vermeer jim hutchins 1

CONTINUOUS SURFACE MINING THE CRUSHING ALTERNATIVE

Author: Jim Hutchins, PhD Applications Engineer, Vermeer

Track Group Specialty Excavation

1510 Vermeer Road East Pella, IA 50219

Telephone: +1 641 780 7439 Facsimile: +1 641 621 7505

Email: [email protected]


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2009 Vermeer Corporation. All Rights Reserved.

CONTINUOUS SURFACE MNING THE CRUSHING ALTERNATIVE

ABSTRACT

Continuous surface mining has the potential to occupy a significant role in open pit mining. A discussion on the merits of continuous surface mining over conventional drilling and blasting are presented. Improvements in technology now allows continuous surface mining of material well above 140MPa efficiently and cost effectively when compared to drilling and blasting / primary crushing. These improvements include the ability to cut vertical highwalls with square corners. Precision mining techniques result in high percentage recoveries. The paper presents several of these techniques for the Vermeer T1255 Terrain Leveler surface excavation machine (SEM), and shows their impact. A detailed parameter study has been conducted into the feasibility of incorporating a built-in loader in the T1255 Terrain Leveler SEM to load the cut material directly onto haul trucks, or to deposit the cut material on the floor behind the machine. The material would then be picked up by a separate loader for hauling to the stockpile. Two different loading techniques are presented in addition to the built-in loader. A comprehensive computer program has been developed to investigate the impact of mine size, production rate, truck size, haul distance, working hours and machine availability on the costs per ton of product produced. In addition, the number, size and types of loaders and haulers can be optimized. The paper presents results of a parameter study for limestone (40 80MPa). Mine sizes from two to 14 million tons per year are used in the study.


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INTRODUCTION

While it is growing in importance, the volume of continuous surface mining is insignificant when compared to the total open-pit mining industry. Most of the tonnage in open-pit mining is produced using conventional drill and blast methods, with continuous surface mining relegated to those mines / quarries where environmental constraints make blasting undesirable or uneconomical. Many continuous surface mining operations are currently in mines where the material is quite soft and easy to cut, such as coal, soft limestone and gypsum. The techniques of drilling and blasting are well known, having been formulated over decades of trial and error and backed up by extensive analytical work carried out by universities and engineering firms. To take advantage of economies of scale most of the equipment used is very large and very expensive. Sufficient operating history has been accumulated to allow maintenance personnel to schedule down time for repair and maintenance of the equipment on a regular, timed basis. The drilling and blasting costs per ton are also well defined. But with increasing regulations and control of explosives throughout the world, these costs can become hostage to political climates especially in remote and possibly unstable, locations. The product size obtained from drilling and blasting operations varies widely from huge rock slabs down to powder. This large size differentiation creates the need for a primary crusher to reduce the product obtained down to manageable size. The size distribution from different types of crushers has been sufficiently documented so that curves of size distribution for various crusher configurations and rock types are available to mine personnel. Any alternative to drilling and blasting/primary crushing, such as continuous surface mining, must be shown to be adaptable to current mines / quarries, and competitive in cost per ton of the product. In some instances, it must also be able to produce a product within certain size-fraction limitations and chemistries. It is also clear that continuous surface mining methodology needs to be refined so that the cost per ton of the entire mining, loading, and hauling operation is optimized. The goal of this paper is to discuss how new and available technologies can make continuous surface mining a strategically important component of the total open-pit mining scenario.

STRATEGIC USE OF CONTINUOUS SURFACE MINING There are several fairly obvious ways to increase the role of continuous surface mining. Increase mining production in existing mines In many cases, the infrastructure in a mine / quarry is fully utilized, and any attempt to increase production to take advantage of spiking spot market prices would take considerable amount of capital and lead time to install a larger primary crusher. Primary crusher capital costs will often be in the tens of millions of dollars, with purchase / commissioning times of 2 4 years. Since continuous surface mining produces a uniform product size with a narrow size fraction, the primary crusher can be by-passed. One iron ore mine in Western Australia was able to greatly


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Figure 1. Gypsum deposit interspersed with clay

increase its annual ore production by using two Vermeer T1255 Terrain Leveler surface excavation machines (SEM) and four portable crushers. Material from the crushers was stockpiled using CAT 988 wheel loaders. This increase in mine production was all put in place within months from the time the equipment order was placed.

