Enhanced solidification/stabilization (S/S) of fluoride in smelting solid waste-based phosphogypsum cemented paste backfill utilizing biochar: Mechanisms and performance assessment.

Phosphogypsum (PG) cemented paste backfill (CPB) is a primary non-hazardous method for treating PG. However, using traditional binders like cement increases global carbon emissions and mining operational costs while complicating the reduction of fluoride leaching risks. This study introduces a novel PG-based CPB treatment method using steel slag (SS) and ground granulated blast furnace slag (GGBFS) as binders, calcium oxide as an exciter, with biochar serving as a fluoride-fixing agent. We investigated the effect of biochar addition on the hydration and solidification/stabilization (S/S) of fluoride in SS and GGBFS-PG-based materials (SSPC). The results indicated that the optimal strength and performance for fluoride S/S were achieved with a biochar addition of 0.2 wt%. Compared to the control group without biochar, the strength increased by 54.3%, and F leaching decreased by 39.4% after 28 days of curing for SSPC. The addition of 0.2 wt% biochar facilitated heterogeneous nucleation and acted as a microfiller, enhancing SSPC's properties. However, excessive biochar reduced the compactness of SSPC. Additionally, the distribution of fluoride was strongly correlated with P, Ca, Fe, and Al, suggesting that fluoride S/S is linked to the formation of stable hydration products like fluorapatite, fluorite, and complexes such as [AlF6]3- and [FeF6]3-. These findings offer a promising approach for the safe treatment of PG and the beneficial reuse of solid waste from SS and GGBFS.

Strength and leaching behavior of tailing-based paste backfill at high water content amended with lime activated ground granulated blast furnace slag and flocculant.

Flocculent is commonly used in mining activities to improve the concentration of tailing slurry by enhancing the sedimentation process of small tailings particles. The presence of flocculent in thickened tailings is unavoidable, and it affects the heavy metal leaching performances and mechanical and rheological characteristics of tailing-based cemented paste backfill (CPB). This study is carried out to investigate the physicochemical and leachability of CPB amended with flocculants and lime-activated ground granulated blast-furnace slag (GGBS). The stabilized samples were subjected to a series of model tests, including toxicity characteristics leaching procedure (TCLP) and pH, unconfined compressive strength (UCS), scanning electron microscopy (SEM), and X-ray diffraction. Moreover, the CPB amended with anionic polyacrylamide (APAM) demonstrated better performance in terms of a decrease in heavy metal leachability besides higher mechanical strength than poly aluminum chloride (PAC) and poly ferric chloride (PFC) samples. Furthermore, the UCS results showed that increasing binder content up to 15% negatively influences strength improvement of all stabilized samples because of weak connections between soil particles and cementitious material, resulting in high leachability of heavy metals. The analysis of XRD and SEM showed that anionic polyacrylamide (APAM) cases exhibited more voluminous hydration products, resulting in a compact stabilized matrix and substantially reduced heavy metal leachability.

Strength development and self-desiccation of saline cemented paste backfill.

Given that many mines around the world are located in areas where fresh water is scarce, and companies are being held to increasingly stringent sustainability and environmental responsibility standards, many mines are looking to use locally available saline groundwater or seawater as mixing water in cemented paste backfill (CPB). However, the impacts of this decision on key engineering properties of CPB (e.g. strength and self-desiccation) that affect its mechanical stability need to be better understood to allow confident selection of this practical and more sustainable solution. Thus, the effect of mixing water salinity and binder type on the strength (unconfined compressive strength, UCS) development and self-desiccation (measured by suction and volumetric water content) of CPB is explored in this research. NaCl concentrations from 0 to 300 g/L were used in CPB made with silica tailings and Portland cement type I (PC). Concentrations of 10 and 35 g/L were found to moderately increase UCS, while a concentration of 100 g/L had comparable UCS to non-saline CPB and a concentration of 300 g/L was found to significantly decrease UCS over all curing times. The overall trend is 10 g/L > 35 g/L > 0 g/L > 100 g/L > 300 g/L. The UCS of the 60-day-old CPB with a NaCl of 300 g/L is significantly lower, registering a 26% decrease compared to the UCS of the 60-day-old CPB without salt. In contrast, the UCS of the 60-day-old CPBs containing 10 g/L and 35 g/L of salt exhibits a notable improvement, being 15% and 10% higher, respectively, than the UCS of the 60-day-old CPB without salt. Water content and suction monitoring were conducted up to 28 days of curing time, and it was found that suction only slightly contributed to UCS gain of the saline CPB, and high salt contents (100 and 300 g/L) significantly inhibited the self-desiccation ability of CPB due to inhibition of cement hydration by the excessive amount of salt. The increase in strength of both saline and non-saline samples was attributed primarily to the increase in cement hydration products, while the increased strength of the samples with salinities of 10 and 35 g/L was mainly attributed to the enhancement of the binder hydration due to the low amount of salt and the presence of Friedel's salt in the pores. The effect of PC replacement by 25 to 75% with slag on CPB with 35 g/L mixing water salinity was also studied. Slag replacement of 50% and higher resulted in significantly higher UCS over most curing times. Suction likely moderately contributed to UCS of the saline CPB with slag, in addition to the presence of Friedel's salt in the pores and the acceleration of cement and slag hydration by the presence of NaCl.

