Key Takeaways & Executive Findings
- •• • 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.
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Abstract
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.
1. Introduction
Ionic rare earth ores in southern China hold a strategic position in medium and heavy rare earth supply, with REEs adsorbed as hydrated or hydroxyhydrated ions on clay minerals. The incumbent commercial route—ammonium sulfate leaching—delivers high ion-exchange efficiency but imposes a severe environmental penalty: 4–6 t of ammonia-nitrogen wastewater per ton of rare earth product and mining-area soil pH depression to 3.5–4.0. These figures are incompatible with tightening discharge limits and the dual-carbon policy framework, yet no ammonium-free alternative has matched the cost-recovery profile of (NH4)2SO4 at industrial scale.
The present work targets the specific bottleneck of ammonium overconsumption rather than attempting full substitution. By pairing low-concentration (NH4)2SO4 (0.020 mol/L) with acetic acid (0.010 mol/L) at pH 4–5, 30 °C, and 1 h, the authors demonstrate 88.92% REE leaching efficiency—13.36% above the single-salt baseline—while cutting ammonium consumption by 33.3% relative to the 0.030 mol/L (NH4)2SO4 required for ~90% efficiency. The mechanism is not merely additive: H+ disrupts RE3+–silicate electrostatic interactions, and CH3COO− sequesters RE3+/Al3+ as soluble complexes, preventing Al(OH)3 passivation. This dual action preserves surface reactivity and offers a drop-in modification to existing leach circuits.
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Shan HU, Lian-jun WU, Jun WANG, Yang LIU, Bing-xuan HE, Guan-zhou QIU (2026). Low-Ammonium Synergistic Leaching of Ionic Rare Earth Ore with Acetic Acid–Ammonium Sulfate System. Transactions of Nonferrous Metals Society of China (中国有色金属学报). https://doi.org/10.1016/S1003-6326(26)67067-7
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Frequently Asked Questions
What is the exact ammonium consumption reduction, and how does it translate to wastewater load at plant scale?
The synergistic system uses 0.020 mol/L (NH4)2SO4 versus 0.030 mol/L for the single-salt baseline achieving ~90% efficiency, a 33.3% reduction. Given the legacy process generates 4–6 t of ammonia-nitrogen wastewater per ton of rare earth, this translates to roughly 1.3–2.0 t less ammonia-nitrogen per ton REE, directly reducing stripping/evaporation duty and discharge compliance costs.
Does the acetic acid co-lixiviant introduce downstream solvent extraction or precipitation interference?
The HAc concentration is only 0.010 mol/L, and CH3COO− functions by forming soluble RE3+/Al3+ complexes that prevent Al(OH)3 passivation. At this low level, acetate is expected to remain in the aqueous raffinate and is readily degraded or washed; the paper reports no adverse effect on REE recovery, with 88.92% leaching efficiency maintained under the optimized 1 h, 30 °C, pH 4–5 window.
Why does the low-ammonium system not suffer the kinetic penalty typical of simply diluting (NH4)2SO4?
The dual mechanism compensates: H+ weakens electrostatic interactions between RE3+ and silicate surfaces, enhancing NH4+–RE3+ exchange, while CH3COO− complexes Al3+ and prevents Al(OH)3 passivation that would otherwise block surface sites. This maintains surface reactivity and delivers 88.92% efficiency—13.36% higher than 0.020 mol/L (NH4)2SO4 alone—within the same 1 h contact time.
Is the pH 4–5 operating window compatible with existing in-situ or heap leach infrastructure, and what are the corrosion implications?
The pH 4–5 range is mildly acidic and less aggressive than the pH 3.5–4.0 soil conditions already observed in legacy mining areas. This reduces acid consumption and corrosion risk relative to stronger acid systems, while remaining within the tolerance of standard HDPE-lined heap and in-situ well materials. The 30 °C, 1 h condition matches ambient seasonal temperatures in southern China, avoiding heating costs.
What is the scalability risk in moving from 0.020 mol/L (NH4)2SO4 + 0.010 mol/L HAc lab conditions to field-scale heap leaching?
The principal scale-up risk is uniform reagent distribution and pH control across heterogeneous clay-rich ore bodies. However, the low reagent concentrations (0.020 mol/L and 0.010 mol/L) reduce the risk of preferential flow and localized over-acidification. The 1 h contact time is short relative to conventional heap leach cycles, suggesting the chemistry is fast-acting; field validation should focus on maintaining pH 4–5 throughout the heap profile to sustain the 88.92% efficiency benchmark.
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