Méthodes expérimentales 7
- Synthèse des échantillons de magnésite 9
- Caractérisation des solides 10
- Analyse chimique 10
- Détermination des surfaces spécifiques 12
Dispositifs expérimentaux utilisés pour les études cinétiques et de solubilité 13
- Réacteur à circulation (mixed flow reactor) 14
- Réacteur fermé (batch reactor) 17
- Cellule potentiométrique-électrodes à hydrogène 18
1999a), where magnesite precipitation rates are proportional to the concentration of >MgOH2+ surface species. The values of the rate constant (kMg!) and the CO32- desorption constant (KCO3) determined in this study as a function of T.
Méthodes d’étude à l’échelle microscopique : le microscope
- L’AFM hydrothermale 22
Méthodes d’analyse des solutions 25
- Analyse du magnésium 25
- Détermination de l’alcalinité 26
La limite de détection de la concentration en magnésium est de l'ordre de 0,020 ppm avec une incertitude de ± 1 %. L'erreur pour chaque détermination est d'environ ± 1% avec une limite de détection de l'ordre de 5.10-5 eq/l.
Introduction 28
Considérations théoriques 28
Si l'on suppose, conformément au principe de microréversibilité, que le complexe précurseur du complexe activé est le même pour la dissolution et la précipitation, l'utilisation du modèle de complexation superficielle, développé dans le cas de dissolution de la magnésite à 25 °C par Pokrovsky et coll. 1999a), nous permet d'établir les conditions suivantes qui peuvent être utilisées pour interpréter nos résultats et vérifier l'applicabilité de ce modèle aux précipitations de magnésite. Dans des conditions où la chimie de surface de la magnésite est dominée par des complexes.
Méthodes expérimentales 30
L'analyse cinétique de relaxation (Eigen et de Maeyer 1963), déjà utilisée dans des études antérieures (par exemple Pines et Huppert, 1983 ; Prabhananda et al., 1987 ; Bénézeth et al., 2008), a permis de décrire éventuellement le changement, en fonction du temps, de la concentration en magnésium ()C/)t selon l'équation.
Résultats principaux de l’étude 31
Les diagrammes des figures 5a et 5b montrent qu'il existe un accord satisfaisant entre les vitesses expérimentales et les vitesses prédites par le modèle, en fonction de 2-. Les taux de précipitation de magnésite mesurés en réacteur fermé ne sont pas compatibles avec un mécanisme de croissance en spirale.
The compositions of the reactive fluids used in this study are listed in Table 2. The logarithm of the solubility product of magnesite obtained in this study by fitting the data (see text).
Introduction 37
For example, it was shown that mineral carbonation proceeds relatively faster and more efficiently when using forsterite (Mg2SiO4) instead of serpentine [(Mg3Si2O5(OH)4] (Gerdemann et al., 2007).Optimal carbonation conditions for processes using Mg silicates (serpentine and olivine), as reported by Gerdermann et al.
Theoretical considerations 39
Thus, within the context of the surface coordination chemistry model, the dissolution rates of magnesite under far-from-equilibrium conditions can be expressed as: where kH and kH2O designate the rate constant with respect to proton-promoted and H2O-promoted dissolution, respectively, and nHand nH2Orepresent the reaction orders with respect to protonated surface carbonate centers and hydrated Mg-centers. The controlling complex >MgOH2+ on the surface of magnesite can be formed by the following reactions: for which the corresponding values of the internal stability constants are given by KOH, KCO3 and KHCO3.
Experimental methods 42
- Magnesite samples 42
- Preparation and analyses of solutions 42
- In-situ pH measurements 46
- Mixed-flow reactor experiments 50
- Batch reactor experiments 51
The pH of the buffer solutions was determined from calculations made with the PHREEQC (Parkhurst and Appelo, 1999) and HCh (GIBBS) software package (Shavrov, 1999; Shvarov and Bastrakov, 1999) using the NaB(OH)4° association constant reported by Pokrovski et al . These experiments exploited the retrograde solubility of magnesite to perturb the saturation state via temperature increases.
Experimental results 52
- Open system reactor experiments 52
- Closed system reactor experiments 58
The rate constants refer to a linear dependence of the precipitation rate on the relative degree of supersaturation (!-1). The measured rates are interpreted within the framework of the surface complexation model developed by Pokrovsky et al.
