Formation Of Magnesium Carbonates Research Articles

Magnesium Isotopes of Carbonate Reveal Seasonal Climate Variation in the Central East Asia During the Middle Eocene

AbstractIt is debated whether there was strong climate seasonality during the Eocene, which provides a close geological analogy for near‐future scenarios of greenhouse gas emissions. Lithological data suggest the existence of a broad arid zone centered around 30°N paleo‐latitude, while a humid climate was supported by palaeobotanic assemblages in East Asia. Here, we report the occurrence of massive primary lacustrine dolomite and magnesite in the central East Asia during the middle Eocene. We provide a novel perspective from magnesium isotopes to link the formation of Mg‐carbonates to seasonal dry‐wet cycles. Rapid magnesium input during the rainy season and intense evaporation in the dry season likely caused the formation of magnesium carbonates in an enclosed lake. These findings provide insights into hydroclimatic seasonality during the Eocene, contributing to our understanding of the hydrological cycle response to a greenhouse climate.

Kinetics of enhanced magnesium carbonate formation for CO2 storage via mineralization at 200 °C

The precipitation of carbonate minerals through chemical reaction between injected CO2 and reactive basaltic rocks can enable a long-term carbon storage solution (carbon mineralization). Magnesite is one of the most stable carbonate precipitates that can contribute to addressing the challenge of long-term carbon storage, and the chemical mechanism that can catalyze its formation is of great interest and practical significance. Here, using batch reactor experiments at 200 ºC and mineralogical characterization, we explore magnesite precipitation kinetics in injected fluids whereby the chemical impact of fluid pH, NaCl, and MgO nanoparticles is investigated. The results show that an alkaline or a saline environment significantly accelerates magnesite formation by enhanced phase transition of hydrated metastable magnesium carbonate to magnesite. Raised CO32− activity in an alkaline environment may promote the transformation of hydromagnesite to magnesite. Competition for hydration water between background ions and magnesite building ions in the presence of NaCl can promote Mg dehydration and magnesite mineralization. Although MgO hydrophilic surfaces may retard magnesite formation at the early stage of the reaction, an alkaline pH environment can subsequently accelerate magnesite growth by enhanced replacement of hydromagnesite. This research provides new insights into the mechanism and kinetics of magnesite precipitation at mineral-fluid interfaces in a range of conditions, and can facilitate the deployment of carbon storage technologies and support defining strategies to accelerate underground CO2 mineralization in deep basalt reservoirs.

Carbonation of fresco mural paintings with a dolomitic mortar

An innovative approach involving a collection of experiments to mimic and assess the different conditions (at selected time-pH-[Mg2+]-humidity) existing during the carbonation process in dolomitic mortars is proposed with a combination of micro-analytical imaging techniques, namely: optical and electron microscopy, micro-Fourier-Transform Infrared spectroscopy, Raman spectroscopy and synchrotron-based micro X-ray diffraction. The carbonation process was studied from short time scale to long time scale and from simple aqueous saturated solutions to fresco mock-up samples. These techniques allowed identifying both crystallized and amorphous components and revealing their in-depth distribution in multi-layered systems. The presence of Mg2+ influences the microstructure and composition of dolomitic mortars, resulting in the presence of aragonite concurrent to calcite. In later stages, i.e. once Ca2+ has transformed into carbonate, brucite can lead to the formation of magnesium carbonates and hydroxycarbonates. The results contribute explaining the lability of old dolomitic mortars observed in historical fresco paintings.

Effect of pH Cycling and Zinc Ions on Calcium and Magnesium Carbonate Formation in Saline Fluids at Low Temperature

The formation of dolomite is very challenging in the laboratory under ambient conditions due to kinetic inhibition. The goal of this study was to test the impact of pH cycling and zinc ions on the formation of magnesium-rich carbonates in saline fluids at a low temperature. Batch reactor experiments were conducted in two series of pH cycling experiments, one without and one with zinc ions, at 43 °C. The results after 36 diel pH cycles indicate a reaction product assemblage of hydromagnesite, aragonite and magnesite in the experiments without zinc ions, and of magnesite and minor aragonite in the experiments with zinc ions. The presence of zinc ions leads to a decrease in the pH in the acid phase of the cycling experiments, which likely plays a role in the reaction product assemblage. Moreover, the hydration enthalpy and other specific ion effects could be additional factors in the formation of magnesium-rich carbonate. The results show a clear evolution towards increasing incorporation of magnesium in the carbonate phase with cycle number, especially in the experiments with zinc ions, reflecting a ripening process that is enhanced by pH cycling. Hence, repeated pH cycling did not lead to more ordered dolomite (from protodolomite), but rather to the formation of magnesite with 92 mol% MgCO3 after 36 cycles, even though geochemical models indicate a higher saturation index for dolomite than for magnesite.

