Search

The cemented paste backfill (CPB) technology has been successfully used for the recycling of mine tailings all around the world. However, its application in coal mines is limited due to the lack of mine tailings that can work as aggregates. In this work, the feasibility of using silts from the Yellow River silts (YRS) as aggregates in CPB was investigated. Cementitious materials were selected to be the ordinary Portland cement (OPC), OPC + coal gangue (CG), and OPC + coal fly ash (CFA). A large number of lab experiments were conducted to investigate the unconfined compressive strength (UCS) of CPB samples. After the discussion of the experimental results, a dataset was prepared after data collection and processing. Deep neural network (DNN) was employed to predict the UCS of CPB from its influencing variables, namely, the proportion of OPC, CG, CFA, and YS, the solids content, and the curing time. The results show the following: (i) The solid content, cement content (cement/sand ratio), and curing time present positive correlation with UCS. The CG can be used as a kind of OPC substitute, while adding CFA increases the UCS of CPB significantly. (ii) The optimum training set size was 80% and the number of runs was 36 to obtain the converged results. (iii) GA was efficient at the DNN architecture tuning with the optimum DNN architecture being found at the 17th iteration. (iv) The optimum DNN had an excellent performance on the UCS prediction of silt-based CPB (correlation coefficient was 0.97 on the training set and 0.99 on the testing set). (v) The curing time, the CFA proportion, and the solids content were the most significant input variables for the silt-based CPB and all of them were positively correlated with the UCS. [ABSTRACT FROM AUTHOR]

Advances in pipe transportation have contributed significantly to the application of cemented paste backfill (CPB). However, the pressure drop during pipe transportation with complex circuit shapes has not been fully investigated. Currently, research in the field is largely experiment-centered, where extensive test loop experiments need to be performed for a specific CPB material. By comparison, computational modeling of the pressure drop during pipe transportation through complex circuits is still in its infancy and is hindered by the challenges of technique availability and data scarcity. In this article, we present a framework for investigating and modeling the pressure drop of CPB during pipe transportation with complex circuit shapes. A test loop system was used to investigate the pressure drop of CPB under different influencing variables. Based on the experimental data, gradient boosting regression tree (GBRT) was utilized to develop a prediction model for the CPB pressure drop in the pipe loop. We first discussed the effect of solids content, cement–tailings ratio, inlet pressure, and circuit shape on the CPB pressure drop. The feasibility of GBRT modeling was demonstrated by comparing the predicted pressure drop with the experimental result on both the training and testing sets. We also investigated the relative importance of influencing variables on the pressure drop of CPB. Harnessing such a modeling approach extends recent efforts to determine the pressure drop during CPB pipe transportation and can significantly accelerate the application of CPB in the future. [ABSTRACT FROM AUTHOR]

This research is an investigation of the feasibility of utilizing phosphogypsum (PG) and phosphate tailings (PTS) for cemented paste backfill. Some experiments were conducted with various combinations of PTS and PG as aggregates, along with slags and/or Portland cement as binders and CaO as an additive. The influence of the PG's ageing time on the consolidation of backfill was also explored. The unconfined compressive strength (UCS), the generated gases and the scanning electron microscope (SEM) were all tested and used in the analysis of backfill characteristics. The results show that (i) the highest UCS of backfill prepared by PG and PTS after curing for either 7 days or 28 days is still less than 1.0 MPa, with a large amount of CO 2 and SO 2 generated; (ii) the slags can improve the UCS by a factor of three, but not without a vast generation of CO 2 , SO 2 , and H 2 S. However, the gases were not produced when CaO was added, but the UCS decreases suddenly to 0.2 or 0.4 MPa after curing for 7 days or 28 days, respectively; (iii) the UCS of backfill increases linearly with increasing cement content. When the CaO was added at 2%, the UCS reached 3.36 MPa after curing for 7 days and 4.44 MPa after curing for 28 days, and no gases were generated; (iv) the influence of the PG's ageing time on the UCS is negligible after 4 days of aging. Based on these results, it was concluded that PG and PTS can be utilized as backfill materials when Portland cement is used as a binder and CaO is used as an additive. [ABSTRACT FROM AUTHOR]

This study demonstrates the feasibility of recycling lead–zinc tailing (LZT) as a cemented paste backfill (CPB) by considering the mechanical properties and environmental effects, thus providing an approach for safe and environmentally friendly treatment of LZT. First, the mechanical properties of CPB samples were tested. When the cement/tailing ratio was 1:6 and the slurry concentration was 70%, the maximum unconfined compressive strength (UCS) of the CPB cured for 28 days reaching 2.05 MPa, which could ensure safe mining. Then, the metals with pollution potential in the backfill slurry were investigated through static leaching. Finally, after adding immobilisation materials to stabilise excessive metals, the environmental stability of the CPB was demonstrated through dynamic leaching and a toxicity characteristic leaching procedure. The results show that the lead leached from the backfill slurry still exceeds the Chinese standard for groundwater quality (GB/T14848-2017 Class III). The addition of 2 mg/L polyaluminium sulfate (PAS) can further improve the strength of the CPB and maintain the environmental friendliness of the CPB. Therefore, the technology of recovering LZT as a CPB proposed in this study is an effective alternative to deal with LZT, which can help lead–zinc mines meet the requirements of cleaner production. [ABSTRACT FROM AUTHOR]

