Paste-like self-flowing transportation backfilling technology based on coal gangue

签MININGS Science DirectSCIENCE ANDTECHNOLOGYELSEVIERMining Science and Technology 19(2009)0137-0143www.elsevier.com/locate/jcumtPaste-like self-flowing transportation backfillingtechnology based on coal gangueWANG Xin-min, ZHAO Bin, ZHANG Chuan-shu, ZHANG Qin-Ii'School of Resources and Safety Engineering Central South University, Changsha,Hunan 410083, chinaSuncun Coal Mine of Xinwen Coal Group, Tai an, Shandong 271219, ChinaAbstract: A paste-like self-flowing pipeline transportation backfillinggy with coal gangue as aggregate is proposed toremove the potential damage caused by coal gangue piles. As well, theproblems of recovering high quality safety coalpillars and deep mining of the Suncun Coal Mine(SCM), Xinwen CShandong are resolved. The physical-chemicalproperties of coal gangue, optimized proportion of materials, backfilling system and craft in the SCM were studied in the laborad then an industrial test was carried out on high quality coal pillars under a town. The results show that finely crushed kaolinizedand fresh gangue with granularity less than 5 mm can be used as aggregate with fly ash to replace part of the cement and a composthan 0.7 MPa at d. The sequence of adding cement, fly ash, water reducer and then coal gangue ensures that the suspended state ofthe slurry, reducing the wear and jam of pipelines. The working face is advancing continuously by the altemating craft of buildingblock walls with coal gangue and backfilling mined-out gobs with paste-like slurry. The recovery rate is as high as 90% with abackfilling cost of 36.9 Yuan/t, good utilization of coal gangue and no subsidence on the surface. This technology provides a goodtheoretical basis and application experience for coal mines, cement backfilling with paste-like slurryKeywords: coal gangue; paste-like; self-flowing: backfilling system; backfilling craft1 Introductionrate of safety coal pillars, restrain surface movementdvance mining safety, as long as a method ofo Coal gangue is a residue resulting from the process backfilling mined-out gobs with coal gangue is usedcoal mining and washing, accounting for This method has found wide application15%-20% of the original coal production. Recently,The Suncun Coal Mine(SCM) of the Xinwen Coalabout 1600 sites of coal gangue hills, adding up to Group, Shandong, China, is the deepest coal mine in4.5xl0't have occupied about 1.5x10 hm? of land our country with a mining depth of 1300 m. Coalin our country and the amount is increasing by gangue hills, occupying 20 hm of land, have exceedthe cancer of coal mining for angue hills are called their limit and continue to increase by 2.0x10 t an-ading and occupying nually, which has led to serious pollution of the envimuch land, resulting in atmospheric and water pollu- ronment and the Xinwen river. Simultaneously, thetion, even engendering geological disasters suchmine has kept many high quality safety coal pillars inhill slides, mud-rock flows, etc. Lately, a number of place(about 1.6x10,), to protect the town.So,it hasdomestic studies have been carried out to make usebeen confronted with a series of difficult problemscoal gangue. These studies have met with some mod- such as rock bursts, high temperatures and dispositionest success. However, the utilization percentage is of pillarsonly about 43%0, far lower than that in developedA paste-like pipeline self-flowing transportationcountries(about 90%)4. Therefore, how to makeckfilling technology, using coal gangue as aggrebetter use of coal gangue is an urgent task of coal gate, is proposed as a remedy at SCM. Paste-like cemines. It will not only remove the damage inflicted mented backfilling is a new technology with a massby coal gangue hills, but also improve the recovery fractiwhose rheological中国煤化工Received 12 July 2008: accepted 23 October 2008Projects 2006BABO2A03 supported by the National Key Technology Research and devCNMHGthe llth Five Year KeyProgram for Science and Technology Development of ChinaComespondingauthorTel+86-731-8879821;e-Mailaddresswxml958@126.comMining Science and TechnologyVol 19 No. 2and backfilling properties are the same as those of composite water reducer to improve the self-flowipaste, i.e., it lends itself to easy self-flowing trans- pipeline capability and reduce pipeline wearportation and avoids the abuse of pipeline wear, de. Meanwhile, #32.5 cement was used as gelling agent.watering difficulties and isolation. This technolgy meets the backfilling capability and syster2.1 Physical-chemical properties of coal ganguesafety requirements at SCM, promises low cost backCoal gangue is a mixture of inorganic substancesfilling and green processes. Furthermore, this tech- and a few organic substances resulting from coalology uses a high mass fraction slurry of self-flow- mining and washing. Three kinds of coaling backfilling in mines with a high pipeline were used in our laboratory study: crushedlength-backfilling depth ratio (3.9 in safety pillars) gangue(KG), coarsely crushed fresh gangue(FGl)well exceeding the limit of about 1. 