Research

Open AccessArticle

Cementitious Backfill with Partial Replacement of Cu-Rich Mine Tailings by Sand: Rheological, Mechanical and Microstructural Properties

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Minerals 2023, 13(3), 437; https://doi.org/10.3390/min13030437 - 18 Mar 2023

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Abstract

The thinning of tailings gradation during ore processing leads to a sizeable fall in the strength of cementitious paste backfill (CPB), increases operational risks, and encourages researchers to use alternative economic products. This study aims to increase the strength performance by improving CPB’s [...] Read more.

The thinning of tailings gradation during ore processing leads to a sizeable fall in the strength of cementitious paste backfill (CPB), increases operational risks, and encourages researchers to use alternative economic products. This study aims to increase the strength performance by improving CPB’s gradation while cutting costs and reducing the sum of the binder employed per unit volume. An evolution of the slump/strength/structural properties of sand-substituted CPBs was explored experimentally. Samples were made with a fixed cement content (7 wt.%), diverse tailings/sand fractions (e.g., 100/0, 90/10, 80/20, 70/30, and 50/50), and diverse solid contents (e.g., 72 and 76 wt.%). After curing for 3–56 days, several experiments, such as slump, uniaxial compressive strength (UCS), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM), were undertaken for the filling samples. The results demonstrate that adding sand to the backfill greatly increases CPB’s strength (up to 99%), but the replacement rate of sand was limited to 30% due to its segregation effect. Microstructural tests reveal that CPB’s void volume decreases as the added amount of sand increases. To sum up, it was concluded that calcareous sand made a major contribution to the filling strength, incorporating the effects of enhancing the fill gradation’s readjustment and reducing the sum of cement being used in the unit volume for CPB manufacturing. Full article

(This article belongs to the Special Issue Solid-Filling Technology in Coal Mining)

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Open AccessArticle

Mechanical Properties and Hydration Mechanism of Coal Flotation Tailing Cemented Filling Materials

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Minerals 2023, 13(3), 389; https://doi.org/10.3390/min13030389 - 10 Mar 2023

Viewed by 317

Abstract

Large-scale application of filled coal mining technology has long been limited by conditions such as the cost of filling. Compared to traditional filling materials, coal flotation tailing filling materials (CFTFM) offers advantages such as low cost and excellent performance. The Box–Behnken response surface [...] Read more.

Large-scale application of filled coal mining technology has long been limited by conditions such as the cost of filling. Compared to traditional filling materials, coal flotation tailing filling materials (CFTFM) offers advantages such as low cost and excellent performance. The Box–Behnken response surface method was used to investigate the influence of flotation tailing properties on the mechanical properties and hydration mechanisms of the filling material. Ash content, blending, and calcination temperature of the flotation tailings were used as the investigating factors, and uniaxial compressive strength (7d and 28d), slump, and the slurry water secretion rate of the filling material as the evaluation indicators. The results showed that the influence of the flotation tailings on the uniaxial compressive strength (28d) of CFTFM followed the order ash > calcination temperature > doping, with the interaction of ash and calcination temperature having a greater influence on the uniaxial compressive strength. The optimized pre-treatment conditions for the flotation tailings were 59% ash, 30% doping, a calcination temperature of 765 °C, and optimum uniaxial compressive strength of 7.02 MPa. The effect of flotation tailings on the exotherm of CFTFM hydration was determined using a TAM Air isothermal microcalorimeter, mainly in the induction and acceleration phases. Combined with SEM electron microscopy and IR FT-IR analysis of the hydration products, a descriptive model of the CFTFM hydration mechanism was established. CFTFM hydration can be described in three phases: diffusion, hydration, and hardening. The CFTFM prepared in this study is applicable to the integrated mining and charging synergistic mining technology, which can effectively reduce gangue lifting energy consumption and washing process waste, reduce the cost of filling, and can effectively achieve harmless, resourceful, and large-scale disposal of coal-based solid waste. Full article

(This article belongs to the Special Issue Solid-Filling Technology in Coal Mining)

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Open AccessArticle

Study on Macro-Meso Deformation Law and Acoustic Emission Characteristics of Granular Gangue under Different Loading Rates

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Minerals 2022, 12(11), 1422; https://doi.org/10.3390/min12111422 - 09 Nov 2022

Cited by 1 | Viewed by 605

Abstract

Bulk gangue is a common backfill material in solid backfill mining. After backfilling into the goaf, bulk gangue serves as the main body to bear the load of overlying strata, and its deformation resistance is the key factor affecting the backfill quality. In [...] Read more.

