Stripping ratio and pit limit

Stripping ratio and pit limit

G.G.Yowa
(July, 2017)

Senior Instructor, Mining Eng. Dept., PNG University of Technology. PNG

      1.    Overview

Extraction of mineral commodity that involves the removal of waste overburden prior to extraction of valuable ore involves determining the stripping ratio. Stripping ratio is expressed broadly as the ratio of waste to ore. It’s simply calculating how much waste has to be removed to expose how much of ore to be mined. Stripping ratio determines where the pit limit will be. 

There are basically three (3) forms of stripping ratios. The overall stripping ratio (average stripping ratio), the maximum allowable stripping (break even stripping ratio), and the instantaneous stripping ratio (incremental stripping ratio).
Fig1. Section of a proposed pit

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The units for stripping ratios can vary and usually depends on the geological properties of the deposit and the commodity mined. It can be expressed in tonnes (t), or cubic meters (m3). Usually for coal, cubic meters (m3) or tonnes (t) are used. However, for hard rock mines where the geology is usually complex with the presence of various rock types, tonnage (t) is usually preferred.

      2.    Instantaneous (incremental) stripping ratio

Instantaneous stripping ratio (SRi) is determined for each bench cut or each cut back (push back). This is purely a physical ratio of waste to ore per bench.


 

[Note: Equation 1 is expressed in tonnes (t), can also be expressed in volume (m3)]

      3.    Overall (average) stripping ratio

Overall stripping ratio (SRo) is the progressive or cumulative ratio of benches in consideration. For instance the overall stripping ratio of two (2) benches will be the ratio of sum of waste to ore.  This is purely a physical ratio of total waste to total ore.

[Note: Equation 2 is expressed in tonnes (t), can also be expressed in volume (m3)]

      4.    Maximum allowable (break-even)stripping ratio

The maximum allowable stripping ratio (SRmax) is significant because it is purely economic ratio and determines the pit limit at current economic scenario. Cost is applied to the physical waste and ore tonnages and that determines the profitability.

Where:

Ore value =           Ore tonnes(t) x Grade(g/t) x Price($/g) x Recovery(%)
 
Operating cost =   Ore unit cost ($/t) x Ore tonnes (t)
 

Stripping cost =     Waste unit cost ($/t) x Waste tonnes (t)

*The operating (production) cost includes the cost of mining, milling, general and administration (G&A). Some additional costs are considered like refining, selling, amortization, depreciation, and mostly depend on the commodity mined and the management decisions.

If SRmax >1, economical mine
If SRmax =1, break-even (marginal) mine
If SRmax <1, uneconomical mine

For instance, in figure 2 below, if the benches 1, 2, and 3 have SRmax >1 and the bench 4 has SRmax <1, then the pit limit will be at bench 3.

 
Fig2. Section of designed benches

The SRmax is sensitive to the commodity price, cost, mill recovery, and geological characteristics. When one of the parameters changes, the mine has to undergo optimisation process again to define the revised pit limit.

In addition, stripping costs may be either categorized as capital or operating expenditure. When capitalized, it is usually amortized and in some cases qualifies for tax rebates.

Stripping ratio and pit limit Stripping ratio and pit limit Reviewed by Gideon Yowa on July 26, 2017 Rating: 5

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