Discussion 60
- Batch experiments 62
Comparison of magnesite precipitation mechanism in batch
Dependence of magnesite precipitation rates on temperature 64
Thus, just as for dissolution, carbonate precipitation rates should be proportional to the exchange rate of water molecules in the coordination sphere of the corresponding metal constituting the carbonate. Moreover, it is interesting to note that while the exchange of water molecules in the Ca coordination sphere takes place via an associatively activated mechanism (the rate-controlling step is the creation of a new bond between the metal and the incoming ligand), the exchange of water molecules in the Mg (and Fe, Co, Ni) sphere proceeds via a dissociatively activated mechanism (rate-controlling step is breaking of an Mg-O bond) (Lincoln and Merbach, 1995).
Concluding remarks and implications for CO 2 mineral sequestration 66
Although the limiting step for the entire process is thought to be the slow rate of Mg ion release, which is also slowed by the rapid development of a silica-rich layer (Pokrovsky and Schott, 2000; Béarat et al., 2006), the rate of magnesite precipitation under conditions where the extent of the carbonization reaction reaches its optimum (high partial pressure of CO2, 150-200 °C) may be relatively slow compared to the mass rates measured for the dissolution of forsterite and extrapolated to higher T (cf. Hänchen et al., 2006). Under alkaline conditions, surface/aqueous complexation of significant amounts of dissolved Mg increasing the concentration of dissolved counterions (HCO3-, CO32- and OH-) would further inhibit the rate of the overall carbonation process.
A list of the thermodynamic properties of magnesite found in the literature (together with those determined in this study) is given in Table 3. Schematic of the analytical system used for the determination of total dissolved inorganic carbon (TDIC).
Etude AFM des vitesses de nucléation et de croissance cristalline de la magnésite 77
Introduction 78
The values of the constants used to describe the dependence of the dissolution rate on the composition of the solution, according to Eq. Magnesite dissolution rates measured at 150 and 200 °C and at neutral to alkaline conditions were interpreted according to the surface complexation model developed by Pokrovsky et al. 1999), taking into account the dependence of the distribution of magnesium surface sites on the composition of the solution.
Méthodes expérimentales 78
Résultats et observations expérimentales 79
This correction introduces a change in the free energy of formation of magnesite equal to -3.82 kJ/mol. Description of the synthesis process and of the samples produced is provided in the magnesite precipitation study by Saldi et al.
Introduction 84
For example, many surface waters are strongly supersaturated with respect to quartz (e.g. Arnorsson et al., 2002). Quartz, however, precipitates rarely, if at all, at Earth surface conditions (Dove and Rimstidt 1994; Gislason et al., 1996).
Theoretical considerations 86
A formalism for quantifying mineral nucleation and growth originates from BCF growth theory (Burton et al. 1951). However, a number of studies have pointed out that although step velocities increase linearly with saturation state under highly saturated conditions, this linear behavior is not observed under near-equilibrium conditions (Teng et al., 1999; Davis et al., 2000).
Experimental methods 88
- HAFM experiments 88
- Mixed flow reactor experiments 90
The stability of the phases formed in the MgO-CO2-H2O system (see Table 1 for a list of names and chemical formulas) has been the subject of several studies. The logarithms of the solubility products taken from Tables 5 and 7 are shown in Fig. 6 (associated with the experimental uncertainties given in these tables) as a function of reciprocal T.
Results 92
- Step generation 94
- Layer formation frequencies and step advancement rates 95
- Bulk magnesite precipitation rates from mixed flow reactor experiments 99
Discussion 99
- Magnesite growth mechanisms 99
- What inhibits magnesite precipitation at 25 °C? 101
Concluding remarks 104
The observations suggest that magnesite precipitation at ambient temperature is limited by slow step propagation, which limits the generation of steps by spiral growth on the magnesite surface. It follows that the dehydration kinetics of aqueous metals can play a critical role in determining whether a mineral can precipitate in natural environments at ambient temperature.
Decrease in magnesite dissolution rates with increasing temperature from 150 to 200 °C can be explained by the decrease of KCO3 and KOH, the constants of formation of the rate controlling species, >MgOH2+. The software used for the acquisition allows the continuous monitoring of the CO2 concentration in the gas carrier flow.
