Controlled rate thermal analysis of hydromagnesite (original) (raw)
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The Journal of Geology, 2008
This study reports the nature of the nesquehonite-to-hydromagnesite transition at 52°C in an aqueous medium hosting magnesian calcite and nesquehonite. The latter mineral occurs with abundant calcite at the floor of the experimental chamber (substrate) and as a film of needles at the interface between the mother liquor and atmosphere (surface film). The experimental vessel was held at 52°C for 336 h and at 60°C for a further 192 h. Precipitates were analyzed by Fourier transform (FT)-Raman, augmented by FT-infrared and x-ray diffraction. At 52°C, hydromagnesite and dypingite occur with abundant quantities of a hitherto unreported transitory magnesium hydrate carbonate (TMHC), together with huntite, magnesian calcite, and traces of nesquehonite and monohydrocalcite. The FT-Raman spectra of the first-formed hydromagnesite crystals contain the Raman forbidden v 2 mode, interpreted to indicate a relaxation in selection rules, caused by rapid precipitation. Hydromagnesite growth at the expense of TMHC was more advanced in the substrate than in the coexisting surface film. Additional heating at 60°C resulted in the loss of TMHC and emergence of a dypingite-and hydromagnesite-rich assemblage, with associated strengthening of selection rules. Transitory magnesium hydrate and hydroxyl carbonates and huntite formed during CO 2 degassing, fueled by the thermally driven decrease in solubility of CO 2 in water and the progressive dissolution of metastable phases. Advancement of the N→HM transition in the substrate most likely reflects greater Ca 2þ ðaqÞ availability to promote acid generation through calcite precipitation, thereby accelerating transitory-phase dissolution.
Chemical Geology, 2013
The application of hydrated Mg-carbonates as CO 2 sequestering media is a pressing environmental challenge, which requires a deep knowledge of the phase transitions occurring in the Mg\CO 2 \H 2 O system as well as the thermal and structural stability of these phases. In this paper we investigate the phase transition of nesquehonite (MgCO 3 ·3H 2 O) to dypingite (Mg 5 (CO 3 ) 4 (OH) 2 · 5H 2 O), occurring after an incubation of months and years in solution, at ambient conditions. However, as the kinetics of this process resulted to be slow, the phase transition of dypingite to hydromagnesite (Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O) was investigated at non-ambient conditions using in situ real-time high-resolution X-ray powder diffraction. Moreover, the thermal behaviour of dypingite and its decomposition have been also investigated with the aim to explore the appropriateness of this carbonate and the products of its decomposition as sinks of anthropogenic carbon dioxide. The results suggest that the dypingite structure remains unaffected up to 438 K. At temperature above this threshold, dypingite transforms into hydromagnesite. A further increase in temperature converts the well-ordered hydromagnesite into a "collapsed form" at 528 K. The heating of dypingite does not produce a loss of CO 2 as the intermediate phases have the same CO 2 :Mg molar ratio. The final product of the heating is periclase MgO. Its nucleation occurs at temperature ranging from 573 to 663 K and it becomes the only phase in the temperature range 633-678 K. These results highlighted that dypingite assures a stable storage of CO 2 in the conditions that prevail at the Earth's surface. Moreover, the transformation of dypingite in more thermodynamically stable hydromagnesite, occurring without release of CO 2 , enhances the safety of carbon dioxide disposal in solid form. Furthermore, the observed volume changes during phase transitions, which in turn could affect the porosity and permeability of the geological reservoir, have been evaluated in order to improve the prediction of the safe and permanent storage of CO 2 in underground.
