Rhombohedral calcite precipitation from CO2-H2O-Ca(OH)2 slurry under supercritical and gas CO2 media

The formation of solid calcium carbonate (CaCO3) from aqueous solutions or slurries containing calcium and carbon dioxide (CO2) is a complex process of considerable importance in the ecological, geochemical and biological areas. Moreover, the demand for powdered CaCO3 has increased considerably recently in various fields of industry. The aim of this study was therefore to synthesize fine particles of calcite with controlled morphology by hydrothermal carbonation of calcium hydroxide at high CO2 pressure (initial PCO2=55 bar) and at moderate and high temperature (30 and 90 degrees C). The morphology of precipitated particles was identified by transmission electron microscopy (TEM/EDS) and scanning electron microscopy (SEM/EDS). In addition, an X-ray diffraction analysis was performed to investigate the carbonation efficiency and purity of the solid product. Carbonation of dispersed calcium hydroxide in the presence of supercritical (PT=90 bar, T=90 degrees C) or gaseous (PT=55 bar, T=30 degrees C) CO2 led to the precipitation of sub-micrometric isolated particles (<1$\mu$m) and micrometric agglomerates (<5$\mu$m) of calcite. For this study, the carbonation efficiency (Ca(OH)2-CaCO3 conversion) was not significantly affected by PT conditions after 24 h of reaction. In contrast, the initial rate of calcium carbonate precipitation increased from 4.3 mol/h in the "90bar-90 degrees C" system to 15.9 mol/h in the "55bar-30 degrees C" system. The use of high CO2 pressure may therefore be desirable for increasing the production rate of CaCO3, carbonation efficiency and purity, to approximately 48 kg/m3h, 95% and 96.3%, respectively in this study. The dissipated heat for this exothermic reaction was estimated by calorimetry to be -32 kJ/mol in the "90bar-90 degrees C" system and -42 kJ/mol in the "55bar-30 degrees C" system

Experimental assessment of CO2-mineral-toxic ion interactions in a simplified freshwater aquifer: Implications for CO2 leakage from deep geological storage

International audienceThe possible intrusion of CO2 into a given freshwater aquifer due to leakage from deep geological storage involves a decrease in pH, which has been directly associated with the remobilization of hazardous trace elements via mineral dissolution and/or via desorption processes. In an effort to evaluate the potential risks to potable water quality, the present study is devoted to experimental investigation of the effects of CO2 intrusion on the mobility of toxic ions in simplified equilibrated aquifers. We demonstrate that remobilization of trace elements by CO2 intrusion is not a universal physicochemical effect. In fact goethite and calcite, two minerals frequently found in aquifers, could successfully prevent the remobilization of adsorbed Cu(II), Cd(II), Se(IV) and As(V) if CO2 is intruded into a drinking water aquifer. Furthermore, a decrease in pH resulting from CO2 intrusion could reactivate the adsorption of Se(IV) and As(V) if goethite and calcite are sufficiently available in underground layers. Our results also suggest that adsorption of cadmium and copper could be promoted by calcite dissolution. These adsorbed ions on calcite are not remobilized when CO2 is intruded into the system, but it intensifies calcite dissolution. On the other hand, arsenite As(III) is significantly adsorbed on goethite, but is partially remobilized by CO2 intrusion

Gas-solid carbonation of Ca(OH)2 and CaO particles under non-isothermal and isothermal conditions by using a thermogravimetric analyzer: Implications for CO2 capture

International audienceThe gas-solid carbonation of alkaline sorbents has been actively investigated as an alternative method to CO2 capture from industrial combustion sources and CO2 contained in the air. This study has a two-fold objective: firstly, quantify the gas-solid carbonation extent and the carbonation kinetics of Ca(OH)2 and CaO; and secondly, propose a reaction mechanism of gas-solid carbonation for CaO under dry conditions (relative humidity close to 0), i.e., when the action of water is negligible. The main results of our study have revealed that a high proportion of Ca(OH)2 nanoparticles were transformed into CaCO3 particles by gas-solid carbonation (carbonation extent, >0.94) under non-isothermal conditions. Moreover, this gas-solid reaction requires low activation energy (Ea≈6kJ/mol) at a constant heating rate of 5 or 10K/min. A similar carbonation extent was determined for gas-solid carbonation of in-situ synthesized CaO under non-isothermal conditions. However, the gas-solid carbonation of CaO takes place in a broader temperature range, implying a more complex thermokinetic behavior (overlapping of carbonation regimes or steps). Concerning the gas-solid carbonation of Ca(OH)2 and CaO under isothermal conditions, a high carbonation extent (>0.9) was determined for CaO at 600 (873K) and 800°C (1073K). Conversely, the gas-solid carbonation of Ca(OH)2 particles was relatively low (<0.56) at 400°C (673K) after 6h of reaction. This case is in agreement with the formation of a dense non-porous layer of carbonate mineral around the core of the reacting Ca(OH)2 particles, thereby limiting the transfer of CO2. Finally, an alternative reaction mechanism is proposed for the gas-solid carbonation of CaO, when the relative humidity is close to 0. This macroscopic control at high temperature avoids CO2 dissociation with molecular water at the CaO-CO2 interface. For these specific conditions, the mineralization of adsorbed CO2 on CaO particles implies a solid state transformation, i.e., CaCO3 formation from CaO-CO2 interactions. This could be explained by an atomic excitation than at high temperature allows the local migration of one oxygen atom from the solid towards the adsorbed CO2 leading to its mineralization into carbonate (porous or non-porous layer) around the reacting particles; chemically the mineralization of CO2 also implies the breaking of one covalent bond in the CO2 molecule