Preparation and strength formation mechanism of alkali-stimulated spontaneous combustion gangue-granulated blast furnace slag backfill.

Spontaneous combustion gangue (SCG) is often used as aggregate in traditional cemented paste backfill (CPB) for mine backfill, but the activation of SCG is insufficient. To stimulate the activity of SCG for the preparation of spontaneous combustion gangue-granulated blast furnace slag backfill (SGB), a new CPB was prepared by activating SCG via a mechanochemical composite activation method and adding ground granulated blast furnace slag (GGBS) to improve its activity. The mixing ratio was optimized by the response surface method and satisfaction function, and the strength formation mechanism was analyzed by scanning electron microscopy-energy dispersive spectrometer (SEM-EDS) and Fourier transform infrared spectroscopy (FTIR). The results showed that SCG had a certain pozzolanic activity, and the optimal grinding time was 30 min. The optimal mix ratio was 82.58% mass concentration, 2.93% alkali content, 30% GGBS content, and 52.92% fine gangue rate. Calcium silicate hydrate (C-S-H) gel and calcium aluminate sulfate hydrate (C-A-S-H) gel were the main reaction products of backfill, and with increasing curing age, C-S-H gel in the reaction system was gradually converted into C-A-S-H gel. FTIR analysis results showed that there were H-O-H, Si-O, and Si-O-T (T was Si or Al) bonds in the product, indicating that C-S-H gel and C-A-S-H gel were formed in the product. A new damage constitutive model was developed. The damage constitutive model could completely describe the backfill stress-strain curve. The study verified the feasibility of preparing cemented paste backfill with SCG and GGBS, which was beneficial to clean coal mine production and environmental protection.

Fluorides immobilization through calcium aluminate cement-based backfill: Accessing the detailed leaching characterization under torrential rainfall.

Urbanization and economic development have increased the demand for fertilizers to sustain food crop yields. Huge amounts of by-products, especially phosphogypsum (PG), are generated during the wet processing of rock phosphate to produce fertilizers. Chronic exposure to fluoride in phosphogypsum in groundwater as a result of the weathering of fluoride-containing waste poses a significant health risk to millions of people. We propose a method for using calcium aluminate cement (CAC) to remediate high fluoride contents in solid waste. Column leaching tests under harsh rainfall conditions confirmed the efficient fluoride immobilization capacity of a CAC binder. Although the fluoride concentrations in leachates during the first 1-2 days (1.25 mg/L) slightly exceeded the threshold of 1.00 mg/L, the concentrations over 3-28 days (ranging from 0.98 to 0.83 mg/L) consistently remained well within the acceptable range. Furthermore, our characterization and geochemical modeling revealed the fluoride retention mechanisms of CAC-stabilized PG under laboratory-simulated conditions of torrential rainfall. During leaching, physical encapsulation prevents fluoride from contacting leachate. However, an unfavorable pH value can cause the release of fluoride from the cement matrix, which is subsequently captured by aluminate hydrate through adsorption or co-precipitation. We quantified the carbon footprint of CAC for immobilizing 1 mg of fluoride in PG, obtaining a remarkably low value of 4.4 kg of CO2, in contrast to the emissions associated with the use of ordinary Portland cement (OPC). The findings suggest a unique opportunity for extensive PG remediation. This opportunity extends the horizons of achieving zero-waste emissions in the phosphorus industry and has practical significance in the context of reducing carbon emissions.

Production of a new type of cemented paste backfill with solid waste from carbide slag, soda residue, and red mud: mechanism, optimization, and its environmental effects.

