Grade Cut Off – Surface Mining

  • What is it?
  • What is is used for?
  • Grade cut-off curves & their uses.

Grade Cut-Off

Or cutoff grade?
A pessimist

  • The grade at which the mineral resource can no longer be processed at profit, or….

An optimist

  • 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?

“Sidetracking”

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

Grade Cut-Off

Grade Cut Off 300x172 Grade Cut Off   Surface Mining

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?

What Goes To The Mill 300x164 Grade Cut Off   Surface Mining

Grade Cut-Off vs. Stripping Ratio

Grade Cut Off vs. Stripping Ratio 300x155 Grade Cut Off   Surface Mining

Grade – Tonnage Curves

What is that?

Grade – Tonnage Curves

Grade – Tonnage Curves 300x181 Grade Cut Off   Surface Mining

Another Example

Another Example 300x186 Grade Cut Off   Surface Mining

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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