1. Planning Surface Mines Mnge 315 ©Dr. B. C. Paul 2003

2. Steps in Planning Determine what it is that you might mine Determine what it is that you might mine –Explore for and define the limits of your deposit Geology Classes and working with Geologists will help you to do this Geology Classes and working with Geologists will help you to do this –Build a model of your mineralization We will look at working with two of the most common types of mineralization models We will look at working with two of the most common types of mineralization models –We’ll dodge how we got them (Geostatistics Course that could be one of your electives)

3. Steps in Planning In fact you have to make some general guesses about how you might mine it – including surface or underground In fact you have to make some general guesses about how you might mine it – including surface or underground Since God and this course both chose surface mining we’ll assume for now it is a surface method Since God and this course both chose surface mining we’ll assume for now it is a surface method Determine how much of your material is ultimately minable by surface methods Determine how much of your material is ultimately minable by surface methods –Called finding the ultimate pit You will get to do some of this in this class - Yippi!!! You will get to do some of this in this class - Yippi!!!

4. Steps in Planning With minable reserve defined With minable reserve defined –Determine your mine size and life Involves economics beyond this class Involves economics beyond this class –An advance surface mining topic (possible elective) –Select a mining sequence and schedule to develop out to your pit limits Our class examples will be with surface methods that have fixed advance direction Our class examples will be with surface methods that have fixed advance direction For some open pits this is an Operations Research Exercise (possibly in an OR elective) For some open pits this is an Operations Research Exercise (possibly in an OR elective) –Tell you about it but not train you to do it

5. Steps in Planning Select an Equipment Fleet Capable of Meeting the Development Plan Select an Equipment Fleet Capable of Meeting the Development Plan –We will do at least the Truck-Shovel parts of that in this class Design mine plants and support infrastructure to meet mines needs Design mine plants and support infrastructure to meet mines needs –Dr. Mohanty’s mineral processing plants –Possibly some stuff from Material Handling –(if this were U.G. might need ventilation but that is the evil competition so we don’t talk about that here)

6. Lets Try an Example of Finding the Ultimate Pit Angle 45 degrees Thickness 20 ft Angle 10 Degrees Where Did This Thing Come From? From the projector. Actually a series of cross sections of the deposit is One of the mineralization models that exploration Team may generate

7. My Objective Define how far down the dip the mine can advance Before the SR becomes uneconomic

8. My Technique Look at each increment of advance for its stripping ratio Until the ratio becomes uneconomic. Last economic increment Defines the ultimate pit limit

9. Selecting my Increment Based on convenience or a guess About production over some Period of time I pick a 50 ft Increment down the dip

10. Investigating My Geometry Ore is a 50 X 20 ft Block Rectangle Area = Length * Width 1000 Square ft. (Ok it’s a parallelogram because haven’t Yet dealt with pit slope) If I have 1 ft unit thickness and phosphate ore is 170 lbs/cubic ft 170,000 lbs / 2,000 lbs/ton = 85 Tons If my phosphate ore makes $27/ton the block is worth $2,295

11. Ok Where Did That Come From How did I know the section was 1 ft thick How did I know the section was 1 ft thick Convenience – If I had worked in cubic yards it would have been 1 yard thick Convenience – If I had worked in cubic yards it would have been 1 yard thick Where did that 170 lbs/cubic foot rock density come from Where did that 170 lbs/cubic foot rock density come from If you’ve sampled the ore you know its density. Can also use typical densities from Surface Mining or Cat Handbooks If you’ve sampled the ore you know its density. Can also use typical densities from Surface Mining or Cat Handbooks Where did the $27/ton value come from Where did the $27/ton value come from If you’ve sampled your ore your probably know how much mineral it contains and what the mineral sells for If you’ve sampled your ore your probably know how much mineral it contains and what the mineral sells for If you’ve taken Dr. Mohanty’s Mineral Processing you probably know about what it costs to process If you’ve taken Dr. Mohanty’s Mineral Processing you probably know about what it costs to process Have an idea what it costs to run equipment to mine the ore Have an idea what it costs to run equipment to mine the ore

