以薄膜電容去離子法回收稀土金屬離子

Recovery of Rare Earth Metal Ions by Membrane Capacitive Deionization

沈毓沂(Yu-Yi Shen)

指導教授 ： 王大銘

共同指導教授 ： 侯嘉洪(Chia-Hung Hou)

國立臺灣大學/工學院/化學工程學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201800291

摘要

稀土金屬因其特殊的物理化學性質、電化學性質、磁性、發光性被視為未來工業中不可或缺的元素。但其礦產資源分布不均且最大出口國中國在2009年開始管制稀土金屬出口，到目前稀土金屬成為各國重要的回收目標。本研究運用電容去離子法回收在硝酸根系統下的三價鑭離子、三價鈰離子。電極使用活性碳混合聚偏二氟乙烯(PVDF)，離子交換膜使用工業強離子交換樹酯混合聚氯乙烯(PVC)。調整進料液體探討其電吸附效率、能源應用與離子選擇性的關係。
首先進行電極分析，確定電極的表面特性及電化學特性及在進行電吸附稀土金屬離子時能有效吸附並無顯著法拉第反應產生。第二部分以電容去離子法回收鑭離子與鈰離子，調整平衡金屬離子濃度與初始氫離子濃度，分析在不同條件下電吸附的效能。第三部分進行薄膜電容去離子法，調整初始氫離子濃度，分析其效能並與電容去離子法比較。整體而言，平衡金屬離子濃度高、氫離子濃度低，能提高電容去離子法的電吸附量與吸附速率。
在電容去離子法與薄膜電容去離子法的比較上，在初始pH值為4，用薄膜電容去離子法進行吸附時，每克電極的電吸附量由6.9 mg提升到8.3 mg，吸附透過速率由2×10-9提升到4×10-9 m/min，電流效率由23%提升到40%，吸附每莫爾離子的能源消耗從4×10-4降到2×10-4 kWh。證實薄膜電容去離子法能有效去除溶液中的鑭離子與鈰離子，且在電吸附效能的表現皆優於電容去離子法。
最後在混合鑭離子與鈰離子的金屬溶液中，薄膜電容去離子法能將溶液中帶相同電性的離子同時回收吸附但無法分離二者，其電吸附表現與吸附單一離子的表現相同。電容去離子法的選擇機制是靠離子價數與水和半徑，若要分離兩條件相近的離子需要靠電極改質增加對單一離子的吸引力。

關鍵字

電容去離子法 ； 薄膜電容去離子法 ； 離子交換膜 ； 鑭離子 ； 鈰離子 ； 電吸附 ； 吸附速率

並列摘要

Rare earth metals have special physical and chemical properties which can make phosphors, laser, permanent magnets, catalysts and other application. It’s considered an indispensable element in the future industry. However, rare earth mineral resources are unevenly distributed and the largest exporter country China has restricted the rare earth metals export amounts since 2009. After that, recycling rare earth metals have become an important issue around the world.
The present work reports on the technique of membrane capacitive deionization(MCDI) to recover two rare earth metal ion, lanthanum ions(III) and cerium ions(III) in nitric acid, by electrosorption. The electrodes were made of activated carbon mixed with hybrid polyvinylidene difluoride (PVDF) and the ion exchange membrane was made of industrial strong ion exchange resin mixed with polyvinyl chloride (PVC).
First, analysis the electrode to make sure it can electrosorb rare earth metal ion without side reaction by surface properties and electrochemical properties analysis. Second, conduct capacitive deionization(CDI) to recover lanthanum ions and cerium ions. The equilibrium metal ion concentration and the initial pH value were adjusted to analyze the electrosorption properties under different conditions. Third, conduct membrane capacitive deionization(MCDI) to recover lanthanum ions and cerium ions. The initial pH value was adjusted to analyze the electrosorption properties under different conditions and compare it with CDI results. Overall, the electrosorption capacity and adsorption rate increase with high equilibrium concentration and high initial pH value.
At an initial pH of 4, electrosorption was carried out using MCDI. In the comparison between CDI and MCDI, the electrosorption capacity per gram electrode increased from 6.9 mg to 8.3 mg, the adsorption rate increased from 2×10-9 to 4×10-9 m/min, the charge efficiency increased from 23% to 40% and the energy consumption reduced from 4×10-4 to 2×10-4 kWh. It is confirmed that MCDI can effectively remove the lanthanum ions and the cerium ions in the solution, and the electrosorption performance is better than that of CDI.
Finally, when the influent contained both Ce3+ and La3+, MCDI can successfully recover both ion from water but cannot separate them. The electrosorption performance of both ions is the same as the electrosorption of single ion.
The selection mechanism of capacitance deionization method depends on the valence number of ion and ion hydrated radius. To separate two ions with similar conditions, the attraction to single ion needs to be improved by electrode modification or membrane modification.

並列關鍵字

Capacitance deionization ； Membrane capacitance deionization ； ion exchange membrane ； lanthanum ； cerium ； electrosorption ； adsorption rate

參考文獻

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