iThe Indian Mineral Industry Journal ENTMS 2008

Mining Strategy for Excavating Steep Coal Seams

Sanjay Singh*

ABSTRACT

Coal reserves are available worldwide with recoverable reserves in 70 countries around the world. With thecurrent rate of consumption of coal in the world, the proven recoverable reserves of coal may only last foranother 147 Years. In contrast, proven oil reserves stand to last for 41 years and gas reserves stand to last for63 year at the current usage rate respectively. Around 90% of the proven coal reserves are available in sixcountries i.e. USA, Russia, China, India, Australia and South Africa. As per World Energy Council (WEC),total world reserves of coal at the end of 2002 were around 909 billion metric tons (By rank world coalreserves on tonnage basis are as - 53% Anthracite & Bituminous Coal, 30% Sub bituminous Coal and 17%Lignite). If that is true then high grade Coal is only available for 78 years if the data published is correct.

The data on coal reserves is not accurately analyzed as the reserves were calculated on the basis that themining will be done with feasible mining areas, as per what is currently being mined. What has not beentaken into consideration is that cost of production of coal that is increasing all over the world because of thedevelopment of new mining areas, difficult geological and local conditions, and additional infrastructurecosts and high fuel costs. Therefore the availability of recoverable reserves of coal is in fact decreasing.

Because of all this data, coal reserves that have been calculated by all the world bodies and organizationsconcerned with such figures are looked at being overestimated amounts and the same is supported by thefact that the world ‘in situ reserve’ has decreased from 10 trillion metric tons of hard coal in 1980 to 4.2 trillionmetric tons in 2005 (60% revision in 25 years as per Energy Watch Group, even though many new miningfields have been explored during that period.).Looking at these facts on reserves of coal around the world,this maybe depleted in less then 78 years because of the increase in consumption of coal as well as theincreasing costs of production.

The savior to this crisis in the future may well be the coal from Indonesia, as long as changes are made intothe thinking of how coal mines should work and new mining methods are adapted.

In this paper the author will focus exactly on that, describing to the reader the development of new miningmethods for steep and high grade coal seams in Indonesia and all around the world and for the recommendationtowards some change in mining strategy for the recovery of high grade coal, so as to maintain if not increasethe world’s proven coal reserves in a time of increasing mining costs and increased coal consumption.

* Production Manager, Luwe Coal Mines, PT.Victor Dua Tiga Mega, Central Kalimantan,Indonesia

INTRODUCTION

Indonesia has 6.97 billion metric tons of proven coal reserves(Around 0.5% of the world’s total proven reserves of coal).Around 83% of reserves are in the category of Lignite andSub bituminous coal and the remaining 17 % are in thecategories of Bituminous and Anthracite i.e. high gradeCoal. Indonesia is the second largest exporter of Coal and7th largest producer in the world. With the currentconsumption rate of Indonesian coal, proven reserves ofcoal in the country may deplete in 37 years, if new miningfields are not developed.

Indonesian coal generally has a low ash and Sulpharpercentage with a high Gross Calorific Value (GCV) and isbest used as steam coal in power generation due to thefact that it causes less pollution than using other types of

coal and also provides better efficiency. Therefore 77% ofIndonesian coal is exported all around the world, to countrieslike Japan, Korea, India, Philippines, Vietnam, Taiwan andChina, to meet their energy requirements every year.

53% of the world’s total reserves of coal is high grade coaland is mainly located in countries (China, USA, Australia)that are themselves thinking of reducing the export of theircoal, so that they can meet their own domestic requirement.(China has already stopped their export). In fact the totalreserve of high grade coal is predominantly located in 2 or3 countries and its fast depletion may threaten future energygeneration in a manner that is both economic and ecofriendly.

In today’s context, developed countries (USA, Japan,Korea, etc) are trying to maintain the best environmentalnorms when trying to produce energy for their domesticuse, pressured mainly by the Kyoto Protocol that was setupto reduce the pollution produced by the developed countries,as they were the biggest environment polluters based on

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their advanced stages of their industrialization. Due to theexternal pressures to industrialize in environmental friendlyways and having the strong financial position, the developedcountries are able to utilize high grade coal, while thedeveloping countries are left to use low grade coal.

As reserves of high grade coal is limited, it has thereforeled to new innovations for power generation, so as to reducethe pollution as well as the use of high grade coal withmethods such as, optimum blending of low and high gradecoal, cleaning of low grade coal, etc. What has been missedout, which can make a big impact on this future problem isthe planned mining method & utilization of high grade coal.

Most of the coal deposits of Indonesia are having steepseam structures with having high quality but with difficulttopographical, geological and environmental conditions.Therefore long term mine plans are very much necessary,with emphasis on how to mine such steep seam structures.

