cms in south africa

7/30/2019 Cms in South Africa

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The use of continuous m iners in SouthAfrican coal m inesby J. D. INCH, J. D . STONE, I. D. BRUMBY,and C. J. BEUKES

A. Continuous m iners, and their operation atBosjesspruit Colliery J. D . INCH*, B.Sc. (M in.), M . I. M in. E. (Visitor)SYNOPSIS

A brief description is given of the m ain types of continuous m iner and their incidence of application in SouthAfrica.D efin itio ns an d de scription s o f t he d esig n p aram eters an d compo nen ts o f t he m ain cu ttin g h ea d are g iv en . to ge the rw ith an outline of the m odifications carried out to im prove m achine cutting perform ance.An account is given of experience with the application of water infusion to suppress airborne dustd ur in g con ti nuou s-min er ope ra ti on s.SAMEVATTING

D ie hooftipes delw ers en die om vang van hulle aanw ending in Suid-A frika w ord kortliks beskryf.D ie hoofsnykopontw erp param eters en kom ponente tesam e m et 'n raamwerk van die resultate van w ysigings w ataangebring is om die m asjien se snyberrigting te verbeter w ord gedefinieer en beskryf.E rvaring m et die aanw ending van infusie tegnieke m et w ater om stof in die lug te bekam p tydens delw erw erkingwo rd b es kr yf .

Types of Continuous M inersContinuous miners designed for operation in coal

m ines fall into four distinct types. Each type can bedistinguished by the action of the cutting components,or cutting head, and a brief description of these actionsis given below .The Borer TypeW ith this type the main cutting components compriserotary boring elements on which are mounted cutterpicks cutting concentric paths. A conventional coal-cutter chain com pletes the periphery of the profile.The Ripper TypeThe main cutting components of this type of machineconsist of a series of conventional coalcutter- type chainsand pickboxes (usually about six chains) driven by acommon sprocket shaft and having a common idlershaft. This arrangement of shafts and chains is termedthe ripp er ba r; being provided w ith elevating hydraulicjacks and mounted on a turntable, the bar can form aroadway with a series of cutting passes as the ripperbar is raised or lowered.The Osc illa tin g-h ea d T yp e

The cutting components of this type of machineconsist of four wheels approximately 1 m in diameter,the periphery of each wheel being equipped with pick-boxes, and each pair of wheels being driven by a com-mon gearbox situated centrally between the wheels. Thetwo in-line combinations of motor, gearbox, and wheelsare suspended from a common boom, and each combina-tion can oscillate from side to side. Thus, a roadway can*B osjesspruit C olliery, Sasol O ne (Proprietary) Ltd, Secunda,Transvaal.

be formed by a series of cutting passes as the machineis advanced and the boom raised or lowered.T he H orizontal-drum T ype

With this type of machine, a horizontal drum about1 m in diameter equipped with an array of pickboxesis mounted on a boom that is capable of being raisedand lowered. The drum is driven by gears or chain-driven sprockets. The chains may also be equipped withpickboxes. A roadway can be formed by a series ofcutting passes as the m achine is advanced.

Introduction to South M rican Coal M inesAll continuous m iners designed for operation in South

African coal m ines are of American origin, and weredesigned essentially for operation in relatively softbituminous types of coal. The introduction of suchmachines into the harder, more abrasive, cleat-freeSouth M rican conditions has no doubt accounted forthe varying degree of application of the machines.Indeed, the successful use of continuous miners inTransvaal coal seams has been demonstrated onlywithin the last four years.

An indication of the introduction of the main typesof machine is given below, and their history is sum.marized in Table I.B orer T yp e

A machine of this type (Goodman borer) was intro-duced at Coalbrook Colliery in 1957, and, as a result ofthe acceptable productivity level (average about 800tons per shift) and the im proved strata control situation,allowing the splitting of pillars and thus improvingthe percentage extraction of reserves, three furtherm ach in es w ere p urc hase d.

The machines operated successfully until 1972. AtJOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1980 5


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TABLE IHISTORY OF CONTINUOUS MINERS IN SOUTH AFRICA

Year Mach ine ty pe Colliery Make1958 B Coalbrook Goodman1959 B R DNC Jo y1960 B R1961 B R1962 B R Sigma1963 B R1964 B R1965 B R1966 B R1967 B R 0 Sigma L ee N orse1968 B R 0 Usutu Lee Norse1969 B R 0 D Coalbrook Jo y1970 B D1971 B D1972 D1973 D1974 D1975 D S. W itbank NMS1976 D1977 D1978 D1979 DB Borer 0 O sc illa tin g h ea dR Ripper D Drumthat time the mine plaIffiing indicated a variable m iningheight in a different coal seam more suitable to othertypes of machine. Alternative types of machines wereavailable, and consequently the borers were phased out.Ripper Type

A machine of this type (Joy 6 CM) was introduced atDurban Navigation Colliery in January 1960, and in1962 two machines were in operation at Sigma Colliery.The latter machines were withdrawn from service in1968-1969.Osci ll at ing -head Type

A machine of this design (Lee Norse) underwenttrials at Sigma Colliery in 1967. The machine was usedprimarily for the driving of development roads forlongwall panels. A sim ilar machine was subjected toe xte nsive trials a t U su tuC ollie ryin 1 968 -69 . P rod uc tiv itylevels of 700 tons per shift were achieved, but the over-all cost per ton of coal produced exceeded that ofconventionally mined coal by unacceptable levels at thetime of the trial, and the machine was withdrawn.Drum Type

A horizontal-drum type of continuous miner (Joy10 CM) was introduced at Coalbrook Colliery in 1969and p erfo rmed s atisfa cto rily .

The first horizontal-drum m achine to operate success-fully in the Transvaal coalfield was on NMS Mariettam iner, which was tested at South W itbank Colliery inMarch 1975. This machine consistently exceeded 1000tons per shift, with a high m achine availability.Drum miners of various makes (Jeffrey, Joy, NMS, andLee-Norse) have since been introduced into SouthA frican m ines.

Operational Experience in South MricaIn a breakdown analysis report for the year 1978, theChamber of Mines' Coal Mining Laboratory indicatedthat by far the largest single cause of the non-availability

of continuous miners was the equipment directly

associated with the cutting operation of the machinecycle. The average annual figure given for non-avail-ability was about 40 per cent. (The non-availability ofthe m achine as a result of conveying, tram ming, grading,and cleaning-up operations never exceeded 9 per centw ithin these individual operational elem ents.)

The vast majority of continuous miners currentlyin operation are of the horizontal-drum , caterpillar-track mounted type, and, in view of the knowledge ofbreakdowns that is now available, it is of interest tolook at the cutting components in more detail.

Cuttin g ComponentsCuttin g D rum

This consists of a steel cylinder on which are mountedblocks, in the form of a scroll or scrolls, carrying clevisboxes or pickboxes. The drum can be caused to rotateby gears or by chain-driven sprockets. (These chainsmay also be equipped with pickboxes to present acontinuous array of picks to the coal face.)C le vis B ox es

These are welded or bolted to the drum and enablethe pickbox to be positioned and fastened by a pinthat passes through the sides of the clevis box and thepickbox. The clevis base forms the seat on which thepic kb ox sits.Pickbox

This is a casting specially shaped to hold picks, andcontains a slot, a hole for the pick shank, a hole for theclevis pin, and a seat.

The pick is located into the pickhole, and the base ofthe pick passes through the slot and rests on the seat.The slot enables fasteners to be fitted to the pick shank,which prevents the pick from being ejected from thebo x durin g op era tio n.Pick

The picks themselves consist of a steel body contain-ing a recess into which a cemented carbide tip is brazed.The cemented carbide tip is the cutting portion of thepick, and consists of two materials, tungsten carbideand cobalt, sintered together to form a matrix of car-bide grains within a cement of fused carbon.