To solve the disposal problems of carbide slag (CS), soda residue (SR), and red mud (RM) solid wastes, a new type of cemented paste backfill (CPB) was prepared with CS, SR, and RM solid wastes. The mixing proportion for the CPB was optimized by combining the Box‒Behnken design (BBD) response surface method and the satisfaction function method. The strength formation mechanism for the CPB was analyzed with physical and mechanical property tests, X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), etc. The safety of the CPB was evaluated with heavy metal leaching testing. The results showed that the 28-day unconfined compressive strength (UCS) of CPB first increased and then decreased with increasing CS/RM (0.2 ~ 0.6) and SR/RM (0.2 ~ 0.6); the optimum mixing ratios were CS/RM = 0.45 and SR/RM = 0.37, and the solid mass concentration was 64.75%; dense calcium silicate (aluminum) hydrate (C-S-H/C-A-S-H) bound to the solid particles of red mud and filled pores to provide early strength for the CPB, laminar interwoven Friedel's salt (Fs), ettringite and portlandite hydration products provided late strength for the CPB; and the leaching concentrations of five heavy metals (Fe, Mn, Cu, Zn, and Cr) in the solidified CPB were greatly reduced and far below the leaching limits specified in China's Quality Standard for Groundwater (GB/T 14848-2017).

Immobilization of Cr(VI)-containing tailings by using slag-cementing materials for cemented paste backfill: influence of sulfate and limestone addition.

Heavy metals in mine tailings lead to serious environmental problems. Cemented paste backfill (CPB) is widely used for treating the mine tailing. The high cost of ordinary Portland cement (OPC) reduces the profit of mine production. The work investigates the treatment of Cr(VI)-containing tailings by using slag-based cementitious materials for CPB. Flue gas desulfurization gypsum (FGDG) and limestone were used to modify the properties of samples. Results showed that the coupling addition of 6 wt% FGDG and 3 wt% limestone (A6L3) led to the highest compressive strength of CPB samples, which also presented satisfactory immobilization effects for Cr(VI). The compressive strength of CPB samples using A6L3 as a binder was comparable to the OPC-based sample, reaching about 5.53 MPa; the immobilization efficiency for Cr(VI) was about 99.5%. The effects of FGDG and limestone were twofold: the addition of FGDG favored the formation of ettringite and then contributed to a more compact structure; besides, incorporating limestone increased the packing density of the CPB system by decreasing the loosening and wedge effect.

Changes in the Strength and Leaching Characteristics of Steel Slag-Oil Shale Residue-Based Filling Paste in a Complex Erosive Environment.

Our research group prepared a new filling paste consisting of steel slag-oil shale residue and no admixtures. It was used as the research object to explore the combined effect of chloride and dry-wet cycling-driven erosion on the long-term stability of a cemented filling paste made of total solid wastes. Macroscopic experiments and microscopic analyses methods were employed. The influence of solutions with different mass fractions of chloride salts and different cycling periods on the uniaxial compressive strength and toxicity of the steel slag-oil shale residue-based filling paste was studied, and the deterioration mechanisms of the steel slag-oil shale residue-based filling paste under combined erosion from chloride and dry-wet cycling were investigated. The test results showed that in the same cycling conditions, the strength of the steel slag-oil shale residue-based filling paste increased first with the increase in the mass fraction of the chloride solution and then decreased with the increase in the mass fraction of the chloride solution after reaching the peak value; the leached concentrations of heavy metal ions decreased with increasing chloride salt mass fraction. With an increase in the number of dry-wet cycles, the compressive strength of the specimens in the chloride salt solution with a mass fraction of 0 (pure water) first increases and then tends to be stable. The strength of samples in 5% and 10% chloride salt solutions increased first and then decreased with an increase in the number of dry-wet cycles. The leached concentrations of heavy metal ions from the samples in all three solutions first decreased and then stabilized. The prehydration products of the steel slag-oil shale residue-based filling paste were C-S-H gels, AFt and Friedel's salt, and these increased with increasing chloride salt mass fraction and the number of dry-wet cycles. However, the hydration reactions of the samples in the 0% chloride solution nearly stopped in the later stages of cycling, and the samples in 5% and 10% chloride salt solutions developed local cracks due to the accumulation of hydration products. The results showed that the number of dry-wet cycles and the chloride salt mass fraction affected the strength and leaching characteristics of the steel slag-oil shale residue-based filling paste by changing the type and amount of erosion products. The test results provide a scientific basis for the promotion and application of backfilling pastes made from total solid wastes.

