CHAPTER 11 Mining Technology - Princeton

CHAPTER 11

Mining Technology

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Contents

PageIntroduction and Overview. . . . . . . . . . . . . .323

Review of Coal Mining TechnologiesCurrently Used at Federal Mines. . ......325

Surface Mining Techniques. . .....,........325Underground Mining Techniques, ,... .. ...328

Analysis of Mining Technology Problems. .333Recovery Ratio. . . . , , , , . . . . . , . , , , . . . . . . . .333Production and Productivity, ,,. . ..,......,334Environmental . . . . . . . . . . . . . . . . ..........335Roof Support. . . . . . . . . . . . . . . . . . . . . , . . . , , .335

Review of Technology Developments inEurope and the Western United States. ...336

Comparison of Mining Conditions in Europeand the Western United States. . ..........336

New Equipment Developments. . . . ..,......337

Considerations for Using ImprovedLongwall Mining Techniques on FederalCoal Leases . . . . . . . . . . . . .............339

PageCapital . . . . . . . . . ........................339Labor . . . . . . . . . . ........................341Production and Productivity. . . . . ....,.....342Environmental . . . . . . . . ..................343Health and Safety. . . . . . . . . . . . . . . . . , . , , , . .343

List of Figures

Figure No, Page49. Western Strip Mining. ....,.,, . .....,..32750. Truck and Shovel Terrace Pit Mine With

Two Overburden Benches and TwoCoal Seams. . . . . . . . . . . . . . . . . .........328

51. Room-and-Pillar Underground Mining. ..33052. Longwall Mining System . . . . . . . . . . . . . . .332

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CHAPTER 11

Mining Technology

The Federal Coal Leasing Amendments Actof 1976 charged OTA to assess the feasibilityof the use of deep-mining technology onleased areas. With the passage of the Sur-face Mining Control and Reclamation Act of1977 congressional interest in the study ofdeep underground mining technology shiftedits principal focus from a concern for the pro-tection of surface resources to a concern formaximum economic recovery and the conser-vation of the resource. Lessees are requiredto mine all coal that can be extracted eco-nomically and within the limits of safety andtechnology so that coal reserves are not left inthe ground where they can deteriorate andnot later be retrieved. Underground miningmethods usually leave a significant portion ofthe coal reserve in the ground, Some under-

ground mines recover only 30 to 50 percent ofthe minable resource, although, averagedover all underground mines in the UnitedStates, the recovery ratio is 63 percent. Sur-face mines, on the other hand, typically re-cover from 70 to 90 percent of the minableresource.

Introduction and Overview

This chapter summarizes OTA’S review ofthe mining technologies currently in use onFederal leases and the potential for commer-cial mining technologies to extract Federalcoal reserves from deep underground seams.The chapter discusses:

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three surface mining techniques that areused in the West: 1) area strip, 2) openpit, and 3) terrace pit;two methods of underground mining inthe West: 1) room and pillar with contin-uous miners, and 2) longwall mining;recent underground mining technologydevelopments in Europe and the West-ern United States that could affect theproduction of coal from Federal leases;andfactors affecting the choice of these un-derground coal mining technologies inthe West, including: 1) capital require-ments, 2) resource recovery, 3) labor,4) production and productivity, 5) envi-

ronmental impacts, and 6) health andsafety.

A number of technological innovationshave been developed recently for under-ground coal mining, but the greatest near-term commercial promise for the expansionof underground coal mining in the WesternUnited States appears to be the implementa-tion of longwall mining techniques developedin Europe. Although longwall mining is usedvirtually exclusively to produce coal from un-derground mines in Europe, it accounts foronly 5 percent of annual underground coalproduction in the United States. Longwall sys-tems have been used in several Europeancountries to extract most of the reserves in30-ft thick seams at depths of 3,000 ft. In theUnited States, on the other hand, such recov-ery of thick, deep underground seams is stillin the development stage,

Some of the largest underground mines inthe West, including several that produce Fed-

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324 . An Assessment of Development and Production Potential of Federal Coal Leases

eral coal, have recently converted to longwallmining. Longwall mining is also scheduled tobe installed at other large underground oper-ations in the West. For these reasons, muchof this chapter compares longwall miningwith the dominant underground mining tech-nology in the West—room-and-pillar miningwith continuous miners.

Federal coal reserves in the Rocky Moun-tain province provide the greatest near-termpotential for the application of longwall min-ing. The first successful longwall operation inthe West was the York Canyon Mine of Kai-ser Steel located near Raton, N. Mex. Al-though the York Canyon Mine is not locatedon Federal leases, New Mexico provides op-portunities for the implementation of longwallmining on Federal land.

Longwall systems have also been intro-duced in several mines with Federal leases inthe Book Cliffs Field of central Utah. Newconcepts are now being implemented andtested to extract coal from thick seams and tomine steeply dipping seams at two mineswith Federal leases in Colorado—the Coal Ba-sin Complex of Midcontinent Resources andthe Snowmass Mine of Snowmass Coal Co.These projects are likely to encourage the useof longwall mining to extract other thick orsteeply pitching coal seams in the area.

Longwall systems are also scheduled to beimplemented at two mines with Federal re-serves in the Hanna basin of southern Wyo-ming. In 1981 a longwall system will be in-troduced at Carbon No. 1 Mine. This systemwill be the highest longwall unit (14 ft) in theWest. Much of the area overlying this opera-tion has already been surface mined. In 1984Energy Development Co. plans to use a long-wall unit at the Vanguard No. 2 Mine.

In the Powder River basin of Wyoming andMontana, where coal is mined inexpensivelyfrom large surface mines, it may be techni-cally possible to extract thick undergroundseams. However, the resource information ondeep underground coal deposits in this areais inadequate to assess the economic feasibil-ity of this. The comparatively low-Btu value of

coal in the Powder River basin and the verylarge, inexpensively minable surface depositsin the basin are economic barriers to under-ground mining in this region at least through-out this decade.

A potential method for recovering energyfrom coal is through in situ gasification ofdeep coal seams that cannot be mined by sur-face mining methods. The thick coal seams inthe Powder River basin are considered at-tractive in their potential for in situ gasifica-tion, and two separate small-scale test siteshave been developed in Campbell County,Wyo., one by the Department of Energy (HoeCreek Site) and another by ARCO Coal Co. Insitu gasification in this country is still in earlyexperimental stages. A major disadvantagewith in situ gasification is that it does not pro-duce pipeline quality gas. Thus, unless thereare industries nearby that could use low- ormedium-Btu gas, a surface facility must beconstructed to upgrade the gas to pipelinequality or perhaps to convert it to methanol.The National Coal Policy Project concludedthat even if in situ gasification experiments inthe Powder River basin are successful, thedistance of the region from centers of de-mand is likely to limit application of the tech-nology. 2 Considering the present state of de-velopment of the technology, in situ gasifi-cation is not likely to be used commercially inthe Powder River basin until the mid-1990’sat the earliest.

To date, the experience with longwall min-ing both in Europe and the Western UnitedStates points to significant potential advan-tages in terms of increased resource recov-ery, higher production and productivity, re-duction in the cost of labor and frequently inthe overall costs per ton of coal, the control ofdifferential subsidence on the surface, and areduction in the number of unintentional rooffalls at the face. One should not conclude,however, that longwall mining will realizethese advantages in all underground miningenvironments or that longwall mining can be

2F. X. Murray (cd.), Where We Agree: Report of the NationalCoal Policy Project (Boulder, Colo.: Westview Press, 1978).

