International Journal of Minerals, Metallurgy, and Materials最新文献

The dendrite growth behavior of high-strength steel during slab continuous casting with a traveling-wave magnetic field was studied in this paper. The morphology of the solidification structure and composition distribution were analyzed. Results showed that the columnar crystals could deflect and break when the traveling-wave magnetic field had low current intensity. With the increase in current intensity, the secondary dendrite arm spacing and solute permeability decreased, and the columnar crystal transformed into an equiaxed crystal. The electromagnetic force caused by the traveling-wave magnetic field changed the temperature gradient and velocity magnitude and promoted the breaking and fusing of dendrites. Dendrite compactness and composition uniformity were arranged in descending order as follows: columnar-to-equiaxed transition (high current intensity), columnar crystal zone (low current intensity), columnar-to-equiaxed transition (low current intensity), and equiaxed crystal zone (high current intensity). Verified numerical simulation results combined with the boundary layer theory of solidification front and dendrite breaking–fusing model revealed the dendrite deflection mechanism and growth process. When thermal stress is not considered, and no narrow segment can be found in the dendrite, the velocity magnitude on the solidification front of liquid steel can reach up to 0.041 m/s before the dendrites break.

The development of solid waste resources as constituent materials for wet shotcrete has significant economic and environmental advantages. In this study, the concept of using tailings as aggregate and fly ash and slag powder as auxiliary cementitious material is proposed and experiments are carried out by response surface methodology (RSM). Multivariate nonlinear response models are constructed to investigate the effect of factors on the uniaxial compressive strength (UCS) of tailings wet shotcrete (TWSC). The UCS of TWSC is predicted and optimized by constructing Gaussian process regression (GPR) and genetic algorithm (GA). The UCS of TWSC is gradually enhanced with the increase of slag powder dosage and fineness modulus, and it is enhanced first and then decreased with the increase of fly ash dosage. The microstructure of TWSC has the highest gray value and the highest UCS when the fly ash dosage is about 120 kg·m^?3. The GPR–GA model constructed in this study achieves high accuracy prediction and optimization of the UCS of TWSC under multi-factor conditions.

Solidification structure is a key aspect for understanding the mechanical performance of metal alloys, wherein composition and casting parameters considerably influence solidification and determine the unique microstructure of the alloys. By following the principle of free energy minimization, the phase-field method eliminates the need for tracking the solid/liquid phase interface and has greatly accelerated the research and development efforts geared toward optimizing metal solidification microstructures. The recent progress in the application of phase-field simulation to investigate the effect of alloy composition and casting process parameters on the solidification structure of metals is summarized in this review. The effects of several typical elements and process parameters, including carbon, boron, silicon, cooling rate, pulling speed, scanning speed, anisotropy, and gravity, on the solidification structure are discussed. The present work also addresses the future prospects of phase-field simulation and aims to facilitate the widespread applications of phase-field approaches in the simulation of microstructures during solidification.

Water-locking flocs formed by ultrafine tailings particles will damage the thickener underflow concentration in the thickening process during paste preparation. The relationship between the mesostructure and seepage characteristics of tail mortar is typically ignored when investigating the deep dehydration stage. A shearing seepage test of an unclassified tailing–sedimentation bed was performed with copper tailings, and the morphology and geometric distribution of micropores were analyzed via X-ray computed tomography. Moreover, the shearing evolution of the micropore structure and seepage channel was investigated to evaluate the dewatering performance of underflow slurry using a three-dimensional reconstruction approach. The results show that porosity decreases considerably under shearing. The connected-pore ratio and the average radius of the throat channel reach peak values of 0.79 and 31.38 µm, respectively, when shearing is applied for 10 min. However, the reverse seepage velocity and absolute permeability in the bed decrease to various extents after shearing. Meanwhile, the maximum flow rate reaches 1.537 µm/s and the absolute permeability increases by 14.16%. Shearing alters the formation process and the pore structure of the seepage channel. Isolated pores connect to the surrounding flocs to form branch channels, which then become the main seepage channel and create the dominant water-seepage flow channel.