Precipitation of ordered dolomite via simultaneous dissolution of calcite and magnesite: New experimental insights into an old precipitation enigma

7International audienceIn the present study, we demonstrate that ordered dolomite can be precipitated via simultaneous dissolution of calcite and magnesite under hydrothermal conditions (from 100 to 200°C). The temperature and high-carbonate alkalinity have significantly co-promoted the dolomite formation. For example, when high-purity water was initially used as interacting fluid, only a small proportion of disordered dolomite was identified at 200°C from XRD patterns and FESEM observations. Conversely, higher proportion of ordered dolomite, i.e. clear identification of superstructure ordering reflections in XRD patterns, was determined when high-carbonate alkalinity solution was initially used in our system at the same durations of reaction. For this latter case, the dolomite formation is favorable therefrom 100°C and two kinetic steps were identified (1) proto-dolomite formation after about five days of reaction, characterized by rounded sub-micrometric particles from FESEM observations and by the absence of superstructure ordering reflections at 22.02 (101), 35.32 (015), 43.80 (021), etc. 2thetha on XRD patterns; (2) proto-dolomite to dolomite transformation, probably produced by a coupled dissolution-recrystallization process. Herein, the activation energy was estimated to 29 kJ/mol by using conventional Arrhenius linear-equation. This study provides new experimental conditions to which dolomite could be formed in hydrothermal systems. Temperature and carbonate alkalinity are particularly key physicochemical parameters to promote dolomite precipitation in abiotic systems

Rapid precipitation of magnesite micro-crystals from Mg(OH)2-H2O-CO2 slurry enhanced by NaOH and a heat-ageing step (from 20 to 90°C)

International audienceThis study proposes a simple and novel synthesis route for rhombohedral single crystals (90°C) and its synthesis requires several days or weeks depending on experimental conditions. For this reason, industrial-scale magnesite production has been limited. The proposed magnesite synthesis method, requiring only 48h and moderate temperature, could easily be extrapolated on an industrial scale. Moreover, a simple and novel synthesis route for the production of fine platy particles of hydromagnesite is reported, with synthesis requiring only 5h. Based on their chemical compositions and textural properties, there are potential applications for both minerals, for example as a mineral filler and/or as a flame-retardant

Gas-solid carbonation as a possible source of carbonates in cold planetary environments

International audienceCarbonates are abundant sedimentary minerals at the surface and sub-surface of the Earth and they have been proposed as tracers of liquid water in extraterrestrial environments. Their formation mechanism is since generally associated with aqueous alteration processes. Recently, carbonate minerals have been discovered on Mars' surface by different orbital or rover missions. In particular, the phoenix mission has measured from 1 to 5% of calcium carbonate (calcite type) within the soil (Smith P.H. et al., 2009). These occurrences have been reported in area were the relative humidity is significantly high (Boynton et al., 2009). The small concentration of carbonates suggests an alternative process on mineral grain surfaces (as suggested by Shaheen et al., 2010) than carbonation in aqueous conditions. Such an observation could rather point toward a possible formation mechanism by dust-gas reaction under current Martian conditions. To understand the mechanism of carbonate formation under conditions relevant to current Martian atmosphere and surface, we designed an experimental setup consisting of an infrared microscope coupled to a cryogenic reaction cell (IR-CryoCell setup). Three different mineral precursors of carbonates (Ca and Mg hydroxides, and a hydrated Ca silicate formed from Ca2SiO4), low temperature (from -10 to +30°C), and reduced CO2 pressure (from 100 to 2000 mbar) were utilized to investigate the mechanism of gas-solid carbonation at mineral surfaces. These mineral materials are crucial precursors to form Ca and Mg carbonates in humid environments (0 < relative humidity < 100%) at dust-CO2 or dust-water ice-CO2 interfaces. Our results reveal a significant and fast carbonation process for Ca hydroxide and hydrated Ca silicate. Conversely, only a moderate carbonation is observed for the Mg hydroxide. These results suggest that gas-solid carbonation process or carbonate formation at the dust-water ice-CO2 interfaces could be a currently active Mars' surface process. To the best of our knowledge, we report for the first time that calcium carbonate can be formed at a negative temperature (-10°C) via gas-solid carbonation of Ca hydroxide. We note that the carbonation process at low temperature (<0°C) described in the present study could also have important implications on the dust-water ice-CO2 interactions in cold terrestrial environments (e.g. Antarctic)

