The carbonation of chrysotile [Mg 3 Si 4 O 10 (OH) 4 ] under subcritical conditions was experimentally investigated in alkali solution, and the chemical composition as well as the morphological and structural changes were discussed. The starting material was hydrothermally treated by aqueous direct carbonation at a temperature of 100 C and a CO 2 partial pressure range of 0.5 MPa$4 MPa at pH 13. Highly crystalline magnesite was synthesized under a CO 2 partial pressure of 3 MPa. The carbonation rate increased at the proportional rate according to the applied CO 2 pressure to approximately 57%. The surface morphology of chrysotile changed from the fibrous form to a round or oval shape at the initial stage and subsequently to magnesite with well-faceted rhombohedral planes.The dissolution rate of Mg was higher than Si, such that the Mg : Si ratio of chrysotile decreased from 1.56 to 0:4$0:6 as the reaction time increased. The resultant silica-rich layer of the reaction product ultimately changed through the Mg-depleted skeletal phase to the amorphous silica phase. The experimental results suggest that carbonation in alkali solution under subcritical conditions is of great significance because an excessive amount of acid and alkali reagents can be eliminated in the carbonation process.
A novel pretreatment method for the mineral serpentine was proposed to develop an effective carbonation process for CO 2 sequestration. Basically, the method involved preheating a mixture of oxalic acid and serpentine and the subsequent aqueous carbonation. The addition of oxalic acid was found to stimulate the decomposition of serpentine during heat treatment. X-ray diffraction analysis revealed that serpentine is transformed into magnesium oxalate and magnesium oxide by heat treatments at 200 C and 500 C, respectively. The addition of oxalic acid was found to enhance the overall carbonation reaction owing to the formation of magnesium oxide during heat treatment. Thermogravimetric analysis showed that approximately 70% of the Mg 2þ was transformed into magnesium oxide. Using this method, a carbonation rate as high as 72% could be obtained by aqueous carbonation at 100 C under 4 MPa. This novel method can potentially reduce the high energy cost and unavoidable use of expensive chemicals for pH control.
The carbonation of chrysotile [Mg 3 Si 2 O 5 (OH) 4 ] was studied at various temperatures in order to examine the carbonation rate and Mg leaching properties in an alkali solution and distilled water using the direct method. For the efficient carbonation reaction, the chrysotile was converted to metachrysotile by heating at 630°C for 2 h. The carbonation rate was found to increase with the reaction temperature: 5 and 31% at 100°C and 45 and 53% at 260°C in distilled water and alkali solution, respectively. The carbonation in the alkali solution was faster producing well-faceted rhombohedral magnesite (MgCO 3 ) in comparision to the case in the distilled water. Highly crystalline magnesite could be obtained after the carbonation for 1 h at 260°C under a CO 2 pressure of 3 MPa in the alkali solution.The additional object of this study was to convert the fibrous chrysotile to a non-hazardous material. During carbonation, the magnesite exhibited the typical rhombohedral morphology while the unreacted chrysotile changed primarily to aggregations of chrysotile crystals. This result has an important implication for the elimination of toxicity through the transformation of chrysotile asbestos into a non-hazardous material.
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