The mesoscale fiber–matrix interfacial transition zone (FM-ITZ) under induced curing pressure plays a key role in the effectiveness of fiber reinforcement and the engineering application of fiber-reinforced cementitious composites (FRCCs). This critical review establishes the link among induced curing pressure (i.e., external loading condition), multiphysics processes (i.e., internal governing mechanism), and interface behavior (i.e., material behavior) for FRCC materials through analysis of the state-of-the-art research findings on the FM-ITZ of FRCC materials. The following results are obtained. For the mechanical process, the induced curing pressure changes the stress state and enhances multicracking behavior, which can strengthen the FM-ITZ. For the hydraulic process, the strengthened seepage of the FM-ITZ under induced curing pressure weakens the effective stress and exaggerates the deficiency in water retention capacity between the bulk matrix and the FM-ITZ. For the thermal process, the induced curing pressure causes a steep temperature gradient in the FM-ITZ and thus influences the temperature evolution and thermally-induced microcracks in the FM-ITZ. For the chemical process, the induced curing pressure enhances hydration kinetics and results in the formation of additional hydration products in the FM-ITZ. Moreover, recommendations are proposed on the basis of findings from this review to facilitate the implementation of fiber reinforcement in cemented paste backfill technology.

Wall slip is a microscopic phenomenon of cemented paste backfill (CPB) slurry near the pipe wall, which has an important influence on the form of slurry pipe transport flow and velocity distribution. Directly probing the wall slip characteristics using conventional experimental methods is difficult. Therefore, this paper established a noncontact experimental platform for monitoring the microscopic slip layer of CPB pipeline transport independently based on particle image velocimetry (PIV) and analyzed the effects of slurry temperature, pipe diameter, solid concentration, and slurry flow on the wall slip velocity of the CPB slurry, which refined the theory of the effect of wall slip characteristics on pipeline transport. The results showed that the CPB slurry had an extensive slip layer at the pipe wall with significant wall slip. High slurry temperature improved the degree of particle Brownian motion within the slurry and enhanced the wall slip effect. Increasing the pipe diameter was not conducive to the formation of the slurry slip layer and led to a transition in the CPB slurry flow pattern. The increase in the solid concentration raised the interlayer shear effect of CPB slurry flow and the slip velocity. The slip velocity value increased from 0.025 to 0.056 m·s⁻¹ when the solid content improved from 55wt% to 65wt%. When slurry flow increased, the CPB slurry flocculation structure changed, which affected the slip velocity, and the best effect of slip layer resistance reduction was achieved when the transported flow rate was 1.01 m³·h⁻¹. The results had important theoretical significance for improving the stability and economy of the CPB slurry in the pipeline.

The use of aeolian sand (AS) as an aggregate to prepare coal mine cemented filling materials can resolve the problems of gangue shortage and excessive AS deposits. Owing to the lack of research on the mechanism of cemented AS backfill (CASB), the response surface method (RSM) was adopted in this study to analyze the influence of ordinary Portland cement (PO) content (x₁), fly ash (FA)–AS (FA–AS) ratio (x₂), and concentration (x₃) on the mechanical and microscopic properties of the CASB. The hydration characteristics and internal pore structure of the backfill were assessed through thermogravimetric/derivative thermogravimetric analysis, mercury intrusion porosimetry, and scanning electron microscopy. The RSM results show that the influence of each factor and interaction term on the response values is extremely significant (except x₁x₃, which had no obvious effect on the 28 d strength). The uniaxial compressive strength (UCS) increased with the PO content, FA–AS ratio, and concentration. The interaction effects of x₁x₂, x₁x₃, and x₂x₃ on the UCS at 3, 7, and 28 d were analyzed. In terms of the influence of interaction items, an improvement in one factor promoted the strengthening effect of another factor. The enhancement mechanism of the curing time, PO content, and FA–AS ratio on the backfill was reflected in the increase in hydration products and pore structure optimization. By contrast, the enhancement mechanism of the concentration was mainly the pore structure optimization. The UCS was positively correlated with weight loss and micropore content but negatively correlated with the total porosity. The R² value of the fitting function of the strength and weight loss, micropore content, and total porosity exceeded 0.9, which improved the characterization of the enhancement mechanism of the UCS based on the thermogravimetric analysis and pore structure. This work obtained that the influence rules and mechanisms of the PO, FA–AS, concentration, and interaction terms on the mechanical properties of the CASB provided a certain theoretical and engineering guidance for CASB filling.

