Salinity effects on sedimentation behavior of kaolin, bentonite, and soda ash mine tailings

Abstract The objective of this study was to evaluate the effects of soluble salt concentration (i.e., salinity) and cation valence on sedimentation behavior of two common clays with differing plasticity, kaolin and bentonite, and saline mine tailings from a soda ash mine in Wyoming, USA. Salinity was controlled in the natural clays via adding salts with different valences (NaCl, CaCl 2 , and FeCl 3 ) to concentrations ranging between 1 and 1000 mM. Salinity in the soda ash tailings was altered via dialysis. Overall, an increase in ionic strength of the sedimentation fluid (i.e., increase in salinity) yielded higher sedimentation rates and final solids contents for bentonite. The initial sedimentation rate in bentonite increased an order of magnitude (0.01–0.02 mm/min to 0.12–3.6 mm/min) and final solids content increased by a factor of 2 to 5 (3% to 6–16%) with an increase in salinity from 10 mM to 1000 mM. In contrast, the sedimentation rate of kaolin was approximately constant for salinity between 1 and 100 mM, and then decreased with an increase in salinity from 100 to 1000 mM in all solutions evaluated (i.e., decreased by factor of 4.4 in NaCl, 9.0 in CaCl 2 , and 7.9 in FeCl 3 ). A salt concentration of 100 mM was identified as a critical concentration whereupon the sedimentation rate of kaolin decreases regardless of salt type, and was attributed to an increase in sedimentation fluid density and viscosity. The final solids content of kaolin was approximately constant regardless of salt type and concentration (solids content = 26–37%). Soda ash mine tailings exhibited sedimentation behavior similar to kaolin, which showed approximately constant initial sedimentation rate and final solids content for the range of salt concentrations evaluated.