Humic acid as a selective depressant for pyrophyllite in CTAC-mediated reverse flotation of quartz: Insights into sequential depressant–collector interactions

The selective depression of pyrophyllite is essential for the reverse flotation separation of quartz from pyrophyllite using cationic collectors. In this study, humic acid (HA) was evaluated as a selective organic depressant for pyrophyllite in a cetyltrimethylammonium chloride (CTAC)-mediated flotation system, with particular emphasis on the sequential interaction between HA pretreatment and subsequent CTAC-induced surface modification. Single-mineral flotation tests showed that CTAC alone induced high floatability of both pyrophyllite and quartz, indicating poor selectivity under depressant-free conditions. In contrast, HA strongly depressed pyrophyllite while maintaining the high floatability of quartz. Pyrophyllite floatability decreased to approximately 10 % or lower at HA dosages of 2,000–3,000 g/t and above, whereas quartz floatability remained approximately 95–100 % under the same conditions. Compared with conventional inorganic depressants, including sodium silicate (SS) and sodium hexametaphosphate (SHMP), HA exhibited the most selective depression effect toward pyrophyllite. Reagent-addition sequence tests further demonstrated that HA pretreatment was critical for selective depression, with the HA → CTAC sequence producing substantially lower pyrophyllite floatability than CTAC → HA or simultaneous HA–CTAC addition while maintaining high quartz floatability. Artificial mixed-mineral flotation further confirmed this selectivity, with the HA + CTAC system achieving a pyrophyllite grade of 95.88 %, pyrophyllite recovery of 93.07 %, and quartz removal efficiency of 96.00 %. X-ray photoelectron spectroscopy (XPS) showed comparatively larger changes in surface carbon concentration and binding energies for HA-treated pyrophyllite than for HA-treated quartz, supporting preferential HA-induced surface modification of the Al-bearing pyrophyllite surface. Zeta potential measurements further demonstrated that HA pretreatment suppressed the CTAC-induced charge reversal of pyrophyllite, whereas quartz remained sufficiently responsive to CTAC after HA pretreatment. These results suggest that HA preferentially modifies pyrophyllite, possibly through interactions between oxygen-containing functional groups and Al-associated hydroxylated surface sites, thereby suppressing CTAC-induced hydrophobization. In contrast, HA-induced modification of quartz is limited, allowing CTAC to maintain quartz floatability. The combined flotation and surface characterization results demonstrate the potential of HA for selective CTAC-mediated reverse flotation separation of quartz from pyrophyllite.