氫氟酸（HF）用於IC，TFT-LCD和太陽能電池製造工藝中，以蝕刻玻璃面板和矽晶片。在台灣每月氫氟酸之用量達到2000 噸（〜49％）。化學沉澱法通常用於通過添加含鈣化學物質(例如CaCl2,Ca(OH)2和CaCO3形成CaF2(S）來去除氟化物。CaF2(S）可用來作冶金工業的材料達到循環再利用。但是，由於氟化鈣顆粒粒徑極小（約0.16 µm）而難以沉澱。而在沉澱過程中需添加聚合物和PAC，使微小的顆粒變重，以增進沉降速率。因此，由於CaF2(S）之高水分含量和低純度，CaF2(S）污泥的再利用受到了限制。
    此研究中設計氣升式結晶反應槽並添加了顆粒作為擔體，利用形成冰晶石（Na3AlF6）達到除去氟的目的。該系統的pH控制在5至6之間，並且通過將Al / F摩爾比的值保持在0.8/6 至1.1/6 的範圍內來添加適量的鈉和氫氧化鋁。結果表明，氣舉式反應槽可去除70％以上的氟，而且形成的冰晶石尺寸在運行5天後，從反應器中取出的固體顆粒的平均尺寸超過1公釐。

Hydrofluoric acid (HF) is used in IC, TFT-LCD, and solar cell manufacturing processes for etching glass panel and silicon wafer. In Taiwan, 2000 m3 of HF (~49%) is used each month. Chemical precipitation method is usually used to remove fluoride by forming CaF2(S) via adding calcium sources, such as CaCl2, Ca(OH)2, and CaCO3. CaF2(S) can be used as material in metallurgical industry. However, the particle of calcium fluoride is hard to precipitate due to the small particle size (ca., 0.16 µm). Polymer and PAC are added in the precipitate process to make tiny particle became heavy and then settle down quickly. So, the CaF2(S) sludge reuse was limited because of high moisture content and low concentration of CaF2(S). 
An air-lift crystallization reactor with pellet added was designed and operated for the removal of fluoride by forming cryolite (Na3AlF6). The system was operated with pH controlled between 5 and 6, and a suitable amount of sodium and aluminum hydroxide were added by maintaining the values of Al/F molar ratio in the range of 0.8/6 to 1.1/6. The result shows that the twin-pipe air-lift circulation reactor could remove more than 70% of fluoride, and the size of cryolite formed is quite big. The average size reaches more than 1 mm for the sample taken from the reactor after a 5-day of operation.

Chapter 1 Introduction	1
Chapter 2 Literature review	5
2.1 Problems associated to current practice for fluoride-containing wastewater treatment	5
2.2 Fluoride removal methods	6
2.2.1 Chemical precipitation	6
2.2.2 Electrocoagulation Process	8
2.2.3 Adsorption	10
2.2.4 Crystallization process for fluoride removal	12
Chapter 3 Materials and methods	14
3.1 Chemicals and materials	14
3.2 Experimental setup and methods	15
3.3 Analytical method	21
3.3.1 Ion chromatography	21
3.3.2 X-ray Diffraction	22
3.3.3 Particle size analysis	22
3.3.4 Turbidity analysis	23
Chapter 4 Results and discussion	24
4.1 Effect of Al/F molar ratio on fluoride removal	24
4.2 Turbidity monitoring as an operational parameter for various Al/F molar ratio	31
4.3 Solid analysis	33
4.3.1 SEM/EDX analysis	33
4.3.2 XRD analysis	36
4.3.3 Particle size analysis	38
4.4 Efficiency of residual fluoride adsorption by hydroxyapatite	40
Chapter 5 Conclusion and suggestions	42
5.1 Conclusions	42
5.2 Recommendations	44

Figure 1. Effects of pH and Al/F molar ratio on the formation of aluminum-containing solid through chemical equilibrium analysis using Mineql+. Conditions for modeling: temperature of 25℃, and fixed concentrations of F- and Na+ at 0.18 and 0.09 mol L-1, respectively [8].	13
Figure 2. Schematic for the experimental setup of Single-pipe circulated crystallization reactor	16
Figure 3. Schematic for the experimental setup of Twin-pipe air-lift circulated reactor integrated with membrane	18
        Figure 4. Schematic for the experimental setup of backwash system	19
        Figure 5. Twin-pipe air-lift reactor integrated with 45° pipe	20
        Figure 6. Standard curve for F analysis using IC.	21
Figure 7. The fluoride removal efficiency of Single-pipe circulation crystallization reactor in different molar ratio and seed-adding. Fixed pH of 0.8/6 to 1/6. HRT= 120 min.	26
Figure 8. The fluoride removal efficiency of Twin-pipe air-lift circulated reactor integrated with membrane under pH value of 5.5 and the Al/F molar ratio of 1/6. HRT= 120 min.	28
        Figure 9. The membrane pressure data of MF-TACR	28
Figure 10.The fluoride removal efficiency of twin-pipe air-lift circulated reactor integrate 45° pipe under pH value of 5.5 and the Al/F molar ratio of 0.8/6, 1.0/6, and 1.1/6, respectively	30
Figure 11. Turbidity result of effluent treated at various Al/F molar ratio. F concentration= 500 mg F/L; Al/F molar ratio= 0.8/6 and 1.1/6, respectively, pH at the range of 5.5 to 6.0	32
Figure 12.SEM figures of cryolite produced with pH 5.5 at Al: F molar ratio of 1:6 by single-pipe circulated crystallization reactor. Initial F concentration= 500 mg/L. (A) Al/F molar ratio of 1.0/6 (B) Commercial cryolite	34
Figure 13.SEM images of cryolite produced with pH 5.5 by. Twin-pipe air-lift reactor integrate 45° pipe. Initial F concentration= 500 mg/L. (A) Al: F molar ratio of 0.8:6 (B) Al:F molar ratio of 1:6 (C) Al:F molar ratio of 1.1:6	35
Figure 14. XRD analysis of solid produced by single-pipe air-lift reactor at solution pH value of 5.5-6.0. Initial F concentration= 500 mg/L. Molar ratio of Na/Al/F= 3/1/6.	37
Figure 15. XRD analysis of solid produced by twin-pipe air-lift reactor at solution pH value of 5.5-6.0. Initial F concentration= 500 mg/L.	37
Figure 16. Particle size distribution of cryolite produced at various molar ratio, F concentration, and system	39
Figure 17. Particle size distribution of cryolite produced at various Al/F molar ratio by using TACR integrate 45° pipe. F concentration = 500 mg/L, pH = 5.5-6.0	39
Figure 18. Adsorption of fluoride by adding hydroxyapatite. F concentration= 150 mg/L; hydroxyapatite: Fluoride= 1: 1	
41
Table.1 Chemical material…………………………………………………….13
Table. 2 The atomic percentage (At%) of various elements at Al/F molar ratio of 1/6……………………………………………………………………………..33
	Table.3 The atomic percentage (At %) of various elements at various Al: F molar ratio at pH=5.5 by OP-TACR……………………………………….………...34
Table.4 Reactors comparison………………………………………………….42

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