Control of surface deformation and overburden movement in coal mine area by an innovative roadway cemented paste backfilling method using mining waste.

Caving mining method could lead to massive waste rocks hauled to surface while leaving a large void in underground. This would eventually result in the surface subsidence and damage to the environment and surface infrastructures. In this study, we proposed three different backfilling methodologies to minimise the surface subsidence being 1) 100 % mining and 100 % backfilling (method 1); 2) leaving one slice of coal between two backfilled slices (method 2) and 3) leaving one slice of coal between one backfilled slice (method 3). The backfilling materials are made of waste rock, fly ash and cement and the optimal ratio has been found through the test program designed based on the orthogonal experiment design method. The strength of the backfilling paste is 3.22 MPa at the axial strain 0.033. The mine scale numerical simulation has also been conducted and it was concluded that the method 1 would lead to 0.098 m roof deformation in underground roadway whereas the method 2 and method 3 only induced a roof deformation around 32.7 % and 17.3 % of that induced by the method 1, respectively. All three methodologies have been approved to minimise the roof deformation and disturbance to the rock by mining operations. At last, the surface subsidence has been scientifically evaluated based on the probability integration method of surface movement. It indicated that the surface subsidence, horizontal movement, inclined movement and curvature of rock surrounding the panel void were all below the minimum value required by regulation. This confirmed that the selected backfilling mining is able to ensure the integrity of the surface infrastructures. This technology provides a new way to control the surface subsidence caused by coal mining.

Development of Cemented Paste Backfill with Superfine Tailings: Fluidity, Mechanical Properties, and Microstructure Characteristics.

Previous studies have shown that the effectiveness of superfine tailings cemented paste backfill (SCPB) is influenced by multiple factors. To optimize the filling effect of superfine tailings, the effects of different factors on the fluidity, mechanical properties, and microstructure of SCPB were investigated. Before configuring the SCPB, the effect of cyclone operating parameters on the concentration and yield of superfine tailings was first investigated and the optimal cyclone operating parameters were obtained. The settling characteristics of superfine tailings under the optimum cyclone parameters were further analyzed, and the effect of the flocculant on its settling characteristics was shown in the block selection. Then the SCPB was prepared using cement and superfine tailings, and a series of experiments were carried out to investigate its working characteristics. The flow test results showed that the slump and slump flow of SCPB slurry decreased with increasing mass concentration, which was mainly because the higher the mass concentration, the higher the viscosity and yield stress of the slurry, and thus the worse its fluidity. The strength test results showed that the strength of SCPB was mainly affected by the curing temperature, curing time, mass concentration, and cement-sand ratio, among which the curing temperature had the most significant effect on the strength. The microscopic analysis of the block selection showed the mechanism of the effect of the curing temperature on the strength of SCPB, i.e., the curing temperature mainly affected the strength of SCPB by affecting the hydration reaction rate of SCPB. The slow hydration process of SCPB in a low temperature environment leads to fewer hydration products and a loose structure, which is the fundamental reason for the strength reduction of SCPB. The results of the study have some guiding significance for the efficient application of SCPB in alpine mines.

Control effect of coal mining solid-waste backfill for ground surface movement in slice mining: a case study of the Nantun Coal Mine.

Management of solid waste and protecting the ecological balance of the region are key challenges that the coal mining industry has to face. This study evaluated the effect of solid waste backfilling mining on the overlying strata movement and surface deformation variation pattern in slice mining. The mechanical characteristics of different cemented paste backfills (CPB) were compared. The CPB specimens were made of coal gangue and cement with or without the addition of fly ash. The experiments showed that the mechanical strength of the CPBs made of coal gangue and cement increased dramatically. A numerical simulation was then performed to analyze the variation patterns of the overlying strata displacement and surrounding rock stress distribution before and after filling the 3lower and 3upper coal seams with CPB. The CPBs reduced the movement of the surface by 95.1% and 95% during the mining of the 3lower and 3upper coal seams, respectively. Finally, we used a mining-induced subsidence prediction and analysis system to predict the influence of the 3lower and 3upper coal seams on the ground surface subsidence. It was found that the ground surface subsidence induced by CPB mining was 1/20 that of the cumulative ground surface subsidence caused by caving mining. CPB mining could effectively control the ground surface subsidence caused by multi-slice mining of the thick coal seam, offering protection for buildings above the ground. Our research provides theoretical and technical support for coal mining under buildings subjected to similar conditions.

