S2: Water Rock Interaction (Special Session)

[S2-04] CO₂ mineralization in andesitic rocks revealed by hydrothermal experiments and thermal analyses

*Otgonbayar DANDAR¹, Atsushi Okamoto ¹, Masaoki Uno¹, Miku Takeya² (1. Tohoku University, 2. INPEX)

Keywords:Mineral carbonation , CO2 storage, Andesite, Hydrothermal reaction experiment, the Nagaoka area, Niigata Prefecture

Mineral carbonation enables long-term stable CO₂ storage. Among rocks, basalts and ultramafic rocks (high Mg and Ca contents) are known to have high potential to store CO₂. Although, andesitic rocks (composition is wide; microstructures are diverse) are widely distributed in Japan (in a subduction zone), their suitability for mineral carbonation and CO₂ storage is not well understood. In this study, we report preliminary results of hydrothermal reaction experiments (exps) using andesitic agglomerate rocks from the Nagaoka area, Niigata Prefecture and thermal analyses of the products.The andesite agglomerate consists mainly of volcanic glass, plagioclase, clinopyroxene, with small amounts of orthopyroxene, hornblende, and iron oxides. Two types (a batch-type vessel and a stirred apparatus) of exps have been conducted with the powder samples (150-250 μm). In the batch exp (filling ratio = 43% and remained space is CO₂ gas), powder sample is placed at two locations: the top (supercritical CO₂ saturated in water) and bottom (reacted with a CO₂ dissolved solution) of the reaction vessel. The batch exps were conducted at 75 ℃, 18 MPa (80 days), 150 ℃, 20 MPa (20 days), and 200 ℃, 25 MPa (20, 40, and 60 days). The solid phase after exp is observed by EPMA, TG, and TPD-MS, and the solutions are analyzed for element contents by ICP-OES. To accelerate the reaction, a stirred reaction apparatus was used for the exp at 200 ℃, 20MPa, and 10 days.In the batch exps at 75 ℃ and 150 ℃ (Ca and Mg+Fe contents = 10-80 mg/kgH₂O), dissolution of volcanic glass is observed only with small dissolution of plagioclase and other minerals. Significant carbonate precipitation was observed in a 60-day batch exp at 200 ℃ (Fe- and Ca-rich magnesite) and a 10-day stirred exp (calcite and dolomite). The precipitation proceeded not only at the bottom (CO₂ dissolved solution), but also at the top (supercritical CO₂), suggesting the reaction accelerated by water vapor. The experimental solution (carbonate precipitated, the pH increased to >7 and the concentrations of Mg and Ca <20 mg/kH₂O), suggests the pH in the reaction vessel increased and reached to condition of carbonate precipitation.Based on the above exps, the andesitic rocks in the Nagaoka area are considered to have sufficient potential for mineral carbonation and CO₂ storage.