MOLD BAG DAMMING TECHNOLOGY WITH FINE TAILINGS AND ITS PRACTICE (*) Hanmin Zhou Director, General Research Institute of Mining & Metallurgy

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1 COMMISSION INTERNATIONALE DES GRANDS BARRAGES VINGT-CINQUIÈME CONGRÈS DES GRANDS BARRAGES Stavanger, Juin MOLD BAG DAMMING TECHNOLOGY WITH FINE TAILINGS AND ITS PRACTICE (*) Hanmin Zhou Director, General Research Institute of Mining & Metallurgy Xiaofei Liu Staff, General Research Institute of Mining & Metallurgy Xuan Cui Staff, General Research Institute of Mining & Metallurgy Yongbo Qie Staff, General Research Institute of Mining & Metallurgy Xinyan Wang Staff, General Research Institute of Mining & Metallurgy CHINA 1. INTRODUCTION Tailings pond is a man-made debris flow with high potential hazards, it is also an important part of mining engineering, so the safe operation of the tailings pond in the whole production process is very important [1]. Large quantities but small-scale, poor management, large number of diseased and dangerous tailings ponds, poor security conditions and more tailings ponds accidents are features of our countries tailings ponds [2]. Tailings that storage in the tailings pond is increasing fine along with the improving level of beneficiation process and recovery of tailings. In this case, traditional method of tailings storage will always (*) Technologie de barrage à partir de sacs de moulage contenant des stériles fins, et applications 103

2 encounter difficulties such as poor dam infiltration and lacked stability. In addition, constructing a new tailings pond could be costly due to increasingly limited land resources for construction. The challenges of damming with fine tailings are eager to solve in view of conservation of resources, protection of environment and sustainable development [3]. 2. FINE PARTICLE TAILINGS DAM 2.1. THE FEATURES OF FINE PARTICLE TAILINGS Currently, there is no strictly definition for the fine particle tailings. Referring to "Tailings facility design Reference" [4] and "Introduction to Chinese Non-ferrous Metal tailings" [5], the average particle size of fine particle tailings is less than 0.03 mm, among which, the content of mm particle is generally greater than 50%, the content of mm particle is less than 10%, the content of mm particle is below 30%. Compared with the general tailings dam, the fine particle tailings dam has the following main characteristics: 1. Poor stability, from the view of soil mechanics point, the more fine soil particles, the worse mechanical properties; 2. Poor permeability, and slow speed of consolidation, cause the rate of increase of the dam does not meet the emissions of tailings; 3. Slow slope of dry beach face which is generally less than 1%, the coarse particles and fine particles deposited irregularly, since the flood control capacity and freeboard cannot meet the requirements, it brings great difficulties for flood prevention; 4. The phreatic line is generally higher than usual, it is extremely detrimental to the stability of the dam; 5. It is hard to take tailings for building sub dam, the slope of deposition beach is slow, the tailings which only a short distance near the crest is dry, it can be taken to build sub dam. The tailings which located in the other parts of the deposition beach are softer. Thus it is unable to take tailings artificially. 6. For the upstream tailings dam, there is a "dam shell" composed of coarser particles which is usually formed at the downstream slope of the tailings dam. The "dam shell" is thin because of the fine particles, so the soft soil composed of more fine tailings is relatively thick. Thus it is adverse for the stability of tailings dam due to the excess pore water pressure caused by poor drainage and hard solidification 2.2. THE DIFFICULTIES OF FINE PARTICLE TAILINGS DAM The difficulties of fine particle tailings dam mainly reflected in the strength and seepage. Because the fine tailings dam built by upstream method is one of the problems encountered in the tailings processing technology, various 104

3 unfavorable conditions make it a problem on technological development of tailings dam, and the research for the physical and mechanical properties of fine particle tailings and the stability of tailings dam is few, there is no substantive progress yet and little examples of tailings high dam [6]. The position of the phreatic line of the tailings dam is an important factor for penetration stability and sliding stability. The problem of high phreatic line widespread in fine tailings dam, it is extremely detrimental to the stability of the dam. The slow consolidation and the low strength of the fine tailings make it a more difficult problem for fine tailings rapid damming. Therefore, this study presents a rapid damming method suitable for the fine tailings dam construction - mold bag method, which was carried out in a tailings dam. 3. A BRIEF INTRODUCTION TO THE MOLD BAG DAMMING 3.1. CONCEPT OF MOLD BAG Mold bag is composed of two high strength geotextile of layers-an upper layer and a lower layer (such as polypropylene, polyester, nylon etc.), and which was knitted to large scale continuous bag material filling full of pumping tail sand and concretion formed; It is also called the geotextile bag or sand bag. Mold bag dam is made of high strength and permeability of geotextile production of large area continuous mold bag by filling tailings to the mold bag which is filled, pressured by drainage and formed a consolidated filling body, then the filling body continuous interleaved embankment dam and corresponding reinforcement and seepage drainage, result in a new method of stacking dam APPLICABLE SCOPE OF MOLD BAG METHOD According to the author s several experiments of different kinds of tailings outdoor large scale irrigation bag consolidation, which showed that can meet storage of fine tailings below 0.074mm in the tailings dam upstream of 80%-90% dam for dam bag method; Compared to traditional upstream method tailing, this method has broaden the dam required particle size range METHOD MECHANISM OF MOLD BAG (1) Consolidated drained extrusion mechanism During the Mold bag filling process, both sides of the bag is airport drainage 105

