Calculating Stripping Ratios for Area Strip Mines Mnge 315

1. Calculating Stripping Ratios for Area Strip MinesMnge 315 ©Dr. B. C. Paul spring 2003 revised 2010 Note – These notes contain information considered common knowledge to those familiar with Strip Mining. Books such as “Elements of Practical Coal Mining” and SME’s “Surface Mining” were specifically referenced

2. Stripping Ratio as a Fundamental • Ability to surface mine depends on the value of the ore being high enough to offset the cost of Overburden Removal • Stripping Ratio determines relative amounts of each to be moved • If coal generates a $5/ton earning and overburden costs 45 cents per cubic yard to move – what is the maximum economic stripping ratio • $5.00/ 0.45 = just over 11 cubic yards per ton • Called a “Break Even Stripping Ratio”

3. Complexities Warning • Units of Stripping Ratio depend on convenience of the industry • Coal overburden is normally light and machines are limited by volume in cubic yards • Coal is sold by the ton • Cubic Yards/ton makes sense • Hard Rock Metal • Both ore and overburden are trucked • Both are heavy and trucks are usually weight limited • Tons/ton makes sense (unit-less)

4. More Warnings • Geometry problems usually measure volumes which convert to tons only if density is the same for all materials • Material may be rehandled • Get operating and geologic stripping ratios • Don’t use 40 cents per cubic yard to figure stripping ratio off of geometry if you have 10% rehandle

5. Fixed and Variable Costs • Example of the Car • If you have a car • Insurance • Licensing Fees • Storage or Parking Space • These Costs constant per unit of time regardless of use of car • Variable Costs • Gas • Tires • Oil Changes • Largely Dependent on mileage you drive

6. Fixed and Variable Costs in Mining • Some mining costs are fixed (many administrative features for example) • Others are variable – direct materials handling cost • What goes into Break Even Stripping Ratio • Direct materials costs go into Break Even • Fixed Costs are charged to the average stripping ratio you mine over the time

7. Simple Stripping Ratios Bottom of a Strip Mine Top of a Strip Mine For a pit 1 mile long and 160 ft across, how significantly Different is the length at the top And bottom of the pit? Volumes are Length * Width * Height Ratio is A/B What happens if the length term in Both A and B is the same number It cancels – Long strip pits can be Analyzed as 2D problems!!! Life is Good

8. The 2D Cross Section Area = Base * Height Overburden Interest Item – Slope Angle Has no effect The problem is 1D Only depends on Thickness and density Base for both coal and the overburden is the same That means the base will cancel!

9. The Coal Area Strip Mine Formula • Strip Ratio = OB Thickness (in ft) * 11.11/ Coal thickness in inches • Assumes coal is 80 lbs/ft^3 (good for bituminous) • Area Strip Mine Geometry • Geologic Stripping Ratio – no accounting for rehandle or less than 100% coal recovery

10. Your Turn • Use What You Have Learned About Calculating Stripping Ratios for Strip Mines and Break Even Limits to Determine the Size of a Coal Reserve • Do Assignment #3

11. A Little Explanation Very seldom is a coal seam the same Distance from the surface always Less overburden means More profit. Project worth is increased if you take the Biggest profits first – thus strip mines start Shallow and work into deeper stuff. Is it possible that eventually the coal Will get deep enough to exceed the break-even Stripping ratio – of course! In your homework you will look at a coal reserve – start shallow and work until it is Deep enough you can’t afford it anymore. Then you’ll tell me how far you could mine.