•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.
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.
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.
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.
Transformation of Mg-silicates and silica throughout different processes studied.
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•Concentration of H+ in solution affects the dissolution rates of Mg-silicates.•Increase in Si ...coordination (Qn) reduces dissolution rate of Mg-silicates.•Mg solubility affects the structure and Mg content of the precipitated phases.•Rate of silica precipitation accelerated by increasing Mg solubility.•Chemical stability of Mg-silicates are essential for CO2 capture and utilisation.
This work investigates chemical stability of Mg-silicates and silica in aqueous systems in order to gain in-depth understanding of their dissolution and precipitation behaviour under different conditions. The aim was to utilise the knowledge gained to develop an engineered carbon mineralisation technology for an efficient and cost competitive CO2 capture and utilisation. The results, based firmly on 29Si solid-state MAS NMR spectroscopy, and complementary techniques, demonstrate the influence of Si coordination (Qn) on the extent of Mg-silicates dissolution, as the increase in the number of neighbouring Si atoms in the structure of Mg-silicates reduced dissolution of Mg-silicates. While 100 and 80% of Q1(3 Mg) Mg-silicate dissolved in pH = 5 and 6.5, the extent of dissolution for Q2(2 Mg) was lower at values of 90 and 65% under the same conditions, while Q3(1 Mg) did not actually dissolve in solutions with pH = 5 and 6.5. The results of precipitation studies indicated the effect of Mg solubility on the structure and Mg content of the precipitated phases. While Mg-silicates with all three structures of Q1(3 Mg), Q2(2 Mg) and Q3(1 Mg) precipitated in the mildly alkaline environment (pH = 8.5) with the ratios of 20, 40 and 40% respectively, in concentrated acidic solutions of pH = 0, only pure silica (no Mg content) with Q3(1H)/Q4(0H, 0 Mg) having a ratio of 35 and 65% precipitated. The results of direct and indirect carbon mineralisation experiments showed that only 40 and 57 wt% of Mg content of thermally treated Mg-silicate was extracted respectively, consistent with 29Si NMR analyses, indicating that only intermediate Mg-silicate phases I and II with Q1(3 Mg) and Q2(2 Mg) structures were reactive, while other Mg-silicate phases remained inert. Another reason for limited Mg extraction in direct and indirect carbon mineralisation experiments is related to precipitation of a silica-rich phase/s on the surface of the reacting particles leading to passivation, again consistent with 29Si NMR analyses. This confirmed precipitation of Mg-silicate with a Q3(1 Mg) structure as well as hydrated silica Q3(1H) and silica Q4(0H, 0 Mg) in aqueous environments, similar to carbonation processes.
Activation and utilization of mine tailings as feedstock for CO2 mineralization and supplementary cementitious materials (SCMs) have attracted great interest in the mining and concrete industries. In ...this work, a state-of-the-art review on the two utilization strategies and associated activation and pre-treatment methods was provided, aiming at promoting the reuse and reprocessing of mine tailings at a commercial scale. Based on the chemical, mineralogical, and physical characteristics, principles for screening available mine tailings as CO2 mineralization feedstock and SCMs were reviewed and discussed first. Then, basic activation methods were highlighted to improve the reactivity of mine tailings as CO2 mineralization feedstock and SCMs, including thermal, mechanical, and chemical activation. The mechanisms behind each activation method were elaborated, and the energy consumption and optimization of activation treatment were also evaluated. Besides, carbonation conditions affecting the carbonation efficiency of mine tailings were reviewed systematically. In addition, the influence of the utilization of the activated mine tailings as CO2 mineralization feedstock and SCMs on the extraction of trace elements was also examined, and key knowledge gaps were identified and discussed. The critical review concluded that after integrated thermal and mechanical activation, silica-rich mine tailings with kaolinite and a high content of Al2O3 + Fe2O3 + SiO2 (over 80%, by mass) could have a high potential as SCMs. Activated (ultramafic) mine tailings with high Mg/Si and Mg/Fe ratios (of more than 1.5) were susceptible to CO2 mineralization. Overall, the optimal thermal and mechanical activation conditions for pre-treatment of mine tailings as CO2 mineralization feedstock and SCMs were found to be approximately the same, which might facilitate the up-scaled utilization and activation.