4 surface, compared to the traditional way of heap dam, under the condition of other factor remain unchanged, since double-sided drainage and take away half the maximum drain the body of the tailings solids, results time is shortened to nearly1/4 of the original; Secondly, the traditional way of heap consolidation pressure is caused by for the own weight of tailings. The consolidation pressure of mold bag tailings dam is created by the weight of the bag, filling pressure and other external loads measures. So mold bag method dam consolidation time can be further shortened the degree of consolidation and has increased than traditional way. (2) Analysis of consolidation mode bag capacity The mold bag tailings backfill with certain specifications and strength into the mold bag formed in the external force, the overall compression deformation occurs, this lead to elongation of the circumference of the bag, resulting in a tension T. Tension T limit tailings in restraint bag, in turn, this makes contact force N among tailings inside mold bag - increases. According to the laws of friction, the contact force N increases, the friction force between the particles F tailings will increase, which means the shear strength of the mold filling body bags inside tailings increases. (3) Tailings from bulk solid to whole body Under the pressure of filling bags and other external loads measure, the role of the mold bag method stack tailings dam will not only accelerate consolidation, but also allow increases in the tailings dam consolidation than traditional methods. Particles in mold bag is tight more closely, so that the tailings dam mold bag could void ratio decreases, increase the density, and increase the intensity. The whole bag dam integrity enhanced by means of the application of well-proportioned mold bag. (4) Increasing the thickness of dam For fine tailings dam upstream heap, dam tailings deposition of coarse particles is very limited, whose "dam shell" is thinner, more unfavorable for the stability of the tailings dam. When using the mold bag damming technology, fine partial taken from tailings pond makes the mold bag stronger; another way of saying, the equivalent of expanding the scope of grit area, making stability calculation draw an arc to move the tailings pond, which will help improve the stability of the dam. 106

5 4. APPLICATION OF MOLD BAG DAM 4.1. ENGINEERING BACKGROUND Dapingzhang mine tailings pond is the original tailings that aimed to support the production of factory for Yunnan Simao Shanshui Copper Co. Ltd.It is a valley type tailings, which has been designed to be a third level mine, and having a capacity of m 3 and 150 meters dam height in total. Dapingzhang mine tailing is put into operation in early The initial dam is rockfill for water dam, later applied the upstream method tailing dam. In the early stage of tailing, the production scale is 1000~2000 t/d, the number of tailings particle size less than millimeter is around 75% of the total. This stage uses upstream method tailing consolidation dam by natural, the height of the dam is 30 meters (Fifteenth Level). The scale has increased to 4000 t/d after rebuilding in While this leads to rapid increases of dam construction. Traditional method is difficult to meet the needs of rising speed, wood heap dam is used to solve the problem, and the dam height is 13 meters. Although this can temporarily solve the problem of rising speed of dam, it caused another problem of slowing down tailing consolidation and higher line elevation of the saturation. The number of tailings particle size which less than millimeter is increased to more than 80% for the reasons of better mineral processing in As a result, wood heap dam has the shortages of ensuring dam rising velocity matching with plant tail, which further highlights the difficulties of dam construction. Overall problems of the Dapingzhang mine tailings can be summarized as follows: (1) difficulties in of dam constructions for the reasons of smaller average size of tailing dam and fast-rising of the sub dam. (2) Poor stability of the dam, since the short dry beach length, small sedimentary slope, and small Safe Super elevation and high line elevation of the saturation ENGINEERING PRACTICE AND APPLICATION EFFECT According to the Dapingzhang tailings sand grain size distribution curve and the field test, mold bag method is selected by aperture of mm dam heap and implemented by dam process according to the specific process (bag placement tailings slurry transportation bag squeezing consolidation bag body staggered rollout of dam seepage drainage measures and reinforcement measures). From December, 2012 to January, 2014, the Dapingzhang tailings has piled up fifth level mould bag dam, actual height of bag method is 20 meters; based on ensuring safety of the dams, which is better meet the rising speed of production requirements. Accumulation status of the dam bag method is shown in Figure

6 Fig.1 Accumulation status of the dam bag method Érection de barrage par la méthode des sacs de moulage 4.3. STABILITY CALCULATION Dapingzhang Tailings pond is the first heightening and expansion project using the technology of mold bag damming in China. Numerical models of the tailings dam heightened using different technologies (mold bag damming and upstream method-tailing) is shown in Figures 2 and 3. Fig. 2 The numerical model of a dam heightened using mold bag technology Modèle numérique d élévation de barrage grâce à la technologie des sacs de moulage Fig. 3 The numerical model of a dam heightened using the upstream tailing method Modèle numérique d élévation de barrage par la méthode des stériles en amont 108

