Reversible Acidification Enables Closed-Loop Total Separation of Olivine

<title>Abstract</title> <p> Silicate minerals such as olivine are abundant in mineral deposits and mine tailings, and contain alkalinity, Mg ²⁺ , Fe ²⁺ , and other trace metals. While historically considered waste, the total processing and separation of these components can generate MgO for CO ₂ capture and storage, high-grade iron ore, and pozzolanic silica. However, many existing approaches to silicate processing are linear, consuming stoichiometric reagents and generating large volumes of waste. While thermochemical hydrometallurgical approaches to closed-loop silicate processing have been proposed, they typically face incomplete reagent recycling and high enthalpic costs. Herein, we address these challenges by employing a pressure-swing approach with SO ₂ , which reversibly modulates the pH and solubility of metal salts. This reversible modulation of pH enables the use of a traditional ‘pH-swing’ approach to separate metal salts but does so without the addition of stoichiometric acid and base. First, we demonstrate that under high SO ₂ pressure of 3.5 atm, olivine dissolves rapidly in aqueous solutions. Next, we show that depressurization of SO ₂ increases pH incrementally leading to sequential precipitation of Fe ²⁺ and then Mg ²⁺ sulfite salts. We find that two cycles of pressurization/depressurization are sufficient to enrich Mg ²⁺ (92.5% to 99.9%) and Fe ²⁺ (8% to 94%). We further show that SO ₂ pressure modulation allows one to fully sample the phase space of separation and develop a framework for leveraging SO ₂ pressure modulation in counter current ion separations. Finally, we demonstrate that the metal sulfites we produce can be thermally decomposed to recycle SO ₂ and form metal oxides. The enthalpic cost of the proposed process is estimated to be 430 kJ/mol MgO, substantially lower than other proposed approaches. This work advances a versatile approach to closed-loop total processing of silicate minerals. </p>