Chemical and isotopic signatures of waters associated with the carbonation of ultramafic mine tailings, Woodsreef Asbestos Mine, Australia

Extensive carbonate crusts have formed on the tailings of the Woodsreef Asbestos Mine, sequestering significant amounts of CO2 directly from the atmosphere. The physico-chemical (pH, T, conductivity), chemical (cations, dissolved inorganic carbon (DIC)) and isotopic (δ2H, δ18O, δ13CDIC, F14C) signatures of waters interacting with the tailings and associated carbonate precipitates provide insight into the processes controlling carbonation. We observe two distinct evolutionary pathways for a set of stream and meteoric-derived water samples, respectively, with both groups generally being characterised as moderately alkaline, bicarbonate-dominated and Mg-rich waters. Stream water samples are supersaturated with CO2 and therefore prone to degassing, which, in combination with evaporation, drives carbonate supersaturation and precipitation. Isotopic signatures indicate soil CO2 as the main carbon source in the stream waters entering the tailings pile, whereas water emerging downstream of the tailings pile may also contain carbon from the dissolution of isotopically light bedrock magnesite in an open system with respect to soil CO2. The evolution of meteoric-derived waters on the other hand, partly occurs under CO2-limited conditions, which results from reduced CO2 ingress at depth and/or a temporal lag between fluid alkalisation and kinetically hindered uptake of CO2 into alkaline solution. A high pH, Mg-rich meteoric water absorbs atmospheric CO2 after discharging into a tunnel within the tailings pile, resulting in high DIC concentrations with atmospheric carbon isotope signature. Evaporation of the water at the discharge point in the tunnel drives precipitation of hydromagnesite (Mg5(CO3)4(OH)2·4H2O), displaying a clear atmospheric isotope signature, broadly consistent with previous estimates of carbon and oxygen isotope fractionation during precipitation of hydrated Mg-carbonate

Atmospheric emission of NOx from mining explosives: A critical review

High-energy materials such as emulsions, slurries and ammonium-nitrate fuel-oil (ANFO) explosives play crucial roles in mining, quarrying, tunnelling and many other infrastructure activities, because of their excellent transport and blasting properties. These explosives engender environmental concerns, due to atmospheric pollution caused by emission of dust and nitrogen oxides (NOx) from blasts, the latter characterised by the average emission factor of 5 kg (t AN explosive)−1. This first-of-its-kind review provides a concise literature account of the formation of NOx during blasting of AN-based explosives, employed in surface operations. We estimate the total NOx emission rate from AN-based explosives as 0.05 Tg (i.e., 5 × 104 t) N per annum, compared to the total global annual anthropogenic NOx emissions of 41.3 × 106 t N y−1. Although minor in the global sense, the large localised plumes from blasting exhibit high NOx concentration (500 ppm) exceeding up to 3000 times the international standards. This emission has profound consequences at mining sites and for adjacent atmospheric environment, necessitating expensive management of exclusion zones. The review describes different types of AN energetic materials for civilian applications, and summarises the essential properties and terminologies pertaining to their use. Furthermore, we recapitulate the mechanisms that lead to the formation of the reactive nitrogen species in blasting of AN-based explosives, review their implications to atmospheric air pollution, and compare the mechanisms with those experienced in other thermal and combustion operations. We also examine the mitigation approaches, including guidelines and operational-control measures. The review discusses the abatement technologies such as the formulation of new explosive mixtures, comprising secondary fuels, spin traps and other additives, in light of their effectiveness and efficiency. We conclude the review with a summary of unresolved problems, identifying possible future developments and their impacts on the environment with emphasis on local and workplace loads

Analogues to mineral sequestration of CO2: Sources of carbon in magnesite of Attunga Magnesite Quarry, NSW, Australia, a stable isotope study