Increase available ore The safety zone for mines which use drilling and blasting mining methods can vary from 100 to 300 meters. In addition, existing conveyors, mine buildings and crushing equipment all can limit the available ore that can be mined. The shock and vibration from continuous surface mining is negligible, and therefore the danger zone becomes a non-issue. Blasting operations can cause a fracture zone on the mine floor that extends well below the surface. In many mines, the amount of material that can be mined from the floor is limited by the location of the surrounding water table. Because of the negative consequences of punching through the mine floor to the water table, often times four to six meters of mineable material is left at the end of the drilling and blasting operations. The T1255 Terrain Leveler SEM has been successfully used to remove an additional four meters or more of the mine floor in a mine in Western Australia that previously have been left behind. Utilize precision mining During conventional drilling and blasting operations, there is no control over how the material fractures and the amount of mixing that occurs during the blast. In some mines / quarries this mixing would result in unacceptable product quality, and increase the cost per ton of the mined product to unacceptable levels. This is the case in a gypsum mine in Oklahoma (Figure 1), and in a copper mine in Africa (Figure 2). As can be seen from Figure 1, the gypsum deposit is interspersed with clay pockets throughout the mine face. The bands that can be seen in Figure 2 represent different copper content in the ore. Mixing up the ore in both cases would result in a lower quality product. The ability to exactly control the cutting surface is what makes continuous surface mining into precision mining.

Figure 2. Copper mine with bands of variable quality ore


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Figure 3. Following a coal seam in Eastern Australia

Figure 4. Vermeer T1255 Terrain Leveler SEM operating in a limestone mine in Puerto Rico

Other applications of precision mining can be found in coal mines where the coal is laid down in thin seams separated by a parting layer. Using a continuous surface miner, one can differentiate between different qualities of coal and between coal and the parting. The operator can adjust the cutting depth of the surface miner to follow the coal seam along its length. This prevents unwanted mixing of the different coal layers or the coal and parting. Figure 3 shows the T1255 Terrain Leveler SEM following a coal seam in Eastern Australia. Precision surface mining also produces a smooth floor which can help minimize wear and tear on mine trucks

and loaders resulting in significant savings on rubber tires. This can also allow use of off-road trucks in place of mine trucks in many cases. Figure 1 shows the type of floor that can be achieved. Using a laser or GPS system to control the digging depth, a one- or two-degree slope can be imposed on the mine floor to allow drainage towards a sump. This will help to keep the maximum amount of product free of moisture a longer time than with a horizontal floor, and alleviate moisture contamination of the fines. Precision surface mining produces uniform material with a tight particle size distribution from the start. As will be explained in a later section, top-down cutting allows some degree of control of product fines and oversize generation (larger than 6 in or 15 cm). Having this small-sized material produced in a uniform configuration allows the material to be handled much more efficiently than the product achieved in drilling and blasting (including savings on truck bodies and excavator buckets). The uniform product size achieved also allows more efficient settings on secondary and tertiary crushing systems so that savings can continue well past the primary crushing stage. In one six-month trial in limestone, a 20 percent improvement in throughput was achieved in a Jeffries hammer mill, and resident time in a third stage ball mill was greatly reduced according to the quarry manager. This trial was conducted during the second half of 2005.

NEW TECHNOLOGIES IN CONTINUOUS SURFACE MINING

Vermeer T1255 Terrain Leveler SEM A photograph of the Vermeer T1255 Terrain Leveler SEM is shown in Figure 4. The view shown is from the rear of the machine. The following innovations in surface mining continuous machine design become immediately apparent. Stability The machine is designed to deposit the cut


