Reactive CO2 Capture and Mineralization of Magnesium Hydroxide to Produce Hydromagnesite with Inherent Solvent Regeneration

Abstract

Valorization of multiple low value streams including CO2 emissions and magnesium-hydroxide bearing mine tailings to produce magnesium carbonate through reactive CO2 capture and mineralization provides a less explored opportunity to manage several gigatons of CO2 emissions. To resolve the feasibility of converting magnesium hydroxide to magnesium carbonate through reactive CO2 capture and mineralization, CO2 capture solvents such as sodium glycinate are harnessed to capture CO2 and react directly with Mg(OH)2 to produce hydromagnesite (Mg5[(CO3)4(OH)2]·4H2O). This approach eliminates the energy-intensive step of producing high purity CO2 associated with regenerating the solvent, and redissolving CO2 to produce magnesium carbonate. Interestingly, while temperatures below 50 °C facilitate CO2 capture, the mineralization kinetics are slow. However, at higher temperatures, accelerated carbon mineralization is favored by the faster kinetics of Mg(OH)2 dissolution and precipitation of magnesium carbonate. Reacting Mg(OH)2 at 90 °C with 15 wt% solids in the presence of 2.5 M sodium glycinate after 3 hours under well-stirred conditions results in an extent of carbon mineralization of 75.5%. The theoretical maximum extent of carbon mineralization when hydromagnesite is formed is 80%. Pre-loading CO2 on the solvent is also an effective approach to ensure that sufficient CO2 is available for reactive CO2 capture and mineralization, particularly when dilute CO2 and N2 mixtures are used. Higher extents of carbon mineralization are associated with an increase in the particle size and a reduction in the cumulative pore volume. These insights unlock the feasibility of harnessing reactive CO2 capture and mineralization as a pathway to convert magnesium-hydroxide bearing resources into industrially relevant magnesium carbonate products.