the mining rate of the benches, pushbacks and the mine. The space .... give different mining rates as a result of different schemes of exploitation. Therefore, the ...
Schemes of Exploitation in Open Pit Mining Felipe Arteaga, Micah Nehring, Peter Knights, and Juan Camus School of Mechanical and Mining Engineering, The University of Queensland, Australia
Abstract. The exploitation of open pit deposits usually takes place through a series of mining phases, commonly known as pushbacks. In open pit metalliferous mining, each pushback considers the extraction of one or more benches simultaneously in a process where the core operational tasks include: drilling, blasting, loading and hauling. In large open pit mines, shovels and front end loaders may be used to carry out loading activities. The type and number of shovels are selected during the planning process and their productivity determines the mining rate of the benches, pushbacks and the mine. The space available for loading is part of the pushback design. This defines the shape and size of the benches where the equipment will be placed. A relevant stage in mine design is the definition of the location and sequence that loading equipment must follow to deplete the benches of each pushback. The deployment of loading equipment in the mine is commonly referred to the scheme of exploitation. This term is widely used in the mining industry but not frequently referenced in the literature. The objective of this paper is to explore the concept of the scheme of exploitation in open pit mining within the context of the strategic mine planning activity. In the first part, the concept is presented through examples where the pushback size is fixed and the number of shovel is changed. The second part includes a discussion of the motivations and constraints that the mine planner may consider in the design. Configurations with several shovels and benches in a same pushback represent a challenge for the scheme design mainly due to the limitation of space for loading. Mathematical and optimisation tools can be useful in these cases; however, the models have to be able to represent the real constraints that will affect the productivity of the shovels in the different levels. In general, aggressive and costly schemes are rarely used by mining companies that seek high performance levels and lower operating costs. However, the selection of an appropriate scheme of exploitation needs to be aligned with the principal objective of the mine planning activity, which is to: create value through the exploitation of a mineral resource. Keywords: open pit mining, mine planning, mine design.
C. Drebenstedt and R. Singhal (eds.), Mine Planning and Equipment Selection, DOI: 10.1007/978-3-319-02678-7_126, © Springer International Publishing Switzerland 2014
1307
1308
1
F. Arteaga et al.
Introduction
Surface mining is characterized as a capital intensive mining method with higher productivities and lower costs compared to underground methods. The material extraction is usually carried out in stages called phases or pushbacks. Each pushback contains waste and ore that are extracted from the mine through layers called benches. The unitary operations in an open pit mine include: drilling, blasting, loading and hauling. At the mine, the capital investments mostly relate to the acquisition of the equipment for each of the unitary operations. In large metalliferous mines, loading may be carried out with shovels and front end loaders that are placed in the different pushbacks of the operation. Planning the exploitation of a mineral deposit is a complex activity. The principal difference with other industries is that the ore body, which corresponds to the main asset of the business, is finite and non-renewable. Additionally, the mine plan has to be developed with uncertain information such as the characteristics of the ore body and the economic drivers (prices and costs) of the mining project. If the mine planning process is divided according to the degree of breadth, it is possible to classify mine planning as strategic and tactical. This proposal was introduced by Camus in 2002 [1] and is based on the impact that the activity has in the value of the mining business. His approach is not based on a timeframe where usually mining planning is divided into short, medium and long term. Strategic mine planning is the activity where the main decisions that govern the exploitation of the mineral deposit are taken and the main objective is value creation. These decisions correspond to the selection of: the mining method, the processing route, the mining sequence, the size of the operation and the cut offs variables that progressively separate the valuable part of the orebody such as the cut off grade at the mine. Tactical mine planning, on the other hand, deals with the routine planning activities aimed at actually capturing the value of the business and as such this has mayor relevance during the mining operation. These activites include the operation ramping up and others such as the creation of medium and short term production plans, preparation of budget, deployment of equipment and production scheduling on a monthly, weekly or daily basis. The literature in the field of strategic mine planning has mainly focused on principal decisions of the mining project. The research in this field has highlighted the complexity of the problem that comes from the interrelation that exists between its variables. For example, it is not possible to determine the mining method without previously determining the mining sequence, the cut offs, the size of the operation and the processing route. Similarly, the other variables cannot be defined without previouly defining the remaining variables. The solution requires, therefore, recursive and iterative methods. To this effect, the problem is usually
Schemes of Exploitation in Open Pit Mining
1309