Mining Planning: Luwe Coal Mines, PT. Victor Dua TigaMega, Central Kalimantan, IndonesiaIn order to participate in the development of non-oil gasenergy sources, particularly coal, PT.VICTOR DUA TIGAMEGA – an Indonesian private company has been allottedan area of 3,464 HA for exploration and exploitationpurposes in the Central Kalimantan region of Indonesia.The following factors were broadly considered during theplanning and designing of the opencast coal mines:

· Dip of Coal seam

· Depth of Coal seam

· Quality of Coal

· Selection of Mining Method

· Selection of Mining Equipments & others

· Pit limits for mining operation.

· Cut-off stripping ratios based on the cost ofproduction & the expected sale price based onthe market conditions for high grade coal.

· Availability of the land from land owners for mining.

The mining operation was planned for full recovery ofcoal up to the optimum depth, which is the point (pit limit)where the cost of production equals the revenue from sales)so that natural resources may be utilized fully and hencethe reserves of coal as well as the life of the mine will beincreased for long term planning.

· Dip of Coal seam: Dip of the seam varies from 24°degree to 47° in the Luwe coal deposit as per currentdetailed report of exploration and mining operation. Dipin between 24 - 45° are termed as steeply dippingdeposits and that restricts the operation of mining indip direction because of the cut off limit of strippingratios decided on the basis of topography contour

survey, thickness of coal seam, dip of coal seam aswell as the cost to mine such seams and the potentialrevenue from the sales of such coal.

· Depth of Coal seam: Luwe mines has a number ofoutcrops at different locations with different thicknessesof 0.2m to 1.2m, note that almost all the deposits ofcoal start from a very shallow depth. Thus themaximum depth of the recoverable coal is 35 - 45mfrom the surface level but that also depends on thetopography and dip of the coal seam.

· Thickness of Coal Seam: The actual thickness ofthe coal seam varies from 0.3 m to 2.7 m. The same isrequired for the calculation of the cut-off stripping ratiobefore starting the mining operation. Generally fourseams has been found in the concession area andthey are usually having the thickness of 0.8m, 0.9m,0.3m and 2.7m as the 1st, 2nd, 3rd and 4th seamrespectively.

· Cut-off Stripping Ratio: Stripping ratio is generallyexpressed as a ratio in the proportion of overburden tocoal (Cum/MT). As the depth of overburden increases,the more amount of money is spent for removal of theoverburden to expose and extract the coal. As thisratio increases there comes a point beyond which thecoal can not be economically extracted out and this iscalled the break even stripping ratio or cut-off strippingratio. Cut-off stripping ratio is decided on the basis ofthe total cost of production versus the expected saleprice of that coal. This cut-off stripping ratio can beincreased if the price of coal increases based on themarket conditions or the costs of production decreasedue to improvements in the mining method and wastagereductions.

· Pit limits for mining operation: Based on the contoursurvey, dip of seam, thickness of coal, depth ofoverburden, length of block in strike direction, & thecut-off stripping ratio, the limit of pit/mine is calculatedin dip direction so that operation can be well planned,maintained within the economical mining point (cut-offpoint) and the smallest virgin area is disturbed for themining operation.

· Nature of Overburden: Overburden above the coalseam mainly included top soil, hard silica sand & greyclay stone. Generally the top soil is soft in nature andmay be excavated directly by excavators but the silicasand and grey clay stone are hard in nature as suchrippers along with dozers are used for loosening of thehard strata instead of using other methods such asblasting because of the local constraints. These local

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constraints usually relate to the local community inthese areas, hence making blasting a non usablemethod, due to the dangers it may bring about in termsof injury, noise and destruction.

· Quality of Coal: Sample of the outcrop coal has beenanalyzed for proximate analysis as received /dry basisbefore starting the mining operation. The same isrequired to predict the limit of stripping ratio as well asthe future selling price. The quality of coal in this areavaries from 5300Kcal /Kg to 6300 Kcal/Kg (Prime HighGrade Coal).

· Land availability for mining: Before starting theexploration and drilling activity as well as miningoperation, the management has to get permission fromthe respective land owners as well as negotiate acompensation rate for using their land to extract thecoal. Note it is not compulsory for any land owner togive his land to the concession owner to mine inIndonesia and it is up to the management’s negotiatingability. Also due to the unmarked lands, it results indisputes between two land owners, thus involving thevillage head as well as the local land department. Thismay at times delay the operation as it takes time tofind the right papers and records in the department’sdata base, as well as come to a conclusion as to whoseland it may actually be. This may at times, even leadto change in planning of the mines, as certain landsmay not be used due to conflicts or the reluctance ofthe land owner to provide the land for mining.

