| dc.description.abstract | Generally speaking, a Printed Circuit Board (PCB) can be manufactured by a dry process or a wet process. When the wet process is used, the waste liquid is typically treated by first separating and collecting the waste liquid produced; then Passing the waste liquid sequentially through a conditioning tank, a fast mixing tank, a neutralization tank, and a slow mixing tank in order to produce a well-blended liquid mixture; Passing the liquid mixture through a sedimentation tank and a second neutralization tank is for precipitation and further conditioning. Then discharging the treated liquid mixture to a receiving body of water, with a view of complying with applicable laws and regulations.
The waste liquid of a PCB manufacturing process is basically a copper-containing waste liquid. The collection, disposal, and recycling of the high-concentration portion are generally outsourced to a contractor not only to meet the requirements of the Waste Disposal Act, but also in consideration of the operational costs involved. The portion with a low concentration of noble metals, on the other hand, is directly released to a wastewater treatment plant for treatment. Thus, the main objective of this study is to retrieve the noble metals in such a low-concentration waste liquid so as to increase the benefit of waste recycling, reduce the operational costs of wastewater treatment plants, and lower the silver ion content of such a waste liquid to that allowed by the effluent standards.
In this study, the silver-containing waste liquid of a PCB manufacturing process is introduced into an electrolysis apparatus in order to be electrolyzed. When electricity is applied to the graphite anode (+) and the stainless steel cathode (-), the silver ions in the waste liquid are reduced to metal silver and deposit on a stainless steel tank (serving as the cathode). According to the experiment results, the optimal electrolysis conditions (under which the removal rate reached 97%) include a 2-hour duration and a 2-A current. The electrolyzed silver-containing waste liquid is then passed to a chemical coagulation system to complete the serial treatment. The treated waste liquid is analyzed with an Atomic Absorption (AA) Spectrophotometer to determine the silver ion content and the removal rate.
The sludge generated by treating a silver-containing waste liquid in the foregoing serial manner is about one fourth of that generated by directly subjecting the waste liquid to chemical coagulation. This is mainly due to the fact that a silver-containing waste liquid to be electrolyzed requires relatively a small amount of additives during chemical coagulation. Moreover, with a removal rate as high as 98.32%, the effluent of the serial treatment meets both the effluent standards (the upper limit being 0.5 mg/L) and the water quality standards of drinking water sources and of drinking water (the upper limit being 0.05 mg/L). Now that noble metals can be extracted from waste liquids and the benefit of waste recycling will also be enhanced (e.g., approximately 2.488 kg of silver can be extracted by electrolysis from a 3,000-L monthly discharge of silver-containing waste liquids, yielding a monetary return of about NT$57,224.).
| en_US |