1-1.2 in a few and finely crushed fresh gangue(FG2mines'5and supplies the gap of backfilling with a The main chemical compositions of coal ganghigh mass fraction paste-like slurry in soft rock, are Sioz, Al2O3 and C, followed by Fe203, Cao,minesMgO, Na2O, K2O, SO3, P2O5, N and H, as well assuch metallic elements as Ti, v, Co and ga!. Proper2 Physical-chemical evaluation of coal ties of chemical compositions are not fixed, but varyby areas. The main chemical compositions, physicalmechanical properties and size distribution ofThe local coal gangue was used as aggregate with gangues (with fly ash)are listed in Tables 1-2 andadded fly ash(from the nearby power plant)and a. Fig. I, respectivelyTable 1 Main chemical compositions of(with fly ash)(%)Backfill materialsNayoL27516521039291.870.13567Table 2 Main physical-mechanical properties of gangue(with fly ash)Backfill materials Density(U/m) Permeability coefficient(em/s) Bulk density (g/cm) Angle of rest out of water() Angle of rest in water (2.0IxI02.59372762×104482×1040.52particles less than 2 mm make up 85%, theirsizes vary from 0. 4 mm(KG)to 0. 45 mm(f(self-flowing properties are better than for FGl:; how斗却丰套 ever, they have higher costs of crushing3) The permeability coefficient of FGl is considerably higher than for FG2 and KG which makesFGI easy to dewater and become rigid. However, thepermeability coefficients of FG2 and KG will meet20060201051080.20.1000200060001the backfilling requirement as long as the mass frac-tion of slurry, with fly ash, reaches 70%.Fig. 1 Size distribution curves of gangueand fly ash)4)Fly ash is not an ideal aggregate for backfilling,owing to its fine particles and low permeability coef-2.2 Quality evaluationficient. However, it can partly replace cement for1)The amount of SiO, in the three kinds of coal good potential binding properties with the high congangue is higher than 50%, with other compositions tents of SiO2(5643%)and Al2 O3(2717% ),whicsuch as Al2O3, Fe2O3 and so on, possessing some improves self-flowing properties reduces pipelineloose strengthWear2)FGI is coarse and does not easily pass througbodie中国煤化工 gth of backfillingipeline on its own, its particles are larger than 2 miCNMHGangues(FG2and accounts for 77% of the volume: its mediated KGare selecfor beingsize corresponds to 3.5 mm and its non-homogeneous transport, when fly ash is added, although itsindex corresponds to more than 50 For FG2 and KG distribution is not idealWANG Xin-min et alPaste-like self-flowing transportation backfilling technology based on coal gangue3 Optimized proportionpress area(m). The results of our laboratory test arelisted in Table 33.1 Study on proportionIn order to fit in with the production environment,slurry was made under conditions of normal tem-peratures, using a standard tri-unit model of 7.07cm×7.07cm×7.07cm, keeping nine samples forevery suggested proportion, and then carrying outuniaxial compressive strength tests after a maintenance period with the Instron 250 kN rigid hydraulicpressure servo machine. The axial compression modeapplied in study is shown in Fig. 2, with a formulaFig. 2 Press test of cubae=P/S, where P represents press load (N)and S theTable 3 Results of laboratory testNo. Cement: fly ash: gangue Mass fraction(%) [IN2] GangueUniaxial compressive strength (MPa)Bleeding rate(96)1:2:160.380571:2:16}152221-8010FGI67891:2:161:2:20z552555352552525}20FGI[01-13 FGI1:2:25[}-150.25o1234561:3:250.2415FG2100}15FG2l:2:204:3:251:4:150.43[1]15FGI1}-15}1.5FG2l:4:15}-1.50.120:1:3]1.5FGI6.25Note: [I], Composite water reducer: [2]. Early hardening accelerator.3.2 Analysis of resulrate for increasing the dosage of FGl. This means1)The strength of the cemented KG samples isthat KG has potential binding capability and FG2 ismuch higher than those of cemented FGI and FG2 much better than FGl as aggregate.(comparing #8 and 14 to #16). It is obvious that FG22)The initial strength increases quickly and theis better than fgl. Under the same conditions theanaphase strength increases slowly as the mass fraccompressive strength of cemented KG samplestion of slurry increases. Fly ash can effectivelyreached 1.15 MPa and 1. 