Ch.. 11—Mining Technology ● 325

used profitably and efficiently in all of thedeep Federal coal seams in the West. The de-cision to implement a particular undergroundmining technology at a particular site can bemade rationally only after the completion ofcomprehensive sit e-specific geological,engineering, economic, and environmental as-sessments,

In spite of the positive experience withlongwall mining both in Europe and theUnited States, many underground coal pro-ducers in the West will be reluctant to installthis mining technology because of its high ini-tial capital cost. The cost of a typical longwall

installation is $9 million; total capital cost ofthe technology per ton of coal mined over thelife of a system is about $1,50. This comparesto a capital cost of $0.40/ton of coal minedover the life of the system for the typicalroom-and-pillar operation in the West, usingcontinuous miners. In many cases, the sav-ings in labor and the other advantages ofusing longwall mining may not be sufficient tooffset this cost differential. Nevertheless,longwall systems are likely to figure prom-inently in mining underground Federal coalreserves in several areas of New Mexico,Utah, Colorado, and Wyoming.

Review of Coal Mining Technologies Currently Usedat Federal Mines

Although underground mining was, at onetime, the principal mining method in parts ofColorado, Montana, Wyoming, and NorthDakota, and continues to be so in Utah, sur-face coal mining now predominates in mostareas of the West. Because coal seams aregenerally thicker and nearer the surface inmost Western States, compared to the East,they are more amenable to recovery by sur-face mining techniques. Surface miningoperations are usually well-suited to themany areas of the West that have not yet ex-perienced the extensive development oftowns, cities, highways, and railroads char-acteristic of the Midwest and the East.

Surface Mining Techniques

Surface mining of coal is characterized bythe use of large, capital-intensive and effi-cient mining equipment. First, the overlyingsoil and rock layers (overburden) are re-moved. The coal is then fractured with explo-sives or machines, and loaded onto vehi-cles for haulage from the mine site. Finally,the disturbed land must be fully reclaimed.Principal considerations in the selection ofsurface mining and reclamation techniquesand equipment include the thickness and

character of the overburden, the dip of theseam, the thickness and number of recov-erable seams, and the physical and chemicalcharacteristics of the coal. The three surfacemining techniques most widely used in theWest are area strip, open pit, and terrace pit.

Area Strip

Area strip is the principal surface miningtechnique used in the United States. The tech-nique was perfected in the coalfields ofIllinois, Indiana, Kentucky, and Ohio. Thecapacity of a dragline, the machine used toremove the overburden in strip mining, variesin size from 10 to over 200 cubic yards. ManyEastern mines use stripping shovels insteadof draglines; and stripping shovels currentlyare being used successfully at several stripmines in the West such as the Rosebud Minein the Montana portion of the Powder Riverbasin which produces Federal coal.

Area strip mining proceeds by first makinga box cut into the earth to uncover the initialstrip of coal that is to be mined. The strip ofcoal uncovered will vary from 100 to 200 ft inwidth and from one-quarter to several milesin length. The actual size of the cut will be de-termined by the thickness of both the over-

326 ● An Assessment of Development and Production Potential of Federal Coal Leases

burden and the coal and the designed produc-tion rate of the mine. After the coal has beenmined from the bottom of the box cut, theoverburden covering the next strip of coal isremoved and placed in the void left by themining of the preceding strip of coal. Miningproceeds with succeeding parallel strippingcuts until the property limits of the miningarea are reached (see fig, 49).

Even the largest draglines have limits onhow much overburden they can remove froma given operating location. A single draglinecan generally remove overburden to depths of100 ft. However, it is possible to extend thestripping limits to as deep as 200 ft by team-ing the principal stripping dragline with addi-tional equipment such as another dragline, astripping shovel, a bucket-wheel excavator,or fleets of trucks and shovels or scrapers.This additional equipment will increase thetotal overburden removal costs and can bejustified only by significant increases in theamount or quality of the additional coal thatcan be recovered.

Open Pit

The open pit mining technique currentlyused in the Western United States was ini-tially developed in the metal mining industry.An open pit mine is characterized by a seriesof benches, the number of which increases asthe mine is deepened. Each one of thesebenches is 40 to 50 ft in height, and excava-tion can proceed to depths of hundreds orthousands of feet.

The use of an open pit for overburden re-moval is justified only where there is an ex-ceptionally thick seam or a series of seamsthat can be mined in sequence, The singleseam mines can be found in the brown coal-fields of Germany, but only the multiseammines are found in the Western UnitedStates. The best example of the latter type isFMC’s Skull Point Mine located on Federalleases in southwestern Wyoming.

Equipment used for overburden removal inan open pit coal mine is currently limited totruck and shovels or scrapers. The truck and

shovel or scraper approach typically pro-vides the most flexibility in the developmentof a bench system and the mining of coalseams. However, lower overburden removalcosts might be achieved if rail or conveyorhaulage systems, similar to those used in thecopper mines of the Southwestern UnitedStates, could be implemented in open pit coalmines,

Terrace Pit

The terrace pit system for surface coalmining has come into use only during the last5 years (see fig. 50). This method, which es-sentially combines the area strip and open pittechniques, is used in the thicker coal bedsof northeastern Wyoming and southeasternMontana. In these two areas, the removal ofoverburden thickness in excess of the 100-ftstripping limit of a dragline is justified eco-nomically by the mining of exceptionally thickcoal seams.

Terrace pit mines have a system ofbenches similar to those designed for open pitmining. However, the overburden depths thatcan be removed economically by the terracepit method are currently limited to between200 and 300 ft. so that the maximum numberof benches will be about seven, Also, unlikethe open pit system, the terrace pit systemdoes not remain in the same location butrather moves across the property in a mannersimilar to area strip mining. The overburdenthat is removed from one side of the pit ishauled to the other side of the pit and dumpedwhere the coal has already been mined. As aresult, the overburden removal operation ofthe terrace pit moves constantly in a spec-ified direction, usually down dip during theinitial years of mining. The removed over-burden is replaced behind the mining opera-tion at a distance determined by the numberand size of the benches.

Equipment used for overburden removaland coal extraction in the terrace pit systemtypically consists of trucks and shovels, al-though draglines are used at several suchmines in the West, including the Rawhide

Ch. 11—Mining Technology ● 327

F igu re 49 .—Wes te rn S t r i p M in ing

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Figure 50.—Truck and Shovel Terrace Pit Mine With Two Overburden Benches and Two Coal Seams

SOURCE U S Bureau of Mines

Mine which is located on Federal leases inthe Wyoming portion of the Powder Riverbasin. A maximum of two to three shovels willtypically be assigned to each overburdenbench together with a sufficient number oftrucks to haul the material excavated by theshovels to the other side of the pit, The num-ber of shovels used in this operation is deter-mined by the length and the rate of devel-opment of the pit. As in open pit mining,other forms of excavation and haulage equip-ment, such as bucket-wheel excavators andconveyors, are being considered for terracepit mining. However, the dynamic aspect ofterrace pit mining makes it more difficult touse equipment that does not have the mobilitycharacteristic of trucks and shovels,

Underground Mining Techniques

Surface mining is generally preferred bymine operators over underground mining.The reasons for this preference includehigher percentage of coal recovery, higherlabor productivity, lower operating costs, andfewer safety and health hazards. Also, someenvironmental impacts of underground min-ing, such as subsidence, acid mine drainage,and the interruption of aquifers can begreater than those of surface mining, All ofthese factors are important and will be dis-cussed in more detail later in this chapter. Incases where coal seams are too deeply buriedto be recovered economically using surfacemining techniques, it is likely that the coal

Ch. 11—Mining Technology “ 329

will be mined using one of the two under-ground mining techniques discussed below,Furthermore, several companies mining Fed-eral coal in the West have had to shift fromsurface to underground mining as their sur-face minable reserves became exhausted.

