Open-Pit Mining

The least expensive type of mining is an open-pit mine, and is always the very first choice of every developer where an orebody is located close to the surface, is large enough and has little overburden.

Open-pit mines seem simple, but every pit has to be taylor-made. The pit walls have to remain up first and foremost, so an engineer in in rock-mechanics has to determine a secure 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 ore that is valuable and the possibility of the pit’s deepness.

The location and size of the first bench of any open-pit mine is critical. It is excavated well into the waste rock that surrounds an orebody. And since each successive bench is a smaller size than the last one taken, the depth to which the pit can be mined is determined by location and size of the frist bench or cut.

The stripping ratio is the name of the amount of waste rock mined relative to the quantity of ore.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 signifies that during the pit’s lifetime there will be three times as much waste rock mined as ore. An open-pit mine must be designed so that the cost of mining the waste rock does not exceed the value of the ore to make it profitable.

The principal cost advantage of open-pit mining is that the miners are able to use bigger and more powerful trucks and shovels – the equipment is not restricted by the size of the opening it has to work in. This permits quicker production, and the lower cost permits lower grades of ore to be mined too.

If an orebody is large, and it extends from great depth to surface, it is very common to start mining close to the surface from an open pit. This gives some early revenue while preparations are being made for mining underground of the orebody’s deeper parts.