Salt precipitation and trapped liquid cavitation in micrometric capillary tubes.

International audienceLaboratory evidence shows that the occurrence of solid salt in soil pores causes drastic changes in the topology of the porous spaces and possibly also in the properties of the occluded liquid. Observations were made on NaCl precipitation in micrometric cylindrical capillary tubes, filled with a 5.5 M NaCl aqueous solution and submitted to drying conditions. Solid plug-shaped NaCl (halite) commonly grows at the two liquid-air interfaces, isolating the inner liquid column. The initially homogeneous porosity of the capillary tube becomes heterogeneous because of these two NaCl plugs, apparently closing the micro-system on itself. After three months, we observed cavitation of a vapor bubble in the liquid behind the NaCl plugs. This event demonstrates that the occluded liquid underwent a metastable superheated state, controlled by the capillary state of thin capillary films persisting around the NaCl precipitates. These observations show, first, that salt precipitation can create a heterogeneous porous medium in an initially regular network, thus changing the transfer properties due to isolating significant micro-volumes of liquid. Second, our experiment illustrates that the secondary salt growth drastically modifies the thermo-chemical properties of the occluded liquid and thus its reactive behavior

In-Pore stress by drying-induced capillary bridges inside porous materials.

International audienceWe present here some evidences that capillary liquid bridges are able to deform micrometric cylindrical pores by tensile stress. Brine-soaked filter membranes are submitted to drying conditions leading to NaCl precipitation inside the 5-10 μm pores. A close examination demonstrated that two forms of NaCl crystallites are successively generated. First, primary cubic crystals grow, driven by the permanent evaporation. When this angular primary solid gets near the pore wall, while the evaporation makes the pore volume to be partly invaded by air, capillary liquid can bridge the nowsmall gap between the halite angles and the pore wall. In a second step, these small capillary bridges are frozen by a secondary precipitation event of concave-shaped NaCl. The proposed interpretation is that the liquid capillary bridges deform the host matrix of the membrane, and the situation is fossilized by the growth of solid capillary bridges. A quantitative interpretation is proposed and the consequences towards the natural media outlined

Ex-situ mineral carbonation: resources, process and environmental assessment (Carmex project)

This article presents the main results of the Carmex project (2009-2012), whose purpose was to review the feasibility of ex-situ mineral carbonation in terms of resource availability, performance of the aqueous mineral carbonation process and life cycle analysis criteria. This collaborative project looked at a wide range of generic issues about this CO2 mitigation option, with particular views on assessing its potential in the context of New-Caledonia. Indeed, insularity and local abundance of 'carbonatable' rocks and industrial wastes (i.e. rich in MgO, CaO, if not Fe(II)O), coupled with significant GHG emissions from first-class nickel pyrometallurgical industries, make it a potential candidate for application of ex-situ mineral carbonation. The project conducted a worldwide analysis of the potential of ex-situ mineral carbonation using a dedicated SIG-based tool. Using a variety of materials the project also reviewed a number of critical issues associated with the aqueous mineral carbonatation process itself, with promising perspectives. Finally, through life cycle analysis of the system as a whole, ex-situ mineral carbonation was compared to mainstream CSC solutions. It was concluded that the viability of this CO2 storage option is located at the level of the process itself and lies with the optimisation of its operating conditions

Thermodynamic evidence of giant salt deposit formation by serpentinization: an alternative mechanism to solar evaporation

International audienceThe evaporation of seawater in arid climates is currently the main accepted driving mechanism for the formation of ancient and recent salt deposits in shallow basins. However, the deposition of huge amounts of marine salts, including the formation of tens of metres of highly soluble types (tachyhydrite and bischofite) during the Aptian in the South Atlantic and during the Messinian Salinity Crisis, are inconsistent with the wet and warm palaeoclimate conditions reconstructed for these periods. Recently, a debate has been developed that opposes the classic model of evaporite deposition and argues for the generation of salt by serpentinization. The products of the latter process can be called "dehydratites". The associated geochemical processes involve the consumption of massive amounts of pure water, leading to the production of concentrated brines. Here, we investigate thermodynamic calculations that account for high salinities and the production of soluble salts and MgCl2-rich brines through sub-seafloor serpentinization processes. Our results indicate that salt and brine formation occurs during serpentinization and that the brine composition and salt assemblages are dependent on the temperature and CO2 partial pressure. Our findings help explain the presence and sustainability of highly soluble salts that appear inconsistent with reconstructed climatic conditions and demonstrate that the presence of highly soluble salts probably has implications for global tectonics and palaeoclimate reconstructions

Thermodynamics of hydration of MX80-Na: an experimental study of the hydration energies