broken down into stages and the calculation begins with preliminary assumptions than are then refined as planning progresses. Pushback design and the loading equipment selection are two mayor activities of the planning activity. Pushback design involves the determination of the size and shape of each pushback and the characteristics of its benches and access routes. On the other hand, the loading equipment selection considers the definition of the type and number of shovels or front end loaders that will be used for the loading activity. The different options considered in the selection of the final pushback design are embedded in the definition of the ultimate pit and the extraction sequence. On the other hand, loading equipment selection is defined mainly during the operation sizing stage where the mill and mine size are determined. Both activities have been widely researched in strategic mine planning literature. During the mine design stage, the location and sequence that the loading equipment must follow to deplete the benches is determined. The deployment of loading equipment in the different benches of each pushback is known as a scheme of exploitation. This concept is widely used in the mining industry however it has been addressed to a lesser extent in the literature. The scheme of exploitation corresponds to the deployment of loading equipment during the depletion of the benches of each pushback. This changes as mining progresses and therefore, it defines an extraction sequence of the benches. The schemes of exploitation design is part of the mine design stage and, therefore, is part of the strategic and tactical mine planning activity. From a strategic point of view, the definition of the schemes of exploitation, the pushbacks design and the loading equipment selection are interrelated activities. Their definition impact directly the mining rate of the deposit. For example, in a certain pushback design, the same loading equipment could give different mining rates as a result of different schemes of exploitation. Therefore, the schemes of exploitation play a crucial role in the estimation of the mining rate and the other key variables of the strategic mine planning process.
1.1
Objective
The aim of this paper is to explore the concept of the scheme of exploitation in open pit mining. The idea is to introduce the concept and discuss the objectives and constrains that govern its proper design. The scope of this paper is metalliferous open pit mines where electrical shovels develop the loading activity. The schemes of exploitation design is part of strategic and tactical mine planning. However, the focus will be on a strategic perspective. Nevertheless, the conclusions and discussions may be extrapolated to the tactical mine planning activities.
1310
1.2
F. Arteaga et al.
Paper Structure
Section 2 includes the open pit mining considerations and terminology used in the paper. Section 3 introduces the concept of scheme of exploitation. Section 4 includes some more discussions related to the scheme of exploitation design, and finally Section 5 presents the conclusion of the paper.
2
Surface Mining
Figure 1 shows a cross section of a typical open pit block model. The exploitation begins at the surface and the material is removed in stages called pushbacks. These are exploited in a sequence before reaching the ultimate pit. The final pit envelop represents the limits of the mine once that the exploitation of the mineral resource has finished.
Fig. 1 Pushbacks in an open pit mine, Smith, J.(2013) [2]
In an open pit mine the objective is to remove the ore from ground and lift it to surface. However, generally the ore is located in the deeper part of the pit, therefore the beginning of each pushback considers predominantly the extraction of waste. Once ore extraction begins, ore and waste are removed from the mine keeping a relation called stripping ratio. In general, surface mining is characterised as a capital intensive industry with a massive material movement during the life of the operation (Bartos, 2007) [3]. Additionally, when this method is compared with underground mining, it is considered highly productive and with lower costs (Calder et al., 1997) [4] . Open pit mining may be especially suitable for large deposits that are relatively close to surface. Examples of this, are the various porphyry copper deposits located in northern Chile.
Schemes of Exploitation in Open Pit Mining
2.1
1311
Open Pit Drivers
The exploitation by open pit methods has particular challenges that are faced during the planning and design stage but also during the operation itself. An open pit operation may, for example, involve the extraction of a large amount of waste during the first years of operation that may delay the returns necessary to recover the capital investments. Other mines may face the challenge of operating with presence of water which has to be constantly drained from the mine or with the presence of numerous fractures that can compromise the maximum slope angles and stability of the benches and pit walls. According to Calder et al. (1997) [4], the key drivers of an open pit operation include: − − − − − − − − − − − − −
Ore grade and tonnage Topography Physical size, shape and structure of the deposit Capital expenditures Economic factor of operating cost Profit Pit limits, cut-off grade and stripping ratio Mining equipment Rate of production Access Mine design (bench heights, road grades, etc.) Geotechnical aspects and Hydrological conditions
Additionally, there is other drivers that may be considered as part of any mining activity such as: people, availability of key supplies (i.e. water and energy), communities, environmental conditions, available technology and the country where the mine is located (political situation, social aspects, taxes, royalties, regulations and laws)
2.2
Terminology
Figure 2 shows the typical terminology used in surface mining. Each pushback is depleted in layers called benches that have a particular height and slope angle. Each layer is separated by the following one by a space called berm. This is designed to keep the stability of the pit wall. The road ramp corresponds to the access to the different levels of the pushback. The height of the benches, the bench slope angle and the width of berms and ramp will define the overall wall slope angle.
