1/16 So where was I going with #mining block models in my previous thread? I can now show you how we use them to optimise for open pit mining and approximate a good UG mine plan as well. To begin, the decision to go open pit or UG very much depends on the shape & size of the ore
2/16 body, and the decision is usually intuitive. Sometimes a combination of OP & UG methods makes more sense. Consider the following theoretical block model representing a rich, vertical ore body. Assume this is a 2D environment, not a 3D one, with coloured units of value. 
3/16 Now with our avatar safely removed w/o any major social justice fallout, we can get on with the open pit mining. 😉 Let's assume that the cost of mining is 1 unit, processing cost 0, what would the value be mining the 1st level by Open Pit methods?
4/16 So to mine the following benches we will assume a slope angle of 45º should be maintained. Again, ignoring the cost of processing and assuming every block costs 1 unit to mine, doing a little arithmetic shows us that in this e.g. we generate 1 unit of profit on the 2nd bench
5/16 Of course we are also assuming that no ramp accesses are required for equipment either! Moving down through the next two benches in a similar way we can calculate the incremental value on each bench by evaluating the cost of material required to be moved to access each block
6/16 The question whether to stop mining on Bench 3 can only be solved only if you look at the incremental and cumulative net value of every ore block in the model. There may be richer blocks lower down that can cover the stripping cost to get to them providing >overall value.
7/16 So Bench 6 is the maximum pit we can mine without actually incurring a cumulative loss, but mining to Bench/Pit 3 is the OPTIMUM (size) pit providing the best VALUE. This is how the "Lerch-Grossman" 3D pit optimization software works in Whittle or Datamine NPV scheduler.
8/16 Say our geotechnical engineers have completed their drilling program and determined that the rock conditions are excellent and the overall pit slope can be increased from 45º to 63º (or 2-1). IOW, we can mine 2 benches down before having to step out horizontally. See figs
9/16 In summary, a steeper slope has the effect of DEEPENING any pit and also a very significant improvement in value. Naturally should be offset against a suitable safety factor and your appetite for risk! Preferentially it is better for a slope to fail after the pit is mined😆
10/16 Optimization software calls each of the bench steps a "theoretical pit" and each pit comes with its own tonnes, grades, metal, revenues, costs profits etc. Cumulative profit is typically plotted on a pit-by-pit graph, with the top of the curve showing us the optimal pit:
11/16 Okay so Pit 10 is our optimum pit: but how do we mine it? Sequence is everything when it comes to #mining #Economics. "Best Case" depletion and "Worst Case" depletion are the two methods we refer to. Why & what is the difference?
12/16 The thing is the Best Case scenario is that the mining widths are impossibly narrow to get equipment onto for mining. Worst Case scenario has the challenge of mining impossibly large amounts of waste early in the schedule means less money is made early on.
13/16 In reality, we can avoid these best/worst case challenges by mining what we call "pushbacks" of "phased pit" mining, where we choose a pushback or mining "cut" representing a design inside the optimum shell that we mine to first before mining to the optimal limit.
14/16 Having selected the optimal pit and push backs (where needed) using the block model, the next step would be to actually design the Cuts to be practical i.e. to include ramps, inter ramp slope angles, berms etc. in them These practical only approximate the optimum shells and
15/16 now need to be re-evaluated and scheduled against the block model once again. The final result is the only way to make a mine extraction sequence practical, and the results from this work give you your reserve which also includes dilution and ore loss factors. The final
16/16 challenge is to actually execute on the plan in the field, but that is another whole topic on its own, which includes equipment selection, short & medium term scheduling, and most importantly dealing with a workforce.
@threadreaderapp unroll on #Pit #Optimisation