Introduction Carbon dioxide sequestration or disposal is an essential component of the international effort to stabilise CO2 emissions to the atmosphere. Of the proposed sequestration schemes, mineral sequestration represents the most geologically stable and environmentally benign method for carbon disposal (Lackner et al. 1995). Mineral carbonation mimics natural silicate weathering processes that bind CO2 in stable carbonate minerals. Ultramafic rocks from ophiolite belts, containing high abundances of magnesia as serpentine and olivine, represent the best potential feedstock for mineral carbonation (Metz et al. 2005). At present, research efforts focus on the development of economically viable and energy efficient processes for large-scale industrial implementation of mineral carbonation. These efforts could be assisted by gaining enhanced understandingand characterisation of the natural carbonation of ultramafic rocks. Earlier studies have shown the outstanding potential of serpentinites of the Great Serpentinite Belt for CO2 sequestration. Based on an RCO2 of 2.46 (the number of tonnes of rock required to sequester one tonne of CO2) and geophysical modelling of part of Great Serpentinite Belt in the northern New South Wales, Davis (2008) concluded that 24 × 10^9 t CO2 could be sequestered, equivalent to 308 y of the total stationary emissions for NSW at 2005 levels. Natural carbonation of the ultramafic rocks of the Great Serpentinite Belt is common and, among others, manifests itself in the development of magnesite deposits and silica carbonate alteration zones (Ashley 1995, 1997). The first step in the understanding of these analogues to mineral sequestration is to identify and trace the reactants. Commonly, cross plots of stable isotopes of carbon (delta13C) and oxygen (delta18O) are used to differentiate between magnesite occurrences and to deduce their sources (Kralik et al. 1989). The sources of carbon cannot always be unequivocally identified, since different processes and mixing can lead to similar, ambiguous carbon and oxygen fingerprints of the magnesite minerals. However, pathways and mechanisms of formation can be constrained if the isotopic fractionations associated with the reaction steps involved are known. In the case of vein deposits of magnesite associated with ultramafic rocks debate has focussed on plant respiration and decay in contrast to metamorphic exhalation as the source of carbon in these deposits (Abu-Jaber and Kimberly 1992). This study seeks to constrain the sources of carbon involved in the carbonation of the ultramafic rocks of the Great Serpentinite Belt that led to the formation of the Attunga magnesite deposit

A comparison of natural carbonation rates and the conditions promoting natural mineral sequestration of CO2 in the Great Serpentinite Belt, NSW, Australia

No abstract availabl

Sequestration of atmospheric CO2 in chrysotile mine tailings of the Woodsreef Asbestos Mine, Australia: Quantitative mineralogy, isotopic fingerprinting and carbonation rates

Since closure of the Woodsreef Asbestos Mine, located in the Great Serpentinite Belt (GSB), New South Wales, Australia, extensive carbonate-rich crusts have formed by recessive weathering of fine-grained material on the surface of the tailings pile. A relationship exists between the mode of carbonate occurrence, the mineralogy and the isotopic fingerprint of carbonates from the tailings pile. Vertical carbonate crusts, covering most of the tailings, predominantly consist of the hydrated Mg-carbonate hydromagnesite (Mg5(CO3)4(OH)2·4H2O), which has precipitated from evaporating meteoric waters incorporating atmospheric CO2, as reflected in high δ18O, δ13C and F14C signatures, respectively. Low and variable concentrations of magnesite, dolomite and calcite represent bedrock carbonate, which has formed during alteration of the serpentinite bedrock before mining and is characterised by moderately high δ18O, low δ13C and F14C, a signature typical for ‘weathering-derived’ magnesite deposits in the GSB. The carbonate fraction of deep cement samples, collected from 70 to 120 cm below the surface, representing the bulk tailings material at depth, predominantly consists of pyroaurite (Mg6Fe2(CO3)(OH)16·4H2O) and, despite stable isotope signatures similar to bedrock, contains significant radiocarbon. This indicates that pyroaurite, forming under different conditions as hydromagnesite, may represent an additional trap for atmospheric CO2 in the Woodsreef mine tailings. The distribution of carbonates and quartz, together with the absence of isotopic mixing trends between bedrock carbonate and atmospheric-derived carbonate, strongly indicates that dissolution and re-precipitation of bedrock carbonate is not a dominant process in the Woodsreef tailings. The cations for carbonate formation are instead derived from the dissolution of serpentine minerals (lizardite and chrysotile) and brucite. The internal standard method and the reference intensity method have been used with X-ray diffraction data to estimate the abundance of the two major carbonate minerals hydromagnesite and pyroaurite, respectively. Considering the formation of hydromagnesite on the outer surface of the tailings pile alone or together with formation of pyroaurite within the tailings pile we conclude that, between 1400 and 70,000 t of atmospheric CO2 have been sequestered in the mine tailings since closure of the mine 29 a ago. Carbonation rates of 27 g C m− 2 y− 1 and 1330 g C m− 2 y− 1 are significantly higher than background rates of CO2 uptake by chemical weathering and demonstrate the potential of passive carbonation of mine tailings as a cost and energy effective alternative for storage of CO2 in carbonate minerals