41 MPa at 7 d and 28 dstraThese strengths are higher than those of KG2 and flow l中国煤化工e, improving thethe bleeding rateKGl by 30%-32%and 65%-70%0, respectively. (itsC Hile, the bleedingCompressive strength will decrease by about 50% rate will exceed 10% without fly ash, with low com-and the bleeding rate will increase by about twice the pressive strength at 7 d and low anaphase compreve strengthng Science and TeVol 19 No. 23)An early hardening accelerator is not adopted for pipeline, causing little wear on the pipeline, with athis difficult craft, although it can advance compres- low bleeding rate and high strengthsive strength to a certain extent. However, a compos-ite water reducer with added cement and fly ash, 4 Binding mechanism of cement-fly ash-amounting to only 1. 5% of total content, improvesgangue systemthe viscosity of the slurry efficiently, with its advanced mass fraction and compressive strength(#16Binding and solidification of the cement-fly ashis higher than #15 by 5%10%)and reduces pipelinestemis actually a multi-stage and multiwearlevel hydration process consisting of three stages4)The residue strength of the backfilling body has1)Hydvery high elasticity and plasticity, so that the backhis stage lasts for two weeks after formation offilling body will be able to bear the weight of the the slurry. It includes a hydrating process of about 3 dmined-out gobs very well after breakage(see the when the slurry cannot flow freely and a calcifyingstress-strain curve in Fig. 3)process of about 10 d when a large number ofCa(OH)z crystals are formed. The variable whichseparates the phases is the ph value107.0During the hydration process of cement, neitherSiO2 nor Ca(OH)2 begins hydration and the surface offly ash particles remains almost unchanged duringthis stage, and the ph value is high. The structure ofslurry is kept primarily by Ca(OH), called a calcify2)Solidifying stageThis stage ranges from 14 to 30 d after formation4.529.043.658.1726of the slurry. It includes a silicifying process when flStrain (%ash and gangue particles are eroded by alkaline layerFig 3 Stress-strain curve of sample( #20, 7 d)and an xCao-SiO2: yH2O(C-S-H coagulation)is pro-duced on the surface of fly ash and gangue particles5)The following parameters are suggested by an by combination between negative silicate ions andeconomic and technical analysis of our experimental Ca". This, is followed by a process of diffusion,results: the aggregate is kaolinized or fresh gangue< where Ca within the c-S-H coagulation penetrates5 mm in size, a mass ratio of cement to fly ash to the fly ash and gangue particles to form some transi-gangue of 1: 4: 15, the mass fraction of slurry varyingtion layerfrom 72% to 75% and a compressive strength at 7 d Fig. 4 shows SEM photographs of samples with>0.7 MPasuggested dosages, showing that glass beads of thefly ash have formed some C-S-H coagulation during3. Evaluation of rheological properties of slurry the solidifying stage. These SEM photographs showthat the C-S-H coagulation of the #20 KG and #22Rheological properties, showing flowing ability of FG2 samples, finely crushed and with a water reducerthe backfilling mixture of slurry by gravity or outside is much more uniform than that of the #18 FGI sam-gobs, are very important characteristics, including reducer. This makes it easy to improve the strength ofslump,a diffusibility-slump ratio and denseness. the backfilling body and cause the solid particles toLaboratory test results of coal gangue cementbe suspended during transportation. Furthermore,slurry with suggested dosage shows that the average C-S-H coagulation of the #20 sample is much moreslump is more than 25.5 cm, belonging to S4 grade, compact than that of #22, which means that the initialmeeting the requirement of self-flowing pipeline strength of the backfilling body with KG is highertransportation. The diffusibility-slump ratio variedthan that of freshfrom 2. 19 to 2.25, meeting the requirement of prepa-3)Increasing and stabilizing stageration and transportation. The average denseness wasThis stage also includes two sub-stages4-7.Onemore than 12.0 cm, ensuring that solid particles wis a cementing sub-stage, which lasts for about 30-9suspended,easily transported and reducing pipeline d when C-S-H is produced and backfilling strengthwearhasTherefore, the rheological properties of backfilling sub-s中国煤化工 Is a stabilIzingereslurry with the suggested dosage are similar to paste noMHGracteristic effect isslurry,called"paste-like"slurry o. It is the best to transtorm among its hydrated materialchoice to backfilling for its ability of self-flowing in aWANG Xin-min et alPaste- like self-flowing transportation backfilling technology based on coal gaFig 4 SEM pictures of samples with suggested dosage5 Backfilling system and craftvat. The cement and fly ash are discharged to the ce-ment and fly ash storeroom by compressed air from5.1 Backfilling systemtank car, stirred with a composite water reducer asrequired to form the cement-fly ash slurry in a stirringThe