| 摘要: | 印刷電路板業(PCB)為製程繁雜用水需求極高的產業，製造過程會排出大量重金屬廢液與廢水，這些廢液與廢水若未經妥善處理即排放，對人體及自然環境皆會造成重大影響。
化學沉澱法是將工業廢水調整至鹼性環境下，使重金屬離子沈澱分離，產生之化學污泥(例如:Ni(OH)2)是屬於廢棄物。本研究利用還原劑dithionite將Ni(II)轉換成金屬鎳或其他有價值的形式，達到回收鎳離子之目的。本研究探討dithionite劑量、pH值及反應溫度對鎳回收的影響。不含螯合劑(例如:檸檬酸鹽)時，鎳的還原效率會隨dithionite:Ni莫耳比增加而上升，dithionite:Ni莫耳比在3:1，可達到99％以上的回收率。pH值對鎳還原效率的影響，與dithionite的標準氧化還原電位(pe0)有關，當pH值增加時，dithionite的還原電位愈大，使得鎳的還原效率愈好。
當含有檸檬酸的環境下，由於Ni-檸檬酸錯合物的形成，鎳的還原反應被嚴重的抑制。提升反應pH值或溫度，可增加dithionite的還原電位，因此可改善鎳的還原效率。
儘管降低溶液的pH值，可使Ni-檸檬酸錯合物解離產生較多的自由鎳離子，然而，pH值下降會使dithionite的還原力降低，並無法改善鎳的還原效率。鎳還原反應的最佳pH值為8.0。此外，以金屬銅顆粒為核種，可改善鎳的去除速率與效率。除了金屬鎳之外，固體物經XRD分析後顯示為NiS和Ni3S2。
實廠含鎳廢液於dithionite:Ni=1.5:1、溫度40℃、pH 4.5-4.8之條件下，分三次添加dithionite，Ni之去除率為57.4%；實廠含鎳廢水以實廠含鎳廢液稀釋10倍進行試驗，dithionite:Ni為4:1、8:1、12:1時，Ni之去除率則分別為84.3%、94.5%、98.1%。
Printed circuit board (PCB) production involves extremely complex processes and demands large amount of water, generating huge amount of heavy metal-containing waste liquids and wastewaters. Without proper wastewater treatment, discharge of these heavy metal-containing waste liquids and wastewaters poses profound impact to human body and the natural environment. Chemical precipitation, a common practice to treat heavy metal-containing industrial wastewater, generates hazardous chemical sludge (e.g., Ni(OH)2) through precipitation of metal ions at alkaline pH values. In this study, the recovery of Ni by converting Ni(II) to its elemental or other valuable forms through chemical reduction using dithionite was investigated, . The effects of dithionite dose, solution pH, and reaction temperature on Ni reduction were investigated.
Synthetic wastewaters containing Ni(II) with and without ligands were studied first. Without the presence of chelators (e.g., citrate), the nickel reduction efficiency increased with increasing dithionite:Ni molar ratio, reaching ~99% at ratios above 3:1. The effect of pH on Ni reduction was in agreement with the standard redox potentials (pe0) of dithionite, which became more negative with an increase in pH, thus leading to greater Ni reduction efficiencies.In the presence of citrate, however, the Ni reduction reaction was severely inhibited due to the formation of Ni-citrate chelates. Elevated pH and temperature increased the reducing power of dithionite and improved nickel reduction. Although a decrease in the solution pH freed nickel ions from the nickel-citrate complex, the reducing power of dithionite decreased as well, and so Ni(II) reduction was not enhanced. The optimal pH value for Ni(II) reduction was thus found to be pH 8. Furthermore, seeding with preformed Cu particles improved the rate and amount of Ni removed. In addition to elemental Ni, both NiS and Ni3S2 were identified in the crystal by X-ray analysis of the resulting solids.
A PCB waste liquid containing Ni(II) and organic ligands was collected from a PCB manufacturer and treated with chemical reduction process. At dithionite:Ni molar ratio of 1.5:1, temperature of 40℃, and without pH adjustment (original pH of 4.5-4.8), the removal efficiency of Ni was about 57.4% with dithionite being divided into three equal portions and dosed 5 min apart. The PCB waste liquid was diluted 10 times to simulate the wastewater generated from PCB manufacturing process. It was found that the removal efficiencies of Ni from the simulated wastewater were 84.3%, 94.5% and 98.1%, respectively, with dithionite:Ni molar ratio of 4:1, 8:1, and 12:1. |
