Conceptual design and experimental validation of a novel pilot-scale crystallizer for Mg(OH)2 recovery from concentrated saline solutions

Reactive crystallization of magnesium hydroxide from concentrated saline solutions represents an effective route for magnesium recovery yet being challenging due to the complexity in controlling crystallization phenomena. This study presents the numerical model–guided design and experimental validation of an innovative pilot-scale crystallizer, the Multiple Feed Plug Flow Reactor (MFPFR). The reactor configuration was optimized through CFD simulations with local supersaturation analysis, identifying the hydrodynamic conditions that minimize local supersaturation peaks, thereby slowing nucleation processes and promoting crystal growth. The prototype was then constructed and tested using concentrated bitterns ([Mg2+] ≈ 2 M) and 0.5 M NaOH solution as alkaline reagent. The produced Mg(OH)₂ suspensions exhibited excellent solid–liquid separation performance, with initial settling velocities up to 440 mm/h and filtration rates above 80 kg/(m2·h), outperforming literature data. The obtained solids showed a high degree of crystallinity, while the overall process achieved magnesium recoveries exceeding 99%. These results demonstrate the effectiveness of the integrated modeling–experimental approach and confirm the potential of the novel MFPFR for high-quality Mg(OH)₂ production from concentrated saline streams.