1312
F. Arteaga et al.
Fig. 2 Open pit nomenclature, Calder et al. (1997) [4]
2.3
Benches
A bench is a section of a pushback whose dimensions are set during the mining design stage. Figure 3 illustrates a typical pushback in an open pit mine. In this case, two benches are being depleted simultaneously and the access to both benches is through the principal ramp that is placed close to the pit wall. Other
Fig. 3 Pushback in an open pit mine
Schemes of Exploitation in Open Pit Mining
1313
configurations could include auxiliary ramps between benches to facilitate the access of loading, hauling and auxiliary equipment between the different benches in operation. The number of shovels to exploit a pushback is variable and depends on the strategy designed by the mine planner.
2.4
Bench Types
The benches of Figure 3 has a typical half-moon shape with one area close to the wall of the pit and another area called free face because it is oriented to the space left by the previous pushback. Pinochet (2004) [5] introduce a classification of the benches according to the shape and the presence and extension of the free face. Five types of benches are identified: The hillside expansion benches are represented in Figure 3 and are characterised by a large free face. The deep hillside expansion is similar to the benches of Figure 3 but the extension of the free face is smaller in comparison to the area close to the wall. The sunken cut and the expansion of the sunken cut benches do not include a free face and are characteristic of the first pushback of an open pit mine. Finally, the cut top benches have only free face and correspond to the benches that can be placed at the top of a hill.
2.5
Macro Zones
Figure 4 shows a top view representation of a hillside expansion bench. In general, it is possible to distinguish four regions using a geometric point of view: the ramp, the control area, the production area and the extremes of the bench. The ramp is built to connect two different levels. There are different kinds of ramps. The final ramp or design ramp is the one that allow access to all the benches of the pushback. Therefore it remains until the exploitation of the next pushback. The auxiliary ramps can be developed as a temporary access to an inferior level. It can be designed for access of trucks or auxiliary equipment such as drill machines and bulldozers. The control area is extended along the pit wall. Drilling and blasting design of this area is developed to keep the stability of the pit wall. Loading material in this area is a challenging activity because the program line must be reached with precision to continue with the extraction of the inferior benches without affecting their shape and size of the pushback. The extreme areas are smaller than the others and are considered areas with a restrictive space for the loading activity. In general the swing angles of shovels increase considerably and therefore, their productivity decrease. The production area in the central region of the bench is the sector with no restrictions to load trucks from a geometric point of view. Shovels can reach their highest level of productivity in this area.
1314
F. Arteaga et al.
Fig. 4 Areas in an open pit bench
Not only the bench geometry and its singularities determine the productivity that loading equipment can reach in the different areas of the bench. The particle size, drilling and blasting design performance, rock type and mineralisation, the presence of water and fractures, and the performance of operators among other variables will affect the productivity of shovels. Arteaga, F. (2007) [6] developed a study to analyse the utilisation and productivity of the shovel fleet in a copper mine in the north of Chile. The author overlaps the drilling and blasting pattern, the rock type and the geometry of the bench to define macro zones or regions in a sunken cut and in a hillside expansion bench. The results of this study show the existence of significant differences in the productivity of the shovel within these macro zones. Moreover, the study identifies the impact of the operator performance in the shovel productivity.