Access to underground mine workings willbe by one of three methods. If the coal seamoutcrops at the surface it is possible to minedirectly into the seam from the surface; thistype of mine is referred to as a drift mine.Most underground mines in the West aredrift mines. If the minable seam is locatedunder shallow cover then it may be possibleto reach the coal bed through the use of an in-clined drift (slope); this type of mine is re-ferred to as a slope mine. If neither of theseforms of access is possible, then it is neces-sary to sink a vertical shaft from the surfaceto the minable seam; this type of mine is re-ferred to as a shaft mine.

The initial capital cost for developing aslope mine may be slightly more than that fordeveloping a shaft mine because, for thesame overburden thickness, a slope is ap-proximately three times longer than the depthof a corresponding vertical shaft. However,over the long term, a slope mine has loweroperating costs because of the relatively lowcost to move men and materials into the mineand coal out. A slope mine is typically moreeconomical when the overburden is less than500 ft; a shaft mine when it is over 1,000 ft. Inthe 500- to 1,000ft range a site-specific eco-nomic evaluation is usually necessary to de-t e rmine wha t type o f mine shou ld bedeveloped.

Room and Pillar

In the room-and-pillar method, the voidsleft by removed coal form the rooms and theunmined coal forms the pillars (see fig. 51).The pillars are left in place to support theweight of the overlying strata. The principalfactors determining the percentage of coalthat can be removed from a seam are thethickness of the seam, the strength of the

seam and the confining rock strata, the pres-ence of faults or fractures, and the depth ofthe seam. The deeper the seam, the greaterthe weight of overlying rock that must be sup-ported; thus the size of the pillars generallywill be greater for deeper mines.

The extraction of the coal in a room-and-pillar mine is accomplished using eitherconventional mining or continuous miners.

Conventional mining declined in popularityduring the 1960’s and most of the 1970’s butcontinues to account for 35 to 40 percent ofthe underground coal product ion in theUnited States. The first step in conventionalmining, which is more labor-intensive thancontinuous mining, is to cut a slot into theseam with a machine that looks like a largechain saw mounted on a large, rubber-tiredvehicle (see fig. 51), Holes are then drilledinto the face and loaded with explosives.After blasting, the coal is fragmented andallowed to drop on the floor of the mine. Aroof-bolting machine is used to drill verticalholes into the roof and install bolts for roofsupport. A loading machine is then used togather up the coal and to load it into rubber-tired shuttle cars which haul the coal fromthe loader to a conveyor belt for transport outof the mine. In a few instances, a series ofbridge conveyors are used in place of theshuttle cars.

Continuous miners are equipped with a ro-tating head with cutting bits that is used tobreak coal from the face (see fig. 51). The de-sign of the cutting head and the form of rota-tion will vary with the manufacturer, but allcontinuous miners typically break the coalfrom the face and load it directly into shuttlecars or onto conveyors. As in conventionalmining, a roof-bolter is used to install boltsfor roof support. The labor requirement forcontinuous miners is at least 10 percent lessthan that of conventional mining systems.While continuous miners generally are moreefficient, conventional mining can be morereadily adapted to certain difficult miningconditions and to large inclusions in thecoalbed.

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Figure 51 .—Room-and-Pillar Underground Mining

I Conventional mining I I

I Continuous mining I

Roof bolting(continuous and conventional)

Plan v iew (contin uous and conventional showing coal support pillars)

SOURCE Office of Technology Assessment


Longwall Mining

The basic longwall system consists of a setof supports that are located parallel to themining face, a conveyor system that runsalong the base of the face, and a machine thatmoves back and forth along the face, cuttingthe coal and loading it onto the face conveyorfor transport out of the face area (see fig. 52),In addition, continuous miners are requiredfor the development of longwall panels. Thelength of the mining face will depend on anumber of factors (discussed in more detaillater in the chapter), but will generally rangefrom 400 to 650 ft, with 500 ft being typical.

The basic longwall support system consistsof hydraulic rams positioned vertically to sup-port the roof when they are extended. De-pending on the size of the mining operation,the rams are arranged into one or more pairslocated in the plane perpendicular to theface, When more than one pair of hydraulicrams are used, they will be structurally con-nected to one or more other pairs to ensurelateral stability. The exact configuration ofthe rams and the method of interconnectingthem will vary according to model or man-ufacturer. For additional support, a shieldsupport system may be employed. This systemuses a protective canopy as a structural partof the support mechanism to protect minersworking along the face from roof falls.

The longwall chain conveyor system ismounted on the mine floor in front of the baseplate used for the roof support rams or onseparate supports, In either case, the con-veyor assembly must be flexible to allow forbending as the supports are advanced oneafter another to keep pace with the advanc-

ing face. As the supports are advanced, theroof is allowed to collapse behind them. Thetypical conveyor mechanism used on a long-wall chain conveyor system has a series ofhorizontal bars or flights that are locatedperpendicular to the longitudinal axis of theconveyor, These flights are spaced approx-imately 1 ft apart and are fastened togetherwith chains connected to their ends or cen-ters, The chain-connected flights move alongthe top and bottom surfaces of the chain con-veyor system through the force of a gearmechanism which engages the chain. Side-boards are mounted along the top surface ofthe system so that coal falling into the troughformed by the sideboards will be pulled alongby the chain-driven flights.

The machine used to cut the coal from theface and load it on the face conveyor iseither a plow or a shearer. Plows are favoredin West Germany because of the thin coalseams and soft coal deposits in that country.The cutting action of a plow is just as thename indicates. The height and the depth ofthe cut will be limited by the amount of pull-ing force that can be applied to the plow. Inthe case of shearers, however, the cuttingforce comes from a rotating drum with cut-ting bits mounted on it. The diameter of thedrum will typically be somewhat greater thanone half the face height so that two passes ofthe drum will be required to mine the fullheight of the face. The top half of the face ismined first. Since a shearer drum is designedto operate only in one direction, a double-ended shearer makes it possible to cut the up-per and lower portions of the face withouthaving to return the shearer to the same endof the face to begin each cut.

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Figure 52.—Longwall Mining System

Elevation view

Overburden

Mining Conveyormachine

Plan view

Conveyor

Suppo

Slumpedoverburden

I Miningrts machine

Tunnel

L Air Main tunnel

SOURCE Off Ice of Technology Assessment

Ch. 11-Mining Technology ● 333

Analysis of Mining Technology Problems

The preceding descriptions of surface andunderground mining techniques currently inuse on Federal coal leases are very general.They are intended to introduce the reader tothe basic differences between surface andunderground coal mining. This section willdiscuss in more detail the problems that areencountered in using these coal mining tech-nologies. These problems will be illustratedwith examples from mines currently operat-ing on Federal coal leases.

