81 research outputs found
Offsetting of CO₂ emissions by air capture in mine tailings at the Mount Keith Nickel Mine, Western Australia: Rates, controls and prospects for carbon neutral mining
No full textThe hydrated Mg-carbonate mineral, hydromagnesite [Mg₅(CO₃)₄(OH)₂•4H₂O], precipitates within mine tailings at the Mount Keith Nickel Mine, Western Australia as a direct result of mining operations. We have used quantitative mineralogical data and δ¹³C, δ¹⁸O and F¹⁴C isotopic data to quantify the amount of CO₂fixation and identify carbon sources. Our radiocarbon results indicate that at least 80% of carbon stored in hydromagnesite has been captured from the modern atmosphere. Stable isotopic results indicate that dissolution of atmospheric CO₂ into mine tailings water is kinetically limited, which suggests that the current rate of carbon mineralization could be accelerated. Reactive transport modeling is used to describe the observed variation in tailings mineralogy and to estimate rates of CO₂ fixation. Based on our assessment, approximately 39,800 t/yr of atmospheric CO₂ are being trapped and stored in tailings at Mount Keith. This represents an offsetting of approximately 11% of the mine's annual greenhouse gas emissions. Thus, passive sequestration via enhanced weathering of mineral waste can capture and store a significant amount of CO₂. Recommendations are made for changes to tailings management and ore processing practices that have potential to accelerate carbonation of tailings and further reduce or completely offset the net greenhouse gas emissions at Mount Keith and many other mines
Magnesite formation in playa environments near Atlin, British Columbia, Canada
No full textThe hydromagnesite–magnesite playas near Atlin, British Columba, Canada are unique Mg-carbonate depositional environments that have formed at Earth’s surface since the end of the last deglaciation. This study elucidates the mechanisms, pathways, and rates of magnesite (MgCO3) formation in these near-surface environments, which are challenging to study in short-duration laboratory experiments because magnesite precipitation is extremely slow at low temperature. The Atlin playas, having formed over millennia, contain abundant magnesite as well as a suite of other Mg- and Ca-carbonate minerals. Mineralogical and textural evidence demonstrate that hydromagnesite [Mg5(CO3)4(OH)2·4H2O] forms at least in part through transformation of more hydrated phases, e.g., lansfordite (MgCO3·5H2O). Deposition of these hydrated Mg-carbonate minerals is limited by the evaporative flux, and thus, is effectively transport-controlled at the scale of the playas. Magnesite is a spatially distinct phase from hydromagnesite and its crystal morphology varies with depth indicating variable crystal growth mechanisms and precipitation rates. Particle size distributions and mineral abundance data indicate that magnesite formation is nucleation-limited. Furthermore, mineralogical data as well as stable and radiogenic isotope data support magnesite formation starting after the majority of hydromagnesite had been deposited likely resulting from long induction times and slow precipitation rates. Hydrated Mg-carbonate minerals precipitate relatively rapidly and control pore water chemistry while magnesite remains highly supersaturated, and thus, is reaction-controlled. This difference in controlling regime allows for magnesite abundance to increase over time without the loss of hydromagnesite such as through its transformation, which the data also does not support. We estimate rates of magnesite formation (nucleation + crystal growth) in the range of 10−17 to 10−16 mol/cm2/s over approximately 8000 years. This study helps to elucidate the geochemical conditions needed to form Mg-carbonate minerals in ancient and modern sedimentary environments and provides insights into facilitating long-term storage of anthropogenic CO2 within Mg-carbonate minerals.We acknowledge the support of the Natural Sciences and
Engineering Research Council of Canada (NSERC) through a
Discovery Grant to G.M. Dipple
A greenhouse-scale photosynthetic microbial bioreactor for carbon sequestration in magnesium carbonate minerals
No full textA cyanobacteria dominated consortium collected from an alkaline wetland located near Atlin, British Columbia, Canada accelerated the precipitation of platy hydromagnesite [Mg5(CO3)4(OH) 2·4H2O] in a linear flow-through experimental model wetland. The concentration of magnesium decreased rapidly within 2 m of the inflow point of the 10-m-long (∼1.5 m2) bioreactor. The change in water chemistry was monitored over two months along the length of the channel. Carbonate mineralization was associated with extra-cellular polymeric substances in the nutrient-rich upstream portion of the bioreactor, while the lower part of the system, which lacked essential nutrients, did not exhibit any hydromagnesite precipitation. A mass balance calculation using the water chemistry data produced a carbon sequestration rate of 33.34 t of C/ha per year. Amendment of the nutrient deficiency would intuitively allow for increased carbonation activity. Optimization of this process will have application as a sustainable mining practice by mediating magnesium carbonate precipitation in ultramafic mine tailings storage facilities
Accelerating Mineral Carbonation Using Carbonic Anhydrase
No full textCarbonic anhydrase (CA) enzymes have
gained considerable attention
for their potential use in carbon dioxide (CO2) capture
technologies because they are able to catalyze rapidly the interconversion
of aqueous CO2 and bicarbonate. However, there are challenges
for widespread implementation including the need to develop mineralization
process routes for permanent carbon storage. Mineral carbonation of
highly reactive feedstocks may be limited by the supply rate of CO2. This rate limitation can be directly addressed by incorporating
enzyme-catalyzed CO2 hydration. This study examined the
effects of bovine carbonic anhydrase (BCA) and CO2-rich
gas streams on the carbonation rate of brucite [Mg(OH)2], a highly reactive mineral. Alkaline brucite slurries were amended
with BCA and supplied with 10% CO2 gas while aqueous chemistry
and solids were monitored throughout the experiments (hours to days).
