光電業含砷廢水處理-化學沉降和流體化床法之評估

Abstract

自從砷化鎵晶圓開始應用於LED業後，含砷廢水之處理與其產生污泥之處置即成為園區之一大課題。由於砷對人體有結殊的累積性毒，所以世界各國對砷之標準即其他污染物來得嚴格。各國近年來砷之飲用水限值降至0.01 mg-As/L即為一例。園區中含砷廢水主要係來自磊晶製程中被含氧化劑洗滌水洗滌下來之砷酸及研磨過程中所產生之砷化鎵砥粒，其溶解砷濃度約界於500~1,500 mg-As/L。若該工廠在磊晶製程中還含有磷化氫氣體，該股廢水中也可能會含有高濃度之磷酸。 該股廢水若使用化學沉降及混凝將目標殘餘溶解砷濃度降至園區納管標準(0.5 mg-As/L)會有一定的困難度。一般工廠會過量加入反應劑，則又會因此產生更多之有害污泥。本研究之主題將朝向兩部分，第一部分為傳統化學沉降及混凝操作之最佳化，第二部分則將使用流體化床處理該股廢水，其中As(V)/(III)之組合物均會進行研究。 化學沉降方面，經添加氯化鈣後針對高濃度之含砷原水有一定的處理效果(過濾後最低可降至約8 mg-As/L)，但離納管標準仍有一段差距。操作pH區間建議需在pH 10以上，雖砷酸鈣沉降特性較氟化鈣為佳，但仍需注意未沉澱去除之顆粒在放流後(pH降低)再溶出之問題。硫鹽對亞砷酸也有一定處理效果。 化學混凝方面，經比較不同混凝劑對中濃度(約20 mg-As/L)之去除效果，得到結果與歷年文獻相倣。氯化鐵不論是可操作之pH區間或是單位添加金屬鹽除砷之效果均較硫酸鋁來得佳。多元氯化鋁(PACl)在除砷之表現與氯化鐵及硫酸鋁相比相差甚多。 流體化床方面，本研究主要利用實驗設計法，探討pH值、鈣鹽(硫鹽)劑量、砷面積負荷及進流砷濃度對含砷廢水處理之影響。砷酸鈣流體化床實驗結果與化學沉澱法得到的結果相近，但若在同一進流砷濃度下比較，流體化床可以較少的鈣鹽劑量達到相同之砷去除效果。控制砷酸鈣流體化床之主要參數以操作pH及砷面積負荷。另外，硫化砷流體化床，處理對象主要為三價砷。研究結果顯示在pH 2、S/As比達1.8；或操作pH 1.4、S/As比達1.75以上均有合格處理之效果。將長晶後之擔體進行顯微照相、XRF、XRD、SEM/EDS及純度分析後，推測硫化砷確實於擔體上成長。且該晶體含水率約僅14%，與化學沉降法相比相對可降低污泥量，若能再提高該結晶物之純度，將可考慮資源再利用。

Arsenic is of environmental concern due to its toxic properties, which normally occurs in trace level. Due that the mobility of electrons in gallium arsenide is higher than silicon device, the gallium arsenide technology has been broadly employed in the applications of communication and optical electronics. That induces the issue of how to treat waste water with high concentration of arsenic. The arsenic pollutants discharged from the HSIP are of two origins. The soluble arsenic is generated from the chemical vapor deposition (CVD), while the solid form is the residual from the chemical mechanical polishing (CMP) process, of which the formal is the focus of treatment. This wastewater is unique in its low flow rate (10 to 100 CMD) and high concentration (500 to 2000 mg/l). Arsenic-containing wastewater has been treated with coagulation, ionic exchange, activated aluminum and membrane filtration. Most companies prefer the chemical precipitation process. In order to meet the maximum discharge of As, 0.5 mg-As/L, large amount of chemical is added. In turn, a large quantity of hazardous sludge is generated, which requires additional disposal cost. Fluidized bed crystallization technology combines the advantages of fluidized bed reactor and crystallization, which has been adopted for water softening and heavy metals removal. Besides the high efficiency, low operation cost and capital investment, fluidized bed crystallization produces no sludge while producing much purer crystallized pollutant ready for resource reuse. Therefore, fluidized bed crystallization technology can be a potential treatment technology for wastewater containing high level of arsenic. In this study, both As(III) and As(V) will be studied. The wastewater will be characterized. The most influential variables will be decided by the residual arsenic concentration. Using these variables, the regression models for the treatment efficiency will be determined by the response surface methodology. The purity and speciation of the crystal will be qualified and quantified. The ultimate goal is to provide a new alternative for high concentration arsenic removal in the direction of volume reduction and resource reuse.