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Presentation Copyright : TAKRAF 2016
AIChE Strategies for Sustainable Water Management for Mining Denver, Colorado October, 2016
Michael Blois Pr. Eng., QP, FIMMM
Presentation Copyright : TAKRAF 2016
Dry Stack Tailings
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Safety Moment
Background to TAKRAF
Why consider Dry Stack Tailings – Risk Management
What is – “The Complete System”
Description of Tailings Types
Overview of Advantages & Disadvantages of Dry Stack Tailings
Design Criteria for Stacking
Stacking Systems
Filtration
Thickening
Presentation Copyright : TAKRAF 2016
Is 99.9% accident free good enough?
What life would be like if 99.9% safe was good enough: • 20,000 incorrect drug prescriptions would be issued each year
• 50 babies dropped by doctors every day
• 500 incorrect surgical operations each week
• We would drink unsafe water for one hour each month
• 22,000 checks would be deducted from the wrong bank account every hour
• Your heart would miss 32,000 beats each year
99.9% isn’t very safe after all.
Safety is at the core of our culture and our way of living and doing things, at work and at home
We are committed to ensuring that all our offices and sites worldwide are safe, clean and healthy environments at all times for all stakeholders
SAFETY FIRST is the safety promise of the Tenova Group
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Presentation Copyright : TAKRAF 2016
Some tailings dam failures since 2009 • Duke Energy, North Carolina 2014
– Cause: failure of pipe below surface – Results: US EPA issues new
regulations mandating placement of flyash as a landfill material, i.e. dry stacking in lined and covered facility
– Future: No tailings storage facilities allowed for power generation facilities
• Mt. Polley, British Columbia 2014 – Cause: failure due to unusual clays in
foundation and pond operating practices
– Conclusion: embankments not designed for water containment; too much water in tailings pond; not allowed to discharge excess water
Why Consider Dry Stack Tailings – Risk Management
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Presentation Copyright : TAKRAF 2016
Some tailings dam failures since 2009 • Samarco, Brazil 2015 • Failure of dams #3, #4 and #5
– Failure of #3 and #4 caused failure of #5
• Cause: Changes in operating practice and approach from original design
• Significant loss of lives, environmental damage and financial loss to Vale & BHP
Why Consider Dry Stack Tailings – Risk Management
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Presentation Copyright : TAKRAF 2016
Why Consider Dry Stack Tailings – Risk Management
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Some tailings dam failures since 2009 (cont.)
• Luoyang Alumina, Henan
Province, China • August 16th , 2016 • 2 million cubic meters of
red mud • 300 villagers evacuated
Presentation Copyright : TAKRAF 2016
Tailings dam failures • Number of tailings dam failures per decade • 193 total failures from 1960 to 2010 • 39 failures from 1990 to 2009
Why Consider Dry Stack Tailings – Risk Management
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Figures courtesy of: Tailings Dam Failures: A Review of the Last One Hundred Years (2010) by Shahid Azam & Qiren Li
• 20 failures across all regions from 2000 to 2009
• Failures since 2009 are not included.
We should be concerned – Average of >1 dam failure per year
Tailings impoundments are technically complex structures
Presentation Copyright : TAKRAF 2016
The DST Concept Is Not New…!!!
The Disposal of Residues at Kalgoorlie - Harry Adams in 1909:
“Originally the conditions in this district varied mainly in the position of the mill…….in most cases the residue was in the form of filter-press cakes, suitable for conveying on belts, or in trucks, and stacking in a high dump.”
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(After Murphy & Caldwell TechnoMine 2012)
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TAKRAF – Complete Dry Stack Tailings System
Guelb II, Mauritania
An alternative to the storing of tailings in impoundments
Dewatering tailings through filtration removes approximately 85% of the water
Produces a ‘cake-like’ material which can be transported by conveyor or truck to a storage site
A low-risk and environmentally-socially responsible alternative to managing waste
Presentation Copyright : TAKRAF 2016
Tailings Range From Wet to “Dry”
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Underflow from Conventional or High Rate Thickener • Pumpable with centrifugal pumps • Yield Stress 15-40 Pa
Underflow from High Density Thickener • Pumpable with centrifugal pumps (shear thinning may be
required) • Yield Stress <180 Pa Underflow from Paste Thickener • Not pumpable with centrifugal pumps • Positive displacement (PD) pumps required • Yield Stress 100 - 300 Pa
Product from Pressure Filter • Not pumpable • Use conveyor systems to transport • Solids content – 75 to 85% solids • Moisture content – 15 to 25% moisture
Presentation Copyright : TAKRAF 2016
Water recovery and reuse required
Filtered tailings would have enough residual moisture to facilitate optimal compaction of filter cake
Reduced land area requirement for dry stack tailings facility as compared to conventional tailings storage facilities.