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material on the floor behind. This allows the T1255 Terrain Leveler SEM to be built with a low center of gravity. A low center of gravity helps make the machine more stable, and allows for safe operation on a wide variety of sloping terrains by reducing the danger of tipping. Maneuverability A two track system allows the T1255 Terrain Leveler SEM to be extremely maneuverable. With the ability to counter-rotate its tracks, the T1255 Terrain Leveler SEM can virtually turn in its own footprint in less than 30 seconds. This maneuverability allows for cost-effective use of the machine in small spaces and narrow benches if necessary. Rear-mounted drum A rear-mounted drum minimizes the contact of the cut material with the machine. The tracks do not pass over the cut material, thereby keeping the original size fractions intact. Access to the drum for changing of picks is quick and easy helping to reduce down time. Drum, supported from the center, wider than the tracks This feature, together with a rear-mounted drum allows cutting vertical high walls and square corners. This can be seen in Figure 5 for a gypsum mine, where the maximum amount of mineable gypsum was obtained. Top-down cutting The T1255 Terrain Leveler SEM cuts down on the rock floor, as shown in Figure 6. . The drum is cutting down on the rock face, with the material passing under the drum out the rear of the machine. The most important benefit of this direct application of force, along with high weight to engine power ratio of the machine, is that the T1255 Terrain Leveler SEM can cut harder rock than those machines that cut from the bottom up. The machine is cost effective in cutting rock in the 125MPa (18,000psi) range, and in some instances has been used to cut limestone in the 200MPa (29,000psi) range. In these cases, drilling and blasting was not viable.

Top-down cutting minimizes contact of the cut material with the drum, which reduces wear on the picks and pick holders. It also minimizes any problems with clogging/arching/bridging of wet, sticky material in the drum while cutting. Another important aspect of down-cutting that can be seen in Figure 6, is that the drum rotation is in the same direction as the transport, resulting in little or no slipping of the tracks on wet or soft surfaces. In bottom-up cutting, the machine has to pull the drum through the cut.

Figure 5. Vertical highwall with square corners in gypsum quarry


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Figure 6. Top-down cutting

Top-down cutting allows some variation in the size fractions obtained during cutting. Since all of the material passes under the drum, and the tooth penetration reduces with depth, the deeper the cut, the smaller the material achieved. Conversely, larger particles can be achieved by reducing the digging depth, and increasing the speed (increased tooth penetration). This is not to say that we can dial in a product size in large part the size fractions obtained are a function of how the rock is laid into the ground. This is especially true for rock laid down in thin layers, where digging depth has little relationship to product size.

Top-down cutting improves the effect of the cutting force on the rock over that achieved from bottom-up cutting. This was observed during the initial development phase of the T1255 Terrain Leveler SEM, which originally cut up (similar to the rock trenchers that Vermeer has been manufacturing for over 40 years). Changing from bottom-up to top-down cutting improved the production rate of the machine by up to 50%. Realistic mining cost calculations In order for precision mining to become a viable player in the surface mining environment, precise calculations of mining production and cost per ton must be available for various types of rock with variable rock properties and abrasiveness. Vermeer rock mechanics lab In an effort to obtain pertinent rock property information, in early 2006 Vermeer established a state-of-the-art rock mechanics laboratory at its factory in Pella, Iowa, USA (Figure 7). Using computer controlled equipment provided by Geotechnical Consulting & Testing Services, Vermeer has tested rock from all over the world to build up a database of rock properties such as unconfined compressive strength (UCS), indirect tension (Brazilian Test), abrasivity (Cerchar Abrasivity Index) and density. To date, the lab has processed rock from over 700 different locations, and performed over 1500 UCS, 1600 Brazilian, and 3100 Cerchar abrasivity tests. Field demonstrations of capability Determining the rock properties is only part of the picture. Over the past three years, the T1255 Terrain Leveler SEM has been tested on numerous sites where the instantaneous production

Figure 7. Rock mechanics laboratory


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rates and pick wear were exactly determined. This has enabled correlation of rock properties with production rates and abrasivity costs. Solutions calculator A computer program has been developed called the Track solutions calculator. The calculator section of the program (Figure 8) presents results in US and metric units, various world currencies, and eight different languages.

