利用廢鋁和電化學程序從工業廢水中去除氟化物並回收冰晶石

Innovative Methodology for Fluoride Removal and Recovery through Cryolite Production from Industrial Wastewater: Integrating Scrap Aluminum and Electrochemical Processes

黎安(Nguyen Le)

指導教授 ： 李奇旺

淡江大學/工學院/水資源及環境工程學系博士班/博士(2025年)

https://doi.org/10.6846/tku202500037

摘要

半導體產業的快速發展導致含氟廢水問題日益嚴重，對環境和健康造成重大威脅。為解決氟污染問題，已開發出多種處理技術，包括混凝、電凝、離子交換、吸附、流化床結晶和膜分離等。每種技術在成本、操作效率、環境永續性及排放標準等方面皆有其優缺點。隨著產業全球化，對有效去除氟離子技術的需求日益迫切，特別是能夠實現資源回收再利用、符合循環經濟原則的技術，例如生產具有經濟價值的副產品冰晶石。本研究旨在通過平衡經濟效益和高效氟處理，以鋁金屬為原料生產冰晶石。研究採用批次和連續式系統進行化學和電化學實驗，探討影響鋁溶解和冰晶石形成的參數。

本研究提出了一種創新的連續式程序，利用廢鋁填充柱，有效去除氟離子並回收製成冰晶石。鋁在氫氟酸中的溶解反應可產生鹼度和氫氣，提升了程序的經濟效益。實驗結果顯示，在低濃度氫氟酸（500 mg/L）和短暫接觸時間（15分鐘）下，當鋁氟莫耳比為2:6時，氟離子去除率可達98\%。溶液的pH值對鋁氟比有顯著影響，pH值低於4時，鋁氟比可大於1。反應為放熱反應，且與氫氟酸濃度高度相關（R2=0.99），因此需控制反應溫度。實驗結果證實，在最佳條件下（分流比1:1，pH 5-5.5），可成功合成冰晶石，氟離子去除率達98%。X射線繞射和掃描式電子顯微鏡-能量分散式X射線光譜分析證實了冰晶石的生成。此方法為工業氟廢水處理、資源回收和清潔氫能生產提供了一種有前景的解決方案。

控制pH值於電化學程序的操作是至關重要的，而pH值是優化鋁溶解及提升氟離子去除效率的關鍵因素。維持pH值在4.0以下可促進較高的鋁氟莫耳比，此為有效去除氟離子所必需。酸性環境可穩定反應環境，並提升鋁在氫氟酸中的溶解度，促進氟離子的去除。本研究探討了利用廢鋁和鋁板在不同條件下進行電化學溶解及結晶程序以去除氟離子並生產冰晶石。本研究專注於探討電流密度對於電化學及電化學-化學聯合處理系統處理工業廢水中鋁溶解及氟離子去除效率的影響。在處理初始氟離子濃度為5000 mg/L之廢水的連續系統中，於電流密度3 A下，70.53分鐘後可達到1:6之最佳鋁氟莫耳比。透過結合電化學與化學溶解程序，在相同電流條件下，可進一步提高莫耳比至最高2.15:6。值得注意的是，氟離子去除效率超過98\%，且在六小時的操作時間內，去除率保持穩定。以含氟離子濃度為1000 mg/L之廢水進行批次實驗結果顯示，電流密度為1 A時，55分鐘後可達到94%之最高氟離子去除效率，而較高的電流密度則因鋁金屬氧化物塗層的形成而阻礙反應動力學，導致去除效率降低。在整個試驗過程中，電壓測量顯示極少發生鈍化現象。能量消耗與電流密度呈現線性關係，突顯了電流密度最佳化對於能源效率的重要性。本研究強調了電流密度與程序設計對於透過電化學結晶法提升氟離子去除效率的重要性。值得注意的是，以廢鋁於初始pH值為3.2下操作，相較於鋁板，可獲得較高的溶解速率，溶解效率可達174% ±8.6%。此外，初始溶液的pH值顯著影響氟離子去除效率，當pH值維持在3.3至3.6之間時，可達到最高84\%的去除效率。對pH值設定點的分析顯示，維持固定的初始pH值3.2可於不同初始氟離子濃度下最佳化氟離子去除效率。添加鈉，其鈉氟莫耳比為3:1，可透過提升pH值穩定性將氟離子去除效率提高至90%，顯示了pH值穩定性在此電化學程序中扮演著關鍵角色。

