Research ArticleCold Load and Storage Functional Backfill for CoolingDeep Mine

MeiWang ,1,2 Lang Liu ,1,2 Liu Chen,1,2 XiaoyanZhang,1,2 BoZhang,1,2 andChangfa Ji1,2

1School of Energy Engineering, Xi’an University of Science and Technology, Xi’an 710054, China2Key Laboratory of Western Mines and Hazard Prevention, Ministry of Education of China,Xi’an University of Science and Technology, Xi’an 710054, China

Correspondence should be addressed to Lang Liu; liulang@xust.edu.cn

Received 23 November 2017; Accepted 29 January 2018; Published 5 July 2018

Academic Editor: Luigi Di Sarno

Copyright © 2018 Mei Wang et al. ,is is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Considering the deep mining heat-hazard problem, the concept and academic idea of cold load and storage (CLS) functionalbackfill applied on deep mine cooling was put forward. Firstly, according to characteristics of filling mining, a method of coolingstopes with CLS phase changing backfill which is made from the backfill material with CLS phase change material (PCM) wasproposed. ,e working process, cooling physics, and the economic and safety benefits of CLS phase changing backfill wereproduced. Secondly, the theory system of cooling with CLS phase changing backfill was built. ,e theoretical basis of the keyproblems involved was investigated and analyzed which concluded heat transfer, fluid mechanics, and backfill mechanics. Lastly,the technology system of cooling with CLS phase changing backfill was established on the basis of the required technical assistance.It includes four parts: the backfill material design, the backfill material conveying way design, the stope temperature controlscheme design, and the strength of cemented tailings backfill analysis.,e idea of applying CLS phase changing backfill on coolingdeep mine stopes and its theory and technology systems provide a scientific research and suitable development direction for deepmine cooling.

1. Introduction

As shallow mineral resources increasingly exhausted, thedeep resource extraction has become normal. ,ere aremore than one hundred fifty 1000m-plus deep metal minesin the world at present. ,e deepest mines are in SouthAfrica and Canada. ,ere are 76 deep mines in South Africawith depth varying from 1524 to 4800m and 32 mines inChina with depth varying from 1000 to 1600m [1]. ,eoriginal rock temperature increases with depth. ,e averagegeothermal gradient is 2.5∼4.0°C/100m [2], even the highestvalue is 7°C/100m. Mponeng Gold Mine in South Africareached 4000m depth, and its geothermal temperature wasup to 66°C [3]. ,e high geothermal temperature circum-stance results in low efficiency of mining staff and highaccident rate. According to the research of the influence ofhigh mining temperature, when working face temperatureraised from 16°C to 32°C, the relative labor efficienciesdecreased 55%, and when this temperature raised from 29°C

to 32°C, the accident rate increased to 68.1% [4]. Meanwhile,high geothermal temperature also caused other threats, suchas the softening of rock mass and the shortening ofequipment service life and mineral combustion. Heat hazardhas become an important problem which constraints theexploiting of the deep mine.

Facing heat hazard, there are three common coolingmethods: ventilation, applying natural cold resource, andmanual refrigeration. Increasing ventilation quantity isa cooling method which has longest history and widestapplication. However, it often cannot meet the coolingdemands of deep mine with high temperature because of therestrains from inlet air temperature and velocity of workingface. If the natural condition allows, applying undergroundcool water or storage ice form winter on cooling is aneconomic way. He and coworkers [5, 6] proposed the HEMScooling system by using mine inflow as the extracted coldenergy resource. It used the refrigeration system to extractcold from mine inflow and supply to air conditioner

