Simultaneous purification of dysprosium and terbium from dysprosium concentrate using 2-ethyl hexyl phosphonic acid mono-2-ethyl hexyl ester as an extractant

During the solvent extraction fractionation of rare earth chloride obtained from monazite, a heavy rare earth fraction assaying 60% Y2O3 is produced. This is purified further to 93% Y2O3 by another solvent extraction cycle. During this step, most of the Dy and Tb are separated to yield a concentrate assaying > 50% Dy2O3, 14% Tb4O7, 10% Gd2O3, 2.4% Ho2O3, and 21% Y2O3. An attempt has been made to process this Dy-rich concentrate to obtain a high grade Dy2O3 by solvent extraction using 2-ethyl hexyl phosphonic acid mono-2-ethyl hexyl ester (PC 88A) in paraffinic kerosene as an extractant. The distribution ratio (D) of Dy was determined as a function of initial concentration of HCl for different initial concentrations of Dy. The separation factors of Dy/Gd, Dy/Tb, Y/Dy, Ho/Dy, and Er/Dy were determined experimentally. Using the experimentally determined D values of Dy and the separation factors, the distribution data for Gd, Tb, Y, Ho, and Er were derived mathematically. Based on these data, empirical mathematical models have been made to predict the concentration of these metal ions in organic and aqueous phases at various initial acidities and metal concentrations. Using these models, a computer program was developed to calculate the concentration of the metal ions in organic and aqueous phases at various stages of extraction and scrubbing in a counter-current solvent extraction cascade. Using this program, the parameters of the process, such as the initial acidity of the feed, the acidities of scrubbing and stripping solutions, the phase ratio, the number of stages in extraction, and the scrubbing sections have been optimised to obtain > 97% purity of Dy. Under these optimised conditions, counter-current extraction and scrubbing tests were carried out using mixer settlers of 50-ml mixer capacity. Mixer settler runs yielded four different products from its four exit points. The raffinate coming from the first exit was essentially Gd-rich solution containing other light rare earths (LRE). The fourth and final exit point was for the skipped solution that contained 85% Y2O3 with 95% recovery. The remaining two exit points, involving scrubbing stages, yielded an 83% pure Tb4O7 concentrate and a 97% pure Dy2O3 concentrate. (C) 2000 Elsevier Science B.V. All rights reserved.