Data from the rock lab and field demonstrations of capability have been built into this program so that production rates and abrasivity can be estimated for various rock types. The program recognizes six different rock hardness categories from to and nine abrasivity categories from to . The program allows calculation of the total fixed and variable costs per hour of all Vermeer track machines. These costs include ownership, maintenance and operating costs. As can be seen in Figure 9, ownership costs are amortized over 30,000 hours. Maintenance costs include engine rebuilds every 10,000 hours, hydrostatic system rebuilds every 5,000 hours, and wear item costs per hour (chain, sprockets, gussets). Operating costs include tooth (pick) costs, fuel costs, machine operator wages, and oil and filter costs per hour. These costs are maintained by the Vermeer track service department, and correspond as closely as possible to actual costs,

TierIIIns_

TRUE

11/09/2009

DEPTH OF CUT WIDTH OF CUT TRACTOR SPEED

(cm) (meters) (mtr/min) meters3/hr (metric tons/hr) ($/meters3) ($/metric ton)

30 3.7 2.8 190.3 495 $2.58 $0.9930 3.7 4.1 271.8 707 $1.80 $0.6930 3.7 4.9 326.2 848 $1.50 $0.58

$ 1,870,015 20,000 $93.50 /hrMAINTENANCE COSTS

$ 41,280 10,000 $4.13 /hr$ 178,560 5,300 $33.69 /hr

$135.25 /hr $135.25 /hr$173.06 /hr

$100.00 /hr $0.14 ($/metric ton) $100.00 /hr$0.66 120.0 $79.26 /hr

$35.00 /hr $35.00 /hr$9.45 /hr $9.45 /hr

$223.71 /hr

ONE (1) USD = 1.0000 ($) ENGLISH

LIMESTONE

MOSTLY MEDIUM ROCK (40-80 MPa)

USA

, DO

LLAR

S

TOTAL MAINTENANCE COST

1.0 to 2.0 -ABRASIVE

OPERATING COST

HYDROSTATIC SYSTEM REBUILD REBUILD HOURS

PERCENT AVAILABILITY 100.0%

2600

DENSITY (kg/m3)

2.60

SPECIFIC GRAVITY

WEAR ITEMS (NO TEETH)

TOOTH COSTS

< INPUT CUSTOMER NAME >

CURRENCY CONVERSION

TOTAL OPERATING COST

USA, DOLLARS

OIL & FILTERS

FUEL COST PER LITERMACHINE OPERATOR

FUEL USAGE (liters/hr)

OWNERSHIP COST:

HOURS

TRACK SOLUTIONS CALCULATORVERSION: 06012009

T1255 TERRAIN LEVELER SEM

ENGINE REBUILD REBUILD HOURS

PRODUCTION

PURCHASE PRICE

$ 490

Note: The costs and production rates as calculated by this solutions calculator are only estimates. The actual amounts will vary based upon your conditions, maintenance, operator experience, site preparation and many other factors. This is only a tool for estimating. The actual production rates and costs will vary.

METRIC UNITS

PURCHASE

EXPLANATION OF TERMSCURRENCY

MAINTENANCE

ROCK TYPE

MACHINES

RETURN

MATERIAL

ABRASIVITY

LANGUAGE SAVE SCREEN

PRODUCTION COST

COMPARISONS

FINANCING COSTS

START HERE

Figure 8. Calculator section of Track solutions calculator


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quantities, and replacement hours. All of the cells colored in yellow can be modified by the user so that actual values currently in effect can be input. In this way the sensitivity of various parameters on cost per ton can be investigated. When a material type is chosen, the program calculates a range in possible production rates per hour. Once this value is calculated, then cost per cubic meter or yard and cost per ton are calculated using the total fixed and variable costs as described above. Because the range in rock properties is quite broad, three different production rates are presented. The first and third values bracket the low and high production rates, and the second value presents what can be expected on average for that type of rock. Choosing the abrasivity category will cause the tooth and abrasivity values to be calculated. It is important to understand that the production and abrasivity values are estimates based upon laboratory and field demonstrations of capability. The actual values will be different from those predicted based upon site conditions, maintenance, operator efficiency and many other factors. The program is only a tool for estimating. If the cost per ton predictions as presented for one machine by the Solutions Calculator are competitive with other mining methods, then multiple machines can produce desired production rates at the same cost per ton.

