J. Cent. South Univ. (2018) 25: 1813-1824

DOI: https://doi.org/10.1007/s11771-018-3871-z

Opencast to underground iron ore mining method

FAN Xiao-ming(范晓明)^{1, 2}, REN Feng-yu(任凤玉)¹, XIAO Dong(肖冬)¹, MAO Ya-chun(毛亚纯)¹

1. College of Resources and Civil Engineering, Northeastern University, Shenyang 110004, China;

2. Anqian Mining Corporation., Ltd., Anshan Iron and Steel Group, Anshan 114043, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract: At the end of the open-pit mining process in large metal mines, the mining model must change from open-pit mining to underground mining, but the mutual interference between the two mining models leads to poor production safety conditions and difficulties in production convergence during the transition period. To solve these technical problems of poor production safety conditions and difficulties in production convergence during the transition period, in this study, based on the case of the Dagu Mountain Mine, a new transition mode of wedge switching for collaborative mining is proposed and established, which is suitable for collaborative mining. This new mining process completely eliminates the boundary pillar and the artificial covering layer, combining the technology of the mining-induced caving method and the technology of deep mining at the bottom of the open-pit. The results show that 1) the optimization of the open-pit boundary reduces the amount of rock stripping, and 2) it achieves a stable transition of collaborative mining capacity. The study shows that the proposed method uses the technologies of the mining-induced caving method in underground mining and deep mining at the bottom of the open pit in open-pit mining, and the method then optimizes the open-pit mining in detail by comparing the advantages of open-pit mining and underground mining. This study provides true and accurate technical support for the transition from open-pit mining to underground mining.

Key words: opencast-to-underground mining; transition mode; wedge-shaped transition mode

Cite this article as: FAN Xiao-ming, REN Feng-yu, XIAO Dong, MAO Ya-chun. Opencast to underground iron ore mining method [J]. Journal of Central South University, 2018, 25(7): 1813-1824. DOI: https://doi.org/10.1007/ s11771-018-3871-z.

1 Introduction

The iron ore deposit at Dagu Mountain is in the last phase of opencast mining and is in urgent need of transforming into an underground mining operation. During the transition period from opencast mining to underground mining, as underground mining changes the mining environment from the open-air ore body while opencast mining limits the time-space conditions of underground mining, these methods conflict with each other. Therefore, poor safety production conditions and difficult capacity connections prevail during opencast-to-underground mining processes in domestic and overseas large-scale metal mines. To solve this problem, many domestic and overseas scholars have carried out in-depth research. These researchers provide theoretical methods and relatively systematic process technologies for opencast-to-underground mining transitions. However, because these methods fail to address the mutual interference between opencast mining and underground mining during the opencast-to-underground mining process, they fail to radically solve technical problems like poor safety production conditions and difficult yield connections.Considering the conditions of iron ore deposits in Dagu Mountain, this paper analyzes four kinds of transition modes. Finally, it focuses on the wedge- shaped transition mode and then builds a combined opencast-to-underground mining mode during the transition period from opencast mining to underground mining at Dagu Mountain. Theoretical research and production practices indicated that this mode could solve the problems of poor safety production conditions and production contraction in the conventional transition mode and could realize production increases from the opencast-to- underground mining of iron ore at Dagu Mountain.

2 State and future development tendency

To address the problems of poor safety production conditions and difficult capacity connections during opencast-to-underground mining processes in large-scale metal mines, many scholars have proposed research methods and achieved good results, including mining modes and technology [1]; the slope monitoring method and stability analysis [2–4]; the structural parameters of the slope [5–8]; the stability of boundary pillars [9–11]; reasonable thickness of the boundary pillar [12]; recovery sequences [13]; comprehensive mining and production capacity [14, 15] rock body stability [16–18]; and the feasibility of combined opencast and underground mining [19–21].

The slope stability of a mine transitioned from opencast to underground mining is a well-known problem. HAN et al [2] proposed a numerical simulation and sensitivity analysis of slope stability in Ref. [2], which implements a sensitivity analysis of the main stress and strain indexes affecting the slope stability by using a variance analysis based on the orthogonal table and based on the rational room structural parameters that can deduce and ensure that the slope stability is determined and preliminarily used in the mining design.

