1 Introduction to Mine Backfill

8/13/2019 1 Introduction to Mine Backfill

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Introduction to Mine Backfill

Mine backfill is defined as the material or materials used to fill the cavities created by mining soas to establish and retain safe working conditions, economically. Recently, mine backfill isemployed in strategies to reduce surface environmental impacts.

Backfill Costs and Benefits

Benefits Costs

ECONOMC! "ackfill allows higher e#traction ratio

"ackfill reduces dilution

$ids in pillar recovery

ECONOMC! "ackfill costs money particularly if

binders are used "ackfill introduces delays in the

mining cycle $dditional manpower and

infrastructure $dditional dewatering costs

%ilution due to backfill

&$'E()! "ackfill improves regional stability in

the mine "ackfill can reduce risk of

rockbursting

&$'E()! Risk due to bulkhead failure and

li*uefaction of tailings Risk due to collapse of consolidated

backfill walls

EN+RONMEN(! "ackfill provides a means of

disposing of waste rock and milltailings, thereby minimiing surfacedisturbance

EN+RONMEN(! -roundwater contamination

Mining Methods Employing Mine Backfill

Mining Methods RequiringBackfill

Mining Methods Employedwith or without Backfill

Mining Methods Employedwithout Backfill

Conventional Cut and 'illMechanied Cut and 'illCaptive Cut and 'ill%rift and 'illndercut and 'ill$voca

/onghole/ongwall MiningRoom and 0illarOpen 0it

&hrinkage"lock Caving&ub1/evel Caving&*uare &etResueing

Conventional Cut and Fill (verhand cut and fill!Cut and fill stoping is performed in steeply dipping orebodies where the hangingwall or footwallis not sufficiently competent to maintain the stable spans re*uired for open stoping methods.(he method involves the incremental development of a horiontal slot shaped e#cavation. 'ill isplaced in the stope after each horiontal slice is taken. Overhand cut and fill can be eithermechanied or captive.

Mechani"ed cut and fillmining refers to cut and fill stopes that are accessible by rubber tirede*uipment from the regular mine access development. Mechanied cut and fill stopes can bevery productive with stope widths in e#cess of 23 m in good ground conditions and stope liftheights of 2 to 4 meters. %rilling is typically carried out using 5 or 6 boom drill 7umbos using abreasting pattern. "ackfill is normally a *uick draining hydraulic sandfill with no binder added.

&ometimes binder is added at the end of a backfill pour so the surface of the fill has the bearingcapacity to support the rubber1tired e*uipment.


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(ypical e*uipment used in mechanied cut and fill stopes includes ! %rill 8umbos, /9%:s, (rucks,&caling Machines, &cissor /ifts, "olting 8umbos

Captive Cut and Fill refers to mining of stopes where e*uipment remains in the stope duringthe life of the stope. Captive cut and fill is generally used in narrow vein type stopes. (he cost ofcaptive cut and fill is very high relative to other methods. %rilling is typically carried out using

7acklegs ;breasting< or with stopers ;upholes


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mineralogyspecific gravitymoisture contentpercent solidsvoid ratioporosity

rheologygrain sie distributionunconfined compressive strengthshear strength

Mineralogy

t is important to understand mineralogy because some minerals ;particularly *uart< are very abrasive and will cause a lot of wear in the

backfill lines some minerals cause break down of the cement over time

flat minerals settle slower than rounded particles

Specific Gravity&pecific -ravity ;&.-.< is the ratio of mass of fill particles to mass of an e*ual volume of water.$n engineer at a mine site can measure the &.-. of tailings in the mine@s lab using the followingtechni*ue.

$pparatus re*uired!5?3 ml volumetric flask, digital scale accurate to 3.3= grams

&tep =! Measure weight of flask A 5?3 ml water ;B= at the -reens Creek Mine. (he flow characteristics of paste fill are not greatly influenced by thematerial greater than 53 mm in sie. 'or e#ample, gravel sie material from a heavy media separationcircuit has been used successfully in a paste fill plant at the -rund Mine in Best -ermany ;"roicher


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Binder Content"inders are used in paste backfill where structural strength is re*uired of the backfill and whereresistance to li*uefaction is necessary. 0ortland ;(ype =3< cement is the most common binder used forthese purposes. Cement addition rates of 5 to J> are often used to achieve typical strengthre*uirements for mine backfill. n addition to cement, blast furnace slag, fly ash, and natural poolanscan be added to the paste fill to partially replace cement and thereby reduce costs.

(he final strength of the cured backfill is influenced by the curing period temperature. 'igure I showsthe typical increase in strength that is developed over a period of 3 days. $ mining operation thate#poses a backfilled stope wall shortly after it is poured must use more binder in the fill than if ite#poses the wall several months after pouring. (his is a mine planning issue which can have a seriousimpact on backfilling costs since the cost of cement represents the single largest cost for most backfillplants.

Bhen carrying out laboratory scale testing of paste backfill samples, the curing temperature andhumidity should simulate conditions e#pected in the mining operation. 'igure 4 illustrates the significantinfluence curing temperature will have on the 541day strength of backfill samples.)pecific *ravity of )olids

(he specific gravity of the material used to make the paste will influence the pulp density that can beachieved. 'or a given slump, a higher specific gravity will yield a higher pulp density.&orosity

(he porosity of the cured backfill product is important in determining the *uantity of backfill that can beplaced in a given volume. 0orosity is defined as the volume of voids in a material to the total volume ofthe material. 0orosity is generally lowest in well1graded mi#tures. (he porosity measured inuncompacted laboratory scale samples will be higher than what is achieved in a stope, since somedegree of self1consolidation of the fill occurs as the stope is filled.

