Influence of mesh design and surface treatments on particle transport and fate in a vibration-enhanced flooded-bed dust scrubber

Respirable coal mine dust (RCMD) is one of the biggest occupational health hazards for underground coal miners. Coal extraction is a main source of RCMD. To prevent challenges related to RCMD from developing, it is vitally important to limit airborne RCMD and stay within the regulatory limitations for mine operators. The flooded-bed dust scrubber (FBS), mounted on a continuous miner, is one of the most widely used dust mitigation technologies in room-and-pillar mining, but it is limited by technical challenges, the most notably being the potential to clog. In this study, the impact of mesh design and surface modification on system efficiency is examined using different vibrating, liquid-coated, stainless-steel mesh panels in a laboratory-scale FBS. Background While FBSs are ubiquitous in continuous-mining applications, they face numerous technical challenges. Most significantly, the units exhibit a fundamental tradeoff between airflow and mesh density. Higher mesh densities typically have higher particle capture rates, but they also produce higher pressure drops, which reduce the amount of air that can be cleaned through the scrubber. Alternatively, low-density meshes can achieve higher air flow rates, but the dust-capture capacity is poor. These issues are further compounded when the filter mesh is clogged, which reduces overall efficiency and necessitates frequent cleaning. Operational downtime caused by the cleaning process slows the production rate because of overall system degradation. Recent attempts have been made to improve system performance using vibration-enhanced filter panels [1-3]. In Uluer et al. (2023), an experimental study validated that adding a vibration-enhanced mesh panel to a laboratory-scale FBS can improve dust collection efficiency over that of a static unit by 6 percent [1]. The present study proposes further modifications to the vibration-enhanced FBS through surface modifications of the mesh filter. Methods An efficiency study was conducted to determine the effects of vibrating filter panels with various mesh designs. Each test was conducted for 5 min, during which pressure drop and air velocity were continuously measured using pitot tubes, hot wire anemometers and digital manometers to determine the level of mesh screen clogging. Each trial was conducted at an initial airspeed of approximately 9.5 m/s, and post-run air velocity measurements were taken to measure air velocity loss. Previously optimized parameters for water flow rate and vibrational parameters were used in the tests (3 L/min for water flow rate, 30 dB for amplifier gain, and 134 Hz for frequency) [1]. Dust collection efficiency was determined by gravimetrically collecting dust-laden air samples upwind and downwind of the filter assembly. The collection efficiency, η, was calculated by:(1) where mds is downstream sample weight and mus is upstream sample weight.To investigate how altered mesh design affects system performance, 10-layer, 20-layer and 30-layer 316-stainless-steel woven mesh were used. The woven filter packages were made with varying surface hydrophobicities. There were three types of screens: an untreated screen, a hydrophilic enhanced screen and a hydrophobicized screen. Results and discussion To evaluate the combined influence of mesh density and induced vibration on system efficiency, several tests were conducted using the bench-scale dust scrubber system. An in-house and purpose-built filter assembly was used to modify the filter package thickness. Dust collection efficiency data from these tests are shown in Fig. 1. The data obtained from the dust collection efficiency calculations showed that regardless of what filter package was used when the mesh screen was enhanced with the vibration, better results were obtained in both wet and dry conditions compared to the tests conducted in static condition. The 30-layer wet vibration-enhanced operational condition was the most efficient, at 92 percent efficiency, followed by the 30-layer wet vibration-free state, at 89 percent efficiency. The 10-layer screen always had the lowest efficiency. While collection efficiency is one evaluation metric, it must be considered alongside other metrics of performance. As such, throughout the same tests, the pre-test and post-test conditions of the downstream airflow were monitored. The difference of these two values shows how much air velocity loss occurs through the test duration. When the detected loss in air velocity is greater, this implies greater mesh clogging and thus lower overall system efficiency. Figure 2 indicates the highest air velocity loss occurred in the test with a dry and vibration-free operational condition with 30-layer screen (38.1 percent). The conditions with the least air loss were the tests with a 10-layer screen in each operational condition. Although vibrating the mesh screen has been promising in many cases, it can be inferred from the combined data that there are a few situations where the static state is more advantageous. When all these data are combined, the performance of the 10-layer screen is remarkable in terms of air loss and dust accumulation on the filter. However, when system efficiency is also considered, significant decreases are observed in the 10-layer screen compared to the higher-layer filter packages in each operational situation. The reason for its lower overall efficiency is that the dust-laden air passes the 10-layer filter screen without getting captured by water droplets more easily than others. As less-dense screens cause an increase in the amount of material that can move downstream of the system, they are negatively affecting the system efficiency. When all the test results are considered together, the data show that the lowest air velocity and pressure loss and the lowest amount of mass accumulated on the screen is the test performed under the wet vibrating operational condition performed with a 20-layer filter. The surface of the filter panels can be coated in different ways to increase wettability and enhance particle-liquid adhesion. Data from different coating tests (Figs. 3 and 4) showed the hydrophilic surfaces imparted the highest collection efficiency, with the differences being more pronounced for the vibration-free conditions as opposed to the vibration-enhanced conditions. As anticipated, the dry conditions showed the highest air velocity loss, and in all cases, the hydrophilic mesh outperformed the other two. Overall, the data follow the same trend as for collection efficiency, with the difference between the various treatments being more pronounced in the vibration-free cases. Conclusions Tests were performed in a laboratory-scale scrubber unit using mesh screens with different surface treatments and filter media densities. The effects of various factors were measured in both wet and dry environments, as well as with and without vibrations. As part of the tests, air velocity, pressure drop, and weight of dust particles accumulated inside the mesh screen were measured. Overall, the results confirm the benefit of a vibrating mesh FBS. Moreover, the results show that the system can be further optimized through control of the mesh density and particularly the mesh hydrophobicity. Results with different mesh thicknesses generally confirm prior studies showing that higher mesh density increased dust collection; however, they also show higher clogging potential as demonstrated through both mass accumulation and pressure drop. Testing with different mesh surface modifications showed that optimal results were obtained with hydrophilic mesh surface, which exhibited reduced clogging, but similar dust collection as compared to the hydrophobic and untreated counterparts. Selected references Uluer ME, Shigo M, Amini SH, Noble A (2023) An exploratory investigation on the effectiveness of a novel vibration-enhanced flooded bed dust scrubber. Min Metall Explor 41:73–90 Lu Z, Rath A, Amini SH, Noble A, Shahab S (2022) A computational fluid dynamics investigation of a novel flooded-bed dust scrubber with vibrating mesh. Int J Min Sci Technol 32(3)525–537 Janjua AN, Shaefer M, Amini SH, Noble CA, Shahab S (2024) Vibrational energy transmission in underground continuous mining: Dynamic characteristics and experimental research of field data. Appl Energy 354(Part A):122220

Influence of mesh design and surface treatments on particle transport and fate in a vibration-enhanced flooded-bed dust scrubber

Authored by: Mahmud Esad Uluer and Aaron Noble

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