ZHAO et al [10] proposed the method to ensure the stability of boundary pillars in the transition from open pit to underground mining. Based on the various parameters of the mine, which include the height of back-filled materials, thickness of the ore body, height of the boundary pillar, dipping angle of the ore body and water pressure, the authors used the limit equilibrium method to calculate all the safety factors for all the pillars. This method provides a useful standard for boundary pillar and stope design in a mine as it transits from an open pit to an underground mine and implements these measures into practice. VYAZMENSKY et al [19], using a combined FEM/DEM-DFN (discrete fracture network) modeling, highlighted the importance of rock mass tensile strength and its influence on caving-induced slope response. BAKHTAVAR et al [21] studied the simultaneous and non-simultaneous exploitation of opencast and underground mines.

Recently, the wedge-shaped transition mode has been proposed in China [2, 3]. This mode organically combines the hanging-wall ore induction caving mining technique and the steep- stope deepening mining technique of ore at the open pit bottom, cancels out the technologies of reserving boundary pillars and man-made formations of covering layers in the traditional mode, and thus improves opencast and underground production capacity by a large margin.

The remainder of this paper is organized as follows. Section 3 introduces the boundary pillar transition mode, covering layer transition mode, three-layer transition mode, wedge-shaped transition mode, and basic conditions for highly efficient mining during the transition period and finally establishes the wedge-shaped transition mode. Section 4 introduces the simulation results analysis and practical application prospects of the wedge-shaped transition mode. Section 5 introduces the research conclusion.

3 Methodology

The iron ore deposit in Dagu Mountain, located at the northwestern margin of the Qianshan Mountain range, belongs to hilly terrain including two ore districts—Dagu Mountain and Xiaogu Mountain. The stratum of the latter is located in chlorite quartzite schist at the handing wall of the ore body of the former, and the closest distance from the Dagu Mountain ore body to the Xiaogu Mountain ore body is only 100 m. At present, the two stopes use the same opencast mining model. The open mining area has been mined to 310 m below the ground with production capacity reaching 3 million t/a; the Xiaogu Mountain mining area has reached the 150 m level with production capacity reaching 3 to 3.5 million t/a and total production capacity in the mining area reaching 6 to 6.5 million t/a. After detailed boundary optimization, the Dagu Mountain mining area has reached the 474 m level, while the Xiaogu Mountain mining area has reached the 282 m level. There are still 29 million tons of reserves remaining in the Dagu Mountain mining area within the boundary, and it can be mined for another 13 years when calculated according to 3 million t/a. There are 9.7 million tons of reserves remaining in the Xiagu Mountain mining area, and it can be mined for another 3.2 years when calculated according to 3 million t/a. There are 16.1 million tons of geological reserves beyond the boundary in the Dagu Mountain mining area, while there are 36.8 tons of geological reserves beyond the boundary in the Xiaogu Mountain mining area, and both require opencast- to-underground mining. For metal mines under opencast-to-underground mining, the selectable transition modes at present are mainly the boundary pillar transition mode, covering layer transition mode, transition mode set with transition layer and wedge-shaped transition mode, which has been proposed in recent years, although the applicability to Dagu Mountain iron ore is different.

3.1 Boundary pillar transition mode

The transition period from opencast-to- underground mining is a period of simultaneous opencast and underground mining. To ensure the safety of opencast and underground mining, the transition mode with reservation or man-made construction of boundary pillars is often used both at home and abroad, namely, reserving or building separating pillars (abbreviated as boundary pillars) with a certain thickness at the final open boundary, which will separate the space of open production from underground production (Figure 1) to eliminate mutual interference of their mining spaces.

Because the boundary pillar transition mining method is simple, practical and applicable to all kinds of ore body conditions, it has been extensively applied in domestic and overseas metal mines. However, this mode needs to protect the stability of boundary pillars, so the open-stope method or filling method are generally used to mine ore bodies under boundary pillars. The shrinkage mining method is the most frequently used; it temporarily retains ore inside the stope, which are used to support the studding and surrounding rocks of ore blocks and maintain stability of the boundary pillars. After the completion of opencast mining or boundary pillar recovery, the reserved ore inside the stope will be removed. However, because opencast mining interacts with underground mining, neither can use an optimal mining method. In the meantime, because the boundary pillar separates the continuity of the ore body, it will usually separate hanging- wall ore bodies from the main ore body at the lower part, and only after recovery of the hanging-wall ore bodies can the main ore body at the lower part be mined. In this way, the hanging-wall ore body can only be taken as small minor ore bodies to be independently recovered. It is difficult to reach optimum production efficiency when large ore bodies are being mined. Regardless, expansion-wall mining is conducted with the opencast mining method, or footrill excavation mining is conducted with the underground mining method. Moreover, a suitable mining method cannot be used for underground ore bodies during the transition period, which will surely give rise to problems of low mining efficiency and insufficient ore production. On the other hand, as boundary pillars constitute a relatively small layer of isolated ore bodies separated from large ore bodies with small ore body thickness, they are not conducive to large-scale mining. In the meantime, in view of the influence of factors such as mining and ore rock weakening in the lower stope on stability, construction safety will be reduced.

