| 摘要: | 本研究利用反應曲面法(Response Surface Method, RSM)之中央合成設計(Central Composite Design, CCD)探討磷酸亞鐵流體化床結晶操作條件，如pH, Fe/P molar ratio, Zn/P molar ratio等影響因子，對結晶程序之影響，並且將結果最佳化後找出最佳結晶條件，以提升結晶效率。
首先，本研究在濃度100 mg-PO4-3-P/L下藉由CCD實驗進行磷酸亞鐵合成廢水化學沉澱試驗，最佳化後結果顯示，當pH = 7.6, Fe/P molar ratio = 2.5時，去除率可達100 %，由此結果可以初步得知在pH中性且Fe/P大於理論條件1.5時，可以有形成成磷酸亞鐵物種固體沉澱物。將上述所得之CCD實驗條件進入結晶試驗階段探討最佳化後實驗操作參數，並且將此結果進行連續穩定試驗。CCD實驗結果顯示，操作因子pH及Fe/P molar ratio明顯影響結晶率，隨著pH及Fe/P molar ratio的增加PC跟著上升，然而在實驗過程中雖然提升pH及Fe/P molar ratio有利於磷酸亞鐵結晶，但是當操作因子提升過量時，因系統中產生大量膠羽，將不利於磷酸亞鐵晶體的形成，對結晶程序產生干擾而導致結晶效率下降。而將CCD實驗結果最佳化分析後顯示當pH=6.8, Fe/P molar ratio=1.9時，結晶率可達60.24 %。並且為了結合實際應用，本研究將某污水處理廠之放流水所得之最佳結晶結果與合成廢水進一步進行比較後發現，實廠廢水最佳結晶率比合成廢水高20 %，推測原因為實際廢水中雜質吸附於晶體後降低晶體表面自由能，又或者是存在陽離子而促使結晶率提升。
另一方面本研究欲探討當在磷酸亞鐵結晶程序中添加微量鋅離子時是否對結晶有影響，藉由CCD實驗進行化學沉澱試驗，結果顯示相較於未添加Zn離子時之實驗，整體磷去除率有上升之現象。然而，結晶批次試驗結果顯示，添加鋅離子後，結晶效率並不佳，最高結晶率也只能達到47 %左右，推測原因為結晶時雜質金屬物質附著於晶體表面會提高能量屏障，使結晶更加困難，且當金屬離子濃度過高時，會導致系統呈現高度過飽和狀態，而大量成核，微晶體容易被出流水帶走，故導致結晶效率相較於未添加時下降許多，結果顯示添加鋅離子對結晶產生極大的干擾。
由上述研究結果可得知找出最佳pH及Fe/P molar ratio有利於提升PC，並且可以避免過量藥劑的浪費，有效控制成本，而所得到的磷酸亞鐵結晶珠可以進行回收再利用，當溶液中存在鋅離子時，不利於磷酸亞鐵結晶，故建議可先去選擇性去除鋅離子後再進行磷酸亞鐵結晶，以提升結晶效率及結晶純度。
;In this study, the Central Composite Design (CCD) of Response Surface Method (RSM) was used to explore the operating conditions of ferrous phosphate fluidized bed crystallization.Influencing factors such as pH, Fe/P molar ratio, Zn/P molar ratio explore their influence on the crystallization process, and find the best crystallization conditions after optimizing the results to improve crystallization efficiency.
First of all, this study conducted a chemical precipitation test of ferrous phosphate synthetic wastewater by CCD experiment at a concentration of 100 mg-PO4-3-P/L. The optimized results show that when pH = 7.6 and Fe/P molar ratio = 2.5, the removal efficiency can reach 100 %. From this result, we can preliminarily know that when the pH is neutral and Fe/P is greater than the theoretical condition 1.5, it can be a solid precipitate formed as ferrous phosphate species. The CCD experimental conditions obtained above were entered into the crystallization experiment stage to discuss the optimized experimental operating parameters, and the results were subjected to continuous and stable experiments. The CCD experiment results show that the operating factors pH and Fe/P molar ratio significantly affect the crystallization efficiency. As the pH and Fe/P molar ratio increase, PC increases.However, during the experiment, increasing the pH and Fe/P molar ratio is beneficial ferrous phosphate crystals, but when the operating factor is increased excessively, a large amount of flocs will be produced in the system, which will not be conducive to the formation of ferrous phosphate crystals, which will interfere with the crystallization process and cause the crystallization efficiency to decrease. The optimized analysis of CCD experiment results showed that when pH=6.8, Fe/P molar ratio=1.9, the crystallization efficiency can reach 60.24 %.
On the other hand, this study intends to explore whether adding a trace amount of zinc ions in the crystallization process of ferrous phosphate has an effect on the crystallization. The chemical precipitation experiment was carried out by CCD experiment. The result showed that compared with the experiment without zinc ions, the overall phosphorus removal efficiency increased presumably. However, the crystallization batch experiment results show that the crystallization efficiency is not good after the addition of zinc ions, and the best high crystallization efficiency can only reach about 47 %. It is inferred that the reason is that impurity metal substances attached to the crystal surface during crystallization will increase the energy barrier and make crystallization more difficult.And when the concentration of metal ions is too high, it will cause the system to be highly supersaturated, and a large number of nucleation, the fine crystals are easily taken away by the outflow water, so the crystallization efficiency is much lower than when it is not added.The result shows that the addition of zinc ions is very interference to crystallization.
From the above research results, it can be known that finding the optimal pH and Fe/P molar ratio is beneficial to increase PC, and can avoid the waste of excessive chemicals, and effectively control the cost. The obtained ferrous phosphate crystals can be recycled and reused. When zinc ions are present in the solution, it is not conducive to the crystallization of ferrous phosphate. Therefore, it is recommended to selectively remove the zinc ions before crystallization of ferrous phosphate to improve crystallization efficiency and crystal purity. |