Formation of Magnesium Carbonates on Earth and Implications for Mars.

Magnesium carbonates have been identified within the landing site of the Perseverance rover mission. This study reviews terrestrial analog environments and textural, mineral assemblage, isotopic, and elemental analyses that have been applied to establish formation conditions of magnesium carbonates. Magnesium carbonates form in five distinct settings: ultramafic rock-hosted veins, the matrix of carbonated peridotite, nodules in soil, alkaline lake, and playa deposits, and as diagenetic replacements within lime—and dolostones. Dominant textures include fine-grained or microcrystalline veins, nodules, and crusts. Microbial influences on formation are recorded in thrombolites, stromatolites, crinkly, and pustular laminites, spheroids, and filamentous microstructures. Mineral assemblages, fluid inclusions, and carbon, oxygen, magnesium, and clumped isotopes of carbon and oxygen have been used to determine the sources of carbon, magnesium, and fluid for magnesium carbonates as well as their temperatures of formation. Isotopic signatures in ultramafic rock-hosted magnesium carbonates reveal that they form by either low-temperature meteoric water infiltration and alteration, hydrothermal alteration, or metamorphic processes. Isotopic compositions of lacustrine magnesium carbonate record precipitation from lake water, evaporation processes, and ambient formation temperatures. Assessment of these features with similar analytical techniques applied to returned Martian samples can establish whether carbonates on ancient Mars were formed at high or low temperature conditions in the surface or subsurface through abiotic or biotic processes. The timing of carbonate formation processes could be constrained by 147Sm-143Nd isochron, U-Pb concordia, 207Pb-206Pb isochron radiometric dating as well as 3He, 21Ne, 22Ne, or 36Ar surface exposure dating of returned Martian magnesium carbonate samples.

Valorization of Kimberlite Tailings by Carbon Capture and Utilization (CCU) Method

In the world of construction, cement plays a vital role, but despite its reputation and affordable prices, the cement industry faces multiple challenges due to pollution and sustainability concerns. This study aimed to assess the possibility of utilizing carbonated kimberlite tailings, a waste product from diamond mining, as a partial cement substitute in the preparation of concrete bricks. This is a unique opportunity to help close the gap between fundamental research in mineral carbonation and its industrial implementation to generate commercial products. Kimberlite was subjected to a mild thin-film carbonation process in a CO2 incubator at varying levels of CO2 concentration (10 vol% and 20 vol% at ambient pressure), kimberlite paste moisture content (10 wt% to 20 wt%), and chamber temperature (35 and 50 °C). The formation of magnesium carbonates, in the form of nesquehonite and lansfordite, was verified by X-ray diffraction analysis, and total CO2 uptake was quantified by thermal decomposition in furnace testing. Carbonated kimberlite tailings were then used to cast bricks. Replacement of cement between 10% and 20% were tested, with a constant water-to-binder ratio of 0.6:1, and a cementitious material-to-sand ratio of 1:3. Initial water absorption and 7- and 28-days compressive strength tests were carried out. The results obtained confirm the possibility of using carbonated kimberlite to replace cement partially, and highlight the benefits of carbonating the kimberlite for such application, and recommendations for future research are suggested. This study demonstrates the potential use of mining tailings to prototype the sequestration of CO2 into sustainable building materials to positively impact the increasing demand for cement-based products.

