Alkaline Earth Carbonates Research Articles

Effect of CaCO3, SrCO3 and BaCO3 additions on the microstructure and electrical characteristics of SnO2-based varistor ceramics

The ceramic varistors SnO2–Co3O4–Nb2O5–Cr2O3 with the addition of CaCO3, SrCO3 or BaCO3 were sintered at temperatures 1250 and 1350°C, then their structures and electrical properties were studied. In all samples, the SnO2 cassiterite phase with a rutile-type structure and the Co2SnO4 phase with a spinel-type structure were observed. All materials exhibited a highly nonlinear dependence of the current density on the electric field with the nonlinearity coefficient 24–50. When alkaline earth metal carbonates were added, the grain size of all samples decreased and the electric field E1 increased. The addition of CaCO3 lead to the decrease of the low-field electrical conductivity of varistors. The lowest low-field conductivities 4·10-13 and 6·10-13 Ω-1 cm-1 were found in the samples with CaCO3 addition baked at 1250 and 1350°C, respectively. The observed effect is due to the increase of the potential barrier height at the SnO2 grain boundaries by 7 and 13%, respectively, and the decrease of the grain size by 26 and 28% compared to SnO2-based varistor ceramics without CaCO3 addition.

Synthesis and crystal structure of acentric anhydrous beryllium carbonate Be(CO3).

The anhydrous alkaline earth metal carbonate Be(CO3) was synthesized in a laser-heated diamond anvil cell at moderate pressures and temperatures (20(2) GPa and 1500(200) K) by a reaction of BeO with CO2. It crystallizes in the acentric, trigonal space group P3121 with Z = 3. The crystal structure was obtained from synchrotron single crystal X-ray diffraction data and confirmed by density functional theory-based calculations in combination with Raman spectroscopy. Second harmonic generation measurements were employed to verify the acentric space group symmetry. The crystal structure of Be(CO3) is characterized by the presence of isolated [CO3]2--groups and BeO4-tetrahedra. This is a new structure type and such a topology has not been observed in carbonates before. Be(CO3) can be recovered to ambient conditions and is not undergoing a phase transition during decompression.

Synthesis of Geopolymers Incorporating Mechanically Activated Fly Ash Blended with Alkaline Earth Carbonates: A Comparative Analysis

Synthesis of Geopolymers Incorporating Mechanically Activated Fly Ash Blended with Alkaline Earth Carbonates: A Comparative Analysis

A high-throughput approach for assessing antiscaling performance during mineral precipitation from seawater and hard water

The undesired precipitation of minerals from solution poses challenges in various industrial and domestic applications, including water treatment, desalination, dishwashers and boilers. To mitigate this, threshold inhibitors - small quantities of water-soluble additives—are commonly employed to inhibit the precipitation of inorganic phases. However, concerns about the persistence of traditional additives like phosph(on)ates) in natural environments and stricter regulations warrant the development of more sustainable alternatives. We present a high-throughput approach using a UV-Vis spectrophotometer and automated data analysis to assess the scale inhibiting potential of numerous candidates and their combinations. The robustness and versatility of this method were validated by measuring the kinetics of alkaline-earth metal carbonates precipitating from simulated hard waters and seawaters across an extended range of experimental parameters. This approach allows for straightforward evaluation and quantification of each antiscaling additive’s effectiveness and operational range, enabling direct comparison of different additives and blends of additives. Moreover, it facilitates the study of scaling processes in both bulk solutions and at liquid/solid interfaces. By providing a rapid and reliable means of screening potential additives and formulations, our versatile toolbox will expedite the identification of effective scale inhibitors, thereby contributing to the advancement of sustainable practices in various industries reliant on water treatment and mineral precipitation control.

