Coal Preparation » Dewatering
Mine tailings, commonly stored in the form of a slurry with high water content within tailings dams, represent a significant environmental and geotechnical stability concern. Tailings dewatering is a critical process aimed at mitigating environmental hazards linked with tailings dams. By reducing water volume, it transforms tailings into a drier form that is more manageable for storage. Electrokinetic dewatering introduces an innovative method by creating an electric field within the tailings sample, effectively decreasing its moisture content. This technique necessitates electrical power to generate the electric field, which can be supplied either through mains power (50 Hz) or battery power sources.
The overall aim of this project was to design a tailing dewatering system (batch mode) using pulsed power technology as an emerging technique for separation of water and solid of mine tailings. The objective was to propose an optimal method for scale up and potential future industrial applications. Pulsed power technology involves the rapid discharge of stored energy from electrical components, typically capacitors, over micro- or nanosecond durations, resulting in high-power pulses. This technique offers precise control over the timing and duration of energy discharges. Tuning pulse parameters such as pulse magnitude, pulse duty cycle, and pulse frequency can enhance efficiency and reliability, particularly in high-power applications. In this project, the pulsed power generator was supplied by a DC source (20 and 30 V). The voltage level of the DC supply directly influences the amplitude of the applied pulses to the tailings. The pulse ON-state duration (pulse width) and the number of applied pulses during dewatering process can be tunned through adjustments in duty cycle and frequency, respectively. By optimizing pulse parameters, the pulsed power generator can deliver high-power outputs in short bursts, enabling compact and efficient energy transmission. The intermittent delivery of energy in short pulses helps mitigate heating effects within the tailings, resulting in reduced average power consumption for dewatering compared to continuous operation using mains power (50 Hz) or battery power sources.
Samples from both synthetic and actual tailings obtained from two mine sites underwent dewatering using the employing electrical pulses instead of relying on battery or mains power sources. This offers safety benefits by minimizing prolonged exposure to high-power levels.
Tests were conducted at two different scales of tailing, 1 L and 5 L to assess the capability of proposed dewatering method in greater scales compared to prior work reviewed in literature. The achieved results indicates that the pulse amplitude directly influences the dewatering rate, suggesting higher voltage settings may be necessary for larger volumes. Additionally, the duty cycle significantly impacts dewatering rates, with higher duty cycles correlating with increased dewatering rates while maintaining optimal energy consumption levels.
In a 1 L tailings dewatering experiment, elevating the applied pulse amplitude from 20 to 30 V revealed a clear link to increased water extraction, resulting in a boost of 8% to 70% in water removal and consequently yielding a drier solid cake. In the dewatering process of 5 L of tailings, increasing the applied pulse duty cycle results in achieving a drier solid cake, with variations in moisture content reduction ranging from 8% to 14%, depending on the magnitude and frequency of the applied pulses.
In both the 1 L and 5 L trials, fine-tuning the pulse parameters enhanced dewatering performance while also improving energy efficiency, achieving energy savings of 8.5% for the 1 L samples and 16% for the 5 L samples. Electrokinetic methods have not been widely adopted as a standard practice for tailings dewatering, particularly in coal preparation plants. However, comparing results with other published lab research on electrokinetic dewatering highlights the potential of this method to manage fine, highly saturated tailings, suggesting economic and sustainable advantages of pulsed power technology in tailings management.
The post-treatment chemical analysis was conducted on the extracted water and the cake to understand the role of electrokinetic theory in the dewatering process when pulsed power is applied and the potential consequences on tailings water quality. The findings concerning the pH factor of the tailing solid cake indicate a general trend towards acidity following the dewatering process across all applied electrical pulse conditions. This is due to removal of ions from the surface of tailings particles' alkalinity. Moreover, the electrical conductivity (EC) results of the cake samples suggest that the application of pulses for dewatering resulted in an elevation of the EC of the cake. Specifically, wider duty cycles, higher frequencies (ranging within 1-75 kHz), and prolonged dewatering durations contributed to an increase in the EC of the solid cake. Enhancing the applied pulse duty cycle corresponded with achieving a drier solid cake, demonstrating as a reduction in moisture content ranging from 8% to 14%. Furthermore, at higher frequency pulses (above 30 kHz), heat played an additional role in the drying process, albeit with the drawback of increased energy consumption (moisture content increased from 7.8% to 15.1% when the frequency increased from 30 kHz to 62 kHz). The chemical composition of the extracted water demonstrated the migration of ions towards the positive and negative ends of the applied electrical field. The chemical examination of the removed water indicated a rise in cation concentrations as the pulse duty cycles increased, regardless of the applied pulse frequency. The application of the proposed dewatering technique in this investigation generally led to an increase in sodium (Na) concentration within the collected water. Monitoring of liquid salinity and electrical conductivity not only aids in understanding how the electrokinetic process contributes to dewatering when pulsed power is applied it also provides the opportunity to assess potential positive or negative effects of this technology on water quality in view of the discharge or reuse the tailings water.