- 學位論文
以結晶軟化法前處理純水再生廢水中二氧化矽之研究－以某光電廠為例
Softening pretreatment for waste stream resulted from ultrapure water generation with silica －a case study for TFT-LCD industry
摘要
本研究之光電廠其純水製造程序所產生之純水再生廢水特性主要含高濃度的鈣、鎂離子及二氧化矽，易於再生廢水的回收處理程序中之RO膜形成結垢，對薄膜造成不可逆之破壞，故需在進膜前先行前處理將其去除。 本研究基於廢水水質特性及回收再利用水質條件，透過軟化法使鈣、鎂離子形成結晶，並藉由沉澱吸附作用與化學反應形成矽酸鹽之沉澱去除水中二氧化矽，共分五部份討論：1.探討二氧化矽去除率VS.薄膜回收率VS.二氧化矽飽和度之關係，並評估薄膜表面濃度極化影響，進而提供實廠控制進入蒸發罐之水量；2.探討應用軟化程序以pH調整實廠水樣，對二氧化矽去除成效之影響；3.比較實廠水樣濁度提升對於沉澱物沉降速率之影響；4.研析實廠水樣中鈣、鎂離子結晶之沉澱物沉澱對二氧化矽吸附之影響；5.應用表面分析儀分析反應後生成之沉澱物種及其表面元素之組成。 研究結果顯示，當SiO2前處理去除率操作在90 %以上時，薄膜回收率可達到90 %，且不致使薄膜阻塞產生結垢；當濃度極化形成時，SiO2前處理去除率則需操作在95 %以上，才可避免薄膜結垢發生。此外，SiO2去除率隨pH上升而增加，在pH > 9時的去除率會大幅提升，而當pH > 11時，其去除率可達97 %以上。添加Al2O3提升濁度能對水中顆粒之沉降速率產生一定之貢獻，其最適添加量在100 mg至200 mg能大幅提升沉降速率。Ca2+、Mg2+產生之沉澱量與SiO2去除效果成正比；每15 mg的Mg(OH)2可去除1 mg的SiO2和63 mg的CaCO3可去除1 mg的SiO2，故水中Mg2+在去除二氧化矽之貢獻度優於Ca2+。 透過XRD對沉澱物表面進行分析，可知水中溶解性SiO2之去除機制發生在低pH值(pH=8)時，是以CaCO3和Mg(OH)2吸附SiO2於固體表面共沉澱而去除；在高pH值(pH>10)時，游離矽酸與Ca2+、Mg2+反應合成固態矽酸鈣(Ca2SiO4)及矽酸鎂(Mg2SiO4)沉澱，進而降低水中SiO2濃度。
並列摘要
The wastewater generated from ultrapure water generation process of the TFT-LCD industry factory includes high calcium, magnesium and silica content, which may result in an irretrievable damage on membranes fouling and pretreatment of these impurities are necessary. In this paper, wastewater was tested in standard jar test using softening for removal of silica to discuss in the following five steps：(1) Development of a removal-saturation-recovery curve to determine the threshold limits of RO recovery and silica removal; (2) Determining the effect of silica removal by softening pretreatment; (3) Effect of turbidity on settling velocity of precipitant and silica removal; (4) Effects of calcium and magnesium crystallization on silica adsorption, and (5) Analyzing precipitating species components and verifying silica removal mechanism by XRD (X-Ray Diffraction). Threshold limit for the RO recovery and required silica removal were firstly determined by the removal-saturation-recovery curve. The result shows, in order to achieve the 90 % recovery, the targeted SiO2 content before entering RO membrane was set up at 90 % removal of silica for the influent to prevent silica scaling. Different pH was used to test the silica removal efficiency as well as the impact of calcium and magnesium. Also the result shows, higher pH was more effective for silica removal, especially when pH > 9. When pH was adjusted to 11, silica removal rate was raised to above 95 %. Adding Al2O3 to increase the turbidity of the water enhanced the particles settling velocity. The presence of magnesium was more effective than the presence of calcium for silica removal process. At low pH (pH=8), silica removal was attributed to co-precipitation/adsorption by CaCO3 and Mg(OH)2; when pH was higher than 10, silica could also be removed by precipitation as Mg2SiO4(s) and Ca2SiO4(s), which was verified by statistical analysis and XRD.