Weathering and diagenesis of Saskatchewan potash tailings
Landine, Patrick Gordon
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The solid wastes or tailings produced by the Saskatchewan potash industry are the topic of this dissertation. The waste is composed primarily of sodium chloride. Brine is generated due to atmospheric precipitation on the tailings piles. The release of brine to the local environment is the prime environmental problem facing the potash mining industry. The main objectives of this research project were to: (1) Observe and document the weathering processes; (2) Observe and measure the changes in physical and chemical properties after deposition; and (3) Identify and describe the processes responsible for changes in the physical and chemical properties. Field reconnaissance of seven tailings piles revealed a geomorphology related to karst topography. Unique features were observed and described, probable mechanisms were suggested for their development. Field cores were recovered at two tailings piles in the Saskatoon region. Laboratory procedures performed on field cores included: water content, insoluble content, dry density, hydraulic conductivity, chemical composition by wet chemistry, petrographic observation, electron microprobe analysis and mercury intrusion porosimetry. Profiles of dry density in recently deposited tailings showed a relatively constant value of about 7.45 Mg/m3, while dry density in older abandoned tailings areas increased significantly with depth, with values as high as 2.07 Mg/m3. Hydraulic conductivity was influenced by the dry density and fabric of the specimens. Low hydraulic conductivity was observed in uniformly dense specimens and specimens with thin laminations of insoluble minerals or high density. Chemical composition profiles showed that potassium chloride (KCl) is removed from the tailings shortly after deposition and that calcium sulphate (CaSO4) is leached from the weathered zone and enriched within fine textured laminations below the weathered zone. Petrographic and microprobe analysis showed: detrital fine grained gypsum in recently deposited tailings, an absence of detrital gypsum in weathered tailings and the presence of authigenic gypsum in fine textured laminations below the weathered zone. The solubility curve for CaSO4 in a sodium chloride brine was used to develop a model explaining the crystallization of gypsum within the tailings profile. The large increase of density after deposition was ascribed to secondary creep of the tailings mass.