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•Solid products of the CO2 mineralization were characterized as a function of time.•A sampling technique was applied to track changes in the carbonation products.•The formation of ...hydromagnesite prior to magnesite was shown experimentally.•Transformation of hydromagnesite to magnesite strongly depends on reaction T and P.
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.
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•Increased iron loading on MFI improved allyl alcohol yield.•Alkaline preparation conditions engendered the zeolite with mesoporosity.•Rubidium deposition enhanced allyl alcohol yield ...and decreased acrolein yield.•Alkali post synthesis modification reduced the concentration of acid sites.
Under most reaction conditions studied, acrolein is reported as the primary product in the conversion of glycerol over zeolites. In such processes, acrolein forms at relatively high yields, with negligible allyl alcohol selectivity. In this contribution, we report the development of ZSM5-supported iron catalysts, modified by rubidium deposition, as stable materials for production of allyl alcohol from glycerol. Our results demonstrate a reduced rate of formation of acrolein over modified catalysts. Both unmodified and modified catalysts were analysed by inductively coupled plasma optical emission spectrometry, nitrogen adsorption, scanning electron microscope, X-ray diffraction, ammonia temperature programmed desorption, X-ray photoelectron spectroscopy and ultraviolet-visible spectroscopy. These techniques revealed that differences in product distribution and catalyst performance are due to the combined effects of iron loading, catalyst acidity and changes in the porosity of the catalyst.
•Side reactions produced serpentine and magnesium silicate hydroxide phases.•Extent of side reactions were elevated when experiments performed under nitrogen.•Side reactions reduced the magnesite ...yield by up to 40%.•Side reactions reduced the efficiency of direct aqueous carbonation process.
This work discloses a possible explanation for the relatively low efficiency and yield observed in direct aqueous carbonation of heat activated serpentine which remained a critical unanswered question during three decades of ex-situ mineral carbonation research and development. The discovery of undesirable side reactions, occurring during direct aqueous carbonation of heat activated serpentine has been reported and investigated in detail. These reactions result in the reformation of crystalline serpentine and precipitation of amorphous magnesium silicate hydroxide phase/s on the surface of reacting feed particles. Reformation of serpentine occurs under relatively mild conditions (in terms of pressure and temperature) and after only a few minutes of reaction which is in stark contrast to the conditions and rates which occur during geological serpentinisation and other laboratory studies. Scanning Electron Microscopy and Energy Dispersive X-ray spectroscopy analyses showed precipitation of amorphous magnesium silicate hydroxide phase/s during carbonation process. Fourier Transform Infrared Spectroscopy and Thermogravimetric analyses identified and quantified free and hydrogen bonded hydroxyls of silanol groups in the structure of the reaction products when heat activated lizardite and antigorite were carbonated. The growth of a crystalline serpentine phase was confirmed and quantified by X-ray Diffraction and Thermogravimetric analyses in the reaction products when heat activated antigorite was used a feed.
•The dissolution kinetics of roasted lizardite in acid buffer solutions were studied.•An initial rapid liberation of Mg2+ followed by a very slow extraction was observed.•The extent of extraction of ...Mg depends on the particle size and solution pH.•The crackling core model (CCM) was successfully fitted to the experimental data.•Amorphous Si re-precipitation at high S/L and pH 6.1 was demonstrated.
The rate-determining step in the aqueous carbonation of serpentine minerals is the dissolution of Mg from serpentine. The dissolution rate of minerals largely depends on the pH of the solution and the size of serpentine particles. In the present work, an experimental method has been developed to study the dissolution rate of heat activated serpentine (lizardite polymorph) in a wide range of pH, solid to liquid ratio and particle size at room temperature. The results allowed us to determine the effect of these variables on the dissolution kinetics of heat activated lizardite, which represents crucial kinetic data for accurately modelling the carbonation rates of serpentinite. Additionally, amorphous Si re-precipitation at high solid to liquid (S/L) ratio and pH 6.1 was demonstrated. These provide essential data for the design and optimisation of industrial mineral carbonation processes. For the first time, the crackling core model (CCM) was applied to model the dissolution kinetics of heat activated lizardite in acidic solutions. Applying the CCM model to a wide range of particle sizes provides useful information on the mechanism of the dissolution of heat activated lizardite and the range of particle size for which the assumptions of the model are valid. Characterising serpentine particles leached under different conditions, along with analysing model parameters, provided a new insight into the mechanism of the dissolution of heat activated lizardite.
