The opencast method used for mining hard coal and rocks requires drilling and blasting of strata which causes considerable investments and also creates an adverse impact on the environment & safety. These disadvantages demand the introduction of improved technology for blasting free, safe, eco-friendly and economic mining. By continuous research and also experience gained from the rock cutting technology with the application of road header etc. it has become possible to do mining without blasting by a continuous surface miner for hard coal and rocks. Krupp Födertechnik, Germany has developed Krupp Surface Miner (KSM 2000) which can work in medium-hard and hard coal and also in overburden very efficiently up to 40 MPa and in special cases like small lenses and/or thin layers up to 120 MPa of compressive strength. The article has been written with a special focus to Krupp Surface Miner (KSM 2000).
PRESENT MINING SCENARIO IN INDIA
India ranks third in terms of coal production in the World today coming after China & USA. While coal production is declining in western countries, in India, it is on an increasing trend. From a level of around 30 million tonnes per annum at the time of independence of the country, the all India coal production has already reached a level of 290 million tonnes in 1998. The trend is likely to continue in the future also. By 2001-02 annual coal demand has been estimated as 400 million tonnes in India. Such a flamboyant growth in coal production can only be met through large-sized opencast mines in the future. Out of the two broad modes of mining i.e. opencast and underground, the emphasis is given mainly on opencast due to its favorable economics, safety, large coal recovery, better output per man shift (O.M.S) and also lower gestation period.
Till now modernization of opencast mines are done mainly by upgradation of sizes for conventional equipment like shovels, dumpers, drills etc. The conventional mining shall continue with drilling and blasting, which is causing serious threats to environment and safety. Blasting is the cause of vibration, dust, noise and noxious Nitrous fumes. Besides, it also requires shifting and rehabilitation of villages situated in and around the mine site. Many of the opencast mines in India have come in close proximity of human habitats and forestland and as such, DGMS has already imposed restrictions against heavy blasting operation. Some of these locations in Mahanadi Coalfields Ltd. (MCL) are indicated below:
i) Lakhanpur OCP – On the West Side, workings have come to a standstill as Ghanmal village has come within 150-m. of the opencast workings. About 1.5 million te of exposed coal is awaiting extraction. Total coal reserves calculated upto 45-m. of the village boundary (which can be conveniently extracted by surface miners) works out to be 2.2 million te.
ii) Lingaraj OCP - The eastern flank of the overburden face has come within 120-m. of village Balugaon and further progress of the mine has been stopped. As estimated, around 1.0 million te of coal can be further extracted within 45-m. of the village boundary.
iii) Belpahar OCP - Daripali village on the north and Chharia village on the south have come within 200-m. of opencast workings. Workings have been severely restricted with about 0.3 million te of exposed coal awaiting extraction.
iv) Jagannath OCP - Balanda village on the north has come within 150-m. and Purnia village within 170-m. of the working face posing serious restrictions on further progress of the mine.
The above are few examples out of many such present and future situations in opencast mines in India. Shifting of villages in India is a big task. This directly increases cost and gestation period of the projects. Sometimes, even after having settlements & accordingly making huge investments for shifting & rehabilitation of villages and also procurement of costly equipment, situation ultimately compels reduction of the size of the mine for not really able to shift the villages from the mining boundary. Mining operations are also interrupted frequently by local villagers for the adverse effect in their houses due to blasting operation in the mine. These factors affect the production heavily. On the otherhand, cost of drilling and blasting are also on the continuous upward trend, whereas quality of coal produced is on the downward trend due to the presence of dirt bands in the coal. Over and above productivity of shovel - dumper is also not upto the expectation due to human factors and also constrained mining condition. It is also difficult to keep proper administrative control over huge no. of operators and equipment also becomes extremely difficult. There may be many more practical problems, which are inherent in today’s method of opencast mining. The above scenario and also the future need to produce quality coal (upto 34% ash) and with competitive price is compelling mining technologists to think for continuous surface mining technology.