Loading considerations In order for continuous surface mining to be a viable mining alternative, the integration of the new mining method into the total mining, loading, and hauling process also needs to be investigated. Of particular interest is how the cut product should be loaded onto the hauling trucks for transport to stockpile locations. Because of the number of variables involved, a loading comparisons computer program was developed. The program uses surface miner production and cost information from the track solutions calculator program, and computes the number of surface miners, loaders and haulers needed, the total annual costs, and cost per ton for each operation. Input variables include annual production requirements, surface miner production rate, truck size, loader size and type, haul distance, shifts, hours, days and weeks of operation, operator labor rate and machine availability percentages. The total fixed and variable costs for each of the loading and hauling machines include the cost of ownership, maintenance, and operation in the same manner as for the surface miner. Idle time and costs are taken into account. This section uses input from the calculator section of the program for the mining production rates and costs per ton. The program has an optimization feature which varies the mining hours to minimize the idle time of the surface miner, and the loading and hauling hours to reduce the loading and hauling costs to the minimum combined values. Four different loading techniques are investigated for the Vermeer Terrain Leveler SEM: 1) Surface miner with a built-in loader; 2) Surface miner with a separate wheel loader; 3) Surface


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miner with a separate scraper loader; and 4) Surface miner with a separate Vermeer concept loader. The output summary screen is shown in Figure 9. An important fact to realize is that the mining/loading/hauling times are equal for the surface miner with a built-in loader. When you are not mining, you are not loading or hauling, and vice

versa. An immediately apparent effect of this coupling is that the mining efficiency of a miner with a built-in loader is dependent upon the amount of time necessary to change trucks. The miner has to stop mining when a truck is fully loaded. The fully loaded truck needs to be replaced with an empty truck, and then the miner has to start mining again. This time delay can be anywhere from 30 seconds to several minutes depending upon the availability of an empty truck. The overall consequence of this coupling over the course of a day is to reduce the effective mining production rate of the miner by a factor of the time delay times the number of truck loads per day. In addition to the truck-changing delay, mining with a built-in loader limits the loading rate to that of the effective mining rate. In mostly medium rock, the wheel loader can load a truck much faster than the miner with a built-in loader can mine and load. This means that a truck is tied up longer getting loaded with a built-in loader than in the case for a separate loader. A longer loading time will either cause an increase in the number of trucks necessary, or an increase in the mining/loading time. This results in an increase in the loading/hauling costs per ton.

Figure 9. Loading comparisons computer program

Loader 2500

TRUE

/hr

13-METER (CAT 992)

LOADER 2500 16 METER

17 2 $1.68 $ 8,382,102 14 2 $1.25 $ 6,260,154 14 2 $1.25 $ 6,260,154 14 2 $1.25 $ 6,260,154

COSTS INCLUDED WITH MINING 20 1 $0.22 $ 1,123,656 19 1 $0.19 $ 952,520 COSTS INCLUDED WITH HAULING

17 6 $1.09 $ 5,425,863 20 3 $0.76 $ 3,785,038 19 3 $0.75 $ 3,737,245 17 6 $1.44 $ 7,224,682

8 $2.76 $ 13,807,965 6 $2.23 $ 11,168,847 6 $2.19 $ 10,949,919 8 $2.70 $ 13,484,835

$0.53 $ 2,639,117 $0.57 $ 2,858,046 $0.06 $ 323,12924% 24% 26% 26% 2% 2%

Note: The costs and production rates, as calculated by this solutions calculator, are only estimates. The actual amounts will vary based upon your conditions, maintenance, operator experience, site preparation and many other factors. This is only a tool for estimating. The actual production rates and costs will vary.

T1255 TERRAIN LEVELER SEM with BUILT-IN LOADER

YEARLY COSTS

HRS/DAY

# MACH

COST PER TON

# MACH

HRS/DAY

VERMEER LOADER

YEARLY PRODUCTION

(metric tons/hr)

(metric tons)5,000,000

690

(2)COMPARISON

TOTALS

YEARLY COSTS

COST PER TON

SPECIFIC GRAVITY: 2.6

COST PER TON

HRS/DAY

YEARLY COSTS

COST PER TON

WHEEL LOADER#

MACH

REGULAR HOURS

LABOR RATE

(2)

1.0 to 2.0 -ABRASIVEMOSTLY MEDIUM ROCK (40-80 MPa)

