Enhanced Coal Depression Using Starch Nanoparticles: Flotation and Optimization Studies

Spherical starch nanoparticles (7.08 ± 3.30 nm) were synthesized and used for coal depression during the flotation of a sub-bituminous coal (SBC) and a low-rank carbonaceous mineral (LRCM). The nanoparticles rejected 29.3% of the coal into the tailings for the SBC and 48.8% for the LRCM, compared to the 21.0% and 33.1% achieved by caustic starch, respectively. Signals associated with starch molecules on the mineral surface were identified via FTIR spectroscopy after conditioning. Zeta potential measurements confirmed surface modification following conditioning. Furthermore, the contact angle of ethanol on the graphite surface changed from 0° to 53.11° when starch nanoparticles were dispersed, thereby increasing its hydrophilicity. The optimization of coal depression was carried out using a Box–Behnken experimental design. This optimization led to an enhancement in coal concentration of 2.2% for the SBC and 0.5% for the LRCM. Ultimately, mineralogy and kinetics were identified as key factors for coal depression in flotation when using starch nanoparticles.