Characterization of calcium carbonate prepared by carbonation of FGD gypsum

국문 초록 (Abstract)

이산화탄소 저감을 위하여 산업부산물인 탈황석고의 직접 탄산화반응 실험을 수행하였다. 탄산화반응은 암모니아 용액에 이산화탄소를 흡착시키면서 진행하였으며 황안(비료)과 탄산칼슘의 두 가지 화합물이 생성되었다. 본 연구에서는 두 번째 생성물인 탄산칼슘에 초점을 두었으며 주로 두 가지 측면이 고려되었다. 첫 번째는 고순도 탄산칼슘의 합성 가능성이며 두 번째는 제조한 탄산칼슘을 저가 흡착제로 이용하여 카드뮴 이온의 제거 가능성을 타진하는 것이었다. 본 연구에서 직접탄산화법으로 제조된 탄산칼슘은 합성조건에 관계없이 바테라이트와 방해석의 혼합물로 관찰되었다. 이산화탄소 유속이 고정된 조건에서 바테라이트의 함량은 탄산화 반응시간이 증가함에 따라 증가하였다. 탄산칼슘은 용해된 형태의 탄산칼슘 이온쌍으로 존재하는 유도기를 거친 후 최종 결정화되는 것으로 나타났으며 X-선 회절분석법, 광전자분광법, 주사전자현미경을 사용하여 확인하였다. 이 때 유도기를 적절히 이용하여 탈황석고에 존재하던 불순물을 분리하고 백색의 고순도 탄산칼슘을 회수할 수 있었다. 고순도 탄산칼슘은 초기에 비정형으로 형성되었다가 시간이 지나면서 바테라이트 상으로 변하였고, 물과 접촉한 후에는 방해석으로 되었다. 고순도 탄산칼슘의 유도기는 이산화탄소 유속이 증가할수록 감소하였다. 한편 고순도 탄산칼슘의 회수 효율은 암모니아 농도가 증가할수록 증가하였다. 용해된 탄산칼슘은 고액 비에 관계없이 78 mM의 최대 용해치를 나타내었다. 최종적으로 불순물을 포함한 방해석을 이용하여 물속에 포함된 카드뮴 제거를 위한 저가 흡착제로서의 사용 가능성을 확인하였다. 이를 위하여 흡착 반응시간, 카드뮴 용액의 초기 pH, 흡착량, 카드뮴 이온 농도를 변화시켰다. 탄산칼슘의 카드뮴 흡착 반응은 Langmuir 흡착등온선 모델과 잘 일치하였다. 이때 최대 흡착 용량은 7.99 mg/g이었으며 제거되는 카드뮴 이온의 양과 용해되는 칼슘 이온의 양이 서로 비례하였다. 시약급 방해석을 이용하여 대조 실험을 수행하였으며 탈황석고 탄산화반응으로 생성된 방해석을 이용한 경우 시약급 방해석보다 칼슘의 용해와 카드뮴에 대한 흡착량이 모두 증가되는 경향을 보였다. 이는 탈황석고에 포함되었던 바사나이트 불순물이 폐방해석의 용해를 촉진시켜 흡착반응이 촉진된 것으로 해석된다. 본 연구의 결과들은 향후 이산화탄소 저감을 위한 탈황석고 광물탄산화법에 경제적 가치를 증가시키는데 기여할 것으로 기대된다.

다국어 초록 (Multilingual Abstract)