The Journal of Geology, 2008
This study reports the nature of the nesquehonite-to-hydromagnesite transition at 52°C in an aqueous medium hosting magnesian calcite and nesquehonite. The latter mineral occurs with abundant calcite at the floor of the experimental chamber (substrate) and as a film of needles at the interface between the mother liquor and atmosphere (surface film). The experimental vessel was held at 52°C for 336 h and at 60°C for a further 192 h. Precipitates were analyzed by Fourier transform (FT)-Raman, augmented by FT-infrared and x-ray diffraction. At 52°C, hydromagnesite and dypingite occur with abundant quantities of a hitherto unreported transitory magnesium hydrate carbonate (TMHC), together with huntite, magnesian calcite, and traces of nesquehonite and monohydrocalcite. The FT-Raman spectra of the first-formed hydromagnesite crystals contain the Raman forbidden v 2 mode, interpreted to indicate a relaxation in selection rules, caused by rapid precipitation. Hydromagnesite growth at the expense of TMHC was more advanced in the substrate than in the coexisting surface film. Additional heating at 60°C resulted in the loss of TMHC and emergence of a dypingite-and hydromagnesite-rich assemblage, with associated strengthening of selection rules. Transitory magnesium hydrate and hydroxyl carbonates and huntite formed during CO 2 degassing, fueled by the thermally driven decrease in solubility of CO 2 in water and the progressive dissolution of metastable phases. Advancement of the N→HM transition in the substrate most likely reflects greater Ca 2þ ðaqÞ availability to promote acid generation through calcite precipitation, thereby accelerating transitory-phase dissolution.
Phase transitions in the system MgO–CO2–H2O during CO2 degassing of Mg-bearing solutions
Geochimica et Cosmochimica Acta, 2012
This study documents the paragenesis of magnesium carbonates formed during degassing of CO 2 from a 0.15 M Mg 2+ aqueous solution. The starting solutions were prepared by CO 2 sparging of a brucite suspension at 25°C for 19 h, followed by rapid heating to 58°C. One experiment was performed in an agitated environment, promoted by sonication. In the second, CO 2 degassing was exclusively thermally-driven (static environment). Electric conductance, pH, and temperature of the experimental solutions were measured, whereas Mg 2+ was determined by atomic absorption spectroscopy. Precipitates were analysed by X-ray diffraction, Fourier transform (FT) mid-infrared, FT-Raman, and scanning electron microscopy.
Dynamic and controlled rate thermal analysis of hydrozincite and smithsonite
Journal of Thermal Analysis and Calorimetry, 2008
The understanding of the thermal stability of zinc carbonates and the relative stability of hydrous carbonates including hydrozincite and hydromagnesite is extremely important to the sequestration process for the removal of atmospheric CO 2. The hydration-carbonation or hydration-and-carbonation reaction path in the ZnO-CO 2-H 2 O system at ambient temperature and atmospheric CO 2 is of environmental significance from the standpoint of carbon balance and the removal of green house gases from the atmosphere. The dynamic thermal analysis of hydrozincite shows a 22.1% mass loss at 247°C. The controlled rate thermal analysis (CRTA) pattern of hydrozincite shows dehydration at 38°C, some dehydroxylation at 170°C and dehydroxylation and decarbonation in a long isothermal step at 190°C.(erre még visszatérni) The CRTA pattern of smithsonite shows a long isothermal decomposition with loss of CO 2 at 226°C. CRTA technology offers better resolution and a more detailed interpretation of the decomposition processes of zinc carbonate minerals via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. The CRTA technology offers a mechanism for the study of the thermal decomposition and relative stability of minerals such as hydrozincite and smithsonite.
Carbon Dioxide Sequestration by Aqueous Mineral Carbonation of Magnesium Silicate Minerals
Greenhouse Gas Control Technologies - 6th International Conference, 2003
The dramatic increase in atmospheric carbon dioxide since the Industrial Revolution has caused concerns about global warming. Fossil-fuel-fired power plants contribute approximately one third of the total human-caused emissions of carbon dioxide. Increased efficiency of these power plants will have a large impact on carbon dioxide emissions, but additional measures will be needed to slow or stop the projected increase in the concentration of atmospheric carbon dioxide. By accelerating the naturally occurring carbonation of magnesium silicate minerals it is possible to sequester carbon dioxide in the geologically stable mineral magnesite (MgCO 3). The carbonation of two classes of magnesium silicate minerals, olivine (Mg 2 SiO 4) and serpentine (Mg 3 Si 2 O 5 (OH) 4), was investigated in an aqueous process. The slow natural geologic process that converts both of these minerals to magnesite can be accelerated by increasing the surface area, increasing the activity of carbon dioxide in the solution, introducing imperfections into the crystal lattice by high-energy attrition grinding, and in the case of serpentine, by thermally activating the mineral by removing the chemically bound water. The effect of temperature is complex because it affects both the solubility of carbon dioxide and the rate of mineral dissolution in opposing fashions. Thus an optimum temperature for carbonation of olivine is approximately 185 o C and 155 o C for serpentine. This paper will elucidate the interaction of these variables and use kinetic studies to propose a process for the sequestration of the carbon dioxide.
