Historic Insights and Modern Challenges in Mixed Thiol Collector Flotation of Cu–Ni–PGM Ores

Low-grade Cu-Ni-PGM concentrators increasingly operate under the combined constraints of declining ore grades, variable process water quality, and the need to optimise reagent suites for sustainable production. This study examines the performance of mixed thiol collectors under controlled inorganic electrolyte conditions representative of modern concentrator water circuits. A comprehensive review of mixed-collector flotation is followed by a bench-scale experimental programme using sodium isobutyl xanthate (SIBX), sodium diethyl dithiophosphate (SEDTP), and their mixtures, tested in synthetic plant water and in CaCl2 and NaCl solutions at fixed ionic strength. Results show that increasing the SEDTP molar fraction significantly enhances froth stability, water recovery, and solids recovery across all water types, driven by stronger surface activity and the presence of surface-active impurities. Ca2+ bearing process water promoted the highest Cu and Ni recoveries but also intensified gangue recoveries at high SEDTP levels, lowering concentrate grades. In contrast, SIBX-rich mixtures yielded superior selectivity, particularly in Na+ containing process water. Mechanistic interpretation shows that combined effects of electrical double-layer compression, mineral activation, mixed-collector adsorption, and froth stabilisation behaviour govern the observed grade–recovery trends. Overall, this study demonstrates that thiol-collector synergy is strongly water-chemistry-dependent, and that optimising collector mixtures requires coordinated control of reagent composition and process water quality. The findings provide a mechanistic basis for water-responsive reagent design in Cu-Ni-PGM flotation circuits.