Converging fluidized-bed seeding systems: Engineering heuristics bridging ex-situ and in-situ seeding for enhanced fluoride mineral recovery

External seeding strategies in a fluidized-bed reactor were initially investigated to overcome limited particle production and predominance of fine solids, thereby improving the generation of fluoride particle recovery. The study further targeted fluoride concentrations typical of industrial effluents, which range from 1000–4000 mg/L, spanning saturation conditions forming suspended solids and settleable sludge. Several exploratory approaches were evaluated, including the use of (1) industrial Fe-waste for classical seeding, (2) CaF₂ sludge nucleated in bulk solution, (3) moderated fluoride concentration, and (4) recovered calcite and discarded alumina while comparing Ca- and Al-precipitants. The use of Fe-waste and CaF₂ sludge did not result in the anticipated granulation. At 2500 mg/L fluoride concentration, CaF₂ granule formation remained limited; nevertheless, saturation was regulated via recirculation, explicitly introducing a more reconciled two-stage nucleation requirement. Ultimately, although alumina and calcite did not function as classical seeds, redirection as bed supports coupled with saturation control enhanced overall particle development, while the use of Al–F solid generated coarser particles than the fluorite counterpart. Distinct from conventional approaches, the study integrates exploratory strategies into engineering heuristics that connect ex-situ and in-situ seeding under industrially relevant fluoride concentrations. In addition, the findings indicate that recovered particle formation in continuous fluoride precipitation systems is governed not only by seed addition but also by the interplay among nucleation pathways, saturation control, particle retention, and reactor hydrodynamics.