Recovery Ratio

Recovery ratio is the percent of minablecoal recovered from the seam. Generally, therecovery ratio for surface mining will behigher than that for underground mining be-cause some coal must be left in place in un-derground mines to support the roof and limitsurface subsidence. According to the Bureauof Mines, the average recovery ratio from allsurface mines in the United States is 83 per-cent; for underground mining the nationalaverage is 63 percent.

Thick single-seam surface mines often haverecovery ratios in excess of 90 percent. Forexample, the thick-seam mines of the PowderRiver basin of Wyoming are achieving recov-ery ratios of 95 percent. The reason for this isthat the coal lost at the seam boundaries in athick seam is a small fraction of the total coalbeing mined. The operator often does not re-cover 6 to 12 inches of coal at the upper andlower seam boundaries, because this coalusually contains significantly more mineralmatter than the remainder of the seam, Theproportionate amount of the seam thicknesslost is much less for a 50- or 100-ft thick seamthan it is for a 5- or 10-ft thick seam.

A multiseam surface operation will oftenhave a lower recovery ratio than a single-seam mine because a certain thickness ofcoal is lost for each boundary between a coalseam and the surrounding material. Hence,even though the cumulative thickness of theseams in a multiseam mine approaches or ex-

ceeds the seam thickness in many single-seammines, the percentage of coal not recoveredwill be greater.

Recovery ratios in surface coal mines alsocan be adversely affected by such factors asextreme seam dip, faults, and characteristicsof the overburden material that interferewith stripping operations. Such problems arecommon in the surface mines of northwesternColorado and southwestern Wyoming. How-ever, mines such as the Colowyo and theTrapper mines encounter a combination ofthese problems but still achieve recoveryratios in excess of 80 percent. Seam dip at theCanadian Strip Mine in north-central Colora-do is so steep that it has been necessary to im-plement what is essentially a contour stripoperation. Here a bench is cut into the hill-side and all of the coal in the bench area ismined. Even though the ratio of waste materi-al removed to coal mined is on the order of15 or 20 to 1, only a small percentage of thecoal is not recovered.

The recovery ratio for underground miningwill usually be less than for surface mining asa certain amount of the coal must be left inplace around access facilities such as shafts,slopes, drifts, main entries, and submain en-tries. Additional coal is also lost that is left inboundary pillars around the perimeter of theproperty and in pillars in the mined out areasto support the roof and prevent or lessen sub-sidence of the surface.

Recovery ratios for room-and-pillar mineswill vary from a low of 20 percent to a high of80 depending on the completeness of the sec-ondary recovery of pillars from the miningpanels. The average recovery ratio for allroom-and-pillar mines in the country is 62percent. Several equally important factorsdetermine the recovery ratio of these minesincluding the dip of the seam, the presence ofigneous or other intrusions in the seam, faults(vertical displacements in the seam), and thedepth of the seam. The depth of the seam isimportant in the West where many mines are

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334 An Assessment of Development and Production Potential of Federal Coal Leases

2,000 to 3,000 ft deep. The greater the depthof the seam, the more overlying rock stratathat must be supported by pillars left aroundaccess facilities and in the mining areas. Toavoid failure of these support pillars due toexcessive loading, they must have largercross-sectional areas, I f the p i l l a r s a relarger, then the amount of coal that can be re-covered will be correspondingly less. Supportproblems caused by the depth of the seam canbe aggravated by the presence of faults, poorcompetency of the rock strata forming thefloor and roof of the mine, and excessivewater. Several of these conditions are foundat mines with Federal coal leases in westernColorado. Conditions became so severe atU.S. Steel’s Somerset Mine that one of themining levels had to be abandoned, Stressesinduced in support pillars at this mine causedthe pillars to fail explosively, However, ac-cording to a spokesman for the Bureau ofMines, this problem has been brought undercontrol at Midcontinent’s Coal Basin complexin Colorado,

Recovery ratios for longwall mining rangefrom a low of 50 percent to a high of 80. Theaverage recovery ratio for all longwall minesin the country is 75 percent. * Longwall min-ing generally results in higher recovery ratiosthan those achieved in room-and-pillar min-ing when the latter does not include full ex-traction of the pillars. A good example of therecovery ratio increases which can be ex-pected from longwall mining when comparedto room-and-pillar mining is found in the DeerCreek-Wilberg mine complex of Utah Power &Light in central Utah. These mines hadopera ted as room-and-pillar mines withrecovery ratios of 55 to 60 percent. The com-

‘Pcrsf)nal [:(JmlmuIlic;i]liorl to O’1’A from John hf. Karhnak,hlarmger of (Jround Contro]. Division of Minerals. Health &%lfety ‘1’echnology, U.S. E3urci~u of hlines, Mar. 31, 1981.

*’rhc average recovery riiti[) ft}r sh~)rtwail m i n i n g i n t h eUnited Stales is even higher [84 percent). As in longwall min-i nx, short w{] U mining uses ch(x:ks for roof” support. However,ronlinuous” m incrs are used in short w:) II opera t ions 10 mine {hec(N]1 ;+nd haul[~ge is done bv shut tlc (:[~r rather I han with a con-tinuous (xlnvey(]r. Shurtwall mining may he used in the futurein the 14’es t in ciiscs where :) 600-fl fi~(’e is not ava il[lble. Per-S( ~n;{ I (Jomm un ir:i t ion t( ) ()’I’A from I):\ n iel J. Sn\’der, 111, Pres i-dent. (I()]{)rild~J-Wcs tnlf)rt; ltlrl(i. In(’., June 15. 1981.

pany has already installed one longwallsystem and plans to install a second. Therecovery ratio for the longwall system at thismine is approximately 80 percent.

Production and Productivity

Although there is no hard and fast relation-ship between the production rate for a mineand its rate of labor productivity, it is oftentrue that those mines with high productionrates also will have relatively high rates oflabor productivity, The reason for this is thatboth surface and underground mines withhigh production rates generally have trainingprograms and equipment that will generatehigher labor productivity y.

The large surface mines of the WesternUnited States have long been characterizedby high rates of labor productivity. The pro-ductivity of some of these mines is as high as30 to 35 tons per worker per day. The aver-age productivity rate for all surface coalmines in the United States is approximately15 tons per worker per day, Since labor is oneof the major costs for any mining operation,productivities on the order of 30 or more tonsper worker per day translate into signifi-cantly reduced unit operating costs. How-ever, high rates of productivity are typicallyachieved as the result of greater investmentsin equipment, so that the reduction in unit op-erating costs will be partially offset by in-creases in the unit capital costs. Further-more, capital costs for initial infrastructuredevelopment and interest charges are themost significant costs in coal mining.