Introduction 114
Cet intérêt découle de la nécessité de connaître la stabilité et la cinétique de dissolution des minéraux carbonatés dans des conditions pertinentes au stockage du CO2 dans les aquifères salins (Talman et al., 1990 ; le but de ce travail est de fournir des données sur la cinétique de la dissolution des minéraux carbonatés). dissolution de la magnésite, dans des conditions de température (150-200 °C) et d'affinité chimique (0 < A < 25 kJ/mol) représentatives de celles rencontrées dans les formations sédimentaires profondes.
Cadre théorique et méthodes expérimentales 114
A similar strong decrease in the magnesite dissolution rate is observed when the measured rates are plotted as a function of the respective solution pH (Fig. 3c–d), suggesting inhibition of dissolution by hydroxyl ions. In this study, the slight decrease in dissolution rate with increasing temperature was accounted for by decreasing the constants of Eqns. 12) and (13), reflecting a decrease in the surface density of the rate-controlling species (>MgOH2+).
Résultats expérimentaux et interprétation 116
Introduction 121
Magnesite dissolution rates were determined under a large range of pH and solution composition at 25 °C ( Chou et al. 1989 ; Pokrovsky and Schott, 1999 ; Pokrovsky et al. 2005 ). At higher temperature, dissolution kinetics were investigated only under acidic to near neutral conditions by batch and stirred flow reactor under high pCO2 (Pokrovsky et al., 2009) and in the presence of various organic ligands ( Pokrovsky et al., 2009a).
Theoretical considerations 122
The temperature dependence of mineral dissolution rate (r+) is generally interpreted within the framework of the Arrhenius equation:. As shown by Pokrovsky et al. 2009), experimental characterization of Ea requires the separation of the overall rate dependence on temperature into two parts: a) contribution of the enthalpy of formation of the rate-controlled surface sites (i.e. >CO3H° and >MgOH2+), and b) the activation energy of the dissolution rate constant (i.e. the activated complex).
Experimental methods 124
- Magnesite samples 124
The chemical composition of the different phases that crystallize in the system MgO-CO2-H2O is illustrated in the ternary plot of Figure. The values of the Gibbs formation energy, enthalpy and entropy are in the range of values reported in the literature (Table 3), in particular from the recommended values of Robie and Hemingway (1995).
Results 134
- Effects of CO 3 2- activity, pH and ionic strength 134
- Effect of temperature on dissolution rates 135
Modeling of magnesite dissolution as a function of solution composition,
- Dependence of magnesite dissolution rates on pH and CO 3 2- activity 138
- Effect of temperature on magnesite dissolution rates 142
Concluding remarks 143
This wide variation of magnesite solubility product has profound consequences for the relative stability of the different mineral phases of the system MgO-CO2-H2O, and different solubility diagrams can be proposed. Most of the solubility studies were performed on natural samples of magnesite and achieved chemical equilibrium from undersaturated conditions.
Détermination du produit de solubilité de la magnésite 153
Introduction 154
Example of the peaks corresponding to the CO2 stripped from the system (purge) before the injection of the sample and acid, which corresponds to the second peak. As shown in Figure 6, the reversibility of the equilibrium has also been verified at 100°C.
Méthodes expérimentales 155
Résultats expérimentaux 156
Introduction 160
- Stability of carbonates in the system MgO-CO 2 -H 2 O 160
- Magnesite solubility and thermodynamics - review of existing data 162
- High temperature Ti-reactor 168
- Hydrogene-electrode concentration cell (HECC) 169
- Solution analyses 170
- Direct determination of TDIC 171
An accurate pH measurement is critical for the accurate determination of solution speciation and the subsequent calculation of the solubility product at a given temperature. The efficiency and repeatability of the analyzes depend on the flow rate of the carrier gas and the volume of the sample to be analyzed (Bandstra et al., 2006).
Experimental results and discussion 174
At this high temperature, equilibrium was reached within a few hours and, as shown in Figure 6, the reversibility of equilibrium starting from supersaturated conditions was verified. However, our heat capacity value is lower compared to those of these authors, which is preferable to the value we obtained from the second derivative of the fit of the solubility product versus the inverse of the temperature.
Conclusion 179
Low-temperature thermodynamic model for the system Na2CO3-MgCO3-CaCO3-H2O. Experimental determination of the reactions magnesium + quartz = enstatite + CO2 and magnesium = periclase + CO2, and the enthalpies of formation of enstatite and magnesite. Stability of carbonates in the CaO-MgO-CO2-H2O system. Stability of carbonates in the MgO-CO2-H2O system. Solubility of calcium carbonate and magnesium carbonate in carbonic acid-free water.
