A continuous-flow process is proposed for the separation of Cs from complex, strongly alkaline, aqueous mixtures. This process, which includes the use of tetraphenylborate (TPB) anions to precipitate cesium from the bulk waste, minimizes the exposure time of TPB to the alkaline mixture by injecting TPB as a countercurrent into a small stream of the bulk waste and immediately following with filtration. The recovered solids are dissolved in propylene carbonate, and cesium is then extracted into an aqueous solution, to be later recovered as CsNO3, in a three-phase liquid-liquid-liquid extraction. Minimizing the time of exposure of TPB to strong (2 M) sodium hydroxide avoids the decomposition of TPB and subsequent release of benzene into the atmosphere above the reaction vessel.