熱水環境における2M型珪灰石の炭酸塩化：炭酸ナトリウム溶液を用いた実験

In order to understand the processes and mechanism for carbonation of wollastonite-2M under carbonate-rich hydrothermal condition, chemical reaction of wollastonite-2M with sodium carbonate solution was experimentally examined. Natural wollastonite-2M grains between 1.00 and 1.18 mm in diameter were sealed in the Teflon bottle with aqueous Na₂CO₃ solution. A series of experiment with 1/450 mineral/solution weight ratio was carried out with changing temperature (100, 125, 150 and 175°C), initial Na₂CO₃ concentration (0.01, 0.10 and 1.00 mol/l), and reaction time (4, 8, 12, 24, 72, 168, 336 and 672 hours). The run products were filtered by 0.20 μm membrane filters, and the filtrate and residual solid were analyzed. In addition, a series of experiment with 1/450 mineral/solution weight ratio with wollastonite-2M cubes with 2.00×3.50×4.00 mm was also carried out for observation of solid profiles.<br>Regardless of the various conditions, the only identified reaction product was calcite. Amount of calcite relative to initial wollastonite-2M increases with the increase in temperature, initial Na₂CO₃ concentration and reaction time, in the range of 1.8% for the condition of initial Na₂CO₃ 0.01 mol/l-125°C-8 hours and 55.9% for 1mol/l-150°C-672 hours. These amounts of calcite seem to be small as compared to those from wollastonite-1A. The pH values after reaction for all runs range from 11.1 to 11.8, which are higher than those of initial Na₂CO₃ solution ranging from 10.6 to 11.3. The crystal habits of calcite on the surface differ with the initial Na₂CO₃ concentration and reaction time: rhombic crystals for shorter runs, platy, columnar and their aggregates for longer runs with the condition of initial Na₂CO₃ 0.01 mol/l, polyhedron crystals for longer runs with 0.10 mol/l and larger rhombic and irregular square bipyramidal crystals for longer runs with 1.00 mol/l. Coarsening with time, smooth edges for smaller crystals and etch pits on the surfaces suggests Ostwald ripening. Calcite also develops as replacement within the position of initial wollastonite-2M grains, with some relicts of wollastonite-2M grains. The depth of calcite layer increases with the increase in temperature, initial Na₂CO₃ concentration and reaction time. The rate of calcite formation from wollastonite-2M for 24 hours increases with the increase of temperature and initial Na₂CO₃ concentration, in the range of 1.0 x 10^-8 mol·s^-1·m^-2 for the condition of initial Na₂CO₃ 0.01 mol/l-100°C and 5.5 x 10^-8 mol·s^-1·m^-2 for 1.00 mol/l-175°C. Thus, carbonation of wollastonite-2M proceeds under carbonate-rich hydrothermal condition even at low temperature.