0aste "ackfill 0reparation and 0lacement

&aste Backfill &reparation and &lacement

ntroduction to Mine "ackfill0aste "ackfill

#e-watering%e1watering is normally achieved in three ways! hydrocyclones, thickening, or filtering. 9ydrocyclonesare an ine#pensive means of de1watering however they will cause some loss of fines which arenecessary to pump the paste. 9ydrocyclones can only be used when there are e#cess fines in thetailings to remove a portion of the fines and thereby increase the filtration rate of the tailings. (his isknown as partial classification. t is also possible to use a thickener to de1water slimes from a cycloneoverflow and then combine the thickener underflow with the cyclone underflow.

"oth conventional and high density thickeners have been used in paste backfill plants to thicken backfillto a pulp density of appro#imately J?> solids prior to filtration. sing high capacity thickeners toproduce a paste directly from a dilute tailings slurry has been investigated and determined to betechnically feasible. (o date, however, no plants have been constructed that use this techni*ue.

(he Canadian Centre for Mining and Energy (echnology ;C$NME(< is researching a process toproduce paste in settling silos that maintain the suspension by in7ecting compressed air into the pastewith special noles. No operating plants use this techni*ue, however a pilot plant is being constructed.

'iltering is carried out using disc filters, drum filters or belt filters. (he selection of a filter is usuallybased on a consideration of capital and operating costs.

Mi1ingMi#ing of the components of a paste backfill mi#ture can be either a batch or continuous process. (he
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filter cake, cement, water, and aggregate must be accurately weighed and rapidly supplied to the mi#er.t is generally recognied that a batch process is easier to control than a continuous system. &crew typemi#ers are used for continuous systems while high intensity mi#ers of the type used in the concreteindustry are used in batch plants.

&umping0 +ransport0 and &lacement

$fter mi#ing, the paste can be either discharged into a concrete pump hopper or through a borehole tothe lower mine workings. (ypically, paste will flow under gravity at an angle greater than 63P , makingselection of the backfill plant location very important. n designing a backfill system, the plant should belocated where a series boreholes and short lateral runs could carry the paste to all stopes under gravity.'or such plants, a positive displacement pump would normally be installed for clearing line blockagesand for cleaning out the pipeline. n most installations, however, this is not possible and a positivedisplacement pump operates continuously to transport the paste.

&ufficient space should be provided around surface and underground borehole collars to allow a drill tobe moved over it to ream the hole should a blockage occur. One mine has reported using liners in the

boreholes, however this is not common practice. "orehole and pipeline diameter is a function of thee#pected pressure losses in the system. 0ractical pumping distance ranges up to = km laterally andunlimited distance vertically depending on the pipe pressure rating. 'low velocity in the pipeline isusually less than = meter per second. 'igure shows typical pressure losses for a medium slump pastein a =?3mm diameter pipeline.

(he placement of the pipeline outlet in the stope is important because the paste will not beach out.Rather it will form a moderate angle of repose ;63P


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Case ,istories(hree case histories that are well documented in the literature are the -rund Mine, the /ouvicourt Mine,and the /ucky 'riday Mine.

*rund MineOne of the first reported applications of paste technology in the mining industry was at the -rund Minein -ermany. $ simplified flowsheet of the -rund Mine backfill plant is shown in 'igure =3. 'lotationtailings were first de1watered using hydrocyclones followed by thickening. (he thickener underflow waspumped to a vacuum belt filter. (he filter cake and aggregate from a heavy media separation circuitwere mi#ed continuously to produce a paste with an 44> pulp density. (he paste was pumped 43 meter

to a shaft and allowed to flow by gravity down ?33m. &topes were located 233 to 5633 m away from theshaft and so an intermediate pumping station was re*uired for more distant stopes. 0ortland cement


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(he total cost of the plant including building, boreholes, e*uipment, and installation was C%N QJ.5million. (able 5 summaries the key operating parameters for the plant and characteristics of the pastebackfill.

Aucky Friday Mine(he /ucky 'riday Mine in daho was the first mine in North $merica to develop a paste backfill system.

$ stiff backfill was necessary for the mechanied underhand cut and fill stoping techni*ue employed bythe mine. (he backfill plant has an average capacity of =62 tonnes per hour. (he paste backfillproduced by the plant has a pulp density of 4?> ;wt< and a slump of .? to =3.? inches. (he cementaddition rate is J14>.

0aste backfill is prepared by de1watering flotation tailings from a lead1inc concentrator. (he tailings arepartially classified using a hydrocyclone to increase thickening and filtration rates. (he classified tailingsare thickened in a .=m diameter conventional thickener. (he thickener underflow is pumped at J?>solids to a drum filter that produces a filter cake with =6> moisture. (he filter cake is stacked in acovered bunker and reclaimed with a bucket chain e#cavator when the backfill plant is operating. $conveyor belt transports the filter cake to a weigh hopper that loads the cake into a mi#er. $ 0/C1controlled batch process is used to mi# the cement, filter cake and water components. (he paste is

pumped to the shaft with a 0utmeister positive displacement pump. (he paste travels by gravity downthe shaft more than =?33m and laterally to the stopes as far as J33m. (he pipeline diameter varies from=33 to =?3 mm. n the =5? mm pipeline, the average velocity is 5.2 mDs and the friction loss is 4 to =3k0a per meter.

(able 6 summaries some of the key operating parameters for the plant and characteristics of the pastebackfill.

+a.le 8' )ummary of Aucky Friday Mine &aste Backfill &lant

"ackfill &ource 0artially Classified Mill (ailings

"inder J14> Cement

0ulp %ensity 4?>

Operating &lump 52 to 5J.? cm

0lant Capacity =62 tonnesDhour

&ie %istribution 6J> minus 65? mesh, 2> minus 533 mesh

Documents

1 Introduction to Mine Backfill