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Structure and properties of CaCl2-CaO-Al2O3-SiO2 glasses

(CaCl2-CaO-Al2O3-SiO2ガラス系の構造と性質)

氏名 Somak Siwadamrongpong
学位の種類 博士(工学)
学位記番号 博甲第324号
学位授与の日付 平成16年12月31日
学位論文題目 Structure and properties of CaCl2-CaO-Al2O3-SiO2 glasses (CaCl2-CaO-Al2O3-SiO2ガラス系の構造と性質)
論文審査委員
 主査 教授 松下 和正
 副査 教授 小松 高行
 副査 教授 佐藤 一則
 副査 助教授 藤原 巧
 副査 柴田 修一

平成16(2004)年度博士論文題名一覧] [博士論文題名一覧]に戻る.

Contents

Chapter 1 Introduction p.1
 1.1 Solid waste p.2
 1.2 Municipal solid waste p.4
 1.2.1 Properties of municipal solid waste p.4
 1.2.2 Reduction of disposed municipal solid waste quantity p.8
 1.2.3 Disposal of municipal waste p.8
 1.3 Incineration p.12
 1.3.1 Bottom ashes p.12
 1.3.2 Fly ashes treatment p.14
 1.3.3 Fly ashes treatment p.15
 1.4 Melting solidification method p.17
 1.4.1 Slag p.20
 1.4.2 Uses of slag p.22
 1.5 Purpose p.23
 References p.25

Chapter 2 Structure of ternary alumino-silicate glasses p.27
 2.1 Introduction p.28
 2.2 Brief review of glass structure p.30
 2.2.1 Silica and binary silicate glass p.30
 2.2.2 Ternary alumino-silicate glasses p.32
 2.2.3 Existence of the triclusters in ternary alumino-silicate glasses p.33
 2.3 Stoichiometry in ternary alumino-silicate glasses p.34
 2.4 Discussion p.41
 2.5 Conclusion p.43
 References p.44

Chapter 3 Chloride solubility in CaO-Al2O3-SiO2 glasses systems p.45
 3.1 Introduction p.46
 3.2 Experimental procedure p.48
 3.2.1 Glass preparation p.48
 3.2.2 Property measurement p.49
 3.3 Results p.53
 3.4 Discussion p.63
 3.4.1 Introduction of chloride into CaO-Al2O3-SiO2 glasses p.63
 3.4.2 NBO in CaCl2-CaO-Al2O3-SiO2 glasses p.63
 3.4.3 Influence of NBO to chroride solubility p.65
 3.4.4 Triclusters in glasses of lower NBO/O region p.66
 3.5 Conclusion p.69
 Appendix A: Calculation of excess Al2O3 p.70
 References p.72

Chapter 4 Electrical conductivity of CaCl2-CaO-Al2O3-SiO2 glasses melts p.75
 4.1 Introduction p.76
 4.2 Review of electrical conductivity and activation energy p.77
 4.3 Measurement of electrical conductivity p.78
 4.4 Experimental procedure p.80
 4.4.1 Measuring cell characterisitics p.80
 4.4.2 Cell constant calibration p.82
 4.4.3 Glass preperation p.85
 4.4.4 Measurement of molten glass conductivity p.87
 4.5 Results p.92
 4.6 Discussion p.97
 4.7 Conclusion p.100
 References p.101
Chapter 5 Summary p.103
Publications p.107

