OPTIMIZING THE NET PRESENT VALUE OF A PROJECT THROUGH MINE PLANNING

INTRODUCTION

Sensitivity Analysis for Mining Projects Evaluation
The scenario analysis option provided in Excel could bring another dimension to the mining project evaluation.

Geometry of the pit walls
Figure 1 identifies the terminology commonly used for pit walls of an open pit mine or quarry.

Open_pit_wall_terminology

PIT SLOPE TYPES
During the life of an open mine pit or a quarry, there are two types of pit walls, one final and the other temporary.

Final pit wall
This is the wall of the ultimate pit. It defines the economically exploitable limits of a deposit. Its envelope is determined during the optimization process of the pit. This process is based on technical parameters (geotechnical, equipment size, geometry of mineralization, etc.) and economic parameters (commodity prices, investments, operating costs, etc.). The resulting pit envelope must meet the criteria of profitability, safety and restoration conditions.

Temporary pit walls

This is a dynamic pit wall, still progressing towards its final position which is the final pit wall. Its envelope is defined during the mining planning. The temporary position of the excavation boundaries is based on economic criteria and aims at optimizing the project NPV. Although the operator can configure it freely, he must nevertheless respect the security and restoration criteria.
Safety criteria: any pit or quarry must ensure, at all times, the safety of people and equipment. The geometry of the walls for a pit being exploited is governed, in Quebec, by section 41 of the Regulation respecting occupational health and safety in mines. This Article covers, among other things, rock competence, drilling and blasting techniques, etc.

Restoration criteria: All operators must adhere to the restoration configuration. The final geometry of the walls of the excavation is governed, in Quebec, by Article 39 of the Regulation on Quarries and Sand Pits. This article deals, among other things, with the height of the benches, the width and frequency of the berms, etc.


MINE PLANNING

Mining planning role

Mining planning is used to establish the exploitation sequences over time, in terms of bank height, width and frequency of berms, push-backs, etc. of a deposit. The operator, with a view to maximizing the NPV (waste rock movement, sales objectives, grade control, etc.), has the flexibility to configure its operations, provided that it complies with the conditions of Articles 41 (in any time) and 39 (when reaching the final pit design).

Mining planning will allow this transition, from the temporary pit wall to the final pit wall, safely and at no additional cost
 


Example of mine planning
The following example illustrates that it is possible to tie bring the temporary pit wall to the final pit design for a mining project that has 5 phases of operation.

Ultimate pit

Figure 2 shows an ultimate pit designed with the Vulcan modelling software.


Final pit

Mine planning approach

The block model shows that the mineralization consists of subhorizontal mineralized zones, 5 to 8 m thick. These zones are separated by waste zones that sometimes exceed 10 m at the centre of the ore body and to the east.

On the one hand, the mine planning will attempt to mine the deposit using 5 m benches in mineralized zones less than 10 m thick, in order to improve selectivity. On the other hand, it will try to mine the thickest zones with 10 m benches, in order to improve the productivity.

At the same time mine planning will have to ensure that the walls of the excavation comply, at all times, with health and safety rules. It must also ensure that restoration conditions are met once arrived at the ultimate pit design.
The long-term mining sequences are presented by the following five phases.
 


Phase 1
Figure 3 shows the position of the pit walls at the end of phase 1.

Mine planning phase 1

Phase 2
Figure 4 shows the position of the pit walls at the end of phase 2.

Mine planning phase 2

Phase 3
Figure 5 shows the position of the pit walls at the end of phase 3.

Mine planning phase 3

Phase 4
Figure 6 shows the position of the pit walls at the end of phase 4.

Mine planning phase 4

Phase 5
Figure 7 shows the position of the pit walls at the end of phase 5.

Mine planning phase 5

CONCLUSION

As long as an open pit or quarry grows inside the ultimate pit shell, the operator can configure the geometry of the excavation to maximize the net present value of the operation, provided that it respects the following two constraints:

  • Ensure, at all times, the stability of the walls and the safety of people and equipment (Section 41 of the Regulation respecting occupational health and safety in mines, Chapter S-2.1, r 14).
  • Ensure, at the end of the operation, that the restoration conditions will be respected (Section 39 of the Quarry and Sand Pit Regulations, Chapter Q-2, r.7).

CONTACT US

Gesmine Inc.
Gestion des ressources minérales
5532, Avenue Philippe
St-Hubert, Quebec  J3Y 6H2
Canada
 

Phone: 
1-450-812-2280



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