Morphology control of magnesium carbonate for CO2 utilization using Mg2+ ions in industrial wastewater depending on length of alkyl chain of primary alkanolamine, reaction temperature, CO2 concentration, and Mg2+/Na+ ratio

Because global warming has been a constant issue in the environmental field, many studies on the mitigation of CO2 have been conducted. This study for carbon dioxide utilization dealt with the characteristics of magnesium carbonate formation using Mg2+ ions separated from seawater-based industrial wastewater and alkanolamine absorbent depending on the alkyl chain length of the primary alkanolamine type absorbent, reaction temperature, CO2 concentration, and Mg2+/Na+ ratio as part of the investigation. First, nesquehonite was formed with a relatively short length and amorphous MgCO3·3H2O was formed with a relatively long length in the alkyl chain length experiment. Second, nesquehonite at low temperature was produced, and hydromagnesite was produced as temperature increased in the reaction temperature experiment. In addition, nesquehonite was only magnesium carbonate depending on CO2 concentration, but each product had different purities. Finally, increasing of Mg2+/Na+ ratio derived production of not nesquehonite but hydromagnesite with high purity of over 98 wt%. Through this study, the morphology and purity control of magnesium carbonate can reach commercialization levels in the particular conditions, and industrial application of carbon capture and utilization processes using wastewater and alkanolamine absorbent would be achieved.

Synthesis of Magnesium Carbonate via Carbonation under High Pressure in an Autoclave

Magnesium carbonate powders are essential in the manufacture of basic refractories capable of withstanding extremely high temperatures and for special types of cement and powders used in the paper, rubber, and pharmaceutical industries. A novel synthesis route is based on CO2 absorption/sequestration by minerals. This combines the global challenge of climate change with materials development. Carbon dioxide has the fourth highest composition in earth’s atmosphere next to nitrogen, oxygen and argon and plays a big role in global warming due to the greenhouse effect. Because of the significant increase of CO2 emissions, mineral carbonation is a promising process in which carbon oxide reacts with materials with high metal oxide composition to form chemically stable and insoluble metal carbonate. The formed carbonate has long-term stability and does not influence the earth’s atmosphere. Therefore, it is a feasible and safe method to bind carbon dioxide in carbonate compounds such as magnesite. The subject of this work is the carbonation of an olivine (Mg2SiO4) and synthetic magnesia sample (>97 wt% MgO) under high pressure and temperature in an autoclave. Early experiments have studied the influence of some additives such as sodium bicarbonate, oxalic acid and ascorbic acid, solid/liquid ratio, and particle size on the carbonation efficiency. The obtained results for carbonation of olivine have confirmed the formation of magnesium carbonate in the presence of additives and complete carbonation of the MgO sample in the absence of additives.

Biomass-facilitated production of activated magnesium oxide nanoparticles with extraordinary CO2 capture capacity

Discovering sustainable materials that can efficiently capture and store CO2 is a crucial step towards mitigating anthropogenic greenhouse gas emissions. The current materials used in post-combustion scenarios have significant drawbacks, including high cost and resulting hazardous byproducts. This study developed a simple and cost-effective method of producing activated MgO nanoparticles that have high CO2 capture efficiency. To optimize CO2 capture, we incorporate biomass during the production to ensure complete and dispersed MgO nanoparticle formation from the decomposition of MgCl2 hydrate. Materials are characterized by SEM, XRD, BET/DFT, ICP, and TGA analysis. Resulting carbon-supported activated MgO nanoparticles efficiently captured CO2 at low temperature, by physisorption and magnesium carbonate formation. Without the incorporation of biomass, MgCl2 hydrate decomposed via low temperature pyrolysis, is still capable of CO2 capture; however, the efficiency is enhanced with the addition of biomass, as this helps support the formation and distribution of MgO nanoparticles. The carbon-supported MgO nanoparticles produced with a 5:2.5 (w/w MgCl2:biomass) ratio was capable of greatest CO2 capture capacity on both a g−1 material (up to 235 mg g−1) and g−1 magnesium (733 mg g−1) basis after 3 h. The carbon-supported MgO nanoparticles have potential to be applied in inexpensive large-scale CO2 capture, as the production can be adjusted based on capacity, resource, or energy constraints.

