Mathematical modelling and simulation of rare earth solvent extraction for the system of (Gd-Tb)CI3-HCI-HEHEHP-Shellsol D70.
Australian Nuclear Science and Technology Organisation
Equations were developed to model equilibrium in the binary rare earth solvent extraction system; (Gd-Tb)CI3-HCI-HEHEHP-Shellsol D70. The mathematical relationship described the equilibrium rare earth concentration in the organic phase as a function of the total rare earth concentration, acidity and mole fraction of one earth in the aqueous phase. The equations were derived from step-by-step regression analysis of data from 36 equilibrium shake-out tests in which variables were measured over the following feed solution ranges: total rare earth concentration 0.2-1.0 M, acidity 0.05-0.80 M, mole fraction of Gd 0.005-0.995. All experiments were carried out at an organic /aqueous phase ratio of 1 using 1.5 M ethylhexyl phosphinic acid mono-2-ethlyhexyl ester (HEH(EHP)) in Shellsol D70. The average difference between the experimental and calculated values was less that <4%. These models can be used to study the extraction behaviour , and to simulate multi-stage extraction and separation of the rare earths. To verify the model, a 14-stage counter-current extraction circuit was experimentally simulated using separating funnels. Excellent agreement was found between the simulation and stage-wise calculations using a computer program written by BGRINM which takes account of changes in acidity that accompanying rare earth extraction and the organic concentration of rare earths as predicted by the mathematical model. Separation of gadolinium and terbium was also demonstrated in a small mixer-settlers circuit comprising of 10 extraction stages and 5 scrubbing stages. The feed solution contained 75% Gd and 25% Tb. The mixer settler circuit was operated for 45 hours to ensure equilibrium was established. The rare earth mass balances agreed within 3%. The product streams comprised a raffinate containing 93.6% Gd and an organic containing 90.4% Tb. Both parties are slightly lower than predicted by the stage-wise calculation and the simulation experiments using separation funnels. The discrepancies were mainly attributed to the fact that stage efficiencies were less than 100%, and/or the fact that a different organic-phase diluent was used in the mixer settler experiment. The good agreement between the calculated and experimented results (especially in the separation funnel experiments where the stage efficiency was 100%) indicates that the model accurately describes the equilibrium conditions over the wide range of concentrations and acidities within a counter-current solvent extraction circuit.
Mathematical models, Rare earths, Solvent extraction, Solvent extraction, Simulation
Ying, W., Huang, W., & Quan, C. (1994). Mathematical modelling and simulation of rare earth solvent extraction for the system of (Gd-Tb)CI3-HCI-HEHEHP-Shellsol D70. (ANSTO/E720). Lucas Heights, NSW: Australian Nuclear Science and Technology Organisation.