Overview of surface miner technology
Till now three types of surface miners have been developed as follows:
1. Milling type - Cutter picks are fitted on drums and having a depth of cut up to 500-600 mm (max). Production capacity is very much less and can be used mainly for selective mining.
2. Boom type - Cutting drum fitted with picks is mounted on a boom, like roadheaders. Due to longer boom the cutting force of the machine is limited.
3. Bucket wheel type - Cutter picks are fitted on various buckets on the cutting drum, which is situated in the front of the machine. The machine can exert huge force in cutting both hard coal and rock. This type of surface miner is basically a mass-production machine.
Comparisons of technical parameters for various surface miners are enclosed with this article (sheet 1 & 2) for ready reference of the readers.
Continuous mining by Bucket Wheel Excavators without drilling and blasting has already been applied successfully in opencast for lignite and marl, which are softer payminerals with compressive strength upto 15 MPa. For decades the rock cutting technology has been applied in underground mining for rocks with a compressive strength in the range of 20 to 120 MPa. However, the attained capacities are considerably lower and the energy requirements are markedly higher compared with opencast mines. Continuous efforts, trial, laboratory tests and also experience gained from underground rock cutting technology have made it possible to develop new type of excavation machine for opencast mines which is able to attain its nominal capacity for compressive strength of 30 to 40 MPa and also can be operated upto 80 MPa with reduced capacity. It is possible for the surface miner to meet the higher compressive strength for the application of many cutting tools placed on it. By its cutting action the product size is normally below 100 mm. The output and power changes in relation to the compressive strength, brittleness, jointing, and abrasiveness of the pay minerals. Due to design and technical features of these machines, the benches are mined in layers from top to bottom and each layer in strips. Surface miner can also do selective mining to remove interband/dirt bands layers for improving the quality of pay minerals.
Krupp Fördertechnik, Germany has successfully developed its surface miner known as Krupp Surface Miner (model KSM 2000R) which is presently in operation at Taldinskij mine in Siberia. The result of the performance test in the mine (June 1996) showed that continuous production during mining of the test block amounted to 1800 bm³/h or 4482 t/h, which was almost 30% more than guaranteed. The mined block essentially consisted of Aleurolithes (siltstone) with a compressive strength of 20 to 30 MPa. The most remarkable event during the performance test was that at the end of the mining block, a Siderite layer with an approx. the thickness of 0.75m with a compressive strength of 60 - 70 MPa was passed through without any problem, whilst maintaining the nominal production rate. The above-mentioned results were confirmed by an inspection committee appointed by the President of the company
The various features of KSM 2000, its method of operation and its advantages in opencast mining have been explained later in this article. In case of operation of surface miner in combination with dumper trucks, the output of the machine is reduced due to frequent stopping of machine for truck change over. The percentage reduction depends on the payload of the truck and time for truck change over. Better productivity of the machine can be achieved in combination with conveyor transport. For higher production, longer haul length and greater depth, conveyor transport can be far superior to dumper transport in all respects. The additional savings for the application of surface miner are also achieved due to elimination of crushing complex in the mining system and also for more recovery of coal.
MECHANISM OF ROCK EXCAVATION
The KSM 2000 operates according to the rock cutting technology. For ready reference of the readers, the mechanism of rock excavation is briefly explained, as below:
The mechanism of primary rock excavation with continuous surface miner is basically penetration of a wedge of some shape into the material to be cut and this action is performed by a heavy duty drum equipped with large number of tungsten carbide tipped cutting tools. The process of digging is commonly specified by the resistance to digging: KF = Pt/b * td (kgf/cm2).
KF = Digging resistance, kgf/cm2
Pt = Tangential resistance force of rock to digging, kgf.
b = Width of excavated block, cm
td = Depth of cut, cm
‘KF’ and ‘Pt’ depend on the compressive (sc), shearing (ssh), tensile (st) strength of rock, its density (g) and also on jointing property of the massif that is characterized by the co-efficient of structural weakening (l) of rocks in the direction of digging. The value of ‘l‘ depends upon average size of elementary joints and number of joints.
