以鎂合金廢棄物回收尿液中磷之研究

Abstract

由於人類活動的介入，使得磷循環無法平衡磷礦資源的消耗，所以磷可視為一種有限資源，因此取代開採磷礦石，使用替代性磷源例如廢水、尿液，就顯得極為重要。為了加速磷資源的循環，我們以化學沉澱法將廢水中的磷回收為可直接再利用鳥糞石，當鎂添加到尿液中，便會和尿液中的磷酸根及銨根反應，形成 鸟粪石沉澱。電化學沉澱法是利用電解鎂電極，使其釋出鎂離子，取代傳統的加藥的方式，其操作較傳統方法簡便。 本研究利用鎂合金製程的廢棄物作為鎂源，以電化學沉澱法回收合成尿液中之磷，研究反應過程中pH、去除效率以及產物純度的隨時間的變化，實驗結果表明，高攪拌速度有利於磷酸根的去除，但如果攪拌速度不夠快，則會造成層流效應，其去除率反而比不攪拌來的低。在初始pH 8時具有最佳的最終去除率，pH 8-10之間具有最佳的去除效率，但由於鎂電極腐蝕剝落物的影響，在最佳去除條件下，產物的最終純度只有30%~40%。為提高產物純度本研究設計一反應裝置，利用電極剝落物和鳥糞石重量的差別，有效分離電極剝落物和鳥糞石，使最終產物純度提升至70%~80%，換算為P2O5含量超過23%，代表回收的鳥糞石不須經過其他的處理過程，可直接作為肥料使用。電化學沉澱法由於成本較低、產生具有經濟價值的產物，且其反應過程中pH隨沉澱物的產生而提升，這個特性有利於工程應用中監控處理的效果，所以電化學沉澱法處理尿液，是一種具有經濟價值和工程潛力的方法。

As mineral resources of phosphorus are limited, the use of alternative phosphorus sources such as wastewater becomes an increasingly important option. In order to reuse the phosphorus as recycled sources and accelerate the small-scale cycle of phosphorus geochemical circulation efficiently, some studies pursued the chemical precipitation to recover the phosphorus. However, electro-chemical sedimentation is an economical and novel method which could release the target metal element from electrode aggregating with alternative phosphate compounds to form a reusable crystal of struvite and easy to operate comparing with other traditional methods. In general, magnesium added to source-separated urine, then struvite precipitate and phosphorus recovered. Magnesium ions were electrochemically dissolved from a sacrificial magnesium electrode which was used as the source of magnesium. In this study, tailings of magnesium alloy was used as the sacrificial electrode. Experimental results showed that the purity of struvite was strongly dependent on the pH and the electric current density. For the optimum controlling condition, the purity of struvite exceeded above 80%, and the phosphorus content was more than 23% as P2O5. It means that the struvite which recovery from urine can be used as fertilizer without any process. Due to the low cost and the capability of generating useful product, recovering phosphorus with sacrificial magnesium electrode is a worthwhile approach for urine treatment.