To achieve the mining production capacity of 15 million tons per annum for the transition of open-pit to underground mining project of Sijiaying Iron Mine, the vertical zonal mining method was adopted, and a horizontal isolated pillar was required between the upper and lower mining areas. For improving the overall stability of the mining area, FLAC3D numerical simulation method was used to investigate the evolution characteristics of the stress field, the development law of deformation and failure, and the influence of the horizontal isolation pillar thickness on the surface settlement deformation of the open pit under five different thicknesses. Finally, the safety thickness of horizontal isolated pillar in this mine was determined to be 18 m. The results are concluded as follows. Firstly, with the increase of the thickness of the horizontal isolation pillar, the concentrated stress in the pillar decreases significantly, the distribution of the maximum principal stress tends to be uniform, and the distribution range of tensile stress decreases significantly, mainly in the stope part of the mining panel. Secondly, in the horizontal isolated pillar, the displacement presents an obvious nonlinear decreasing change law with the increase of the pillar thickness. The larger displacement occurs at the position where the pillar is close to the hanging wall of the orebody. When the pillar thickness is 18 m, the maximum displacement is 36.2 mm. Thirdly, the volume of the plastic zone in the pillar shows a nonlinear decreasing trend as the thickness of the horizontal isolation pillar increases. When the pillar thickness is 16 m, the plastic zone is no longer connected. In comparison, when the pillar thickness reaches 18 m, only a small amount of plastic failure occurs locally, and the isolated pillar is in a safe state. Finally, the deformation and settlement amount of the open pit surface gradually decreases with the increase of the thickness of the horizontal isolated pillar, and the position where the maximum displacement occurs is the same as the one where the maximum displacement occurs in the isolated pillar. When the pillar thickness is 18 m, the surface settlement amount of the open pit is 10.8 mm, indicating a good stability.

To address the problems of the determination of the safe thickness and stability of isolated pillars during open-pit to underground mining, taking a domestic metal mine with open-pit to underground mining as the engineering background, dynamic simulation and comparative analysis were carried out on the stopes under the conditions of different isolated pillar thicknesses and different stoping sequences based on the particle safety factor calculation method derived from the twin-shear unified strength theory and numerical simulation. The results are shown as follows. Firstly, when the mining room width is 20 m, the optimal safe thickness of the isolated pillar in the mining room is 39.37 m. At this time, the minimum value of the particle safety factor at the bottom surface of the isolated pillar is 1, which meets the engineering safety requirements. Considering the actual engineering conditions, the isolated pillar thickness is determined to be 40 m. Secondly, the particle safety factor at the bottom surface of the isolated pillar shows a nonlinear growth trend with the increase of thickness, with similar spatial distribution characteristics: the safety factor in the contact area with the hanging wall surrounding rock is lower than that of the footwall, the safety factor at the center of the bottom edge in the width direction of the roof is lower than that at both ends, and the safety factor shows an increasing trend from west to east in the length direction of the roof. Thirdly, mining disturbance leads to the deterioration of the stability of isolated pillars. As the exposed area of the goaf increases, the dangerous range expands accordingly. The closer the particles are to the goaf, the smaller their particle safety factor will be, and the dangerous area migrates from the center of the bottom surface to the midpoint of the long side with the advance of stoping. The proposed thickness optimization method for isolated pillars based on the critical safe thickness threshold and dynamic risk evolution can provide a reference for the problem of determining the safe thickness of isolated pillars in open-pit to underground mining mines.

In response to the mining technical problems of high-grade pillars caused by insufficient preparation in mining levels and stopes, as well as significant fluctuations in ore output grade in a copper-nickel mine, three mining methods were proposed, namely panel mechanized upward drift filling mining method, downward drift filling mining method, and combined mining method of upward drift and strip medium-length hole. The panel mechanized upward drift filling mining method was determined to be the optimal solution with comparative analysis of the advantages, disadvantages, technical and economic indexes of these methods. The classical Janssen theory of granular pressure was used to calculate the overburden load of pillars. The mechanical models of simply supported beam and soft support weak plate were adopted to analyze the relationship between the thickness of artificial false roof and the maximum compressive stress. By comprehensively considering the construction form of artificial false roof and the mechanical properties of cementitious materials, the filling and construction parameters of artificial false roof were determined as follows: the thickness of the artificial false roof is 7.0 m, and the 28-day strength of the filling body is 4.5 MPa. To address the quality, safety and cost problems in the construction of artificial false roof, by optimizing the construction plan, the Optimization Scheme Ⅰ was confirmed as the optimal solution, that is, the artificial false roof serves as the direct roof of the 10 m roof pillar in the lower stope. The artificial false roof adopted a reinforcement design scheme of bottom reinforcement, waist reinforcement and top reinforcement combined. Application results show that the proposed pillar mining scheme is practical and feasible. The panel production capacity of pillar stopes increases to 276 t/d, the pillar recovery rate reaches 82%, and the stope ground pressure activities remain stable. The construction technology of artificial false roof is highly operable, safe and reliable. This scheme has realized the safe and efficient recovery of pillar resources, providing technical reference for pillar mining in similar mines.