Waters associated with the carbonation of ultramafic mine tailings, woodsreef asbestos mine, Australia

The Woodsreef Asbestos Deposit, New South Wales, Australia, is a chrysotile mineralisation hosted in the ultramafic rocks of the Great Serpentinite Belt, predominantly consisting of schistose and massive serpentinite, as well as partially serpentinised harzburgite. Chrysotile has been extracted from the deposit intermittently between 1906 and 1983, producing 24.2 Mt of ultramafic tailings. The tailings result from dry-grinding of chrysotile ore and are stored above ground on an area covering about 0.5 km2. Extensive carbonate crusts have formed on the tailings pile since the closure of the mine. Natural weathering dissolves Mg-silicate minerals present in the tailings and precipitates Mg-carbonate minerals in the form of crusts and cements. Isotopic signatures of the carbonate minerals (δ13C, δ18O, F14C) indicate that carbonate crusts consisting of hydromagnesite predominantly incorporate CO2 of atmospheric origin. Estimation of the carbonate content has shown that large amounts of CO2 have been sequestered in the tailings at Woodsreef at rates significantly elevated above the background CO2 uptake rate by chemical weathering of coherent silicate rocks. There is potential to further enhance the rates of CO2 sequestration by optimizing the tailings storage for this purpose. Natural weathering of ultramafic tailings thus represents a viable option for low-energy, low-cost sequestration of CO2, directly from the atmosphere. Since the carbonation of mine tailings during weathering occurs in the aqueous phase additional information on the process can be unlocked by investigating the chemistry and isotopic composition of waters interacting with the tailings. The isotopic composition of these waters also represents an intermediate step in the pathway of the sequestered carbon and can thus serves to better constrain isotopic fractionation during formation of hydrated Mg-carbonates in these settings. In this contribution we consider the chemistry and isotopic signatures of natural waters that are associated with the carbonation of the tailings of the Woodsreef Asbestos Mine. Measurements of pH, T, conductivity, cation content, δ2H, δ13CDIC, δ18O and F14C of water samples are presented and used to discuss the interaction of these waters with the tailings materia

Geogenic and anthropogenic lead isotope signatures in the urban environment of Natal (NE-Brazil)

In this study the effect of anthropogenic emissions on the lead isotopic composition of sediments from the Potengi-Jundiai river system near the fast growing city of Natal, NE-Brazil, is investigated. The lead isotope signatures of sediments from the region of Natal were measured by ICP QMS and can be discussed in terms of three different end members of lead. Two geogenic lead endmembers can be distinguished and also be spatially separated, as higher lead isotope ratios occur in the vicinity of the town of Macaiba while the lead isotope ratios decrease towards the city of Natal and the mouth of the estuary. Proterozoic rocks of different age are potential lead sources as Paleoproterozoic rocks occur in the catchment of Jundiai river and younger, Neoproterozoic rocks predominate towards the mouth of the river. The lead isotope signatures of the anthropogenically affected samples deviate from the signatures of the unaffected samples indicating the existence of a third, anthropogenic source of lead. This source represents the lead isotope signature of anthropogenic emitters like waste- and coal-combustion which is also revealed by other geochemical studies conducted in Brazil

Study of thermally conditioned and weak acid-treated serpentinites for mineralisation of carbon dioxide