This research work was an exploration of the feasibility of utilizing a lithium slag (LS) and fly ash (FA)-based binder for cemented fine tailings backfill (CFTB). Extensive experiments were conducted with different combinations of LS and ordinary Portland cement (OPC), along with FA as an additive. The unconfined compressive strength (UCS), micromorphology and slump values were analyzed. The results showed that (i) the LS and FA had a significant influence on the strength of binders. The OPC-LS-FA ratio of 2:1:1 appeared to be optimal with the highest strength and was referred as the LS and FA-based binder (LFB). (ii) The LFB significantly improved the UCS of the CFTB. The UCS values of CFTB specimens curing for 7,28 and 56 days reached 0.95 MPa,2.28 MPa and 3.37 MPa, respectively, with a 10 wt% content of LFB. The strength satisfied the strength requirement of backfill for supporting the surrounding rock of stopes in the Yinshan lead-zinc mine (0.8 MPa, 2.0 MPa, 3.0 MPa). (iii) The pore-filling effect of the secondary hydration products, which was mainly produced by LFB, played a significant role in the early stage (<7 days), while the pozzolanic activity worked mostly in the mid-long period (>28 days). (iv) The LFB reduced the slump value of CFTB slurry by 2.6%–9.4% compared with OPC when the mass concentration increased from 58% to 64%, which was acceptable to satisfy the requirements of better fluidity and less transportation resistance in the Yinshan lead-zinc mine. Therefore, the LFB could be utilized as an alternative cementitious material for CFTB, which also provides a safe and economical approach to recycle LS and FA in an underground mine. • The OPC-LS-FA ratio of 2:1:1 appears to be optimal with the highest strength. • LFB significantly improves the UCS of the CFTB. • LFB has a slightly negative effect on the workability of the CFTB slurry. • LFB utilized as an alternative cementitious material has economic and environmental benefits. [ABSTRACT FROM AUTHOR]

Recycling phosphogypsum (PG) for cemented paste backfill (CPB) has been widely used at phosphate mines in China. However, the impurities in PG prolong the setting time and reduce the uniaxial compressive strength (UCS), limiting the engineering application of PG. This paper aims to investigate the feasibility of treated PG (TPG) washed repeatedly using deionised water (DW) for CPB. A water-washing pre-experiment was first conducted to find the proportion with the least DW demand and the effects of water-washing on ordinary PG (OPG). Then, based on the PG:DW ratio obtained from the pre-experiment, the properties of the OPG-based CPB (OCPB) and TPG-based CPB (TCPB) were tested using slump tests, UCS tests, and microstructural analysis. The results show that (1) after 11 water-washings at the PG:DW ratio of 1:1.75, the pH of the supernatant (pH = 6.328) meets the requirements of Chinese standard GB 8978-1996. (2) Water-washing improves the particle gradation quality of PG and removes the soluble impurities adsorbed at the surface of PG crystals. (3) The initial slump values of TCPB are 0.19–1.15 cm higher than that of OCPB, furthermore, the diffusivity values of TCPB are better than the performance of OCPB, with 0.61–1.68 cm of superiority. (4) The UCS values of TCPB are up to 0.838 MPa, 1.953 MPa, and 2.531 MPa, after curing for 7, 14, and 28 days. These are 0.283 MPa, 0.823 MPa, and 0.881 MPa higher than that of OCPB, respectively. It can be concluded that water-washing pre-treatment greatly improves the workability and mechanical property of PG-based CPB. These results are of great value for creating a reliable and environmentally superior alternative for the recycling of PG and for safer mining production. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Cemented paste backfill (CPB) plays an important role in minerals engineering. • Hydration in CPB relies on structural and electronic properties of cement phases. • C 2 F, C 4 AF and C 2 A were analysed and compared using first-principles calculations. • Lattice parameters, bond length and angle were analysed for structural properties. • PDOS, Bader charge and charge distribution were studied for electronic properties. Abstract Cemented paste backfill (CPB) plays a vital role in mineral engineering and understanding the cement hydration is crucial for its successful application. As an important constituent in ordinary Portland cement (5–15 wt%), ferrite is heavily used in CPB without being fully demonstrated due to its composition complexity. In this paper, the structural and electronic properties of Ca 2 Fe 2 O 5 (C 2 F), Ca 2 AlFeO 5 (C 4 AF) and Ca 2 Al 2 O 5 (C 2 A) were investigated using first-principles calculations. The results indicated that lattice parameters and bond length decreased with increasing substitution of the smaller Al3+ for the larger Fe3+. The chemical reactivity of the unit cell was derived mainly from O 2 p , Fe 3 p , Fe 3 d , and Al 3 p orbitals. The O atoms were prone to electrophilic attack with cations such as H+. In contrast, the Fe/Al atoms were prone to nucleophilic attack with anions such as OH– and the Fe atoms were more reactive than the Al atoms. This study provides new insight into the structural and electronic properties of ferrite at the atomic level, which will provide a better understanding of cement hydration and promote the application of CPB. [ABSTRACT FROM AUTHOR]