Taking a copper mine as the engineering background, optimization research on the parameters of stope mining drifts was carried out to realize the safe and efficient mining of horizontal pillars in underground mines and ensure the stability of stopes. Based on the structural mechanical model of simply supported beam, the law of tensile stress of drift roof varying with drift width was analyzed, and the drift width and reserved roof thickness were initially determined. Further, FLAC3D numerical simulation was used to compare the influence of drift width on the overall stability of stope under different reserved roof thicknesses. The results show that based on the structural mechanical model of simply supported beam, the optimized structural parameters of the drift are a reserved roof thickness of 2 m and a drift width of 4 m. Taking Scheme 3 (drift width of 3.5 m, drift height of 4 m) as an example, theoretical calculation shows that the maximum tensile stress of its roof is 0.37 MPa, and the safety coefficient reaches 1.86.In the numerical simulation, the maximum tensile stress is 0.49 MPa, the stope roof displacement is small, the roof basically does not undergo plastic failure, and the structure remains stable. The method combining the simply supported beam theory and numerical simulation can accurately predict the load changes and stress distribution in the mining of horizontal pillars, and provide a basis for the optimization of structural parameters of mining drift for similar mine projects.

The strength design of filling body is often determined by indoor proportioning test or engineering experience method, but the influences of stope structure parameters such as stope height, stope length and stope width on the strength of required filling body are ignored. Taking a copper mine as the engineering background, based on the L-L strength analysis method, the theoretical strength values of filling body in one-step stope and two-step stope under different structural parameters were calculated by orthogonal tests. The results show that the influence weights of stope structure parameters on the theoretical strength of filling body are ordered as follows: stope height>stope width>stope length. The strength prediction model of filling body in one-step stope and two-step stope was constructed, combined with the existing filling technology, the stope structure parameters were optimized reversely. Finally, the optimal stope structure parameters are recommended as follows: stope height of 5-25 m, stope width of 9-12.5 m, and stope length of 80-120 m. The structural parameters of the stope have been used for on-site production, with strictly controlling the construction specifications of the stope, as well as eliminating the phenomenon of over-excavation and under-excavation. The practical results show that the collapse phenomenon caused by unreasonable structural parameters of the stope and insufficient strength of the filling body basically disappears. The research results can provide reference for the theoretical strength prediction of filling body and the optimization of stope structure parameters.

In order to study the competition mechanism of particle migration and settlement behavior in the process of pipeline transportation of unclassified tailings paste slurry, Electrical Resistance Tomography (ERT) was used to carry out pipeline transportation tests on paste slurry with mass concentration of 60% and coarse particle content of 0, 5%, 10% and 15%, respectively. Based on the Maxwell equation, the conductivity-mass concentration mapping model was established. The plunger flow structure was reconstructed by the conductivity distribution, and the regression equation of the mass concentration and the coarse particle content in the plunger area was established. The results show that the conductivity and slurry mass concentration in the central area of the pipeline are higher than those in the side wall area, and the particle migration behavior forms a typical plunger flow structure. With the increase of coarse particle content, there are "optimal point" (4.3%) and "equilibrium point" (9.6%) of coarse particle content. When the content of coarse particles is 4.3%, the particle migration effect is the best, forming a stable particle structure flow of "fine particles carrying coarse particles". When the content of coarse particles exceeds 9.6%, the settlement effect begins to dominate, and the accumulation of particles at the bottom of the pipeline increases significantly. The ERT technology has successfully realized the dynamic visualization of the concentration field of paste pipeline transportation, revealed the dynamic balance law of particle migration-settlement, improved the stability of "plunger structure" and reduced the risk of plugging, which has a reference value for the safe transportation of paste filling.