關鍵字

持續性氟化物去除 ； 廢鋁增值利用 ； 冰晶石回收 ； 氫氣共同生產 ； 放熱過程設計 ； 電化學過程

並列摘要

Rapid technological advancements in the semiconductor industry significantly contribute to wastewater generation, particularly containing high levels of fluoride, which poses serious environmental and public health risks. Various treatment technologies have been developed to address fluoride contamination, including coagulation, electrocoagulation, ion exchange resin, adsorption, fluidized bed crystallization, and membrane-based methods. Each technique presents unique advantages and challenges related to cost, operational efficiency, environmental sustainability, and compliance with discharge standards. As industries expand globally, the need for effective fluoride removal technologies becomes increasingly critical, especially those that align with the principles of circular economy by enabling resource recovery and reuse, such as the production of economically valuable by-products, such as cryolite. This comprehensive approach not only aims to mitigate fluoride pollution, but also promotes sustainable industrial practices. In order to balance the financial advantages and efficient fluorine treatment for purity, aluminum, a crucial component of cryolite, needs to be carefully chosen as a raw material. Through chemical and electrochemical process studies in batch and continuous systems, this study investigates the parameters that influence the dissolution of aluminum and cryolite formation using aluminum scrap and aluminum plate as sources.
Using a scrap aluminum-packed column, this study introduces a unique continuous technique for fluoride removal and resource recovery as cryolite. The effective dissolution of aluminum in hydrofluoric acid (HF) produces cost-effective alkalinity and hydrogen gas. At low HF concentrations (500 mg/L) and short empty bed contact times (15 minutes), the procedure achieves high efficiency (Al/F molar ratio of 2/6). The pH of the solution is important; for pH values below 4, Al/F ratios greater than 1 can be achieved. With a strong correlation (R2=0.99) between HF concentration and heat generation, the exothermic process requires temperature control. The generation of hydrogen is consistent with the theory. A 1:1 bypass ratio and adjusted pH (5-5.5) were used to test cryolite synthesis, which resulted in 98% fluoride elimination. The production of cryolite was confirmed by XRD and SEM-EDS analyses. This approach offers a promising solution for industrial fluoride management, resource recovery, and clean hydrogen production.

Electrochemical processes are essential for controlling pH, which is critical for optimizing aluminum dissolution and enhancing fluoride removal efficiency. Maintaining a pH below 4 facilitates higher Al:F molar ratios necessary for effective fluoride treatment, stabilizing the reaction environment and maximizing aluminum solubility in HF while promoting efficient fluoride ion interactions. This study examines the electrochemical dissolution and crystallization processes for the removal of fluorides and the production of cryolite using scrap aluminum and plate aluminum under varying conditions. The purpose of this study is to investigate specifically the impact of current intensity on aluminum dissolution and fluoride removal efficiency in both electrochemical and combined electrochemical-chemical processes for the treatment of industrial wastewater. In a continuous system treating wastewater with a fluoride concentration of 5000 mg/L, an optimal Al:F molar ratio of 1:6 was achieved at a current intensity of 3A after 70.53 minutes, while a combined electrochemical and chemical dissolution process reached a maximum ratio of 2.15:6 under the same current conditions. The fluoride removal efficiency of the process was notably high, achieving 98% and maintaining stable performance for six hours. Batch experiments carried out with wastewater containing 1000 mg/L fluoride demonstrated that a current intensity of 1A yielded the highest fluoride removal efficiency of 94\% after 55 minutes, while higher currents resulted in decreased effectiveness due to the formation of aluminum coatings that hindered reaction kinetics. Voltage measurements throughout the trials exhibited minimal passivation effects, and a linear correlation between energy consumption and current intensity emphasized the necessity for optimization. Overall, this research underscores the significant role of current intensity and process design in enhancing fluoride removal efficiency through electrochemical crystallization methods. Notably, operating at an initial pH of 3.2 with scrap aluminum resulted in superior dissolution rates compared to aluminum plates, with a dissolution efficiency reaching 174%±8.6%. Furthermore, the initial pH solution markedly influenced fluoride removal, achieving up to 84% efficiency when maintained between pH 3.3 and 3.6. The analysis of pH set points revealed that maintaining a fixed initial pH of 3.2 optimized fluoride removal across varying initial fluoride concentrations, while sodium addition with a Na:F ratio of 3:1 improved fluoride removal to 90% through enhanced pH stability, demonstrating the importance of pH stabilization in this electrochemical process.

並列關鍵字

Continuous Fluoride Removal ； Scrap Aluminum Valorization ； Cryolite Recovery ； Hydrogen Co-Production ； Exothermic Process ； Electrochemical Processes

參考文獻

[1] M. F. Chang and J. C. Liu, (2007) “Precipitation removal of fluoride from semi- conductor wastewater” Journal of Environmental Engineering 133 : 419–

425. DOI: 10.1061/(ASCE)0733-9372(2007)133:4(419).

[2] Y. Qiu, L.-F. Ren, L. Xia, J. Shao, Y. Zhao, and B. Van der Bruggen, (2022) “In- vestigation of fluoride and silica removal from semiconductor wastewaters with a clean coagulation-ultrafiltration process” Chemical Engineering Journal 438 : 135562. DOI: https://doi.org/10.1016/j.cej.2022.135562.

[3] X. Cao, J. Peng, W. Dong, Y. Li, Y. Wang, and Y. Di, (2024) “Review on the Preparation of Cryolite from Industry Waste Containing Fluorine” Journal of Sustainable Metallurgy 10 : 419–434. DOI: 10.1007/s40831-024-00835-0.

[4] S. J. Ouedraogo, Y. Vinh, P. ( Noophan, V. Naddeo, and C.-W. Li, (2023) “Dissolution of aluminum hydroxide to provide Al and to neutralize acidity for the removal and recovery of fluoride through cryolite crystallization” Journal of Cleaner Production 404 : 136987. DOI: 10.1016/J.JCLEPRO.2023.136987.