In-situ remediation of phosphogypsum in a cement-free pathway: Utilization of ground granulated blast furnace slag and NaOH pretreatment.

In-situ remediating phosphogypsum (PG) for cemented paste backfill (CPB) in the contaminated site is economic management for promoting sustainable developments in the phosphate industry. This study concerns the combined use of NaOH pretreatment and ground-granulated blast furnace slag (GGBFS) additives to promote the solidification/stabilization of PG with a lower carbon footprint pathway. According to physico-chemical analyses, the NaOH pretreatment effectively removed approximately 95% of F within the PG, which may originally be present as sparingly soluble fluorides or coexisting with silicates. The micro mineralogical characterization illustrates that the pretreatment can accelerate the early age hydration, with more hydration products observed, including calcium silicate hydrates and ettringite, effective F and P retention candidates. Whereas the incorporation of GGBFS plays an essential role in promoting the generation of additional cement hydrates at the following stages. The macro mechanical performance analysis indicates that the mixtures of pretreated-PG-OPC-GGBFS exhibit an excellent mechanical performance satisfying the design criteria. Subsequent elemental mapping and toxicity characteristic leaching procedures demonstrate that this combined approach has a competitive F and P immobilization ability compared to the typical OPC binder and individual GGBFS addition. The newly formed phases effectively controlled the concentration of F and P through adsorption, incorporation, or encapsulation. Objectively, the proposed methodology can be a promising candidate pathway for extrapolating the in-situ immobilization of PG. This study opens up new perspectives for synergetically recycling PG and GGBFS in a profitable and low carbon footprint way.

Synergistic effect of activator nature and curing temperature on time-dependent rheological behavior of cemented paste backfill containing alkali-activated slag.

Cemented paste backfill (CPB) that contains alkali-activated slag (AAS) produces more desirable properties and performance (enhanced fluidity, higher strength, lower cost, and limited carbon emission) as compared with CPB made with cement. Significant efforts have been devoted to the study of the effect of the individual factor on the rheology of AAS-CPB. However, the synergistic effect of curing temperature, time, and activator nature is still unclear. Therefore, the current research aims to investigate the time-dependent rheology of AAS-CPB under the combined influence of curing temperature, silica modulus (Ms), and activator concentration (AC). The findings revealed that a higher curing temperature results in a reduction in fluidity and an increase in the thixotropy of CPB. The evolution of rheological parameters of AAS-CPB is more insensitive to the curing temperature as compared to that of OPC-CPB. During the initial 2 h, higher AC can weaken the rheological parameter. However, a more rapid growth rate of rheological properties was observed after 2 h. The rheological parameters of AAS-CPB with higher Ms are always lower than those of AAS-CPB with lower Ms at all temperatures studied. In addition, the discrepancy in the linear correlation between thixotropy and plastic viscosity for OPC-CPBs and AAS-CPBs indicates the different hydration rates of slag and Portland cement. These findings are beneficial in guiding the mix proportion design of AAS-CPB in mines with various underground temperatures.

Analysis on mechanical properties and mesoscopic acoustic emission characteristics of prefabricated fracture cemented paste backfill under different loading rates.

The mechanical characteristics of cemented paste backfill (CPB) are significantly influenced by the loading rate (LR) and initial defects. Therefore, the CPB with prefabricated fracture (PF, PFCPB) was prepared, and uniaxial compressive strength (UCS) tests considering LR and acoustic emission (AE) monitoring were performed. The particle flow code (PFC2D) was introduced to analyze the mesoscopic crack evolution of the filling body, and the moment tensor theory was used to invert the AE signals characteristics. The results show that as the PF angle increased, the UCS and elastic modulus (EM) of PFCPB decreased and then increased, and the 30° PF was the turning point. The mechanical properties of PFCPB were deteriorated by the presence of PF. Meanwhile, the mechanical properties of PFCPB were positively correlated with the LR. The stress-strain curve of PFCB (excluding 90°) showed bimodal curves. After the UCS test, the macro crack of PFCPB sprouted at the tip of PF or converged toward PF. PFCPB mainly suffered from shear failure, accompanied by a few tensile failures. Numerical simulation results showed that the crack initiation stress of PFCPB was reduced by the PF. The number of cracks first dropped and then gradually increased when the PF angle was enhanced, while gradually increased with the LR increased. Meanwhile, the mesoscopic AE characteristics of CPB were strongly in line with the test results. The AE events of 0 ~ 60° PFCPB experienced two slow rising periods and rapid rising periods. The temporal and spatial distribution characteristics of AE corresponded to the crack evolution trend. The PF was prone to stress concentration, especially at the tip and upper and lower surfaces of 0 ~ 45° PF, resulting in rapid crack initiation and reducing the energy storage limit and mechanical behavior of 0 ~ 45° PFCPB. The increasing LR (within a certain range) was in favor of improving the mechanical behavior of the filling body. The research results can provide a basic reference for the stability evaluation of the filling body with initial defects.

