Feature Focus

March 20, 2009

Heap leach operations have gained popularity in the past around the world as a reasonable method for extracting metals such as gold, copper, and nickel from earth material. In such heap operations, however, the success of the leaching operation depends on the flow regime established by the sprinklers at the top of each lift and the drainage system at the bottom.

Each lift represents a unique unsaturated flow system, which is established in order to provide a targeted saturation range and to allow the presence of oxygen in the heap layer. Through the three-dimensional saturated and unsaturated flow software features in SVFlux, it is now possible to determine how to optimize the speed of the drainage through the use of a drainage pipe system in the over-liner material. There are two primary types of design, which can be analyzed for this pipe system: a herringbone type design, and a parallel pipe design.

This particular problem is more appropriate for 3D and cannot be reasonably modeled using a 2D system, due to the lateral aspects of the flow regime.

A herringbone type of drainage structure, as well as a parallel type of drainage pipe structure can be set up as shown in Figures 1 and 2, shown on the right.

Solution is then applied as a flux boundary condition to the top of the numerical model and the model is then wet up to operating conditions. After a steady-state condition is achieved with the numerical model, the input flux is then turned off and the model is allowed to drain down to residual conditions. The performance of each drainage system can then be evaluated in the numerical model:

Figure 3, on the left, shows the model mesh after analysis. Figure 4 on the right shows the location of the herringbone drains. Figure 5, below, illustrates the zones of saturation developing at the downslope corners of the base of the heap leach material when ksat is 197 ft/hr. All three figures contain a 3X vertical exaggeration to make visualization easier.

This type of analysis is not traditionally done with many heap leach operations. This numerical model illustrates the value of a simple 3D numerical model in order to provide a fiscal basis for judgments made regarding the drainage for such heap leach operations.

Such an analysis yields insight between the relationship of the application rate and hydraulic conductivity of the heap material. Saturated zones must be avoided at all costs, as they reduce the recovery process and introduce the potential poor slope stability issues within heap. Once the flow regime is established, then estimates of recovery can be obtained.

Extensions to traditional numerical modeling methods are easy with SVFlux. In particular, the numerical model can be used to model spatial variability and preferential flow, the influence of unsaturated soil properties on proper application rates, as well as the development of potential saturation areas in a heap leach facility. The development of potential saturation zones can then be evaluated with the SVSlope slope stability software package.

If you are interested in this type of numerical model, please contact us for more information regarding the evaluation of your present design.

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