Paste Pipeline Transportation of Pumping Backfill Technology with Long Distance and High Stowing Gradient in Cold and High-Altitude Areas

A paste pipeline transportation of pumping backfill technology with long distance and high stowing gradient is proposed to solve the problem of filling slurry transportation with low concentration, the filling body poor quality, and the transportation difficulties with long distance and high stowing gradient in Heiniudong copper mine (HCM). The physical and chemical properties of the backfill material, backfill proportion test, circular pipe experiment, and backfill system analysis evaluation were studied in the laboratory and outdoor, and the application in HCM was carried out to evaluate the technology. The research results show the feasibility of considering classified tailings and binder as backfill aggregates, and the optimum proportion of cement-binder-classified tailings applied in the stope and goaf is 1 : 4 : 8 and 1 : 4 : 15, respectively, with paste rheological properties of mass fractions of both being 74%∼76% and the backfill strength of about 1.5 MPa at 28 d. Furthermore, when backfill proportions and rate of flow are 1 : 4 : 8 and 50 m3/h, the pressure loss of the pipeline is around 0.4 MPa/100 m, and the backfill pump meets the backfill requirements. On this condition, the technology is capable of obvious economic benefits with the backfill cost of only 25.56 yuan/t, remnant ore recovery rate of 80%, and new output value of 1.28 billion. It creates a precedent for the paste pumping backfill technology with long distance and high stowing gradient in cold and high-altitude areas. The technology also provides reference mining experience for similar mines.

Safety Analysis of Synergetic Operation of Backfilling the Open Pit Using Tailings and Excavating the Ore Deposit Underground

The transition from open pit mining to underground mining is essential for mineral resources to achieve deep excavation. Recently, cemented paste backfill (CPB) has been proposed as a novel technology to achieve open pit backfill (OPB). The proposed method not only eliminates the danger of the open-pit slope but also reduces the disposal of waste tailings. In order to ensure safe mining during the synergetic operation of OPB and underground mining, it is of great significance to improve this technology. In the present study, an open-pit metal mine in Anhui Province was taken as the research object. Then, the safety of underground stope roofs, underground backfill pillars, and open-pit slopes was evaluated during OPB. To this end, numerical simulations were performed and experiments were conducted on a similar physical model. Accordingly, the backfill mechanical parameters were optimized. The obtained results show that backfill height exerts the most significant effect on the safety of roofs and underground backfill pillars, accompanied by small displacements along the vertical direction during the backfill process. Moreover, concentration was observed at the foot of the slope, while the overall structure remained stable with no considerable displacement. The overall safety factors met the safety requirements. Based on the obtained results, the optimal foundation strength, foundation height, backfill strength and backfill height were 4 MPa, 10 m, 1.5 MPa, and 120 m, respectively. Moreover, it was concluded that displacements in the abovementioned three regions tend to be stable when the backfill height exceeds 150 m without damage. The present article provides a certain theoretical and application guideline for OPB practices in similar metal mines and suggests possibilities for cleaner production.

Development of Coal Mine Filling Paste with Certain Early Strength and Its Flow Characteristics

In coal mine paste filling technology, geomaterials like coal gangue and fly ash are used as the main component, and cement is applied as the cementing material. In the mining production, mining-and-filling is a cyclic work, where the filling immediately after mining and mining immediately after filling. Long solidification time after filling will affect mining; consequently, the paste should have early strength. In addition, the prepared paste will be conveyed to goaf through the pipeline. The paste flow characteristics will change to some extent in the conveying process, and there is uncertainty about whether the paste can meet the requirements of pumpability and strength. Therefore, the influence of pipeline conveying on flow characteristics of paste before filling the goaf should be taken into consideration. Based on the above two points, this paper studies the paste strength, backfill strength, and pumpability parameters in coal mine paste filling and determines the early and later strength of coal mine paste, as well as the pumpability parameters such as slump degree, segregation degree, setting time, and paste gradation. With the determined mass proportion of coal gangue, fly ash, and silicate cement, the orthogonal test was carried out with three factors including gypsum content, the content of early strength agent (Na2SO4), and the mass concentration, and at three levels. The factors affecting paste flow characteristics were determined by range analysis, and the factors affecting the paste’s early strength were determined by the XRD test and SEM test on its microstructure. With paste proportioning and pipeline conveying simulation system, taking slump, segregation degree, backfill strength, and other parameters as indicators, we obtain the influence law of pipeline conveying on the flow characteristics of paste. The research has great theoretical and practical significance for developing coal paste with early strength and its flow characteristics.