SYNTHESIS AND PROPERTIES OF NdMe<sup>II</sup>TeO4.5 (MeII – Ca,Ba) TELLURITES

The aim of this work was to synthesize and investigate the roentgenographic and electro-physical properties of new phases, namely double tellurites of neodymium. Oxotellurites of the composition NdMeIITeO4.5(MeII - Ca, Ba) were synthesized by the method of ceramic technology with high-temperature interaction of Nd2O3, TeO2 oxides with carbonates of alkaline earth metals in the interval of 800°С - 1200°С. Roentgenograms of the synthesized compounds were obtained using an Empyrean powder diffractometer by PANalytical and indication was determined using the X’Pert HighScore Plus program. It was found that the synthesized tellurites were crystallized in tetragonal syngony. The unit cell parameters, roentgen and pycnometric densities of the compounds were determined. It was revealed that with increasing ionic radii from Ca to Ba, the lattice parameters and unit cell volumes of synthesized tellurites increase. The assumed structure of the synthesized double tellurites of neodymium is a distorted perovskite structure. A study of the temperature dependence of the dielectric permittivity and electrical resistance of tellurites of neodymium-alkaline earth metals established that they can possess semiconducting properties at 293 K - 483 K. Temperature coefficients of resistance and forbidden zone width of tellurites were calculated.

Enhanced mechanical strength of micro-porous ceramics through the removal of alkaline earth carbonates from Moroccan red clay for membrane support application

The influence of the removal of carbonates on the ceramic properties made of Moroccan red clay as a main material and tea waste as pore-forming agent, has been investigated. ATD-TG, dilatometry, dimensional changes, weight loss, water absorption, open porosity, bulk density, flexural and indirect tensile strengths were assessed at different firing temperatures within the range of 900–1150 °C. SEM was used to evaluate the microstructures of the fired samples, while XRD was used to study the phase evolution. At varying firing temperatures, the materials which contains the carbonate (CC) exhibit greater weight loss, water absorption, and porosity compared to the materials without carbonate (NCC). On the other hand, NCC materials has greater mechanical strength than that CC materials. The microstructural analysis conducted through SEM revealed distinct changes in the fired samples when comparing observations at 1100 °C to those at 900 and 1000 °C. At 1100 °C, there was a noticeable increase in particle contacts and layered structures, which can be attributed to the presence of dehydroxylated clay minerals (illite and chlorite), quartz particles, and pores formed during firing. Subsequently, at 1150 °C/ 2 h, further alterations in the microstructures were observed due to a higher degree of vitrification in the fired samples. This resulted in material consolidation, interparticle and neck contacts, leading to the formation of vitrified bridges. Firing has resulted in the formation of closed and open pores of varying sizes, in the vitrified structures associated with anorthite, hematite, and quartz, a thin crystal precipitation of small particles was observed. This alteration in microstructure made it possible to conclude that the flexural and indirect tensile strength increased during firing, reaching its pinnacle at 1150 °C. As a result of the firing process, the ceramic supports underwent heightened sintering, leading to a gradual decline in open porosity. The present inquiry proved to be intriguing as it led to a deeper understanding of the utilization of Moroccan red clay as a ceramic’s raw material.

Carbonates of alkaline earth metals: Impact of the cation size on strengths of interionic interactions and polymorphic preferences

The energies of all types of pair-wise interionic interactions between atoms of the representative (MCO3)13 clusters sampled from X-ray structures of rhombohedral magnesite (M = Mg) and calcite (M = Ca), on one hand, and orthorhombic aragonite (M = Ca) and cerussite (M = Pb), on the other hand, are evaluated theoretically to ascertain how the size of a divalent metal cation governs the preferable existence of rhombohedral or (and) orthorhombic polymorph (polymorphs) at ambient temperature and pressure. The carbonates studied and their clusters are considered as those consisting of (i) columns of carbonate anions (CAs) separated by trios of cations, (ii) ribbons and layers of CAs, and (iii) rods and layers of cations. For each cluster, coordination energies of six CAs by a cation and six cations by a CA are evaluated. Energies of interactions of C and O atoms of columns and ribbons with cations and energies of inter-anionic C···O, O···O, and C···C interactions in column and ribbons of clusters are compared. Strengths of cation – cation repulsions are also evaluated. Analyses of the interaction energies, interionic distances in the carbonates studied, and thermodynamic properties of carbonates of alkaline earth metals allow one to understand how the size of a particular divalent metal cation affects relative thermodynamic stabilities and the very existence of its rhombohedral or (and) orthorhombic crystalline phase (phases).

