Presented by Mike Gowan Principal CODISPOSAL Presented by Mike Gowan Principal

Co-disposal involves the combining of these waste streams DEFINITION In mining and mineral processing, materials are separated according to their particle size and mineralogy The wastes produced fall into Coarse-grained (waste/rejects); & Fine-grained (tailings) Conventionally disposed of separately Co-disposal involves the combining of these waste streams

MINE WASTES -1 Tailings - rock flour resulting from the crushing and or grinding of mine ore - <1mm Rejects - washery waste resulting from the processing of coal - >1 to 120 mm Spoil/Waste - rock separated in the mining process and not processed - 0 to >1 m

CODISPOSAL WITH MINING PRODUCTS Tailings disposed as a slurry has a high porosity (>40%), with water-filled voids. Rejects/waste has a high porosity (>30%), with largely air-filled voids. Codisposal - some of the tailings can be made to settle in the voids in the coarse waste.

POROSITIES Tailings Rejects Waste SG 1.8 2.2 2.7 Dry Density 0.9 1.2   Tailings Rejects Waste SG 1.8 2.2 2.7 Dry Density 0.9 1.2 Void Ratio 1.000 0.833 0.500 Porosity 50% 45% 33%

TYPICAL WASTE

CONCRETE - IDEAL CODISPOSAL Aggregate, sand, cement & water mixed together No air voids Coarse aggregate suspended in fines mixture Aggregate Sand/Cement

CONCRETE - IDEAL CODISPOSAL MODEL Using concrete as the model: Products need to be: Nearly dry Well mixed before placement Minimum water added Coarse:fine ratio not critical Low energy placement to reduce risk of segregation

IDEAL CODISPOSAL Tailings needs to be dewatered to paste or cake Tailings and rejects need to be mixed together Mixture then pumped, trucked or conveyed to disposal Expensive operations, dictated by circumstances

MIXED CODISPOSAL Used successfully: Trialled at Dartbrook Wollongong by BHP Westcliff coal mine Trialled at Dartbrook

CODISPOSAL Co-mingling Co-placement Co-disposal

CO-MINGLING The coarse and fine products are transported separately and allowed to mix together within the disposal site after deposition. An example of this the dumping of rock and the deposition of tailings at Kidston Gold Mine.

CO-PLACEMENT The coarse and fine products are transported separately and mixed together just prior to or on placement in the disposal site. An example of this is the mixing of slimes and tailings used at the Argyle Diamond Mine.

CO-DISPOSAL Coarse and fine waste products are mixed together before they are transported to the disposal site. An example of this is the pumped codisposal practice carried out in Australian coal mines.

CO-MINGLING at KIDSTON AIM – to fill a pit and produce a stable landform at closure Materials available tailings and waste rock Reviewed many codisposal systems: Autogenous mixing Active mixing Winrowing Tailings cells Selected co-mingling Other systems too costly

AUTOGENOUS MIXING Tailings Discharge

ACTIVE MIXING Tailings Discharge

WINROWING - 1 Tailings Deposition

WINROWING - 2 Tailings filling between Windrows Tailings Spigot Pipeline Tailings/Waste Windrows

TAILINGS CELLS - 1 Waste cells Tailings deposition

TAILINGS CELLS - 2 Waste Cell Tailings Waste cover/mixture Mixed Tailings/Waste

deposited into pit pond KIDSTON DETAILS Thickened tailings deposited into pit pond Waste rock end-dumped into pit

Eventually Waste extended over Tailings to produce a Closure Cover VIEW OF KIDSTON PIT Thickened tailings Waste Eventually Waste extended over Tailings to produce a Closure Cover

Tarong – Comingled Reject & Tailings

CO-PLACEMENT-ARGYLE Problem – very fine slimes that would not settle Solution – mix the two materials Slimes & Tailings mixed at disposal area Slimes pumped Tailings conveyed

NE USA Mixing Rejects & Dewatered Tailings Placing and Spreading

DEVELOPMENT OF CODISPOSAL Tried in The UK in 1960’s South Africa in 1980’s Tailings slurry spread over layer of rejects Penetration up to 300 mm Costly to operate Thin layers of rejects Moving tailings pipeline Spreading tailings

SOUTH AFRICA TRIALS

AUSTRALIAN TRIALS Tested placing rejects over tailings Some penetration of rejects Problems: Development of Bow-wave Slow advancement rate

REJECTS INTO & OVER TAILINGS Bow-wave

WASTE PLACEMENT OVER 10 m TAILINGS

CODISPOSAL IN AUSTRALIA Confined to Coal Mines Idea developed at Jeepropilly Now used at: Hail Creek Kestrel North Goonyella Mooranbah Coppabella Moorevale Stratford Others???

COAL CODISPOSAL - 1 Tailings & Reject mixed at CHPP Pumped to disposal site Slurry solids 27 to 35% Flow velocities 2.7 to +4 m/sec Single point full pipe discharge Clean water recovery

LIMITATIONS OF CODISPOSAL 3 Stage pumping reaches ~2 km Steel pipe for high heads High pipe wear Limited tailings encapsulation

2 STAGE PUMPING

CERAMIC LINED STEEL PIPE

TYPICAL CODISPOSAL BEACH Rejects only Beach Well Mixed Codisposal & Encapsulated Tailings

COAL CODISPOSAL BEACH Codisposal beach Tailings beach Decant Pond

TAILINGS BEACH TOTAL Coarse Fine Waste tph 500 357 143 SG 2.2 1.8   TOTAL Coarse Fine Waste tph 500 357 143 SG 2.2 1.8 C:F Ratio 2.5 1 Split 71% 29% Slurry solids 27% Water 1,352 Mean Density t/cum 1.4 0.8 Void Ratio 0.57 1.25 Porosity 36% 56% TAILINGS BEACH   Total Coarse Fine Beach Pond TOTAL 500 100% 86% Tailings 48% Waste tph 357 143 431 69 Dry density t/cum 1.40 0.80 1.69 1.47 Total Volume cum 255 179 86 341 Void Ratio 0.57 1.25 Solids volume 162 79 Void volume 93 99 Moisture Content 26% 69% 11.9% 19.8% Water contained 1,352 52 48 Return water 1,253 93%

EFFECT OF C:F RATIO

ADVANTAGES OF CODISPOSAL Pumping lower cost than trucking No transport fleet required Stable landform made by beach Tailings contained by beach High water return

TRAFFICABLE BEACH Generally cannot drive easily over rejects, but can over upper codisposal beach

STABLE CODISPOSAL – Despite Wall Failure Stable Codisposal Wall Clay starter- wall failure

BEARING CAPACITY LIMITATIONS Codisposal beach Tailings

WATER RETURN Tails 143 tph SG 1.8 Slurry 35% Water 551 cum/hr   Slurry 35% Water 551 cum/hr Dry Density 0.8 t/cum MC 69% Retained 99 Return water 82% Rejects 8% 29 TOTAL 128 Codisposal

SUMMARY Codisposal difficult but not impossible in metalliferous mines Codisposal works for coal mines There is a tailings pond that needs to be managed Water losses are no higher than for separate reject:tailings disposal systems

ACKNOLEDGEMENTS The many mines mentioned Assoc. Prof. David Williams of The U of Q

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