Hydration properties of swelling clay minerals may be very variable depending on the chemical composition of the clay, on the nature of the interlayer cations and on the interlayer charge (Berend et al., 1995; Vieillard et al., 2011). The Wyoming smectite has been largely studied, notably for assessing its hydration behavior as a function of the interlayer cations, in connection with its structural characteristics (Ferrage et al., 2005; Salles et al., 2007). In the present work, carried out as part of a collaborative Andra/BRGM/HydrASA research program for ThermoChimie project, we propose an original experimental study, based on adsorption and desorption isotherms performed on MX80 clay samples. The goal is to determine energetic contributions to the reactions of hydration, which have been revealed to be non-negligible with respect to the stability of the clay minerals (Gailhanou et al., submitted). In particular, the present work addresses the problems of the hysteresis loop between adsorption and desorption isotherms and of the irreversibility of hydration reactions. This is directly related to the application of classical thermodynamics to the hydration reactions of clay minerals. In a first stage, an experimental study is dedicated to better understand the origin of the hysteresis loop which is systematically observed for the adsorption-desorption isotherms at 25°C. The development of the hysteresis loop has been studied by considering several kinetically related parameters: stabilization periods, temperatures (from 25°C to 60°C) and hydration steps (Figure 1). No sensible change was observed in the hysteresis loop. Therefore, the amount of adsorbed water depends on the followed reaction pathway (adsorption or desorption). The variations in microstructures and in the distribution of hydration layers (0/1/2 water layers; Ferrage et al., 2005) as a function of relative humidity (RH) could provide a possible explanation for this phenomenon

Ex situ mineral carbonation for CO2 mitigation: Evaluation of mining waste resources, aqueous carbonation processability and life cycle assessment (Carmex project)

This article presents the main outputs from the multidisciplinary Carmex project (2009–2012), which was concerned with the possibility of applying ex situ mineral carbonation concepts to mafic/ultramafic mining wastes. Focus points of the project included (i) matching significant and accessible mining wastes to large CO2 emitters through a dedicated geographical information system (GIS), (ii) analysis of aqueous carbonation mechanisms of mining waste and process development and (iii) environmental assessment of ex situ mining waste carbonation through life cycle assessment (LCA) methodology. With a number of materials associated with the mining sector, the project took a close look at the aqueous carbonation mechanisms for these materials and obtained unexpected carbonation levels (up to 80%) by coupling mechanical exfoliation and reactive carbonation. Results from this work support the possibility of processing serpentine-rich peridotites without applying the classical first step of heat activation. Perspectives are also given for the carbonation of Ni-pyrometallurgical slag available closed to ultramafic mining residues. LCA of the mining waste carbonation system as a whole made it clear that the viability of this CO2 storage option lies with the carbonation process itself and optimisation of its operating conditions. By combining the body of knowledge acquired by this project, it is concluded that New Caledonia, with its insularity and local abundance of ‘carbonable’ rocks and industrial wastes coupled with significant greenhouse gas (GHG) emissions from world-class nickel pyro and hydrometallurgical industries stands out as a strong potential candidate for application of ex situ mineral carbonation

Modelling the porewater chemistry of the Callovian–Oxfordian formation at a regional scale

International audienceIn ANDRA's studies to characterize the Callovian-Oxfordian formation, porewater chemistry is a key topic. Indeed, chemistry determines the durability of the repository materials (bentonite, concrete, metals, nuclear glass) and the speciation (and thus the mobility) of radionuclides. The method developed in the frame of the THERMOAR project enables the acquisition of a complete set of data from core samples to model the porewater chemistry. The method requires a detailed mineralogical study, a model of free-water/bound-water distribution, leaching experiments, adsorbed ion measurements, ion-exchange constant acquisition, and CO2 partial-pressure measurements. These experiments and measurements were done on samples from the site of the Meuse/Haute-Marne laboratory and from ANDRA's regional boreholes. The regional stability of a great number of parameters can be observed, except for a decrease of the Na and Cl concentration following a NE-SW axis passing through the laboratory. The water/rock equilibrium model makes it possible to calculate the chemical composition of interstitial waters of the formation

A revised description of the binary CaCl2-H2O chemical system up to solution-mineral equilibria and temperatures of 250°C using Pitzer equations. Extension to the multicomponent HCl-LiCl-NaCl-KCl-MgCl2-CaCl2-H2O system

International audienceCalcium chloride is a highly soluble chemical compound involved in variable amounts in many natural and industrial environments. The description of its chemical properties and mineral-solution equilibrium conditions must cover a wide range of temperatures and chemical compositions, including saline solutions. This article reports an improved model for the thermodynamic accurate description of the CaCl2-H2O chemical system according to the Pitzer formalism from 25 to 250°C, over the whole concentration range between pure water and the solubility of salts (up to 30 M). It accounts for the aqueous speciation of the CaCl2 electrolyte, according to the partial molar properties of the Ca2+, CaCl+, CaCl20 and Cl- aqueous species described by the HKF theory. The numerical stability, resulting from the lowered ionic strength in comparison to full dissociation, facilitates the development of temperature- dependent models for ternary systems containing the main major cations: H+, Li+, Na+, K+, and Mg2

Darapskite solubility in basic solutions at 25 °C: A Pitzer model for the Na NO 3 SO 4 OH H 2 O system

International audienc