Sampling and Mechanical Testing of Backfill in Large Mined-Out Area

To investigate the physicomechanical properties of stope backfill and to explore the mining conditions for an adjacent pillar, four boreholes, namely, GZK1, GZK2, GZK3, and GZK4, were constructed for taking the backfill core in the test stope. During borehole sampling, it is found that the strength of backfill is usually lower than that of the rock and ordinary concrete, and its resistance to tensile and compressive loads is limited. Therefore, the drilling speed should not be too fast, and a small amount of water is needed to continue drilling smoothly. For backfill with high strength, the sampling process is relatively smooth, and the backfill samples are relatively complete. GZK1 is located on the upper part of the stope near the footwall of the orebody, and the test results show that the backfill quality of this part is poor; thus, a complete backfill core cannot be obtained. GZK2 is located at the bottom of the stope close to the footwall of the orebody, GZK3 is located at the bottom of the stope close to the hanging wall of the orebody, and GZK4 is located at the top of the stope close to the hanging wall of the orebody. The average compressive strength and average tensile strength of the backfill samples obtained from the three boreholes, namely, GZK2, GZK3, and GZK4, are 2.928 to 3.583 MPa and 0.328 to 0.523 MPa, respectively, indicating that the backfill near the upper part and bottom close to the hanging wall of the orebody is good, while the backfill near the upper part close to the footwall of the orebody is poor. Special attention should be paid to the backfill with the range of GZK1 in the future second-step pillar mining process, and the sublevel method can be adopted to ensure the safety of the mining process. The backfill samples in the large goaf of No.17 room were obtained by geological drilling. Segregation occurred in the upper part of the No.17 room near the area of the footwall. The concentration and flow rate of the filling slurry were reasonably adjusted and controlled with the improvement of backfill quality. Therefore, the backfill strength of the No.17 room is generally good, which can meet the requirements of pillar mining, and also creates a good condition for the resource utilization of waste tailings of Caolou Iron Mine.

Influencing Factors on Strength of Waste Rock Tailing Cemented Backfill

Tailing cement filling is an important development direction in mine filling, as it is a green and environmentally friendly method for efficiently treating solid waste in mines. Adding a certain amount of waste rock can effectively improve the backfill strength and better meet the filling strength requirements. To address the use of waste rock tailings in cemented filling materials, a uniaxial compression test was carried out on backfills with different cement/sand ratios and waste rock contents, and the influence of the cement/sand ratio and waste rock content on the strength of the backfill was studied. This study found that when the waste rock content is certain, the strength of the backfill increases with the increase in the cement/sand ratio, and the increase in strength slows with the increase in the cement/sand ratio until the strength of the backfill reaches a limit and no longer increases. When the cement/sand ratio is constant, the strength of the backfill first increases and then decreases as the waste rock content increases. When the cement content is constant, the addition of a certain amount of waste rock reduces the specific surface area of the solid materials in the backfill, increases the amount of cement per unit area, and improves the strength of the backfill. When the waste rock content is too high, due to the large particle size of the waste rock, the tailings cannot completely wrap around the waste rock, resulting in a weakening of the cement in the backfill, which reduces the strength of the backfill. This study found that the waste rock content and the cement/sand ratio in the backfill have a significant impact on backfill damage. The damage is mainly caused by insufficient cement strength. The presence of waste rock will change the original direction of crack propagation, resulting in more crack bifurcation, and the form of the destruction surface on the backfill is complicated and diverse.

Numerical investigation of the geomechanical response of adjacent backfilled stopes

Backfilling of mine stopes helps provide a safe workplace underground. The interaction between the backfill and surrounding rock mass has to be evaluated to ensure the secure application of backfill. This critical issue has led to much research on the stress state in single (isolated) backfilled stopes. However, the stress distribution in multiple openings that interact with each other has not yet been investigated as thoroughly. In this paper, the authors are using numerical simulations to evaluate the response of two adjacent backfilled stopes created in sequence, with a new assumption that is based on an explicit relationship between Poisson’s ratio and the internal friction angle of the backfill; as shown here, the use of this relationship can significantly modify the stress state in backfilled stopes. The simulation results, presented in terms of stresses, displacements, and strains, illustrate the influence of different parameters including backfill strength, pillar width, stope depth, rock mass stiffness, natural stress state, and excavation and filling sequence. Complementary aspects are also considered. A discussion follows on some of the characteristics and limitations of this investigation.

Analytical solution for determining the required strength of a side-exposed mine backfill containing a plug

Backfilling is well known in the mining industry. A basic issue associated with mine backfill design is the determination of the strength of the backfill required to maintain the stability of the backfill body upon removal of a side wall when an adjacent secondary stope is excavated. This task is usually accomplished using a solution proposed by Mitchell et al. in the early 1980s. Recently, this solution has been reviewed and a modified solution has been proposed. These existing solutions, however, were developed for a uniform backfill. In practice, mine stopes are usually divided into primary and secondary stopes. Primary stopes are often backfilled in two stages: a plug pour and a final pour. In many cases, the cement content of the plug pour is higher than that of the final pour. The positive effect of the stronger plug is neglected in both the original and the modified Mitchell et al. solutions. Here, a new analytical solution is developed for estimating the required strength of a laterally exposed backfill by taking the plug into account. The proposed solution reduces to the modified Mitchell et al. solution if the plug pour and final pour have the same cement content. If the plug pour has a cement content higher than that of the final pour, the required backfill strength calculated with the proposed solution is lower than the values estimated with the modified Mitchell et al. solution; the same conclusion can be drawn to the comparison between the proposed solution and the original Mitchell et al. solution when the bond cohesion along the side walls is close to the cohesion of the backfill. Furthermore, the proposed solution indicates that an optimal cohesion ratio between the plug and final pours exists: there is no benefit in using excessively high cement content in the plug pour. It is noted that the proposed solution is valid only for high, narrow backfilled stopes, solutions for low and wide backfilled stopes are under development.