Because there are fewer operating con-straints, surface mining will generally pre-sent fewer problems with respect to achiev-ing high production rates and concomitantlyhigh labor productivity rates. The achieve-ment of these goals in underground coal min-ing, however, requires good mining conditionsas well as good management and a willing-ness to invest in the appropriate equipment.A good example of the high level of productiv-ity that can be achieved in underground min-ing is the Soldier Canyon Mine of California

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Portland Cement Co. in central Utah. Using alongwall system, this mine is achieving alabor productivity of 23.5 tons per worker perday and an annual production rate of over 1million tons. This productivity rate is not onlymuch higher than the national average of 8.6tons per worker per day for underground coalmining but also exceeds that achieved bymost surface coal mines,

It is generally accepted that longwallmining represents a better opportunity forachieving higher production rates, higherlabor productivity rates, and better safety inunderground mining. A room-and-pillar oper-ation using continuous miners can readilyachieve shift production rates of 350 to 400tons. Rates in excess of 700 tons per shift areexceptional, In the case of longwall mining,shift production rates in excess of 1,000 tonsare common in the West and rates of 1,500 to2,000 tons per shift have been achieved at anumber of longwall operations.

Environmental

Environmental problems resulting fromcoal mining operations in the arid West aremore likely to be associated with surface min-ing than underground mining. Environmentalissues are discussed in detail in chapter 10.Although the siting of surface facilities forunderground mining operations will generallyhave limited environmental impacts, a poten-tially greater problem associated with in-creased underground mining on Federal coalleases is surface subsidence. * The impactfrom subsidence depends on the location ofthe mine and is greatest in highly built-up ur-ban areas. This problem is most often asso-ciated with cities and towns in the EasternUnited States but has also occurred in West-ern towns such as Rock Springs, Wyo., wherebuildings and other facilities located inseveral areas of the city have been endan-gered.

*Acid mine drainage, which can be an acute problem inmany mining areas in the East, is not a significant problem inmost arid regions of the West.

Surface subsidence can also be a problemin rural and unpopulated areas. Differentialsubsidence can break through to the surfacein the form of fractures and sinkholes. Moretypically, subsidence will manifest itself inthe form of a generally lowered surface ele-vation, This will not be a problem unless itoccurs under a stream, railroad, highway,building, or dam. Even then, the adverse ef-fects of subsidence can be minimized if themine is properly planned. In several Euro-pean countries, especially West Germany,where the art of deploying planned subsid-ence is well developed, entire towns havebeen lowered as a result of underground min-ing without adverse effects.

It is becoming apparent that longwall min-ing is usually preferred to room-and-pillarmining where surface subsidence may be aproblem. The reason for this is that longwallmining is more likely to produce more uniformand predictable subsidence. Concern aboutthe potential impact of surface subsidence onsprings and ground water hydrology has beenexpressed by local residents in mining areasin Utah and Colorado. Subsidence tends to bedifferential in room-and-pillar mining andmay not occur until years after mining opera-tions have been completed. It may then occurwithout warning and with potentially catas-trophic results. However, full recovery of thepillars will usually result in the more uniformsubsidence comparable to that achieved withlongwall mining.

Roof Support

Deeper mines require that larger supportstructures be left in place in the undergroundworkings. Greater support can be accom-plished by leaving the coal in place or by re-placing it with substitute support structures,For longwall mining there is increasing evi-dence that control led subsidence of themined areas reduces support stresses onboundary pillars that are left in place alongmain and submain entries. In some longwall

-.


mines it has been necessary to install supple- much less than that required to support thementary supports such as packs or cribbing, full weight of the overlying rock strata.but the amount of support provided has been

Review of Technology Developments in Europe and theWestern United States

This section reviews mining technology de-velopments in Europe and the United Statesand discusses possible solutions of the miningtechnology problems described in the preced-ing section.

Because surface mining techniques andequipment in the United States are relativelyadvanced compared to underground coalmining technology in the United States, onlynew underground technology developmentswill be discussed in this section. As thenumber of underground mining operations in-creases in the West, improvements in under-ground mining technology can lead to signifi-cant improvements in coal production, recov-ery ratios, productivity rates, and safety.

Comparison of Mining Conditions inEurope and the Western United States

Mining conditions on Federal coal leasesvary from nearly ideal to some of the mostdifficult conditions anywhere in the world.For example, in certain locations in north-western Colorado and in northwestern NewMexico the minable seams are a few hundredfeet deep and from 6 to 30 ft thick, the dips ofthe seams range from the horizontal to a fewdegrees, there are few faults, and the floorand roof rocks are nearly ideal for support. Incontrast, the conditions found at some exist-ing mines with Federal leases in western Col-orado and central Utah include depths ofcover that are over 3,000 ft, seams that rangefrom 4 to over 40 ft in thickness, seam dipsthat approach 350, extreme fracturing andfaulting of both the coal seams and the confin-ing rock strata, and floor and roof rocks ofvery poor competency. Many mines in Colo-

rado and Utah extract coal from seams thatare over 1,000 ft deep.

Mining conditions vary in other areaswhere there are substantial Federal lease-holdings. For example, in the Powder Riverbasin of northeastern Wyoming the depth ofcoal seams ranges from a few hundred feet toan undesirable 2,000 to 3,000 ft. However,most other mining conditions are good in thisarea. Difficult mining conditions in the StarLake-Bisti area of New Mexico include seamdips up to zoo and faults. In Oklahoma,high concentrations of methane gas and un-dulating, thin seams cause extremely difficultmining conditions on most Federal leases.

Direct comparisons of coal mining in Euro-pean countries with coal mining in the West-ern United States can be misleading becausemany mining conditions are different in thesetwo areas. The principal coal mining coun-tr ies of Europe—France, Great Brit ian,Czechoslovakia, the Soviet Union, Poland, andWest Germany— are currently mining coalfrom seams that would be considered unmin-able in the United States. A condition which ispresent in all of these countries, but whichhas not been observed in the Western UnitedStates, is extreme folding of the coal seams.In the case of folding, it is necessary to dealnot only with steeply dipping seams but ver-tically oriented seams as well. Most of thecoal mined in England is extracted fromseams that are deeply buried, thin, folded,and faulted. Both France and Poland are cur-rently operating deep mines with almost fullextraction of coal seams 20 to 30 ft thick.However, even though many mining condi-tions in Europe differ from those in theWestern United States, the use of longwall

Ch. 11-Mining Technology ● 337

mining in Europe to extract deep, thick seamsis relevant to assessing the technical poten-tial for extracting deep, thick seams in theWestern United States.

New Equipment Developments

The equipment developments discussed be-low include both refinements to existing un-derground mining technology and new equip-ment concepts that are still in the prototypeand testing stage. New equipment and con-cepts could solve many of the problems de-scribed in the section on mining technologyproblems.

Longwall Mining Improvements

Modern longwal l mining technologyemerged in its present form in Germany, theSoviet Union, and Great Britain during thelatter part of the 1950’s after more than 20years of continuous development in thesecountries. Refinement and improvement oflongwall equipment and techniques havesince continued in these countr ies . TheUnited States has been a late entrant into theuse of longwall mining, and the equipmentmanufacturers in this country have made rel-atively few contributions to the technologyduring the past 10 years.

Many of the recent developments in long-wall technology have dealt with improve-ments in reliability and production capability,although there also has been considerable ef-fort to improve safety conditions associatedwith longwall mining. The discussion of theseimprovements of longwall mining technologywill be organized according to the three prin-cipal components of a longwall system; roofsupport, the face conveyor, and coal cuttingand loading.

