conclusions 102
• 국 문 초 록 104