熱水環境におけるソーダ珪灰石の炭酸塩化

炭酸ナトリウム溶液と炭酸水素ナトリウム溶液を用いた実験

抄録

In order to understand the processes and mechanism for carbonation of pectolite under hydrothermal condition, chemical reaction of pectolite with sodium carbonate and bicarbonate solution was experimentally examined. In the experiment, crashed and sieved pectolite grains between 1.00 and 1.18 mm in diameter were sealed in the polypropylene bottle with sodium carbonate or bicarbonate solution, and left for fixed duration in time. A series of experiment with 1/450 mineral/solution weight ratio was carried out with changing temperature (110-150°C), concentration of sodium carbonate or bicarbonate solution (0.01, 0.1 and 1 mol/l), and reaction time (7 to 56 days). The run products were filtered by 0.20 μm membrane filters, and the filtrated solution and residual solid were analyzed.
In the residual solid for all runs, the one and only identified reaction product was calcite. Calcite grains grew on surfaces of initial pectolite grains. Amount of calcite increases with the increase of temperature, initial Na₂CO₃ or NaHCO₃ concentration and reaction time, in the range of 88.6% calcite/total Ca (150°C-1 mol/l) and 17.9% (110°C-0.01 mol/l) for NaHCO₃ runs of 56 days, and of 60.2 % (150°C-1 mol/l) and 18.2% (130°C-0.01 mol/l) for Na₂CO₃ runs of 56 days. The size of calcite grains increases with the increase of reaction time, suggesting Ostwald ripening. Residual silica layers cannot be recognized on surfaces. The Si concentration in the filtrated solution increases with the increase of temperature, initial Na₂CO₃ or NaHCO₃ concentration and reaction time. The stoichiometric relationship between the Si concentration in solution and amount of calcite and no Si-rich solid layer on pectolite suggest that dissolution of pectolite proceeds congruently. The rate of calcite formation from pectolite for 7 days increases with the increase of temperature and initial concentration, in the range of 2.21 x 10^-8 mol·s^-1·m^-2 for the condition of initial Na₂CO₃ 0.1 mol/l-130°C and 1.10 x 10^-7 mol·s^-1·m^-2 for the condition of initial Na₂CO₃ 0.1 mol/l-150°C for Na₂CO₃ runs, and of 2.30 x 10^-8 mol·s^-1·m^-2 for the condition of initial Na₂CO₃ 0.01 mol/l-130°C and 1.34 x 10^-7 mol·s^-1·m^-2 for the condition of initial Na₂CO₃ 1 mol/l-150°C for NaHCO₃ runs. Thus, carbonation of pectolite proceeds easily under hydrothermal carbonate-rich condition.