Control of Fluoride Pollution in Cemented Phosphogypsum Backfill by Citric Acid Pretreatment.

Using phosphogypsum (PG) as the aggregate of cemented backfill is an economical and effective method of PG utilization. However, the stability and performance of cemented backfill are challenged by the rich fluoride content in PG. In this study, the effects of citric acid pretreatment on PG defluorination, backfill performance and environmental behavior were investigated by washing PG with different concentrations of citric acid and washing times. The results showed that the citric acid pretreatment could significantly reduce the fluoride content in PG and promote the hydration reaction with the binder, thus greatly reducing the usage and cost of the binder in actual production. Considering the efficiency of defluorination, the optimal citric acid concentration and washing times were determined to be 4% and 7-8 times, respectively. In addition, after citric acid pretreatment, the viscosity and setting time of the backfill slurry and the porosity of the backfill reduced, and the strength of the backfill improved, which was conducive to slurry pipeline transportation and underground mine stability. Finally, a further analysis of environmental behavior was conducted and it was found that the citric acid washing greatly reduced the content of fluoride in the bleeding water of slurry and the backfill leachate, which met the integrated wastewater discharge standard in China. The results of this study can provide important guidance for the large-scale recycling and environmental management of PG.

An extra-chelator-free fenton process assisted by electrocatalytic-induced in-situ pollutant carboxylation for target refractory organic efficient treatment in chemical-industrial wastewater.

For traditional Fenton processes, the quenching behavior of radical contenders (e.g., most aliphatic hydrocarbons) on hydroxyl radicals (·OH) usually hinders the removal of target refractory pollutants (aromatic/heterocyclic hydrocarbons) in chemical industrial wastewater, leading to excess energy consumption. Herein, we proposed an electrocatalytic-assisted chelation-Fenton (EACF) process, with no extra-chelator addition, to significantly enhance target refractory pollutant (pyrazole as a representative) removal under high ·OH contender (glyoxal) levels. Experiments and theoretical calculations proved that superoxide radical (·O2-) and anodic direct electron transfer (DET) effectively converted the strong ·OH-quenching substance (glyoxal) to a weak radical competitor (oxalate) during the electrocatalytic oxidation process, promoting Fe2+ chelation and therefore increasing radical utilization for pyrazole degradation (reached maximum of ∼43-fold value upon traditional Fenton), which appeared more obviously in neutral/alkaline Fenton conditions. For actual pharmaceutical tailwater treatment, the EACF achieved 2-folds higher oriented-oxidation capability and ∼78% lower operation cost per pyrazole removal than the traditional Fenton process, demonstrating promising potential for future practical applications.

Fluoride immobilization and release in cemented PG backfill and its influence on the environment.

Waste recycling must consider secondary pollution, which is affected by recycling methods. Cemented phosphogypsum (PG) backfill is a cost-effective method for PG recycling. However, due to dynamic environmental conditions, the impurity fluoride is challenging to stabilize. In this study, we investigated the immobilization and release of fluoride and its influence on backfill strength. The results showed that the fluoride was temporarily immobilized by PG. However, when the binder was mixed with PG to make the backfill, immobilized fluoride was re-released into the backfill slurry due to the increased pH caused by binder hydration. Therefore, simply converting fluoride into CaF precipitation cannot avoid the risk of fluoride exceeding the Chinese standard (GB8978-1996) (10 mg/L). Furthermore, fluoride deteriorated strength development by inhibiting binder hydration and weakening the backfill structure. The fluoride content in the slurry, rather than in PG, directly affected the backfill strength. Considering the recycling of PG as aggregate for backfill, fluoride should be removed in advance or immobilized in other low-solubility forms instead of CaF precipitation. These results were of great significance for the large-scale resource recycling and safety management of PG.

Research of Dynamic Tensile Properties of Five Rocks under Three Loading Modes Based on SHPB Device.

The validity of calculating the dynamic tensile strength of rock materials based on dynamic Brazilian tests is problematic. In order to gain a deeper understanding of the effects of three typical loading methods on the damage mechanism of rock specimens in the dynamic Brazilian tests, five different rocks were selected for the study. In the constant incident energy dynamic Brazilian test, the loading modes had a significant effect on the loading rate and dynamic tensile strength of the specimen, with the highest loading rate and tensile strength of the specimens under mode-III loading, followed by mode-I loading and mode-II loading. A high-speed camera and the digital image correlation (DIC) technique were used to successfully capture the rupture process of the Brazilian disc during impact loading. The evolution of the displacement and strain fields of the specimen was obtained by DIC technique, and four typical failure patterns and two rupture characteristics in the dynamic Brazilian test were summarized. The loading mode determined the crack initiation position of the specimen in the dynamic Brazilian test. The results showed that the mode-III loading is the most consistent with the Brazilian test theory, while the mode-II loading violates the test principle.

Reuse of phosphogypsum pretreated with water washing as aggregate for cemented backfill.

Phosphogypsum (PG) is reused as aggregate in the cemented backfill, which effectively improves the PG reutilization efficiency. However, the massive impurities contained in aggregate PG would adversely affect the hydration of binder, and therefore deteriorate the strength development of backfill. This research starts with the feasibility study on pretreating PG with the water washing method. Based on the most economical principle of the water demand, the optimal conditions for washing PG were determined at a stirring time of 5 min and a solid-liquid ratio of 1:0.5. Then, the original and pretreated PG were made into the backfill. Compared to using the original PG, the backfill slurry using the pretreated PG had better fluidity performance, such as the lower slurry viscosity and the higher bleeding rate. Furthermore, with the pretreated aggregate PG, the backfill strength was significantly enhanced by more than 8 times. Finally, the environmental behavior of the cemented backfill was investigated. Using the pretreated PG as aggregate, concentrations of PO43- and F- in the bleeding water and backfill leachates could meet the Chinese standard for integrated wastewater discharge. The results extend the reuse of PG as aggregate in a more environmental-friendly way, meeting the needs for sustainable mines.

Development and Application of Gas Production Measurement System of Coal-Rock under Temperature-Pressure Coupling.

In this study, a measurement system for gas generation of coal-rock under temperature-pressure coupling was developed by adding gas extraction, collection, and flow-monitoring devices to the original stainless-steel liquid seepage pipeline of an MTS-815 rock triaxial testing machine, which can be used to study the production mechanism of coalbed methane in a real geological environment. The system has the functions of axial loading, confining pressure loading, continuous heating, gas gathering, etc., and has the advantages of good air tightness, high accuracy and stability, long-term loading and heating, and controllable single variables. The preliminary test for the gas production of anthracite in the Shaanxi Formation of the Qinshui Basin under temperature-pressure coupling was carried out by the developed test system. The results show that the test system can provide accurate and effective measurement means for the study of gas production by coal-rock deformation and is expected to provide effective help for the control and exploitation of coalbed methane.