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Elemental Mercury Adsorption and Recovery by Electrothermal Swing System with Gold Electrodeposited Activated Carbon Fiber Cloth
fluorescent lamps recycling,mercury adsorption and recovery,gold electrodeposition,activated carbon fiber cloth,electrothermal swing system,
|Publication Year :||2019|
|Abstract:||由於汞的高危害性，近年來各國法規對於含汞產品的使用提高限制，大量含汞產品如液晶顯示器中的螢光燈管進入回收體系，造成每年平均約有5,000公噸廢棄汞螢光燈管需回收破碎；然而市面上仍存在無法取代金屬汞添加之產品，因此回收破碎過程逸散的汞蒸氣具有去除與回收的必要性。現今台灣採用之廢棄汞螢光燈管回收流程中低濃度汞蒸氣之吸附，其活性碳材一般採用含硫活性碳(sulfur-impregnated activated carbon, SAC)，利用其表面的硫官能基捕捉尾氣中的汞蒸氣。然而，SAC雖具有較佳的汞蒸氣去除效果，高穩定性的硫化汞形成導致使用後之SAC難以再生而被歸類為有害事業廢棄物並須進行固化掩埋的處理。
本研究使用電沉積技術將黃金沉積於活性碳纖維布(activated carbon fiber cloth, ACFC)表面以提升汞蒸氣吸附能力，並可有效進行吸脫附汞以達汞回收與活性碳再生目的。結果顯示，金電沉積活性碳纖維布(gold-electrodeposited activated carbon fiber cloth, GE-ACFC)之飽和吸附量可由48±5 μg/g提升最高至408±13 μg/g，其中以電沉積成長時間1200秒效果最佳，成長時間過長會使金沉積物太大並降低汞之接觸效率而減少汞吸附量，而ACFC之多孔纖維結構造成電阻分布不均，進而影響金沉積物之散佈。為了瞭解汞蒸氣之吸附型態，應用程序升溫脫附技術(temperature-programmed desorption, TPD)以獲得特徵峰進行判斷。由ACFC於93.7oC與164.5oC之特徵峰判斷汞蒸氣主要以吸附材孔洞之物理吸附與C=O官能基之化學吸附為主；而GE-ACFC於149.4oC至163.5oC出現單一特徵峰，則判斷以汞齊形式之物理吸附為主。然而本研究之特徵峰溫度相較文獻有偏低的情形，其原因為升溫速率較慢造成特徵峰有趨向低溫的狀況。本研究使用150、200和250oC進行三循環之電熱再生實驗，結果顯示GE-ACFC於不同再生溫度之吸附效率皆能穩定維持在90%以上；ACFC在150和200oC進行再生之吸附效率介於72.5至78.5%，250oC進行再生則介於58.6至62.6%。ACFC吸附效率下降之原因可能為通電時生成熱點使材料孔洞崩壞或纖維結構破損造成短流現象，熱點生成原因為ACFC之電阻分布不均，分別測量各再生溫度下之熱點溫度，ACFC相差最高可達153oC，而GE-ACFC由於經過金電沉積處理使導電性與導熱性有所提升，使溫度相差最高僅42oC。電熱再生效率結果顯示ACFC與GE-ACFC若施加足夠再生溫度皆可達到接近100%之再生效率，然而與TPD結果比較，ACFC之再生效率高於TPD推導結果，其原因為ACFC之熱點溫度較高造成汞脫附量上升；再生濃度結果顯示ACFC之最大再生濃度可達初始吸附濃度之4‒8倍，而GE-ACFC可達3.5‒6.5倍，表示GE-ACFC對於汞之鍵結強度較大且受熱點影響小，本研究進行脫附動力模式分析並提出表面反應機制解釋汞蒸氣吸脫附現象。
Owing to the high toxicity of mercury (Hg), the restriction of Hg-containing products in different countries is getting rigorous. The huge amounts of Hg-containing products such as fluorescent lamps of liquid-crystal display are discarded and need to be recycled annually. Because Hg in several commercial products is still indispensable, it is necessary to remove and recover Hg released during disassembling process. The aims of this study are to develop a sustainable approach to adsorb and recover low-concentration elemental Hg (Hg0) in the tail gas of the fluorescent lamps recycling process. Activated carbon fiber cloth (ACFC) with Au electrodeposition was utilized to improve the Hg0 adsorption ability and regenerability of ACFC. Gold-electrodeposited ACFC with the Au growth time of 1200 s (i.e., GE-ACFC-1200s) exhibited the largest Hg0 adsorption capacity. Owing to the size development of Au electrodeposited at a high growth time, the accessibility of Hg0 was reduced and lowered the Hg0 adsorption capacity. Non-uniform distribution of Au electrodeposition in ACFC was also observed, resulting from the various resistance of ACFC caused by the porous and fibrous structure. Hg temperature-programmed desorption (Hg-TPD) experiments showed that the Hg adsorption of untreated ACFC (RAW-ACFC) was mainly controlled by physical adsorption by the adsorbent’s pores or by chemical adsorption related to carbonyl groups (C=O). In contrast, the Hg adsorption of GE-ACFC appeared to be controlled by physical adsorption by Au amalgamation. Notably, the Hg characteristic peak temperatures of this study were lower than those shown in previous results due to different heating rates; the characteristic peaks shifted to higher temperature when heating rate increased. The adsorption efficiency of GE-ACFC-1200s was above 90% and showed a high stability at different regeneration temperatures in 3-cycle electrothermal swing system experiments. The adsorption efficiencies of RAW-ACFC were in the range of 72.5 to 78.5% after 150 and 200oC, respectively, and decreased to 60% after 250oC regeneration due to the formation of electrothermal hot spots in ACFC, which might result in the collapse of pores on ACFC and short circulation caused by the structural damage. Because of Au electrodeposition, the thermal and electrical conductivity of GE-ACFC increased and the effect caused by electrothermal hot spots in GE-ACFC-1200s was relatively minor. Higher regeneration efficiency occurred in the latter cycle, resulting in the greater desorption of residual Hg adsorbed in the former cycle. The regeneration efficiency of both ACFCs could approach 100% if the regeneration temperature was high enough. The regeneration efficiency of ACFC at different temperatures in the first cycle was much greater than the expected efficiency integrated from Hg-TPD, which might be resulted from the formation of hot spots in electrothermal processes. The regeneration concentration of RAW-ACFC was 6 to 8 times higher than the initial inlet Hg0 concentration while that of GE-ACFC-1200s was 3.5 to 6.5 times higher due to the formation of hot spots and the difference of Hg binding strength. The kinetic model was adopted to simulate the behavior of Hg0 desorption and the mechanism was purposed to explain Hg0 adsorption/desorption on GE-ACFC.
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