SEPARATION OF DYSPROSIUM AND YTTRIUM FROM YTTRIUM CONCENTRATE USING ALKYLPHOSPHORIC ACID (DEHPA) AND ALKYLPHOSPHONIC ACID (EHEHPA - PC 88A) AS EXTRACTANTS

Abstract

ABSTRACT Separation of Dy and Y from Y-concentrate (60% Y2O3) a fraction obtained in the process of fractionation of rare earths from monazite, is a difficult task due to low separation factors between adjacent rare earths. In this paper an attempt has been made to develop solvent extraction processes for the separation of DY2O3 (∼97% purity) and high grade concentrate of Y2O3 (93% purity) by using the extractants di-(2-ethylhexyl) phosphoric acid (DEHPA) and 2-ethylhexyl 2-ethylhexyI phosphonic acid (EHEHPA - PC 88A). The extraction of rare earths depends on the acidity of the solution as the extractants are liquid cation exchangers. Hence, distribution data, distribution coefficient (D) as a function of initial acidity (Hi) of the solution at varying initial concentrations (Ci) of rare earths that are present in Y-concentrate, have been determined. In fact such data were experimentally determined for Gd by the author of this paper and the data for various other rare earths have been derived from the data of Gd. These distribution data have been utilised to derive the mathematical models representing the extraction behaviour of rare earths. Incorporating these models in the computer program, the concentrations of rare earths in organic and aqueous phases for any aqueous to organic volume ratio, any acidity of the feed or scrub in any compartment of solvent extraction cascade can be estimated. Using these models, the parameters such as phase ratio, acidity of the feed and the scrub, concentration of the feed etc. could be optimised to get the desired purity and recovery of Dy and Y. With these optimised parameters the countercurrent solvent extraction processes were run using mini mixer-settlers and substantial quantities of Dy of 97% purity with 93% recovery and Y of 93% purity with 95% recovery could be produced. This paper summarises the basic approach of utilisation of the experimental data in terms 9f mathematical models for development of solvent extraction processes.