<p>A novel process of acid leaching polymerization was initiated for the preparation of magnetic polyiron cation flocculant (MPCF) based on the reutilization of steel mills solid waste. Two kinds of raw material steel slag and red dust were combined in specific proportions, then a Box-Behnken design (BBD) optimization experiment was carried out to optimize the preparation process by adjusting hydrochloric acid concentration, quantity of hydrochloric acid, acid leaching time, and maturing time. The flocculant and raw materials were characterized using XRD, SEM, FT-IR, and XRF. The coagulation performance was evaluated using caprolactam. The optimal preparation conditions were determined as follows: the hydrochloric acid concentration was 7.2%, the amount of hydrochloric acid was 2&#xa0;mL/g, the acid leaching time was 4&#xa0;h, the maturing time was 3&#xa0;days, and the ratio of red dust to steel slag was 1:1. The results indicated that MPCF formed new complex compounds, specifically iron hydroxyl polymeric groups, with γ-Fe₂O₃ being the primary component responsible for the magnetic effect. Zeta potential measurements confirmed the mechanism of charge neutralization. Observations of floc morphology and fractal dimension studies during the flocculation process revealed that adsorption bridging also plays a significant role. The precipitation mesh trapping mechanism was enhanced by an external magnetic field. Under optimal flocculation conditions, with an addition amount of 0.5&#xa0;g/L and pH ranging from 8 to 12, the turbidity removal rate reached 46.22%, and the COD removal rate exceeded 12.71%.</p> Graphical abstract <p></p>