2.6
Shovel Mining Methods
The shovel mining methods defines the way in which the material will be extracted from each bench of the mine. According to there are four mayor methods and they are defined considering the shovel set-up with respect to the benches face and the trucks set-up during loading. These four shovel mining methods are: Double back-up methods, single back-up methods, drive-by methods and modified drive-by methods. The back-up methods consider to the shovel and the cable oriented perpendicular to the muckpile or blasted bench face. In the double back-up the shovel can load from both sides following the configuration shown in Figure 5. In the single back-up, the shovel can load from one side and it is designed for reduced loading areas such as Figure 6.
Schemes of Exploitation in Open Pit Mining
1315
Fig. 5 Double back-up methods, Calder et al. (1997) [4]
Fig. 6 Single back-up methods, Calder et al. (1997) [4]
In the drive by methods, the shovel is placed parallel to the muckpile. The cable is also parallel to the muckpile and can only load from one side. The drives by methods have lower productivities than the back-up methods because larger swing angles. Moreover, drive-by methods are not selective and shovel and cable are constantly in risk of falling rocks from the bench face.
1316
F. Arteaga et al.
In general the backs up methods are commonly used in metalliferous open pit mines and drive by mainly in coal mines however; the selection is not based in the type of material. According to the selection of the method is mainly based on factor such as: the bench shape, the mining are available, the ore grading constraint and the experience of the operators.
3
Schemes of Exploitation
3.1
Scheme of Exploitation with One Bench and One Shovel
Schemes of exploitation correspond to the deployment of loading equipment in mine pushbacks. If only one shovel is positioned to extract the bench, it could follow a sequence as illustrated in Figure 7 where the numbers represents the regions to be extracted. The exploitation of the bench begins with the ramp that is associated with the number 1. This is followed by the control region 2, then production region 3 and so on.
Fig. 7 Scheme of exploitation with one bench and one shovel
3.2
Scheme of Exploitation with Two Benches and Two Shovels
The mine design including more than one shovel per bench could have a scheme of exploitation as the one shown in Figure 8. In this case both shovels will follow the sequence illustrated with the arrows that indicate that both shovels will extract the bench in opposite directions at the beginning of region 3. An alternative scheme of exploitation could consider both shovels working in the same direction. This configuration would give rise to different schemes of exploitation although the number of shovels is the same.
Schemes of Exploitation in Open Pit Mining
1317
Fig. 8 Scheme of exploitation with one bench and two shovels
3.3
Strategy with Simultaneous Benches
In schemes with more than one shovel, two or more benches may be extracted simultaneously. Figure 3 shows a hillside expansion bench with two benches in exploitation. The width of a hillside expansion bench is generally smaller that the width of sunken cut benches. The displacement between equipment in different levels is necessary to avoid safety problems associated with falling rocks from higher levels.
4
The Schemes of Exploitation Design
The scheme of exploitation represented in Figure 8 uses two shovels. Shovel 1 has to create the access to the bench through the extraction of the ramp and then section 2 and 3. Shovel 2 can enter in operation on the same bench once there is enough space for the operation of both shovels. The primary element to consider in the pushback design is, therefore, the area available for mining. The simplest case is when the scheme is designed with only one shovel in a pushback. Figure 7 illustrates a scheme where the only shovel in operation begins with the extraction of the ramp, and then it opens an access to the bench extracting section 2 and 3. Finally the shovel choose to extract the right side of the bench first and the go to the left side. A different scheme may consider the extraction of the left side of the bench first and then go to the right side. However, this new configuration will have the same results refard to the final time used to extract the whole bench. Figure 9 corresponds to the same case presented in Figure 7 but the regions 3 and 6 have been divided into sections. Once the shovel opens the access to the
1318
F. Arteaga et al.
bench in region 3 it could finish the extraction of region 3 or instead go to region 6. However, another alternative could be to extract part of region 3, then go to region 6, extract part of region 6 and return to region 3.
