Cracking, salinity and evaporation in mesoscale experiments on three types of tailings

Abstract

Evaporation is a phenomenon useful in assisting in the dewatering and stabilisation of various mineral wastes. This paper summarises findings on the influence of cracking and salinity on evaporation in mesoscale (1·0 m by 0·7 m in plan) deposition experiments on three different mineral slurries: thickened gold tailings, thickened oil sands tailings and oil sands tailings modified by in-line polymer flocculation. Each tailings exhibited substantially different evaporation related phenomena. In the two finer-grained oil sands tailings, crack development correlated with apparent actual evaporation rates larger than the potential rate, which ceased once crack volume stopped increasing. Total suction at the surface was dominated by osmotic suction in the thickened oil sands tailings, whereas total suction was largely matric in the other two tailings. In the gold tailings, no strong signal from cracks on evaporation could be detected. The gold tailings exhibited 'declining stage I' evaporation, which has been recently described from idealised drying experiments on sands. The relatively unique behaviour of each tailings type with respect to evaporation highlights the importance of considering larger scale effects when assessing tailing dewatering by evaporation. NotationA E actual evaporation (mm/d) A EV air entry value (kPa) a soil-dependent parameter in the soil surface resistance equation C a specific heat of air (MJ/(kg°C)) D xx percent (%) of particles less than XX microns in size G flux of heat into the soil away from the soil surface (MJ/(m 2 d))L L liquid limit (%) P E potential evaporation (mm/d) P L plastic limit (%) R universal gas content R E relative evaporation R H relative humidity R Ha relative humidity of the air above the evaporating surface R n net short and long wave radiation (MJ/(m 2 d)) r a density of air (kg/m 3 )r s , r a soil surface and aerodynamic resistance factors S L shrinkage limit (%) T temperature (K) v s , v a vapour pressures at the soil surface and in the air (kPa) W molecular weight of water (kg/mol) w geotechnical gravimetric water content (%) g psychometric constant (kPa/°C) D slope of vapour pressure-temperature relationship (kPa/°C) q ref reference volumetric water content at an arbitrary depth used to calculate soil surface resistance factor q top actual water content at the same depth as the reference volumetric water content L latent heat of evaporation (MJ/kg) y total suction (kPa) IntroductionDrying tailings to increase density and strength is often used to assist stabilisation and reclamation of tailings impoundments. The aluminium industry relies heavily on evaporation to stabilise 'red muds' (Doucet and Paradis, 2010), while thickened tailings deposition in a variety of climates takes advantage of evaporation to reduce tailings volumes (Simms, 2016). Certain tailings management methods in the Canadian oil sands industry, such as Suncor's tailings reduction operations (Caldwell et al., 2014) and Shell's atmospheric fine drying technologies , use evaporation to reduce tai...

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