| Summary: | 碩士 === 國立交通大學 === 環境工程所 === 91 === The treatment of the fluoride-containing wastewater and the enormous amount of sludge generated has been a challenge for the HSIP. Fluoride-containing wastewater is generated during the etching and clean processes of semiconductor production. The range of fluoride concentration is usually between 200 and 500 mg/l. However, concentrations as high as 1000 to 2000 mg/l have been reported.
In Taiwan, the most popular way of treating the fluoride-containing wastewater is the precipitation/coagulation/sedimentation process. The fluoride is first precipitated as calcium fluoride at pH around 8.0 to 8.5. Because of its small size, calcium fluoride particles are hard to remove by sedimentation. In this study, two methods for improvement were experimented. In the first method, the fluoride is removed from the liquid phase through a co-precipitation of “Calcium carbonate-Calcium fluoride”. The second method uses membrane filtration of the coagulated calcium fluoride particles.
Optimum dosages of calcium salt and sodium carbonate and the optimum Ca/F molar ratio were determined. The dosing sequence and optimal pH were evaluated through the particle size and its settleability. The following are concluded from the six procedures tested: the pH must
first be adjusted to 8, the carbonate must be added before the calcium chloride and its dosage must be adequate to avoid affecting the pH of the reaction.
The suspensions were then coagulated with the PACl before the dead-end filtration through a 0.5μm MF membrane. The effects of the coagulant dosage, pH, and the permeate flux on the performance of solid/liquid separation were evaluated.
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