The most important physical properties of thecemented carbide are hardness and toughness. Thevalue of both these properties can be varied by the

Toughness

T ou h ne ssH or ness

Hardness

" C ob al tFig. I-Variation in the hardness and toughness of cementedcarbide with amount of cobalt present6 JANUARY 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY


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2 I Iw

--i~46 ~I

i 81-0 --. III L-F --r---

--. !1 r

-----,--Ll

amount of cobalt present, as shown in Fig. I. If thecarbide is too hard, premature fracturing will occur,and, if it is too soft, the material will wear away tooquickly. Thus, for optimum cutting performance, abalance between the two properties is necessary, de-pendent upon the quality of the coal being cut.

Design Definitions of Cutting HeadsA tta ck A ngle

The angle of attack (Fig. 2) of a pick can be definedas the angle formed by a line drawn longitudinallythrough the shank and point of the pick and a lineforming the radius of the pick point (through the centreof the drum and the point of the pick). Some manu-facturers use the 'complementary angle' as their defini-tion of attack angle.

Radius......

--

...... ......\ --\'-'Instantaneausdirection of cutting

Fig. 2-Attack angle

Clearance Ang leIn a point-attack pick with a point 'cone angle' of,

say, 60 degrees, the sum of the back and front clearanceangles will be equal to 30 degrees. The individual valuesof the clearance angles will depend upon the attackangle in use at the time (Fig. 3).

" Back clearance ,~Radius

'" 7g1e ;. ;./

- - - -Attackangle

/ ";."'< \;. . ~F t \ Instontaneousrond" f;.;. clearance', ~rech?n 0

"';. angle 'cuthng

~Fig. 3-Clearance angles

P ic k S pa cin gFor a single scroll, the position of the picks on anauger is usually determined as shown in Fig. 4. Multi-

scroll drums can be made by the interspersing of pick-boxes to the required pattern. Thus, more than onepick per line can be made available.P ick 'O ff-set' A ngle

With point-attack picks, the self-sharpening capa-bility of the picks, due to normal rotation, can be in-creased by the setting of each pickbox slightly out ofline by a small amount, say 2 to 3 degrees. This inducesrotation, and hence self-sharpening of the tips.

0 14

IDrum circumference

1

60 "120"180"240"300"360" 1 Pick

spacingFig. 4-Position of picks on an auger

Continuous Miners at Bosjesspruit CollieryThe continuous miners in operation at Bosjesspruit

Colliery are Jeffrey 120-H2 Helim iners. The mainspecifications of the m achines w ere originally as follow s:

Mass (approx.) 50000 kgMotors 2 X 3300 V X 205 kWMining height (max.) 3,66 mCutting width 3,30 mCutting-head diameter 933 mmCutting-head speed 67 or 50 r/m inPick spacing 57,1 mm.Three machines have been deployed since the com-pletion of shaft-sinking operations in 1977 to mineprimary, secondary, and tertiary development in seam

heights ranging from 2,5 to 3,5 m:P rim ary develo pm ent. This consists of 25 or 28 m

square pillars formed by 6,0 m wide roads driven asnine roa d sec tio ns.S ec on da ry d eve lo pmen t. 'Fftig, IS mainly 25 or 28 m

square pillars formed by 6,0 m wide roads driven assev en roa d se ctio ns.

Ter tia ry d ev elo pmen t. So far this has consisted ofdevelopment for longwall units, being 100 m X 35 mpillars (CL) formed by 5,0 m roads driven as two roadsections,

Basic Geology at BosjesspruitThe no. 4 seam at Bosjesspruit Colliery displays the

normal characteristics of South African coals, beingharder, more abrasive, and containing fewer cleatsthan the normal D.S. or European coals. In addition,patches of the seam have been encountered that containabnormally large numbers of carbonate nodules andlayers of pyritic lenses. The nodules have uniaxial com-pressive strengths in the region of 30000 IbJin2, andoccur interm ittently in tw o distinct bands approxim atelyI m and 2 m from the seam floor. The patches of highincidence of nodules are apparently randomly distri-buted, and, so far, geological investigations have indi-cated no m eans of predicting their distribution.

The seam also contains a sandstone band occurringabout 0,5 m from the seam roof and of variable thick-ness from 0 to 0,3 m . The planned mining height assumesthe machine will m ine up to the sandstone band, but inweak areas this must be cut, and, in conjunction with a

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1980 7


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high incidence of nodules, difficult cutting conditionsare experienced. The floor comprises a competentsandstone.

O perational Perform ance at BosjesspruitThe design of the continuous-miner cutting head

originally incorporated pin-on type pickboxes withplumbob point-attack picks spaced at 57 mm on adouble-scroll pattern with an attack angle of 37,5degrees. The drum speed was 50 r/m in, and the width ofthe cutting head 3,3 m .

During the first few months of operation, the pro-duction perform ances were inconsistent, and the cuttingefficiency of the machines was seriously affected by theincidence of nodules. Physical evidence of the difficultcutting conditions w ere as follows:- high rates of pick damage: pick shanks were snappedand bent- high wear of pick tips: tips were ripped out of there cesse s or c hipp ed- damage to clevis brackets: damage occurred atweld ed jo in ts- wear between pick, pickbox, and clevis pins: ad.jacent picks could be displaced towards each otherby as much as 25 mm when grasped with the hand.

M odifications to Im prove Perform ancePicks

Several small-scale tests were conducted on pickssupplied by various manufacturers, and it becameobvious that a simple bullet-type of pick gave the bestresults. The picks in the pickboxes of the scrolls werechanged immediately, and those in the trim chains ofthe machines were changed as the chains became wornand were replaced. Pick damage was reduced, but anexcessive degree of w ear rem ained in the pickbox-clevisarea.Trim Chains

After some time, the amount of wear in the pickboxeswas such that as many picks were being ejected fromthe boxes as were being consumed by normal usage. Thelines of picks were therefore increased from five to seven,i.e. the pick spacing was reduced to about 37 mm, and anim provem ent was effected.Clev is Bracke ts

In order to reduce wear in this area, these were re-moved, and the pickboxes were welded direct to thedrum blocks. A special jig was constructed in the mineworkshops to assist with the modifications, and its useresulted in the disclosure that some of the picks of thecutting head were not describing the same concentriccutting circle. All the drums are now checked on thisjig before being sent underground, and this has resultedin much reduced machine vibration during normalcuttin g ope ra tio ns .Cu tt ing- he ad Spe edTests were conducted at two speeds: 50 and 67 r/m in.At the higher speed, more dust was produced, but noappreciable difference in cutting performance could beclaimed.8 JANUARY 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Pick Attack AngleThe machines were tested with picks covering a range

of attack angles between 37,5 and 45 degrees. Thesete sts are co ntin uing .Summary of Test Results(I) The simple bullet type of pick has given the best

re su lts a t B os je ss pru it.(2) An increase in the number of lines of picks reducedthe wear on trim chains.(3) Improved performance was achieved by the weldingof pickboxes direct to the drum blocks.(4) At higher drum speeds, the amount of airbornedust produced increased dramatically, W ith only aslight increase in production rate.(5) The tests on various pick attack angles have beeninconclusive and are continuing.(6) The use of a simple jig to check the modification hasresulted in reduced m achine vibration during under-g ro un d o pe ra tio n.

ProductivityAlthough the output of individual shifts has been in

excess of 1000 tons, the average section productivity atBosjesspruit (on a 3-shift-per-day basis) lies between 600and 700 tons per shift. It should be noted that, at anyone time, two of the three machines have been engagedin longwall development, i.e. m ining two roads withlO Om (chain) pillar centres.

