半導體業之氟系廢水處理仍以添加鈣鹽，產生氟化鈣(CaF2)為主，處理過程中會產生大量高含水率的污泥。雖然上述傳統處理流程可將氟離子濃度降至法定排放標準(<15 mg/L)，但目前台灣面臨現有的掩埋空間明顯不足，導致污泥廢棄置成本大幅提高，因此亟需思考其他可行的處理方式。 本研究從資源再利用的角度，以結晶方式，利用不同鋁鹽提供合成無機化合物冰晶石（Na3AlF6）所需的鋁離子，與含氟廢液混合後，希望可有效去除水中氟離子以及達到含氟廢液回收再利用之目的。實驗中藉由瓶杯(Jar-Test)試驗方式，探討溶液中氟離子初始濃度、Al/F及Na/F莫耳比、pH值、溫度、反應時間及陰離子濃度，對氟離子去除效率影響。 實驗結果顯示，於適當的pH範圍下，固定Al/F莫耳比進行反應可有效合成冰晶石，其氟離子回收效率高達89%。如果鋁離子添加過量，容易導致水中多餘Al3+與OH-產生凝膠狀的氫氧化鋁，影響生成冰晶石純度的問題。鋁離子在水中對其它陰離子吸引順序為F->SO42->>Cl->NO3-，因為不同的鋁鹽解離於溶液中的陰離子不同，導致去除氟離子之效率也不同。 以實廠氫氟酸廢水進行冰晶石結晶實驗，發現分批次添加鋁鹽可以更有效合成冰晶石及回收廢液中的氟離子，其固體產物經X射線繞射儀分析後，含有冰晶石(Na3AlF6)、錐冰晶石(Na5Al3F14)、氟矽酸鈉(Na2SiF6)。在調整pH值時，須注意溶液內會產生放熱反應，添加液鹼時須緩慢加入，避免產生氫氟酸氣體及氟離子濃度減少等問題。 Changing fluoride solubility in solution with addition of calcium salt to precipitate calcium fluoride solid is the most common practice to treat fluoride wastewater generated from semiconductor industry. As the result, large volume of sludge with high water content was produced. Although the above-mentioned traditional treatment process is capable of lowering fluoride concentration below the regulatory discharge limit of 15 mg/L in Taiwan, alternative treatment process is needed due to substantial increase in sludge-handling cost associated with limited landfill space. In this study, various aluminum salts were used as aluminum source for the synthesis of cryolite by crystallization in the hope of removing, recycling, and reusing fluoride from fluoride-containing wastewater. Several process parameters such as initial concentration of fluoride, Al/F and Na/F molar ratios, pH, temperature, reaction time and anionic concentration which might affect fluoride removal efficiency were investigated in Jar-Test experiments. The results showed that synthetic cryolite was formed effectively within appropriate Al/F molar ratio and pH ranges and fluoride recovery efficiency of up to 89% could be achieved. Excessive addition of aluminum ions with Al/F ratios of higher than 1/6 leads to formation of gelatinous aluminum hydroxide which will affect the purity of synthetic cryolite. Addition of different aluminum salts achieved different fluoride removal efficiency due to the attraction of aluminum ions with various anions which is in the order of F- > SO42- >> Cl- > NO3-. Recovery of fluoride by cryolite crystallization was conducted with wasted hydrofluoric acid obtained from an optoelectronic manufacturer. Aluminum salt added stepwisely was more effective than addition of aluminum salt at once for producing synthetic cryolite and removing fluoride. The precipitates solids contain cryolite (Na3AlF6), chiolite (Na5Al3F14), fluorine sodium silicate (Na2SiF6) confirmed by XRD analysis. Cares should be exercised when adding caustic soda to adjust pH of wasted hydrofluoric acid. Heat will be generated due to exothermic nature of the reaction.
