• Economic and Environmental Geology
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• v.41 no.2
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• pp.165-172
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• 2008

Kaolinite [$Al_2Si_2O_5(OH)_4$] used in these experiments was synthesized at 250$^{\circ}C$ for 36 hrs by a hydrothermal process from amorphous $Al(OH)_3$ and $SiO_2$. The change of the mineralogical properties of the phase synthesized were observed in the pH range 2 to 9. The synthetic kaolinite were characterized by the analytical methods of XRD, IR, DIA, and FE-SEM. Kaolinite was obtained in a wide range of pH. The phases with high- to midium- defect kaolinite with high thermal stability were obtained from the acidic conditions and high-defect kaolinite with low thermal stability from the basic conditions. These variations of kaolinite properties appears to be related to the pH dependence of kaolinite surface speciation. The peaks intensity and resolution of the kaolinite decrease according to the alkalinity of the solution by the results of the IR testing. And the peak intensity increases in the 60 to 70$^{\circ}C$ range due to dehydration reaction observed by TG-DTA. Such phenomena was the result of increase of unreacted amorphous materials in the high pH condition, which could be identified by FE-SEM.

• Economic and Environmental Geology
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• v.53 no.1
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• pp.1-9
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• 2020

Magnesium silicate minerals such as serpentine [Mg₃Si₂O₅(OH)₄] have a high potential for the sequestration of CO₂; thus, their reactivity toward dissolution under CO₂-free and CO₂-containing conditions in acidic solvents is a critical process with respect to their carbonation reactions. To examine the carbonation efficiency and dissolution mechanism of serpentine, hydrothermal treatment was performed to the starting material via a modified direct aqueous carbonation process at 100 and 150℃. The serpentine dissolution experiments were conducted in H₂SO₄ solution with concentration range of 0.3-1 M and at a CO₂ partial pressure of 3 MPa. The initial pH of the solution was adjusted to 13 for the carbonation process. Under CO₂-free and CO₂-containing conditions, the carbonation efficiency increased in proportion to the concentration of H₂SO₄ and the reaction temperature. The leaching rate under CO₂-containing conditions was higher than that under CO₂-free conditions. This suggests that shows the presence of CO₂ affects the carbonation reaction. The leaching and carbonation efficiencies at 150℃ in 1 M H₂SO₄ solution under CO₂-containing conditions were 85 and 84%, respectively. The dissolution rate of Mg was higher than that of Si, such that the Mg : Si ratio of the reacted serpentine decreased from the inner part (approximately 1.5) to the outer part (less than 0.1). The resultant silica-rich layer of the reaction product ultimately changed through the Mg-depleted skeletal phase and the pseudo-serpentine phase to the amorphous silica phase. A passivating silica layer was not observed on the outer surface of the reacted serpentine.