a. Types and definitions
b. What does a SR measure ?
c. What to use the SR for ?
On Terminology
1. Overburden O/B
2. Coal / ore
3. Interburden (or parting), I/B
4. Waste
Sometimes:
Waste = O/B + I/B
(look at the context)
Waste (O/B + I/B) needs to be removed to get access to coal / ore
What Is Stripping Ratio ?
a. Measures relative deposit depth
Q: How important is depth in surface mining?
* Consider Chuquicamata at 850 m depth, SR = 0.5
* Consider Capital Quarries , Rolla, at 25 m depth, SR = 0.5
b. To surface mine we need to remove some waste in addition to ore / coal
c. SR measures how much waste needs to be removed so that a unit of ore/coal can be mined
* This means “relative depth”
Geologists vs. Miners
a. How do the Geologists measure SR?
* Look @ sections and measure thickness of waste and ore / coal
* Express SR the ratio of thickness, eg: m/m, ft/ft
b. How do the miners differ
* We know that the slopes are not vertical
* We calculate the amount of waste that needs to be removed to get an amount of ore/coal
Miners Only ….
a. SR measures the amount of waste that needs to be removed to get an amount of ore, just as we mine it.
Note the “amount”!
b. This will differ depending on time span under consideration
* Initial studies (feasibility) – overall stripping ratio
* Current month – incremental stripping ratio
What is an “Amount”?
Depends…..
a. Coal miners measure waste in BCM, or bcy, and coal in tonnes or tons
* Thus in coal SR is the number of BCM of waste that needs to be removed to uncover 1 t of coal
* Some applies in other bedded deposits
b. Ore miners measure both waste and ore in tons or tonnes. Definition of ore/waste changes!
* Thus in ore SR is the number of tonnes of waste that need to be removed to uncover one tonne of ore
More Complications
a. Units in use differ between countries
b. Each country uses its own units to define stripping ratios
* Consider McIntyre Coal: BCM/long tons
* What is a long ton? Some coal & iron ore trades internationally in long tons!
* Consider Bingham Canyon: tons / tons
* Consider Rheinbraun: BCM/ tonnes
Uses of Stripping Ratio
a. Definition of pit limits
* At what SR can we still make money
* Unit value of ore minus cost of ore AND waste mining gives us profit or loss estimate
b. Production scheduling
* Mine the areas with low SR first
* To maximize cash flow
c. Definition of mine feasibility / profitability
* Rough economics if no better data available
Incremental or Overall SR ?
a. Most evaluations / plans are done based on incremental stripping ratio
* The initial money was sunk already
* Of concern is what it would take to get that extra tons
b. Overall SR used in early evaluations of mines
c. Overall SR is normally “capitalized”
* What does that mean?
* Does the Taxman have something to do here?
“MOTHERHOOD STATEMENT”
If you work in a mine, ALWAYS use incremental stripping ratio
Use overall stripping ratio only on capital projects
SURFACE OR UNDERGROUND ?
a. Which type of mining is better for your deposit?
b. If it is surface mining, when do you stop mining?
A Brief Summary
a. Surface mining is much cheaper
* may result in more profit, but……
b. Surface mining requires (most often) waste removal
* waste removal cost money and lowers profit
c. Break even point exists at which both surface and u/g mining are equally (un)profitable
* Remember the “Motherhood” statements?
d. Stripping ratio allows definition of this “breakeven” point
Switch to underground mining if your stripping ratio is more than BESR
a. May or may not be economically feasible
* Consider value of ore
b. Major expense/effort is required to switch
* Major capital expenditure: how to handle this?
* Is there enough ore left to pay for the switch?
* Do we have expertise to mine underground?
* Are there other opportunities to make more $$?
c. A decision may be made not to mine underground even if it would be profitable
Break Even Point: Discussion
a. Cost of mining – you control it!
* Relative difference between underground, surface
* Relative difference between ore, waste
b. Economics usually determined by:
* Geology of the deposit
* Compare: Escondida mine, Chile vs. a Kansas quarry
* Efficiency and effectiveness of mining methods
* Economic considerations:
Interest rates, discount rates, other