Improved drill shroud capture of respirable dust utilizing air nozzles underneath the drill deck

This study tests the ability of a new air nozzle system, located under the drill deck shroud, to improve the dust capture of the drill deck shroud. The system consists of three air nozzles supplied with regulated pressurized air and located midway down the shroud in the off-inlet corners of the shroud. Laboratory testing produced operating curves for the system that showed an optimum operating point of 207 kPa (30 psi) and generated respirable dust reductions of 48% to 52% from the drill deck shroud. Introduction Surface mine blasthole drills have been shown to generate large amounts of respirable dust during dry drilling. A past NIOSH study documented that time-weighted average dust concentrations from area sampling in the vicinity of the drill shroud can potentially range from 8.68 to 95.15 mg/m3 (Organiscak and Page, 1995). Another study corroborated these results, showing area sampling respirable dust concentrations ranging from 1.04 to 52.30 mg/m3 for dry drilling (Listak and Reed, 2007). These concentrations are highly variable and are dependent on the effectiveness of the dust controls used on the drill, with higher dust concentrations resulting from poor dust control methods. To put these dust concentrations into perspective, 2.0 mg/ m3 is the maximum allowable respirable dust concentration for exposure at coal mines. If the silica in the respirable dust sample is >5%, then the maximum allowable concentration is reduced to the quotient of 10 divided by the percent silica in the sample. Generally, emissions from drilling operations also contain silica, which continues to be an ongoing concern in the mining industry, as exposure to respirable crystalline silica dust can lead to silicosis. Silicosis is a respiratory disease that is often fatal and has no cure, except through its prevention (Porter and Kaplan, 2007). A review of the Mine Safety and Health Administration (MSHA) database of respirable dust samples containing silica from 2001 to 2004 shows that drilling operators and their helpers have some of the highest exposures to respirable silica dust, with overexposure rates of 19% and 14%, respectively. Additionally, the blaster or shotfirer, who often works near operating surface mine blasthole drills, has an overexposure rate of 9% for respirable silica 1 � dust (Joy, 2005). Wet drilling is an effective method to reduce dust concentrations, but wet drilling has disadvantages, such as water-freezing issues in northern climates and rotary bit life reductions caused by the use of water during drilling. Therefore, many drilling operations use a dust collector system to control dust emissions. When operating properly, dust collector systems are very efficient. However, as operating parameters change or if the system is poorly maintained, there can be a significant reduction in system efficiency (Listak and Reed, 2007). Therefore, additional dust control methods for dry drilling are being investigated. Dust from drilling operations generally emanates from three sources on the drill rig: the dust collector dump, the drill stem and the drill shroud (Maksimovic and Page, 1985). The focus of this work was directed toward eliminating dust emissions from the drill shroud. Prior research in this area focused on maximizing dust capture from the drill shroud through the evaluation of collector-to-bailing airflow ratios. This work also showed the importance of minimizing leakage from the drill shroud. Collector-to-bailing airflow ratios of 4:1 (i.e., a collector airflow four times the amount of bailing airflow) were shown to efficiently reduce dust concentrations from the shroud area. However, most drills operate with collector-to-bailing airflow ratios of 2:1 (Page and Organiscak, 2004). Therefore, further investigations were conducted to find additional methods to reduce concentrations. Based on the results of this research, a new method of using air nozzles underneath the drill deck shows promising results for minimizing dust concentrations caused by leakage from the shroud without having to modify the collector airflow. Background of the problem The drill deck or table, shown in Fig. 1, is located adjacent to the operator’s cab, which allows the operator access to perform maintenance on the drilling apparatus. Figure1—Drilldeckwithshroudshowing leakagebetween the bottom of the shroud and ground surface. This platform is generally 0.91 m (3 ft) above the ground level. The gap between the bottom of the drill deck and the ground surface, which can range from 0.61 to 1.22 m (2 to 4 ft), is enclosed using conveyor belting or similar material. This enclosure is part of the drill dust collection system, which helps to contain the dust generated during drilling. The dust collector, connected to the drill deck by large diameter wire-reinforced hose, removes dust from under the shroud. The inlet to the dust collection system is generally located on the drill deck at the back corner of the side opposite the operator’s cab. Dust leakage from the drill deck enclosure can occur at three different sources. Two of the sources that were evaluated in this study are leakage from gaps between the bottom of the shroud and the ground and from gaps at the corners of the enclosure. The other source of dust is from leakage around the table bushing where the drill steel goes through the drill deck. During the drilling operation, bailing air is sent through the center of the drill steel, exiting at openings on the drill bit. This bailing air is used to cool the drill bit and to flush the drill cuttings from the hole (Brantly, 1961). The air and the cuttings exit the drill hole at a high velocity. Once the air enters the drill deck enclosure, its velocity is reduced and the large drill cuttings drop out of the air stream. The respirable dust, because of its small size, continues with the airstream, as the reduction of velocity is generally not enough to allow these smaller particles to fall out. The suspended respirable dust leaks from the shroud, resulting in respirable dust emissions. This study evaluated the use of air nozzles for directing dusty air to the scrubber inlet to improve dust capture and reduce the 2 � amount of dust leaking from the shroud. Different air nozzle configurations underneath the shroud were tested on a drill shroud simulator that was constructed at NIOSH labs. Test facility NIOSH’s test facility used for the air nozzle testing has been thoroughly described in previous publications (Page and Organiscak, 2004; Organiscak and Page, 2005). This facility, which simulates the dust collection system of a