本研究探討利用鋁犧牲陽極於含氟廢水中產生冰晶石的新穎結晶方法去除氟化物並將其回收。藉由形成冰晶石而不是氟化鈣去除氟化物，不但能產生更具有價值的產物，也能減少污泥的體積。實驗中探討使用不同的鈉源、pH值、Al / F莫耳比、初始氟濃度和電流密度等參數對新穎結晶方法去除氟化物的影響。實驗結果顯示，在控制pH於最佳的條件下，Al / F莫耳比小於1/6時能有效產出冰晶石，與化學平衡模擬預測的結果一致。
針對各參數對於去除氟的影響，可得以下結果:利用NaCl / NaHCO3莫耳比為1：1的混合電解質時，除可以消除電極塗層問題，也可以保持pH的穩定；當pH 值為5和5.5，以Al / F> 1/6的比率進行實驗，氟的去除率會降低。此乃因為AlFn3-n物質的形成，但過多的Al所產生的Al（OH）3有助於吸附的方式去除氟；當初始濃度為75和150 mM時，會根據形成冰晶石的化學計量去產生冰晶石，且氟的去除率可達到97％；當氟之初始濃度低時，會形成氟化鋁複合物，並導致氟的去除率降低；於電流密度之實驗顯示，氟化物去除效率與電流密度無關。最後分析化學結晶與電結晶之操作成本，發現化學結晶成本低於電結晶方法，而主要的花費為能源的消耗。

Removal of fluoride through the formation of cryolite rather than the formation of calcium fluoride has the benefit of generating valuable products and reducing sludge volume. In this study, a novel electro-crystallization process using aluminum as a sacrificial anode was developed to recover fluoride as cryolite from F-containing wastewater. The effects of sodium source, solution pH, Al/F molar ratio, initial F concentration, and current density were investigated. The experimental results revealed that the cryolite was successfully produced under well-controlled pH conditions and Al/F molar ratio of less than 1/6. The results consistent with the prediction by the chemical equilibrium modeling.
The mixed electrolyte of NaCl and NaHCO3 with the molar ratio of 1:1 could eliminate electrode-coating problem and maintain the stable pH condition. The
removal of F decreased at the ratio of Al/F > 1/6 for pH 5 and 5.5 conditions due to the formation of AlFn3-n species. Adsorption of F onto Al(OH)3(s) contributed the removal. The formation of cryolite at the initial concentration of 75 and 150 mM followed well with the stoichiometry for cryolite formation and the F removal reached 97%. At the low fluoride initial concentrations, the removal of fluoride is low due to the formation of aluminum fluoride species. The fluoride removal efficiency was independent of the current density. The operation cost of chemical-crystallization process was less than that of electro-crystallization process. Energy consumption was the major cost for the electro-crystallization process.

Table of content
List of Figure	III
List of Table	VII
Chapter 1    Introduction	1
Chapter 2    Literature review	4
2.1	Sources of fluoride wastewater and recovery issue	4
2.2	Fluoride removal method	5
2.2.1	Chemical precipitation	6
2.2.2	Fluidized bed crystallization reactor for fluoride removal with sand pellets……	10
2.2.3	Chemical crystallization for cryolite formation without sand pellets (homogeneous crystallization)	12
2.2.4	Electrocoagulation Process for F removal	15
2.2.5	Chemical equilibrium modeling	16
Chapter 3    Materials and methods	18
3.1	Chemicals and materials	18
3.2	Experimental setup and methods	19
3.3	Analytical method	22
Chapter 4    Results and discussion	25
4.1	Effect of electrolyte	25
4.2	Effect of pH	29
4.3	The effect of initial fluoride concentration on fluoride removal	36
4.4	Effects of Current density	41
4.5	The solid analysis	46
4.5.1	The XRD analysis	46
4.5.2	The SEM/EDX analysis	48
4.5.3	The XPS analysis	52
4.5.4	Particle size analysis	53
4.6	Operation cost	54
Chapter 5    Conclusion and suggestions	58
5.1	Conclusions	58
5.2	Recommendations	59
Reference	60