12. Now for the Overburden 45º Overburden Block is A Trapezoid Height Base 1 Base 2 Area = (Base 1 + Base 2)/2 * Height

13. The Dreaded Pit Slope Question Enters My sketch implies that Footwall of the Mineralization will be The final slope on that side Will the Footwall rock stand at 45º ? This information is obtained from rock strength And fracture patterns plotted on stereo nets. Wait for Dr. Kreoger’s Rock Mechanics For Our Example We Will Say That Dr. Kroeger Gave Us an Ok

14. Next We Worry About the Highwall Let us suppose that the highwall Slope is limited to 35º

15. Fun With Trapezoid Geometry 45º 35º 100º I’m setting this up to get the Height of my Trapezoid

16. Continuing with the Excitement 35º 55º 50 ft 10º 90º Height of Trapezoid Side Adjacent = Hypotenuse * Cos (ß) Height of Trapazoid = 50 * cos (10º) 49.24 ft

17. For My Next Mathemagic Trick 45º 35º Get the Increase in slope length for each 50 ft of advance down dip 49.24 ft 45º 90º 49.24 ft

18. Chugging on those Increments Base 1 = 0 Base 2 = 49.24 Height = 49.24 Plug and Chug (49.24 + 0)/2 * 49.24 = 1212.31 ft^2 Assume 1 ft thick and 152 lbs/ft^3 for Rock density 31.52 Tons First Stripping Ratio 31.52/ 85 = 0.371:1

19. Next Increment New Trapezoid Base 1 = 49.24 Base 2 = 98.48 Height = 49.24 Plug and Chug Area = (49.24 + 98.48)/2 * 49.24 3636.87 ft^2 at one ft thickness and 152 lb/ft^3 276.4 Tons Stripping Ratio 3.25:1 = 276.4/ 85

20. So How Long Do I Keep Doing This? Repetitive task is obviously a candidate for spreadsheet work Repetitive task is obviously a candidate for spreadsheet work How do I know when ultimate pit is reached How do I know when ultimate pit is reached Need a cost for O.B. removal so we can get the break even stripping ratio so that when the stripping ratio reaches that limit we can call it our ultimate pit Need a cost for O.B. removal so we can get the break even stripping ratio so that when the stripping ratio reaches that limit we can call it our ultimate pit

21. Overburden Removal Cost Let the OB removal cost be $1.2/ton Let the OB removal cost be $1.2/ton –In practice this would be an estimate of the cost based on operations similar to the one you are considering Break Even Stripping Ratio is Break Even Stripping Ratio is –$27/ 1.2 = 22.5 to 1 To determine the ultimate pit we simply keep calculating increments till we reach a 22.5 to 1 ratio To determine the ultimate pit we simply keep calculating increments till we reach a 22.5 to 1 ratio

22. Warning on Average and Incremental Stripping Ratio Break-Even Stripping Ratios are intended to prevent you from mining material where you will loose money Break-Even Stripping Ratios are intended to prevent you from mining material where you will loose money –When you start loosing money you stop –When the next increment of material looses money you stop Average Stripping Ratio for the entire pit will still be less than break-even Average Stripping Ratio for the entire pit will still be less than break-even –We do need to make some money and cover our indirect and other costs –DO NOT keep going until the average stripping ratio equals break even –If you made money would you keep on going till you lost it all?

23. Your Assignment Use the Conepit Spread Sheet to find the ultimate pit limits for the following ore body Use the Conepit Spread Sheet to find the ultimate pit limits for the following ore body –The ore body is a vertical cylinder 700 feet in diameter –The Ore weight is 4700 lb/cubic yard –The Overburden weight is 4200 lb/cubic yard –The bench height will be 35 feet –The Over-all slope will stand at 42 degrees –The break even stripping ratio is 5:1 Use Conepit to determine how deep the ore body can be mined (ie the ultimate pit) Use Conepit to determine how deep the ore body can be mined (ie the ultimate pit)

24. The Spread Sheet Input Fields are yellow Critical Output Fields are Red

25. The Inputs Present Depth is the Depth the pit has reached (obviously to start with That will be 0) The increase in depth is Your bench height Pit can only go down in Integer numbers of benches (ie 35 ft, 70 ft, 105 ft etc)