HindawiAdvances in Civil EngineeringVolume 2018, Article ID 5435214, 8 pageshttps://doi.org/10.1155/2018/5435214

underground. Bu et al. [7] designed the ice making systemwhich took advantages of the cold temperature in winter. Icewas produced in winter and stored in ice storage device andthen used in summer. Shi et al. [8] proposed a spray icemaking system and analyzed the feasibility of this methodapplying on mine cooling. Using natural cold energy is aneconomic and environmental method, but it had a limitedapplication range because of the constraints of naturalcondition. ,e vast majority of mine adopted the coolingmethods which are similar to the ground cooling. ,ere aretwo typical systems: refrigeration equipment set in groundand underground. ,ough the chiller locations of the twotypes are different, there are some similarities. First, the coldload fluid pipe need to be laid, and the length of this pipe inthe equipment centralized on ground type could be thou-sand meters. Second, some cooling equipment was set un-derground, especially the equipment centralized inunderground type. ,e construction and management iscomplex, and the heat is difficult to exclude. ,e great in-vestment of equipment and pipes, the complex constructionand management, and the high running costs are thecommon problems in mine cooling engineering.

,e implementation of the mine cooling techniqueseffectively mitigates the heat-hazard problem but also leadsto the great increase of investment and energy consumption.According to some statistics, the power consumption ofmine cooling system is about a quarter of the total minepower consumption [9]. In order to save energy and reducecontaminant, innovation of the cooling method based onexploiting characteristics could be a new way. Ghoreishi-Madiseh et al. [10] proposed a new method of using backfillto absorb underground heat to realize temperature controlof stopes and geothermal energy utilization. ,e heattransfer model of extracting heat from underground byusing the heat exchange pipe laid in backfill was built bynumerical simulation and experimental research. Consid-ering the cost of investment, this method is suitable for thecase with high geothermal temperature and valuable ingeothermal energy exploitation. For the lower geothermaltemperature cases which are not suitable for geothermalenergy exploitation, adding the cooling function to fillingmaterial and cooling stopes with functional backfill is anappropriate solution. ,erefore the concept of cold load andstorage (CLS) backfill cooling was proposed in this research.

2. Cold Load and Storage (CLS)Functional Backfill

2.1.+eConventional Cold Load and StorageMediumUsed inMine Cooling. ,e role of cold load and storage medium istransferring and storing cold from refrigerator to air con-ditioner. In the conventional mining cooling system, water,ice, and glycol solution were applied as cold load and storagemedium usually. ,e system using water as coolant pro-duced low-temperature chilled water by the refrigerationunit and then was pumped to air cooler exchanging heatwith air. ,e cool wind was transported to working face tocool stopes. ,e system using ice as coolant produced icecube, flake ice, or ice slurry by refrigerator on the ground.

And the ice was transported to the melt pool undergroundby wind or water power. When the ice was melt to coldwater, it was surveyed to the working face, applying to aircooler or spray thrower for cooling stopes. ,e system usingglycol solution as coolant produced low-temperature glycolsolution by refrigerator, and then it was surveyed to heatexchanger underground to cool the water. ,e cold waterwas transported to terminal air cooler for cooling air. All theabove cooling mediums were transported by a series ofindependent cooling system. Considering the energy savingand contaminant reduction, seeking a new coolant mediumwhich combined with the mining method is a breakthroughway of mine cooling.

2.2. Basic Contempt of CLS Functional Backfill. Fillingmining is one of the common mining methods. ,e backfillmaterial was surveyed to gob and formed to backfill as mineworking faces propelled. ,e volume could be hundreds ofthousands of cubic meters. For the deep mine with seriousheat-hazard problem and applying filling method, the hugebackfill could be applied sufficiently as cooling material. Byadding the CLS medium to the backfill material, a new CLSfunctional backfill material was created. ,e material wastransported by backfill material survey pipe to gobs, andthe stopes nearby were cooled through ground or wall’sradiant cooling. ,is new cooling method leaves out thecoolant transportation system and air conditioner ter-minal units.

2.3. Cooling Process and Period of CLS Functional Backfill.Taking the upward sublevel filling method as an example, asFigure 1 shows, when the CLS functional backfill materialwas surveyed to gobs, the heat was absorbed from stopesnearby and above. In the preliminary stage of cooling, thecooling potential of CLS functional backfill was large andwas named period I (Figure 1(a)). When the cold energyreleased a lot to the stope, the cooling potential decreasedand was named period II (Figure 1(b)). When the stopenearby above was exploited and gob was formatted com-pletely, the cooling task of CLS backfill accomplished and theheat transfer between the stope and the backfill finished. AsFigure 1(c) shows, this stage was named period III. ,en thenew gob was filled with CLS backfill material and a newperiod of cooling began.