Preparation and performance of composite activated slag-based binder for cemented paste backfill.

The utilization of blast furnace slag (BFS) and fly ash (FA) to replace ordinary portland cement (OPC) has become a hot topic in the preparation of low-cost cemented paste backfill (CPB). This study has prepared a composite activated slag-based binder (CASB) using BFS and FA as the basic raw materials and desulfurization gypsum (DG) and cement clinker (CC) as the activator. The optimum ratio of CASB was determined based on the orthogonal test and the efficacy coefficient method. The hydration products and hydration mechanism of CASB materials were further investigated using XRD, TG, and SEM tests; on this basis, the compressive strength of hardened CASB-CPB under different working conditions and the rheological properties of fresh slurry were investigated, and the cost analysis and environmental effects of CASB were carried out. The results show that the optimum ratio of CASB was 15:12:13:60 for FA: CC: DG: BFS; the hydration mechanism of CASB was the coupled alkali-sulfate activation of CC and DG, and the main hydration products were hydrated calcium silicate gels (C-S-H gels) and ettringite (AFt); increasing the mass concentration (Cw) at a constant cement-aggregate ratio (C/A), which caused a significant improvement in the compressive strength at 7 and 28 d while reduced the flowability of the slurry; CASB considerably reduced the filling cost compared to OPC, and effectively immobilization the heavy metals in the tailings. This paper has developed a cement alternative binder of CASB, which has considerable significance for the comprehensive utilization of solid waste, reduction of filling costs, and improvement of economic and ecological benefits of the mine.

Investigation of Fluidity and Strength of Enhanced Foam-Cemented Paste Backfill.

To solve the problems of high cement dosage and poor fluidity of conventional cemented paste backfill (CPB) materials, the fluidity and strength properties of foam-cemented paste backfill (FCPB) were studied in combination. Based on determining the optimum contents of a foaming agent and a foam stabilizer, FCPB density was measured. To investigate the fluidity and strength of FCPB under different foam contents (0%, 5%, 10%, 15%, 20%, 25%, 30%, and 40%), different solid contents (75 wt.% and 77 wt.%), and different cement-tailing ratios (1:4 and 1:5), spread tests and unconfined compressive strength (UCS) tests were conducted. In addition, the FCPB microstructure was analyzed by scanning electron microscopy (SEM). The results indicate that the optimum combination dosages of sodium lauryl sulfate (K12) and sodium carboxymethyl cellulose (CMC) are 0.5 g/L and 0.2 g/L. The density decreases with the foam content (FC), but the fluidity and strength of the FCPB increase first and then decrease with the FC. In addition, the microstructure analysis explains the enhanced strength of FCPB by adding foam. These results contribute to further understanding the effect of foam content on the fluidity and strength of the FCPB.

The Slope Safety, Heavy Metal Leaching, and Pollutant Diffusion Prediction Properties under the Influence of Unclassified Cemented Paste Backfill in an Open Pit.

Open-pit unclassified cemented paste backfilling (OPUCPB) methods have not only addressed the disposal problems of tailings but also eliminated geological hazards of high and steep open pit slopes and created conditions for ecological restoration of the open pit in the future. In this paper, slope safety simulations, heavy metal leaching, groundwater monitoring, and pollutant diffusion predictions were examined to evaluate the slope safety pattern and environmental protection enabled by OPUCPB. The results showed that: (1) The safety factor of the open pit slope was proportional to the height of OPUCPB operation. Under the condition of seismic force and seepage field, the safety factor of slope B was increased from 1.188 to 1.574 by OPUCPB. (2) The toxic and harmful components in tailings were significantly stabilized by the OPUCPB. Under the conditions of acid leaching and water leaching, the quality of the leaching solution met the requirements of the class III limit of groundwater (GB/T14848-2017). (3) The monitoring results of groundwater quality around the open pit showed that the OPUCPB had no effect on groundwater, and the water quality met the requirements of the class III limit of groundwater (GB/T14848-2017). (4) Considering the diffusion prediction of pollutants and groundwater under extreme conditions, it was found that the pollution process is slow, and the shortest time required for pollutants to reach the standard limit is 232 d at a distance of 50 m from the leakage point. Therefore, the influence of OPUCPB can be controlled, and this method can achieve improved reclamation of open pits and safety treatment of tailings. It was worth popularizing and applying in mining enterprises.

