Caleb Cook, Kappes, Cassiday & Associates

Dan Kappes, Kappes, Cassiday & Associates

Conveying and Stacking Systems Design for Heap Leach Applications

Kappes, Cassiday & Associates7950 Security Circle Reno, Nevada 89506

Telephone: (775) 972-7575www.kcareno.com

Heap Leaching: Permeability is Key to Success

Permeability of top Working Surface

Uniform Permeability of the HeapPermeability

of Intermediate Layers Between LiftsPermeabilit

y of Drainage Layer Below Heap

Truck Stacking Conveyor Stacking• Crushed ores and ores high

in clay• Lower operating cost

compared to truck stacking• Flexibility to add moisture or

chemicals before stacking• Less compaction (generally)

of top of heap

• Operation flexibility

• When conveying options become too large

• ROM applications

Stacking MethodsVs.

Four Main Configurations:• Discharge from a tripper on a bridge conveyor which

straddles the heap (small tonnage, on-off heaps)• Discharge from a tripper on a long traveling belt supported

on bulldozer tracks running on the heap• Retreat stacking• Top advance stacking

Conveyor Stacking of Heaps

Bridge Conveyor & “Racetrack” Design Stacking: Thin Layer Heap LeachingPros:• On-off style leaching• Good for low permeability

heaps• Fast recoveries• Useful for acid leaches

Cons:• Generally low lift heights

(1-4 m)• Not suited for multi-lift

heaps• For “racetrack” design –

very capital intensive

Bridge Conveyor & “Racetrack” Design Stacking: Thin Layer Heap Leaching - Continued

“Racetrack” design on-off heap leach at Round Mountain

Permanent Heap

New Heap Being Stacked

Old Heap Being Removed

Pros:• Similar to bridge conveyor

arrangement• Can climb slopes and

construct multi-lift heaps• Results in a very

permeable heap

Rahco-type Stacker for Permanent Heaps

Cons:• Capital intensive

• Very large machines which require regular geometries or large area for turning

Rahco-type Stacker for Permanent Heaps

Rahco stacker at Chuquicamata, Chile – copper heap leach

Rahco stacker building “racetrack” style heap. Tracks are on the “permeable” heap base. Alternatively, the stacker can stack on top of the lift and advance forward, avoiding traveling on saturated ore.

• Most common type of conveyor stacking system

• Easy to operate for single lifts up to 8 m

• Can be used for multiple lifts, but may result is a reduction of ore permeability at the working surface

Retreat Stacking

Retreat stacking system at Yatela operation in Mali

• Ideal for multiple lift heaps

• Well suited for valley fill heap leaching

• If designed properly should be lighter in weight compared to retreat stacker

• No limit to lift height

Top Advance Stacking

Top advance stacking at Ocampo in Mexico

Top advance stacking at Ocampo in Mexico

Top advance stacking at Ocampo in Mexico

• Often not considered with the ore type or overall process in mind• Generally left to the stacker supplier with minimal design criteria• Ideally project metallurgist or design engineer should work with

stacker supplier to properly select a stacker• Stacker design should include considerations for:• Ore type, heap configuration, size, etc.• Design for minimal need for support equipment (cranes, loaders, dozers)• As light weight as possible (while maintaining structural integrity)• Low ground pressures

Stacking Equipment Design

• Insufficient or over designed equipment structural strength

• Excessive equipment weight / ground pressure of tires or tracks

• Poor design of systems used for equipment movement on heap

Deficiencies of Conveyor Stacking Systems

Index Conveyor at Ocampo – tipped while moving

• Manufacturer generally biased towards making equipment too heavy• Good for equipment durability, not necessarily for heap leaches• Equipment overdesigned due to the fear of equipment failure on

leach pad• Too heavy equipment = increased chance of failure of the heap.

• Can result in excessive use of mobile equipment to aid in positioning• Ideal design requires No mobile equipment on the heap.

Insufficient or over designed structural strength and equipment weight

• Heap surfaces are irregular with water saturated material

• System should travel on the heap without leaving visible wheel or track ruts

• Ground pressure of 10,000 kg/m2 or less is generally acceptable

• Large flotation tires or caterpillar-type tracks are ideal

Excessive Ground Pressure of Tires and Tracks

Stacker tires with tracks at Ruby Hill mine

• Stacking system must continually move on heap up to several meters per day

• Movement should be done by equipment itself to avoid having any more additional equipment (bulldozers, loaders, cranes) on the heap than necessary

• Excessive equipment movement on heap will inevitably result in some lost recovery

• Equipment drives should be powerful enough so that the equipment can extract itself when it runs into depressions in the surface

Poor Design of Systems for Moving Equipment

• Stackers should be equipped with stingers to minimize movement on the heap

• Correctly designed equipment can build a flat heap

Poor Design of Systems for Moving Equipment - Continued

Comco Silver Heap Leach – Potosi, Bolivia

Comco Silver Heap Leach – Potosi, Bolivia

• There are several different ways to place ore onto heaps

• Not all of these are optimal in all circumstances

• Poor choices may result in loss of recovery

Conclusion