BAP or BAT? A review of some dramatic changes in tailings management in the recent past and potential changes in the future

Transcription

1 BAP or BAT? A review of some dramatic changes in tailings management in the recent past and potential changes in the future Andy Fourie School of Civil, Environmental & Mining Engineering 1

2 BAP or BAT? A review of some dramatic changes in tailings management in the recent past and potential changes in the future Andy Fourie School of Civil, Environmental & Mining Engineering 2

3 Outline Contrast conventional tailings with thicker tailings Provide examples; what worked, what perhaps did not work so well The use of filtered tailings Concluding comments 3

4 Unacceptable rates of accidents with TSFs In 2014 alone: Mount Polley Itabirito, Minais Gerais, Brazil two fatalities Cananea, Sonora, Mexico Dan River Steam Station 4

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6 Effects of Mount Polley failure Scrutiny of mining industry in B.C. and in Canada generally Increased 3 rd party review advocated Scrutiny of geotechnical design??????? 6

7 Conventional Tailings Disposal Systems Conventional Tailings Engineering Properties Low density thickened slurry Newtonian flow behaviour (hydraulic transport) Segregating (hydraulic sorting of particles) Relative particle size d50/d50 (max) Distance from point of deposition (m) 7

8 The thickening continuum Methods of disposal 8

9 The thickened tailings continuum Yield Strength (Pa) Slurry Gel Point Thickened Tailings Paste Cake Solids Concentration (%) 9

10 The original concept: Robinsky, 1975 AF Introduction to P&TT 10

11 Kidd Creek

12 Paste and TT Disposal Systems Paste and Thickened Tailings Engineering Properties High density and high yield stress slurry Non-Newtonian flow behaviour Non-Segregating (lava flow) Non-segregating Low water loss Low Seepage 12

13 Paste and TT Impoundment Types: 1. Central Thickened Discharge (CTD) De Beers Kimberley CTP, South Africa 13

14 Paste and TT Impoundment Types: Central Thickened Discharge (CTD) De Beers Kimberley CTP, South Africa 14

15 Central Thickened Discharge (CTD) Very small embankment, if any Simple operations If single point: cannot guarantee thin layers Large area results (closure issues?) Stormwater management

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17 CTD: option 2 Multi-point risers As previous, except thin layers are possible. Mt Keith

18 Mt Keith

19 Paste and TT Impoundment Types: Down Valley Discharge (DVD) Miduk, Iran 19

20 Down valley discharge (DVD) Construct engineered embankment across valley Deposit from head of valley May not need to move deposition point Layer thickness control difficult Examples: Iran, Century

21 Paste and TT Impoundment Types: In-pit using existing pit 21 Ekapa Mine, Kimberley, South Africa

22 In-pit No embankment required Water management simple No moving of deposition point Layer thickness not critical

23 Paste and TT Impoundment Types: Ring-dyke facility Hillendale, South Africa

24 Hillendale TSF 24

25 Spigots, Ring-dyke Characterised by retaining embankment Rotation of deposition point Requires movement of deposition point Allows deposition in thin layers

26 Field performance Site Ore type Tonnage (Mtpa) Solids (%) Beach slope (%) Kidd Creek Cu, Zn (2.5) Peak Au (2.5+) Mount Keith Ni Sunrise Au CTP Diamond (1.5) Osborne Cu, Au (1.5) Alcoa Pinjara Bauxite (1.2) See Williams et al (2008) 26

27 The new technologies have proven to be more than just wishful thinking However, 27

28 Perceived benefits 1. Similar capital and reduced operating cost to wet disposal 2. Costs accrued during operations 3. Increased deposit strength 4. Decreased land footprint by at least doubling practical stacking height 5. Decreased demand for borrow materials for construction 6. Reduced risk of leachate seepage 7. Reduces or eliminates ponding and low strength mud deposits 8. Prompt creation of firm, convex draining surface at completion 9. Earlier, better surface leaching and drainage 10. Potentially large reductions in water use 11. Reduced potential for liquefaction 12. Potentially reduced heating, lower water demand 13. Reduced reagent requirements

29 Water Savings Water Loss (m^3/t headfeed) Yield Stress (Pa) Underflow Solids Conc. (%vol) Mt Polley: rely on BAP for the remaining active life 29

30 Perceived benefits 1. Similar capital and reduced operating cost to wet disposal 2. Costs accrued during operations 3. Increased deposit strength 4. Decreased land footprint by at least doubling practical stacking height 5. Decreased demand for borrow materials for construction 6. Reduced risk of leachate seepage 7. Reduces or eliminates ponding and low strength mud deposits 8. Prompt creation of firm, convex draining surface at completion 9. Earlier, better surface leaching and drainage 10. Potentially large reductions in water use 11. Reduced potential for liquefaction 12. Potentially reduced heating, lower water demand 13. Reduced reagent requirements

31 So not all perceived benefits have been realised, but perhaps overoptimism is sometimes the only way to move technology forward 31

32 Alternative and emerging technologies In-line flocculation High-volume filtration

33 In-line flocculation The process Limited flocculation using conventional thickener may be included turbulent transportation possible High concentration of flocculant added close to end of deposition pipe High intensity mixing produces heavily flocculated product Very rapid release of water

34 High volume filtration Logical extension of thickened tailings

35 Examples Cerro Lindo: around tonnes per day La Coipa: to tonnes per day Mantos Blancos: tonnes per day Rosemount (proposed): tonnes per day

36 Mantos Blancos La Coipa El Penon Cerro Lindo Country Chile Chile Chile Peru Production (ktpd) Ore type Au Au Pb-Cu-Zn Pb-Zn Transportation Conveyors Conveyors Truck Truck Filter type Belt Belt Belt Belt Solids content (%) Percent fines (%) Specific gravity Compacted? No No No Yes P (mm/yr) <50 <50 < E p (mm/yr) >2 000 >2 000 > After Lara and Leon (2011) 36

37 Deposition strategy May be governed by compaction characteristics Could become a very large, conventional earth moving operation 37

38 Perceived advantages of filtered, stacked TSF Regulatory expedience Cheaper than surface paste? Applicable to range of climates Closure costs significantly reduced 38

39 Experience to date suggests (for filtered tailings): Well suited to rugged terrain (e.g. where slopes are >20%) Appropriate for highly seismic regions (>0.25g) Difference between produced moisture content and compaction requirements is crucial Rainfall during transport and placement can be major drawback Moisture content should be < Liquid Limit 39

40 Example of satisfactory performance: Cerro Lindo facility: after Pisco earthquake in 2007 (magnitude 7.8), there was no evidence of cyclic mobility and no permanent deformation. 40

41 Methods of disposal 41

42 What constitutes BAT? There is some indicati0n that filtered tailings are considered to be the new sure-fix Filtered and dry stacked tailings Is there an upper limit on tonnage that can be managed? Can operations continue during inclement weather? Is there a limitation on the particle size distribution that can be managed? Can appropriate compaction QA/QC be ensured? Can the total tailings stream be managed using filtered tailings? Dust generation 42

43 Concluding comments The danger of oversimplification In the aftermath of the Mount Polley failure there is understandable public concern and anger Our challenge is to respond in a responsible way, without succumbing to apparent panaceas Clearly improved standards of design, operation and stewardship is an industry imperative 43