Dust S up pr essio nThe production of airborne dust by coal-cuttingmachines has been a problem since the development of

mechanized coal m ining. W ith continuous miners, theproblem of dust production is amplified by the fact thatthe machine is relatively static in one roadway duringnorm al operations, and, if it is producing coal consistentlyat a high rate, the local environment undergoes littlevariation compared with that in a conventional mech-anized section. In the latter, a regular change of activitiesand machines in a heading can generate a variety ofairborne dust levels, varying from maximum to mini-mum with a time interval of hours between thesevalues. The continuous miner, however, may be en-gaged in non-dust-producing activities for only a fewseconds.Apart from the direct health hazards associated withthe presence of dust in the working environment (pneu-m oconiosis, eye irritation, eye strain, spitting, etc.), theindirect effects can be just as dangerous. For example, acontinuous miner, when leaving a coal roof, can cutthrough a slip so smoothly that the presence of slipsis indicated only by a small crack. In a very dustyatmosphere, this crack can easily evade detection, andwithin a few minutes the crack can be directly above thearea normally occupied by the driver, cable handler,supervisor, etc. and create a potentially serious situation.Reduced visibility during the m anoeuvring of m achinesalso obviously induces potential accident situations.

From a productivity point of view, the presence of alot of airborne dust prevents the driver from steering


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correctly, with a consequent waste of time duringrealignment, and maximizes the errors associated withroof and floor control. The constant presence of airbornedust with its dimming effects on machine lights andcap lamps, and the copious amounts of water that haveto be used via sprays, culm inating in mud and slurrythroughout the area, are bad for morale and expensiveto a ll ev ia te .Modific ation s to D ust-sup pre ssio n E quipm ent

The original dust-suppression equipment suppliedwith the continuous miners consisted of an array ofwater sprays covering the cutting head and an exhaustscru bb er system .

The scrubber system consisted of scrubber jets anda fan unit situated at the rear of the machine, on theside opposite to the driver's cab, and a system of steelducting, by which air was drawn from air intakessituated on the top side of the boom. These air intakeswere positioned so that the orifices faced forwardtowards the cutting head. The capacity of this systemwas 1,3 m3/s, the air and slurry being discharged behindthe fan unit and at right angles to the machine body.

After tests had been conducted on different types ofsprays and spray positions, it was decided that thesystem should be redesigned to increase its effectiveness.

The scrubber unit, fan, and discharge point wererelocated above the conveyor, in line with the hingepoint of the boom, and the air intakes were repositionedto take in air vertically from the side of the boom.Additional scrubber sprays were installed, and, as aresult of the reduced length of ducting, the capacity ofthe system was increased to 1,5 m3/s. The systemfunctioned approximately 20 per cent more effectivelytha n be fo re.Water I nfu sion

Despite the improvement, dust remained a problem ,and it was decided to inject water into the coal seamahead of the continuous miners and to measure theeffects on dust production. This method of injection, orwater infusion, has been known in Europe for someyears, but is normally practised on longwall faces insoft, broken coal.

Since the specialized equipment for this technique isnormally imported and potentially expensive, it wasdecided to use conventional cem ent-grouting equipm entto determ ine the water pressure, water flow, and drill-hole positions required for efficient infusion. A hole wasdrilled with a diamond drill into the centre of a 25 msquare pillar, and, by use of a compressed-air pump,water was infused into the coal. W ater appeared on thesides of the pillar at various horizons, and the infusionpressure was measured as approximately 3 M Pa.

The results were encouraging, and it was decided to

introduce the equipment into an area in which a con-tinuous miner was operating. Again, a diamond drillwas used to drill a hole compatible with the groutingequipment, and an area of about 200 m by 100 m wasinfused with approximately 14000 litres of water. Dustsamples were taken, according to standard procedures,as the continuous miner progressed through the area,and the results indicated a substantial reduction inairborne dust compared with that when the samemachine was used in an untreated area. The resultsindicated reductions in dust counts of approximately50 per cent.T he sp ec ia liz ed drillin g e quipmen t an d w ater-inje ctio nequipment necessary to achieve these results werereplaced by an imported injection gun, which can oper-ate in a small hole drilled by a hand-held drillingmachine. An area ahead of a continuous miner wasinfused with water, and dust samples were taken in thedriver's cab as the machine traversed the area. Theresults again showed a substantial reduction in airbornedust.

This technique will now be developed further andincorporated into the mine operational plan for con-tinuous-m iner sections. It is expected that the techniquecan also be utilized with longwall m ining and con-ventional m echanized m ining for dust suppression and asa potential aid to productivity.

Optim ization of Continuous Miner PerformanceThe normal mining horizon at Bosjesspruit for the

next few years will be between a hard sandstone band,which forms the roof, and a hard micaceous sandstonefloor. O perator error in controlling roof and floor horizoncan, of course, amplify pool'


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B. The selection and perform ance of continuous m iners atMatla Coalby J. D . STONE*, B.Sc. Eng. (M in.,) M .B.L. (G raduate)

SYNOPSISThe reasons for the choice of continuous m iners for the developm ent of accesses and the winning of coal in am odern underground coal m ine are discussed. It is show n that, at relatively Iow depths, the extraction rate as w ellas the costs are m ore favourable than those of proved total extraction m ethods.The selection of a continuous m iner is largely dictated by factors such as production rate, capital and runningcosts, im ported content and the extent to w hich local content and m anufacturing are to be increased, m anpow errequirem ents, seam thickness, expected floor and roof conditions, and m axim um cutting height.T he present inherent shortcom ings of continuous m iners, such as inefficient continuous transportation of coalaw ay from the m iner and the lack of an infinitely variable traction to m atch the sum ping speed with the feasiblepenetration speed, are pointed out.Specific problem s encountered and the m ethods of solving them , as w ell as a planned trial w itn'a heavy-dutyc on tin uo us m in er , a re d is cu ss ed .

SAMEVATTINGD ie redes vir die keuse van aaneendclw ers in die ontw ikkeling van toegange en die ontginning van steenkool in'n m oderne ondergrondse steenkoolm yn w ord bespreek.D aar w ord aangedui dat die tem po van ontginning sow el as die koste op relatiew e lae dieptes m eer voordelig isa s d ie v an b ewes e to ta le v er ha lin gsme to de s.D ie keuse sow el as tipe van 'n aaneendelw er w ord hoofsaaklik bepaal deur faktore soos produksietem po, kapitaalen bedryfskoste, ingevoerde inhoud en die m ate w aarin die plaaslike inhoud en vervaardiging beplan w ord om tev erm eerde r, m an nek ra gv ereistes, laag dik te, v erw ag te v lo er- en d ak om stan digh ed e e n m ak sim ale sny ho og te .D ie huidige inherente tekortkom inge van aaneendelw ers w aaronder die ondoeltreffende deurlopende vervoervan steenkool wag van die delwer en die gebrek aan 'n oneindige varieerbare trekkrag sodat die delfspoed diemoon tlik e in drin gspo ed ew en aar, w ord to eg elig .Spesifieke problem e w at teegekom w ord en die m etodes om hulle op te los sow el as die beplande proefnem ingm et 'n sw aardienstipe aaneendelw er w ord ook bespreek.

