Lecture 4 Calculating Stripping Ratios for Area Strip Mines

7/29/2019 Lecture 4 Calculating Stripping Ratios for Area Strip Mines

1/11

Calculating Stripping Ratios forArea Strip Mines

Mnge 315

Dr. B. C. Paul spring 2003 revised 2010

Note These notes contain information considered commonknowledge to those familiar with Strip Mining. Books such as

Elements of Practical Coal Mining and SMEs SurfaceMining were specifically referenced


2/11

Stripping Ratio as a Fundamental

Ability to surface mine depends on the value ofthe ore being high enough to offset the cost ofOverburden Removal

Stripping Ratio determines relative amounts of eachto be moved

If coal generates a $5/ton earning andoverburden costs 45 cents per cubic yard tomove what is the maximum economicstripping ratio

$5.00/ 0.45 = just over 11 cubic yards per ton Called a Break Even Stripping Ratio


3/11

Complexities Warning

Units of Stripping Ratio depend on convenienceof the industry

Coal overburden is normally light and machines arelimited 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)


4/11

More Warnings

Geometry problems usually measurevolumes which convert to tons only if

density is the same for all materials Material may be rehandled

Get operating and geologic stripping ratios

Dont use 40 cents per cubic yard to figurestripping ratio off of geometry if you have 10%rehandle


5/11

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


6/11

Fixed and Variable Costs in Mining

Some mining costs are fixed (manyadministrative features for example)

Others are variable direct materialshandling 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


7/11

Simple Stripping RatiosBottom of a

Strip MineTop 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


8/11

The 2D Cross Section

Overburden

Area = Base * Height

Interest Item Slope Angle

Has no effect

Base for both coal and the overburden is the same

That means the base will cancel!

The problem is 1D

Only depends on

Thickness and density


9/11

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 forbituminous)

Area Strip Mine Geometry

Geologic Stripping Ratio no accounting forrehandle or less than 100% coal recovery


10/11

Your Turn

Use What You Have Learned AboutCalculating Stripping Ratios for Strip Mines

and Break Even Limits to Determine theSize of a Coal Reserve

Do Assignment #3


11/11

A Little Explanation

Very seldom is a coal seam the sameDistance from the surface always

Less overburden meansMore profit. Project worth is increased if you take the

Biggest profits first thus strip mines startShallow and work into deeper stuff.

Is it possible that eventually the coalWill get deep enough to exceed the break-evenStripping ratio of course!

In your homework you will look at a coal reserve start shallow and work until it isDeep enough you cant afford it anymore. Then youll tell me how far you could mine.

Documents

Lecture 4 Calculating Stripping Ratios for Area Strip Mines