9.
Open pit mining
9.1
Introduction
9.1.1
Basic descri description ption of open pit mining
Open pit mining is applied to the extraction of nearsurface deposits. Overburden removal (stripping) and mining are carried out systematically from a series of benches of benches (steps) as the pit is progressively deepened. The bench layout is designed to produce an overall slope angle that is compatible with slope stability so that an open pit resembles an inverted cone. As the base of the pit is deepened, the upper benches are pushed out so as to maintain the required slope angle. Overburden is stripped from benches to uncover the deposit and transported to a dump at some point remote from the operation itself. As the depth increases the ratio of overburden to volume of ore extracted steadily increases and, at a certain point, the cost of overburden removal makes the operation uneconomic. The remainder of the deposit might then be worked by underground mining. Advantages of surface mining compared with underground mining. Higher Productivity due to: greater degree of mechanisation, larger equipment can be used economies of scale, fewer personnel required Lower operating costs per tonne due to: higher productivity, concentration of production, less constraint on production level (easier materials handling) Lower grade deposits can be mined Reduced development time (generally). Therefore more favourable cash flow and quicker repayment of capital investment. Greater geological certainty. Safer operations •
•
• •
• •
Disadvantages of surface mines: Large proportion of waste to ore. High level of environmental impact. Affected by climatic conditions. Depth limit • • • •
Fig. 9.1: Palabora open pit copper mine, South Africa 59530725.doc
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9.1.2
Pit Limits
The pit limits are the vertical and lateral extent to which the open pit mining may be economically conducted. Establishment of the pit limits is the first stage in mine planning (Laurich 1990). They determine the: amount of economically recoverable ore, metal content, volume of waste to be excavated and moved, location of waste dumps, tailings lagoons, processing plant, access roads and all other surface facilities. • • • •
9.2
Stripping ratio
9.2.1
Definitions
The pit limits and sequence of mining are determined ultimately by economics. The concept of stripping ratio (SR) is the method of analysis used. It is a measure of the amount of waste that must be removed in order to mine one unit of ore. Grade Cutoff grade Ore Waste
The content of valuable metal (%, g/t or kg/t) in a mineral. The grade at which the value of the metal equals the cost of mining and processing the mineral. Mineral that is above the cutoff grade Mineral that is below the cutoff grade
The SR at any level of the pit is defined as: SR =
volume of waste removed tonnes of ore recovered
m 3 t
SR is also sometimes expressed as tonnes/tonne (tonnage of ore removed per tonne of ore). However, the first definition is more convenient as the costs of waste removal are directly related to its volume and the revenue from the ore is directly related to its mass. The Pit Limit is defined by the economic stripping ratio SRecon. This value is the stripping ratio at which the costs of mining and processing the ore and stripping the waste are equal to the revenue from the ore. SR econ (Bm 3 /t)
9.2.2
=
revenue from ore($/t) - ore mining cost ($/t) - ore processing cost ($/t) waste stripping cost ($/Bm 3 )
Simple stripping ratio calculation
To determine the pit limit for a simple orebody (Figure 9.2) Calculate the economic SR: Multiply by ore density to convert to m3/m3 (multiplying top line of SR equation to obtain net value of ore in $/Bm3 • •
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•
• •
•
Estimate on the orebody section where the pit limit may lie and draw in line ABC at the required slope angle The SR at this point = AB/BC m /m Compare with economic SR; if lower, move line down to, say A 1B1C1; if higher, move back up to A2B2C2 Continue until SR represented by line on section equals economic SR 3
3
In practice, the procedure is much more complicated; for example, the ore grade will usually vary throughout the orebody. A simple solution is to express the economic SR as a function of grade then draw a straight line graph of economic SR vs grade. For any grade value in the orebody, the corresponding economic SR can be read off the graph.
