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Background to the Study
There is widespread recognition that naturally occurring ultramafic rock formations, with high concentrations of Mg and Ca silicates, react with atmospheric CO2 to form Mg and Ca carbonates. Significant concentrations of key industrial mineral resources are often present in ultramafic formations, notably nickel, PGE, Chromium. Consequently, there are large accumulations of mine tailings with the potential to react and sequester CO2 either as a solid phase carbonate or modified into a useful material. Countries with mineral resources hosted in ultramafic formations such as Canada, the USA, South Africa, Finland and Norway have been actively investigating the potential to use mine tailings as a means of ex-situ CO2 sequestration and the potential for reuse or additional mineral recovery.
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Key Messages
- Three decades of research and development work on accelerated mineral carbonation (AMC) has demonstrated that ultramafic, magnesium rich minerals in mine waste materials has the potential to sequestrate CO2 via mineralisation. However, despite three decades of R&D most concepts have not advanced beyond Technology Readiness Level (TRL) 4.1
- Although the stockpiled amounts of material are vast, the suitability of these materials is highly dependent upon specific conditions imposed by mineralogy, geochemistry, petrology, permeability and hydrology. AMC treatment and effectives is therefore highly site-specific.
- The limited scale-up from laboratory-scale investigation is partly due to the energy requirements and chemical kinetics to a timeframe of minutes rather than hours or days, as is still commonly being reported.
- Carbonation of mine wastes using CO2 divided into two broad groups: direct carbonation, which often ends in struggling with a passivating silicate layer that prevents rapid chemistry and high conversion levels; while the second group focusses on stepwise extraction and conversion aimed at reaching higher conversion levels in shorter times.
- Extraction methods can produce materials of marketable value. The evidence from this review is that there are very few AMC approaches that could be economically viable at an industrial CO2 mineralisation scale in a single facility.
- Although most AMC technology related to Mg rich waste streams is still at TRL 4 a few have progressed to TRL 6 or 7. One company, Mineral Carbon International (MCi), reported in 2021, that its aqueous process has advanced from TRL 6 to TRL 7. The company has received support to develop the technology subject to the results of final pilot studies and engineering designs.
- Calcium-based AMC has niche deployment opportunities as a result of the large markets for calcium carbonate based products. The relative reactive properties of calcium compared with magnesium means that the chemistry is less challenging than for conversions that produce magnesium carbonate.
- There have been very few cost assessments to determine the potential for commercial AMC deployment, even when possible revenues from the sales of metals or other by-products have been included. The most promising candidates for marketable products from AMC using magnesium silicate-type rock feedstock is nesquehonite (MgCO3·3H2O). However, large-scale production could lower the price to uneconomic levels.
- There is a lack of reported evidence on the economic effectiveness of metal recovery from mine waste.
- Adverse environmental impacts caused by handling Mg-rich silicate host rocks include toxic metallic by-product streams, and other problematic solid, liquid or gaseous effluents, that can be produced by processing mine wastes. This aspect has received less attention.
- Life cycle assessment (LCA) tools are being increasingly used to quantify the environmental footprint of AMC. The technique does require impacts such as land use, water use and resource depletion as well as the benefits from a reduction in global warming potential.
- Deployment of large AMC facilities presents a public acceptance challenge which has yet to be adequately addressed. Experience from CarbFix does show that positive engagement with the public can produce favourable attitudes to power generation and associated environmental impacts. In this case subsurface in-situ carbonation.
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