The thermal decomposition of huntite and hydromagnesite—A review
Thermochimica Acta, 2010
Naturally occurring mixtures of hydromagnesite and huntite are important industrial minerals. Their endothermic decomposition over a specific temperature range, releasing water and carbon dioxide, has lead to such mixtures being successfully used as fire retardants, often replacing aluminium hydroxide or magnesium hydroxide. The current understanding of the structure and thermal decomposition mechanism of both minerals and their combination in natural mixtures is reviewed. The crystalline structure of both minerals has been fully characterised. The thermal decomposition of huntite has been characterised and is relatively simple. However, the thermal decomposition mechanism of hydromagnesite is sensitive to many factors including rate of heating and the composition of the atmosphere. The partial pressure of carbon dioxide significantly affects the decomposition mechanism of hydromagnesite causing magnesium carbonate to crystallise and decompose at a higher temperature instead of decomposing directly to magnesium oxide.
The thermal decomposition of natural mixtures of huntite and hydromagnesite
Thermochimica Acta, 2012
The thermal decomposition of natural mixtures of huntite and hydromagnesite has been investigated. Hydromagnesite decomposes endothermically giving off water and carbon dioxide, the mechanism is dependent on heating rate. Mass losses measured by TGA are consistent with the loss of four water molecules, from the loss of water of crystallisation, and one water molecule from the decomposition of the hydroxide ion, followed by the loss of four carbon dioxide molecules from the decomposition of the carbonate ions. The magnesium carbonate, remaining after the dehydration of hydromagnesite, recrystallises exothermically in response to higher heating rates. This causes the decomposition of the carbonate ions to split into two stages with the second stage moving to a higher temperature. The magnitude of each stage is dependent on the heating rate. Huntite decomposes endothermically, at a higher temperature, giving off carbon dioxide in two stages. Mass losses measured by TGA are consistent with the loss of three carbon dioxide molecules, from the decomposition the carbonate ions associated with the three magnesium ions, followed by the loss of a single carbon dioxide molecule associated with the decomposition of the carbonate ion associated with the calcium ion.
Journal of Hazardous Materials, 2010
In order to gauge the appropriateness of CO 2 reaction with Mg chloride solutions as a process for storing carbon dioxide, the thermal behaviour and structural stability of its solid product, nesquehonite (MgCO 3 ·3H 2 O), were investigated in situ using real-time laboratory parallel-beam X-ray powder diffraction. The results suggest that the nesquehonite structure remains substantially unaffected up to 373 K, with the exception of a markedly anisotropic thermal expansion acting mainly along the c axis. In the 371-390 K range, the loss of one water molecule results in the nucleation of a phase of probable composition MgCO 3 ·2H 2 O, which is characterized by significant structural disorder. At higher temperatures (423-483 K), both magnesite and MgO·2MgCO 3 coexist. Finally, at 603 K, periclase nucleation starts and the disappearance of carbonate phases is completed at 683 K. Consequently, the structural stability of nesquehonite at high temperatures suggests that it will remain stable under the temperature conditions that prevail at the Earth's surface. These results will help (a) to set constraints on the temperature conditions under which nesquehonite may be safely stored and (b) to develop CO 2 sequestration via the synthesis of nesquehonite for industrial application.