Novel Mg metal production from MgO by molten salt electrolysis and vacuum distillation = 용융염전해 및 진공증류를 통한 산화마그네슘으로부터 마그네슘 금속 신제련법

다국어 초록 (Multilingual Abstract)

Magnesium (Mg) metal is an attractive material in various industries due to its superior properties of lightweight and high specific strength. Currently, thermal reduction and electrolytic processes are used in commercial Mg metal production, and approximately 87 % of global Mg metal is produced using a thermal reduction process, the Pidgeon process, in China. However, because of the global warming potential of the Pidgeon process, electrolytic processes have attracted attention. Industrial electrolytic processes that use anhydrous magnesium chloride (MgCl2) feedstock have a lower impact on global warming. However, toxic chlorine (Cl2) gas is generated during metal production, and energy-intensive feedstock preparation is necessary.
Under these circumstances, many studies have been carried out on the development of electrolytic processes for environmentally sound Mg metal production. Among these, an electrolytic process using magnesium oxide (MgO) feed is promising. By using MgO as the feedstock, the need for the production of anhydrous MgCl2 is not required. In addition, Cl2 gas is not generated during Mg metal production. However, the developed processes generally exhibited a relatively low current efficiency of 60 %.
In this thesis, a novel and efficient process for producing Mg metal from MgO is suggested. The process consists of two stages: (1) Electrolysis of MgO in magnesium fluoride (MgF2) – lithium fluoride (LiF) molten salt at 1053 – 1083 K using a high-density metal such as copper (Cu), silver (Ag), or tin (Sn) as a cathode to produce Mg alloy. (2) Vacuum distillation of Mg alloy obtained from electrolysis at 1200 – 1300 K to recover high-purity Mg metal.
For the development of the novel and efficient Mg metal production process, a fundamental study on the developed electrolytic process was conducted. The influence of electrolytic parameters on the current (or production) efficiency was investigated with thermodynamic considerations. In addition, the feasibility of producing Mg metal from primary and secondary resources containing MgO was investigated to enhance the robustness of the developed Mg metal production process. In consideration of the potential accumulation of impurities in the molten salt when using secondary MgO resources, the recovery of high-purity MgO from the secondary resource and also the subsequent production of Mg metal using the recovered MgO were also investigated. Furthermore, a scale-up study on the developed process was conducted in consideration of commercialization.

국문 초록 (Abstract)

마그네슘(Mg)은 구조용 금속 중에서 가장 가벼운 금속으로 높은 비강도 등의 우수한 특성으로 인해 다양한 분야에서 널리 사용되고 있다. 상용 Mg 금속 제련기술은 크게 열환원법과 전해법으로 구분되며, 전세계 Mg 금속의 약 87 %는 열환원법 중 하나인 Pidgeon 공정을 이용하여 중국에서 생산된다. 그러나 Pidgeon 공정은 에너지 소모 및 환경부하가 크기 때문에 전해법이 주목받고 있다. 무수 염화마그네슘(MgCl2) 원료를 사용하는 상용 전해법은 Pidgeon 공정 대비 환경부하가 적지만, 무수 MgCl2 제조공정이 에너지 집약적이고 용융염전해 시 독성의 염소(Cl2) 가스가 발생하는 단점이 있다.
상용 Mg 금속 제련기술의 단점을 개선하기 위한 친환경적 Mg 금속 제련기술 개발을 위해 많은 연구가 진행되어 왔으며, 산화마그네슘(MgO)을 원료로 사용하는 전해공정이 유망하다. 원료로 MgO을 사용하면 무수 MgCl2을 제조할 필요가 없고, 용융염전해 시 Cl2 가스가 발생하지 않는 장점이 있다. 하지만 개발된 공정들은 일반적으로 약 60 %의 비교적 낮은 전류 효율을 보였다.
본 논문에서는 MgO으로부터 Mg 금속을 제조하는 친환경∙ 고효율 Mg 금속 신제련법을 제안한다. 제안된 공정은 구리(Cu), 은(Ag), 또는 주석(Sn)과 같은 고밀도 금속을 음극으로 사용하여 1053 – 1083 K에서 불화마그네슘(MgF₂) – 불화리튬(LiF) 용융염에 용해된 MgO의 전해를 통해 Mg 합금을 제조한다. 이후 전해 후 회수된 Mg 합금을 1200 – 1300 K에서 진공증류하여 고순도 Mg 금속으로 제조한다.
