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Precious metal heap leaching projects in the United States have been located in the western states, primarily in Nevada. What appears to be a regional location
preference is attributable to a variety of factors. First and foremost among these is the presence of large, low grade deposits amenable to leaching. However important secondary factors include topography and climate suitable for simple heap leaching design and limited land use and regulatory constraints. More recent applications of heap leaching technology have resulted in the development of projects in all areas of the U.S., under wet and cold climatic conditions, and in mountains terrains. Although operations in these areas require rather specialized designs, successful implementation has and is occurring in areas traditionally considered unsuitable. Even so, the operation of heap leaching facilities is often restricted to a range of moderate climates. Low and freezing temperatures limit operation more through the formation of ice than through depression of chemical leach reaction.
The hydrology of a site is related to the precipitation regime of the site and has a very definite effect on how well the process works. In order to effectively and economically leach the precious metals, a relatively narrow range of solution concentrations exists. The maintenance of a chemically constant solution (apart from making chemical additions) is dependent upon the hydrology of the project site. Arid climates with high evaporation rates can result in very large requirements of make-up water to maintain the volume of water needed within the leaching circuit. Conversely, areas of high net precipitation can result in the continuous build up of solution volumes in the leaching circuit. This may necessitate the bleed, neutralization and discharge of chemically active solution from the circuit.
Factors such as these must be taken into account during the project design phase. In order to evaluate and identify the appropriate method of heap leaching for a particular project, as well as, the techniques to be applied to optimize the operation, it is necessary to assimilate the results of metallurgical testing, topographic and climatic site conditions, general geotechnical and geologic site conditions, and the methods and rate of mining.
Irrespective of the
specifics of any particular project, a heap leaching operation consists of the following components:
- Mine/ore source;
- Ore preparation;
- Heap and pad;
- Solution application/collection system;
- Pregnant solution containment;
- Metal recovery circuit; and
- Barren solution containment.
A schematic of a heap leach system is given on Figure 1.2. Some details of the heap leach components are described in the subpart that follows.
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