Open pit mine occupies a large proportion in the country's iron ore mining industry, the early part of this century, open-air production accounts for about 80% of underground production capacity, but with the sharp rise in nearly a decade on Mining, Open Pit Mine is currently a lot of superficial The resources are exhausted and urgently need to be transferred to underground production.
It is estimated that around 2025, the existing open-pit iron ore production capacity will disappear by about 50%. The open-pit to underground mining project is not a simple conversion method of production mode. From the technical point of view, it involves two sets of production processes in the open and underground, and also takes into account the interconnection of the two. The engineering problems are cumbersome, and the transition process is often There are many complicated security issues involved.
The open-air to underground cover is a safety isolation layer between the open pit and the underground stop. It is a safety engineering facility that must be installed in the mine that uses the caving method in the underground process. It can prevent the slope from colliding. Pressure, block water penetration, reduce air leakage, prevent cold and keep warm, prevent downhole debris flow and provide mining and ore mining conditions, etc., to ensure the safety and stability of the transition to the underground transition period. In addition, because the overburden layer is directly overlaid on the underground ore, there is a close relationship between the fluidity, mechanical properties and grade changes of the ore-bearing rock mass.
1 Theoretical study on the foundation of open-pit to underground cover
1.1 The basic research of the cover layer theory The basis of the study of the cover layer theory is the study of the sublevel caving method without the bottom column. The core of the research is the theoretical study of the ore deposit theory. The theoretical study of ore-mining can be traced back to the former Soviet Union in the 1930s. At that time, it was only in the laboratory to establish a simple ore-mining model to observe the flow of caving rock, and the initial experimental conclusions of the rock flow were obtained. The prototype of the ore mining study. On the basis of the experimental conclusions, scholar И. M. In the 1950s, Marahof proposed the theory of ellipsoidal ore-mining, which provided theoretical support for the determination and selection of the structural parameters of the stope. This theory also provided a solid foundation for later scholars at home and abroad to study the theory of ore mining and geotechnical properties. . Since then, the theory of the ore-concentrating system has been continuously improved and a new theory has been proposed. In the 1960s and 1970s, the theory of the ore-out angle and the theory of the ore-leaching funnel and the theory of the triaxial ellipsoid were gradually proposed. The numerical simulation scheme was proposed by loose medium mechanics and random ore-mining theory. The theory of discrete element in the 1980s and 1990s began to be introduced into the field of ore mining research. The theory of discrete element is based on the theory of probability of particle motion, which has promoted the numerical study of the cover layer and achieved considerable The research progress; after the 21st century, the layered structure of the overlay layer is proposed. By utilizing the difference of the internal coupling mechanism of each layer, a complete overlay protection system is formed, and the upper and lower layers are combined to better form a safe and reasonable cover protection. The loose body is a new milestone in the theoretical research system of the cover layer.
The study of the overburden layer is based on an underground mine with a sublevel caving method without a bottom column, and is only applicable to mines that are mined by a sublevel caving method without a bottom column. The sublevel caving method without pillars was introduced into China in the 1960s, and has been rapidly and widely applied since its advantages of simple structure, large amount of mining, and convenient operation. Open-pit mines require underground mining methods with large amount of mining during the transition period to reduce the production caused by the reduction of open pit mining and achieve a stable production transition, while the sub-column sublevel caving method is the best solution for underground mining.
The study of the sublevel caving method without pillars is based on the theory of stochastic medium ore and the theory of ellipsoidal ore mining. The most commonly used research theory for the study of bulk fluid flow is stochastic medium theory. The theory of stochastic media is the Polish scholar J. Litwiniszyn was proposed in the 1950s when studying the movement of rock formations [1]. In China, it was introduced by Academician Liu Baozhen of the Chinese Academy of Engineering. He is an explorer and practitioner of the development of stochastic media theory in China. The principle is to simply consider the bulk as a continuous medium of continuous flow, and use the probability theory method to calculate the theoretical system of the velocity and displacement of the bulk particles during the movement.