Seepage rates significantly lower than impoundments
Concurrent reclamation of the stack reduces shutdown time
Dry stack tailings facility can be viewed as easier to permit
Significantly cheaper to “bond” with the authorities.
Potential Advantages of Dry Stacked Tailings
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Presentation Copyright : TAKRAF 2016
Water saving alone is seldom a justification for “dry stacking”.
If water loses can jeopardize plant performance, or makes the water resource unsustainable.
Technical Background - “We want to save water” – a common misconception
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Presentation Copyright : TAKRAF 2016
Considerations for Tailings Storage Facility -- Seepage or Catastrophic Failure
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• Ground water seepage
• River water contamination
• Ocean contamination
Presentation Copyright : TAKRAF 2016
Should not be regarded as a universal panacea.
Filtration performance, similar to thickening performance, is subject to
changes in ore mineralogy and processing characteristics, which can have a significant impact on the filtration rate and/or the moisture level achievable.
May be necessary to incorporate additional equipment for variance in
mineralogy thereby increasing capital and operating cost.
Potential Disadvantages of Dry Stacked Tailings
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Dry Stack Tailings: The Complete System
Guelb II, Mauritania
An alternative to the storing of tailings in impoundments.
Dewatering tailings through filtration removes
approximately 85% of the water
Produces a ‘cake-like’ material which can be transported
by conveyor or truck to a storage site.
A low-risk and environmentally-socially responsible
alternative to managing waste.
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The Complete System: A Process Perspective !
An alternative to the storing of tailings in impoundments.
Dewatering tailings through filtration removes
approximately 85% of the water
Produces a ‘cake-like’ material which can be transported
by conveyor or truck to a storage site.
A low-risk and environmentally-socially responsible
alternative to managing waste.
Presentation Copyright : TAKRAF 2016
15 000 to 18 000 metric ton/day Gold Leach where Au solution recovery is with Vacuum Belt Filters Reason for dry stack – gold and cyanide recovery and topography Product – filter cake at 19 – 22% moisture In situ moisture is 13%, generated by compaction & evaporation Operating cost – not specifically known; paid for by cyanide reagent recovered Plant water usage – 0.27 m3/ton ROM
Dry stack example - La Coipa
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Presentation Copyright : TAKRAF 2016
Design Criteria for a Stack
Location
• Topography
• Geohydrology, geochemistry & ARD potential
• Climate
Rheology of Feed Material
• Mineralogy of the particles
• Solids density
• Solids concentration
• Particle size distribution
• Particle morphology
Degree of compaction
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Conveyability ??
Bearing Pressure
Presentation Copyright : TAKRAF 2016
Design Criteria for a Stack - Which are Controllable?
Location
• Topography
• Geohydrology, geochemistry & ARD potential
• Climate
Rheology of Feed Material
• Mineralogy of the particles
• Solids density
• Solids concentration
• Particle size distribution
• Particle morphology
Degree of compaction
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Conveyability ??
Bearing Pressure
Presentation Copyright : TAKRAF 2016
Trucking
Transportation and placement of dry stack tailings
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Transportation and placement of dry stack tailings
Conveying & Stacking
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Economics in Stacking Methods
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Shiftable / Extendable Conveyors
Mobile Stacking Bridges
Grasshopper / Portable Conveyors
Trucking / Dozing
OPEX CAPEX
Spreaders
Presentation Copyright : TAKRAF 2016
Stacking Technologies
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Presentation Copyright : TAKRAF 2016
Material Handling - Equipment Selection
1. Allowable ground pressure needs to be taken into consideration • Rheological information is
necessary in selecting/dimensioning equipment
• Expect lower allowable ground pressure on a DST system when compared to a waste rock system
2. Crest to equipment’s track distance
• Also from rheological information
• Compared to waste rock stacking, expect to see bigger distances from equipment to dump area crest
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Mobile Conveyor Bridge
• Grasshopper type conveyors are most commonly tire mounted: higher bearing pressures.
