concentration was determined where the maximum SF was obtained. The optimum HCl concentrations were 0.10M, 0.14M, and 0.15M in the La/Ce, Ce/Pr, and Pr/Nd systems, respectively.
The stripping percentage of LREEs changed significantly, affected by the OA volumetric ratio. The stripping percentage decreased with an increase of the OA ratio, since the amount of H+ ions reacting with the LREE-extractant complex diminished. These experimental data were used to draw a McCabe–
Thiele diagram of stripping. By plotting the LREE concentrations in the aqueous phase versus those in the organic phase, it was possible to determine the optimum OA ratio and the number of stages in the counter-current process.
The structure of the LREE-extractant complex and the chemical reaction equation of stripping were examined using the slope analysis method.
Approximately three moles of H+ ions participated in the stripping reaction with one mole of the LREE-extractant complex in the Pr/Nd systems, while 2.7 moles of H+ ions reacted in the La/Ce and Ce/Pr systems. The difference of the stoichiometric number of H+ ions participating in the stripping reaction can be explained by HSAB theory.
The optimization was also conducted in the four LREE-component system, and the results were compared with the data obtained in the two LREE-
stripping were applied to the counter-current process. In the extraction step, four stages of extraction were conducted at an OA ratio of 4 with 0.5M PC88A, maintaining 89.8% purity and 42.2% remaining of La in the aqueous phase while other LREEs were extracted. To remove the extracted La completely in the organic phase, the first selective stripping stage was adopted, three stages at an OA ratio 7:4 with 0.10M HCl. Seven stages of stripping at an OA ratio of 3:2 with 0.15M HCl was followed to strip extracted Ce and Pr, maintaining the high purity of Nd in the organic phase. After the process, 34.9% of Nd was left in the organic phase with 90.3% purity. Finally, 0.5M of HCl was used to strip Nd to the aqueous phase.
Although solvent extraction of LREEs are actively discussed and studied, sufficient research about stripping has not been performed. Since this is the first study about the selective stripping of four LREEs, it has significant implications.
Because stripping is a requisite process after extraction, the efficiency of the overall process can be enhanced if the LREEs are separated not only in the extraction step, but also in the stripping step. Before a design of a process, the optimization of stripping conditions should be preceded, considering five factors including extractant type, extractant concentration, LREE concentration ratio in the feed solution, acid concentration, and OA volumetric ratio. After the optimization experiments, the possibility of the selective stripping of LREEs was verified and a process including selective stripping was designed in this research. In the future, the selective stripping of other REEs and the mechanism of the stripping reaction should be investigated.
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초 록
경희토류 원소는 현대 사회의 산업에서 널리 사용되고 있는 중요한 자원이다. 이 원소들은 유사한 화학적, 물리적 성질을 가지고 있기 때문에, 운영적 효율성과 경제성을 고려한 능률적인 분리 공정을 설계하는 것이 중요하다. 용매 추출법은 경희토류의 분리에 가장 많이 사용되는 방법이나, 수 십 단에서 수 백 단에 이르는 많은 추출 단수 및 탈거 과정에서 높은 농도의 산과 염기가 필요하다는 단점을 가지고 있다. 따라서 이 연구에서는 선택적 탈거를 통한 새로운 경희토류 분리 공정을 개발하였다.
네 종류의 경희토류 원소 계에 선택적 탈거를 도입하기에 앞서, 주기율표 상에서 인접하게 위치한 두 개의 경희토류 조합들에 대하여 탈거 과정에서의 최적화를 실시하였다. 추출제 종류, 추출제의 농도, 유입 용액에서 경희토류의 농도 비, 유기상과 수용액상의 부피 비, 그리고 산의 농도가 경희토류의 탈거에 미치는 영향을 조사하였으며, 이러한 변수들의 최적 조건을 각각의 두 경희토류 조합에 대하여 결정하였다. 추출제로는 0.5M PC88A가 선택되었으며, La 및 Ce 용액에 대하여 0.10M HCl을 사용해
Nd 용액에서는 0.15M HCl로 유기상과 수용액상의 부피 비를 3:2로 둔 상태에서 7단의 탈거를 실시하는 것이 최적의 조건임을 확인하였다.
경희토류와 추출제 간의 복합체 구조를 알아내고 추출 및 탈거 과정에서의 화학량론을 조사하기 위하여 기울기 해석 방법이 적용되었다. 그 결과, 경희토류는 유기상에서 염화 이온을 포함하며
PC88A 추출제와 복합체를 이루며, 원자번호가 작아 가벼운 희토류
원소일수록 추출제와의 복합체를 이룰 때 염화 이온을 더 많이 포함한다는 것을 알 수 있었다.
동일한 변수들에 대하여 네 개의 경희토류 계에서 탈거 과정의 최적화 조건을 알아보기 위한 실험을 진행하였으며, 그 결과를 두 개의 경희토류 계에서 얻은 값과 비교하였다. 또한, McCabe-Thiele 법을 사용하여 선택적 탈거를 위한 역류형 탈거 과정의 단수 및 유기상과 수용액상의 부피 비를 결정하였다.
최적화 실험 결과와 McCabe-Thiele 도표를 바탕으로, 선택적 탈거를 이용한 반연속성 역류 과정을 네 개의 경희토류 계에 도입하였다. 이 과정은 세 개의 단계로 구성되어 있으며, La를 수용액상에서 분리하기 위한 4단 추출, 추출된 La를 유기상으로 탈거시키기 위한 3단 탈거, 그리고 추출된 Ce와 Pr 을 탈거시키고 Nd만 수용액 상에 남기기 위한 7단 탈거가 그것이었다. 이 과정을 통해, 수용액 상에는 89.8% 순도의 La를 얻을 수 있었으며, 유기상에서는 90.3% 순도의 Nd를 획득할 수 있었다.
본 연구에서는 경희토류 계에서 탈거의 최적 조건을 찾아내었으며 선택적 탈거를 이용한 새로운 반연속성 역류 공정을