Solubility-consistent force field simulations for aqueous metal carbonate systems using graphical processing units.

Crystallization of alkaline earth metal carbonates from water is important for biomineralization and environmental geochemistry. Here, large-scale computer simulations are a useful approach to complement experimental studies by providing atomistic insights and even by quantitatively determining the thermodynamics of individual steps. However, this is dependent on the existence of force field models that are sufficiently accurate while being computationally efficient enough to sample complex systems. Here, we introduce a revised force field for aqueous alkaline earth metal carbonates that reproduces both the solubilities of the crystalline anhydrous minerals, as well as the hydration free energies of the ions. The model is also designed to run efficiently on graphical processing units thereby reducing the cost of such simulations. The performance of the revised force field is compared against previous results for important properties relevant to crystallization, including ion-pairing and mineral-water interfacial structure and dynamics. This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.

ПРИМЕНЕНИЕ МЕХАНОАКТИВАЦИИ ДЛЯ ПОЛУЧЕНИЯ ГЕОПОЛИМЕРНЫХ МАТЕРИАЛОВ НА ОСНОВЕ ЗОЛООТХОДОВ СЖИГАНИЯ УГЛЯ С ДОБАВКАМИ КАРБОНАТНЫХ МИНЕРАЛОВ

The results of studies on the synthesis of geopolymers based on mechanically activated mixtures of fly ash from Apatitskaya Thermal Power Plant with magnesium, calcium, strontium, and barium carbonates are presented. It has been shown that in terms of geopolymer strength, calcite CaCO3 is the most effective additive to the fly ash. The influence of the carbonate addition to the fly ash on the physical-mechanical properties of geopolymers has been considered from the standpoint of the reactivity of alkaline earth metal carbonates with respect to the alkaline agent.

Compressional behavior of the aragonite-structure carbonates to 6 GPa

The behaviors of aragonite (CaCO_3), strontianite (SrCO_3), cerussite (PbCO_3), and witherite (BaCO_3) at increasing pressure have been studied up to 6 GPa using density functional theory with plane waves. A parallelism of the orthorhombic carbonates with the closed-packed AsNi structure is considered in our analysis, being the CO_3^{2-} groups not centered in the interstice of the octahedron. The decomposition of the unit-cell volume into atomic contributions using the Quantum Theory of Atoms in Molecules has allowed the analysis of the bulk modulus in atomic contributions. The bulk, axes, interatomic distances, and atomic compressibilities are calculated. The largest compression is on the c crystallographic axis, and the c linear modulus has a linear function with the mineral bulk modulus (K_0). Many of the interatomic distances moduli of the alkaline earth (AE) carbonates show linear functions with the bulk modulus; however, the whole series (including cerussite) only gives linear functions when K_0 is related either with the CC distances modulus or the modulus of the distances of the C to the faces of the octahedron perpendicular to c. These last distances are the projections of the Metal–Oxygen (MO) distances to the center of the octahedron. K_{0AE} carbonates also show linear functions with the atomic moduli of their cations. However, the whole series show a linear relation with the atomic modulus of C atoms. Therefore, the whole series highlight the importance of the C atoms and their interactions in the mechanism of compression of the orthorhombic carbonate series.

Interface channels accelerating ion transport through alkaline earth metal carbonate - Gd0.1Ce0.9O1.95 heterostructure

Designing the interface proton channel between different phases to accelerate the proton ion transport is an effective way to realize the high proton conduction for the low-temperature ceramic fuel cell (CFC). Cerium based materials coated with molten carbonate has been widely demonstrated for high performance CFCs. Here, we prepared alkaline earth metal carbonate - Gd0.1Ce0.9O1.95 (GDC) heterostructure composites in various compositions by precipitation method using NH4HCO3 and NaHCO3 as the deposit. The samples prepared using NH4HCO3 as the electrolyte, the cell can deliver an even higher power output of 811 mW cm−2. The results are much higher than that reported in the literature for the GDC electrolyte fuel cells. The ion conduction on the interface between GDC and solid carbonate particles is proposed. The ionic conductivity is determined to be 0.13 S cm−1 at 500 °C; while GDC as reported in literature is 0.005 S cm−1 at the same temperature. This proposed solid carbonate-GDC heterostructure method has succeeded in enhancing ionic conductivity and the CFC performance, which presents a new way to develop high proton conducting materials and advanced ceramic fuel cells at low temperatures (<550 °C).