Isotopic evidence of passive mineral carbonation in mine wastes from the Dumont Nickel Project (Abitibi, Quebec)

Natural weathering of ultramafic rocks in mine tailings captures atmospheric CO2 through the formation of magnesium carbonates. The Dumont Nickel Project (DNP) is of particular interest as it will generate 1.7 Gt of ultramafic residues. A field experiment has been conducted at the DNP site in order to understand the process of natural CO2 sequestration. Two experimental cells were built using waste rock and mineral processing tailings and were instrumented with gas sampling ports and probes to monitor water saturation and suction. A decrease of the interstitial gas-phase CO2 concentration in both cells, from atmospheric values (∼390ppmv) near the surface to ∼100ppmv near the bottom, reflects active CO2 consumption by the residues. The total carbon content of the weathered DNP mine waste ranges from 0.2wt% to 6.5wt%C. Hydrotalcites supergroup minerals (pyroaurite-3R, brugnatellite, pyroaurite 2-H), aragonite, nesquehonite, dypingite and hydromagnesite were absent from the unweathered residues and precipitated in the cells during passive mineral carbonation. In situ measurements using Wavelength-Scanned Cavity Ring Down Spectroscopy reveal an increase of δ13C(air) from −8‰ near the surface of the cells to ∼2‰ near the bottom that is correlated with the decrease in CO2 concentration. This trend is explained by kinetic carbon isotope fractionation during dissolution of atmospheric CO2 in interstitial water (ΔDIC-CO2=−11.2‰). Secondary carbonates, precipitated from the interstitial water, are characterized by a moderately high δ18O and low δ13C. These isotopic compositions of the carbonates are consistent with precipitation in an evaporative environment where the kinetic carbon fractionation during atmospheric CO2 dissolution produces interstitial water depleted in 13C. Moreover, isotopic compositions of hydrotalcite supergroup minerals and other carbonate minerals are consistent with modern precipitation from the weathering of mining residue. These observations demonstrate the atmospheric source for the sequestered CO2 and help constrain a conceptual model of the carbonation reaction in the residues.

Evidence of a Role for Aerobic Bacteria in High Magnesium Carbonate Formation in the Evaporitic Environment of Dohat Faishakh Sabkha in Qatar

Dolomite (MgCa(CO3)2) is an important petroleum reservoir rock mineral common in ancient sedimentary rocks which is infrequently found in modern environments. The mechanism of dolomite formation remains poorly understood, although recent research has focused on the contribution of microbial processes. Sabkha is the Arabic term for saline mudflats occurring in regions characterized by extreme environmental conditions (high temperature, salinity, light intensity, and aridity), where diverse halophilic and extremophilic microorganisms are found. The dynamic evaporitic systems characteristic of sabkhas are crucial for the precipitation of minerals and a role for microorganisms in sabkhas in the process of mineralization has been proposed. In this study the Dohat Faishakh Sabkha in Qatar was investigated for evidence of the role for aerobic bacteria in mediating the formation of high magnesium carbonates and dolomite, two minerals that commonly occur in the sabkha sediments. 29 strains of aerobic microbes were obtained through inoculation on agar plates from two different cores sampled from the sabkha and identified by 16S rRNA gene sequencing as belonging to the genera Bacillus, Salinivibrio, Staphylococcus and, primarily, Virgibacillus. All strains examined caused the pH of an artificial growth medium to increase from 7 to 8.5; however, not all were capable of mediating mineral formation. Only Salinivibrio and Virgibacillus spp. isolates mediated the formation of detectable solid phases within the agar plates. Light microscopy, scanning electron microscopy energy dispersive X-ray (SEM/EDX), and X-ray diffraction (XRD) analyses indicate that the solid phase produced in the presence of these bacterial strains is MgCa(CO3)2 with a MgCO3 mol% varying from 0% to 40%. The results of these laboratory experiments suggested that, in the Dohat Faishakh Sabkha, aerobic bacteria may contribute in the formation of very high Mg calcite, a mineral that is considered the precursor of ordered dolomite.

In Situ Natural Abundance 17O and 25Mg NMR Investigation of Aqueous Mg(OH)2 Dissolution in the Presence of Supercritical CO2.

