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. D uring 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 a ttempt has been made to process this Dy-rich concentrate to obtain a high g rade Dy2O3 by solvent extraction using 2-ethyl hexyl phosphonic acid mono-2 -ethyl hexyl ester (PC 88A) in paraffinic kerosene as an extractant. The di stribution ratio (D) of Dy was determined as a function of initial concentr ation of HCl for different initial concentrations of Dy. The separation fac tors 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 fac tors, the distribution data for Gd, Tb, Y, Ho, and Er were derived mathemat ically. Based on these data, empirical mathematical models have been made t o predict the concentration of these metal ions in organic and aqueous phas es at various initial acidities and metal concentrations. Using these model s, a computer program was developed to calculate the concentration of the m etal ions in organic and aqueous phases at various stages of extraction and scrubbing in a counter-current solvent extraction cascade. Using this prog ram, 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 optimi sed to obtain > 97% purity of Dy. Under these optimised conditions, counter -current extraction and scrubbing tests were carried out using mixer settle rs of 50-ml mixer capacity. Mixer settler runs yielded four different produ cts 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 scrubbin g stages, yielded an 83% pure Tb4O7 concentrate and a 97% pure Dy2O3 concen trate. (C) 2000 Elsevier Science B.V. All rights reserved.