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• Receives material from Mobile Conveyor Bridge (MCB) or from Tripper car
• Dumps material from a safe distance into the low cast/high cast dumping location
• Adds flexibility to dump where fixed stackers can’t
Spreader
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Stacking Scheme
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• Less maintenance intensive
• Greater uptime • Less relocation time
• Less flexibility • Needs a relatively flat
original terrain
Mobile Stacking Bridge
Presentation Copyright : TAKRAF 2016
Trucking
Transportation and placement of dry stack tailings
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Transportation and placement of dry stack tailings
Conveying - Conventional
• Example – El Abra, Chile • Capacity: 6,000 tonnes / hour • Length: 9,500 m (31,173 ft) • Elev. change: -510 m (-1,670 ft) • Speed: 6.1 m/s (1,200 ft / min) • Regenerative power: 5,500 kW
(7,400 hp)
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Transportation and placement of dry stack tailings
Conveying – Pipe Conveyors
• Eliminates transfer towers • Requires less space • Simultaneous two-way
transport possible • Tight horizontal and vertical
curve radii • Up to 10,000 t/h material
transfer • Inclines up to 18 degrees
TAKRAF’s electronic training panel eliminates manual training of the belt. The 3-piece panel design reduces downtime during scheduled maintenance.
Presentation Copyright : TAKRAF 2016
Filtercake Moisture Content ?
Filter Presses: typically 8 – 15% moisture
Vacuum Belt Filters: typically 12 – 20% moisture
Is it really necessary to achieve the lowest possible moisture content?
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Presentation Copyright : TAKRAF 2016
Filter cycle has “dead times” when pressure chambers are filled and emptied, i.e. batch cyclic operation.
Relatively low unit capacity More complex cake handling system Capital and operating costs per ton can be high but moistures produced are low
Pressure filters – Delkor F.A.S.T. Filter
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Presentation Copyright : TAKRAF 2016
Filters - Delkor
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Delkor Horizontal Vacuum Belt Filter • Simplified cake discharge & material handling • Simpler to operate
Presentation Copyright : TAKRAF 2016
Filter Feed Conditions ?
Dependent on the viscosity of the feed slurry
Vacuum Belt Filters: • Need uniform belt coverage and cake thickness
Pressure Filters
• Higher feed pressures may cause greater pump wear • Batch process: storage capacity needed & agitator power requirements
Is it really necessary to achieve the lowest possible moisture content of the thickener underflow?
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Presentation Copyright : TAKRAF 2016
Do not produce a conveyable product Capital and operating costs are low. Product rheological properties (medium to high Yield Stress) – lower than paste; still easy to
pump Large diameter units can be used; fewer machines, lower capital More consistent production of final product with a Yield Stress of 50 to 150Pa.
High Rate/Density Thickeners - Delkor
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Presentation Copyright : TAKRAF 2016
TAKRAF / DELKOR Systems and Equipment
Process Systems Required for Dry Stack Tailings
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Sedimentation – Thickeners • Conventional
Thickeners • High Rate Thickener • High Compaction / High
Density Thickeners • Paste Thickeners
Filtration • Horizontal Vacuum Belt
Filter • F.A.S.T. Pressure Filter
Material Handling
• Overland Conveyor • Bridge Conveyors • Stacker Conveyors
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TAKRAF – Complete Dry Stack Tailings System
Guelb II, Mauritania
An alternative to the storing of tailings in impoundments
Dewatering tailings through filtration removes approximately 85% of the water
Produces a ‘cake-like’ material which can be transported by conveyor or truck to a storage site
A low-risk and environmentally-socially responsible alternative to managing waste
Presentation Copyright : TAKRAF 2016
Reduced footprint – land availability and cost, environmental and social impacts
Safety – reduced water storage and high dams, creation of stable landform less susceptible to liquefaction.
Expedite/progressive/concurrent rehabilitation Increased water recovery/recycle and decreased consumption
• Cost and environmental impacts of water consumption • Water availability and quality • Recovery of valuables from process water • Virtually eliminate evaporative water loss
Eliminate groundwater contamination Significantly cheaper to “bond” with the authorities.
Conclusion – Dry Stack Tailings
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Thank you