본 논문에서는 제안된 고밀도 금속 음극을 사용한 MgO의 용융염전해에 대한 기초연구를 진행하였으며, 전해변수가 전류효율에 미치는 영향을 열역학적 분석과 함께 조사하였다. 또한, 보다 친환경적인 Mg 금속 신제련법을 위하여 MgO을 함유한 1차 및 2차자원으로부터 고순도 Mg 금속 제조 가능성을 조사하였다. 장기적인 관점에서 2차자원 사용 시 전해셀 내 불순물 축적을 고려하여 2차자원으로부터 고순도 MgO 회수 및 개발된 공정을 통한 Mg 금속 제조 가능성을 조사하였다. 더 나아가, 상용화를 고려하여 스케일업 연구를 진행하였다.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1 Current status of Mg metal 1
• 1.2 Mg resources 2
• 1.3 Primary Mg metal production processes 5
• 1.4 Molten salt electrolysis of MgO 25
• 1.5 Objective 31
• 1.6 Thesis outline 32
• Chapter 2. Mechanism of a novel and green Mg metal production process 80
• 2.1 Introduction 80
• 2.2 Molten salt electrolysis of MgO using high-density metal cathode 80
• 2.2.1 Supporting electrolyte 80
• 2.2.2 Cathode material 82
• 2.2.3 Determination of amount of Mg metal to electrodeposit 84
• 2.3 Vacuum distillation of Mg alloy produced after electrolysis 84
• Chapter 3. Molten salt electrolysis of MgO using Cu cathode for Mg metal production 93
• 3.1 Introduction 93
• 3.2 Mechanism 95
• 3.3 Experimental 96
• 3.3.1 Cyclic voltammetry 96
• 3.3.2 Molten salt electrolysis 98
• 3.3.3 Analysis 99
• 3.4 Results and discussion 100
• 3.4.1 Cyclic voltammetry 100
• 3.4.2 Molten salt electrolysis of MgO using Cu cathode 102
• 3.4.2.1 Influence of the electrolysis temperature 102
• 3.4.2.2 Influence of the concentration of Mg in Mg alloy 105
• 3.4.2.3 Influence of the size of the Cu pellets for the cathode 107
• 3.4.2.4 Influence of the cathodic current density 108
• 3.4.3 Stability of Al2O3 crucible during electrolysis 109
• 3.4.4 Vacuum distillation of Mg alloy 110
• 3.5 Conclusion 110
• Chapter 4. Molten salt electrolysis of MgO using Ag cathode and vacuum distillation for Mg metal production 138
• 4.1 Introduction 138
• 4.2 Mechanism 140
• 4.3 Experimental 142
• 4.3.1 Molten salt electrolysis 142
• 4.3.2 Vacuum distillation of Mg alloy 144
• 4.3.3 Analysis 145
• 4.4 Results and discussion 145
• 4.4.1 Molten salt electrolysis of MgO using Ag cathode 145
• 4.4.1.1 Influence of the electrolysis temperature 145
• 4.4.1.2 Influence of the concentration of Mg in Mg alloy 149
• 4.4.2 Vacuum distillation of Mg alloy 151
• 4.5 Conclusion 153
• Chapter 5. Mg metal production from various MgO resources through molten salt electrolysis using Cu cathode and vacuum distillation 176
• 5.1 Introduction 176
• 5.2 Mechanism 179
• 5.3 Experimental 181
• 5.3.1 Preparation of feedstock before the use 181
• 5.3.2 Molten salt electrolysis of various MgO resources 182
• 5.3.3 Vacuum distillation of Mg alloy 184
• 5.3.4 Analysis 184
• 5.4 Results and discussion 185
• 5.4.1 Feedstocks used for molten salt electrolysis 185
• 5.4.2 Molten salt electrolysis of primary MgO resources 187
• 5.4.2.1 Electrochemical reactions 187
• 5.4.2.2 Mg alloys obtained after electrolysis 188
• 5.4.2.3 Stability of Al2O3 crucible during electrolysis 189
• 5.4.2.4 Characteristics of electrolysis 190
• 5.4.3 Molten salt electrolysis of secondary MgO resources 190
• 5.4.3.1 Electrochemical reactions 190
• 5.4.3.2 Mg alloys obtained after electrolysis 192
