The separation of light lanthanoids through pertraction

Abstract

The light lanthanoid elements (57-60) are the most common of the f-block elements. These elements are plentiful and upon their discovery, the specified application of individual elements have grown alongside our ability to separate them. The growing demand for these elements is driven by their irreplaceability within modern society. Most noteworthy, by the modern green technologies associated with cerium and neodymium. It is the occupation of the f-orbitals which separate these elements from the rest. These valence orbitals are shielded by the 5th orbital shell from the environment. This results in unique magnetic- and electronic properties but, by the same grace, these metals exhibit nearly identical chemical behaviours especially within the trivalent cationic state which they all adopt in aqueous media. Therefore, chemically separating them becomes inefficient. The many benefits of pertraction has seen to its rise as the hydrometallurgical separation process of the future. In this light the separation of the light lanthanoid elements through this process is evaluated during this investigation. Within the light lanthanoid group, it is only cerium which is relatively stable in the tetravalent state and this has become the common starting point within their separation effort. In this study a green alternative to cerium oxidation is presented through hydrogen peroxide (H2O2). Initially an efficient, environmentally sound solvent extraction process was identified, followed by the novel liquid-liquid oxidation of cerium(III) to cerium(IV). This is shown through UV/Vis-spectroscopic analysis. The simultaneous optimization of extraction parameters for both oxidized and un-oxidized separation processes illustrates the effect of this species manipulation. It is shown that within the oxidized system the extraction of cerium is supressed resulting in increased separation efficiency, and ultimately within the pertraction application, the oxidation process results in a decrease in metal transfer rate. In this continuous circulating batch application with a Hollow-Fibre membrane contactor, the mass transfer coefficients for each metal is calculated in order to evaluate pertraction as feasible separation process. It is shown in this study that the separation of the LLn elements through pertraction is viable at the very least to effectively separate lanthanum and, with the novel oxidation process, also cerium from the valuable neodymium.