Open-Pit Mining Methods
An open-pit mine is the least expensive of all mining methods, and is every developer’s first choice where an orebody is situated close to surface, is big enough, and has little overburden.
Open-pit mines look simple, but every pit needs to be tailor-made. First and foremost, the pit walls have to stay up, so a rock-mechanics engineer has to determine a safe slope for the pit. There is also a delicate balance between how much waste rock can be mined in order to gain access to the valuable ore and how deep a pit can be.
The size and location of the first bench of any open-pit mine is critical. It is excavated well into the waste rock surrounding an orebody. And since each successive bench is smaller than the last one taken, the depth to which the pit can be mined is determined by the size and location of the first cut or bench.
The amount of waste rock mined relative to the amount of ore mined is called the stripping ratio. In most cases, this ratio is high for the first bench and decreases steadily with each successive bench. A stripping ratio of 3 to 1 means that during the life of the pit, there will be three times as much waste rock mined as ore. To be profitable, an open-pit mine must be designed so that the cost of mining the waste rock does not exceed the value of the ore.
The main cost advantage of open-pit mining is that the miners can use larger and more power shovels and trucks — the equipment is not restricted by the size of the opening it must work in. This allows faster production, and the lower cost also permits lower grades of ore to be mined.
If an orebody is large, and extends from surface to great depth, it is common to start mining near the surface from an open pit. This provides some early revenue while preparations are made for underground mining of the deeper parts of the orebody.