‘KF’ is reduced by multi- bucket excavators fitted with sharp cutting edge and also by increasing the number of simultaneously cutting elements and decreasing the spacing of cutting elements. ‘KF’ is also reduced for semi-free /free digging than blocked digging.
Depth of cut is determined from the diameter of the cutting drum. Overall cross-section of the cut depends upon size of cutting drum, power of the machine, weight of the machine and also on the configuration, design and material of cutting elements.
The KSM 2000 is a continuously operating mobile opencast mining machine, which cuts consolidated soils and semi-solid rocks having compressive strength up to 80 MPa, without drilling and blasting, in special cases like small lenses and/or thin layers up to 120 MPa. The KSM 2000 attains its nominal capacity in rock up to a compressive strength of 40 MPa. While cutting, the material is pre-crushed to the size (-100 mm) which is very much suitable for belt conveying. The excavated material is transferred to downstream means of transportation either by dumpers or by conveyors.
The specialty of KSM 2000 is provision of parallel bucket wheels in front of the machine that is attached to main frame without a boom. The cutters of bucket wheels are equipped with point attack picks, which cut the mining face during the continuous advance of two-crawler track assembly.
For design reasons there is a space for the bucket wheel suspension between the inner and outer bucket wheels. During mining, short ribs may be left over in these gaps. Two ripper teeth operate in the gaps in order to break up and remove the remaining rock ribs between the outer and inner bucket wheels. A dozer blade is arranged behind the bucket wheels, which is constantly pressed to the ground during the cutting operation. Thus the remaining rock ribs are removed and the spillage of the bucket wheel discharge is returned to the bucket wheel loading area. This ensures a smooth & level track for easy movement of dumper trucks. The machine is built for high capacity digging upto 2.9-m cutting depth and a block width of 7.1-m. KSM 2000 is primarily a production machine, which can be compared with the present conventional mining system. The design of KSM 2000 also permits the selective mining of changing coal/rock layers of multi-layer deposits with small percentage of fines. In case of selective mining the machine is able to maintain its nominal capacity upto a depth of cut of 800 mm by suitably increasing its advancing speed. However, the depth of cut can be further reduced to a height of 500 mm for selective mining with somewhat reduced capacity of the machine, beyond which selective mining may be un-economic, since, production will be very much reduced. The output of KSM 2000 amounts to 1400 bm3/h in continuous cuttingoperation under normal conditions upto a compressive strength of 40 MPa (400 kg/cm2). The output changes in relation to the compressive strength, brittleness, jointing and abrasiveness of the rocks. KSM 2000 is able to mine at a maximum production rate of 125% of nominal output in easily mineable materials. KSM 2000 is able to excavate pay mineral of compressive strength upto 80 MPa (800 kg/cm2) with reduced output and even small lenses with compressive strength upto 120 MPa (1200 kg/cm2) with very much reduced capacity.
By turning the bucket wheels in front of the machine and continuously advancing the crawlers, the face in front of the bucket wheels is mined by the picks. The buckets pick up the cut material directly from the mining face and discharge it at the rear side of the bucket wheels by gravity on to chain conveyors. The conveying system of KSM 2000 consists of two short lead-in chain conveyors, the main belt conveyor and the belt conveyor of the discharge boom. The discharge conveyor is provided with slewing (± 90°), luffing (high 12.6m at +25° & low 3.9m at -8°) arrangements for facilitating discharge either onto dumpers or on conveyor systems.
In order to perform the necessary operating movements, the superstructure is flexibly mounted onto the undercarriage. The undercarriage is having a two-crawler assembly. Each crawler is driven by individual DC motors with infinitely variable speed. The position of the superstructure in relation to the undercarriage is adjusted by means of two hydraulic cylinders moving around in pivot point for ramping in and out of the cut and for the correction of cutting depth. The arrangement also allows the superstructure to be transversely inclined and to cut a transverse track level by the bucket wheels. Longitudinal inclination in normal operation can be up to 6° and up to 8.5° when cutting ramps. The transverse inclination in operation and also traveling can be up to 5°.