blasthole drill rig, is used to evaluate dust control techniques for surface drills in a controlled laboratory environment. The laboratory environment includes a full-scale model of a drill deck and shroud as shown in Fig. 2. Figure 2 — Drill deck testing facility at NIOSH. The simulator is contained in a large dust chamber, which serves as a containment and monitoring area for the dust that escapes the shroud. To prevent dust from leaking outside of the chamber, static pressure in the chamber is controlled and maintained by a series of louvered vents located on the top of the chamber. The simulated drill deck and shroud, whose dimensions are 1.52 m wide, 1.22 m deep and 1.22 m high (5 by 4 by 4 ft), are located in the center of the dust chamber. This shroud size is within the range found on medium-sized rock drills (rubber-tired or track-mounted) that drill holes 127 to 203 mm (5 to 8 in.) in diameter, with about 178 to 222 kN (40,000 to 50,000 lbs) of drill pulldown pressure. The base of the shroud is fitted with hinged plywood slats that can be adjusted to simulate gaps between the ground and the shroud, from which dust escapes. The gaps can be set at a range of 51 to 356 mm (2 to 14 in.). Compressed air, using a Kaiser DSD125 air compressor capable of delivering 326 L/s at 758 kPa (690 cfm at 110 psi), is piped into the simulated drill steel to represent the bailing air of the drill rig during the drilling process. A 152-mm (6in.) steel pipe, which extends from 0.61 m (2 ft) above the simulated drill deck to 0.91 m (3 ft) beneath the floor of the test chamber, is used to simulate the drill steel. This 152-mm (6-in.) pipe is concentrically located in a 203-mm (8-in.) steel pipe. This 203-mm (8-in.) pipe simulates the drill hole with its opening at the chamber floor surface; it extends from the floor to 0.91 m (3 ft) beneath the floor of the chamber. The pressure and flow rate can be regulated to provide accurate bailing air velocities. To simulate the dust emissions from the borehole, a Vibra-Screw feeder delivers the amount of limestone rock dust (approximately 25 g/min) at a constant rate into a compressed air eductor, operating on a small separate split of air at 11.8 L/s (25 cfm). The limestone dust has a particle size distribution similar to the bulk dust at the dust collector dump point. Figure 3 shows the mass frequency of the particle size of the test material with the bulk dust collector material and illustrates the similarities in particle size distributions. Figure 3 — Mass frequency of particle diameters of limestone test material (dashed line) compared with bulk dust collector material (solid line). This dust is mixed with the remaining bailing airflow near the top of the 152-mm (6-in.) pipe and blown out the concentric opening between the 152-mm (6-in.) pipe and the 203-mm (8-in.) pipe. The respirable dust concentrations escaping the shrouded area over this simulated drill hole is the response variable measured during these experiments, while maintaining a constant feed rate with the Vibra-Screw feeder. As with a rotary drill rig, the simulator’s dust collector provides the negative pressure within the shroud to collect the dust as it is emitted from the hole at the base of the chamber. The dust collector was simulated using a baghouse connected through fiberglass tubing. The final connection to the drill deck was accomplished using 203-mm(8-in.-) diameter wire-reinforced tubing, which connected using a metal transition from 203mm(8-in.-) diameter to a 457by 190.5-mm (18by 7.5-in.) rectangular opening in the drill deck. The collector is capable of providing variable flow rates up to a maximum flow of 944 L/s (2,000 cfm). The collector’s inlet is located in the corner of the simulator, similar to those found on drills in the field. Preliminary testing Preliminary experiments were conducted to determine the viability and the optimum placement of the air nozzles for maximized dust capture of the drill shroud and to narrow these variable ranges for a factorial design experiment. Variables investigated included spray location, orientation, quantity, type and pressure. The air nozzles evaluated were flat-fan and hollow-cone plastic air nozzles (Spraying Systems Co. WindJet Models AA727-11 and AA707-11, respectively). These nozzles were chosen because they do not require large quantities of air from the compressor and thus are able to generate airflow without affecting the bailing airflow quantity. Dust measurements were made with two RAM sampling devices located outside of the shroud on the front (on-inlet) and back (off-inlet) sides. The locations are shown in Fig. 4 and are labeled as RAM-Front and RAM-Back. Figure 4— Plan view of shroud inside the drill deck testing facility showing the locations of the gravimentric sampling locations(numberedlocations)andinstantaneoussampling locations (RAM-Front and RAM-Back). The locations of the gravimetric sampling, which was not conducted during the preliminary testing, are also shown and are numbered consecutively from 1 to 8. The RAM data were fed into a chart recorder and a data recorder to provide an instantaneous visual representation of dust concentrations and to allow downloading of data for post-test analysis. The preliminary tests followed an ABAB pattern, where A was the spray-off condition and B was the spray-on condition, with each segment lasting 10 minutes. Initially, horizontal spray bars containing four in-line nozzles were tested at various locations underneath the shroud, as shown in Fig. 5 (a). The variables considered in this evaluation were one and two spray bars (four or eight nozzles, respectively), hollow-cone and flat-fan nozzles and air pressures ranging from 34 to 276 kPa (5 to 40 psi). None of the locations tested produced consistent significant reductions of respirable dust outside of the shroud. An attempt was made to push all the airflow from the off-inlet side towards the collector by using 20 air nozzles in a grid pattern on the off-inlet wall, which is opposite the collector inlet, shown in Fig. 5 (b). This evaluation utilized hollow-cone sprays with air pressures ranging from 138 to 552 kPa (20 to 80 psi). This configuration also resulted in unsatisfactory results, sometimes producing more respirable dust emissions, especially at higher pressures. Another configuration placed a set of four hollow-cone nozzles directed into the collector inlet, shown in Fig. 5 (c), at pressures ranging from 34 to 276 kPa (5 to 40 psi). These operating pressures produced dust reductions at the 69 and 138 kPa (10 and 20 psi) levels. However, these reductions were inconsistent and varied from 3.0% to 26.0%, with the best reductions