Separation of Fluoride and Phosphorus from Acidic Steel Pickling Wastewater and Its Sludge Via Electrocoagulation and Precipitation

Sammanfattning

The depletion of commercial fluorspar reserves requires the recovery of fluoride from secondary industrial sources, including stainless steel acidic wastewater (AWW) and pickling sludge. Selectively separating fluoride from phosphorus in these liquid and solid wastes is technically difficult due to competing ion affinities. To address this, the present study establishes a thermodynamic framework based on the 2-pK Constant Capacitance Surface Complexation Model (SCM) to predict and control phase separation behavior. Modeling results demonstrated that maintaining a pH of 4.0–4.1 maximizes phosphorus adsorption onto hydrous ferric oxide (HFO), while the intentional introduction of aluminum ions forms competitive complexes with fluoride, effectively preventing its co-precipitation with the iron matrix. Based on these thermodynamic parameters, a sequential electrocoagulation method was implemented. A specific charge-driven pH shift was applied to isolate phosphorus into an Fe-gelatin sludge, followed by Ca(OH)2 addition to recover a CaF2 and AlF2(OH) co-precipitate with approximately 70% purity. Additionally, sludge leaching kinetics were analyzed using a Shrinking Core Model. The experimental data indicated a surface chemical reaction-controlled dissolution characterized by an instantaneous initial extraction phase, ultimately yielding a solid product with 71.15% phase purity. The recovered materials naturally form a CaO-Al2O3-CaF2 complex, which can serve as achemically active, direct substitute for traditional metallurgical fluxes in steelmaking refining slags