2.4.CLSFunctionalBackfillMaterial. CLS functional backfillmaterial is based on solid waste of mining, added with somephase change material (PCM) which has the cold load andstorage ability and some cementitious material.

Sensible heat transfer and latent heat transfer are the twoheat absorption types. ,e latent heat energy produced byphase change is 5∼14 times of sensible heat under the samevolume condition [11]. PCM is very suitable for choosing ascold load and storage material by the advantage of its highcold storage density. CLS functional phase change backfillwas made by mixing the PCM with conventional backfillmaterial. ,e common PCM for cold storage consists of

2 Advances in Civil Engineering

organic and inorganic types (Table 1).�e organic type PCMmainly includes pure substance (e.g., ice) and eutectic salt(e.g., Na2SO4·10H2O). It is widely used with mature tech-nology; however, it has a salient phenomenon of supercooling and phase separation. �e inorganic-type PCMmainly includes single component material (e.g., glycol) andeutectic mixture (e.g., para�n oil). It has a wide phasechange temperature range and good chemical properties buta lower cold storage density. �e phase change materialwhich has good performance on thermodynamic property(suitable phase change temperature, high latent heat, highheat capacity, and high heat conductivity), chemical prop-erty (good stability, noncorrosion, nonburning, nontoxic,and nonpolluting), and economic e�ciency (cheap in ma-terial and charging cold) should be chosen as cold load andstorage material.

�e proportion of CLS functional back�ll materialshould be determined by the consideration of �ow char-acteristic, rheology property, strength characteristic, andcold supply capacity.

2.5. Advantages. Comparing with the conventional minecooling method, the newmethod of applying CLS functionalback�ll on cooling has the following obvious advantages:

(i) No special pipes surveying cold load medium needto be laid. �e transportation system of �lling slurrytakes the task of surveying cold load medium.

(ii) No air-conditioning equipment in the mine needsto be arranged. �e radiant cooling was applied bythe �oor or wall of stopes.

(iii) No complex heat transfer between air-conditioningequipment exists, and the energy e�ciency is high.

(iv) For the deposits with spontaneous combustionproperty, the whole low-temperature environmentcreated by CLS functional back�ll is bene�cial indecreasing �re probability and enhancing miningsecurity.

�ese advantages could sharply decrease the investmentand running costs of mining cooling, reduce the con-struction and management of workload, and lower the �reprobability. �e CLS functional back�ll cooling method hassigni�cant economic bene�t and safety e�ect.

3. Theories of CLS Functional Backfill Cooling

3.1. Heat Transfer. Heat transfer (heat conduction, heatconvection, and radiation) was driven by the temperaturedi�erence between CLS functional back�ll and the sur-roundings. Figure 2 shows the total thermal equilibriumwhich consists of six terms of heat:Qf ,l, latent cold producedby PCM fusion in the CLS functional back�ll; Qf ,s, sensiblecold produced by low-temperature CLS functional back�ll;Qcond andQrad, heat conduction and radiation from ore bodyto CLS functional back�ll; Qconv, heat convection from

Ore body

Stope

Backfill periodI

Backfill periodIII

(a)

Backfill periodII

Ore body

Stope

Backfill periodIII

(b)

Ore body

Stope

Backfill periodI

Backfill periodIII

Backfill periodIII

(c)

Figure 1: Cooling period of CLS back�ll applied in upward sublevel �lling method. (a) Preliminary period, (b) interim period, and (c) �nalperiod.

Table 1: Conventional cold storage PCM.

Name of PCM Phase changetemperature (°C)

Latent heat offusion (kJ/kg) Reference

Ice 0 335 [12]Na2SO4·10H2O 32 249 [13]Na2SO4·10H2O(adding NH4CL)

8 — [14]

Na2HPO4·12H2O 36 265 [13]CaCl2·6H2O 29∼39 174 [13]Glycol −13∼−11 187 [15]Amino ethanolaqueous solution 6∼9.2 155∼196 [16]

Para�n oil/water 9.5 157 [17]

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stopes to CLS functional backfill; and Qh, hydration heatproduced by hydration reaction in the CLS functionalbackfill.