We report an in situ high-pressure NMR capability that permits natural abundance 17O and 25Mg NMR characterization of dissolved species in aqueous solution and in the presence of supercritical CO2 fluid (scCO2). The dissolution of Mg(OH)2 (brucite) in a multiphase water/scCO2 fluid at 90 atm pressure and 50 °C was studied in situ, with relevance to geological carbon sequestration. 17O NMR spectra allowed identification and distinction of various fluid species including dissolved CO2 in the H2O-rich phase, scCO2, aqueous H2O, and HCO3-. The widely separated spectral peaks for various species can all be observed both dynamically and quantitatively at concentrations as low as 20 mM. Measurement of the concentrations of these individual species also allows an in situ estimate of the hydrogen ion concentration, or pCH+ values, of the reacting solutions. The concentration of Mg2+ can be observed by natural abundance 25Mg NMR at a concentration as low as 10 mM. Quantum chemistry calculations of the NMR chemical shifts on cluster models aided in the interpretation of the experimental results. Evidence for the formation of polymeric Mg2+ clusters at high concentrations in the H2O-rich phase, a possible critical step needed for magnesium carbonate formation, was found.

Clumped-isotope thermometry of magnesium carbonates in ultramafic rocks

Magnesium carbonate minerals produced by reaction of H2O–CO2 with ultramafic rocks occur in a wide range of paragenetic and tectonic settings and can thus provide insights into a variety of geologic processes, including (1) deposition of ore-grade, massive-vein cryptocrystalline magnesite; (2) formation of hydrous magnesium carbonates in weathering environments; and (3) metamorphic carbonate alteration of ultramafic rocks. However, the application of traditional geochemical and isotopic methods to infer temperatures of mineralization, the nature of mineralizing fluids, and the mechanisms controlling the transformation of dissolved CO2 into magnesium carbonates in these settings is difficult because the fluids are usually not preserved. Clumped-isotope compositions of magnesium carbonates provide a means to determine primary mineralization or (re)equilibration temperature, which permits the reconstruction of geologic processes that govern magnesium carbonate formation. We first provide an evaluation of the acid fractionation correction for magnesium carbonates using synthetic magnesite and hydromagnesite, along with natural metamorphic magnesite and low-temperature hydromagnesite precipitated within a mine adit. We show that the acid fractionation correction for magnesium carbonates is virtually indistinguishable from other carbonate acid fractionation corrections given current mass spectrometer resolution and error. In addition, we employ carbonate clumped-isotope thermometry on natural magnesium carbonates from various geologic environments and tectonic settings. Cryptocrystalline magnesite vein deposits from California (Red Mountain magnesite mine), Austria (Kraubath locality), Turkey (Tutluca mine, Eskişehir district) and Iran (Derakht-Senjed deposit) exhibit broadly uniform Δ47 compositions that yield apparent clumped-isotope temperatures that average 23.7±5.0°C. Based on oxygen isotope thermometry, these clumped-isotope temperatures suggest mineralization at shallow crustal depths in the presence of meteoric water. Hydrous magnesium carbonates from a 400-km latitudinal transect along the serpentinized-peridotite bodies of the California Coast Ranges record clumped-isotope temperatures between 14.2 and 22.7±2.8°C, in agreement with historical maximum temperatures during the rainy season for California. Talc–carbonate alteration of ultramafic rocks in Greenland (Isua Supracrustal Belt) and Vermont (Ludlow) yields clumped-isotope alteration temperatures of magnesite and dolomite between 326 and 490°C, broadly consistent with paragenesis and thermodynamic analysis for CO2 metasomatism of ultramafic rocks. These metamorphic carbonates extend the applicability of clumped-isotope thermometry to high-temperature magnesium carbonate systems and indicate equilibrium blocking temperatures for magnesite of ∼490°C. Our study demonstrates the applicability of the clumped isotope approach to provide information on the formation of magnesium carbonates as ore resources, surface records of climate, and metamorphic assemblages.

Experimental study on the precipitation of magnesite from thermally activated serpentine for CO2 sequestration

