Abstract

Accelerated carbonation of ultramafic mine tailings has the potential to offset CO2 emissions produced by mining ores from Cu-Ni-platinum group element, podiform chromite, diamondiferous kimberlite, and historical chrysotile deposits. Treatments such as acid leaching, reaction of tailings with elevated concentrations of gaseous CO2, and optimization of tailings pore water saturation have been shown to enhance CO2 sequestration rates in laboratory settings. The next challenge is to deploy treatment technologies on the pilot and field scale while minimizing cost, energy input, and adverse environmental impacts. Implementation of accelerated tailings carbonation at field scale will ideally make use of in situ treatments or modified ore-processing routes that employ conventional technology and expertise and operate at close to ambient temperatures and pressures. Here, we describe column experiments designed to trial two geochemical treatments that address these criteria: (1) direct reaction of partially saturated ultramafic tailings with synthetic flue gas from power generation (10% CO2 in N2) and (2) repeated heap leaching of ultramafic tailings with dilute sulfuric acid. In the first experiment, we report rapid carbonation of brucite [Mg(OH)2] in the presence of 10% CO2 gas within tailings sampled from the Woodsreef chrysotile mine, New South Wales, Australia. Within four weeks, we observe a doubling of the amount of CO2 stored within minerals relative to what is achieved after three decades of passive mineral carbonation via air capture in the field. Our simulated heap leaching experiments, treated daily with 0.08 M H2SO4, produce high-Mg leachates that have the potential to sequester 21.2 kg CO2 m–2 y–1, which is approximately one to two orders of magnitude higher than the rate of passive carbonation of the Woodsreef mine tailings. Although some nesquehonite (MgCO3 · 3H2O) forms from these leachates, most of the Mg is precipitated as Mg sulfate minerals instead. Therefore, an acid other than H2SO4 could be used; otherwise, sulfate removal would be required to maximize CO2 sequestration potential from acid heap leaching treatments. Reactive transport modeling (MIN3P) is employed to simulate acid leaching experiments and predict the effects of heap leaching for up to five years. Finally, our synchrotron X-ray fluorescence microscopy results for leached tailings material reveal that valuable trace metals (Fe, Ni, Mn, Co, Cr) become highly concentrated within secondary Fe (hydr)oxide minerals at the pH neutralization horizon within our column experiments. This discrete horizon migrates downward, and our reactive transport models indicate it will become increasingly enriched in first-row transition metals in response to continued acid leaching. Acid-leaching treatments for accelerated mineral carbonation could therefore be useful for ore processing and recovery of base metals from tailings, waste rock, or low-grade ores.

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