Offsetting of CO2 emissions by air capture in mine tailings at the Mount Keith Nickel Mine, Western Australia: Rates, controls and prospects for carbon neutral mining

Abstract

The hydrated Mg-carbonate mineral, hydromagnesite [Mg 5 (CO 3) 4 (OH) 2 •4H 2 O], precipitates within mine tailings at the Mount Keith Nickel Mine, Western Australia as a direct result of mining operations. We have used quantitative mineralogical data and  13 C,  18 O and F 14 C isotopic data to quantify the amount of CO 2 fixation and identify carbon sources. Our radiocarbon results indicate that at least 80% of carbon in hydromagnesite is sourced from the modern atmosphere. Stable isotopic results indicate that dissolution of atmospheric CO 2 into mine tailings water is kinetically limited, which suggests that the passive rate of carbon mineralization could be accelerated. Reactive transport modeling is used to describe the observed variation in tailings mineralogy and to estimate rates of CO 2 fixation. Based on our assessment, approximately 39 800 t/yr of atmospheric CO 2 are being trapped and stored in tailings at Mount Keith. This represents an offsetting of approximately 11% of the mine's annual greenhouse gas emissions. Thus, passive sequestration via enhanced weathering of mineral waste can capture and store a significant amount of CO 2. Implementation of geoengineering strategies that accelerate the uptake of CO 2 into tailings water could further reduce or completely offset the net greenhouse gas emissions at Mount Keith and many other mines.

Strategizing Carbon-Neutral Mines: A Case for Pilot Projects

“…These mines have formed the basis of field, laboratory, and geochemical modeling studies aimed at developing the capacity to better utilize mine wastes as carbon sinks [11,13,[15][16][17][18][19][20][21]28,[37][38][39][40].…”

“…We refer to this process as passive carbonation because it occurs without human mediation and is not an intended outcome of the TSF design. Passive carbonation has been documented at Clinton Creek [18,20], Diavik [17,21], and Mount Keith [11,19] as well as other abandoned and active mine sites in Canada, United States, Australia, and Norway [24,26,30,42].…”

“…Here, reaction rates are limited by the TSF design, which has the majority of tailings submerged in a pond. Passive carbonation rates of 2400 g CO 2 /m 2 /year are estimated at Mount Keith in an operational central discharge TSF [19], which offsets annual mine emissions by ~11% [19]. Rates are likely faster at Mount Keith than at Diavik because of its warm, dry climate (i.e., high evaporation rates), saline water chemistry (i.e., greater cation concentrations), and a TSF design that allows for subaerial exposure of tailings enabling greater ingress of atmospheric CO 2 and evapoconcentration of pore waters.…”

“…Specifically, numerous studies have explored the use of ultramafic mine wastes as a potentially valuable feedstock for carbon mineralization [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. Ultramafic and mafic mines generate vast quantities of mine tailings that offer a readily available, fine-grained feedstock for carbonation.…”

“…Although these strategies have proven effective in laboratory-scale studies (e.g., [13,15,19,28,32,33]), pilot projects are crucially needed to evaluate strategies for accelerating carbon mineralization, which if successful and cost effective could be incorporated into TSF design with the purpose of sequestering CO 2 . Discussion of these strategies is followed by the proposal of two scenarios for pilot projects, which if implemented at the mine-scale could render some mining operations carbon-neutral.…”