Why ore conditioning matters in heap leaching
Agglomeration is an ore-conditioning step used before heap construction. Crushed ore is mixed with controlled moisture, reagents and, where required, a binder so that fine particles attach to larger particles and form more stable agglomerates. The objective is not to manufacture perfectly uniform pellets. It is to create a material structure that can retain useful void space, resist excessive fines migration and allow leach solution to move through the heap.
This distinction matters because heap leaching depends on both chemistry and hydraulics. The leach solution must reach the mineral surfaces, but it also needs connected flow paths through the stacked ore. If clay and fine particles migrate into those paths, parts of the heap may receive less solution while other areas carry preferential flow.
Why fines and clay can restrict a heap
Crushing increases mineral exposure, but it can also generate fine particles. Some ores also contain naturally occurring clay. When loose fines move through the pore spaces between larger particles, they can obstruct flow channels and reduce the ability of leach solution to percolate through the ore bed.
Low permeability may present as slow drainage, ponding, slumping or uneven wetting. Preferential flow can then develop, with solution following easier paths instead of contacting the ore mass consistently. Agglomeration is one method used to manage this physical risk before the ore reaches the pad.
How heap leach agglomeration works
The exact flowsheet varies by ore and leach chemistry, but the functional sequence is usually:
- Crushing and, where required, screening establish the feed size distribution.
- Moisture, process reagents and any selected binder are added in controlled quantities.
- Mixing and rolling bring fine particles into contact with larger particles.
- Liquid bridges and binder interactions help the fines attach to the coarser ore.
- The agglomerates may be cured before further handling, depending on the selected system.
- The conditioned ore is conveyed and stacked on the heap leach pad.
Rotary drums are widely used because they provide tumbling and rolling action, but agglomeration can also be achieved through other engineered mixing arrangements. Equipment selection should follow test work and the required level of mixing, residence time, moisture control and throughput.
Agglomeration is not the same as pelletising
Agglomeration in heap leaching does not normally aim to produce a narrow, uniform product size like a conventional pelletising plant. The practical objective is to attach fines to larger particles and produce agglomerates that remain sufficiently stable during conveying, stacking and irrigation.
Appearance alone is therefore not enough to judge performance. An agglomerate may look acceptable when it leaves the drum but break down during transfer, stacking, curing or exposure to leach solution. Testing needs to consider how the material behaves under representative hydraulic, chemical and loading conditions.
The role of moisture, binders and reagents
Moisture
Moisture enables particle adhesion and supports the liquid bridges involved in agglomerate formation. Too little moisture may produce weak or incomplete agglomeration. Too much moisture can create handling problems and may reduce the quality or stability of the product. The correct operating range depends on the ore, fines and clay content, absorption characteristics and process chemistry.
Binders
Binders may be used where moisture and mechanical mixing alone do not produce agglomerates with adequate stability. Cement and lime are established options in alkaline precious-metal systems, while acid leach environments require materials that remain compatible with acidic conditions. Binder selection cannot be transferred blindly from one ore or process to another.
Process reagents
Agglomeration may also be used to distribute process reagents through the ore before stacking. Examples include alkaline reagents in cyanide systems and acid addition in some copper applications. The metallurgical programme must determine compatibility, dosage and the effect on downstream leaching.
Agglomeration, conveying and stacking form one system
Agglomeration quality can be degraded after the drum. Multiple transfer points, excessive drop heights, impact, belt loading and uncontrolled stacking can break agglomerates or separate coarse and fine material. Material handling design should therefore protect the conditioned ore from the agglomeration stage to final placement on the pad.
Controlled conveyor stacking can reduce the dependence on repeated truck dumping and dozer rehandling in the active placement area. However, conveyors do not automatically eliminate segregation. Chute geometry, transfer velocity, drop height, feed consistency, stacker operation and lift design still require engineering attention.
What good agglomeration should achieve
- Fines attached to larger particles rather than remaining freely mobile.
- Agglomerates strong enough for the expected conveying, transfer and stacking route.
- A stacked ore bed with connected flow paths for leach solution.
- Controlled moisture and reagent addition within the validated operating range.
- Repeatable field quality that can be monitored during operation.
The final objective is not agglomerate strength in isolation. It is acceptable heap behaviour under the intended stacking load, irrigation rate and chemical environment.
How agglomeration requirements are established
A reliable design starts with representative ore samples and a test programme that reflects expected variability. The test work may assess particle-size distribution, fines and clay content, moisture demand, binder type and dosage, curing requirements, agglomerate degradation, slump, percolation, hydraulic behaviour and column leach performance.
Comparative tests such as percolation tests can support binder and operating-condition selection, but a single test does not replace broader permeability and metallurgical evaluation. Laboratory results also need to be considered against field stresses, scale, transfer handling and irrigation conditions.
Common causes of poor agglomeration performance
- Feed variability that is not matched by moisture or binder control.
- Incorrect moisture addition or poor distribution of water and reagents.
- Insufficient mixing, rolling action or residence time.
- Binder selection that is incompatible with the leach chemistry.
- Inadequate curing where curing is required.
- Agglomerate damage at conveyor transfers or during stacking.
- Segregation after agglomeration.
- Relying on visual appearance without hydraulic and metallurgical testing.
Is agglomeration always required?
No. The need for agglomeration depends on the ore and the required heap performance. Coarser, competent ores with limited problematic fines may not require the same treatment as clay-rich or finely crushed feeds. Agglomeration should be justified by ore characterisation, column testing, permeability assessment and the selected leach and stacking strategy.
Why agglomeration matters to project design
Agglomeration affects more than the process plant. It influences water and reagent addition, equipment selection, transfer design, conveyor loading, stacking method, curing arrangements, irrigation strategy and operating control. Treating the drum as an isolated machine can leave important interfaces unresolved.
For project development, the better question is not simply whether an agglomeration drum is required. The project team should ask what physical condition the ore must retain from preparation through stacking and irrigation, then design the agglomeration and material handling system around that requirement.
Frequently Asked Questions
Here are some common questions we get asked at Carmaky.
What is agglomeration in heap leaching?
Agglomeration is the conditioning of crushed ore with moisture, reagents and, where required, a binder so that fine particles attach to larger particles before stacking.
Why is agglomeration used?
It is used where fines or clay could migrate, obstruct flow paths or contribute to poor solution percolation through the heap.
Does all heap leach ore need agglomeration?
No. The requirement depends on the ore’s particle-size distribution, clay and fines content, physical behaviour, leach chemistry and the results of representative test work.
What is added during agglomeration?
Water or process solution is commonly added. Depending on the ore and chemistry, lime, cement, acid or another validated binder or reagent may also be used.
What equipment is used for agglomeration?
Rotary agglomeration drums are common, although other engineered mixing and rolling arrangements may be used where they meet the process requirements.
How is agglomerate quality checked?
Quality assessment may include moisture control, visual checks, size distribution, degradation or soak testing, slump and percolation testing, and representative column tests. The selected method must reflect the project risk and process conditions.
Conclusion
Agglomeration is the interface between ore preparation and heap construction. When it is required and correctly designed, it helps control fines and supports the permeability needed for solution movement through the heap. Its success depends on more than the drum: ore variability, moisture, chemistry, binder selection, curing, conveying, transfers and stacking all influence the final result.
The correct agglomeration strategy must therefore be based on representative metallurgical and physical test work, then carried through into the design of the material handling and stacking system.
Related resources: heap leach stacking systems · Heap Leach Resource Centre