SCRAPERYEARLY COSTS

HRS/DAY

# MACH

NUMBER SHIFTSLIMESTONE MINE

100 TON

10DAYS/WEEK

WEEKS/YEAR7

52$35.00

OPERATORS WORKING

INSTANTANEOUS PRODUCTION RATE

LOADING COMPARISONS - MINING

HAUL DISTANCE (km)

2

4.00

AUSTRALIA, DOLLARS - METRIC UNITS

LABOR DISTRIBUTION

ADJUSTMENTS

MINING

LOADING

HAULING

SAVE SCREENEXPLANATION OF

TERMS RETURN

OPTIMIZE ON OPERATORS

CHOOSE

LOAD DIRECT TO TRUCK

CHOOSE CHOOSE

TRUCK SIZE

WHOLE SHIFT MACHINE AVAILABILITY

BREAKEVEN DISTANCE

MINING PLAN


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With a separate loader, the mining time is independent of the loading and hauling time, so both can be optimized separately. Finally, it takes a certain amount of the available horsepower to gather and load the cut material for a miner with a built-in loader. For purposes of the parameter study below, this was set to 40 percent.

Efficient material handling procedures A parameter study capability was built into the loading comparisons program so that the effect of variables such as haul distances and miner production rates could be studied. Of particular interest to Vermeer was to determine the most cost effective method of loading and hauling the cut material to the final processing location. Haul distance The effect of varying hauling distance from the mine face to the stockpile of a limestone mine

Figure 10. Haul distance parameter study results

TRUE TRUE TRUE FALSE

SHIFTS HOURS/ DAYDAYS/ WEEK

WEEKS/ YEAR

2 10 7 50

MINER TRUCK WHEEL LOADER SCRAPER

80% 90% 85% 80%10.00%1.0000 America,

WHEEL LOADER LOADING POWEROne (1) USD =

SCRAPER LABOR RATE ?35.00

9-METER (CAT 988)

MACHINE AVAILABILITY

SPECIFIC GRAVITY:

100 TON

2.4 LIMESTONE

VARIABLE

24 METER


PLOT PARAMETERSANNUAL MINE

PRODUCTION (tons) 4,000,000

TRUCK SIZE

PRODUCTION RATE (tph) HAUL DISTANCE (km)

800

EFFECT OF HAUL DISTANCE ON TOTAL COST PER TON

$0.00

$1.00

$2.00

$3.00

0 1 1 2 3 4 5

HAUL DISTANCE (km)

CO

ST P

ER T

ON

MINER WITH BUILT-INLOADER

MINER WITH WHEELLOADER

MINER WITHSCRAPER LOADER


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from 0.25 to 5km is shown in Figure 10. The set of bar graphs shows the cost per ton variation for the different loader types. From the bar graphs, it can be seen that the cost per ton for all loading/hauling methods increases with increasing haul distance. In addition, it can be seen that the cost per ton for both of the miners with separate loaders is well below that of the miner with a built-in loader for all haul distances shown. In comparing the performance of the wheel loader vs. the scraper loader, it can be seen that the scraper loader becomes less efficient as the haul distance increases. This relationship is the result of the increasing number of scrapers necessary for loading and hauling as haul distance is increased much beyond a kilometer. When this increase in cost per ton is combined with the necessity of the scraper to run over the cut material during the loading process, it would appear as though loading with a scraper loader is only viable for a limited number of cases. For this reason only the miner with a built-in loader and the miner with a wheel loader are studied further in this paper. The parameter study has been carried out for all sizes of trucks, wheel loaders, scrapers, and haul distances with similar conclusions. Size of mine Figure 11 shows the results of increasing mine production from 2 to 14 million tons per year. Since the total cost of all equipment (ownership, maintenance, and operating costs) is used to determine the cost per ton, increasing mine production does not result in a dramatic change in the cost per ton. The top graph shows a slight increase in cost per ton of the miner with a built-in loader as the mine production increases. This is caused by the coupling effect between mining, hauling and loading which prevents individual optimization of the process. This coupling effect is more dramatically revealed when looking at the bottom set of graphs. For each case, the number of continuous miners with built-in loaders is considerably greater than that when a separate loader is used, or the loading hours per day is considerably greater or both. The trend in cost per ton for a miner using a wheel loader decreases slightly as the mine size increases. The reason for this is included in the bottom set of bar graphs and curves showing the effect of mine production on number of miners. The loading hours per day (right hand scale) is shown to be fairly independent of mine production, even though the number of miners is increasing. One important conclusion that can be reached from this parameter study is that the solution to increasing mine production is increasing the number of surface miners. When the mining is decoupled from the loading, the cost per ton actually decreases with larger mines. Using multiple numbers of smaller continuous surface miners provides great flexibility in mine planning, and also improves overall mine output through increased redundancy of equipment.


