Recovery of Rare-metal Elements from their EDTA Complex Solution by means of Electrodialysis Accompanied by Metal Substitution Reaction (「日中電気透析シンポジウム」小特集) Recovery of Rare-metal Elements from their EDTA Complex Solution by means of Electrodialysis Accompanied by Metal Substitution Reaction
EDTA-レアメタル錯体からの金属の回収を目的とした新しい電気透析法を提案した. 実験で使用した電気透析槽は, 陽イオン交換膜および陰イオン交換膜で仕切られた陽極室, フィード室, 反応室, ストリップ室及び陰極室の5室からなる回分循環型である。この手法は, フィード室に塩化銅水溶液, 反応室にレアメタル-EDTA錯体を流し, 反応室で銅とレアメタル錯体との金属置換反応を生じさせ, EDTAから解離した金属をさらにストリップ室で回収を行うものであり, 実験に際してはコバルト, ランタン, ガドリニウム, イットリウムおよびニッケルのEDTA錯体を用いた。コバルト, ランタン, ガドリニウム, イットリウムのEDTA単成分錯体を用いて実験を行った結果, EDTA-金属錯体と銅との金属置換反応は比較的速やかに進行し,それぞれの金属単成分として回収され, 各イオンの膜透過流束は電流密度の増加に伴い増加した. しかし, ニッケル-EDTA錯体の場合は銅イオンとの混合物として回収され, 金属種によって金属置換反応速度に違いがあることが示唆された. これらの結果を基に, 分光光度法によりEDTA-金属錯体と銅との金属置換反応速度を測定し, 速度定数を決定した.<BR>その結果, 各金属種における金属置換反応速度はLa-EDTA, Gd-EDTA, Y-EDTA, Ni-EDTAの順序で減少した. また, 得られた各速度定数はそれぞれの金属-EDTA錯体の安定度定数との間に良好な相関関係が見られた. 以上の結果を基に, 本手法における金属の回収のメカニズムについて考察を行った.
A novel electrodialysis process has been developed for the recovery of rare-metal elements from their EDTA complexes. The electrodialyzer used for the experiments was composed of five compartments divided by a cation-exchange membrane, C, and an anion-exchange membrane, A. The compartments were set in the order of Anode, Feed, Reaction, Strip, and Cathode compartments, and were partitioned with the A, C, C and A membranes, respectively. The Feed solution, the Reaction solution, and the Strip solution, each of which flowed through the corresponding compartment, contained CuCl<SUB>2</SUB>, rare metal-EDTA complex, and HCl solution, respectively. The electrodialysis for the recovery of rare-metal elements was expected to proceed as follows: When voltage is applied to the electrodialyzer, Cu<SUP>2+</SUP> should proceed from the Feed compartment to the Reaction compartment, and act as the substitute for rare-metal elements in the EDTA complexes. The resultant free ions of rare-metal elements move to the Strip compartment, and are recovered.<BR>In the experiments, Co, La, Gd, Y, and Ni were selected as rare-metal elements. In the electrodialysis experiment for the Co-EDTA system, Cu<SUP>2+</SUP> from the Feed compartment was immediately substituted for Co, and part of the resultant Co<SUP>2+</SUP> moved to the Strip compartment. No permeation of Cu-EDTA and free Cu<SUP>2+</SUP> was observed during the dialysis. The flux of Co<SUP>2+</SUP> increased with an increase in current density. For the systems of La-EDTA, Gd-EDTA, and Y-EDTA, the permeation behavior of metal ions was almost the same as that for the Co-EDTA system. For the Ni-EDTA system, however, Cu<SUP>2+</SUP>as well as Ni<SUP>2+</SUP> proceeded to the Strip compartment: This suggests that for the Ni-EDTA system the metal substitution reaction is much slower than that for the other metal elements. Moreover, the substitution reaction rates of Cu for the rare-metal elements in the EDTA complexes were measured spectroscopically and analyzed with due consideration of dinuclear complex intermediates. The rate constants were smaller in decreasing order-La-EDTA, Gd-EDTA, Y-EDTA, and Ni-EDTA and were correlated with the stability constants for EDTA.
日本海水学会誌 51(4), 219-227, 1997-09-01