Placement of Thickened Tailings: Adoption of a Rheology-Oriented Model for Slope Predictions

Abstract: Thickened tailings or “paste” disposal with deposition slopes varying from 2 to 4% results in steeper slopes compared to those of conventional placement (less than 0.5%). As a consequence, thickened tailings disposal ends up with filling more volume per storage area and thus avoids costly & frequent dam raises. Reported observations describe that the discharged slurry typically flows down the slope or “beach” in a confined, self-formed channel, in a macroscopic equilibrium between erosion and sedimentation, defining an overall slope and then spreads out & deposits on a broader area (Simms, et al., 2011).
The objective of this study is to describe the slope forming elements and adopt the model for beach slope predictions by Fitton (2007) together with a discussion of simulated results and the effects of rheological & hydraulic parameters.
Typical thickened tailings have average particle sizes of 25 to 75 μm with maximum particles of about 0.5 mm. To obtain a conceptually even slope with practically no segregation of particles occurring and no drainage of water taking place, the concentration by volume often needs to exceed 40% (even 45% for some cases). It is reported that these slurries show non-Newtonian behaviour and often found in a supercritical open channel flow state located in the transitional or turbulent zone. When the slurry spreads out, the flow attains a sheet-like, laminar state where particles settle out. The process repeat itself in a cyclic manner and the overall beach profiles are created as a result.
Fitton’s model, based on open channel hydraulics (Darcy-Weisbach friction loss concept) and sediment transport approach, considers the self-confined open channel flow to be turbulent, based on field and large scale flume observations. The model relates the deposition velocity to maintain the channelized flow, to a Bingham rheological model parameter.
The model is utilized to simulate the depositional behavior of thickened tailings slurries having a volumetric solids concentration of 46% (71% mass). This value is used to represent homogeneous (non-segregating) conditions for tailings with an average particle size of 50 μm and a solids density of 2900 kg/m3. Simulation results for slurry flow rates
from 25 to 400 m3/h correspond to slopes of about 6 and 2%, respectively, expressing a flow rate dependence at yield stresses and Bingham viscosities of up to 30 Pa and 0.1 Pa.s, respectively. Reynolds numbers were from about 500 to 94500 for which the effect of channel roughness on predicted slopes was practically negligible within values from 0.3 mm to smooth conditions at 0.05 mm.
Calculated slopes were nearly insensitive to the channel shape as long as the width/depth ratio remained constant about 5.5. A rectangular cross-section was used for the calculations but a parabolic channel shape reflects field observations better.
Equilibrium yield stress requirement for the flow to come to rest and the required shear strength (cohesion) to be developed upon drying for stability of slopes formed by stacked layers of various thicknesses are demonstrated in schematic examples.