Recovery of Fluoride as Cryolite from Acidic Semiconductor Waste Liquor: Crystallization Behavior and Continuous-Flow Evaluation

Acidic fluoride-rich waste liquor from oxide etching in liquid-crystal display and integrated-circuit manufacturing is commonly treated by calcium precipitation, generating fine, impurity-bearing sludge with limited resource value. This study investigated a thermodynamics-guided route for the recovery of cryolite from acidic semiconductor waste liquor using batch experiments and continuous-flow assessment. Visual MINTEQ was used to screen aqueous speciation and solid saturation, while batch tests examined the Al/F molar ratio, pH, aging time, and temperature. Although cryolite was predicted to be supersaturated over a broad pH range, favorable fluoride removal occurred only within a narrower empirical window. At an Al/F molar ratio of 1.0:6, pH 6.5, an aging time of 2 h, and 20 °C, fluoride removal reached 79.6%. Under recirculating fluidized-bed operation, the principal cryolite phase was retained, and the SEM-derived mean particle size increased from 0.57 ± 0.10 μm in the batch system to 0.82 ± 0.15 μm in the continuous-flow system. These results demonstrate that dissolved fluoride in real acidic oxide-etching waste liquor can be converted into a separable cryolite-enriched solid.