The materials in a heap leach pad constitute a heterogeneous, anisotropic mass. Material hydraulic conductivities vary greatly from point to point. This random variation from point to point of hydraulic conductivity is the result of the inherent in suit variability in the ore being mined, variations in the comminution of the ore as a result of blasting, loading, and dumping, and segregation and blinding that occur during placement…

While it is tempting to think of seepage of leach solution and lixiviants as a uniform vertically downward flow regime, this simply is not the case. Simply put, the paths that the solution will take as it flows down through the mass of heap leach material will depend on these, and probably many other factors:

  • The heterogeneity of the mass, and hence the presence and pattern of channels or paths of greater permeability.
  • The moisture content of the ore which depend on the moisture content as mines, as placed, and as resulting from ambient conditions including antecendent rainfall percolation
  • The rate and pattern of application of the solution and lixiviant.

As O’Kane notes:

Layers of coarse and fine textured ore inevitably develop within heap and dump leach piles as natural processes segregate coarse and fine material during material placement. Segregation of heap leach material will occur regardless of whether the material is agglomerated or non-agglomerated…... Under such conditions leaching solution flows preferentially in the more conductive layer, potentially leaving areas within the heap unleached. The preferred flow path is not dependent entirely on the physical properties of each layer, but also on the stress state and resulting degree of saturation, and therefore the solution application rate. For this reason either the coarse or the finer material can be the preferred flow path.

Thus it is not quiet as simple as multiplying the area of the pad by the saturated hydraulic conductivity if you want to establish the maximum possible application rate.

If you do succeed in applying enough solution to the top of the pad to create fully saturated flow through the heap leach materials, you will certainly be getting lots of solution through the materials, but you may not be getting the metal recovery you seek or could achieve by less aggressive solution application.

To quote O’Kane again:

Column testing revealed that solution application rates greater than the saturated hydraulic conductivity of the finer material resulted in preferential flow in the coarser layer. The preferred flow path became the finer textured material when application rates were less than the saturated hydraulic conductivity of the fine material.

In other words, to get the metal out of the finer materials and into the solution, you need to get the solution to seep through these finer materials. And that only happens best when the material is partially saturated, and the seepage retreats, as it were, into the finer channels.

This leads to the counterintuitive conclusion: to increase recovery, it may be better to reduce solution application rates, rather than increase them.

Keep in mind also if you increase application rates too much you may create a saturated zone near the base of the pad, and that could induce slope failure.

Here is a random result from the web that supports the greater recovery from finer material concept:

Samples were subjected to leach ranging from 6", 4", 2" and 1" top crush size. The samples were subjected to various amounts and rates of acid addition to the column leaches. The following results have been reported by Plenge Labs for some selected samples. 1" top size intrusive mineralization containing 0.33% total copper of which 53.5% was cyanide soluble copper was subjected to an acid cure and leached in a ferric sulfate environment for 295 days. The intrusive sample leached well and 72.9% of the total copper was leached. The sample consumed 25.7 kg/t sulfuric acid. Non-cyanide soluble appeared to leach well and the mineralization was still producing copper at the time the leach was terminated. 4" top size intrusive mineralization containing 0.33% total copper, of which 53.5% was cyanide soluble copper, was subjected to a leach using an application rate of leach solution of 15 L/hr-m2 of 5 gpL H2SO4. The rock was leached for 247 days and yielded 53.5% total copper recovery. An identical sample crushed to 6" top size is still running and the total copper recovery has exceeded 60%. The copper is still leaching in the 6" column. Total acid consumption of the 4" material was 21.9 kg/t.

Maybe it is time to get a consultant to help. Here is the description of services from one:

WMC developed its proprietary Heap Leach Dynamic Technology (HLDT) to assist mines in designing, operating and closing heap leach facilities. HLDT uses a combination of geophysical, laboratory, field and modeling techniques to fully characterize and optimize the heap leaching process. With its wide understanding and experience in water flow and solute transport, WMC is able to assist mine metallurgists in optimizing key design and operational conditions that affect the hydrodynamics of the leaching process. This gives the operator control of solution-to-rock interaction, thereby greatly increasing the potential for higher yields during the leaching operation. WMC has successfully improved leaching efficiency, increased recovery and lowered costs through application of this technology at mines worldwide. Results include operational costs reduced by up to 30%, expansion of economically leachable reserves, ability to leach ores with very high fine contents and significantly increase recoveries have been demonstrated. Services offered for HLDT include:

· Electrical resistivity tomography (ERT) surveys of heap leach facilities

· Laboratory testing of unsaturated flow parameters affecting heap leaching in our Tucson laboratory facility

· Field testing and instrumentation of heap leach facilities to monitor flow and leach efficiency

· Integration of hydrodynamics with standard column tests

· Numerical modeling of heap leach hydrodynamics

· Evaluation of existing heap facilities for overall efficiency

· Determination of optimum leaching application rates and dripper spacing

· Design of agglomerates to improve hydrodynamics and recovery

· Development of material handling and placement criteria to improve recovery

· Studies of optimum leach cycles including variable application rates

· Heap leach closure