The direct aqueous carbonation of flue gas desulfurization (FGD) gypsum, an industrial waste product, was performed through CO2 sorption into ammonia solution containing FGD gypsum for CO2 sequestration. Mineral carbonation of FGD gypsum produces two products: ammonium sulfate (a fertilizer) and calcium carbonate. In this study, the focus was laid on the calcium carbonate. The possibility of synthesizing impurity-free calcium carbonate crystals and the feasibility of using the final carbonated product as a low-cost adsorbent for cadmium removal from water were considered. The direct carbonation produced a mixture of vaterite and calcite phases under all conditions. The amount of vaterite phase in the mixture was found to increase with carbonation time for a fixed CO2 flow rate. An induction period before the precipitation of physically detectable calcium carbonate crystals was assessed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and field-emission scanning electron microscopy (FE-SEM). For the CO2 flow rate of 1 L/min, the C1s peak of CaCO3 became clear after carbonation took place for longer than 15 min. Impurity-free calcium carbonate were precipitated from the solution extracted during the induction period. Its initial phase was amorphous and turned into round-shaped vaterite, and afterward calcite in contact with water. The induction time for pure precipitated calcium carbonate (PCC) decreased as the CO2 flow rate (1—3 L/min) increased. The maximum formation efficiency for pure PCC was seen to increase linearly with the ammonia content (4—12%). The maximum solubility limitation of dissolved calcium carbonate was detected as 78 mM at a condition (ammonia content of 12% and CO2 flow rate of 1 L/min) without regard to solid-to-liquid ratio (50—300 g/L). The final carbonated product, waste calcite, was evaluated as low-cost adsorbent for Cd(II) removal from wastewater. Batch experiments were performed in aqueous solution varying contact time, initial pH of Cd(II) solution, adsorbent dose, and Cd(II) concentration. The adsorption of Cd(II) was found to fit with the Langmuir model and the maximum adsorption capacity was 7.99 mg/g. The amount of removed Cd(II) was found to be proportionally related to dissolved Ca(II) reflecting the exchange nature of the adsorption. The reagent-grade calcite particles were also examined for a comparative study. The use of waste calcite instead of reagent-grade calcite enhanced Ca(II) dissolution and thereby Cd(II) uptake significantly. The enhanced Ca(II) dissolution and Cd(II) uptake by the waste calcite were attributed to the fast-dissolving bassanite phase which provides the substantial quantity of Ca(II) and sulfate ions to the calcite/solution interface. These findings could increase the economic value of mineral carbonation of FGD gypsum for CO2 sequestration.

목차 (Table of Contents)

• Abstract i
• Contents iii
• List of Figures vii
• List of Tables xi
• Chapter I: Introduction 1
• 1. Background 2
• 2. Carbon capture and storage 2
• 3. Previous studies 5
• 3.1. Mineral carbonation 5
• 3.2. Gypsum waste 9
• 4. Objective 10
• References 12
• Chapter II: Precipitation of calcium carbonate during direct aqueous carbonation of flue gas desulfurization gypsum 17
• Abstract 18
• 1. Introduction 18
• 2. Experimental 20
• 2.1. Materials 20
• 2.2. Carbonation experiments 21
• 2.3. Determination of phase composition 23
• 3. Results and discussion 24
• 3.1. Characterization of the FGD-gypsum 24
• 3.2. Carbonation of FGD gypsum 25
• 3.3. Polymorphs of calcium carbonate 30
• 3.4. Induction period 33
• 3.5. Precipitation of high-purity calcite 36
• 4. Conclusions 37
• References 38
• Chapter III: Factors affecting the precipitation of pure calcium carbonate during the direct aqueous carbonation of flue gas desulfurization of gypsum 41
• Abstract 42
• 1. Introduction 42
• 2. Experimental 44
• 2.1. Materials 44
• 2.2. Carbonation experiments 45
• 3. Results and discussion 46
• 3.1. Scheme for pure PCC 46
• 3.2. Polymorphs of pure PCC 47
• 3.3. Effect of CO2 flow rate 49
• 3.4. Effect of ammonia content 52
• 3.5. Effect of solid-to-liquid ratio 54
• 4. Conclusions 56
• References 58
• Chapter IV: Adsorption of Cd(II) on waste calcite produced by the carbonation of flue gas desulfurization gypsum 61
• Abstract 62
• 1. Introduction 62
• 2. Experimental 64
• 2.1. Preparation of adsorbent 64
• 2.2. Preparation of Cd(II) solution 65
• 2.3. Sorption experiments 65
• 3. Results and discussion 66
• 3.1. Effect of adsorbent-sorbate contact time 66
• 3.2. Effect of solution pH 68
• 3.3. Effect of adsorbent dosage 70
• 3.4. Sorption rate 70
• 3.5. Sorption isotherms 72
• 4. Conclusions 76
• References 78
• Chapter V: Enhanced Cd(II) uptake by the bassanite phase contained in waste calcite produced via the carbonation of flue gas desulfurization gypsum 81
• Abstract 82
• 1. Introduction 82
• 2. Experimental 85
• 2.1. Materials 85
• 2.2. Macroscopic experiments 85
• 3. Results and discussion 86
• 3.1. Characterization of waste calcite 86
• 3.2. Uptake of Cd(II) 90
• 3.3. Dissolution of Ca(II) 92
• 3.4. Dissolution of bassanite 94
• 4. Conclusions 96
• References 98
• CONCLUSIONS 101
• Abstract (in Korean) 104
• Acknowledgement (in Korean) 106