Hydraulic calculation of gravity transportation pipeline system for backfill slurry

Transcription

1 J. Cent. South Uni. Technol. (008) 15: DOI: /s x Hydraulic calculation of graity transportation pipeline system for backfill slurry ZHANG Qin-li( 张钦礼 ), HU Guan-yu( 胡冠宇 ), WANG Xin-min( 王新民 ) (School of Resources and Safety Engineering, Central South Uniersity, Changsha , China) Abstract: Taking cemented coal gangue pipeline transportation system in Suncun Coal Mine, Xinwen Mining Group, Shandong Proince, China, as an example, the hydraulic calculation approaches and process about graity pipeline transportation of backfill slurry were inestigated. The results show that the backfill capability of the backfill system should be higher than 74.4 m 3 /h according to the mining production and backfill times in the mine; the minimum elocity (critical elocity) and practical working elocity of the backfill slurry are 1.44 and 3.8 m/s, respectiely. Various formulae gie the maximum ratio of total length to ertical height of pipeline (L/H ratio) of the backfill system of 5.4, and then the reliability and capability of the system can be ealuated. Key words: hydraulic calculation; critical elocity; working elocity; pipe length-backfill depth ratio; backfill capability 1 Introduction Backfill materials are commonly transported into underground stopes by weight force of backfill slurry within ertical drill hole or pipe in mines [1]. Key point is the choice and determination of graity pipeline transportation parameters in order to transport solid filling materials effectiely and safely with the minimum transportation energy loss []. Related hydraulic calculation approaches and process about graity pipeline transportation system were discussed in this work by using data of cemented coal gangue pipeline transportation system in Suncun Coal Mine (SCM), Xinwen Mining Group, Shandong Proince, China. SCM is one of the deepest coal mines with more than 100 years mining history and m of mining depth. Gangue site with an area of m has reached limit. Gangue is still increased by 00 kt eery year, but no spared space can accommodate gangue newly produced. Cemented gangue backfill is the best way to recoer land occupied by old gangue hill and to treat new gangue. Laboratory test about recipe of cemented gangue backfill suggested that sizes of coal gangue particles should be less than 5 mm so that coal gangue could be smoothly transported by graity pipeline transportation system. The result also indicated that high compressie strength could be obtained if mass ratio of Portland cement to fly ash to gangue was, and mass fraction of solid materials aried between 7% and 75%. If 1.0% 1.5% of water-reducing agent to sum of cement and fly ash was added, the rheological properties of backfill slurry such as slump, diffusibility and denseness could be improed obiously [3]. The backfill slurry with suggested dosage was like paste backfill in rheological property, here named as paste-like backfill. It is characterized by excellent floatation of solid materials in pipe, low dewatering in stope, and little wear to pipe wall [4]. Graity pipeline transportation parameters, such as the capability and the maximum transportation distance of backfill system, should be clarified by hydraulic calculation of the system, in order to obtain good results mentioned aboe. It is generally necessary to determine such hydraulic transportation parameters as fluid elocity, pipe pressure and transportation resistance when the pipeline transportation system of natural gas, oil and polluted water is designed. These parameters can be calculated or estimated by theoretical analysis, numeral simulation and semi-empirical equations [5 8] since the transported slurry is usually uniform single phase or two-phase flow [9 10]. The hydraulic transportation parameters of coarse backfill aggregates, howeer, are commonly estimated by empirical equations because backfill slurry is non-uniform solid-liquid flow in most cases [11 1]. Data preparation.1 Backfill system in SCM The graity pipeline transportation system for cemented coal gangue backfill in SCM is shown in Fig.1. Foundation item: Project( ) supported by the National Natural Science Foundation of China Receied date: ; Accepted date: Corresponding author: ZHANG Qin-li, Professor, PhD; Tel: ; zhangqinli@mail.csu.edu.cn