Highly-efficient fluoride retention in on-site solidification/stabilization of phosphogypsum: Cemented paste backfill synergizes with poly-aluminum chloride activation.

Phosphogypsum (PG) is a massively generated hazardous by-product in the phosphorus industry. Large-scale, efficient, profitable on-site recycling is an emerging topic for promoting sustainable phosphorus circularity and mitigating potential human exposure. In this work, we integrated a green and low-cost additive polymeric aluminum chloride (PAC) into the binder design of PG immobilization. The overall experimental results illustrate that the incorporation of PAC can efficiently promote the cement hydration reaction, with amorphous phases increased from 25.9 wt% (control group) to 27.5 wt% (with 2 g/L PAC). The macro-investigations indicate that the PAC optimized the porosity and mechanical properties of specimens, facilitating a mechanically stable solidified matrix for extrapolating its field engineering application. The detailed micrographs and elemental mapping demonstrate that apart from co-existing with the hydration products, the PAC agent plays a role in the immobilization of fluoride. Herein, the combined optimization enhanced the fluoride retention capacity due to the precipitated additional hydration products, comparable encapsulation, and high adsorption ability of PAC agents. Therefore our design of PAC-augmented binders can open up a new field of PG on-site solidification/stabilization application that ensures efficient fluoride retention in a technically feasible and financially profitable methodology.

Strength prediction and application of cemented paste backfill based on machine learning and strength correction.

Cemented paste backfill (CPB) is wildly used in mines production practices around the world. The strength of CPB is the core of research which is affected by factors such as slurry concentration and cement content. In this paper, a research on the UCS is conducted by means of a combination of laboratory experiments and machine learning. BPNN, RBFNN, GRNN and LSTM are trained and used for UCS prediction based on 180 sets of experimental UCS data. The simulation results show that LSTM is the neural network with the optimal prediction performance (The total rank is 11). The trial-and-error, PSO, GWO and SSA are used to optimize the learning rate and the hidden layer nodes for LSTM. The comparison results show that GWO-LSTM is the optimal model which can effectively express the non-linear relationship between underflow productivity, slurry concentration, cement content and UCS in experiments ( R = 0.9915 , RMSE = 0.0204, VAF = 98.2847 and T = 16.37 s). The correction coefficient (k) is defined to adjust the error between predicted UCS in laboratory (UCSM) and predicted UCS in actual engineering (UCSA) based on extensive engineering and experimental experience. Using GWO-LSTM combined with k, the strength of the filling body is successfully predicted for 153 different filled stopes with different stowing gradient at different curing times. This study provides both effective guidance and a new intelligent method for the support of safety mining.

Effects of slag-based cementitious material on the mechanical behavior and heavy metal immobilization of mine tailings based cemented paste backfill.

Slag-based cementitious material was synthesized from blast furnace slag, clinker, gypsum, and activator to replace cement in cemented paste backfill (CPB). We researched the influence of slag-based cementitious material dosages and curing times on the properties of CPB, including unconfined compressive strength tests, leachate toxicity and chemical speciation of heavy metal as well as microstructural tests and analyses. The results indicated that the addition of slag-based cementitious material improved the compressive strength of the CPB, which attained the compressive strength requirements (≥1.0 MPa) at 28 days. The leachate concentrations of Pb, Cr, Cu, and Cd in CPB decreased as the slag-based cementitious material dosage and curing period increased, which met the standard (GB 5085.3-2007). The dosage of 10% slag-based cementitious material could effectively immobilize the heavy metals in the tailings, and the immobilization performance was similar to that of 20% cement, which indicated the amount of slag-based cementitious material was only half the quantity of cement in CPB. Microstructural analysis showed the hydration products included calcium silicate hydrate, ettringite, and portlandite, which could enhance the bonding force between the tailing grains.