Figure 1 Traditional boundary pillar transition mode

Recovery methods used in boundary pillars mainly include 1) the medium-length hole one-time caving method, which conducts tunneling and drilling at the footwall of ore bodies and then uses medium-length hole concentrated break blasting, as shown in Figure 2(a). This method must dig specialized rock drilling projects and channels, which increases the amount of work of preliminary mining. Moreover, the footwall tunnel for drilling in the neighboring dead zone will be influenced by the underneath dead zone, so it will lower the safety levels of tunneling, drilling and blasting work. In addition, because of the tunneling work of drilling tunnels and footrills, as well as drilling, blasting and combined recovery procedures, the recovery efficiency is always low, and this will seriously affect the production of underground mining. For the method that uses reserved ores inside the stope as a working platform, drills medium-length holes upward and uses cut-raise blasting constructed in advance, as shown in Figure 2(b), both cut-raise and medium-length hole construction will weaken the roof strength, thus leading to poor safety conditions and low efficiency. 2) The shallow hole expanding hopper recovery method drills upward from the stope formed by the shrinkage recovery method after opencast mining is finished. When the cut raise and open stope can pass through, it expands the cross section of the cut raise by drilling shallow holes downward from the open stope to form a hopper-shaped recovery working plane, and then recovery of boundary pillars is completed step by step. This method has disadvantages of poor construction safety conditions and low efficiency.3) The open descending and translational push recovery method first drills a filling shaft interlinked with an open stope inside the stope and then lets out ores inside the stope through the filling shaft, finally filling dead rocks into the stope through the filling shaft. After the stope is filled with dead rocks, opencast mining will conduct translational pushing in normal order, or it conducts translational pushing and recovery of boundary pillars after benches descend, as shown in Figure 2(c). This method exhibits poor safety construction conditions, increased ore dilution rates, large quantities of filling shafts, large working amounts and difficult construction.

This mode separates and diminishes the open exploitable scope of the Dagu Mountain ores and makes protecting the stability of boundary pillars an additional condition for exploitable ore bodies. During the transition period, recovery efficiency is low, boundary pillar recovery is difficult and production linkage is hard, thus seriously restricting the simultaneous opencast and underground mining production efficiency of Dagu Mountain.

Figure 2 Schematic diagram of common recovery methods of boundary pillars:

3.2 Covering layer transition mode

To solve the difficult problems in boundary pillar recovery and ensure the safety of underground mining, a covering layer transition mode is proposed in China. This mode, after opencast mining proceeds to the boundary, uses dead rock backfilling, slope surrounding rock blasting or deep-hole drilling and ore caving at the open pit bottom to form bulk solid covering layers with thicknesses no less than 40 m and then conducts underground mining under the protection of covering layers.

The advantage of the covering layer transition mode lies in that it negates the bottom boundary pillars with great recovery difficulty. In the meantime, it directly uses the highly efficient underground mining method under a covering layer, which is convenient for the recovery work of open-pit bottom ores and good for rapidly improving underground production capacity. However, this mode still requires mining hanging- wall ores first. After the mining of hanging-wall ores is finished, the underground mining method is used to mine underneath the ore bodies. The iron ore deposit in Dagu Mountain takes quite a long time to mine, regardless of whether opencast mining or underground mining or a combined method of the two is used. During this period, limited by the mining conditions of the ore body, mining efficiency will be reduced and production linkage will be difficult, so reduced production will occur during the transition period.