Mineral carbonation has the potential to store billions of tonnes of CO2 safely and permanently. Enhancement of the kinetics of the formation of magnesium carbonate from magnesium-bearing silicate minerals has been the subject of numerous research studies. However, significant progress is yet to be achieved. This is, in part, due to a lack of understanding of the mechanism of the formation of magnesite in the presence of additives and under mineral carbonation conditions. In this work, an in-depth study was performed to investigate the precipitation of magnesium carbonate during single step high pressure, high temperature carbonation of thermally activated serpentine in an aqueous bicarbonate solution. Slurry samples were obtained throughout the duration of the carbonation experiments, enabling analysis of both the aqueous and solid compositions over time, providing insight into the reaction mechanism. Additionally, the effect of operating temperature on the formation of various magnesium carbonate species was examined. TGA-MS, in combination with XRD and SEM, confirmed the formation of hydromagnesite in the absence of carbon dioxide (CO2) during the reactor heat up period, owing to a reaction with the sodium bicarbonate (NaHCO3) carrier solution. Hydromagnesite was transformed to magnesite over time, with the rate of this phase transformation highly dependent on the reaction temperature. At 185°C all hydromagnesite converted to magnesite in a few minutes whereas at 120°C even after 90min hydromagnesite remained in the reactor. PHREEQC thermodynamic software was used to assess the observed formation of carbonate species. The model prediction was consistent with the experimental results obtained in this work.

New tools for stimulating dissolution and carbonation of ultramafic mining residues

The carbonation of chrysotile and nickel mining residues was studied under ambient laboratory conditions to assess their response under various potential field conditions (pore saturation, watering episodes, temperature, CO2 diffusion, and dissolved oxygen) and to a variety of natural and chemical enhancers (sulfide minerals, brucite, chelate ligands, ionic liquids, and carbonic anhydrase enzyme). Watering of the residues to achieve partial pore saturation was critical for optimal ambient CO2 sequestration by mediating magnesium leaching and CO2 absorption, and by favouring diffusion of gaseous CO2 in pores deep inside the residues. Increasing temperature stimulated CO2 uptake whereas dissolved oxygen triggered undesirable oxidative precipitation and passivation by iron (III) hydroxides. The latter effect was attenuated through addition of depassivation chelates, which impeded iron precipitation and enhanced carbonation under ambient conditions. Pyrite and pyrrhotite, as natural acid generators, failed to improve ambient carbonation by fostering iron passivation to the detriment of mining residue dissolution whereas the use of carbonic anhydrase inhibited formation of magnesium carbonates. Ionic liquids were found suitable for magnesium extraction under alkaline conditions but inefficient for inducing carbonate precipitation. The lower rate of carbonation with hydrophobic ionic liquid‐water mixtures was ascribed to a slower gas‐liquid mass transfer of CO2 to the liquid, which primarily prompted a loss of gas‐liquid interfacial area due to viscous effects and bubble coalescence.

Study of Corrosion Inhibitors for Mg Alloy AZ31B with Variations of PCO2 and Electrolyte Layer Thickness

Magnesium alloys have been proven as useful, lightweight, engineering materials in several applications such as aerospace transmission casings and enclosure frames for general consumer electronics. Automotive industry has also shown much interest in this material both in the past and recent years. Yet, the full applicability of magnesium alloys has not yet been achieved due to their active corrosion in common media. One way to act upon the corrosion of magnesium alloys is by corrosion inhibiting chemical compounds. Such compounds, when effective at low concentrations, are ideal candidates for coatings-based inhibitor systems provided that they can be stored and released by ion-exchange pigment additions to the coating. To date, corrosion inhibitor studies for magnesium alloys have almost entirely relied on techniques based on immersion of specimen in bulk solution. In the present work, it is shown that the corrosion product and effectiveness of several inorganic inhibitor compounds on AZ31B, such as sodium metavandate, vary with both electrolyte layer thickness and partial pressure of CO2. Characterization of corrosion product layers indicates tendency for the formation of magnesium carbonates rather than Mg(OH)2 (brucite) when specimen are exposed to electrolyte layers as thick as 250µm compared to test conditions of the same CO2 partial pressure in bulk solution. Furthermore, it is shown that in some cases inhibitor compounds exhibit change in speciation with the changing parameters. It is well known that corrosion inhibition by inorganic ions is often particular to oxidation state and speciation of the compound such as is the case for tetrahedral coordinated vanadate species. Hence corrosion inhibitor effectiveness is altered by accessibility of CO2 and electrolyte layer thickness. Lastly, synergistic effects of exposure cnditions with corrosion inhibitor compounds is discussed and examples given. These conclusions highlight the importance of replication of likely field exposure environment in design of lab experiments for magnesium alloys. Such information is crucial to the future development of coatings-based inhibitor systems.