Fig. 9 Scheme of exploitation with one shovel and several movements
Apparently, this last case represents a scheme of exploitation that is less efficient than the previus examples for the continuous movements between regions. This is valid at least when efficiency is defined in terms of the equipment performance. The result of this strategy will be decrease in productivity and utilisation of the shovel and ultimately an increase the time necessary to extract the bench. The schemes presented in Figure 9 could be used in cases where the movement of the shovel between areas is required by the selectivity of the mining stage. For example, in the ore loading when the shovel must extract specific materials to achieve the mix required by the mill. If there are no indications that demand a specific direction in the loading activity then the scheme of exploitation may be designed to maximise the performance of the shovels that consequently will maximise the productivity of the pushback. In schemes with one shovel the productivities of loading equipment and pushbacks are directly related. It means that an improvement in the performance of the shovel represents and improvement in the productivity of the pushback. The three schemes presented as options in the extraction of the bench of Figure 7 represent different design strategies. These examples demonstrate that different schemes of exploitation can give rise to different mining rates even though they are designed considering the same pushback design and the same type and quantity of shovels. Figure 10 illustrates the interrelation that exists between the pushback designs, the loading equipment selection (size, type and quantity) and the scheme of
Schemes of Exploitation in Open Pit Mining
1319
exploitation design. In mine planning, different configurations of these three variables are evaluated to reach the material requirement of the mine. Figure 10 additionally includes the shovel mining method. This can be seen as a constraint because the selection of the shovel mining method may restrict the scheme of exploitation design. However, it is included in the diagram because it is a decision variable that can be modified according to the requirement of the operation.
Fig. 10 Variables considered on the scheme of exploitation design
At the beginning of this section was highlighted the relevance of the space available for mining in the scheme of exploitation design. Figure 10 includes the key variables in the definition of the available space for loading. Firstly, the pushback design defines the size and shape of the benches. The number of shovels and their size limits the space inside the area defined by the bench design. Finally the shovel mining method may limit the access to certain areas depending of the space restriction of each shovel mining method. The arrows of Figure 10 represent the recursive character of these variables. Therefore, in the same way that in the general strategic mine planning problem, an initial assumption may be made at the beginning and then refined as the evaluation progresses. Assuming a certain pushback design, a certain shovel fleet and a selected shovel mining method, the options available for the scheme of exploitation design will be restricted to the space previously defined by the other variables. Figure 11 shows the representation of the minimum available space for loading presented by Ahumada (1980) [7] . The diagram represents a section of a pushback where the only shovel in the area is prepared to load under the double back-up method. According to the available space is also restricted by an additional variable: the mechanical behaviour of the rock after blasting.
1320
F. Arteaga et al.
Fig. 11 Mining width necessary for loading operation, Ahumada, A. (1980) [7]
4.1
Complex Schemes of Exploitation
Figure 8 represents a scheme of exploitation with two shovels. However, complex schemes may include three or more shovels in different benches of a same pushback. The interaction between shovels is a challenge in the scheme design. Ideally, the space is sufficient for the operation of the entire shovel fleet without delays for lack of space. Schemes where equipment are not fully utilised are not considered as options in the design stage. In a configuration of shovels working in parallel in the same bench it is necessary to establish the minimum distance between shovels. Under a double back-up method the distance between shovels may be equal to the diameter of operation of each one. This limit distance could be different if the shovels are working with a certain displacement. See Figure 12.
Fig. 12 Double back-up loading sequence, Calder et al. (1997) [4]
Schemes of Exploitation in Open Pit Mining
1321
In configurations with multiple benches in operation, special safety consideration need to be taken. The shovels should be working with certain displacement to avoid the operation of one shovel over the other and the consequences of possible rocks falling from the upper level.
4.2
Discussion
The discussion presented in this paper highlight the maximisation of productivity and utilisation of the loading equipment as one the main objectives in the scheme of exploitation design. This analysis is aligned to the proposal of Marek, J. et al., (1985) [8] that state that a mine plan has to consider an appropriate space for the operation of the loading equipment. However, the scheme design should consider other variables and decisions of the strategic mine planning activity. Different schemes of exploitation under similar pushback designs and loading equipment could result in different mining rates. In this regard, the mining rate will be affected by a variable within the mine planning process whose objective is the maximisation of the equipment performance. The definition of the mining rate is part of the sizing the operation stage whose objective is the maximisation of business value. If the process is seen from an opposite view point and the mining rate is defined first, the scheme of exploitation that matches this mining rate could be one that will not match the objective of maximising the equipment utilisation. Therefore, the objective durign the design of the schemes should be done from a strategic point of view. With this perspective, complex schemes, with lower equipment performance can be valid options that have to be evaluated in the process to maximise the value that can be possible to obtain through the exploitation of the mineral deposit.
