Erkki Levänen

Carbon mineralization is expected to play a key role in the mitigation of climate change, as viable and efficient carbon capture and utilization (CCU) pathway. Indeed, the process has the advantage of enabling the recycling of waste-streams such as mine tailings or desalination brine, as well as the prospect of large-scale uptake of carbon dioxide emissions. However, the applications of the produced carbonates still hinder the commercial feasibility of the existing CCU routes, especially when hydrated Mg carbonates (HMCs) are obtained. HMCs are thermodynamically unstable, which poses potential risks in long-term stability. Nesquehonite (NQ, MgCO₃·3H₂O) is the major product of Mg carbonation in most aqueous reaction settings at moderate temperatures (15–50 °C), which has demonstrated suitable properties for producing construction materials. At somewhat higher temperatures (50–100 °C) hydromagnesite is obtained (4MgCO₃·Mg(OH)₂·4H₂O). Yet, the final applications are not feasible as NQ often converts to hydromagnesite or other HMCs over time causing liberation of CO₂ and volume instability. A key scientific gap remains on the relationship between the morphology of NQ with the operational settings of the carbonation reaction. In turn, such understanding is needed to enable tuning NQ applications in construction materials. Therefore, the current work reports the observed features of NQ via three different synthetic routes, showing the effect of two additives (Mg acetate, and sodium dodecyl sulphate), and overpressure CO₂ on the physico-chemical features of NQ formed from magnesium chloride or sulphate solutions and from brucite-water system and sCO₂ conditions.

Wollastonite (CaSiO₃) is the most researched and well-defined mineral in the field of CO₂ mineralization, but it is also a sought-after process mineral and thus, not easily justified for large scale ex situ carbon sequestration, which requires an energy-intensive step of comminution to increase reactivity. Wollastonite-rich mine tailings are a side stream with an already fine particle size resulting from the extractive process, but their effective utilization is problematic due to legislation, logistics, a high number of impurities, and chemical inconsistency. In this study, the accelerated weathering (aqueous carbonation) of high-calcite (CaCO₃) wollastonite tailings was studied under elevated temperatures and high partial pressures of CO₂ to determine the carbon sequestration potential of those tailings compared to those of pure reference wollastonite originating from the same quarry. The main process variables were pressure (20–100 bar), temperature (40 °C–60 °C), and time (10 min–24 h). Despite consisting largely of non-reactive silicates and primary calcite, very fine tailings showed promise in closed-chamber batch-type aqueous carbonation, achieving a conversion extent of over 28% in one hour at 100 bar and 60 °C.