Simultaneous precipitation of magnesite and lizardite from hydrothermal alteration of olivine under high-carbonate alkalinity

13 pagesInternational audienceThe present study reports original experiments in order to investigate the simultaneous serpentinization and carbonation of olivine with relevance in Earth systems (e.g. functioning of hydrothermal fields) or in engineered systems (e.g. ex-situ and in-situ mineral sequestration of CO2). For this case, specific experimental conditions were examined (200°C, saturated vapor pressure ≈ 16bar, solution/solid weight ratio = 15, olivine grain size < 30µm and high-carbonate alkalinity ≈ 1M NaHCO3). Under these conditions, competitive precipitation of magnesite and serpentine (preferentially lizardite type) were clearly determined by using conventional analytic tools (XRD, FESEM, FTIR and TGA); excluding the fate of the iron initially contained in olivine, the alteration reaction for olivine under high-carbonate alkalinity can be expressed as follows: 2〖Mg〗_2 SiO_4+2H_2 O+H〖CO〗_3^-→Mg〖CO〗_3+〖Mg〗_3 〖Si〗_2 O_5 〖(OH)〗_4+〖OH〗^- This reaction mechanism implied a dissolution process, releasing Mg and Si ions into solution until supersaturation of solution with respect to magnesite and/or serpentine. The released iron contained in the olivine has not implied any precipitation of iron oxides or (oxy)hydroxides; in fact, the released iron was partially oxidized (about 50%) via a simple reduction of water (2〖Fe〗^(2+)+〖2H〗_2 O→2〖Fe〗^(3+)+H_2+2〖OH〗^-). In this way, the released iron was incorporated in serpentine (Fe(II) and Fe(III)) and in magnesite (Fe(II). This latter was clearly determined by FESEM/EDS chemical analysis on the single magnesite crystals. The nucleation and epitaxial growth processes at the olivine-fluid interfaces cannot be excluded in our investigated system. The experimental kinetic data fitted by using a kinetic pseudo-second-order model have revealed a retarding process of serpentine formation with respect to magnesite (about three times slower); in fact, the magnesite seems to reach an apparent stabilization after about 20 days of reaction while the serpentine follows a progressive slower evolution. We assumed that the magnesite has reached a fast apparent equilibrium with solution because the available carbonate species are not renewed from fluid phase as typically constrained in aqueous carbonation experiments where a given CO2 pressure is imposed in the system. On the other hand, the reactivity of serpentinized olivine (chrysotile+brucite+small amount of residual olivine) and high-purity chrysotile at the same above investigated conditions; and the olivine serpentinization in initial acid pH ≈ 0.66 are also reported as complementary information in this study. These novel experimental results concerning simultaneous serpentinization and aqueous carbonation of olivine expand the thermodynamic conditions where serpentine and magnesite can simultaneously precipitate; this could contribute to a better understanding of fluid-rock interactions in natural active hydrothermal fields on Earth

A memetic algorithm based on Artificial Bee Colony for optimal synthesis of mechanisms

