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Basic Planning and Design Considerations. Tailings Facilities - An Introduction from an Insurance Perspective. Typical Mine Site Layout. Procedures in Tailings Production. Crushing Grinding Leaching Concentration Heating Dewatering Tailings Slurry Disposal.
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Basic Planning and Design Considerations Tailings Facilities - An Introduction from an InsurancePerspective
Typical Mine Site Layout CIM ‘99
Procedures in Tailings Production Crushing Grinding Leaching Concentration Heating Dewatering Tailings Slurry Disposal CIM ‘99
Tailings Grain Size Curves CIM ‘99
Tailings Impoundments - Operating Components • Tailings Delivery - pumps, pipelines, and bridges • Tailings Impoundment - retention dams, tailings, tailings pond, diversion ditches, and seepage control • Pond Water Return - pump barge, decant conduits, CIM ‘99
Tailings Impoundment Life Cycle Planning Construction Operation Closure CIM ‘99
Key Elements in Tailings Impoundment Design • Siting Studies • Retention Dam Design • Water Management • Tailings Management • Contamination - Cyanide and Acid Rock Drainage CIM ‘99
Location and elevation relative to the mill Topography Hydrology and catchment area Geology Groundwater Length of tailings and return-water pipelines; Capital and operating costs for the pumps Embankment layout; Embankment fill requirements Diversion feasibility Long-term water accumulation; Flood-handling requirements Availability of natural borrow type and volume; Seepage losses; Foundation stability Rate and direction of seepage movement; Contamination potential Factors Influencing Impoundment Siting CIM ‘99
Catchment Diversion Planning Runoff diverted by dyke Runoff diverted by conduit beneath tailings(rare); usually have diversion ditches CIM ‘99
A Cross-Valley Downstream Dam • 200 ft. high downstream dam in a semi-tropical area • Decant conduit pond discharge through right abutment • Dam raised by added slices of fill on downstream slope CIM ‘99
A Cross-Valley Tailings Dam • Tailings is discharged to storage area upstream of the dam. • Clearwater pond is retained by the dam • Dam and decant are raised as the tailings level rises CIM ‘99
Basic Tailings Dam Section Types A1 UPSTREAM A2, A2=2A1 CENTRELINE A3, A3=3A1 CIM ‘99 DOWNSTREAM
Upstream Tailings Dams CIM ‘99
Poderosa Tailings Dam Peru, 1998 CIM ‘99
INCO R4 - Modified Upstream CIM ‘99
Centreline Construction CIM ‘99
Brenda Centreline Tailings Dam CIM ‘99
Downstream Tailings Dam CIM ‘99
Downstream Tailings Dam CIM ‘99
Downstream Construction CIM ‘99
Impoundment must be stable under all static and transient loads; must identify failure modes Design criteria from CDA; FOS of 1.5 for long term; FOS of 1.3 for short term Impoundment must be stable for seismic loading and must pass design flood Transient load design magnitudes are selected from consequence-based criteria such as CDA High consequences mean design to MCE and PMF Surface and groundwater quality must meet regional and global water quality standards Tailings Dam Design Criteria CIM ‘99
CLASSIFICATION BASIS CIM ‘99
Typical Dam Zonation - Earthfill CIM ‘99
Internal Seepage Control CIM ‘99
Water Balance • Water accumulates in tailings impoundment from tailings transport and runoff • Return water to mill for processing circuit • Design impoundment water handling so that impoundment is not overtopped • Want a closed system with no loss to environment without treatment CIM ‘99
SchematicWater Balance > _ Inflows Outflows - Precipitation - Surface Water Runoff - Tailings Process Water - Evaporation - Reclaim Water To Mill SPILLWAY DISCHARGE Seepage Losses Pond Storage Storage In Tailings Voids CIM ‘99
Tailings Management • The rate of rise of a tailings impoundment depends on mill production and design of tailings distribution systems within the pond • Must be able to predict storage requirements with time so that retention dam construction can stay ahead of rising tailings surface CIM ‘99
Tailings Volume Balance CIM ‘99
Tailings Deposition Planning Construct digital terrain model Run terrain filling model by spigoting from different locations CIM ‘99
Tailings Discharge Techniques Spigotting Open Discharge CIM ‘99
Single Discharge Point Tailings discharged into middle of pond forming beach at slopes less than 1 % CIM ‘99
Water Quality • Chemical factors affecting water quality are pH, salinity, and toxicity • Mill effluents that are products of the leaching process most likely to cause contamination. Acid leaching causes low pH increasing heavy metal solubility • Tailings and waste rock with pyrite may result in low pH due to production of sulphuric acid by oxidation. This is ARD (Acid Rock Drainage). Low pH increases heavy metal solubility which are toxic at low concentrations. • Cyanide used in gold extraction is highly toxic but is unstable and rapidly degrades with time CIM ‘99
Closure Design • All tailings impoundments need to be designed from the outset so that they can be closed efficiently “ Designing for Closure” • Closure design is driven by water management considerations - passing floods and water quality • Low pH water containing heavy metals persist in perpetuity ; must treat with lime before release or prevent ARD by dry and/or wet covers • Cyanide is not a closure issue because of its instability CIM ‘99
Tailings Design Optimization • Unlike water retention dams, tailings dams are constructed and operated simultaneously • Can optimize performance and reduce cost by monitoring • Monitoring programs key to improvements but usually neglected • Technology exists to construct safe tailings impoundments (same failure rate as water retaining dams) CIM ‘99
Trends in Tailings Impoundment Design • Tailings dams are becoming some of the highest in the world; Los Leones in Chile is over 200 m high; Antamina is scheduled to be 230 m high • Closure and abandonment of mine sites in North America is expensive • The ARD problem is not yet satisfactorily solved so that mine sites can be abandoned with confidence • Mining companies are acutely aware of tailings and environmental problems CIM ‘99