Waste tailings are increasingly being recycled as cemented paste backfill (CPB) for the cleaner production of mineral resources. In most cases, waste tailings cannot be fully recycled and the stability of underground stopes and open pits cannot be ensured. This research proposed a new procedure for recycling waste tailings as CPB to both underground stopes and open pits. Firstly, the physical, geotechnical and chemical tests required before the application of such procedure were discussed. Then, the new procedure was introduced in detail, including the process parameters determination and the backfill implementation. Finite element method and the three-zone theory were used to determine the process parameters. The application of this new procedure to an engineering instance, the Shirengou Iron Mine (SIM), was expatiated. The process parameters of recycling waste tailings as CPB in SIM were determined as follows. The solids content of CPB was 72%, the cement-tailings ratios were 1:8, 1:10, 1:20, the total height of open pit backfill was 80 m (with four backfilling stages), the stratification height was 1.5 m, and the cement-tailing ratios for four open-pit backfilling stages were 1:8 (stage 1 and 2), 1:10 (stage 3), and 1:20 (stage 4). It is shown that this new procedure achieved 100% recycling of waste tailings in SIM, which is of great environmental significance during mining operations. Harnessing such procedure extends recent efforts to recycle waste tailings as CPB, and can significantly promote the cleaner and safer production of minerals resources worldwide. [ABSTRACT FROM AUTHOR]

The recycling of waste tailings as cemented paste backfill (CPB) has attracted worldwide attention because of the increasing environmental awareness during mineral resources excavation. However, lots of mechanical tests are required to understand the strength development of CPB and its prediction under the combined effect of influencing variables is almost an unexplored field. This study proposes a strength prediction model integrating boosted regression trees (BRT) and particle swarm optimization (PSO), where the BRT algorithm was used for modelling the non-linear relationship between inputs and outputs and PSO was used for the BRT hyper-parameters tuning. An extensive mechanical experiment was performed to provide the dataset for the PSO-BRT model. This dataset contained unconfined compressive strength (UCS) results of 585 CPB specimens produced with a different combination of influencing variables, including the physical and chemical characteristics of tailings, the cement-tailings ratio, the solids content, and the curing time. 10-fold cross validation was used as the validation method, and performance measures were chosen as the mean squared error and the correlation coefficient. The results show that PSO was efficient in the hyper-parameters tuning of the BRT. The optimum BRT model was very accurate at predicting CPB strength. The relative importance of influencing variables was investigated, in which the cement-tailings ratio was found to be the most significant variable for CPB strength. This research indicates that more efficient reuse of waste tailings as CPB can be achieved by reducing the required number of mechanical experiments during engineering applications. [ABSTRACT FROM AUTHOR]

Cemented paste backfill (CPB) has been widely used to prevent and mitigate hazards produced during the excavation of underground stopes. In practice, the strength of CPB is often an essential parameter for successful stope design. We propose an intelligent technique in this study for predicting the unconfined compressive strength (UCS) of CPB. This intelligent technique is a combination of the artificial neural network (ANN) and particle swarm optimization (PSO). The ANN was used for non-linear relationships modelling and PSO was used for the ANN architecture-tuning. Inputs of the ANN were selected to be the tailings type, the cement-tailings ratio, the solids content, and the curing time. A total of 396 CPB specimens under different combination of influencing variables were tested for the preparation of the dataset. The results showed that PSO was efficient for the ANN architecture-tuning. Also, comparison of the predicted UCS values with experimental values showed that the optimum ANN model was very accurate at predicting CPB strength. [ABSTRACT FROM AUTHOR]