Bulk gangue is a common backfill material in solid backfill mining. After backfilling into the goaf, bulk gangue serves as the main body to bear the load of overlying strata, and its deformation resistance is the key factor affecting the backfill quality. In this study, the laterally confined compression test of broken gangue was designed, the compaction deformation characteristics of gangue specimens under different loading rates were studied, the acoustic emission (AE) energy characteristics of gangue specimens under compression were analyzed, and the relationship model between macroscopic deformation of broken gangue under compression and AE energy was established. The particle flow numerical software PFC2D was used to stimulate the particle breakage in the gangue compaction process, and the coal gangue particle model was established through particle cluster units. The particle force chain distribution and fracture evolution characteristics of gangue specimens in the compression process were studied, and the macroscopic deformation mechanism was revealed from the mesoscopic perspective. The results showed that: the porosity variation of the gangue specimen increases with the increase of loading rate; the porosity increases with the decrease in the strain, the porosity decreases with the increase in the stress, and the relationship between porosity and stress is monotonously decreasing. With the increase of loading rate, the AE signals produced by particle breakage become stronger, while the influence of the loading rate on the maximum strain, fragmentation and AE signal of the specimen is gradually weakened. Under different loading rates, the “instability-optimization” of the skeleton force chain structure of the gangue model and the crushing-recombination of cracks are the main reasons for the compaction deformation of gangue specimens at the early stage of loading. The research results are of great significance to reveal the deformation mechanism of coal gangue as backfill materials under compression. Full article

(This article belongs to the Special Issue Solid-Filling Technology in Coal Mining)

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Open AccessArticle

Stability Influencing Factors and Control Methods of Residual Coal Pillars with Solid Waste Materials Backfilling Method

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Minerals 2022, 12(10), 1285; https://doi.org/10.3390/min12101285 - 13 Oct 2022

Cited by 1 | Viewed by 475

Abstract

Affected by coal mining activities, the remaining coal pillars are very likely to be destabilized and cause safety accidents. The backfilling of the remaining goaf can maintain the stability of the coal pillar well, but the coal pillar in the unfilled zone may [...] Read more.

Affected by coal mining activities, the remaining coal pillars are very likely to be destabilized and cause safety accidents. The backfilling of the remaining goaf can maintain the stability of the coal pillar well, but the coal pillar in the unfilled zone may still be unstable. In this paper, the effect of backfilling materials on coal pillars and the reinforcement method are discussed using numerical simulation, statistical mathematics, elastic mechanics, and mechanical test methods. The results show that: backfilling with solid waste materials and reinforcing the coal pillar could maintain the stability of the bottom goaf, where the backfill body height is the main factor in the strength of the coal pillar. The propagation of the confining stress of the backfill body on the pillar in the unfilled zone is the primary way to influence the coal pillar strength. Changing the backfill body height filling can affect the coal pillar strength. By analyzing the propagation law of confining stress in the coal pillar, the minimum backfill body height is determined to be 7 m. Combined with mechanical tests and the Mohr–Coulomb criterion, the minimum confining pressure required to maintain the coal pillar stability under the peak ground pressure is analyzed. The ratio of solid waste materials is determined based on this. Field tests have proved that the coal pillar remains stable when the goaf is not filled, and the cement/fly ash ratio is 1:4, which can ensure product safety. The research has significant value and significance for the governance of the remaining coal pillars and production safety. Full article

(This article belongs to the Special Issue Solid-Filling Technology in Coal Mining)

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Open AccessArticle

Prediction of the Mechanical Performance of Cemented Tailings Backfill Using Ultrasonic Pulse Velocity Measurement

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Xichun Tian

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Wenbin Xu

Minerals 2022, 12(8), 986; https://doi.org/10.3390/min12080986 - 03 Aug 2022

Cited by 2 | Viewed by 629

Abstract

Cemented tailings backfill (CTB), prepared by a mixture of tailings, binder, and water in a certain proportion, is widely applied to mines worldwide for ground support and tailings disposal. The prediction of the mechanical properties of CTB during the whole consolidation process is [...] Read more.