In order to solve the problem of the functional mechanism and formation method of the open-to-ground cover, experts and scholars in the mining industry have studied the formation of the cover layer by various methods based on the theory of the dispersion dynamics. In terms of theoretical research on overburden foundation, there are mainly stratified control of North China University of Technology - the theory of high-step formation, the theory of induced landslide formation in Northeastern University, and the mechanism of prevention of debris flow in Beijing University of Science and Technology. In addition, the Maanshan Mine Research Institute and other units have also conducted a lot of research on the movement law, material composition and loss control of the cover layer.
Beijing University of Science and Technology Zhu Zhigen and Wu Aixiang based on the study of the sublevel caving method without pillars, using the theory of bulk mechanics, the indoor physical experiment of the interaction between the bulk and the caving ore of different block sizes. [2] Studies have shown that the size of the particle block has an important influence on the shape and size of the released ore body. The conclusion also has certain guiding significance for the site.
The Gandeqing of North China University of Science and Technology takes the Shougang Xingshan Iron Mine as the research background. From the analysis of the dynamics of the backing layer and the influencing factors of the motion state and natural classification, theoretical analysis, physical simulation test, field dump test and so on. Means, in-depth analysis of the natural grading of backfill to the underground cover, and finally verified the feasibility of using the inverted row dumping waste rock to form a cover layer with reasonable structure and thickness [3].
Du Cuifeng of Beijing University of Science and Technology conducted a physical experiment on the air leakage caused by defects in the overburden design during the underground mining of Daye Iron Mine [4]. The air leakage problem of Daye Iron Mine is very serious. Difficulties in ventilation make the underground sewage can not be discharged from the roadway and endanger underground construction personnel. In the experiment, four kinds of granular particles were used to form the covering layer, and the relationship between the porosity of the four kinds of granular grades and the height of the covering layer was studied to design the optimal bulk and cover thickness to prevent surface leakage.
In addition, a large number of studies have also studied the influence of the bulk layer on the ore-mining of the overburden layer [5-7], the study of the law of the dispersion of the bulk [8], the formation method of the reasonable cover layer [9-13], seepage ventilation The research [14-16] and the optimization study of capacity convergence and smooth transition scheme [17-19] have made basic research exploration and discovery.
1.2 Research on the migration law of open-pit to underground cover. Researchers at North China University of Science and Technology have carried out a lot of research work in the open-pit to underground mining process, and proposed and developed basic theories such as open-pit to underground joint mining methods. Process technology. In the early research, Gan Deqing and his research team combined with the engineering practice of mines such as Shirengou, conducted some preliminary explorations and experiments on the functional mechanism and structural thickness determination of the overburden, through computer simulation, physical experiments and on-site. Experiments were carried out to observe the movement law of the ore-rock interface along the ore deposit (Fig. 1), and based on the results of physical simulation experiments, the overburden leakage model was preliminarily constructed. These early researches explored the cover layer proposed later. The hierarchical control theory provides the basis.
During the “Eleventh Five-Year†National Science and Technology Support Program, “Study on the Key Technologies of Safe and Efficient Mining in Open-pit to Undergroundâ€, the research project will use the Shougang Xingshan Iron Mine as a demonstration mine to cover the underground. A large number of experimental studies have been carried out on the transport law of the layers. Based on the stochastic medium ore-mining theory, Professor Gan Deqing built a similar simulation platform and numerical simulation model. The project starts from the open-to-subsurface coverage safety function requirements and structural characteristics, and studies the overall movement law of the overlay and its movement process. Structural changes (Figure 2). The experimental results show that the migration process of the cover layer is a gradual process of releasing the ellipsoid and the loose ellipsoid. Under the premise of uniform mining, the cover layer migration has a layered law, that is, the cover layer in the loose ellipsoid It is an unstable structural layer (flowing layer); the covering layer outside the influence of the loose ellipsoid is a stable structural layer (the overall downward moving layer), as shown in FIG. The particle moving speed of the ellipsoidal inner cover layer has a pulsed characteristic. The fine particle migration speed is greater than the coarse particle velocity, and the rapid migration of the fine particles downward is the main reason for the lead depletion.
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