Continuous annular sorption is a recent innovation that uses a slowly rotating annular sorbent bed to continuously separate components as a function of angular position. The current application of this technology, continuous ion exchange (CIEx), utilizes eluent in a much more efficient manner than the previous chromatographic applications. The efficient recovery of semidilute levels (10-20 mM) of lanthanide mixtures by CIEx is demonstrated experimentally, and a mathematical model of the process is used to predict equipment performance. Results from batch shake tests and fixed-bed runs support the CIEx modeling effort and are useful separative benchmarks. The implications of the results for scaleup and economic evaluation are examined.