4.3
Operational Constraints
The principal operational constraint is related to the minimum space available for loading. It is especially relevant in the complex configuration with multiple shovels. Additionally, it is necessary to consider other aspects such as the type of material (ore or waste) and the impact of the macro zones in the shovel productivity.
4.4
Additional Considerations
Designs that consider multiple shovels could be affected by other operational issues that can be considered in the evaluation of the operating time as a result of the scheme. The cable movement in the area may be higher with several shovels in operation. It will be especially relevant if the shovel mining method is double back-ups that is characterised by several movements of the shovel in the loading area. The drilling and blasting activity can cause delays if they are not coordinated
1322
F. Arteaga et al.
properly. More shovels in operation means more drilling and blasting and therefore, more movements to clear the blasting area. If the shovels are working in close proximity and the blasting is not programmed together, the time to clear the area for blasting could be doubled. Finally, schemes with several shovels in areas with limited space can create congestion forcing the trucks to wait far from the shovels. This in turn, increases the parking time in the loading area and also the shovel cycle time.
4.5
An Example in the Literature
Ahumada (1980) [7] evaluated two different schemes of exploitation for the Chuquicamata copper mine in northern Chile. The study is an example of the interdependence of the variable presented in Figure 10. The objective of this research was to solve the problem of the introduction of new and bigger trucks to exploit the copper mine. The original design considered schemes of exploitation with one shovel per bench and a route design according to the size of the trucks in operation. The change in the truck fleet has the implicit consequence of modifying the dimension of the roads and ramps and therefore, in the pushback design. The analysis was a technical and economic evaluation of two alternative schemes of exploitation for the new pushback design. The first option was to change the size of the benches and the number of shovels per bench. The second option considered maintaining the size of the benches assuming a change in the pushback as a result of the bigger roads and therefore a change in the final pit slope angle. The final decision was the second option with a change in the pushback design but keeping the scheme of exploitation of one shovel per bench.
5
Conclusions
Design of schemes of exploitation is a challenging activity that involves a deep knowledge of the parameters that govern the operation. The experience and creativity of the mine planner is fundamental to create schemes that address the principal objective of value creation and also respect the constraints of the operation. This paper has explored the concept of the scheme of exploitation, proposing a formal definition and a discussion about the considerations that are relevant in the design. For complex scenarios with multiple shovels in a reduced space, the aid of mathematical and optimisation tools can be useful to deal with the scheme design. However, the optimisation models have to be able to represent the design objective and the operational constraints. Further research could be focused on the search of existing and new mathematical methods that can be useful to optimise the design of the schemes of exploitation in open pit mining.
Schemes of Exploitation in Open Pit Mining
1323
References [1] Camus, J.: Management of Mineral Resources: Creating Value in the Mining Business. Society for Mining, Metallurgic and Exploration Inc., Colorado (2002) ISBN: 0-87335216-5 [2] Smith, J.: Module Learning Guide: Production Scheduling. In: University Course Material, Mine Planning and Design. The University of Queensland, Australia (2013) [3] Bartos, P.: Is mining a high-tech industry?: Investigations into innovation and productivity advance. Resource Policy Journal 32(4), 149–158 (2007) [4] Calder, P., et al.: Planificacion Operativa de Minas a Cielo Abierto – International Specialization Program in Mining Technology. Pontificia Universidad Catolica de Chile (1997) (unpublished material) [5] Pinochet, M.: Efecto de las singularidades de los bancos en la productividad de las palas en una mina a cielo abierto, final year thesis, Mining Center, Engineering School, Pontificia Universidad Catolica de Chile, Santiago, Chile (2004) [6] Arteaga F.: Estudio de Utilizacion y Productividad de la Flota de Palas de Mina Escondida norte, final year thesis, Mining Center, Engineering School, Pontificia Universidad Catolica de Chile, Santiago, Chile (2007) [7] Ahumada, F.: Esquemas de Explotacion del Rajo Chuquicamata y su Respectiva Evaluacion Economica, final year thesis, Faculty of Science, Physics and Mathematics, The University of Chile, Santiago, Chile (1980) [8] Marek, J., et al.: Cutoff grade – A balancing act, for presentation at SME- AIME Fall Meeting, Albuquerque New Mexico. Society of Mining Engineers of AIME, Colorado (1985)