Cemented tailings backfill (CTB), prepared by a mixture of tailings, binder, and water in a certain proportion, is widely applied to mines worldwide for ground support and tailings disposal. The prediction of the mechanical properties of CTB during the whole consolidation process is of great practical importance. The objective of this paper focuses on the investigation of the prediction of the mechanical performance of CTB based on the ultrasonic pulse velocity (UPV) method. The CTB samples prepared with different binder-to-water (b/w) ratios, as well as solid content were monitored by the UPV method during the curing age of 28 days. The evolution of dynamic shear modulus and dynamic elasticity modulus properties of CTB samples were studied by UPV monitoring. Meanwhile, uniaxial compressive strength (UCS) and microstructure tests were performed on CTB samples at curing times of 3, 7, and 28 days. The results showed that the UPV development follows a trend that increases fast at early curing ages and then becomes stable at the 10 d curing age. UPV and UCS increased with the increase in b/w, solid content, and curing age. From the results of microstructure tests, the increase in UPV is attributed to the low porosity and compact structure due to the increase in the b/w ratio and solid content. For the purpose of predicting the UCS of CTB utilizing UPV monitoring, the empirical equations for the relationship between UCS and UPV of CTB with variation b/w ratios and solid content were regression analyses. F-tests, as well as t-tests, were used to check the validity of the equations, which indicate that higher calculated values for CTB to predicted UCS by means of the UPV method. The main finding of this paper shows that the UPV monitoring method can be an effective way to predict the mechanical property of CTB in the field and is non-destructive and effective. Full article

(This article belongs to the Special Issue Solid-Filling Technology in Coal Mining)

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Open AccessArticle

Research on the Stability Mechanism of the Surrounding Rock of Gob-Side Entry Retaining by Roof Cutting in Dianping Coal Mine

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Minerals 2022, 12(8), 965; https://doi.org/10.3390/min12080965 - 29 Jul 2022

Cited by 2 | Viewed by 578

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According to the movement characteristics of the surrounding rock of the gob-side entry retaining by roof cutting (GERRC), the structural evolution mechanism and stability control countermeasures are studied in this paper. Taking the roof cutting and bulking as the core point, the movement [...] Read more.

According to the movement characteristics of the surrounding rock of the gob-side entry retaining by roof cutting (GERRC), the structural evolution mechanism and stability control countermeasures are studied in this paper. Taking the roof cutting and bulking as the core point, the movement process of the surrounding rock of GERRC is divided into five stages: roof cutting, dynamic pressure support, gangue bulking, double arch roadway support, and roadway formation. Combined with mechanical analysis, the roof pressure of the mechanical model of the short-arm beam formed by roof cutting is calculated, and the roof breaking criterion is obtained, which provides a basis for roof control design and reasonable support strength calculation. A structural model of double-arch roadway protection under the action of hydrostatic roof cutting and gangue bulking was established, and the mechanism of roadway formation stability was studied. The gob-side bulking gangue and short-arm beam are formed by roof cutting, and the solid coal seam forms the immediate balance arch, the bulking gangue in goaf, articulated transfer rock beam, and solid coal seam forms the main balance arch. The two together are called double balance arch. With the support of the double-balanced arch on the overlying rock strata, the roadway below is located in the low-stress zone, which ensures the stability of the roadway surrounding rock. Through mechanical analysis and field observation, the law of bulking and deformation and mechanical characteristics of gob-side gangue in the above structural model are analyzed, and the reasonableness of the structural model of the double-arch guard lane is verified through numerical simulation of the cut top into lanes. and the reasonableness of the structural model of double-arch protection in GERRC is verified by using numerical simulation. In this paper, a mechanical double-arch model has been developed that allows us to understand the mechanism of stress transfer of roof cutting roadway from the perspective of a dynamic balance of roadway surrounding rock and overlying rock strata, helping us to specify efficient support countermeasures. Full article

(This article belongs to the Special Issue Solid-Filling Technology in Coal Mining)

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Open AccessArticle

Prediction of the Adaptability of Using Continuous Extraction and Continuous Backfill Mining Method to Sequestrate CO₂-A Case Study

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Minerals 2022, 12(8), 936; https://doi.org/10.3390/min12080936 - 25 Jul 2022

Cited by 3 | Viewed by 704

Abstract

The consumption of coal resources has caused an increase in CO₂ emissions. A scientific concept that can realize CO₂ sequestration, the harmless treatment of solid wastes, and coal extraction under buildings, railways, and water bodies (BRW) is proposed. First, a novel [...] Read more.

