Séquestration du CO₂ associée aux phénomènes de minéralisation passive du carbone dans les résidus miniers du Projet Dumont Nickel (Abitibi-Témiscamingue, Québec, Canada)

Authors: Gras, Antoine
Advisor: Beaudoin, GeorgesMolson, JohnPlante, Benoît
Abstract: The implication of anthropogenic carbon dioxide (CO2) emissions in climate change is now widely accepted and solutions are emerging in order to limit the accumulation of CO2. Carbon mineralization, which allows the sequestration of CO2 through carbonate precipitation, stable minerals over geological time scales, is one of the options considered. Among the proposed carbon mineralization pathways, passive carbon mineralization in ultramafic mining residues can potentially lead to developing carbon-neutral mines. However, the impacts on leachate water quality and evolution of sequestration capacity in natural conditions, on medium and large scales, are still poorly documented. RNC Minerals plans to mine a nickel deposit located in the northwestern part of Quebec. The operation at the Dumont Nickel Project (DNP) would produce approximately 1.7 Gt of ultramafic mining residues. Several factors which influence the carbon sequestration capacity of the DNP residues have been studied in the laboratory, at variable CO2 concentrations. In this study, the processes of passive carbon mineralization in the DNP mining residues are described and the atmospheric CO2 sequestration capacity is estimated, at the experimental cell scale, under natural conditions. In order to study the impacts of meteoric weathering of the DNP residues, two experimental cells were built and instrumented. The first cell EC-1, contains the ultramafic waste rock, and the second EC-2, was filled with milling residues (Tailings). The hydrogeological properties and surface area of the residues contained in the two cells are different whereas the mineralogy is similar. The main minerals in the residues are chrysotile, lizardite, brucite, chlorite and magnetite. Between 2011 and 2015, changes in CO2 concentrations, mineralogy, and chemical composition of leachate waters were recorded. Monitoring of CO2 concentrations showed a decrease in CO2 concentration from the surface (~ 390 ppmv) to the bottom of the cells (~ 100 ppmv). At the same time, the carbon content in the weathered residues increased and the mineralogical analyses revealed precipitation of several magnesium carbonates such as hydromagnesite. These observations indicate that passive mineral carbonation of the mining residues is occurring within the experimental cells, for which three potential sources of CO2 can be identified : (1) the atmosphere, (2) the CO2(g) produced from organic matter oxydation, and (3) CO2(g) produced from carbonate dissolution. The isotopic compositions of CO2(g) and newly formed carbonates were measured. Using these isotopic compositions it was possible to demonstrate that dissolution of CO2(g) in interstitial water limits the sequestration capacity and that atmospheric CO2 is the main source of the CO2 sequestered. Despite the differences between the two experimental cells the same processes control CO2 sequestration. A conceptual model of the carbon mineralization reactions, including evolution of the isotopic compositions, is proposed. The leachate water sampled at the bottom of the experimental cells, between May and November since 2011, is characterized by an alkaline pH (~9.5), a high alkalinity (~90 to ~750 mg/L CaCO3) and a high concentration of magnesium (~50 at ~750 mg/L). This composition is consistent with weathering of ultramafic rocks in a system open to CO2. Since 2012, the chemical composition of the leachate water was evolved seasonnaly. These seasonal variations are explained by: (1) recharge and temeprature variations over the year and (2) increased carbonate precipitation between May and July. The seasonal decrease of alkalinity and magnesium concentrations, caused by increased carbonate precipitation, induces undersaturation of carbonate minerals. Therefore carbonate precipitation self-limits carbon sequestration through a negative feed-back loop. Since 2011, an estimated 13 kg of atmospheric CO2 was sequestered in the milling residues from EC-2, which corresponds to a mean rate of 1,4 (+/- 0.3) kgCO2/tonne/year. Using this mean rate, during the mining operation the milling residues will sequester about 21 kt of atmospheric CO2 each year, which will represents one quarter of the 127,700 tonnes of CO2 emitted. Using MIN3P, a numerical model which allow to simulate multi-component and multiphase reactive transport in unsaturated porous media, the carbon mineralization reactions were simulated in 1D at the center of cell EC-2. The data collected during the 4 years of monitoring were used to calibrate the numerical model. However, none of the simulations allowed to reproduce the evolution of the leachate water geochemistry and the CO2 concentrations observed in the experimental cell. Several simplifications of the conceptual model could explain the differences with the observed data.
Document Type: Thèse de doctorat
Issue Date: 2018
Open Access Date: 3 July 2018
Permalink: http://hdl.handle.net/20.500.11794/30265
Grantor: Université Laval
Collection:Thèses et mémoires

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