Principal improvements in the longwallsupport system have included increases inthe load-carrying capability of the hydraulicrams, increased maneuverability of the sup-ports, better stability, and refinements in thecontrols. Most manufacturers of longwallsupports now offer units that have maximumyield loads of 1,000 tons or more, The in-

creases in the maximum yield loads also havebeen accompanied by the development ofshields and chock-shields that offer greaterstability and more protection to the miner.Advancement and alinement of the supportsnow can be done remotely which not only re-duces the time required for advancing thesupport system, but also allows locating min-ers away from the moving support and out ofthe way of falling material caused by move-ment of the support.

The transport of the cut coal away fromthe longwall face by the face conveyor stillremains a major potential bottleneck in thelongwall mining system. A breakdown in theconveyor can result from broken fl ightchains; minor delays also are caused by over-sized lumps of coal becoming stuck in con-veyor transfer points. The broken flight chainproblem has been partially solved by the useof a “twin-inboard” chain at the center of theflight which reduces stress on the flightchains as the flight conveyor bends aroundcurves. The need for a transfer point at theheadgate where the face conveyor meets thepanel belt conveyor in the headgate entry hasbeen eliminated by the recent innovation ofthe roller curve in West Germany. The rollercurve allows the face conveyor to turn the900 corner at the headgate and to dump coaldirectly onto the panel gate conveyor or into afeeder-breaker which reduces the size ofoversize lumps and then feeds the coal ontothe panel belt conveyor.

The majority of the longwall cutter-loadersinstalled in the United States are shearersrather than plows. Two major hazards asso-ciated with the use of shearers have beenhigh coal dust concentrations created by thecutting action of the rotating shearer drumsand the danger from a break in the chain thatis used to pull the shearer back and forthalong the face. Significant reduction of dustconcentrations has been reported through theuse of a “Shearer-Clearer” water spray sys-tem which was developed by Foster-MillerAssociates in conjunction with the U.S. Bu-reau of Mines. This system partitions the air-flow around the shearer into a clean split


through the use of water sprays. The coaldust cloud is confined to the vicinity of thecoal face while the shearer operators remainin the clean split on the support side of thecutting machine.

The broken chain hazard has been solvedby equipment manufacturers in Great Brit-ain, West Germany, and the United Stateswhich now offer chainless drives, These drivesystems are all based on a variation of therack and pinion gear system. Initial indica-t ions a re tha t t hese cha in le s s sys t emsalready have gained wide acceptance by theoperators.

One additional development in longwalltechnology is a system designed specificallyfor steeply dipping seams. The system iscalled the “Troika” and is offered by Hem-scheidt America Corp. It is designed for usein seams that dip up to 750 and is scheduledto be used to extract Federal coal from aseam pitching 330 at the Snowmass Mine inColorado. It consists of three shields con-nected to a central structural beam by adouble-acting ram assembly. The beam is con-nected mechanically to the center shield andto the outer shields by the rams. The centershield, which has no double-acting ram, ismoved by the outer shields with the ramsthrough the beam. The outer shields followthis beam during movement.

The new equipment developments for long-wall mining are potentially important, butbetter use of available equipment is an equal-ly important aspect of technology develop-ment for this system. An example of the latteris the use of available longwall technology toextract the maximum thickness possible fromseams that are thicker than the approximately12-ft seams currently being mined with con-ventional longwall systems.

One approach to thick seam extraction inunderground mining is the double lift long-wall method currently being developed tomine Federal reserves at the Coal Basin Com-plex of Midcontinent Resources under a cost-sharing contract with the Department ofEnergy. With this system, the coal seam is ex-

tracted by taking two successive passes ofthe longwall. Total thickness of the coal seamis 25 ft; by taking two 10- to 12-ft lifts, all but5 ft of the seam will be recovered. Althoughthis is less than the total seam thickness atCoal Basin, it is a significant improvementover the extraction of 8 to 10 ft of coalachieved with the single lift approach. Con-ceptually, this method could be extended toextract the full coal seam, using three ormore lifts.

Another approach to thick-seam miningusing a longwall system, developed in France,uses a single-lift longwall operation under thebottom of the sea. The roof supports on thissystem have been modified to allow the por-tions of the seam located above the longwallto cave in behind the supports under the over-lying broken roof rock. The broken coal isthen collected on a conveyor belt running be-hind the supports.

Room-and= Pillar Mining Improvements

For some time there has been an aware-ness that the cutting action used by existingcontinuous miners is less efficient and pro-duces more dust than alternative cutting ac-tions. Tests of the linear-cutting miner experi-mental concept developed by the U.S. Bureauof Mines have indicated that deeper cuts atconstant depth in the coal face can be mademore efficiently, while reducing the respir-able dust that is normally generated by con-tinuous miners. It is estimated that this newcutting concept would produce three timesthe amount of coal with one-third of the dustand would use one-third to two-thirds lesselectrical power, depending on the cuttingdepth. However, the development of the com-mercial machine is not likely for another 20years.

A concept for extracting more coal frompillars during secondary recovery operationsis the underground auger miner developed byFMC under contract to the U.S. Departmentof Energy. In addition to the recovery of coalin pillars, the system shows a potential formining out prepared panels of coal at signifi-cantly lower cost than conventional methods.

Ch. 11l—Mining Technology 339

The system consists of an undergroundaugering machine, a two-stage coal conveyor,and auxiliary ventilation and rock-dustingequipment. The auger miner excavates coalby drilling a series of large holes side by sideinto [he coal seam. The conveyor carries thecoal to the mine’s face haulage system, Thein-hole ventilation and rock-dusting equip-ment is used to dilute methane gas and coaldust to nonexplosive concentrations,

The availability of efficient and reliablehaulage systems has long been one of thegoals of underground mining technology de-velopment. Existing rubber-belt conveyor sys-tems have gone a long way to satisfy this re-quirement, but major deficiencies remain atthe face. The principal need is for a contin-uous haulage system which is sufficientlyflexible to adapt to the multitude of configu-rations experienced during the developmentand product ion from a coal panel. Besides theneed for flexible components, there is also aneed for an efficient method for transferringcoal from one component to another, e.g.,from shuttle cars to panel belt conveyors. *

* This need is also eveident for panel development in longwallmining

Long-Airdox has introduced a continuoushaulage concept that is based on the use ofmobile bridge carriers and piggyback bridgeconveyors. A piggyback bridge is atttacheddirectly to the boom of the continuous minerat one end and is supported by a dolly at theother end. This dolly is designed to move free-ly along rails mounted on top of the sides of aseparate mobile bridge. Coal flows from theminer to a piggyback bridge to a mobilebridge to a piggyback bridge, and so on, untilit reaches the last piggyback bridge’s dollyand is dumped onto the panel belt. Fourstandard mobile conveyors (each 30 ft) cou-pled with five standard piggyback conveyors(each 3 to 41 ft) can provide an effectivereach of over 300 ft in a seven-entry miningprojection.

Other concepts of continuous haulage thathave been tested include the use of air andwater as the carrying media for crushed coalin pipelines. Although this approach offerssome advantages, it is still more difficult toimplement than are concepts based on theuse of conveyor belts. A slurry pipeline sys-tem has been installed in one Eastern mine,however,

Considerations for Using Improved Longwall MiningTechniques on Federal Coal Leases

The preceding sections of this chapterhave provided an overview of mining systemscurrently in use on Federal coal leases andhave considered several of the technologyproblems associated with the use of these sys-tems. This section will address: 1 ) the cost incapital and labor and the time needed to im-plement longwall mining and 2) the compara-tive production advantages and some of thephysical, environmental, and social conse-quences of using this technology.

