Influence of ballasted material properties in enhancing the separation of high concentration suspended solids in coal mining water

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Abstract

With the continuous improvement and economic development of China’s coal mining industry, research on the characteristics of wastewater discharge has gained attraction in recent years. Ballasted flocculation technology, has been widely used in recent years. However, its effect on the settling velocity of the flocs in wastewater has not been quantified. This study aimed to screen optimal reaction condition for ballasted flocculation and to derive the settling velocity of the ballasted flocs under these conditions. The results showed that, the size of the ballasted material had no significant effect on the turbidity and suspended solids removal rate; however, the settling time was effectively shortened, and the turbidity removal rate could reach more than 95% within 15 s. Owing to the optimization of the reaction conditions, 90% of the flocs sizes were distributed in the 80–200 µm range. Moreover, the difference between the theoretical and the measured value of the ballasted flocculation was small, indicating that the derived formula could effectively quantify the settling velocity of the ballasted flocs under optimal conditions. These findings may serve as a critical reference for the treatment of high-suspended solids in coal mining water and have significant potential for engineering application.

Introduction

Coal, a primary energy source, accounts for approximately 70% of China’s primary energy production and consumption (National Bureau of Statistics, 2018). In the coal mining process, approximately 2.1 tons of water is discharged for every ton of coal mined in China (Kevin et al., 2005). Moreover, the re-utilization rate of water is approximately 60%, resulting in a massive wastage of coal mining water (CMW) resources (He et al., 2018). Meanwhile, with the continuous improvement in the industrialization of coal mining in China, the widespread use of comprehensive mining equipment has led to an increase in productivity, resulting in more suspended solids in water. Solids suspended from mining activity are characterized by small sizes and centralized distributions (He et al., 2010), resulting in a traditional coagulation-sedimentation that leads to multiple challenges, such as deterioration of effluent quality and floc deposition in the tube of the inclined-tube sedimentation tank (He and Li, 2010, Zhou et al., 2000, Chen and Guo, 2004). These problems cause sedimentation tank collapse and engineering accidents.

In recent years, new technologies such as the ballasted flocculation process, which is characterized by a high hydraulic load and strong resistance to impact load, have been implemented in the coal mining industry.

Ballasted flocculation uses high-density insoluble materials such as fine sand or magnetic powder as the core of the floc, resulting in a high precipitation efficiency, small footprint, and strong resistance to impact load (Johnny G et al., 2012; Christian et al., 2002a). Moreover, coagulation between the dosage material and suspended solids in the water accelerate floc growth and facilitate their removal by gravity or magnetic forces (US EPA, 2003; Jason and Sun, 2002). Since the 1980s, numerous studies have been conducted on the application of ballasted flocculation in various fields of environmental protection(Christian et al., 2002b; Dianous and Dernaucourt, 1991). Christian et al. (Christian et al., 2002b) determined that the turbidity removal rate of the ballasted flocculation process reaches a high level in a short time (less than 15 s) and remains stable, and a partial factorial analysis was used to select the significant factors. Dianous and Dernaucourt (Dianous and Dernaucourt, 1991) found that the maximum G value tolerated by ballasted flocs was 10 times more that of ordinary flocs, indicating that the ballasted flocs are stronger and are more robust in resisting shear force. The authors also reported that the optimal size of the ballasted material should be less than 160 µm. In CMW treatment, ballasted flocculation has many practical applications. Mackie and Walsh (Allison and Margaret, 2015) compared the effects of traditional and ballasted flocculation for the removal of suspended solids and suggested that ballasted flocculation was the most effective method with the lowest turbidity of the outlet. Zhang et al. (2019) studied the effects of various reaction factors of ballasted flocculation on the removal rate of turbidity and suspended solids (SS) of CMW. They reported that the dosage of ballasted material did not show a correlation with the efficacy of turbidity or SS removal; however, the ballasted material could improve the density of flocs, and accelerate the sedimentation speed effectively. Further, Zheng et al. (2016) suggested that ballasted flocculation could reduce the dosage of coagulant/polymer and effectively decreased the hydraulic retention time. Overall, existing research on ballasted flocculation has predominantly focused on optimizing reaction factors and comparing the effects of different treatment processes on a beaker-scale or small scale, and the factors that most significantly impact the treatment still remain unknown. Moreover, the specific mechanism of ballasted flocculation in the effective treatment of high suspended solids in the CMW is still not clear.

In this study, ballasted flocculation methods were selected to treat the CMW. To explore the change in floc morphology and the mechanism of accelerating floc settlement by dosing the ballasted material, we used different experimental designs to determine the optimal reaction conditions and characterize the relationship between the factors and outlet quality. Then, theoretical formulas for the settling velocity of ballasted flocs were derived and verified experimentally. The research presented in this study will serve as a critical reference for the treatment of high-suspended solids in coal mining water.

Section snippets

Raw water and reagents

The pilot-scale experimental apparatus used in this study required approximately 2 m3/h of raw water; however, this quantity could not realistically be collected and transported from a coal mining area. Therefore, synthetic water made up of a mixture of coal slime and tap water was used. The coal slime was collected from the bottom of the equalization tank of a large wastewater treatment plant in the coal mining region of Handan City, Hebei Province, China. A comparison between the water

Selection of approximate optimal range for each factor

In this study, the effects of different operating conditions on the turbidity and SS removal rate were investigated. The experimental results are presented in Fig. 6. When the dosage of PAC was 160 mg/L, the dosage of FeCl3 was 180 mg/L, the dosage of Al2(SO4)3 was 220 mg/L, and that of PFS was 80 mg/L, the turbidity removal rate reached a maximum at 75.5%, 72.3%, 64.8% and 64.5%, respectively. Similarly, the SS removal rate (SRR) of the PAC was higher than that of the others. The better

Conclusions

Most of the existing research has focused on application of ballasted flocculation used in different environmental fields and, to the best of our knowledge, the dosage-related effects of ballasted material on floc coagulation remain unknown. In this study, a single factor experiment and PB experimental design were used to determine the optimal reaction conditions for flocs coagulation. Then, theoretical formulas for the settling velocity of ballasted flocs were analyzed and derived. Under

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by the Water Pollution Control Center, China University of Mining & Technology. State Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University and in part by grants from the Hebei Province Science and Technology Project as a “Key Technology and Engineering Demonstration of Optimal Allocation of Coal mining water Resources” (Grant No. 15274006D) and the National Key Research and Development as a “High Efficiency and Low Consumption

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