Numerical simulation of freeze-thaw and large strain consolidation of thickened tailings

Nam H. Pham, David C. Sego, Abdelkader Hammami, Jianmin Kan

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Uranium Tailings Management

ABSTRACT: Freeze-thaw changes the micro-structure of tailings in a positive way. It creates micro channels for fluid flow that enhance the hydraulic conductivity and decrease the compressibility of tailings. As a result, tailings consolidation is accelerated, and higher solids content are reached. A MATLAB based numerical model was developed at the University of Alberta to assess the effect of freeze-thaw on tailings settlement. The MATLAB model simulations indicate that for a 12m thickened tailings deposit filled over 3 years, it is expected to observe an additional settlement of 1m as a result of freeze-thaw. In addition, the model indicates that lower excess pore water pressure (PWP) values should be expected for the frozen and thawed deposit when compared to unfrozen one. The model also shows that the solids content at the bottom 3m of the deposit are at or greater than 77% and the undrained shear strength islarger than 5 kPa (Junqueira et al., 2011). 1 INTRODUCTION Due to the significant amount of tailings produced by the oil sands industry, storage and disposal of tailings presents a challenge for mining companies from both an operational and environmental perspective. Consolidation of mature fine tailings due to its weight is slow, therefore it is a challenge to reach the required strength for reclamation. As a result, it is important to find an economical solution that can enhance the consolidation process. One natural process to consolidate and increase the solid content of tailings is freeze-thaw. It is well known that the freeze-thaw process change the micro structure of fine grain soils and tailings in positive ways, such as increasing solid content, hydraulic conductivity and decreasing compressibility. Therefore, post-thaw tailings will consolidate much faster than tailings that have not experienced freeze-thaw. In areas where air temperature during winter is cold and prolonged, freeze-thaw dewatering may be applicable. Freeze-thaw dewatering has been used to dewater many industrial waste slurries such as municipal sewage treatment sludges. Using freeze-thaw, Stanczynk et al. (1971) reported an increase of 35 % in solid content of phosphatic clay waste which is a byproduct of phosphate fertilizer production. Elliott (1975) used chemical amendment and freeze-thaw to increase the solid content of Mature Fine Tailings (MFT) from 10% to 26%. To examine the influence of freeze-thaw cycles on the post-thaw solid content of MFT both Sego (1992) and Johnson et al. (1993) indicated that the rate of solid content increase is largest during the first cycle. Sego (1992) (Figure 1) and Johnson et al. (1993) achieved a solid content of around 50 % from an initial solid content of 30 % after three freeze-thaw cycles. The initial solid content of tailings is important as it directly reflects on the results of the freeze-thaw mechanism. Higher initial solid content results in lower solid content increase. Dawson et al. (1999) showed that at low freezing rates the post-thaw solid content increases for a given initial solid content. However, the freezing rate in the field may be difficult to control. Figure 1. Freeze-thaw test results for Suncor MFT (data from Sego, 1992) Multilayer freeze-thaw experiments were reported by Proskin et al. (2010). The experiment consisted of 5 layers of 0.2m each with an initial solid content around 33% and the total time to freeze is 69 hours. Immediately after thaw was completed, a thaw strain of 55% was recorded and the solid content increased to 59%. Accounting for post-thaw consolidation and surface desiccation, the final solid content increased to 68%. In this paper, numerical simulations were used to demonstrate the effectiveness of freeze-thaw in increasing solid content of thickened tailings (TT). Equations of heat conduction and large strain consolidation were solved in a coupled manner using Finite Element Method (FEM). Numerical results and all

RÉSUMÉ: ical simulation of freeze-thaw and large

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Cite this article:
Nam H. Pham; David C. Sego; Abdelkader Hammami; Jianmin Kan (2014) Numerical simulation of freeze-thaw and large strain consolidation of thickened tailings in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper347,author = Nam H. Pham; David C. Sego; Abdelkader Hammami; Jianmin Kan,title = Numerical simulation of freeze-thaw and large strain consolidation of thickened tailings,year = 2014}