In comparison to controls, brucite carbonation using BCA was accelerated
by up to 240%. Nesquehonite [MgCO3·3H2O]
precipitation limited the accumulation of hydrated CO2 species,
apparently preventing BCA from catalyzing the dehydration reaction.
Geochemical models reproduce observed reaction progress in all experiments,
revealing a linear correlation between CO2 uptake and carbonation
rate. Data demonstrates that carbonation in BCA-amended reactors remained
limited by CO2 supply, implying further acceleration is
possible
Prospects for CO2 mineralization and enhanced weathering of ultramafic mine tailings from the Baptiste nickel deposit in British Columbia, Canada
No full textThe impact of evolving mineral–water–gas interfacial areas on mineral–fluid reaction rates in unsaturated porous media
No full textA depositional model for hydromagnesite-magnesite playas near Atlin, British Columbia, Canada
No full textThis study formulates a comprehensive depositional model for hydromagnesite-magnesite playas. Mineralogical, isotopic and hydrogeochemical data are coupled with electron microscopy and field observations of the hydromagnesite-magnesite playas near Atlin, British Columbia, Canada. Four surface environments are recognized: wetlands, grasslands, localized mounds (metre-scale) and amalgamated mounds composed primarily of hydromagnesite [Mg(CO)(OH)·4HO], which are interpreted to represent stages in playa genesis. Water chemistry, precipitation kinetics and depositional environment are primary controls on sediment mineralogy. At depth (average ≈ 2 m), Ca-Mg-carbonate sediments overlay early Holocene glaciolacustrine sediments indicating deposition within a lake post-deglaciation. This mineralogical change corresponds to a shift from siliciclastic to chemical carbonate deposition as the supply of fresh surface water (for example, glacier meltwater) ceased and was replaced by alkaline groundwater. Weathering of ultramafic bedrock in the region produces Mg-HCO groundwater that concentrates by evaporation upon discharging into closed basins, occupied by the playas. An uppermost unit of Mg-carbonate sediments (hydromagnesite mounds) overlies the Ca-Mg-carbonate sediments. This second mineralogical shift corresponds to a change in the depositional environment from subaqueous to subaerial, occurring once sediments 'emerged' from the water surface. Capillary action and evaporation draw Mg-HCO water up towards the ground surface, precipitating Mg-carbonate minerals. Evaporation at the water table causes precipitation of lansfordite [MgCO·5HO] which partially cements pre-existing sediments forming a hardpan. As carbonate deposition continues, the weight of the overlying sediments causes compaction and minor lateral movement of the mounds leading to amalgamation of localized mounds. Radiocarbon dating of buried vegetation at the Ca-Mg-carbonate boundary indicates that there has been ca 8000 years of continuous Mg-carbonate deposition at a rate of 0·4 mm yr. The depositional model accounts for the many sedimentological, mineralogical and geochemical processes that occur in the four surface environments; elucidating past and present carbonate deposition
Potential for offsetting diamond mine carbon emissions through mineral carbonation of processed kimberlite: an assessment of De Beers mine sites in South Africa and Canada
No full textDe Beers kimberlite mine operations in South Africa (Venetia and Voorspoed) and Canada (Gahcho Kué, Victor, and Snap Lake) have the potential to sequester carbon dioxide (CO2) through weathering of kimberlite mine tailings, which can store carbon in secondary carbonate minerals (mineral carbonation). Carbonation of ca. 4.7 to 24.0 wt% (average = 13.8 wt%) of annual processed kimberlite production could offset 100% of each mine site’s carbon dioxide equivalent (CO2e) emissions. Minerals of particular interest for reactivity with atmospheric or waste CO2 from energy production include serpentine minerals, olivine (forsterite), brucite, and smectite. The most abundant minerals, such as serpentine polymorphs, provide the bulk of the carbonation potential. However, the detection of minor amounts of highly reactive brucite in tailings from Victor, as well as the likely presence of brucite at Venetia, Gahcho Kué, and Snap Lake, is also important for the mineral carbonation potential of the mine sites.</p
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