Review of Carbonate-Based Systems for Thermochemical Energy Storage for Concentrating Solar Power Applications: State-of-the-Art and Outlook

Thermochemical energy storage (TCS) systems are receiving increasing research interest as a potential alternative to molten salts in concentrating solar power (CSP) plants. In this framework, alkaline-earth metal carbonates are very promising candidates since they can rely on wide availability, low cost, high volumetric density (>1 GJ m–3), relatively high operating temperatures (>800 °C), nontoxic and noncorrosive chemical nature, and no occurrence of any side reactions involving the production of undesired byproducts. Therefore, their reversible calcination/carbonation reaction with CO2 can be used to store/release energy in CSP plants. However, in spite of these promising features, the TCS research field is relatively new, and most of it is still limited to the lab-scale. Therefore, great research efforts are needed to bridge the gap from fundamental research to real-scale application and implementation of TCS-CSP systems. This manuscript reviews the state-of-the-art of carbonate-based systems for TCS in CSP plants. In particular, the literature has been analyzed in-depth, paying attention to (i) the materials development, with a focus on the solutions available to improve the durability of the materials (namely, the ability to withstand repeated carbonation/calcination cycles); and (ii) the design of the reactor configuration for both the solar-driven endothermic calcination and the exothermic carbonation reaction, focusing on the optimization of the reactor concept, based on the physicochemical properties and working temperatures of the reagents.

Development of boring solutions produced from using compositions of clay minerals and surface-active substances from raw fatty acids of cotton soap stock

The article studies the physical and chemical properties of water glycerin to obtain various types of surface-active substances for use as additives to boring solutions. The addition of technical glycerin to the composition of surface-active substances obtained from raw fatty acids of cotton soap stock at oil and fat enterprises contributes to a significant improvement in their surface-active properties. A water solution of glycerin to a certain extent increases the surface-active properties of the developed soap-like surface-active substances. The qualitative indicators of the composition of surfactants obtained with different ratios of a water solution of glycerol of liquid and ointment fractions of raw fatty acids of cotton soap stock were studied for their use in the process of drilling oil and gas wells. Analyzes of the developed compositions of were carried out in the preparation of drilling fluids from alkaline bentonite, alkaline earth bentonite and carbonate palygorskite, as well as double and triple compositions. A comparative analysis of the main indicators of drilling fluids, as well as surface-active substances synthesized from the greasy fraction of raw fatty acids of cotton soap stock, showed the greatest stability of boring solutions at high temperatures and salinity of formation waters.

Experimental Study of Sulfur Solubility in Ca–Mg Carbonate Melt under P–T Parameters of Lithospheric Mantle

Abstract —Modeling the compositions, properties, and conditions of generation of natural agents of mantle metasomatism is one of the most topical subjects in experimental petrology. Particular attention is paid to the study of C- and S-bearing metasomatic agents and their role in the global carbon and sulfur cycles and in the processes of natural diamond formation. Experimental studies aimed at the estimation of sulfur solubility in carbonate melts under lithospheric mantle conditions were carried out on a multianvil high-pressure apparatus of the “split-sphere” type (BARS) in the carbonate–sulfur ((Mg,Ca)CO3–S) and carbonate–pyrite ((Mg,Ca)CO3–FeS2) systems at 6.3 GPa and 1050–1550 °C for 20–60 h. It has been experimentally established that the main processes occurring in the carbonate–sulfur system are the recrystallization of carbonate in a sulfur melt (1050–1350 °C) and the generation of a high-calcium carbonate melt with dissolved sulfur (5.0–6.5 wt.%) (1450–1550 °C) as well as graphite recrystallization and the initial stage of diamond growth (1550 °C) in this melt. The work demonstrates that the carbonate–pyrite interaction is accompanied by the recrystallization of carbonates and pyrite (1050–1250 °C) and the generation of two immiscible melts (sulfide one with dissolved oxygen and carbonate one with dissolved sulfur (1.7–2.5 wt.%) (1350–1550 °C)) along with the formation of graphite and the growth of diamond on seed crystals (1550 °C). It has been found that the solubility of sulfur in carbonate melts increases with temperature, which goes together with a decrease in CaO (±FeO) concentrations and an increase in MgO concentrations. The present study has shown for the first time that melts of alkaline-earth carbonates are capable of dissolving up to 6.5 wt.% sulfur and they are probable sulfur concentrators under the conditions of the lithospheric mantle.

