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Gold Heap Leach Methods
Numerous variations exist on the three heap construction methods identified previously (i.e. reusable pad, expanding pad, and valley leach). In fact, various c ombinations of certain aspects of each method may be valid for a project, as may be the use of more than one method. The following subparts discuss the general concept of each method. Again, no single method is suited to all projects; development of the technique adopted for a project requires site and project specific evaluation.
Reusable pad method.
The reusable pad method involves the construction of a series of durable pads onto which the prepared ore is loaded, leached, washed, neutralized (if necessary), and unloaded for disposal in a spent ore pile. See Figure 1.4.
The major requirements and construction involved in the design of a reusable pad are listed below:
- Needs predictable/short leach cycle;
- Needs ore of consistent leachability;
- Needs limited flat land;
- Needs suitable waste site;
- Needs durable high-stress environmental liner;
- Can tolerate arid to balance climate;
- May allow for covering in wet climates due to limited active area;
- Requires smaller ponds due to limited impact of flood events;
- Requires double handling;
- Provides no conditions for ore to mature;
- Can be used in sites of limited flat terrain.
The design of a reusable pad system requires the evaluation of the leach recovery and the consistency of the ore. The size of the pads and number of pad segments is dictated both by the rate at which ore is mined and by the length of the leach cycle needed to optimize metal extraction. The ore is required to leach with a relatively constant leach cycle.
Generally leach times less than 60 days are preferable. Longer leach times require larger pad area, as residence time for a fixed production rate is longer. Scheduling of the leaching cycle for a fixed production also requires that a limit be set for the time the ore can be leached. Variable ores may, therefore, not be at the required recovery within the limit of the leach cycle.
A specific drawback to the reusable method is the limited flexibility in the leach time. Once ore is removed from the pad, residual values are lost. No opportunity to releach the ore or allow the ore to mature in the heap is available.
Construction of the pad involves a relatively limited area of suitable ground in comparison to the expanding pad method. Site topography for the pad should be flat; however, as a result of the limited areas required, excavation or construction of the pad can be viable. Intrinsic to the reusable pad method is a suitable location for disposal of the leached or spent ore.
The loading and unloading of the pas necessitates a durable liner system capable of tolerating the working stress involved. Typical liner systems such as asphalt, concrete, etc. are used. Liner systems are primarily chosen to accommodate the excavation method used to unload the ore. If accurate level control is achievable so that a layer of liner cover is left in place, membrane liners could be feasible.
Significant advantages do exist for this method and are primarily related to the limited area in which the ore is leached. The per ton cost of the initial pad construction decreases the more times the pad are reused. The size of solutions ponds is subject only to the area of the reusable pad. This limits the impact of the hydrologic cycle and provides a high level of solution control.
In summary, the reusable method is best suited to: ores of limited duration and consistent leach characteristics, projects with limited suitable leach pad areas, and those with a suitable spent ore disposal site.