In order to achieve efficient and stable continuous preparation of filling slurry, the computational fluid dynamics numerical simulation method was used to compare and analyze the influences of traditional rigid paddle and flexible-guide composite paddles on the flow field structure, velocity distribution and mixing characteristics of multi-state materials during continuous stirring. The action mechanism of the flexible-guide composite paddles on the mixing uniformity, mixing time and energy consumption of the slurry was explored. The technical superiority of the flexible-guide composite paddles in the homogenization stirring of multi-state materials was verified by laboratory tests and industrial applications were carried out. The results show that the flexible-guide composite paddles significantly expand the turbulent kinetic energy distribution range of the slurry through elastic deformation and rigid-flexible coupling, form an axial flow-radial diffusion stirring flow field, and realize the wide-area dispersion of multi-state materials. Compared with the traditional rigid paddles, the particle dispersion efficiency under the flexible-guide composite paddles is increased by 51.8%. Under the equivalent mixing effect, the power of the flexible-guide composite paddles is reduced by 15.3%. Under the equivalent power, the stirring effect of the flexible-guide composite paddles is significantly better than that of the traditional rigid paddles. At present, the flexible-guide composite paddles has been successfully applied to mine production, and the slurry homogenization effect after stirring is excellent. The mass concentration of slurry is stable at 70%, and the fluctuation range is less than 1%. The research can provide a new method for the preparation of fine-grained slurry.

To reveal the synergistic mechanism of mechanical vibration on tailings thickening and sand discharge, based on the mechanism of tailings slurry vibration thickening, an experimental system for vibration thickening independently developed was used to conduct discontinuous thickening and sand discharge experiments under vibration. The technical feasibility of mechanical vibration to improve tailings slurry thickening efficiency and promote underflow activation and sand discharge was verified. The results show that the accumulation of tailings particles in a conical structure within the thickener is the key to improving the mass concentration of the underflow. The eccentric vibration mechanism transmits energy in a sinusoidal harmonic motion, and the excitation range of the vibrating rod is related to the slurry density and vibrator mass. A cylindrical activation region is formed around the vibrating rod during vibration, with a larger radius when the yield stress is lower and the plastic viscosity is higher. Compared with the no-vibration condition, with the increase of feeding velocity under vibration, the underflow mass concentration is increased by an average of 7.74%, the solid content of overflow water is increased by an average of 10.11%, while the increase of the overflow water rising velocity is not obvious. The research results provide a basis for the development of mechanical vibration-assisted high-efficiency thickening equipment.

The security of shaft safety pillar is an important guarantee for safe mining in mines. The ground pressure activities in the mining area around the safety pillar of a gold mine were investigated and analyzed. Combined with the main risk factors, a security evaluation model considering multiple influencing factors was proposed, and the current safety evaluation of the mining area around the shaft safety pillar was evaluated. The research shows that the shallow ground pressure activity is caused by the combined action of geological factors and mining disturbance. The deep local deformation and failure are caused by the comprehensive influences of mining depth, in-situ stress increase and geological factors. The security of the safety pillar is affected by three types of factors, such as natural conditions, mining disturbance and production management, involving 11 influencing factors. The security evaluation of 41 stopes show that the safety level of most stopes is grade II, and the overall safety situation is good. The occurrence conditions of ore bodies, geological structure, mining methods and position of shaft safety pillars are the main factors affecting the security of pillars. In addition, stoping areas farther away from the shaft safety pillars exert less mining disturbance on the shaft zone, resulting in higher security of the safety pillars.

Taking Qidashan open-pit stope as the research object, field investigations were carried out. Fixed-point and vehicle-mounted mobile continuous monitoring was conducted during the drilling, blasting, loading and transportation stages, with meteorological parameters recorded and the dust migration process photographed. Based on the field monitoring data, an analysis was conducted on the dust dispersion characteristics and influence mechanism in different stages during the production process of the open-pit mine. The results show that there is a significant correlation between PM_2.5 and PM₁₀ concentrations. The cubic polynomial curve model has the highest goodness of fit, with a coefficient of determination of 0.945, and exhibits the strongest explanatory power for this relationship. The average dust emission concentrations and diffusion ranges differ significantly among different production stages. The blasting stage has the highest dust emission concentration in a single operation, reaching 324.4 μg/m³, and its dust diffusion range is the widest. The dust emission concentrations of a single operation in the drilling, loading and transportation stages are 59.8 μg/m³, 88.2 μg/m³ and 61.8 μg/m³, respectively. In addition, the dust concentration in the pit bottom area is significantly higher than that at the bench crest, with its mean value being approximately five times that of the dust concentration at the bench crest. The research results reveal the variation laws of dust concentration in open-pit stopes and can provide theoretical support for dust prevention and control.