Utilising the byproducts of mineral carbonation processes contributes to rendering the technology environmentally benign and enhances the economically viability of the process. In this work we ...synthesised, characterised and investigated the technical feasibility and environmental benefits of utilising feed and byproducts of mineral carbonation technology as Portland cement substitutes. These materials, with and without pre-treatment, were used to substitute 5, 10 and 20 wt% of Portland cement in mortars. Pozzolanic activity tests indicated that acid treated silica enriched residue (ATSER) displayed pozzolanic activity. At 5% cement replacement all materials showed compressive strengths comparable to the control. When 10 wt% of cement was replaced, only heat activated lizardite showed strength results similar to the control. The compressive strength of mortars containing other samples with 10 wt% or greater cement replacement showed that the extra water demand outweighed any pozzolanic contribution of mineral carbonation materials and resulted in a lowering of the compressive strength of these mortars compared to controls, in particular when in excess of 10 wt% of cement is replaced. An environmental analysis showed that in addition to significant CO2 emissions reduction, the economic and environmental costs of waste disposal were avoided when mineral carbonation byproducts substituted Portland cement.
•Results of Si extraction at pH∼13.5 indicated ATSER had the fastest rate of Si extraction even higher than silica fume.•Pozzolanic activity tests showed that ATSER displayed some level of pozzolanic activity slightly less than silica fume.•5 wt% cement replacement with mineral carbonation materials showed acceptable compressive strength for all subsitutes.•10 wt% or higher cement replacement resulted in lower strength compared to the control.
This research aims to provide insight into the structure and reaction mechanism of silica-rich phases formed as byproducts in direct aqueous carbonation of heat-activated lizardite. In undertaking ...this work, we employed analytical techniques such as thermogravimetric analysis (TGA), X-ray powder diffraction (XRPD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), 29Si solid-state nuclear magnetic resonance (29Si SS NMR), inductively coupled plasma-optical emission spectrometry (ICP-OES), and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) to characterize carbonation products and to understand the mechanism of formation and the structure of silica-rich byproducts. Thermodynamic analysis predicts the formation of magnesite and amorphous silica in the process of direct aqueous carbonation of heat-activated lizardite under the experimental conditions studied. Characterization of carbonation products disclosed the presence of magnesite, amorphous silica, and magnesium silicate phases. Analysis of supernatant solutions obtained from direct aqueous carbonation by MALDI spectroscopy showed the presence of silica polymers, which precipitate during the carbonation experiments. The precipitated amorphous silica on the surface of reacting particles was found to subsequently adsorb the dissolved magnesium (Mg) from the solution to form a magnesium silicate phase.
Vast reserves of peridotite and serpentinite rocks can be utilised for the safe and permanent sequestration of global CO2 emissions via aqueous mineral carbonation. These, and indeed most feedstocks ...used in mineral carbonation require ultrafine grinding and/or heat-activation, to engender significantly enhanced reactivity in the rock such that it can then be carbonated. Both activation processes are energy intensive and present significant obstacles to the commercial application of mineral carbonation. Here we show that these limitations can be addressed, at least in part, through the application of a concurrent or in operando grinding technique which does not require feedstocks which have been subjected to prior ultrafine grinding nor heat-activation.
Concurrent grinding is shown to result in a significant increase in magnesite yields for non-heat activated feedstock, prepared such that fines (<20 μm particles) were excluded from the feed. We assert that concurrent grinding may be a suitable technique for the processing of feedstocks such as those containing significant proportions of forsterite and pyroxene, minerals which are unresponsive to thermal activation for use in aqueous mineral carbonation. This study also investigates the effect of different grinding media particle size on reducing the particle size distribution (PSD) of the feed. Optimum ratio of grinding media size to feed particle size, optimum grinding media and slurry concentrations, optimum time for grinding and optimum impeller designs are determined for the system under study. The quantitative effect of grinding media concentration, slurry concentration, pressure and temperature on magnesite yield has been investigated.
