Mineral sequestration of carbon dioxide: Enhancing carbonation reactivity of brucite and forsterite

Gas-phase brucite dehydroxylation and simultaneous dehydroxylation/carbonation processes were investigated to better understand the fundamental mechanisms involved and the role of different parameters governing them. Dehydroxylation was found to generally precede carbonation as a distinct but interrelated process. Above the minimum CO2 pressure for brucite carbonation, reactivity for both carbonation and dehydroxylation decreases with increasing CO2 pressure. Low-temperature dehydroxylation prior to carbonation can form porous intermediate materials with enhanced carbonation reactivity at reduced temperature and pressure. Major morphological changes observed are: dimensional changes, blister formation, cracking, delamination, and crystal growth. The extent and rate of these changes are driven by two groups of forces: (i) evolution of water vapor inside the crystal due to dehydroxylation and simultaneous formation of MgO in the Mg(OH)2 matrix that leads to high strain and surface energy and (ii) exterior CO2 pressure and formation of carbonate passivating layers. Carbonation reactivity of the material is therefore the resultant effect of both groups of forces. Understanding the formation mechanisms and roles of the above morphological changes is crucial to enhancing carbonation reactivity of the material and reducing its carbonation process cost.; Aqueous-solution olivine mineral carbonation process was investigated to better understand its mechanisms. Key mechanisms that impact carbonation reactivity include: passivating silica layer formation, cracking, and exfoliation; silica surface migration; etch pit formation; particle-particle and particle-wall abrasion; and nucleation and growth of magnesite crystals on/in the silica/olivine reaction matrix. Fe present in the olivine is carried into the product carbonate, along with Mg. Magnesite crystals appear to grow both into and away from the reaction matrix. Reaction interface regions have been observed to contain a structurally disrupted reaction front, which precedes mineral carbonation, underscoring the importance of olivine structural disruption during mineral carbonation. In an intensely stirred medium, the extent of carbonation increases with olivine solids weight ratio up to a maximum and then decreases. For both natural and synthetic forsterite materials, optimum solids weight ratio for carbonation is ∼15%. Passivating layer cracking and particle abrasion may play substantial roles in enhancing carbonation reactivity, whereas the persistence of passivating layers and interparticle agglomeration may hinder it.