 
List of Figure
Figure 1. Chemical speciation of fluoride in the solution modeled by Mineql+. Modeling conditions: Temperature of 25 °C; Fixed concentrations of F− and Ca2+ at 10 mM and 5 mM, respectively	7
Figure 2. The two-step treatment process from [22]	9
Figure 3. New Fluorine Treatment Process [22]	10
Figure 4. Crystallization phase diagram Adapted from [26].	11
Figure 5. 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 °C, and fixed concentrations of F− and Na+ at 0.18 and 0.09 mol L−1, respectively. Copied from ref. [7].	13
Figure 6. Fluoride removal efficiency as function of Na/F in the solution modeled by Mineql+. Conditions for modeling: Temperature = 25 °C, pH = 4, and fixed Al/F = 1/6. Initial F concentration = 0.225 M.	14
Figure 7. Log C vs. pH for aluminum species modeled by Mineql+. Total Al concentration = 0.002 M.	15
Figure 8. The percentage of F in cryolite as function of Na/F and Al/F molar ratios modeled using Mineql+ (A) pH 5 (B) pH 5.5 (C) pH 6 (D) pH 6.5	17
Figure 9. Schematic for the experimental setup	20
Figure 10. Experimental parameters and methods	22
Figure 11. Standard curve for F analysis using IC.	23
Figure 12. (A) The voltage profile as a function of reaction time for various electrolytes. (B) The fluoride removal efficiency as a function of the theoretic Al/F molar ratio. Experiment condition: initial F concentration = 150 mM; pH of 5.5; Electric current intensity = 1.1 A; Current density = 27.16 mA/cm2; NaHCO3 system: Al electrodes for both anode and cathode; NaCl and NaCl+NaHCO3 systems: Al plate was used in anode only.	28
Figure 13. The photos of the virgin and used electrodes.	29
Figure 14. Effect of Al/F molar ratio on fluoride removal under various pH values. Experimental condition: F concentration = 150 mM; electrolytes NaCl/NaHCO3 ratio = 1:1; current density = 27.16 mA/cm2. Mechanic mixing = 100 rpm. Mineql+ was used for modeling the corresponding systems.	31
Figure 15. Effect of Al/F molar ratio on fluoride removal under various pH values. Experimental condition: F concentration = 150 mM; electrolytes NaCl concentration = 75 mM; current density = 27.16 mA/cm2 mechanic mixing = 100 rpm. Mineql+ was used for modeling the corresponding systems.	32
Figure 16. Al species, % of total concentration and F species, % of total concentration under various pH values by Mineql+, condition: F concentration=150 mM, Na/F mole ratio=9/6	35
Figure 17. Adsorption of AlFx species by pre-formed Al(OH)3 flocs. Experiment condition: Al/F molar ratio=1/6, pH=5.	36
Figure 18. Effect of initial F concentration the fluoride removal efficiency as a function of reaction time at pH of 6. Experiment condition: electrolyte F concentration = 25 to150 mM; electrolytes NaCl+NaHCO3 molar ratio = 1:1; current density = 27.16 mA/cm2 mechanic mixing = 100 rpm.	39
Figure 19. Effect of initial F concentration the fluoride removal efficiency as a function of Al/F molar ratio for fixed pH of 6 from experimental and Mineql+ modeling results. Experiment condition: electrolyte F concentration = 25 to 150 mM; electrolytes NaCl+NaHCO3 molar ratio = 1:1; current density = 27.16 mA/cm2; mechanic mixing = 100 rpm.	40
Figure 20. Percent of Al species as function of Al/F molar ratio at pH of 6.	40
Figure 21. Different current density (A) as function of time (B) as function of Al/F molar ratio, pH 5.5. Experiment condition: F concentration = 75 mM, electrolytes NaCl+NaHCO3=1:1 concentration = 37.5 mM, mechanic mixing = 100 rpm.	43
Figure 22. (A) Energy consumption vs. current density. (B) voltage as function of time. Experiment condition: fixed Al/F molar, F concentration = 75 mM, electrolytes NaCl+NaHCO3=1:1 concentration = 37.5 mM, mechanic mixing = 100 rpm.	44
Figure 23. The effect of the different current density on the electrode surface. (A) 6.17 mA/cm2, (B) 12.34 mA/cm2, (C) 15.51 mA/cm2, and (D) 27.16 mA/cm2.	45
Figure 24. XRD analysis of solid produced at various solution pH values of (A) 5, (B) 5.5, and (C) 6. Initial F concentration = 150 mM. Electrolytes: Electrolytes concentration: NaHCO3 = 37.5 mM and NaCl = 37.5 mM, Al/F= 1/6.	46
Figure 25. XRD analysis of solid produced at different Al/F ratio. Initial F concentration = 150 mM; Electrolytes concentration: pH = 6; NaHCO3 = 37.5 mM and NaCl = 37.5 mM (A) Al/F= 0.5/6, (B) Al/F= 1/6, (C) Al/F= 2/6, and (D) Al/F= 3/6	47
Figure 26. SEM images of precipitates produced with various pH values at Al:F molar ratio of 1:6 for experiments with combined electrolye. Initial F concentration = 150 mM. Electrolytes: NaHCO3 = 37.5 mM. NaCl = 37.5 mM. (A) pH 5 (B) pH 5.5 (C) pH6 (D) Commercial cryolite	50
Figure 27. SEM images of precipitates produced with various pH at Al:F molar ratio of 1:6 for NaCl electrolye. Initial F concentration = 150 mM. Electrolytes: NaCl =75 mM. (A) pH 5 (B) pH 5.5 (C) pH 6 (D) Commercial cryolite	51
Figure 28. SEM images of precipitates produced at different Al:F molar ratios. Initial F concentration = 150 mM. Electrolytes NaCl/NaHCO3 ratio = 1:1. (A) 0.5/6 (B) 1/6 (C) 2/6 (D) 3/6 (E) Commercial cryolite	52
Figure 29. XPS analysis of solid colleted at different conditions. Initial F concentration = 150 mM. Electrolytes NaCl/NaHCO3 ratio = 1:1, (A) Al/F=1/6; pH 5.5 (B) Al/F=3/6; pH 5.5 (C) Al/F=1/6; pH 6 (D) Al/F=3/6; pH 6 (E) Commercial cryolite	53
Figure 30. Particle size distribution of cryolite produced at various reaction cycles. F concentration = 150 mM; Electrolytes NaCl/NaHCO3 ratio = 1:1, pH 5.5	54
Figure 31. Total operation costs for electro-crystallization processes with various Na salts and chemical crystallization process.	57
 
List of Table
Table 1. The specifications of the man-made fluorite required for low-temperature production of aluminum and its alloys in Dragon Steel Corporation (information obtained from personal communication)	5
Table 2. Chemical materials	18
Table 3. The atomic percentage of various elements at different pH.	50
Table 4 The atomic percentage of various elements at different pH.	51
Table 5. The atomic percentage of various elements at different Al/F molar ratio	52
Table 6. Operation costs (USD/mole F) for electro-crystallization process and chemical crystallization	56
Table 7. The conductivity of different Na salts system	57

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