The benches are mined in layers from top to bottom and each layer in strips. Depending on the initial state in the opencast, each layer is mined by terrace cut or the KSM 2000 initially cuts a channel and subsequently removes the rest of the layer with terrace cuts. The width of the cut is 7.1m. The depth of cut may be varied from 0.5m to 2.9m. The normal cut dimension is 7.1m by 2.4m. Cut height and advance speed of KSM 2000 can be continuously adjusted. The machine operator selects the preset values in accordance with mine condition. Travel speed during cutting can be adjusted between 0.37 to 3.0m/min. Relocating speed in level terrain may be varied between 1.36 to 31.9m/min. In normal operation, KSM 2000 advances with 1.36m/min. The speed of the bucket wheel is fixed at 6 r.p.m. During scheduled operation, KSM 2000 moves straight ahead. For correcting the advance direction, the machine can be steered in the cut. Steering movements are carried out by controlling the speed and direction of each crawler.
In channel cut operation, the machine is automatically guided along a plane by means of its ‘cut control’ system. Deviations from the ideal line are only a few centimeters. In terrace cut operation, the track level of channel cut is followed and copied by means of the control unit. The control system can also be used for selective mining.
KSM 2000 is suitable for mining of compacted soils and rocks upto a compressive strength of approx. 70 - 80 MPa without drilling and blasting and loading of the mine masses on downstream transport systems. The application of the KSM 2000 offers advantages whenever the following conditions are met:
Mining of huge masses.
1. Selective mining of changing rock layers with thickness exceeding 0.5 m.
2. Selective mining with a high separation accuracy.
3. Production of lump sizes suitable for belt conveying with less fines.
4. Use of cost-beneficial belt conveyors.
The spectrum of mineable materials ranges from soils with semi-solid consistency such as compacted clay and loam to claystone, marl, siltstone, and medium-solid sandstone. Pay minerals such as lignite and hard coal or limestone can also be mined. In view of the applied mining technology, the KSM is not suitable for the mining of soils with plastic consistency such as clay or loam, whose natural water content exceeds the plastic limit. Material characteristics to be investigated in order to determine the diggability and application of a continuous mining machine are :
Uniaxial / unconfined compressive strength UCS (MPa or psi).
Tensile strength (MPa or psi).
Ratio between compressive and tensile strength (ductility).
Point load strength (MPa or psi).
Fracture spacing (m or in).
Rock quality designation RQD (%).
A sample calculation has been given below showing the production estimates in coal with various dumpers. Annual production hours can be more, however, this has been kept matching the norms usually followed in India. The calculation shows that it is possible to achieve an annual production of 7.0 to 8.0 million tonnes of coal by one KSM 2000.
|Shifts per day||3||(shifts/day)|
|Hours per shift||8||(hrs/shift)|
|Total available time per day||24||(hrs/day)|
|Working days per week||7||(days/week)|
|Working weeks per year||52||(weeks/a)|
|Holidays per Year||15||(days/a)|
|Total available days / year||350||(days/a)|
|Total available working time||8,400||(hrs/a)|
|Total available calendar time||8,760||(hrs/a)|
|EXPECTED LOSS TIME (during available working time)|
|Shiftchange + teabreak||hrs/shift||0.5||480||(hrs/a)|
|AVAILABLE WORKING TIME||6,096||(hrs/a)|
|Truck changing time/lack of trucks||min/h||10.0||1,016||(hrs/a)|
|(also maneuver of machine in new block)|
|Unexpected loss time||%||10.0||610||(hrs/a)|
|(machine failures, geological problems etc.)|
|EFFECTIVE LOADING TIME||4,470||(hrs/a)|
|Possible coal production/hour||1,670||tonne|
|with 85 Te dumper (considering|
|30 sec. for truck change over)|
|ANNUAL PROD. WITH 85 TE DUMPER||7.465568||million tonne|
|Possible coal production/hour||1760||tonne|
|with 110 Te dumper (considering|
|30 sec. for truck change over)|
|ANNUAL PROD.WITH 110 TE DUMPER||7.867904||million tonne|
Possible coal production /hour has been calculated considering truck changing time as indicated, job efficiency & operator efficiency. Variations are possible & shall depend upon application data. Productivity shall increase with conveyor as out by transport.