Groundwater Quality and Potential Pollution in the Southern Shimabara Peninsula, Japan

Nitrate pollution in groundwater is a severe problem in Shimabara Peninsula, Nagasaki Prefecture, Japan. Previous studies have investigated water quality characteristics in the northern part of the peninsula and shown serious effects of nitrate pollution in the groundwater. The present study aimed to investigate the groundwater quality in the southern areas of the peninsula for improved understanding of the water quality status for the entire peninsula. Groundwater samples were collected at 56 locations in Minami-Shimabara City from 28 July to 4 August 2021. The spatial distribution of water quality constituents was assessed by Piper-trilinear and Stiff diagrams for major ion concentrations. One agricultural area in the western parts exceeded Japanese recommended standards for water. According to the Piper-trilinear diagram, 44 sampling sites (78.6%) were classified as alkaline earth carbonate type, nine sites (16.1%) as alkaline earth non-carbonate type, and three sites (5.3%) as alkaline carbonate type. Stiff diagrams displayed Ca-HCO3 water type for most of the sites. Na-HCO3 and Mg-HCO3 types were found in coastal areas. Principal component analyses showed that the first component corresponded to dissolved constituents in groundwater and denitrification, the second effects of ion exchange and low nitrate pollution, and the third effects of severe nitrate pollution. Hierarchical cluster analysis was used to classify the groundwater into five groups. The first group included sites with relatively high nitrate concentration. The second group had relatively low ion concentration, distributed from center to eastern parts. The third group included intermediate ion concentration, distributed at lower altitudes along the coastal line. The fourth and fifth groups had a higher ion concentration, especially characterized by high sodium and bicarbonate concentration.

Mineral precipitation and hydrochemical evolution through evaporitic processes in soda brines (East African Rift Valley)

Soda lakes of the East African Rift Valley are hyperalkaline, hypersaline lakes extremely enriched in Na+, K+, Cl−, CO32−, HCO3−, and SiO2. In this paper, we investigate the chemical evolution in these lakes and the production of chemical sediments by salt precipitation via evaporation. Water samples from tributary springs and three lakes (Magadi, Nasikie Engida and Natron) have been experimentally studied by in-situ X-ray diffraction during evaporation experiments to characterize the sequence of mineral precipitation. These data are complemented by ex-situ diffraction studies, chemical analyses and thermodynamic hydrochemical calculations producing detailed information on the activity of all solution species and the saturation state of all minerals potentially generated by the given composition. Major minerals precipitating from these samples are sodium carbonates/bicarbonates as well as halite. The CO3/HCO3 ratio, controlled by pH, is the main factor defining the Na‑carbonates precipitation sequence: in lake brines where CO3/HCO3 > 1, trona precipitates first whereas in hot springs, where CO3/HCO3 ≪ 1, nahcolite precipitates instead of trona, which forms later via partial dissolution of nahcolite. Precipitation of nahcolite is possible only at lower pH values (pCO2 higher than −2.7) explaining the distribution of trona and nahcolite in current lakes and the stratigraphic sequences. Later, during evaporation, thermonatrite precipitates, normally at the same time as halite, at a very high pH (>11.2) after significant depletion of HCO3− due to trona precipitation. The precipitation of these soluble minerals increases the pH of the brine and is the main factor contributing to the hyperalkaline and hypersaline character of the lakes. Villiaumite, sylvite, alkaline earth carbonates, fluorapatite and silica are also predicted to precipitate, but most of them have not been observed in evaporation experiments, either because of the small amount of precipitates produced, kinetic effects delaying the nucleation of some phases, or by biologically induced effects in the lake chemistry that are not considered in our calculations. Even in these cases, the chemical composition in the corresponding ions allows for discussion on their accumulation and the eventual precipitation of these phases. The coupling of in-situ and ex-situ experiments and geochemical modelling is key to understanding the hydrogeochemical and hydroclimatic conditions of soda lakes, evaporite settings, and potentially soda oceans of early Earth and other extraterrestrial bodies.