preparation plant for the backfilling system vat through an asteroid conveyor, the plate flow-me-was established on a slope, near the coal gangue hills, ter and the single spire conveyor to the main stirringvat. All the materials are mixed in the main stirringhe underground mine. The flowchart of the process vat to form qualified paste- like slurry which is trans-of backfilling and preparing the system is shown in ported by gravity through a pipeline to the mined-outgobs. The best sequence of adding material is first toal gangues are crushed to appropriate granular- add cement, fly ash and the composite water reducerity by the crushing plant, established at the foot of the then add gangue as required after 3 min, making surecoal gangue hills, conveyed to coal gangue piles by a of the suspended state of particles in both the mainhigh frame conveyor, transported to the cushion fillerstirring vat and the pipeline, improving the quality ofby an electric slide buried under the coal gangue piles, backfilling, reducing pipeline wear and avoidingsupplied to the vibrating griddle with a fixed amountby the disk conveyor and finally to the main stirringFly aCoal gangueDischarge by compressive Discharge by compressive airair from tank carm tank carCement storeroomFly ash storeroomCoal gangue hillsAsteroid conveyorAsteroid conveyorElectrical gliderSpire conveyorCushion fillerplate flow-meterdisk conveyorBelt conveyorL单Main stiring vatig. 5 Flowchart of process of backfilling中国煤化工5.2 Backfilling craftforody to possessingCNMHGore, appropriateThe backfilling craft in coal seams is very difficult, block walls must be constructed at the bottom of thegiven the contradiction between the continuous ad- mined-out gobs for thof mining safety, givvancement of the working face and the time required that inclined workingare so long that the bigMining Science and Technologypressure of slurry acts on the block.with reserves of 1. 6x10t.The craft of alternating of synthesizing the backAccording to our industrial test, this technologyfilling of constructing a block wall with gangue only has the following characteristics: low cost, goodand then backfilling the conjoint part with the flowing capability, convenient transportation, highpaste-like slurry, is carried out depending on the in- strength and environmental protection. The entireclined working faces and the long wall caving method budgetary estimate of the investment is 7151.7 thou-of SCM(Fig. 6). During the process of constructing sand Yuan, while the synthesizing cost is 336.9the block wall, seven rows of supports are first set to Yuan/t, including a backfilling cost of 36.9 Yuan/t andcontrol the mined-out gobs, followed by a sequence mining cost of 300 Yuan/t An amount of 1.44x10 tof inner to outer and underside to top rows recovery; coal was reclaimed with a recovery rate of 90% in thethe edge row consists of many temporary supports, test section. The upper town remained safe withoutfixed by bamboos with two deck gauzes. Finally, coal any subsidence. The entire production process wasgangues are transported by a conveyor from the un- valued as high as 648 million Yuan(given the coalderground storeroom to the upper part of working price of 2005)and profits of 162. 864 million Yuanfaces and unloaded into mined-out gobs, which have The payback period of this backfilling system is onlya width of 1.6 m. during the process of paste-like 2 months with an annual profit of 45.24 million Yuanslurry cemented backfilling, between the block walls, Meanwhile, about 40 million Yuan is gained from thesoft plastic pipelines with a diameter of 75 mm are release of land from the coal gangue hills. So, thefixed on the upper part of mined-out gobs to transport economic benefit will be enhanced if the technologye slurry and backfill the space all at once or at is used in the deep part of SCMtages, given the 4.8 m width of this space. Afterbackfilling with slurry, the remaining slurry in the 7 Conclusionspipelines must be washed with water, after which thewash water needs to be drained. at the same time. the1)The method of backfilling coal ganguworking face isadvancing continuously and the al- mined-out gobs, not only resolves the problem of coalmating backfilling craft are repeated, within a width gangue hills, environmental pollution etc, but alsoof 9.6 mimproves the recovery rate of safety pillars and sup-Ventilation entry Conveyor for gangueplying low-cost backfilling materials for deep mines.2) The capability of cemented slurry pipelinetransportation will be clearly advanced by adding flyash and a composite water reducer. The optimizedmass ratio of cement to fly ash to gangue is 1: 4: 15,with a mass fraction of 70%-72%, a composite waterreducer content of 1.0%6-1.5% added to the combinedBlock