Atmospheric Carbon Mineralization in an Industrial-Scale Chrysotile Mining Waste Pile

Our plain-English summary of Atmospheric Carbon Mineralization in an Industrial-Scale Chrysotile Mining Waste Pile, by A. Nowamooz, J.C. Dupuis, G. Beaudoin, et al (Environmental Science & Technology, 2018).

The study

• Authors: A. Nowamooz, J.C. Dupuis, G. Beaudoin, J. Molson, J.M. Lemieux, M. Horswill, R. Fortier, F. Larachi, X. Maldague, M. Constantin, J. Duchesne, R. Therrien
• Journal: Environmental Science & Technology
• Published: 2018;52(14):8050-8057
• Cited by: 5 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Magnesium-rich minerals that are abundant in ultramafic mining waste have the potential to be used as a safe and permanent sequestration solution for carbon dioxide (CO2). Our understanding of thermo-hydro-chemical regimes that govern this reaction at an industrial scale, however, has remained an important challenge to its widespread implementation. Through a year-long monitoring experiment performed at a 110 Mt chrysotile waste pile, we have documented the existence of two distinct thermo-hydro-chemical regimes that control the ingress of CO2 and the subsequent mineral carbonation of the waste. The experimental results are supported by a coupled free-air/porous media numerical flow and transport model that provides insights into optimization strategies to increase the efficiency of mineral sequestration at an industrial scale. Although functioning passively under less-than-optimal conditions compared to laboratory-scale experiments, the 110 Mt Thetford Mines pile is nevertheless estimated to be sequestering up to 100 tonnes of CO2 per year, with a potential total carbon capture capacity under optimal conditions of 3 Mt. Annually, more than 100 Mt of ultramafic mine waste suitable for mineral carbonation is generated by the global mining industry. Our results show that this waste material could become a safe and permanent carbon sink for diffuse sources of CO2.

In plain English

This study concerned storing carbon dioxide (CO2) in mining waste, not people or disease. From January 2016 to January 2017, the authors monitored a 92 m borehole drilled into a 110 Mt pile of chrysotile mining waste at the Black Lake Mine, Thetford Mines, Quebec. Temperature and pressure were logged from January 2016, CO2 from April 2016, and carbon isotopes three times from May to October 2016. They also built a computer model. Core samples contained 0.16 to 0.71 wt.% carbon. The authors described two seasonal regimes. In cold weather the pile was over-pressured and its CO2 fell to 15 to 75 ppm. In warm weather outside air flowed in, losing 40% of its CO2 before reaching the borehole. The authors interpreted heat at depth as strong evidence of active carbonation reactions. The model estimated about 100 t of CO2 stored per year, lower than an earlier estimate of 600 t. Separately, using optimized laboratory results, the authors put the pile's potential total capacity at about 3 Mt. The authors concluded that passive uptake was inefficient, limited by CO2 and water supply.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Year-long field monitoring experiment in a mining waste pile, combined with a 2D numerical flow and transport model (environmental geochemistry study; no human subjects)
• Who was studied: A 110 Mt chrysotile mining waste pile at the Black Lake Mine, Thetford Mines, Quebec, Canada. It was studied through a 92 m borehole (TH-01) drilled in August 2014 and monitored from January 2016 to January 2017. Pressure was monitored from January 2016, CO2 concentration measurements started in April 2016, and carbon isotopes were measured three times from May to October 2016.
• Exposure: No human exposure was studied. The material was chrysotile mining waste, made up of lizardite and chrysotile with smaller amounts of other minerals including antigorite, brucite and magnesium carbonates, reacting with atmospheric CO2.
• Main results:
  • In 40 core samples from the 92 m borehole, carbon content ranged from 0.16 to 0.71 wt.% C.
  • The thermal gradient was 18 °C/km in the upper 50 m and 42 °C/km in the lower portion of the pile.
  • The authors stated there is strong evidence that the heat sources at depth, which seasonal variation alone could not explain, are caused by active exothermic carbon mineralization reactions.
  • In cold-weather periods the pile was pressurized and CO2 was consumed until the reactions were effectively starved of CO2 (15 to 75 ppm).
  • In warm-weather periods a pressure depression let CO2 permeate the pile; by the time air reached the monitoring borehole it had lost 40% of its CO2 content.
  • Water saturation in the first 40 m of the pile ranged from 2% to 30%.
  • The model suggested approximately 80 kg of CO2 stored annually per transverse meter of pile width.
  • Extrapolated to the whole pile (1200 m transverse width), approximately 100 t of CO2 are stored annually.
  • This estimate is lower than the 600 t per year previously estimated for the same pile.
  • Carbon capture capacities from optimized reactions in earlier laboratory studies varied from 0.022 to 0.034 tonnes of CO2 per tonne of mining residue, much more than the pile captured passively.
  • Using those laboratory capacities, not the field data or the pile model, the authors estimated the potential carbon capture capacity of the pile at on the order of 3 Mt CO2.
• The authors' conclusion: The authors concluded that the roughly 100 tonnes per year captured passively in the pile results from an inefficient process limited by CO2 and water input to reaction sites. They suggested that waste piles could be designed to favor air and water infiltration.
• Limitations the authors state:
  • The heat production estimate should be treated with caution because the model did not consider the heterogeneity of the pile.
  • Model results were computed for steady-state conditions and do not include transient atmospheric fluctuations.
  • Frozen conditions (snow cover or frozen ground) were not included in the model.
  • Monitoring data are missing from November to December because of maintenance and upgrade of the power system.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile.

Medical terms in this study, explained in our Lexicon: Chrysotile, Thetford Mines.

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