According to the first law of thermodynamics, the totalthermal equilibrium equation was given:

Qf ,l + Qf ,s � Qcond + Qconv + Qrad + Qh. (1)

With the cold releasing from CLS functional backfill,PCM fused and the temperature difference between back-fill and surroundings diminished gradually. At last, thebackfill temperature is consistent with surroundings andthe cooling process finished. So the cooling process isa special process of cooling capacity decayed gradually. ,ecooling period and the decaying characteristics are thecritical factors which influences on stope cooling effect.Equation (1) was taken a the derivative with respect to timeto get (2) which expressed the gradient of releasing cold ofCLS backfill to time.

dQf ,l

dt+

dQf ,s

dt�

dQcond

dt+

dQconv

dt+

dQrad

dt+

dQh

dt. (2)

,e cooling process decaying characteristics of CLSfunctional backfill was calculated by studying the laws ofheat transfer terms varying with time. ,e heat absorbedfrom surroundings through heat conduction, convection,and radiation depended on temperature difference betweenfillings and surroundings. As the temperature differencereduced, the heat transfer rate decreased.,e hydration heatrelease rate related to the composition, hydration time, andtemperature. According to the different composition, cor-responding hydration products generated and the hydrationheat released with the progress of hydration reaction. Inparticular, the hydration temperature is significant in thiscase. Owing to the low temperature of CLS filling slurry, thehydration reaction was restrained at the preliminary periodobviously.

3.2. Fluid Mechanics

3.2.1. CLS Filling Slurry Transportation. CLS functionalfilling slurry composed of waste rock and tailings, PCMparticles (take ice particles for instance following) and water.,e diameter of solid particles is large, and the volume ratiois high; therefore, the filling slurry should be regarded asnon-Newton fluid. According to the fluid regime supposedby Doron and Barnea [18] which researched water and sandtwo phase flow, there are three fluid regimes: static bed,fluidized bed, and fully suspended bed. When the averagevelocity of CLS filling slurry is large, strong turbulence floweffect resulted in a large component of pulse velocity which isperpendicular to the flow direction. Consequently, the forceacting on the solid particles overcame the gravity of the solidparticles which formed the fully suspended flow regime.When the flow speed decreased, the turbulence decreasedwhich results in the phenomenon of waste rock and tailingsflow in the bottom of the pipe and ice particles flow in thetop of the pipes owing to the different densities. ,us thefluidized bed formed. When the flow speed decreased fur-ther, the static bed was formed which has the distribution ofthe ice particles in the top and the waste rock and tailings inthe bottom (Figure 3).

,e critical velocity and pressure drop researches of CLSfilling slurry depend on the conventional filling slurry andice slurry, on account of which the CLS filling slurrycomponents are the conventional filling slurry and ice slurry(Table 2). However, the unique flow mechanism charac-teristics of CLS filling slurry should be noted: (1) ,e flow isaccompanied by phase change which resulted in the gradientof slurry changed along the path, and therefore, the flowcharacteristics changed. (2) ,e hydration speed changedbecause of the rising temperature of slurry which absorbedheat from surroundings and the increasing water ratiocaused by ice melting in the transportation process. So thegradient of slurry and flow characteristics changed. (3) ,ediameter of waste rock and tailings changed caused byhydration and the diameter of ice particles changed bymelting and aggregation influenced the flow characteristicsand even caused the pipe blocking.

