| Summary: | 碩士 === 國立屏東科技大學 === 環境工程與科學系所 === 95 === In Taiwan, Cr(Ⅵ)–containing wastewater produced from etching process of TFT-LCD manufactures was usually treated by chemical reduction/ precipitation method. However it was found that the settling time of precipitates was very long. The objective of this research was to characterize the settling behavior of chemical reduction/precipitation sludge of TFT-LCD etching wastewater. The results showed that the sludge settling behavior was type IV (compressed settling). The sludge from a simulated Ce(IV) wastewater exhibited a settling pattern similar to that of TFT-LCD sludge. On the ther hand, the sludge settling behavior of a simulated Cr(Ⅵ) solution was characterize as type III. These results revealed that when using chemical reduction/precipitation procedure to treat TFT-LCD etching wastewater, the Ce ion in the wastewater was simultaneously reduced and settled, resulting in an adverse settling effect. Moreover, nitric acid, another major ingredient of the etching wastewater, would also cause slightly negative effect on the settling of the chemical reduction/precipitation sludge. Nevertheless, Cr(Ⅵ) removal was not affected by the presence of Ce(Ⅳ) in the wastewater.
By the dilution of 1/10, the settling of etching wastewater sludge was shifted to zone settling, and the sludge settling speeded up significantly. Similar results were obtained for simulated Ce(Ⅳ) solutions at lower concentrations (36 mM、91 mM、182 mM、274 mM). For the sludge produced from 36-182 mM of Ce(Ⅳ) solutions, the settling was characterized as zone settling (type Ⅲ) before 6 hrs, and a compressed settling (type IV) was found afterward. At higher concentrations (274 and 364 mM), Ce(Ⅳ) sludge exhibited a compressed settling for the entire settling period.
The particle diameter and SEM analyses showed that the average particle diameter of Ce(Ⅳ) sludge was smaller at higher Ce(Ⅳ) concentrations. The SEM images also showed that the sludge particles for high concentrations of Ce(Ⅳ) solutions was small, solid and compact, without extensive compression, the Ce(Ⅳ) sludge of lower concentrations looked flaky, porous and large.
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