The consumption of coal resources has caused an increase in CO₂ emissions. A scientific concept that can realize CO₂ sequestration, the harmless treatment of solid wastes, and coal extraction under buildings, railways, and water bodies (BRW) is proposed. First, a novel CO₂ mineralized filling body (CMFB) is developed by employing CO₂ gas, fly ash, silicate additives, and cement. It is then injected into the mined-out mining roadways (MRs) of the continuous extracting and continuous backfill (CECB) mining method to ameliorate the overburden migration and thus extract the coal body under the BRW. The AHP-fuzzy comprehensive evaluation method was employed to construct a prediction model for the suitability of this concept. Subsequently, the evaluation model is generalized and applied to the Yu-Shen mining area. Each indicator affecting adaptability is plotted on a thematic map, and the corresponding membership degree is determined. The aptness for 400 boreholes distributed in the entire area was determined and a zoning map which divides the whole area into good, moderate, slightly poor, and extremely poor suitability was drawn. This paper puts forward a mathematical model for predicting the suitability of using CECB and CMFB to sequestrate CO₂. Research results can provide references for determining the site of CO₂ sequestration under the premise of maximizing the economic and ecological benefits, which is conducive to constructing ecological, green, and sustainable coal mines. Full article

(This article belongs to the Special Issue Solid-Filling Technology in Coal Mining)

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Open AccessArticle

Ground Stress Analysis and Automation of Workface in Continuous Mining Continuous Backfill Operation

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Seun A. Ajayi

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Liqiang Ma

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Anthony J. S. Spearing

Minerals 2022, 12(6), 754; https://doi.org/10.3390/min12060754 - 14 Jun 2022

Cited by 6 | Viewed by 1218

Abstract

The cost, complexity, lack of filling space and time create challenges in the longwall backfill operation, resulting in poor subsidence control and reduced productivity. This paper proposes an automated continuous mining and continuous backfill (CMCB) method by examining its key requirements and investigates [...] Read more.

The cost, complexity, lack of filling space and time create challenges in the longwall backfill operation, resulting in poor subsidence control and reduced productivity. This paper proposes an automated continuous mining and continuous backfill (CMCB) method by examining its key requirements and investigates the optimum sequence of coal panel (such as drifts) excavation to ensure ground strata control at relatively high productivity. The automated CMCB adopts the highwall mining technique underground, which enables easier automation at the workface. A numerical simulation of the Changxing coal mine in China was undertaken, and five different sequences of coal excavation were investigated, using the automated CMCB excavation parameters (assuming a 4 m width cut, 5 m mining height for a 200 m long coal slice) to determine the optimum sequence of resource excavation. The plastic zones and vertical displacement across the five models were analyzed. Simulation results of the 5 m high coal seam excavation show that the odd-even slice (OES) mining sequence, which has a vertical ground displacement of 74 mm, is the most efficient excavation method, due to its effective stress redistribution and lower induced ground displacement. Full article

(This article belongs to the Special Issue Solid-Filling Technology in Coal Mining)

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Open AccessArticle

Aging Stability Analysis of Slope Considering Cumulative Effect of Freeze–Thaw Damage—A Case Study

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Minerals 2022, 12(5), 598; https://doi.org/10.3390/min12050598 - 09 May 2022

Cited by 2 | Viewed by 847

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The change of physical and mechanical properties of slope rock mass in open-pit mines in seasonally frozen area under the action of freeze–thaw cycles is one of the main reasons for slope instability. In this paper, taking the mechanical parameters of coal seam [...] Read more.

The change of physical and mechanical properties of slope rock mass in open-pit mines in seasonally frozen area under the action of freeze–thaw cycles is one of the main reasons for slope instability. In this paper, taking the mechanical parameters of coal seam and sandstone layer in the Beitashan Pasture Open-Pit Mine in Xinjiang as the research object, considering the combined effect of the frost-heave tensile stress in the crack perpendicular to the crack surface and the three-dimensional confining pressure in the crack, the criterion for cracking of fractured rock mass under freeze-thaw condition is determined by applying the principle of stress superposition and the theory of strain energy density factor, and the theoretical frost-heave stress required for cracking is deduced. On this basis, the sensitivity analysis of the fixed factors and variable factors to the theoretical frost-heave stress was performed, respectively. Finite element analysis was utilized to analyze the slope stability under the attenuation of five groups of different rock mass mechanical properties and to determine the slope angle required for the slope stability. Seven different slope angles of sidewall mining ranging from 36° to 51° are analyzed. The results of finite element analysis show that considering the timeliness difference of rock mass parameters with time, the safety factor of slope is reduced from the original 1.70 to 1.18, and 91,500 tons of coal resources can be recovered every year, with remarkable economic benefits. Full article

(This article belongs to the Special Issue Solid-Filling Technology in Coal Mining)

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Open AccessArticle

Influence of Layered Angle on Dynamic Characteristics of Backfill under Impact Loading

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Minerals 2022, 12(5), 511; https://doi.org/10.3390/min12050511 - 20 Apr 2022

Cited by 11 | Viewed by 1042

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In order to explore the relationship between the filling surface angle and the dynamic characteristics of the filling body under impact load, this paper uses the Hopkinson pressure bar (SHPB) test system to study the influence of different filling surface angles, different cement–sand [...] Read more.