Fig. 9.2: Simple pit limit estimation 9.3
Overburden stripping strategies
Described below are the four basic types of stripping schedule. The first two are extreme cases and would not be applied in practice. (BucyrusErie 1979, Fourie & Dohm,1992) 9.3.1
Declining Stripping Ratio Method (Figure 9.3)
As each bench of ore is mined, all the waste on that bench is removed to the pit limit. Advantages: good operating space good accessibility to ore on next bench, all equipment working on same level, no contamination from waste blasting above the ore, equipment requirements a minimum towards the depletion of the orebody. operating costs tend to be constant in later years as the increased mining cost with depth is offset by the decreased stripping ratio. • • • • • •
Disadvantage: overall operating costs are maximum during the initial years when maximum profits are required to handle interest charges and repay the project capital investment.
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Fig. 9.3: Reducing stripping ratio method 9.3.2
Increasing Stripping Ratio Method (Figure 9.4)
Only sufficient stripping required to uncover the ore is carried out. This method allows for maximum profit in the initial years of operation and greatly reduces the investment risk in waste removal for ore to be mined at a later date. It may be applied where the economics of the operation and cutoff stripping ratio is liable to change on very short notice. The main disadvantage is the impracticability of operating a large number of stacked narrow benches simultaneously to meet regular production requirements.
Fig. 9.4: Increasing stripping ratio method 9.3.3
Constant Stripping Ratio Method (Figure 9.5)
Waste is removed at a rate approximately equal to the overall stripping ratio. The method is a compromise that removes the extreme conditions of the former two methods described. Equipment fleet size and labour requirements are relatively constant.
Fig. 9.5: Constant stripping ratio method 59530725.doc
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9.3.4
Phased Mining Sequence (Figure 9.6)
In practice, the optimum stripping sequence for a large deposit would feature a low stripping ratio in the initial and final years of operation. This plan has the following advantages. • •
• •
• •
A high level of profit can be generated at the outset to improve the cash flow. The labour and equipment fleet can be built up to maximum size over a period of time. This approach is also advantageous from a cashflow point of view. Labour and equipment requirements decrease gradually towards the end of the mine life. Distinct mining and stripping areas can be operated simultaneously, allowing for flexibility in planning. The number of mining and stripping faces required is not too high. In a large orebody, the mining and stripping areas are sufficiently wide to create good operational conditions.
Fig. 9.6: Phased mining sequence 9.4
Bench design
9.4.1
Bench height
Bench height is the most important parameter as it largely determines the other dimensions. Values range from about 2.5 m for small gold mines to 20 m for large open pits. The final bench height may be subdivided for extraction purposes into a number of subbenches or flitches. Bench height is influenced by: 1. excavating equipment dimensions (reach, operating height) 2. size and geometry of orebody small benches used for narrow lodes or lenses in order to minimise dilution and facilitate good grade control. Bench height is no longer limited by drilling depth. The prime determining parameter is the maximum digging height dimension of a shovel. Table 9.1 gives some advantages and disadvantages of maximizing the bench height. Table 9.1: Features of high wide benches Advantages High productivity and efficiency Can use large scale equipment Larger blasts Fewer equipment moves and setups Facilitates more effective supervision
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Disadvantages Less selectivity More dilution Fewer working places, therefore less flexibility Flatter working slopes (larger shovels)
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Generally, it is more advantageous, in terms of drilling and shovel efficiency, to design benches as high as possible. a)
Drilling Efficiency