Implications of recently derived thermodynamic data and specific ionic interaction theory parameters for (Mg/Ca)nUO2(CO3)3(4-2n)- complexes on the predominance of the Mg2+–Ca2+–UO22+–OH−–CO32− systems, and application to natural and legacy-mine waters

The formation of alkaline earth(II)triscarbonatouranyl(VI) (AenUO2(CO3)3(4-2n)-) species that have been evidenced both in laboratory and in-field studies, is important from slightly acidic pH up to near degraded cementitious in carbonated waters. They are also showing distinctive luminescence properties with a hypsochromic shift relative to UO22+. The conditions of pH, activities of alkaline earth(II) free ions (mostly Mg2+ and Ca2+) and carbonate ions (HCO3−) can be predicted from the thermodynamic functions and constants. The predictive validity of the activity of major alkaline ions (mostly Na+) is determined from the models used to describe the ionic strength comportment of these species, particularly using coefficients from the specific ion interaction theory (SIT). The stability domains of these species are better defined as a function of the activity of the constituents, and applied to natural waters. In this work, using recently obtained complete thermodynamic data and SIT coefficients, we will draw the stability domains of the AenUO2(CO3)3(4-2n)- species in combinations of activities of H+, HCO3−, Mg2+, Ca2+, and Na+ for a wide selection of water compositions from the literature. Water samples were collected near a French mining legacy-site (Site du Bosc, Lodève, France). After determining the major ion compositions, we will verify that the luminescence signal of uranium is in agreement with the predicted speciation in the stability domains.

Malic Acid Production by Aspergillus Oryzae: the Effect of Alkaline-earth Carbonate Buffer Identity

Malic Acid Production by Aspergillus Oryzae: the Effect of Alkaline-earth Carbonate Buffer Identity

(Digital Presentation) Modern State and Prospects of Electrochemical CO2 Conversion in Molten Salts