•Feedstocks can be carbonated using concurrent grinding without prior pre-treatment.•Different grinding medias and media mixtures were investigated.•Different impeller designs were investigated.•Quantitative effect of operating variables was determined.•62% magnesite yield was obtained using raw dunite rock.
A number of iron−ZSM-5 catalysts have been prepared and characterized by X-ray absorption spectroscopy using fluorescence detection, electron spectroscopy, temperature-programmed reduction, infrared ...spectroscopy, and electron microscopy. Iron has been introduced by aqueous exchange, by a novel method recently proposed by Feng and Hall (Catal. Lett. 1996, 41, 45), by exchange from a rigorously dried methanolic solution accompanied by agitation with ultrasound, and by a method intended to promote solid-state exchange. The degree of interaction with the zeolite framework has been probed by examining the effect on the zeolite proton OH band in the infrared spectrum. Less than 30% of the protons were exchanged from aqueous solution, but almost 80% exchange was achieved using ultrasound, as well as by the method reported by Feng and Hall (FH). Initially, both methods exhibited mainly isolated metal ions; however, calcination of the samples prepared according to FH exhibited rather large oxide clusters. After aqueous exchange and activation, most of the iron is present in the form of small oxygen-containing nanoclusters within the zeolite matrix, with EXAFS measurements indicating an average composition of Fe4O4, although electron microscopy identifies some larger particles at the external surface of the zeolite. Depending on the preparation methods, isolated cationic species within the zeolite matrix were also found. The small Fe4O4 type clusters cannot be reduced to the metallic state, even by hydrogen at 1100 K, although interconversion between Fe(II) and Fe(III) is facile. When the zeolite was exposed to nitric oxide, stretching vibrations corresponding to adsorption on the different iron species present could be identified by infrared spectroscopy. It is proposed that the ultrastable iron−oxygen nanoclusters have structures similar either to the iron−sulfur compounds ferredoxin II of desulfovibrio Gigas or to the cubanes of high-potential iron protein (HIPIP). Reactivity of these Fe−ZSM-5 materials in the selective catalytic reduction of NO x by propene in oxygen/helium differs significantly, depending irreversibly on whether they are initially activated in oxygen or in an inert atmosphere. Correlations between catalytic activity and the infrared spectroscopy results for adsorbed NO indicate that the nanoclusters are more active (per iron atom) in the SCR reaction than the isolated cations.
•The dissolution kinetics of roasted lizardite at different T and pH were studied.•An initial rapid liberation of Mg2+ followed by a very slow extraction was observed.•Increasing temperature caused ...an increase in the rate of Mg extraction initially.•The extent of extraction of Mg2+ decreased due to precipitation of silica.•Silica re-precipitation at high temperature and S/L was demonstrated.
The dissolution of magnesium silicate minerals such as serpentine in aqueous solutions saturated or near saturated with carbon dioxide (CO2) enables its subsequent reaction to form magnesium carbonate, a process called aqueous mineral carbonation. The dissolution rate of magnesium ions (Mg2+) from thermally activated serpentine and the factors influencing the rate and extent of dissolution have been studied in our research group. The current contribution focuses on the effect of temperature and pH on the dissolution of heat activated lizardite (a polymorph of serpentine). The extent of dissolution of thermally activated lizardite was measured experimentally as a function of temperature (25 °C ≤ T ≤ 75 °C) and pH (1.2 ≤ pH ≤ 9.8). It was found that at higher temperatures the level of Mg extraction is greater during the initial stage of dissolution but is then hindered by the re-precipitation of amorphous silica. Thermodynamic modelling was used to assess the susceptibility of solid phase formation and confirmed the likelihood of re-precipitation of amorphous silica from the solutions. For the first time, in this work, the crackling core model (CCM) was used to model experimental data at different pH values.
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