Grade Cut Off – Surface Mining
- What is it?
- What is is used for?
- Grade cut-off curves & their uses.
Or cutoff grade?
- The grade at which the mineral resource can no longer be processed at profit, or….
- The minimum grade at which the ore can be processed at profit.
A break-even grade?
Cut-Off Grade Calculation
Plot of grade vs. net ore value is a straight line
Need to calculate the net ore value at two points and
- plot the grade-profit line
- define the equation of this line
Example: consider a copper deposit…..
Problem (Hustrulid & Kuchta, pp. 373 & following)
Calculate net profit at 0.55 and 0.35% Cu
Consider saleable copper obtained from 1 t of ore and define its value
Consider technology: recoveries at all stages lower the amount of copper
- Mill, concentrator, smelter, refinery, other
Consider all costs:
- Mining, milling, concentrating, smelting, refining, transport, sales
Do we want to include overheads? Fixed costs? Main office costs?
Not always do we produce the final product
- Sell a copper concentrate to Japan? Trona mine?
But we need to know “cash value” of product for BEG
- Cash value: all costs, fixed and variable, incurred in cash
- Include cost of refining smelting even if done by others
Sometimes we have by-products
- Example Bingham Canyon: Cu, Au, Ag, +++
- Related credits deducted from gross cash costs
Steps In Defining Each Point…
- Define the profit (loss) per tonne of ore
- Do the same for another ore grade
- Construct a net value vs. grade plot
- Determine the cut-off grade
Cut-off grade is where the line crosses “0 profit” line
How does the stripping ratio fits here?
What Goes To The Mill?
Grade Cut-Off vs. Stripping Ratio
Grade – Tonnage Curves
What is that?
Grade – Tonnage Curves
Define the relation between tons of ore and the grade cut-off; usually:
The lower the grade, the higher the tonnage
The higher the grade, the lower the tonnage
Consider accuracy of grade / tons estimates
Allow for optimization of cut-off grades
Higher grade means small mine & mill, little product
Low grade means more capital cost but more product
Remember: cut-off grade is a function of changing economics!
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