Cost reduction by optimization of the primary crusher position

Category Mine
Group GSI.IR
Location 20th WORLD MINING CONGRESS 2005
Author H. Tudeshki* - T. Hardebusch - R. Bagherpour
Holding Date 02 January 2006
ABSTRACT
 
The selection of the position of the primary crusher is one of the main factors affecting the economic efficiency of raw mineral mining in hard rock open pit mines. One of the most important objectives of mine planning is, therefore, to identify the optimum primary crusher position with regard to the lowest possible transport costs for a certain mining period. Theoretically, the optimum position of the primary crusher is to be found at the centre of mass of a deposit. In reality, however, the optimization is confined by the fact that the primary crusher can only be placed on an existing mining level.
 The goal is, therefore, to place the primary crusher as close to the centre of mass of the deposit as possible, or onto the same level, respectively.This paper illustrates, by way of example of a model open pit mine, the optimization potentials of the selection of the primary crusher position. This is done by computer-aided investigations into different primary crusher positions in the model open pit mine. For the different scenarios, a discontinuous material transport and a combination of discontinuous and continuous material transport are investigated. Mining trucks and a new type of long-distance continuous conveyor - the Rope Con-System- has been selected as means for material transport.
Key words: in-pit crushing, cost, optimization, pit, transport, rope con, primary crusher, model, truck.
 
 

INTRODUCTION
 
The combination of loading, transport and Primary crushing equipment has significant effect on the economic efficiency of raw minerals mining operations, with transportation being the most influencing factor due to its high cost intensity.
 
While in the past, mainly discontinuous Transport equipment has been used for moving bulk solid materials in hard rock mining, continuous transport systems (belt conveyor systems) become more and more common today.
 
To use belt conveyors as the main means of transportation in hard rock mining, however, it is necessary to have a primary crusher installed upstream to the conveyor to crush the mined material down to a size Suitable for belt conveying. To follow the progress of the actual mining operations, this crusher has to be relocatable.
 
Relocatable primary crushing plants are differentiated between mobile and moveable (Semi-mobile) types. While mobile plants do have their own travelling device, semi-mobile plants are relocated by means of auxiliary equipment, Such as heavy crawler vehicles, for example.
 
Due to safety requirements, a minimum distance has to be kept between quarry face and the equipment during blasting. If a stationary belt conveyor is used, this distance has to be bridged by either a movable belt conveyor (continuous transport) or wheel loaders or heavy trucks (discontinuous transport).
 
In the first case the primary crushing plant is located right at the loading point and the mined material is fed directly to the primary crusher by the loading equipment without intermediate transport. After crushing, the raw material is transported to the processing plant by means of the movable and the stationary belt conveyor. Here, the previously described transport equipment fulfils all transportation needs without the need for mining trucks.
 
In the second case an either mobile or semi-mobile primary crusher (followed by a stationary belt conveyor) is located at the minimum safety distance to the blasting operations on the level where the raw material is mined. In single-level mining mode normally wheel loaders are used for intermediate transport (load & carry), whereas in multi-level mining mode intermediate transport is performed by means of mining trucks.
 
In both cases the stationary belt conveyor has to be relocated and/or extended when a certain mining progress has been reached in order to keep abreast with the mining operations rate of advance.
 
If a stationary primary crusher has already been installed in an operating mine, investigations into the economic efficiency have to focus on the loading and transport processes. However, in case of a totally new open pit mine or the extension of an existing open pit mine, investigations into the total system, consisting of loading and transportation equipment and the primary crushing plant, has to be integrated into the mine planning process. Of the many selections that have to be made during this process, the selection of the position of the primary crusher is one of the most important factors due to its great impact on the overall economic efficiency of the mine.
 