7 The materials of the dam include mold bags, tail silty-sand, tail silt, tail silty-clay and tail clay are shown in Table 1. Table 1 The mechanics parameters of materials of tailing dam materials natural saturated horizontal k x vertical k y shear strength density density (cm/s) (cm/s) C(kPa) F( ) mold bags tail silty-sand tail silt tail silty-clay tail clay the initial dam There are two conditions to be considered: normal condition and flooding condition. According to the design code for tailings facility system and the result of the flood routing, the dry beach length in normal condition was identified in 200 meters and the dry beach length in flood condition is identified in 70 meters. And the corresponding water levels depend on the slope of deposited beach. Boundary conditions: bottom is settled as impervious boundary, upstream and downstream slope are settled as head boundaries, downstream slope of the initial dam is settled as free spill boundary. The calculation result of seepage field is shown as in Figures 4 and 5. Fig. 4 The saturation line of the tailing dam in normal conditions Ligne de saturation du barrage de stériles en conditions normales 109

8 Fig. 5 The saturation line of the tailing dam in flood conditions Ligne de saturation du barrage de stériles en conditions de crue The whole stability of the tailing dam means by using mold bag and sliding of whole tailing, calculate the slip surfaces and the tail silty clay and the tail of clay, applied simplified Bishop method of the conventional limit equilibrium method to get a stability calculation. As a result, the same mechanics parameters in two calculation models are the same. And the different stability safety factors between heightened using mold bag technology and upstream method-tailing are compared in Table 2. Table 2 Stability safety factors of the tailing dam normal condition flood condition mold bag technology upstream method-tailing It is obverse from the above analysis of stability, in the late of heightening the tailings dam which built by the method of mold bag, it is limited relative to the total height of the dam. The calculation results showed that the most dangerous slip line of the tailings dam does not pass through the mold bag. Furthermore the mold bag method can take fine particle size tailings for filling the dam. The average width of the bottom of the sub dam built by the mold bag method is 42 meters, in result, each partition line of tailing moves backward. The mechanical parameters of the tailings in the region of the slip bands across are improved, so the tailings dam built by the method of mold bags is a litter higher than the usual method of upstream. 5. CONCLUSIONS The author used mold bag method. according to the problems in damming with fine tailings. Mold bag cloth has characteristics such as permeable 110

9 impervious pulp, rapid drainage consolidation, beneficial to fast damming, high strength that damming with it and making tailings became a whole. Besides with water seepage drainage and reinforced engineering measures, mold bag method is a new way to solve fine tailings damming problems. This method has prodigious popularization value. According to the engineering practice of mold bag method, the following cognations are obtained: (1) Compared with usual backfilling dam directly, mold bag damming has the advantage of the damming with high intensity, fast drainage consolidation, technology maturity, Simple operation, and simultaneous production and damming, no effect to production, which is specially applied to solve the problem with the slow consolidating velocity of tailings Dam made of Fine Tailings and the difficulty. (2) To tailings dam made of fine tailings, the effectiveness of drainage installations is also critical to dam stability. Using suitable drainage installations to keep the seepage line in a low level is important to protect the safety of dam in the working process. (3) Mold bag damming process method can be extended the application to similar mine tailings ponds to solve problems such as difficult later damming and consolidation, short dry beach length and closure of tailings ponds engineering. The method has great economic and social benefits. REFERENCES [1] CHEN SHOUYI. Some Superficial Views on the Problem of Building Fine Grain Tailings Fill Dams by Means of Up-stream Method. Rock and Soil Mechanics, 1995(3), [2] ZHOU HANMIN. New technology and development of partial fine-grained tailings dam. Nonferrous Metals (Mining Section), 2011(5), 1-3, 7. [3] ZHOU HANMIN, LIU XIAOFEI, CUI XUAN. New rapid method of damming with partial fine tailings. Modern Mining, 2011(11), [4] WRITING GROUP OF TAILINGS FACILITY DESIGN REFERENCE. Tailings facility design reference, Beijing: Metallurgical Industry Press, [5] EDITORIAL BOARD OF INTRODUCTION TO CHINA NONFERROUS METALS TAILINGS. Introduction to China nonferrous Metals Tailings, Beijing: China Nonferrous Metals Industry Corporation, [6] SUN GUOWEN, YU GUO, YIN GUANGZHI. Factors affecting the stability of fine tailings dam safety and countermeasures. Mining safety & Environmental Protection, 2006(33),

10 SUMMARY This paper analyzes the characteristics of fine tailings along with the difficulties of dam construction with these fine tailings. A new method called the mold bag method, suitable for fine tailings dams, is proposed. The scope of application and the mechanism of the mold bag method are described. Project examples and computational analysis are used to demonstrate the effectiveness of the method. RÉSUMÉ Cet article analyse les caractéristiques des stériles fins et les difficultés de construction de barrages avec ces stériles. Une nouvelle méthode appelée méthode des sacs de moulage, appropriée pour les barrages de stériles fins est proposée. Le champ d'application et le mécanisme de la méthode des sacs de moulage sont décrits. Des exemples de projets et leur analyse informatique sont utilisés pour démontrer l'efficacité de la méthode. 112