Over the last century, anthropogenic activity has led to a significant increase in the concentration of carbon dioxide in the Earth's atmosphere. Today, the annual increase in CO2 is 3200-3600 million tons. Carbon dioxide together with other greenhouse gases (water vapor H2O, methane CH4, nitrous oxide N2O, sulfur hexafluoride SF6, etc.) causes global warming, which leads to serious environmental degradation - increasing the acidity of sea water, melting glaciers, strengthening the water cycle, which in some regions leads to heavy rainfall and floods, and in others - to extreme droughts. Therefore, the effective carbon dioxide utilization is becoming an urgent scientific and environmental task.One of the ways of carbon dioxide utilization is the electrochemical method. It consists in the electrochemical reduction of CO2 at the cathode. Depending on the conditions of electrolysis (composition of the electrolytic bath, temperature, current density, voltage in the bath, electrode materials), the chemical composition of cathode products changes dramatically.Report is a review of the present state of research concerning the electrochemical conversion of carbon dioxide from conducting liquid and solid media of different chemical composition. They include solid, aqueous, non-aqueous (organic and ionic liquids), and molten salt electrolytes. It contains a comparative analysis of the effectiveness of using these electrolytes, as well as products obtained by the electrochemical splitting of CO2 and the prospects for their use. Special emphasis has been made on the electrochemical decomposition of carbon dioxide from molten salts, several variants of decomposition have been considered, the advantages and disadvantages of each variant have been analyzed. An analysis of literature data on this subject as well as research results of the author on the direct electrochemical conversion of CO2 into valuable solid-phase chemicals with added value is presented.A feature of the electrochemical reduction of CO2 dissolved in molten salts, in contrast to aqueous, organic, solid electrolytes and ionic liquids is the possibility of forming a solid carbon phase at the cathode. Knowledge of the laws of electrode processes involving CO2 is necessary to create a scientific basis and control for the processes of electrochemical synthesis of nanosized carbon-containing inorganic compounds - nanotubes, nanofibers, graphene, amorphous carbon, carbides of refractory metals and numerous composites based on them.The voltammetric study of carbon electrowinning mechanism from carbon dioxide under excessive pressure and lithium carbonate in different gas media (air, argon, carbon dioxide) dissolved in chloride melts at temperatures of 550–800 °С was conducted. The characterization of the chemical and phase compositions, morphological and structural features of nanosized carbon phases obtained by electrolysis in the studied melts has been carried out; a correlation between the structure of the products and the synthesis conditions is established.Thermodynamic analysis of possible ways of electroreduction of CO2 and carbonates of alkali and alkaline earth metals in the temperature range 700 - 1000 K showed that the cathode product can be: (1) - solid carbon phase; (2) - carbon monoxide; (3) - carbide phase. At temperatures below 1000 K, the first path will be thermodynamically advantageous.Electrochemical behavior of CO2 in molten salt mixtures NaCl–KCl (0.5: 0.5 mol%); NaCl–KCl–CsCl (0.3: 0.245: 0.455 mol%); NaCl–KCl–NaF (0.423: 0.423: 0.154 mol.%) under gas excess pressure (1-17 atm.) was studied by cyclic voltammetry at different potential scan rates (0.02-10 Vs-1) and reverse potentials at Pt, Au and GC electrodes. It was established that CO2 electroreduction takes place in two stages, the electrode process is controlled by the charge transfer rate and the rate of CO2 diffusion in the melts. The solubility of CO2 in the melts of Na,K|Cl and Na,K|Cl,F was determined and the implementation of Henry's law in the studied pressure range was shown. An ECE (electrochemical–chemical–electrochemical) mechanism of cathodic reaction has been proposed for the cathode discharge of CO2. Oxidation of oxide- and carbonate- anions takes place at the anode, with the release of oxygen.Electrochemical reduction of Li2CO3 in molten equimolar mixture of Na,K|Cl in various gaseous media (air, argon, CO2) was studied by CV. Reduction of Li2CO3 to carbon in air occurs through the stage of preliminary chemical reaction of acid-base type with the formation of two electrochemically active particles of CO2 and {LixCO3}2-x at potentials -0.8 and -1.7 V, respectively, relative to Pb|PbCl2 reference electrode . Both processes are irreversible, and the limiting stage of reduction {LixCO3}2-x is the diffusion of the depolarizer to the electrode surface. In the atmosphere of argon and CO2, only {LixCO3}2-x participates in the electroreduction process.Promising areas of application of electrolytic carbon are presented.

Quench Products of K-Ca-Mg Carbonate Melt at 3 and 6 GPa: Implications for Carbonatite Inclusions in Mantle Minerals

Alkali-rich carbonate melts are found as inclusions in magmatic minerals, mantle xenoliths, and diamonds from kimberlites and lamproites worldwide. However, the depth of their origin and bulk melt composition remains unclear. Here, we studied quench products of K-Ca-Mg carbonate melt at 3 and 6 GPa. The following carbonates were detected at 3 GPa: K2CO3, K2Ca(CO3)2 bütschliite (R3¯2/m), o-K2Ca3(CO3)4 (P212121), K2Ca2(CO3)3 (R3), K2Mg(CO3)2 (R3¯m), Mg-bearing calcite, dolomite, and magnesite. At 6 GPa, the variety of quench carbonate phases includes K2CO3, K2Ca(CO3)2 bütschliite (R3¯2/m), d-K2Ca3(CO3)4 (Pnam), K2Mg(CO3)2 (R3¯m), aragonite, Mg-bearing calcite, dolomite, and magnesite. The data obtained indicate that alkali-bearing carbonate melts quench to the alkaline earth and double carbonates that are thermodynamically stable at quenching pressure and can be used as markers reflecting the pressure of their entrapment. Further, in this study, we established the fields of melt compositions corresponding to the distinct quench assemblages of carbonate minerals, which can be used for the reconstruction of the composition of carbonatitic melts entrapped by mantle minerals.