Closure Planning for a Tailings Storage Facility in Western Australia

Transcription

1 Closure Planning for a Tailings Storage Facility in Western Australia K. Bonstrom, D. Chapman, D. Swain, and M. O Kane 5 th International Conference on Mine Closure Session 11: Tailings Deposits Closure 26 November 2010 Vina Del Mar, Chile

2 Project Overview Strategy for closure of a TSF with objectives: To provide geotechnical and geomorphic stability, To blend visually to the surrounding natural landscape, and To provide water management (surface water and groundwater quality).

3 Scope of Project Established base metal mine in Kimberley Region of WA. Requires additional capacity to allow operations to continue beyond the current life of mine Plan: Increase current TSF capacity Tailings to remain in situ following cessation of mining

4 Site Overview Valley fill Tailings Storage Facility (TSF) Main tailings embankment Water storage facility Waste rock pile TSF Surface Area ~20 ha

5 Climate Conditions Strong seasonality in rainfall 82% of average annual in December to March period Potential Evaporation (PE) ~ 2,000mm 100-year ave. annual rainfall = 636 mm

6 Key Issues Key issues identified through consultation and risk assessment process Key Issues: Geochemistry of tailings and cover materials Surface water and groundwater quality Mine closure and rehabilitation Monitoring, contingencies, and relinquishment

7 Geochemistry of Materials Tailings Predominantly pyrrhotite Classified as PAF Forms a trafficable hardpan surface O 2 consuming? Remain relatively fresh below hardpan Reducing/anoxic environment Do not appear to generate acid conditions after an extended storage period: ABA data Water quality seepage data Visual appearance L-T seepage anticipated to remain neutral with elevated SO 4 and EC

8 Geochemistry of Materials Potential Cover Materials Waste rock Non-Acid Forming (NAF) Well-graded (some gap grading) Low erodibility potential Topsoil and subsoil Finer textured Comparatively higher erodibility potential

9 Percent Passing 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% > 40% Passing #4 sieve (4.75 mm) Waste Rock PSD Relatively Well Graded Material with Some Gap Grading 0% Particle Size (mm)

10 Erodibility Parameters WR / Subsoil Subsoil Topsoil

11 Cover System Design Site-specific climate assuming bare surface conditions (i.e. no vegetation) Store and Release: Two Layers (~2.3m) Waste rock ~ 2m Varies to achieve surface runoff management objectives and address consolidation issues Topsoil/waste rock mixture of ~ 0.3m at surface Thick Growth Medium Does not include benefits (if any) of hardpan tailings

12 TSF Seepage Assessment Overall: Hydrological behaviour of the TSF Specific Questions Asked: Operations? 1D Operations Draindown rate? 1D Seepage Settlement? 1D Consolidation Areal Seepage? 2D Seepage Phreatic Surface? 2D Seepage Cell 1 2D 10% Infiltration

13 D/S Surface and GW Quality Seepage Water Quality SO 4 (indicator of salinity) values in GW and surface water not expected to significantly increase beyond current operational levels Analytes are diluted by recharge and mixing with groundwater in storage. Aquatic fauna studies: minimal differences in species richness between near mine and reference sites. reduction in sensitive macro-invertebrate species however these species are expected to re-colonise following mine closure.

14 Current Landform Angle of Repose WRD Slope Single Deposition Area Main Embankment

15 Final Landform Tri-Linear Concave Slope Store and Release Cover System Design with Management of Surface Runoff for Large Rainfall Events Multiple Catchment Areas Spillways Armored Drainage Channels

16 Final Landform Design Landform

17 Final Landform 200 years

18 Rehabilitation and Closure Main embankment Designed to Category 1, High hazard rating Risk of embankment failure low Tailings consolidation Well understood Current tailings > 90% consolidated Anticipated future consolidation < 0.5m Landform (at closure) Stable greater than 200 years Assumes no vegetation to assist with erosion protection Cover system (at closure) Moisture store-and-release (no vegetation) Management of surface runoff for large rainfall events (including run-on from natural areas) Required net percolation rate linked to longterm GW and solute transport prediction

19 Staged Approach to Closure of TSF Completion of a single area prior to complete TSF filling Development of a watershed scale field trial to evaluate cover system design Evaluation of tailings consolidation and surface water management Provides opportunities for adaptive management and development of a final design prior to completion tailings deposition For subsequent cells at cessation of tailings deposition: Allows for staged material placement to strategically enhance consolidation to assist with implementing final landform for surface water management

20 Field Trials and Monitoring? Or. Why a Watershed? A Watershed is the Major Building Block of Landscapes Majority of Questions asked about Landscape Performance can be Addressed at the Watershed Scale Monitoring at a Watershed Scale Demands Thought about Interactions It is Manageable

21 Field Trials and Monitoring

22 Key Points Appropriate strategy for closure of TSF identified Conservative modelling and design parameters Cover system design (Cell 1) watershed scale field monitoring trial Ongoing monitoring (geochemistry, geotechnical, aquatic, GW, surface water, etc.) Develop (with stakeholders) measurable relinquishment objectives Contingency during closure leading up to relinquishment Cover construction QA/QC Monitor cover performance, stability, maintenance (as required) Enact remedial measures as required for cover

23 Thank You