KSM 2000 executes several operating steps in opencast mining e.g. cutting, shoveling & primary crushing. Therefore all the operating steps must be considered while comparing with shovel/dumper and in-pit crushing/conveyor systems. It has been found that in case of a block height of 15m; KSM 2000 system attains a result, which is better than shovel/dumper system and very close to in-pit crushing/conveyor system. But for lower block height of 2m, KSM 2000 is far superior to all other systems. Besides, due to quality improvement by selective mining and also due to better exploitation of the deposits in comparison with conventional mining by drilling/blasting, KSM 2000 system shall offer further cost advantages. The estimated quantum of coal, which can further be recovered as described in the previous chapter of this article, gives an indication of such additional cost advantage for application of surface miners.
A comparative statement (sheet -3) is provided with this article, in respect of the estimated operating & owning cost of the machine (in Rs/te for coal or Rs/bm3 for overburden) and the cost for drilling/blasting, shoveling and primary crushing of coal needed in case of conventional mining. Operating & owning cost as shown in sheet-3 is tentative but it will give an idea of cost for application of KSM 2000.
The KSM 2000 offers the following advantages over conventional mining and also other milling type of smaller surface miners.
1.Higher productivity and lower mining cost.
2. Elimination of drilling and blasting. It avoids the chance of dilution of pay minerals and also offers more safety. It will also create the possibility of mining the areas where administrative regulations are imposed against blasting.
3. Saving of cost due to elimination of crushing system.
4. Higher yield of pay minerals.
5. Flexibility of its operation with both dumpers and conveyors. For higher production, longer haul distance & higher depth, conveyor transport shall be more economic than dumper transport, which can be easily catered by surface miner. In case of conventional mining, due to bigger output size (more than 800-1000 mm) dumper transport is essential. Conventional mining shall need additional in-pit crushing system to cope up with conveyor transport. In dumper transport the ratio of service weight to payload is much higher than conveyor transport, since, empty dumpers on return movement do not carry any payload. Moreover, annual wear & spareparts cost on account of tires etc. for dumper transport are also higher than conveyor transport. Cost for setting up of workshop for maintenance of Heavy Earth Moving Equipment is also huge and can be saved in case of surface miner/conveyor system. Hence, combination of surface miner and conveyor shall be more economic. Besides, it will be also easier to increase the production of mine in future, if needed, with such combination.
6. Large-scale cutting area and low advancing speed of KSM 2000 makes it possible to load dumpers more conveniently. Dumpers shall remain static at the time of loading by KSM 2000. Smaller surface miner moves at a faster speed and hence dumpers shall also have to be moved suitably at the time of loading. This is a special advantage of KSM 2000 over other surface miners.
7. Since, discharge conveyor of surface miner load the dumpers evenly from a suitable height, dumpers are subjected to better filling, which ensures greater capacity utilization and also it reduces wear of dumpers compared with loading by shovels.
8. The cutting drum of KSM 2000 is in the front. Its diameter is large (4.8m) and speed is low (6 r.p.m). Hence the cutting elements of the drum run at a slower speed and get more time to cool. This phenomenon increases the life of the cutting tools. This is an advantage over other surface miners, since, other surface miners are having smaller cutting drum diameter and rotates with higher speed and hence, cutting tools get much less time to cool. It is also easier to change the cutter picks, since the cutting drum is located in the front.
9. Due to bigger drum diameter the KSM 2000 can go for bigger depth of cut (2.9 m) and more production in comparison with other surface miners. KSM 2000 has more no. of cutting elements (540), much more power (total 3200 kW) and more weight (540 tonne). As a result it can offer much more cutting force and is able to operate in much tougher condition in comparison with other surface miners.
