Desiccation in dewatering and strength development of high density hard rock tailings

1. Desiccation in dewatering and strength development of high density hard rock tailings P. Simms, S. Sivathayalan, F. Daliri Carleton University

2. Stress history of high density tailings stack layers : Effect of degree of desiccation Settling o r s/w consolidation

3. Stress history of thickened tailings stack layers: Geotechnical Stability? Geotehcnical Stability? Cost? Acid Rock Drainage? Settling Settling 4 ICOLD and UNEP (2001) Drying to Shrinkage Limit? 1 2 3

4. Conceptual model of factors influencing “drying time” Effect of salts? Cracks? Thickness of fresh-layer Rheology Site topography Consolidation

5. Within a range, initial solids concentration does not affect initial degree of settling (column test data)

6. How we predict the rate of drying? • Hypothesis: We can use standard unsaturated flow codes, that are used in soil cover design, to predict rate of drying • Models such as SoilCover, SVFlux, Hydrus… • Complicating factors with tailings: salts, cracking, significant volume change

7. Matric suctiondata from a two-layer test Placement of Second layer Initial period of hindered settling

8. So we have found that this approach has a certain skill,what happens when we extrapolate to deep layers of tailings? • Hypothetical analyses performed to analyze what happens when a fresh layer is deposited over several (1m to 10 m) of previously desiccated tailings • We used a range of parameters (Water retention curves, saturated hydraulic conductivity) typical to many hard rock tailings (air-entry value from 50-500 kPa, saturated hydraulic conductivity between 10-6 and 10-8 m/s) • Potential evaporation rate varied between 2 and 10 mm/day. • Drainage and no-flow bottom boundary conditions applied

9. Influence of fresh layer thickness on drying rate PE = 5 mm /day These and similar predictions published in 2010 Paste and thickened tailings conference

10. Multilayer Deposition in a Drying Box Senix Distance Sensors Tensiometers Volumetric Water Content Sensors Scale

11. Desiccation Process in the Drying Box Day 5 Day 2 Day 6 Day 1 Day 3 Day 4 Evolution of Cracks

12. Drying Time

13. Modelling of drying box

14. Modelling of drying box

15. Modelling of drying box

16. Generic predictions compared to field and drying box results Predictions from Paste 2010 paper Drying box – only 1 m of tailings

17. Monotonic Results of Desiccated/Rewetted Samples

18. Vane Shear Results

19. The influence of stress history on shear strength Settling Desiccation History 4 1 2 3

20. If only desiccation to 25 % instead of 20% (SL) required, significant reduction in needed drying time

22. Important Outcomes: • Time to reach end of SW consolidation, for lifts up to 1 m and as low as 65% solids concentration, seems to be within 48 hours. Only small influence of initial solids concentration on final void ratio post-consolidation and post-drying • Samples experiencing settling without desiccation exhibited strain softening response. A relatively small degree of desiccation changes the response to strain hardening. • Though desiccation to SL and beyond increases the shear strength, it requires substantially longer drying time. • The degree of desiccation should be determined based on required site specific shear strength.

23. Acknowledgements • Former and current graduate students who have contributed to this work: BereketFisseha, Julio Henriquez, Rachel Bryan, AdedejiDunmola, ShabnamMizani, Adrian Manlagnit, ParvizHeidarian, Mary-Ellen Gleeson, and FarzadDaliri • Collaborator Siva Sivathayalan (co-supervisor of FarzadDaliri) • Thank you GolderAssociates, Musselwhite Mine, and Barrick Gold

24. Actual evaporation in drying box

25. Preliminary Results 8 Days 11 Days 5 Days