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석회석 충진컬럼을 이용한 불산폐수의 처리

석회석 충진컬럼을 이용한 불산폐수의 처리
Other Titles
Treatment of fluoirde wastewater using calcite packed column
Alternative Author(s)
Kim, Hyo-Seon
Issue Date
Electronic products are major exporting items representing industrial Korea. Process of manufacturing display panels and semiconductors consumes large amount of fluorine compounds. Fluoride is used for the process that can clearly remove the dust attached on the surface of the board and a metallic oxide. Thereby large amount of fluoride wastewater has been emitted. It contains high concentration of fluoride and other contaminants like chloride, nitrate, phosphate and sulfate. Conventional methods have been used to remove fluoride from wastewater, namely, alum coagulation, ion exchange, lime precipitation, and adsorption using rare earth metal like lanthanum. But such complicated multi-step treatment involves many problems such as large sludge production, scale formation, and higher chemical costs. Thus we began investigation to find new method that can remove these pollutants in more economical and eco-friendly ways. In this point of view the calcite (CaCO₃) is suitable agent for fluoride removal. It is low-cost material that exists on the natural world in spots. In this paper, calcite was studied as alternative fluoride removal agent. Calcite contains calcium oxide that precipitates fluoride ions by forming fluorite (CaF₂). To revive a real system we obtained calcite used in the industrial process and crushed it down and sieved under 150㎛. First, we conducted the batch experiments to evaluate capacity of calcite and to investigate the fluoride removal rate by precipitating CaF₂. Initial fluoride concentration in the real wastewater was 536 mg/L. This process reached a state of equilibrium within 12 hours. After the reaction fluoride concentration of treated water is under 10 mg/L. The screening tests revealed the possibility of calcite as alternative fluoride removal agent. Based on this screening experiment we performed column experiments to investigate efficiency of fluoride removal using calcite, to observe a tendency of other ions, and to check applicability to real process. A fixed bed reactor filled with granular calcite (1~2mm) was used to treat real wastewater ,from a semiconductor making plant, containing 460 mg/L of fluoride, 1300 mg/L of phosphate, 530 mg/L of sulfate and pH of 2.5. Removal rate of fluoride as well as other ions was investigated in column test. As soon as the real wastewater passes through the column, the concentration of fluoride and phosphate decreased below 1 mg/L and 100 mg/L, respectively these results were caused by precipitating with fluoride ions and calcium oxides. By following equation 2F- + Ca^(2+) = CaF₂, fluoride ions in the wastewater was precipitated to the fluorite (CaF₂). The result of column experiments indicated the influent of pH 2.1 reached neutral range around 6~7. Fluoride removal in calcite column was affected by pH. When pH range is neutrality treated fluoride concentration reached to the minimum. Initial fluoride concentration of 460 mg/L immediately decreased under 1 mg/L before the breakthrough that began around 500 pore. 1300 mg/L of phosphate decreased below 200 mg/L because phosphate was precipitates to apatite (Ca^(5)(PO₄)₃). Initial calcium concentration was around 3 mg/L but after reaction, residual calcium concentration increased 200 mg/L that was permanent hardness. We make it crystallize. CaCO₃ was produced. In comparison with conventional method, this system has more neutral pH, highly fluoride removal rate, and re-available residual calcium.
나날이 증가하는 불산 폐수를 처리하기 위해 기존에 쓰고 있는 lime 침전 공정은 용해도가 낮아 과량 주입함으로 스케일이 생기고 pH 중화에 따른 2차 처리가 필요하여 많은 비용이 드는 실정이다. 이에 저렴하고 추가적인 공정이 필요 없는 대체 매질의 개발하고자 calcite를 선정하여 실험을 수행하였다. 회분식 실험에서 본 결과 calcite는 99%의 불소 제거율을 보이며 94%의 제거율을 갖는 lime 보다 높은 불소 제거율을 보이고 pH가 중성 영역의 범위를 나타내는 것으로 보아 lime의 대체 매질로 적절한 매질임을 알게 되었다. 매질의 주입량에 따른 영향을 살펴보기 위하여 실시한 평형 실험에서 반응하는 데 필요한 양 이상의 주입량에서는 모두 비슷한 불소 제거율을 보였고 그 이하의 주입량에서는 불소 제거율이 낮았는데 이는 반응에 충분하지 못한 양을 주입하여 반응이 다 일어나지 못한 것으로 보여졌다. 또한 주입양이 증가함에 따라 처리수에서 나타나는 잔류 칼슘의 농도가 증가하였다. 입자 크기에 따른 영향은 입도가 작을수록 비표면적이 넓어져 반응성이 커짐으로 제거율이 높아지는 것을 알 수 있었다. Calcite의 연속적인 처리가 가능한지 알아보기 위하여 컬럼 실험을 수행하였다. Channeling 현상을 막기 위해 폐수를 상향류식으로 유입시켰고, 1~2 mm 입자 크기의 calcite를 컬럼에 충진시켰다. CaF₂ 침전 반응에 의해 99% 이상의 F^(-)가 제거되고 Ca^(2+)의 양이 증가하는 것을 알 수 있었다. 파과점이 나타나기 전까지 배출수 허용기준 아래의 농도를 나타내었고 중성영역의 pH를 나타내어 별다른 추가 공정이 필요 없어 실제 공정에 적용가능하다고 판단되었다. Apatite의 생성으로 1500 mg/L 가량의 고농도의 PO₄^(3-)가 90% 이상 제거되고 SO₄^(2-)와 NO₃^(-) 가 소량 제거되었다. 중화적정법을 이용하여 폐수내의 알칼리도를 측정한 결과 실폐수에서는 측정되지 않았으나 처리수내에서는 알칼리도가 측정되었다. 이 총알칼리도는 CaCO₃ 재생에 사용될 수 있을 것이라 여겨진다. 따라서 본 연구 결과 calcite는 불산 폐수 실제 공정에 이용한다면 기존 lime 공법보다 경제적인 매질이 될 것이라 판단되어진다.
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