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Production rate One of the coupling factors for a miner with a built-in loader is that the loading rate is the same as the production rate. It would seem as though there would be a point where increasing continuous surface miner production rates could overcome the coupling effect. If so, this point could be where the surface miner production capacity reached or exceeded that of a wheel loader. In practice, this could be the case from mining in soft material such as coal compared to mining in harder and harder limestone. The results of a parameter study where the continuous surface

miner production rate varies from 800 to 1800 tons per hour is shown in Figure 12. The total cost per ton for both loading cases generally decreases with increasing mine production. In addition, the difference between the cost per ton for each loading configuration decreases as the miner production increases. But it can be seen that the cost per ton for a miner with a built in loader is always greater than that for a miner using a separate wheel loader for all practical mining production rates.

Figure 11. Mine size parameter study results

Parameter FALSE TRUE FALSE FALSE

TRUE TRUE TRUE FALSE

SHIFTS HOURS/ DAYDAYS/ WEEK

WEEKS/ YEAR

2 10 7 50

MINER TRUCK WHEEL LOADER SCRAPER

85% 95% 90% 85%40.00%1.0000

WHEEL LOADER LOADING POWEROne (1) USD =

SCRAPER LABOR RATE $35.00

9-METER (CAT 988)

MACHINE AVAILABILITY

SPECIFIC GRAVITY:

100 TON

2.4 LIMESTONE

America, Dollars

2.00

24 METER


PLOT PARAMETERSANNUAL MINE

PRODUCTION (tons) VARIABLE

TRUCK SIZE

PRODUCTION RATE (tph) HAUL DISTANCE (km)

800

EFFECT OF MINE PRODUCTION ON TOTAL COST PER TON

$0.00

$1.00

$2.00

$3.00

2 4 6 8 10 12 14

ANNUAL PRODUCTION (millions of tone)

CO

ST P

ER T

ON



EFFECT OF MINE PRODUCTION ON NUMBER OF MINERS

0

2

4

6

8

2.0 4.0 6.0 8.0 10.0 12.0 14.0ANNUAL PRODUCTION (millions of tons)

NU

MBER O

F M

INERS

0

5

10

15

20

25

LOAD

ING

H

OU

RS P

ER D

AY

MINER WITH BUILT-IN LOADER MINER WITH WHEEL LOADERMINER WITH BUILT-IN LOADER MINER WITH WHEEL LOADER


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The reason for this is the second coupling effect of mining with a built-in loader - which is the time delay between changing trucks. This has made the cost per ton of a miner with a built-in loader greater than that of a similar miner using a separate wheel loader in all of the cases investigated.

Efficient mining methodology This paper has addressed the concept of decoupling the mining from the loading and hauling processes. The next step is to address how the mining, loading, and hauling should be interfaced. Two different configurations are presented below: Mining, loading, and hauling in three sections This scenario would set up three different sections, one for mining, one for loading and hauling, and one for sampling. The area of each section would be defined as that area able to be mined in one shift or work day by the surface miner. This configuration would minimize interference of the mining and loading/hauling processes. The methodology can use one or more surface miners per pit depending upon the area available. The copper mine in Africa shown in Figure 2 utilizes this configuration. Figure 13 shows a possible layout of three section mining with one wheel loader servicing two continuous miners. Mining, loading and hauling in two sections Where sampling of the cut material is not required, then only mining and loading/hauling sections need to be established. The area of each section is defined above as that area able to be mined in one shift or work day by the surface miner. This mine setup is one used in an iron ore mine in Western Australia. Figure 14 shows a possible two-section mining configuration with one loader servicing one continuous miner.