2 646 J. Cent. South Uni. Technol. (008) 15: Fig.1 Graity pipeline transportation system of cemented coal gangue backfill in SCM The inner diameter and wall thickness of ceramiclined pipe used for transporting the backfill slurry by graity are 100 and 10 mm, respectiely. The ertical depth from surface to the backfilling leel is about 385 m, the maximum horizontal transportation distance is m in the first stage, and the L/H ratio is consequently Designed capability of backfill system in SCM The yearly productiity (T y ) of ore that is mined using the backfilling method is 4 Mt/a and the necessary backfilling olume required (V a ) is calculated as follows: Ty 5 Va = Z K1 = m 3 /a (1) o where o denotes the density of ore, here o =1.34 g/cm 3 ; Z is the olumetric proportion of backfill to the emptied area left by mining actiities, here Z=0.8; and K 1 is the spare coefficient, here K 1 =1.15. The necessary capability (Q r ) of the backfill system is Va Q r = K K 3 = 89.5 m 3 /d () T d or 74.4 m 3 /h if operational time is designed as 330 d/a (T d ) or 1 h/d, and where K and K 3 are the compression and settlement ratio, and the loss factor of backfill mixture while dewatering from stope to be backfilled, here they are 1.05 and 1.0, respectiely..3 Physical and mechanical parameters of backfill slurry The physical and mechanical parameters of the suggested cemented coal gangue backfill are summarized in Table 1. Table 1 Physical and mechanical parameters of backfill slurry in SCM Factor Value Density of cement/(g cm 3 ) 3.00 Unit weight of cement/(g cm 3 ) 1.30 Density of gangue/(g cm 3 ).65 Unit weight of gangue/(g cm 3 ) 1.41 Density of fly ash/(g cm 3 ).44 Unit weight of fly ash/(g cm 3 ) 1.0 Density of mixture/(g cm 3 ).6 Unit weight of mixture/(g cm 3 ) 1.4 Mediate size of mixture*/mm 0.3 Aerage size of mixture/mm 0.83 Density of backfill slurry/(g cm 3 ) 1.77 Mass fraction of backfill slurry/% 7.00 Volume fraction of slurry/% Mass ratio of solid-liquid of slurry.57 Volumetric solid-liquid ratio of slurry 0.98 Volumetric liquid-solid ratio of slurry 1.0 * Particles whose diameters are less than the mediate size make up half of total backfill mixture. 3 Critical and working elocities of backfill slurry The critical elocity of the backfill slurry is defined as the minimum elocity at which particles are kept in a suspended state within the pipe and the flow resistance of the slurry is minimized. The critical elocity l can be computed according to the following equation [13] : m 1 l = Fl gd (3) 1

3 J. Cent. South Uni. Technol. (008) 15: where F l is the elocity factor related to the mediate size of the backfill mixture and to the mass fraction of backfill slurry, here it is about 1.15; D and g are the inner diameter of the pipe and the graity acceleration, respectiely; m is the density of the mixture; and l is the density of the transportation media composed of water and fine solid particles with sizes less than 100 µm, and here it is 1.46 g/cm 3. By substituting alues of the parameters mentioned aboe into Eqn.(3), the critical elocity of this system is obtained to be about 1.44 m/s. In other words, the working elocity of backfill slurry in SCM should be higher than 1.44 m/s in order to get stable and reliable transportation of backfill slurry. The stable working elocity() depends on many factors, such as the inner diameter(d) of the pipe, L/H ratio and the resistance of the slurry, and it can be estimated by the following empirical equation: Fig. Diagram showing relationship between hydraulic gradient and elocity of backfill slurry fraction of slurry, and with decreasing diameter and installation quality of pipe. (1 + N) = 3.3 gd 3 (4) XN where X and N represents olumetric ratio of liquid to solid in slurry and L/H ratio, respectiely. By substituting alues of the related parameters into Eqn.(4), the working elocity of the graity pipeline transportation system is calculated to be 3.8 m/s, which is about.7 times as large as that of the critical elocity, implying that the system is reliable in flowability. 4 Hydraulic gradient of backfill slurry Graity transportation reliability of backfill slurry depends greatly on hydraulic gradient of backfill slurry, including turbulence resistance in slurry and friction resistance between backfill slurry and pipe. Hydraulic gradient of the backfill slurry must be estimated before backfill system is designed. 4.1 Influencing factors of hydraulic gradient The elocity of backfill slurry is the most important factor influencing the hydraulic gradient. Hydraulic gradient of backfill slurry increases initially with the increasing elocity of the slurry and then decreases until the elocity reaches its critical alue l (point A in Fig.). If the elocity of backfill slurry continues increasing after point A, the gradient will increase gradually again between points A and B and proportionally after point B. Reasonable elocity of backfill slurry should, therefore, lie between points A and B as shown in Fig.. Hydraulic gradient of the backfill slurry also increases with increasing sizes of solid particles and mass 4. Hydraulic gradient of backfill slurry in SCM Thinking of a fact that hydraulic gradient is affected by many factors, hydraulic gradient of the backfill slurry in SCM can only be approximated by the following empirical equations Jinchuan s equation Jinchuan Non-ferrous Metal Corporation, a world famous nickel and copper mine situated in the northwest of China, proposed the following empirical equation to calculate the hydraulic gradient of backfill slurry with coarse particles [14] : i = i C 3.96 gd( j 1) C x 1.1 (5) where i denotes the hydraulic gradient of backfill slurry; C is the olume fraction of slurry; j is the density of backfill slurry; i 0 and C x are the hydraulic gradient of fresh water and the settlement resistance factor of solid particles, respectiely. The following equation can be used to approximate i 0 : i0 = λ (6) gd where λ is the friction resistance factor estimated as follows: K 4K 5 λ = = (7) D ( lg ) where K 4 and K 5 are the installation and joint quality factors of the pipeline, each taken as 1.1 here.