Mechanistic Study of Magnesium Carbonate Semibatch Reactive Crystallization with Magnesium Hydroxide and CO2

This work investigates semibatch precipitation of magnesium carbonate at ambient temperature and pressure using Mg(OH)2 and CO2 as starting materials. A thermal analysis method was developed that reflects the dissolution rate of Mg(OH)2 and the formation of magnesium carbonate. The method was successfully used to determine the composition of solids precipitated from the reaction. Concentration profiles of Mg2+ and total carbon over time were determined from the liquid phase. The influence of CO2 flow rate and stirring rate on precipitation was analyzed based on comprehensive information from the liquid and solid phases. A stirring rate of 650 rpm was found to be the optimum speed as the flow rate of CO2 was 1 L/min. Precipitation rate increased with gas flow rate, which indicates that mass transfer of CO2 plays a critical role in this precipitation case. Magnesium carbonate trihydrate (nesquehonite) was formed as a bunch of needle-like primary crystals.

Influence of steam partial pressures in the CO2 capture capacity of K-doped hydrotalcite-based sorbents for their application to SEWGS processes

The performance of three different alkali-promoted hydrotalcite-based materials has been studied under conditions typical of sorption-enhanced WGS process to investigate the influence of steam partial pressure in the feed gas stream in the CO2 capture mechanisms and system efficiency. The sorbents were prepared by calcination and a minimum temperature of 600°C was required to guarantee that all CO2 has been released from the structure. The presence of steam in the feed gas was found to be very beneficial for the capture process efficiency even at very low water content. In order to gain insight into the capture mechanisms involved in the CO2 capture process, XRD analysis were performed and the results showed that under low steam contents in the feed gas formation of K-dawsonite takes place while under high steam contents (35% v/v) reconstruction of hydrotalcite structure and formation of some carbonates are detected and CO2 capture values between 4mol/kg and 9mol/kg have been measured for the three sorbents when tested under isothermal conditions (300°C) at PH2O=4.5 bar and PCO2=0.34 bar. Under these operating conditions formation of magnesium carbonate is suspected to occur although it could not be demonstrated.

Treatment of high-manganese mine water with limestone and sodium carbonate

Manganese is one of the most difficult elements to remove from mine waters, due to its high solubility in both acid and neutral conditions; thus it can be found in quite high concentrations, depending on the rock’s mineralogy. Metal carbonate precipitation can be an effective way for its removal, as manganese carbonate has been detected in net alkaline mine waters. However, limestone is effective in removing manganese only if the metal content is low. This research sought to study manganese precipitation from high-manganese (140 mL) content mine waters applying sodium carbonate and limestone mixtures. It was observed that besides the total carbonate concentration, pH plays a key role on manganese carbonate formation. Provided the pH solution is above 8.5, 99.9% manganese removal can be achieved with carbonate ions. Although not required for manganese precipitation, limestone acts as a solid substrate for the nucleation of fine manganese carbonate grains. Infrared spectroscopy showed manganese carbonate precipitation on the limestone surface. Magnesium was also removed from the mine water but magnesium carbonate formation was not observed.

Thermodynamic and Kinetic Study of Leaching Magnesia from Natural Magnesites by Carbon Dioxide

The applicability of carbon dioxide pressure leaching for the extraction of high purity magnesia from natural magnesites has been investigated. Factors which have influence on the magnesia recovery such as time, temperature, pressure, particle size and calcination temperature have been studied. Optimum values were determined: calcination temperature was in the range 650 – 750 0C, time of the calcination was approximately 5 – 3 hours; carbon dioxide partial pressure leaching for the extraction of magnesia was about 3 bar; particle size was between 0,2 – 0,6 mm. A kinetic study of carbonation of MgO has been investigated at leaching temperature from 00C to 50 0C. Two processes were observed: the formation of a magnesium bicarbonate solution and the precipitation of magnesium carbonate. Our research has determined that the concentration of impurities in magnesium carbonate is virtually no different from their concentration in natural magnesites. Apparently, the formation of magnesium carbonate proceeds without destruction of structure. So the extraction of high purity magnesia from natural magnesites by carbon dioxide is only possible using magnesium bicarbonate. It is necessary to optimize the translation process of magnesium in solution and the technology of extraction of pure MgO from it to generate high purity magnesium on an industrial scale.