•Screening of tailings as CO2 feedstock and SCMs was critically reviewed•Carbonation characteristics and pozzolanic properties of tailings were summarized•Leaching of trace elements of activated tailings upon utilization was assessed•Optimal activation conditions and associated energy consumption were judged•Strategies for promoting up-scaled reuse and activation of tailings were recognized
•Mg extractions almost doubled (71 % vs 39 %).•58 % Mg extraction achieved after 1 h concurrent grinding.•Multi-stage grinding increased Mg extractions.•Similar extractions using 30 wt% media and ...reference experiments.•Agglomeration increased particle size during second occurrence of grinding.
Single-stage mineral carbonation is undertaken at relatively high pressures (up to 18 MPa) and high temperatures (up to 185 °C), where in contrast, two-stage mineral carbonation operates at low pressures (<1 MPa) and mild temperatures (<100 °C). This article describes the development of concurrent grinding for application during the initial dissolution stage of the two-stage aqueous mineral carbonation, using heat activated lizardite as feed ore. Various grinding media sizes and concentrations were studied, with 60 wt% grinding media being determined as optimum and engendering 65 % of the Mg in the feed ore being extracted in 2 h compared to 38 % in reference experiments. Mg extraction using a 30 wt% grinding media and in reference experiments were essentially identical, indicating that the use of 30 wt% media is ineffective. Silica-rich layers were observed to be present in the product produced in experiments using 30 wt% media, explaining why the extent of Mg extraction was similar in both experiments.
Multi-stage grinding was investigated in an effort to obtain a higher level of Mg extraction. However, it was observed that the average particle size of the ground product increased during the second stage of multi-stage grinding. It is suggested this increase in the average particle size limited the extent of extraction during the second phase of grinding. The particle size increase was determined to be due to the agglomeration of fines and subsequently polyacrylic acid, a commercially available grinding media aid was applied in order to reduce the extent of agglomeration.
•Concurrent grinding results six times higher magnesite yields.•Concurrent grinding disintegrate silica-rich layers.•Zirconia and stainless steel are better grinding media than alumina.•Silica ...re-precipitation on grinding media observed.•Dunite is more reactive compared to olivine and lizardite.
Formation of silica-rich passivation layers formed on the periphery of reacting feed particles is one of the primary obstacles in obtaining high magnesite yields during direct aqueous mineral carbonation of peridotites and serpentinites. The disruption of the silica-rich layer around partially reacted grains as a result of concurrent grinding on the degree of carbonation (magnesite yield) was investigated in this work. Three types of naturally-occurring magnesium silicate feedstocks, dunite, olivine and lizardite, as well as three types of grinding media, were examined.
Discrete size fractions of feed samples, with and without grinding media, were carbonated. SEM readily disclosed the formation of a silica-rich shell around a magnesium rich core during carbonation. EDS analysis was employed to study the elemental composition of reacted particles’ shell and core. The method confirmed that during concurrent grinding these silica-rich layers were removed and continuously produced a fresh surface available for further reaction. The removal of the silica-rich layer was shown to significantly improve magnesite yields up to a 600 % increase in yield. Among the three different grinding media used in this work, zirconia and stainless steel media resulted in similar and highest magnesite yields, which is believed to be due to a combination of their high densities and hardness. The findings of this research showed that enhanced magnesite yields could be achieved for all feedstock without the need for energy intensive pre-treatment steps (e.g. ultrafine grinding and heat-activation). Moreover, concurrent grinding resulted in a magnesite yield when raw lizardite was carbonated.
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