IntroductionThe objective in the establishment of Matla Coal

Limited is to supply Escom 's new 3600 MW Matlapower station with coaL The colliery will consist of threemines of approximately equal capacity within a radiusof 8 km from the power station. The power station andmining complex is situated almost 30 km south of Ogiesin the Bethal district of the eastern Transvaal. Thepower station, when fully commissioned, will requirearound 10 M t of coal per annum.The coalfield from which the Matla power station willbe supplied is known as block IV of the Highveld Coal-field. The area to be exploited is bounded to the eastby the Anglo Power colliery, which supplies Kriel powerstation, to the north by the South W itbank and Tavi-stock collieries, and to the west by Delmas colliery,and it stretches south-west towards Leslie. The reservescontained in the three mineable seams in the miningrights area are approximately 1183 Mt. These seams,from the top down, are no. 5 with an average thicknessof 1,4 m at an average depth of 48 m, no. 4 with anaverage thickness of 4,8 m at an average depth of 76 m,and no. 2 with an average thickness of 4,2 m at an aver-age depth of 105 m.Over the eastern portion of the coalfield, the roof

of the no. 5 seam consists of dark-brown shale, which isvery susceptible to dehydration. The resultant de-hydration cracks cause the shale to fall out between theroofbolts. Over the remainder of the coalfield, the roofconsists of dark-grey micaceous sandstone. Micaceouspartings in this sandstone are sporadically very welldeveloped and cause about 15 cm of sandstone to fallas soon as it is exposed.*M atla C oal L td, K riel, T ransvaal.10 JANUARY 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

The immediate floor of the no. 5 seam consists ofsoft shale underlain by very fine-grained micaceoussandstone. Heavy machinery tends to break this shaleup, with the result that the floor is very uneven andcovered with debris, which hinders movement andcontam inates the product.The roof of the mining excavations in the no. 4 coalseam consists of inferior-quality coal and carbonaceousshale. Under normal conditions, the roof tends to bevery good. Well-developed fracture planes and theintrusion of dolerites tend to result in a deterioration ofroof conditions in certain areas.The floor of the no. 4 seam consists of fine- to medium-grained sandstone, which proves to be very stable.The roof and floor of the no. 2 coal seam consists of acompetent sandstone, and it is expected that generally

excellent m ining conditions will prevail. Of the threecoal seams, the no, 5 seam has proven metallurgicalqualities, and, from investigations, it was concludedthat a market for this coal should be obtained as soonas possible. Escom stands to benefit through the econom yof scale with respect to both working cost and capitalrequirements, and Matla will at the same time increaseits profit through increased sales. The coal will beproduced by means of 17 bord-and-pillar continuous-m iner and tw o longw all sections. O f these, six continuous-m iner sections and one longw all section w ill be operatingin the relatively low no, 5 coal seam .

Choice of Continuous MinersDuring the initial planning stage, a comparative cost

projection was done, which indicated that by 1980,owing to the expected rapid escalation of wages, theworking costs for a continuous-miner section would be


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less than that of a conventional mechanized section. Ata capital charge of 20 per cent, the projected workingcosts for a continuous-miner section would be 412 centsper ton, compared with 424 cents per ton for a conven-tional mechanized section. The predicted saving ofapproximately 12 cents per ton in 1980, which wouldincrease to 30 cents per ton in 1983, was one of the mainmotivating factors for the choice of continuous miners.(See Fig. 1.)

CO ""'l'O USI"R , -LEe,"",CO>1VE'TIO'A'. . h __ _d _h -..lQQ

--.f&Q

;00

zc~'"~~ --1illD I

-2Q9""

2OO

TO O~ ~

~ :\: ~. ~ - -rERIOD

Fig. I-Comparative cost projection for continuous-minerand conventional m echanized sections

Present-day working costs, taken from mines withinthe group operating under sim ilar conditions, indicatethat, by the operation of continuous-miner sectionsinstead of conventional mechanized sections, a savingof 5,7 cents per ton in no. 4 seam and 8 cents per ton inno. 5 seam can be realized. Where the depth of no. 5seam exceeds 50 m, a combination of longwall m iningand development with continuous miners will be ex-ploited to increase the extraction rate of the mineablereserves from 57 per cent to an expected 80 per cent forthe three seam s.Until more accurate information regarding the extent

of surface damage created by longwall m ining is estab-lished, all reserves having a cover of less than 50 m willbe m ined by the bord-and-pillar m ethod with continuousminers. From the experience that will be gained fromsubsidences caused by total extraction methods at no.2 m ine, further consideration will be given to the possibleextraction of pillars. Until this is known, there is adanger of ventilation disruption and spontaneous

com bustion if severe surface subsidences are experienced.Based on the present standard safety factors of 2,2

for m ain developm ent, 1,8 for secondary developm ent,and 1,6 for panel developm ent, a satisfactory extractionrate of more than 80 per cent of the mineable reservescan be obtained from the no. 5 seam , which has anaverage cover of 40 m. The no. 4 seam , however, has anaverage cover of 75 m and an expected extraction rateof 57 per cent. An investigation into the introductionof ash-filling is being conducted at present; if the panelsafety factor were reduced to 1,0, the extraction ratecould be increased to 67 per cent at an estimated addi-tional cost of 62 cents per ton. Judging from the poorroof conditions of the no. 4 seam in the neighbouringK riel Colliery, it w as considered that blasting operationswould have to be avoided where possible.At this stage, it appears that, from both a cost and

an operational point of view, the continuous miner hasd ef in ite advan tage s.

S ele ctio n o f M achin esTenders were received from four companies and, at

the time of tender, two companies had machines operat-ing in South Africa. Since prospective customers hadthe opportunity of seeing these machines operating inlocal conditions, as well as being able to assess the per-form ances being achieved immediately, these com panieshad an advantage over the others. In the selection of thebest machines, the following factors were taken intoconsideration.

~0

Ability to Produce CoalThere seemed to be virtually no difference betweenthe machines' ability to produce coal. It was decided to

select the machine with the most powerful cutting head,since experience in this country with longwall shearershad indicated that it was wise to choose the most power-ful machine available. American experience had alsoindicated that less maintenance is required for a morepowerful machine doing the same job.Cuttin g Heig htWith the no. 4 and 2 seams varying from 4 to 6 m inheight, it was preferable to take the machine that could

extract the most coal in one pass, especially for mainand secondary development, and so avoid top coalingat a later date. The poorer-quality coal is found mainlyin the upper portions of the seam .For the no. 5 seam , a large portion of the reserves

would be mined at 1,2 m. The tramming height of eachmachine was studied, and the lowest was selected toensure adequate clearance for maintenance purposes.Because of the systematic support necessary and theprobability of having to leave a thin layer of coal toprotect the soft floor, it was essential to aim for maximumclearance.Techn ica l Cons idera ti onsSo that maintenance costs would be kept as low aspossible, it was considered that the machines should

have the follow ing:(1) no m echanical clutches where possible,JOURNA L O F TH E SOU TH A FR IC AN IN STIT UT E O F M IN ING AND META LLURGY JANUARY 1980 1 1


8/17

/'---- /' ~\ /v \ /\

(2) fixed cutting heads rather than those incorporatingchains,

(3) electric power supply for traction rather than hy-draulic,

(4) the highest sumping speed without producing slipon a soft floor,

(5) sufficient mass to avoid slip when sumping.F inancia l Con sid erati on sT he least expensive m achine m ost closely approaching

the above specifications would be chosen as long as thecost and availability of spares w ere satisfactory.Loc al Con te ntAt the time of the selection, the amount of local

content was not considered a serious problem . Sincethen, ow ing to the increasing probability of internationalem bargoes and the consequences for the m ining industry,an increased local content and m anufacture of equipm entbecome more and more important. Subsequent to thepurchase of the m achines, the suppliers w ere approachedin this regard, and assurance was given that by 1981almost all the equipment will be manufactured locally.

F in al S ele ctionThe machines required for nos. 4 and 5 seams toconform with the above specifications were considered

to be the best available at the time of selection.The average progressive machine availability and

section productivity for the continuous miners in theno. 4 seam are shown in Figs. 2 and 3. Unnecessarymechanical problems are still being encountered, but itis envisaged that these will be elim inated shortly. Sincemining commenced, extraction has been in main roadsto provide safe access over the long term .