4 648 By substituting alues of the friction resistance factor(λ), the working elocity() and other parameters into Eqn.(6), the hydraulic gradient of fresh water 3.8 i0 = λ = = gd is obtained. The settlement resistance factor of solid particles C x can be estimated by the following equation: C x 1308( j 1) dcp = (8) ω where d cp is the aerage size of the mixture; ω is the aerage settlement rate of solid particles and can be calculated according to the temporary parameter A= /( j 1) = Since d cp =0.083 is greater than A but less than 4.5A, ω can be estimated by following equation: ω= 10.71d cp ( j 1) 0.7 =7.1 cm/s and consequently C x =1.66. By substituting alues of parameters mentioned aboe into Eqn.(5), the hydraulic gradient of backfill slurry with coarse particles, i, is about Northwest Hydraulic Research Institute s equation Experts of Northwest Hydraulic Research Institute, China, put forward the following empirical equation to calculate the hydraulic gradient of backfill slurry [15] : 1/6 m w j i = 1.96 =0.7 (9) w gd 100 where w is the density of water. In this work, the maximum of two alues estimated by Eqns.(5) and (9), i.e. 0.7, is taken as the hydraulic gradient of the backfill slurry in SCM. 5 Estimation of maximum transportation distance of backfill slurry The maximum distance of pipeline transportation by graity flow of backfill slurry can be estimated by the allowable maximum L/H ratio, N max, which is approximated by the following empirical equation: N K 6 j max = (10) Ki 7 where K 6 is the filled fraction of slurry in the ertical pipe, K 6 =0.9; K 7 is the coefficient of local resistance, K 7 =1.1; i is the hydraulic gradient of the backfill slurry in SCM, i=0.7. Eqn.(10) gies the maximum L/H ratio N max of 5.4. Since the current L/H ratio N=3.9<N max =5.4, the graity flow transportation is belieed to be smooth and reliable. J. Cent. South Uni. Technol. (008) 15: The maximum distance of pipeline transportation by graity flow of backfill slurry can reach 079 m, assuming that the ertical backfill depth is 385 m. 6 Examination of capability of backfill system The backfill capability of the system (Q j ) can be estimated by the working elocity of slurry as follows: π = D = m 3 /h (11) 4 Q j The fact that Q j is much greater than Q r means that the backfill system can satisfy the requirement of the necessary backfill olume. Practical elocity of backfill slurry and backfill capability of the system reach 3.5 m/s and 100 m 3 /h in site test, implying that the hydraulic calculation approaches used in this work are of enough accuracy. 7 Conclusions 1) Hydraulic calculation process of graity transportation pipeline system for backfill slurry is demonstrated by a case study of Suncun Coal Mine, Shandong Proince, China, where goaf left by coal extraction is filled by cemented coal gangue. ) The necessary capability of the graity transportation pipeline system for cemented coal gangue backfill at the Suncun Coal Mine, is about 74.4 m 3 /h according to the mining production and working times in the mine. The critical elocity of the backfill slurry defined as the minimum elocity for reliable transportation within the pipe is about 1.44 m/s. The working elocity and the maximum L/H ratio of the backfill system are 3.8 m/s and 5.4, respectiely and the probable backfill capability of the system is m 3 /h. 3) The results of hydraulic calculations for the backfill transportation system show that the designed backfill system is reliable in flowability and sufficient for the needs of backfill capability. Acknowledgments Many thanks are due to President DI Kuang-you and General Engineer ZHANG Dian-zhen of SCM for their support of this research work. References [1] YU Bin. Computation and measurement of hydraulic parameters for graity pipeline transportation [J]. Jiangxi Non-ferrous Metals, 1999, 13(4): 1 3, 10. (in Chinese) [] WANG Pei-xun. Hydraulic gradient calculation of mine filling paste [J]. Non-ferrous Mines, 003, 3(1): (in Chinese)

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