以UF配合鎂鹽前處理移除CMP廢水中矽酸之研究

Abstract

現今高科技晶圓產業中，化學機械研磨(CMP)製程所產出的廢水其主要成分為二氧化矽，其中溶解性矽(亦稱矽酸)之濃度約為70 ~ 95 mg/L。利用薄膜處理系統中含有矽酸的原水，矽酸濃度過高時將析出沉積在薄膜表面，造成無機性結垢，導致薄膜通量下降、壓力增加、薄膜壽命降低等問題。一般RO處理系統中可接受低矽酸濃度的進流水，但較高的矽酸濃度會促使其聚合作用，導致矽酸沉積在薄膜表面，而可能會有結垢的問題存在，所以必須有適當的前處理將水中矽酸去除，以解決薄膜結垢問題。 本研究以氧化層CMP廢水為主要處理對象，利用氧化層CMP研磨液配製人工原水，利用瓶杯試驗決定出最適操作條件後，進行混凝前處理並直接進流薄膜處理，並利用程式模擬出RO的回收分配比率。實驗結果發現，鎂鹽對矽酸的移除效果比PACl及Alum有較佳的效果，尤其是在高pH下(pH 10 ~ 11)更具有明顯的去除效果，因鎂鹽在高pH值下會形成帶正電荷之Mg(OH)2固體物，此時與水中帶負電的矽酸離子相互吸引並行脫水聚合作用後，藉共沈降機制從水中予以去除。隨著鎂鹽劑量的增加，矽酸的去除也有增加的比率，且當前處理對矽酸的移除效果越好，則經薄膜過濾時越不易在薄膜上發生結垢問題，但對於直接進流薄膜過濾的通量有下降的情形發生。另外以鎂鹽配合高純度聚氯化鋁(EPACl)來混沉處理，可將水中顆粒放大，且對於矽酸的去除也有其幫助，但對於Cross-flow薄膜過濾通量衰減率卻較單獨鎂鹽處理者高；然而，以Dead-end式薄膜過濾，兩者的通量卻是相反的結果。ROSA程式模擬的結果得知，若能利用前處理來提高矽酸的移除率，則所能達到的RO處理水回收率也會增加，較不會造成薄膜結垢的問題，進而降低在實廠操作上成本的開銷。

Silica is the major component of wastewater from chemical mechanical polishing (CMP) process in semi-conductor manufacturing. In the wastewater, the concentration of the dissolved silica (silicates) is between 70 to 95 mg/l. Normally, RO system can handle wastewater containing low concentration of silicates. When the silicate concentration of the feed water exceeds its solubility (120 mg/l), silicate will deposit on the RO membrane and silica scaling will happen, leading to flux decline and the increase in the trans-membrane pressure, which will shorten the membrane life. Therefore, the silicate must be removed before the membrane process. The objective of this study was to evaluate the feasibility of pretreatment by magnesium salts for membrane process. In this study, the synthetic wastewater was prepared from the CMP slurry to simulate the oxide CMP wastewater. Batch tests were performed to determine the optimal coagulation condition. Silicate removal of the pretreatment with magnesium salt and membrane separation was evaluated. The RO recovery rate was modeled from a software program. Results showed that magnesium salts is more effective than PACl and Alum in removing silicates, especially in the pH range around 10 to 11. At high pH, the negatively charged silicate ions are attracted to the positively charged magnesium hydroxide precipitates. After dehydration-copolymerization, silicate ions were removed from the water. Silicate removal increased with the dosage of magnesium salt and reduced the degree of membrane scaling during membrane filtration. However, the filtration flux declined substantially with the increasing magnesium dosage. Combining the EPACl coagulation with magnesium pretreatment enlarged the flocs, which enhanced the silicate removal. The application of EPACL reduced the flux when cross-flow mode was adopted. However, for dead-end mode, EPACL application improved the performance of magnesium salts alone. Result from the ROSA calculation indicated that the enhancement of silicate removal by pretreatment also improved the RO recovery and reduced membrane scaling.