Low-Ammonium Synergistic Leaching of Ionic Rare Earth Ore with Acetic Acid–Ammonium Sulfate System

• • 88.92% REE leaching efficiency at 0.020 mol/L (NH4)2SO4 + 0.010 mol/L HAc, pH 4–5, 30 °C, 1 h — a 13.36% absolute gain over single 0.020 mol/L (NH4)2SO4, enabling higher recovery from the same ore feed without additional ammonium input. • • 33.3% ammonium consumption reduction versus the 0.030 mol/L (NH4)2SO4 baseline required for ~90% efficiency — directly lowers ammonia-nitrogen wastewater load (currently 4–6 t per ton REE) and associated treatment capex/opex. • • Acetate complexation of RE3+/Al3+ prevents Al(OH)3 passivation and maintains surface reactivity, while H+ weakens RE3+–silicate electrostatic binding — this dual mechanism explains why the low-ammonium system does not suffer the kinetic decay typical of simple dilution. • • Operating window of pH 4–5, 30 °C, and 1 h is compatible with existing in-situ and heap leach infrastructure, avoiding the soil acidification (pH 3.5–4.0) associated with legacy ammonium sulfate practice and reducing regulatory exposure under total-amount mining controls.