In order to explore the relationship between the filling surface angle and the dynamic characteristics of the filling body under impact load, this paper uses the Hopkinson pressure bar (SHPB) test system to study the influence of different filling surface angles, different cement–sand ratios and different strain rates on the dynamic peak strength, dynamic strength growth factor and failure mode of the filling body. The results show that with the increase in the average strain rate, the dynamic peak strength and dynamic strength growth factor of the layered filling body increase gradually. With the increase in the filling surface angle, the static and dynamic peak strength of the layered filling body decreases gradually, but there is no obvious change law linking the dynamic strength growth factor and filling surface angle. According to the failure mode analysis and the LS-DYNA numerical simulation results for the layered filling body, with the increase in the filling surface angle, the failure mode of the layered filling body changes from splitting failure under tension to shear failure. The research results in this paper can provide theoretical support for the stability of underground layered filling bodies. Full article

(This article belongs to the Special Issue Solid-Filling Technology in Coal Mining)

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Open AccessArticle

Study on Fluid–Solid Characteristics of Grouting Filling Similar-Simulation Materials

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Kaidan Zheng

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Dayang Xuan

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Jian Li

Minerals 2022, 12(5), 502; https://doi.org/10.3390/min12050502 - 19 Apr 2022

Cited by 3 | Viewed by 995

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The mining-induced overburden bed separation grouting technique can control surface subsidence through the high-pressure grouting and filling into the bed separation during mining. The physical simulation method can be used to objectively reproduce the dynamic migration process of filling slurry in the bed [...] Read more.

The mining-induced overburden bed separation grouting technique can control surface subsidence through the high-pressure grouting and filling into the bed separation during mining. The physical simulation method can be used to objectively reproduce the dynamic migration process of filling slurry in the bed separation but the traditional similar-simulation materials are not suitable for the simulation of bed separation grouting. Considering the water disintegration, weak water storage capacity, and poor permeability of traditional simulation materials, the existing similar-simulation materials were modified in this study. The improved similar-simulation materials have adjustable physical and mechanical parameters, stable properties in a water-filled environment, and high water storage and permeability, and the reasonable ratio of similar-simulation materials was determined for hard rock, medium-hard rock, and soft rock. The similarity simulation function suitable for bed separation grouting was deduced and the time similarity coefficient and permeability similarity coefficient of the bed separation grouting simulation were obtained to judge the similarity and applicability of similar-simulation materials with specific proportions. This study provides a reliable experimental simulation scheme for the physical simulation of mining-induced bed separation grouting and provides a theoretical basis for the improvement of similar-simulation materials with fluid–solid characteristics. Full article

(This article belongs to the Special Issue Solid-Filling Technology in Coal Mining)

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Open AccessArticle

Development of Backfill Concrete Including Coal Gangue and Metakaolin and Prediction of Compressive Strength by Extreme Learning Machine

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Minerals 2022, 12(3), 330; https://doi.org/10.3390/min12030330 - 07 Mar 2022

Cited by 1 | Viewed by 1283

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The main aim of this investigation is to develop backfill concrete including coal gangue and metakaolin to reduce solid waste. For this purpose, a total of 30 concrete mixtures were designed by the inclusion of 0%, 25%, 50%, 75% and 100% coal gangue [...] Read more.

The main aim of this investigation is to develop backfill concrete including coal gangue and metakaolin to reduce solid waste. For this purpose, a total of 30 concrete mixtures were designed by the inclusion of 0%, 25%, 50%, 75% and 100% coal gangue as coarse aggregates and 0%, 10% and 20% metakaolin as binder at 0.55 and 0.45 water to cement ratios. The compressive strength was tested after 3, 7 and 28 days for a total of 90 samples. Meanwhile, the influences of coal gangue and metakaolin on the elastic modulus, ultrasonic pulse velocity, rebound number and open porosity were explored. Then, the relationship between physical and mechanical properties was revealed by design code expressions and empirical models. Furthermore, an extreme learning machine was developed to predict compressive strength by concrete mixtures. The results show that the inclusion of coal gangue results in a poor performance in physical and mechanical properties of concrete. However, the drawbacks of concrete containing coal gangue can be compensated by metakaolin. The predicted results of design code expressions and empirical models are closed to the experiment results, with a 10% error. In addition, the findings reveal that the extreme learning machine offers significant potential to predict the compressive strength of concrete with high precision. Full article

(This article belongs to the Special Issue Solid-Filling Technology in Coal Mining)

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