The role of relative humidity in mitigating fugitive dust emission from mine tailings
Damilare Ogungbemide, Cheryl McKenna Neuman
Trent University, Peterborough ON, Canada
Mining companies routinely invest heavily in dust control strategies, inclusive of planting vegetation to increase aerodynamic roughness and provide shelter, and applying various commercial products to bind and protect particularly emissive surfaces. However, frequent application of water to increase the cohesion of haul roads, tailings and stockpiles remains the favoured treatment in most locations, but can be problematic where either water resources are limited or there is a high risk of contamination of the aquifer. The relative humidity (RH) of the ambient air governs both the rate of evaporation of water applied to the surface, and the tension (Ψ) at which water is adsorbed onto particle surfaces. However, few measurements of the relation between humidity and the vertical dust flux exist for mine tailings having varied physical properties. Most important, there are no guidelines for mines located in temperate settings that identify humidity ranges for which PM emission is expected to be negligible. The primary focus remains on wind speed, although under varied climate conditions it may well play a secondary role.
This study reports on a series of laboratory experiments, supplemented with data from direct field measurements carried out at a local nepheline syenite (feldspar) mine, in which the influence of relative humidity on PM emission by size fraction was quantified and modelled. The experiments were conducted in the climate-controlled, Trent Environmental Wind Tunnel for test surfaces comprised of sub-samples of the mine tailings. More than 350 runs were conducted at various combinations of temperature (T) and relative humidity (RH). The operating free-stream wind velocities ranged from 6.0 ms-1 to 12.0 ms-1, with a stepwise increment of 0.5 ms-1. The selected temperature range was 0-300C at 100C intervals, while the relative humidity ranged from 0-70% at 10% intervals. Wind velocity and dust concentration were measured using a Laser Doppler Anemometer (LDA) system and TSI DustTrakTM aerosol monitors, respectively. For the field component of the study, a 3-meter high weather station equipped with aerosol monitors was set up over one of the tailings ponds.
Preliminary results show that PM concentration is not a monotonic function of RH. Rather, there exists a complex relationship between the two variables. The vertical dust flux is found to vary as a logarithmic function of the matric potential for water adsorbed onto the PM, calculated from RH and T using the Kelvin equation. Using regression analysis, the lower limit for PM10 emission is estimated to occur around 80% RH (Ψ ~32.96 MPa); beyond this value, dust emission becomes negligible over the range of windspeed considered.