A Thermal Imaging Methodology To Study Evaporation Kinetics in Mine Tailings

Abstract

<p>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. As a case study, thermal imaging is demonstrated here to be a valuable tool to study the evaporation mechanisms and rates from bauxite residues as a function of their thickness and physicochemical properties, as well as environmental conditions. These investigations reveal their that late stage drying rates are limited by gas phase diffusion through the interstitial air within their internal microporosity. The smallness of the effective diffusion coefficient indicates that water adsorption on bauxite residues surfaces is the dominant phenomenon responsible for their slow water vapour transport kinetics, a phenomenon that ultimately controls their late stage drying rates, that is when dust scattering is most likely to occur. As such, application of this thermal imaging methodology in the field may also contribute to improve the accuracy of risk assessment protocols, support intervention and mitigation strategies, underpin optimization efforts for mining residues management, and improve forecasting of fugitive dust emissions from mine tailings by enabling more accurate predictions of the evolution in their surface drying state.</p>

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 fractions reported in this range might be significantly underestimated, thereby explaining the lower Δ albedo reported for prepared samples compared to those observed for evaporating samples having solid fractions greater than 70%.…”

“…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 half-times from stage II evaporation ( t evap , indicated by X on the kinetics traces of figure 4 a , 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 4 b ) and NIA ( figure 4 d ) kinetics traces plots.…”

“…Previously reported methods to estimate soil moisture content include in situ measurements (gravimetric, dielectric measurements, etc.) [ 12 – 16 ], as well as remote sensing approaches, at radar [ 17 – 25 ], infrared [ 26 , 27 ] and optical [ 18 , 28 – 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 – 38 ].…”

“…revisit period) to continuously monitor the evolution of surface TMC at the scale of individual plots [ 44 – 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 ].…”