Ø Mines Working:

· As coal deposits are outcropped and the gradientis steep, therefore the opencast mining method isthe best and most economic method that shouldbe used.

· As 90% of the strata is hard clay stone, the heightof the overburden bench may be kept up to 8mwithout problems of any slope failure.

· Height of benches is restricted by a maximum 8mand if the height of the overburden is greater thenits negotiated by making a number of benches,each time leaving a bench width of 1.5 - 2m tomaintain an optimum ultimate pit slope of 85°.

· The ultimate pit slope as well as the bench slope,considered up to 85° as at the end of the limit,allows the reclamation process to start early.

· Steep ultimate pit slopes are considered becauseof the hard clay stone in overburden, the hardnessof the strata facilitates the full recovery of coalreserves from the benches.

· Height of benches, slope angle and width ofbenches are kept as follows:

§ Bench Height - 8 m Max

§ Bench Width - 1 . 5 – 2 mMin.(For Ultimate Slope)- 4 m(During working)

§ Bench slope - 70°

§ Ultimate pit slope - 85°

§ Number of benches- Depends on depth of working.

· Normally the mines are developed in a strikedirection and then progress is made in the dipdirection for optimum production and productivity.

Ø Haul road Design: As the deposit is of a steepgradient and as such the development in each minebecomes deeper and deeper, therefore the gradient ofthe haul road and its location also becomes critical,hence the following precautions are taken during thedesign of the haul road:

· Design of permanent haul road at end of pit i.e. pitlimit.

· Gradient of haul road kept to a maximum of 1:10at some places to recover the coal at greaterdepths.

· Initial road gradient is maintained up to a ratio of1:16 to 1:20.

· Proper drainage system has to be made so thatthe roads are not under water when it rains, basedon the topographical data which is taken duringthe exploration stage.

Ø Overburden Disposal area: Overburden is dumpedin the disposal area outside the excavation area of themine. All the overburden material is dumped in thedesignated area with the help of Dozers and Loaders.

· Disposal area is selected near the mine pit andnot having any coal deposits.

· The angle of inclination of disposal area ismaintained at a maximum of up to 37 ° or up to thenatural slope to avoid slope failure.

· Height of the bench is restricted by a maximum of8m and the heightening of the disposal area is doneby leaving a 4 - 5m width, for the proper stability ofthe disposal area.

· Height of the disposal area may be maintained upto 30 - 35m, based on the size of the disposalarea and by making 4 benches by spiral dumpprocess, with the gradient of the haul road at aratio of 1: 10.

· In the case of there being a shortage of externaldisposal area, the mining method maybe changed,so that the mine is developed in the dip directionand then progress is made in the strike direction.Thus the reclamation of the mined out area is donesequentially using the most optimum area fordisposal.

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· The top soil is preserved at a specified location forits spreading on the top of the mined out area forrehabilitation of the area.

Ø Mine Drainage & Pumping: The area of the Luwemines has a number of small rivers that disturb theoverall mine working and layout. In most of the casesit is very difficult to divert the rivers because of thetopography as well as getting the permission of thelocal land owners (who may use the river for variousthings). Therefore suitable precaution has to be takento protect the mine from such rivers. Normally the wateraccumulates only in the monsoon period, because ofthe continuation and intensity of the constant rain. Thismonsoon period, in the central Kalimantan regionranges from September to April (8 months). Averagerain fall in this area is 446 mm per month and averagerainfall per month is around 9 days (estimates).Thereforethe following controlling measures are adopted to controlthe surface and rain water during this period:

· Most of the rain water should be diverted toother places by following the natural drainagecontours. In any case the pit floor of the minesand other areas are well sloped (2°) and gradedto divert the rain water from the mine pit.

· In the case of an area having hillocktopography, the mine is developed in a strikedirection to divert the rain water by making asuitable drain that is lower down the elevation,hence draining the rain water away with thehelp of gravity and without using any pumps.

· As the size of the mine working area increases,the catchments area will also increase andthus making it difficult to avoid to a certainamount of rain water based on the high rainfallin the monsoon period. Therefore suitablesumps are made at the lowest point of thepits, to collect all the water in one area ratherthan have it spread all over the pit area.

· Pump of suitable capacity is selected andinstalled in the sump for dewatering of the pit.

· Normally centrifugal diesel pumps are installedin the pits, because of the ease in operatingprocedure and frequent shifting.

· Mine areas that are near active rivers areplanned to be excavated out in the dry period,so as to not take a risk in terms of having theriver break the banks and flood the mine area,affecting the equipment and personnel.