Recently, municipal waste has become an important problem in many countries. Since incineration is the method employed to treat the municipal waste, incineration fly ash has become another environmental issue because of significant concentration of heavy metal and chloride in the fly ash. One of possible methods to deal with the incineration fly ash is to solidify it with bottom ash to be glassy slag. It has been reported that large amount of chloride in incineration fly ash can be retained in the slag by melting solidification method under reduced atmosphere conditions. The main components of the slag are SiO2, Al2O3 and CaO. However, only little reports are available on slag containing chloride. The aim of present work was to study slag-like SiO2-Al2O3-CaO glass system containing chloride and to discuss the glass properties based on structural basis.
Generally, ternary alumino-silicate glass system, SiO2-Al2O3-(MO,M2O) where M is alkali or alkali-earth metal species, basically consists of Si-O-Si and Si-O-Al linkages because of thermodynamic unfavorable of Al-O-Al linkage. The modifier cations are compensated with AlO2- tetrahedra. The excess cations will break the network continuity and compensate with non-bridging oxygen (NBO). However, in the ternary alumino-silicate glasses that contain relatively high content of Al2O3, the Al-O-Al linkage has been reported to exist, known as triclusters. The triclusters is defined as structure consists of three-coordinated oxygen associated with three tetrahedra. It was proposed that initiation of the triclusters may due to an excess of Al2O3, which results from the complex influences of [MO,M2O]/[Al2O3] ratio, [SiO2]/[Al2O3] ratio and NBO amount in the network. The existence of the triclusters conditions was concluded, stoichiometrically, as either [MO, M2O] <[Al2O3] or 2[SiO2] < 3[Al2O3] + [MO, M2O].
This study deals with SiO2-Al2O3-CaO glass system of various NBO contents, in which CaCl2 was substituted for CaO. The glass was melted under reduced atmosphere conditions at 1,450 ℃ for 2 hours and cast on an iron plate in open atmosphere. The glass properties such as composition, density and Tg were investigated by XRF, pycnometer and TMA, respectively. The electrical conductivity of glass melts was measured by 2-electrode method; the measuring cell was calibrated by KCl solution and obtained the cell constant approximately 1.12 cm.
It was found that chloride could be retained in the glass network around 90-100% of batch composition. Glass density and Tg were found to decrease, and thermal expansion coefficient and molar volume were found to increase with introduction of chloride into the glass network. It could be considered the more open glass network due to substitution of one oxygen ion by two larger chloride ions. It was found that chloride could be introduced into SiO2-Al2O3-CaO glass network in limited concentration, depended on NBOcl content in the glass network, [NBOcl] = 2{[CaO]-[Al2O3]-[CaCl2]}. The NBOcl is signified the NBO available for occupying the chloride ion, in which obtained by subtract the Cl-occupied NBO (O----Ca2+---Cl-) from the total NBO amount, [NBO] = 2{[CaO]-[Al2O3]}. The further introduction of chloride over such limitation will lead to CaCl2 salt layer separation and decrease of chloride content in the network. In some low NBOcl content glass compositions, there show higher-than-expected chloride solubility compared to their NBOcl content. It was accounted for by presence of the triclusters as conditions described above. The NBOcl content of those compositions was re-calculated presuming existence of the triclusters based on stoichiometric method. The re-calculated result shows better agreement between chloride solubility and NBOcl content. It is suggested that chloride solubility in this glass system could be determined by NBOcl content, and the higher-than-expected chloride solubility in low NBOcl content composition might be resulted from existence of the triclusters in the network.
The conductivity of the molten glass was found to increase with increasing of chloride content. The Arrhennius-type relation was used to fit the relation between conductivity and temperature. Average activation energy for conduction was found to slightly decrease with introduction of chloride. It was also found that the conductivity of molten Cl-free base glasses is clearly higher in glass, which contains higher NBO content. In case of molten glass containing chloride, although NBO content is decrease, conductivity is increase with increase of chloride content. It is implied that the increase of conductivity should result from increase of charge carrier concentration. It is suggested that charge carriers in molten glasses containing chloride are both of Ca2+ and Cl- ions.
From these results, it could be concluded that NBO plays an important role to determine glass properties and also influences the existence of the triclusters by proposed conditions based on stoichiometric basis. Chloride ion in this glass system is proposed to act as charge carrier at least in the melt state. It is also postulated that chloride ions in this glass network might occupy the NBO site in a bonded state with Ca2+ ions as O----Ca2+---Cl-.

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