Dynamic Modeling for the Separation of Rare Earth Elements Using Solvent Extraction: Predicting Separation Performance Using Laboratory Equilibrium Data

Industrial rare earth element (REE) separation facilities utilize acidic cation exchange ligands such as 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC88A) for solvent extraction processes. REE separations are costly and difficult due to their chemical similarities and subsequent low separation factors. Several empirical correlations are available in the literature to predict steady state extraction equilibria for various solvent systems. However, complete solvent extraction flow sheet design for REE separations requires complex scrubbing and stripping circuits to separate and produce individual pure species. Furthermore, dynamic modeling of extraction, scrubbing, and stripping in REE separations circuits will aid in process design, optimization, and management of process fluctuations. A dynamic MATLAB/SIMULINK REE equilibrium model has been coupled with dynamic acid balances to predict REE solvent extraction processes using laboratory equilibrium data. The model was used to predict a flow sheet that produced high purity neodymium from a 25 wt % praseodymium and 75 wt % neodymium feed. Laboratory mixer–settlers were used to verify and validate model performance. Results indicated that the model reasonably predicts the dynamic behavior of a countercurrent REE separation process, and accurately predicts the steady state REE concentration profiles across the cascade. Transient concentration predictions exhibit more deviation from experimental results due to the initial assumption of a homogeneous, well-mixed stage. The model was revised to account for variations in mixer–settler holdup volumes for future validation efforts. Current model limitations assume complete equilibrium is achieved in each stage. The model can be applied to any REE separation or solvent system provided adequate laboratory equilibrium data are available.