Liquid-liquid extraction of neodymium.

Abstract

Neodymium is classified as a rare earth element (REE). These elements possess a unique set of optical, electrochemical and magnetic properties that allow for their use in electronics manufacturing, medicine, catalysis and clean technologies. The global neodymium supply from primary source mining is isolated to a few countries, therefore developing technologies to recover neodymium and other rare earth elements from electronic waste is an emerging research area with economic incentive. The readiness of these technologies for industrial implementation is dependent on data for the extraction of neodymium from aqueous acidic solution into an organic phase for recovery. The available literature on these processes is limited. To address the gaps in the available literature, in this study, the distribution coefficient of neodymium in liquid-liquid equilibrium systems was measured across a range of nitric acid concentrations (0.1 – 2.9 M). The distribution coefficient is a measure of the affinity of a solute for the organic solvent to the aqueous phase. The extractant solutions used were composed of various concentrations of phosphorous acid diluted with n-dodecane. The tested extractant solutions are 0.1, 0.5 and 1 M of di(-2-ethylhexyl)phosphoric acid in n-dodecane. To investigate possible enhancements to the performance of the extractant, trace amounts of the ionic liquids (ILs) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (at concentrations of 0.019 and 0.19 M) and tributylmethylphosphonium methyl sulfate (at concentrations of 0.01, 0.1 and 0.25 M) were added to the organic extractant (0.5 M HDEHP in n-dodecane). The distribution coefficient data obtained for an extractant concentration of 0.5 M HDEHP was also used to determine its performance in a liquid-liquid extraction column by way of elementary mass balance calculations. The experiments performed in this study were undertaken using a bank of 6 stirred equilibrium cells immersed in a water bath maintained at a temperature of 298.15 K. Each vessel was filled with 5 ml of the aqueous and the organic solutions and mixed vigorously for 12 hours before being allowed to gravimetrically settle for 8 hours. Samples of the aqueous phase were withdrawn from the vessels, diluted using de-ionised water and analysed by way of inductively coupled plasma optical emission spectroscopy (ICP- OES). The equilibrium acid concentration of these samples was measured using acid-base titrations with 0.1 M sodium hydroxide solution. In this work the distribution coefficient data of 10 unique systems are presented, 2 test systems to validate the experimental method and 8 unique configurations of nitric acid concentration and extractant composition. The analysis of the distribution coefficient of neodymium showed that neodymium has an inversely proportional relationship to the aqueous [H+] concentration, established by the nitric acid concentration. For the HDEHP in n-dodecane extractant, the maximum distribution coefficient calculated was 274.26 at a nitric acid concentration of 0.2701 M with the 1.0 M HDEHP in n-dodecane. In the ionic liquid doped systems the maximum calculated distribution coefficients were 158.70 at a nitric acid [H+] concentration of 0.1161 M 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide doped extractant and 23.454 at an aqueous acid concentration of 0.0974 M when neodymium was extracted with the 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide doped extractant. The degree of extraction achievable by the addition of ILs is either decreased or increased, based on the concentration and species of ionic liquid. It was found that when ILs are used to enhance phosphorus acid extractant solutions, phase separation within the extractant occurs readily, decreasing the precision of measurements by more than 10%. The calculations for a liquid-liquid extraction column were performed based on a solution of neodymium and iron in nitric acid media extracted using 0.5 M HDEHP in n-dodecane. The results showed that neodymium with a solvent free purity of 96.75% by mass could be obtained using a column in which the extractant to volumetric feed flow ratio is 3.2. It is recommended that further distribution coefficient studies be undertaken to provide insight into the distribution behaviour of multiple ion-containing systems when extracted with IL containing synergistic extractant solutions.