Technical Stream | Operations | Mining Talking Points II | Room 110
Abstract
Open pit mine design is often done in two stages. A more analytical/strategic stage followed by detailed/tactical stage. In the first stage, fundamental decisions about the project are made, with a focus on investment security assurance. Pit optimization, strategic mine planning, and pit size selection are done in this first stage. At the end of this stage, the shape and size of the ultimate pit, production rate and viability of the project are known with some reasonable uncertainty. The subsequent stage of detailed mine design, mine sequencing and production scheduling determines how the reserve is mined throughout the project life. The production schedule with detailed mine plans, along with equipment requirements, are the core of this stage of work. At the end of this stage, reliable estimations of equipment, personnel, consumables, and supplies, along with subsequent costing are made.
The main drivers in strategic mine planning are to: a) advance the revenues as much as possible and b) delay mining unnecessary waste while honouring technical constraints such as minimum mining width, safe slopes and realistic mining rates.
Pushbacks are designed to have quick access to the high-grade zones of the deposit to improve revenues in the early years as much as possible. While this is the main purpose of designing the pushbacks it also helps to reduce the risk of investment. Mining using pushbacks provide a safety zone for the projects so that when the market is not favourable, the operation could be aborted with minimum loss.
In theory, maximizing the number of pushbacks will help boost the economics of the project. However, due to operational and economical constraints there is a limited number of pushbacks that we can realistically select for the life of mine. Mine designs often contain too many pushbacks. Although they provide quick access to high-grade ore, they are not executable and eventually are mined together.
It is important to know that pushbacks are effective only if the number of active pushbacks in each period is limited. In author(s) view, the number of pushbacks should be limited based on the life of mine. Each pushback should provide ore for 3 to 7 years of operation. Anything less than 3 years is not practical, and more than 7 years increases the risk for the operation.
Parameters to consider when selecting the number of pushbacks are, the life of mine, shape of orebody and grade distribution, smooth and realistic strip ratio throughout the mine life, if applicable the interaction with the underground operation, safety, and the minimum mining width.
This paper uses real examples of mine design in different situations to describe some of the critical issues related to pushback definition in open pit mines.
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