Reversible Acidification Enables Closed-Loop Total Separation of Olivine

Technical Summary: Reversible Acidification Enables Closed-Loop Total Separation of Olivine

Problem Statement
Silicate minerals like olivine are abundant reservoirs of alkalinity, magnesium (Mg²⁺), iron (Fe²⁺), and silica. While historically viewed as waste or mine tailings, their total processing could yield valuable products: high-grade iron ore, pozzolanic silica for cement, and magnesium oxides for CO₂ capture and soil remediation. However, existing hydrometallurgical approaches are largely linear, relying on stoichiometric inputs of strong acids (e.g., sulfuric acid) and bases (e.g., lime or ammonia) to dissolve minerals and precipitate products. This generates significant stoichiometric waste and incurs high environmental and economic costs.

Closed-loop thermochemical alternatives have been proposed, such as ammonium sulfate roasting and "nitrate looping" (using nitric acid). However, these face critical limitations: ammonium sulfate processes suffer from reagent losses (SO₂, NO) and water evaporation costs, while nitrate looping requires the thermal decomposition of highly soluble magnesium nitrate hexahydrate. This decomposition is highly endothermic (630 kJ/mol Mg), and the prior evaporation of water to isolate the solid salt incurs additional energy costs (610 kJ/mol Mg). Consequently, current closed-loop methods struggle to meet the competing requirements of rapid mineral dissolution, selective separation of metal ions, and energy-efficient acid regeneration.

Methodology
This work proposes a "reversible acidification" approach using sulfur dioxide (SO₂) gas to modulate pH and anion speciation without stoichiometric reagents. The process relies on the pressure-dependent solubility of metal sulfites. The methodology involves four main stages:

1. High-Pressure Leaching: Natural olivine is exposed to aqueous solutions under elevated SO₂ partial pressures (up to 3.5 atm) at 80 °C. The dissolved SO₂ forms sulfurous acid, dissolving the silicate to release Mg²⁺ and Fe²⁺ as bisulfites while leaving silica as a solid residue.
2. Pressure-Swing Separation: The system utilizes the differential solubility of iron and magnesium sulfites. By progressively depressurizing the SO₂, the pH increases, and the solubility of metal sulfites decreases. Because FeSO₃ is less soluble than MgSO₃, Fe²⁺ precipitates first as a solid sulfite, followed by Mg²⁺ at lower pressures. This allows for the sequential separation of cations via solid-liquid separation.
3. Counter-Current Process Modeling: To optimize separation efficiency, the authors mapped the phase space of the MgO-FeO-SO₂-H₂O system. They developed a framework for multi-stage counter-current precipitation, using experimental solubility data (Roozeboom plots) to predict the necessary SO₂ pressures for specific separation targets (purity vs. yield).
4. Thermal Decomposition and Recycling: The separated metal sulfites (FeSO₃ and MgSO₃) are thermally calcined. This step regenerates SO₂ gas for reuse in the leaching stage and produces purified metal oxides (Fe₂O₃ and MgO).

Key Results

• Rapid Dissolution: Under 3.5 atm SO₂ at 80 °C, olivine dissolves rapidly, achieving ~80% dissolution in 8.5 hours and ~100% in 48 hours. The dissolution rates are comparable to those achieved with nitric acid (HNO₃) at equivalent acid concentrations.
• Pure Silica Recovery: The solid residue after leaching is identified as highly purified, amorphous silica gel. It is free of crystalline olivine or pyroxene impurities and demonstrates high reactivity as a pozzolan, fully reacting with CaO to form calcium silicate hydrates.
• Selective Cation Separation: Pressure modulation successfully separates Mg²⁺ and Fe²⁺.
  • At high SO₂ pressures (e.g., 15.8 atm), both ions remain largely in solution.
  • As pressure drops to ~5.8 atm, Fe²⁺ preferentially precipitates, enriching the solid phase to ~66% Fe²⁺.
  • Further depressurization allows for the isolation of Mg²⁺.
  • Two cycles of pressurization/depressurization were sufficient to enrich Mg²⁺ from 92.5% to 99.9% and Fe²⁺ from 8% to 94% in the respective product streams.
• Thermal Recycling: Calcination of the precipitated sulfites at 800–950 °C successfully regenerates SO₂ and yields pure MgO and Fe₂O₃. The resulting MgO shows rapid reactivity with CO₂ and water, confirming its utility for carbon mineralization.
• Energy Efficiency: The estimated minimum enthalpic cost for the SO₂ recycling process is 430 kJ/mol MgO. This is substantially lower than the ~1,240 kJ/mol MgO estimated for the nitrate looping approach. The authors attribute this efficiency to the moderate acidity of SO₂, which avoids the high endothermic costs associated with regenerating strong acids like HNO₃, while still providing sufficient acidity for rapid dissolution due to the leveling effect of water.

Significance and Claims
The paper claims to advance a versatile, closed-loop approach for the total processing of silicate minerals. The fundamental contribution is the demonstration that acid gas pressure modulation can replace stoichiometric acid/base addition to achieve a "pH-swing." This enables:

1. Reversible modulation of metal salt solubility and anion speciation.
2. Separation of metal cations based on differential solubility without water evaporation or the addition of external reagents.
3. Energy efficiency, offering a significantly lower enthalpic cost compared to existing thermochemical closed-loop methods.

The authors posit that this framework establishes foundational principles for efficient hydrothermal processing of multicomponent minerals, potentially applicable to other ion separations and pH-modulating gases, provided the specific solubility and pressure relationships are characterized. The work is presented as a proof-of-concept that overcomes the trade-off between rapid dissolution and energy-efficient reagent recycling in silicate valorization.