10. KSM 2000 permits block operation. The KSM 2000 can cut whole block on its own. Cutting can be carried out in any direction. Its operation also offers better slope stability.
11. Milling type of surface miners produce more fines (70% of output is around minus 35 mm), whereas, output size from KSM 2000 remains within 75 to 100 mm. This ensures better price realization of coal produced by KSM 2000.
12. KSM 2000 shall offer better economy in respect of cost/tonne or cost/m3 for coal & overburden. The reason is the machine is having longer life (20 years) and one machine is able to produce around 7.5 million tonne/year of coal even in combination with dumpers (please see the paragraph “Productivity of KSM 2000).
Surface miner being a new technology is yet to be adopted by mining technologists for medium hard/hard coal and overburden. Due to various disadvantages and hazards of the present conventional mining method, as explained in this article, more & more interests are growing towards this new technology. Many coal mines in India are suitable for applying this technology due to higher production demand, favorable seam gradient, long strike length, multi-seam mining and also presence of villages in close vicinity of the mines. It may not be prudent to use surface miners only for the purpose of selective mining due to the following reasons:
Use of surface miners only for the purpose of selective mining of thinner bands within coal seams shall reduce overall production of mine due to low output for such selective mining. The extent of such reduction shall depend upon the nature and thickness of the interbands and the production capacity of the surface miners. Hence, it should be necessary to assess, whether there will be grade improvement of coal (not only improvement in kcal/kg of output) for such selective mining. Grade improvement of output may prove economic viability to compensate the investment for such surface miners and also for the loss of overall production.
Due to smaller output size (< 100 mm), conveyor can be adopted with surface miner. The various advantages of conveyor system have been explained above. Dumper transport, on the other hand, essential for conventional mining for carrying bigger lumps. Mixing of two mining system, i.e. selective mining by surface miner and main production by conventional method in the same mining block, shall compel using dumper transport for surface miner also. This will obviously reduce the productivity of surface miner.
Surface miner needs different kind of planning for mining coal seams in layers from top to bottom & each layer in various strips. The slope of negotiating ramps shall be different for surface miner. Conventional mining, on the otherhand, follows benches of more height. Height and width of benches depend upon size of shovels & dumpers. Hence, it may be difficult to make proper mining layout for adopting two different kinds of mining methods in the same mining block.
Though milling type (smaller capacity) surface miners are in use in limestone and also in lignite coal for quite sometime, but bigger surface miner for mass production has been recently launched by Krupp Fördertechnik, Germany and yet to become popular. But it shall prove its worth in near future. Adopting this new technology in Indian coalmines shall open a new era in coal mining. It shall be proven as a boon in mining technology in the next millennium.
Opencast Mining (Unit Operations) - V.V. Rzhevsky.
Article (Non-Explosive Mining) - Mr. M.P. Mishra, published in MINE SAFE’97.
Article (New Technology in Opencast Mining) - Mr. S.K.Bag, published in Opencast’98 of MGMI.
Article (New Technology in Opencast Mining) - Mr. S.K.Bag, published in Scuim’99 of MGMI.
VIDEO LINK OF WORKING OF KRUPP SURFACE MINER AT TALDINSKIJ, SIBERIA
The above article had been written by the undersigned in 1998 along with his colleague Mr. Detlev Schroeder as his co-author, during his tenure in Krupp Foedertechnik, Germany. Incidentally, after the merger of Krupp with Thyssen in 2000 and also undersigned left Krupp Foedertechnik, further work on Krupp Surface Miner had been discontinued by Krupp Foedertechnik. Subsequently, Krupp had abandoned this product and the reasons are yet not known to the undersigned. This surface miner could have been ideal for working in the overburden had technological up-gradation could have continued by Thyssen-Krupp. The Krupp Surface Miner being the most strong machine could have been very demanding in the present scenario.
Merchentus India Exim International LLP
Dated, 17th Sept, 2020