In the storage or transportation process of ice, the iceparticle size evolution behavior of nucleation and growth,aggregation, and fragmentation influenced the diameter ofice particles to change a lot from tens of to hundreds ofmicrometers which influenced the flow and heat transfercharacteristics. Researchers have done a lot of work based oncrystallization kinetic. Hansen et al. [25] indicated thatthe ice particle size changed in the ice storage tank even onthe condition of thermal equilibrium. Under the Gibbs–,omson effect and Ostwald ripening effect, the small iceparticles melt and get smaller, while the big particles growand get bigger. Pronk et al. [26, 27] concluded that thedistribution of ice particles changed because of the abrasion,aggregation, and Ostwald ripening effect. Especially theOstwald ripening effect is the main reason of the ice particlesgrowth. Furthermore, the influences of solution concen-tration and type on ripening were studied. Kanetoshiand Ken [28] introduced a set of measurement to detect

Discharged cold (Qf,l + Qf,s)Conductive heat (Qcond)

Convective heat (Qconv)

Radiant heat (Qrad)

Hydration heat( Qh)Ore bodyFunctional backfillwith iced particlesStope

Figure 2: ,e total thermal equilibrium diagram of CLS phasechange backfill cooling.

4 Advances in Civil Engineering

aggregation by the electrical conductivity di�erence betweenice and water. �e experiment results showed that the biggerclump of ice was formed by aggregating the smaller iceparticles and the low concentration of additives could re-strain the aggregation. Xu et al. [29] developed the pop-ulation balance model to study what factors in�uence the icecrystal distribution characteristics and its evolution duringthe process of ice slurry storage. �e results indicated thatwith the decrease of the mass fraction of the additive so-lution, the growth rate of ice crystal size decreases.�e lowerIPF and higher dissipation of ice slurry slow the growth ofice crystal size in ice slurry solution. �ese researchesestablished the foundation for the determination of iceparticle diameter.

3.2.2. CLS Back�ll Seepage. Some of water which consists ofwater in slurry and water changed from ice participated inhydration and the rest of water became the seepage. �eseepage characteristics of CLS back�ll is the basis of de-termination of drainage time, analysis of back�ll strength,and the plan of continuous work. Mao et al. [30] studied theseepage law of upward sublevel hydraulic back�ll. �emathematical model of dewatering time was established onthe parameters of permeability coe�cient, speci�c storagecoe�cient, and thickness. �e dewatering coe�cient wascalculated, and the dewatering coe�cient curves of di�erentparameters were obtained. Wu et al. [31] coupled the hy-draulic equations and thermodynamic equations, applying

COMSOL Multiphysics software and experiments to solvethe seepage problem of cemented tailings back�ll. �e re-search indicated that with the higher initial temperature andcuring temperature and the higher cement-sand mass ratio,the water permeability decreased. �e analysis of initialtemperature has signi�cance for the research of CLS back�ll.With higher temperature, the more hydration products weregenerated because hydration was enhanced. �e hydrationproducts aggregated in the pore structures, and the poreswere blocked gradually which ended the seepage of back�ll.�erefore, the higher the initial temperature of back�ll was,the higher the compactness of structure was and the lowerthe permeability was [31]. In the periods I and II of the CLSback�ll cooling process, the back�ll temperature was verylow which restrains the hydration. �e less hydrationproducts resulted in little change of the pore structure andhigh permeability. While in the period III, the rising tem-perature of back�ll enhanced the hydration, and lots ofhydration heat was generated which raises the temperaturefurther. �e lots of hydration products generated and thepore structure changed a lot which resulted in high per-meability. So the di�erent seepage characteristics of the CLSfunctional back�ll are shown in the di�erent periods ofcooling process.

3.3. Back�llMechanics. �emechanical functions of back�llsupport the unsteady surrounding rock of stope and play therole of arti�cial pillar in exploiting the large ore body [32].Many researches of in�uences of mixture ratio on tensilestrength and compressive strength have been done [33, 34].It is generally recognized that the factors of the cementedback�ll strength are the cement type and addition material,�lling concentration, tailings chemical composition andgradation, mixing time, curing age, and condition [35]. Inthe case of CLS �lling slurry, both the solid properties of iceand the adhesive of ice surface enhance the strength. �eresearches of concrete proved this. Suzuki et al. [36] de-veloped the method of using crushed ice instead of water formixing concrete which having the two times of strength thanconventional concrete. Furthermore, by considering thenegative e�ect of hydration heat, the casting temperature ofconcrete was limited by standards of construction. Forexample, the upper limit of casting temperature is 18°C [37].�is limit was set to decrease the thermal stress and theconcrete cracks resulted from nonuniform temperature

Wasterock

Tailings

Ice particle

Static bedFully suspended bed Fluidized bed

Figure 3: �ree �ow regime of CLS back�ll slurry.

