Influence of activation mode and aeration rate on ferritization efficiency, phase formation, and sediment stability in spent etching solution treatment

The growing volume of spent etching solutions generated by the metallurgical industry represents a significant environmental challenge due to their high acidity and high concentrations of dissolved iron. Developing efficient treatment methods that ensure both decontamination and resource recovery is a priority for sustainable industrial wastewater management. Ferritization is a process that converts dissolved iron into stable ferrite phases and thus offers an effective pathway for transforming hazardous waste into environmentally safe and technologically valuable materials. This study evaluates the ferritization of diluted sulfuric acid under three activation conditions: thermal, ultrasonic, and alternating magnetic field. Experiments were conducted with air oxygen bubbling rate of 0.02–0.06 dm³/s and reaction durations of 30–75 min. The resulting precipitates were characterized using X-ray diffraction to determine their phase composition, while leaching tests assessed the stability of the ferrite products in aqueous environments. The results show that thermal activation at 75 °C combined with the highest aeration rate significantly enhances the conversion of iron oxyhydroxides into magnetite (Fe₃O₄). Under optimal conditions, magnetite content approached 100%, iron removal reached 99.99%, and leaching of iron ions did not exceed 0.2 mg/dm³. Overall, the study demonstrates that ferritization is an efficient and environmentally viable method for treating spent pickling solutions while obtaining valuable iron-containing products.