A greater bench height reduces setup time per meter drilled. Also, for a given blast design, the subgrade drilling required is independent of the bench height. This means that the greater the bench height, the greater the ‘tonnage yield’ per meter drilled or per kg of explosive used. Consider bench heights of 10m and 12m, each to be drilled on a 5m x 5m pattern with a 1m subgrade. The respective drilling yields are: Drilling Y ield =
Burden x S pacing x B ench Heigh t x Densit y Hole Depth
Assuming rock density = 2.5 tonnes/m 3, for a 10m bench: Drilling Y ield =
5 x 5 x 10 x 2.5 10 + 1
=
56.8 tonnes / m
=
57.7 tonnes / m
For a 12m bench: Drilling Y ield =
5 x 5 x 12 x 2.5 12 + 1
The yield for the 12 m bench represents an increase in drilling yield of some 1.6%. Although seemingly small, for 10,000m of drilling, it would result in an extra 9000 tonnes production. Similarly, drilling costs per tonne are reduced as bench height is increased b)
Shovel Efficiency
Increased bench height also improves overall productivity of shovels, FELs, or excavators. The number of rows in a blasting pattern is generally governed by the hole diameter and explosive type. If these parameters are fixed for a given operation, the total volume of bench that can be blasted at once depends on the bench height. The greater the volume of broken ground, the lower the number of times a shovel has to be moved in order for blasting operations to be carried out. 9.4.2
Bench width
Figure 9.7a shows the common terminology for open pit slopes. A bench is a horizontal ledge from which drilling, blasting, excavation and loading of ore or waste is carried out. A Working bench is one that is in the process of being mined. The width extracted from the working bench is called the cut . The working bench width is determined by the dimensions of trucks and the required reach of excavating equipment. Figure 9.7b illustrates a slope profile cutting across an operating bench. It shows a narrow bench width of only some 3m, not sufficiently wide to accommodate equipment. However, each bench is systematically mined from one end, giving adequate room for drilling rigs, shovels and trucks. After the cut has been removed a bench of width typically 2.53.0 m is left to catch and collect material, which slides down from upper benches. Normally, the bench slope angle is 7580 o and a berm every second or third bench is sufficient. 59530725.doc
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9.4.3
Bench angle
Bench faces are normally mined as steeply as possible. The steeper the bench angle, the smaller the stripping ratio. Safe angles are determined by: •
•
geotechnical considerations, taking into account the cohesive and frictional properties of the rock and the character, spacing and orientation of joints and bedding planes. the dip of the orebody.
There are two angles which define a bench design: Overall slope angle
The angle consistent with slope stability over the full height/depth of the mine. Usually lies between 45o and 60o. The overall angle is a function of the bench face angle and the bench width. Note that a haul road on a pit slope will flatten the overall slope angle.
Bench face angle
The maximum angle consistent with stability of a single bench (say, 5 to 10 m in height). Typical values lie between 60o and 80o.
The overall slope angle is less than the bench face angle because the larger the slope, the more planes of weakness it has.
Fig. 9.7a: Pit slope crosssection, with typical dimensions
9.5
Fig. 9.7b Geometry of working bench (Atkinson 1992)
Method of working benches
Operations on each bench are conducted in cycles; typically: 1. 2. 3. 4. 5. 6.
Grade control mark out ore zones with tape or survey staffs Drill blastholes Charge holes Fire holes Excavate blasted material and load into trucks for haulage out of pit Clean bench and prepare for drilling
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9.5.1
Mining direction
Orebody lenses may be excavated in either a transverse or longtudinal direction. A transverse mining direction (digging in a direction normal to the ore vein) is more suitable for thin lenses. It allows better grade control and less dilution. With a thicker lens, it may be possible with longitudinal extraction to blast and load the ore, leaving the waste temporarily insitu. The number of working faces is determined by the required production rate and equipment capacities. Figure 9.8 illustrates how excavation can take place simultaneously and on multiple levels.