Figure 3 Covering layer transition mode

3.3 Three-layer transition mode

In the 1970s, former Soviet Union experts Professor JELINCHEV and Professor AGOSKOF et al proposed a three-layer transition mode—an opencast mining layer, a combined mining transition layer and an underground mining layer [22]. When opencast mining engineering reaches the final depth at one wing of a deposit, mining engineering advances towards the center ore deposit along the horizontal direction, then the open working slope will be pushed ahead for 150–200 m. A raise is drilled near the terminal of the non-working slope to connect the bottom of the open stope and horizontal underground tunnel. This raise is taken as a cut raise to form a cutting slot traversing the full-thickness of the ore body. In the transition layer, drilling-blasting is then conducted from the bottom of the open stope downward, and ores formed by blasting can be released from the underground tunnel (Figure 4).

Figure 4 Schematic diagram of three-layer transition mode

The three-layer (namely, open mining layer, combined mining transition layer and underground mining layer) transition mining mode requires long underground preparation time, and caving ores exhibit good conditions of underground recycling pace. Because the underneath ore bodies of the iron ore deposits in Dagu Mountain are gently inclined, and it is difficult to control ore loss and dilution of caving ores, these deposits do not have the spatial conditions for three-layer transition.

3.4 Wedge-shaped transition mode

To give full play to the technological characteristics of opencast and underground mining, ZHAO et al [10] proposed the wedge-shaped transition mode. This mode demarcates opencast and underground mining by slope. Open steep slope mine ore quantities at the footwall side and residual ore quantities at the hanging side are mined with the induction caving method. This mode can satisfy ore body conditions of the iron ore deposit in Dagu Mountain during the transition period very well; the mining method of the iron ore deposit in Dagu Mountain developed on the basis of this mode can expand space and time under simultaneous opencast and underground mining, radically solving difficult problems, such as poor safety production conditions and low production capacity, and providing a new highly efficient mining mode from opencast mining to underground mining.

3.5 Basic conditions for highly efficient mining during transition period

1) Opencast steep-slope mining is a transition period from opencast mining to underground mining. At this point, the open stope has entered the deep mining period. To improve the mining efficiency, it is necessary to implement steep-slope mining techniques. Steep-slope mining is mining-stripping technology used to enlarge the angle of the working slope in order to cut down the stripping ratio, reduce production costs, balance the production stripping ratio and save investment during the development process of opencast mining and stripping technology. Compared with a relatively gentle slope, the steep-slope expanding method can execute stripping operations by using methods such as combined bench and strip dividing on the condition that the working slope angle is large (25°–35°).

The combined-bench slope expanding method divides the slope expanding phase into several groups. Generally, three to six benches constitute one group, and each group consists of one working bench and several temporary non-working benches, as shown in Figure 5.

Figure 5 Schematic diagram of combined-bench

In the figure, B_p is the width of the working platform; B is the one-time pushing width of the combined bench; b is the width of the security platform; H₁ and H₂ are heights of the combined bench; and h is the bench height. A single horizontal operation from top to bottom is used within each group of benches, where when one group of benches push to scheduled temporary non-working slopes, a stripping circulation is completed. As the number of benches in the combined bench is under dynamic change, the above circulation will not be obvious, so an interlaced combination is usually implemented according to the practical situation.

Inclined-strip slope expansion divides the slope expanding area into one or several strips along the inclining direction, where only the top of one strip is reserved with a wide working platform, and the slope expanding is conducted on benches one by one from the top to bottom by using mining, loading and transporting equipment. To accelerate the descending speed of slope expansion, the tail-after operation can be used between the upper and lower benches, as shown in Figure 6.

Figure 6 Schematic diagram of slope expansion from top to bottom of tail-after operating plane

Because the transition period is short (generally two to four years), the working slope angle should be enlarged up as much as possible with steep-slope mining techniques, on the condition that stope safety is ensured according to the stability of ore rocks during the production process, to realize the mining goal of less rock stripping and more ore production.

2) Underground high-efficiency mining. Because the caving method is generally used in opencast mining during the transition period, an underground stope is gradually formed during this process. In order to improve mining efficiency, underground mining should select a high-efficiency mining method.

The domestic underground mining practice of metal ore indicates that the mining efficiency of the big structural-parameter caving method is commonly high, from the sublevel caving method to the phased natural caving method, and can be flexibly selected according to ore body conditions. In particular, the induction caving method proposed by Northeastern University, which uses an organic combination of mining technologies of ore rock caving and the sublevel caving method, has characteristics of simple structure and flexible application of the sublevel caving method as well as characteristics of ground pressure ore crushing, big mining strength and low cost of the phased natural caving method.