100

=J~~

"~

80

70

~~~~~g~~~~~~~~~~~MACHINE AVAILAB ILITY

IN SH IFTP

' . : N"" { (PLA .NNED MAINTENA .NC ' ~ " N ELANNED MA INTENANC E X lOO) } .",IA"E.OO-~ , -.AVAILA8ILITY SHIFTTIMEFig. 2-Availability of continuous miners in no. 4 seam

12 JANUARY 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

l O U D

00 0

'"80 0~

-,. ...j-""~~ 70 0JJJi-1-jl~>"

~ ~ ~ ~ ~ g ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~PERIOD

F ig. 3 -A vera ge p ro gressiv e p ro du ctiv ity o f co ntin uo us-m in ersectio n in n o;).4 se am

Shortcom ings in Use of Continuous M inersBecause of the varying nature and hardness of thecoal in South Africa, recommendations from overseashave proved to be of little value. For the optimumcutting and loading rate to be achieved by a machine,the pick spacing, drum-rotation and sumping speed,pick angle, and type of picks have to be selected so thatthe machine is suitable for the coal being cut.From studies conducted at Matla in the no. 4 seam ,

it was found that the total utilization of the cutting timefor the machine is in the region of 59 per cent. To date,no problems have been encountered in cutting coal. As aresult, no experiments have been carried out to improvethe production rate of the machine, and all efforts havebeen directed towards an im provem ent in the utilizationof the available cutting time. The following areas havebeen singled out.(;y The time lost in waiting for shuttle cars to man-

oeuvre into position before loading and cutting cancommence accounts for approximately l() per centof the lost cutting time. An efficient means ofcontinuous transportation of coal away from them iner could elim inate this lost tim e.

(b) So that the maximum sumping depth can beobtained, it is essential to first sump in on the floorto make room for the loading shovel. The machineis then reversed, the drum is lifted, and sumpingat the desired height can com mence. The possibilityof shortening the shovel is being evaluated to


9/17

reduce this lost time, which accounts for approxi-mately 8 per cent.(c) The changing and checking of picks at hourlyintervals were found to be essential if the cuttingcapacity of 8 to 11 t/m in is to be maintained. Toovercome this, several experiments involving pickspacing, sum ping speeds, drum -rotation speeds, andtypes of picks are still to be carried out to obtainmaximum cutting capacity with the minimumnumber of pick changes. Approximately 4 percent of the lost time can be attributed to changingo f p ic ks .(d) Bords in the no. 4 seam are worked at 6 m widthswith a cutting drum 3,3 m wide. During the firstpass, the drum is fully utilized to a depth of ap-proximately 8 m, roof conditions permitting. Onthe second pass, only 80 per cent of the drumcapacity is used, reducing the tonnage per minuteand lengthening the time taken to fill a shuttlecar. By the introduction of ash-filling into thepanels, it is envisaged that the bords in the panelscan be widened to 6,6 m to overcome this problem .(e) Under normal conditions with a competent roofand floor, tramming accounts for approximately9 per cent of the lost cutting time. Where poor roofconditions are encountered, the depth of each passis considerably reduced, increasing the amount oftramming and thus reducing the effective cuttingtime.(f) Unplanned maintenance accounts for approxi-mately 10 per cent of the planned cutting time.By a reduction in the unproductive times, it isenvisaged that the present average of 1084 t pershift can be increased to 1375 t per shift at a targetof 75 per cent effective cutting time.

In the no. 5 seam , a specific problem of a soft shalefloor varying in thickness from 0,2 to 0,5 m was en-countered. To maintain the floor while cutting was not aproblem , but the traction exerted by the cat-tracks ofthe machine while sumping resulted in the breaking upof the floor. The slip on the cat-tracks was 72 per cent,and it was decided that experim ents should be conductedon ways of elim inating this.Planned Trial w ith Heavy-duty Continuous MinerThe nos. 4 and 2 seams at Matla Coal are up to 6 mthick in som e areas. The present continuous m iners are

capable of cutting up to a height of 3,6 m , which necessi-tates top coaling to improve the extraction rate, with adetrimental effect on cost. For this reason, it waSdecided to obtain a Dosco TB.600 twin-boom miner on atrial basis so that cutting to a height of 4,5 m in onepass could be evaluated. Should the trials prove success-ful, the company has the option to purchase the machine.

The Dosco is designed to excavate a tunnel 6 m wideby 4,5 m high without repositioning, which will decreasethe manoeuvring time experienced with the Marietta.The production rate of the machine is to be modifiedto achieve an average of 1400 tons per shift with acutting capacity sim ilar to that of the Marietta 5012(8 to 11 tons per minute).

The dim ensions and specifications of the heavy-dutym iner are as follow s:Dimen8ions

M achine length w ithout conveyorM achine length w ith conveyorW idth over apronWidth over main frameD ischarge height of conveyorCutting w idth (from one position)C uttin g h eig htSum ping depthM ass of m achine

11,95m13,69m4,Om3,2m1,5m6,Om4,5m0,74m

81,3tSpecifications

Voltage 1000, 3-phase 50 cyclesCutting motors 2 X 187 kWPower pack motors 1 X 187 kWTracks Caterpillar 235Power take off for two hydraulic drills and boomsE le ctric a ll flamep ro ofS te ko h ose fittin gsThe machine was due to arrive at the mine towards

the end of 1979, when tests were to be carried out.Acknowledgements

The author thanks the management of the Coal Divi-sion of General M ining and Finance Corporation forpermission to publish this information, and the staffof M atla Coal Lim ited, for their assistance in the prepara-tion of this paper.

JO UR NAL O F THE SO UTH AFR ICA N INSTITU TE O F M IN ING AND M ETA LLUR GY JANUARY 1980 13


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C. Continuous m ining w ithin the Tavistock G roupof collierie s

I. D . BRUMBY*.D ip . M in . (V isito r)SYNPOSISThe Tavistock G roup of collieries is situated som e 25 km south of W itbank. The three collieries com prising theG roup are Tavistock C olliery, P hoenix C olliery, and South W itbank C oal M ines.C ontinuous m ining is practised to som e degree at all three collieries. This system of m ining w as introduced atPhoenix during 1976, South W itbank during 1978, and Tavistock during 1979. For the tw elve m onths ended June1979, continuous m ining accounted for 1,7 M t of the 4,5 M t hauled from underground. A ll the continuous m inersintroduced to date have been of the rotary-drum type.

SAMEVATTINGD ie Tavistock G roep steenkoolm yne is 25 km suid van W itbank gelee. D ie drie steenkoolm yne w aaruit die G roepb estaa n is d ie T av istoc k stee nk oolm yn , die P ho en ix steen koo lm yn en d ie S uid-W itb an k k oo lm yne .O ntginning m et aaneendelw ers w ord in 'n sekere m ate deur al drie steenkoolm yne beoefen. H ierdie sisteem vanontginning w as by P hoenix gedurende 1976, Suid-W itbank gedurende 1978 en T avistock gedurende 1979 ingestel.V ir die tw aalf m aande geeindig Junie 1979 is ontginning deur aaneendelw ers verantw oordelik vir 1,70 m iljoen tonvan die 4,5 miljoen ton wat ondergronds ontgin is. AI die aaneendelwers wat tot op datum verkry is, is van dieroterende trom tipe.

IntroductionThe Tavistock Group of Collieries is situated some

25 km south of W itbank, the three collieries comprisingthe Group being Tavistock Colliery, Phoenix Colliery,and South W itbank Coal Mines.Trials with a rotary-drum continuous miner were