· Mine areas are planned, so that they areprotected from the river by making a suitableembankment. Though sometimes it mayberestricted because of the flow of water, thereforeit is planned that a minimum 5m area alongthe river will be left without mining so that the

problem of inrushing water in to the pit may beavoided.

· In the case of a river existing at the centre ofthe coal deposit and there being no chancesof diverting the river due to topographicalreasons, then the mine operation needs tobecome quick and concentrated so as toexcavate the maximum reserve in the dryerperiod, before the start of the monsoon period.

· Submersible pumps as well as centrifugalpumps of higher capacity may also beenplanned for fast dewatering of the pits in thefuture as the size of mine area increases.

Ø Method of working:

· Removal of all trees or bushes that are presentover the site of the planned mining pit, eitherwith the help of a dozer or by excavators.

· A box cut is made over the top soil or hardclay stone present over the coal outcrop. Thisbox cut is made along the strike direction fromthe outcrop of Coal deposits and to be touchedup to the ends of the boundary of the mine.

· Box cut is made directly by Back hoeExcavators in case of loose strata or with thehelp of Dozer cum Ripper in case of Hardstrata.

· Permanent haul road is made along the pitlimit location.

· Selection of site for disposal area as well asTop soil storage and its leveling, dozing andCarpeting by Clay stone before starting ofmining operation.

· Deployment of Backhoe excavators andSuitable DT at planned location on the basisof pit limits area, production plan and mineworking plan.

· Normally around 25-30m area allotted for oneexcavator with suitable deployment of DTbased on lead from pit to disposal area, andcapacity of excavators in Overburden strata.

· Dozer cum Ripper is mainly used for ripping inhard clay stone to improve the loading efficiencyof excavators as well as disposal area.

· Selection of disposal area is mainly donenearby mine pit with maximum lead of 1 km..

· Mines are developed in strike and thenprogress in Dip direction.

· Sequence of mining and reclamation arefollowed one by one, after recovery of all Coalfrom one mine pit, that area used as disposalarea of next pit adjacent to the same pit.

Ø Land Reclamation and Rehabilitation:

After total winning the Coal the mined out area is

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planned to be backfilled with the Overburden andarea is well leveled with help of Dozer, Loader,Grader and Vibro as required and top soil is spreadon the same for normal vegetation.

The mined out area is leveled and dozed, as requiredby particular land owner and the same land isreturned to particular land owner after completionof mining.

Some times the mined out area is converted intowater pond, fish firming or water reservoir asrequired by particular land owner. Conversion ofmined out area in to water reservoir facilitate thecharging of water level in nearby area, generateearning to owner by making fish firming, facilitatethe water requirement for greenery of nearby plantsand tree in dry periods. The conversion of minedout area into Water reservoir may also be plannedas per land owner requirements.

Following Points may be considered by MiningAuthority for making their strategy of Mines operations:

1. Feasibility of underground mine/Coal gasificationfor winning of high grade after certain depth Coalmay be review after completion of mining operationby opencast mine.

2. Reclamation process of Opencast mine may bereview whether it will affect the operation ofUnderground mine/Gasification plant in future ornot.

3. Disposal of Overburden in front of deposit after pitlimit should not be encouraged, as the same Coalis available at greater depth may be mines dependson the sale price of Coal.

4. Employment of competent person in MiningDepartment to review mining operation and plan in

totality and complain of NGO, locals etc shouldbe review by technical person as most of thecomplains all around the world is for personal gainby making pressure on mine management.

5. Schedule Inspection of Mining authority should befor done solving the field mining problem astechnocrats and not like police –Theft relationships.Mining authorities should promote and motivateeconomic and eco friendly mining operationtechnically in long term with field Mining Engineer.

6. More strict rules should be framed andimplemented for Illegal Miners.

7. Land owner in mining lease area are makingdifficulties for Exploration and Exploitationoperation and in some cases systematic operationis not possible because of non availability of theland, necessary support should be provided to minemanagement for systematic mining operation.

CONCLUSION

Indonesia has lot of mineral resources and their economyis also growing because of improvement in production ofminerals but they are in preliminary stage of making &implementation in their mining policy,& legislation foreffective and economic mining operation.

They have to increase their exploration activities, Investmentin power plant run on low grade Coal, opening of low gradeCoal mine for domestic use, faming & implementation Minesdraft, Motivate foreign investors by considering the all aspectof market as well as their domestic requirement may leadIndonesia to new era in this world.

ACKNOWLEDGEMENTS

The view given in this paper is solely of author and not theview of company in which he works. Author is thankful toMr. Mohit Maheshwari and Mr. David - Directors of the Minesfor their approval to publish this paper.

With Best Compliments From:

Luwe Coal Mines, PT. Victor Dua Tiga Mega,

Central Kalimantan, Indonesia.