Tailored oxidised starch derivatives for enhanced selectivity of pyrrhotite depression in gold-copper flotation

• Starch was systematically oxidised using H 2 O 2 and NaClO to tailor its functionality. • H 2 O 2 oxidation produced carbonyl-rich while NaClO generated carboxyl-rich starch. • Flotation showed NaClO oxidised starch as the most effective pyrrhotite depressant. • Synch-XRD and DFT showed depression depends on functional groups, not pyrrhotite type. • Carboxyl groups play the key role in achieving strong and selective pyrrhotite depression. Pyrrhotite, a problematic gangue in gold–copper ores, lowers concentrate grade and raises SO 2 emissions when unintentionally floated. Selective depression is difficult due to the coexistence of multiple pyrrhotite superstructures (4C, 5C, 6C), each with distinct surface reactivity. This study investigates oxidised starch (OS) derivatives as sustainable depressants, focusing on how functional groups and collector competition influence selectivity across pyrrhotite types. Native wheat starch (NWS) was oxidised using NaClO or H 2 O 2 to produce carboxyl-rich and carbonyl-rich products, respectively. Flotation results showed a consistent pyrrhotite flotation trend: baseline > NWS > H 2 O 2 -OS > NaClO-OS. Synchrotron-XRD combined with Rietveld analysis confirmed that total pyrrhotite recovery dropped from ∼ 39.6% (baseline) to 21.4% (H 2 O 2 -OS) and 10.7% (NaClO-OS). Each superstructure, magnetic 4C and non-magnetic 5C and 6C, responded almost similarly to each depressant, with slightly stronger depressive effect for non-magnetic types. It was found that depressant performance was governed more by functional group chemistry than by crystallography. NaClO-OS consistently outperformed H 2 O 2 -OS across all surfaces, highlighting the crucial role of carboxylate groups in enhancing surface binding. DFT calculations supported this, showing that NaClO-OS exhibited stronger adsorption than H 2 O 2 -OS, NWS, and even the xanthate collector, driven by a combination of multidentate Fe-O coordination and strong electrostatic attraction between the anionic polymer and positively polarised Fe surface sites. These findings demonstrate the potential of NaClO-oxidised starch, rich in carboxyl groups, as an effective, selective, and sustainable depressant for all pyrrhotite types, particularly valuable for depressing non-magnetic pyrrhotite, which poses the greatest challenge in the flotation of complex sulphide ores.