 
Figure 1: The open pit mine model
 
 
OPTIMIZATION OF THE PRIMARY CRUSHER POSITION
 
The selection of the position of the primary crusher in hard rock open pit mining is influenced by numerous factors. The main factors that have to be taken into account during the selection process are shape, content and position of the material to be mined and, thus, the accessibility of the deposit.
Theoretically, the optimum position of the primary crushing plant, that guarantees a permanent reduction in transport costs, is to be found at the centre of mass of the deposit. In reality, however, the primary crushing plant can only be placed on an already existing surface of the deposit. Because the centre of mass is always located inside the deposit, the theoretically ideal position cannot be taken into account. It must, therefore, be the goal of the planning process to determine a position for the primary crushing plant, which offers the lowest possible increase of transport work compared to the case that the primary crusher is located at the centre of mass of the deposit.
 
 It can be stated that, in principle, a displacement between the Primary crushing plant and the centre of mass in vertical direction has a more negative effect on the economic efficiency than a displacement in horizontal direction. In general, a horizontal Displacement in the direction of the processing plant is to be preferred.
 
Shape and content of a deposit, especially the quality distribution of the mine able material, determine the starting point of a mining operation, the mining method and the future extension of an open pit mine. The process of optimizing the primary crusher position, therefore, has to be integrated into the early design phase of open pit mining operations.  
 
For large-scale open pit mines with a long operating life, the optimization can be carried out for only a part of the deposit. In this case, however, the life span of this part of the deposit should be adjusted to the expected service life of the primary crushing plant or the calculated relocation cycle, respectively.
 
The interval between two relocations is calculated as the minimum of costs of the relocation and the increasing transport costs, which increase with increasing transport distances.
In the following chapters, the effect of the primary crusher position on the economic efficiency of a hard rock open pit mine will be illustrated by exemplary investigation of different primary crusher positions in a model open pit mine. For every position - selected in dependence of the mining progress - different types of transport equipment are selected and dimensioned.
 
Alternatively, a solution with only discontinuous transport equipment and a combination of continuous and discontinuous transport   equipment is investigated, and the respective investment and operating costs are calculated. Mining Trucks have been selected as a means of discontinuous transport, for continuous transport the authors selected the new Rope Con-System jointly developed by Doppelmayr, Doubrava and Conti Tech.
 
THE MODEL OPEN PIT MINE
 
The model open pit mine used as an example in this investigation is a hard rock open pit iron ore mine. In this example, the ore is mined by extension of the pit into the vertical direction. The processing plant - including the primary crusher - is located on the surface. Up to the date of investigation, the mine has reached a depth of 130 m below the surface. The depth is reached in 13 steps of 10 m level height, each. The levels are connected by a system of ramps with an inclination ratio of 10 %. The overburden layer on top of the ore body has a thickness of 40 m. Consequently, ore mining takes place on levels 5 to 13. Investigations are carried out for the following three scenarios of future mine development and different alternative positions of the primary crushing plant.
SCENARIO 1
 
Horizontal extension of the mine, ore mining takes place on levels 5 to 13.
a) Primary crusher remains at its current position at the border of the open pit, Discontinuous transport by mining trucks.
b) Relocation of the primary crusher to the 5th level. Combination of discontinuous truck transport and Rope Con- System (length: 150 m).
c) Relocation of the primary crusher to the level of the centre of mass on level 8. Combination of discontinuous truck transport, Rope Con- System (length: 300 m).
d) Relocation of the primary crusher to level 10. Combination of discontinuous truck transport and Rope Con- System (length: 350 m).
SCENARIO 2
 
Horizontal and vertical extension of the mine, ore mining takes place on levels 5 to 18.
a) Primary crusher remains at its current position at the border of the open pit and Discontinuous transport by mining trucks.
b) Relocation of the primary crusher to level 5.
Combination of discontinuous truck transport and Rope Con- System (length: 150 m).
c) Relocation of the primary crusher to the level of the centre of mass of the ore body on level 10.
Combination of discontinuous truck transport and Rope Con- System (length: 350 m).
 