held in the no. 4 seam at South W itbank Coal Minesduring 1975. These trials, conducted by Anglo AmericanCorporation and Dowson and Dobson Ltd, proved thatthis type of equipment could cut the no. 4 seam, thecutting rates being high enough to warrant considera-tion of the equipment for production purposes pro-vided that fairly continuous operation could be achieved.These trials had a significant influence on Tavistock'sfuture plans regarding mechanization programmes andthe replacem ent of existing m echanized equipment.

Mechanization in the Tavistock Groupo f Co lli eri esPho en ix Collie ryDuring 1975 a programme of mechanization was com-menced at Phoenix. Initially, no. 4 seam was developedon a conventional basis, but the weak roof conditionsthat were encountered were a contributing factor whena decision was taken to introduce a continuous minerduring 1976. A Marietta 3080 model was installed inthe no. 4 seam , and a second such machine was broughtinto use during 1977. To date, these machines haveextracted some 2 M t of run-of-m ine coal.S ou th W itb an k

South W it bank commenced production during 1946.The mine was mechanized from the beginning and wasone of the first collieries in South Africa to be mechan-ized. The mining conditions, generally speaking, arevery favourable for mechanization. During the mid-seventies, when it had become necessary to considerreplacing outdated equipment, the management de-cided to introduce continuous mining. At the beginningof 1978, a Joy 12 CM 6 was brought into use in the no. 4* Tav isto ck C ollieries L td , O gies, T ra nsv aal.14 JANUARY 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

seam , and a second such machine was installed during1979. To date, these machines have extracted some1,25 M t of coal.T avistock C olliery N o. 2 SectionUntil the beginning of 1979, continuous m ining within

the Tavistock Group had been practised only in theno. 4 seam . The no. 2 seam was considered to be a moredifficult proposition for continuous m ining owing to thephysical characteristics of the seam and a much moreun du la tin g flo or asso cia te d w ith sto ne ro lls. N ev erth ele ss,a decision was taken to do the primary development inthe no. 2 seam at Tavistock no. 2 section using a narrow-head M arietta 5012 continuous m iner. There were indica-tions that weak roof conditions would be encounteredinitially at this new section, and it was considered thatthe drivages formed would be much more stable if theywere machine-cut and the use of explosives elim inated.Furthermore, valuable experience would be gained. Theresults to date indicate, at this somewhat early stage,that an output of around 60 000 t per month can beobtained from a continuous miner operating on a treble-shift basis. This compares favourably with the 65000 tper month currently obtained from a conventionalmechanized section in the no. 2 seam at Tavistock no. 1section.

Size Distribution of the ProductContinuous miners produce finer coal than does the

conventional mechanized system that utilizes coal-cutters, drills, and explosives. The output of the Tavi-stock Group is marketed by the Transvaal Coal Owners'Association, all of the coal being for the inland market.The m ajor custom ers are thermal power stations situatedthroughout the Republic and Mocambique, and thechemical and cement industries. The coal is marketed onthe basis of calorific value and size. It is therefore veryimportant that coal is produced that satisfies thecustomer not only in calorific value but also in percen-t ag e s iz e d is tr ib utio n.Table I compares the size distribution of coal pro-

duced from a continuous-miner section and that pro-


11/17

Cumulative CumulativeSize, mm % Size, mm %+76,2 30,8 +76,0 5,0-76,2 +38,1 47,1 -76,0 +31,5 22,4-38,1 +25,4 54,8 -31,5 +25,0 28,1-25,4 +6,4 79,1 -25,0 +6,3 67,2-6,4 100,0 -6,3 100,0

26 Ma25 Ap21hODescription Hours

Available 59 4Stoppages 203,8Production 390,2Stoppages 203,8D ow ntim e continuousminer 75,3Picks 54,8Maintenance 42,3Shuttiecar 13Stockpile or plant 4, 8B elt e xte ns io n 4, 0V isito rs film ing 3Water 2, 5Belt 1,5M iner absent 1M ine power 0, 9Feeder 0, 5Reporte81 2Measure79 4

Hours %562,8 10 0162,.5 29400,3 71162,5 10 0.56,4 3531,8 2038,0 231l,1 62,7 2,04,0 2, 05,4 3, 09,0 6,03,2 2,00,9 1,0

Reported tons81 767M easured tons85306

duced from a conventional mechanized section in thesame seam (no. 4) at the same colliery (South W itbank).At this particular colliery, the demand for coal above75 mm in size is negligible, and the plus 75 mm materialhas to be crushed. In recent years, there has been anincreased demand for smaller sizes of coal and lessdemand for the larger sizes. It is interesting to note thatcoal produced by the continuous miner at Tavistock inthe no. 2 seam contains no more than 20 per cent minus6,4 mm material after the plus 100 mm material hasbeen crushed to 100 mm.

TABLE ISIZE DISTRIBUTION OF COAL PRODUCED FROM A CONVENTIONALMECHANIZED SECTION AND A CONTINUOUS-MINER SECTION OF

SEAM NO. 4, TAVISTOCKConventional mechanized Continuous m ining

Although continuous miners produce smaller coalthan the conventional mechanized system , there areindications that modifications can be made to thecutting parameters so that less fine coal is produced bythis type of equipment. W ithin the Tavistock Group,the overall effect of continuous mining on coal sizinghas not proved to be a problem .Production Potential of the Continuous MinerExperience in the Tavistock Group indicates thatcutting rates of up to 300 tjh can be achieved. In an

attempt to maximize on this potential, efforts have beendirected at the operation of the machines with theminimum of interruptions. Treble-shift operation ispractised, with the first production shift of the week

TABLE IITIME DISTRIBUTION IN A CONTINUOUS-MINER SECTION26 Apr. to26 Mayat 05hOO

r. tor. at0

d tons28d tons13

commencing at 21hOO on Sunday evening and the lastshift of the week finishing at 17hOO on Saturday. Themain breaks in production are caused by the followingfactors:(I) scheduled daily maintenance for a period of ap-

proxim ately 3 hours,(2) unscheduled downtime on the continuous miner dueto b reakdown,(3) extension of the belt conveyor when necessary,

(4) replacement of the cutting tools,(5) delays when the continuous miner is being moved

from place to place, shuttle-car change-out time,and interruption of water or power supply,(6) stoppages outbye of the section affecting thes ec tio n conveyo r.

Table II gives an example of the time distribution in ac on tin uo us -m in er s ec tio n.

M ode of OperationFig. I shows a typical geological section through

South W itbank, and ]'ig. 2 shows the face-cuttings eq uenc e th ere .Fig. 3 indicates the mining sequence in the headingsand splits or cross-cuts. Headings are mined in rotationfrom left to right. The 60 degree split from the rightbarrier heading is preferred to a 90 degree split becausethe cutting time for this particular split is reduced.One example of a continuous-miner panel layout is

shown in Fig. 4. It is not suggested that this is theoptimum layout for this particular m ine, but merelythat, at this stage, it has proved to be convenient.

%

Advantages and Disadvantages of ContinuousMiningThe following are some of the more apparent advan-

tages of this system of mining.(a) More stable pillars and drivages are formed thanwhen explosives are used. This could possibly leadto an im provem ent in percentage extraction.(b) Reserves that w ould have been difficult or im possibleto extract by conventional mechanized methodscan possibly be exploited.(c) Fewer people are exposed to face operations.