Capital

During the 20-year period that reliablelongwall mining systems have been on the

market, an important reason for the reluc-tance of coal producers to use this technologyhas been the initial cost of installation. Theinstallation of a complete longwall miningsystem, not including the cost of developmentworkings and other mine infrastructure, re-quires the expenditure of a single large lumpsum of capital, usually $1.5 million per 100 ftof face length. This cost includes: 1 ) the facesupport subsystem, 2) the face conveyor sub-system, and 3) the coal cutting-loading sub-system. The total installed cost of the threelongwall subsystems will vary significantlyfrom mine to mine. The more important var-iables which determine the cost of a specificlongwall system include the length of the

——. . . —


face, the height of the coal to be cut, the geo-logical conditions of the seam, the thicknessand quality of the roof rocks (which deter-mine the capacity of the face support subsys-tem required), and the rated capacity of thesystem.

The typical longwall system now being in-stalled in the Western United States has a de-signed face length of 600 ft and a rated pro-duction capacity of 1,250 tons of coal pershift. However, the actual production capac-ity obtained varies from 700 to 1,500 tons pershift because of variations in mining condi-tions and in the ability of producers to op-erate longwall systems. This compares with anational average of 400 tons per shift forlongwall systems in 1980.

The cost of this typical longwall installationin 1980 dollars is estimated at approximately$9 million. Assuming that the mine operates250 days per year and two shifts per day, therated annual production capacity of the typ-ical longwall system will be 625,000 tons. * Ifthe longwall system is assumed to have a pro-ductive life of 10 years, which is not unreal-istic if the system is adequately maintainedand there is selective replacement of themore expendable system components, thenthe total production capacity over the 10-yearlife will be 6,250,000 tons of coal. Hence, thetotal installed capital cost of the longwallsystem over the life of the system will be ap-proximately $1.50/ton of coal mined.

The significance of capital cost for long-wall mining can be illustrated by the follow-ing example. Utah Power & Light installed alongwall system in its Deer Creek mine nearHuntington, Utah, in April 1979. The systemwas designed for a 480-ft face and a 10-ftthick seam. The initial production capacity ofthe system was 1,500 tons per shift. However,after only 3 months of operation longwall pro-duction reached an average of 2,500 tons pershift, Using a schedule of two shifts per day,the initial 3,000-ft long panel was mined out

*This figure takes in!{) arc[)unt the am(mnl of time the sys[emis m )1 i n opera I i ( )n [iu ring I his pf?ri(d hcra use () f m:] in Iena rice,t?t c.

in a period of 6 months for an average pro-duction of 2,200 tons per shift.

The impact of the success of this initiallongwall system on the entire Deer Creek op-eration is dramatic. Prior to the installationof the longwall unit, daily production ca-pacity at the mine was 7,000 tons. To achievethis production rate it was necessary to useas many as 10 continuous miner sections.With the implementation of the longwallsystem the daily production has increased toin excess of 10,000 tons and the monthly pro-duction to 220,000 tons. Of this 220,000 tons,a total of 115,000 tons is produced by thesingle longwall unit and the balance by eightcontinuous miner sections. With the instal-lation of a second longwall, the company ex-pects the requirement for continuous minersto drop to four sections. Two of these sectionswill be used for longwall panel developmentand two to extract pillars from sections of themine which have already been mined usingthe room-and-pillar technique.

Prior to the installation of the first longwallunit the Deer Creek Mine was producing 1.75million tons of coal per year, This rate wasachieved through the use of 10 continuousminer sections. Assuming an average invest-ment of $700,000 per section, the total invest-ment in mining equipment was $7 million.Based on a 10-year life for the continuousminers and auxillary equipment, the totalcapital cost per ton of coal mined over the 10-year period was $0.40. Assuming an installedcost of $9 million per longwall unit, a similarcalculation for the 2-longwall and 4-contin-uous miner production system now producing2.64 million tons of coal per year results in acapital investment of $0.80/ton of coal pro-duced over a lo-year period, assuming thelongwall units and the continuous minershave productive lives of 10 years. Consider-ing the cost of money at 20 percent and totalfinancing of the equipment, the all continuousminer system cost becomes $0.85/ton and thecost for the mixed system $1,65/ton.

Although the capital cost per ton of coalproduced is higher for the longwall system,labor costs are reduced. Assuming that the

Ch. 11—Mining Technology 341

longwall and the continuous miner sectionsboth require 10-person crews for their opera-tion and that the size of the maintenance sup-port staff is the same for both the all contin-uous miner mine and the combined longwalland continuous miner mine, the combined op-eration will require 80 fewer hourly employ-ees to operate on a two shift per day basis.Based on a direct hourly rate of $9/hour and afringe benefit rate of 35 percent, each ofthese 80 employees costs approximately$25,000 per year. Therefore, the total savingsin labor costs for the mine configuration usingthe combined systems is $2 million per year,This sum is 14 percent of the difference incapital cost between the combined operation($21 million) and the all continuous mineroperation ($7 million). When the fact that thecombined operation produces 900,000 addi-tional tons of coal per year is factored in,then the payback period becomes of the orderof 2 years.

There a re a number o f ex i s t ing andplanned underground mines on Federal coalleases that could use development strategiessimilar to that of the Deer Creek Mine. TheSkyline Mine of Coastal States Energy Co.,which is located near Price, Utah, is sched-uled to open in 1982 with an initial productionrate of 437,000 tons per year, This rate ul-timately will be increased to 5.4 million tonsper year. Over 40 percent of this production,2.3 million tons, is scheduled for three long-wall units ranging in capacity from 702,000to 864,000 tons per year. Because of extremevariations in seam thickness and the pres-ence of faulting on the property, problemswhich do not exist at the Deer Creek Mine,the balance of the annual production will bemined with continuous miners. It is estimatedthat 14 continuous miners will be required toproduce their 3. l-million-ton share of the an-nual production.

Labor

As the above section shows, increased cap-ital costs for longwall mining can be offset byreduced labor costs and increased produc-tion, Because the costs of capital and labor

will be the major inputs into any mining sys-tem, both surface and underground, therealways will be some tradeoff between thetwo, which will vary from mining system tomining system. However, several aspects oflabor requirements for longwall systems dif-fer from the labor requirements for room-and-pillar systems. These differences cannotbe readily quantified in terms of direct cost.

Western coal mines usually recruit theirminers from the general labor force. Someworkers may come to the industry with abackground in construction or some other re-lated occupation, but few have any mining ex-perience. Therefore, if a mine using longwallmining hires a new employee, there is fre-quently no need to retrain an individual re-cruited from room-and-pillar operations. Thisability of the Western coal miner to adaptreadily to the longwall mining environmenthas been noted by a number of companies. Asindicated above in the discussion of the DeerCreek Mine, the production rate from its new-ly installed longwall face increased from1,500 tons per shift to 2,500 tons per shift inlittle more than 3 months of operation. Theonly experiences longwall miners at the timethe longwall unit was installed were the threeshift foremen. The Coal Basin Mine longwalloperation in western Colorado and the Sun-nyside Mine in central Utah also have hadgood experiences with the installation of theirfirst longwall units. Like Deer Creek, thesemines have achieved production rates muchhigher than those obtained at many of thelongwall units located in Eastern coal mines.

