Accelerating Mineral Carbonation in Ultramafic Mine Tailings via Direct CO2 Reaction and Heap Leaching with Potential for Base Metal Enrichment and Recovery

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.

Enhanced weathering to capture atmospheric carbon dioxide: Modeling of a trickle‐bed reactor

“…Currently, the weathering of rock by carbon dioxide and water, a natural process, absorbs about 1.1 Gt CO 2 per year from the atmosphere, mainly stored as bicarbonate in the ocean 6 . Many types of carbonate and silicate mineral show CO 2 sequestration ability 7‐11 . An advantage of the EW approach to CDR is that the CO 2 is captured in the form of carbonate or bicarbonate ions, removing the need for CO 2 compression and underground storage.…”

“…Moreover, it is known that rates of natural weathering are relatively low, and it is not certain that these can be enhanced to an extent sufficient to make EW useful in countering climate change in this century. For the implementation of EW, suggestions include spreading crushed minerals on the soil 8,9 or in coastal environments, 10 and heap leaching 11 . Exploring the potential of a more “engineered” approach, the motivation for the modeling study presented here is to quantify capture rates and water and energy requirements for an EW process taking place in a chemical reactor.…”

Global Carbon Dioxide Removal Potential of Waste Materials From Metal and Diamond Mining

“…Engineered solutions for enhancing weathering rates could involve utilising enclosed reactor systems or heap leaching (Darton and Yang, 2020;Hamilton et al, 2020;Xing et al, 2021). For on-site operations and reactor-based weathering systems, several mine-specific considerations need to be addressed for successful deployment in addition to the chemical and kinetic barriers to implementation.…”

Carbon Mineralization with North American PGM Mine Tailings—Characterization and Reactivity Analysis

“…This could be realized by using low-cost, ambient temperature and pressure conditions with elevated CO 2 concentration (10%). This method has had success at the lab [18][19][20] and field scale [21] with brucite-rich alkaline feedstock, but has not yet been used for PGM mine tailings.…”