Estimation of stressses within the tailings slurry during self-weight consolidation is a critical issue for cost-effective barricade design and efficient backfill planning in underground mine stopes. This process requires a good understanding of self-weight consolidation behaviors of the tailings slurry within practical stopes, where many factors can have significant effects on the consolidation, including drainage condition and cement addition. In this paper, the prepared tailings slurry with different cement contents (0, 4.76wt%, and 6.25wt%) was poured into 1.2 m-high columns, which allowed three drainage scenarios (undrained, partial lateral drainage near the bottom part, and full lateral drainage boundaries) to investigate the effects of drainage condition and cement addition on the consolidation behavior of the tailings slurry. The consolidation behavior was analyzed in terms of pore water pressure (PWP), settlement, volume of drainage water, and residual water content. The results indicate that increasing the length of the drainage boundary or cement content aids in PWP dissipation. In addition, constructing an efficient drainage boundary was more favorable to PWP dissipation than increasing cement addition. The final stable PWP on the column floor was not sensitive to cement addition. The final settlement of uncemented tailings slurry was independent of drainage conditions, and that of cemented tailings slurry decreased with the increase in cement addition. Notably, more pore water can drain out from the cemented tailings slurry than the uncemented tailings slurry during consolidation.

Detecting a pipeline’s abnormal status, which is typically a blockage and leakage accident, is important for the continuity and safety of mine backfill. The pipeline system for gravity-transport high-density backfill (GHB) is complex. Specifically designed, efficient, and accurate abnormal pipeline detection methods for GHB are rare. This work presents a long short-term memory-based deep learning (LSTM-DL) model for GHB pipeline blockage and leakage diagnosis. First, an industrial pipeline monitoring system was introduced using pressure and flow sensors. Second, blockage and leakage field experiments were designed to solve the problem of negative sample deficiency. The pipeline’s statistical characteristics with different working statuses were analyzed to show their complexity. Third, the architecture of the LSTM-DL model was elaborated on and evaluated. Finally, the LSTM-DL model was compared with state-of-the-art (SOTA) learning algorithms. The results show that the backfilling cycle comprises multiple working phases and is intermittent. Although pressure and flow signals fluctuate stably in a normal cycle, their values are diverse in different cycles. Plugging causes a sudden change in interval signal features; leakage results in long variation duration and a wide fluctuation range. Among the SOTA models, the LSTM-DL model has the highest detection accuracy of 98.31% for all states and the lowest misjudgment or false positive rate of 3.21% for blockage and leakage states. The proposed model can accurately recognize various pipeline statuses of complex GHB systems.

Cemented paste backfill (CPB) is one of the effective methods for resource utilization of tailings, but the high cost of ordinary Portland cement (OPC) limits its utilization. Considering the poor performance of Na₂CO₃-activated binders, in this work, supplementary materials, including CaO, MgO, and calcined layered double hydroxide (CLDH), were used to modify their properties with the aim of finding an alternative binder to OPC. Isothermal calorimetry, X-ray diffraction, and thermogravimetric analyses were conducted to explore the reaction kinetics and phase assembles of the binder. The properties of the CPB samples, such as flowability, strength development, and heavy metal immobilization effects, were then investigated. The results show that the coupling utilization of MgO and CLDH showed good performance. The strength of the Mg₂-CLDH₃ sample was approximately 2.94 MPa after curing for 56 d, which was higher than that of the OPC-based sample. Moreover, the cost of the modified Na₂CO₃-activated binder was lower than that of the OPC-based binder. Modified sample showed satisfactory heavy metal immobilization effects. These findings demonstrate that carbonate-activated binder modified by supplementary materials can be suitable in CPB.