# Chromatographic Separation of Rare Earth Elements as MGDA Complexes on Anion Exchange Resins
## 1 Introduction
These kinds of complex compounds include amino(poly)carboxylic acids (EDTA (ethylenediaminettetraacetic acid), IDA (iminodiacetic acid, NTA (nitrilotriacetic acid), MGDA) and (poly)carboxylic acids (citric acid, acetic acid). Separation can be achieved with both cation and anion resins, depending on the charge of the complex formed. A suitable eluent combined with the right resin enables selective separation of REEs with a known order relative to their complex stabilities and properties [24; 25]. REE mixtures can be introduced to the chromatographic system either as free ions or complexes. Cation exchange resins can be used when the feed contains REE as free cations or positively charged complexes. Free cations are sorbed to the resin, then released from the resin with suitable eluent (complexing agent, acid, etc.). Anion resins can be used in the opposite situation, where the REE feed or the separation material has been pre-treated with a complexing agent. Preferably these negatively charged species are usually pushed through the chromatographic system using a complexing agent as the mobile phase [26; 27]. This kind of separation usually requires large quantities of the mobile phase relative to the amount of complex present in the system [28]. Table 1 reviews several methods from the literature that achieved partial or complete REE separation. In most of the previous methods, separation was achieved using HPLC equipment and/or eluents that are expensive or highly concentrated. In this work, the separation of REEs in the form of the MGDA complex is studied. The study proposes a novel process to separate anionic MGDA-REE complexes with dilute acid as mobile phase. MGDA-REE complexes originated from resin-in-leach recovery of REE from phosphogypsum [32]. In addition to the complexation of REE with MGDA, the combination of anion IX resin as a sorbent and the dilute acid as the mobile phase make the studied unit process novel for the separation of pure REEs from their mixture. Due to the anionic nature of MGDA-REE complexes, the cation IX resins were not considered as a separation material. Dilute acid as the mobile phase in this separation can reduce the chemical load compared to systems with alkaline complexing agents (e.g., EDTA). The effect of feed composition, resin type, acid type, and temperature are explored with a synthetic MGDA-REE solution. The purpose of synthetic feed is to determine the factors that are affecting the separation of the MGDA-REE complex without the presence of impurities. The effect of impurities for separation efficiency is not discussed, although in realistic case minor levels of impurities would exist in the feed. The separation of MGDA-REE complexes with a conventional packed resin bed without the use of HPLC technology will enable us to proceed towards techno-economically efficient REE recovery from phosphogypsum. The novelty of this work is that the separation of REEs is achieved with dilute HCl while the anion IX resin acts as a separation material and not with the use of a complex forming ligand in the mobile phase. This results in a new method for REE separation, which could be a more economically viable option for separation since it uses dilute HCl as the eluent, which is cheap compared to the ligand-based (for example MGDA) mobile phases.