SCENARIO 3
 
Vertical extension of the mine, ore mining takes place on levels 13 to 18.
a) Primary crusher remains at its current position at the border of the open pit .Discontinuous transport by mining trucks.
b) Relocation of the primary crusher to level 13.
Combination of discontinuous truck transport and Rope Con- System (length: 500 m).
 
THE ROPE CON-SYSTEM
 
The Rope Con-System is a new continuous bulk material conveyor system, that incorporates the advantages of aerial ropeway transport and traditional belt conveyor systems.
 
The main element of the Rope Con-System is a flat belt with flexible belt walls attached to both sides. These belt walls guarantee a controlled bulk material transport and, at the same time, allow for a high volumetric capacity. Transversal bars with cable guide rolls mounted at both sides are fixed to the belt in regular intervals, such that the upper and lower belt strands are guided by two ropeways each.
 
Similar to traditional belt conveyor systems, the belt acts as the tractive element of the Rope Con-System and is driven by pulleys at either the loading and/or the unloading station of the conveyor.
 
The ropes are completely sealed and act as a supporting member. They are anchored at both ends and are supported by Ropeway towers, with a maximum distance between two towers of about 1 000 m. In combination with an inclination angle of up to 35°, existing obstacles - such as building structures or high slopes - can be easily crossed without additional work. Other than traditional belt conveyor systems, the Rope Con does not need any kind of track, thus making it possible to set up a straight connection between the primary crusher and the processing plant even in very deep open pit mines (see Fig. 3).
 
On the bottom line, the new innovative concept and design of the Rope Con-System, with its guided and closed-loop belt, offers totally new applications for continuous bulk material transport. Due to its design, the system is also characterized by low wear rates and maintenance costs.
 
Some of the general advantages of the Rope Con-System are: 
• Conveyance across long distances in only one section (no need for additional transfer and drive stations or intermediate drives).
• Low energy consumption (reduced belt resistances, low rolling resistances comparable to ropeway systems).
• High-performance conveyor belt with belt speeds up to 12 m/s.
• Positive belt guidance due to cable guide rolls prevents lateral belt displacement.
 
EQUIPMENT SELECTION AND CALCULATION OF COST
 
The  following  software  applications  have  been  used during
the investigation for the dimensioning of the loading and transport Equipment and for the calculation of costs:
• Auto PLAN 2000, Dohmen, Herzog & Partner GmbH
• Fleet Production and Cost Analysis FPC, Caterpillar, Inc.
• XTRUCKTOR, X graphic engineering company GmbH
These software applications are based on large numbers of investigations into open pit mining operations and their databases are up-dated on a regular basis.
 
 
   Figure 2: Long-distance continuous conveyor Rope Con
 
In addition to the determination of performance related data, such as equipment size, number of equipment, and loading or transport rates, and all these applications can also be used to calculate the specific costs for loading and transport operations. This makes it possible to easily compare the costs of the different alternatives for loading and transport.
 
The necessary equipment for the previously described different primary crusher locations is selected based on the following basic data:
• Annual output                               4.2 million t
• Annual operating time                   2000 h/a
(Single-shift operation)
• Effective annual operating time     1667 h/a
• Necessary output per hour             2500 t/h
• ore density (solid ore)                    3.26 t/m3
• ore density (loose ore)                   2.79 t/m3
• Availability of equipment                 83%
 
In accordance to this data, a combination of mining trucks with a working load of 140 t and hydraulic excavators has been selected, on which the calculation of the economic efficiency is based. Dimensioning and selection of the appropriate Rope Con-System has been conducted by Doppelmayr.
 
