Optimizing milling energy for enhancement of solid-state magnesium sulfate (MgSO4) thermal extraction for permanent CO2 storage

Abstract

Mineral carbonation of Mg-silicates via the indirect-dry route is among the most appealing technical approaches for permanent CO₂ storage. It brings about the possibility of recycling heat released during exothermic carbonation and it offers a higher rate of conversion through two separate stages: producing of reactive compounds, mainly Mg(OH)₂, through solid reactions between Mg-silicates and ammonium sulfates and subsequent carbonation of reactive compounds. Milling is essential to enhance the solid-state reaction rate and increase the conversion percentage. The milling energy, being the major energy consumption of the entire carbonation process, needs to be minimized without sacrifice of its activation purpose. This study focuses on enhancing the kinetics of solid-state magnesium sulfate (MgSO₄) thermal extraction from the solid–solid reaction of olivine ((Fe,Mg)₂SiO₄) and ammonium sulfate ((NH₄)₂SO₄), with optimized milling energy input. This process constitutes the first stage of Mg(OH)₂ production for indirect CO₂ storage purposes. The mechanical activation of reactants via a high-energy magneto ball milling with a controlled energy input is achieved. The variation of structural parameters such as the particle size, specific surface area (SSA), pore volume, and crystallite size and strain are characterized as a function of milling energy input and the correlation between structural factors and activation energy of extraction is investigated. In addition, the variation in the apparent activation energy of solid-state extraction is examined as a function of milling energy. The optimal amount of milling energy input for increasing the reaction kinetics of MgSO₄ extraction is estimated to be about 27.6 kJ g⁻¹ which causes around 34% reduction in the activation energy of MgSO₄ solid-state extraction.