Fig. 9.8: Mining on multiple benches (Hustrulid & Kuchta 1995) 9.5.2
Selection of excavating equipment
In a mine, certain production requirements have to be satisfied and in a civil construction project the operation will have to comply with the project schedule. As equipment is very expensive in terms of capital or contract and operating costs, its utilisation should be maximised in order to minimise the unit costs of earth moving. Table 9.2: Guidelines for selection of excavating equipment Type of machine Electric rope shovel Hydraulic excavator (front end loader) Back hoe sits on top of bench, digging down Ripper , Impact ripper Mechanical cutter (drum laced with picks)
Application Large benches
Small benches only (< 5.0 m) Mediumhard material, low depth Mediumhard material Permits very selective mining Soft material, low depth
Bulldozer, Scraper
In rock and earthmoving projects, planners are usually concerned with insitu volumes as the quantities can readily be determined from excavation or mine plans. The basic unit of measurement is termed the bank cubic meter (BCM or Bm3). However, manufacturers give equipment bucket capacities in terms of the nominal bucket capacity , the volume enclosed within the perimeter of the bucket. The first task in a productivity determination exercise is to convert this measurement into BCM units. The tables given in this section are from Atkinson (1992), unless
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otherwise stated. Figure 9.9 shows the two alternative methods of excavating a bench and of spotting trucks.
a) Parallel cut
b) Front cut
Fig. 9.9: Methods of excavating benches (Hustrulid & Kuchta 1995) 9.6
Haul road layout
The form of haul roads may be spiral or switchback (zigzag). They may also be either temporary or permanent, depending on the configuration of the orebody. Where benches are being systematically worked all round the pit as it is deepened, haul roads will be mined through and new ones formed as the pit develops. Often, however, it is possible to construct permanent haul roads at one side of the pit. This would be the case for a dipping orebody, where the permanent haul road could be located at the footwall and extended as the pit deepened (figure 9.10). Note that the inclusion of a haul road in a pit wall will lower the overall slope angle and hence increase the stripping ratio. Where the orebody dips at a shallower angle than the stable pit slope, constructing the haul road as a switchback on the footwall will take the overall pit slope close to the orebody dip. The hanging wall slope, formed without a haul road, can be made as steep as possible, consistent with slope stability.
Fig. 9.10: Illustration of how permanent haul roads can be established in a footwall Factors determining selection of layout include the following. (Atkinson 1992) 1. The switchback layout allows a permanent haul road to be located at one side of the pit. 2. In large pits, a spiral layout can result in a haulage distance that is too great. 3. Areas where potential slope stability hazards exist should be avoided, possibly eliminating the spiral option. 59530725.doc
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4. The pit walls may be too steep to allow suitable bends to be formed for a switchback layout without greatly increasing the stripping ratio. 5. Tight bends associated with a switchback may be detrimental to truck and tyre life. 9.7
Haul road Construction
9.7.1
Road base
Good haul road design and construction promotes lower haulage costs and improved safety. Roads are constructed with three or four layers (figure 9.10): 1. 2. 3. 4.
Subgrade Subbase (optional) Base Wearing surface
The subgrade is the foundation layer, usually comprising compacted rock or soil. It must be strong enough to bear the loads associated with vehicles, which are transmitted from the road surface. A subbase may or may not be present, depending on local conditions. It is used where there is very weak subgrade material or in areas subject to severe frost. It is generally constructed from a clean, granular material.
Fig. 9.11: Haul road construction (Hustrulid & Kuchta 1995) The base is a layer of very high stability and density. Its main purpose is to distribute the load from vehicle tyres. It also serves to insulate the subgrade from frost penetration and protect the upper wearing surface from any swelling or softening of the subgrade. The top road layer is the wearing surface , which should provide traction, reduce rolling resistance, and resist abrasion, raveling and shear. It is formed usually of crushed rock. 9.7.2
Straight sections
The crosssection of an open pit haul road features a one or twoway travel lane, a safety berm and a drainage ditch (figure 9.11). For determination of lane width, a number of rules of thumb can be applied, in which the widest vehicles determine the road width. Three of these rules are: • •
The clearance on each side of a truck should be equal to about half the truck width. For 2way traffic, the lane width should be grater than or equal to 4 x the truck width.
