利用煉鋼廠廢棄耐火磚回收廢水中的磷酸與氨氮之研究

Abstract

煉鋼廠的廢棄耐火磚中鎂含量很高，而鎂是以鳥糞石（MgNH4PO4．6H2O）沉澱回收磷酸（PO43--P）和氨氮（NH3-N）不可或缺的一部分。在酸性條件下，鎂離子(Mg2+)、鈣離子(Ca2+)和少數其他金屬離子如：鐵離子(Fe3+)、亞鐵離子(Fe2+)、鋁離子(Al3+)等會從廢棄耐火磚礫石中溶出。鎂離子、氨氮、磷酸的摩爾比和pH值是促進鳥糞石沉澱的重要參數，而鈣離子則會抑制鳥糞石沉澱。本研究中以下順序形成三種沉澱物以去除回收模擬廢水中的磷酸和氨氮：1.在廢棄耐火磚礫石溶出液中加入亞硫酸根(SO32-)，在[SO32-]0/[Ca2+]0 = 1.2和pH 8.9下，和亞硫酸根和鈣離子產生亞硫酸鈣（CaSO3）沉澱，以去除溶出液中的鈣，同時維持鎂離子濃度。2.混和去除鈣後之含鎂溶出液和模擬廢水，在[Mg2+]0 : [NH3-N]0 : [PO43-P]0 = 2 : 1 : 2和pH 9.5下，利用鳥糞石沉澱回收模擬廢水中的氨氮和部分磷酸。3.以未除鈣之溶出液，在[Ca2+]0/[PO43--P]0 = 1.67和pH 9.5 下藉由磷酸鈣（Ca3(PO4)2）沉澱去除水中殘留磷酸。利用表面形態和礦物分析儀器包括XRD和SEM-EDS顯示， XRD圖譜中亞硫酸鈣與鳥糞石均與標準圖譜吻合。在SEM照片中，亞硫酸鈣和鳥糞石的形態一致，而在EDS中，亞硫酸鈣和鳥糞石的元素組成比與理想的亞硫酸鈣和鳥糞石的組成比相同。本研究建立了一個從廢水中回收磷酸和氨氮的流程，最終廢水中的磷酸和氨氮能達放流水標準。

Spent refractory brick from steel industry contains high content of magnesium (Mg), which is an indispensable source of Mg for phosphate (PO43--P) and ammonia nitrogen (NH3-N) recovery from wastewater via struvite (MgNH4PO4．6H2O) precipitation. In acidic condition, Mg2+, Ca2+ and other metal ions (Fe3+, Fe2+ and Al3+) were leached from spent refractory brick gravel. The molar ratio of [Mg2+]0 : [NH3-N]0 : [PO43--P]0 and pH value are important parameters regulating the precipitation of struvite, while the presence of Ca2+ is known to inhibit struvite precipitation.. Three different precipitates were formed in the proposed process to recover PO43--P and NH3-N in synthetic wastewater: 1. The addition of SO32- in the spent refractory brick leachate with a molar ratio of [SO32-]0/[Ca2+]0 = 1.2 at pH 8.9 to sequester Ca2+ via calcium sulfite (CaSO3) precipitation. 2. The addition of Mg2+-rich leachate after Ca2+ removal in synthetic wastewater with a molar ratio of [Mg2+]0 : [NH3-N]0 : [PO43-P]0 = 2 : 1 : 2 at pH 9.5 to initiate struvite precipitation to recover NH3-N and PO43--P. 3. The addition of spent refractory brick leachate before Ca2+ removal to synthetic wastewater after step 2 with a molar ratio of [Ca2+]0/[PO43--P]0 = 1.67 at pH 9.5 to initiate calcium phosphate (Ca3(PO4)2) precipitation to remove residual PO43--P. The surface morphology and mineralogy of precipitates were characterized using XRD and SEM-EDS. The XRD patterns of precipitates matched the standard ones of both CaSO3 and struvite. In SEM images showed characteristic morphologies of both CaSO3 and struvite and the EDS showed identical elemental composition proportion for both CaSO3 and struvite. Overall, a process was developed for the recovery of PO43--P and NH3-N using the Mg2+-rich leachate from spent refractory brick and the final effluent could meet the discharge standards.