En este documento se presenta una propuesta novedosa de un algoritmo híbrido modular, como herramienta para resolver problemas de ingeniería del mundo real. Se implementa y aplica un algoritmo memético, MemMABC, para la solución de dos casos de diseño de mecanismos, con el fin de evaluar su eficiencia y rendimiento. El algoritmo propuesto es simple y flexible debido a su modularidad; estas características lo vuelven altamente reutilizable para ser aplicado en una amplia gama de problemas de optimización. Las soluciones de los casos de estudio también son modulares, siguiendo un esquema de programación estructurada que incluye el uso de variables globales para la configuración, y de subrutinas para la función objetivo y el manejo de las restricciones. Los algoritmos meméticos son una buena opción para resolver problemas duros de optimización, debido a la sinergia derivada de la combinación de sus componentes: una metaheurística poblacional para búsqueda global y un método de refinamiento local. La calidad en los resultados de las simulaciones sugiere que el MemMABC puede aplicarse con éxito para la solución de problemas duros de diseño en ingeniería.In this paper a novel proposal of a modular hybrid algorithm as a tool for solving real-world engineering problems is presented. A memetic algorithm, MemMABC, is implemented with this approach and applied to solve two case studies of mechanism design, in order to evaluate its efficiency and performance. Because of its modularity, the proposed algorithm is simple and flexible; these features make it quite reusable to be applied on different optimization problems, with a wide scope. The solutions of the optimization problems are also modular, following a scheme of structured programming that includes the use of global variables for configuration, and subroutines for the objective function and the restrictions. Memetic algorithms are a good option to solve hard optimization problems, because of the synergy derived from the combination of their components: a global search population-based metaheuristic and a local refinement method. The quality of simulation results suggests that MemMABC can be successfully applied to solve hard problems in engineering design.Peer Reviewe

Study of dolomite dissolution at various temperatures – Evidence for the formation of nanocrystalline secondary phases at dolomite surface and influence on dolomite interactions with other minerals

International audienceIn most clay-rock geological formation studied for the storage of nuclear waste,pore water compositions are expected to be at equilibrium with carbonate minerals, which are always included in predictive models for pore water composition calculations [1]. Among the carbonates known to be present, dolomite may be problematic in the pore water composition calculation because its solubility spans a large range of values as a function of its crystallinity in thermodynamic databases. In addition, the composition of dolomite minerals observed in clay-rock formations such as Callovian-Oxfordian or Opalinus clay formation differs from this of a pure dolomite: the Ca/Mg stoichiometry is not ideal, and the minerals contain minor amounts of Fe and traces of many other elements [2]. To understand the influence of secondary phases precipitation during dolomite dissolution on pore water chemistry, the dissolution of monocrystals of dolomite were investigated at 25 °C and at 80 °C in a pH range 3 to 8 for various time periods (30 minutes to 21 days) in sealed PTFE reactors. Solution analyses evidenced a stoichiometric release of Ca and Mg in solution during dolomite dissolution. Scanning Electron Microscopy (SEM), Raman and X-Ray Diffraction (XRD) analyses did not evidence secondary Mg-bearing minerals precipitation, but revealed the formation of Fe-bearing particles on the dolomite surface. Morphological characterizations performed with Small-angle X-ray scattering (SAXS)evidenced that the precipitation occurs along a specific crystallographic plane of the dolomite monocrystal. Thus, the precipitated nanoparticles clustered on specific surface sites, and are made of Fe-rich phases poorly crystallized (carbonates, oxides and hydroxides)

Study of dolomite dissolution at various temperatures – Evidence for the formation of nanocrystalline secondary phases at dolomite surface and influence on dolomite interactions with other minerals

International audienceIn most clay-rock geological formation studied for the storage of nuclear waste,pore water compositions are expected to be at equilibrium with carbonate minerals, which are always included in predictive models for pore water composition calculations [1]. Among the carbonates known to be present, dolomite may be problematic in the pore water composition calculation because its solubility spans a large range of values as a function of its crystallinity in thermodynamic databases. In addition, the composition of dolomite minerals observed in clay-rock formations such as Callovian-Oxfordian or Opalinus clay formation differs from this of a pure dolomite: the Ca/Mg stoichiometry is not ideal, and the minerals contain minor amounts of Fe and traces of many other elements [2]. To understand the influence of secondary phases precipitation during dolomite dissolution on pore water chemistry, the dissolution of monocrystals of dolomite were investigated at 25 °C and at 80 °C in a pH range 3 to 8 for various time periods (30 minutes to 21 days) in sealed PTFE reactors. Solution analyses evidenced a stoichiometric release of Ca and Mg in solution during dolomite dissolution. Scanning Electron Microscopy (SEM), Raman and X-Ray Diffraction (XRD) analyses did not evidence secondary Mg-bearing minerals precipitation, but revealed the formation of Fe-bearing particles on the dolomite surface. Morphological characterizations performed with Small-angle X-ray scattering (SAXS)evidenced that the precipitation occurs along a specific crystallographic plane of the dolomite monocrystal. Thus, the precipitated nanoparticles clustered on specific surface sites, and are made of Fe-rich phases poorly crystallized (carbonates, oxides and hydroxides)