258
2016, Ortiz, 2017) were published in the last few years. These papers found optimum solutions for waste management throughout mixed integer programming (MIP), to solve optimum waste and tailings sequencing in long-term planning, considering multiple dump sites
and potential acid drainage issues (Li et al. 2012) and reducing haulage distances. However,
in all studies the dumps locations were already defined, and waste rock sequencing would
avoid material re-handling in the future.
This paper proposes a new approach for waste disposal called a Multi Stage Dumping
Sequence (MSDS), which allocates temporary waste dumps along the way to the final dump
destination. This method proposes a planned material re-handling at the beginning of operation, providing additional time to design, prepare and license the final waste dump site
(temporary dumps sites also may require license in certain cases, depending on its location),
besides reducing waste haulage cost in the first years. Although an increase in unity haulage
cost is considered to re-handle material from/to temporary dumps, the NPV increment in the
first years, by reducing haulage distances, is still potentially higher. The methodology uses an
integrated mine planning and waste sequencing and it is carried out an application to a real
case study to demonstrate its applicability.
2 METHODOLOGY
The proposed methodology consists of defining temporary dump sites between the open pit
and final dump location. Some variables must be observed to define the number of stages to
be used. Variables like haulage and material re-handle costs, distance from pit exit to final
dump site and temporary dumps, distance between temporary dumps and waste rate production are considered in this preliminary study.
For explanation purposes and to focus on the differences between conventional and MSDS
approaches let’s consider a fixed distance “X” from pit exit to final dump location as shown
in Figure 1. In conventional approach waste rock would be hauled the whole “X” distance
during mine’s life, keeping fixed haulage distances outside pit. In MSDS approach two stages
will be considered to cover the distance “X”, thus haulage distance will be incremental along
the mine’s life from “Y” to “X”. For example, in Figure 1 (where P is the period from mine
life) waste rock from P1 (W1) and P2 (W2) will be dumped in a distance “Y” from pit exit.
In the P3 (example’s last period), W3 is sent all the way to final dump (distance “X”) and
W1 and W2, located in the first temporary dump stage, are also re-handled to that position
(“X–Y” distant).
In the end of the process, total haulage distance is the same for both methods, however in
MSDS approach total haulage distance in P1 and P2 (sum of distances) is lesser than conventional method for the same period. The distance reduction in the first periods decreases
total haulage cost, even considering an incremental factor for re-handling, which results in
NPV increment compared to the base case. Although distance haulage costs reduction in earlier stages will cause an increment in this parameter in later stages, and as NPV is penalized
by period (the further is the period, the greater is the penalty), the effect of this increment
in operational cost will also be attenuated by period penalization, resulting in higher final
NPV.
As a case study, the MSDS methodology was applied to a phosphate mine in Brazil. A block
model, from a mine sector, was used to calculate the profit function, which was imported to
NPV Scheduler software to build a mathematical open pit and scheduling. Haulage costs
were not considered to build the profit function and pit optimization in order to maintain
the same number of blocks and NPV potential for comparison purpose. Haulage costs were
included afterwards using an Excel spreadsheet, where four MSDS different scenarios were
evaluated. Given the waste management purpose of this study and considering that all ore
blocks are sent to a fixed processing plant (which represent the same revenue for any tested
scenario), all results shown here represent only costs related to waste block.
A base case was considered as the conventional approach. A pit exit and final dump
location were established. Inside pit haulage distance and cost were calculated using blocks
2016, Ortiz, 2017) were published in the last few years. These papers found optimum solutions for waste management throughout mixed integer programming (MIP), to solve optimum waste and tailings sequencing in long-term planning, considering multiple dump sites
and potential acid drainage issues (Li et al. 2012) and reducing haulage distances. However,
in all studies the dumps locations were already defined, and waste rock sequencing would
avoid material re-handling in the future.
This paper proposes a new approach for waste disposal called a Multi Stage Dumping
Sequence (MSDS), which allocates temporary waste dumps along the way to the final dump
destination. This method proposes a planned material re-handling at the beginning of operation, providing additional time to design, prepare and license the final waste dump site
(temporary dumps sites also may require license in certain cases, depending on its location),
besides reducing waste haulage cost in the first years. Although an increase in unity haulage
cost is considered to re-handle material from/to temporary dumps, the NPV increment in the
first years, by reducing haulage distances, is still potentially higher. The methodology uses an
integrated mine planning and waste sequencing and it is carried out an application to a real
case study to demonstrate its applicability.
2 METHODOLOGY
The proposed methodology consists of defining temporary dump sites between the open pit
and final dump location. Some variables must be observed to define the number of stages to
be used. Variables like haulage and material re-handle costs, distance from pit exit to final
dump site and temporary dumps, distance between temporary dumps and waste rate production are considered in this preliminary study.
For explanation purposes and to focus on the differences between conventional and MSDS
approaches let’s consider a fixed distance “X” from pit exit to final dump location as shown
in Figure 1. In conventional approach waste rock would be hauled the whole “X” distance
during mine’s life, keeping fixed haulage distances outside pit. In MSDS approach two stages
will be considered to cover the distance “X”, thus haulage distance will be incremental along
the mine’s life from “Y” to “X”. For example, in Figure 1 (where P is the period from mine
life) waste rock from P1 (W1) and P2 (W2) will be dumped in a distance “Y” from pit exit.
In the P3 (example’s last period), W3 is sent all the way to final dump (distance “X”) and
W1 and W2, located in the first temporary dump stage, are also re-handled to that position
(“X–Y” distant).
In the end of the process, total haulage distance is the same for both methods, however in
MSDS approach total haulage distance in P1 and P2 (sum of distances) is lesser than conventional method for the same period. The distance reduction in the first periods decreases
total haulage cost, even considering an incremental factor for re-handling, which results in
NPV increment compared to the base case. Although distance haulage costs reduction in earlier stages will cause an increment in this parameter in later stages, and as NPV is penalized
by period (the further is the period, the greater is the penalty), the effect of this increment
in operational cost will also be attenuated by period penalization, resulting in higher final
NPV.
As a case study, the MSDS methodology was applied to a phosphate mine in Brazil. A block
model, from a mine sector, was used to calculate the profit function, which was imported to
NPV Scheduler software to build a mathematical open pit and scheduling. Haulage costs
were not considered to build the profit function and pit optimization in order to maintain
the same number of blocks and NPV potential for comparison purpose. Haulage costs were
included afterwards using an Excel spreadsheet, where four MSDS different scenarios were
evaluated. Given the waste management purpose of this study and considering that all ore
blocks are sent to a fixed processing plant (which represent the same revenue for any tested
scenario), all results shown here represent only costs related to waste block.
A base case was considered as the conventional approach. A pit exit and final dump
location were established. Inside pit haulage distance and cost were calculated using blocks