This contribution assesses the dissolution behaviour of serpentinite specimens, featuring distinct stages of serpentinisation, by treating the specimens with aqueous solutions of formic acid. We have observed a marked improvement in the extraction of magnesium when the samples were finely ground and thermally conditioned before treatment with formic acid. An extraction of 42% for -25 μm particles activated at 700 °C (29% residual OH) could be obtained from the forsterite-lizardite bearing specimen whereas 66% of magnesium was leached out of the fully serpentinised antigorite mineral, which was crushed to a particle size of -53 μm and baked at 720 °C (36% residual OH). Combined results derived from FTIR and XRD indicate that heat activation between 500 and 720 °C results in a reorganisation of lizardite and antigorite to amorphised material, forsterite and silica. Unreactive enstatite forms from the amorphised material and silica once the heating temperature exceeds 800 °C. Semi-quantitative XRD analysis yields an estimate of the crystalline and non-crystalline (forsterite) fractions of the activated material, permitting approximation of relative rates of dissolution of amorphous and forsterite phases. Although FTIR provides important information on forsterite and silica formation, it cannot detect the amorphous material. Forsterite and amorphous phases alike dissolve in the weak acid but the formation of skins of the amorphous silica limits the overall magnesium yield on a laboratory time scale. The material that constitutes the skins originates from two sources: (i) silica formed in forsterisation of serpentine minerals undergoing heat treatment, and (ii) silica produced during extraction of Mg by a weak acid from amorphous and forsterite phases. Heat activation also leads to the formation of andradite and modified chlorite minerals that exhibit less solubility than forsterite and amorphous phases in weakly acidic medium

Mg isotope signatures for tracing of natural carbonation reactions

[No abstract available

Mineralisation of atmospheric CO2 in hydromagnesite in ultramafic mine tailings – Insights from Mg isotopes

In this study we present the first Mg isotope data that record the fate of Mg during mineralisation of atmospheric CO2 in ultramafic mine tailings. At the Woodsreef Asbestos Mine, New South Wales, Australia, weathering of ultramafic mine waste sequesters significant amounts of CO2 in hydromagnesite [Mg5(CO3)4(OH)2·4H2O]. Mineralisation of CO2 in above-ground, sub-aerially stored tailings is driven by the infiltration of rainwater dissolving Mg from bedrock minerals present in the tailings. Hydromagnesite, forming on the surface of the tailings, has lower δ26Mg (δ26MgHmgs = −1.48 ± 0.02‰) than the serpentinised harzburgite bedrock (δ26MgSerpentinite = −0.10 ± 0.06‰), the bulk tailings (δ26MgBulk tailings = −0.29 ± 0.03‰) and weathered tailings containing authigenic clay minerals (δ26MgWeathered tailings = +0.28 ± 0.06‰). Dripwater (δ26MgDripwater = −1.79 ± 0.02‰) and co-existing hydromagnesite (δ26MgHmgs = −2.01 ± 0.09‰), forming in a tunnel within the tailings, and nodular bedrock magnesite [MgCO3] (δ26MgMgs = −3.26 ± 0.10‰) have lower δ26Mg than surficial fluid (δ26Mg = −0.36‰) and hydromagnesite. Complete dissolution of source minerals, or formation of Mg-poor products during weathering, is expected to transfer Mg into solution without significant alteration of the Mg isotopic composition. Aqueous geochemical data and modelling of saturation indices, along with Rayleigh distillation and mixing calculations, indicate that the 26Mg-depletion in the drip water, relative to surficial water, is the result of brucite dissolution and/or precipitation of secondary Mg-bearing silicates and cannot be assigned to bedrock magnesite dissolution. Our results show that the main mineral sources of Mg in the tailings (silicate, oxide/hydroxide and carbonate minerals) are isotopically distinct and that the Mg isotopic composition of fluids and thus of the precipitating hydromagnesite reflects both isotopic composition of source minerals and precipitation of Mg-rich secondary phases. The consistent enrichment and depletion of 26Mg in secondary silicates and carbonates, respectively, underpins the use of the presented hydromagnesite and fluid Mg isotopic compositions as a tracer of Mg sources and pathways during CO2 mineralisation in ultramafic rocks