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•
For a straight, even grade, onelane haul road, the minimum road width is 2 times truck width; for two lanes, 3.5 times truck width. The road crosssection should also be raised or ‘crowned’ slightly, to facilitate water runoff. The height of the crown is expressed in mm per meter of road width. A figure of 45 mm/m is typical.
Fig. 9.12: Typical 2way road section (Hustrulid & Kuchta 1995) Road Grade is determined from the truck performance charts with respect to speed and braking. Gradients of 4.5 to 6 o (8 to 10%) are usually adopted, with reardump trucks being the preferred haulage unit. A 12% grade may be used for trolleyassist trucks. (Atkinson 1992) 9.7.3
Curves
For sharp curves, additional width must be included, both on the curve and the tangent to the curve, to cover the front and rear overhangs of the vehicle and the difficulty of negotiating the curve. A recommended additional allowance for a reardump truck on a 6m radius is 125% and a 45m radius 118%. (Atkinson 1992.) Table 9.3 gives minimum turning radii for a range of trucks, classified according to gross weight. These radii can then be used in table 9.4 to find the recommended design width for single and doublelane curves of a particular minimum radius. Table 9.3: Minimum truck turning radius (adapted from Hustrulid & Kuchta 1995) Vehicle weight class Gross vehicle weight (tonnes) Minimum turning radius (m) 1 2 3 4
<45.5 45.5 – 91 91 – 181 >181
5.8 7.3 9.4 11.9
Table 9.4: Design widths for curves – rigid body trucks (adapted from Hustrulid & Kuchta 1995) Radius on inner Singlelane road Doublelane road edge of road (m) Truck category Truck category Minimum 7.6 15.2 30.5 45.7 61.0 59530725.doc
1 8.8 8.2 7.6 7.3 7.3 7.0
2 10.4 10.4 9.4 8.8 8.8 8.8
3 13.7 13.4 12.5 11.9 11.9 11.6 18
4 21.3 20.7 19.2 18.0 17.7 17.4
1 15.5 14.6 13.4 12.8 12.5 12.2
2 18.3 18.3 16.5 15.5 15.2 14.6
3 24.1 23.2 21.9 21.0 20.7 19.8
4 37.5 36.3 33.5 31.4 30.8 29.9 8/3/2011
Tangent
7.0
8.5
11.3
17.1
12.2
14.6
19.8
29.9
Depending on vehicle speeds and bend radius, a curve may also have to be banked (super elevation). Typical super elevations for mine haul roads and trucks are around 40mm per meter of road width (Hustrulid & Kuchta 1995). The distance required to make the transition from the normal crossslope section to the super elevated section and back again (super elevation runout) also needs to be considered. 9.8
Equipment
The following diagrams illustrate some modern equipment currently operating in Australian mines. Fig. 9.13: Hitachi 20 m 3 hydraulic shovel loading into Komatsu 240 tonne truck
Fig. 9.14: The KOMATSU DEMAG H655S is the worlds largest proven hydraulic shovel at over 685t gross weight and 35m3 bucket.
Fig. 9.15: The KOMATSU HAULPAK 930E is the largest truck in mining today and was the first to use AC drive. Over 100 930E’s are operating worldwide, providing production up to 320t per cycle.
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9.9
References
Atkinson T. Design and layout of haul roads. SME Mining Engineering Handbook, Vol 2, Chapt 13.4, pp13341342. SME (1992). BucyrusErie Company. Mine Planning. Surface Mining Supervisory Training Programme, Chapt 3. BucyrusErie Co. (1979). Fourie GA, Dohm GC. Open pit planning and design. SME Mining Engineering Handbook, Vol 2, Chapt 13.1, pp12741297. SME (1992). Hartman HL. Introductory Mining Engineering. Wiley (1987). Hustrulid W, Kuchta M, (1995), Open Pit Mine Planning and Design, A A Balkema, Rotterdam. Laurich R. Ultimate pit definition. Surface Mining, 2nd Edition, pp465469. SME (1990).
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