Regarding the necessary investments, it has been assumed that the complete equipment has to be newly bought. The following primary investment is standard values:
• Mining truck with 140 t pay load EUR 1.3 million
• Rope Con-System, 150 m length EUR 1.5 million
• Rope Con-System, 350 m length EUR 3.0 million
• Rope Con-System, 500 m length EUR 4.0 million
 
The necessary capital costs include the calculated payment of interest with an assessed interest rate of 7 % and the calculated depreciation. To calculate the depreciation the expected life time of the equipment has to be known. In this paper the calculation of the depreciation is based on a linear depreciation with the following life cycle times:
• Heavy trucks:                  16000 h
• Rope Con-System:           30000 h
 
Calculation of operating costs for all three scenarios comprises the determination of the specific operating costs for material transport to the primary crusher by either mining trucks only or the combination of mining trucks and Rope Con-System in EUR/t. The operating costs to be determined Include:
• Labour costs (including indirect labour
Costs, e.g. taxes and insurances)
• Energy costs
• Costs of tires
• Maintenance costs
 
Tables 1 to 3 and Figs. 4 to 6 provide an exemplary overview of the calculated specific investment and operating costs as well as the total costs of the respective alternatives.
 
In all three scenarios, the relocation of the primary crusher will create a reduction in costs. The reduction in costs increases with decreasing distance between the primary crusher and the centre of mass of the ore body.
 
In case of a mine extension in horizontal direction (Scenario 1), relocating the primary crusher away from the border of the mine onto level 5 will produce a reduction in costs of approx. EUR 0.074 per ton of mine able mineral. The maximum difference of EUR 0.089 per ton of mine able mineral is gained by placing the primary crusher onto level 8. If the crusher is placed onto level 10, the reduction in costs will decrease down to EUR 0.78 per ton of mine able mineral. This decrease can be explained by the increasing displacement between the primary crusher and the centre of mass of the ore body.
 
A similar behaviour of the specific costs can be observed in case of a horizontal and vertical extension of the mine (Scenario 2). If the mining operations are extended into the horizontal direction while, at the same time, the depth of the mine is increased to 18, the relocation of the primary crusher onto level 5 will create a  reduction in costs of EUR 0.078 per ton of mine able mineral. The best result, however, can be achieved by placing the primary crusher onto level 10, which corresponds to the position of the centre of mass of the ore body. In this case the reduction in costs will amount to EUR 0.101 per ton of mine able mineral. The analysis of Scenario 3, where the mine is extended into the vertical direction only (from the 13th level down the 18th level), shows that  relocating the primary crusher onto level 13 will create a reduction in costs of EUR 0.163 per ton of mineable mineral.
 
CONCLUSIONS
 
The selection of the position of the primary crusher is one of the main factors affecting the economic efficiency of raw mineral mining in hard rock open pit mines. One of the most important objectives of mine planning is, therefore, to identify the optimum primary crusher position with regard to the lowest possible transport costs for a certain mining period.
 
Theoretically, the optimum position of the primary crusher is to be found at the centre of mass of a deposit. In reality, however, the optimization is confined by the fact that the primary crusher can only be placed on an existing mining level. The goal is, therefore, to place the primary Crusher as close to the centre of mass of the deposit as possible, or onto the same level, respectively.  This paper illustrates, by way of example of a model open pit mine, the optimization potentials of the selection of the primary crusher position. This is done by Computer aided investigations into different Primary crusher positions in the model open pit mine. For the different scenarios, a discontinuous material transport and a combination of discontinuous and continuous material transport are investigated. Mining trucks and a new type of long-distance continuous conveyor - the Rope Con-System - have been selected as means for material transport.
 
