The surface ore deposits are extracted using open pit mines until this operation is economically convenient. When the economical ore begins to deepen the underground mining becomes more profitable. Then the operation of the mine goes through a transition from open pit to underground.

Nowadays, this transition is implemented in mines such as Chuquicamata (Chile), in the future, a larger number of mines will change their exploitation method from open pit to underground mine.

Historically, the beginning of the transition was decided analysing the operational costs of open pit and underground mines. Later, this methodology was improved valuing the block model with differential costs and applying algorithms for optimizing open pit design. However, both methodologies do not consider the investments and the value of money over time.

The aim of this paper is to describe, with a comprehensive view, the main aspects and criteria to be considered in making this transition, and a meth-odology for defining the economic limit between the two exploitation methods.

The proposed methodology starts defining the economical boundary of open pit through marginal analysis of the push-backs or phases of the mine plan, which includes all the push-backs of the open pit and considering the underground mine exploitation like another phase.

Once the open pit optimum is defined, we must design the underground mining method feasible for the ore deposit and its main parameters (in the case of Bc: The level of sinking, the maximum column height and exploita-tion boundary).

When the economic limit and the period to which the open pit operation must be reached is defined, you must verify that the start of underground production matches the term of the open pit production, allowing continuity of supply to plant. This check should be performed putting together an implementation plan of the underground mine, which should include all activities necessary to put in underground mine production.

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A methodology to define transition from open pit to underground operation of a mine

Antonio Navarro, Vctor Encina and Leonardo Fenner. JRI Ingeniera S.A., Chile

INTRODUCTION

The mineralization which outcrops ore deposits close to surface are usually exploited by open pit

(OP) method and its limits are defined by the last phase that provides economically NPV. When

these deposits contain mineralization at depth beyond the economic limit of the pit, the natural

continuation of the exploitation is by underground mining (UG).

The previous situation involves making a transition to move from one open pit mine to an

underground mine. This situation has been presented in significant deposits, and this challenge

must be overcome by currently operating pits.

Historically, the boundary between open pit and underground mining was defined using the rate

of change of methods, which basically compares operating costs of both methods. Later, thanks to

the evolution software, some authors proposed an equivalent calculation valuing block models

with differential costs and applying optimisation algorithms to the pits. Both methods did not

consider the cost or investment of money over time.

The aim of this paper is to describe conceptually the main subject and criteria to be considered for

studying this transition, and to find a methodology to define the economic limit between open pit

mine and underground mine.

METHODOLOGY

The proposed methodology to define the economic limit between both open pit mines and

underground mines is the marginal analysis phase. This analysis is determined by the final limit of

the pit and the underground mining is considered as another phase.

For incorporating underground mining it is necessary to define that the method is feasible for

operating the reservoir and its main parameters (for example for Block Caving it would be

necessary to define the level of collapse, the maximum column height and the operating limits).

The methodology starts determining the total NPV considering the latest economic phases of the

pit, and a preliminary design for the underground mine adopting a height of the column measure

from the bottom of the pit. Subsequently, the calculation of the total NPV is repeated, moving

upstream the floor of the pit and the floor of underground mine maintaining constant increasing

the column height from the floor of the new phase pit.

The calculation is repeated to determine the highest total NPV, which determines the last phase to

exploit as an open pit and the time when the underground mine should be begin to exploit.

Due to the high levels of OP production, the underground mine should be consider massive

method of exploitation as Block Caving or Sub Level Caving. The level of production of these

methods is similar to the open pit mines.

For other different types of mineral deposits, such as ore veins, your analysis of the transition can

be made in a similar way, i.e. seeking higher total VPN the both methods. In these cases, methods

exist different underground exploitation and this methods can be coexistent whit open pit, so that

the underground mine has no restrictions for starting your production when end the open pit

production.

Once the economical limit is determined, the underground mine master plan should be verify to

ensure that the production of the underground mine begin with the end of the open pit exploitation

(see Figure 4). This check should be done considering an underground mine plan, which include

all the activities necessary for the development of the underground mine production.

The plan should also consider the detailed mining ramp up of underground mine production, and

the gradual decline in the production of the pit to ensure the continuity of mineral supply to the

plant.

Finally, the operational interference between construction of the underground mine and the open

pit operation should be review for safety assurance.

