Open Pit (Open Cast) MiningOpen pit, or open cast, mines excavate ore from the surface creating a hole in ground. Open pit mining is generally less expensive than underground mining. Whether or not a deposit can be open pit mined will depend on the shape and depth of the ore deposit. The main issue for an open pit mine design is the strip ratio. This ratio indicates the amount of waste (rock without gold) that must be extracted to access ore rock. For example, ore rock that is mined directly from the surface will have no strip ratio, because no waste rock needs to be mined in order to excavate the ore. If we need to excavate some waste rock to access the ore, for example 100t of waste rock to remove 100t of ore, the strip ratio would be 1:1. This scenario may exist but only for a short time, as quickly the need to remove more waste will arise. As an open pit gets deeper, the stripping ratio will become larger because more and more waste rock will need to be removed to access the ore. When this ratio gets so high that it costs more to mine an area of ore than the revenue extracted from it, then the pit will be at it's final depth. While the stripping ratio of an open pit mine changes through the mine life, a single ratio will be quoted in feasibility studies that represents the ratio of total waste to total ore through the life of the project. |
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