Low-Ammonium Synergistic Leaching of Ionic Rare Earth Ore with Acetic Acid–Ammonium Sulfate System
Conventional ammonium sulfate leaching of ionic rare earth ores generates 4–6 t of ammonia-nitrogen wastewater per ton of rare earth and drives mining-area soil pH to 3.5–4.0, creating an acute environmental compliance risk. This study evaluates a low-ammonium synergistic lixiviant comprising 0.020 mol/L (NH4)2SO4 and 0.010 mol/L acetic acid (HAc) at pH 4–5, 30 °C, and 1 h contact time. Comparative leaching experiments establish a rare earth element (REE) leaching efficiency of 88.92%, a 13.36% absolute increase over single 0.020 mol/L (NH4)2SO4 leaching. To achieve the same ~90% efficiency benchmark, the conventional single-salt system requires 0.030 mol/L (NH4)2SO4; the synergistic system therefore reduces ammonium consumption by 33.3%. Surface characterization indicates a dual mechanism: H+ attenuates electrostatic interactions between RE3+ and silicate surfaces, enhancing NH4+–RE3+ exchange, while CH3COO− forms soluble RE3+/Al3+ complexes that prevent Al(OH)3 passivation and sustain surface reactivity. The protocol offers a directly deployable route to cut reagent cost and ammonia-nitrogen load without sacrificing recovery, addressing the principal bottleneck restraining sustainable ionic rare earth ore exploitation under China's dual-carbon and rare earth total-amount control policies.