(d) Under normal circumstances, blasting and itsassociated dangers are removed from the worksituation. In addition, the legal responsibility ofall the persons normally involved in the storage,handling, and use of explosives is considerablyreduced.(e) Greater flexibility exists where restricted m ining hasto b e pra ctise d.(f) A considerable improvement in the productivity ofthe unskilled labour force is possible. Im provem entsof up to 225 per cent have been recorded in theT avistoc k G roup .(g) The mined product is in a much more convenients iz e fo r h an dlin g.The more apparent disadvantages are as follows.

(I) In the event of a breakdown on the continuousminer, production from the section ceases com-pletely.(2) To date these machines and a high proportion of the

10 0346610 0372721622212

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY 15ANUARY 1980


12/17

Nt . Seam3 .4 to 4 ,2mthick

16 JANUARY 1980

1"' ;>X~ v,>


13/17

. . ~in. . .ewerB . . . . .RRI . . . . .R . . . . .. . . . .ig. 3-Panel-mining sequence at South W itbank

II I~. . .ENTILATION f.o-Qg~ feeder TEMPORARY

B II' .~ . . ileA""" BA AR 1 RR RII ~EE . . . RR . 13,5. J1~Sm---I. . . .,6.. . . . . tR E T U R N AI R slight leakage of fresh aj r INTAKE AIR 30rrysF ig. 4-L ayout of a continuous-m iner panel at South W itbankJOURNAL DF T HE S OU TH A FR IC AN IN ST ITU TE O F M IN IN G AND METALLURGY JANUARY 1980 17


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spares have been imported. Local manufacture doesappear to have started, but the extent is uncertainat this stage.(3) Stone intrusions within the seam can have anadverse affect on production rates and on themach in e its elf.(4) Frequent moves of the continuous miner may benecessary where systematic support is required.This can considerably reduce the time availablefor p roducti on .(5) Spillage left by the continuous miner necessitatesthe use of hand labour or additional mechanizedequipm ent for clean-up operations.

ConclusionsDuring the past three years, all the continuous

miners introduced in the Tavistock Group have pri-

m arily been used for production purposes. W here specialdevelopm ent or underm ining work arose, these m achinesproved to be a valuable asset. The production ratesachieved to date have exceeded expectations, and forthe twelve months ended June 1979, continuous miningaccounted for 1,7 Mt of the 4,5 Mt hauled from under-ground. During 1977 at Phoenix Colliery a worldrecord was broken when one continuous miner producedmore than 70 000 t in one month. Recently this recordwas broken when 85 000 t was produced by one machinein a single month at South W itbank.This system of mining has proved to be very efficient

and safe. It is very likely that the percentage of coalm ined by continuous miners within the Group willincrease at the expense of the conventional mechanizedsections. This will apply in both the no. 2 and no. 4seams.

D . Pillar extraction at U sutu Collieries usingcontinuous minersby c. J. B EUKES* (V isitor)

SYNOPSISIn 1969 Usutu Col li er ies began p illar e xtractio n w ith co nv en tion al m echa nize d eq uip ment. T his w as v ery su cce ss-f ul, b ut c er ta in d is ad va nta ge s c ou ld be e lim in ate d if c o ntin uo us m in ers w ere u se d in ste ad . In 1 97 6 t he f ir st cont inuou sm iner was introduced for this purpose; this proved so successful that a second unit was introduced in 1977, an dpi ll ar ex tract ion by conventional equipm ent w as discontinued. B y the end of 1978, 1,3 Mt had been mined fromp il la r ex tr act ion by c on tin uo us m in ers.T his paper describes the m ethod in som e detail, an d indicates that the operational costs are low er than for con-ventional bord-and-pillar m ining.

SAMEVATTINGDie U sutu Steenkoolm yne het in 1969 m et pilaarafbouing deur m iddel van konvensionele gem eganiseerde toe-rusting begin. Alhoewel baie suksesvol kon sekere nadele egter deur m iddel van die gebruik van aaneendelwersu it d ie w eg g eru im w ord. Die ee rste aan een delw er is in 1 97 6 v ir h ierd ie do el in ge bruik ge nee m; d it w as so suk sesvo lda t 'n tweede eenheid in 19 77 verkry is en pila arafbo uing m et ko nven sion ele to erustin g g estaak is . T een d ie e in deva n 1978 w as 1 ,3 m iljoe n to n v an p ilaarafb ou in g d eur m id de l v an a anee nd elw ers reed s o ntgin .H ie rd ie re fe ra at b es kr yf hierdie metode en dui aan dat die bedryfskoste laer is as vir konvensionele kam er enpilaar afbou.

IntroductionUsutu Collieries is situated 20 km south of Ermelo,

the mine being contracted to supply approximately5,0 M t per annum to Escom 's Camden Power Station.In order to increase the percentage extraction of coal

reserves at Usutu, pillar extraction was introduced in1969. This was done using available conventionalmechanized equipment, i.e. coal cutter, loader, shuttlecars, and face drills. The method of extraction consistedof splitting the pillar and extracting the two fenders.This method proved very successful but had the follow-ing disadvantages.(a) Blasting caused the immediate shale roof to deterio-

rate.(b) A minimum of 5 pillars had to be mined in variousstages of extraction at the same time. This causedthe span of supervision to be excessive.(c) The extraction was too slow, and resulted in a lossof pillars and created a safety hazard.During 1976 it was decided to use a Marietta 3080

* Usu tu C ollieries L td, E rm elo, T ran svaa l.18

continuous m iner instead of the conventional equipm entand technique. This proved to be so successful that asecond unit was put into operation during 1977.Table I shows the production statistics for the years

1969 to 1978.GeologyFig. 1 shows a typical section through a borehole.

The thickness of the overlying dolerite varies consider-ably (i.e. from 1 to 90 m), depending on the degree ofsurface erosion. In general, the overlying stratum isa very competent formation. The immediate roof abovethe B seam, which is being extracted at. present, ispredominantly shale with sandstone. This has to besupported during primary mining with roofbolts an-chored in the sandstone above the B-upper seam .The surface is fairly flat with no significant water-

courses. The overlying farm land is used for grazing andma ize p roductio n.

Panel D esignA minimum safety factor of 1,8 is adhered to. Thismay be exceeded when the pillar centres are being kept

JANUARY 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY


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TABLE IPRODUCTION A T U SU TU COLLIERIES

Average number Average number Total tonnage Total tonnageo f convent ional o f cont inuou s from pillar from pillarsections on m iner sections extraction extractionpillar extraction on pillar expre3sed as aextractions percentage oftotal m inedYear Tonnage Tonnage tonnage1969 1,0 1 01 4 68 1 01 4 68 5,821970 1,8 557 081 557081 13,361971 2,0 629 092 629 092 13,171972 1,4 441 556 441 556 8,991973 1,0 410331 410331 8,791974 0,4 201 072 201 072 3,891975 0,7 177 563 177 563 3,551976 0,8 154 939 0,8 241 960 396899 8,041977 0,4 39 321 1,5 443 773 483 094 10,201978 1,8 601 042 601 042 11,71

--Total 2712423 1 286775 3999 198 8,84

',72 SOil

+ +-+ ++ +

+- ++- +

+ ++ ++ +

t- t- 57 ,51 OO lER ITE+ +

+- ++- +

+ +-+ ,+- ,+ +

+ ++ +

+- +

'" SANDS TONE

COClAP ElEV "",35

~~//NU:;~EJ B/M P )j88 G

~rB 0 0~~f B BOOfa': EJ 0 0 0 0////"fH~JBEJDDDD/UN,:~EJ0 EJ ODD 0 0~Ef'EJ EJ 0 0 0 0 0 0nO'",,!BEJBDDDDDDDEJEJDDDDDDDDEJDDDDDDDDD

ST OO PIN G LIN E '5LEFT TO R IG H T

ROOF

\giEJ~8~'EH ~GSl 8 "J'~L.:::J ~0 G 8 "0'~0 0 ~ B "EJ'~~0 DOG E]"EH0 0 0 0 EJ B ~'B~ODD DOG ~ ~'B~0 0 0 0 0 0 EJ G ~'EH0 0 0 0 0 0 0 EJ EJ~'[~j"DDDDDDDDEJEJ

ST OO PIN G L IN E '5RIG H T TO LEF T

10,80 S H A L E

17,32 S A N D S T O N E

15 " ,880 ,3 1 8 -U PP ER S E A M1 , 7 0 S A N ~ S T O N E W IT H S H A L E B A N D S2 ,1 1 B - S E A M

S A N D S T O N EN OT TO SCA LE

Fig. I-Section through borehole W.IO/69

to a specific pattern. The pillar centres and road widthsare as follow s:Pillar centres 17 mRoad width 6 mPillar size II m .All the panels have been developed at a width of