In all three scenarios, the relocation of the primary crusher into the pit, leads to a transport cost reduction, compared to a transport system to a primary crusher located at the border of the mine. In case of a horizontal extension of the mine only, a maximum reduction of 0,089 EUR per ton can be realized. If the mine is extended both horizontal and vertical the maximum cost reduction is calculated at 0,101 EUR per ton. A vertical extension of the pit only, leads to a reduction of transport costs of 0,163 EUR per ton
 
REFERENCES
 
1.        Caterpillar Inc, 2003, Fleet Production and Cost Analysis Version 3.04r
 
2.        Dohmen, Herzog & Partner GmbH, 2003, Benutzerhandbuch Auto PLAN 2003
 
3.        H. Frühstück, 2004, Rope Con – Die Innovation in der Fِrdertechnik Kolloquium Fِrdertechnik im Bergbau, Lehrstuhl für Maschinelle Betriebsmittel in Bergbau und Geotechnik am Institut für Bergbau der TU Clausthal, Tagungsband 2004, 91-106
 
4.        R.D. Stoll, H. Tudeshki, R. Zühlsdorf , 1996, Einsatzmِglichkeiten der EDV im Tagebau, Braunkohle/Surface Mining, Jg. 45, Nr. 1,
 
5.        W. Platzek, 1994, Untersuchungen zur Optimierung von Primنrbrechanlagen in Natursteinwerken über den Abbauzeitraum, Dissertation 1994, RWTH Aachen
 
6.        X Graphics , 2003, Ingenieur-GmbH, XTRUCKTOR2,Ein von X Graphic entwickeltes Programmsystem zur Planung und  Simulation von Transportvorgنngen im Tagebau

 


Figure 3: Schematic illustration of the Rope con-system

Figure 4: Scenario1: mining on levels 5to 13 –specific transport costs for horizontal mine extension
 
 
Figure 5:  Scenario 2: mining on levels 5 to 18 – specific transport costs for horizontal and vertical mine extension
 
 
Figure 6: Scenario 3: mining on levels 13 to 18 – specific transport costs for vertical mine extension
Primary crusher position system
 
Border of mine only trucks
Level  5
trucks & Ropecon
Level 10        trucks & Ropecon
Average transport rate ,trucks
t/Bh
417
592
736
Specific capital costs, trucks
EUR/t
0.239
0.168
0.135
Specific operating costs, trucks
EUR/t
0.204
0.144
0.115
Specific costs, trucks
EUR/t
0.443
0.311
0.250
Specific capital costs ,Ropecon
EUR/t
-
0.028
0.056
Specific operating costs,Ropecon
EUR/t
-
0.025
0.035
Specific costs, Ropecon
EUR/t
-
0.053
0.091
Total specific costs
EUR/t
0.443
0.365
0.342
 
Table 1:  Scenario 1: mining on levels 5 to 13 – specific transport costs in case of horizontal mine extension
 
 
 
 
Primary crusher position system
 
Border of mine only trucks
Level  5
trucks & Ropecon
Level 10        trucks & Ropecon
Average transport rate ,trucks
t/Bh
462
677
803
Specific capital costs, trucks
EUR/t
0.215
0.147
0.124
Specific operating costs, trucks
EUR/t
0.184
0.125
0.106
Specific costs, trucks
EUR/t
0.399
0.272
0.230
Specific capital costs ,Ropecon
EUR/t
-
0.028
0.056
Specific operating costs, Ropecon
EUR/t
-
0.025
0.035
Specific costs, Ropecon
EUR/t
-
0.053
0.091
Total specific costs
EUR/t
0.399
0.326
0.321
 
Table 2: Scenario 2: mining on levels 5 to 18 – Specific transport costs for horizontal and vertical mine extension
 
 
 
 
Primary crusher position system
 
Border of mine only trucks
Level  13
trucks & Ropecon
Average transport rate ,trucks
t/Bh
342
709
Specific capital costs, trucks
EUR/t
0.291
0.140
Specific operating costs, trucks
EUR/t
0.248
0.120
Specific costs, trucks
EUR/t
0.539
0.260
Specific capital costs ,Ropecon
EUR/t
-
0.076
Specific operating costs, Ropecon
EUR/t
-
0.040
Specific costs, Ropecon
EUR/t
-
0.116
Total specific costs
EUR/t
0.539
0.376
 
Table 3: Scenario 3: mining on levels 13 to 18 – Specific transport costs for vertical mine extension

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