電結晶程序從工業廢水中去除與回收磷酸鹽及氰化物之機制與效率

Electrochemical crystallization for Removal and Recovery of Phosphorus and Cyanide from Industrial Wastewater: Mechanisms and Efficiencies

馬娜莎(Martin Natacha Viviane Patricia)

指導教授 ： 李奇旺

淡江大學/工學院/水資源及環境工程學系博士班/博士(2021年)

https://doi.org/10.6846/TKU.2021.00165

摘要

有鑑於「循環經濟」在未來趨勢的發展，積極的開發回收工業廢水污染物（例如 : 磷和氰化物），並將廢物作為有價值的產品進行了回收再利用是對永續發展之主軸。磷酸鐵，即 vivianite (Fe3II(PO4)2) 和 strengite (FeIIIPO4) ，和亞鐵氰化鐵，即 Prussian Blue (Fe4III[FeII(CN)6]3)，皆為有價值的產品，可在鋰的合成過程中重複使用磷酸鐵鋰分別用於鋰離子二次電池以及塗料和油墨著色劑的生產。
「磷酸鐵」和「亞鐵氰化」鐵的形成高度依賴於pH值、鐵的形態和鐵的用量；溶氧(DO)含量和pH值是分析鐵形態之關鍵參數。本研究分別著重於磷和氰化物的去除和回收，從合成廢水和工業廢水中研究該過程，方法其將磷酸鐵結晶為磷、普魯士藍結晶為氰化物。透過使用犧牲性鐵陽極在各種電流強度、Fe:污染物摩爾比、DO含量、氣逸法 (air sparging)、機械混合和氮氣沖洗 (nitrogen purging)、pH值(初始和控制pH)等情況下，進行電化學結晶實驗，探索其回收率和污染程度。收集污泥，乾燥並通過XRD和SEM-EDS進行分析。
在高溶氧的情況下有效去除了磷，但是由於電解過程中pH的升高，除磷的機制是吸附在氫氧化鐵上。在酸性的條件下研究了高壓電化學系統，並在pH為4.5和Fe∶P摩爾比為1的條件下除去60%的磷，這就是磷酸鐵的化學計量摩爾比。固體分析表明，水合磷酸鐵有菱鐵礦的沉澱。在該系統中未回收到高質量的硬石膏。在合成氣和工業廢水的氮氣沖洗 (nitrogen purging) 條件下，磷得到了有效去除，並且磷去除率隨著Fe:P摩爾比的增加而提高,當Fe:P摩爾比（磷酸亞鐵的比例）等於1.5時，磷的去除率達到100%。磷酸亞鐵顆粒迅速沉降，並確認其晶體結構為工業廢水中的堇青石。在pH為 6的情況下獲得的最終產物由82%的堇青石組成。該工藝的成本約為2.3美元/千克磷，與化學結晶法（2.34美元/千克）相比要低一些。
對合成的含氰化物廢水的研究表明，在pH為5-7範圍內觀察到沉澱出類似於普魯士藍的清澈藍色沉澱。固體分析證實了普魯士藍的沉澱。CN-的去除率隨初始CN-初始濃度的增加而提高，從而導致含Fe：CN-$的廢水中殘留的CN-濃度為8、7.5和12 mg/L。初始濃度分別為10、50和100 mg CN-/L時，CN-的摩爾比為0.8。進行了H2O2氧化的拋光處理，以降低殘留的CN-濃度並達到<1 mg CN-/L的排放極限/升包括電化學結晶和拋光處理在內的工藝成本為3.04美元/千克CN-，與鹼式氯化相比，可節省6%的成本。由於電鍍廢水的COD含量高，在pH值為7且Fe:CN-摩爾比為10的情況下，僅從工業廢水中去除了55%的CN-。固體分析未顯示出普魯士藍的存在。工業廢水的特徵為COD濃度高，會干擾普魯士藍的沉澱，COD去除與氰化物去除競爭。

關鍵字

磷酸鹽 ； Vivianite ； 氰化物 ； Prussian Blue ； 電結晶

並列摘要

With the circular economy in mind, recovery of industrial wastewater pollutants, such as phosphorus and cyanide, is actively developed and waste are recovered as valuable products. Iron phosphate, i.e. vivianite (Fe3II(PO4)2) and strengite (FeIIIPO4), and iron ferrocyanide, i.e. Prussian Blue (PB, Fe4III[FeII(CN)6]3), are valuable products for their reuse in the synthesis of lithium iron phosphate for Li-ion secondary battery and in the production of coatings and ink colorants, respectively. The formation of iron phosphate and iron ferrocyanide highly depends on pH, iron speciation and iron dosage. Dissolved oxygen (DO) level and pH are the key parameters for iron speciation. The present study focused on the phosphorus and cyanide removal and recovery separately.

The process was studied from synthetic and industrial wastewater by crystallization of iron phosphate for phosphorus and Prussian Blue for cyanide. The recovery was explored by means of electrochemical crystallization using a sacrificial iron anode under various current intensities, Fe:contaminant molar ratio, DO levels (air sparging, mechanical mixing, and nitrogen purging), pH values (both initial and fixed pH), and initial pollutant levels. Sludge were collected, dried and analyzed by XRD and SEM-EDS to assess the quality of the solid produced. Phosphorus was efficiently removed under high DO levels but due to the increasing of pH during the electrolytic process, the mechanism of phosphorus removal was the adsorption onto ferric hydroxides. High pressure electrochemical system was investigated in acidic conditions and removed 60% of P at the pH of 4.5 and the Fe:P molar ratio of 1, i.e. the stoichiometric molar ratio of ferric phosphate. Solid analysis showed the precipitation of giniite, which is a hydrated iron phosphate. High quality of strengite was not recovered in this system. Phosphorus was efficiently removed under the nitrogen purging condition for synthetic and industrial wastewater, and the P removal linearly increased with increasing of the Fe:P molar ratios, reaching 100% when the Fe:P molar ratio equals 1.5, which is the stoichiometric molar ratio of ferrous phosphate. The ferrous phosphate particles quickly settled and the crystalline structure was confirmed as vivianite in the industrial wastewater. The final product obtained at pH 6 was composed of 82 wt% of vivianite. The cost of the process was about 2.3 USD/kg P, which is a bit lower compare to chemical crystallization (2.34 USD/kg).

The study on the synthetic cyanide wastewater showed the precipitation of a clear blue precipitate, similar to Prussian Blue, observed in the pH range of 5-7. Solid analysis confirmed the precipitation of Prussian Blue. CN- removal increases with increasing initial CN- concentration, resulting in residual CN- concentrations of 8, 7.5 and 12 mg/L in the effluent with the Fe:CN- molar ratio of 0.8 for initial concentrations of 10, 50 and 100 mg CN-/L, respectively. A polishing treatment with H2O2 oxidation was employed to lower the residual CN- concentration and meet the discharge limit of < 1 mg CN-/L. The cost of the process including electrochemical crystallization and polishing treatment was 3.04 USD/kg CN-, saving 68% of the cost compare to alkaline chlorination. Only 55% of CN- were removed from the industrial wastewater at pH 7 and the Fe:CN- molar ratio of 10, due to high COD content of the electroplating wastewater. Solid analysis did not show the presence of Prussian Blue. The characteristics of industrial wastewater, i.e. high COD concentration, interferes with the precipitation of Prussian Blue and the COD removal competes with the cyanide removal.

並列關鍵字

Phosphate ； Vivianite ； Cyanide ； Prussian Blue ； Electrochemical crystallization

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