180 m regardless of pillar centres, no exception beingmade for panels that are to be stooped. However,investigations are being conducted to attempt to relate

Fig. 2-Sequence o f p illa r e xtr ac ti onthe design width of the panels to the thickness of theoverlying dolerite, the depth below surface, etc.

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1980 19


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A panel normally consists of 10 rows of pillars at 17 mcentres.Roofbolting is done in all the roadways on a standard

pattern of 3 rows of roofbolts 1,5 m apart and 2 mspacing between rows. No roofbolts are used duringpillar-extraction operations. Only timber supports ofminimum 100 mm diameter are used for roof support.

Pillar E xtractionThe stooping line is kept at 45 degrees to the panel

direction. Various stooping lines were tried, but the 45degree line remains the most suitable approach forUsutu's conditions. The sequence of pillar extractionis shown in Fig. 2. Pillar-extraction operations areconfined to one pillar at a time.The sequence of cutting and support installation is

indicated in Fig. 3. The sequence of activities is asfollows:Stage 1Breakerlines BL 1 and BL 2 as well as fingerlinesFL 1 and FL 2 are installed.

Stage 2Cut C 1 is completed up to the goaf (i.e., a singlepass). The machine is moved back, and cut C 2 is taken.

The machine is now moved back to position BL 4 forpick inspection. Fingerline FL 3 is installed during thisperiod. In the event of poor roof conditions, fingerlineFL 3 is installed between the split and goaf before cutC 2 commenc es.Stage 3Cut C 3 is taken, followed by cut C 4. The machine

///////~N OT T O S CA LE : ~D gogogogog lO A F ~L = B R E:A K E:R L IN E : ( TIM B E:R ) / /F L -F IN GE :R LIN E: (T IM BE :R ) / //00/: ( / / / / {{JfO 000~ 000 ~~~ o ~ : ' : :D l/ ~ 0// @ 00 ::0GOAF :/;00 00 '< 0 000000 . d",/ 00 0:%~~~o~' 00 gg/~jD:;?~~OLl~b D l,0,0, 0 Fig. 3-Pillar extraction by continuous m iner

20 JANUARY 1980

is now moved back to BL 4 position for pick inspection.Fingerline FL 4 is installed during this period.Stage 4Finally, cut C 5 is taken, after which the machine is

moved to the next pillar on the 45 degree line, (i.e.,Pillar B). Breakerlines BL 3 and BL 4 are installed.Stage 5A long sling is put around BL 1, BL 2, FL 1, FL 2,FL 3, and FL 4 and hooked on to a shuttle car in oneop eration. A bou t 50 p er cent o f the tim bers are reclaim edfor r e-use.

Problems Encountered in Pillar Ex1!tactionThe machine must move in a straight line. Serious

cat-track problem s were encountered in the experim entalstages when direction changes were attempted withinthe pillar while cutting. A cat-track break during thefinal stages of a pillar-extraction operation can causes er io us p roblems.The first stage of goafing normally occurs within 30

minutes after the timbers have been drawn. On twooccasions, the continuous miner was totally coveredw hen unscheduled goafing occurred, fortunately w ithoutinjury or loss of life. In both instances the machine wasrecovered.A breakdown, especially in the tram ming m echanism ,

can be hazardous in pillar extraction. This is overcomeby a proper maintenance programm e.At the start of every shift the machine is checked and

lubricated. A fitter and electrician are on shift in thesection as part of the production team .Monthly maintenance is done over a full shift. Thisnormally includes welding, for which permission mustbe obtained from the Inspector of M ines.M inor sub-assemblies are kept underground. Major

sub-assemblies, Le. traction motor, cutting motor, etc.,are kept in a sub-assembly store on surface. Whenrequired, any sub-assembly unit is normally deliveredto the section at short notice.

Equipment and PersonnelThe equipm ent consists of 1 M arietta 3080 continuous

miner and 3 Torkar Model 48 AC transshuttle cars.The section personnel employed are as follows (per

shift) : ShiftbossMinerFitterElectrician

1.21113t

Team LeaderC ontinuous M iner O peratorC ab le Han dlin gShuttle C ar D riversD isch arg e P oin tSupport

12231615

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY


17/17

Electr ic ian A id esF itte r A id esLubrication2226

Total 24 t

Costs and EfficienciesThe average stores costs are as follows (section only):

Picks 8,7 cftSupport 9,5 cftM iscellaneous (Mining) 1,9 cftC ontinuous M inerMaintenance

LubricationS hu ttle C ar M ainte na nc eT ota l S tore s

48,3 cft1,7cft2 4,2 c ft

94,3 cft

Ignoring the added cost advantage of reduced geo-graphic expansion and excluding capital charges, theoperating cost compared with that of conventionalbord-and-pillar m ining is approximately 10 per centlower. .

T he efticiencies are as follow s:Tons per pick 33,3Tons per shift 730Tons per hour 105Tons per month ~ 30 0 00 average

Average cutting time per shift (drum-turning time)=4 hours.A ve ra ge co ntin uo us m in er av aila blity =8 2% .

Aver ag e s ec tio n ava ilabi lity=65% .N.B . Continuous miner availability is calculated on

the machine's downtime only. Section availability iscalculated on the total time available and the totaldow ntim e, w hich includes external dow ntim e.The section extraction rate is 95 per cent.

ConclusionThe continuous miner as a pillar-extraction unit hascome to stay at Usutu. The advantages of the machine

in terms of greater recovery of coal reserves, costs,productivity, and safety have assured its continuedapplication for this purpose.

AcknowledgementsThe author thanks the management of the Coal

Division of General Mining and Finance Corporationfor permission to publish this information, and the staffofU sutu Collieries for their assistance in the preparationof this paper.

Defect solid stateA one-day multidisciplinary symposium entitled 'The

Defect Solid State '80' will be held under the auspicesof the Solid State Physics and Materials Science Sub-committee of the South African Institute of Physics atthe University of the W itwatersrand on Monday, 14thJuly, 1980, following the Twenty-fifth Annual Con-ference of the South African Institute of Physics. TheSymposium is the second in a series (the first having beenheld under the auspices of the NCRLfCSIR and the SouthAfrican Chemical Institute in 1978) and is intended toprovide a forum for scientists and engineers of variousdisciplines and backgrounds interested in the field ofmicroscopic defects and defect-controlled processes inm aterials. Em inent overseas speakers will be taking part.There will be both invited and contributed papers.Those providing a broad perspective of recent develop-

ments and techniques with a description of their futurescientific and industrial potential will be particularlywelcome. In order to provide a suitably balanced pro-

gramme within the lim ited period of the symposium ,papers will be selected both on scientific merit and fortheir general appeal to a m ultidisciplinary audience.A reas of interest w ould include

- Point and extended defects, their production, pro-p ertie s, a nd in te ra ctio ns .- M aterials m odification and developm ent - ion im-plantation, chem ical defects, and doping, etc.- Surface defects, catalysis, etc.- Materials with device potential that use defectproperties - sem iconductors, solid electrolytes, etc.- Instrum ental m ethods and techniques of productionand observation of defects.For further information contact Dr J. D . Comins,

Department of Physics, University of the W itwaters-rand, Johannesburg. Tel. 39-4011 Ext. 8195.ORDr R. G. Copperthwaite, NCRL, CSIR , Pretoria.

Tel. 86-9211 Ext. 2645.JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1980 21

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