Introduction
The stripping ratio in mining refers to the amount of waste material (overburden) that must be removed to extract a unit of ore. It is a critical metric in surface or open-pit mining, as it directly impacts the economic viability of a mining operation. That said, stripping ratios are not only about the volume of unwanted material present at a site; they also take into account the types of material that must be removed to reach the ore. After all, moving lightweight material like sand or dirt is simpler than moving hard rock (Stripping Ratios, n.d.).
How to calculate stripping ratios?
An article published in (Stripping Ratios, n.d.) clearly explained it as indicated in the lines below.
At the most basic level, the strip ratio formula involves dividing overburden thickness by ore thickness. For example, an overburden thickness of 100 meters and an ore thickness of 50 meters would yield a strip ratio of 2:1. That means mining 1 cubic meter of ore would require mining 3 cubic meters of overburden.
The stripping ratio of a deposit may be used, in part, to gauge how profitable it may be. The lower the strip ratio the better, since a low strip ratio translates into lower mining costs and good prospects for profitability.
Conversely, a project with a very high strip ratio likely will not be profitable. In this case, the unwanted material is much greater than the amount of ore that can potentially be extracted, making it too expensive to mine.
Ore quality is another consideration in stripping ratios in mining. That’s because if a deposit contains low-quality ore, more of it must be mined in order to achieve a return on investment.
It should be noted that every deposit is different, and a project that benefits from another factor — for example, having high grades — can potentially support a higher strip ratio. Generally speaking, there is an inverse relationship between reserve grade and strip ratio.
With all of this in mind, mining companies calculate strip ratios for open-pit projects well before they enter development and production and tend to seek out projects with relatively low strip ratios. Even so, given all the factors involved in calculating a strip ratio, it’s difficult to determine an overall ideal figure. In the case of a “typical” large, low-grade copper porphyry deposit, a strip ratio below 3:1 is generally considered good.
What are the types of stripping ratios in mining?
The major types of stripping ratios are overall, instantaneous, and break-even.
Overall stripping ratio
The overall stripping ratio in mining is a measure used to evaluate the economic feasibility of an open-pit mining operation. It represents the amount of waste material (overburden) that must be removed to extract a given amount of ore. The stripping ratio is expressed as a ratio or number, such as 3:1, meaning three units of waste are removed for every unit of ore mined. A high-grade deposit with a low strip ratio might attract investment, heralding robust returns, while a lower-grade deposit with a high strip ratio might give pause, signaling a more challenging financial journey (Mining 101: Strip Ratio – the Key to Open-Pit Mining | TSX-V:LL, n.d.).
Instantaneous stripping ratio
The instantaneous stripping ratio (ISR) in mining refers to the amount of waste material that must be removed for a given amount of ore in a specific pushback or slice of material from a pit wall (3.2.5: Instantaneous Stripping Ratio | MNG 230: Introduction to Mining Engineering, n.d.). It helps determine if a pit can be made larger by evaluating the waste extracted each time it deepens (3.2.5: Instantaneous Stripping Ratio | MNG 230: Introduction to Mining Engineering, n.d.). ISR focuses on a small slice of material removed from the pit, considering the length of waste and ore in that specific section. The ISR is used in a series of slices to evaluate pit expansion, assuming parameters like density, weight, thickness, and ore length remain constant, with only the waste segment length changing.
Break-even stripping ratio
The break-even stripping ratio (BESR), also known as the cut-off stripping ratio, is a crucial economic indicator in open-pit mining operations. It represents the point at which the cost of mining ore and waste is equal to the revenue generated from that ore (Open Pit Mining Terminology, Break-Even Stripping Ratio | Lecture Notes Mining Engineering | Docsity, n.d.). It serves as a quick method to evaluate a mine’s or a design’s economic value. A lower BESR is generally more desirable, as it indicates that less waste needs to be removed to access profitable ore. The BESR helps in determining the economic viability of open-pit mining versus underground mining methods. It is a controlling factor in the comparative cost analysis between these two mining approaches.
Conclusion
The stripping ratio is a key metric in open-pit mining, measuring the volume of waste removed relative to ore extracted. A lower stripping ratio generally means lower mining costs and higher project profitability, while a higher ratio may signal economic challenges. The ratio is influenced by factors such as overburden type, ore grade, and deposit characteristics. There are three main types: overall, instantaneous, and break-even stripping ratios, each helping assess different aspects of mine design and feasibility. While lower ratios are preferable, high-grade deposits may still be viable even with higher ratios. Ultimately, stripping ratios play a critical role in evaluating and optimizing mining projects.
Image credits: Christopher Halloran / Shutterstock
Reference
3.2.5: Instantaneous Stripping Ratio | MNG 230: Introduction to Mining Engineering. (n.d.). Retrieved March 24, 2025, from https://www.e-education.psu.edu/mng230/node/859
Mining 101: Strip ratio—The key to open-pit mining | TSX-V:LL. (n.d.). Retrieved March 24, 2025, from https://www.proactiveinvestors.co.uk/companies/news/1032093/mining-101-strip-ratio-the-key-to-open-pit-mining-1032093.html
Open Pit Mining Terminology, Break-even Stripping Ratio | Lecture notes Mining Engineering | Docsity. (n.d.). Retrieved March 24, 2025, from https://www.docsity.com/en/docs/open-pit-mining-terminology-break-even-stripping-ratio/11159907/
Stripping Ratios: What Are They and Why Are They Important? | INN. (n.d.). Retrieved March 24, 2025, from https://investingnews.com/daily/resource-investing/base-metals-investing/copper-investing/strip-ratio-western-copper-gold-nemaska-lithium/