26. Watching the Output Critical answers are in the Incremental stripping ratio. When weight based SR Reaches 5:1 the pit is done

27. The Planning Function Continued Ultimate pits can be calculated using a variety of methods reviewed Ultimate pits can be calculated using a variety of methods reviewed –Assignment used an analytical model In the slides example, Pit volumes are calculated by doing average end area methods on each of the cross-sections In the slides example, Pit volumes are calculated by doing average end area methods on each of the cross-sections The plan view of the pit is prepared by smoothing between the pit advance from each of the cross-sections The plan view of the pit is prepared by smoothing between the pit advance from each of the cross-sections

28. The Geometry Freak-Out I had uniform thick seams with a constant dip, with a land surface with a uniform tilt I had uniform thick seams with a constant dip, with a land surface with a uniform tilt –The geometry probably still brings back bad memories from your math classes Real world geometry is often too hard to work out mathematically like in the example Real world geometry is often too hard to work out mathematically like in the example Real world – Cross Sections of the potential pit area are prepared from geologic data Real world – Cross Sections of the potential pit area are prepared from geologic data

29. Digitizers to the Rescue You trace around the ore and waste Increments with a digitizer (could be An Autocad Area feature) and the Area will be computed digitally.

30. Where We Are Now Note that we used our model of our mineralized ground and worked out to our economic ultimate pit limit. Note that we used our model of our mineralized ground and worked out to our economic ultimate pit limit. –With this done I have a minable tonnage I will use this information to set a crude idea of how rapidly I will mine I will use this information to set a crude idea of how rapidly I will mine –I will use this info to pick a logical increment of pit advance planning I will then plan my mine advance sequence to the ultimate pit limit I will then plan my mine advance sequence to the ultimate pit limit

31. The Advance Sequence Problem In my example I have an open pit which by design must advance down dip In my example I have an open pit which by design must advance down dip –I have relatively little freedom in my advance Other open pit scenarios such as the porphyry copper deposit have much greater freedom in sequencing Other open pit scenarios such as the porphyry copper deposit have much greater freedom in sequencing –We’ll look at some of the more powerful tools for that situation

32. Why is Sequencing Such a Big Deal? Some may have had engineering economics (Engr 361) and know about discounted cash flow analysis Some may have had engineering economics (Engr 361) and know about discounted cash flow analysis For financial planning you have a series of expenses and earnings that will come in over the life of the mine For financial planning you have a series of expenses and earnings that will come in over the life of the mine –These cash flows can be put on a time line

33. The Discounted Cash Flow Engine People value money sooner more than money later People value money sooner more than money later –If you get it now you can earn interest –Inflation may diminish the buying power of future dollars –The further it is in the future the greater the chance that you might not get it People develop weighting factors for money depending on how soon they get it People develop weighting factors for money depending on how soon they get it Add up the weighted dollars for NPV Add up the weighted dollars for NPV

34. NPV Effects Big NPVs are good Big NPVs are good Make your cash flow have higher NPV by putting off expenses (so the money will have a low weighting factor) and move your money making forward in time (so the good money will have high weighting factor) Make your cash flow have higher NPV by putting off expenses (so the money will have a low weighting factor) and move your money making forward in time (so the good money will have high weighting factor)

35. How to Make it Happen In case of the open pit that can expand out to limit in any direction In case of the open pit that can expand out to limit in any direction Go after ore with a low stripping ratio first – its puts off high stripping ratio costs till in the future Go after ore with a low stripping ratio first – its puts off high stripping ratio costs till in the future Go after high grade ore first – it moves your biggest profits forward in time Go after high grade ore first – it moves your biggest profits forward in time

36. How Not to Sequence Pay Dirt Slice #1 Slice #2 Slice #3 Slice #4 Slice #5 Slice #6 Slice #7 Slice #8

37. Why Did That Not Work? We took all our stripping expenses and took them up front We took all our stripping expenses and took them up front We delayed making any money till as late in the game as possible We delayed making any money till as late in the game as possible The order we go it does make a difference The order we go it does make a difference We need more powerful models to help us plan our elliptical type open pits We need more powerful models to help us plan our elliptical type open pits –We’ll review the computer aided models next