The tailings and rice straw that are produced in large quantities each year in the mining and agricultural industries, respectively, have significant effects on the ecological environment. This study aimed to explore the mechanical properties of cemented tailings backfill (CTB) mixed with alkalized rice straw (ARS) of different lengths. A series of uniaxial compressive strength (UCS) and indirect tensile strength (ITS) tests were conducted on the CTB. The results indicated that as the length of the ARS increased from 3 to 15 mm, the UCS and ITS values initially increased and then decreased. The critical length of the ARS was 12 mm, for which the effect of strength increase was the most significant. From the overall analysis, the UCS of CTB samples with ARS (9 and 12 mm) demonstrated the better improvement (increased by 10.0 and 14.7%, respectively) at 28 d curing age, and the improvement effect of the CTB samples with ARS of other lengths was not ideal. The ITS of CTB samples with ARS increased (except for an ARS length of 3 mm) regardless of the curing age; the maximum increase was approximately 24.2% at 28 d. The integrity, residual strength, and toughness of CTB sample with the ARS (12 mm) were the largest after the UCS test. Scanning electron microscopy (SEM) tests indicated that the surface of the ARS was covered with cement hydration products, and the interior of the ARS was filled with cement tailings, which produced stronger adhesion between the ARS (12 mm) and CTB matrix; the ARS performed a bridging role and suppressed crack propagation, which effectively improving the mechanical properties of CTB. Significantly short ARS exhibited a lower adhesive force with the matrix, and significantly long ARS exhibited a lower filling rate. Thus, while improving the mechanical properties of CTB, ARS provides a new method for treating rice straw and decreasing its combustion pollution. • The toughness of cemented tailings backfill (CTB) with ARS is increased. • The UCS and ITS of CTB with ARS (critical length) are significantly improved. • CTB with ARS (critical length) has the highest integrity after the USC test. • The strength of CTB was particularly sensitive to the length of ARS. [ABSTRACT FROM AUTHOR]

Highlights • A GP-based method was employed for strength forecasting of CPB. • An enlarged dataset was prepared using 1545 UCS tests on 11 types of tailings. • A sensitivity study was conducted and the relative variable importance was studied. • The GP performance was verified and compared with three ML techniques. • The generalisation capability of GP modelling to entirely new tailings was studied. Abstract As cemented paste backfill (CPB) plays an increasingly important role in minerals engineering, forecasting its mechanical properties becomes a necessity for efficient CPB design. Machine learning (ML) techniques have previously demonstrated remarkable successes in such task by providing black-box predictions. To express the non-linear relationship in an explicit and precise way, we employed genetic programming (GP) for the uniaxial compressive strength (UCS) prediction of CPB. The influence of sampling method, training set size and maximum tree depth on the GP performance was investigated. A detailed analysis was conducted on a representative GP model and the relative variable importance was investigated using the relative variable frequency, partial dependence plots and relative importance scores. The statistical parameters show that a satisfactory performance was obtained by the GP modelling (R 2 > 0.80 on the testing set). Results of this study indicate that cement-tailings ratio, solids content and curing time were the most three important variables for the UCS prediction. The predictive performance of GP modelling was comparable to well-recognised ML techniques, and the trained GP model can be generalised to entirely new tailings with satisfactory performance. This study indicates that the GP-based method is capable of providing explicit and precise forecasting of UCS, which can serve as a reliable tool for quick, inexpensive and effective assessment of UCS in the absence of adequate experimental data. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • PP fibre can improve the UCS, stiffness, ductility and stability of CPB. • Best fibre parameter levels are a fibre content of 0.15% and a fibre length of 6 mm. • PP fibre bring much more improvement to the early strength of CPB. • Fibre can bridge the cracks of CPB. Abstract This experimental study was carried out to investigate the influence of polypropylene (PP) fibre on the compressive behavior and microstructural properties of tailings (classified as CL – lean clay) cemented paste backfill (CPB). The compressive behavior was determined by the unconfined compressive strength (UCS), and structural changes in CPB were evaluated by macrostructural failure analysis and microstructural tests with scanning electron microscopy (SEM) analysis. Orthogonal tests were designed to research the influences and significance of the cement content, solid mass concentration, fibre content and fibre length on CPB. The results indicate that although the fibre parameters are not as significant as the cement content and solid mass concentration for CPB strength, they are also important factors for improving the UCS of CPB, and the effect of PP fibre on the early (3 days curing time) and later (28 days curing time) stage strength of the backfill is much more obvious than that on the medium stage (7 days curing time) strength. And fibre can enhance the stiffness and ductility of CPB with increasing strain and E 50. In addition, the fibre content is much more important than the fibre length, there is no linear relationship between UCS and the fibre parameters (fibre content and fibre length), and the best fibre parameter levels are a fibre content of 0.15% and a fibre length of 6 mm in this study. Moreover, macrostructural failure analysis and SEM microstructural tests indicated that PP fibre tend to bridge the cracks, which can reduce the porosity of the CPB matrix and improve its compactness, integrity and residual strength with some calcium silicate hydrate (C-S-H) gelling trapped on its surface. [ABSTRACT FROM AUTHOR]