wallcement and fly ash Slurry is like a paste slurry meetBA with gangueing the requirements of self-flowing pipeline trans-portation and backfilling. The slurry has a low bleed-ing rate and high strength3)The essential function of the cement-fly ashgangue combination is a multi-stage and multi- levelPaste-likebackfillhydration process among cement, fly ash and coaltial strength, a solidifying stage to produce a C-S-HTransportation entrytransition layer, an increasing and stabilizing stage toproduce C-S-H and to give advanced strengthFig 6 Sketch of synthesizing gangue backfillinguence of adding cement, fly ash, wareducer and then coal gangue ensures that the sus-6 Industrial testpended state of the slurry reduces the wear of andjams in the pipeline. The working face is advanceIn order to ensure the feasibility of the backfilling continuously by the craft of building the block wallsystem and craft, an industrial test was carried out. with coal gangue and alternately with backfillingThe section for the industrial test was the area of high mined-out gobs with the paste -like slurryquality safety coal pillars under the town, with X co5)ordinates between 20561. 4-20562.0 and Y coordi-中国煤化工cn1mnates between 3972.0-3972.6 at levels from -210 totoCNMH Gder-three-objects-400 m, with a pipeline length-backfilling depth ratio ore boent, resolvIng a series of problemsf 3.9. The main coal seams were typical"urof coal gangue hills, providing an effective way inder-three-objects"ore bodies located in the 13 and deep mines. Meanwhile, it fills the gap of cementedWANG Xin-min et alPaste-like self-flowing transportation backfilling technology based on coal gangubackfilling with a high mass fraction paste-like slurry (7 JiYs,Yuan Y s, Yuan G L, Hao w, Xue HExperimen-in soft rock mines in the world of mining and offerstal research on composite portland cement with fly ash totheoretical considerations for similar mines.mprove concrete performance. Journal of China Uni-versity of mining Technology, 2006, 35(3): 306-310n Chinese)Acknowledgements[8] Liu Z x, Li X B Reconstruction of chaotic time seriesfor backfill deformation and prediction with neutralThis study is sponsored by the National Keynetwork. Mining and Metallurgical Engineering 2005nology Research and Development Program25(1):16-192006BAB02A03)and the llth Five Year Key[9] Sivakumar B, Femando T M. River flow forecasting: usegram for Science and Technology Development ofof phase-space reconstruction and artificial neural neorks approaches. Journal of Hydrology, 2002 (265)China(No. 2006BA02B05). Also we express our sir225-245cere gratitude to the entire staff of SCM[10] Sun HH, Huang Y C, Yang B G Cementation Backfilling Technology in Recent Year. Beijing: Metallurgy Pub-Referencesshing House, 2002.( In Chinese[Il] Zhou H Q, Quan Y H, Zheng B C, Zhao C Z. Proporioning of paste backfilling materials to moisture and its[1 Zhou S X, Chen xwS. Prediction of com-allowable measurement error. Joumalpressive strength ofortars with fly ash and ac-Safery Engineering, 2007, 24(3): 270-273. (In Chinese)tivated coal gangueof Southeast University, [12] Zhou S X, Chen Y M, Zhang WS. Prediction of com-2006,22(4):549-552pressive strength of cement mortars with fly ash and ac-[2] Zhou H Q, Hou C J, Sun X K, Qu Q D, Chen D J Solidtivated coal gangue. Journal of Southeast University.waste paste filling for none-village-relocation coal min-2006,22(4):549552.ing. Journal of China University of Mining& Technol- [13] Hou G H, Shen X D, Xu ZZ. Composition design forgy,2004,33(2):154158. (n Chinese)C3s cement clinker and its mineral formation[3] Chang Q H, Zhou H Q, Hou C J Using particle swarmJournal of wuhan University of Technology, 2007, 22(1):optimization algorithm in an artificial neural network to5660.forecast the strength of paste filling material. Journal of [14] Gao Y L, Zhou S Q. Influence of ultra-fine fly ash onChina University of Mining Technology, 2008, 18(4)ydration shrinkage of cement paste. Journal of CentralSouth Universiry of Technology, 2005, 12(5): 596-60[4] Miao x, Zhang JX, Feng MM. Waste-filling in[15] Zhang J R, Cao X. Stabilization of expansive soil bymechanized coal mining and its application. Journme and fly ash. Journal of wuhan Universiry of TechChina Universiry of Mining Technology, 2008,nology,2002,17(4):73-77.479482[16] Lu X Y, Zhu X Y. Present situation and developing[5] Wang X M, Xiao w G zhang Q L Deep Well Mineprospect of comprehensive utilize of fly ashes. JoumalSouth University Press, 2005. (In Chinese)[17] Zhang Q L Wang X M. Performance of cemented coal[6] Qian JS. Fly Ash Characteristic and Fly Ash Concretegangue backfill. Joumal of Central South University ofBeijing: Science Press, 2002. (In ChineseTechnology,2007,14(2):216-219中国煤化工CNMHG