This method can divide the ore body into three areas according to workable conditions, namely, an induction caving area, a normal recovery area and a bottom recycling area; the stope structure is shown in Figure 7.

In the induction caving areas, only one layer of recovery access (called induction engineering) is set, and the gob provided after the recovery of this layer of access is used to induce natural caving of the upper ores. The equivalent circle area (effective induction caving area) in the gob shall not be less than the continuous caving area of the ores. In terms of medium-stable ore body, its continuous caving area is 1000–4000 m². The recovery of induction engineering mainly implements complete caving penetration of the supporting pillars between caving accesses to form a continuous recovery space and induce natural caving of the upper ores. Because the ground pressure for induction engineering mining is high, spacing of its accesses requires being appropriately enlarged when compared with that of the normal recovery area, and therefore, the net height of the gob must satisfy the requirements for caving and bulking of the upper ores.

In normal recovery areas, in addition to mining ore quantities in this sublevel, it is also necessary to receive upper caving ores. To reduce the dead rock mixing rate, structural parameters and the ore drawing mode of the stope should adapt to the flow law of ore bulk solids. In addition, the size of the access sections in the normal recovery area should not only consider the use demand for excavation equipment but also the disposing convenience of large blocks of caving ores. During the ore drawing process in this area, large blocks of ores will gather inside the mine mouth and continuously and simultaneously flow out. If there are multiple layers of access in the normal recovery area, large blocks of ores will become stuck inside the mine mouth and can be appropriately transferred to the next sublevel to be drawn out. Through the attrition crushing of 1-to-2-sublevel bulk solid movement fields, the lumpiness of large blocks of ores will be reduced along with the block rate.

Figure 7 Stope structure chart of induction caving method:

In the bottom recycling area, recovery engineering mainly recycles the residual ore quantities of the stope as well as recovers of ore quantities at the nearby baseboard (or footwall). As none of the mining ore quantities borne by all accesses in the area have conditions for transferring and recycling towards lower sublevels or continuous moving spatial conditions for receiving ore quantities transferred by the upper sublevel, the ores released through the access ports cannot be effectively mined. Therefore, it is necessary to reasonably design the location of each access and recycle the ores at each step pitch to improve the recovery rate. The arrangement form of access in this area depends on the relationship between the sublevel location as well as an economical and reasonable rock excavation height. The layout forms 1) set the compacted access as shown in Figure 8, replenish the access between the two bottom recovery accesses arranged according to a normal rhombus, of which this access is known as compacted access, thus shortening the interspacing of accesses by half. Compacted access is equivalent to elevating the recovery access of the next sublevel to the upper sublevel for arrangement. When the rock excavation height (calculated from access roof) of the recovery access of the next sublevel exceeds an economical and reasonable maximum rock excavation height, it should be elevated to the last sublevel as compacted access for arrangement. In the figure, h_j refers to an economical and reasonable maximum rock excavation height. 2) A layer of bottom recovery accesses that takes recycling residual ore bodies at the spine are arranged in the baseboard (or footwall) of the surrounding rock. When the altitude difference of the baseboard (or footwall) boundary of the ore body when related to the sublevel is large but not greater than an economical and reasonable maximum rock excavation height, or when the ore body dip angle is gentle and can be adjusted to the location of recycle engineering along the direction of the height, it is suitable for arranging the recovery access. 3) Compacted access and recycle access are used jointly, and they constitute bottom recycle engineering.

Figure 8 Schematic diagram of layout conditions for compacted and recycle accesses

When accesses are arranged, access spacing is diminished by half, which weakens the bearing capacity of studding significantly and will reduce the stability of neighboring accesses. Therefore, pressure relief should be conducted in advance, and it is necessary to dig compacted accesses after engineering recovery and pressure relief are conducted.

During the recovery process of the three areas of the induction caving method, the mining ground pressure of the induction caving area can crush ores, thus saving preliminary mining, drilling and blasting expenses and improving ore caving intensity. In the meantime, as this method can release caving ores at high quality, it can improve mining strength, reduce dead rock mixing quantity and then improve the ore recovery rate. As shown in Figure 7, experimental studies in Beiminghe iron ore and Hemushan iron ore indicate that the induction caving method has prominent advantages of big mining strength, high efficiency, low dilution ratio and strong adaptability to ore body conditions, and it is one of highest-efficiency mining methods with the most development prospects in opencast- to-underground mining.

