Polymetallic separation in chlorination titanium waste acid: A solubility-driven cascade precipitation strategy

Chlorinated titanium dioxide waste acid (CTWA) is a representative chemical waste stream, characterized by high-volume generation and complex chemical composition. In this study, the speciation of metal ions in the aqueous solution is systematically analyzed to determine the pH-dependent sequence of hydrolytic precipitation. Based on this precipitation sequence, a three-stage stepwise precipitation process was developed to recover resources from the target effluent. In Stage I (pH = 1.5), CaCO 3 was used to selectively precipitate Ti, achieving a recovery rate of 98.71%. In Stage II (pH = 3.0–5.0), CaO was used as a precipitant to induce the coprecipitation of Al, V, Sc, and Nb, with precipitation yields of 97.43%, 98.53%, 96.9%, and 99.97%, respectively. In Stage III (pH = 10), Fe, Mn, and Y were efficiently co-precipitated via NaOH-mediated pH adjustment, with recovery rates exceeding 97.61% for all three elements. As a result, Ti-rich slag, V-Al-Sc-Nb-rich slag, and Fe-Mn-Y slag were obtained, respectively, enabling effective metal pre-separation from CTWA. The underlying mechanism of this cascade precipitation is selective hydrolysis driven by differences in solubility product constants (Ksp), which enables precise control over the hydrolysis sequence and precipitation morphology of various metal ions via systematic pH adjustment. Compared with existing CTWA treatment technologies, the developed strategy avoids Fe 2+ pre-oxidation and achieves group-wise enrichment of multiple target metals directly from the highly acidic waste acid. This process offers advantages including operational simplicity, controllable cost, and high recovery rates, providing a feasible technical route for the resource-oriented treatment of CTWA.