Figure 12. Production rate parameter study results

EFFECT OF PRODUCTION RATE ON TOTAL COST PER TON

$0.00

$1.00

$2.00

$3.00

800 900 1000 1200 1400 1600 1800

MINER PRODUCTION RATE (tph)

CO

ST P

ER T

ON




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Mining, loading and hauling in the same section This configuration shown in Figure 15 can be used in situations where the cut material would permanently coalesce in the presence of moisture. The continuous surface miner would always be two rows ahead of the loading and hauling operations. So in the event of a downpour, the amount of cut ore adversely effected by the moisture would be limited to acceptable quantities. This method would allow decoupling of the mining and loading/hauling operations in that the

Figure 13. Two miners and one loader in three sections in same pit

Figure 14. One miner and one loader in two sections in same pit


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mining could continue without the loading and hauling operations, but stoppage of the mining operations would eventually stop loading and hauling in the long run. The gypsum mine shown in Figure 1 uses this configuration. Laser/GPS control The T1255 Terrain Leveler SEM can be outfitted with one or two laser/GPS receivers. The system is pre-wired to accept either type of system to control digging depth. These systems can be used to obtain a smooth, flat floor angled horizontally, in a single plane if using one

laser/GPS or in two angled planes if two guidance instruments are used. Tilting the mine floor toward a sump will allow rain water to drain off the cut face, preventing moisture contamination of the cut product. This is especially important while mining those materials which produce sticky messes in combination with water or where a dry product is necessary for down-stream processing. Tilt/two directional leveling is shown in Figure 16.

Cutting depth optimization There is a relationship between cutting depth and product size, as mentioned in the section on top-down cutting. There is also a relationship between cutting depth and the production rate of the Vermeer T1255 Terrain Leveler SEM. This relationship was determined in trials conducted in coal parting material in a coal mine in Eastern Australia. The relationship between cutting

Figure 16. Tilt / Two directional leveling

Figure 15. One miner and one loader in same section


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depth and production rate has been input into the calculator section of the Track solutions calculator. This relationship between decreasing the cutting depth and total mine cost per ton is shown in Figure 17. The only change in input parameters for the graphs is the cutting depth was decreased in increments from the maximum of 68cm down to 15cm. Figure 17 shows that the production rate for the material being mined increased from 710 tons per hour up to about 830 ton per hour over the range in cutting depth. It shows a slight increase in loading cost per ton caused by the longer travel time of the loader to achieve a full bucket as the cutting depth decreased. The hauling cost per ton remained unchanged, while there was a significant decrease in the mining cost per ton. This decrease over-shadowed the increase in loading cost per ton so that the overall total cost per ton was decreased with a decrease in cutting depth.

There is a point where a decrease in cutting depth may make the product size distribution unacceptable, or where the loading becomes inefficient. This point can best be determined through trials conducted onsite in actual mining conditions.

Figure 17. Cutting depth parameter study results

EFFECT OF DECREASING CUTTING DEPTH

$0.00

$0.40

$0.80

$1.20

$1.60

$2.00

1520253041516169

CUTTING DEPTH (cm)

CO

ST PER TO

N

700

750

800

850

PRO

DU

CTI

ON

RA

TE (T

PH)

TOTAL COST/TON MINING HAULING LOADING PRODUCTION RATE


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CONCLUSIONS Continuous surface mining can be cost effective for a much broader range of mines than

suggested earlier in this paper. Those mines/quarries which currently use conventional drill and blast methods, but whose primary crusher is approaching its useful life may benefit from continuous surface mining strategies.

Improvements in continuous surface mining methods now allows cost effective mining in

much harder rock and mine geometries than heretofore. The most cost-effective method of loading cut material onto haul trucks is with a separate

loader. Miners with built-in loaders require more trucks, increased mining/loading/ hauling hours, or both.

Tools are now available to help mine planners determine the total cost per ton, and cost

effective equipment configurations for a wide variety of mines/quarries. Bigger machines are not necessarily better in terms of overall costs and reliability.

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