3.6 Wedge-shaped transition mode

During the transition period of simultaneous opencast mining and underground mining, in order to improve the production capacity as much as possible, the ideal method is to completely abandon boundary pillars and man-made formations of covering layer and release the productivity of opencast mining and underground mining on the condition that basic conditions for high-efficiency opencast and underground mining are met. Therefore, it is necessary to conduct an organic combination of hanging-wall ore induction caving mining techniques and steep-stope deepening mining techniques of ore quantities at the open-pit bottom in order to build a brand-new transition mode of opencast mining and underground mining.

To ensure that both opencast mining and underground mining can realize high-efficiency mining, the open stope must maintain continuous ore body mining conditions while avoiding the depression harm of underground mining. Underground mining must have a recovery area required by induction caving while not endangering open blasting and oscillation. Hence, the open stope design can deepen along the reasonable slope angle to the recovery working plane, and the width of the mining working plane shall not be less than the minimum width of the working platform. As the underground recovery area gradually expands, it is beneficial to induce the natural caving of upper ores and the reasonable recycling of caving ore quantities. In consideration of opencast mining systems used to rapidly conduct underground exploitation and preliminary mining work, the open stope should be located at the lower part of the underground stope. The mining mode of “opencast mining is below while underground mining is upper” has been formed, but the open stope should be lower than the underground stope; thus, we propose a separated mining boundary of an inclined cutting ore body, as shown in Figure 9.

Figure 9 Schematic diagram of opencast and underground mining boundary dividing

According to the separated mining boundary in Figure 9, the width diminishes from top to bottom, while that of the underground stope enlarges. Finally, the open stope disappears, and the entire ore body will be transitioned into underground mining. During this transition mode, as the mining depth increases, the scope of open mining gradually diminishes and disappears, while the scope of underground mining will gradually expand to the entire ore body. In this way, a smooth transition from opencast mining to underground mining is realized, and we call this transition mode the wedge-shaped opencast-to-underground transition mode, which is simplified as the wedge-shaped transition mode.

The wedge-shaped transition mode eliminates the difficulties caused by boundary pillars and realizes high-efficiency hanging-wall ore induction caving mining. Bottom ore quantities can conduct deepening mining with the opencast steep-stope mining method, thus laying a foundation for simultaneous and high-efficiency opencast and underground mining during the transition period.

4 Result analysis and application

4.1 Result analysis

As shown in Figure 10, for the wedge-shaped transition mode, the open stope is adjacent to the underground stope in order to realize safe and high-efficiency mining. It is first necessary to control the direction of slope rock movement, eliminate any harm of slope rock movement and ensure opencast and underground production safety; second, high-efficiency mining process technology (open steep-slope mining, mining with underground induction caving method) should be used; third, measures should be taken to accelerate the mining progress of hanging-wall ores and rapidly form and improve underground production capacity. In this way, a combined opencast and underground mining method is established.

Because the wedge-shaped transition mode can totally negate the need for boundary pillars and man-made formations of the covering layer and simplify the constraint conditions for simultaneous opencast and underground mining, under this transition mode, the hanging wall is mined with the induction caving method. A sufficiently large collapse can be formed when the gob penetrates out of the earth’s surface by adjusting the recovery sequence and height of induction engineering to absorb bulk solids sliding from the slope. In this way, the direction of the slope rock movement can be controlled, and rocks will point at the collapse pit but not slide into the open stope. This breakthrough from traditional protection of slope stability to a slope rock movement control technique allowing slope collapse radically weakens the restrictive relation of simultaneous opencast and underground mining. In the meantime, the induction caving method is used to mine underground hanging-wall ores, steep-slope mining technology is used to mine ores at the pit bottom, and detailed boundary mining by opencast deepening is optimized through a comparison in advantages of opencast mining and underground mining technologies. In this way, the technological advantages of opencast and underground mining can be fully used to improve the mining efficiency of ore bodies during the transition period. The above advanced process technologies, when combined, can radically solve the problems of poor safe production conditions and difficult production linkage while realizing smooth transitions and production increase linkage from opencast mining to underground mining.