Table 2: �e critical velocity formulas of slurry.

Researchers Formulas Application

Durand[19, 20] FL

����������2gD(1− S)√ Filling

slurry, iceslurry

Knorroz [21] 0.85(0.35 + 1.36���CD3

√)β Ice slurry

GuilpartandFournasion[22]

2.8���������gD(1− S)√

Ice slurry

BechtelCompany[23]

K(��������(ρS − ρ)/ρ√

)D1/3(d95/η)1/4e(1+42CV)

Fillingslurry

Fei [24] [(2gDSV(ρs − ρm))/(esfmρm)]1/3 Filling

slurry

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distribution of material. However, the low initial tempera-ture could decrease the strength by retaining hydration. Liuet al. [38] studied the cementing waste rock strength ofa goldmine in cold region through experiment. It was shownthat the curing temperature had obvious impact on strength.,e strength increased by rising curing temperature espe-cially in early age than in later period.

According to the analysis above, in the early age, thereare two different effects of low temperature on CLS backfillstrength, while in the later period, the temperature rises andits impact fades away.

4. Technical System of CLS FunctionalBackfill Cooling

,e implement of CLS functional backfill involves the recipeof the filling material, transport scheme of filling material,stope temperature controlling scheme, and strength analysisof backfill. According to the aspects above, the technologysystem of CLS functional backfill was built (Figure 4).

4.1. +e Recipe of the CLS Filling Material. CLS filling ma-terial is a mixture of waste rock and tailings, PCM with coldload and storage function, and cemented material. ,e se-lection of CLS PCM and the optimization of the ratio ofcomponents are the two key issues.

,e selection of CLS PCM is based on the principles ofhigh cold load density, nonhazardous, stable in chemicalcomposition, accessible, and low material cost and coldstorage cost. ,e theory analysis of selecting ice as CLS PCMwas discussed above.Whether the other PCMmaterial couldbe applied should be analyzed and identified ulteriorly.

,ere are three ways to optimize the ratio of compo-nents. First, orthogonal experimental design which usedpartial experiments instead of overall experiments is themost common method. It is the main method of factorialdesign. Wu et al. [39] applied this method on design ofcemented fillingmaterial and obtained the optimized ratio of

components by range analysis. Second, uniform experimentdesign: this method considers how to distribute the designpoints evenly within the scope of the experiment in order toensure the experiment points have the statistical propertiesof uniform distribution. Hu et al. [40] used this method andgot the optimized ratio of filling slurry components. ,ird,formulation experiment design: this method is a specialmixture experiment design. Yang et al. [41] applied thismethod and built a fitting model of the strength with theratio of every component for cement-steel slag-mineral slagternary material. ,e research of the optimized ratio of CLSfunctional backfill material should consider the flow char-acteristics and mechanics and potential of heat absorption.

4.2. +e Transportation of the CLS Filling Material.Besides the conventional transportation problems, the pipeheat preservation and pipe blocking problem in CLS fillingmaterial transportation system should be paid attention to.

In order to decrease the cold loss in the CLS fillingmaterial transportation process, the PCM with suitablephase change temperature could be selected which changedthe phase only under the temperature condition of goaf.However, the goaf temperature changes variably withmining depth, and it is close to transportation temperature.So this method is hard to realize. ,e feasible method isretrofitting the heat preservation layer to the filling slurrypipe. Considering the requirement of fire proofing of mine,glass wool or rock wool could be used as heat preservationlayer material.