• 5.4.3.3 Stability of Al2O3 crucible during electrolysis 193
• 5.4.4 Vacuum distillation of Mg alloy 193
• 5.5 Conclusion 194
• Chapter 6. Investigation on the solubility and electrochemical behavior of Mg2SiO4 in MgF2 – LiF molten salt at 1053 K 226
• 6.1 Introduction 226
• 6.2 Experimental 229
• 6.2.1 Solubility of Mg2SiO4 in MgF2 – LiF molten salt 229
• 6.2.2 Electrochemical behavior of Mg2SiO4 in MgF2 – LiF molten salt 230
• 6.2.3 Analysis 232
• 6.3 Results and discussion 233
• 6.3.1 Validation of solubility evaluation using an inert gas fusion-infrared absorption analyzer 233
• 6.3.2 Solubility of Mg2SiO4 in MgF2 – LiF molten salt 234
• 6.3.3 Estimation of reduction sequence of ions of Mg2SiO4 in MgF2 – LiF molten salt 235
• 6.3.4 CV measurement in MgF2 – LiF molten salt containing Mg2SiO4 236
• 6.3.5 Electrolysis using the three-electrode method 238
• 6.3.6 Electrolysis using the two-electrode method 239
• 6.4 Conclusion 241
• Chapter 7. Mg metal production from dolomite through molten salt electrolysis using Cu cathode and vacuum distillation 261
• 7.1 Introduction 261
• 7.2 Mechanism 263
• 7.3 Experimental 265
• 7.3.1 Preparation of feedstock 265
• 7.3.2 Molten salt electrolysis 265
• 7.3.3 Vacuum distillation of Mg alloy 267
• 7.3.4 Analysis 268
• 7.4 Results and discussion 269
• 7.4.1 Preparation of feedstock 269
• 7.4.2 Molten salt electrolysis 270
• 7.4.2.1 Influence of the electrolysis temperature 270
• 7.4.2.2 Influence of the concentration of Mg in Mg alloy 273
• 7.4.2.3 Behavior of CaO during electrolysis 274
• 7.4.2.4 Influence of the amount of Ca2+ ions in the molten salt 275
• 7.4.2.5 Influence of cathodic current density 278
• 7.4.2.6 Influence of the use of Pt anode 279
• 7.4.3 Vacuum distillation of Mg alloy 279
• 7.5 Conclusion 280
• Chapter 8. Mg metal production from sludge cake from a Mg-based flue gas desulfurization process 310
• 8.1 Introduction 310
• 8.2 Experimental 312
• 8.2.1 Recovery of MgO from the sludge cake 312
• 8.2.2 Molten salt electrolysis of recovered MgO 314
• 8.2.3 Analysis 316
• 8.3 Results and discussion 316
• 8.3.1 Water leaching of the sludge cake 316
• 8.3.2 Recovery of MgO from the leachate obtained by water leaching 318
• 8.3.3 Molten salt electrolysis of recovered MgO 320
• 8.3.3.1 Verification of electroreduction of MgO 320
• 8.3.3.2 Influence of the electrolysis temperature 321
• 8.3.3.3 Mg alloy obtained after electrolysis 323
• 8.3.4 Vacuum distillation of Mg alloy 324
• 8.4 Conclusion 324
• Chapter 9. Scale-up study of molten salt electrolysis using Cu or Ag cathode and vacuum distillation for Mg metal production 350
• 9.1 Introduction 350
• 9.2 Experimental 352
• 9.2.1 Molten salt electrolysis 352
• 9.2.2 Vacuum distillation of Mg alloy 353
• 9.2.3 Analysis 354
• 9.3 Results and discussion 354
• 9.3.1 Scale-up electrolysis of MgO using Cu or Ag cathode 354
• 9.3.2 Stability of Al2O3 cell during electrolysis 358
• 9.3.3 Vacuum distillation of Mg alloy 359
• 9.4 Conclusion 360
• Chapter 10. Conclusions 384
• 10.1 Summary 384
• 10.2 Original contributions to knowledge 391
• List of publications 393
• 국 문 초 록 395