具分散反萃取相支撐式液膜、氧化與沉澱法分離回收鑭鈰稀土金屬離子

Abstract

因稀土元素具高經濟價值，且最大宗出產國中國限制出口配額造成稀土金屬供給危機，故近年來稀土金屬的分離回收技術已成為國內外重要經濟及科技研究議題。本研究使用具分散反萃取相支撐式液膜分離搭配氧化、沉澱技術，對拋光粉廢液中的鑭鈰稀土離子進行分離與回收。 本研究提出四種程序組合來分離鑭鈰離子，並評估各種組合的分離效能及鑭離子損失率。第一種為兩階段式具分散反萃取相支撐式液膜分離法，先以第一階段液膜移除四價鈰離子，再以第二階段液膜移除三價鈰離子；第二種為氧化程序結合兩階段液膜分離法，先將大部分三價鈰離子氧化成四價，以第一階段液膜移除四價鈰離子，再以第二階段液膜移除剩餘的少量三價鈰離子；第三種為沉澱結合液膜分離法，調整pH值讓四價鈰離子沉澱，再以液膜法移除三價鈰離子；第四種方法結合氧化、沉澱及液膜法，先將大部分三價鈰離子氧化成四價，調整pH值讓四價鈰離子沉澱，最後再以液膜法移除三價鈰離子。四種程序組合均可將拋光粉廢液中的鈰離子移除，亦可將拋光粉廢液中的少量鎵、釹、鐠等離子移除，可得到高純度的鑭離子溶液。四種組合的鑭離子損失率分別為24%、14%、35%及23%，第二種方式的鑭損失率最低，為14%，但需兩步液膜操作，分離成本較高；第三種方式的分離操作最簡單，但損失最高；而第四種方式雖然鑭離子損失率高於第二種方式，但可以省下一步液膜操作，分離成本會降低。我們認為：日後放大製程時，使用第三種或第四種方式因只需一步液膜操作，成本會較低，選用第四種方式雖然損失率較第三種方式低，但因使用溴酸鈉氧化程序，須額外考量後續鹵素廢液處理問題，選用第三種方式程序上雖然簡便，但鑭離子損失率偏高。選用第三或第四種方式，各有其優劣，需進一步進行成本分析，才能明確判斷選用何者較有利。

The separation and recovery of rare-earth metals from waste streams have gained much research attention because of the increasing demands and the strategic control of supply by China. In this study, supported liquid membranes with strip dispersion (SLMSD), together with oxidation and precipitation processes, were applied to the recovery and separation of ceriumand lanthanum ions from the waste streams of polishing industries. We proposed four separation schemes and evaluated their efficiencies and the resulted lanthanum loss. The methods include: (1) two SLMSD processes; to remove cerium(IV) ions first with the first SLMSD process and then cerium(III) with the second one; (2) oxidation and two SLMSD processes, to oxidize first most of the cerium(III) ions into cerium(IV) and then to adopt two SLMSD processes to remove cerium(IV) and the remaining cerium(III); (3) precipitation and SLMSD, to precipitate cerium(IV) ions first by adjusting pH and then to remove cerium(III) with SLMSD; (4) oxidation-precipitation-SLMSD, to oxidize most of the cerium(III) ions into cerium(IV), then to precipitate cerium(IV) ions by adjusting pH, and to remove cerium(III) with SLMSD. With these separation schemes, we could not only remove almost all cerium ions, but also eliminate most of the impurity ions such as neodymium, praseodymium and gallium. Lanthanum ions with high purity can thus be obtained. The loss percentages of lanthanum ions with the four methods were 24%, 14%, 35% and 23%, respectively. The second scheme gave the lowest loss of lanthanum ions, but the separation cost would be high because two SLMSD processes were needed. The third scheme was simpler, compared with the other schemes, but the loss of lanthanum ions was the highest. The lanthanum loss of the fourth scheme was higher, compared with the second scheme, but the cost would be lower since it required only one SLMSD process. For future scale up, we suggest the use of the third or fourth methods because they are more feasible than the other two. The third and fourth schemes have their own advantages and disadvantages and it needs more investigations to find out which one has the edge for pratical applications.