A thermal imaging methodology to study evaporation kinetics in mine tailings

Abstract

Predicting why, how, and when mine tailings disposal sites become prone to dust scattering events is often hampered by our limited understanding of the factors that affect the drying rates from their surface layers.

Monitoring moisture content and evaporation kinetics from mine slurries through albedo measurements to help predict and prevent dust emissions

“…As previously reported in the literature [15,27,59,60], water evaporation from porous media can be limited by various water transport mechanisms depending on the samples drying state. Indeed, at the higher moisture contents, supernatant water evaporates freely from the residues surface leading to an elevated drying rate which is relatively independent of solid fraction or time.…”

“…Furthermore, evaporation should have a greater effect on sample surface moisture content in the range of solid fraction where hydraulic continuity is disrupted thereby reducing the effectiveness of capillary transport of liquid water to the sample surface. Previous measurements of the evaporative fluxes from wet bauxite residues have been found to vary between 0.2 and 0.6 g m −2 s −1 from quantitative evaluation of evaporation rates as function of sample thickness at an ambient air relative humidity of about 40% [27]. Given the surface area of the Petri dish of 7.85 × 10 −3 m 2 , the mass of water evaporated per minute should range from 0.1 g to values as high as approximately 0.28 g. Therefore, this supports the hypothesis that the solid royalsocietypublishing.org/journal/rsos R. Soc.…”

“…An estimate of the effective vapour diffusion coefficient, describing water transport through the bauxite residues interconnected pore structure, can be obtained by correlating the evaporation halftimes from stage II evaporation (t evap , indicated by X on the kinetics traces of figure 4a,c; see definition of t evap in Maurais et al [27]) with the sample thickness squared, d 2 [55,59]. This straightforward analysis follows a protocol described previously for the analysis of drying kinetics using a novel thermal imaging methodology [27] and reproduced here in the insets to the normalized Δ albedo (figure 4b) and NIA (figure 4d ) kinetics traces plots. The linear relationship between t evap and d 2 strongly suggests drying rates become increasingly limited by slow vapour diffusion in the later stages of the evaporation process [9,15,58,67,68].…”

“…Previously reported methods to estimate soil moisture content include in situ measurements (gravimetric, dielectric measurements, etc.) [12][13][14][15][16], as well as remote sensing approaches, at radar [17][18][19][20][21][22][23][24][25], infrared [26,27] and optical [18,[28][29][30][31][32][33][34][35] wavelengths that are known to probe water absorption bands. Unfortunately, most in situ methods either lack the sensitivity or selectivity towards the surface layer, or cannot be easily deployed for continuous real-time monitoring and field applications [13,[36][37][38].…”

“…revisit period) to continuously monitor the evolution of surface TMC at the scale of individual plots [44][45][46][47]. Indeed, disposal sites can have areas of up to a few square kilometres and present dramatic changes in their surface drying state over just a few hours time, and a few meters distance, depending on specific local tailings physico-chemical properties, along with instantaneous meteorological conditions as well as their recent history [27].…”