Efficient purification of graphite industry wastewater by a combined neutralization-coagulation-flocculation process strategy: Performance of flocculant combinations and defluoridation mechanism

The wastewater generated by the graphite industry exhibits high acidity and contains elevated concentrations of toxic ions, posing challenges for its efficient and cost-effective treatment. This study presents a novel approach for treating graphite industry wastewater, involving a synergistic process that combines NaOH neutralization, Ca(OH)2 mid-stage coagulation, and compound agent flocculation. The results showed that when pH = 2 was adjusted with NaOH and continued to pH = 8 with Ca(OH)2, the amount of precipitation produced was 62% lower than that of the conventional Ca(OH)2 coagulation process. Based on the defluoridation mechanism, polyaluminum chloride sulfate (PACS) and poly(dimethyldiallylammonium chloride) (PDMDAAC) were identified as the best compound flocculants. The turbidity and fluoride removal efficiencies of raw water reached 99.37% and 99.80%, respectively, when the mass ratio was m(PACS:PDMDAAC) = 9:1, the stirring rate was 150 rpm, and the coagulant dosage was 170 mg/L. By using X-ray powder diffractometer (XRD), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS) characterization, the flocculation-defluoridation mechanism of PACS in combination with PDMDAAC was examined for its synergistic effect. The flocculation-defluoridation mechanism confirmed that the adsorption complexation of Al3+ on F- is significant in the removal of low concentrations of fluoride ions. Practical applications have proved that this treatment strategy can effectively solve the problems of excessive sludge mass and low removal rate of fluoride and metal ions in graphite industrial wastewater treatment. This process provides a new idea to promote the application of combined flocculants in the large-scale treatment of industrial wastewater.