,e pipe blocking problem of CLS filling material ismore serious than conventional filling material because ofthe PCM. Taking water, for example, some water in theslurry may freeze under undercooling condition. ,e wasterock and tailings in slurry contribute to the phase changeowing to them providing a lot of surface for crystal nu-cleation. Furthermore, the ice surface melts slightly. When itcontacts with other surface, cementation occurs underundercooling condition and large size ice particles are

Technical system of CLS functional backfill cooling

Recipe of the CLS fillingmaterial

Transportation of the CLSfilling material

Stope temperaturecontrol scheme

Backfill strengthanalysis

PCMselectRatio

optimization

Pipe heatpreservation Pipe blocking

PCMratiodetermination

Temperaturecontrolmeasure

Lowtemperature PCM ratio

Figure 4: ,e technical system of CLS functional backfill cooling.

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created. To avoid the ice block problem, three interventionmeasures should be taken: (1) Optimal design of the slurrytransportation pipe; (2) Initiative melting based on moni-toring ice block location; and (3) Interior wall of pipe coatingfor decreasing interfacial energy and improving flow ca-pacity of the pipe [42].

4.3. Stope Temperature Control Scheme. By insuring thestope temperature meets the requirement in a whole periodof stope exploiting, there are two aspects should be paidattention to. First, the total cooling load should be calculatedaccording to the cold load condition of stope and totalexploiting time of a layer so that the PCM ratio could bedetermined. Second, the influence factors of stope tem-perature should be analyzed comprehensively to obtain thetemperature field change trend. And the temperature controlscheme was established on the trend.

,e CLS functional backfill cooling involves the backfillregion, ore body region, and stope region. Heat transfersbetween three regions. ,e cooling capacity of CLS backfilldecayed which results in cold discharge rate changes ina cooling period. So, it is need to weaken heat transfer in theinitial stage of a cooling period and enhance heat transfer inthe later stage. For weakening heat transfer, thermal con-ductivity resistance could be increased through layingtemporary thermal insulation layer on the ground of thestope at the preliminary period of cooling. For enhancingheat transfer, the convective heat transfer coefficient be-tween air and ground of the stope could be increasedthrough adding local forced heat convection by fans. Bytaking this scheme, the stope temperature could be keptnearly constant.

4.4. Backfill StrengthAnalysis. ,e backfill strength is relatedto density, porosity, cementitious material ratio, and so on.On account of the PCM, the composition proportions ofCLS backfill differs from conventional backfill a lot. And itwould influence the backfill strength. ,ere are two aspectsfor analysis of CLS backfill strength: low-temperature in-fluence and composition proportion of PCM influence.Analogy, experience formulas, physical simulation, nu-merical analysis, and probabilistic method could be used onthis research.

5. Conclusions

According to this study, four conclusions are underlined asfollows:

(i) ,e concept of the CLS functional backfill coolingmethod was proposed. ,e cooling process andprinciple was introduced by taking the upper wardfilling method as an example.

(ii) ,e Conventional mine cooling method was sum-marized, and the advantages of the CLS backfillcooling method on investment, running cost, andsafety were listed. A conclusion that this method is

very suitable for high temperature deep minecooling was achieved by comparison.

(iii) ,eories of CLS functional backfill cooling werediscussed. Heat transfer, flow mechanics, andbackfill mechanics which involved in this methodwere analyzed through the literature research.

(iv) ,e technical system of CLS functional backfillcooling was built which consists of the recipe andtransportation of the CLS filling material, stopetemperature control scheme, and backfill strengthanalysis.

,is research indicated the direction of research anddevelopment of CLS functional backfill cooling.

Conflicts of Interest

,e authors declare that there are no conflicts of interestregarding the publication of this paper.

Authors’ Contributions

Mei Wang designed the research framework and wrote themanuscript. Lang Liu conceived the new method. XiaoyanZhang collected the data of literatures. Liu Chen, Bo Zhang,and Changfa Ji analyzed the feasibility of the new method.

Acknowledgments

,e authors are grateful for the support provided by theNatural Science Basic Research Plan in Shaanxi Province(no. 2018JQ5183) through the Program “,e deep minecooling mechanism and the temperature controlling effectand efficiency of cold storage PCM backfill,” the Foundationof Shaanxi Provincial Department of Education (no.18JK528), and the National Natural Science Foundation ofChina (nos. 51674188, 51504182, 51404191, and 51504188).

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