Another benefit of longwall mining fromthe point of view of the mine operator is thereduced labor requirement when comparedto room-and-pillar systems. This factor is anadvantage both with respect to f indingenough qualified employees to staff an opera-tion and in reducing the amount of infra-structure needed where a mine is remotelylocated and requires development of a sup-porting community infrastructure to serveemployees, Since minable coal deposits in theWest often are located in sparsely populatedareas, the availability and the housing of


employees are two major considerations. Inthe case of a mine such as Skyline, which islocated in a large rural area of Utah thatalready has seen the expansion and develop-ment of a number of large mines, attractingskilled labor becomes a problem. Thus, reduc-tion in the total number of operating employ-ees required from 480 for a mine using allcontinuous miners to 340 for a mine using acombination of continuous miners and long-wall units is significant,

There are other advantages to the mine op-erator of reduced overall manpower require-ments. These include the reduced potentialfor personnel turnover, less need for em-ployee training, and an overall reduction inpersonnel support costs. Operating with asmaller labor force also means that fewerpeople will be exposed to the hazards of un-derground coal mining per ton of coal pro-duced. This will be discussed in greater detailin the section on health and safety,

Production and Productivity

As has been discussed in preceding sec-tions of this chapter, production and pro-ductivity are related but separate opera-tional considerations. An increased produc-tion rate is of interest to the operator who hasthe reserves to support large market commit-ments but who is not able to produce the coalrequired of these commitments because of de-ficiencies in the production system. In con-trast, productivity is of interest to all mine op-erators, Productivity is usually discussed interms of coal output in tons per unit of laborexpended, but is equally applicable in termsof coal output per unit of machine time ex-pended. Whether stated in terms of labor pro-ductivity or equipment productivity, the ob-jective is to maximize coal production perunit of resource used.

The question of improved productivity andlongwall mining has been addressed. In thediscussion of the capital cost of longwall min-ing, the potential for improving labor produc-tivity through the installation of longwallequipment was illustrated by two examples.It was also shown that even though the im-

proved labor productivi ty was achievedthrough the use of longwall equipment whichwas more expensive than the continuousminers it replaced, the overall effect was areduction in the total cost per ton of coalmined.

The underground coal mining industry inthe United States has undergone significantchange in the past 10 years. A 500,000-ton-per-year mine, formerly considered a largemine, is now considered of small to averagesize. With this change, production equipmentand the scheduling of equipment have becomemore complex. Whereas a 50(),0()0-ton minecould operate with two to three continuousminers on a two shift-per-day schedule, minessuch as the Skyline Mine discussed abovewould require 24 continuous miners to sus-tain its 5,4-million-ton-per-year productionrate, Coping with the production schedulingin a mine where there are 24 operating sec-tions which require the assignment of man-power, equipment, maintenance, and trans-portation is an extensive and demanding task.The Skyline operation is not unique in this re-spect. Other existing and planned under-ground mines on Federal leases that even-tually will be producing in the 5-million-ton-per-year range include the Price River CoalCo. mine (formerly Braztah) in Utah and theLoma Complex in Colorado. In the 2-million-to 3-million-ton-per-year range there will beSUFCo in Utah, Mt. Gunnison in Colorado,Energy Development in Colorado, and poten-tially La Ventana in New Mexico, The minesin the l-million- to 2-million-ton-per-yearrange are even more numerous.

Nearly all of these mines will install long-wall units, One of the major considerations inarriving at the decision to use longwall min-ing at these mines has been the need toachieve high production rates. A single long-wall unit can produce 750,000 tons of coalper year. The longwalls at the Deer CreekMine are producing in excess of 1 million tonsper year per unit, The Wilberg Mine, a sistermine of Deer Creek, is installing a longwallunit that is rated at 3,000 tons per shift. Thistranslates into 1.5 million tons per year,

Ch. 1 l—Mining Technology ● 343

Coastal States Energy Co. is currentlystudying the feasibility of using longwall min-ing at its SUFCo Mine which is located onFederal leases near Salina, Utah. The minableseam, the Hiawatha, varies from 6 to 15 ft inthickness. However, the average thickness is12 ft and with the exception of thinning onone part, the lease tends to maintain this aver-age over extensive areas. There is little dip tothe seam and little noticeable faulting. Insum, these conditions suggest that installa-tion of at least one longwall unit to replacesome of the existing 10 continuous minerunits would simplify the operation and mightreduce the cost of mining.

In summary, Western underground minesneed to continue to pursue methods for in-creasing their production rates and improv-ing their productivity. These combined goalswill lower their unit mining costs and therebyenable them to compete more effectively withthe coal mines in the Eastern United Statesand the large surface mines in the West.Other than reduced mining costs, an addi-tional advantage of the increased productionrates could be the ability to use unit-traintransportation to move coal to markets andthereby recover some of the transportationcost penalty associated with supplying coal toMidwestern and Eastern markets.

Environmental

In the arid West, the impact of under-ground coal mining on the environment is gen-erally less than that resulting from surfacecoal mining. Longwall mining can furtherreduce the environmental impacts normallyresulting from room-and-pillar operations.

Subsidence from underground mining cantake the form of either a wide-area loweringof the ground surface or sinkholes and frac-tures that break through to the surface. Theresults of several studies, conducted both inthe United States* and Europe, indicate that

longwall mining is most likely to result inwide-area subsidence and to cause little dif-ferential subsidence that can result in breaksin the ground surface. These studies alsohave indicated that surface subsidence isgenerally limited to an amount equal to one-half the thickness of the coal being extracted,although it can exceed 50 percent and can goas high as 90 percent of the seam thickness.For the thicker seam longwall operations, thiswould generally mean a subsidence of about6 ft. A lowering of the surface elevation ofthis magnitude could be a problem in theWestern United States and would have to behandled on a case-by-case basis. However,the areawide form of subsidence likely toresult from longwall operations is preferableto the differential subsidence that is morelikely to result from room-and-pillar opera-tions,

Health and Safety

Regardless of the type of mine, under-ground coal mining takes place in a hazar-dous environment. Fewer workers will be ex-posed to these hazards in a longwall opera-tion than in other underground coal produc-tion met hods for a given level of production.

Because of the fundamental differences be-tween room-and-pillar and longwall miningsystems it is difficult to make a direct com-parison as to which provides a healthier orsafer environment for the worker. The basichazards—coal dust, methane gas, sponta-neous combustion, roof falls, bumps, andmoving machinery in confined spaces—arepresent in both types of mining. In some casesthere are tradeoffs in hazards, The control ofexcessive coal dust at the longwall face is aproblem that must be solved, although thereare some potential breakthroughs. On theother hand, however, the protection from rooffall hazards provided by longwall supportsis superior to that available to the continuousminer operator.

Equipment advances that improve the un-derground mining environment with respect


to worker health and safety will continue. tivity and a concomitant reduction in theHowever, the most direct results will be ob- number of workers that must be exposed totained through increases in labor produc- the dangers of underground coal mining.

CHAPTER 11 Mining Technology - Princeton

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