According to the methodology proposed to carry out the transition from an open pit (OP) to an

underground operation (UG), should be perform the following planning activities:

Marginal analysis phase OP

This analysis provides the economical limit of the open pit operation determining the NPV of each

phase and the depth of the exploitable pit. This analysis allows obtain the last period of

exploitation, which in turn determines the period that underground mining will start production.

In Figure 1, below shows an example of marginal analysis phase.

Figure 1 NPV Open Pit mining

Figure 1 shows a decrease of NPV at phase N. Then, the end of the open pit should be the phase N-

1.

Preliminary design of underground mining

The project should include an underground mining ore zone considered in the final stages of open

pit, and an underground mine to compete with. In this area, if the method is the Block Caving, mine

design should be consider:

The level of the bottom of the first undercut is defined based on a global analysis study of the

underground mine.

The maximum column height is defined by the bottom of the final pit and by the first undercut

level.

A production plan considering the production rate of OP should be develop to estimate the

capacity of the process plant.

The plan of underground mine construction including the access, the ventilation levels, the

haulage levels, and infrastructure for services should be developed.

The capital expenditures (CAPEX) and operating expense (OPEX) and NPV of underground

mines should be calculated.

And the Total NPV of open pit and underground mine should be calculated.

In Figure 2, it is shown schematically the column heights of underground mine for the different

final phase of the OP.

Figure 2 Preliminary design of underground mine level

Determination of economic limit and time of transition

The NPV Total (OP + UG) is calculated considering different column heights changing the final

phase of the open pit. From this calculation is obtained a maximum value of NPV. For example, in

Figure 3 it is shown that the highest Total NPV is obtained for the Phase N-2 of the OP. Then, this

phase is defined as the economic limit for the open pit operation. At this time the underground

mine should be operating.

Figure 3 NPV Open Pit + Underground mining

Project Execution Plan

This plan should be consider all activities and milestones for assure that underground mine

production begins with the ending open pit production. This plan includes the following:

Project Description.

Strategy Execution: Awards, Pre-feasibility, Feasibility, DIA, Early Works, Contracts

Strategy.

Major Projects in Execution.

Organisation for implementation phase.

Master Plan. The helpfulness of this plan consists of verifying the feasibility of coincidence of

the end of open pit production with the beginning of underground mine production. The

Figure 4 shows a general master plan.

Figure 4 Underground Mine Master Plan

In the case that some milestone delays, it is possible to develop Fast Track Pans to reduce losses of

NPV due to send less minerals to process plant with less, or to process mineral with lower grades or

higher costs of exploitation.

Mine Planning

A detailed Mine Planning should be develop considering the time up to production rate (ramp

up) for the underground mine and the time during the decrease of the production at the ended of

the open pit. This program allows identifying any difficulties arising in this period.

RESULTS AND DISCUSSION

The methodology presented has made some assumptions that can be reviewed or changed

according to the particular case being studied:

In estimating the production plan of the UG, it can be assumed that the reserves that are

competing with the pit are mined in the same period as this.

For this production plan, in addition, these reserves could be affected by dilution, such as in the

case study.

The first panel of the UG was defined with a fixed transport layer and a column height that

varied, but this may be limited to a maximum, forcing to vary the height of the transport layer.

After the economic limit and the transition period, it is recommended to make a detailed plan

with the Ramp up the UG and the end of production of the OP. This ramp-up period should be

maintained at full capacity in the plant feed.

CONCLUSION

Depending on the complexity of the operations facing the transition, it may be advisable to treat it

as a special project whose target is to:

Ensure that the production in this period will generate alternative or contingency plans.

Coordinate the closing of the OP and the opening of the UG, generating the conversion plans of

staff, infrastructure and other disarmament.

Study and resolve potential interference.

Update the determination of economic threshold for transition and implementation plans of the

UG in its engineering stages.

REFERENCES

Pea, J.C. (2003) Transicin Minera Rajo Abierto a Minera Subterrnea, XIII Simposium de Ingeniera de Mina.

Andrs A. Solar D., Enrique Rubio E. & Alexandra Newman (2010) Transicin rajo subterrnea, una oportunidad en el valor a largo plazo, Taller de planificacin minera Delphos 2010, Universidad de

Chile.