Figure 10 Schematic diagram of combined mining method surrounding rocks:

4.2 Application of opencast-to-underground iron ore mining method

Because the iron ore deposit in the Dagu Mountain mining occurs in the same open pit at the same time, the mining process has entered the final phase of mining and faces the technology problems of open pit mining transitioning into underground mining. Therefore, this paper addresses the difficult problems of poor safety conditions during the transition period and the production convergence in the process of open pit mining into underground mining to study the collaborative mining technology under the existing conditions of the iron ore deposit at Dagu Mountain, focusing on the problems of high stripping ratio, the small quantity of hanging ore, and the production convergence present at the independent deep mining from the 222 m level of iron ore stope in Xiaogu Mountain, which reduces the quantity of the waste iron and creates favorable conditions for high ore quantities.

Thus, we apply the SURPAC mining engineering software to establish the geological and mining model of the iron ore deposit in Dagu Mountain, analyze the technical methods of high- efficiency mining, insist on the cooperative mining principle of “open Steep Slope Mining Orebody footwall side, left the side wall of the hanging wall ore to underground induced caving mining” and propose the cooperative mining model of open pit and underground mining of the iron ore deposit in Dagu Mountain. In the later phase of the iron ore in the Dagu Mountain deep extended mining area, we apply the caving mining method to mine the Xiaogu Mountain hanging wall ore and the hanging wall ore in Dagu Mountain mining area and ensure the stability yield of the transition period. According to the ore body conditions and domestic sublevel caving production experience, the Dagu Mountain iron mine open pit to underground shows reasonable production capacity reaching 7 million t/a. The smooth transition of the capacity from 6 million t/a to approximately 7 million t/a in the underground process is given.

The economic benefits of this research are mainly reflected in the following two points: 1) The open pit boundary optimization reduces the quantity of rock stripping. According to the open pit and underground mining method of collaborative optimization of the detailed pit, Dagu Mountain decreases the stripping waste rock by 23890000 tons, and the mine at Xiaogu Mountain decreases the stripping waste rock by 7300000 tons, a total reduction of 31190000 tons of stripping rock volume. 2) Economic benefits of stable production during the transition period of collaborative mining. With the application of the collaborative mining method in Dagu Mountain iron, the yield can achieve 6 million tons of production capacity, which creates more significant economic benefits.

5 Conclusions

At the end of open-pit mining, the mining model must change from open-pit mining to underground mining, but the mutual interference between the two mining models leads to poor production safety conditions and difficulties in production convergence during the transition period. To solve these technical problems, a new transition mode of wedge switching for collaborative mining is proposed and established that combines the technology of the mining-induced caving method and the technology of deep mining at the bottom of the open-pit. The conclusions of this study are as follows: 1) the mining process completely eliminates the boundary pillar and the artificial covering layer and releases the production capacity. 2) The study combines the technology of the mining-induced caving method and the technology of deep mining at the bottom of the open-pit and implements stable capacity of the mining transition. A new transition mode of wedge switching for collaborative mining expands the production of the mining space and provides a new reference to solve the problem of poor production conditions during the transitional period. However, other factors in the mine may affect the accuracy of the simulation model. Therefore, the popularization and application of the open-pit to underground model should be based on the real-time monitoring and analysis of relevant data.

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(Edited by HE Yun-bin)

中文导读

露天与地下协同开采模式与方法研究

摘要：在大型金属矿山露天开采的末期，需要经历露天开采转地下开采的过渡时期，但两者存在着相互干扰的问题, 造成过渡期安全生产条件差和产能衔接困难。为了解决过渡期开采安全条件差和产量衔接困难等技术难题。本文以大孤山矿山为例，针对过渡期矿体条件，提出并建立了适合协同开采的楔形转接新型过渡模式，其完全取消了境界矿柱以及人工形成覆盖层的工艺，并将挂帮矿诱导冒落法开采技术与露天底部矿量陡帮延深开采技术有机结合，用于实际开采过程中。结果显示，1）露天境界细部优化减少岩石剥离量，2）实现协同开采过渡期产能稳定衔接。研究表明，该方法地下采用诱导冒落法开采挂帮矿，露天采用陡帮开采工艺开采坑底矿，按露天与地下开采工艺的优势对比优化露天延深开采的细部境界，本研究在一定程度上为过渡期矿山露天转地下开采提供了真实准确的技术支撑。

关键词：露天转地下开采；过渡模式；楔形过渡模式

Foundation item: Projects(41371437, 61473072, 61203214) supported by the National Natural Science Foundation of China; Projet(N160404008) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2017-10-12; Accepted date: 2018-01-09

Corresponding author: FAN Xiao-ming, PhD, Engineer; Tel: +86–13516023788; E-mail: fxm19860806@163.com