Control technology and characteristics of toxic air pollutants from valuable metals recovery in waste

• Main Authors: Yi-Chieh Lai, 賴怡潔
• Other Authors: Wen-Jhy Lee
• Format: Others
• Language: en_US
• Online Access: http://ndltd.ncl.edu.tw/handle/35831289695400965909

博士 === 國立成功大學 === 環境工程學系碩博士班 === 96 === The spent printed circuit boards (P-CB) and spent hydrodesulfurization (HDS) catalysts are two kinds of wastes with high metal contents. From the viewpoints of environmental protection and resource utilization, the disposal of wastes which contain valuable metals is of great concern. Incineration has been recently considered as a predominate technology of the waste treatment. However, some undesirable toxic air pollutants i.e. volatile metals, dioxins, and polycyclic aromatic hydrocarbons may be generated during the thermal treatment. The goal of this study is to develop feasible process for valuable metals recovery and to investigate control technologies for toxic air pollutants, generating during metal recovering from spent P-CBs and spent HDS catalysts. For spent P-CB containing halogens flame retardants were pyrolyzed in a high-temperature melting system to observe the formation behaviours of chlorinated (PCDD/PCDF) and brominated (PBDD/PBDF) dibenzo-p-dioxins and dibenzofurans. Results showed that the formation of PCDD/Fs and PBDD/Fs during pyrolysis can be destroyed under controlled primary combustion conditions. There are two significant factors that influenced the extent of PCDD/Fs and PBDD/Fs formation. The first factor was temperature. The result showed that, both the total-PCDD/F and PBDD/F content in the bottom ash and the total-PCDD/F and PBDD/F emission factor from the flue gas decrease by approximately 50% with the increase of pyrolysis temperature from 850 to 1200 ˚C. The second factor was the addition of CaO. The possible mechanism involves the reaction between CaO and HCl/HBr to form the solid phase product (CaCl2/CaBr2). Thus, the addition of CaO is effective in absorbing HCl and HBr, resulting in the inhibition of PBDD/Fs synthesis by more than 80%, and further prevents the acid gases (HCl and HBr) that corrode the equipment. Then, the acid-leachate of P-CBs was investigated to recover Cu using a fluidized-bed electrolysis process equipped with a glass bead medium, an iridium oxide mesh anode, and a stainless steel plate cathode. The effects of electrolysis parameters (initial electrolyte pH and electrolytic time) on the electrolytic recoveries of Cu were investigated. For 2hr electrolysis at initial electrolyte pH of 2.5–4.5 under constant current 2 A, the maximum electrolytic recovery of Cu (~43%) was obtained at the initial electrolyte pH of 4.5. Under this initial electrolyte pH, it was found that the recoveries of Cu dramatically increased with electrolytic time and the maximum recovery of Cu (77%) was obtained at 6 hr. Furthermore, XRD patterns of the electrodeposited compounds on cathode show that Cu is the predominant one on surface of cathode, mainly indicating a fluidized-bed electrolysis process is feasible for the Cu recovery. However, further study is necessary to improve the metal recovery. For spent HDS catalysts, thermal treatment was performed to remove contaminants (residual oil, carbon and sulfur) present on the surface of HDS catalysts. Results show that total-PAH content in treated residues decreased with the pyrolysis temperature of the primary furnace, while those generated in flue gases were destroyed by the afterburner at an efficiency of approximately 95%. In addition, the thermal process converts high molecular weight PAHs to low molecular weight PAHs, and the afterburner temperature involved (1200 ˚C) was high enough to prohibit the generation of high molecular weight PAHs (HM-PAHs), leading to the domination of low molecular weight PAHs (LM-PAHs) in flue gases, while treated residues were dominated by HM-PAHs. Finally, information on metal contents and their concentrations in the Toxicity Characteristic Leaching Procedure in spent HDS catalyst and thermal treated residues are examined as an index of the potential for metal recovery. An acid-leaching and fluidized-bed electrolysis combined process was then performed to recover valuable metals from spent HDS catalysts. It was found that an acid solution consisting of concentrated HNO3/H2SO4/HCl with a volume ratio of = 2:1:1 was found to be better than the other (HNO3/H2SO4 = 1:1) to leach the metals, the better solid/liuid ratio and leaching time were 40 g/L and 1 hr, respectively, at 70 °C. Under this condition, the leaching yields of target metals (Mo, Ni, and V) in the 1st-stage leaching reached 90, 99, and 99%, respectively, much higher than those in the 2nd/3rd-/4th-stage. When this acid leachate was electrolyzed for 2-hr at 2 A constant current (current density = ~35.7 mA/cm2), a stable cell voltage of 5 V was observed and the electrolytic recoveries of Mo, Ni, and V were ~15, 61, and 66%, respectively, but extending the electrolysis time from 2 to 4 hrs did not apparently increased the recoveries. For such an operation, the total recoveries (leaching yield × electrolytic recovery) of Mo, Ni, and V were ~14, 60, and 65%, respectively.